MSEC Working Group B. Weis
Internet-Draft Cisco Systems
Intended status: Standards Track G. Gross
Expires: August 22, 2008 IdentAware Security
D. Ignjatic
Polycom
February 22, 2008
Multicast Extensions to the Security Architecture for the Internet
Protocol
draft-ietf-msec-ipsec-extensions-08.txt
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Copyright (C) The IETF Trust (2008).
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. This document describes how the IPsec security services
are applied to IP multicast packets. These extensions are relevant
only for an IPsec implementation that supports multicast.
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Table of Contents
1. Introduction.....................................................3
1.1 Scope.........................................................3
1.2 Terminology...................................................4
2. Overview of IP Multicast Operation...............................6
3. Security Association Modes.......................................6
3.1 Tunnel Mode with Address Preservation.........................7
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)......................11
4.1.3 Group Peer Authorization Database (GPAD).................11
4.2 Group Security Association (GSA).............................13
4.3 Data Origin Authentication...................................16
4.4 Group SA and Key Management..................................16
4.4.1 Co-Existence of Multiple Key Management Protocols........16
4.4.2 New Security Association Attributes......................17
5. IP Traffic Processing...........................................17
5.1 Outbound IP Traffic Processing...............................17
5.2 Inbound IP Traffic Processing................................18
6. Security Considerations.........................................21
6.1 Security Issues Solved by IPsec Multicast Extensions.........21
6.2 Security Issues Not Solved by IPsec Multicast Extensions.....21
6.2.1 Outsider Attacks.........................................22
6.2.2 Insider Attacks..........................................22
6.3 Implementation or Deployment Issues that Impact Security.....23
6.3.1 Homogeneous Group Cryptographic Algorithm Capabilities...23
6.3.2 Groups that Span Two or More Security Policy Domains.....23
6.3.3 Source-Specific Multicast Group Sender Transient Locators23
7. IANA Considerations.............................................24
8. Acknowledgements................................................24
9. References......................................................24
9.1 Normative References.........................................24
9.2 Informative References.......................................24
Appendix A - Multicast Application Service Models..................27
A.1 Unidirectional Multicast Applications........................27
A.2 Bi-directional Reliable Multicast Applications...............27
A.3 Any-To-Any Multicast Applications............................28
Author's Address...................................................29
Full Copyright Statement...........................................29
Intellectual Property..............................................30
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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 OPTIONAL 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 that result 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.
An IPsec implementation that does not supports multicast is not
required to support these extensions.
Throughout this document, RFC 4301 semantics remain unchanged by
the presence these multicast extensions unless specifically noted
to the contrary.
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] group addresses are supported. These extensions are used
when management policy requires IP multicast packets protected by
IPsec to remain IP multicast packets. When management policy
requires that the IP multicast packets are encapsulated as IP
unicast packets (e.g., because the network connected to the
unprotected interface does not support IP multicast), the
extensions in this document are not used.
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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
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.
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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.
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
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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 delivered to all
members of the group with either "best-efforts" reliability
[RFC1112], or as part of a reliable stream (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 "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 of [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 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.
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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", which are
described as follows.
- When an IP multicast packet is received by a host or router the
destination address of the packet is compared to the local IP
multicast state. If the destination of an IP multicast packet is
changed the host or router receiving the IP multicast packet
will not process it properly. Therefore, an IPsec host or
security gateway needs to preserve the multicast IP destination
address after IPsec tunnel encapsulation.
- IP multicast routing protocols typically create multicast
distribution trees based on the source address as well as the
group address. If an IPsec security gateway changes the source
address of an IP multicast packet (to its own IP address), the
resulting IPsec protected packet may fail Reverse Path
Forwarding (RPF) checks performed by other routers. A failed RPF
check may result in the packet being dropped. To accommodate
routing protocol RPF checks, the security gateway implementing
the IPsec multicast extensions SHOULD preserve the original
packet IP source address. However, it should be noted that a
security gateway performing source address preservation will not
receive ICMP PMTU or other messages intended for the security
gateway. Security gateway applications not requiring source
address preservation will be able to receive ICMP PMTU messages
and process them as described in section 6.1 of RFC 4301.
