MSEC Working Group B. Weis
Internet-Draft Cisco Systems
Intended status: Standards Track G. Gross
Expires: June 9, 2008 IdentAware Security
D. Ignjatic
Polycom
December 9, 2007
Multicast Extensions to the Security Architecture for the Internet
Protocol
draft-ietf-msec-ipsec-extensions-07.txt
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Copyright (C) The IETF Trust (2007).
Abstract
The Security Architecture for the Internet Protocol describes
security services for traffic at the IP layer. That architecture
primarily defines services for Internet Protocol (IP) unicast
packets. It also defines services for manually keyed Security
Associations (SAs) matching IP multicast traffic selectors. This
document further defines the security services for manually and
dynamically keyed SAs matching IP multicast traffic selectors
within that Security Architecture.
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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.............................................8
4.1 Major IPsec Databases.........................................8
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).............................11
4.3 Data Origin Authentication...................................13
4.4 Group SA and Key Management..................................13
4.4.1 Co-Existence of Multiple Key Management Protocols........13
4.4.2 New Security Association Attributes......................14
5. IP Traffic Processing...........................................14
5.1 Outbound IP Multicast Traffic Processing.....................14
5.2 Inbound IP Multicast Traffic Processing......................15
6. Security Considerations.........................................15
6.1 Security Issues Solved by IPsec Multicast Extensions.........15
6.2 Security Issues Not Solved by IPsec Multicast Extensions.....15
6.2.1 Outsider Attacks.........................................16
6.2.2 Insider Attacks..........................................16
6.3 Implementation or Deployment Issues that Impact Security.....17
6.3.1 Homogeneous Group Cryptographic Algorithm Capabilities...17
6.3.2 Groups that Span Two or More Security Policy Domains.....17
6.3.3 Network Address Translation..............................17
7. IANA Considerations.............................................20
8. Acknowledgements................................................20
9. References......................................................20
9.1 Normative References.........................................20
9.2 Informative References.......................................21
Appendix A - Multicast Application Service Models..................24
A.1 Unidirectional Multicast Applications........................24
A.2 Bi-directional Reliable Multicast Applications...............24
A.3 Any-To-Any Multicast Applications............................25
Author's Address...................................................26
Full Copyright Statement...........................................27
Intellectual Property..............................................27
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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 candidate 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
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multicast routing protocols (e.g., PIM-SM [RFC4601]) or multicast
admission protocols (e.g., IGMP [RFC3376].
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this 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 need to 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.
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Group Member
An IPsec device that belongs to a group. A Group Member is
authorized to be a Group Sender and/or a Group Receiver.
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 4.4.1.1. 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 Sender.
Group Sender
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].
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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 "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 of each interface for which
there are known receivers.
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 or 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 address.
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
needs to 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. These semantics are due to the following unique
requirements of IP multicast routing protocols (e.g., PIM-SM
[RFC4601]). This document describes these new header construction
semantics as "tunnel mode with address preservation", and is
described as follows.
- 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 needs to
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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 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 gateway.
- 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 dropped.
To accommodate routing protocol RPF checks, the GKM Subsystem on
a security gateway implementation implementing the IPsec
multicast extensions needs to preserve the original packet IP
source address as follows. Firstly, the GSPD-S entry for the
traffic selectors sets the Source Address PFP flag. 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 followed.
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
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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
The following sections describe the GKM Subsystem and IPsec
extension interactions with the 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 Sender GSPD policy might trigger on a match
with a specified multicast application packet. The ensuing Group
Sender registration exchange would setup the Group Sender's
outbound SAD entry that encrypts the multicast application's data
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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 Sender. 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).
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 may allow a GCKS to minimize exposure of the
group's secret key by re-keying a group on a group membership
change event. It also minimizes cost on a GCKS for those groups
that maintain member state.
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
Sender and the destination address is the multicast group address.
An inbound multicast SA MUST be configured with the source
addresses of each Group Sender 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) is extended in order to
accommodate peers that may take on specific roles in the group.
Such roles can be GCKS, Group Sender or a Group Receiver. A peer
can have multiple roles. The PAD may also contain root certificates
for PKI used by the group.
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4.1.3.1 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.
. 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 Sender role authorization rules. In
some groups the action of sending protected packets is
restricted to a subset of group members. A GCKS uses these rules
to declare which systems are authorized to be a Group Sender for
a given group.
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.
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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 need
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:
- There are multiple Group Senders 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 Senders.
- The life spans of a Group Sender'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 Sender 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 Sender 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 new associated keying
material. For a short period after the GCKS multicasts the RKE, a
Group Sender does not yet transmit data using the leading edge
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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
Sender 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 period that a Group Sender waits before starting its first
leading edge SA transmission is a GKM/IPsec policy parameter.
