draft-ietf-behave-nat-udp-03.txt   draft-ietf-behave-nat-udp-04.txt 
BEHAVE F. Audet, Ed. BEHAVE F. Audet, Ed.
Internet-Draft Nortel Networks Internet-Draft Nortel Networks
Expires: January 16, 2006 C. Jennings Expires: March 10, 2006 C. Jennings
Cisco Systems Cisco Systems
July 15, 2005 September 6, 2005
NAT Behavioral Requirements for Unicast UDP NAT Behavioral Requirements for Unicast UDP
draft-ietf-behave-nat-udp-03 draft-ietf-behave-nat-udp-04
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2005).
Abstract Abstract
This document defines basic terminology for describing different This document defines basic terminology for describing different
types of NAT behavior when handling Unicast UDP and also defines a types of NAT behavior when handling Unicast UDP and also defines a
set of requirements that would allow many applications, such as set of requirements that would allow many applications, such as
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Developing NATs that meet this set of requirements will greatly Developing NATs that meet this set of requirements will greatly
increase the likelihood that these applications will function increase the likelihood that these applications will function
properly. properly.
Table of Contents Table of Contents
1. Applicability Statement . . . . . . . . . . . . . . . . . . . 3 1. Applicability Statement . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Network Address and Port Translation Behavior . . . . . . . . 5 4. Network Address and Port Translation Behavior . . . . . . . . 5
4.1 Address and Port Mapping . . . . . . . . . . . . . . . . . 5 4.1. Address and Port Mapping . . . . . . . . . . . . . . . . . 5
4.2 Port Assignment . . . . . . . . . . . . . . . . . . . . . 8 4.2. Port Assignment . . . . . . . . . . . . . . . . . . . . . 8
4.2.1 Port Assignment Behavior . . . . . . . . . . . . . . . 8 4.2.1. Port Assignment Behavior . . . . . . . . . . . . . . . 8
4.2.2 Port Parity . . . . . . . . . . . . . . . . . . . . . 10 4.2.2. Port Parity . . . . . . . . . . . . . . . . . . . . . 10
4.2.3 Port Contiguity . . . . . . . . . . . . . . . . . . . 10 4.2.3. Port Contiguity . . . . . . . . . . . . . . . . . . . 10
4.3 Mapping Refresh . . . . . . . . . . . . . . . . . . . . . 11 4.3. Mapping Refresh . . . . . . . . . . . . . . . . . . . . . 11
5. Filtering Behavior . . . . . . . . . . . . . . . . . . . . . . 12 4.4. Conflicting Internal and External IP Address Spaces . . . 12
5.1 Filtering of Unsolicited Packets . . . . . . . . . . . . . 12 5. Filtering Behavior . . . . . . . . . . . . . . . . . . . . . . 13
5.2 NAT Filter Refresh . . . . . . . . . . . . . . . . . . . . 14 6. Hairpinning Behavior . . . . . . . . . . . . . . . . . . . . . 15
6. Hairpinning Behavior . . . . . . . . . . . . . . . . . . . . . 14 7. Application Level Gateways . . . . . . . . . . . . . . . . . . 16
7. Application Level Gateways . . . . . . . . . . . . . . . . . . 15 8. Deterministic Properties . . . . . . . . . . . . . . . . . . . 17
8. Deterministic Properties . . . . . . . . . . . . . . . . . . . 15 9. ICMP Destination Unreachable Behavior . . . . . . . . . . . . 18
9. ICMP Destination Unreachable Behavior . . . . . . . . . . . . 16 10. Fragmentation of Outgoing Packets . . . . . . . . . . . . . . 19
10. Fragmentation of Outgoing Packets . . . . . . . . . . . . . 17 10.1. Smaller Adjacent MTU . . . . . . . . . . . . . . . . . . . 19
10.1 Smaller Adjacent MTU . . . . . . . . . . . . . . . . . . . 17 10.2. Smaller Network MTU . . . . . . . . . . . . . . . . . . . 19
10.2 Smaller Network MTU . . . . . . . . . . . . . . . . . . . 18 11. Receiving Fragmented Packets . . . . . . . . . . . . . . . . . 20
11. Receiving Fragmented Packets . . . . . . . . . . . . . . . . 18 12. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 21
12. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 19 13. Security Considerations . . . . . . . . . . . . . . . . . . . 22
13. Security Considerations . . . . . . . . . . . . . . . . . . 20 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . 21 15. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 24
15. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 21 16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 22 17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 23 17.1. Normative References . . . . . . . . . . . . . . . . . . . 25
17.1 Normative References . . . . . . . . . . . . . . . . . . . 23 17.2. Informational References . . . . . . . . . . . . . . . . . 25
17.2 Informational References . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 24 Intellectual Property and Copyright Statements . . . . . . . . . . 28
Intellectual Property and Copyright Statements . . . . . . . . 25
1. Applicability Statement 1. Applicability Statement
The purpose of this specification is to define a set of requirements The purpose of this specification is to define a set of requirements
for NATs that would allow many applications, such as multimedia for NATs that would allow many applications, such as multimedia
communications or on-line gaming, to work consistently. Developing communications or on-line gaming, to work consistently. Developing
NATs that meet this set of requirements will greatly increase the NATs that meet this set of requirements will greatly increase the
likelihood that these applications will function properly. likelihood that these applications will function properly.
The requirements of this specification apply to Traditional NATs as The requirements of this specification apply to Traditional NATs as
described in RFC 2663 [3]. described in RFC 2663 [8].
This document is meant to cover NATs of any size, from small This document is meant to cover NATs of any size, from small
residential NATs to large Enterprise NATs. However, it should be residential NATs to large Enterprise NATs. However, it should be
understood that Enterprise NATs normally provide much more than just understood that Enterprise NATs normally provide much more than just
NAT capabilities: for example, they typically provide firewall NAT capabilities: for example, they typically provide firewall
functionalities. Firewalls are specifically out-of-scope for this functionalities. Firewalls are specifically out-of-scope for this
specification; however, this specification does cover the inherent specification; however, this specification does cover the inherent
filtering aspects of NATs. filtering aspects of NATs.
Approaches using directly signaled control of middle boxes such as Approaches using directly signaled control of middle boxes such as
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This document only covers the UDP Unicast aspects of NAT traversal This document only covers the UDP Unicast aspects of NAT traversal
and does not cover TCP, IPSEC, or other protocols. Since the and does not cover TCP, IPSEC, or other protocols. Since the
document is for UDP only, packet inspection above the UDP layer document is for UDP only, packet inspection above the UDP layer
(including RTP) is also out-of-scope. (including RTP) is also out-of-scope.
2. Introduction 2. Introduction
Network Address Translators (NATs) are well known to cause very Network Address Translators (NATs) are well known to cause very
significant problems with applications that carry IP addresses in the significant problems with applications that carry IP addresses in the
payload RFC 3027 [5]. Applications that suffer from this problem payload RFC 3027 [10]. Applications that suffer from this problem
include Voice Over IP and Multimedia Over IP (e.g., SIP [6] and H.323 include Voice Over IP and Multimedia Over IP (e.g., SIP [12] and
[20]), as well as online gaming. H.323 [20]), as well as online gaming.
Many techniques are used to attempt to make realtime multimedia Many techniques are used to attempt to make realtime multimedia
applications, online games, and other applications work across NATs. applications, online games, and other applications work across NATs.
Application Level Gateways [3] are one such mechanism. STUN [7] Application Level Gateways [8] are one such mechanism. STUN [17]
describes a UNilateral Self-Address Translation (UNSAF) mechanism describes a UNilateral Self-Address Translation (UNSAF) mechanism
[2]. UDP Relays have also been used to enable applications across [15]. UDP Relays have also been used to enable applications across
NATs, but these are generally seen as a solution of last resort. ICE NATs, but these are generally seen as a solution of last resort. ICE
[16] describes a methodology for using many of these techniques and [18] describes a methodology for using many of these techniques and
avoiding a UDP Relay unless the type of NAT is such that it forces avoiding a UDP Relay unless the type of NAT is such that it forces
the use of such a UDP Relay. This specification defines requirements the use of such a UDP Relay. This specification defines requirements
for improving NATs. Meeting these requirements ensures that for improving NATs. Meeting these requirements ensures that
applications will not be forced to use UDP media relay. applications will not be forced to use UDP media relay.
As pointed out in UNSAF [2], "From observations of deployed networks, As pointed out in UNSAF [15], "From observations of deployed
it is clear that different NAT boxes' implementation vary widely in networks, it is clear that different NAT box implementations vary
terms of how they handle different traffic and addressing cases." widely in terms of how they handle different traffic and addressing
This wide degree of variability is one factor in the overall cases." This wide degree of variability is one factor in the overall
brittleness introduced by NATs and makes it extremely difficult to brittleness introduced by NATs and makes it extremely difficult to
predict how any given protocol will behave on a network traversing predict how any given protocol will behave on a network traversing
NAT. Discussions with many of the major NAT vendors have made it NAT. Discussions with many of the major NAT vendors have made it
clear that they would prefer to deploy NATs that were deterministic clear that they would prefer to deploy NATs that were deterministic
and caused the least harm to applications while still meeting the and caused the least harm to applications while still meeting the
requirements that caused their customers to deploy NATs in the first requirements that caused their customers to deploy NATs in the first
place. The problem NAT vendors face is that they are not sure how place. The problem NAT vendors face is that they are not sure how
best to do that or how to document how their NATs behave. best to do that or how to document how their NATs behave.
