Network Working Group S. Bellovin
Request for Comments: 3554 J. Ioannidis
Category: Standards Track AT&T Labs - Research
A. Keromytis
Columbia University
R. Stewart
Cisco
July 2003
On the Use of Stream Control Transmission Protocol (SCTP) with IPsec
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.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document describes functional requirements for IPsec (RFC 2401)
and Internet Key Exchange (IKE) (RFC 2409) to facilitate their use in
securing SCTP (RFC 2960) traffic.
1. Introduction
The Stream Control Transmission Protocol (SCTP) is a reliable
transport protocol operating on top of a connection-less packet
network such as IP. SCTP is designed to transport PSTN signaling
messages over IP networks, but is capable of broader applications.
When SCTP is used over IP networks, it may utilize the IP security
protocol suite [RFC2402][RFC2406] for integrity and confidentiality.
To dynamically establish IPsec Security Associations (SAs), a key
negotiation protocol such as IKE [RFC2409] may be used.
This document describes functional requirements for IPsec and IKE to
facilitate their use in securing SCTP traffic. In particular, we
discuss additional support in the form of a new ID type in IKE
[RFC2409] and implementation choices in the IPsec processing to
accommodate for the multiplicity of source and destination addresses
associated with a single SCTP association.
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1.1. Terminology
In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
"recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
described in [RFC-2119].
2. SCTP over IPsec
When utilizing the Authentication Header [RFC2402] or Encapsulating
Security Payload [RFC2406] protocols to provide security services for
SCTP frames, the SCTP frame is treated as just another transport
layer protocol on top of IP (same as TCP, UDP, etc.)
IPsec implementations should already be able to use the SCTP
transport protocol number as assigned by IANA as a selector in their
Security Policy Database (SPD). It should be straightforward to
extend existing implementations to use the SCTP source and
destination port numbers as selectors in the SPD. Since the concept
of a port, and its location in the transport header is
protocol-specific, the IPsec code responsible for identifying the
transport protocol ports has to be suitably modified. This, however
is not enough to fully support the use of SCTP in conjunction with
IPsec.
Since SCTP can negotiate sets of source and destination addresses
(not necessarily in the same subnet or address range) that may be
used in the context of a single association, the SPD should be able
to accommodate this. The straightforward, and expensive, way is to
create one SPD entry for each pair of source/destination addresses
negotiated. A better approach is to associate sets of addresses with
the source and destination selectors in each SPD entry (in the case
of non-SCTP traffic, these sets would contain only one element).
While this is an implementation decision, implementors are encouraged
to follow this or a similar approach when designing or modifying the
SPD to accommodate SCTP-specific selectors.
Similarly, SAs may have multiple associated source and destination
addresses. Thus an SA is identified by the extended triplet ({set of
destination addresses}, SPI, Security Protocol). A lookup in the
Security Association Database (SADB) using the triplet (Destination
Address, SPI, Security Protocol), where Destination Address is any of
the negotiated peer addresses, MUST return the same SA.
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RFC 3554 SCTP with IPsec July 2003
3. SCTP and IKE
There are two issues relevant to the use of IKE when negotiating
protection for SCTP traffic:
a) Since SCTP allows for multiple source and destination network
addresses associated with an SCTP association, it MUST be possible
for IKE to efficiently negotiate these in the Phase 2 (Quick Mode)
exchange. The straightforward approach is to negotiate one pair of
IPsec SAs for each combination of source and destination addresses.
This can result in an unnecessarily large number of SAs, thus wasting
time (in negotiating these) and memory. All current implementations
of IKE support this functionality. However, a method for specifying
multiple selectors in Phase 2 is desirable for efficiency purposes.
Conformance with this document requires that implementations adhere
to the guidelines in the rest of this section.
