< draft-ietf-behave-nat-udp-06.txt   draft-ietf-behave-nat-udp-07.txt >
BEHAVE F. Audet, Ed. BEHAVE F. Audet, Ed.
Internet-Draft Nortel Networks Internet-Draft Nortel Networks
Expires: November 7, 2006 C. Jennings Expires: December 2, 2006 C. Jennings
Cisco Systems Cisco Systems
May 6, 2006 May 31, 2006
NAT Behavioral Requirements for Unicast UDP NAT Behavioral Requirements for Unicast UDP
draft-ietf-behave-nat-udp-06 draft-ietf-behave-nat-udp-07
Status of this Memo Status of this Memo
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This Internet-Draft will expire on November 7, 2006. This Internet-Draft will expire on December 2, 2006.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2006). Copyright (C) The Internet Society (2006).
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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9. ICMP Destination Unreachable Behavior . . . . . . . . . . . . 18 9. ICMP Destination Unreachable Behavior . . . . . . . . . . . . 18
10. Fragmentation of Outgoing Packets . . . . . . . . . . . . . . 19 10. Fragmentation of Outgoing Packets . . . . . . . . . . . . . . 19
11. Receiving Fragmented Packets . . . . . . . . . . . . . . . . . 19 11. Receiving Fragmented Packets . . . . . . . . . . . . . . . . . 19
12. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 20 12. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 20
13. Security Considerations . . . . . . . . . . . . . . . . . . . 22 13. Security Considerations . . . . . . . . . . . . . . . . . . . 22
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
15. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 23 15. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 23
16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24 16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24 17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
17.1. Normative References . . . . . . . . . . . . . . . . . . . 24 17.1. Normative References . . . . . . . . . . . . . . . . . . . 24
17.2. Informational References . . . . . . . . . . . . . . . . . 24 17.2. Informational References . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28
Intellectual Property and Copyright Statements . . . . . . . . . . 28 Intellectual Property and Copyright Statements . . . . . . . . . . 29
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 [8]. described in [RFC2663].
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 which may resemble firewall operation.
Approaches using directly signaled control of middle boxes such as Approaches using directly signaled control of middle boxes are out of
Midcom, UPnP, or in-path signaling are out of scope. scope.
UDP Relays are out-of-scope. UDP Relays (e.g., TURN [I-D.ietf-behave-turn]) are out-of-scope.
Application aspects are out-of-scope, as the focus here is strictly Application aspects are out-of-scope, as the focus here is strictly
on the NAT itself. on the NAT itself.
This document only covers the UDP Unicast aspects of NAT traversal This document only covers aspects of NAT traversal related to Unicast
and does not cover TCP, IPSEC, or other protocols. Since the UDP [RFC0768] over IP [RFC0791] and their dependencies on other
document is for UDP only, packet inspection above the UDP layer protocols.
(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 [10]. Applications that suffer from this problem payload (see [RFC3027]). Applications that suffer from this problem
include Voice Over IP and Multimedia Over IP (e.g., SIP [12] and include Voice Over IP and Multimedia Over IP (e.g., SIP [RFC3261] and
H.323 [20]), as well as online gaming. H.323 [ITU.H323]), 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 [8] are one such mechanism. STUN [17] Application Level Gateways [RFC2663] are one such mechanism. STUN
describes a UNilateral Self-Address Translation (UNSAF) mechanism [I-D.ietf-behave-rfc3489bis] describes a UNilateral Self-Address
[15]. UDP Relays have also been used to enable applications across Fixing (UNSAF) mechanism [RFC3424]. Teredo [RFC4380] describes an
UNSAF mechanism consisting of tunnelling IPv6 [RFC2460] over UDP/
IPv4. 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
[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
the use of such a UDP Relay. This specification defines requirements
for improving NATs. Meeting these requirements ensures that
applications will not be forced to use UDP media relay.
As pointed out in UNSAF [15], "From observations of deployed [I-D.ietf-mmusic-ice] describes a methodology for using many of these
techniques and 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 for improving NATs. Meeting these requirements
ensures that applications will not be forced to use UDP relay.