The goals of this document are to define a set of common terminology The goals of this document are to define a set of common terminology
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providers of services that have NAT traversal issues to make providers of services that have NAT traversal issues to make
statements about where their applications will work and where they statements about where their applications will work and where they
will not, as well as to specify their own NAT requirements. will not, as well as to specify their own NAT requirements.
3. Terminology 3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1]. document are to be interpreted as described in RFC 2119 [1].
Readers are urged to refer to RFC 2263 [3] for information on NAT Readers are urged to refer to RFC 2263 [8] for information on NAT
taxonomy and terminology. Traditional NAT is the most common type of taxonomy and terminology. Traditional NAT is the most common type of
NAT device deployed. Readers may refer to RFC 3022 [4] for detailed NAT device deployed. Readers may refer to RFC 3022 [9] for detailed
information on traditional NAT. Traditional NAT has two main information on traditional NAT. Traditional NAT has two main
varieties - Basic NAT and Network Address/Port Translator (NAPT). varieties - Basic NAT and Network Address/Port Translator (NAPT).
NAPT is by far the most commonly deployed NAT device. NAPT allows NAPT is by far the most commonly deployed NAT device. NAPT allows
multiple internal hosts to share a single public IP address multiple internal hosts to share a single public IP address
simultaneously. When an internal host opens an outgoing TCP or UDP simultaneously. When an internal host opens an outgoing TCP or UDP
session through a NAPT, the NAPT assigns the session a public IP session through a NAPT, the NAPT assigns the session a public IP
address and port number so that subsequent response packets from the address and port number, so that subsequent response packets from the
external endpoint can be received by the NAPT, translated, and external endpoint can be received by the NAPT, translated, and
forwarded to the internal host. The effect is that the NAPT forwarded to the internal host. The effect is that the NAPT
establishes a NAT session to translate the (private IP address, establishes a NAT session to translate the (private IP address,
private port number) tuple to (public IP address, public port number) private port number) tuple to (public IP address, public port number)
tuple and vice versa for the duration of the session. An issue of tuple and vice versa for the duration of the session. An issue of
relevance to peer-to-peer applications is how the NAT behaves when an relevance to peer-to-peer applications is how the NAT behaves when an
internal host initiates multiple simultaneous sessions from a single internal host initiates multiple simultaneous sessions from a single
(private IP, private port) endpoint to multiple distinct endpoints on (private IP, private port) endpoint to multiple distinct endpoints on
the external network. In this specification, the term "NAT" refers the external network. In this specification, the term "NAT" refers
to both "Basic NAT" and "Network Address/Port Translator (NAPT)". to both "Basic NAT" and "Network Address/Port Translator (NAPT)".
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This document uses the term "session" as defined in RFC 2663: "TCP/ This document uses the term "session" as defined in RFC 2663: "TCP/
UDP sessions are uniquely identified by the tuple of (source IP UDP sessions are uniquely identified by the tuple of (source IP
address, source TCP/UDP ports, target IP address, target TCP/UDP address, source TCP/UDP ports, target IP address, target TCP/UDP
Port)." Port)."
This document uses the term "address and port mapping" as the This document uses the term "address and port mapping" as the
translation between an external address and port and an internal translation between an external address and port and an internal
address and port. Note that this is not the same as an "address address and port. Note that this is not the same as an "address
binding" as defined in RFC 2663. binding" as defined in RFC 2663.
RFC 3489 used the terms "Full Cone", "Restricted Cone", "Port RFC 3489 [8] used the terms "Full Cone", "Restricted Cone", "Port
Restricted Cone" and "Symmetric" to refer to different variations of Restricted Cone" and "Symmetric" to refer to different variations of
NATs applicable to UDP only. Unfortunately, this terminology has NATs applicable to UDP only. Unfortunately, this terminology has
been the source of much confusion as it has proven inadequate at been the source of much confusion as it has proven inadequate at
describing real-life NAT behavior. This specification therefore describing real-life NAT behavior. This specification therefore
refers to specific individual NAT behaviors instead of using the refers to specific individual NAT behaviors instead of using the
Cone/Symmetric terminology. Cone/Symmetric terminology.
4. Network Address and Port Translation Behavior 4. Network Address and Port Translation Behavior
This section describes the various NAT behaviors applicable to NATs. This section describes the various NAT behaviors applicable to NATs.
4.1 Address and Port Mapping 4.1. Address and Port Mapping
When an internal endpoint opens an outgoing session through a NAT, When an internal endpoint opens an outgoing session through a NAT,
the NAT assigns the session an external IP address and port number so the NAT assigns the session an external IP address and port number so
that subsequent response packets from the external endpoint can be that subsequent response packets from the external endpoint can be
received by the NAT, translated, and forwarded to the internal received by the NAT, translated, and forwarded to the internal
endpoint. This is a mapping between an internal IP address and port endpoint. This is a mapping between an internal IP address and port
IP:port and external IP:port tuple. It establishes the translation IP:port and external IP:port tuple. It establishes the translation
that will be performed by the NAT for the duration of the session. that will be performed by the NAT for the duration of the session.
For many applications, it is important to distinguish the behavior of For many applications, it is important to distinguish the behavior of
the NAT when there are multiple simultaneous sessions established to the NAT when there are multiple simultaneous sessions established to
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It is important to note that these three possible choices make no It is important to note that these three possible choices make no
difference to the security properties of the NAT. The security difference to the security properties of the NAT. The security
properties are fully determined by which packets the NAT allows in properties are fully determined by which packets the NAT allows in
and which it does not. This is determined by the filtering behavior and which it does not. This is determined by the filtering behavior
in the filtering portions of the NAT. in the filtering portions of the NAT.
REQ-1: A NAT MUST have an "External NAT mapping is endpoint REQ-1: A NAT MUST have an "External NAT mapping is endpoint
independent" behavior. independent" behavior.
Justification for REQ-1: In order for UNSAF methods to work, REQ-1 Justification: In order for UNSAF methods to work, REQ-1 needs to be
needs to be met. Failure to meet REQ-1 will force the use of a met. Failure to meet REQ-1 will force the use of a Media Relay
Media Relay which is very often impractical. which is very often impractical.
Some NATs are capable of assigning IP addresses from a pool of IP Some NATs are capable of assigning IP addresses from a pool of IP
addresses on the external side of the NAT, as opposed to just a addresses on the external side of the NAT, as opposed to just a
single IP address. This is especially common with larger NATs. Some single IP address. This is especially common with larger NATs. Some
NATs use the external IP address mapping in an arbitrary fashion NATs use the external IP address mapping in an arbitrary fashion
(i.e. randomly): one internal IP address could have multiple external (i.e. randomly): one internal IP address could have multiple external
IP address mappings active at the same time for different sessions. IP address mappings active at the same time for different sessions.
These NATs have an "IP address pooling" behavior of "Arbitrary". These NATs have an "IP address pooling" behavior of "Arbitrary".
Some large Enterprise NATs use an IP address pooling behavior of Some large Enterprise NATs use an IP address pooling behavior of
"Arbitrary" as a means of hiding the IP address assigned to specific "Arbitrary" as a means of hiding the IP address assigned to specific
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with the same internal IP address. These NATs have an "IP address with the same internal IP address. These NATs have an "IP address
pooling" behavior of "Paired." NATs that use an "IP address pooling" pooling" behavior of "Paired." NATs that use an "IP address pooling"
behavior of "arbitrary" can cause issues for applications that use behavior of "arbitrary" can cause issues for applications that use
multiple ports from the same endpoint but do not negotiate IP multiple ports from the same endpoint but do not negotiate IP
addresses individually (e.g., some applications using RTP and RTCP). addresses individually (e.g., some applications using RTP and RTCP).
REQ-2: It is RECOMMENDED that a NAT have an "IP address pooling" REQ-2: It is RECOMMENDED that a NAT have an "IP address pooling"
behavior of "Paired". Note that this requirement is not behavior of "Paired". Note that this requirement is not
applicable to NATs that do not support IP address pooling. applicable to NATs that do not support IP address pooling.
