One document matched: draft-ietf-msec-ipsec-extensions-00.txt
Internet Engineering Task Force Brian Weis (Cisco)
INTERNET-DRAFT George Gross (IdentAware)
draft-ietf-msec-ipsec-extensions-00.txt Dragan Ignjatic (Polycom)
Expires: December, 2005 June, 2005
Multicast Extensions to the Security Architecture for the Internet
Protocol
Status of this Memo
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Abstract
The Security Architecture for the Internet Protocol [RFC2401BIS]
describes security services for traffic at the IP layer. That
architecture primarily defines services for Internet Protocol (IP)
unicast packets, as well as manually configured IP multicast packets.
This document further defines the security services for IP multicast
packets within that Security Architecture.
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Table of Contents
1.0 Introduction.......................................................2
1.1 Scope...........................................................3
1.2 Terminology......................................................3
2.0 Overview of IP Multicast Operation.................................4
3.0 Security Association Modes.........................................4
4.0 Security Association...............................................5
4.1 Major IPsec Databases............................................5
4.1.1 SPD..........................................................5
4.1.2 SAD..........................................................6
4.1.3 PAD..........................................................6
4.1.4 GSA..........................................................8
4.2 Data Origin Authentication.......................................9
4.3 Group SA and Key Management.....................................10
4.3.1 Co-Existence of Multiple Key Management Protocols...........10
5.0 IP Traffic Processing.............................................10
5.1 Outbound IP Multicast Traffic Processing........................10
5.2 Inbound IP Multicast Traffic Processing.........................11
5.0 Networking Issues.................................................11
5.1 Network Address Translation.....................................11
5.1.1 SPD Losses Synchronization with Internet Layer's State......11
5.1.2 Secondary Problems Created by NAT Traversal.................12
5.1.3 Avoidance of NAT Using an IP-v6 Over IP-v4 Network..........14
5.1.4 GKMP/IPsec Multi-Realm IP-v4 NAT Architecture...............14
6.0 Security Considerations...........................................20
7.0 Acknowledgements..................................................20
8.0 Appendix A - Multicast Application Service Models.................20
8.1 Unidirectional Multicast Applications...........................21
8.2 Bi-directional Reliable Multicast Applications..................21
8.3 Any-To-Any Multicast Applications...............................22
9.0 References........................................................22
9.1 Normative References............................................22
9.2 Informative References..........................................22
Author's Address......................................................24
Full Copyright Statement..............................................24
Intellectual Property.................................................24
1.0 Introduction
The Security Architecture for the Internet Protocol [RFC2401BIS]
provides security services for traffic at the IP layer. It describes
a base architecture for IPsec compliant systems, and a set of
security services for the IP layer. These security services primarily
describe services and semantics for IP packets that carry a unicast
address in the IP destination field. Those security services can also
be used to tunnel IP multicast packets, where the tunnel is a
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pairwise tunnel between two IPsec devices. Some support for IP
packets with a multicast address in the IP destination field is
supported, but only with manual keying.
This document describes extensions to [RFC2401BIS] that further
define the IPsec security architecture for packets with a multicast
address in the IP destination field to remain IP multicast packets.
[NOTE TO THE READER: The scope of the extensions proposed has not
been finalized. For example, there are varying opinions as to the
extent that this document must accommodate interoperability between
different group key management and policy systems, which may occur in
very large groups. Comments regarding matters of scope are
solicited.]
1.1 Scope
The IPsec extensions described in this document support for IPsec
Security Associations used with both Any-Source Multicast (ASM) and
Source-Specific Multicast (SSM) [RFC3569, RFC3376] groups.
They extensions also support Security Associations with IPv4
Broadcast addresses, and Anycast addresses [RFC2526], since there are
be multiple receivers defined for a packet sent to those addresses.
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
multicast routing protocols (e.g., PIM-SM [RFC2362]) or multicast
admission protocols (e.g., IGMP [RFC3376] to participate in IPsec.
All implementation models of IPsec (e.g., "bump-in-the-stack", "bump-
in-the-wire") are supported.
1.2 Terminology
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.
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.
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2.0 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 with either a "best-effort" reliability to all members of
the group [RFC1112], or reliably (e.g., NORM) [RFC3940].
A sender to an IP multicast group sets the destination of the packet
to an IP address allocated to be used for IP multicast. Allocated IP
multicast addresses are defined in RFC 3171 [RFC3171]. Potential
receivers of the packet "join" the IP multicast group by registering
with a network routing device, 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) [RFC2362].
Multicast routing protocols maintain state regarding which devices
have registered to receive packets for a particular IP multicast
group. When a router receives an IP multicast packet, it forwards a
copy of the packet out each interface for which there are known
receivers.
3.0 Security Association Modes
IPsec supports two modes of use: transport mode and tunnel mode. In
transport mode, AH and ESP 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
support both modes to encapsulate an IP multicast packet. These
processing rules are identical to the rules described in [RFC2401BIS,
Section 4.1]. 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
[RFC2401BIS, Section 4.1]. In particular, the security gateway must
use tunnel mode to encapsulate incoming fragments.
New header construction semantics are required when tunnel mode is
used to encapsulate IP multicast packets that are to remain IP
multicast packets. This is due to unique requirements of IP multicast
routing protocols (such as PIM-SM [RFC2362]).
