Network Working Group D. Wing
Request for Comments: 5135 T. Eckert
BCP: 135 Cisco Systems, Inc.
Category: Best Current Practice February 2008
IP Multicast Requirements for a Network Address Translator (NAT)
and a Network Address Port Translator (NAPT)
Status of This Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Abstract
This document specifies requirements for a for a Network Address
Translator (NAT) and a Network Address Port Translator (NAPT) that
support Any Source IP Multicast or Source-Specific IP Multicast. An
IP multicast-capable NAT device that adheres to the requirements of
this document can optimize the operation of IP multicast applications
that are generally unaware of IP multicast NAT devices.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology Used in This Document . . . . . . . . . . . . . . 2
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. NATing IP Multicast Data Packets . . . . . . . . . . . . . 5
4.1.1. Receiving Multicast Data Packets . . . . . . . . . . . 5
4.1.2. Sending Multicast Data Packets . . . . . . . . . . . . 5
4.2. IGMP Version Support . . . . . . . . . . . . . . . . . . . 6
4.2.1. IGMPv1 or IGMPv2 . . . . . . . . . . . . . . . . . . . 7
4.2.2. IGMPv3 . . . . . . . . . . . . . . . . . . . . . . . . 7
4.3. Any Source Multicast Transmitters . . . . . . . . . . . . 8
5. Requirements Summary . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Application Considerations . . . . . . . . . . . . . 14
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1. Introduction
In order for IP multicast applications to function well over NATs,
multicast UDP must work as seamlessly as unicast UDP. However, NATs
have little consistency in IP multicast operation, which results in
inconsistent user experiences and failed IP multicast operation.
This document targets requirements intended to enable correct
operations of Any Source Multicast and Source-Specific Multicast in
devices running Internet Group Management Protocol (IGMP) proxy
routing and NAT and without applying NAT to IP multicast group
addresses. This profile of functionality is the expected best
practice for residential access routers, small branch routers, or
similar deployments.
Most of the principles outlined in this document do also apply when
using protocols other than IGMP, such as Protocol Independent
Multicast - Sparse Mode (PIM-SM), or when performing NAT between
multiple "inside" interfaces, but explicit consideration for these
cases is outside the scope of this document.
This document describes the behavior of a device that functions as a
NAT for unicast flows and also forwards IP multicast traffic in
either direction ('inside' to 'outside', or 'outside' to 'inside').
This allows a host 'inside' the NAT to both receive multicast traffic
and to source multicast traffic. Hosts on the 'inside' interface(s)
of a NAT indicate their interest in receiving an IP multicast flow by
sending an IGMP message to their local interface. An IP multicast-
capable NAT will see that IGMP message (IGMPv1 [RFC1112], IGMPv2
[RFC2236], IGMPv3 [RFC3376]), possibly perform some functions on that
IGMP message, and forward it to its upstream router. This causes the
upstream router to send that IP multicast traffic to the NAT, which
forwards it to those 'inside' segment(s) with host(s) that had
previously sent IGMP messages for that IP multicast traffic.
Out of scope of this document are PIM-SM [RFC4601] and IPv6
[RFC2460]. The IGMP Proxy devices that are scoped in this document
do not forward PIM-SM. IPv6 is out of scope because NAT is not
considered necessary with IPv6.
This document is a companion document to "NAT Behavioral Requirements
for Unicast UDP" [RFC4787].
2. Terminology Used in This Document
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].
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In this document, the term "NAT" applies to both Network Address and
Port Translator (NAPT) as well as a NAT that does not translate
ports.
The term 'inside' refers to the interface(s) on a NAT that contain
hosts that wish to source or receive IP multicast traffic. The term
'outside' refers to the interface(s) that the NAT forwards IGMP
membership messages to, and where the NAT routes IP multicast traffic
that originates from hosts on its 'inside' interface.
3. Background
When a NAT isn't used, a host might be connected to the Internet in a
configuration such as this:
+-------------+
+------+ | DSL modem | +------------+
| host +---+ or +-//-+ WAN Router |
+------+ | cable modem | +------------+
+-------------+
Figure 1: Network without NATing IGMP Proxy
If instead of a single host as shown in Figure 1, one or more LANs
with potentially multiple hosts are to be connected, with the same
type of service termination on the DSL or cable modem, a NAT device
is added as shown in Figure 2. This device, in general, perform
routing and NAT functions such that it does look like a single host
towards the DSL/cable modem.
