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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 5374

MSEC Working Group                                              B. Weis
Internet-Draft                                            Cisco Systems
Intended status: Standards Track                               G. Gross
Expires: August, 2007                               IdentAware Security
                                                            D. Ignjatic
                                                         February, 2007

    Multicast Extensions to the Security Architecture for the Internet

Status of this Memo

   By submitting this Internet-Draft, each author represents that
   any applicable patent or other IPR claims of which he or she is
   aware have been or will be disclosed, and any of which he or she
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   BCP 79.

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   The Security Architecture for the Internet Protocol [RFC4301]
   describes security services for traffic at the IP layer. That
   architecture primarily defines services for Internet Protocol (IP)
   unicast packets, as well as manually configured IP multicast packets.
   This document further defines the security services for manually and
   dynamically keyed IP multicast packets within that Security

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Table of Contents

1. Introduction........................................................3
  1.1 Scope............................................................3
  1.2 Terminology......................................................4
2. Overview of IP Multicast Operation..................................5
3. Security Association Modes..........................................6
  3.1 Tunnel Mode with Address Preservation............................6
4. Security Association................................................7
  4.1 Major IPsec Databases............................................7
    4.1.1 Group Security Policy Database (GSPD)........................8
    4.1.2 Security Association Database (SAD)..........................9
    4.1.3 Peer Authorization Database (PAD)............................9
  4.2 Group Security Association (GSA)................................10
  4.3 Data Origin Authentication......................................12
  4.4 Group SA and Key Management.....................................12
    4.4.1 Co-Existence of Multiple Key Management Protocols...........13
    4.4.2 New Security Association Attributes.........................13
5. IP Traffic Processing..............................................13
  5.1 Outbound IP Multicast Traffic Processing........................14
  5.2 Inbound IP Multicast Traffic Processing.........................14
6. Security Considerations............................................14
  6.1 Security Issues Solved by IPsec Multicast Extensions............14
  6.2 Security Issues Not Solved by IPsec Multicast Extensions........15
    6.2.1 Outsider Attacks............................................15
    6.2.2 Insider Attacks.............................................15
  6.3 Implementation or Deployment Issues that Impact Security........16
    6.3.1 Homogeneous Group Cryptographic Algorithm Capabilities......16
    6.3.2 Groups that Span Two or More Security Policy Domains........16
    6.3.3 Network Address Translation.................................17
7. IANA Considerations................................................19
8. Acknowledgements...................................................19
9. References.........................................................20
  9.1 Normative References............................................20
  9.2 Informative References..........................................20
Appendix A - Multicast Application Service Models.....................23
  A.1 Unidirectional Multicast Applications...........................23
  A.2 Bi-directional Reliable Multicast Applications..................23
  A.3 Any-To-Any Multicast Applications...............................24
Author's Address......................................................25
Full Copyright Statement..............................................26
Intellectual Property.................................................26

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1. Introduction

   The Security Architecture for the Internet Protocol [RFC4301]
   provides security services for traffic at the IP layer. It describes
   an architecture for IPsec compliant systems, and a set of security
   services for the IP layer. These security services primarily describe
   services and semantics for IPsec Security Associations (SAs) shared
   between two IPsec devices. Typically, this includes SAs with traffic
   selectors that include a unicast address in the IP destination field,
   and results in an IPsec packet with a unicast address in the IP
   destination field. The security services defined in RFC 4301 can also
   be used to tunnel IP multicast packets, where the tunnel is a
   pairwise association between two IPsec devices.  RFC4301 defined
   manually keyed transport mode IPsec SA support for IP packets with a
   multicast address in the IP destination address field. However,
   RFC4301 did not define the interaction of an IPsec subsystem with a
   Group Key Management protocol or the semantics of a tunnel mode IPsec
   SA with an IP multicast address in the outer IP header.

   This document describes extensions to RFC 4301 that further define
   the IPsec security architecture for groups of IPsec devices to share
   SAs. In particular, it supports SAs with traffic selectors that
   include a multicast address in the IP destination field, and results
   in an IPsec packet with an IP multicast address in the IP destination
   field. It also describes additional semantics for IPsec Group Key
   Management (GKM) subsystems. Note that this document uses the term
   "GKM protocol" generically and therefore it does not assume a
   particular GKM protocol.

1.1 Scope

   The IPsec extensions described in this document support IPsec
   Security Associations that result in IPsec packets with IPv4 or IPv6
   multicast group addresses as the destination address. Both Any-Source
   Multicast (ASM) and Source-Specific Multicast (SSM) [RFC3569]
   [RFC3376] group addresses are supported.

   These extensions also support Security Associations with IPv4
   Broadcast addresses that result in an IPv4 link-level broadcast
   packet, and IPv6 Anycast addresses [RFC2526] that result in an IPv6
   Anycast packet. These destination address types share many of the
   same characteristics of multicast addresses because there may be
   multiple receivers of a packet protected by IPsec.

   The IPsec architecture does not make requirements upon entities not
   participating in IPsec (e.g., network devices between IPsec
   endpoints). As such, these multicast extensions do not require
   intermediate systems in a multicast enabled network to participate in
   IPsec. In particular, no requirements are placed on the use of
   multicast routing protocols (e.g., PIM-SM [RFC4601]) or multicast
   admission protocols (e.g., IGMP [RFC3376].

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   All implementation models of IPsec (e.g., "bump-in-the-stack", "bump-
   in-the-wire") are supported.

   This version of the multicast IPsec extension specification requires
   that all IPsec devices participating in a Security Association are
   homogeneous. They MUST share a common set of cryptographic transform
   and protocol handling capabilities. The semantics of an "IPsec
   composite group" [COMPGRP], a heterogeneous IPsec cryptographic group
   formed from the union of two or more sub-groups, is an area for
   future standardization.

1.2 Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   The following key terms are used throughout this document.

   Any-Source Multicast (ASM)
      The Internet Protocol (IP) multicast service model as defined in
      RFC 1112 [RFC1112]. In this model one or more senders source
      packets to a single IP multicast address. When receivers join the
      group, they receive all packets sent to that IP multicast address.
      This is known as a (*,G) group.