As pointed out in UNSAF [RFC3424], "From observations of deployed
networks, it is clear that different NAT box implementations vary networks, it is clear that different NAT box implementations vary
widely in terms of how they handle different traffic and addressing widely in terms of how they handle different traffic and addressing
cases." 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
for describing the behavior of NATs and to produce a set of for describing the behavior of NATs and to produce a set of
requirements on a specific set of behaviors for NATs. The requirements on a specific set of behaviors for NATs.
requirements represent what many vendors are already doing, and it is
not expected that it should be any more difficult to build a NAT that
meets these requirements or that these requirements should affect
performance.
This document forms a common set of requirements that are simple and This document forms a common set of requirements that are simple and
useful for voice, video, and games, which can be implemented by NAT useful for voice, video, and games, which can be implemented by NAT
vendors. This document will simplify the analysis of protocols for vendors. This document will simplify the analysis of protocols for
deciding whether or not they work in this environment and will allow deciding whether or not they work in this environment and will allow
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 [RFC2119].
Readers are urged to refer to RFC 2263 [8] for information on NAT Readers are urged to refer to [RFC2663] 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 [9] for detailed NAT device deployed. Readers may refer to [RFC3022] 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
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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 [13] used the terms "Full Cone", "Restricted Cone", "Port This document uses IANA terminology for port ranges, i.e., "Well
Known Ports" is 0-1023, "Registered" is 1024-49151, and "Dynamic
and/or Private" is 49152-65535, as defined in
http://www.iana.org/assignments/port-numbers.
STUN [RFC3489] 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.
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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 "Endpoint Independent Mapping" behavior. REQ-1: A NAT MUST have an "Endpoint Independent Mapping" behavior.
Justification: In order for UNSAF methods to work, REQ-1 needs to be Justification: In order for UNSAF methods to work, REQ-1 needs to be
met. Failure to meet REQ-1 will force the use of a Media Relay met. Failure to meet REQ-1 will force the use of a UDP 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
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Justification: This will allow applications that use multiple ports Justification: This will allow applications that use multiple ports
originating from the same internal IP address to also have the originating from the same internal IP address to also have the
same external IP address. This is to avoid breaking peer-to-peer same external IP address. This is to avoid breaking peer-to-peer
applications that are not capable of negotiating the IP address applications that are not capable of negotiating the IP address
for RTP and the IP address for RTCP separately. As such it is for RTP and the IP address for RTCP separately. As such it is
envisioned that this requirement will become less important as envisioned that this requirement will become less important as
applications become NAT-friendlier with time. The main reason why applications become NAT-friendlier with time. The main reason why
this requirement is here is that in a peer-to-peer application, this requirement is here is that in a peer-to-peer application,
you are subject to the other peer's mistake. In particular, in you are subject to the other peer's mistake. In particular, in
the context of SIP, if my application supports the extensions the context of SIP, if my application supports the extensions
defined in RFC 3605 [16] for indicating RTP and RTCP addresses and defined in [RFC3605] for indicating RTP and RTCP addresses and
ports separately, but the other peer does not, there may still be ports separately, but the other peer does not, there may still be
breakage in the form of the stream losing RTCP packets. This breakage in the form of the stream losing RTCP packets. This
requirement will avoid the loss of RTP in this context, although requirement will avoid the loss of RTP in this context, although
the loss of RTCP may be inevitable in this particular example. It the loss of RTCP may be inevitable in this particular example. It
is also worth noting that RFC 3605 is unfortunately not a is also worth noting that RFC 3605 is unfortunately not a
mandatory part of SIP (RFC 3261). This requirement will therefore mandatory part of SIP (RFC 3261). This requirement will therefore
address a particularly nasty problem that will prevail for a address a particularly nasty problem that will prevail for a
significant period of time. significant period of time.
4.2. Port Assignment 4.2. Port Assignment
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Justification: This requirement must be met in order to enable two Justification: This requirement must be met in order to enable two
applications on the internal side of the NAT both to use the same applications on the internal side of the NAT both to use the same
port to try to communicate with the same destination. NATs that port to try to communicate with the same destination. NATs that
implement port preservation have to deal with conflicts on ports, implement port preservation have to deal with conflicts on ports,
and the multiple code paths this introduces often result in and the multiple code paths this introduces often result in
nondeterministic behavior. However, it should be understood that nondeterministic behavior. However, it should be understood that
when a port is randomly assigned, it may just randomly happen to when a port is randomly assigned, it may just randomly happen to
be assigned the same port. Applications must therefore be able to be assigned the same port. Applications must therefore be able 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 the discussion of Network File System
port expectations in [RFC2623] 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 [14] rule that RTP use odd port. This behavior respects the [RFC3550] 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 [16]. However, some separately, for example using [RFC3605]. 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
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port to make it even. The same considerations as per the IP port to make it even. The same 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 problem with glare 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
considering that a NAT cannot reliably distinguish between RTP over considering that a NAT cannot reliably distinguish between RTP over
UDP and other UDP packets where there is no contiguity rule. For UDP and other UDP packets where there is no contiguity rule. For
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,
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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, unless REQ-5a applies. minutes, unless REQ-5a applies.