+----+ +-------------+
|host+---+ +---------+ | +-----------+
+----+ | |Multicast| | | DSL modem | +------------+
| | Proxy | +--+ or +-//-+ WAN Router |
'inside' | +---------+ | |cable modem| +------------+
interfaces | | +-----------+
| +------+ |
+----+ | | NAT | | 'outside'
|host+---+ +------+ | interfaces
+----+ +-------------+
IGMP Proxy NAT Device
Figure 2: Network with NATing IGMP Proxy
In IP multicast, IGMP is the protocol used by hosts, such as the one
shown in Figure 1. For the NAT device in Figure 2 to look like the
single host for IP multicast services towards the DSL/cable modem and
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RFC 5135 NAT IP Multicast Requirements February 2008
to forward IP multicast traffic from and to the multiple hosts in the
picture, it needs to perform so called "IGMP Proxying" [RFC4605] --
but within the context of also performing NAT. NAT is not covered by
[RFC4605]. Adding NAT to IGMP proxying does not need to change the
processing of the IGMP messages as defined in RFC 4605:
IGMP messages are never logically forwarded by the IGMP proxying
device, but rather sourced or received by it. In general, receipt
of IGMP messages by the device updates the device's IGMP state.
The updated state changes the device's forwarding of multicast
messages or triggers the sending of IGMP messages. "Forwarding"
of IGMP protocol messages may thus only happen implicitly by
implementation optimizations that create shortcuts in this
machinery.
This specifically means that IGMP protocol packets sent by the NAT
device will always use the IP address of the interface ('inside' or
'outside') from which they are sent, but because those packets are
logically "sourced" and not "forwarded", NAT does not have any impact
on this.
Unlike unicast flows, packets with a multicast destination IP address
do not have their destination IP address or destination port changed
by a NAT. However, their source IP address (and source UDP port, in
some cases with a NAPT) is changed if the packet goes from an
'inside' interface of a NAT to the 'outside' interface of a NAT --
similar to the behavior of a unicast packet across those same
interfaces.
Adding NAT to IGMP proxying changes the processing of IP multicast
data packets forwarded across the IGMP proxying device as described
in the following sections. These changes actually simplify the
ability to deploy IGMP proxying over a device that does *not* perform
NAT.
With an IGMP Proxy NAT Device, IP multicast data traffic sourced from
hosts on the 'inside' is NATed such that it will look like it is
being sourced from a host directly connected to the WAN router, thus
eliminating all non-standard PIM-SM concerns/configurations described
in Section 3.2 of [RFC4605].
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4. Requirements
4.1. NATing IP Multicast Data Packets
4.1.1. Receiving Multicast Data Packets
REQ-1: For IP multicast packets that are forwarded to a host(s) on
its 'inside' interface(s), a NAT MUST NOT modify the
destination IP address or destination port of the packets.
If a NAT were to modify the destination IP or port addresses, the
NAT would also need to modify session announcements (e.g.,
electronic program guides, Session Announcement Protocol (SAP))
and session establishment and control (e.g., SIP, Real Time
Streaming Protocol (RTSP)) messages. Such modifications of
application messages are not considered a best practice.
Furthermore, a NATed multi-homed network would need to coordinate
such rewriting between its NATs.
REQ-2: A NAT MUST forward IP multicast UDP datagrams from its
'outside' interface to multicast receivers on its 'inside'
interface(s).
REQ-3: A NAT SHOULD forward IP multicast non-UDP protocols (e.g.,
Pragmatic General Multicast (PGM) [RFC3208], Resource
Reservation Protocol (RSVP) [RFC2205]) from its 'outside'
interface to IP multicast receivers on its 'inside'
interface(s).
4.1.2. Sending Multicast Data Packets
The following requirement is normal NAT behavior for unicast packets,
as described in [RFC4787], and is extended here to provide support
for IP multicast senders behind the NAT.
REQ-4: A NAT MUST modify the source IP address of packets that
arrive from an 'inside' interface towards the 'outside'
interface so that those packets use the NAT's 'outside' IP
address(es).
a: If the NAT also performs port translation (that is, it
is a NAPT), the NAT MUST also create a mapping to allow
responses to that IP multicast packet to be received by
the appropriate host. For Any Source Multicast, also
see Section 4.3.