   Group Controller Key Server (GCKS)
      A Group Key Management (GKM) protocol server that manages IPsec
      state for a group. A GCKS authenticates and provides the IPsec SA
      policy and keying material to GKM group members.

   Group Key Management (GKM) Protocol
      A key management protocol used by a GCKS to distribute IPsec
      Security Association policy and keying material. A GKM protocol is
      used when a group of IPsec devices require the same SAs. For
      example, when an IPsec SA describes an IP multicast destination,
      the sender and all receivers must have the group SA.

   Group Key Management Subsystem
      A subsystem in an IPsec device implementing a Group Key Management
      protocol. The GKM subsystem provides IPsec SAs to the IPsec
      subsystem on the IPsec device. Refer to RFC 3547 [RFC3547] and RFC
      4535 [RFC4535] for additional information.

   Group Member
      An IPsec device that belongs to a group. A Group Member is
      authorized to be a Group Speaker and/or a Group Receiver.

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   Group Owner
      An administrative entity that chooses the policy for a group.

   Group Security Association (GSA)
      A collection of IPsec Security Associations (SAs) and GKM
      Subsystem SAs necessary for a Group Member to receive key updates.
      A GSA describes the working policy for a group. Refer to RFC 4046
      [RFC4046] for additional information.

   Group Security Policy Database (GSPD)
      The GSPD is a multicast-capable security policy database, as
      mentioned in RFC3740 and RFC4301 section Its semantics
      are a superset of the unicast SPD defined by RFC4301 section 
      4.4.1. Unlike a unicast SPD-S in which point-to-point traffic
      selectors are inherently bi-directional, multicast security
      traffic selectors in the GSPD-S introduce a "sender only",
      "receiver only" or "symmetric" directional attribute. Refer to
      section 4.1.1 for more details.

   Group Receiver
      A Group Member that is authorized to receive packets sent to a
      group by a Group Speaker.

   Group Speaker
      A Group Member that is authorized to send packets to a group.

   Source-Specific Multicast (SSM)
      The Internet Protocol (IP) multicast service model as defined in
      RFC 3569 [RFC3569]. In this model, each combination of a sender
      and an IP multicast address is considered a group. This is known
      as an (S,G) group.

   Tunnel Mode with Address Preservation
      A type of IPsec tunnel mode used by security gateway
      implementations when encapsulating IP multicast packets such that
      they remain IP multicast packets. This mode is necessary for IP
      multicast routing to correctly route IP multicast packets
      protected by IPsec.

2. Overview of IP Multicast Operation

   IP multicasting is a means of sending a single packet to a "host
   group", a set of zero or more hosts identified by a single IP
   destination address. IP multicast packets are UDP data packets
   delivered to all members of the group with either "best-effort"
   [RFC1112], or reliable delivery  (e.g., NORM) [RFC3940].

   A sender to an IP multicast group sets the destination of the packet
   to an IP address that has been allocated for IP multicast. Allocated
   IP multicast addresses are defined in RFC 3171, RFC 3306, and RFC
   3307 [RFC3171] [RFC3306] [RFC3307]. Potential receivers of the packet

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   "join" the IP multicast group by registering with a network routing
   device [RFC3376] [RFC3810], signaling its intent to receive packets
   sent to a particular IP multicast group.

   Network routing devices configured to pass IP multicast packets
   participate in multicast routing protocols (e.g., PIM-SM) [RFC4601].
   Multicast routing protocols maintain state regarding which devices
   have registered to receive packets for a particular IP multicast
   group. When a router receives an IP multicast packet, it forwards a
   copy of the packet out each interface for which there are known

3. Security Association Modes

   IPsec supports two modes of use: transport mode and tunnel mode.  In
   transport mode, IP Authentication Header (AH) [RFC4302] and IP
   Encapsulating Security Payload (ESP) [RFC4303] provide protection
   primarily for next layer protocols; in tunnel mode, AH and ESP are
   applied to tunneled IP packets.

   A host implementation of IPsec using the multicast extensions MAY use
   either transport mode and tunnel mode to encapsulate an IP multicast
   packet. These processing rules are identical to the rules described
   in Section 4.1 or [RFC4301]. However, the destination address for the
   IPsec packet is an IP multicast address, rather than a unicast host

   A security gateway implementation of IPsec using the multicast
   extensions MUST use a tunnel mode SA, for the reasons described in
   Section 4.1 of [RFC4301]. In particular, the security gateway must
   use tunnel mode to encapsulate incoming fragments, since IPsec cannot
   directly operate on fragments.

3.1 Tunnel Mode with Address Preservation

   New header construction semantics are required when tunnel mode is
   used to encapsulate IP multicast packets that are to remain IP
   multicast packets. This is due to the following unique requirements
   of IP multicast routing protocols (e.g., PIM-SM [RFC4601]).

   - IP multicast routing protocols compare the destination address on
     a packet to the multicast routing state. If the destination of an
     IP multicast packet is changed it will no longer be properly
     routed. Therefore, an IPsec security gateway must preserve the
     multicast IP destination address after IPsec tunnel encapsulation.

     The GKM Subsystem on a security gateway implementing the IPsec
     multicast extensions preserves the multicast IP address as
     follows. Firstly, the GKM Subsystem sets the Remote Address PFP
     flag in the GSPD-S entry for the traffic selectors. This flag
     causes the remote address of the packet matching IPsec SA traffic

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     selectors to be propagated to the IPsec tunnel encapsulation.
     Secondly, the GKM Subsystem needs to signal that destination
     address preservation is in effect for a particular IPsec SA. The
     GKM protocol MUST define an attribute that signals destination
     address preservation to the GKM Subsystem on an IPsec security

   - IP multicast routing protocols also typically create multicast
     distribution trees based on the source address. If an IPsec
     security gateway changes the source address of an IP multicast
     packet (e.g., to its own IP address), the resulting IPsec
     protected packet may fail Reverse Path Forwarding (RPF) checks on
     other routers. A failed RPF check may result in the packet being

     To accommodate routing protocol RPF checks, the GKM Subsystem on a
     security gateway implementation implementing the IPsec multicast
     extensions must preserve the original packet IP source address as
     follows. Firstly, the GSPD-S entry for the traffic selectors must
     have the Source Address PFP flag set. This flag causes the remote
     address to be propagated to the IPsec SA. Secondly, the GKM
     Subsystem needs to signal that source address preservation is in
     effect for a particular IPsec SA. The GKM Subsystem MUST define a
     protocol attribute that signals source address preservation to the
     GKM Subsystem on an IPsec security gateway.

