PIM-SM Link Security: Authentication and Confidentiality in Protocol Independent Multicast Sparse Mode (PIM-SM) Link-Local Messages

Internet Engineering Task Force (IETF)                         W. Atwood
Request for Comments: 5796                      Concordia University/CSE
Updates: 4601                                                   S. Islam
Category: Standards Track                                        IRS-EMT
ISSN: 2070-1721                                                 M. Siami
                                              Concordia University/CIISE
                                                              March 2010


                 Authentication and Confidentiality in
Protocol Independent Multicast Sparse Mode (PIM-SM) Link-Local Messages

Abstract

   RFC 4601 mandates the use of IPsec to ensure authentication of the
   link-local messages in the Protocol Independent Multicast - Sparse
   Mode (PIM-SM) routing protocol.  This document specifies mechanisms
   to authenticate the PIM-SM link-local messages using the IP security
   (IPsec) Encapsulating Security Payload (ESP) or (optionally) the
   Authentication Header (AH).  It specifies optional mechanisms to
   provide confidentiality using the ESP.  Manual keying is specified as
   the mandatory and default group key management solution.  To deal
   with issues of scalability and security that exist with manual
   keying, optional support for an automated group key management
   mechanism is provided.  However, the procedures for implementing
   automated group key management are left to other documents.  This
   document updates RFC 4601.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5796.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................4
      1.1. Goals and Non-Goals ........................................4
   2. Terminology .....................................................5
   3. Transport Mode versus Tunnel Mode ...............................5
   4. Authentication ..................................................5
   5. Confidentiality .................................................6
   6. IPsec Requirements ..............................................6
   7. Key Management ..................................................8
      7.1. Manual Key Management ......................................8
      7.2. Automated Key Management ...................................8
      7.3. Communications Patterns ....................................9
      7.4. Neighbor Relationships ....................................10
   8. Number of Security Associations ................................11
   9. Rekeying .......................................................12
      9.1. Manual Rekeying Procedure .................................13
      9.2. KeyRolloverInterval .......................................14
      9.3. Rekeying Interval .........................................14
   10. IPsec Protection Barrier and SPD/GSPD .........................14
      10.1. Manual Keying ............................................14
           10.1.1. SAD Entries .......................................14
           10.1.2. SPD Entries .......................................14
      10.2. Automatic Keying .........................................15
           10.2.1. SAD Entries .......................................15
           10.2.2. GSPD Entries ......................................15
           10.2.3. PAD Entries .......................................15
   11. Security Association Lookup ...................................16
   12. Activating the Anti-Replay Mechanism ..........................16
   13. Implementing a Security Policy Database per Interface .........17
   14. Extended Sequence Number ......................................17
   15. Security Considerations .......................................18
   16. Acknowledgements ..............................................18
   17. References ....................................................19
      17.1. Normative References .....................................19
      17.2. Informative References ...................................19

1.  Introduction

   All the PIM-SM [RFC4601] control messages have IP protocol number
   103.  Some control messages are unicast; the rest are multicast with
   Time to Live (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 (224.0.0.13 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 [RFC4601].
   The effects of these forged messages are outlined in Section 6.1 of
   [RFC4601].  Some of the effects are very severe, whereas some are
   minor.

   PIM-SM version 2 was originally specified in RFC 2117 [RFC2117], and
   revised in RFC 2362 [RFC2362] and RFC 4601.  RFC 4601 obsoletes RFC
   2362, and corrects a number of deficiencies.  The "Security
   Considerations" section of RFC 4601 is based primarily on the
   Authentication Header (AH) specification described in RFC 4302
   [RFC4302].

   Securing the unicast messages can be achieved by the use of a normal
   unicast IPsec Security Association (SA) between the two communicants.

   This document focuses on the security issues for link-local messages.
   It provides some guidelines to take advantage of the new permitted AH
   functionality in RFC 4302 and the new permitted ESP functionality in
   RFC 4303 [RFC4303], and to bring the PIM-SM specification into
   alignment with the new AH and ESP specifications.  In particular, in
   accordance with RFC 4301, the use of ESP is made mandatory and AH is
   specified as optional.  This document specifies manual key management
   as mandatory to implement, i.e., that all implementations MUST
   support, and provides the necessary structure for an automated key
   management protocol that the PIM routers may use.

