Network Working Group G. Pelletier
Request for Comments: 5225 K. Sandlund
Category: Standards Track Ericsson
April 2008
RObust Header Compression Version 2 (ROHCv2):
Profiles for RTP, UDP, IP, ESP and UDP-Lite
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document specifies ROHC (Robust Header Compression) profiles
that efficiently compress RTP/UDP/IP (Real-Time Transport Protocol,
User Datagram Protocol, Internet Protocol), RTP/UDP-Lite/IP (User
Datagram Protocol Lite), UDP/IP, UDP-Lite/IP, IP and ESP/IP
(Encapsulating Security Payload) headers.
This specification defines a second version of the profiles found in
RFC 3095, RFC 3843 and RFC 4019; it supersedes their definition, but
does not obsolete them.
The ROHCv2 profiles introduce a number of simplifications to the
rules and algorithms that govern the behavior of the compression
endpoints. It also defines robustness mechanisms that may be used by
a compressor implementation to increase the probability of
decompression success when packets can be lost and/or reordered on
the ROHC channel. Finally, the ROHCv2 profiles define their own
specific set of header formats, using the ROHC formal notation.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Background (Informative) . . . . . . . . . . . . . . . . . . 7
4.1. Classification of Header Fields . . . . . . . . . . . . . 7
4.2. Improvements of ROHCv2 over RFC 3095 Profiles . . . . . . 8
4.3. Operational Characteristics of ROHCv2 Profiles . . . . . 10
5. Overview of the ROHCv2 Profiles (Informative) . . . . . . . . 10
5.1. Compressor Concepts . . . . . . . . . . . . . . . . . . . 11
5.1.1. Optimistic Approach . . . . . . . . . . . . . . . . . 11
5.1.2. Tradeoff between Robustness to Losses and to
Reordering . . . . . . . . . . . . . . . . . . . . . 11
5.1.3. Interactions with the Decompressor Context . . . . . 13
5.2. Decompressor Concepts . . . . . . . . . . . . . . . . . . 14
5.2.1. Decompressor State Machine . . . . . . . . . . . . . 14
5.2.2. Decompressor Context Management . . . . . . . . . . . 17
5.2.3. Feedback Logic . . . . . . . . . . . . . . . . . . . 19
6. ROHCv2 Profiles (Normative) . . . . . . . . . . . . . . . . . 19
6.1. Channel Parameters, Segmentation, and Reordering . . . . 19
6.2. Profile Operation, Per-context . . . . . . . . . . . . . 20
6.3. Control Fields . . . . . . . . . . . . . . . . . . . . . 21
6.3.1. Master Sequence Number (MSN) . . . . . . . . . . . . 21
6.3.2. Reordering Ratio . . . . . . . . . . . . . . . . . . 21
6.3.3. IP-ID Behavior . . . . . . . . . . . . . . . . . . . 22
6.3.4. UDP-Lite Coverage Behavior . . . . . . . . . . . . . 22
6.3.5. Timestamp Stride . . . . . . . . . . . . . . . . . . 22
6.3.6. Time Stride . . . . . . . . . . . . . . . . . . . . . 22
6.3.7. CRC-3 for Control Fields . . . . . . . . . . . . . . 23
6.4. Reconstruction and Verification . . . . . . . . . . . . . 23
6.5. Compressed Header Chains . . . . . . . . . . . . . . . . 24
6.6. Header Formats and Encoding Methods . . . . . . . . . . . 25
6.6.1. baseheader_extension_headers . . . . . . . . . . . . 26
6.6.2. baseheader_outer_headers . . . . . . . . . . . . . . 26
6.6.3. inferred_udp_length . . . . . . . . . . . . . . . . . 26
6.6.4. inferred_ip_v4_header_checksum . . . . . . . . . . . 26
6.6.5. inferred_mine_header_checksum . . . . . . . . . . . . 27
6.6.6. inferred_ip_v4_length . . . . . . . . . . . . . . . . 28
6.6.7. inferred_ip_v6_length . . . . . . . . . . . . . . . . 28
6.6.8. Scaled RTP Timestamp Compression . . . . . . . . . . 29
6.6.9. timer_based_lsb . . . . . . . . . . . . . . . . . . . 30
6.6.10. inferred_scaled_field . . . . . . . . . . . . . . . . 31
6.6.11. control_crc3_encoding . . . . . . . . . . . . . . . . 32
6.6.12. inferred_sequential_ip_id . . . . . . . . . . . . . . 33
6.6.13. list_csrc(cc_value) . . . . . . . . . . . . . . . . . 34
6.7. Encoding Methods with External Parameters as Arguments . 38
6.8. Header Formats . . . . . . . . . . . . . . . . . . . . . 40
6.8.1. Initialization and Refresh Header Format (IR) . . . . 40
6.8.2. Compressed Header Formats (CO) . . . . . . . . . . . 41
6.9. Feedback Formats and Options . . . . . . . . . . . . . . 100
6.9.1. Feedback Formats . . . . . . . . . . . . . . . . . . 100
6.9.2. Feedback Options . . . . . . . . . . . . . . . . . . 102
7. Security Considerations . . . . . . . . . . . . . . . . . . . 104
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 105
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 105
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 106
10.1. Normative References . . . . . . . . . . . . . . . . . . 106
10.2. Informative References . . . . . . . . . . . . . . . . . 107
Appendix A. Detailed Classification of Header Fields . . . . . 108
A.1. IPv4 Header Fields . . . . . . . . . . . . . . . . . . . 109
A.2. IPv6 Header Fields . . . . . . . . . . . . . . . . . . . 112
A.3. UDP Header Fields . . . . . . . . . . . . . . . . . . . 113
A.4. UDP-Lite Header Fields . . . . . . . . . . . . . . . . . 114
A.5. RTP Header Fields . . . . . . . . . . . . . . . . . . . . 115
A.6. ESP Header Fields . . . . . . . . . . . . . . . . . . . . 117
A.7. IPv6 Extension Header Fields . . . . . . . . . . . . . . 117
A.8. GRE Header Fields . . . . . . . . . . . . . . . . . . . . 118
A.9. MINE Header Fields . . . . . . . . . . . . . . . . . . . 119
A.10. AH Header Fields . . . . . . . . . . . . . . . . . . . . 120
Appendix B. Compressor Implementation Guidelines . . . . . . . 121
B.1. Reference Management . . . . . . . . . . . . . . . . . . 121
B.2. Window-based LSB Encoding (W-LSB) . . . . . . . . . . . 121
B.3. W-LSB Encoding and Timer-based Compression . . . . . . . 122
1. Introduction
The ROHC WG has developed a header compression framework on top of
which various profiles can be defined for different protocol sets or
compression requirements. The ROHC framework was first documented in
[RFC3095], together with profiles for compression of RTP/UDP/IP
(Real-Time Transport Protocol, User Datagram Protocol, Internet
Protocol), UDP/IP, IP and ESP/IP (Encapsulating Security Payload)
headers. Additional profiles for compression of IP headers [RFC3843]
and UDP-Lite (User Datagram Protocol Lite) headers [RFC4019] were
later specified to complete the initial set of ROHC profiles.
This document defines an updated version for each of the above
mentioned profiles, and the definitions depend on the ROHC framework
as found in [RFC4995]. The framework is required reading to
understand the profile definitions, rules, and their role.
Specifically, this document defines header compression schemes for:
o RTP/UDP/IP : profile 0x0101
o UDP/IP : profile 0x0102
o ESP/IP : profile 0x0103
o IP : profile 0x0104
o RTP/UDP-Lite/IP : profile 0x0107
o UDP-Lite/IP : profile 0x0108
Each of the profiles above can compress the following type of
extension headers:
o AH [RFC4302]
o GRE [RFC2784][RFC2890]
o MINE [RFC2004]
o IPv6 Destination Options header[RFC2460]
o IPv6 Hop-by-hop Options header[RFC2460]
o IPv6 Routing header [RFC2460]
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
This document is consistent with the terminology found in the ROHC
framework [RFC4995] and in the formal notation for ROHC [RFC4997].
In addition, this document uses or defines the following terms:
Acknowledgment Number
The Acknowledgment Number identifies what packet is being
acknowledged in the RoHCv2 feedback element (See Section 6.9).
The value of this field normally corresponds to the Master
Sequence Number (MSN) of the header that was last successfully
decompressed, for the compression context (CID) for which the
feedback information applies.
Chaining of Items
A chain of items groups fields based on similar characteristics.
ROHCv2 defines chain items for static, dynamic and irregular
fields. Chaining is achieved by appending an item to the chain
for each header in its order of appearance in the uncompressed
packet. Chaining is useful to construct compressed headers from
an arbitrary number of any of the protocol headers for which a
ROHCv2 profile defines a compressed format.
CRC-3 Control Fields Validation
The CRC-3 control fields validation refers to the validation of
the control fields. This validation is performed by the
decompressor when it receives a Compressed (CO) header that
contains a 3-bit Cyclic Redundancy Check (CRC) calculated over
control fields. This 3-bit CRC covers controls fields carried in
the CO header as well as specific control fields in the context.
In the formal definition of the header formats, this 3-bit CRC is
labeled "control_crc3" and uses the control_crc3_encoding (See
also Section 6.6.11).
Delta
The delta refers to the difference in the absolute value of a
field between two consecutive packets being processed by the same
compression endpoint.
Reordering Depth
The number of packets by which a packet is received late within
its sequence due to reordering between the compressor and the
decompressor, i.e., reordering between packets associated with the
same context (CID). See the definition of sequentially late
packet below.
ROHCv2 Header Types
ROHCv2 profiles use two different header types: the Initialization
and Refresh (IR) header type, and the Compressed (CO) header type.
Sequentially Early Packet
A packet that reaches the decompressor before one or several
packets that were delayed over the channel, where all of the said
packets belong to the same header-compressed flow and are
associated to the same compression context (CID). At the time of
the arrival of a sequentially early packet, the packet(s) delayed
on the link cannot be differentiated from lost packet(s).
Sequentially Late Packet
A packet is late within its sequence if it reaches the
decompressor after one or several other packets belonging to the
same CID have been received, although the sequentially late packet
was sent from the compressor before the other packet(s). How the
decompressor detects a sequentially late packet is outside the
scope of this specification, but it can for example use the MSN
for this purpose.
Timestamp Stride (ts_stride)
The timestamp stride (ts_stride) is the expected increase in the
timestamp value between two RTP packets with consecutive sequence
numbers. For example, for a media encoding with a sample rate of
8 kHz producing one frame every 20 ms, the RTP timestamp will
typically increase by n * 160 (= 8000 * 0.02), for some integer n.
Time Stride (time_stride)
The time stride (time_stride) is the time interval equivalent to
one ts_stride, e.g., 20 ms in the example for the RTP timestamp
increment above.
3. Acronyms
This section lists most acronyms used for reference, in addition to
those defined in [RFC4995].
AH Authentication Header.
ESP Encapsulating Security Payload.
GRE Generic Routing Encapsulation.
FC Full Context state (decompressor).
IP Internet Protocol.
LSB Least Significant Bits.
MINE Minimal Encapsulation in IP.
MSB Most Significant Bits.
MSN Master Sequence Number.
NC No Context state (decompressor).
OA Optimistic Approach.
RC Repair Context state (decompressor).
ROHC Header compression framework (RFC 4995).
ROHCv2 Set of header compression profiles defined in this document.
RTP Real-time Transport Protocol.
SSRC Synchronization source. Field in RTP header.
CSRC Contributing source. The RTP header contains an optional
list of contributing sources.
TC Traffic Class. Field in the IPv6 header. See also TOS.
TOS Type Of Service. Field in the IPv4 header. See also TC.
TS RTP Timestamp.
TTL Time to Live. Field in the IPv4 header.
UDP User Datagram Protocol.
UDP-Lite User Datagram Protocol Lite.
4. Background (Informative)
This section provides background information on the compression
profiles defined in this document. The fundamentals of general
header compression and of the ROHC framework may be found in sections
3 and 4 of [RFC4995], respectively. The fundamentals of the formal
notation for ROHC are defined in [RFC4997]. [RFC4224] describes the
impacts of out-of-order delivery on profiles based on [RFC3095].
4.1. Classification of Header Fields
Section 3.1 of [RFC4995] explains that header compression is possible
due to the fact that there is much redundancy between field values
within the headers of a packet, especially between the headers of
consecutive packets.
Appendix A lists and classifies in detail all the header fields
relevant to this document. The appendix concludes with
recommendations on how the various fields should be handled by header
compression algorithms.
The main conclusion is that most of the header fields can easily be
compressed away since they never or seldom change. A small number of
fields however need more sophisticated mechanisms.
These fields are:
- IPv4 Identification (16 bits) - IP-ID
- ESP Sequence Number (32 bits) - ESP SN
- UDP Checksum (16 bits) - Checksum
- UDP-Lite Checksum (16 bits) - Checksum
- UDP-Lite Checksum Coverage (16 bits) - CCov
- RTP Marker ( 1 bit ) - M-bit
- RTP Sequence Number (16 bits) - RTP SN
- RTP Timestamp (32 bits) - TS
In particular, for RTP, the analysis in Appendix A reveals that the
values of the RTP Timestamp (TS) field usually have a strong
correlation to the RTP Sequence Number (SN), which increments by one
for each packet emitted by an RTP source. The RTP M-bit is expected
to have the same value most of the time, but it needs to be
communicated explicitly on occasion.
For UDP, the Checksum field cannot be inferred or recalculated at the
receiving end without violating its end-to-end properties, and is
thus sent as-is when enabled (mandatory with IPv6). The same applies
to the UDP-Lite Checksum (mandatory with both IPv4 and IPv6), while
the UDP-Lite Checksum Coverage may in some cases be compressible.
For IPv4, a similar correlation as that of the RTP TS to the RTP SN
is often observed between the Identifier field (IP-ID) and the master
sequence number (MSN) used for compression (e.g., the RTP SN when
compressing RTP headers).
4.2. Improvements of ROHCv2 over RFC 3095 Profiles
The ROHCv2 profiles can achieve compression efficiency and robustness
that are both at least equivalent to RFC 3095 profiles [RFC3095],
when used under the same operating conditions. In particular, the
size and bit layout of the smallest compressed header (i.e., PT-0
format U/O-0 in RFC 3095, and pt_0_crc3 in ROHCv2) are identical.
There are a number of differences and improvements between profiles
defined in this document and their earlier version defined in RFC
3095. This section provides an overview of some of the most
significant improvements:
Tolerance to reordering
Profiles defined in RFC 3095 require that the channel between
compressor and decompressor provide in-order delivery between
compression endpoints. ROHCv2 profiles, however, can handle
robustly and efficiently a limited amount of reordering after the
compression point as part of the compression algorithm itself. In
addition, this improved support for reordering makes it possible
for ROHCv2 profiles to handle prelink reordering more efficiently.
Operational logic
Profiles in RFC 3095 define multiple operational modes, each with
different updating logic and compressed header formats. ROHCv2
profiles operate in unidirectional operation until feedback is
first received for a context (CID), at which point bidirectional
operation is used; the formats are independent of what operational
logic is used.
IP extension header
Profiles in RFC 3095 compress IP Extension headers using list
compression. ROHCv2 profiles instead treat extension headers in
the same manner as other protocol headers, i.e., using the
chaining mechanism; it thus assumes that extension headers are not
added or removed during the lifetime of a context (CID), otherwise
compression has to be restarted for this flow.
IP encapsulation
Profiles in RFC 3095 can compress at most two levels of IP
headers. ROHCv2 profiles can compress an arbitrary number of IP
headers.
List compression
ROHCv2 profiles do not support reference-based list compression.
Robustness and repairs
ROHCv2 profiles do not define a format for the IR-DYN packet;
instead, each profile defines a compressed header that can be used
to perform a more robust context repair using a 7-bit CRC
verification. This also implies that only the IR header can
change the association between a CID and the profile it uses.
Feedback
ROHCv2 profiles mandate a CRC in the format of the FEEDBACK-2,
while this is optional in RFC 3095. A different set of feedback
options is also used in ROHCv2 compared to RFC 3095.
4.3. Operational Characteristics of ROHCv2 Profiles
Robust header compression can be used over different link
technologies. Section 4.4 of [RFC4995] lists the operational
characteristics of the ROHC channel. The ROHCv2 profiles address a
wide range of applications, and this section summarizes some of the
operational characteristics that are specific to these profiles.
Packet length
ROHCv2 profiles assume that the lower layer indicates the length
of a compressed packet. ROHCv2 compressed headers do not contain
length information for the payload.
Out-of-order delivery between compression endpoints
The definition of the ROHCv2 profiles places no strict requirement
on the delivery sequence between the compression endpoints, i.e.,
packets may be received in a different order than the compressor
has sent them and still have a fair probability of being
successfully decompressed.
However, frequent out-of-order delivery and/or significant
reordering depth will negatively impact the compression
efficiency. More specifically, if the compressor can operate
using a proper estimate of the reordering characteristics of the
path between the compression endpoints, larger headers can be sent
more often to increase the robustness against decompression
failures due to out-of-order delivery. Otherwise, the compression
efficiency will be impaired from an increase in the frequency of
decompression failures and recovery attempts.
5. Overview of the ROHCv2 Profiles (Informative)
This section provides an overview of concepts that are important and
useful to the ROHCv2 profiles. These concepts may be used as
guidelines for implementations but they are not part of the normative
definition of the profiles, as these concepts relate to the
compression efficiency of the protocol without impacting the
interoperability characteristics of an implementation.
5.1. Compressor Concepts
Header compression can be conceptually characterized as the
interaction of a compressor with a decompressor state machine, one
per context. The responsibility of the compressor is to convey the
information needed to successfully decompress a packet, based on a
certain confidence regarding the state of the decompressor context.
This confidence is obtained from the frequency and the type of
information the compressor sends when updating the decompressor
context from the optimistic approach (Section 5.1.1), and optionally
from feedback messages (See Section 6.9), received from the
decompressor.
5.1.1. Optimistic Approach
A compressor always uses the optimistic approach when it performs
context updates. The compressor normally repeats the same type of
update until it is fairly confident that the decompressor has
successfully received the information. If the decompressor
successfully receives any of the headers containing this update, the
state will be available for the decompressor to process smaller
compressed headers.
If field X in the uncompressed header changes value, the compressor
uses a header type that contains an encoding of field X until it has
gained confidence that the decompressor has received at least one
packet containing the new value for X. The compressor normally
selects a compressed format with the smallest header that can convey
the changes needed to achieve confidence.
The number of repetitions that is needed to obtain this confidence is
normally related to the packet loss and out-of-order delivery
characteristics of the link where header compression is used; it is
thus not defined in this document. It is outside the scope of this
specification and is left to implementors to decide.
5.1.2. Tradeoff between Robustness to Losses and to Reordering
The ability of a header compression algorithm to handle sequentially
late packets is mainly limited by two factors: the interpretation
interval offset of the sliding window used for lsb encoded fields
[RFC4997], and the optimistic approach (See Section 5.1.1) for seldom
changing fields.
lsb encoded fields:
The interpretation interval offset specifies an upper limit for
the maximum reordering depth, by which is it possible for the
decompressor to recover the original value of a dynamically
changing (i.e., sequentially incrementing) field that is encoded
using a window-based lsb encoding. Its value is typically bound
to the number of lsb compressed bits in the compressed header
format, and thus grows with the number of bits transmitted.
However, the offset and the lsb encoding only provide robustness
for the field that it compresses, and (implicitly) for other
sequentially changing fields that are derived from that field.
This is shown in the figure below:
<--- interpretation interval (size is 2^k) ---->
|------------------+---------------------------|
v_ref-p v_ref v_ref + (2^k-1) - p
Lower Upper
Bound Bound
<--- reordering --> <--------- losses --------->
where p is the maximum negative delta, corresponding to the
maximum reordering depth for which the lsb encoding can recover
the original value of the field;
where (2^k-1) - p is the maximum positive delta, corresponding
to the maximum number of consecutive losses for which the lsb
encoding can recover the original value of the field;
where v_ref is the reference value, as defined in the lsb
encoding method in [RFC4997].