   Some applications of address preservation may only require the
   destination address to be preserved. For this reason, the
   specification of destination address preservation and source address
   preservation are separated in the above description.

   Address preservation is applicable only for tunnel mode IPsec SAs
   that specify the IP version of the encapsulating header to be the
   same version as that of the inner header. When the IP versions are
   different, tunnel processing semantics described in RFC 4301 MUST be

   In summary, retaining both the IP source and destination addresses of
   the inner IP header allow IP multicast routing protocols to route the
   packet irrespective of the packet being protected by IPsec. This
   result is necessary in order for the multicast extensions to allow a
   security gateway to provide IPsec services for IP multicast packets.
   This method of RFC 4301 tunnel mode is known as "tunnel mode with
   address preservation".

4. Security Association

4.1 Major IPsec Databases

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   The following sections describe the GKM Subsystem and IPsec extension
   interactions with the major IPsec databases. The major IPsec
   databases needed expanded semantics to fully support multicast.

4.1.1 Group Security Policy Database (GSPD)

   The Group Security Policy Database is a security policy database
   capable of implementing both unicast security associations as defined
   by RFC4301 and the multicast extensions defined by this
   specification. A new Group Security Policy Database (GSPD) attribute
   is introduced: GSPD entry directionality. Directionality can take
   three types. Each GSPD entry can be marked "symmetric", "sender only"
   or "receiver only". "Symmetric" GSPD entries are the common entries
   as specified by RFC 4301. "Symmetric" SHOULD be the default
   directionality unless specified otherwise. GSPD entries marked as
   "sender only" or "receiver only" SHOULD support multicast IP
   addresses in their destination address selectors. If the processing
   requested is bypass or discard and a "sender only" type is configured
   the entry SHOULD be put in GSPD-O only. Reciprocally, if the type is
   "receiver only", the entry SHOULD go to GSPD-I only. SSM is supported
   by the use of unicast IP address selectors as documented in RFC 4301.

   GSPD entries created by a GCKS may be assigned identical SPIs to SAD
   entries created by IKEv2 [RFC4306]. This is not a problem for the
   inbound traffic as the appropriate SAs can be matched using the
   algorithm described in RFC 4301 section 4.1. In addition, SAs with
   identical SPI values but not manually keyed can be differentiated
   because they contain a link to their parent SPD entries. However, the
   outbound traffic needs to be matched against the GSPD selectors so
   that the appropriate SA can be created on packet arrival. IPsec
   implementations that support multicast MUST use the destination
   address as the additional selector and match it against the GSPD
   entries marked "sender only".

   To facilitate dynamic group keying, the outbound GSPD MUST implement
   a policy action capability that triggers a GKM protocol registration
   exchange (as per Section 5.1 of [RFC4301]). For example, the Group
   Speaker GSPD policy might trigger on a match with a specified
   multicast application packet. The ensuing Group Speaker registration
   exchange would setup the Group Speaker's outbound SAD entry that
   encrypts the multicast application's data stream. In the inverse
   direction, group policy may also setup an inbound IPsec SA.

   At the Group Receiver endpoint(s), the GSPD policy might trigger on a
   match with the multicast application packet sent from the Group
   Speaker. The ensuing Group Receiver registration exchange would setup
   the Group Receiver's inbound SAD entry that decrypts the multicast
   application's data stream. In the inverse direction, the group policy
   may also setup an outbound IPsec SA (e.g. when supporting an ASM
   service model).

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   The IPsec subsystem MAY provide GSPD policy mechanisms (e.g. trigger
   on detection of IGMP/MLD leave group exchange) that automatically
   initiate a GKM protocol de-registration exchange. De-registration
   minimizes exposure of the group's secret key. It also minimizes cost
   for those groups that incur cost on the basis of membership duration.

   Additionally, the GKM subsystem MAY setup the GSPD/SAD state
   information independent of the multicast application's state. In this
   scenario, the group's Group Owner issues management directives that
   tells the GKM subsystem when it should start GKM registration and de-
   registration protocol exchanges. Typically the registration policy
   strives to make sure that the group's IPsec subsystem state is
   "always ready" in anticipation of the multicast application starting
   its execution.

4.1.2 Security Association Database (SAD)

   The Security Association Database (SAD) can support multicast SAs, if
   manually configured. An outbound multicast SA has the same structure
   as a unicast SA. The source address is that of the Group Speaker and
   the destination address is the multicast group address. An inbound
   multicast SA must be configured with the source addresses of each
   Group Speaker peer authorized to transmit to the multicast SA in
   question. The SPI value for a multicast SA is provided by a GCKS, not
   by the receiver as occurs for a unicast SA.  Other than the SPI
   assignment and the inbound packet de-multiplexing described in
   RFC4301 section 4.1, the SAD behaves identically for unicast and
   multicast security associations.

4.1.3 Peer Authorization Database (PAD)

   The Peer Authorization Database (PAD) needs to be extended in order
   to accommodate peers that may take on specific roles in the group.
   Such roles can be GCKS, Group Speaker (in case of SSM) or a Group
   Receiver. A peer can have multiple roles. The PAD may also contain
   root certificates for PKI used by the group. GKM/IPsec Interactions with the PAD

   The RFC 4301 section 4.4.3 introduced the PAD. In summary, the PAD
   manages the IPsec entity authentication mechanism(s) and
   authorization of each such peer identity to negotiate modifications
   to the GSPD/SAD. Within the context of the GKM/IPsec subsystem, the
   PAD defines for each group:

   . For those groups that authenticate identities using a Public Key
     Infrastructure, the PAD contains the group's set of one or more
     trusted root public key certificates. The PAD may also include the
     PKI configuration data needed to retrieve supporting certificates
     needed for an end entity's certificate path validation.

