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Network Working Group S. Bellovin Request for Comments: 3554 J. Ioannidis Category: Standards Track AT&T Labs - Research

                                                          A. Keromytis
                                                   Columbia University
                                                            R. Stewart
                                                             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.


 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.

Bellovin, et. al. Standards Track [Page 1] RFC 3554 SCTP with IPsec July 2003

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
 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.

Bellovin, et. al. Standards Track [Page 2] 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.

Bellovin, et. al. Standards Track [Page 3] RFC 3554 SCTP with IPsec July 2003

  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
 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

Bellovin, et. al. Standards Track [Page 4] RFC 3554 SCTP with IPsec July 2003

 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.

Bellovin, et. al. Standards Track [Page 5] 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.

Bellovin, et. al. Standards Track [Page 6] RFC 3554 SCTP with IPsec July 2003

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

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.

Bellovin, et. al. Standards Track [Page 7] RFC 3554 SCTP with IPsec July 2003

Authors' Addresses

 Steven M. Bellovin
 AT&T Labs - Research
 180 Park Avenue
 Florham Park, New Jersey 07932-0971
 Phone: +1 973 360 8656
 John Ioannidis
 AT&T Labs - Research
 180 Park Avenue
 Florham Park, New Jersey 07932-0971
 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
 Randall R. Stewart
 24 Burning Bush Trail.
 Crystal Lake, IL 60012
 Phone: +1-815-477-2127

Bellovin, et. al. Standards Track [Page 8] RFC 3554 SCTP with IPsec July 2003

Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
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 or assist in its implementation may be prepared, copied, published
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 followed, or as required to translate it into languages other than
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assignees.
 This document and the information contained herein is provided on an


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Bellovin, et. al. Standards Track [Page 9]

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