There is thus a tradeoff between the robustness against reordering
and the robustness against packet losses, with respect to the
number of MSN bits needed and the distribution of the
interpretation interval between negative and positive deltas in
the MSN.
Seldom changing fields
The optimistic approach (Section 5.1.1) provides the upper limit
for the maximum reordering depth for seldom changing fields.
There is thus a tradeoff between compression efficiency and
robustness. When only information on the MSN needs to be conveyed to
the decompressor, the tradeoff relates to the number of compressed
MSN bits in the compressed header format. Otherwise, the tradeoff
relates to the implementation of the optimistic approach.
In particular, compressor implementations should adjust their
optimistic approach strategy to match both packet loss and reordering
characteristics of the link over which header compression is applied.
For example, the number of repetitions for each update of a non-lsb
encoded field can be increased. The compressor can ensure that each
update is repeated until it is reasonably confident that at least one
packet containing the change has reached the decompressor before the
first packet sent after this sequence.
5.1.3. Interactions with the Decompressor Context
The compressor normally starts compression with the initial
assumption that the decompressor has no useful information to process
the new flow, and sends Initialization and Refresh (IR) packets.
Initially, when sending the first IR packet for a compressed flow,
the compressor does not expect to receive feedback for that flow,
until such feedback is first received. At this point, the compressor
may then assume that the decompressor will continue to send feedback
in order to repair its context when necessary. The former is
referred to as unidirectional operation, while the latter is called
bidirectional operation.
The compressor can then adjust the compression level (i.e., what
header format it selects) based on its confidence that the
decompressor has the necessary information to successfully process
the compressed headers that it selects.
In other words, the responsibilities of the compressor are to ensure
that the decompressor operates with state information that is
sufficient to successfully decompress the type of compressed
header(s) it receives, and to allow the decompressor to successfully
recover that state information as soon as possible otherwise. The
compressor therefore selects the type of compressed header based on
the following factors:
o the outcome of the encoding method applied to each field;
o the optimistic approach, with respect to the characteristics of
the channel;
o the type of operation (unidirectional or bidirectional), and if in
bidirectional operation, feedback received from the decompressor
(ACKs, NACKs, STATIC-NACK, and options).
Encoding methods normally use previous value(s) from a history of
packets whose headers it has previously compressed. The optimistic
approach is meant to ensure that at least one compressed header
containing the information to update the state for a field is
received. Finally, feedback indicates what actions the decompressor
has taken with respect to its assumptions regarding the validity of
its context (Section 5.2.2); it indicates what type of compressed
header the decompressor can or cannot decompress.
The decompressor has the means to detect decompression failures for
any compressed (CO) header format, using the CRC verification.
Depending on the frequency and/or on the type of the failure, it
might send a negative acknowledgement (NACK) or an explicit request
for a complete context update (STATIC-NACK). However, the
decompressor does not have the means to identify the cause of the
failure, and in particular the decompression of what field(s) is
responsible for the failure. The compressor is thus always
responsible for determining the most suitable response to a negative
acknowledgement, using the confidence it has in the state of the
decompressor context, when selecting the type of compressed header it
will use when compressing a header.
5.2. Decompressor Concepts
The decompressor normally uses the last received and successfully
validated (IR packets) or verified (CO packets) header as the
reference for future decompression.
The decompressor is responsible for verifying the outcome of every
decompression attempt, to update its context when successful, and
finally to request context repairs by making coherent usage of
feedback once it has started using feedback.
Specifically, the outcome of every decompression attempt is verified
using the CRC present in the compressed header; the decompressor
updates the context information when this outcome is successfully
verified; finally, if the decompressor uses feedback once for a
compressed flow, then it will continue to do so for as long as the
corresponding context is associated with the same profile.
5.2.1. Decompressor State Machine
The decompressor operation may be represented as a state machine
defining three states: No Context (NC), Repair Context (RC), and Full
Context (FC).
The decompressor starts without a valid context, the NC state. Upon
receiving an IR packet, the decompressor validates the integrity of
its header using the CRC-8 validation. If the IR header is
successfully validated, the decompressor updates the context and uses
this header as the reference header, and moves to the FC state. Once
the decompressor state machine has entered the FC state, it does not
normally leave; only repeated decompression failures will force the
decompressor to transit downwards to a lower state. When context
damage is detected, the decompressor moves to the repair context (RC)
state, where it stays until it successfully verifies a decompression
attempt for a compressed header with a 7-bit CRC or until it
successfully validates an IR header. When static context damage is
detected, the decompressor moves back to the NC state.
Below is the state machine for the decompressor. Details of the
transitions between states and decompression logic are given in the
sub-sections following the figure.
CRC-8(IR) Validation
+----->----->----->----->----->----->----->----->----->----->----+
| CRC-8(IR) |
| !CRC-8(IR) or CRC-7(CO) or or CRC-7(CO) |
| PT not allowed CRC-8(IR) or CRC-3(CO) |
| +--->---+ +--->----->----->----->---+ +--->---->---+ |
| | | | | | | |
| | v | v | v v
+-----------------+ +----------------------+ +--------------------+
| No Context (NC) | | Repair Context (RC) | | Full Context (FC) |
+-----------------+ +----------------------+ +--------------------+
^ ^ Static Context | ^ !CRC-7(CO) or | ^ Context Damage | |
| | Damage Detected | | PT not allowed | | Detected | |
| +--<-----<-----<--+ +----<------<----+ +--<-----<-----<--+ |
| |
| Static Context Damage Detected |
+--<-----<-----<-----<-----<-----<-----<-----<-----<---------+
where:
CRC-8(IR) : Successful CRC-8 validation for the IR header.
!CRC-8(IR) : Unsuccessful CRC-8 validation for the IR header.
CRC-7(CO) and/or
CRC-3(CO) : Successful CRC verification for the decompression
of a CO header, based on the number of CRC bits
carried in the CO header.
!CRC-7(CO) : Failure to CRC verify the decompression of a CO
header carrying a 7-bit CRC.
PT not allowed : The decompressor has received a packet type (PT)
for which the decompressor's current context does
not provide enough valid state information to
decompress the packet.
Static Context Damage Detected: See definition in Section 5.2.2.
Context Damage Detected: See definition in Section 5.2.2.
5.2.1.1. No Context (NC) State
Initially, while working in the No Context (NC) state, the
decompressor has not yet successfully validated an IR header.
Attempting decompression:
In the NC state, only packets carrying sufficient information on
the static fields (i.e., IR packets) can be decompressed.
Upward transition:
The decompressor can move to the Full Context (FC) state when the
CRC validation of an 8-bit CRC in an IR header is successful.
Feedback logic:
In the NC state, the decompressor should send a STATIC-NACK if a
packet of a type other than IR is received, or if an IR header has
failed the CRC-8 validation, subject to the feedback rate
limitation as described in Section 5.2.3.
5.2.1.2. Repair Context (RC) State
In the Repair Context (RC) state, the decompressor has successfully
decompressed packets for this context, but does not have confidence
that the entire context is valid.
Attempting decompression:
In the RC state, only headers covered by an 8-bit CRC (i.e., IR)
or CO headers carrying a 7-bit CRC can be decompressed.
Upward transition:
The decompressor can move to the Full Context (FC) state when the
CRC verification succeeds for a CO header carrying a 7-bit CRC or
when validation of an 8-bit CRC in an IR header succeeds.
Downward transition:
The decompressor moves back to the NC state if it assumes static
context damage.
Feedback logic:
In the RC state, the decompressor should send a STATIC-NACK when
CRC-8 validation of an IR header fails, or when a CO header
carrying a 7-bit CRC fails and static context damage is assumed,
subject to the feedback rate limitation as described in
Section 5.2.3. If any other packet type is received, the
decompressor should treat it as a CRC verification failure to
determine if NACK is to be sent.
5.2.1.3. Full Context (FC) State
In the Full Context (FC) state, the decompressor assumes that its
entire context is valid.
Attempting decompression:
In the FC state, decompression can be attempted regardless of the
type of packet received.
Downward transition:
The decompressor moves back to the RC state if it assumes context
damage. If the decompressor assumes static context damage, it
moves directly to the NC state.
Feedback logic:
In the FC state, the decompressor should send a NACK when CRC-8
validation or CRC verification of any header type fails and if
context damage is assumed, or it should send a STATIC-NACK if
static context damage is assumed; this is subject to the feedback
rate limitation described in Section 5.2.3.
5.2.2. Decompressor Context Management
All header formats carry a CRC and are context updating. A packet
for which the CRC succeeds updates the reference values of all header
fields, either explicitly (from the information about a field carried
within the compressed header) or implicitly (fields inferred from
other fields).
The decompressor may assume that some or the entire context is
invalid, when it fails to validate or to verify one or more headers
using the CRC. Because the decompressor cannot know the exact
reason(s) for a CRC failure or what field caused it, the validity of
the context hence does not refer to what specific part(s) of the
context is deemed valid or not.
Validity of the context rather relates to the detection of a problem
with the context. The decompressor first assumes that the type of
information that most likely caused the failure(s) is the state that
normally changes for each packet, i.e., context damage of the dynamic
part of the context. Upon repeated decompression failures and
unsuccessful repairs, the decompressor then assumes that the entire
context, including the static part, needs to be repaired, i.e.,
static context damage. Failure to validate the 3-bit CRC that
protects control fields should be treated as a decompression failure
when the decompressor asserts the validity of its context.
Context Damage Detection
The assumption of context damage means that the decompressor will
not attempt decompression of a CO header that carries only a 3-bit
CRC, and will only attempt decompression of IR headers or CO
headers protected by a CRC-7.
Static Context Damage Detection
The assumption of static context damage means that the
decompressor refrains from attempting decompression of any type of
header other than the IR header.
How these assumptions are made, i.e., how context damage is detected,
is open to implementations. It can be based on the residual error
rate, where a low error rate makes the decompressor assume damage
more often than on a high rate link.
The decompressor implements these assumptions by selecting the type
of compressed header for which it will attempt decompression. In
other words, validity of the context refers to the ability of a
decompressor to attempt (or not) decompression of specific packet
types.
When ROHCv2 profiles are used over a channel that cannot guarantee
in-order delivery, the decompressor may refrain from updating its
context with the content of a sequentially late packet that is
successfully decompressed. This is to avoid updating the context
with information that is older than what the decompressor already has
in its context.
5.2.3. Feedback Logic
ROHCv2 profiles may be used in environments with or without feedback
capabilities from decompressor to compressor. ROHCv2 however assumes
that if a ROHC feedback channel is available and if this channel is
used at least once by the decompressor for a specific context, this
channel will be used during the entire compression operation for that
context (i.e., bidirectional operation).
The ROHC framework defines 3 types of feedback messages: ACKs, NACKs,
and STATIC-NACKs. The semantics of each message is defined in
Section 5.2.4.1. of [RFC4995]. What feedback to send is coupled with
the context management of the decompressor, i.e., with the
implementation of the context damage detection algorithms as
described in Section 5.2.2.
The decompressor should send a NACK when it assumes context damage,
and it should send a STATIC-NACK when it assumes static context
damage. The decompressor is not strictly expected to send ACK
feedback upon successful decompression, other than for the purpose of
improving compression efficiency.
When ROHCv2 profiles are used over a channel that cannot guarantee
in-order delivery, the decompressor may refrain from sending ACK
feedback for a sequentially late packet that is successfully
decompressed.
The decompressor should limit the rate at which it sends feedback,
for both ACKs and STATIC-NACK/NACKs, and should avoid sending
unnecessary duplicates of the same type of feedback message that may
be associated with the same event.
6. ROHCv2 Profiles (Normative)
6.1. Channel Parameters, Segmentation, and Reordering
The compressor MUST NOT use ROHC segmentation (see Section 5.2.5 of
[RFC4995]), i.e., the Maximum Reconstructed Reception Unit (MRRU)
MUST be set to 0, if the configuration of the ROHC channel contains
at least one ROHCv2 profile in the list of supported profiles (i.e.,
the PROFILES parameter) and if the channel cannot guarantee in-order
delivery of packets between compression endpoints.
6.2. Profile Operation, Per-context
ROHCv2 profiles operate differently, per context, depending on how
the decompressor makes use of the feedback channel, if any. Once the
decompressor uses the feedback channel for a context, it establishes
the feedback channel for that CID.
The compressor always starts with the assumption that the
decompressor will not send feedback when it initializes a new context
(see also the definition of a new context in Section 5.1.1. of
[RFC4995], i.e., there is no established feedback channel for the new
context. At this point, despite the use of the optimistic approach,
decompression failure is still possible because the decompressor may
not have received sufficient information to correctly decompress the
packets; therefore, until the decompressor has established a feedback
channel, the compressor SHOULD periodically send IR packets. The
periodicity can be based on timeouts, on the number of compressed
packets sent for the flow, or any other strategy the implementer
chooses.
The reception of either positive feedback (ACKs) or negative feedback
(NACKs or STATIC-NACKs) from the decompressor establishes the
feedback channel for the context (CID) for which the feedback was
received. Once there is an established feedback channel for a
specific context, the compressor can make use of this feedback to
estimate the current state of the decompressor. This helps to
increase the compression efficiency by providing the information
needed for the compressor to achieve the necessary confidence level.
When the feedback channel is established, it becomes superfluous for
the compressor to send periodic refreshes, and instead it can rely
entirely on the optimistic approach and feedback from the
decompressor.
The decompressor MAY send positive feedback (ACKs) to initially
establish the feedback channel for a particular flow. Either
positive feedback (ACKs) or negative feedback (NACKs or STATIC-NACKs)
establishes this channel. Once it has established a feedback channel
for a CID, the decompressor is REQUIRED to continue sending feedback
for the lifetime of the context (i.e., until it receives an IR packet
that associates the CID to a different profile), to send error
recovery requests and (optionally) acknowledgments of significant
context updates.
Compression without an established feedback channel will be less
efficient, because of the periodic refreshes and the lack of feedback
to trigger error recovery; there will also be a slightly higher
probability of loss propagation compared to the case where the
decompressor uses feedback.
6.3. Control Fields
ROHCv2 defines a number of control fields that are used by the
decompressor in its interpretation of the header formats received
from the compressor. The control fields listed in the following
subsections are defined using the formal notation [RFC4997] in
Section 6.8.2.4 of this document.
6.3.1. Master Sequence Number (MSN)
The Master Sequence Number (MSN) field is either taken from a field
that already exists in one of the headers of the protocol that the
profile compresses (e.g., RTP SN), or alternatively it is created at
the compressor. There is one MSN space per context.
The MSN field has the following two functions:
o Differentiating between reference headers when receiving feedback
data;
o Inferring the value of incrementing fields (e.g., IPv4
Identifier).
There is one MSN field in every ROHCv2 header, i.e., the MSN is
always present in each header type sent by the compressor. The MSN
is sent in full in IR headers, while it can be lsb encoded within CO
header formats. The decompressor always includes LSBs of the MSN in
the Acknowledgment Number field in feedback (see Section 6.9). The
compressor can later use this field to infer what packet the
decompressor is acknowledging.
For profiles for which the MSN is created by the compressor (i.e.,
0x0102, 0x0104, and 0x0108), the following applies:
o The compressor only initializes the MSN for a context when that
context is first created or when the profile associated with a
context changes;
o When the MSN is initialized, it is initialized to a random value;
o The value of the MSN SHOULD be incremented by one for each packet
that the compressor sends for a specific CID.
6.3.2. Reordering Ratio
The control field reorder_ratio specifies how much reordering is
handled by the lsb encoding of the MSN. This is useful when header
compression is performed over links with varying reordering
characteristics. The reorder_ratio control field provides the means
for the compressor to adjust the robustness characteristics of the
lsb encoding method with respect to reordering and consecutive
losses, as described in Section 5.1.2.
6.3.3. IP-ID Behavior
The IP-ID field of the IPv4 header can have different change
patterns: sequential in network byte order, sequential byte-swapped,
random or constant (a constant value of zero, although not conformant
with [RFC0791], has been observed in practice). There is one IP-ID
behavior control field per IP header. The control field for the
IP-ID behavior of the innermost IP header determines which set of
header formats is used. The IP-ID behavior control field is also
used to determine the contents of the irregular chain item, for each
IP header.
ROHCv2 profiles MUST NOT assign a sequential behavior (network byte
order or byte-swapped) to any IP-ID but the one in the innermost IP
header when compressing more than one level of IP headers. This is
because only the IP-ID of the innermost IP header is likely to have a
sufficiently close correlation with the MSN to compress it as a
sequentially changing field. Therefore, a compressor MUST assign
either the constant zero IP-ID or the random IP-ID behavior to
tunneling headers.
6.3.4. UDP-Lite Coverage Behavior
The control field coverage_behavior specifies how the checksum
coverage field of the UDP-Lite header is compressed with RoHCv2. It
can indicate one of the following encoding methods: irregular,
static, or inferred encoding.
6.3.5. Timestamp Stride
The ts_stride control field is used in scaled RTP timestamp encoding
(see Section 6.6.8). It defines the expected increase in the RTP
timestamp between consecutive RTP sequence numbers.
6.3.6. Time Stride
The time_stride control field is used in timer-based compression
encoding (see Section 6.6.9). When timer-based compression is used,
time_stride should be set to the expected difference in arrival time
between consecutive RTP packets.
6.3.7. CRC-3 for Control Fields
ROHCv2 profiles define a CRC-3 calculated over a number of control
fields. This 3-bit CRC protecting the control fields is present in
the header format for the co_common and co_repair header types.
The decompressor MUST always validate the integrity of the control
fields covered by this 3-bit CRC when processing a co_common or a
co_repair compressed header.
Failure to validate the control fields using this CRC should be
considered as a decompression failure by the decompressor in the
algorithm that assesses the validity of the context. However, if the
decompression attempt can be verified using either the CRC-3 or the
CRC-7 calculated over the uncompressed header, the decompressor MAY
still forward the decompressed header to upper layers. This is
because the protected control fields are not always used to
decompress the header (i.e., co_common or co_repair) that updates
their respective value.
The CRC polynomial and coverage of this CRC-3 is defined in
Section 6.6.11.
6.4. Reconstruction and Verification
Validation of the IR header (8-bit CRC)
The decompressor MUST always validate the integrity of the IR
header using the 8-bit CRC carried within the IR header. When the
header is validated, the decompressor updates the context with the
information in the IR header. Otherwise, if the IR cannot be
validated, the context MUST NOT be updated and the IR header MUST
NOT be delivered to upper layers.
Verification of CO headers (3-bit CRC or 7-bit CRC)
The decompressor MUST always verify the decompression of a CO
header using the CRC carried within the compressed header. When
the decompression is verified and successful, the decompressor
updates the context with the information received in the CO
header; otherwise, if the reconstructed header fails the CRC
verification, these updates MUST NOT be performed.
A packet for which the decompression attempt cannot be verified
using the CRC MUST NOT be delivered to upper layers.
Decompressor implementations may attempt corrective or repair
measures on CO headers prior to performing the above actions, and
the result of any decompression attempt MUST be verified using the
CRC.
6.5. Compressed Header Chains
Some header types use one or more chains containing sub-header
information. The function of a chain is to group fields based on
similar characteristics, such as static, dynamic, or irregular
fields.
Chaining is done by appending an item for each header to the chain in
their order of appearance in the uncompressed packet, starting from
the fields in the outermost header.
In the text below, the term <protocol_name> is used to identify
formal notation names corresponding to different protocol headers.