Define a new type of ID, ID_LIST, that allows for recursive inclusion
of IDs. Thus, the IKE Phase 2 Initiator ID for an SCTP association
MAY be of type ID_LIST, which would in turn contain as many
ID_IPV4_ADDR IDs as necessary to describe Initiator addresses;
likewise for Responder IDs. Note that other selector types MAY be
used when establishing SAs for use with SCTP, if there is no need to
use negotiate multiple addresses for each SCTP endpoint (i.e., if
only one address is used by each peer of an SCTP flow).
Implementations MUST support this new ID type.
ID_LIST IDs cannot appear inside ID_LIST ID payloads. Any of the ID
types defined in [RFC2407] can be included inside an ID_LIST ID.
Each of the IDs contained in the ID_LIST ID must include a complete
Identification Payload header.
The following diagram illustrates the content of an ID_LIST ID
payload that contains two ID_FQDN payloads.
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ID Type ! Protocol ID ! Port !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ID Type ! Protocol ID ! Port !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ FQDN 1 Identification Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ID Type ! Protocol ID ! Port !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ FQDN 2 Identification Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Next Payload field in any of the included IDs (for FQDN 1 and
FQDN 2) MUST be ignored by the Responder. The Payload Length, ID
Type, Protocol ID, and Port fields of the included Payloads should be
set to the appropriate values. The Protocol ID and Port fields of
the ID_LIST Payload should be set to zero by the Initiator and MUST
be ignored by the Responder.
Different types of IDs (e.g., an ID_FQDN and an ID_IPV4_ADDR) can be
included inside the same ID_LIST ID. If an ID type included in an
ID_LIST ID payload is invalid in the context the ID_LIST ID is used,
the whole ID_LIST should be considered to be at fault, e.g., if an
ID_LIST ID payload that contains an ID_FQDN and an ID_IPV4_ADDR is
received during an IKE Quick Mode exchange, the Responder should
signal a fault to the Initiator and stop processing of the message
(the same behavior it would exhibit if simply an ID_FQDN was received
instead).
The IANA-assigned number for the ID_LIST ID is 12.
b) For IKE to be able to validate the Phase 2 selectors, it must be
possible to exchange sufficient information during Phase 1.
Currently, IKE can directly accommodate the simple case of two peers
talking to each other, by using Phase 1 IDs corresponding to their IP
addresses, and encoding those same addresses in the SubjAltName of
the certificates used to authenticate the Phase 1 exchange. For more
complicated scenarios, external policy (or some other mechanism)
needs to be consulted, to validate the Phase 2 selectors and SA
parameters. All addresses presented in Phase 2 selectors MUST be
validated. That is, enough evidence must be presented to the
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Responder that the Initiator is authorized to receive traffic for all
addresses that appear in the Phase 2 selectors. This evidence can be
derived from the certificates exchanged during Phase 1 (if possible);
otherwise it must be acquired through out-of-band means (e.g., policy
mechanism, configured by the administrator, etc.).
In order to accommodate the same simple scenario in the context of
multiple source/destination addresses in an SCTP association, it MUST
be possible to:
1) Specify multiple Phase 1 IDs, which are used to validate Phase
2 parameters (in particular, the Phase 2 selectors). Following
the discussion on an ID_LIST ID type, it is possible to use the
same method for specifying multiple Phase 1 IDs.
2) Authenticate the various Phase 1 IDs. Using pre-shared key
authentication, this is possible by associating the same shared
key with all acceptable peer Phase 1 IDs. In the case of
certificates, we have two alternatives:
a) The same certificate can contain multiple IDs encoded in
the SubjAltName field, as an ASN.1 sequence. Since this is
already possible, it is the preferred solution and any
conformant implementations MUST support this.
b) Multiple certificates MAY be passed during the Phase 1
exchange, in multiple CERT payloads. This feature is also
supported by the current specification. Since only one
signature may be issued per IKE Phase 1 exchange, it is
necessary for all certificates to contain the same key as
their Subject. However, this approach does not offer any
significant advantage over (a), thus implementations MAY
support it.
In either case, an IKE implementation needs to verify the
validity of a peer's claimed Phase 1 ID, for all such IDs
received over an exchange.
