Internet DRAFT - draft-atwood-pim-sm-linklocal


PIM Working Group                                              W. Atwood
Internet-Draft                                                  S. Islam
Expires: December 25, 2006                      Concordia University/CSE
                                                           June 23, 2006

             Security Issues in PIM-SM Link-local Messages

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   Copyright (C) The Internet Society (2006).


   This document proposes some additions to the specification of the
   Protocol Independent Multicast - Sparse Mode (PIM-SM) Protocol
   regarding security issues of its link-local messages.  Although the
   new specifications for IPsec architecture (RFC 4301) and
   Authorization Header (RFC 4302) permit the use of anti-replay, they
   counsel against its use for multi-sender, multicast Security
   Associations.  This makes PIM-SM vulnerable to Denial of Service
   (DoS) attack.  In this document, a new proposal is presented to

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   protect PIM link-local messages while activating the anti-replay
   mechanism as well.  This proposal builds on the new Security
   Association lookup method that has been specified in RFC 4301 and RFC

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1.  Introduction

   All the PIM-SM [1] control messages have IP protocol number 103.
   These messages are either unicast, or multicast with TTL = 1.  The
   source address used for unicast messages is a domain-wide reachable
   address.  For the multicast messages, a link-local address of the
   interface on which the message is being sent is used as the source
   address and a special multicast address, ALL_PIM_ROUTERS (
   in IPv4 and ff02::d in IPv6) is used as the destination address.
   These messages are called link-local messages.  Hello, Join/Prune and
   Assert messages are included in this category.  A forged link-local
   message may be sent to the ALL_PIM_ROUTERS multicast address by an
   attacker.  This type of message affects the construction of the
   distribution tree [1].  The effects of these forged messages are
   outlined in section 6.1 of [1].  Some of the effects are very severe,
   whereas some are minor.

   PIM-SM version 2 was originally specified in RFC 2117, and revised in
   RFC 2362.  A PIM-SM Internet Draft [1] has been approved by the IESG
   for publication as an RFC.  It is intended to obsolete RFC 2362, and
   to correct a number of deficiencies.  The Security Considerations
   section of the PIM-SM Internet Draft is based primarily on the new
   Authentication Header (AH) specification described in RFC 4302 [2].

   Securing the unicast messages can be achieved by the use of a normal
   unicast IPsec Security Association between the two communicants.
   Securing the user data exchanges is covered in RFC 3740 [4].  This
   document focuses on the security issues of link-local messages.  It
   provides some guidelines to take advantage of the new permitted AH
   functionality, and to bring the PIM-SM Internet Draft into alignment
   with the new AH specification.

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2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 and
   indicate requirement levels for compliant PIM-SM implementations.

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3.  Authentication according to the PIM-SM Internet-Draft

   In the PIM-SM Internet Draft, IP Security (IPsec) [[3] transport mode
   using Authentication Header (AH) [2] is recommended to prevent
   attacks generated by forged control messages.  The Network
   Administrator will configure the specific AH authentication algorithm
   and parameters, including the choice of authentication algorithm and
   the choice of keys.  The PIM-SM Internet Draft does not specify
   protocols for establishing Security Associations.  The PIM-SM
   Internet Draft assumes that manual configuration of Security
   Associations will be performed, although it does not preclude the use
   of a negotiation protocol such as the Internet Key Exchange (IKE) [2]
   to establish Security Associations.  Once the Security Associations
   have been established, all the control messages should go through the
   IPsec authentication process.  A PIM-SM router should authenticate a
   control message before processing it, and should reject any
   unauthorized PIM protocol messages.

   Given that IPsec [3] provides protection against replayed unicast and
   multicast messages, the PIM-SM Internet Draft further states that the
   IPsec anti-replay option SHOULD be enabled for these Security
   Associations.  Unfortunately, the AH specification notes as follows:

      "If the key used to compute an ICV is manually distributed,
      correct provision of the anti-replay service would require correct
      maintenance of the counter state at the sender, until the key is
      replaced, and there would likely be no automated recovery
      provision if counter overflow were imminent.  Thus, a compliant
      implementation SHOULD NOT provide this service in conjunction with
      SAs that are manually keyed."