The mapping between these is defined in the following table:
+----------------------------------+---------------+
| Protocol | protocol_name |
+----------------------------------+---------------+
| IPv4 RFC 0791 | ipv4 |
| IPv6 RFC 2460 | ipv6 |
| UDP RFC 0768 | udp |
| RTP RFC 3550 | rtp |
| ESP RFC 4303 | esp |
| UDP-Lite RFC 3828 | udp_lite |
| AH RFC 4302 | ah |
| GRE RFC 2784, RFC 2890 | gre |
| MINE RFC 2004 | mine |
| IPv6 Destination Option RFC 2460 | dest_opt |
| IPv6 Hop-by-hop Options RFC 2460 | hop_opt |
| IPv6 Routing Header RFC 2460 | rout_opt |
+----------------------------------+---------------+
Static chain:
The static chain consists of one item for each header of the chain
of protocol headers that is compressed, starting from the
outermost IP header. In the formal description of the header
formats, this static chain item for each header type is labeled
<protocol_name>_static. The static chain is only used in the IR
header format.
Dynamic chain:
The dynamic chain consists of one item for each header of the
chain of protocol headers that is compressed, starting from the
outermost IP header. In the formal description of the header
formats, the dynamic chain item for each header type is labeled
<protocol_name>_dynamic. The dynamic chain is only used in the IR
and co_repair header formats.
Irregular chain:
The structure of the irregular chain is analogous to the structure
of the static chain. For each compressed header that uses the
general format of Section 6.8, the irregular chain is appended at
a specific location in the general format of the compressed
headers. In the formal description of the header formats, the
irregular chain item for each header type is a format whose name
is suffixed by "_irregular". The irregular chain is used in all
CO headers, except for the co_repair format.
The format of the irregular chain for the innermost IP header
differs from the format used for the outer IP headers, because the
innermost IP header is part of the compressed base header. In the
definition of the header formats using the formal notation, the
argument "is_innermost", which is passed to the corresponding
encoding method (ipv4 or ipv6), determines what irregular chain
items to use. The format of the irregular chain item for the
outer IP headers is also determined using one flag for TTL/Hop
Limit and TOS/TC. This flag is defined in the format of some of
the compressed base headers.
ROHCv2 profiles compress extension headers as other headers, and thus
extension headers have a static chain, a dynamic chain, and an
irregular chain.
ROHCv2 profiles define chains for all headers that can be compressed,
i.e., RTP [RFC3550], UDP [RFC0768], ESP [RFC4303], UDP-Lite
[RFC3828], IPv4 [RFC0791], IPv6 [RFC2460], AH [RFC4302], GRE
[RFC2784][RFC2890], MINE [RFC2004], IPv6 Destination Options header
[RFC2460], IPv6 Hop-by-hop Options header [RFC2460], and IPv6 Routing
header [RFC2460].
6.6. Header Formats and Encoding Methods
The header formats are defined using the ROHC formal notation. Some
of the encoding methods used in the header formats are defined in
[RFC4997], while other methods are defined in this section.
6.6.1. baseheader_extension_headers
The baseheader_extension_headers encoding method skips over all
fields of the extension headers of the innermost IP header, without
encoding any of them. Fields in these extension headers are instead
encoded in the irregular chain.
This encoding is used in CO headers (see Section 6.8.2). The
innermost IP header is combined with other header(s) (i.e., UDP, UDP-
Lite, RTP) to create the compressed base header. In this case, there
may be a number of extension headers between the IP headers and the
other headers.
The base header defines a representation of the extension headers, to
comply with the syntax of the formal notation; this encoding method
provides this representation.
6.6.2. baseheader_outer_headers
The baseheader_outer_headers encoding method skips over all the
fields of the extension header(s) that do not belong to the innermost
IP header, without encoding any of them. Changing fields in outer
headers are instead handled by the irregular chain.
This encoding method, similarly to the baseheader_extension_headers
encoding method above, is necessary to keep the definition of the
header formats syntactically correct. It describes tunneling IP
headers and their respective extension headers (i.e., all headers
located before the innermost IP header) for CO headers (see
Section 6.8.2).
6.6.3. inferred_udp_length
The decompressor infers the value of the UDP length field as being
the sum of the UDP header length and the UDP payload length. The
compressor must therefore ensure that the UDP length field is
consistent with the length field(s) of preceding subheaders, i.e.,
there must not be any padding after the UDP payload that is covered
by the IP Length.
This encoding method is also used for the UDP-Lite Checksum Coverage
field when it behaves in the same manner as the UDP length field
(i.e., when the checksum always covers the entire UDP-Lite payload).
6.6.4. inferred_ip_v4_header_checksum
This encoding method compresses the header checksum field of the IPv4
header. This checksum is defined in RFC 791 [RFC0791] as follows:
Header Checksum: 16 bits
A checksum on the header only. Since some header fields change
(e.g., time to live), this is recomputed and verified at each
point that the internet header is processed.
The checksum algorithm is:
The checksum field is the 16 bit one's complement of the one's
complement sum of all 16 bit words in the header. For purposes
of computing the checksum, the value of the checksum field is
zero.
As described above, the header checksum protects individual hops from
processing a corrupted header. As the data that this checksum
protects is mostly compressed away and is instead taken from state
stored in the context, this checksum becomes cumulative to the ROHC
CRC. When using this encoding method, the checksum is recomputed by
the decompressor.
The inferred_ip_v4_header_checksum encoding method thus compresses
the header checksum field of the IPv4 header down to a size of zero
bits, i.e., no bits are transmitted in compressed headers for this
field. Using this encoding method, the decompressor infers the value
of this field using the computation above.
The compressor MAY use the header checksum to validate the
correctness of the header before compressing it, to avoid processing
a corrupted header.
6.6.5. inferred_mine_header_checksum
This encoding method compresses the minimal encapsulation header
checksum. This checksum is defined in RFC 2004 [RFC2004] as follows:
Header Checksum
The 16-bit one's complement of the one's complement sum of all
16-bit words in the minimal forwarding header. For purposes of
computing the checksum, the value of the checksum field is 0.
The IP header and IP payload (after the minimal forwarding
header) are not included in this checksum computation.
The inferred_mine_header_checksum encoding method compresses the
minimal encapsulation header checksum down to a size of zero bits,
i.e., no bits are transmitted in compressed headers for this field.
Using this encoding method, the decompressor infers the value of this
field using the above computation.
The motivations for inferring this checksum are similar to the ones
explained above in Section 6.6.4.
The compressor MAY use the minimal encapsulation header checksum to
validate the correctness of the header before compressing it, to
avoid processing a corrupted header.
6.6.6. inferred_ip_v4_length
This encoding method compresses the total length field of the IPv4
header. The total length field of the IPv4 header is defined in RFC
791 [RFC0791] as follows:
Total Length: 16 bits
Total Length is the length of the datagram, measured in octets,
including internet header and data. This field allows the
length of a datagram to be up to 65,535 octets.
The inferred_ip_v4_length encoding method compresses the IPv4 Total
Length field down to a size of zero bits, i.e., no bits are
transmitted in compressed headers for this field. Using this encoding method,
EID 1421 (Verified) is as follows:Section: 6.6.6, p.28
Original Text:
| The inferred_ip_v4_length encoding method compresses the IPv4 header
| checksum down to a size of zero bits, i.e., no bits are transmitted
in compressed headers for this field. [...]
Corrected Text:
| The inferred_ip_v4_length encoding method compresses the IPv4 Total
| Length field down to a size of zero bits, i.e., no bits are
transmitted in compressed headers for this field. [...]
Notes:
Apparently, a copyedit flaw (text copied from 6.6.4 insufficiently modified).
the decompressor infers the value of this field by counting in octets
the length of the entire packet after decompression.
6.6.7. inferred_ip_v6_length
This encoding method compresses the payload length field in the IPv6
header. This length field is defined in RFC 2460 [RFC2460] as
follows:
Payload Length: 16-bit unsigned integer
Length of the IPv6 payload, i.e., the rest of the packet
following this IPv6 header, in octets. (Note that any
extension headers present are considered part of the payload,
i.e., included in the length count.)
The "inferred_ip_v6_length" encoding method compresses the payload
length field of the IPv6 header down to a size of zero bits, i.e., no
bits are transmitted in compressed headers for this field. Using
this encoding method, the decompressor infers the value of this field
by counting in octets the length of the entire packet after
decompression.
IPv6 headers using the jumbo payload option of RFC 2675 [RFC2675]
will not be compressible with this encoding method since the value of
the payload length field does not match the length of the packet.
6.6.8. Scaled RTP Timestamp Compression
This section provides additional details on encodings used to scale
the RTP timestamp, as defined in the formal notation in
Section 6.8.2.4.
The RTP timestamp (TS) usually increases by a multiple of the RTP
Sequence Number's (SN's) increase and is therefore a suitable
candidate for scaled encoding. This scaling factor is labeled
ts_stride in the definition of the profile in the formal notation.
The compressor sets the scaling factor based on the change in TS with
respect to the change in the RTP SN.
The default value of the scaling factor ts_stride is 160, as defined
in Section 6.8.2.4. To use a different value for ts_stride, the
compressor explicitly updates the value of ts_stride to the
decompressor using one of the header formats that can carry this
information.
When the compressor uses a scaling factor that is different than the
default value of ts_stride, it can only use the new scaling factor
once it has enough confidence that the decompressor has successfully
calculated the residue (ts_offset) of the scaling function for the
timestamp. The compressor achieves this by sending unscaled
timestamp values, to allow the decompressor to establish the residue
based on the current ts_stride. The compressor MAY send the unscaled
timestamp in the same compressed header(s) used to establish the
value of ts_stride.
Once the compressor has gained enough confidence that both the value
of the scaling factor and the value of the residue have been
established in the decompressor, the compressor can start compressing
packets using the new scaling factor.
When the compressor detects that the residue (ts_offset) value has
changed, it MUST NOT select a compressed header format that uses the
scaled timestamp encoding before it has re-established the residue as
described above.
When the value of the timestamp field wraps around, the value of the
residue of the scaling function is likely to change. When this
occurs, the compressor re-establishes the new residue value as
described above.
If the decompressor receives a compressed header containing scaled
timestamp bits while the ts_stride equals zero, it MUST NOT deliver
the packet to upper layers and it SHOULD treat this as a CRC
verification failure.
Whether or not the scaling is applied to the RTP TS field is up to
the compressor implementation (i.e., the use of scaling is OPTIONAL),
and is indicated by the tsc_indicator control field. In case scaling
is applied to the RTP TS field, the value of ts_stride used by the
compressor is up to the implementation. A value of ts_stride that is
set to the expected increase in the RTP timestamp between consecutive
unit increases of the RTP SN will provide the most gain for the
scaled encoding. Other values may provide the same gain in some
situations, but may reduce the gain in others.
When scaled timestamp encoding is used for header formats that do not
transmit any lsb-encoded timestamp bits at all, the
inferred_scaled_field encoding of Section 6.6.10 is used for encoding
the timestamp.
6.6.9. timer_based_lsb
The timer-based compression encoding method, timer_based_lsb,
compresses a field whose change pattern approximates a linear
function of the time of day.
This encoding uses the local clock to obtain an approximation of the
value that it encodes. The approximated value is then used as a
reference value together with the num_lsbs_param least-significant
bits received as the encoded value, where num_lsbs_param represents a
number of bits that is sufficient to uniquely represent the encoded
value in the presence of jitter between compression endpoints.
ts_scaled =:= timer_based_lsb(<time_stride_param>,
<num_lsbs_param>, <offset_param>)
The parameters "num_lsbs_param" and "offset_param" are the parameters
to use for the lsb encoding, i.e., the number of least significant
bits and the interpretation interval offset, respectively. The
parameter "time_stride_param" represents the context value of the
control field time_stride.
This encoding method always uses a scaled version of the field it
compresses.
The value of the field is decoded by calculating an approximation of
the scaled value, using:
tsc_ref_advanced = tsc_ref + (a_n - a_ref) / time_stride.
where:
- tsc_ref is a reference value of the scaled representation
of the field.
- a_n is the arrival time associated with the value to decode.
- a_ref is the arrival time associated with the reference header.
- tsc_ref_advanced is an approximation of the scaled value
of the field.
The lsb encoding is then applied using the num_lsbs_param bits
received in the compressed header and the tsc_ref_advanced as
"ref_value" (as per Section 4.11.5 of [RFC4997]).
Appendix B.3 provides an example of how the compressor can calculate
jitter.
The control field time_stride controls whether or not the
timer_based_lsb method is used in the CO header. The decompressor
SHOULD send the CLOCK_RESOLUTION option with a zero value, if:
o it receives a non-zero time_stride value, and
o it has not previously sent a CLOCK_RESOLUTION feedback with a non-
zero value.
This is to allow compression to recover from the case where a
compressor erroneously activates timer-based compression.
The support and usage of timer-based compression is OPTIONAL for both
the compressor and the decompressor; the compressor is not required
to set the time_stride control field to a non-zero value when it has
received a non-zero value for the CLOCK_RESOLUTION option.
6.6.10. inferred_scaled_field
The inferred_scaled_field encoding method encodes a field that is
defined as changing in relation to the MSN, and for which the
increase with respect to the MSN can be scaled by some scaling
factor. This encoding method is used in compressed header formats
that do not contain any bits for the scaled field. In this case, the
decompressor infers the unscaled value of the scaled field from the
MSN field. The unscaled value is calculated according to the
following formula:
unscaled_value = delta_msn * stride + reference_unscaled_value
where "delta_msn" is the difference in MSN between the reference
value of the MSN in the context and the value of the MSN decompressed
from this packet, "reference_unscaled_value" is the value of the
field being scaled in the context, and "stride" is the scaling value
for this field.
For example, when this encoding method is applied to the RTP
timestamp in the RTP profile, the calculation above becomes:
timestamp = delta_msn * ts_stride + reference_timestamp
6.6.11. control_crc3_encoding
The control_crc3_encoding method provides a CRC calculated over a
number of control fields. The definition of this encoding method is
the same as for the "crc" encoding method specified in Section 4.11.6
of [RFC4997], with the difference being that the data covered by the
CRC is given by a concatenated list of control fields.
In other words, the definition of the control_crc3_encoding method is
equivalent to the following definition:
control_crc_encoding(ctrl_data_value, ctrl_data_length)
{
UNCOMPRESSED {
}
COMPRESSED {
control_crc3 =:=
crc(3, 0x06, 0x07, ctrl_data_value, ctrl_data_length) [ 3 ];
}
}
where the parameter "ctrl_data_value" binds to the concatenated
values of the following control fields, in the order listed below:
o reorder_ratio, 2 bits padded with 6 MSB of zeroes
o ts_stride, 32 bits (only for profiles 0x0101 and 0x0107)
o time_stride, 32 bits (only for profiles 0x0101 and 0x0107)
o msn, 16 bits (not applicable for profiles 0x0101, 0x0103, and
0x0107)
o coverage_behavior, 2 bits padded with 6 MSB of zeroes (only for
profiles 0x0107 and 0x0108)
o ip_id_behavior_outer, one octet for each IPv4 header except the
innermost in the compressible header chain starting from the outermost
header. Each octet consists of 2 bits padded with 6 MSBs of zeroes.
o ip_id_behavior_innermost, one octet if the innermost header is an
IPv4 header. The octet consists of 2 bits padded with 6 MSBs of zeroes.
EID 2703 (Verified) is as follows:Section: 6.6.11
Original Text:
o ip_id_behavior, one octet for each IP header in the compressible
header chain starting from the outermost header. Each octet
consists of 2 bits padded with 6 MSBs of zeroes.
Corrected Text:
o ip_id_behavior_outer, one octet for each IPv4 header except the
innermost in the compressible header chain starting from the outermost
header. Each octet consists of 2 bits padded with 6 MSBs of zeroes.
o ip_id_behavior_innermost, one octet if the innermost header is an
IPv4 header. The octet consists of 2 bits padded with 6 MSBs of zeroes.
Notes:
There is no control field called ip_id_behavior in the document. There are two control fields related to IP-ID behavior, ip_id_behavior_innermost and ip_id_behavior_outer. For IPv6, only the ip_id_behavior_innermost field exists and its value is always IP_ID_BEHAVIOR_RANDOM according to the FN. This makes it impossible to include ip_id_behavior_outer when calculating the crc for IPv6 headers. Furthermore, since the ip_id_behavior_innermost is constant it makes no sense to include it in the crc calculation.
This errata has been verified based on discussion on the ROHC mailing list involving the authors in February, 2011.
The "ctrl_data_length" binds to the sum of the length of the control
field(s) that are applicable to the specific profile.
The decompressor uses the resulting 3-bit CRC to validate the control
fields that are updated by the co_common and co_repair header
formats; this CRC cannot be used to verify the outcome of a
decompression attempt.
This CRC protects the update of control fields, as the updated values
are not always used to decompress the header that carries them and
thus are not protected by the CRC-7 verification. This prevents
impairments that could occur if the decompression of a co_common or
of a co_repair succeeds and the decompressor sends positive feedback,
while for some reason the control fields are incorrectly updated.
6.6.12. inferred_sequential_ip_id
This encoding method is used with a sequential IP-ID behavior
(sequential or sequential byte-swapped) and when there are no coded
IP-ID bits in the compressed header. In this case, the IP-ID offset
from the MSN is constant, and the IP-ID increases by the same amount
as the MSN (similar to the inferred_scaled_field encoding method).
The decompressor calculates the value for the IP-ID according to the
following formula:
IP-ID = delta_msn + reference_IP_ID_value
where "delta_msn" is the difference between the reference value of
the MSN in the context and the uncompressed value of the MSN
associated to the compressed header, and where
"reference_IP_ID_value" is the value of the IP-ID in the context.
For swapped IP-ID behavior (i.e., when ip_id_behavior_innermost is
set to IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED), "reference_IP_ID_value"
and "IP-ID" are byte-swapped with regard to the corresponding fields
in the context.
If the IP-ID behavior is random or zero, this encoding method does
not update any fields.
6.6.13. list_csrc(cc_value)
This encoding method compresses the list of RTP CSRC identifiers
using list compression. This encoding establishes a content for the
different CSRC identifiers (items) and a list describing the order in
which they appear.
The compressor passes an argument (cc_value) to this encoding method:
this is the value of the CC field taken from the RTP header. The
decompressor is required to bind the value of this argument to the
number of items in the list, which will allow the decompressor to
correctly reconstruct the CC field.
6.6.13.1. List Compression
The CSRC identifiers in the uncompressed packet can be represented as
an ordered list, whose order and presence are usually constant
between packets. The generic structure of such a list is as follows:
+--------+--------+--...--+--------+
list: | item 1 | item 2 | | item n |
+--------+--------+--...--+--------+
When performing list compression on a CSRC list, each item is the
uncompressed value of one CSRC identifier.
The basic principles of list-based compression are the following:
When initializing the context:
1) The complete representation of the list of CSRC identifiers is
transmitted.
Then, once the context has been initialized:
2) When the list is unchanged, a compressed header that does not
contain information about the list can be used.
3) When the list changes, a compressed list is sent in the compressed
header, including a representation of its structure and order.
Previously unknown items are sent uncompressed in the list, while
previously known items are only represented by an index pointing
to the item stored in the context.
6.6.13.2. Table-based Item Compression
The table-based item compression compresses individual items sent in
compressed lists. The compressor assigns a unique identifier,
"Index", to each item "Item" of a list.
Compressor Logic
The compressor conceptually maintains an item table containing all
items, indexed using "Index". The (Index, Item) pair is sent
together in compressed lists until the compressor gains enough
confidence that the decompressor has observed the mapping between
items and their respective index. Confidence is obtained from the
reception of an acknowledgment from the decompressor, or by
sending (Index, Item) pairs using the optimistic approach. Once
confidence is obtained, the index alone is sent in compressed
lists to indicate the presence of the item corresponding to this
index.
The compressor MAY reset its item table upon receiving a negative
acknowledgement.