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   . A set of one or more group membership authorization rules. The GCKS
     examines these rules to determine a candidate group member's
     acceptable authentication mechanism and to decide whether that
     candidate has the authority to join the group.

   . A set of one or more GCKS role authorization rules. A group member
     uses these rules to decide which systems are authorized to act as a
     GCKS for a given group. These rules also declare the permitted GCKS
     authentication mechanism(s).

   . A set of one or more Group Speaker role authorization rules. In
     some groups the group members allowed to send protected packets is

   Some GKM protocols (e.g. GSAKMP [RFC4535]) distribute their group's
   PAD configuration in a security policy token [RFC4534] signed by the
   group's policy authority, also known as the Group Owner (GO). Each
   group member receives the policy token (using a method not described
   in this memo) and verifies the Group Owner's signature on the policy
   token. If that GO signature is accepted, then the group member
   dynamically updates its PAD with the policy token's contents.

   The PAD MUST provide a management interface capability that allows an
   administrator to enforce that the scope of a GKM group's policy
   specified GSPD/SAD modifications are restricted to only those traffic
   data flows that belong to that group. This authorization MUST be
   configurable at GKM group granularity. In the inverse direction, the
   PAD management interface MUST provide a mechanism(s) to enforce that
   IKEv2 security associations do not negotiate traffic selectors that
   conflict or override GKM group policies.

   This document refers to re-key mechanisms as being multicast because
   of the inherent scalability of IP multicast distribution. However,
   there is no particular reason that re-key mechanisms must be
   multicast. For example, [ZLLY03] describes a method of re-key
   employing both unicast and multicast messages.

4.2 Group Security Association (GSA)

   As stated in Section 4 of [RFC3740] an IPsec implementation
   supporting these extensions has a number of security associations:
   one or more IPsec SAs, and one or more GKM SAs used to download IPsec
   SAs. These SAs are collectively referred to as a Group Security
   Association (GSA).

4.2.1 Concurrent IPsec SA Life Spans and Re-key Rollover

   During a cryptographic group's lifetime, multiple IPsec group
   security associations can exist concurrently. This occurs principally
   due to two reasons:

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  - There are multiple Group Speakers authorized in the group, each
     with its own IPsec SA that maintains anti-replay state. A group
     that does not rely on IP Security anti-replay services can share
     one IPsec SA for all of its Group Speakers.

  - The life spans of a Group Speaker's two (or more) IPsec SAs are
     allowed to overlap in time, so that there is continuity in the
     multicast data stream across group re-key events. This capability
     is referred to as "re-key rollover continuity".

   Each group re-key multicast message sent by a GCKS signals the start
   of a new Group Speaker time epoch, with each such epoch having an
   associated IPsec SA. The group membership interacts with these IPsec
   SAs as follows:

  - As a precursor to the Group Speaker beginning its re-key rollover
     continuity processing, the GCKS periodically multicasts a Re-Key
     Event (RKE) message to the group. The RKE multicast contains group
     policy directives, and new IPsec SA policy and keying material. In
     the absence of a reliable multicast transport protocol, the GCKS
     may re-transmit the RKE a policy defined number of times to improve
     the availability of re-key information.

  - The RKE multicast configures the group's GSPD/SAD with the new
     IPsec SAs. Each IPsec SA that replaces an existing SA is called a
     "leading edge" IPsec SA. The leading edge IPsec SA has a new
     Security Parameter Index (SPI) and its associated keying material
     keys it. For a short period after the GCKS multicasts the RKE, a
     Group Speaker does not yet transmit data using the leading edge
     IPsec SA. Meanwhile, other Group Members prepare to use this IPsec
     SA by installing the new IPsec SAs to their respective GSPD/SAD.

  - After waiting a sufficiently long enough period such that all of
     the Group Members have processed the RKE multicast, the Group
     Speaker begins to transmit using the leading edge IPsec SA with its
     data encrypted by the new keying material. Only authorized Group
     Members can decrypt these IPsec SA multicast transmissions. The
     time delay that a Group Speaker waits before starting its first
     leading edge SA transmission is a GKM/IPsec policy parameter. This
     value SHOULD be configurable at the Group Owner management
     interface on a per group basis.

  - The Group Speaker's "trailing edge" SA is the oldest security
     association in use by the group for that speaker. All authorized
     Group Members can receive and decrypt data for this SA, but the
     Group Speaker does not transmit new data using the "trailing edge"
     SA after it has transitioned to the "leading edge SA". The trailing
     edge SA is deleted by the group's endpoints according to group
     policy (e.g., after a defined period has elapsed)"

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   This re-key rollover strategy allows the group to drain its in
   transit datagrams from the network while transitioning to the leading
   edge SA. Staggering the roles of each respective IPsec SA as
   described above improves the group's synchronization even when there
   are high network propagation delays. Note that due to group
   membership joins and leaves, each Group Speaker time epoch may have a
   different group membership set.

   It is a group policy decision whether the re-key event transition
   between epochs provides forward and backward secrecy. The group's re-
   key protocol keying material and algorithm (e.g. Logical Key
   Hierarchy) enforces this policy. Implementations MAY offer a Group
   Owner management interface option to enable/disable re-key rollover
   continuity for a particular group. This specification requires that a
   GKM/IPsec implementation MUST support at least two concurrent IPsec
   SA per Group Speaker and this re-key rollover continuity algorithm.

4.3 Data Origin Authentication

   As defined in [RFC4301], data origin authentication is a security
   service that verifies the identity of the claimed source of data. A
   Message Authentication Code (MAC) is often used to achieve data
   origin authentication for connections shared between two parties. But
   MAC authentication methods are not sufficient to provide data origin
   authentication for groups with more than two parties. With a MAC
   algorithm, every group member can use the MAC key to create a valid
   MAC tag, whether or not they are the authentic originator of the
   group application's data.

   When the property of data origin authentication is required for an
   IPsec SA distributed from a GKCS, an authentication transform where
   the originator keeps a secret should be used. Two possible algorithms
   are TESLA [RFC4082] or RSA digital signature [RFC4359].