IP multicast routing protocols use the destination address on a
packet to decide to where the packet should be routed. If the
destination of an IP multicast packet is changed it will no longer be
properly routed. To accommodate this routing requirement, the GKMP
Subsystem may specify two actions. Firstly, the SPD-S entry for the
traffic selectors must have the Remote Address PFP flag set. This
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flag causes the remote address to be propagated to the IPsec SA.
Secondly, a new IPsec SA attribute must be specified by the GKMP
Subsystem that causes the tunnel mode header construction process to
copy the remote address in the SA into the tunnel header remote
address.
IP multicast routing protocols also typically create multicast
distribution trees based on the source address. An IPsec security
gateway that changes the source address of an IP multicast packet may
cause RPF checks on other routers to return a different result than
the original plaintext IP multicast packet. As a result, multicast
routing may drop the packet. To accommodate this routing requirement,
the GKMP Subsystem may specify two actions. Firstly, the SPD-S entry
for the traffic selectors must have the Source Address PFP flag set.
This flag causes the remote address to be propagated to the IPsec SA.
Secondly, a new IPsec SA attribute must be specified by the GKMP
Subsystem that causes the tunnel mode header construction process to
copy the source address in the SA into the tunnel header remote
address.
Some applications of address preservation may only require the remote
address to be preserved. For this reason, the specification of remote
address preservation and source address preservation are separated in
the above description.
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 IPsec protected. This result
is necessary in order for the multicast extensions to allow a
security gateway to provide IPsec services for IP multicast packets.
This method of tunnel mode is known as tunnel mode with address
preservation.
4.0 Security Association
4.1 Major IPsec Databases
The following sections describe the GKMP Subsystem and IPsec
extension interactions with the major IPsec databases. Major IPsec
databases need to be expanded in order to fully support multicast.
4.1.1 SPD
A new SPD attribute is introduced - SPD entry directionality.
Directionality can take three types. Each SPD entry can be marked
symmetric, sender or receiver only. Symmetric SPD entries are the
common entries as specified by RFC2401bis. Symmetric SHOULD be the
default directionality unless specified otherwise. SPD entries marked
as sender 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
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SHOULD be put in SPD-O only. Reciprocally, if the type is receiver
only, the entry SHOULD go to SPD-I only. SSM is supported by the use
of unicast IP address selectors as documented in IPsec RFCs.
SPD entries created by a GCKS may have identical SPIs as some of the
IKE created ones. This is not a problem for the inbound traffic as
the appropriate SA's can be matched using the algorithm described in
RFC2401bis and SA's contain a link to their parent SPD entries if
such an entry exists (i.e. they are not manually keyed in). However,
the outbound traffic needs to be matched against the SPD selectors so
that the appropriate SA can be created on packet arrival. IPsec
implementations that support multicast SHOULD use the destination
address as the additional selector and match it against the SPD
entries marked sender only.
4.1.2 SAD
The 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 sender and the destination address is
the multicast group address. An inbound, multicast SA must be
configured with the source addresses of each peer authorized to
transmit to the multicast SA in question. The SPI value for a
multicast SA is provided by a multicast group controller, not by the
receiver, as for a unicast SA. Because an SAD entry may be required
to accommodate multiple, individual IP source addresses that were
part of an SPD entry (for unicast SAs), the required facility for
inbound, multicast SAs is a feature already present in an IPsec
implementation. However, SPD needs provisions for accommodating
multicast entries in order to enable automatic multicast SA creation.
PAD needs to be extended in order to accommodate peers that may take
on specific roles in the group. Such roles can be GCKS, speaker (in
case of SSM) or just a member. It may also contain root certificates
for PKI used by the group.
4.1.2.1 Anti-Replay for Multi-Sender SAs
TBD
4.1.3 PAD
4.1.3.1 GKMP/IPsec Interactions with the PAD
The RFC2401-bis section 4.4.3 introduced the Peer Authorization
Database (PAD). In summary, the PAD manages the IPsec entity
authentication mechanism(s) and authorization of each such peer
identity to negotiate modifications to the SPD/SAD. Within the
context of the GKMP/IPsec subsystem, the PAD defines for each group:
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. 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 GKCS role authorization rules. A group member
uses these rules to decide which systems are authorized to act as a
GCKS for a given group. These rules also declare the permitted GCKS
authentication mechanism(s).
. A set of one or more Group Speaker role authorization rules. A GCKS
uses these rules to authorize candidate group members that request
the speaker privilege. For an authorized speaker, the GCKS creates
a GSA description, and a subsequent RKE multicast configures that
speaker's GSA in the group SPD/SAD.
Some GKMP (e.g. GSAKMP) distribute their group's PAD configuration in
a security policy token [COREPT] signed by the group's policy
authority, also known as the "Group Owner" (GO). The GCKS re-key
multicast includes the current policy token. At each of the group's
endpoints, the GKMP subsystem is statically pre-configured with the
Group Owner's signature public key certificate or else the
information needed to acquire that certificate. All authorized group
members receive the GCKS re-key multicast and verify the Group
Owner's signature on the revised policy token. If that GO signature
is accepted, then all group members dynamically update their
respective PAD with the policy token's contents.
