BEHAVE Working Group D. Wing
Internet-Draft T. Eckert
Intended status: Best Current Cisco Systems, Inc.
Practice June 20, 2007
Expires: December 22, 2007
Multicast Requirements for a Network Address (and port) Translator (NAT)
draft-ietf-behave-multicast-07
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document specifies requirements for a Network Address (and port)
Translator (NAT) that supports any source multicast or source
specific IP multicast. A multicast-capable NAT device that adheres
to the requirements of this document can optimize the operation of
multicast applications that are generally unaware of multicast NAT
devices.
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Table of Contents
1. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Conventions Used in this Document . . . . . . . . . . . . . . 5
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. NATting of IP Multicast Packets . . . . . . . . . . . . . 5
4.2. IGMP Versions . . . . . . . . . . . . . . . . . . . . . . 6
4.2.1. IGMPv1 or IGMPv2 . . . . . . . . . . . . . . . . . . . 7
4.2.2. IGMPv3 . . . . . . . . . . . . . . . . . . . . . . . . 7
4.3. Any Source Multicast Transmitters . . . . . . . . . . . . 8
4.4. Transport Protocol Support . . . . . . . . . . . . . . . . 9
5. Requirements Summary . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . . 12
9.2. Informational References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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1. Problem Statement
In order for multicast applications to function well over NATs,
multicast UDP must work as seamlessly as unicast UDP. However, NATs
have little consistency in multicast operation which results in
inconsistent user experiences and failed multicast operation.
2. Introduction
This document describes the requirements of an IP multicast-capable
NAT. These requirements allow existing UDP any source IP multicast
[RFC1112] applications or source specific IP multicast [RFC4607]
applications to function without awareness of the multicast-capable
NAT device. Additionally, non-UDP IP multicast applications can be
received.
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').
Hosts on the 'inside' interface(s) of a NAT indicate their interest
in receiving a multicast flow by sending an IGMP message to their
local interface. A 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
multicast traffic to the NAT, which forwards it to those inside
segment(s) with host(s) that had previously sent IGMP messages for
that 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].
2.1. Background
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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 NATting 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
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 RFC4605:
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 updated IGMP state maintained by
the device and either those changes or timers trigger the sending
of IGMP messages. "Forwarding" of IGMP protocol messages may thus
only happen implicitly by implementation optimizations that create
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shortcuts in this machinery.
This specifically means that IGMP protocol packets sent by the NAT
device will always use IP address of th interface (inside or outside)
to which they are sent, but because those packets are logically
"sourced" and not "forwarded" , NAT does not have any impact into
this.
Adding NAT to IGMP proxying does change the processing of IP
multicast data packets forwarded across the IGMP proxying device as
described in the following sections. These changes do 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 being
sourced from a directly connected host to the WAN router, thus
eliminating all non-standard PIM-SM concerns/configurations described
in section 3.2 of [RFC4605].
3. Conventions 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].
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 which contain
hosts that wish to send or receive multicast traffic. The term
'outside' refers to the interface(s) the NAT forwards IGMP membership
messages to, and where the NAT routes multicast traffic that
originates from hosts on its 'inside' interface.
4. Requirements
4.1. NATting of IP Multicast Packets
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
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interfaces.
REQ-1: For IP multicast packets that are forward to a host(s) on its
inside interface(s), a NAT MUST NOT modify the destination IP
address or destination port of the packets.
Note: If a NAT were to violate this requirement and modify the
destination IP or port addresses, the NAT would also need to
modify session announcements (e.g., electronic program guides,
SAP) and session establishment and control (e.g., SIP, RTSP)
messages. Such modification is not considered a best practice.
Note: This behavior is required for UDP, but has a useful side-
effect that it permits other, non-UDP multicast protocols across a
NAT (e.g., PGM [RFC3208], RSVP [RFC2750]).
The following requirement is normal NAT behavior for unicast packets,
as described in [RFC4787], and provides support for multicast senders
behind the NAT:
REQ-2: 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 public 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 multicast packet to be received by the
appropriate host. For any source multicast, also see
Section 4.3. For source specific multicast, also see
Section 4.2.2.
b: To support learning their public transport 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 a multicast address.
4.2. IGMP Versions
REQ-3: A NAT MAY support IGMPv1 (although IGMPv1 is considered
obsolete).
REQ-4: A NAT MUST support IGMPv2.
REQ-5: A NAT SHOULD support IGMPv3.
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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-6: 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: 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 a 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
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 a multicast
data packet on the outside ('public') interface and forward the
multicast packet to inside ('internal') interfaces 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:
REQ-7: If a NAT supports IGMPv3, the NAT MUST:
a: implement IGMP/MLD Proxying [RFC4605]. Such compliance
causes the NAT to aggregate the IGMPv3 membership reports
and report only the aggregated information upstream, and;
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b: support any source multicast listeners and transmitters
(Section 4.3), and;
c: support source specific multicast listeners and
transmitters ([RFC4604], section 4.2 of [RFC4607]).