Justification for REQ-2: This will allow applications that use Justification: This will allow applications that use multiple ports
multiple ports originating from the same internal IP address to originating from the same internal IP address to also have the
also have the same external IP address. This is to avoid breaking same external IP address. This is to avoid breaking peer-to-peer
peer-to-peer applications which are not capable of negotiating the applications that are not capable of negotiating the IP address
IP address for RTP and the IP address for RTCP separately. As for RTP and the IP address for RTCP separately. As such it is
such it is envisioned that this requirement will become less envisioned that this requirement will become less important as
important as applications become NAT-friendlier with time. The applications become NAT-friendlier with time. The main reason why
main reason why this requirement is here is that in a peer-to-peer this requirement is here is that in a peer-to-peer application,
application, you are subject to the other peer's mistake. In you are subject to the other peer's mistake. In particular, in
particular, in the context of SIP, if my application supports the the context of SIP, if my application supports the extensions
extensions defined in RFC 3605 [9] for indicating RTP and RTCP defined in RFC 3605 [16] for indicating RTP and RTCP addresses and
addresses and ports separately, but the other peer does not, there ports separately, but the other peer does not, there may still be
may still be breakage in the form of letting the stream loose the breakage in the form of the stream losing RTP packets. This
RTP packets. This requirement will avoid the loss of RTP in this requirement will avoid the loss of RTP in this context, although
context, although the loss of RTCP may be inevitable in this the loss of RTCP may be inevitable in this particular example. It
particular example. It is also worth noting that RFC 3605 is is also worth noting that RFC 3605 is unfortunately not a
unfortunately not a mandatory part of SIP (RFC 3261). This mandatory part of SIP (RFC 3261). This requirement will therefore
requirement will therefore address a particularly nasty problem address a particularly nasty problem that will prevail for a
that will prevail for a significant amount of time. significant period of time.
4.2 Port Assignment 4.2. Port Assignment
4.2.1 Port Assignment Behavior 4.2.1. Port Assignment Behavior
This section uses the following diagram for reference. This section uses the following diagram for reference.
E E
+-------+ +-------+ x +-------+ +-------+ x
| Y1 | | Y2 | t | Y1 | | Y2 | t
+---+---+ +---+---+ e +---+---+ +---+---+ e
| Y1:y1 Y2:y2 | r | Y1:y1 Y2:y2 | r
+---------+ +---------+ n +---------+ +---------+ n
| | a | | a
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X':x2') is referred to as "No port preservation". X':x2') is referred to as "No port preservation".
Some NATs use "Port overloading", i.e. they always use port Some NATs use "Port overloading", i.e. they always use port
preservation even in the case of collision (i.e., X'=X1'=X2' and preservation even in the case of collision (i.e., X'=X1'=X2' and
x=x1=x2=x1'=x2', or a mapping of X1:x to X':x, and X2:x to X':x). x=x1=x2=x1'=x2', or a mapping of X1:x to X':x, and X2:x to X':x).
These NATs rely on the source of the response from the external These NATs rely on the source of the response from the external
endpoint (Y1:y1, Y2:y2) to forward a packet to the proper internal endpoint (Y1:y1, Y2:y2) to forward a packet to the proper internal
endpoint (X1 or X2). Port overloading fails if the two internal endpoint (X1 or X2). Port overloading fails if the two internal
endpoints are establishing sessions to the same external destination. endpoints are establishing sessions to the same external destination.
Most applications fail in some cases with "Port Overloading". It is Most applications fail in some cases with "Port overloading". It is
clear that "Port Overloading" behavior will result in many problems. clear that "Port overloading" behavior will result in many problems.
For example it will fail if two internal endpoints try to reach the For example it will fail if two internal endpoints try to reach the
same external destination, e.g., a server used by both endpoints such same external destination, e.g., a server used by both endpoints such
as a SIP proxy, or a web server, etc.) as a SIP proxy, or a web server, etc.
When NATs do allocate a new source port, there is the issue of which When NATs do allocate a new source port, there is the issue of which
IANA-defined range of port to choose. The ranges are "well-known" IANA-defined range of port to choose. The ranges are "well-known"
from 0 to 1023, "registered" from 1024 to 49151, and "dynamic/ from 0 to 1023, "registered" from 1024 to 49151, and "dynamic/
private" from 49152 through 65535. For most protocols, these are private" from 49152 through 65535. For most protocols, these are
destination ports and not source ports, so mapping a source port to a destination ports and not source ports, so mapping a source port to a
source port that is already registered is unlikely to have any bad source port that is already registered is unlikely to have any bad
effects. Some NATs may choose to use only the ports in the dynamic effects. Some NATs may choose to use only the ports in the dynamic
range; the only down side of this practice is that it limits the range; the only down side of this practice is that it limits the
number of ports available. Other NAT devices may use everything but number of ports available. Other NAT devices may use everything but
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Other NATs preserve the port range if it is in the well-known range. Other NATs preserve the port range if it is in the well-known range.
It should be noted that port 0 is reserved and must not be used. It should be noted that port 0 is reserved and must not be used.
REQ-3: A NAT MUST NOT have a "Port assignment" behavior of "Port REQ-3: A NAT MUST NOT have a "Port assignment" behavior of "Port
overloading". overloading".
a) If the host's source port was in the range 1-1023, it is a) If the host's source port was in the range 1-1023, it is
RECOMMENDED the NAT's source port be in the same range. If the RECOMMENDED the NAT's source port be in the same range. If the
host's source port was in the range 1024-65535, it is host's source port was in the range 1024-65535, it is
RECOMMENDED that the NAT's source port be in that range. RECOMMENDED that the NAT's source port be in that range.
Justification for REQ-3: This requirement must be met in order to Justification: This requirement must be met in order to enable two
enable two applications on the internal side of the NAT both to applications on the internal side of the NAT both to use the same
use the same port to try to communicate with the same destination. port to try to communicate with the same destination. NATs that
NATs that implement port preservation have to deal with conflicts implement port preservation have to deal with conflicts on ports,
on ports, and the multiple code paths this introduces often result and the multiple code paths this introduces often result in
in nondeterministic behavior. However, it should be understood nondeterministic behavior. However, it should be understood that
that when a port is randomly assigned, it may just randomly happen when a port is randomly assigned, it may just randomly happen to
to be assigned the same port. Applications must therefore be able be assigned the same port. Applications must therefore be able to
to deal with both port preservation, and no port preservation. deal with both port preservation and no port preservation.
a) Certain applications expect the source UDP port to be in the a) Certain applications expect the source UDP port to be in the
well-known range. See RFC 2623 for an example. well-known range. See RFC 2623 for an example.
4.2.2 Port Parity 4.2.2. Port Parity
Some NATs preserve the parity of the UDP port, i.e., an even port Some NATs preserve the parity of the UDP port, i.e., an even port
will be mapped to an even port, and an odd port will be mapped to an will be mapped to an even port, and an odd port will be mapped to an
odd port. This behavior respects the RFC 3550 [8] rule that RTP use odd port. This behavior respects the RFC 3550 [14] rule that RTP use
even ports, and RTCP use odd ports. RFC 3550 allows any port numbers even ports, and RTCP use odd ports. RFC 3550 allows any port numbers
to be used for RTP and RTCP if the two numbers are specified to be used for RTP and RTCP if the two numbers are specified
separately, for example using RFC 3605 [9]. However, some separately, for example using RFC 3605 [16]. However, some
implementations do not include RFC 3605 and do not recognize when the implementations do not include RFC 3605 and do not recognize when the
peer has specified the RTCP port separately using RFC 3605. If such peer has specified the RTCP port separately using RFC 3605. If such
an implementation receives an odd RTP port number from the peer an implementation receives an odd RTP port number from the peer
(perhaps after having been translated by a NAT), and then follows the (perhaps after having been translated by a NAT), and then follows the
RFC 3550 rule to change the RTP port to the next lower even number, RFC 3550 rule to change the RTP port to the next lower even number,
this would obviously result in the loss of RTP. NAT-friendly this would obviously result in the loss of RTP. NAT-friendly
application aspects are outside the scope of this document. It is application aspects are outside the scope of this document. It is
expected that this issue will fade away with time, as implementations expected that this issue will fade away with time, as implementations
improve. Preserving the port parity allows for supporting improve. Preserving the port parity allows for supporting
communication with peers that do not support explicit specification communication with peers that do not support explicit specification
of both RTP and RTCP port numbers. of both RTP and RTCP port numbers.
REQ-4: It is RECOMMENDED that a NAT have a "Port parity preservation" REQ-4: It is RECOMMENDED that a NAT have a "Port parity preservation"
behavior of "Yes". behavior of "Yes".
Justification for REQ-4: This is to avoid breaking peer-to-peer Justification: This is to avoid breaking peer-to-peer applications
applications which do not explicitly and separately specify RTP which do not explicitly and separately specify RTP and RTCP port
and RTCP port numbers and which follow the RFC 3550 rule to numbers and which follow the RFC 3550 rule to decrement an odd RTP
decrement an odd RTP port to make it even. The same port to make it even. The same considerations as per the IP
considerations as per the IP address pooling requirement apply. address pooling requirement apply.
4.2.3 Port Contiguity 4.2.3. Port Contiguity
Some NATs attempt to preserve the port contiguity rule of RTCP=RTP+1. Some NATs attempt to preserve the port contiguity rule of RTCP=RTP+1.
These NATs do things like sequential assignment or port reservation. These NATs do things like sequential assignment or port reservation.
Sequential port assignment assumes that the application will open a Sequential port assignment assumes that the application will open a
mapping for RTP first and then open a mapping for RTCP. It is not mapping for RTP first and then open a mapping for RTCP. It is not
practical to enforce this requirement on all applications. practical to enforce this requirement on all applications.