All compliant IPsec subsystems MUST provide a trusted mechanism for a
GKMP subsystem to update the PAD's per group configuration as
described in the above list. The details of that trusted mechanism
are implementation-specific and they are outside the scope of this
standardization.
The PAD MUST provide a management interface capability that allows an
administrator to enforce that the scope of a GKMP group's policy
specified SPD/SAD modifications are restricted to only those traffic
data flows that belong to that group. This authorization MUST be
configurable at GKMP group granularity. In the inverse direction, the
PAD management interface MUST provide a mechanism(s) to enforce that
IKE-v2 security associations do not negotiate traffic selectors that
conflict or override GKMP group policies. An implementation SHOULD
offer PAD configuration capabilities that authorize the GKMP policy
configuration mechanism to set security policy for other aspects of
an endpoint's SPD/SAD configuration, not confined to its group
security associations. This capability allows the group's policy to
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inhibit the creation of back channels that might otherwise leak
confidential group application data.
This document refers to re-key mechanisms as being multicast because
of the inherent scalability of IP multicast distribution. However,
there is no particular reason that re-key mechanisms must be
multicast. For example, [ZLLY03] describes a method of re-key
employing both unicast and multicast messages.
4.1.4 GSA
A IPsec implementation supporting these extensions has a number of
security associations: one or more IPsec SAs, and one or more group
key management SAs used to download IPsec SAs [RFC3740, Section 4].
These SAs are collectively referred to as a GSA.
4.1.4.1 Concurrent GSA Life Spans and Re-key Rollover
During a cryptographic group's lifetime, multiple group security
associations can exist concurrently. This occurs principally due to
two reasons:
- There are multiple Group Speakers authorized in the group, each
with its own GSA that maintains anti-replay state. A group that
does not rely on IP Security anti-replay services can share one
GSA for all of its Group Speakers.
- The life spans of a Group Speaker's two (or more) GSA are allowed
to overlap in time, so that there is continuity in the multicast
data stream across group re-key events. This capability is
referred to as "re-key rollover continuity".
Each group re-key multicast message sent by a GCKS signals the start
of a new Group Speaker time epoch, with each such epoch having an
associated GSA. The group membership interacts with these GSA as
follows:
- As a precursor to the Group Speaker beginning its re-key rollover
continuity processing, the GCKS periodically multicasts a Re-Key
Event (RKE) message to the group. The RKE multicast contains group
membership management directives (e.g. LKH), a new Group Traffic
Protection Key (GTPK), and for some GKMP the RKE may include a
revised group policy token. 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 SPD/SAD with the new
"leading edge" GSA state information. The leading edge GSA
allocates a new Security Parameter Index and it is keyed by the
GTPK distributed by the most recent RKE multicast. For a short
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period after the GCKS multicasts the RKE, a Group Speaker does not
transmit data using the leading edge GSA. Meanwhile, the Group
Receiver endpoints prepare to use this GSA by installing the RKE
directed changes to their respective SPD/SAD.
- After waiting a sufficiently long enough period such that all of
the Group Receiver endpoints have processed the RKE multicast, the
Group Speaker begins to transmit using the leading edge GSA with
its data encrypted by the new GTPK. Only authorized Group Members
can decrypt these GSA multicast transmissions. The time delay that
a Group Speaker waits before starting its first leading edge GSA
transmission is a GKMP/IPsec policy parameter. This value SHOULD be
configurable at the Group Owner management interface on a per group
basis.
- The Group Speaker's "trailing edge" GSA is the oldest group
security association in use by the group for that speaker. All
authorized Group Receiver endpoints can receive and decrypt data
for this GSA, but the Group Speaker does not transmit new data
using the "trailing edge" GSA after it has transitioned to the
"leading edge GSA". The trailing edge GSA 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 GSA. Staggering the roles of each respective GSA as described
above improves the group's synchronization even when there are high
network propagation delays. Note that due to group membership joins
and leaves, each Group Speaker time epoch may have a different group
membership set.
It is a group policy decision whether the re-key event transition
between epochs provides forward and backward secrecy. The group's re-
key protocol keying material and algorithm (e.g. Logical Key
Hierarchy) enforces this policy. Implementations MAY offer a Group
Owner management interface option to enable/disable re-key rollover
continuity for a particular group This specification requires that a
GKMP/IPsec implementation MUST support at least two concurrent GSA
per Group Speaker and this re-key rollover continuity algorithm.
4.2 Data Origin Authentication
As defined in [RFC2401BIS], data origin authentication is a security
service that verifies the identity of the claimed source of data.
While HMAC authentication methods are to used to provide data origin
authentication, they are not sufficient to provide data origin
authentication for groups. With an HMAC, every group member can use
the HMAC key to create a valid authentication tag whether or not they
are the authentic origin.
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When the property of data origin authentication is required for an
IPsec SA distributed from a GKMP, an authentication transform where
the originator keeps a secret should be used. Two possible algorithms
are TESLA [RFC4082] or RSA [W05].
In some cases, (e.g., RSA) 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 an HMAC authentication algorithm.
However, doing so requires the packet to be sent across the IPsec
boundary for additional inbound processing [RFC2401BIS, Section 5.2].