1.1.  Goals and Non-Goals

   The primary goal for link-local security is to provide data origin
   authentication for each link-local message.  A secondary goal is to
   ensure that communication only happens between legitimate peers
   (i.e., adjacent routers).  An optional goal is to provide data
   confidentiality for the link-local messages.

   The first goal implies that each router has a unique identity.  It is
   possible (but not mandatory) that this identity will be based on the
   unicast identity of the router.  (The unicast identity could be, for
   example, based on some individually configured property of the
   router, or be part of a region-wide public key infrastructure.)  The
   existence of this unique identity is assumed in this specification,
   but procedures for establishing it are out of scope for this
   document.

   The second goal implies that there is some form of "adjacency matrix"
   that controls the establishment of Security Associations among
   adjacent multicast routers.  For manual keying, this control will be
   exercised by the Administrator of the router(s), through the setting
   of initialization parameters.  For automated keying, the existence of
   this control will be reflected by the contents of the Peer
   Authorization Database (PAD) (see RFC 4301 [RFC4301]) or the Group
   Security Policy Database (GSPD) (see RFC 5374 [RFC5374]) in each
   router.  Procedures for controlling the adjacency and building the
   associated PAD and GSPD are out of scope for this document.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].
   They indicate requirement levels for compliant PIM-SM
   implementations.

3.  Transport Mode versus Tunnel Mode

   All implementations conforming to this specification MUST support SA
   in transport mode to provide required IPsec security to PIM-SM link-
   local messages.  They MAY also support SA in tunnel mode to provide
   required IPsec security to PIM-SM link-local messages.  If tunnel
   mode is used, both destination address preservation and source
   address preservation MUST be used, as described in Section 3.1 of RFC
   5374 [RFC5374].

4.  Authentication

   Implementations conforming to this specification MUST support
   authentication for PIM-SM link-local messages.  Implementations
   conforming to this specification MUST support HMAC-SHA1 [RFC2404].

   In order to provide authentication of PIM-SM link-local messages,
   implementations MUST support ESP [RFC4303] and MAY support AH
   [RFC4302].

   If ESP in transport mode is used, it will only provide authentication
   to PIM-SM protocol packets excluding the IP header, extension
   headers, and options.

   If AH in transport mode is used, it will provide authentication to
   PIM-SM protocol packets, selected portions of the IP header,
   extension headers and options.

   Note: when authentication for PIM-SM link-local messages is enabled,

   o  PIM-SM link-local packets that are not protected with AH or ESP
      will be silently discarded by IPsec, although the implementation
      of IPsec may maintain a counter of such packets.

   o  PIM-SM link-local packets that fail the authentication checks will
      be silently discarded by IPsec, although the implementation of
      IPsec may maintain a counter of such packets.

5.  Confidentiality

   Implementations conforming to this specification SHOULD support
   confidentiality for PIM-SM.  Implementations supporting
   confidentiality MUST support AES-CBC [RFC3602] with a 128-bit key.

   If confidentiality is provided, ESP MUST be used.

   Since authentication MUST be supported by a conforming
   implementation, an implementation MUST NOT generate the combination
   of NON-NULL Encryption and NULL Authentication.

   Note: when confidentiality for PIM-SM link-local packets is enabled,

   o  PIM-SM packets that are not protected with ESP will be silently
      discarded by IPsec, although the implementation of IPsec may
      maintain a counter of such packets.

6.   IPsec Requirements

   In order to implement this specification, the following IPsec
   capabilities are required.

   Transport mode
      IPsec in transport mode MUST be supported.

   Multiple Security Policy Databases (SPDs)
      The implementation MUST support multiple SPDs with an SPD
      selection function that provides an ability to choose a specific
      SPD based on interface.

   Selectors
      The implementation MUST be able to use source address, destination
      address, protocol, and direction as selectors in the SPD.

   Interface ID tagging
      The implementation MUST be able to tag the inbound packets with
      the ID of the interface (physical or virtual) on which they
      arrived.

   Manual key support
      It MUST be possible to use manually configured keys to secure the
      specified traffic.

   Encryption and authentication algorithms
      Encryption and authentication algorithm requirements described in
      RFC 4835 [RFC4835] apply when ESP and AH are used to protect
      PIM-SM.  Implementations MUST support ESP-NULL, and if providing
      confidentiality, MUST support the ESP transforms providing
      confidentiality required by [RFC4835].  However, in any case,
      implementations MUST NOT allow the user to choose a stream cipher
      or block mode cipher in counter mode for use with manual keys.