Furthermore, there is a glaring problem if many applications (or Furthermore, there is a glaring problem if many applications (or
endpoints) are trying to open mapping simultaneously. Port endpoints) are trying to open mapping simultaneously. Port
preservation is also problematic since it is wasteful, especially preservation is also problematic since it is wasteful, especially
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those reasons, it would be too complex to attempt to preserve the those reasons, it would be too complex to attempt to preserve the
contiguity rule by suggesting specific NAT behavior, and it would contiguity rule by suggesting specific NAT behavior, and it would
certainly break the deterministic behavior rule. certainly break the deterministic behavior rule.
In order to support both RTP and RTCP, it will therefore be necessary In order to support both RTP and RTCP, it will therefore be necessary
that applications follow rules to negotiate RTP and RTCP separately, that applications follow rules to negotiate RTP and RTCP separately,
and account for the very real possibility that the RTCP=RTP+1 rule and account for the very real possibility that the RTCP=RTP+1 rule
will be broken. As this is an application requirement, it is outside will be broken. As this is an application requirement, it is outside
of the scope of this document. of the scope of this document.
4.3 Mapping Refresh 4.3. Mapping Refresh
NAT mapping timeout implementations vary but include the timer's NAT mapping timeout implementations vary but include the timer's
value and the way the mapping timer is refreshed to keep the mapping value and the way the mapping timer is refreshed to keep the mapping
alive. alive.
The mapping timer is defined as the time a mapping will stay active The mapping timer is defined as the time a mapping will stay active
without packets traversing the NAT. There is great variation in the without packets traversing the NAT. There is great variation in the
values used by different NATs. values used by different NATs.
REQ-5: A NAT UDP mapping timer MUST NOT expire in less than 2 REQ-5: A NAT UDP mapping timer MUST NOT expire in less than 2
minutes. minutes.
a) The value of the NAT UDP mapping timer MAY be configurable. a) The value of the NAT UDP mapping timer MAY be configurable.
b) A default value of 5 minutes for the NAT UDP mapping timer is b) A default value of 5 minutes for the NAT UDP mapping timer is
RECOMMENDED. RECOMMENDED.
Justification for REQ-5: This requirement is to ensure that the Justification: This requirement is to ensure that the timeout is long
timeout is long enough to avoid too frequent timer refresh enough to avoid too frequent timer refresh packets.
packets.
a) Configuration is desirable for adapting to specific networks a) Configuration is desirable for adapting to specific networks
and troubleshooting. and troubleshooting.
b) This default is to avoid too frequent timer refresh packets. b) This default is to avoid too frequent timer refresh packets.
Some NATs keep the mapping active (i.e., refresh the timer value) Some NATs keep the mapping active (i.e., refresh the timer value)
when a packet goes from the internal side of the NAT to the external when a packet goes from the internal side of the NAT to the external
side of the NAT. This is referred to as having a NAT Outbound side of the NAT. This is referred to as having a NAT Outbound
refresh behavior of "True". refresh behavior of "True".
Some NATs keep the mapping active when a packet goes from the Some NATs keep the mapping active when a packet goes from the
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referred to as having a NAT Inbound Refresh Behavior of "True". referred to as having a NAT Inbound Refresh Behavior of "True".
Some NATs keep the mapping active on both, in which case both Some NATs keep the mapping active on both, in which case both
properties are "True". properties are "True".
REQ-6: The NAT mapping Refresh Direction MUST have a "NAT Outbound REQ-6: The NAT mapping Refresh Direction MUST have a "NAT Outbound
refresh behavior" of "True". refresh behavior" of "True".
a) The NAT mapping Refresh Direction MAY have a "NAT Inbound a) The NAT mapping Refresh Direction MAY have a "NAT Inbound
refresh behavior" of "True". refresh behavior" of "True".
Justification for REQ-6: Outbound refresh is necessary for allowing Justification: Outbound refresh is necessary for allowing the client
the client to keep the mapping alive. to keep the mapping alive.
a) Inbound refresh may be useful for applications with no outgoing a) Inbound refresh may be useful for applications with no outgoing
UDP traffic. However, allowing inbound refresh may allow an UDP traffic. However, allowing inbound refresh may allow an
application to keep a mapping alive indefinitely. This may be application to keep a mapping alive indefinitely. This may be
a security risk. Also, if the process is repeated with a security risk. Also, if the process is repeated with
different ports, over time, it could use up all the ports on different ports, over time it could use up all the ports on the
the NAT. NAT.
4.4. Conflicting Internal and External IP Address Spaces
Many NATs, particularly consumer-level devices designed to be
deployed by nontechnical users, routinely obtain their external IP
address, default router, and other IP configuration information for
their external interface dynamically from an external network such as
an upstream ISP. The NAT in turn automatically sets up its own
internal subnet in one of the private IP address spaces assigned to
this purpose in RFC 1918 [7], typically providing dynamic IP
configuration services for hosts on this internal network.
Auto-configuration of NATs and private networks can be problematic,
however, if the NAT's external network is also in RFC 1918 private
address space. In a common scenario, an ISP places its customers
behind a NAT and hands out private RFC 1918 addresses to them. Some
of these customers in turn deploy consumer-level NATs, which in
effect act as "second-level" NATs, multiplexing their own private RFC
1918 IP subnets onto the single RFC 1918 IP address provided by the
ISP. There is no inherent guarantee in this case that the ISP's
"intermediate" privately-addressed network and the customer's
internal privately-addressed network will not use numerically
identical or overlapping RFC 1918 IP subnets. Customers of consumer-
level NATs further cannot be expected to have the technical knowledge
to prevent this scenario from occurring by manually configuring their
internal network with non-conflicting RFC 1918 subnets.
NAT vendors need to design their NATs to ensure that they function
correctly and robustly even in such problematic scenarios. One
possible solution is for the NAT to ensure that whenever its external
link is configured with an RFC 1918 private IP address, the NAT
automatically selects a different, non-conflicting RFC 1918 IP subnet
for its internal network. A disadvantage of this solution is that if
the NAT's external interface is dynamically configured or re-
configured after its internal network is already in use, then the NAT
may have to renumber its entire internal network dynamically if it
detects a conflict.
An alternative solution is for the NAT to be designed so that it can
translate and forward traffic correctly even when its external and
internal interfaces are configured with numerically overlapping IP
subnets. In this scenario, for example, if the NAT's external
interface has been assigned an IP address P in RFC 1918 space, then
there might also be an internal node I having the same RFC 1918
private IP address P. An IP packet with destination address P on the
external network is directed at the NAT, whereas an IP packet with
the same destination address P on the internal network is directed at
node I. The NAT therefore needs to maintain a clear operational
distinction between "external IP addresses" and "internal IP
addresses" to avoid confusing internal node I with its own external
interface. In general, the NAT needs to allow all internal nodes
(including I) to communicate with all external nodes having public
(non-RFC 1918) IP addresses or having private IP addresses that do
not conflict with the addresses used by its internal network.
REQ-7: A NAT device whose external IP interface can be configured
dynamically MUST either (1) automatically ensure that its internal
network uses IP addresses that do not conflict with its external
network, or (2) be able to translate and forward traffic between
all internal nodes and all external nodes whose IP addresses do
not numerically conflict with the internal network.
Justification: If a NAT's external and internal interfaces are
configured with overlapping IP subnets, then there is of course no
way for an internal host with RFC 1918 IP address Q to initiate a
direct communication session to an external node having the same
RFC 1918 address Q, or to other external nodes with IP addresses
that numerically conflict with the internal subnet. Such nodes
can still open communication sessions indirectly via NAT traversal
techniques, however, with the help of a third-party server such as
a STUN server having a public, non-RFC 1918 IP address. In this
case, nodes with conflicting private RFC 1918 addresses on
opposite sides of the second-level NAT can communicate with each
other via their respective temporary public endpoints on the main
Internet, as long as their common first-level NAT (e.g., the
upstream ISP's NAT) supports hairpinning behavior as described in
Section 6.
5. Filtering Behavior 5. Filtering Behavior
This section describes various filtering behaviors observed in NATs. This section describes various filtering behaviors observed in NATs.
5.1 Filtering of Unsolicited Packets
When an internal endpoint opens an outgoing session through a NAT, When an internal endpoint opens an outgoing session through a NAT,
the NAT assigns a filtering rule for the mapping between an internal the NAT assigns a filtering rule for the mapping between an internal
IP:port (X:x) and external IP:port (Y:y) tuple. IP:port (X:x) and external IP:port (Y:y) tuple.
The key behavior to describe is what criteria are used by the NAT to The key behavior to describe is what criteria are used by the NAT to
filter packets originating from specific external endpoints. filter packets originating from specific external endpoints.
Endpoint Independent Filtering: Endpoint Independent Filtering:
The NAT filters out only packets not destined to the internal The NAT filters out only packets not destined to the internal
address and port X:x, regardless of the external IP address and address and port X:x, regardless of the external IP address and
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Address and Port Dependent Filtering: Address and Port Dependent Filtering:
This is similar to the previous behavior, except that the This is similar to the previous behavior, except that the
external port is also relevant. The NAT filters out packets external port is also relevant. The NAT filters out packets
not destined for the internal address X:x. Additionally, the not destined for the internal address X:x. Additionally, the
NAT will filter out packets from Y:y destined for the internal NAT will filter out packets from Y:y destined for the internal
endpoint X:x if X:x has not sent packets to Y:y previously. In endpoint X:x if X:x has not sent packets to Y:y previously. In
other words, for receiving packets from a specific external other words, for receiving packets from a specific external
endpoint, it is necessary for the internal endpoint to send endpoint, it is necessary for the internal endpoint to send
packets first to that external endpoint's IP address and port. packets first to that external endpoint's IP address and port.