4.3 Group SA and Key Management
4.3.1 Co-Existence of Multiple Key Management Protocols
Often, the GKMP will be introduced to an existent IPsec subsystem as
a companion key management protocol to IKE-v2 [IKE-v2]. A fundamental
GKMP IP Security subsystem requirement is that both the GKMP and IKE-
v2 can simultaneously share access to a common Security Policy
Database and Security Association Database. The mechanisms that
provide mutually exclusive access to the common SPD/SAD data
structures are a local matter. This includes the SPD-outbound cache
and the SPD-inbound cache. However, implementers should note that
IKE-v2 SPI allocation is entirely independent from GKMP SPI
allocation because group security associations are qualified by a
destination multicast IP address and may optionally have a source IP
address qualifier. See [RFC2406-bis] section 2.1 for further
explanation.
The Peer Authorization Database does require explicit coordination
between the GKMP and IKE-v2. Section X.Y describes these
interactions. This document discusses the coordination of multiple
GKMP group owner and endpoint local management systems in section
4.11.
5.0 IP Traffic Processing
Processing of traffic follows [RFC2401BIS, Section 5], 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 3.0), 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
[RFC2401BIS]. Similarly, If the destination address is marked as
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being preserved, during header construction the "dest address" header
field MUST be "copied from inner hdr" rather than "constructed".
5.2 Inbound IP Multicast Traffic Processing
If an IPsec SA is marked as supporting tunnel mode with address
preservation (as described in Section 3.0), 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
[RFC2401BIS, Section 5.2]. In particular the IPsec system MUST
discard the packet, as well as treat the inconsistency as an
auditable event.
5.0 Networking Issues
5.1 Network Address Translation
With the advent of NAT and mobile Nodes, IPsec multicast applications
must overcome several architectural barriers to their successful
deployment. This section surveys those problems and identifies the
SPD/SAD state information that the GKMP must synchronize across the
group membership.
5.1.1 SPD Losses Synchronization with Internet Layer's State
The most prominent problem facing GKMP IPsec is that the GKMP group
security policy mechanism can inadvertently configure the group's SPD
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
multicast speaker's source IP address without signaling the GKMP. The
absence of a SPD synchronization mechanism can cause the group's data
traffic to be discarded rather than processed correctly.
5.1.1.1 Mobile Multicast Care-Of Address Route Optimization
Both Mobile IP-v4 [RFC3344] and Mobile IP-v6 [MIPV6] provide
transparent unicast communications to a mobile Node. However,
comparable support for secure multicast mobility management is not
specified by these standards. The goal is the ability to maintain an
end-to-end transport mode group SA between a Group Speaker mobile
node that has a volatile care-of-address and a Group Receiver
membership that also may have mobile endpoints. In particular, there
is no secure mechanism for route optimization of the triangular
multicast path between the correspondent Group Receiver Nodes, the
home agent, and the mobile Node. Any proposed solution must be secure
against hostile re-direct and flooding attacks.
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5.1.1.2 NAT Translation Mappings Are Not Predictable
The following spontaneous NAT behaviors adversely impact source-
specific secure multicast groups. When a NAT gateway is on the path
between a Group Speaker endpoint residing behind a NAT and a public
IP-v4 multicast Group Receiver, the NAT gateway alters the private
source address to a public IP-v4 address. This translation must be
coordinated with every Group Receiver's inbound Security Policy
Database (SPD) multicast entries that depend on that source address
as a traffic selector. One might mistakenly assume that the GC/KS
could set up the group endpoints with a SPD 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 SPD selectors at the group endpoints.
- The GC/KS 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, an endpoint's
address may change due to a DHCP assigned address lease expiration.
- Alternate paths may exist between a given pair of group endpoints.
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 GC/KS can not be pre-
configured with NAT mappings, as the SPD at the group's endpoints
will lose synchronization as soon as a NAT mapping changes due to any
of the above events. In the worst case, group endpoints in different
sections of the Internet will see different NAT mappings, because the
multicast packet traversed multiple NAT gateways.
5.1.2 Secondary Problems Created by NAT Traversal
5.1.2.1 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 Internet. Mobility changes a Node'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
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source IP address makes it infeasible for transit multicast routers
in the public Internet to know which SSM speaker originated the
multicast packet, which in turn selects the correct multicast
forwarding policy.
5.1.2.2 ESP Cloaks Its Payloads from NAT Gateway
When traversing NAT, application layer protocols that contain IP-v4
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 IP-v4
addresses into equivalent public IP-v4 addresses. However, when
encrypted by end-to-end ESP, such payloads are opaque to application
layer gateways.
When multiple Group Speaker endpoints reside behind a NAT with a
single public IP-v4 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 [UDPESP] avoids this problem. However, this
capability must be configured at the GC/KS as a group policy, and it
must be supported in unison by all of the group endpoints within the
group, even those that reside in the public Internet.
5.1.2.3 UDP Checksum Dependency on Source IP Address
A GKMP/IPsec multicast application that uses UDP within an ESP
payload will encounter NAT induced problems. The original IP-v4
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 IP-4 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 IKE-v2 [IKE-v2], this information is exchanged
between the endpoints by a NAT-OA payload (NAT original address). See
also reference [IPSECNAT]. A comparable facility must be exist in a
GKMP payload that defines the multicast application GSA attributes
for each Group Speaker endpoint.