   Encapsulation of ESP packets
      IP encapsulation of ESP packets MUST be supported.  For
      simplicity, UDP encapsulation of ESP packets SHOULD NOT be used.

   If the automatic keying features of this specification are
   implemented, the following additional IPsec capabilities are
   required:

   Group Security Policy Database (GSPD)
      The implementation MUST support the GSPD that is described in RFC
      5374 [RFC5374].

   Multiple Group Security Policy Databases
      The implementation MUST support multiple GSPDs with a GSPD
      selection function that provides an ability to choose a specific
      GSPD based on interface.

   Selectors
      The implementation MUST be able to use source address, destination
      address, protocol and direction as selectors in the GSPD.

7.  Key Management

   All the implementations MUST support manual configuration of the
   Security Associations (SAs) that will be used to authenticate PIM-SM
   link-local messages.  This does not preclude the use of a negotiation
   protocol such as the Group Domain Of Interpretation (GDOI) [RFC3547]
   or Group Secure Association Key Management Protocol (GSAKMP)
   [RFC4535] to establish these SAs.

7.1.  Manual Key Management

   To establish the SAs at PIM-SM routers, manual key configuration will
   be feasible when the number of peers (directly connected routers) is
   small.  The Network Administrator will configure a router manually.
   At that time, the authentication method and the choice of keys SHOULD
   be configured.  The parameters for the Security Association Database
   (SAD) will be entered.  The Network Administrator will also configure
   the Security Policy Database of a router to ensure the use of the
   associated SA while sending a link-local message.

7.2.  Automated Key Management

   All the link-local messages of the PIM-SM protocol are sent to the
   destination address, ALL_PIM_ROUTERS, which is a multicast address.
   By using the sender address in conjunction with the destination
   address for Security Association lookup, link-local communication
   turns into a Source-Specific Multicast (SSM) or "one-to-many"
   communication.

   The procedures for automated key management are not specified in this
   document.

   One option is to use Group Domain Of Interpretation (GDOI) [RFC3547],
   which enables a group of users or devices to exchange encrypted data
   using IPsec data encryption.  GDOI has been developed to be used in
   multicast applications, where the number of end users or devices may
   be large and the end users or devices can dynamically join/leave a
   multicast group.  However, a PIM router is not expected to join/leave
   very frequently, and the number of routers is small when compared to
   the possible number of users of a multicast application.  Moreover,
   most of the PIM routers will be located inside the same
   administrative domain and are considered to be trusted parties.  It
   is possible that a subset of GDOI functionalities will be sufficient.

   Another option is to use the Group Secure Association Key Management
   Protocol (GSAKMP) [RFC4535].

7.3.  Communications Patterns

   Before discussing the set of Security Associations that are required
   to properly manage a multicast region that is under the control of a
   single administration, it is necessary to understand the
   communications patterns that will exist among the routers in this
   region.  From the perspective of a speaking router, the information
   from that router is sent (multicast) to all of its neighbors.  From
   the perspective of a listening router, the information coming from
   each of its neighbors is distinct from the information coming from
   every other router to which it is directly connected.  Thus, an
   administrative region contains many (small) distinct groups, all of
   which happen to be using the same multicast destination address
   (e.g., ALL_PIM_ROUTERS, see Section 11), and each of which is
   centered on the associated speaking router.

   Consider the example configuration as shown in Figure 1.

   R2    R3    R4    R5    R6
   |     |     |     |     |
   |     |     |     |     |
   ---------   ---------------
          |     |
          |     |
           \   /
             R1
           /   \
          |     |
          |     |
   ---------    --------------------
         |       |    |    |    |
         |       |    |    |    |
        R7      R8   R9   R10  R11
         |       |    |    |    |
                      |
                      |
                  -------------
                   |    |    |
                   |    |    |
                  R12  R13  R14

         Figure 1: Set of router interconnections

   In this configuration, router R1 has four interfaces, and is the
   speaking router for a group whose listening routers are routers R2
   through R11.  Router R9 is the speaking router for a group whose
   listening routers are routers R1, R8, and R10-R14.

   From the perspective of R1 as a speaking router, if a Security
   Association SA1 is assigned to protect outgoing packets from R1, then
   it is necessary to distribute the key for this association to each of
   the routers R2 through R11.  Similarly, from the perspective of R9 as
   a speaking router, if a Security Association is assigned to protect
   the outgoing packets from R9, then it is necessary to distribute the
   key for this association to each of the routers R1, R8, and R10
   through R14.