Because some applications of address preservation may require only
the destination address to be preserved, specification of
destination address preservation and source address preservation
are separated in the above description. Destination address
preservation and source address preservation attributes are
described in the Group Security Policy Database (GSPD) (defined
later in this document), and are copied into corresponding SAD
entries.
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, IP multicast packets can be encapsulated using a tunnel
interface, for example as described in [RFC4891], where the tunnel
is also treated as an interface by IP multicast routing protocols.
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In summary, retaining both the IP source and destination addresses
of the inner IP header allows IP multicast routing protocols to
route a packet properly when the packet is protected by IPsec.
This result is necessary in order for the multicast extensions to
allow a host or 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
The following sections describe the GKM Subsystem and IPsec
extension interactions with the IPsec databases. The major IPsec
databases need 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 supporting both unicast security associations as
defined by RFC 4301 and the multicast extensions defined by this
specification. The GSPD is considered to be the SPD, with the
addition of the semantics relating to the multicast extensions
described in this section.
This document describes a new "Address Preservation" (AP) flag
indicating that tunnel mode with address preservation is to be
applied to a GSPD entry. The AP flag has two attributes: AP-L used
in the processing of the local tunnel address, and AP-R used in the
processing of the remote tunnel process. This flag is added to the
GSPD "Processing info" field of the GSDP. The following text
reproduced from Section 4.4.1.2 of RFC 4301 includes this
additional processing. (Note: for brevity, only the Processing info
related to tunnel processing has been reproduced.)
o Processing info -- which action is required -- PROTECT,
BYPASS, or DISCARD. There is just one action that goes
with all the selector sets, not a separate action for each
set. If the required processing is PROTECT, the entry
contains the following information.
- IPsec mode -- tunnel or transport
- (if tunnel mode) local tunnel address -- For a non
mobile host, if there is just one interface, this is
straightforward; if there are multiple interfaces, this
must be statically configured. For a mobile host, the
specification of the local address is handled
externally to IPsec. If tunnel mode with address
preservation is specified for the local tunnel address,
the AP-L attribute is set to TRUE for the local tunnel
address and the local tunnel address is unspecified.
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The presence of the AP-L attribute indicates that the
inner IP header source address will be copied to the
outer IP header source address during IP header
construction for tunnel mode.
- (if tunnel mode) remote tunnel address -- There is no
standard way to determine this. See 4.5.3, "Locating a
Security Gateway". If tunnel mode with address
preservation is specified for the remote tunnel
address, the AP-R attribute is set to TRUE for the
remote tunnel address and the remote tunnel address is
unspecified. The presence of the AP-R attribute
indicates that the inner IP header destination address
will be copied to the outer IP header destination
address during IP header construction for tunnel mode.
This document describes unique directionality processing for GSPD
entries with a remote IP multicast address. Since an IP multicast
address must not be sent as the source address of an IP packet
[RFC1112], directionality of Local and Remote address and ports is
maintained during incoming SPD-S and SPD-I checks rather than
being swapped. Section 4.4.1 of RFC 4301 is amended as follows:
Representing Directionality in an SPD Entry
For traffic protected by IPsec, the Local and Remote
address and ports in an SPD entry are swapped to
represent directionality, consistent with IKE
conventions. In general, the protocols that IPsec
deals with have the property of requiring symmetric
SAs with flipped Local/Remote IP addresses. However,
SPD entries with a remote IP multicast address do not
have their Local and Remote address and ports in an
SPD entry swapped during incoming SPD-S and SPD-I
checks.
A new Group Security Policy Database (GSPD) attribute is
introduced: GSPD entry directionality. The following text is added
to the bullet list of SPD fields described in Section 4.4.1.2 of
RFC 4301.
o Directionality -- can one of three types: "symmetric",
"sender only" or "receiver only". "Symmetric" indicates
that a pair of SAs are to be created (one in each
direction as specified by RFC 4301). GSPD entries marked
as "sender only" indicate that one SA is to be created in
the outbound direction. GSPD entries marked as "receiver
only" indicate that one SA is to be created in the inbound
direction. 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.
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Reciprocally, if the type is "receiver only", the entry
SHOULD go to GSPD-I only.
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. However, the
outbound traffic needs to be matched against the GSPD selectors so
that the appropriate SA can be created.