The compressor MAY reassign an existing index to a new item by re-
establishing the mapping using the procedure described above.
Decompressor Logic
The decompressor conceptually maintains an item table that
contains all (Index, Item) pairs received. The item table is
updated whenever an (Index, Item) pair is received and
decompression is successful (CRC verification, or CRC-8
validation). The decompressor retrieves the item from the table
whenever an Index is received without an accompanying Item.
If an index is received without an accompanying Item and the
decompressor does not have any context for this index, the
decompressor MUST NOT deliver the packet to upper layers.
6.6.13.3. Encoding of Compressed Lists
Each item present in a compressed list is represented by:
o an Index into the table of items, and a presence bit indicating if
a compressed representation of the item is present in the list.
o an item (if the presence bit is set).
If the presence bit is not set, the item must already be known by the
decompressor.
A compressed list of items uses the following encoding:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Reserved |PS | m |
+---+---+---+---+---+---+---+---+
| XI_1, ..., XI_m | m octets, or m * 4 bits
/ --- --- --- ---/
| : Padding : if PS = 0 and m is odd
+---+---+---+---+---+---+---+---+
| |
/ Item_1, ..., Item_n / variable
| |
+---+---+---+---+---+---+---+---+
Reserved: MUST be set to zero; otherwise, the decompressor MUST
discard the packet.
PS: Indicates size of XI fields:
PS = 0 indicates 4-bit XI fields;
PS = 1 indicates 8-bit XI fields.
m: Number of XI item(s) in the compressed list. Also, the value
of the cc_value argument of the list_csrc encoding (see
Section 6.6.13).
XI_1, ..., XI_m: m XI items. Each XI represents one item in the
list of items of the uncompressed header, in the same order as
they appear in the uncompressed header.
The format of an XI item is as follows:
0 1 2 3
+---+---+---+---+
PS = 0: | X | Index |
+---+---+---+---+
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
PS = 1: | X | Reserved | Index |
+---+---+---+---+---+---+---+---+
X: Indicates whether the item is present in the list:
X = 1 indicates that the item corresponding to the Index is
sent in the Item_1, ..., Item_n list;
X = 0 indicates that the item corresponding to the Index is
not sent.
Reserved: MUST be set to zero; otherwise, the decompressor MUST
discard the packet.
Index: An index into the item table. See Section 6.6.13.4
When 4-bit XI items are used, the XI items are placed in octets
in the following manner:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| XI_k | XI_k + 1 |
+---+---+---+---+---+---+---+---+
Padding: A 4-bit Padding field is present when PS = 0 and the
number of XIs is odd. The Padding field MUST be set to zero;
otherwise, the decompressor MUST discard the packet.
Item 1, ..., item n: Each item corresponds to an XI with X = 1 in
XI 1, ..., XI m. Each entry in the Item list is the uncompressed
representation of one CSRC identifier.
6.6.13.4. Item Table Mappings
The item table for list compression is limited to 16 different items,
since the RTP header can only carry at most 15 simultaneous CSRC
identifiers. The effect of having more than 16 items in the item
table will only cause a slight overhead to the compressor when items
are swapped in/out of the item table.
6.6.13.5. Compressed Lists in Dynamic Chain
A compressed list that is part of the dynamic chain must have all of
its list items present, i.e., all X-bits in the XI list MUST be set.
All items previously established in the item table that are not
present in the list decompressed from this packet MUST also be
retained in the decompressor context.
6.7. Encoding Methods with External Parameters as Arguments
A number of encoding methods in Section 6.8.2.4 have one or more
arguments for which the derivation of the parameter's value is
outside the scope of the ROHC-FN [RFC4997] specification of the
header formats.
The following is a list of encoding methods with external parameters
as arguments, from Section 6.8.2.4:
o udp(profile_value, reorder_ratio_value)
o udp_lite(profile_value, reorder_ratio_value,
coverage_behavior_value)
o esp(profile_value, reorder_ratio_value)
o rtp(profile_value, ts_stride_value, time_stride_value,
reorder_ratio_value)
o ipv4(profile_value, is_innermost, outer_ip_flag,
ip_id_behavior_value, reorder_ratio_value))
o ipv6(profile_value, is_innermost, outer_ip_flag,
reorder_ratio_value))
o iponly_baseheader(profile_value, outer_ip_flag,
ip_id_behavior_value, reorder_ratio_value)
o udp_baseheader(profile_value, outer_ip_flag, ip_id_behavior_value,
reorder_ratio_value)
o udplite_baseheader(profile_value, outer_ip_flag,
ip_id_behavior_value, reorder_ratio_value)
o esp_baseheader(profile_value, outer_ip_flag, ip_id_behavior_value,
reorder_ratio_value)
o rtp_baseheader(profile_value, ts_stride_value, time_stride_value,
outer_ip_flag, ip_id_behavior_value, reorder_ratio_value)
o udplite_rtp_baseheader(profile_value, ts_stride_value,
time_stride_value, outer_ip_flag, ip_id_behavior_value,
reorder_ratio_value, coverage_behavior_value)
The following applies for all parameters listed below: At the
compressor, the value of the parameter is set according to the
recommendations for each parameter. At the decompressor, the value
of the parameter is set to undefined and will get bound by encoding
methods, except where otherwise noted.
The following is a list of external arguments with their respective
definition:
o profile_value:
Set to the 16-bit number that identifies the profile used to
compress this packet. When processing the static chain at the
decompressor, this parameter is set to the value of the profile
field in the IR header (see Section 6.8.1).
o reorder_ratio_value:
Set to a 2-bit integer value, using one of the constants whose
name begins with the prefix REORDERING_ and as defined in
Section 6.8.2.4.
o ip_id_behavior_value:
Set to a 2-bit integer value, using one of the constants whose
name begins with the prefix IP_ID_BEHAVIOR_ and as defined in
Section 6.8.2.4.
o coverage_behavior_value:
Set to a 2-bit integer value, using one of the constants whose
name begins with the prefix UDP_LITE_COVERAGE_ and as defined
in Section 6.8.2.4.
o outer_ip_flag:
This parameter is set to 1 if at least one of the TOS/TC or
TTL/Hop Limit fields in outer IP headers has changed compared
to their reference values in the context; otherwise, it is set
to 0. This flag may only be set to 1 for the "co_common"
header format in the different profiles.
o is_innermost:
This boolean flag is set to 1 when processing the innermost of
the compressible IP headers; otherwise, it is set to 0.
o ts_stride_value
The value of this parameter should be set to the expected
increase in the RTP Timestamp between consecutive RTP sequence
numbers. The value selected is implementation-specific. See
also Section 6.6.8.
o time_stride_value
The value of this parameter should be set to the expected
inter-arrival time between consecutive packets for the flow.
The value selected is implementation-specific. This parameter
MUST be set to zero, unless the compressor has received a
feedback message with the CLOCK_RESOLUTION option set to a non-
zero value. See also Section 6.6.9.
6.8. Header Formats
ROHCv2 profiles use two different header types: the Initialization
and Refresh (IR) header type, and the Compressed header type (CO).
The CO header type defines a number of header formats: there are two
sets of base header formats, with a few additional formats that are
common to both sets.
6.8.1. Initialization and Refresh Header Format (IR)
The IR header format uses the structure of the ROHC IR header as
defined in Section 5.2.2.1 of [RFC4995].
Header type: IR
This header format communicates the static part and the dynamic
part of the context.
The ROHCv2 IR header has the following format:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and (CID != 0)
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 1 0 1 | IR type octet
+---+---+---+---+---+---+---+---+
: :
/ 0-2 octets of CID / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
| Profile | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
| |
/ Static chain / variable length
| |
- - - - - - - - - - - - - - - -
| |
/ Dynamic chain / variable length
| |
- - - - - - - - - - - - - - - -
CRC: 8-bit CRC over the entire IR-header, including any CID fields
and up until the end of the dynamic chain, using the polynomial
defined in [RFC4995]. For purposes of computing the CRC, the CRC
field is zero.
Static chain: See Section 6.5.
Dynamic chain: See Section 6.5.
6.8.2. Compressed Header Formats (CO)
6.8.2.1. Design Rationale for Compressed Base Headers
The compressed header formats are defined as two separate sets for
each profile: one set for the headers where the innermost IP header
contains a sequential IP-ID (either network byte order or byte-
swapped), and one set for the headers without sequential IP-ID
(either random, zero, or no IP-ID). There are also a number of
common header formats shared between both sets. In the description
below, the naming convention used for header formats that belong to
the sequential set is to include "seq" in the name of the format,
while similarly "rnd" is used for those that belong to the non-
sequential set.
The design of the header formats is derived from the field behavior
analysis found in Appendix A.
All of the compressed base headers transmit lsb-encoded MSN bits and
a CRC.
The following header formats exist for all profiles defined in this
document, and are common to both the sequential and the random header
format sets:
o co_common: This format can be used to update the context when the
established change pattern of a dynamic field changes, for any of
the dynamic fields. However, not all dynamic fields are updated
by conveying their uncompressed value; some fields can only be
transmitted using a compressed representation. This format is
especially useful when a rarely changing field needs to be
updated. This format contains a set of flags to indicate what
fields are present in the header, and its size can vary
accordingly. This format is protected by a 7-bit CRC. It can
update control fields, and it thus also carries a 3-bit CRC to
protect those fields. This format is similar in purpose to the
UOR-2-extension 3 format of [RFC3095].
o co_repair: This format can be used to update the context of all
the dynamic fields by conveying their uncompressed value. This is
especially useful when context damage is assumed (e.g., from the
reception of a NACK) and a context repair is performed. This
format is protected by a 7-bit CRC. It also carries a 3-bit CRC
over the control fields that it can update. This format is
similar in purpose to the IR-DYN format of [RFC3095] when
performing context repairs.
o pt_0_crc3: This format conveys only the MSN; it can therefore only
update the MSN and fields that are derived from the MSN, such as
IP-ID and the RTP Timestamp (for applicable profiles). It is
protected by a 3-bit CRC. This format is equivalent to the UO-0
header format in [RFC3095].
o pt_0_crc7: This format has the same properties as pt_0_crc3, but
is instead protected by a 7-bit CRC and contains a larger amount
of lsb-encoded MSN bits. This format is useful in environments
where a high amount of reordering or a high-residual error rate
can occur.
The following header format descriptions apply to profiles 0x0101 and
0x0107.
o pt_1_rnd: This format can convey changes to the MSN and to the RTP
Marker bit, and it can update the RTP timestamp using scaled
timestamp encoding. It is protected by a 3-bit CRC. It is
similar in purpose to the UO-1 format in [RFC3095].
o pt_1_seq_id: This format can convey changes to the MSN and to the
IP-ID. It is protected by a 3-bit CRC. It is similar in purpose
to the UO-1-ID format in [RFC3095].
o pt_1_seq_ts: This format can convey changes to the MSN and to the
RTP Marker bit, and it can update the RTP Timestamp using scaled
timestamp encoding. It is protected by a 3-bit CRC. It is
similar in purpose to the UO-1-TS format in [RFC3095].
o pt_2_rnd: This format can convey changes to the MSN, to the RTP
Marker bit, and to the RTP Timestamp. It is protected by a 7-bit
CRC. It is similar in purpose to the UOR-2 format in [RFC3095].
o pt_2_seq_id: This format can convey changes to the MSN and to the
IP-ID. It is protected by a 7-bit CRC. It is similar in purpose
to the UO-2-ID format in [RFC3095].
o pt_2_seq_ts: This format can convey changes to the MSN, to the RTP
Marker bit and it can update the RTP Timestamp using scaled
timestamp encoding. It is protected by a 7-bit CRC. It is
similar in purpose to the UO-2-TS format in [RFC3095].
o pt_2_seq_both: This format can convey changes to both the RTP
Timestamp and the IP-ID, in addition to the MSN and to the Marker
bit. It is protected by a 7-bit CRC. It is similar in purpose to
the UOR-2-ID extension 1 format in [RFC3095].
The following header format descriptions apply to profiles 0x0102,
0x0103, 0x0104, and 0x0108.
o pt_1_seq_id: This format can convey changes to the MSN and to the
IP-ID. It is protected by a 7-bit CRC. It is similar in purpose
to the UO-1-ID format in [RFC3095].
o pt_2_seq_id: This format can convey changes to the MSN and to the
IP-ID. It is protected by a 7-bit CRC. It is similar in purpose
to the UO-2-ID format in [RFC3095].
6.8.2.2. co_repair Header Format
The ROHCv2 co_repair header has the following format:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and CID 1-15
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 0 1 1 | discriminator
+---+---+---+---+---+---+---+---+
: :
/ 0, 1, or 2 octets of CID / 1-2 octets if large CIDs
: :
+---+---+---+---+---+---+---+---+
|r1 | CRC-7 |
+---+---+---+---+---+---+---+---+
| r2 | CRC-3 |
+---+---+---+---+---+---+---+---+
| |
/ Dynamic chain / variable length
| |
- - - - - - - - - - - - - - - -
r1: MUST be set to zero; otherwise, the decompressor MUST discard
the packet.
CRC-7: A 7-bit CRC over the entire uncompressed header, computed
using the crc7 (data_value, data_length) encoding method defined
in Section 6.8.2.4, where data_value corresponds to the entire
uncompressed header chain and where data_length corresponds to the
length of this header chain.
r2: MUST be set to zero; otherwise, the decompressor MUST discard
the packet.
CRC-3: Encoded using the control_crc3_encoding method defined in
Section 6.6.11.
Dynamic chain: See Section 6.5.
6.8.2.3. General CO Header Format
The CO header format communicates irregularities in the packet
header. All CO formats carry a CRC and can update the context. All
CO header formats use the general format defined in this section,
with the exception of the co_repair format, which is defined in
Section 6.8.2.2.
The general format for a compressed header is as follows:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and CID 1-15
+---+---+---+---+---+---+---+---+
| first octet of base header | (with type indication)
+---+---+---+---+---+---+---+---+
: :
/ 0, 1, or 2 octets of CID / 1-2 octets if large CIDs
: :
+---+---+---+---+---+---+---+---+
/ remainder of base header / variable length
+---+---+---+---+---+---+---+---+
: :
/ Irregular Chain / variable length
: :
--- --- --- --- --- --- --- ---
The base header in the figure above is the compressed representation
of the innermost IP header and other header(s), if any, in the
uncompressed packet. The base header formats are defined in
Section 6.8.2.4. In the formal description of the header formats,
the base header for each profile is labeled
<profile_name>_baseheader, where <profile_name> is defined in the
following table:
+------------------+----------------+
| Profile number | profile_name |
+------------------+----------------+
| 0x0101 | rtp |
| 0x0102 | udp |
| 0x0103 | esp |
| 0x0104 | ip |
| 0x0107 | udplite_rtp |
| 0x0108 | udplite |
+------------------+----------------+
6.8.2.4. Header Formats in ROHC-FN
Notes:
checksum_coverage_dynchain(behavior) compression method (page 66) may compress the checksum_coverage field to 0 bits if behavior is set to UDP_LITE_COVERAGE_INFERRED.
This field is expected to have the value two and the field is therefore classified as STATIC-KNOWN.
