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Network Working Group                                     S. M. Bellovin
Internet Draft                                              J. Ioannidis
draft-ietf-ipsec-sctp-01.txt                        AT&T Labs - Research
                                                         A. D. Keromytis
                                                     Columbia University
                                                           R. R. Stewart
                                                                   Cisco

                      On the Use of SCTP with IPsec

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other
   documents at any time.  It is inappropriate to use Internet- Drafts
   as reference material or to cite them other than as "work in
   progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
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Abstract

   This document describes functional requirements for IPsec [RFC2401]
   and IKE [RFC2409] to facilitate their use for securing SCTP
   [RFC2960] traffic.


1.  Introduction

   The Stream Control Transmission Protocol (SCTP) is a reliable
   transport protocol operating on top of a connectionless 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 presents some considerations on the use of SCTP in
   conjunction with IPsec.  In particular, we discuss some additional
   support in the form of a new ID type in IKE [RFC2409] and some
   implementation choices in the IPsec processing to accomodate for
   the multiplicity of source and destination addresses associated
   with a single SCTP association.


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 possible 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 suitable modified.

   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 accomodate 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 accomodate 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.

   When operating in tunnel mode, the question of what to use as the
   tunnel destination address (for the `outer' header) arises.  We
   distinguish three cases: where the end hosts are also the tunnel
   endpoints; where neither host is a tunnel endpoint (the tunnel
   endpoints are security gateways); and where only one of the hosts is
   a tunnel endpoint (the usual case for the `road warrior' talking to
   a security gateway).  In the first case, the outer addresses MUST be
   the same as the inner addresses of the tunnel.  In the second case
   (security gateways) there is no special processing; address
   selection proceeds as it would for two distinct sets of end hosts.
   In the third case, the `road warrior' uses the security gateway's
   address as the tunnel destination address, and MUST use the same
   source address as that of the inner packet.  Symmetrically, the
   security gateway uses its own address as the source address of the
   tunnel, and MUST use the the same destination address in the outer
   header as that of the inner packet.  An implementation will probably
   structure the code so that if, during SA setup, the inner and outer
   address of either side is the same, rather than explicitely store the
   corresponding address of the tunnel, it sets a flag that marks the SA
   to use the same address in the tunnel header as in the inner header.


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.  Thus, a method for
   specifying multiple selectors in Phase 2 is desirable.  There are
   two ways of accomplishing this:

      1) Allow for multiple Initiator/Responder ID payloads in Phase
         2.  The advantage of this approach is that no new payloads or
         ID types are necessary.  The disadvantage is that it changes
         the actual wire protocol.  While it is possible to use a
         new ISAKMP minor version number to indicate ability to
         negotiate multiple Initiator/Responder ID payloads, we do not
         believe this approach scales well in the long term.

      2) Define a new type of ID, IPSEC_ID_RECURSE, that allows for
         recursive inclusion of IDs.  Thus, the IKE Phase 2 Initiator
         ID for an SCTP association could be of type IPSEC_ID_RECURSE,
         which would in turn contain as many IPSEC_ID_IPV4_ADDR IDs as
         necessary to describe Initiator addresses; likewise for
         Responder IDs.

         Two issues pertinent to the new ID are whether arbitrary
         recursion should be allowed, and whether Phase 2 IDs other
         than IPSEC_ID_IPV4_ADDR can be specified inside an
         IPSEC_ID_RECURSE.  The authors' opinion with respect to the
         first issue is that arbitrary recursion is unnecessary (and
         would complicate the code); with respect to the second issue,
         we believe other payloads should be allowed as it may prove
         useful in the context of a Phase 1 exchange.

   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 accomodate the simple case of two
   machines 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.

   In order to accomodate 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, that can be used to validate
         Phase 2 parameters (in particular, the Phase 2 selectors).
         Following the discussion on an IPSEC_ID_RECURSE 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 (at least in theory), it is the
            preferred solution and any compliant 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.  As usual, there exist two solutions to this problem:

      1) 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.

      2) Alternately, the Initiator may suggest only its own Selectors
         in the Quick Mode exchange.  The Responder then adds its own
         Selectors in its response (second message in the Quick Mode
         exchange).  The Initiator MUST verify that the Responder's
         Selectors comply with local policy.  While this approach is
         more general and flexible, it requires changes to the IKE
         specification.  It is only included here for completeness.


4.  Security Considerations

   This documents discusses the use of a security protocol (IPsec) in
   the context of a new transport protocol (SCTP).

5.  IANA Considerations

   No actions by IANA are required (yet).


References:

   [Bel96]    Steven M. Bellovin, "Problem Areas for the IP Security
              Protocols", Proceedings of the Sixth Usenix Unix Security
              Symposium, July, 1996.

   [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.

   [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.

   [UDE]      Xie, Q., Stewart, R. R., Sharp, C. and I. Rytina, "SCTP
              Unreliable Data Mode Extension", work in progress,
              draft-xie-usctp-sigtran-01.txt


Authors' addresses:

   Steven M. Bellovin
   AT&T Labs - Research
   180 Park Avenue
   Florham Park, New Jersey 07932-0971

   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: +1 215 573 3639
   Email: angelos@cs.columbia.edu

   Randall R. Stewart
   24 Burning Bush Trail.
   Crystal Lake, IL 60012
   USA

   Phone: +1-815-477-2127
   EMail: rrs@cisco.com


Expiration and File Name

   This draft expires in January 2002

   Its file name is draft-ietf-ipsec-sctp-01.txt


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