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 entering the
implementation via the protected interface, or emitted by the
implementation on the protected side of the boundary and directed
toward the unprotected interface.. The ensuing Group Sender
registration exchange would set up the Group Sender's outbound SAD
entry that encrypts the multicast application's data stream. In
the inverse direction, group policy may also set up an inbound
IPsec SA.
At the Group Receiver endpoint(s), the IPsec subsystem MAY use
GSPD policy mechanisms (e.g., trigger on detection of IGMP/MLD
join group exchange) that initiate a GKM protocol registration
exchange. The ensuing Group Receiver registration exchange would
set up the Group Receiver's inbound SAD entry that decrypts the
multicast application's data stream. In the inverse direction, the
group policy may also set up an outbound IPsec SA (e.g., when
supporting an ASM service model).
The IPsec subsystem MAY provide GSPD policy mechanisms that
automatically initiate a GKM protocol de-registration exchange.
De-registration allows 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. One such policy mechanism could be the
detection of IGMP/MLD leave group exchange. However, a security
gateway Group Member would not initiate a GKM protocol de-
registration exchange until it detects that there are no more
receivers behind a protected interface.
Additionally, the GKM subsystem MAY set up the GSPD/SAD state
information independent of the multicast application's state. In
this scenario, the group's Group Owner issues management
directives that tell 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.
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4.1.2 Security Association Database (SAD)
The SAD contains an item describing whether tunnel or transport
mode is applied to traffic on this SA. The text in RFC 4301 Section
4.4.2.1 is amended to describe Address Preservation.
o IPsec protocol mode: tunnel or transport. Indicates which
mode of AH or ESP is applied to traffic on this SA. When
tunnel mode is specified, the data item also indicates
whether or not address preservation is applied to the
outer IP header. Address preservation MUST NOT be
specified when the IP version of the encapsulating header
and IP version of the inner header do not match. The local
address, remote address, or both addresses MAY be marked
as being preserved during tunnel encapsulation.
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.
4.1.3 Group Peer Authorization Database (GPAD)
The multicast IPsec extensions introduce a new data structure
called the Group Peer Authorization Database (GPAD). Analogous to
the PAD (refer to RFC 4301 section 4.4.3), the GPAD provides a link
between a Group Key Management protocol and the SPD. The GPAD
contains three group role authorization rule sets.
A group role authorization rule set specifies which peers are
authorized to participate in a group in a given Group Role. The
GCKS GKM subsystem examines these rules to determine whether a
candidate Group Member has the authority to join the group. A Group
Member GKM subsystem uses these rules to decide which systems are
authorized to act as a GCKS for a given group. The third group role
authorization rule set determines which Group Members are
authorized to send data to the group.
When making a group role authorization decision, the GKM subsystem
queries the GPAD with the 3-tuple {Group Identifier, Group Role,
GKM ID}. These three GPAD search indices are defined as follows:
. Group Identifier - The Group Identifier is an opaque byte string
of IKE ID type Key ID that identifies a secure multicast group.
The Group Identifier byte string MUST be at least four bytes
long and less than 256 bytes long. The GPAD administrator MUST
assign each Group Identifier a unique value within the scope of
all secure multicast groups administered by the GPAD's security
domain.
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. Group Role - There are three types of group roles: Group Member,
Group Sender (i.e., a Group Member authorized to be a multicast
sender), and GCKS. A Group Member may be authorized to act in
more than one role within a given group.
. GKM ID - The GKM ID is the identifier used in the GKM protocol's
identification payload. The GKM protocol MUST support the six
IKE ID payload types specified in RFC 4301 section 4.4.3.1.
A Group Role authorization rule set is an ordered list of GPAD
Entry Identifiers. GPAD Entry Identifiers have the same syntax and
pattern matching semantics as the PAD Entry Identifiers defined in
RFC 4301 section 4.4.3.1. Every group has three group role
authorization rule sets, one group role authorization rule set per
Group Role. Within the GPAD, the group authorization rule sets are
indexed by the 2-tuple {Group Identifier, Group Role}.
When the GKM subsystem needs to make an authorization decision, it
linearly searches the selected group role authorization rule set
until either it acquires a GPAD Entry Identifier match to the GKM
ID, or it reaches the end of the rule set. A match authorizes the
GKM ID to act in the specified Group Role. The no find condition
indicates the GKM ID is not authorized for the requested Group
Role.