pad_bit [ 1 ];
extension [ 1 ];
cc [ 4 ];
marker [ 1 ];
payload_type [ 7 ];
sequence_number [ 16 ];
timestamp [ 32 ];
ssrc [ 32 ];
csrc_list [ cc.UVALUE * 32 ];
}
CONTROL {
ENFORCE(profile == profile_value);
ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
ENFORCE(time_stride_value == time_stride.UVALUE);
ENFORCE(ts_stride_value == ts_stride.UVALUE);
dummy_field =:= field_scaling(ts_stride.UVALUE,
ts_scaled.UVALUE, timestamp.UVALUE, ts_offset.UVALUE) [ 0 ];
}
INITIAL {
ts_stride =:= uncompressed_value(32, TS_STRIDE_DEFAULT);
time_stride =:= uncompressed_value(32, TIME_STRIDE_DEFAULT);
}
DEFAULT {
ENFORCE(msn.UVALUE == sequence_number.UVALUE);
pad_bit =:= static;
extension =:= static;
cc =:= static;
marker =:= static;
payload_type =:= static;
sequence_number =:= static;
timestamp =:= static;
ssrc =:= static;
csrc_list =:= static;
ts_stride =:= static;
time_stride =:= static;
ts_scaled =:= static;
ts_offset =:= static;
}
COMPRESSED rtp_static {
ssrc =:= irregular(32) [ 32 ];
}
COMPRESSED rtp_dynamic {
reserved =:= compressed_value(1, 0) [ 1 ];
reorder_ratio =:= irregular(2) [ 2 ];
list_present =:= irregular(1) [ 1 ];
tss_indicator =:= irregular(1) [ 1 ];
tis_indicator =:= irregular(1) [ 1 ];
pad_bit =:= irregular(1) [ 1 ];
extension =:= irregular(1) [ 1 ];
marker =:= irregular(1) [ 1 ];
payload_type =:= irregular(7) [ 7 ];
sequence_number =:= irregular(16) [ 16 ];
timestamp =:= irregular(32) [ 32 ];
ts_stride =:= sdvl_or_default(tss_indicator.CVALUE,
TS_STRIDE_DEFAULT) [ VARIABLE ];
time_stride =:= sdvl_or_default(tis_indicator.CVALUE,
TIME_STRIDE_DEFAULT) [ VARIABLE ];
csrc_list =:= csrc_list_dynchain(list_present.CVALUE,
cc.UVALUE) [ VARIABLE ];
}
COMPRESSED rtp_irregular {
}
}
/////////////////////////////////////////////
// UDP-Lite Header
/////////////////////////////////////////////
checksum_coverage_dynchain(behavior)
{
UNCOMPRESSED {
checksum_coverage [ 16 ];
}
COMPRESSED inferred_coverage {
ENFORCE(behavior == UDP_LITE_COVERAGE_INFERRED);
checksum_coverage =:= inferred_udp_length [ 0 ];
}
COMPRESSED static_coverage {
ENFORCE(behavior == UDP_LITE_COVERAGE_STATIC);
checksum_coverage =:= irregular(16) [ 16 ];
}
COMPRESSED irregular_coverage {
ENFORCE(behavior == UDP_LITE_COVERAGE_IRREGULAR);
checksum_coverage =:= irregular(16) [ 16 ];
}
}
checksum_coverage_irregular(behavior)
{
UNCOMPRESSED {
checksum_coverage [ 16 ];
}
COMPRESSED inferred_coverage {
ENFORCE(behavior == UDP_LITE_COVERAGE_INFERRED);
checksum_coverage =:= inferred_udp_length [ 0 ];
}
COMPRESSED static_coverage {
ENFORCE(behavior == UDP_LITE_COVERAGE_STATIC);
checksum_coverage =:= static [ 0 ];
}
COMPRESSED irregular_coverage {
ENFORCE(behavior == UDP_LITE_COVERAGE_IRREGULAR);
checksum_coverage =:= irregular(16) [ 16 ];
}
}
udp_lite(profile_value, reorder_ratio_value, coverage_behavior_value)
{
UNCOMPRESSED {
ENFORCE((profile_value == PROFILE_RTP_0107) ||
(profile_value == PROFILE_UDPLITE_0108));
src_port [ 16 ];
dst_port [ 16 ];
checksum_coverage [ 16 ];
checksum [ 16 ];
}
CONTROL {
ENFORCE(profile == profile_value);
ENFORCE(coverage_behavior.UVALUE == coverage_behavior_value);
ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
}
DEFAULT {
src_port =:= static;
dst_port =:= static;
coverage_behavior =:= static;
}
COMPRESSED udp_lite_static {
src_port =:= irregular(16) [ 16 ];
dst_port =:= irregular(16) [ 16 ];
}
COMPRESSED udp_lite_endpoint_dynamic {
ENFORCE(profile_value == PROFILE_UDPLITE_0108);
reserved =:= compressed_value(4, 0) [ 4 ];
coverage_behavior =:= irregular(2) [ 2 ];
reorder_ratio =:= irregular(2) [ 2 ];
checksum_coverage =:=
checksum_coverage_dynchain(coverage_behavior.UVALUE) [ 16 ];
checksum =:= irregular(16) [ 16 ];
msn =:= irregular(16) [ 16 ];
}
COMPRESSED udp_lite_regular_dynamic {
ENFORCE(profile_value == PROFILE_RTP_0107);
coverage_behavior =:= irregular(2) [ 2 ];
reserved =:= compressed_value(6, 0) [ 6 ];
checksum_coverage =:=
checksum_coverage_dynchain(coverage_behavior.UVALUE) [ 16 ];
checksum =:= irregular(16) [ 16 ];
}
COMPRESSED udp_lite_irregular {
checksum_coverage =:=
checksum_coverage_irregular(coverage_behavior.UVALUE) [ 0, 16 ];
checksum =:= irregular(16) [ 16 ];
}
}
/////////////////////////////////////////////
// ESP Header
/////////////////////////////////////////////
esp(profile_value, reorder_ratio_value)
{
UNCOMPRESSED {
ENFORCE(profile_value == PROFILE_ESP_0103);
ENFORCE(msn.UVALUE == sequence_number.UVALUE % 65536);
spi [ 32 ];
sequence_number [ 32 ];
}
CONTROL {
ENFORCE(profile == profile_value);
ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
}
DEFAULT {
spi =:= static;
sequence_number =:= static;
}
COMPRESSED esp_static {
spi =:= irregular(32) [ 32 ];
}
COMPRESSED esp_dynamic {
sequence_number =:= irregular(32) [ 32 ];
reserved =:= compressed_value(6, 0) [ 6 ];
reorder_ratio =:= irregular(2) [ 2 ];
}
COMPRESSED esp_irregular {
}
}
///////////////////////////////////////////////////
// Encoding methods used in the profiles' CO headers
///////////////////////////////////////////////////
// Variable reordering offset used for MSN
msn_lsb(k)
{
UNCOMPRESSED {
master [ VARIABLE ];
}
COMPRESSED none {
ENFORCE(reorder_ratio.UVALUE == REORDERING_NONE);
master =:= lsb(k, 1);
}
COMPRESSED quarter {
ENFORCE(reorder_ratio.UVALUE == REORDERING_QUARTER);
master =:= lsb(k, ((2^k) / 4) - 1);
}
COMPRESSED half {
ENFORCE(reorder_ratio.UVALUE == REORDERING_HALF);
master =:= lsb(k, ((2^k) / 2) - 1);
}
COMPRESSED threequarters {
ENFORCE(reorder_ratio.UVALUE == REORDERING_THREEQUARTERS);
master =:= lsb(k, (((2^k) * 3) / 4) - 1);
}
}
ip_id_lsb(behavior, k)
{
UNCOMPRESSED {
ip_id [ 16 ];
}
CONTROL {
ip_id_nbo [ 16 ];
}
COMPRESSED nbo {
ENFORCE(behavior == IP_ID_BEHAVIOR_SEQUENTIAL);
ENFORCE(ip_id_offset.UVALUE == ip_id.UVALUE - msn.UVALUE);
ip_id_offset =:= lsb(k, ((2^k) / 4) - 1) [ k ];
}
COMPRESSED non_nbo {
ENFORCE(behavior == IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED);
ENFORCE(ip_id_nbo.UVALUE ==
(ip_id.UVALUE / 256) + (ip_id.UVALUE % 256) * 256);
ENFORCE(ip_id_nbo.ULENGTH == 16);
ENFORCE(ip_id_offset.UVALUE == ip_id_nbo.UVALUE - msn.UVALUE);
ip_id_offset =:= lsb(k, ((2^k) / 4) - 1) [ k ];
}
}
ip_id_sequential_variable(behavior, indicator)
{
UNCOMPRESSED {
ip_id [ 16 ];
}
COMPRESSED short {
ENFORCE((behavior == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(behavior == IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
ENFORCE(indicator == 0);
ip_id =:= ip_id_lsb(behavior, 8) [ 8 ];
}
COMPRESSED long {
ENFORCE((behavior == IP_ID_BEHAVIOR_SEQUENTIAL) ||
(behavior == IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
ENFORCE(indicator == 1);
ENFORCE(ip_id_offset.UVALUE == ip_id.UVALUE - msn.UVALUE);
ip_id =:= irregular(16) [ 16 ];
}
COMPRESSED not_present {
ENFORCE((behavior == IP_ID_BEHAVIOR_RANDOM) ||
(behavior == IP_ID_BEHAVIOR_ZERO));
}
}
dont_fragment(version)
{
UNCOMPRESSED {
df [ 0, 1 ];
}
COMPRESSED v4 {
ENFORCE(version == 4);
df =:= irregular(1) [ 1 ];
}
COMPRESSED v6 {
ENFORCE(version == 6);
unused =:= compressed_value(1, 0) [ 1 ];
}
}
pt_irr_or_static(flag)
{
UNCOMPRESSED {
payload_type [ 7 ];
}
COMPRESSED not_present {
ENFORCE(flag == 0);
payload_type =:= static [ 0 ];
}
COMPRESSED present {
ENFORCE(flag == 1);
reserved =:= compressed_value(1, 0) [ 1 ];
payload_type =:= irregular(7) [ 7 ];
}
}
csrc_list_presence(presence, cc_value)
{
UNCOMPRESSED {
csrc_list;
}
COMPRESSED no_list {
ENFORCE(presence == 0);
csrc_list =:= static [ 0 ];
}
COMPRESSED list_present {
ENFORCE(presence == 1);
csrc_list =:= list_csrc(cc_value) [ VARIABLE ];
}
}
scaled_ts_lsb(time_stride_value, k)
{
UNCOMPRESSED {
timestamp [ 32 ];
}
COMPRESSED timerbased {
ENFORCE(time_stride_value != 0);
timestamp =:= timer_based_lsb(time_stride_value, k,
((2^k) / 2) - 1);
}
COMPRESSED regular {
ENFORCE(time_stride_value == 0);
timestamp =:= lsb(k, ((2^k) / 4) - 1);
}
}
// Self-describing variable length encoding with reordering offset
sdvl_sn_lsb(field_width)
{
UNCOMPRESSED {
field [ field_width ];
}
COMPRESSED lsb7 {
discriminator =:= '0' [ 1 ];
field =:= msn_lsb(7) [ 7 ];
}
COMPRESSED lsb14 {
discriminator =:= '10' [ 2 ];
field =:= msn_lsb(14) [ 14 ];
}
COMPRESSED lsb21 {
discriminator =:= '110' [ 3 ];
field =:= msn_lsb(21) [ 21 ];
}
COMPRESSED lsb28 {
discriminator =:= '1110' [ 4 ];
field =:= msn_lsb(28) [ 28 ];
}
COMPRESSED lsb32 {
discriminator =:= '11111111' [ 8 ];
field =:= irregular(field_width) [ field_width ];
}
}
// Self-describing variable length encoding
sdvl_lsb(field_width)
{
UNCOMPRESSED {
field [ field_width ];
}
COMPRESSED lsb7 {
discriminator =:= '0' [ 1 ];
field =:= lsb(7, ((2^7) / 4) - 1) [ 7 ];
}
COMPRESSED lsb14 {
discriminator =:= '10' [ 2 ];
field =:= lsb(14, ((2^14) / 4) - 1) [ 14 ];
}
COMPRESSED lsb21 {
discriminator =:= '110' [ 3 ];
field =:= lsb(21, ((2^21) / 4) - 1) [ 21 ];
}
COMPRESSED lsb28 {
discriminator =:= '1110' [ 4 ];
field =:= lsb(28, ((2^28) / 4) - 1) [ 28 ];
}
COMPRESSED lsb32 {
discriminator =:= '11111111' [ 8 ];
field =:= irregular(field_width) [ field_width ];
}
}
sdvl_scaled_ts_lsb(time_stride)
{
UNCOMPRESSED {
field [ 32 ];
}
COMPRESSED lsb7 {
discriminator =:= '0' [ 1 ];
field =:= scaled_ts_lsb(time_stride, 7) [ 7 ];
}
COMPRESSED lsb14 {
discriminator =:= '10' [ 2 ];
field =:= scaled_ts_lsb(time_stride, 14) [ 14 ];
}
COMPRESSED lsb21 {
discriminator =:= '110' [ 3 ];
field =:= scaled_ts_lsb(time_stride, 21) [ 21 ];
}
COMPRESSED lsb28 {
discriminator =:= '1110' [ 4 ];
field =:= scaled_ts_lsb(time_stride, 28) [ 28 ];
}
COMPRESSED lsb32 {
discriminator =:= '11111111' [ 8 ];
field =:= irregular(32) [ 32 ];
}
}
variable_scaled_timestamp(tss_flag, tsc_flag, ts_stride, time_stride)
{
UNCOMPRESSED {
scaled_value [ 32 ];
}
COMPRESSED present {
ENFORCE((tss_flag == 0) && (tsc_flag == 1));
ENFORCE(ts_stride != 0);
scaled_value =:= sdvl_scaled_ts_lsb(time_stride) [ VARIABLE ];
}
COMPRESSED not_present {
ENFORCE(((tss_flag == 1) && (tsc_flag == 0)) ||
((tss_flag == 0) && (tsc_flag == 0)));
}
}
variable_unscaled_timestamp(tss_flag, tsc_flag)
{
UNCOMPRESSED {
timestamp [ 32 ];
}
COMPRESSED present {
ENFORCE(((tss_flag == 1) && (tsc_flag == 0)) ||
((tss_flag == 0) && (tsc_flag == 0)));
timestamp =:= sdvl_lsb(32);
}
COMPRESSED not_present {
ENFORCE((tss_flag == 0) && (tsc_flag == 1));
}
}
profile_1_7_flags1_enc(flag, ip_version)
{
UNCOMPRESSED {
ip_outer_indicator [ 1 ];
ttl_hopl_indicator [ 1 ];
tos_tc_indicator [ 1 ];
df [ 0, 1 ];
ip_id_behavior [ 2 ];
reorder_ratio [ 2 ];
}
COMPRESSED not_present {
ENFORCE(flag == 0);
ENFORCE(ip_outer_indicator.CVALUE == 0);
ENFORCE(ttl_hopl_indicator.CVALUE == 0);
ENFORCE(tos_tc_indicator.CVALUE == 0);
df =:= static;
ip_id_behavior =:= static;
reorder_ratio =:= static;
}
COMPRESSED present {
ENFORCE(flag == 1);
ip_outer_indicator =:= irregular(1) [ 1 ];
ttl_hopl_indicator =:= irregular(1) [ 1 ];
tos_tc_indicator =:= irregular(1) [ 1 ];
df =:= dont_fragment(ip_version) [ 1 ];
ip_id_behavior =:= irregular(2) [ 2 ];
reorder_ratio =:= irregular(2) [ 2 ];
}
}
profile_1_flags2_enc(flag)
{
UNCOMPRESSED {
list_indicator [ 1 ];
pt_indicator [ 1 ];
time_stride_indicator [ 1 ];
pad_bit [ 1 ];
extension [ 1 ];
}
COMPRESSED not_present{
ENFORCE(flag == 0);
ENFORCE(list_indicator.UVALUE == 0);
ENFORCE(pt_indicator.UVALUE == 0);
ENFORCE(time_stride_indicator.UVALUE == 0);
pad_bit =:= static;
extension =:= static;
}
COMPRESSED present {
ENFORCE(flag == 1);
list_indicator =:= irregular(1) [ 1 ];
pt_indicator =:= irregular(1) [ 1 ];
time_stride_indicator =:= irregular(1) [ 1 ];
pad_bit =:= irregular(1) [ 1 ];
extension =:= irregular(1) [ 1 ];
reserved =:= compressed_value(3, 0) [ 3 ];
}
}
profile_2_3_4_flags_enc(flag, ip_version)
{
UNCOMPRESSED {
ip_outer_indicator [ 1 ];
df [ 0, 1 ];
ip_id_behavior [ 2 ];
}
COMPRESSED not_present {
ENFORCE(flag == 0);
ENFORCE(ip_outer_indicator.CVALUE == 0);
df =:= static;
ip_id_behavior =:= static;
}
COMPRESSED present {
ENFORCE(flag == 1);
ip_outer_indicator =:= irregular(1) [ 1 ];
df =:= dont_fragment(ip_version) [ 1 ];
ip_id_behavior =:= irregular(2) [ 2 ];
reserved =:= compressed_value(4, 0) [ 4 ];
}
}
profile_8_flags_enc(flag, ip_version)
{
UNCOMPRESSED {
ip_outer_indicator [ 1 ];
df [ 0, 1 ];
ip_id_behavior [ 2 ];
coverage_behavior [ 2 ];
}
COMPRESSED not_present {
ENFORCE(flag == 0);
ENFORCE(ip_outer_indicator.CVALUE == 0);
df =:= static;
ip_id_behavior =:= static;
coverage_behavior =:= static;
}
COMPRESSED present {
ENFORCE(flag == 1);
reserved =:= compressed_value(2, 0) [ 2 ];
ip_outer_indicator =:= irregular(1) [ 1 ];
df =:= dont_fragment(ip_version) [ 1 ];
ip_id_behavior =:= irregular(2) [ 2 ];
coverage_behavior =:= irregular(2) [ 2 ];
}
}
profile_7_flags2_enc(flag)
{
UNCOMPRESSED {
list_indicator [ 1 ];
pt_indicator [ 1 ];
time_stride_indicator [ 1 ];
pad_bit [ 1 ];
extension [ 1 ];
coverage_behavior [ 2 ];
}
COMPRESSED not_present{
ENFORCE(flag == 0);
ENFORCE(list_indicator.CVALUE == 0);
ENFORCE(pt_indicator.CVALUE == 0);
ENFORCE(time_stride_indicator.CVALUE == 0);
pad_bit =:= static;
extension =:= static;
coverage_behavior =:= static;
}
COMPRESSED present {
ENFORCE(flag == 1);
reserved =:= compressed_value(1, 0) [ 1 ];
list_indicator =:= irregular(1) [ 1 ];
pt_indicator =:= irregular(1) [ 1 ];
time_stride_indicator =:= irregular(1) [ 1 ];
pad_bit =:= irregular(1) [ 1 ];
extension =:= irregular(1) [ 1 ];
coverage_behavior =:= irregular(2) [ 2 ];
}
}
////////////////////////////////////////////
// RTP profile
////////////////////////////////////////////
rtp_baseheader(profile_value, ts_stride_value, time_stride_value,
outer_ip_flag, ip_id_behavior_value,
reorder_ratio_value)
{
UNCOMPRESSED v4 {
ENFORCE(msn.UVALUE == sequence_number.UVALUE);
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
ip_version =:= uncompressed_value(4, 4) [ 4 ];
header_length =:= uncompressed_value(4, 5) [ 4 ];
tos_tc [ 8 ];
length =:= inferred_ip_v4_length [ 16 ];
ip_id [ 16 ];
rf =:= uncompressed_value(1, 0) [ 1 ];
df [ 1 ];
mf =:= uncompressed_value(1, 0) [ 1 ];
frag_offset =:= uncompressed_value(13, 0) [ 13 ];
ttl_hopl [ 8 ];
next_header [ 8 ];
ip_checksum =:= inferred_ip_v4_header_checksum [ 16 ];
src_addr [ 32 ];
dest_addr [ 32 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
src_port [ 16 ];
dst_port [ 16 ];
udp_length =:= inferred_udp_length [ 16 ];
udp_checksum [ 16 ];
rtp_version =:= uncompressed_value(2, 2) [ 2 ];
pad_bit [ 1 ];
extension [ 1 ];
cc [ 4 ];
marker [ 1 ];
payload_type [ 7 ];
sequence_number [ 16 ];
timestamp [ 32 ];
ssrc [ 32 ];
csrc_list [ VARIABLE ];
}
UNCOMPRESSED v6 {
ENFORCE(ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_RANDOM);
ENFORCE(msn.UVALUE == sequence_number.UVALUE);
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
ip_version =:= uncompressed_value(4, 6) [ 4 ];
tos_tc [ 8 ];
flow_label [ 20 ];
payload_length =:= inferred_ip_v6_length [ 16 ];
next_header [ 8 ];
ttl_hopl [ 8 ];
src_addr [ 128 ];
dest_addr [ 128 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
src_port [ 16 ];
dst_port [ 16 ];
udp_length =:= inferred_udp_length [ 16 ];
udp_checksum [ 16 ];
rtp_version =:= uncompressed_value(2, 2) [ 2 ];
pad_bit [ 1 ];
extension [ 1 ];
cc [ 4 ];
marker [ 1 ];
payload_type [ 7 ];
sequence_number [ 16 ];
timestamp [ 32 ];
ssrc [ 32 ];
csrc_list [ VARIABLE ];
df =:= uncompressed_value(0,0) [ 0 ];
ip_id =:= uncompressed_value(0,0) [ 0 ];
}
CONTROL {
ENFORCE(profile_value == PROFILE_RTP_0101);
ENFORCE(profile == profile_value);
ENFORCE(time_stride.UVALUE == time_stride_value);
ENFORCE(ts_stride.UVALUE == ts_stride_value);
ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
ENFORCE(ip_id_behavior_innermost.UVALUE == ip_id_behavior_value);
dummy_field =:= field_scaling(ts_stride.UVALUE,
ts_scaled.UVALUE, timestamp.UVALUE, ts_offset.UVALUE) [ 0 ];
}
INITIAL {
ts_stride =:= uncompressed_value(32, TS_STRIDE_DEFAULT);
time_stride =:= uncompressed_value(32, TIME_STRIDE_DEFAULT);
}
DEFAULT {
ENFORCE(outer_ip_flag == 0);
tos_tc =:= static;
dest_addr =:= static;
ttl_hopl =:= static;
src_addr =:= static;
df =:= static;
flow_label =:= static;
next_header =:= static;
src_port =:= static;
dst_port =:= static;
pad_bit =:= static;
extension =:= static;
cc =:= static;
// When marker not present in packets, it is assumed 0
marker =:= uncompressed_value(1, 0);
payload_type =:= static;
sequence_number =:= static;
timestamp =:= static;
ssrc =:= static;
csrc_list =:= static;
ts_stride =:= static;
time_stride =:= static;
ts_scaled =:= static;
ts_offset =:= static;
reorder_ratio =:= static;
ip_id_behavior_innermost =:= static;
}
// Replacement for UOR-2-ext3
COMPRESSED co_common {
ENFORCE(outer_ip_flag == outer_ip_indicator.CVALUE);
discriminator =:= '11111010' [ 8 ];
marker =:= irregular(1) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
flags1_indicator =:= irregular(1) [ 1 ];
flags2_indicator =:= irregular(1) [ 1 ];
tsc_indicator =:= irregular(1) [ 1 ];
tss_indicator =:= irregular(1) [ 1 ];
ip_id_indicator =:= irregular(1) [ 1 ];
control_crc3 =:= control_crc3_encoding [ 3 ];
outer_ip_indicator : ttl_hopl_indicator :
tos_tc_indicator : df : ip_id_behavior_innermost : reorder_ratio
=:= profile_1_7_flags1_enc(flags1_indicator.CVALUE,
ip_version.UVALUE) [ 0, 8 ];
list_indicator : pt_indicator : tis_indicator : pad_bit :
extension =:= profile_1_flags2_enc(
flags2_indicator.CVALUE) [ 0, 8 ];
tos_tc =:= static_or_irreg(tos_tc_indicator.CVALUE, 8) [ 0, 8 ];
ttl_hopl =:= static_or_irreg(ttl_hopl_indicator.CVALUE,
ttl_hopl.ULENGTH) [ 0, 8 ];
payload_type =:= pt_irr_or_static(pt_indicator) [ 0, 8 ];
sequence_number =:=
sdvl_sn_lsb(sequence_number.ULENGTH) [ VARIABLE ];
ip_id =:= ip_id_sequential_variable(
ip_id_behavior_innermost.UVALUE,
ip_id_indicator.CVALUE) [ 0, 8, 16 ];