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b: To allow hosts to learn the NAT's 'outside' interface
address, the NAT MUST have "Endpoint-Independent
Mapping" behavior (REQ-1 of [RFC4787]), no matter if the
destination IP address is a unicast address or an IP
multicast address.
c: If the NAT has multiple public IP addresses, the NAT
SHOULD have an address pooling behavior of "Paired" (as
described in Section 4.1 of [RFC4787]) for its IP
multicast mappings as well as for its unicast UDP
mappings. This allows a multicast source to discover
the NAT's public IP address using a unicast address
discovery mechanism (e.g., [ICE]) and communicate that
discovered IP address to a multicast receiver.
REQ-5: A NAT MUST forward IP multicast UDP datagrams from its
'inside' interface(s) to its 'outside' interface.
a: NATs that support the above requirement MUST also
provide a configuration option to disable this feature.
Otherwise, a multihomed network would cause duplicate
instances of the multicast data traffic on the public
network.
As many NATs are located adjacent to bandwidth-constrained access
links, it is important that IP multicast senders communicating with
IP multicast receivers behind the NAT not have their flows consume
bandwidth on the access link. This is accomplished by applications
using administratively scoped IP addresses. Similarly, link-local
multicast traffic isn't supposed to be routed off the local network.
REQ-6: The NAT's default configuration MUST NOT forward
administratively scoped IP multicast traffic (239.0.0.0/8)
[RFC2365] from its 'inside' interface(s) to its 'outside'
interface.
REQ-7: The NAT MUST NOT forward Local Network Control Block
(224.0.0/24) [RFC3171] (also known as "link-local
multicast") traffic from its 'inside' interface(s) to its
'outside' interface.
4.2. IGMP Version Support
REQ-8: A NAT MAY support IGMPv1 (although IGMPv1 is considered
obsolete).
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REQ-9: A NAT MUST support IGMPv2.
REQ-10: A NAT SHOULD support IGMPv3.
4.2.1. IGMPv1 or IGMPv2
For IGMPv1 and IGMPv2, a NAT can successfully operate by merely
forwarding IGMP membership reports and queries between the interested
hosts (on its internal interface) towards its external interface.
REQ-11: If a NAT supports IGMPv1 and/or IGMPv2 (but not IGMPv3), the
NAT MAY simply receive IGMP membership reports on the
'inside' interface, NAT them, and relay the IGMP membership
report, and do the same function in the opposite direction
to the IGMP listeners. That is, the NAT does not need to do
any aggregation of IGMP messages.
a: If a NAT relays IGMPv1 or IGMPv2 messages in this
manner, it MUST NOT decrement the TTL of the IGMP
messages, as they are already sent with TTL=1.
b: However, it is RECOMMENDED that such a NAT implement
IGMP/MLD Proxying [RFC4605], because IGMP aggregation
provides a useful optimization.
4.2.2. IGMPv3
When an IGMPv3 proxying device receives an IGMP membership on an
'inside' interface, it creates its own IGMP proxying membership state
and its own IGMP forwarding table. It then creates an independent
IGMP membership report on its 'outside' interface reporting the IP
multicast groups/channels -- but there is no direct relationship or
"forwarding" of IGMP membership reports or queries across the
interfaces. The NAT device will subsequently receive an IP multicast
data packet on the 'outside' interface and forward the IP multicast
packet to the 'inside' interface(s) based on its IGMP forwarding
table.
By performing NAT on IGMPv3 membership reports, the membership
reports appear to originate from a single IGMPv3 reporter instead of
different reporters. Because IGMPv3 has different types of
membership reports differentiating between status (IS_INCLUDE,
IS_EXCLUDE) and change indication (e.g., TO_INCLUDE, TO_EXCLUDE), if
a NAT were to interleave reports from two or more reporters (joining
and leaving the same groups), the NAT would create a sequence of
packets that are not compliant with an IGMPv3 reporter [RFC3376].
For this reason, the following requirements are specified:
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REQ-12: If a NAT supports IGMPv3, the NAT MUST implement IGMP/MLD
Proxying [RFC4605]. Such compliance causes the NAT to
aggregate the IGMPv3 membership reports and report only the
aggregated information upstream.