   In some cases, (e.g., digital signature authentication transforms)
   the processing cost of the algorithm is significantly greater than an
   HMAC authentication method. To protect against denial of service
   attacks from device that is not authorized to join the group, the
   IPsec SA using this algorithm may be encapsulated with an IPsec SA
   using a MAC authentication algorithm. However, doing so requires the
   packet to be sent across the IPsec boundary for additional inbound
   processing (see Section 5.2 of [RFC4301]). This use of ESP
   encapsulated within ESP accommodates the constraint that an ESP
   trailer defines an Integrity Check Value (ICV) for only a single
   authenticator transform. Relaxing this constraint on the use of the
   ICV field is an area for future standardization.

4.4 Group SA and Key Management

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4.4.1 Co-Existence of Multiple Key Management Protocols

   Often, the GKM subsystem will be introduced to an existent IPsec
   subsystem as a companion key management protocol to IKEv2 [RFC4306].
   A fundamental GKM protocol IP Security subsystem requirement is that
   both the GKM protocol and IKEv2 can simultaneously share access to a
   common Group Security Policy Database and Security Association
   Database. The mechanisms that provide mutually exclusive access to
   the common GSPD/SAD data structures are a local matter. This includes
   the GSPD-outbound cache and the GSPD-inbound cache. However,
   implementers should note that IKEv2 SPI allocation is entirely
   independent from GKM SPI allocation because group security
   associations are qualified by a destination multicast IP address and
   may optionally have a source IP address qualifier. See [RFC4303,
   Section 2.1] for further explanation.

   The Peer Authorization Database does require explicit coordination
   between the GKM protocol and IKEv2. Section 4.1.3 describes these

4.4.2 New Security Association Attributes

   A number of new security association attributes are defined to convey
   extensions defined in this document. Each GKM protocol supporting
   this architecture MUST support the following list of attributes
   described elsewhere in this document.

   - Address Preservation (Section 3.1). This attribute describes
   whether address preservation is to be applied to the SA on the source
   address, destination address, or both source and destination

   - Directional attribute (Section 4.1.1). This attribute describes
   whether a pair of SAs (one in each direction) are to be installed (to
   match the "symmetric" SPD directionality), only in the outbound
   direction (to match "receiver only" SPD directionality), or only in
   the inbound direction (to match "sender only" SPD directionality).

   - Any of the cryptographic transform-specific parameters and keys
   that are sent from the GCKS to the Group Members (e.g. data origin
   authentication parameters as described in section 4.3).

   - Re-key rollover procedure time intervals (section The
   time that the Group Receiver IPsec subsystems will wait after
   creating the leading edge IPsec SA before they will retire the
   trailing edge IPsec SA. Also, the time that the Group Speaker will
   delay before it starts transmitting on the leading edges IPsec SA.

5. IP Traffic Processing

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   Processing of traffic follows Section 5 of [RFC4301], with the
   additions described below when these IP multicast extensions are

5.1 Outbound IP Multicast Traffic Processing

   If an IPsec SA is marked as supporting tunnel mode with address
   preservation (as described in Section 3.1), either or both of the
   outer header source or destination addresses is marked as being
   preserved. If the source address is marked as being preserved, during
   header construction the "src address" header field MUST be "copied
   from inner hdr" rather than "constructed" as described in [RFC4301].
   Similarly, if the destination address is marked as being preserved,
   during header construction the "dest address" header field MUST be
   "copied from inner hdr" rather than "constructed".

5.2 Inbound IP Multicast Traffic Processing

   If an IPsec SA is marked as supporting tunnel mode with address
   preservation (as described in Section 3.1), the marked address (i.e.,
   source and/or destination address) on the outer IP header MUST be
   verified to be the same value as the inner IP header. If the
   addresses are not consistent, the IPsec system MUST treat the error
   in the same manner as other invalid selectors, as described in
   Section 5.2 of [RFC4301]. In particular the IPsec system MUST discard
   the packet, as well as treat the inconsistency as an auditable event.

6. Security Considerations

   The IP security multicast extensions defined by this specification
   build on the unicast-oriented IP security architecture [RFC4301].
   Consequently, this specification inherits many of the RFC4301
   security considerations and the reader is advised to review it as
   companion guidance.

6.1 Security Issues Solved by IPsec Multicast Extensions

   The IP security multicast extension service provides the following
   network layer mechanisms for secure group communications:

  - Confidentiality using a group shared encryption key.

  - Group source authentication and integrity protection using a group
     shared authentication key.

  - Group Speaker data origin authentication using a digital signature,
     TESLA, or other mechanism.

  - Anti-replay protection for a limited number of Group Speakers using
     the ESP (or AH) sequence number facility.

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  - Filtering of multicast transmissions by those group members who are
     not authorized by group policy to be Group Speakers. This feature
     leverages the IPsec state-less firewall service.

   In support of the above services, this specification enhances the
   definition of the SPD, PAD, and SAD databases to facilitate the
   automated group key management of large-scale cryptographic groups.

6.2 Security Issues Not Solved by IPsec Multicast Extensions

   As noted in RFC4301 section 2.2, it is out of scope of this
   architecture to defend the group's keys or its application data
   against those attacks that do not originate in the network. However,
   it should be noted that the risk of these attacks is magnified to the
   extent that the group keys are shared across a large number of

   The security issues that are left unsolved by the IPsec multicast
   extension service divide into two broad categories: outsider attacks,
   and insider attacks.

6.2.1 Outsider Attacks

   The IPsec multicast extension service does not defend against an
   Adversary outside of the group who has:

  - The capability to launch a multicast flooding denial-of-service
     attack against the group, originating from a system whose IPsec
     subsystem does not filter the unauthorized multicast transmissions.

  - Compromised a multicast router, allowing the Adversary to corrupt
     or delete all multicast packets destined for the group endpoints
     downstream from that router.

  - Captured a copy of an earlier multicast packet transmission and
     then replays it to a group that does not have the anti-replay
     service enabled. Note that for a large-scale any source multicast
     group, it is impractical for the Group Receivers to maintain an
     anti-replay state for every potential Group Speaker. Group policies
     that require anti-replay protection for a large-scale any-source-
     multicast group should consider an application layer total order
     multicast protocol.