ts_scaled =:= variable_scaled_timestamp(tss_indicator.CVALUE,
tsc_indicator.CVALUE, ts_stride.UVALUE,
time_stride.UVALUE) [ VARIABLE ];
timestamp =:= variable_unscaled_timestamp(tss_indicator.CVALUE,
tsc_indicator.CVALUE) [ VARIABLE ];
ts_stride =:= sdvl_or_static(tss_indicator.CVALUE) [ VARIABLE ];
time_stride =:= sdvl_or_static(tis_indicator.CVALUE) [ VARIABLE ];
csrc_list =:= csrc_list_presence(list_indicator.CVALUE,
cc.UVALUE) [ VARIABLE ];
}
// UO-0
COMPRESSED pt_0_crc3 {
discriminator =:= '0' [ 1 ];
msn =:= msn_lsb(4) [ 4 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
timestamp =:= inferred_scaled_field [ 0 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// New format, Type 0 with strong CRC and more SN bits
COMPRESSED pt_0_crc7 {
discriminator =:= '1000' [ 4 ];
msn =:= msn_lsb(5) [ 5 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
timestamp =:= inferred_scaled_field [ 0 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// UO-1 replacement
COMPRESSED pt_1_rnd {
ENFORCE(ts_stride.UVALUE != 0);
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_ZERO));
discriminator =:= '101' [ 3 ];
marker =:= irregular(1) [ 1 ];
msn =:= msn_lsb(4) [ 4 ];
ts_scaled =:= scaled_ts_lsb(time_stride.UVALUE, 5) [ 5 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
}
// UO-1-ID replacement
COMPRESSED pt_1_seq_id {
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator =:= '1001' [ 4 ];
ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 4) [ 4 ];
msn =:= msn_lsb(5) [ 5 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
timestamp =:= inferred_scaled_field [ 0 ];
}
// UO-1-TS replacement
COMPRESSED pt_1_seq_ts {
ENFORCE(ts_stride.UVALUE != 0);
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator =:= '101' [ 3 ];
marker =:= irregular(1) [ 1 ];
msn =:= msn_lsb(4) [ 4 ];
ts_scaled =:= scaled_ts_lsb(time_stride.UVALUE, 5) [ 5 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// UOR-2 replacement
COMPRESSED pt_2_rnd {
ENFORCE(ts_stride.UVALUE != 0);
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_ZERO));
discriminator =:= '110' [ 3 ];
msn =:= msn_lsb(7) [ 7 ];
ts_scaled =:= scaled_ts_lsb(time_stride.UVALUE, 6) [ 6 ];
marker =:= irregular(1) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
}
// UOR-2-ID replacement
COMPRESSED pt_2_seq_id {
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator =:= '11000' [ 5 ];
msn =:= msn_lsb(7) [ 7 ];
ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 5) [ 5 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
timestamp =:= inferred_scaled_field [ 0 ];
}
// UOR-2-ID-ext1 replacement (both TS and IP-ID)
COMPRESSED pt_2_seq_both {
ENFORCE(ts_stride.UVALUE != 0);
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator =:= '11001' [ 5 ];
msn =:= msn_lsb(7) [ 7 ];
ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 5) [ 5 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ts_scaled =:= scaled_ts_lsb(time_stride.UVALUE, 7) [ 7 ];
marker =:= irregular(1) [ 1 ];
}
// UOR-2-TS replacement
COMPRESSED pt_2_seq_ts {
ENFORCE(ts_stride.UVALUE != 0);
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator =:= '1101' [ 4 ];
msn =:= msn_lsb(7) [ 7 ];
ts_scaled =:= scaled_ts_lsb(time_stride.UVALUE, 5) [ 5 ];
marker =:= irregular(1) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
}
////////////////////////////////////////////
// UDP profile
////////////////////////////////////////////
udp_baseheader(profile_value, outer_ip_flag, ip_id_behavior_value,
reorder_ratio_value)
{
UNCOMPRESSED v4 {
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
ip_version =:= uncompressed_value(4, 4) [ 4 ];
header_length =:= uncompressed_value(4, 5) [ 4 ];
tos_tc [ 8 ];
length =:= inferred_ip_v4_length [ 16 ];
ip_id [ 16 ];
rf =:= uncompressed_value(1, 0) [ 1 ];
df [ 1 ];
mf =:= uncompressed_value(1, 0) [ 1 ];
frag_offset =:= uncompressed_value(13, 0) [ 13 ];
ttl_hopl [ 8 ];
next_header [ 8 ];
ip_checksum =:= inferred_ip_v4_header_checksum [ 16 ];
src_addr [ 32 ];
dest_addr [ 32 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
src_port [ 16 ];
dst_port [ 16 ];
udp_length =:= inferred_udp_length [ 16 ];
udp_checksum [ 16 ];
}
UNCOMPRESSED v6 {
ENFORCE(ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM);
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
ip_version =:= uncompressed_value(4, 6) [ 4 ];
tos_tc [ 8 ];
flow_label [ 20 ];
payload_length =:= inferred_ip_v6_length [ 16 ];
next_header [ 8 ];
ttl_hopl [ 8 ];
src_addr [ 128 ];
dest_addr [ 128 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
src_port [ 16 ];
dst_port [ 16 ];
udp_length =:= inferred_udp_length [ 16 ];
udp_checksum [ 16 ];
df =:= uncompressed_value(0,0) [ 0 ];
ip_id =:= uncompressed_value(0,0) [ 0 ];
}
CONTROL {
ENFORCE(profile_value == PROFILE_UDP_0102);
ENFORCE(profile == profile_value);
ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
ENFORCE(ip_id_behavior_innermost.UVALUE == ip_id_behavior_value);
}
DEFAULT {
ENFORCE(outer_ip_flag == 0);
tos_tc =:= static;
dest_addr =:= static;
ip_version =:= static;
ttl_hopl =:= static;
src_addr =:= static;
df =:= static;
flow_label =:= static;
next_header =:= static;
src_port =:= static;
dst_port =:= static;
reorder_ratio =:= static;
ip_id_behavior_innermost =:= static;
}
// Replacement for UOR-2-ext3
COMPRESSED co_common {
ENFORCE(outer_ip_flag == outer_ip_indicator.CVALUE);
discriminator =:= '11111010' [ 8 ];
ip_id_indicator =:= irregular(1) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
flags_indicator =:= irregular(1) [ 1 ];
ttl_hopl_indicator =:= irregular(1) [ 1 ];
tos_tc_indicator =:= irregular(1) [ 1 ];
reorder_ratio =:= irregular(2) [ 2 ];
control_crc3 =:= control_crc3_encoding [ 3 ];
outer_ip_indicator : df : ip_id_behavior_innermost =:=
profile_2_3_4_flags_enc(
flags_indicator.CVALUE, ip_version.UVALUE) [ 0, 8 ];
tos_tc =:= static_or_irreg(tos_tc_indicator.CVALUE, 8) [ 0, 8 ];
ttl_hopl =:= static_or_irreg(ttl_hopl_indicator.CVALUE,
ttl_hopl.ULENGTH) [ 0, 8 ];
msn =:= msn_lsb(8) [ 8 ];
ip_id =:= ip_id_sequential_variable(ip_id_behavior_innermost.UVALUE,
ip_id_indicator.CVALUE) [ 0, 8, 16 ];
}
// UO-0
COMPRESSED pt_0_crc3 {
discriminator =:= '0' [ 1 ];
msn =:= msn_lsb(4) [ 4 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// New format, Type 0 with strong CRC and more SN bits
COMPRESSED pt_0_crc7 {
discriminator =:= '100' [ 3 ];
msn =:= msn_lsb(6) [ 6 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// UO-1-ID replacement (PT-1 only used for sequential)
COMPRESSED pt_1_seq_id {
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator =:= '101' [ 3 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
msn =:= msn_lsb(6) [ 6 ];
ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 4) [ 4 ];
}
// UOR-2-ID replacement
COMPRESSED pt_2_seq_id {
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator =:= '110' [ 3 ];
ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 6) [ 6 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
msn =:= msn_lsb(8) [ 8 ];
}
}
////////////////////////////////////////////
// ESP profile
////////////////////////////////////////////
esp_baseheader(profile_value, outer_ip_flag, ip_id_behavior_value,
reorder_ratio_value)
{
UNCOMPRESSED v4 {
ENFORCE(msn.UVALUE == sequence_number.UVALUE % 65536);
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
ip_version =:= uncompressed_value(4, 4) [ 4 ];
header_length =:= uncompressed_value(4, 5) [ 4 ];
tos_tc [ 8 ];
length =:= inferred_ip_v4_length [ 16 ];
ip_id [ 16 ];
rf =:= uncompressed_value(1, 0) [ 1 ];
df [ 1 ];
mf =:= uncompressed_value(1, 0) [ 1 ];
frag_offset =:= uncompressed_value(13, 0) [ 13 ];
ttl_hopl [ 8 ];
next_header [ 8 ];
ip_checksum =:= inferred_ip_v4_header_checksum [ 16 ];
src_addr [ 32 ];
dest_addr [ 32 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
spi [ 32 ];
sequence_number [ 32 ];
}
UNCOMPRESSED v6 {
ENFORCE(msn.UVALUE == (sequence_number.UVALUE % 65536));
ENFORCE(ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_RANDOM);
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
ip_version =:= uncompressed_value(4, 6) [ 4 ];
tos_tc [ 8 ];
flow_label [ 20 ];
payload_length =:= inferred_ip_v6_length [ 16 ];
next_header [ 8 ];
ttl_hopl [ 8 ];
src_addr [ 128 ];
dest_addr [ 128 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
spi [ 32 ];
sequence_number [ 32 ];
df =:= uncompressed_value(0,0) [ 0 ];
ip_id =:= uncompressed_value(0,0) [ 0 ];
}
CONTROL {
ENFORCE(profile_value == PROFILE_ESP_0103);
ENFORCE(profile == profile_value);
ENFORCE(ip_id_behavior_innermost.UVALUE == ip_id_behavior_value);
ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
}
DEFAULT {
ENFORCE(outer_ip_flag == 0);
tos_tc =:= static;
dest_addr =:= static;
ttl_hopl =:= static;
src_addr =:= static;
df =:= static;
flow_label =:= static;
next_header =:= static;
spi =:= static;
sequence_number =:= static;
reorder_ratio =:= static;
ip_id_behavior_innermost =:= static;
}
// Replacement for UOR-2-ext3
COMPRESSED co_common {
ENFORCE(outer_ip_flag == outer_ip_indicator.CVALUE);
discriminator =:= '11111010' [ 8 ];
ip_id_indicator =:= irregular(1) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
flags_indicator =:= irregular(1) [ 1 ];
ttl_hopl_indicator =:= irregular(1) [ 1 ];
tos_tc_indicator =:= irregular(1) [ 1 ];
reorder_ratio =:= irregular(2) [ 2 ];
control_crc3 =:= control_crc3_encoding [ 3 ];
outer_ip_indicator : df : ip_id_behavior_innermost =:=
profile_2_3_4_flags_enc(
flags_indicator.CVALUE, ip_version.UVALUE) [ 0, 8 ];
tos_tc =:= static_or_irreg(tos_tc_indicator.CVALUE, 8) [ 0, 8 ];
ttl_hopl =:= static_or_irreg(ttl_hopl_indicator.CVALUE,
ttl_hopl.ULENGTH) [ 0, 8 ];
sequence_number =:=
sdvl_sn_lsb(sequence_number.ULENGTH) [ VARIABLE ];
ip_id =:= ip_id_sequential_variable(ip_id_behavior_innermost.UVALUE,
ip_id_indicator.CVALUE) [ 0, 8, 16 ];
}
// Sequence number sent instead of MSN due to field length
// UO-0
COMPRESSED pt_0_crc3 {
discriminator =:= '0' [ 1 ];
sequence_number =:= msn_lsb(4) [ 4 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// New format, Type 0 with strong CRC and more SN bits
COMPRESSED pt_0_crc7 {
discriminator =:= '100' [ 3 ];
sequence_number =:= msn_lsb(6) [ 6 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// UO-1-ID replacement (PT-1 only used for sequential)
COMPRESSED pt_1_seq_id {
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator =:= '101' [ 3 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
sequence_number =:= msn_lsb(6) [ 6 ];
ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 4) [ 4 ];
}
// UOR-2-ID replacement
COMPRESSED pt_2_seq_id {
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator =:= '110' [ 3 ];
ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 6) [ 6 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
sequence_number =:= msn_lsb(8) [ 8 ];
}
}
////////////////////////////////////////////
// IP-only profile
////////////////////////////////////////////
iponly_baseheader(profile_value, outer_ip_flag, ip_id_behavior_value,
reorder_ratio_value)
{
UNCOMPRESSED v4 {
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
ip_version =:= uncompressed_value(4, 4) [ 4 ];
header_length =:= uncompressed_value(4, 5) [ 4 ];
tos_tc [ 8 ];
length =:= inferred_ip_v4_length [ 16 ];
ip_id [ 16 ];
rf =:= uncompressed_value(1, 0) [ 1 ];
df [ 1 ];
mf =:= uncompressed_value(1, 0) [ 1 ];
frag_offset =:= uncompressed_value(13, 0) [ 13 ];
ttl_hopl [ 8 ];
next_header [ 8 ];
ip_checksum =:= inferred_ip_v4_header_checksum [ 16 ];
src_addr [ 32 ];
dest_addr [ 32 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
}
UNCOMPRESSED v6 {
ENFORCE(ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_RANDOM);
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
ip_version =:= uncompressed_value(4, 6) [ 4 ];
tos_tc [ 8 ];
flow_label [ 20 ];
payload_length =:= inferred_ip_v6_length [ 16 ];
next_header [ 8 ];
ttl_hopl [ 8 ];
src_addr [ 128 ];
dest_addr [ 128 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
df =:= uncompressed_value(0,0) [ 0 ];
ip_id =:= uncompressed_value(0,0) [ 0 ];
}
CONTROL {
ENFORCE(profile_value == PROFILE_IP_0104);
ENFORCE(profile == profile_value);
ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
ENFORCE(ip_id_behavior_innermost.UVALUE == ip_id_behavior_value);
}
DEFAULT {
ENFORCE(outer_ip_flag == 0);
tos_tc =:= static;
dest_addr =:= static;
ttl_hopl =:= static;
src_addr =:= static;
df =:= static;
flow_label =:= static;
next_header =:= static;
reorder_ratio =:= static;
ip_id_behavior_innermost =:= static;
}
// Replacement for UOR-2-ext3
COMPRESSED co_common {
ENFORCE(outer_ip_flag == outer_ip_indicator.CVALUE);
discriminator =:= '11111010' [ 8 ];
ip_id_indicator =:= irregular(1) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
flags_indicator =:= irregular(1) [ 1 ];
ttl_hopl_indicator =:= irregular(1) [ 1 ];
tos_tc_indicator =:= irregular(1) [ 1 ];
reorder_ratio =:= irregular(2) [ 2 ];
control_crc3 =:= control_crc3_encoding [ 3 ];
outer_ip_indicator : df : ip_id_behavior_innermost =:=
profile_2_3_4_flags_enc(
flags_indicator.CVALUE, ip_version.UVALUE) [ 0, 8 ];
tos_tc =:= static_or_irreg(tos_tc_indicator.CVALUE, 8) [ 0, 8 ];
ttl_hopl =:= static_or_irreg(ttl_hopl_indicator.CVALUE,
ttl_hopl.ULENGTH) [ 0, 8 ];
msn =:= msn_lsb(8) [ 8 ];
ip_id =:= ip_id_sequential_variable(ip_id_behavior_innermost.UVALUE,
ip_id_indicator.CVALUE) [ 0, 8, 16 ];
}
// UO-0
COMPRESSED pt_0_crc3 {
discriminator =:= '0' [ 1 ];
msn =:= msn_lsb(4) [ 4 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// New format, Type 0 with strong CRC and more SN bits
COMPRESSED pt_0_crc7 {
discriminator =:= '100' [ 3 ];
msn =:= msn_lsb(6) [ 6 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// UO-1-ID replacement (PT-1 only used for sequential)
COMPRESSED pt_1_seq_id {
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator =:= '101' [ 3 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
msn =:= msn_lsb(6) [ 6 ];
ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 4) [ 4 ];
}
// UOR-2-ID replacement
COMPRESSED pt_2_seq_id {
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator =:= '110' [ 3 ];
ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 6) [ 6 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
msn =:= msn_lsb(8) [ 8 ];
}
}
////////////////////////////////////////////
// UDP-lite/RTP profile
////////////////////////////////////////////
udplite_rtp_baseheader(profile_value, ts_stride_value,
time_stride_value, outer_ip_flag,
ip_id_behavior_value, reorder_ratio_value,
coverage_behavior_value)
{
UNCOMPRESSED v4 {
ENFORCE(msn.UVALUE == sequence_number.UVALUE);
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
ip_version =:= uncompressed_value(4, 4) [ 4 ];
header_length =:= uncompressed_value(4, 5) [ 4 ];
tos_tc [ 8 ];
length =:= inferred_ip_v4_length [ 16 ];
ip_id [ 16 ];
rf =:= uncompressed_value(1, 0) [ 1 ];
df [ 1 ];
mf =:= uncompressed_value(1, 0) [ 1 ];
frag_offset =:= uncompressed_value(13, 0) [ 13 ];
ttl_hopl [ 8 ];
next_header [ 8 ];
ip_checksum =:= inferred_ip_v4_header_checksum [ 16 ];
src_addr [ 32 ];
dest_addr [ 32 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
src_port [ 16 ];
dst_port [ 16 ];
checksum_coverage [ 16 ];
udp_checksum [ 16 ];
rtp_version =:= uncompressed_value(2, 2) [ 2 ];
pad_bit [ 1 ];
extension [ 1 ];
cc [ 4 ];
marker [ 1 ];
payload_type [ 7 ];
sequence_number [ 16 ];
timestamp [ 32 ];
ssrc [ 32 ];
csrc_list [ VARIABLE ];
}
UNCOMPRESSED v6 {
ENFORCE(ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_RANDOM);
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
ip_version =:= uncompressed_value(4, 6) [ 4 ];
tos_tc [ 8 ];
flow_label [ 20 ];
payload_length =:= inferred_ip_v6_length [ 16 ];
next_header [ 8 ];
ttl_hopl [ 8 ];
src_addr [ 128 ];
dest_addr [ 128 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
src_port [ 16 ];
dst_port [ 16 ];
checksum_coverage [ 16 ];
udp_checksum [ 16 ];
rtp_version =:= uncompressed_value(2, 2) [ 2 ];
pad_bit [ 1 ];
extension [ 1 ];
cc [ 4 ];
marker [ 1 ];
payload_type [ 7 ];
sequence_number [ 16 ];
timestamp [ 32 ];
ssrc [ 32 ];
csrc_list [ VARIABLE ];
df =:= uncompressed_value(0,0) [ 0 ];
ip_id =:= uncompressed_value(0,0) [ 0 ];
}
CONTROL {
ENFORCE(profile_value == PROFILE_RTP_0107);
ENFORCE(profile == profile_value);
ENFORCE(time_stride.UVALUE == time_stride_value);
ENFORCE(ts_stride.UVALUE == ts_stride_value);
ENFORCE(coverage_behavior.UVALUE == coverage_behavior_value);
ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
ENFORCE(ip_id_behavior_innermost.UVALUE == ip_id_behavior_value);
dummy_field =:= field_scaling(ts_stride.UVALUE,
ts_scaled.UVALUE, timestamp.UVALUE, ts_offset.UVALUE) [ 0 ];
}
INITIAL {
ts_stride =:= uncompressed_value(32, TS_STRIDE_DEFAULT);
time_stride =:= uncompressed_value(32, TIME_STRIDE_DEFAULT);
}
DEFAULT {
ENFORCE(outer_ip_flag == 0);
tos_tc =:= static;
dest_addr =:= static;
ttl_hopl =:= static;
src_addr =:= static;
df =:= static;
flow_label =:= static;
next_header =:= static;
src_port =:= static;
dst_port =:= static;
pad_bit =:= static;
extension =:= static;
cc =:= static;
// When marker not present in packets, it is assumed 0
marker =:= uncompressed_value(1, 0);
payload_type =:= static;
sequence_number =:= static;
timestamp =:= static;
ssrc =:= static;
csrc_list =:= static;
ts_stride =:= static;
time_stride =:= static;
ts_scaled =:= static;
ts_offset =:= static;
reorder_ratio =:= static;
ip_id_behavior_innermost =:= static;
}
// Replacement for UOR-2-ext3
COMPRESSED co_common {
ENFORCE(outer_ip_flag == outer_ip_indicator.CVALUE);
discriminator =:= '11111010' [ 8 ];
marker =:= irregular(1) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
flags1_indicator =:= irregular(1) [ 1 ];