   From the perspective of R1 as a listening router, all packets
   arriving from R2 through R11 need to be distinguished from each
   other, to permit selecting the correct Security Association in the
   SAD.  (Packets from each of the peer routers (R2 through R11)
   represent communication from a different speaker, with a separate
   sequence-number space, even though they are sent using the same
   destination address.)  For a multicast Security Association, RFC 4301
   permits using the source address in the selection function.  If the
   source addresses used by routers R2 through R11 are globally unique,
   then the source addresses of the peer routers are sufficient to
   achieve the differentiation.  If the sending routers use link-local
   addresses, then these addresses are unique only on a per-interface
   basis, and it is necessary to use the Interface ID tag as an
   additional selector, i.e., either the selection function has to have
   the Interface ID tag as one of its inputs or separate SADs have to be
   maintained for each interface.

   If the assumption of connectivity to the key server can be made
   (which is true in the PIM-SM case), then the Group Controller/Key
   Server (GC/KS) that is used for the management of the keys can be
   centrally located (and duplicated for reliability).  If this
   assumption cannot be made (i.e., in the case of adjacencies for a
   unicast router), then some form of "local" key server must be
   available for each group.  Given that the listening routers are never
   more than one hop away from the speaking router, the speaking router
   is the obvious place to locate the "local" key server.  As such, this
   may be a useful approach even in the PIM-SM case.  This approach has
   the additional advantage that there is no need to duplicate the local
   key server for reliability, since if the key server is down, it is
   very likely that the speaking router is also down.

7.4.  Neighbor Relationships

   Each distinct group consists of one speaker, and the set of directly
   connected listeners.  If the decision is made to maintain one
   Security Association per speaker (see Section 8), then the key server
   will need to be aware of the adjacencies of each speaker.  Procedures
   for managing and distributing these adjacencies are out of scope for
   this document.

8.  Number of Security Associations

   The number of Security Associations to be maintained by a PIM router
   depends on the required security level and available key management.

   This SHOULD be decided by the Network Administrator.  Two different
   ways are shown in Figures 2 and 3.  It is assumed that A, B, and C
   are three PIM routers, where B and C are directly connected with A,
   and there is no direct link between B and C.

                                       |
            +++++                      |
            + B + SAb     ------------>|
            +   + SAa     <------------|
            +++++                      |
                                       |
            +++++ SAb     <------------|
            +   +                 ---->|
            +   +                /
            + A + SAa     -------
            +   +                \
            +   +                 ---->|
            +++++ SAc     <------------|
                                       |
            +++++                      |
            + C + SAc     ------------>|
            +   + SAa     <------------|
            +++++                      |
                                       |
                          Directly connected network

         Figure 2: Activate unique Security Association for each peer

   The first method, shown in Figure 2, SHOULD be supported by every
   implementation.  In this method, each node will use a unique SA for
   its outbound traffic.  A, B, and C will use SAa, SAb, and SAc,
   respectively, for sending any traffic.  Each node will include the
   source address when searching the SAD for a match.  Router A will use
   SAb and SAc for packets received from B and C, respectively.  The
   number of SAs to be activated and maintained by a PIM router will be
   equal to the number of directly connected routers, plus one for
   sending its own traffic.  Also, the addition of a PIM router in the
   network will require the addition of another SA on every directly
   connected PIM router.  This solution will be scalable and practically
   feasible with an automated key management protocol.  However, it MAY
   be used with manual key management, if the number of directly
   connected routers is small.

                                       |
            +++++                      |
            + B + SAo     ------------>|
            +   + SAi     <------------|
            +++++                      |
                                       |
            +++++ SAi     <------------|
            +   +                 ---->|
            +   +                /
            + A + SAo     -------
            +   +                \
            +   +                 ---->|
            +++++ SAi     <------------|
                                       |
            +++++                      |
            + C + SAo     ------------>|
            +   + SAi     <------------|
            +++++                      |
                                       |
                          Directly connected network

         Figure 3: Activate two Security Associations

   The second method, shown in Figure 3, MUST be supported by every
   implementation.  In this simple method, all the nodes will use two
   SAs, one for sending (SAo) and the other for receiving (SAi) traffic.
   Thus, the number of SAs is always two and will not be affected by
   addition of a PIM router.  Although two different SAs (i.e., SAo and
   SAi) are used in this method, the SA parameters (keys, Security
   Parameter Index (SPI), etc.) for the two SAs are identical, i.e., the
   same information is shared among all the routers in an administrative
   region.  This document RECOMMENDS this second method for manual key
   configuration.  However, it MAY also be used with automated key
   configuration.