6.2.2 Insider Attacks

   For large-scale groups, the IP security multicast extensions are
   dependent on an automated Group Key Management protocol to correctly
   authenticate and authorize trustworthy members in compliance to the
   group's policies. Inherent in the concept of a cryptographic group is
   a set of one or more shared secrets entrusted to all of the group's

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   members. Consequently, the service's security guarantees are no
   stronger than the weakest member admitted to the group by the GKM
   system. The GKM system is responsible for responding to compromised
   group member detection by executing a group key recovery procedure.
   The GKM re-keying protocol will expel the compromised group members
   and distribute new group keying material to the trusted members.
   Alternatively, the group policy may require the GKM system to
   terminate the group.

   In the event that an Adversary has been admitted into the group by
   the GKM system, the following attacks are possible and they can not
   be solved by the IPsec multicast extension service:

  - The Adversary can disclose the secret group key or group data to an
     unauthorized party outside of the group. After a group key or data
     compromise, cryptographic methods such as traitor tracing or
     watermarking can assist in the forensics process. However, these
     methods are outside the scope of this specification.

  - The insider Adversary can forge packet transmissions that appear to
     be from a peer group member. To defend against this attack for
     those Group Speaker transmissions that warrant the overhead, the
     group policy can require the Group Speaker to multicast packets
     using the data origin authentication service.

  - If the group's data origin authentication service uses digital
     signatures, then the insider Adversary can launch a computational
     resource denial of service attack by multicasting bogus signed

6.3 Implementation or Deployment Issues that Impact Security

6.3.1 Homogeneous Group Cryptographic Algorithm Capabilities

   The IP security multicast extensions service can not defend against a
   poorly considered group security policy that allows a weaker
   cryptographic algorithm simply because all of the group's endpoints
   are known to support it. Unfortunately, large-scale groups can be
   difficult to upgrade to the current best in class cryptographic
   algorithms. One possible approach is the deployment of composite
   groups that can straddle heterogeneous groups [COMPGRP]. A standard
   solution for heterogeneous groups is an activity for future
   standardization. In the interim, synchronization of a group's
   cryptographic capabilities could be achieved using a secure and
   scalable software distribution management tool.

6.3.2 Groups that Span Two or More Security Policy Domains

   Large-scale groups may span multiple legal jurisdictions (e.g
   countries) that enforce limits on cryptographic algorithms or key
   strengths. As currently defined, the IPsec multicast extension

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   service requires a single group policy per group. As noted above,
   this problem remains an area for future standardization.

6.3.3 Network Address Translation

   With the advent of NAT and mobile nodes, IPsec multicast applications
   must overcome several architectural barriers to their successful
   deployment. This section surveys those problems and identifies the
   GSPD/SAD state information that the GKM protocol must synchronize
   across the group membership. GSPD Losses Synchronization with Internet Layer's State

   The most prominent problem facing GKM protocols supporting IPsec is
   that the GKM protocol's group security policy mechanism can
   inadvertently configure the group's GSPD traffic selectors with
   unreliable transient IP addresses. The IP addresses are transient
   because of either node mobility or Network Address Translation (NAT),
   both of which can unilaterally change a Group Speaker's source IP
   address without signaling the GKM protocol. The absence of a GSPD
   synchronization mechanism can cause the group's data traffic to be
   discarded rather than processed correctly. Mobile Multicast Care-Of Address Route Optimization

   Both Mobile IPv4 [RFC3344] and Mobile IPv6 provide transparent
   unicast communications to a mobile Node. However, comparable support
   for secure multicast mobility management is not specified by these
   standards. The goal is the ability to maintain an end-to-end
   transport mode group SA between a Group Speaker mobile node that has
   a volatile care-of-address and a Group Receiver membership that also
   may have mobile endpoints. In particular, there is no secure
   mechanism for route optimization of the triangular multicast path
   between the correspondent Group Receiver nodes, the home agent, and
   the mobile node. Any proposed solution must be secure against hostile
   re-direct and flooding attacks. NAT Translation Mappings Are Not Predictable

   The following spontaneous NAT behaviors adversely impact source-
   specific secure multicast groups. When a NAT gateway is on the path
   between a Group Speaker residing behind a NAT and a public IPv4
   multicast Group Receiver, the NAT gateway alters the private source
   address to a public IPv4 address. This translation must be
   coordinated with every Group Receiver's inbound GSPD multicast
   entries that depend on that source address as a traffic selector. One
   might mistakenly assume that the GCKS could set up the Group Members
   with a GSPD entry that anticipates the value(s) that the NAT
   translates the packet's source address. However, there are known

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   cases where this address translation can spontaneously change without

  - NAT gateways may re-boot and lose their address translation state

  - The NAT gateway may de-allocate its address translation state after
     an inactivity timer expires. The address translation used by the
     NAT gateway after the resumption of data flow may differ than that
     known to the GSPD selectors at the group endpoints.

  - The GCKS may not have global consistent knowledge of a group
     endpoint's current public and private address mappings due to
     network errors or race conditions. For example, a Group Member's
     address may change due to a DHCP assigned address lease expiration.

  - Alternate paths may exist between a given pair of Group Members. If
     there are parallel NAT gateways along those paths, then the address
     translation state information at each NAT gateway may produce
     different translations on a per packet basis.