REQ-7: If application transparency is most important, it is REQ-8: If application transparency is most important, it is
RECOMMENDED that a NAT have an "Endpoint independent filtering" RECOMMENDED that a NAT have an "Endpoint independent filtering"
behavior. If a more stringent filtering behavior is most behavior. If a more stringent filtering behavior is most
important, it is RECOMMENDED that a NAT have an "Address dependent important, it is RECOMMENDED that a NAT have an "Address dependent
filtering" behavior. filtering" behavior.
a) The filtering behavior MAY be an option configurable by the a) The filtering behavior MAY be an option configurable by the
administrator of the NAT. administrator of the NAT.
OPEN ISSUE: Should REQ-7a be a SHOULD instead of a MAY? Justification: The recommendation to use Endpoint Independent
Filtering is aimed at maximizing application transparency, in
Justification for REQ-7: The recommendation to use Endpoint particular for applications that receive media simultaneously from
Independent Filtering is aimed at maximizing application multiple locations (e.g., gaming), or applications that use
transparency, in particular for applications that receive media rendezvous techniques. However, it is also possible that in some
simultaneously from multiple locations (e.g., gaming), or circumstances, it may be preferable to have a more stringent
applications that use rendezvous techniques. However, it is also filtering behavior. Filtering independently of the external
possible that in some circumstances, it may be preferable to have endpoint is not as secure: an unauthorized packet could get
a more stringent filtering behavior. Filtering independently of through a specific port while the port was kept open if it was
the external endpoint is not as secure: an unauthorized packet lucky enough to find the port open. In theory, filtering based on
could get through a specific port while the port was kept open if both IP address and port is more secure than filtering based only
it was lucky enough to find the port open. In theory, filtering on the IP address (because the external endpoint could in reality
based on both IP address and port is more secure than filtering be two endpoints behind another NAT, where one of the two
based only on the IP address (because the external endpoint could endpoints is an attacker): however, such a policy could interfere
in reality be two endpoints behind another NAT, where one of the with applications that expect to receive UDP packets on more than
two endpoints is an attacker): however, such a policy could one UDP port. Using Endpoint Independent Filtering or Address
interfere with applications that expect to receive UDP packets on Dependent Filtering instead of Address and Port Dependent
more than one UDP port. Using Endpoint Independent Filtering or
Address Dependent Filtering instead of Address and Port Dependent
Filtering on a NAT (say NAT-A) also has benefits when the other Filtering on a NAT (say NAT-A) also has benefits when the other
endpoint is behind a non-BEHAVE compliant NAT (say NAT-B) which endpoint is behind a non-BEHAVE compliant NAT (say NAT-B) that
doesn't support REQ-1. When the endpoints use ICE, if NAT-A uses does not support REQ-1. When the endpoints use ICE, if NAT-A uses
Address and Port Dependent Filtering, connectivity will require a Address and Port Dependent Filtering, connectivity will require a
Media Relay. However, if NAT-A uses Endpoint Independent Media Relay. However, if NAT-A uses Endpoint Independent
Filtering or Address Dependent Filtering, ICE will ultimately find Filtering or Address Dependent Filtering, ICE will ultimately find
connectivity without requiring a Media Relay. Having the connectivity without requiring a Media Relay. Having the
filtering behavior being an option configurable by the filtering behavior being an option configurable by the
administrator of the NAT ensures that a NAT can be used in the administrator of the NAT ensures that a NAT can be used in the
widest variety of deployment scenarios. widest variety of deployment scenarios.
5.2 NAT Filter Refresh
The time for which a NAT filter is valid can be refreshed based on
packets that are inbound, outbound, or going either direction.
6. Hairpinning Behavior 6. Hairpinning Behavior
If two hosts (called X1 and X2) are behind the same NAT and If two hosts (called X1 and X2) are behind the same NAT and
exchanging traffic, the NAT may allocate an address on the outside of exchanging traffic, the NAT may allocate an address on the outside of
the NAT for X2, called X2':x2'. If X1 sends traffic to X2':x2', it the NAT for X2, called X2':x2'. If X1 sends traffic to X2':x2', it
goes to the NAT, which must relay the traffic from X1 to X2. This is goes to the NAT, which must relay the traffic from X1 to X2. This is
referred to as hairpinning and is illustrated below. referred to as hairpinning and is illustrated below.
NAT NAT
+----+ from X1:x1 to X2':x2' +-----+ X1':x1' +----+ from X1:x1 to X2':x2' +-----+ X1':x1'
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More formally, a NAT that supports hairpinning forwards packets More formally, a NAT that supports hairpinning forwards packets
originating from an internal address, X1:x1, destined for an external originating from an internal address, X1:x1, destined for an external
address X2':x2' that has an active mapping to an internal address address X2':x2' that has an active mapping to an internal address
X2:x2, back to that internal address X2:x2. Note that typically X1' X2:x2, back to that internal address X2:x2. Note that typically X1'
is the same as X2'. is the same as X2'.
Furthermore, the NAT may present the hairpinned packet with either an Furthermore, the NAT may present the hairpinned packet with either an
internal or an external source IP address and port. The hairpinning internal or an external source IP address and port. The hairpinning
NAT behavior can therefore be either "External source IP address and NAT behavior can therefore be either "External source IP address and
port" or "Internal source IP address and port". "Internal source IP port" or "Internal source IP address and port". "Internal source IP
address and port" may cause problems by confusing an implementation address and port" may cause problems by confusing implementations
that is expecting an external IP address and port. that expect an external IP address and port.
REQ-8: A NAT MUST support "Hairpinning". REQ-9: A NAT MUST support "Hairpinning".
a) A NAT Hairpinning behavior MUST be "External source IP address a) A NAT Hairpinning behavior MUST be "External source IP address
and port". and port".
Justification for REQ-8: This requirement is to allow communications Justification: This requirement is to allow communications between
between two endpoints behind the same NAT when they are trying two endpoints behind the same NAT when they are trying each
each other's external IP addresses. other's external IP addresses.
a) Using the external IP address is necessary for applications a) Using the external IP address is necessary for applications
with a restrictive policy of not accepting packets from IP with a restrictive policy of not accepting packets from IP
addresses that differ from what is expected. addresses that differ from what is expected.
7. Application Level Gateways 7. Application Level Gateways
Certain NATs have implemented Application Level Gateways (ALGs) for Certain NATs have implemented Application Level Gateways (ALGs) for
various protocols, including protocols for negotiating peer-to-peer various protocols, including protocols for negotiating peer-to-peer
sessions such as SIP. sessions such as SIP.
Certain NATs have these ALGs turned on permanently, others have them Certain NATs have these ALGs turned on permanently, others have them
turned on by default but let them be be turned off, and others have turned on by default but let them be be turned off, and others have
them turned off by default but let them be turned on. them turned off by default but let them be turned on.
NAT ALGs may interfere with UNSAF methods or protocols that try to be NAT ALGs may interfere with UNSAF methods or protocols that try to be
NAT-aware and must therefore be used with extreme caution. NAT-aware and must therefore be used with extreme caution.
REQ-9: If a NAT includes ALGs, it is RECOMMENDED that all of those REQ-10: If a NAT includes ALGs that affect UDP, it is RECOMMENDED
ALGs (except for DNS [19] and FTP [18]) be disabled by default. that all of those ALGs be disabled by default.
a) If a NAT includes ALGs, it is RECOMMENDED that the NAT allow a) If a NAT includes ALGs, it is RECOMMENDED that the NAT allow
the NAT administrator to enable or disable each ALG separately. the NAT administrator to enable or disable each ALG separately.
Justification for REQ-9: NAT ALGs may interfere with UNSAF methods. Justification: NAT ALGs may interfere with UNSAF methods.
a) This requirement allows the user to enable those ALGs that are a) This requirement allows the user to enable those ALGs that are
necessary to aid in the operation of some applications without necessary to aid in the operation of some applications without
enabling ALGs which interfere with the operation of other enabling ALGs which interfere with the operation of other
applications. applications.
8. Deterministic Properties 8. Deterministic Properties
The classification of NATs is further complicated by the fact that The classification of NATs is further complicated by the fact that
under some conditions the same NAT will exhibit different behaviors. under some conditions the same NAT will exhibit different behaviors.
This has been seen on NATs that preserve ports or have specific This has been seen on NATs that preserve ports or have specific
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Non-deterministic NATs generally change behavior when a conflict of Non-deterministic NATs generally change behavior when a conflict of
some sort happens, i.e. when the port that would normally be used is some sort happens, i.e. when the port that would normally be used is
already in use by another mapping. The NAT mapping and External already in use by another mapping. The NAT mapping and External
Filtering in the absence of conflict is referred to as the Primary Filtering in the absence of conflict is referred to as the Primary
behavior. The behavior after the first conflict is referred to as behavior. The behavior after the first conflict is referred to as
Secondary and after the second conflict is referred to as Tertiary. Secondary and after the second conflict is referred to as Tertiary.