9.  Rekeying

   An analysis of the considerations for key management is provided in
   RFC 4107 [RFC4107].

   In PIM-SM deployments it is expected that secure sessions will be
   relatively long-lived, and it is not expected that keys will be
   significantly exposed through normal operational activity.  Manual
   keying is judged acceptable in the light of the relatively low rate
   of change that is required.

   To maintain the security of a link, the authentication and encryption
   key values SHOULD be changed periodically, to limit the risk of
   undetected key disclosure.  Keys SHOULD also be changed when there is
   a change of trusted personnel.

   Manual keying offers the ability to change keys in a coordinated way,
   but it has several drawbacks in PIM-SM systems.  Some of these are
   listed in Section 15 ("Security Considerations") of this document.

   According to an analysis in line with RFC 4107 [RFC4107], PIM-SM
   would benefit from automated key management and roll over because all
   the disadvantages of manual keys listed in Section 15 would be
   eliminated.  However, suitable techniques for automated key
   management do not currently exist.  Work is in hand in the IETF to
   develop suitable solutions.  In the meantime, implementations MUST
   support manual rekeying as described below.  Implementers and
   deployers need to be aware of the requirement to upgrade to support
   automated key management as soon as suitable techniques are
   available.

9.1.  Manual Rekeying Procedure

   In accordance with the requirements of RFC 4107 [RFC4107], the
   following three-step procedure provides a possible mechanism to rekey
   the routers on a link without dropping PIM-SM protocol packets or
   disrupting the adjacency, while ensuring that it is always clear
   which key is being used.

   1.  For every router on the link, create an additional inbound SA for
       the interface being rekeyed using a new SPI and the new key.

   2.  For every router on the link, replace the original outbound SA
       with one using the new SPI and key values.  The SA replacement
       operation MUST be atomic with respect to sending PIM-SM packets
       on the link, so that no PIM-SM packets are sent without
       authentication/encryption

   3.  For every router on the link, remove the original inbound SA.

   Note that all routers on the link MUST complete step 1 before any
   begin step 2.  Likewise, all the routers on the link MUST complete
   step 2 before any begin step 3.

   One way to control the progression from one step to another is for
   each router to have a configurable time constant KeyRolloverInterval.
   After the router begins step 1 on a given link, it waits for this
   interval and then moves to step 2.  Likewise, after moving to step 2,
   it waits for this interval and then moves to step 3.

   In order to achieve smooth key transition, all routers on a link MUST
   use the same value for KeyRolloverInterval and MUST initiate the key
   rollover process within this time period.

   At the end of this time period, all the routers on the link will have
   a single inbound and outbound SA for PIM-SM with the new SPI and key
   values.

9.2.  KeyRolloverInterval

   The configured value of KeyRolloverInterval needs to be long enough
   to allow the Administrator to change keys on all the PIM-SM routers.
   As this value can vary significantly depending on the implementation
   and the deployment, it is left to the Administrator to choose an
   appropriate value.

9.3.  Rekeying Interval

   In keeping with the goal of reducing key exposure, the encryption and
   authentication keys SHOULD be changed at least every 90 days.

10.  IPsec Protection Barrier and SPD/GSPD

10.1.  Manual Keying

10.1.1.  SAD Entries

   The Administrator must configure the necessary Security Associations.
   Each SA entry has the source address of an authorized peer, and a
   Destination Address of ALL_PIM_ROUTERS.  Unique SPI values for the
   manually configured SAs MUST be assigned by the Administrator to
   ensure that the SPI does not conflict with existing SPI values in the
   SAD.

10.1.2.  SPD Entries

   The Administrator must configure the necessary SPD entries.  The SPD
   entry must ensure that any outbound IP traffic packet traversing the
   IPsec boundary, with PIM as its next layer protocol and sent to the
   Destination Address of ALL_PIM_ROUTERS, is protected by ESP or AH.
   Note that this characterization includes all the link-local messages
   (Hello, Join/Prune, Bootstrap, Assert).