Failure to implement IGMP aggregation ([RFC4605]) will cause
undesired temporary blackholing of 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].
Primarily due to NATs functioning as IGMP proxies with multiple
receivers behind the NAT, multicast applications are encouraged to
use identifiers, rather than IP addresses and UDP ports, to identify
specific multicast receivers (e.g., [I-D.ietf-avt-rtcpssm] encourages
SSM applications to not rely exclusively on transport addresses for
collision detection). As compared to any source multicast, the use
of such receiver identifiers removes the need for the NAT to have
long mapping timers; instead, the timers in [RFC4787] are used when a
host transmits to a source specific IP multicast address.
Note: SSM requires listeners to know the SSM channel (S,G), which
is comprised of the IP source address (S) and the multicast group
(G). An SSM sender needs to communicate its IP address in its SSM
session establishment message (e.g., SDP). 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 public
IP address of the NAT. This public address needs to be included
in the SSM session establishment message (e.g., SDP) so that
listeners on the public side of the NAT can receive the SSM
channel.
If there are SSM listeners on both the public and private side of
the NAT, it may be valuable to consider using ICE
[I-D.ietf-mmusic-ice] in the session advertisement; the full scope
of the interaction between SSM and ICE is beyond the scope of this
document
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,
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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 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 a multicast group of
approximately 300 members with a normal 50kbps audio RTP stream.
REQ-8: 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]).
4.4. Transport Protocol Support
REQ-9: A NAT MUST support transport of multicast UDP with both
multicast receivers and with multicast transmitters on the
'inside' interface(s) of the NAT.
REQ-10: A NAT SHOULD support transport of multicast non-UDP
protocols (e.g., PGM [RFC3208], RSVP [RFC2750]) with
multicast receivers on the 'inside' interface(s) of the NAT.
5. Requirements Summary
This section summarizes the requirements; if there is a difference in
this summary and the text in the main body of the document, the main
body takes precedence.
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REQ-1: For IP multicast packets that are forward 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 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 public 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 multicast packet to be received by the
appropriate host. For any source multicast, also see
Section 4.3. For source specific multicast, also see
Section 4.2.2.
b: To support learning their public transport 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 a multicast address.
REQ-3: A NAT MAY support IGMPv1 (although IGMPv1 is considered
obsolete).
REQ-4: A NAT MUST support IGMPv2.
REQ-5: A NAT SHOULD support IGMPv3.
REQ-6: 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: However, it is RECOMMENDED that such a NAT implement
IGMP/MLD Proxying [RFC4605], because IGMP aggregation
provides a useful optimization.
REQ-7: If a NAT supports IGMPv3, the NAT MUST:
a: implement [RFC4605]. Such compliance causes the NAT to
aggregate the IGMPv3 membership reports and report only
the aggregated information upstream, and;
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b: support any source multicast listeners and transmitters
(Section 4.3), and;
c: support source specific multicast listeners and
transmitters ([RFC4604], section 4.2 of [RFC4607]).
REQ-8: 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]).
REQ-9: A NAT MUST support transport of multicast UDP with both
multicast receivers and multicast transmitters on the
'inside' interface(s) of the NAT.
REQ-10: A NAT SHOULD support transport of multicast non-UDP
protocols (e.g., PGM [RFC3208], RSVP [RFC2750]) with
multicast receivers on the 'inside' interface(s) of the NAT.
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 multicast
session no matter the size of the multicast host or periodicity of
the host's RTCP transmissions.
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7. IANA Considerations
This document does not require any IANA registrations.
8. Acknowledgments
Thanks to Yiqun Cai, Stephen Casner, Remi Denis-Courmont, Alfred
Hines, Prashant Jhingran, Albert Manfredi, Marcus Maranhao, Bryan
McLaughlin, Pekka Savola, and Magnus Westerlund for their assistance
in writing this document.
9. References
9.1. 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.
[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.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
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9.2. Informational References
[I-D.ietf-avt-rtcpssm]
Chesterfield, J., "RTCP Extensions for Single-Source
Multicast Sessions with Unicast Feedback",
draft-ietf-avt-rtcpssm-13 (work in progress), March 2007.
[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-16 (work in progress), June 2007.
[IANA-ALLOC]
Internet Assigned Numbers Authority, "Internet Multicast
Addresses",
<http://www.iana.org/assignments/multicast-addresses>.
[RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, August 1989.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2750] Herzog, S., "RSVP Extensions for Policy Control",
RFC 2750, January 2000.
[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.
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", RFC 4607, August 2006.
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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
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