   All the link-local messages of the PIM-SM protocol are sent to the
   destination address, ALL_PIM_ROUTERS, which is a multicast address.
   The Security Policy Database (SPD) within IPsec (see section 4.4.2 of
   RFC 4301 [3]) is not capable of representing a policy for a multicast
   Security Association.  RFC 4301 provides no specification for an
   automated way to create SAD entries for a multicast, inbound SA.
   Only manually configured SAD entries can be created to accomodate
   inbound, multicast traffic.  As a result, the anti-replay option must
   be disabled while using the IPsec AH protocol for security of link-
   local messages.

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4.  Proposed Authentication Technique

   The authentication mechanism [6][7] for PIM link-local messages
   presented in this document has following two criteria to achieve:

   o  The anti-replay mechanism of Authetication Header protocol will be
      activated while sending/receiving any PIM link-local message.

   o  To attain more flexibility, a PIM router will be able to deploy a
      different authentication method for each directly connected PIM
      router if necessary.  In that case, a PIM router will maintain a
      separate Security Association per peer PIM router.

4.1.  Security Association Lookup

   For an SA that carries unicast traffic, three parameters (SPI,
   destination address and security protocol type (AH or ESP)) are used
   in the Security Association lookup process for inbound packets.  The
   SPI is sufficient to specify an SA.  However, an implementation may
   use the SPI in conjunction with the IPsec protocol type (AH or ESP)
   for the SA lookup process.  According to RFC 4301 [3] and the AH
   specification [2], for multicast SAs, in conjunction with the SPI,
   the destination address or the destination address plus the sender
   address may also be used in the SA lookup.  The security protocol
   field is not employed for a multicast SA lookup.

   The reason for the various prohibitions in the IPsec RFCs concerning
   multisender multicast SAs lies in the difficulty of coordinating the
   multiple senders.  However, if the use of multicast for link-local
   messages is examined, it may be seen that in fact the communication
   need not be coordinated---from the prospective of a receiving router,
   each peer router is an independent sender.  In effect, link-local
   communication is an SSM communication that happens to use an ASM
   address (which is shared among all the routers).  Two possibilities
   may be envisaged:

   1.  The address ALL_PIM_ROUTERS can be specified to operate as a set
       of SSM Security Associations, when IPsec is enabled;

   2.  Secure Link-local communication can be specified to occur on an
       SSM address, instead of ALL_PIM_ROUTERS.

   Given that the sender address of an incoming packet will be
   (globally) unique for a specific sender and in conjunction with the
   SPI it will be possible for a receiver to sort out the associated SA
   for that sender from all the SAD entries (even if a single SAD is
   maintained regardless of the number of interfaces), we propose that
   the SPI and the sender address MUST be used in the SA lookup process.

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4.2.  Activating the Anti-replay Mechanism

   Although link-level messages on a link constitute a multiple-sender,
   multiple-receiver group, the use of the sender address for SA lookup
   essentially resolves the communication into a separate SA for each
   sender/destination pair.  Therefore, the statement in the AH RFC
   (section 2.5 of [2]) that "for a multi-sender SA, the anti-replay
   features are not available" becomes irrelevant to PIM-SM link-local
   message exchange.

   To activate the anti-replay mechanism in a unicast communication, the
   receiver uses the sliding window protocol and it maintains a sequence
   number for this protocol.  This sequence number starts from zero.
   Each time the sender sends a new packet, it increments this number by
   one.  In a multi-sender multicast group communication, a single
   sequence number for the entire group would not be enough.

   The whole scenario is different for PIM link-local messages.  These
   messages are sent to local links with TTL = 1.  A link-local message
   never propagates through one router to another.  Given that the
   number of peer routers is small, and given that the use of the sender
   address for SA lookup converts the relationship from a multiple-
   sender group to multiple single-sender associations, the anti-replay
   mechanism SHOULD be activated while sending PIM link-local messages,
   and at that time a PIM router MUST maintain a different sliding
   window for each directly connected sender.