10.2.  Automatic Keying

   When automatic keying is used, the SA creation is done dynamically
   using a group key management protocol.  The GSPD and PAD tables are
   configured by the Administrator.  The PAD table provides the link
   between the IPsec subsystem and the group key management protocol.

   For automatic keying, the implementation MUST support the multicast
   extensions described in [RFC5374].

10.2.1.  SAD Entries

   All PIM routers participate in an authentication scheme that
   identifies permitted neighbors and achieves peer authentication
   during SA negotiation, leading to child SAs being established and
   saved in the SAD.

10.2.2.  GSPD Entries

   The Administrator must configure the necessary GSPD entries for
   "sender only" directionality.  This rule MUST trigger the group key
   management protocol for a registration exchange.  This exchange will
   set up the outbound SAD entry that encrypts the multicast PIM control
   message.  Considering that this rule is "sender only", no inbound SA
   is created in the reverse direction.

   In addition, the registration exchange will trigger the installation
   of the GSPD entries corresponding to each legitimate peer router,
   with direction "receiver only".  Procedures for achieving the
   registration exchange are out of scope for this document.

   A router SHOULD NOT dynamically detect new neighbors as the result of
   receiving an unauthenticated PIM-SM link-local message or an IPsec
   packet that fails an SAD lookup.  An automated key management
   protocol SHOULD provide a means of notifying a router of new,
   legitimate neighbors.

10.2.3.  PAD Entries

   The PAD will be configured with information to permit identification
   of legitimate group members and senders (i.e., to control the
   adjacency).  Procedures for doing this are out of scope for this
   document.

11.  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 [RFC4301], 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.  This applies to both ESP and AH.  The security
   protocol field is not employed for a multicast SA lookup.

   Given that, from the prospective of a receiving router, each peer
   router is an independent sender and given that the destination
   address will be the same for all senders, the receiving router MUST
   use SPI plus destination address plus sender address when performing
   the SA lookup.  In effect, link-local communication is an SSM
   communication that happens to use an Any-Source Multicast (ASM)
   address (which is shared among all the routers).

   Given that it is always possible to distinguish a connection using
   IPsec from a connection not using IPsec, it is recommended that the
   address ALL_PIM_ROUTERS be used, to maintain consistency with present
   practice.

   Given that the sender address of an incoming packet may be only
   locally unique (because of the use of link-local addresses), it is
   necessary for a receiver to use the interface ID tag to determine the
   associated SA for that sender.  Therefore, this document mandates
   that the interface ID tag, the SPI, and the sender address MUST be
   used in the SA lookup process.

12.  Activating the Anti-Replay Mechanism

   Although link-level messages on a link constitute a multiple-sender,
   multiple-receiver group, the use of the interface ID tag and sender
   address for SA lookup essentially resolves the communication into a
   separate SA for each sender/destination pair, even for the case where
   only two SAs (with identical SA parameters) are used for the entire
   administrative region.  Therefore, the statement in the AH RFC
   (Section 2.5 of [RFC4302]) that "for a multi-sender SA, the anti-
   replay features are not available" becomes irrelevant to the 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.  The use of the
   sender address and the interface ID tag for SA lookup converts the
   relationship from a multiple-sender group to multiple single-sender
   associations.  This specification RECOMMENDS activation of the anti-
   replay mechanism only if the SAs are assigned using an automated key
   management procedure.  If manual key management is used, the anti-
   replay SHOULD NOT be activated.

   If an existing router has to restart, in accordance with RFC 4303
   [RFC4303], the sequence-number counter at the sender MUST be
   correctly maintained across local reboots, etc., until the key is
   replaced.

13.  Implementing a Security Policy Database per Interface

   RFC 4601 suggests that it may be desirable to implement a separate
   Security Policy Database (SPD) for each router interface.  The use of
   link-local addresses in certain circumstances implies that
   differentiation of ambiguous speaker addresses requires the use of
   the interface ID tag in the SA lookup.  One way to do this is through
   the use of multiple SPDs.  Alternatively, the interface ID tag may be
   a specific component of the selector algorithm.  This is in
   conformance with RFC 4301, which explicitly removes the requirement
   for separate SPDs that was present in RFC 2401 [RFC2401].

14.  Extended Sequence Number

   In the [RFC4302], 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 are 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 for a small number of PIM
   routers, 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 protocol.  In addition,
   when an ESN is used with a manually keyed SA, it MUST be saved over a
   reboot, along with an indication of which sequence numbers have been
   used.

