[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 RFC 3554

Network Working Group                                     S. M. Bellovin
Internet Draft                                              J. Ioannidis
draft-ietf-ipsec-sctp-05.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
   http://www.ietf.org/shadow.html.


Abstract

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

   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 explicitly 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.  All current
   implementations of IKE support this functionality.  However, a
   method for specifying multiple selectors in Phase 2 is desirable
   for efficiency purposes.  Compliance 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
   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 [TBD].

   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
   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.  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 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.

   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 must assign a number 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.


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.

   [RFC2407]  D. Piper, "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

   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


Expiration and File Name

   This draft expires in October 2003

   Its file name is draft-ietf-ipsec-sctp-05.txt


Html markup produced by rfcmarkup 1.109, available from https://tools.ietf.org/tools/rfcmarkup/