4.3.  Implementing a Security Association Database per Interface

   According to RFC 2401 [5], there is nominally a different Security
   Association Database (SAD) for each router interface.  However, RFC
   4301 explicitly removes this requirement.  The PIM-SM Internet Draft,
   however, notes the possible utility of this feature.  The proposal
   above to use the source address to resolve the SAs implies that the
   use of an SAD per interface is not necessary.

4.4.  Manual Key Configuration

   To establish the SAs at PIM-SM routers, manual key configuration will
   be feasible, since the number of peers will be small.  The Network
   Administrator will configure a router manually during its boot up
   process.  At that time, the authentication method and the keys per
   sender basis for each peer router SHOULD be configured.  The SAD
   entry for each sender connected with this router will be created.
   The Network Admin will also configure the Security Policy Database of
   a router to ensure the use of the associated SA while sending a link-
   local message.

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   The addition of a new router to the set visible from a particular
   router will clearly require a re-configuration of that router.

   A negotiation protocol such as the Internet Key Exchange [2] MAY also
   be used to negotiate and establish a suitable authentication method
   and keys for the SA between two routers.  However, a PIM router is
   not expected to join/leave very frequently, so it is doubtful that
   the overhead of automatic key configuration will be justified.  In
   any case, it will still be necessary to manually configure the basic
   information that will allow the router to trust its peers.  For these
   reasons, manual key configuration SHOULD be used to establish SAs.

   Unfortunately, the use of manual keying is called out in RFC 4302 as
   a specific reason why anit-replay should be prohibited.  It will be
   necessary to either

   1.  explicitly override RFC 4302, or

   2.  design a negotiation protocol to deal with the case of counter

   Once the WG decides that the present proposal is an acceptable
   direction to follow, the authors are prepared to work on the
   development of such a negotiation protocol.

4.5.  Extended Sequence Number

   In the [2], there is a provision for a 64-bit Extended Sequence
   Number (ESN) as the counter of the sliding window used in the anti-
   replay protocol.  Both the sender and the receiver maintain a 64-bit
   counter for the sequence number, although only the lower order 32
   bits is sent in the transmission.  In other words, it will not affect
   the present header format of AH.  If ESN is used, a sender router can
   send 2^64 -1 packets without any intervention.  This number is very
   large, and from a PIM router's point of view, a PIM router can never
   exceed this number in its lifetime.  This makes it reasonable to
   permit manual configuration, since the sequence number will never
   roll over.  For this reason, when manual configuration is used, ESN
   SHOULD be deployed as the sequence number for the sliding window

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5.  Security Considerations

   The whole document considers the security issues of PIM link-local
   messages and proposes a mechanism to protect them.

6.  References

   [1]  Fenner, B., "Protocol Independent Multicast-Sparse Mode
        (PIM-SM): Protocol Specification(Revised),
        draft-ietf-pim-sm-v2-new-12.txt", March 2006.

   [2]  Kent, S., "IP Authentication Header", RFC 4302, December 2005.

   [3]  Kent, S. and K. Seo, "Security Architechture for the Internet
        Protocol", RFC 4301, December 2005.

   [4]  Hardjono, T. and B. Weis, "The Multicast Group Security
        Architecture", RFC 3740, March 2004.

   [5]  Kent, S. and R. Atkinson, "Security Architecture for the
        Internet Protocol", RFC 2401, November 1998.

   [6]  Islam, S., "Security Issues in PIM-SM Link-local Messages,
        Master's Thesis, Concordia University", December 2003.

   [7]  Islam, S., "Security Issues in PIM-SM Link-local Messages,
        Proceedings of LCN 2004", November 2004.

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Authors' Addresses

   J. William Atwood
   Concordia University/CSE
   1455 de Maisonneuve Blvd, West
   Montreal, QC  H3G 1M8

   Phone: +1(514)848-2424 ext3046

   Salekul Islam
   Concordia University/CSE
   1455 de Maisonneuve Blvd, West
   Montreal, QC  H3G 1M8

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