For long lived large-scale groups, there are operational and
security benefits to having an automated and distributed group
security policy management facility. The security policy changes
can be rapidly propagated across the whole group membership rather
than at the pace of manual administration at individual systems. In
the context of the GPAD, the management facility promotes the
timely and accurate introduction of newly authorized Group Members,
updates to existing role authorizations, and the revocation of a
compromised Group Member's role authorizations. The latter
capability intercepts a compromised Group Member from re-joining
the group.
For example, some GKM protocols (e.g., GSAKMP [RFC4535]) distribute
their group's GPAD, GSPD, and SAD configuration in a security
policy token [RFC4534] signed by the group's policy authority, also
known as the Group Owner (GO). Each Group Member GSAKMP subsystem
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's GSAKMP
subsystem dynamically updates the GPAD, GSPD, and SAD with the
policy token's contents. Alternatively, the group's security policy
management facility could be based on a trusted configuration
management protocol rather than a GKM protocol.
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A group security policy management facility is not mandated by this
specification. The GPAD MUST have a trusted management interface
that allows the manual creation, editing, and deletion of the group
role authorization rule sets. The GPAD, GSPD, and SAD databases MAY
be configured by a group security policy management facility
through that same management interface. How an IPsec subsystem
implementation coordinates the manual and automated database
modifications to avoid conflicts is a local matter outside the
scope of this specification.
The GPAD 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 is 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 GPAD management interface MUST provide a
mechanism(s) to ensure that IKEv2 security associations do not
negotiate traffic selectors that conflict or override GKM group
policies.
4.2 Group Security Association (GSA)
An IPsec implementation supporting these extensions will support a
number of security associations: one or more IPsec SAs, and one or
more GKM SAs used to download IPsec SAs [RFC3740]. 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 secure multicast 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 which 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".
The rekey continuity rollover algorithm depends on an IPsec SA
management interface between the GKM subsystem and the IPsec
subsystem. The IPsec subsystem MUST provide management interface
mechanisms for the GKM subsystem to add IPsec SAs and to delete
IPsec SAs. For illustrative purposes, this text defines the rekey
rollover continuity algorithm in terms of two timer parameters
that govern IPsec SA lifespans relative to the start of a group
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rekey event. However, it should be emphasized that the GKM
subsystem interprets the group's security policy to direct the
correct timing of IPsec SA activation and deactivation. A given
group policy may choose timer values that differ from those
recommended by this text. The two rekey rollover continuity timer
parameters are:
1. Activation Time Delay (ATD) - The ATD defines how long after the
start of a rekey event to activate new IPsec SAs. The ATD
parameter is expressed in units of seconds. Typically, the ATD
parameter is set to the maximum time it takes to deliver a
multicast message from the GCKS to all of the group's members.
For a GCKS that relies on a Reliable Multicast Transport
Protocol (RMTP), the ATD parameter could be set equal to the
RTMP protocol's maximum error recovery time. When a RMTP is not
present, the ATD parameter might be set equal to the network's
maximum multicast message delivery latency across all of the
group's endpoints. The ATD is a GKM group policy parameter. This
value SHOULD be configurable at the Group Owner management
interface on a per group basis.
2. Deactivation Time Delay (DTD) - The DTD defines how long after
the start of a rekey event to deactivate those IPsec SAs that
are destroyed by the rekey event. The purpose of the DTD
parameter is to minimize the residual exposure of a group's
keying material after a rekey event has retired that keying
material. The DTD is independent of and should not to be
confused with the IPsec SA soft lifetime attribute. The DTD
parameter is expressed in units of seconds. Typically, the DTD
parameter would be set to the ADT plus the maximum time it takes
to deliver a multicast message from the Group Sender to all of
the group's members. For a Group Sender that relies on a RMTP,
the DTD parameter could be set equal to ADT plus the RTMP
protocol's maximum error recovery time. When a RMTP is not
present, the DTD parameter might be set equal to ADT plus the
network's maximum multicast message delivery latency across all
of the group's endpoints. A GKM subsystem MAY implement the DTD
as a group security policy parameter. If a GKM subsystem does
not implement the DTD parameter then other group security policy
mechanisms MUST determine when to deactivate an IPsec SA.