5.1.2.4 Can Not 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.
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5.1.3 Avoidance of NAT Using an IP-v6 Over IP-v4 Network
A straightforward and standards-based architecture that effectively
avoids the GKMP interaction with NAT gateways is the IP-v6 over IP-v4
transition mechanism [RFC2529]. In IP-v6 over IP-v4 (a.k.a.
"6over4"), the underlying IP-v4 network is treated as a virtual
multicast-capable Local Area Network. The IP-v6 traffic tunnels over
that IP-v4 virtual link layer.
Applying GKMP/IPsec in a 6over4 architecture leverages the fact that
an administrative domain deploying GKMP/IPsec would already be
planning to deploy IP-v4 multicast router(s). The group's IP-v6
multicast routing can execute in parallel to IP-v4 multicast routing
on that same physical router infrastructure. In particular, the NAT
gateways at administrative domain public/private boundaries are
replaced by IP-v6 multicast routers operating with 6over4 mode
enabled on their network interfaces.
Within the GKMP, all references to IP addresses are IP-v6 addresses
for all security association endpoints and these addresses do not
change over the group's lifetime. This yields a substantial reduction
in complexity and error cases over the NAT-based approaches. This
reduction in complexity can translate into better security.
Reliable scalable GKMP/IPsec based on 6over4 deployment is far more
practical than an IP-v4 with NAT deployment. In particular, new
GKMP/IPsec multicast applications SHOULD prefer IP-v6 native mode.
However, the GKMP/IPsec architecture supports either choice. The
following factors may weigh against the decision to deploy GKMP/IPsec
using 6over4:
- A drawback of the GKMP/IPsec 6over4 approach is that the secure
multicast application must be (re-)written to an IP-v6 multicast
socket API or equivalent, and it must interact with the Multicast
Listener Discovery (MLD) API [RFC3590] [RFC3678] rather than IGMP.
In addition, the application layer protocol itself must embed
references to IP-v6 addresses rather than IP-v4 addresses within
its payloads. For new applications, this may not be of consequence;
it usually only becomes an issue if the application and its
protocol has an embedded base.
- An embedded base of GKMP/IPsec IP-v4 multicast applications that
are only available in binary form will not be able to migrate to
these transitional IP-v6 mechanisms.
- The secondary drawbacks of GKMP/IPsec using 6over4 are that the IP
hosts must be upgraded to dual-stack, the attendant overlay IP-v6
multicast network operational costs, and the perceived difficulty
of deploying commercial wide-area IP-v6 multicast services.
5.1.4 GKMP/IPsec Multi-Realm IP-v4 NAT Architecture
In a multi-realm group, GKMP/IPsec security association endpoints may
straddle any combination of IP-v4 public addresses and private
addresses [RFC1918]. In such cases, transport layer endpoint
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identifiers when resolved to their underlying private or public IP-v4
addresses entangle the GKMP protocol with NAT gateway behaviors. The
NAT translation of IP-v4 header addresses impacts the GKMP
registration SA, the GKMP re-key GSA, and the secure multicast
application GSA.
This section overviews the GKMP/IPsec mechanisms that partially
mitigate the inherent complexity spawned by IP-v4 NAT and Network
Address Protocol Translation (NAPT) traversal. However, the attendant
Group Owner configuration procedures are labor-intensive, prone to
configuration mismatch errors between the GC/KS and NAT gateways, and
they do not scale well to large groups. Given the large number of
documented NAT problems and its erosion of end-to-end security, new
GKMP/IPsec applications and deployments SHOULD strongly prefer the
use of IP-v6. Section X.Y offers IP-v4 to IP-v6 transitional guidance
in support of that objective.
5.1.4.1 GKMP/IPsec IP-v4 NAT Architectural Assumptions
To make the multi-realm GKMP/IPsec IP-v4 NAT interaction problem
tractable to a solution, this specification requires the following
simplifying assumptions:
- The secure multicast group destination address is a statically
allocated public IP-v4 multicast address known to all group
endpoints.
- Wherever they are present in the GKMP, group endpoint addresses are
expressed as permanent IP-v6 "6to4" addresses [RFC3056] to assure
that the group endpoints that refer to hosts assigned private IP-v4
addresses are globally unique. In this context, a "permanent" 6to4
address means that the address is constant for the group's
lifetime.
- Each private IP-v4 address space has one or more NAT gateways
directly connected to the IP-v4 public Internet, and a packet does
not have to traverse multiple private networks to reach the public
Internet. This can be thought of as a "spoke and hub" configuration
wherein the public Internet is the hub.
- A GC/KS may reside within one of the private networks, but it also
MUST have a permanent public IP-v4 address on at least one of its
network interfaces. This requirement applies to both the Primary-
GC/KS and all of the group's Subordinate-GC/KS.
- GKMP/IPsec group security associations are end-to-end transport
mode. However, since the one or more GC/KS are constrained to
straddle a public/private network boundary, they effectively
terminate the GSA at a combined NAT/security gateway [RFC2709].
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- The GC/KS domain name RR record should point to that public IP-v4
address, and it is recommended that it be protected by DNS-SEC.