   The consequence of this problem is that the GCKS can not be pre-
   configured with NAT mappings, as the GSPD at the Group Members will
   lose synchronization as soon as a NAT mapping changes due to any of
   the above events. In the worst case, Group Members in different
   sections of the network will see different NAT mappings, because the
   multicast packet traversed multiple NAT gateways. SSM Routing Dependency on Source IP Address

   Source-Specific Multicast (SSM) routing depends on a multicast
   packet's source IP address and multicast destination IP address to
   make a correct forwarding decision. However, a NAT gateway alters
   that packet's source IP address as its passes from a private network
   into the public network. Mobility changes a Group Member's point of
   attachment to the Internet, and this will change the packet's source
   IP address. Regardless of why it happened, this alteration in the
   source IP address makes it infeasible for transit multicast routers
   in the public Internet to know which SSM speaker originated the
   multicast packet, which in turn selects the correct multicast
   forwarding policy. ESP Cloaks Its Payloads from NAT Gateway

   When traversing NAT, application layer protocols that contain IPv4
   addresses in their payload need the intervention of an Application
   Layer Gateway (ALG) that understands that application layer protocol
   [RFC3027] [RFC3235]. The ALG massages the payload's private IPv4
   addresses into equivalent public IPv4 addresses. However, when

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   encrypted by end-to-end ESP, such payloads are opaque to application
   layer gateways.

   When multiple Group Speakers reside behind a NAT with a single public
   IPv4 address, the NAT gateway can not do UDP or TCP protocol port
   translation (i.e. NAPT) because the ESP encryption conceals the
   transport layer protocol headers. The use of UDP encapsulated ESP
   [RFC3948] avoids this problem. However, this capability must be
   configured at the GCKS as a group policy, and it must be supported in
   unison by all of the group endpoints within the group, even those
   that reside in the public Internet. UDP Checksum Dependency on Source IP Address

   An IPsec subsystem using UDP within an ESP payload will encounter NAT
   induced problems. The original IPv4 source address is an input
   parameter into a receiver's UDP pseudo-header checksum verification,
   yet that value is lost after the IP header's address translation by a
   transit NAT gateway. The UDP header checksum is opaque within the
   encrypted ESP payload. Consequently, the checksum can not be
   manipulated by the transit NAT gateways. UDP checksum verification
   needs a mechanism that recovers the original source IPv4 address at
   the Group Receiver endpoints.

   In a transport mode multicast application GSA, the UDP checksum
   operation requires the origin endpoint's IP address to complete
   successfully. In IKEv2, this information is obtained from the Traffic
   Selectors associated with the exchange [RFC4306, Section 2.23]. See
   also reference [RFC3947]. A facility that obtains the same result
   must exist in a GKM protocol payload that defines the multicast
   application GSA attributes for each Group Speaker. Cannot Use AH with NAT Gateway

   The presence of a NAT gateway makes it impossible to use an
   Authentication Header, keyed by a group-wide key, to protect the
   integrity of the IP header for transmissions between members of the
   cryptographic group.

7. IANA Considerations

   This document has no actions for IANA.

8. Acknowledgements

   The authors wish to thank Pasi Eronen and Tero Kivinen for their
   helpful comments.

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   The "Guidelines for Writing RFC Text on Security Considerations"
   [RFC3552] was consulted to develop the Security Considerations
   section of this memo.

9. References

9.1 Normative References

   [RFC1112] Deering, S., "Host Extensions for IP Multicasting," RFC
             1112, August 1989.

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

   [RFC3552] Rescorla, E., et. al., "Guidelines for Writing RFC Text on
             Security Considerations", RFC 3552, July 2003.

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

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

   [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
             4303, December 2004.

9.2 Informative References

   [COMPGRP] Gross G. and H. Cruickshank, "Multicast IP Security
             Composite Cryptographic Groups", draft-gross-msec-ipsec-
             composite-group-01.txt, work in progress, September 2006.

   [RFC2526] Johnson, D., and S. Deering., "Reserved IPv6 Subnet Anycast
             Addresses", RFC 2526, March 1999.

   [RFC2914] Floyd, S., "Congestion Control Principles", RFC 2914,
             September 2000.

   [RFC3027] Holdrege, M., and P. Srisuresh, "Protocol Complications
             with the IP Network Address Translator", RFC 3027, January

   [RFC3171] Albanni, Z., et. al., "IANA Guidelines for IPv4
             Multicast Address Assignments", RFC 3171, August 2001.

   [RFC3235] Senie, D., "Network Address Translator (NAT)-Friendly
             Application Design Guidelines", RFC 3235, January 2002.

   [RFC3306] Haberman B. and D. Thaler, " Unicast-Prefix-based IPv6
             Multicast Addresses", RFC3306, August 2002.

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   [RFC3307] Haberman B., " Allocation Guidelines for IPv6 Multicast
             Addresses", RFC3307, August 2002.

   [RFC3344] Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
             August 2002.

   [RFC3376] Cain, B., et. al., "Internet Group Management Protocol,
             Version 3", RFC 3376, October 2002.

   [RFC3547] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
             Group Domain of Interpretation", RFC 3547, December 2002.

   [RFC3569] Bhattacharyya, S., "An Overview of Source-Specific
             Multicast (SSM)", RFC 3569, July 2003.

   [RFC3740] Hardjono, Tl, and B. Weis, "The Multicast Group Security
             Architecture", RFC 3740, March 2004.

   [RFC3810] Vida, R., and L. Costa, "Multicast Listener Discovery
             Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.

   [RFC3940] Adamson, B., et. al., "Negative-acknowledgment (NACK)-
             Oriented Reliable Multicast (NORM) Protocol", RFC 3940,
             November 2004.

   [RFC3947] Kivinen, T., et. al., "Negotiation of NAT-Traversal in the
             IKE", RFC 3947, January 2005.

   [RFC3948] Huttunen, A., et. al., "UDP Encapsulation of IPsec ESP
             Packets", RFC 3948, January 2005.

   [RFC4046] Baugher, M., Dondeti, L., Canetti, R., and F. Lindholm,
             "Multicast Security (MSEC) Group Key Management
             Architecture", RFC4046, April 2005.

   [RFC4082] Perrig, A., et. al., "Timed Efficient Stream Loss-Tolerant
             Authentication (TESLA): Multicast Source Authentication
             Transform Introduction", RFC 4082, June 2005.

   [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC
             4306, December 2005.

   [RFC4359] Weis, B., "The Use of RSA/SHA-1 Signatures within
             Encapsulating Security Payload (ESP) and Authentication
             Header (AH)", RFC 4359, January 2006.

   [RFC4534] Colegrove, A., and H. Harney, "Group Security Policy Token
             v1", RFC 4534, June 2006.