Each group re-key multicast message sent by a GCKS signals the
start of a new Group Sender IPsec SA time epoch, with each such
epoch having an associated set of two IPsec SAs. Note that 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-keying mechanisms must be
multicast. For example, [ZLLY03] describes a method of re-key
employing both unicast and multicast messages.
The group membership interacts with these IPsec SAs as follows:
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- 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 MAY contain
group policy directives, new IPsec SA policy, and group keying
material. In the absence of a RMTP, the GCKS may re-transmit the
RKE a policy-defined number of times to improve the availability
of re-key information. The GKM subsystem starts the ATD and DTD
timers after it receives the last RKE retransmission.
- The GKM subsystem interprets the RKE multicast to configure 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 time period of
ATD seconds in duration after the GCKS multicasts the RKE, a
Group Sender 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 for the ATD period, such that all of the Group
Members have received and processed the RKE message, the GKM
subsystem directs the Group Sender to begin 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 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
IPsec SA after it has transitioned to the leading edge IPsec SA.
The trailing edge IPsec SA is deleted by the group's GKM
subsystems after the DTD time period has elapsed since the RKE
transmission.
This re-key rollover strategy allows the group to drain its in
transit datagrams from the network while transitioning to the
leading edge IPsec 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 IPsec SA
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, refer to [RFC2627] and Appendix A of [RFC4535]) enforces
this policy. Implementations MAY offer a Group Owner management
interface option to enable/disable re-key rollover continuity for a
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particular group. This specification requires that a GKM/IPsec
implementation MUST support at least two concurrent IPsec SA per
Group Sender 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.
However, 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 a 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 a
second time for additional inbound processing (see Section 5.2 of
[RFC4301]). This use of AH or ESP encapsulated within AH or ESP
accommodates the constraint that AH and ESP define an Integrity
Check Value (ICV) for only a single authenticator transform.
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
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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
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) is 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 edge's 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 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 are marked as being
preserved.
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Header construction for tunnel mode is described in Section 5.1.2
of RFC 4301. The first bullet of that section is amended as
follows:
o If address preservation is not marked in the SAD entry for
either the outer IP header Source Address or Destination
Address, the outer IP header Source Address and
Destination Address identify the "endpoints" of the tunnel
(the encapsulator and decapsulator). If address
preservation is marked for the IP header Source Address,
it is copied from the inner IP header Source Address. If
address preservation is marked for the IP header
Destination Address, it is copied from the inner IP header
Destination Address. The inner IP header Source Address
and Destination Addresses identify the original sender and
recipient of the datagram (from the perspective of this
tunnel), respectively. Address preservation MUST NOT be
marked when the IP version of the encapsulating header and
IP version of the inner header do not match.
Note (3) regarding construction of tunnel addresses in Section
5.1.2.1 of RFC 4301 is amended as follows:
(3) Unless marked for address preservation Local and Remote
addresses depend on the SA, which is used to determine
the Remote address, which in turn determines which Local
address (net interface) is used to forward the packet.
If address preservation is marked for the Local address,
it is copied from the inner IP header. If address
preservation is marked for the Remote address, that
address is copied from the inner IP header.
5.2 Inbound IP Traffic Processing
IPsec-protected packets generated by an IPsec device supporting
these multicast extensions may (depending on its GSPD policy)
preserve a destination address such that the IP destination address
is not an IPsec device. This requires an IPsec device supporting
these multicast extensions to process IP traffic that is not
addressed to the IPsec device itself. The following additions to
IPsec inbound IP traffic processing are necessary.
For compatibility with RFC 4301, the phrase "addressed to this
device" is taken to mean packets with a unicast destination address
belonging to the system itself, and multicast packets that are
received by the system itself. However, multicast packets not
received by the IPsec device are not considered addressed to this
device.
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The discussion of processing Inbound IP Traffic described in
Section 5.2 of RFC 4301 is amended as follows. The first dash in
item 2 is amended as follows:
- If the packet appears to be IPsec protected and it is
addressed to this device, or appears to be IPsec protected
and is addressed to a multicast group, an attempt is made
to map it to an active SA via the SAD.