- Each of an administrative domain's NAT gateways are explicitly
configured with static private/public address translation mappings
for the GC/KS's GKMP re-key multicast ESP protected UDP packets
inbound from the public Internet [RFC2588].
- The NAT gateways/firewalls are explicitly configured with stateless
filter rules that simply pass through without any address
translation the group's inbound multicast application packets
arriving from the public Internet. The NAT gateway does not
translate the multicast application packet's public multicast IP
destination address into a private IP multicast address.
- In the outbound direction, NAT gateways generally translate the
multicast application packet's private source IP address into a
dynamically selected public IP address. Exceptions to this policy
for source specific multicast are noted in subsequent sections.
- Within each administrative domain, a multicast routing protocol
domain routes packets based on the group's destination multicast
public IP-v4 address. The multicast routers will distribute the
group's packets to all of the group's Group Receiver endpoints
residing in that administrative domain.
- The border routers of each of the administrative domains spanned by
the group do cross-realm multicast routing and distribution on
behalf of the group. The IP-v4 multicast routers that exchange
reachability information regarding the group across trust
boundaries authenticate that information.
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"A" Admin . ISP Admin . "B" Admin
Domain . Domain . Domain
. .
+---------------.--------------.-------------------+
| . . |
| P U B L I C . I P - v 4 . I N T E R N E T |
| . . |
+------/\-------.-----A-----A--.----/\--------/\---+
|| public. | | . || public ||
|| IP-v4 . | | . || IP-v4 ||
+------\/------+. | | .+---\/---+ +--\/---+
|Grp.Z |NAT "A"|. | | .|Group Z | |NAT "B"|
|S-GCKS|gateway|. | | .|P-GC/KS | |gateway|
+---A--+---A---+. | | .+---A----+ +--A----+
| | .registratn | . | |
regist. SA| . SA | . regist. SA |
| | . | | . | |
+-V-+ | . +-V-+ | . +-V-+ |
|GM1| | . |GM2| | . |GM3| |
+-A-+ | . +-A-+ | . +-A-+ |
| | . | | . | |
Group data SA . Group data SA. Group data SA
rekey SA . rekey SA . and rekey SA
| | . | | . | |
+-V------V--+ . +---V-----V-+.+---V---------V-+
| Group "Z" | . | Group "Z" |.| Group "Z" |
| multicast | . | multicast |.| multicast |
| routing | . | routing |.| routing |
| domain | . | domain |.| domain |
+-----------+ . +-----------+.+---------------+
. .
Figure 2 Representative GKMP/NAT architecture
5.1.4.2 Representative GKMP Multi-Realm Configuration
Figure 2 illustrates a representative group "Z" wherein a GKMP/IPsec
group security association spans two private IP-v4 networks and the
public IP-v4 Internet. The Group "Z" GC/KS has two network
interfaces, one attached to the public Internet and the other
interface attached to the administrative domain "B" private network.
The group member GM1 resides within the administrative domain "A"
private network. It communicates with the group Z Group Speaker
endpoint(s) through the administrative domain "A" NAT gateway. When
GM1 multicasts application SA traffic to the group Z public multicast
address, the Group Z peer members (i.e. GM2, and GM3) receive that
multicast with the source address translated by NAT gateway "A"
processing. In the inverse direction, the administrative domain "A"
NAT gateway/firewall must be configured to allow Group Z multicast
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application GSA traffic to enter the private network "A" from the
public Internet (e.g. a multicast originating from GM3).
The group member GM3 resides within the administrative domain "B"
private network. Its interactions with Group Z are very similar to
those discussed for member GM1. It uses private addresses when
communicating with the P-GC/KS, as it is in private network "B".
The group member GM2 is in a public Internet administrative domain
operated by an ISP. It communicates with the P-GC/KS using public IP-
v4 addresses without passage through a NAT gateway. When GM2
multicasts application SA traffic to the group Z public multicast
address, the Group Z peer members behind NAT gateways receive that
multicast with the source address unchanged by NAT processing.
Each administrative domain operates an IP-v4 multicast routing domain
instance. The multicast routers distribute both GKMP re-key messages
and multicast application GSA data traffic. The multicast routing for
group "Z" peers between these three multicast routing domains.
5.1.4.3 Registration Security Association NAT Traversal
The GKMP registration protocol's unicast messages are exchanged
between a GC/KS in the public IP-v4 Internet and a candidate Group
Member that may be in a private network.
A group member sends a registration SA GKMP message to the GC/KS
public IP-v4 address and the GKMP reserved port number. The group
member assigns a unique GKMP UDP source port number for each GKMP
registration SA that it participates in. The group member SHOULD send
the GKMP UDP packet without a checksum to avoid NAT alterations to
that field. The UDP packet's transmission error detection depends on
the GKMP signature within the payload. A NAT gateway on the path
leading to the GC/KS translates the private source IP address and
source UDP port number into a public address and a temporary UDP port
number (assuming NAPT), then forwards the packet to the GC/KS. The
NAT gateway creates state information for that public/private address
mapping so it can do the inverse translation on the GKMP messages
sent from the GC/KS to that group member.
The GC/KS must process the GKMP messages that it receives from group
members originating from any source IP address or source port number,
even if those two values have changed since the last time that the
GC/KS had interacted with a given group member. To identify the group
member, the GC/KS MUST use the GKMP signature payload's identifying
information and validate the message's digital signature.