flags2_indicator =:= irregular(1) [ 1 ];
tsc_indicator =:= irregular(1) [ 1 ];
tss_indicator =:= irregular(1) [ 1 ];
ip_id_indicator =:= irregular(1) [ 1 ];
control_crc3 =:= control_crc3_encoding [ 3 ];
outer_ip_indicator : ttl_hopl_indicator :
tos_tc_indicator : df : ip_id_behavior_innermost : reorder_ratio
=:= profile_1_7_flags1_enc(flags1_indicator.CVALUE,
ip_version.UVALUE) [ 0, 8 ];
list_indicator : pt_indicator : tis_indicator : pad_bit :
extension : coverage_behavior =:=
profile_7_flags2_enc(flags2_indicator.CVALUE) [ 0, 8 ];
tos_tc =:= static_or_irreg(tos_tc_indicator.CVALUE, 8) [ 0, 8 ];
ttl_hopl =:=
static_or_irreg(ttl_hopl_indicator.CVALUE, 8) [ 0, 8 ];
payload_type =:= pt_irr_or_static(pt_indicator.CVALUE) [ 0, 8 ];
sequence_number =:=
sdvl_sn_lsb(sequence_number.ULENGTH) [ VARIABLE ];
ip_id =:= ip_id_sequential_variable(ip_id_behavior_innermost.UVALUE,
ip_id_indicator.CVALUE) [ 0, 8, 16 ];
ts_scaled =:= variable_scaled_timestamp(tss_indicator.CVALUE,
tsc_indicator.CVALUE, ts_stride.UVALUE,
time_stride.UVALUE) [ VARIABLE ];
timestamp =:= variable_unscaled_timestamp(tss_indicator.CVALUE,
tsc_indicator.CVALUE) [ VARIABLE ];
ts_stride =:= sdvl_or_static(tss_indicator.CVALUE) [ VARIABLE ];
time_stride =:= sdvl_or_static(tis_indicator.CVALUE) [ VARIABLE ];
csrc_list =:=
csrc_list_presence(list_indicator.CVALUE,
cc.UVALUE) [ VARIABLE ];
}
// UO-0
COMPRESSED pt_0_crc3 {
discriminator =:= '0' [ 1 ];
msn =:= msn_lsb(4) [ 4 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
timestamp =:= inferred_scaled_field [ 0 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// New format, Type 0 with strong CRC and more SN bits
COMPRESSED pt_0_crc7 {
discriminator =:= '1000' [ 4 ];
msn =:= msn_lsb(5) [ 5 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
timestamp =:= inferred_scaled_field [ 0 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// UO-1 replacement
COMPRESSED pt_1_rnd {
ENFORCE(ts_stride.UVALUE != 0);
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_ZERO));
discriminator =:= '101' [ 3 ];
marker =:= irregular(1) [ 1 ];
msn =:= msn_lsb(4) [ 4 ];
ts_scaled =:= scaled_ts_lsb(time_stride.UVALUE, 5) [ 5 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
}
// UO-1-ID replacement
COMPRESSED pt_1_seq_id {
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator =:= '1001' [ 4 ];
ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 4) [ 4 ];
msn =:= msn_lsb(5) [ 5 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
timestamp =:= inferred_scaled_field [ 0 ];
}
// UO-1-TS replacement
COMPRESSED pt_1_seq_ts {
ENFORCE(ts_stride.UVALUE != 0);
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator =:= '101' [ 3 ];
marker =:= irregular(1) [ 1 ];
msn =:= msn_lsb(4) [ 4 ];
ts_scaled =:= scaled_ts_lsb(time_stride.UVALUE, 5) [ 5 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// UOR-2 replacement
COMPRESSED pt_2_rnd {
ENFORCE(ts_stride.UVALUE != 0);
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_ZERO));
discriminator =:= '110' [ 3 ];
msn =:= msn_lsb(7) [ 7 ];
ts_scaled =:= scaled_ts_lsb(time_stride.UVALUE, 6) [ 6 ];
marker =:= irregular(1) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
}
// UOR-2-ID replacement
COMPRESSED pt_2_seq_id {
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator =:= '11000' [ 5 ];
msn =:= msn_lsb(7) [ 7 ];
ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 5) [ 5 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
timestamp =:= inferred_scaled_field [ 0 ];
}
// UOR-2-ID-ext1 replacement (both TS and IP-ID)
COMPRESSED pt_2_seq_both {
ENFORCE(ts_stride.UVALUE != 0);
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator =:= '11001' [ 5 ];
msn =:= msn_lsb(7) [ 7 ];
ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 5) [ 5 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ts_scaled =:= scaled_ts_lsb(time_stride.UVALUE, 7) [ 7 ];
marker =:= irregular(1) [ 1 ];
}
// UOR-2-TS replacement
COMPRESSED pt_2_seq_ts {
ENFORCE(ts_stride.UVALUE != 0);
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator =:= '1101' [ 4 ];
msn =:= msn_lsb(7) [ 7 ];
ts_scaled =:= scaled_ts_lsb(time_stride.UVALUE, 5) [ 5 ];
marker =:= irregular(1) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
}
////////////////////////////////////////////
// UDP-lite profile
////////////////////////////////////////////
udplite_baseheader(profile_value, outer_ip_flag, ip_id_behavior_value,
reorder_ratio_value, coverage_behavior_value)
{
UNCOMPRESSED v4 {
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
ip_version =:= uncompressed_value(4, 4) [ 4 ];
header_length =:= uncompressed_value(4, 5) [ 4 ];
tos_tc [ 8 ];
length =:= inferred_ip_v4_length [ 16 ];
ip_id [ 16 ];
rf =:= uncompressed_value(1, 0) [ 1 ];
df [ 1 ];
mf =:= uncompressed_value(1, 0) [ 1 ];
frag_offset =:= uncompressed_value(13, 0) [ 13 ];
ttl_hopl [ 8 ];
next_header [ 8 ];
ip_checksum =:= inferred_ip_v4_header_checksum [ 16 ];
src_addr [ 32 ];
dest_addr [ 32 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
src_port [ 16 ];
dst_port [ 16 ];
checksum_coverage [ 16 ];
udp_checksum [ 16 ];
}
UNCOMPRESSED v6 {
ENFORCE(ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_RANDOM);
outer_headers =:= baseheader_outer_headers [ VARIABLE ];
ip_version =:= uncompressed_value(4, 6) [ 4 ];
tos_tc [ 8 ];
flow_label [ 20 ];
payload_length =:= inferred_ip_v6_length [ 16 ];
next_header [ 8 ];
ttl_hopl [ 8 ];
src_addr [ 128 ];
dest_addr [ 128 ];
extension_headers =:= baseheader_extension_headers [ VARIABLE ];
src_port [ 16 ];
dst_port [ 16 ];
checksum_coverage [ 16 ];
udp_checksum [ 16 ];
df =:= uncompressed_value(0,0) [ 0 ];
ip_id =:= uncompressed_value(0,0) [ 0 ];
}
CONTROL {
ENFORCE(profile_value == PROFILE_UDPLITE_0108);
ENFORCE(profile == profile_value);
ENFORCE(coverage_behavior.UVALUE == coverage_behavior_value);
ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
ENFORCE(ip_id_behavior_innermost.UVALUE == ip_id_behavior_value);
}
DEFAULT {
ENFORCE(outer_ip_flag == 0);
tos_tc =:= static;
dest_addr =:= static;
ttl_hopl =:= static;
src_addr =:= static;
df =:= static;
flow_label =:= static;
next_header =:= static;
src_port =:= static;
dst_port =:= static;
reorder_ratio =:= static;
ip_id_behavior_innermost =:= static;
}
// Replacement for UOR-2-ext3
COMPRESSED co_common {
ENFORCE(outer_ip_flag == outer_ip_indicator.CVALUE);
discriminator =:= '11111010' [ 8 ];
ip_id_indicator =:= irregular(1) [ 1 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
flags_indicator =:= irregular(1) [ 1 ];
ttl_hopl_indicator =:= irregular(1) [ 1 ];
tos_tc_indicator =:= irregular(1) [ 1 ];
reorder_ratio =:= irregular(2) [ 2 ];
control_crc3 =:= control_crc3_encoding [ 3 ];
outer_ip_indicator : df : ip_id_behavior_innermost :
coverage_behavior =:=
profile_8_flags_enc(flags_indicator.CVALUE,
ip_version.UVALUE) [ 0, 8 ];
tos_tc =:= static_or_irreg(tos_tc_indicator.CVALUE, 8) [ 0, 8 ];
ttl_hopl =:= static_or_irreg(ttl_hopl_indicator.CVALUE,
ttl_hopl.ULENGTH) [ 0, 8 ];
msn =:= msn_lsb(8) [ 8 ];
ip_id =:= ip_id_sequential_variable(ip_id_behavior_innermost.UVALUE,
ip_id_indicator.CVALUE) [ 0, 8, 16 ];
}
// UO-0
COMPRESSED pt_0_crc3 {
discriminator =:= '0' [ 1 ];
msn =:= msn_lsb(4) [ 4 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// New format, Type 0 with strong CRC and more SN bits
COMPRESSED pt_0_crc7 {
discriminator =:= '100' [ 3 ];
msn =:= msn_lsb(6) [ 6 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
ip_id =:= inferred_sequential_ip_id [ 0 ];
}
// UO-1-ID replacement (PT-1 only used for sequential)
COMPRESSED pt_1_seq_id {
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator =:= '101' [ 3 ];
header_crc =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
msn =:= msn_lsb(6) [ 6 ];
ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 4) [ 4 ];
}
// UOR-2-ID replacement
COMPRESSED pt_2_seq_id {
ENFORCE((ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior_innermost.UVALUE ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator =:= '110' [ 3 ];
ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 6) [ 6 ];
header_crc =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
msn =:= msn_lsb(8) [ 8 ];
}
}
6.9. Feedback Formats and Options
6.9.1. Feedback Formats
This section describes the feedback format for ROHCv2 profiles, using
the formats described in Section 5.2.3 of [RFC4995].
The Acknowledgment Number field of the feedback formats contains the
least significant bits of the MSN (see Section 6.3.1) that
corresponds to the reference header that is being acknowledged. A
reference header is a header that has been successfully CRC-8
validated or CRC verified. If there is no reference header
available, the feedback MUST carry an ACKNUMBER-NOT-VALID option.
FEEDBACK-1
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Acknowledgment Number |
+---+---+---+---+---+---+---+---+
Acknowledgment Number: The eight least significant bits of the
MSN.
A FEEDBACK-1 is an ACK. In order to send a NACK or a STATIC-NACK,
FEEDBACK-2 must be used.
FEEDBACK-2
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
|Acktype| Acknowledgment Number |
+---+---+---+---+---+---+---+---+
| Acknowledgment Number |
+---+---+---+---+---+---+---+---+
| CRC |
+---+---+---+---+---+---+---+---+
/ Feedback options /
+---+---+---+---+---+---+---+---+
Acktype:
0 = ACK
1 = NACK
2 = STATIC-NACK
3 is reserved (MUST NOT be used for parsability)
Acknowledgment Number: The least significant bits of the MSN.
CRC: 8-bit CRC computed over the entire feedback payload including
any CID fields but excluding the feedback type, the 'Size' field,
and the 'Code' octet, using the polynomial defined in Section
5.3.1.1 of [RFC4995]. If the CID is given with an Add-CID octet,
the Add-CID octet immediately precedes the FEEDBACK-1 or
FEEDBACK-2 format. For purposes of computing the CRC, the CRC
field is zero.
Feedback options: A variable number of feedback options, see
Section 6.9.2. Options may appear in any order.
A FEEDBACK-2 of type NACK or STATIC-NACK is always implicitly an
acknowledgment for a successfully decompressed packet, which
corresponds to a packet whose LSBs match the Acknowledgment Number of
the feedback element, unless the ACKNUMBER-NOT-VALID option (see
Section 6.9.2.2) appears in the feedback element.
The FEEDBACK-2 format always carries a CRC and is thus more robust
than the FEEDBACK-1 format. When receiving FEEDBACK-2, the
compressor MUST verify the information by computing the CRC and
comparing the result with the CRC carried in the feedback format. If
the two are not identical, the feedback element MUST be discarded.
6.9.2. Feedback Options
A feedback option has variable length and the following general
format:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Opt Type | Opt Len |
+---+---+---+---+---+---+---+---+
/ Option Data / Opt Len (octets)
+---+---+---+---+---+---+---+---+
Opt Type: Unsigned integer that represents the type of the
feedback option. Section 6.9.2.1 through Section 6.9.2.4
describes the ROHCv2 feedback options.
Opt Len: Unsigned integer that represents the length of the Option
Data field, in octets.
Option Data: Feedback type specific data. Present if the value of
the Opt Len field is set to a non-zero value.
6.9.2.1. The REJECT Option
The REJECT option informs the compressor that the decompressor does
not have sufficient resources to handle the flow.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Opt Type = 2 | Opt Len = 0 |
+---+---+---+---+---+---+---+---+
When receiving a REJECT option, the compressor MUST stop compressing
the packet flow, and SHOULD refrain from attempting to increase the
number of compressed packet flows for some time. The REJECT option
MUST NOT appear more than once in the FEEDBACK-2 format; otherwise,
the compressor MUST discard the entire feedback element.
6.9.2.2. The ACKNUMBER-NOT-VALID Option
The ACKNUMBER-NOT-VALID option indicates that the Acknowledgment
Number field of the feedback is not valid.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Opt Type = 3 | Opt Len = 0 |
+---+---+---+---+---+---+---+---+
A compressor MUST NOT use the Acknowledgment Number of the feedback
to find the corresponding sent header when this option is present.
When this option is used, the Acknowledgment Number field of the
FEEDBACK-2 format is set to zero. Consequently, a NACK or a STATIC-
NACK feedback type sent with the ACKNUMBER-NOT-VALID option is
equivalent to a STATIC-NACK with respect to the type of context
repair requested by the decompressor.
The ACKNUMBER-NOT-VALID option MUST NOT appear more than once in the
FEEDBACK-2 format; otherwise, the compressor MUST discard the entire
feedback element.
6.9.2.3. The CONTEXT_MEMORY Option
The CONTEXT_MEMORY option informs the compressor that the
decompressor does not have sufficient memory resources to handle the
context of the packet flow, as the flow is currently compressed.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Opt Type = 9 | Opt Len = 0 |
+---+---+---+---+---+---+---+---+
When receiving a CONTEXT_MEMORY option, the compressor SHOULD take
actions to compress the packet flow in a way that requires less
decompressor memory resources or stop compressing the packet flow.
The CONTEXT_MEMORY option MUST NOT appear more than once in the
FEEDBACK-2 format; otherwise, the compressor MUST discard the entire
feedback element.
6.9.2.4. The CLOCK_RESOLUTION Option
The CLOCK_RESOLUTION option informs the compressor of the clock
resolution of the decompressor. It also informs whether or not the
decompressor supports timer-based compression of the RTP TS timestamp
(see Section 6.6.9). The CLOCK_RESOLUTION option is applicable per
channel, i.e., it applies to any context associated with a profile
for which the option is relevant between a compressor and
decompressor pair.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Opt Type = 10 | Opt Len = 1 |
+---+---+---+---+---+---+---+---+
| Clock resolution (ms) |
+---+---+---+---+---+---+---+---+
Clock resolution: Unsigned integer that represents the clock
resolution of the decompressor expressed in milliseconds.
The smallest clock resolution that can be indicated is 1 millisecond.
The value zero has a special meaning: it indicates that the
decompressor cannot do timer-based compression of the RTP Timestamp.
The CLOCK_RESOLUTION option MUST NOT appear more than once in the
FEEDBACK-2 format; otherwise, the compressor MUST discard the entire
feedback element.
6.9.2.5. Unknown Option Types
If an option type other than those defined in this document is
encountered, the compressor MUST discard the entire feedback element.
7. Security Considerations
Impairments such as bit errors on the received compressed headers,
missing packets, and reordering between packets could cause the
header decompressor to reconstitute erroneous packets, i.e., packets
that do not match the original packet, but still have a valid IP, UDP
(or UDP-Lite), and RTP headers, and possibly also valid UDP (or UDP-
Lite) checksums.
The header compression profiles defined herein use an internal
checksum for verification of reconstructed headers. This reduces the
probability that a header decompressor delivers erroneous packets to
upper layers without the error being noticed. In particular, the
probability that consecutive erroneous packets are not detected by
the internal checksum is close to zero.
This small but non-zero probability remains unchanged when integrity
protection is applied after compression and verified before
decompression, in the case where an attacker could discard or reorder
packets between the compression endpoints.
The impairments mentioned above could be caused by a malfunctioning
or malicious header compressor. Such corruption may be detected with
end-to-end integrity mechanisms that will not be affected by the
compression. Moreover, the internal checksum can also be useful in
the case of malfunctioning compressors.
Denial-of-service attacks are possible if an intruder can introduce
(for example) bogus IR or FEEDBACK packets onto the link and thereby
cause compression efficiency to be reduced. However, an intruder
having the ability to inject arbitrary packets at the link layer in
this manner raises additional security issues that dwarf those
related to the use of header compression.
8. IANA Considerations
The following ROHC profile identifiers have been assigned by the IANA
for the profiles defined in this document:
Identifier Profile
---------- -------
0x0101 ROHCv2 RTP
0x0102 ROHCv2 UDP
0x0103 ROHCv2 ESP
0x0104 ROHCv2 IP
0x0107 ROHCv2 RTP/UDP-Lite
0x0108 ROHCv2 UDP-Lite
9. Acknowledgements
The authors would like to thank Mark West, Robert Finking, Haipeng
Jin, and Rohit Kapoor for serving as committed document reviewers,
and also for constructive discussions during the development of this
document. Thanks to Carl Knutsson for his extensive contribution to
this specification, as well as to Jani Juvan and Anders Edqvist for
useful comments and feedback. Thanks also to Elwyn Davies for his
review as the General Area Review Team (Gen-ART) reviewer, and to
Stephen Kent for his review on behalf of the IETF security
directorate, during IETF last-call. Finally, thanks to the many
people who have contributed to previous ROHC specifications and
supported this effort.
10. References
10.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC2004] Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
March 2000.
[RFC2890] Dommety, G., "Key and Sequence Number Extensions to GRE",
RFC 2890, September 2000.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
G. Fairhurst, "The Lightweight User Datagram Protocol
(UDP-Lite)", RFC 3828, July 2004.
