On the Use of Stream Control Transmission Protocol (SCTP) with IPsec

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.

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.

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.

    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
   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.

   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.

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
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   The IETF invites any interested party to bring to its attention any
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   this standard.  Please address the information to the IETF Executive
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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.

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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