After processing a message from a group member that requires a GC/KS
response, the GC/KS creates the GKMP UDP message destined for the
same IP-v4 address and UDP port that the GC/KS found in the candidate
Group Member message's source IP address and UDP source port.
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5.1.4.4 GKMP Re-key GSA NAT Traversal
The GKMP Re-Key GSA is considerably simplified by the constraint that
every Subordinate-GC/KS and Primary-GC/KS MUST straddle a public
Internet/private network boundary adjacent to wherever it has Group
Members behind a NAT gateway. Consequently, a GC/KS may have Group
Members on either side of that boundary, but there is no intervening
NAT gateway tampering with the GC/KS transmissions.
The GC/KS multicasts the GKMP re-key message to the Re-Key GSA in an
ESP protected UDP|GKMP packet addressed to its (sub-)group's
destination public IP-v4 multicast address. The UDP destination port
is set to the GKMP-UDP reserved port number. The group keyed ESP
authenticator protects the UDP payload, so a UDP checksum MUST NOT be
used.
A multi-realm IP-v4 GKMP/IPsec group operates in autonomous
distributed mode. Therefore, each of the group's Subordinate-GC/KS
must relay to their respective sub-group membership the GTEK (and
policy token, if any) that it extracts from the Primary-GC/KS rekey
multicast. The S-GC/KS sends its re-key message to its sub-group
membership from its public Internet interface.
5.1.4.5 Multicast Application GSA NAT Traversal
Unlike the GKMP rekey message multicast to the Re-Key GSA, a
multicast application message sent to the group may originate from a
Group Speaker endpoint located behind a NAT gateway. Since the
application's message is encrypted within an ESP payload, the
transport layer protocol header port fields are concealed from NAT
gateways and they can not participate in NAPT. The multicast
application GSA must be handled differently depending on whether the
application requires source-specific multicast.
If the application requires source-specific multicast routing, then
there must be a separate public IP-v4 address statically reserved at
the NAT gateway for each Group Speaker endpoint private/public
address mapping. This constraint allows the GC/KS to specify at every
Group Member the inbound SPD traffic selector with a pre-determined
public source address for each Group Speaker endpoint in the group.
The traffic selector's public source address in combination with the
group's destination multicast address and SPI selects the inbound SA.
Keeping the NAT gateway's source address mapping static rather than
dynamic also allows the multicast routers along the packet's path to
apply source-specific routing policies. Note that the use of a static
source address mapping NAT avoids the need for the group's policy
token to specify UDP encapsulated ESP. The drawback of this approach
is that the GC/KS SPD/SAD configuration database must be kept
synchronized with the group's NAT gateway address mapping
configurations. These operational procedures can be labor-intensive
and error-prone, making large-scale group deployments difficult. A
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more sophisticated GKMP may sidestep this problem by dynamically
setting the Group Receiver endpoint's SPD/SAD entry traffic selector
rather than relying on static GC/KS configuration.
If the application requires the any-source multicast service model,
then the NAT gateway's source address translation can use dynamically
allocated public IP-v4 addresses rather than statically allocated IP-
v4 addresses. However, unless the group uses UDP encapsulated ESP,
then the NAT gateway must have a pool of public IP-v4 addresses
reserved that is at least as large as the number of Group Speaker
endpoints within its private network. The public IP address pool
allows the NAT gateway to do a one-to-one mapping from every Group
Speaker endpoint's private source address to a dynamically allocated
public source address. In this case, the use of NAPT rather than NAT
is not an option, since the transport layer protocol is within an
opaque ESP payload. The GC/KS specifies the SPD/SAD traffic selector
as the combination of the group's destination multicast address and
the SPI.
In some deployments, the number of public IP-v4 addresses assigned to
a NAT gateway is very limited (e.g. only one public IP-4 address).
Also, it may be difficult to predict how many Group Speaker endpoints
will reside within the private network before the group begins its
operation. For these cases, the group MAY use UDP encapsulated ESP.
The NAT gateway applies NAPT to the UDP header's source port field,
sidestepping the constraint of its limited public IP-v4 address pool.
The Group Owner modifies the group policy token to specify that the
outbound SPD processing must pre-append a UDP header in front of the
ESP header. When a Group Speaker endpoint originates a multicast
application packet, it inserts a UDP header in front of the ESP
header, as per reference [UDPESP].
6.0 Security Considerations
[TBD]
7.0 Acknowledgements
[TBD]
8.0 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 Protocol (GKMP) Subsystem and
the IPsec subsystem MUST be able to configure the SPD/SAD security
policies to match these dominant usage scenarios. The SPD/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 GKMP Subsystem management interface MAY
include mechanisms to configure the security policies for service
models not identified by this standard.
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8.1 Unidirectional Multicast Applications
Multi-media content delivery multicast applications that do not have
congestion notification or retransmission error recovery mechanisms
are inherently unidirectional. RFC2401-bis only defines bi-
directional unicast security associations (as per sections 4.4.1 and
5.1 with respect to security association directionality). The GKMP
Subsystem requires that the IPsec subsystem MUST support
unidirectional Group Security Associations (GSA). 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 SPD configuration is optionally setup to discard unauthorized
attempts to transmit unicast or multicast packets to the group.