A new item is added to the list between items 3a and 3b to describe
processing of IPsec packets with destination address preservation
applied:
3aa. If the packet is addressed to a multicast group and AH
or ESP is specified as the protocol, the packet is looked
up in the SAD. Use the SPI plus the destination or SPI
plus destination and source addresses, as specified in
Section 4.1. If there is no match, the packet is directed
to SPD-I lookup. Note that if the IPsec device is a
security gateway, and the SPD-I policy is to PYPASS the
packet, a subsequent security gateway along the routed
path of the multicast packet may decrypt the packet.
Figure 3 in RFC 4301 is updated to show the new processing path
defined in item 3aa.
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Unprotected Interface
|
V
+-----+ IPsec protected
------------------->|Demux|-------------------+
| +-----+ |
| | |
| Not IPsec | |
| | IPsec protected not |
| V addressed to device |
| +-------+ +---------+ and not in SAD |
| |DISCARD|<---|SPD-I (*)|<------------+ |
| +-------+ +---------+ | |
| | | |
| |-----+ | |
| | | | |
| | V | |
| | +------+ | |
| | | ICMP | | |
| | +------+ | |
| | | V
+---------+ | +-----------+
....|SPD-O (*)|............|...................|PROCESS(**)|...IPsec
+---------+ | | (AH/ESP) | Boundary
^ | +-----------+
| | +---+ |
| BYPASS | +-->|IKE| |
| | | +---+ |
| V | V
| +----------+ +---------+ +----+
|--------<------|Forwarding|<---------|SAD Check|-->|ICMP|
nested SAs +----------+ | (***) | +----+
| +---------+
V
Protected Interface
Figure 1. Processing Model for Inbound Traffic
(amending Figure 3 of RFC 4301)
The discussion of processing Inbound IP Traffic described in
Section 5.2 of RFC 4301 is amended to insert a new item 6 as
follows.
6. If an IPsec SA is marked as supporting tunnel mode with
address preservation (as described in Section 3.1), the
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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 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 identified with a source
address of systems that are not authorized by group policy to be
Group Senders. This feature leverages the IPsec state-less
firewall service (i.e., SPD-I and/or SDP-O entries with a packet
disposition specified as DISCARD).
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 attacks targeting vulnerabilities of the operating
environment in which the IPsec implementation executes. However, it
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.
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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 replayed 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 re-key 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
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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 Source-Specific Multicast Group Sender Transient Locators
A Source Specific Multicast (SSM) Group Sender's source IP address
can dynamically change during a secure multicast group's lifetime.
Examples of the events that can cause the Group Sender's source
address to change include but are not limited to NAT, a mobility
induced change in the care-of-address, and a multi-homed host
using a new IP interface. The change in the Group Sender's source
IP address will cause those GSPD entries related to that multicast
group to become out of date with respect to the group's multicast
routing state. In the worst case, there is a risk that the Group
Sender's data originating from a new source address will be BYPASS
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processed by a security gateway. If this scenario was not
anticipated, then it could leak the group's data. Consequently, it
is recommended that SSM secure multicast groups have a default
DISCARD policy for all unauthorized Group Sender source IP
addresses for the SSM group's destination IP address.
7. IANA Considerations
This document has no actions for IANA.
8. Acknowledgements
The authors wish to thank Steven Kent, Russ Housley, 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.
[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 2005.
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.
[RFC2627] Wallner, D., Harder, E. and R. Agee, "Key Management for
Multicast: Issues and Architectures", RFC 2627,
September 1998.
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[RFC2914] Floyd, S., "Congestion Control Principles", RFC 2914,
September 2000.
[RFC3171] Albanni, Z., et al., "IANA Guidelines for IPv4
Multicast Address Assignments", RFC 3171, August 2001.
[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.
[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.
[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, T., 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.
[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.
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[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.
[RFC4891] Graveman R., et al., "Using IPsec to Secure IPv6-in-IPv4
Tunnels", RFC 4891, May 2007.
[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
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
Association (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 set up to discard
unauthorized attempts to transmit unicast or multicast packets to
the group.
The GKM Subsystem's management interface MUST have the ability to
set up 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 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 set up 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 an "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
set up 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
Full Copyright Statement
Copyright (C) The IETF Trust (2008).
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.
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This document and the information contained herein are provided on an
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