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

   [RFC4601] Fenner, B., et. al., "Protocol Independent Multicast -
             Sparse Mode (PIM-SM): Protocol  Specification (Revised)",
             RFC 4601, August 2006.

   [ZLLY03] Zhang, X., et. al., "Protocol Design for Scalable and
             Reliable Group Rekeying", IEEE/ACM Transactions on
             Networking (TON), Volume 11, Issue 6, December 2003. See

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Appendix A - Multicast Application Service Models

   The vast majority of secure multicast applications can be catalogued
   by their service model and accompanying intra-group communication
   patterns. Both the Group Key Management (GKM) Subsystem and the IPsec
   subsystem MUST be able to configure the GSPD/SAD security policies to
   match these dominant usage scenarios. The GSPD/SAD policies MUST
   include the ability to configure both Any-Source-Multicast groups and
   Source-Specific-Multicast groups for each of these service models.
   The GKM Subsystem management interface MAY include mechanisms to
   configure the security policies for service models not identified by
   this standard.

A.1 Unidirectional Multicast Applications

   Multi-media content delivery multicast applications that do not have
   congestion notification or retransmission error recovery mechanisms
   are inherently unidirectional. RFC 4301 only defines bi-directional
   unicast traffic selectors (as per sections 4.4.1 and 5.1 with respect
   to traffic selector directionality). The GKM Subsystem requires that
   the IPsec subsystem MUST support unidirectional SPD entries, which
   cause a Group Security Associations (GSA)to be installed in only one
   direction. Multicast applications that have only one group member
   authorized to transmit can use this type of group security
   association to enforce that group policy. In the inverse direction,
   the GSA does not have a SAD entry, and the GSPD configuration is
   optionally setup to discard unauthorized attempts to transmit unicast
   or multicast packets to the group.

   The GKM Subsystem's management interface MUST have the ability to
   setup a GKM Subsystem group having a unidirectional GSA security

A.2 Bi-directional Reliable Multicast Applications

   Some secure multicast applications are characterized as one group
   speaker to many receivers, but with inverse data flows required by a
   reliable multicast transport protocol (e.g. NORM). In such
   applications, the data flow from the speaker is multicast, and the
   inverse flow from the group's receivers is unicast to the speaker.
   Typically, the inverse data flows carry error repair requests and
   congestion control status.

   For such applications, it is advantageous to use the same IPsec SA
   for protection of both unicast and multicast data flows. This does
   introduce one risk: the IKEv2 application may choose the same SPI for
   receiving unicast traffic as the GCKS chooses for a group IPsec SA
   covering unicast traffic. If both SAs are installed in the SAD, the
   SA lookup may return the wrong SPI as the result of an SA lookup. To
   avoid this problem, IPsec SAs installed by the GKM SHOULD use the 2-
   tuple {destination IP address, SPI} to identify each IPsec SA. In

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   addition, the GKM SHOULD use a unicast destination IP address that
   does not match any destination IP address in use by an IKE-v2 unicast
   IPsec SA. For example, suppose a Group Member is using both IKEv2 and
   a GKM protocol, and and the group security policy requires protecting
   the NORM inverse data flows as described above. In this case, group
   policy SHOULD allocate and use a unique unicast destination IP
   address representing the NORM Group Speaker. This address would be
   configured in parallel to the Group Speaker's existing IP addresses.
   The GKM subsystems at both the NORM Group Speaker and Group Receiver
   endpoints would install the IPsec SA protecting the NORM unicast
   messages such that the SA lookup uses the unicast destination address
   as well as the SPI.

   The GSA SHOULD use IPsec anti-replay protection service for the
   speaker's multicast data flow to the group's receivers. Because of
   the scalability problem described in the next section, it is not
   practical to use the IPsec anti-replay service for the unicast
   inverse flows. Consequently, in the inverse direction the IPsec anti-
   replay protection MUST be disabled. However, the unicast inverse
   flows can use the group's IPsec group authentication mechanism. The
   group receiver's GSPD entry for this GSA SHOULD be configured to only
   allow a unicast transmission to the speaker Node rather than a
   multicast transmission to the whole group.

   If an ESP digital signature authentication is available (E.g., RFC
   4359), source authentication MAY be used to authenticate a receiver
   Node's transmission to the speaker. The GKM protocol MUST define a
   key management mechanism for the group speaker to validate the
   asserted signature public key of any receiver Node without requiring
   that the speaker maintain state about every group receiver.

   This multicast application service model is RECOMMENDED because it
   includes congestion control feedback capabilities. Refer to [RFC2914]
   for additional background information.

   The GKM Subsystem's Group Owner management interface MUST have the
   ability to setup a symmetric GSPD entry and one group speaker. The
   management interface SHOULD be able to configure a group to have at
   least 16 concurrent authorized speakers, each with their own GSA
   anti-replay state.

A.3 Any-To-Any Multicast Applications

   Another family of secure multicast applications exhibits a "any to
   many" communications pattern. A representative example of such an
   application is a videoconference combined with an electronic

   For such applications, all (or a large subset) of the Group Members
   are authorized multicast speakers. In such service models, creating a
   distinct IPsec SA with anti-replay state for every potential speaker

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   does not scale to large groups. The group SHOULD share one IPsec SA
   for all of its speakers. The IPsec SA SHOULD NOT use the IPsec anti-
   replay protection service for the speaker's multicast data flow to
   the Group Receivers.

   The GKM Subsystem's management interface MUST have the ability to
   setup a group having an Any-To-Many Multicast GSA security policy.

Author's Address

   Brian Weis
   Cisco Systems
   170 W. Tasman Drive,
   San Jose, CA 95134-1706

   Phone: +1-408-526-4796
   Email: bew@cisco.com

   George Gross
   IdentAware Security
   82 Old Mountain Road
   Lebanon, NJ 08833

   Phone: +1-908-268-1629
   Email: gmgross@identaware.com

   Dragan Ignjatic
   1000 W. 14th Street
   North Vancouver, BC V7P 3P3

   Phone: +1-604-982-3424
   Email: dignjatic@polycom.com

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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an

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   Administrative Support Activity (IASA).

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