The GKMP Subsystem's Group Owner management interface MUST have the
ability to setup a GKMP Subsystem group having a unidirectional GSA
security policy.
8.2 Bi-directional Reliable Multicast Applications
Some secure multicast applications are characterized as one group
speaker to many receivers, but with inverse data flows required by a
reliable multicast transport protocol (e.g. NORM). In such
applications, the data flow from the speaker is multicast, and the
inverse flow from the group's receivers is unicast to the speaker.
Typically, the inverse data flows carry error repair requests and
congestion control status.
For such applications, the GSA SHOULD use IPsec anti-replay
protection service for the speaker's multicast data flow to the
group's receivers. Because of the scalability problem described in
the next section, it is not practical to use the IPsec anti-replay
service for the unicast inverse flows. Consequently, in the inverse
direction the IPsec anti-replay protection MUST be disabled. However,
the unicast inverse flows can use the group's IPsec group
authentication mechanism. The group receiver's SPD entry for this GSA
SHOULD be configured to only allow a unicast transmission to the
speaker Node rather than a multicast transmission to the whole group.
If ESP RSA signature mechanism is available, source authentication
MAY be used to authenticate a receiver Node's transmission to the
speaker. The GKMP MUST define a key management mechanism for the
group speaker to validate the asserted signature public key of any
receiver Node without requiring that the speaker maintain state about
every group receiver.
This multicast application service model is RECOMMENDED because it
includes congestion control feedback capabilities. Refer to [RFC2914]
for additional background information.
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The GKMP Subsystem's Group Owner management interface MUST have the
ability to setup a GKMP Subsystem GSA having a bi-directional GSA
security policy and one group speaker. The management interface
SHOULD be able to configure a group to have at least 16 concurrent
authorized speakers, each with their own GSA anti-replay state.
8.3 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.
For such applications, all (or a large subset) of the group's
endpoints are authorized multicast speakers. In such service models,
creating a distinct GSA with anti-replay state for every potential
speaker does not scale to large groups. The group SHOULD share one
GSA for all of its speakers. The GSA SHOULD NOT use IPsec anti-replay
protection service for the speaker's multicast data flow to the
group's listeners.
The GKMP Subsystem's Group Owner management interface MUST have the
ability to setup a group having an Any-To-Many Multicast GSA security
policy.
9.0 References
9.1 Normative References
[AH] Kent, S., "IP Authentication Header", draft-ietf-ipsec-
rfc2402bis-10.txt, December 2004.
[ESP] Kent, S., "IP Encapsulating Security Payload (ESP)", draft-
ietf-ipsec-esp-v3-09.txt, September 2004.
[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.
[RFC2401BIS] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", draft-ietf-ipsec-rfc2401bis-06.txt, March, 2005.
[RFC3552] E. Rescorla, et. al., "Guidelines for Writing RFC Text on
Security Considerations", RFC 3552, July 2003.
9.2 Informative References
[IKEV2] C. Kaufman, "Internet Key Exchange (IKEv2) Protocol", draft-
ietf-ipsec-ikev2-17.txt, September 23, 2004.
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[RFC2526] D. Johnson, S. Deering., "Reserved IPv6 Subnet Anycast
Addresses", RFC 2526, March, 1999.
[RFC2914] S.Floyd, "Congestion Control Principles", RFC2914,
September 2000.
[RFC3171] Z. Albanni, et. al., "IANA Guidelines for IPv4 Multicast
Address Assignments", RFC 3171, August, 2001.
[RFC2362] Estrin, D., et. al., "Protocol Independent Multicast-Sparse
Mode (PIM-SM): Protocol Specification", RFC 2362, June, 1998.
[RFC3376] B. Cain, et. al., "Internet Group Management Protocol,
Version 3", RFC 3376, October, 2002.
[RFC3547] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
Group Domain of Interpretation", RFC 3547, December 2002.
[RFC3569] S. Bhattacharyya, "An Overview of Source-Specific
Multicast (SSM)", RFC 3569, July, 2003.
[RFC3940] B. Adamson, et. al., "Negative-acknowledgment (NACK)-
Oriented Reliable Multicast (NORM) Protocol", RFC 3940, November,
2004.
[RFC4082] A. Perrig, et. al., "Timed Efficient Stream Loss-Tolerant
Authentication (TESLA): Multicast Source Authentication Transform
Introduction", RFC 4082, June 2005.
[W05] B. Weis, "The Use of RSA/SHA-1 Signatures within ESP and AH",
draft-ietf-msec-ipsec-signatures-06.txt, June 2005.
[ZLLY03] X. Zhang, 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.pdf.
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Author's Address
Brian Weis
Cisco Systems
170 W. Tasman Drive,
San Jose, CA 95134-1706, USA
(408) 526-4796
bew@cisco.com
George Gross
IdentAware Security
82 Old Mountain Road
Lebanon, NJ 08833
908-268-1629
gmgross@identaware.com
Dragan Ignjatic
Polycom
1000 W. 14th Street
North Vancouver, BC V7P 3P3
Canada
tel: +1 604 982 3424
email: dignjatic@polycom.com
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Weis Expires December, 2005 [Page 25]
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