15.  Security Considerations

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

   Limitations of manual keys:

   The following are some of the known limitations of the usage of
   manual keys.

   o  If replay protection cannot be provided, the PIM routers will not
      be secured against all the attacks that can be performed by
      replaying PIM packets.

   o  Manual keys are usually long lived (changing them often is a
      tedious task).  This gives an attacker enough time to discover the
      keys.

   o  As the Administrator is manually configuring the keys, there is a
      chance that the configured keys are weak (there are known weak
      keys for DES/3DES at least).

   Impersonation attacks:

   The usage of the same key on all the PIM routers connected to a link
   leaves them all insecure against impersonation attacks if any one of
   the PIM routers is compromised, malfunctioning, or misconfigured.

   Detailed analysis of various vulnerabilities of routing protocols is
   provided in RFC 4593 [RFC4593].  For further discussion of PIM-SM and
   multicast security, the reader is referred to RFC 5294 [RFC5294], RFC
   4609 [RFC4609], and the Security Considerations section of RFC 4601
   [RFC4601].

16.  Acknowledgements

   The structure and text of this document draw heavily from RFC 4552
   [RFC4552].  The authors of this document thank M. Gupta and N. Melam
   for permission to do this.

   The quality of this document was substantially improved after SECDIR
   pre-review by Brian Weis, and after AD review by Adrian Farrel.

17.  References

17.1.  Normative References

   [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", RFC 4601, August 2006.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              December 2005.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

   [RFC4835]  Manral, V., "Cryptographic Algorithm Implementation
              Requirements for Encapsulating Security Payload (ESP) and
              Authentication Header (AH)", RFC 4835, April 2007.

   [RFC5374]  Weis, B., Gross, G., and D. Ignjatic, "Multicast
              Extensions to the Security Architecture for the Internet
              Protocol", RFC 5374, November 2008.

   [RFC2404]  Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within
              ESP and AH", RFC 2404, November 1998.

   [RFC3602]  Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
              Algorithm and Its Use with IPsec", RFC 3602,
              September 2003.

17.2.  Informative References

   [RFC2117]  Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering,
              S., Handley, M., Jacobson, V., Liu, C., Sharma, P., and L.
              Wei, "Protocol Independent Multicast-Sparse Mode (PIM-SM):
              Protocol Specification", RFC 2117, June 1997.

   [RFC2362]  Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering,
              S., Handley, M., and V. Jacobson, "Protocol Independent
              Multicast-Sparse Mode (PIM-SM): Protocol Specification",
              RFC 2362, June 1998.

   [RFC2401]  Kent, S. and R. Atkinson, "Security Architecture for the
              Internet Protocol", RFC 2401, November 1998.

   [RFC4535]  Harney, H., Meth, U., Colegrove, A., and G. Gross,
              "GSAKMP: Group Secure Association Key Management
              Protocol", RFC 4535, June 2006.

   [RFC3547]  Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
              Group Domain of Interpretation", RFC 3547, July 2003.

   [RFC4593]  Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
              Routing Protocols", RFC 4593, October 2006.

   [RFC5294]  Savola, P. and J. Lingard, "Host Threats to Protocol
              Independent Multicast (PIM)", RFC 5294, August 2008.

   [RFC4609]  Savola, P., Lehtonen, R., and D. Meyer, "Protocol
              Independent Multicast - Sparse Mode (PIM-SM) Multicast
              Routing Security Issues and Enhancements", RFC 4609,
              October 2006.

   [RFC4552]  Gupta, M. and N. Melam, "Authentication/Confidentiality
              for OSPFv3", RFC 4552, June 2006.

   [RFC4107]  Bellovin, S. and R. Housley, "Guidelines for Cryptographic
              Key Management", BCP 107, RFC 4107, June 2005.

Authors' Addresses

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

   Phone: +1(514)848-2424 ext3046
   EMail: bill@cse.concordia.ca
   URI:   http://users.encs.concordia.ca/~bill


   Salekul Islam
   INRS Energie, Materiaux et Telecommunications
   800 de La Gauchetiere, Suite 800
   Montreal, QC  H5A 1K6
   Canada

   EMail: Salekul.Islam@emt.inrs.ca
   URI:   http://users.encs.concordia.ca/~salek_is


   Maziar Siami
   Concordia University/CIISE
   1455 de Maisonneuve Blvd. West
   Montreal, QC  H3G 1M8
   Canada

   EMail: mzrsm@yahoo.ca