REQ-13: If a NAT supports IGMPv3, the NAT MUST implement Source-
Specific Multicast (SSM) for IP [RFC4607] and IGMPv3/MLDv2
for SSM [RFC4604].
Failure to implement IGMP aggregation [RFC4605] will cause undesired
temporary black holing of IP multicast traffic. For example,
consider two hosts behind the same NAT. If one host is joining a
session at the same time another is leaving the session, and the NAT
were to merely relay the join and leave upstream, the session will be
terminated, and the join and leave announcements would not comply
with Section 5 of [RFC3376].
4.3. Any Source Multicast Transmitters
Any Source Multicast (ASM) uses the IP addresses in the 224/8 through
231/8, and 233/8 through 239/8 range [IANA-ALLOC].
When a host both receives an ASM stream and sends traffic into it,
using RTP [RFC3550], there is a potential problem if a NAT merely
followed the requirements of [RFC4787]. The problem is that RTP uses
the source transport address (source IP address and source UDP port)
and the Real-time Transport Protocol / RTP Control Protocol (RTP/
RTCP) SSRC value to identify session members. If a session member
sees the same SSRC arrive from a different transport address, that
session member will perform RTP collision detection (Section 8.2 of
[RFC3550]). If a NAT merely followed the requirements of [RFC4787]
and timed out a UDP session after 2 minutes of inactivity and RTCP
receiver reports are sent less often than every 2 minutes, RTP
collision detection would be performed by other session members
sharing the same SSRC, complicating diagnostic tools and potentially
interfering with jitter buffer algorithms. This situation can occur,
for example, with an IP multicast group of approximately 300 members
with a normal 50 Kbps audio RTP stream.
Source-Specific Multicast does not need this long timer because
application feedback reports are unicast (rather than IP multicast)
and identifiers, rather than IP addresses and UDP ports, are used to
identify a specific IP multicast receiver (e.g., [RTCPSSM].
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RFC 5135 NAT IP Multicast Requirements February 2008
REQ-14: If a host on the 'inside' interface of a NAT belongs to an
Any Source Multicast host group and the host sends a UDP
packet to the same group, the NAT SHOULD have a UDP mapping
timer of 60 minutes for that mapping.
a: This UDP mapping SHOULD be destroyed when the host
leaves that host group. The NAT is aware of this
through receipt of an IGMP message from the host.
b: If a NAT has exhausted its resources, the NAT MAY time
out that mapping before 60 minutes have elapsed, but
this is discouraged. Note that even in a situation with
resource exhaustion, a NAT is still required to follow
the minimum mapping duration of 2 minutes (REQ-5 of
[RFC4787]).
5. Requirements Summary
This section summarizes the requirements.
REQ-1: For IP multicast packets that are forwarded to a host(s) on
its 'inside' interface(s), a NAT MUST NOT modify the
destination IP address or destination port of the packets.
REQ-2: A NAT MUST forward IP multicast UDP datagrams from its
'outside' interface to multicast receivers on its 'inside'
interface(s).
REQ-3: A NAT SHOULD forward IP multicast non-UDP protocols (e.g.,
PGM [RFC3208], RSVP [RFC2205]) from its 'outside' interface
to IP multicast receivers on its 'inside' interface(s).
REQ-4: A NAT MUST modify the source IP address of packets that
arrive from an 'inside' interface towards the 'outside'
interface so that those packets use the NAT's 'outside' IP
address(es).
a: If the NAT also performs port translation (that is, it
is a NAPT), the NAT MUST also create a mapping to allow
responses to that IP multicast packet to be received by
the appropriate host. For Any Source Multicast, also
see Section 4.3.
b: To allow hosts to learn the NAT's 'outside' interface
address, the NAT MUST have "Endpoint-Independent
Mapping" behavior (REQ-1 of [RFC4787]), no matter if the
destination IP address is a unicast address or an IP
multicast address.
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c: If the NAT has multiple public IP addresses, the NAT
SHOULD have an address pooling behavior of "Paired" (as
described in Section 4.1 of [RFC4787]) for its IP
multicast mappings as well as for its unicast UDP
mappings. This allows a multicast source to discover
the NAT's public IP address using a unicast address
discovery mechanism (e.g., [ICE]) and communicate that
discovered IP address to a multicast receiver.