Although SCTP does not currently support modification of the
addresses associated with an SCTP association (while the latter is in
use), it is a feature that may be supported in the future. Unless
the set of addresses changes extremely often, it is sufficient to do
a full Phase 1 and Phase 2 exchange to establish the appropriate
selectors and SAs.
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RFC 3554 SCTP with IPsec July 2003
The last issue with respect to SCTP and IKE pertains to the initial
offer of Phase 2 selectors (IDs) by the Initiator. Per the current
IKE specification, the Responder must send in the second message of
the Quick Mode the IDs received in the first message. Thus, it is
assumed that the Initiator already knows all the Selectors relevant
to this SCTP association. In most cases however, the Responder has
more accurate knowledge of its various addresses. Thus, the IPsec
Selectors established can be potentially insufficient or inaccurate.
If the proposed set of Selectors is not accurate from the Responder's
point of view, the latter can start a new Quick Mode exchange. In
this new Quick Mode exchange, the roles of Initiator and Responder
have been reversed; the new Initiator MUST copy the SA and Selectors
from the old Quick Mode message, and modify its set of Selectors to
match reality. All SCTP-supporting IKE implementations MUST be able
to do this.
4. Security Considerations
This documents discusses the use of a security protocol (IPsec) in
the context of a new transport protocol (SCTP). SCTP, with its
provision for mobility, opens up the possibility for
traffic-redirection attacks whereby an attacker X claims that his
address should be added to an SCTP session between peers A and B, and
be used for further communications. In this manner, traffic between
A and B can be seen by X. If X is not in the communication path
between A and B, SCTP offers him new attack capabilities. Thus, all
such address updates of SCTP sessions should be authenticated. Since
IKE negotiates IPsec SAs for use by these sessions, IKE MUST validate
all addresses attached to an SCTP endpoint either through validating
the certificates presented to it during the Phase 1 exchange, or
through some out-of-band method.
The Responder in a Phase 2 exchange MUST verify the Initiator's
authority to receive traffic for all addresses that appear in the
Initiator's Phase 2 selectors. Not doing so would allow for any
valid peer of the Responder (i.e., anyone who can successfully
establish a Phase 1 SA with the Responder) to see any other valid
peer's traffic by claiming their address.
5. IANA Considerations
IANA has assigned number 12 for ID_LIST (defined in Section 3) in the
"IPSEC Identification Type" registry from the Internet Security
Association and Key Management Protocol (ISAKMP) Identifiers table.
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6. Intellectual Property Rights Notice
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Normative References
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC2402] Kent, S. and R. Atkinson, "IP Authentication Header", RFC
2402, November 1998.
[RFC2406] Kent, S. and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC 2406, November 1998.
[RFC2407] Piper, D., "The Internet IP Security Domain of
Interpretation for ISAKMPD", RFC 2407, November 1998.
[RFC2408] Maughan, D., Schertler, M., Schneider, M. and J. Turner,
"Internet Security Association and Key Management
Protocol", RFC 2408, November 1998.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
Zhang, L. and V. Paxson, "Stream Control Transmission
Protocol", RFC 2960, October 2000.
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Authors' Addresses
Steven M. Bellovin
AT&T Labs - Research
180 Park Avenue
Florham Park, New Jersey 07932-0971
Phone: +1 973 360 8656
EMail: smb@research.att.com
John Ioannidis
AT&T Labs - Research
180 Park Avenue
Florham Park, New Jersey 07932-0971
EMail: ji@research.att.com
Angelos D. Keromytis
Columbia University, CS Department
515 CS Building
1214 Amsterdam Avenue, Mailstop 0401
New York, New York 10027-7003
Phone: +1 212 939 7095
EMail: angelos@cs.columbia.edu
Randall R. Stewart
24 Burning Bush Trail.
Crystal Lake, IL 60012
Phone: +1-815-477-2127
EMail: rrs@cisco.com
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Bellovin, et. al. Standards Track [Page 9]