No NATs have been observed that change on further conflicts but it is No NATs have been observed that change on further conflicts but it is
certainly possible that they exist. certainly possible that they exist.
REQ-10: A NAT MUST have deterministic behavior, i.e., it MUST NOT REQ-11: A NAT MUST have deterministic behavior, i.e., it MUST NOT
change the NAT mapping or the External Filtering Behavior at any change the NAT mapping or the External Filtering Behavior at any
point in time or under any particular conditions. point in time or under any particular conditions.
Justification for REQ-10: Non-deterministic NATs are very difficult Justification: Non-deterministic NATs are very difficult to
to troubleshoot because they require more intensive testing. This troubleshoot because they require more intensive testing. This
non-deterministic behavior is the root cause of much of the non-deterministic behavior is the root cause of much of the
uncertainty that NATs introduce about whether or not applications uncertainty that NATs introduce about whether or not applications
will work. will work.
9. ICMP Destination Unreachable Behavior 9. ICMP Destination Unreachable Behavior
When a NAT sends a packet towards a host on the other side of the When a NAT sends a packet towards a host on the other side of the
NAT, an ICMP message may be sent in response to that packet. That NAT, an ICMP message may be sent in response to that packet. That
ICMP message may be sent by the destination host or by any router ICMP message may be sent by the destination host or by any router
along the network path. The NAT's default configuration SHOULD NOT along the network path. The NAT's default configuration SHOULD NOT
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and the ICMP payload) and forwarded to the appropriate internal or and the ICMP payload) and forwarded to the appropriate internal or
external host. The NAT needs to perform this function for as long as external host. The NAT needs to perform this function for as long as
the UDP mapping is active. Receipt of any sort of ICMP message MUST the UDP mapping is active. Receipt of any sort of ICMP message MUST
NOT destroy the NAT mapping. A NAT which performs the functions NOT destroy the NAT mapping. A NAT which performs the functions
described in the paragraph above is referred to as "support ICMP described in the paragraph above is referred to as "support ICMP
Processing". Processing".
There is no significant security advantage to blocking ICMP There is no significant security advantage to blocking ICMP
Destination Unreachable packets. Additionally, blocking ICMP Destination Unreachable packets. Additionally, blocking ICMP
Destination Unreachable packets can interfere with application Destination Unreachable packets can interfere with application
failover, UDP Path MTU Discovery (see RFC1191 [10] and RFC1435 [15]), failover, UDP Path MTU Discovery (see RFC1191 [3] and RFC1435 [4]),
and traceroute. Blocking any ICMP message is discouraged, and and traceroute. Blocking any ICMP message is discouraged, and
blocking ICMP Destination Unreachable is strongly discouraged. blocking ICMP Destination Unreachable is strongly discouraged.
REQ-11: Receipt of any sort of ICMP message MUST NOT destroy the NAT REQ-12: Receipt of any sort of ICMP message MUST NOT destroy the NAT
mapping. mapping.
a) The NAT's default configuration SHOULD NOT filter ICMP messages a) The NAT's default configuration SHOULD NOT filter ICMP messages
based on their source IP address. based on their source IP address.
b) It is RECOMMENDED that a NAT support ICMP Destination b) It is RECOMMENDED that a NAT support ICMP Destination
Unreachable messages. Unreachable messages.
Justification for REQ-11: This is easy to do, is used for many things Justification: This is easy to do, is used for many things including
including MTU discovery and rapid detection of error conditions, MTU discovery and rapid detection of error conditions, and has no
and has no negative consequences. negative consequences.
10. Fragmentation of Outgoing Packets 10. Fragmentation of Outgoing Packets
When sending a packet, there are two situations that may cause IP When sending a packet, there are two situations that may cause IP
fragmentation for packets from the inside to the outside. It is fragmentation for packets from the inside to the outside. It is
worth noting that many IP stacks do not use Path MTU Discovery with worth noting that many IP stacks do not use Path MTU Discovery with
UDP packets. UDP packets.
10.1 Smaller Adjacent MTU 10.1. Smaller Adjacent MTU
The first situation is when the MTU of the adjacent link is too The first situation is when the MTU of the adjacent link is too
small. This can occur if the NAT is doing PPPoE, or if the NAT has small. This can occur if the NAT is doing PPPoE, or if the NAT has
been configured with a small MTU to reduce serialization delay when been configured with a small MTU to reduce serialization delay when
sending large packets and small higher-priority packets, or for other sending large packets and small higher-priority packets, or for other
reasons. reasons.
The packet could have its Don't Fragment bit set to 1 (DF=1) or 0 The packet could have its Don't Fragment bit set to 1 (DF=1) or 0
(DF=0). (DF=0).
If the packet has DF=1, the NAT SHOULD send back an ICMP message If the packet has DF=1, the NAT SHOULD send back an ICMP message
"fragmentation needed and DF set" message to the host as described in "fragmentation needed and DF set" message to the host as described in
RFC 792 [13]. RFC 792 [2].
If the packet has DF=0, the NAT SHOULD fragment the packet and send If the packet has DF=0, the NAT SHOULD fragment the packet and send
the fragments, in order. This is the same function a router performs the fragments, in order. This is the same function a router performs
in a similar situation RFC 1812 [14]. in a similar situation RFC 1812 [5].
NATs that operate as described in this section are described as NATs that operate as described in this section are described as
"Supports Fragmentation" (abbreviated SF). "Supports Fragmentation" (abbreviated SF).
10.2 Smaller Network MTU 10.2. Smaller Network MTU
The second situation is when the MTU on some link in the middle of The second situation is when the MTU on some link in the middle of
the network that is not the adjacent link is too small. If DF=0, the the network that is not the adjacent link is too small. If DF=0, the
router adjacent to the small-MTU segment will fragment the packet and router adjacent to the small-MTU segment will fragment the packet and
forward the fragments as specified in RFC 1812 [14]. forward the fragments as specified in RFC 1812 [5].
If DF=1, the router adjacent to the small-MTU segment will send the If DF=1, the router adjacent to the small-MTU segment will send the
ICMP message "fragmentation needed and DF set" back towards the NAT. ICMP message "fragmentation needed and DF set" back towards the NAT.
The NAT needs to forward this ICMP message to the inside host. The NAT needs to forward this ICMP message to the inside host.
The classification of NATs that perform this behavior is covered in The classification of NATs that perform this behavior is covered in
Section 9. Section 9.
REQ-12: A NAT MUST support fragmentation of packets larger than link REQ-13: A NAT MUST support fragmentation of packets larger than link
MTU. MTU.
Justification for REQ-12: Fragmented packets become more common with Justification: Fragmented packets become more common with large video
large video packets and should continue to work. Applications can packets and should continue to work. Applications can use MTU
use MTU discovery to work around this problem. discovery to work around this problem.
11. Receiving Fragmented Packets 11. Receiving Fragmented Packets
For a variety of reasons, a NAT may receive a fragmented packet. The For a variety of reasons, a NAT may receive a fragmented packet. The
IP packet containing the header could arrive in any fragment IP packet containing the header could arrive in any fragment
depending on network conditions, packet ordering, and the depending on network conditions, packet ordering, and the
implementation of the IP stack that generated the fragments. implementation of the IP stack that generated the fragments.
A NAT that is capable only of receiving fragments in order (that is, A NAT that is capable only of receiving fragments in order (that is,
with the header in the first packet) and forwarding each of the with the header in the first packet) and forwarding each of the
skipping to change at page 19, line 5 skipping to change at page 20, line 40
A NAT that is capable of receiving fragments in or out of order and A NAT that is capable of receiving fragments in or out of order and
forwarding the individual packets (or a reassembled packet) to the forwarding the individual packets (or a reassembled packet) to the
internal host is referred to as "Receive Fragments Out of Order". internal host is referred to as "Receive Fragments Out of Order".
See the Security Considerations section of this document for a See the Security Considerations section of this document for a
discussion of this behavior. discussion of this behavior.
A NAT that is neither of these is referred to as "Receive Fragments A NAT that is neither of these is referred to as "Receive Fragments
None". None".
REQ-13: A NAT MUST support receiving in order fragments, so it MUST REQ-14: A NAT MUST support receiving in order and out of order
be "Received Fragment Ordered" or "Received Fragment Out of fragments, so it MUST have "Received Fragment Out of Order"
Order". behavior.
a) A NAT MAY support receiving fragmented packets that are out of a) A NAT's out of order fragment processing mechanism MUST be
order and be of type "Received Fragment Out of Order". designed so that fragmentation-based DoS attacks do not
compromise the NAT's ability to process in-order and
unfragmented IP packets.
Justification for REQ-13: See Security Considerations. Justification: See Security Considerations.
12. Requirements 12. Requirements
The requirements in this section are aimed at minimizing the The requirements in this section are aimed at minimizing the
complications caused by NATs to applications such as realtime complications caused by NATs to applications such as realtime
communications and online gaming. The requirements listed earlier in communications and online gaming. The requirements listed earlier in
the document are consolidated here into a single section. the document are consolidated here into a single section.