REQ-5: A NAT MUST forward IP multicast UDP datagrams from its
'inside' interface(s) to its 'outside' interface.
a: NATs that support the above requirement MUST also
provide a configuration option to disable this feature.
Otherwise, a multihomed network would cause duplicate
instances of the multicast data traffic on the public
network.
REQ-6: The NAT's default configuration MUST NOT forward
administratively scoped IP multicast traffic (239.0.0.0/8)
[RFC2365] from its 'inside' interface(s) to its 'outside'
interface.
REQ-7: The NAT MUST NOT forward Local Network Control Block
(224.0.0/24) [RFC3171] (also known as "link-local
multicast") traffic from its 'inside' interface(s) to its
'outside' interface.
REQ-8: A NAT MAY support IGMPv1 (although IGMPv1 is considered
obsolete).
REQ-9: A NAT MUST support IGMPv2.
REQ-10: A NAT SHOULD support IGMPv3.
REQ-11: If a NAT supports IGMPv1 and/or IGMPv2 (but not IGMPv3), the
NAT MAY simply receive IGMP membership reports on the
'inside' interface, NAT them, and relay the IGMP membership
report, and do the same function in the opposite direction
to the IGMP listeners. That is, the NAT does not need to do
any aggregation of IGMP messages.
a: If a NAT relays IGMPv1 or IGMPv2 messages in this
manner, it MUST NOT decrement the TTL of the IGMP
messages, as they are already sent with TTL=1.
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b: However, it is RECOMMENDED that such a NAT implement
IGMP/MLD Proxying [RFC4605], because IGMP aggregation
provides a useful optimization.
REQ-12: If a NAT supports IGMPv3, the NAT MUST implement IGMP/MLD
Proxying [RFC4605]. Such compliance causes the NAT to
aggregate the IGMPv3 membership reports and report only the
aggregated information upstream.
REQ-13: If a NAT supports IGMPv3, the NAT MUST implement Source-
Specific Multicast (SSM) for IP [RFC4607] and IGMPv3/MLDv2
for SSM [RFC4604].
REQ-14: If a host on the 'inside' interface of a NAT belongs to an
Any Source Multicast host group and the host sends a UDP
packet to the same group, the NAT SHOULD have a UDP mapping
timer of 60 minutes for that mapping.
a: This UDP mapping SHOULD be destroyed when the host
leaves that host group. The NAT is aware of this
through receipt of an IGMP message from the host.
b: If a NAT has exhausted its resources, the NAT MAY time
out that mapping before 60 minutes have elapsed, but
this is discouraged. Note that even in a situation with
resource exhaustion, a NAT is still required to follow
the minimum mapping duration of 2 minutes (REQ-5 of
[RFC4787]).
6. Security Considerations
The Security Considerations sections of IGMPv3 [RFC3376] and IGMP
Proxying [RFC4605] apply to a device complying with this document.
When a host is using RTP and participating in an Any Source Multicast
session, the host's periodic RTCP receiver reports cause the NAT to
create a mapping. When the group size is less than approximately
300, the RTCP reports are sent frequently enough that a NAT's mapping
will always be kept open. When the group size is larger than
approximately 300, the RTCP reports are sent less frequently. The
recommendation in Section 4.3 causes the NAT mapping to be kept open
for the duration of the host's participation in that IP multicast
session no matter the size of the multicast host or periodicity of
the host's RTCP transmissions.
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7. Acknowledgments
Thanks to Jari Arkko, Yiqun Cai, Stephen Casner, Remi Denis-Courmont,
Lars Eggert, Gorry Fairhurst, Alfred Hines, Prashant Jhingran, Bharat
Joshi, Francois Le Faucheur, Albert Manfredi, Marcus Maranhao, Bryan
McLaughlin, Chris Newman, Tim Polk, Pekka Savola, Mark Townsley,
Magnus Westerlund, and Stig Venaas for their assistance in writing
this document.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2236] Fenner, W., "Internet Group Management Protocol,
Version 2", RFC 2236, November 1997.
[RFC2365] Meyer, D., "Administratively Scoped IP Multicast",
BCP 23, RFC 2365, July 1998.