This transmit delay period value for a new "leading edge" IPsec
SA SHOULD be configurable at the Group Owner management interface
on a per group basis. The period duration will typically be
measured in seconds, and should take account for the time that
the RKE message takes to reach all Group Members, as well as the
time Group Members need to process the RKE message. If group
policy includes retransmitting RKE messages for availability, the
period should include the retransmission times as well. Periods
of longer duration (e.g, to pre-distribute future SAs) are also
possible, but since the SA lifetimes will begin decrementing
immediately the effective useful lifetime of the SA may be
greatly reduced.
- The Group Sender's "trailing edge" SA is the oldest security
association in use by the group for that sender. All authorized
Group Members can receive and decrypt data for this SA, but the
Group Sender 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)"
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 Sender 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 SAs per Group Sender and this re-key rollover continuity
algorithm.
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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 typical MAC authentication methods using a single shared
secret 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
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.
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The Peer Authorization Database does require explicit coordination
between the GKM protocol and IKEv2. Section 4.1.3 describes these
interactions.
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 addresses.
- 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 4.2.1). 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 Sender will
delay before it starts transmitting on the leading edges IPsec SA.
5. IP Traffic Processing
Processing of traffic follows Section 5 of [RFC4301], with the
additions described below when these IP multicast extensions are
supported.
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".
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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 Sender data origin authentication using a digital
signature, TESLA, or other mechanism.
- Anti-replay protection for a limited number of Group Senders
using the ESP (or AH) sequence number facility.
- Filtering of multicast transmissions by those group members who
are not authorized by group policy to be Group Senders. 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 against many aspects of the operating
environment in which the IPsec implementation executes. However, it
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should be noted that the risk of attacks originating by an
adversary in the network is magnified to the extent that the group
keys are shared across a large number of systems.
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 Sender. 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 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:
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- 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 Sender transmissions that merit the overhead, the
group policy can require the Group Sender 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
packets.
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 to solving many of these problems
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
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 need to overcome several architectural barriers to
their successful deployment. This section surveys those problems
and identifies the GSPD/SAD state information that the GKM
protocol supporting NAT and mobile nodes need to synchronize
across the group membership.
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6.3.3.1 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 Sender'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.
6.3.3.2 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 Sender 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 needs to be secure against
hostile re-direct and flooding attacks.
6.3.3.3 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 Sender 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 needs to
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 cases where this address translation can spontaneously
change without warning:
- NAT gateways may re-boot and lose their address translation
state information.
- 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.
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- 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.
6.3.3.4 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 sender
originated the multicast packet, which in turn selects the correct
multicast forwarding policy.
6.3.3.5 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 encrypted by end-to-end ESP, such payloads are
opaque to application layer gateways.
When multiple Group Senders 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 needs to be configured at the GCKS as a group policy,
and it needs to be supported in unison by all of the group
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endpoints within the group, even those that reside in the public
Internet.
6.3.3.6 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 needs to exist in a GKM protocol payload that defines
the multicast application GSA attributes for each Group Sender.
6.3.3.7 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.
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.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Level", BCP 14, RFC 2119, March 1997.
[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
2005.
[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-ietf-msec-ipsec-
composite-group-01.txt, work in progress, February 2007.
[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 2001.
[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.
[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.
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Internet-Draft Multicast Extensions to RFC 4301 December 2007
[RFC3552] Rescorla, E., et. al., "Guidelines for Writing RFC Text
on Security Considerations", RFC 3552, July 2003.
[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.
[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
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Networking (TON), Volume 11, Issue 6, December 2003. See
http://www.cs.utexas.edu/users/lam/Vita/Cpapers/ZLLY01.p
df.
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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
policy.
A.2 Bi-directional Reliable Multicast Applications
Some secure multicast applications are characterized as one Group
Sender 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 sender is multicast, and the
inverse flow from the group's receivers is unicast to the sender.
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
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GKM SHOULD use the 2-tuple {destination IP address, SPI} to
identify each IPsec SA. In 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 Sender. This address would be
configured in parallel to the Group Sender's existing IP
addresses. The GKM subsystems at both the NORM Group Sender 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
sender'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 sender 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 sender. The GKM protocol MUST define a
key management mechanism for the Group Sender to validate the
asserted signature public key of any receiver Node without
requiring that the sender 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 Sender. The
management interface SHOULD be able to configure a group to have at
least 16 concurrent authorized senders, 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
whiteboard.
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For such applications, all (or a large subset) of the Group Members
are authorized multicast senders. In such service models, creating
a distinct IPsec SA with anti-replay state for every potential
sender does not scale to large groups. The group SHOULD share one
IPsec SA for all of its senders. The IPsec SA SHOULD NOT use the
IPsec anti-replay protection service for the sender'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
USA
Phone: +1-408-526-4796
Email: bew@cisco.com
George Gross
IdentAware Security
977 Bates Road
Shoreham, VT 05770
USA
Phone: +1-908-268-1629
Email: gmgross@identaware.com
Dragan Ignjatic
Polycom
1000 W. 14th Street
North Vancouver, BC V7P 3P3
Canada
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
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Acknowledgement
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
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