It should be understood, however, that applications normally do not It should be understood, however, that applications normally do not
know in advance if the NAT conforms to the recommendations defined in know in advance if the NAT conforms to the recommendations defined in
this section. Peer-to-peer media applications still need to use this section. Peer-to-peer media applications still need to use
normal procedures such as ICE [16]. normal procedures such as ICE [18].
A NAT that supports all of the mandatory requirements of this A NAT that supports all of the mandatory requirements of this
specification (i.e., the "MUST"), is "compliant with this specification (i.e., the "MUST"), is "compliant with this
specification." A NAT that supports all of the requirements of this specification." A NAT that supports all of the requirements of this
specification (i.e., included the "RECOMMENDED") is "fully compliant specification (i.e., including the "RECOMMENDED") is "fully compliant
with all the mandatory and recommended requirements of this with all the mandatory and recommended requirements of this
specification." specification."
REQ-1: A NAT MUST have an "External NAT mapping is endpoint REQ-1: A NAT MUST have an "External NAT mapping is endpoint
independent" behavior. independent" behavior.
REQ-2: It is RECOMMENDED that a NAT have an "IP address pooling" REQ-2: It is RECOMMENDED that a NAT have an "IP address pooling"
behavior of "Paired". Note that this requirement is not behavior of "Paired". Note that this requirement is not
applicable to NATs that do not support IP address pooling. applicable to NATs that do not support IP address pooling.
REQ-3: A NAT MUST NOT have a "Port assignment" behavior of "Port REQ-3: A NAT MUST NOT have a "Port assignment" behavior of "Port
overloading". overloading".
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RECOMMENDED the NAT's source port be in the same range. If the RECOMMENDED the NAT's source port be in the same range. If the
host's source port was in the range 1024-65535, it is host's source port was in the range 1024-65535, it is
RECOMMENDED that the NAT's source port be in that range. RECOMMENDED that the NAT's source port be in that range.
REQ-4: It is RECOMMENDED that a NAT have a "Port parity preservation" REQ-4: It is RECOMMENDED that a NAT have a "Port parity preservation"
behavior of "Yes". behavior of "Yes".
REQ-5: A NAT UDP mapping timer MUST NOT expire in less than 2 REQ-5: A NAT UDP mapping timer MUST NOT expire in less than 2
minutes. minutes.
a) The value of the NAT UDP mapping timer MAY be configurable. a) The value of the NAT UDP mapping timer MAY be configurable.
b) A default value of 5 minutes for the NAT UDP mapping timer is b) A default value of 5 minutes for the NAT UDP mapping timer is
RECOMMENDED. RECOMMENDED.
REQ-6: The NAT mapping Refresh Direction MUST have a "NAT Outbound REQ-6: The NAT mapping Refresh Direction MUST have a "NAT Outbound
refresh behavior" of "True". refresh behavior" of "True".
a) The NAT mapping Refresh Direction MAY have a "NAT Inbound a) The NAT mapping Refresh Direction MAY have a "NAT Inbound
refresh behavior" of "True". refresh behavior" of "True".
REQ-7: If application transparency is most important, it is
REQ-7 A NAT device whose external IP interface can be configured
dynamically MUST either (1) Automatically ensure that its internal
network uses IP addresses that do not conflict with its external
network, or (2) Be able to translate and forward traffic between
all internal nodes and all external nodes whose IP addresses do
not numerically conflict with the internal network.
REQ-8: If application transparency is most important, it is
RECOMMENDED that a NAT have "Endpoint independent filtering" RECOMMENDED that a NAT have "Endpoint independent filtering"
behavior. If a more stringent filtering behavior is most behavior. If a more stringent filtering behavior is most
important, it is RECOMMENDED that a NAT have "Address dependent important, it is RECOMMENDED that a NAT have "Address dependent
filtering" behavior. filtering" behavior.
a) The filtering behavior MAY be an option configurable by the a) The filtering behavior MAY be an option configurable by the
administrator of the NAT. administrator of the NAT.
OPEN ISSUE: Should REQ-7a be a SHOULD instead of a MAY? REQ-9: A NAT MUST support "Hairpinning".
REQ-8: A NAT MUST support "Hairpinning".
a) A NAT Hairpinning behavior MUST be "External source IP address a) A NAT Hairpinning behavior MUST be "External source IP address
and port". and port".
REQ-9: If a NAT includes ALGs, it is RECOMMENDED that all of those REQ-10: If a NAT includes ALGs that affect UDP, it is RECOMMENDED
ALGs (except for DNS [19] and FTP [18]) be disabled by default. that all of those ALGs be disabled by default.
a) If a NAT includes ALGs, it is RECOMMENDED that the NAT allow a) If a NAT includes ALGs, it is RECOMMENDED that the NAT allow
the NAT administrator to enable or disable each ALG separately. the NAT administrator to enable or disable each ALG separately.
REQ-10: A NAT MUST have deterministic behavior, i.e., it MUST NOT REQ-11: A NAT MUST have deterministic behavior, i.e., it MUST NOT
change the NAT mapping or the External External Filtering Behavior change the NAT mapping or the External External Filtering Behavior
at any point in time or under any particular conditions. at any point in time or under any particular conditions.
REQ-11: Receipt of any sort of ICMP message MUST NOT destroy the NAT REQ-12: Receipt of any sort of ICMP message MUST NOT destroy the NAT
mapping. mapping.
a) The NAT's default configuration SHOULD NOT filter ICMP messages a) The NAT's default configuration SHOULD NOT filter ICMP messages
based on their source IP address. based on their source IP address.
b) It is RECOMMENDED that a NAT support ICMP Destination b) It is RECOMMENDED that a NAT support ICMP Destination
Unreachable messages. Unreachable messages.
REQ-12: A NAT MUST support fragmentation of packets larger than link REQ-13: A NAT MUST support fragmentation of packets larger than link
MTU. MTU.
REQ-13: A NAT MUST support receiving in order fragments, so it MUST REQ-14: A NAT MUST support receiving in order and out of order
be "Received Fragment Ordered" or "Received Fragment Out of fragments, so it MUST have "Received Fragment Out of Order"
Order". behavior.
a) A NAT MAY support receiving fragmented packets that are out of a) A NAT's out of order fragment processing mechanism MUST be
order and be of type "Received Fragment Out of Order". designed so that fragmentation-based DoS attacks do not
compromise the NAT's ability to process in-order and
unfragmented IP packets.
13. Security Considerations 13. Security Considerations
NATs are often deployed to achieve security goals. Most of the NATs are often deployed to achieve security goals. Most of the
recommendations and requirements in this document do not affect the recommendations and requirements in this document do not affect the
security properties of these devices, but a few of them do have security properties of these devices, but a few of them do have
security implications and are discussed in this section. security implications and are discussed in this section.
This work recommends that the timers for mapping be refreshed only on This work recommends that the timers for mapping be refreshed only on
outgoing packets and does not make recommendations about whether or outgoing packets and does not make recommendations about whether or
skipping to change at page 21, line 15 skipping to change at page 23, line 19
private address space could mitigate this by cleaning any mappings private address space could mitigate this by cleaning any mappings
when a DHCP lease expired. For unicast UDP traffic (the scope of when a DHCP lease expired. For unicast UDP traffic (the scope of
this document), it may not seem relevant to support inbound timer this document), it may not seem relevant to support inbound timer
refresh; however, for multicast UDP, the question is harder. It is refresh; however, for multicast UDP, the question is harder. It is
expected that future documents discussing NAT behavior with multicast expected that future documents discussing NAT behavior with multicast
traffic will refine the requirements around handling of the inbound traffic will refine the requirements around handling of the inbound
refresh timer. Some devices today do update the timers on inbound refresh timer. Some devices today do update the timers on inbound
packets. packets.
This work recommends that the NAT filters be specific to the external This work recommends that the NAT filters be specific to the external
IP only and not the external IP and port. It can be argued that this IP only and not to the external IP and port. It can be argued that
is less secure than using the IP and port. Devices that wish to this is less secure than using the IP and port. Devices that wish to
filter on IP and port do still comply with these requirements. filter on IP and port do still comply with these requirements.
Non-deterministic NATs are risky from a security point of view. They Non-deterministic NATs are risky from a security point of view. They
are very difficult to test because they are, well, non-deterministic. are very difficult to test because they are, well, non-deterministic.
Testing by a person configuring one may result in the person thinking Testing by a person configuring one may result in the person thinking
it is behaving as desired, yet under different conditions, which an it is behaving as desired, yet under different conditions, which an
attacker can create, the NAT may behave differently. These attacker can create, the NAT may behave differently. These
requirements recommend that devices be deterministic. requirements recommend that devices be deterministic.
The work requires that NATs have an "external NAT mapping is endpoint The work requires that NATs have an "external NAT mapping is endpoint
skipping to change at page 21, line 43 skipping to change at page 23, line 47
packets are out of order so that the initial fragment does not arrive packets are out of order so that the initial fragment does not arrive
first, many systems simply discard the out of order packets. first, many systems simply discard the out of order packets.