[RFC3171] Albanna, Z., Almeroth, K., Meyer, D., and M. Schipper,
"IANA Guidelines for IPv4 Multicast Address
Assignments", BCP 51, RFC 3171, August 2001.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol,
Version 3", RFC 3376, October 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC4604] Holbrook, H., Cain, B., and B. Haberman, "Using
Internet Group Management Protocol Version 3 (IGMPv3)
and Multicast Listener Discovery Protocol Version 2
(MLDv2) for Source-Specific Multicast", RFC 4604,
August 2006.
[RFC4605] Fenner, B., He, H., Haberman, B., and H. Sandick,
"Internet Group Management Protocol (IGMP) / Multicast
Listener Discovery (MLD)-Based Multicast Forwarding
("IGMP/MLD Proxying")", RFC 4605, August 2006.
[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast
for IP", RFC 4607, August 2006.
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RFC 5135 NAT IP Multicast Requirements February 2008
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP",
BCP 127, RFC 4787, January 2007.
8.2. Informative References
[IANA-ALLOC] Internet Assigned Numbers Authority, "Internet
Multicast Addresses",
<http://www.iana.org/assignments/multicast-addresses>.
[ICE] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", Work
in Progress, October 2007.
[RFC1112] Deering, S., "Host extensions for IP multicasting",
STD 5, RFC 1112, August 1989.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G.,
and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, February 1996.
[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version
1 Functional Specification", RFC 2205, September 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version
6 (IPv6) Specification", RFC 2460, December 1998.
[RFC3208] Speakman, T., Crowcroft, J., Gemmell, J., Farinacci,
D., Lin, S., Leshchiner, D., Luby, M., Montgomery, T.,
Rizzo, L., Tweedly, A., Bhaskar, N., Edmonstone, R.,
Sumanasekera, R., and L. Vicisano, "PGM Reliable
Transport Protocol Specification", RFC 3208,
December 2001.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP:
Session Description Protocol", RFC 4566, July 2006.
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601,
August 2006.
[RTCPSSM] Ott, J., Chesterfield, J., and E. Schooler, "RTCP
Extensions for Single-Source Multicast Sessions with
Unicast Feedback", Work in Progress, January 2008.
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Appendix A. Application Considerations
SSM requires listeners to know the SSM channel (S,G), which is
comprised of the IP source address (S) and the IP multicast group
(G). An SSM source needs to communicate its IP address in its SSM
session establishment message (e.g., in its Session Description
Protocol (SDP) [RFC4566]). When the SSM sender is behind a NAT and
the SSM receiver(s) are on the other side of that NAT, the SSM sender
will need to determine its IP source address relevant to the SSM
receivers; generally, this will be the 'outside' IP address of the
NAT. This 'outside' address needs to be included in the SSM session
establishment message (e.g., SDP) so that listeners on the 'outside'
of the NAT can receive the SSM channel.
If there are SSM listeners on both the 'outside' and 'inside' of the
NAT, it may be valuable to consider using ICE [ICE] in the session
advertisement; the full scope of the interaction between SSM and ICE
is beyond the scope of this document.
If multiple SSM sources on the 'inside' of a NAT choose the same
multicast group address, those sources are uniquely identifiable
because their IP addresses are unique. However, if their multicast
traffic is NATed and sent on the NAT's public interface, the traffic
from those individual sources is no longer uniquely identifiable.
This will cause problems for multicast receivers, which will see an
intermixing of traffic from those sources. Resolution of this issue
is left for future study. In the meantime, applications that source
SSM multicast traffic are encouraged to allow the user to modify the
multicast SSM address so that users can avoid this problem if that
application is placed behind a NAT.
A multicast source that wants its traffic to not traverse a router
(e.g., leave a home network) may find it useful to send traffic with
IP TTL=1. Both ASM and SSM sources may find this useful.
As many NATs use the same private address space (e.g.,
192.168.0.0/16, [RFC1918]), RTP stacks are encouraged to generate
CNAMEs properly (see end of Section 6.5.1 of [RFC3550].)
Wing & Eckert Best Current Practice [Page 14]
RFC 5135 NAT IP Multicast Requirements February 2008
Authors' Addresses
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA
EMail: dwing@cisco.com
Toerless Eckert
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA
EMail: eckert@cisco.com
Wing & Eckert Best Current Practice [Page 15]
RFC 5135 NAT IP Multicast Requirements February 2008
Full Copyright Statement
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Wing & Eckert Best Current Practice [Page 16]
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