[RFC4019] Pelletier, G., "RObust Header Compression (ROHC): Profiles
for User Datagram Protocol (UDP) Lite", RFC 4019,
April 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4995] Jonsson, L-E., Pelletier, G., and K. Sandlund, "The RObust
Header Compression (ROHC) Framework", RFC 4995, July 2007.
[RFC4997] Finking, R. and G. Pelletier, "Formal Notation for RObust
Header Compression (ROHC-FN)", RFC 4997, July 2007.
10.2. Informative References
[RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
RFC 2675, August 1999.
[RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le,
K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header
Compression (ROHC): Framework and four profiles: RTP, UDP,
ESP, and uncompressed", RFC 3095, July 2001.
[RFC3843] Jonsson, L-E. and G. Pelletier, "RObust Header Compression
(ROHC): A Compression Profile for IP", RFC 3843,
June 2004.
[RFC4224] Pelletier, G., Jonsson, L-E., and K. Sandlund, "RObust
Header Compression (ROHC): ROHC over Channels That Can
Reorder Packets", RFC 4224, January 2006.
Appendix A. Detailed Classification of Header Fields
Header compression is possible due to the fact that most header
fields do not vary randomly from packet to packet. Many of the
fields exhibit static behavior or change in a more or less
predictable way. When designing a header compression scheme, it is
of fundamental importance to understand the behavior of the fields in
detail.
In this appendix, all fields in the headers compressible by these
profiles are classified and analyzed. The analysis is based on
behavior for the types of traffic that are expected to be the most
frequently compressed (e.g., RTP field behavior is based on voice
and/or video traffic behavior).
Fields are classified as belonging to one of the following classes:
INFERRED - These fields contain values that can be inferred from
other values, for example the size of the frame carrying the packet,
and thus do not have to be included in compressed packets.
STATIC - These fields are expected to be constant throughout the
lifetime of the flow; in general, it is sufficient to design a
compressed format so that these fields are only updated by IR
packets.
STATIC-DEF - These fields are expected to be constant throughout the
lifetime of the flow and their values can be used to define a flow.
They are only sent in IR packets.
STATIC-KNOWN - These fields are expected to have well-known values
and therefore do not need to be communicated at all.
SEMISTATIC - These fields are unchanged most of the time. However,
occasionally the value changes but will revert to its original value.
For ROHCv2, the values of such fields do not need to be possible to
change with the smallest compressed packet formats, but should be
possible to change via slightly larger compressed packets.
RARELY CHANGING (RACH) - These are fields that change their values
occasionally and then keep their new values. For ROHCv2, the values
of such fields do not need to be possible to change with the smallest
compressed packet formats, but should be possible to change via
slightly larger compressed packets.
IRREGULAR - These are the fields for which no useful change pattern
can be identified and should be transmitted uncompressed in all
compressed packets.
PATTERN - These are fields that change between each packet, but
change in a predictable pattern.
A.1. IPv4 Header Fields
+------------------------+----------------+
| Field | Class |
+------------------------+----------------+
| Version | STATIC-KNOWN |
| Header Length | STATIC-KNOWN |
| Type Of Service | RACH |
| Packet Length | INFERRED |
| Identification | |
| Sequential | PATTERN |
| Seq. swap | PATTERN |
| Random | IRREGULAR |
| Zero | STATIC |
| Reserved flag | STATIC-KNOWN |
| Don't Fragment flag | RACH |
| More Fragments flag | STATIC-KNOWN |
| Fragment Offset | STATIC-KNOWN |
| Time To Live | RACH |
| Protocol | STATIC-DEF |
| Header Checksum | INFERRED |
| Source Address | STATIC-DEF |
| Destination Address | STATIC-DEF |
+------------------------+----------------+
Version
The version field states which IP version is used and is set to
the value four.
Header Length
As long as no options are present in the IP header, the header
length is constant with the value five. If there are options, the
field could be RACH or STATIC-DEF, but only option-less headers
are compressed by ROHCv2 profiles. The field is therefore
classified as STATIC-KNOWN.
Type Of Service
For the type of flows compressed by the ROHCv2 profiles, the DSCP
(Differentiated Services Code Point) and ECN (Explicit Congestion
Notification) fields are expected to change relatively seldom.
Packet Length
Information about packet length is expected to be provided by the
link layer. The field is therefore classified as INFERRED.
IPv4 Identification
The Identification field (IP-ID) is used to identify what
fragments constitute a datagram when reassembling fragmented
datagrams. The IPv4 specification does not specify exactly how
this field is to be assigned values, only that each packet should
get an IP-ID that is unique for the source-destination pair and
protocol for the time the datagram (or any of its fragments) could
be alive in the network. This means that assignment of IP-ID
values can be done in various ways, but the expected behaviors
have been separated into four classes.
Sequential
In this behavior, the IP-ID is expected to increment by one for
most packets, but may increment by a value larger than one,
depending on the behavior of the transmitting IPv4 stack.
Sequential Swapped
When using this behavior, the IP-ID behaves as in the
Sequential behavior, but the two bytes of IP-ID are byte-
swapped. Therefore, the IP-ID can be swapped before
compression to make it behave exactly as the Sequential
behavior.
Random
Some IP stacks assign IP-ID values using a pseudo-random number
generator. There is thus no correlation between the ID values
of subsequent datagrams, and therefore there is no way to
predict the IP-ID value for the next datagram. For header
compression purposes, this means that the IP-ID field needs to
be sent uncompressed with each datagram, resulting in two extra
octets of header.
Zero
This behavior, although not a legal implementation of IPv4, is
sometimes seen in existing IPv4 stacks. When this behavior is
used, all IP packets have the IP-ID value set to zero.
Flags
The Reserved flag must be set to zero and is therefore classified
as STATIC-KNOWN. The Don't Fragment (DF) flag changes rarely and
is therefore classified as RACH. Finally, the More Fragments (MF)
flag is expected to be zero because IP fragments will not be
compressed by ROHC and is therefore classified as STATIC-KNOWN.
Fragment Offset
Under the assumption that no fragmentation occurs, the fragment
offset is always zero and is therefore classified as STATIC-KNOWN.
Time To Live
The Time To Live field is expected to be constant during the
lifetime of a flow or to alternate between a limited number of
values due to route changes.
Protocol
This field will have the same value in all packets of a flow and
is therefore classified as STATIC-DEF.
Header Checksum
The header checksum protects individual hops from processing a
corrupted header. When almost all IP header information is
compressed away, there is no point in having this additional
checksum; instead, it can be regenerated at the decompressor side.
The field is therefore classified as INFERRED.
Source and Destination addresses
These fields are part of the definition of a flow and must thus be
constant for all packets in the flow.
A.2. IPv6 Header Fields
+----------------------+----------------+
| Field | Class |
+----------------------+----------------+
| Version | STATIC-KNOWN |
| Traffic Class | RACH |
| Flow Label | STATIC-DEF |
| Payload Length | INFERRED |
| Next Header | STATIC-DEF |
| Hop Limit | RACH |
| Source Address | STATIC-DEF |
| Destination Address | STATIC-DEF |
+----------------------+----------------+
Version
The version field states which IP version is used and is set to
the value six.
Traffic Class
For the type of flows compressed by the ROHCv2 profiles, the DSCP
and ECN fields are expected to change relatively seldom.
Flow Label
This field may be used to identify packets belonging to a specific
flow. If it is not used, the value should be set to zero.
Otherwise, all packets belonging to the same flow must have the
same value in this field. The field is therefore classified as
STATIC-DEF.
Payload Length
Information about packet length (and, consequently, payload
length) is expected to be provided by the link layer. The field
is therefore classified as INFERRED.
Next Header
This field will have the same value in all packets of a flow and
is therefore classified as STATIC-DEF.
Hop Limit
The Hop Limit field is expected to be constant during the lifetime
of a flow or to alternate between a limited number of values due
to route changes.
Source and Destination addresses
These fields are part of the definition of a flow and must thus be
constant for all packets in the flow. The fields are therefore
classified as STATIC-DEF.
A.3. UDP Header Fields
+------------------+-------------+
| Field | Class |
+------------------+-------------+
| Source Port | STATIC-DEF |
| Destination Port | STATIC-DEF |
| Length | INFERRED |
| Checksum | |
| Disabled | STATIC |
| Enabled | IRREGULAR |
+------------------+-------------+
Source and Destination ports
These fields are part of the definition of a flow and must thus be
constant for all packets in the flow.
Length
Information about packet length is expected to be provided by the
link layer. The field is therefore classified as INFERRED.
Checksum
The checksum can be optional. If disabled, its value is
constantly zero and can be compressed away. If enabled, its value
depends on the payload, which for compression purposes is
equivalent to it changing randomly with every packet.
A.4. UDP-Lite Header Fields
+--------------------+-------------+
| Field | Class |
+--------------------+-------------+
| Source Port | STATIC-DEF |
| Destination Port | STATIC-DEF |
| Checksum Coverage | |
| Zero | STATIC-DEF |
| Constant | INFERRED |
| Variable | IRREGULAR |
| Checksum | IRREGULAR |
+--------------------+-------------+
Source and Destination Port
These fields are part of the definition of a flow and must thus be
constant for all packets in the flow.
Checksum Coverage
The Checksum Coverage field may behave in different ways: it may
have a value of zero, it may be equal to the datagram length, or
it may have any value between eight octets and the length of the
datagram. From a compression perspective, this field is expected
to either be entirely predictable (for the cases where it follows
the same behavior as the UDP Length field or where it takes on a
constant value) or to change randomly for each packet (making the
value unpredictable from a header-compression perspective). For
all cases, the behavior itself is not expected to change for this
field during the lifetime of a packet flow, or to change
relatively seldom.
Checksum
The information used for the calculation of the UDP-Lite checksum
is governed by the value of the checksum coverage and minimally
includes the UDP-Lite header. The checksum is a changing field
that must always be sent as-is.
A.5. RTP Header Fields
+----------------+----------------+
| Field | Class |
+----------------+----------------+
| Version | STATIC-KNOWN |
| Padding | RACH |
| Extension | RACH |
| CSRC Counter | RACH |
| Marker | SEMISTATIC |
| Payload Type | RACH |
| Sequence Number| PATTERN |
| Timestamp | PATTERN |
| SSRC | STATIC-DEF |
| CSRC | RACH |
+----------------+----------------+
Version
This field is expected to have the value two and the field is
therefore classified as STATIC-KNOWN.
Padding
The use of this field is application-dependent, but when payload
padding is used, it is likely to be present in most or all
packets. The field is classified as RACH to allow for the case
where the value of this field changes.
Extension
If RTP extensions are used by the application, these extensions
are often present in all packets, although the use of extensions
is infrequent. To allow efficient compression of a flow using
extensions in only a few packets, this field is classified as
RACH.
CSRC Count
This field indicates the number of CSRC items present in the CSRC
list. This number is expected to be mostly constant on a packet-
to-packet basis and when it changes, change by small amounts. As
long as no RTP mixer is used, the value of this field will be
zero.
Marker
For audio, the marker bit should be set only in the first packet
of a talkspurt, while for video, it should be set in the last
packet of every picture. This means that in both cases the RTP
marker is classified as SEMISTATIC.
Payload Type
Applications could adapt to congestion by changing payload type
and/or frame sizes, but that is not expected to happen frequently,
so this field is classified as RACH.
RTP Sequence Number
The RTP Sequence Number will be incremented by one for each packet
sent.
Timestamp
In the audio case:
As long as there are no pauses in the audio stream, the RTP
Timestamp will be incremented by a constant value, which
corresponds to the number of samples in the speech frame. It
will thus mostly follow the RTP Sequence Number. When there
has been a silent period and a new talkspurt begins, the
timestamp will jump in proportion to the length of the silent
period. However, the increment will probably be within a
relatively limited range.
In the video case:
Between two consecutive packets, the timestamp will either be
unchanged or increase by a multiple of a fixed value
corresponding to the picture clock frequency. The timestamp
can also decrease by a multiple of the fixed value for certain
coding schemes. The change in timestamp value, expressed as a
multiple of the picture clock frequency, is in most cases
within a limited range.
SSRC
This field is part of the definition of a flow and must thus be
constant for all packets in the flow. The field is therefore
classified as STATIC-DEF.
Contributing Sources (CSRC)
The participants in a session, who are identified by the CSRC
fields, are usually expected to be unchanged on a packet-to-packet
basis, but will infrequently change by a few additions and/or
removals.
A.6. ESP Header Fields
+------------------+-------------+
| Field | Class |
+------------------+-------------+
| SPI | STATIC-DEF |
| Sequence Number | PATTERN |
+------------------+-------------+
SPI
This field is used to identify a distinct flow between two IPsec
peers and it changes rarely; therefore, it is classified as
STATIC-DEF.
ESP Sequence Number
The ESP Sequence Number will be incremented by one for each packet
sent.
A.7. IPv6 Extension Header Fields
+-----------------------+---------------+
| Field | Class |
+-----------------------+---------------+
| Next Header | STATIC-DEF |
| Ext Hdr Len | |
| Routing | STATIC-DEF |
| Hop-by-hop | STATIC |
| Destination | STATIC |
| Options | |
| Routing | STATIC-DEF |
| Hop-by-hop | RACH |
| Destination | RACH |
+-----------------------+---------------+
Next Header
This field will have the same value in all packets of a flow and
is therefore classified as STATIC-DEF.
Ext Hdr Len
For the Routing header, it is expected that the length will remain
constant for the duration of the flow, and that a change in the
length should be classified as a new flow by the ROHC compressor.
For Hop-by-hop and Destination options headers, the length is
expected to remain static, but can be updated by an IR packet.
Options
For the Routing header, it is expected that the option content
will remain constant for the duration of the flow, and that a
change in the routing information should be classified as a new
flow by the ROHC compressor. For Hop-by-hop and Destination
options headers, the options are expected to remain static, but
can be updated by an IR packet.
A.8. GRE Header Fields
+--------------------+---------------+
| Field | Class |
+--------------------+---------------+
| C flag | STATIC |
| K flag | STATIC |
| S flag | STATIC |
| R flag | STATIC-KNOWN |
| Reserved0, Version | STATIC-KNOWN |
| Protocol | STATIC-DEF |
| Checksum | IRREGULAR |
| Reserved | STATIC-KNOWN |
| Sequence Number | PATTERN |
| Key | STATIC-DEF |
+--------------------+---------------+
Flags
The four flag bits are not expected to change for the duration of
the flow, and the R flag is expected to always be set to zero.
Reserved0, Version
Both of these fields are expected to be set to zero for the
duration of any flow.
Protocol
This field will have the same value in all packets of a flow and
is therefore classified as STATIC-DEF.
Checksum
When the checksum field is present, it is expected to behave
unpredictably.
Reserved
When present, this field is expected to be set to zero.
Sequence Number
When present, the Sequence Number increases by one for each
packet.
Key
When present, the Key field is used to define the flow and does
not change.
A.9. MINE Header Fields
+---------------------+----------------+
| Field | Class |
+---------------------+----------------+
| Protocol | STATIC-DEF |
| S bit | STATIC-DEF |
| Reserved | STATIC-KNOWN |
| Checksum | INFERRED |
| Source Address | STATIC-DEF |
| Destination Address | STATIC-DEF |
+---------------------+----------------+
Protocol
This field will have the same value in all packets of a flow and
is therefore classified as STATIC-DEF.
S bit
The S bit is not expected to change during a flow.
Reserved
The reserved field is expected to be set to zero.
Checksum
The header checksum protects individual routing hops from
processing a corrupted header. Since all fields of this header
are compressed away, there is no need to include this checksum in
compressed packets and it can be regenerated at the decompressor
side.
Source and Destination Addresses
These fields can be used to define the flow and are not expected
to change.
A.10. AH Header Fields
+---------------------+----------------+
| Field | Class |
+---------------------+----------------+
| Next Header | STATIC-DEF |
| Payload Length | STATIC |
| Reserved | STATIC-KNOWN |
| SPI | STATIC-DEF |
| Sequence Number | PATTERN |
| ICV | IRREGULAR |
+---------------------+----------------+
Next Header
This field will have the same value in all packets of a flow and
is therefore classified as STATIC-DEF.
Payload Length
It is expected that the length of the header is constant for the
duration of the flow.
Reserved
The value of this field will be set to zero.
SPI
This field is used to identify a specific flow and only changes
when the sequence number wraps around, and is therefore classified
as STATIC-DEF.
Sequence Number
The Sequence Number will be incremented by one for each packet
sent.
ICV
The ICV is expected to behave unpredictably and is therefore
classified as IRREGULAR.
Appendix B. Compressor Implementation Guidelines
This section describes some guiding principles for implementing a
ROHCv2 compressor with focus on how to efficiently select appropriate
packet formats. The text in this appendix should be considered
guidelines; it does not define any normative requirement on how
ROHCv2 profiles are implemented.
B.1. Reference Management
The compressor usually maintains a sliding window of reference
headers, which contains as many references as needed for the
optimistic approach. Each reference contains a description of which
changes occurred in the flow between two consecutive headers in the
flow, and a new reference is inserted into the window each time a
packet is compressed by this context. A reference may for example be
implemented as a stored copy of the uncompressed header being
represented. When the compressor is confident that a specific
reference is no longer used by the decompressor (for example by using
the optimistic approach or feedback received), the reference is
removed from the sliding window.
B.2. Window-based LSB Encoding (W-LSB)
Section 5.1.1 describes how the optimistic approach impacts the
packet format selection for the compressor. Exactly how the
compressor selects a packet format is up to the implementation to
decide, but the following is an example of how this process can be
performed for lsb-encoded fields through the use of Window-based LSB
encoding (W-LSB).
With W-LSB encoding, the compressor uses a number of references (a
window) from its context. What references to use is determined by
its optimistic approach. The compressor extracts the value of the
field to be W-LSB encoded from each reference in the window, and
finds the maximum and minimum values. Once it determines these
values, the compressor uses the assumption that the decompressor has
a value for this field within the range given by these boundaries
(inclusively) as its reference. The compressor can then select a
number of LSBs from the value to be compressed, so that the LSBs can
be decompressed regardless of whether the decompressor uses the
minimum value, the maximum value or any other value in the range of
possible references.
B.3. W-LSB Encoding and Timer-based Compression
Section 6.6.9 defines decompressor behavior for timer-based RTP
timestamp compression. This section gives guidelines on how the
compressor should determine the number of LSB bits it should send for
the timestamp field. When using timer-based compression, this number
depends on the sum of the jitter before the compressor and the jitter
between the compressor and decompressor.
The jitter before the compressor can be estimated using the following
computation:
Max_Jitter_BC =
max {|(T_n - T_j) - ((a_n - a_j) / time_stride)|,
for all headers j in the sliding window}
where (T_n - T_j) is the difference in the timestamp between the
currently compressed header and a reference header and (a_n - a_j) is
the difference in arrival time between those same two headers.
In addition to this, the compressor needs to estimate an upper bound
for the jitter between the compressor and decompressor
(Max_Jitter_CD). This information may for example come from lower
layers.
A compressor implementation can determine whether the difference in
clock resolution between the compressor and decompressor induces an
error when performing integer arithmetics; it can then treat this
error as additional jitter.
After obtaining estimates for the jitters, the number of bits needed
to transmit is obtained using the following calculation:
ceiling(log2(2 * (Max_Jitter_BC + Max_Jitter_CD + 2) + 1))
This number is then used to select a packet format that contains at
least this many scaled timestamp bits.
Authors' Addresses
Ghyslain Pelletier
Ericsson
Box 920
Lulea SE-971 28
Sweden
Phone: +46 (0) 8 404 29 43
EMail: ghyslain.pelletier@ericsson.com
Kristofer Sandlund
Ericsson
Box 920
Lulea SE-971 28
Sweden
Phone: +46 (0) 8 404 41 58
EMail: kristofer.sandlund@ericsson.com
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