Moreover, since some networks deliver small packets ahead of large Moreover, since some networks deliver small packets ahead of large
ones, there can be many out of order fragments. NATs that are ones, there can be many out of order fragments. NATs that are
capable of delivering these out of order packets are possible but capable of delivering these out of order packets are possible but
they need to store the out of order fragments, which can open up a they need to store the out of order fragments, which can open up a
DoS opportunity if done incorrectly. Fragmentation has been a tool DoS opportunity if done incorrectly. Fragmentation has been a tool
used in many attacks, some involving passing fragmented packets used in many attacks, some involving passing fragmented packets
through NATs and others involving DoS attacks based on the state through NATs and others involving DoS attacks based on the state
needed to reassemble the fragments. NAT implementers should be aware needed to reassemble the fragments. NAT implementers should be aware
of RFC 3128 [12] and RFC 1858 [11]. of RFC 3128 [11] and RFC 1858 [6].
14. IANA Considerations 14. IANA Considerations
There are no IANA considerations. There are no IANA considerations.
15. IAB Considerations 15. IAB Considerations
The IAB has studied the problem of "Unilateral Self Address Fixing", The IAB has studied the problem of "Unilateral Self Address Fixing",
which is the general process by which a client attempts to determine which is the general process by which a client attempts to determine
its address in another realm on the other side of a NAT through a its address in another realm on the other side of a NAT through a
collaborative protocol reflection mechanism [2]. collaborative protocol reflection mechanism [15].
This specification does not in itself constitute an UNSAF This specification does not in itself constitute an UNSAF
application. It consists of a series of requirements for NATs aimed application. It consists of a series of requirements for NATs aimed
at minimizing the negative impact that those devices have on peer-to- at minimizing the negative impact that those devices have on peer-to-
peer media applications, especially when those applications are using peer media applications, especially when those applications are using
UNSAF methods. UNSAF methods.
Section 3 of UNSAF lists several practical issues with solutions to Section 3 of UNSAF lists several practical issues with solutions to
NAT problems. This document makes recommendations to reduce the NAT problems. This document makes recommendations to reduce the
uncertainty and problems introduced by these practical issues with uncertainty and problems introduced by these practical issues with
skipping to change at page 22, line 32 skipping to change at page 24, line 37
ones widely deployed today. The "fix" helps constrain the ones widely deployed today. The "fix" helps constrain the
variability of NATs for true UNSAF solutions such as STUN. variability of NATs for true UNSAF solutions such as STUN.
Arch-2: This will exit at the same rate that NATs exit. It does not Arch-2: This will exit at the same rate that NATs exit. It does not
imply any protocol machinery that would continue to live imply any protocol machinery that would continue to live
after NATs were gone or make it more difficult to remove after NATs were gone or make it more difficult to remove
them. them.
Arch-3: This does not reduce the overall brittleness of NATs but will Arch-3: This does not reduce the overall brittleness of NATs but will
hopefully reduce some of the more outrageous NAT behaviors hopefully reduce some of the more outrageous NAT behaviors
and make it easer to discuss and predict NAT behavior in and make it easer to discuss and predict NAT behavior in
given situations. given situations.
Arch-4: This work and the results [17] of various NATs represent the Arch-4: This work and the results [19] of various NATs represent the
most comprehensive work at IETF on what the real issues are most comprehensive work at IETF on what the real issues are
with NATs for applications like VoIP. This work and STUN with NATs for applications like VoIP. This work and STUN
have pointed out more than anything else the brittleness NATs have pointed out more than anything else the brittleness NATs
introduce and the difficulty of addressing these issues. introduce and the difficulty of addressing these issues.
Arch-5: This work and the test results [17] provide a reference model Arch-5: This work and the test results [19] provide a reference model
for what any UNSAF proposal might encounter in deployed NATs. for what any UNSAF proposal might encounter in deployed NATs.
16. Acknowledgments 16. Acknowledgments
The editor would like to acknowledge Bryan Ford, Pyda Srisuresh and The editor would like to acknowledge Bryan Ford, Pyda Srisuresh and
Dan Kegel for the their multiple contributions on peer-to-peer Dan Kegel for the their multiple contributions on peer-to-peer
communications across a NAT. Dan Wing contributed substantial text communications across a NAT. Dan Wing contributed substantial text
on IP fragmentation and ICMP behavior. Thanks to Rohan Mahy, on IP fragmentation and ICMP behavior. Thanks to Rohan Mahy,
Jonathan Rosenberg, Mary Barnes, Melinda Shore, Lyndsay Campbell, Jonathan Rosenberg, Mary Barnes, Melinda Shore, Lyndsay Campbell,
Geoff Huston, Jiri Kuthan, Harald Welte, Steve Casner, Robert Geoff Huston, Jiri Kuthan, Harald Welte, Steve Casner, Robert
Sanders, Spencer Dawkins, Saikat Guha, Christian Huitema, Yutaka Sanders, Spencer Dawkins, Saikat Guha, Christian Huitema, Yutaka
Takeda and Paul Hoffman for their contributions. Takeda and Paul Hoffman for their contributions.
17. References 17. References
17.1 Normative References 17.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[2] Daigle, L. and IAB, "IAB Considerations for UNilateral Self- 17.2. Informational References
Address Fixing (UNSAF) Across Network Address Translation",
RFC 3424, November 2002.
17.2 Informational References [2] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[3] Srisuresh, P. and M. Holdrege, "IP Network Address Translator [3] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[4] Knowles, S., "IESG Advice from Experience with Path MTU
Discovery", RFC 1435, March 1993.
[5] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812,
June 1995.
[6] Ziemba, G., Reed, D., and P. Traina, "Security Considerations
for IP Fragment Filtering", RFC 1858, October 1995.
[7] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E.
Lear, "Address Allocation for Private Internets", BCP 5,
RFC 1918, February 1996.
[8] Srisuresh, P. and M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations", RFC 2663, August 1999. (NAT) Terminology and Considerations", RFC 2663, August 1999.
[4] Srisuresh, P. and K. Egevang, "Traditional IP Network Address [9] Srisuresh, P. and K. Egevang, "Traditional IP Network Address
Translator (Traditional NAT)", RFC 3022, January 2001. Translator (Traditional NAT)", RFC 3022, January 2001.
[5] Holdrege, M. and P. Srisuresh, "Protocol Complications with the [10] Holdrege, M. and P. Srisuresh, "Protocol Complications with the
IP Network Address Translator", RFC 3027, January 2001. IP Network Address Translator", RFC 3027, January 2001.
[6] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., [11] Miller, I., "Protection Against a Variant of the Tiny Fragment
Attack (RFC 1858)", RFC 3128, June 2001.
[12] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002. Session Initiation Protocol", RFC 3261, June 2002.
[7] Rosenberg, J., Huitema, C., and R. Mahy, "STUN - Simple [13] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN
Traversal of User Datagram Protocol (UDP) Through Network - Simple Traversal of User Datagram Protocol (UDP) Through
Address Translators (NATs)", draft-ietf-behave-rfc3489bis (work Network Address Translators (NATs)", RFC 3489, March 2003.
in progress), February 2003.
[8] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, [14] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", "RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003. RFC 3550, July 2003.
[9] Huitema, C., "Real Time Control Protocol (RTCP) attribute in [15] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-
Session Description Protocol (SDP)", RFC 3605, October 2003. Address Fixing (UNSAF) Across Network Address Translation",
RFC 3424, November 2002.
[10] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[11] Ziemba, G., Reed, D., and P. Traina, "Security Considerations
for IP Fragment Filtering", RFC 1858, October 1995.
[12] Miller, I., "Protection Against a Variant of the Tiny Fragment
Attack (RFC 1858)", RFC 3128, June 2001.
[13] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[14] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, [16] Huitema, C., "Real Time Control Protocol (RTCP) attribute in
June 1995. Session Description Protocol (SDP)", RFC 3605, October 2003.
[15] Knowles, S., "IESG Advice from Experience with Path MTU [17] Rosenberg, J., "Simple Traversal of UDP Through Network Address
Discovery", March 1993. Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-02
(work in progress), July 2005.
[16] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A [18] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
Methodology for Network Address Translator (NAT) Traversal for Methodology for Network Address Translator (NAT) Traversal for
the Session Initiation Protocol (SIP)", Offer/Answer Protocols", draft-ietf-mmusic-ice-05 (work in
draft-ietf-mmusic-ice-04 (work in progress), February 2005. progress), July 2005.
[17] Jennings, C., "NAT Classification Results using STUN",
draft-jennings-behave-test-results-00 (work in progress),
February 2005.
[18] Postel, J. and J. Reynolds, "FILE TRANSFER PROTOCOL (FTP)",
RFC 959, October 1985.
[19] Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION AND [19] Jennings, C., "NAT Classification Test Results",
SPECIFICATION", RFC 1035, November 1987. draft-jennings-behave-test-results-01 (work in progress),
July 2005.
[20] "Packet-based Multimedia Communications Systems", ITU- [20] "Packet-based Multimedia Communications Systems", ITU-
T Recommendation H.323, July 2003. T Recommendation H.323, July 2003.
Authors' Addresses Authors' Addresses
Francois Audet (editor) Francois Audet (editor)
Nortel Networks Nortel Networks
4655 Great America Parkway 4655 Great America Parkway
Santa Clara, CA 95054 Santa Clara, CA 95054
 End of changes. 

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