a) A NAT MAY have UDP mapping timers that have much shorter a) For specific destination ports in the well-known port range
timers, but only for specific ports in the well-known port (ports 0-1023), a NAT MAY have shorter UDP mapping timers that
range (i.e., ports 0-1023) where the IANA- registered protocol are specific to the IANA-registered application running over
is strictly a request/response protocol, such as for example that specific destination port.
DNS over UDP/53.
b) The value of the NAT UDP mapping timer MAY be configurable. b) The value of the NAT UDP mapping timer MAY be configurable.
c) A default value of 5 minutes for the NAT UDP mapping timer is c) A default value of 5 minutes or more for the NAT UDP mapping
RECOMMENDED. timer is RECOMMENDED.
Justification: This requirement is to ensure that the timeout is long Justification: This requirement is to ensure that the timeout is long
enough to avoid too frequent timer refresh packets. enough to avoid too frequent timer refresh packets.
a) Some UDP protocols using UDP use very short-lived connections. a) Some UDP protocols using UDP use very short-lived connections.
There can be very many such connections; keeping them all in a There can be very many such connections; keeping them all in a
connections table could cause considerable load on the NAT. connections table could cause considerable load on the NAT.
Having shorter timers for these specific applications is Having shorter timers for these specific applications is
therefore an optimization technique. It is important that the therefore an optimization technique. It is important that the
shorter timers applied to specific protocols be used sparingly, shorter timers applied to specific protocols be used sparingly,
and only for protocols using well-known destination port that and only for protocols using well-known destination port that
are known to have a shorter timer, and that are known not to be are known to have a shorter timer, and that are known not to be
used by any applications for other purposes. used by any applications for other purposes.
b) Configuration is desirable for adapting to specific networks b) Configuration is desirable for adapting to specific networks
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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: Outbound refresh is necessary for allowing the client Justification: Outbound refresh is necessary for allowing 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 external attacker or misbehaving application to keep a mapping
a security risk. Also, if the process is repeated with alive indefinitely. This may be a security risk. Also, if the
different ports, over time it could use up all the ports on the process is repeated with different ports, over time it could
NAT. use up all the ports on the NAT.
4.4. Conflicting Internal and External IP Address Spaces 4.4. Conflicting Internal and External IP Address Spaces
Many NATs, particularly consumer-level devices designed to be Many NATs, particularly consumer-level devices designed to be
deployed by nontechnical users, routinely obtain their external IP deployed by nontechnical users, routinely obtain their external IP
address, default router, and other IP configuration information for address, default router, and other IP configuration information for
their external interface dynamically from an external network such as their external interface dynamically from an external network such as
an upstream ISP. The NAT in turn automatically sets up its own an upstream ISP. The NAT in turn automatically sets up its own
internal subnet in one of the private IP address spaces assigned to internal subnet in one of the private IP address spaces assigned to
this purpose in RFC 1918 [7], typically providing dynamic IP this purpose in [RFC1918], typically providing dynamic IP
configuration services for hosts on this internal network. configuration services for hosts on this internal network.
Auto-configuration of NATs and private networks can be problematic, Auto-configuration of NATs and private networks can be problematic,
however, if the NAT's external network is also in RFC 1918 private however, if the NAT's external network is also in RFC 1918 private
address space. In a common scenario, an ISP places its customers address space. In a common scenario, an ISP places its customers
behind a NAT and hands out private RFC 1918 addresses to them. Some behind a NAT and hands out private RFC 1918 addresses to them. Some
of these customers in turn deploy consumer-level NATs, which in of these customers in turn deploy consumer-level NATs, which in
effect act as "second-level" NATs, multiplexing their own private RFC effect act as "second-level" NATs, multiplexing their own private RFC
1918 IP subnets onto the single RFC 1918 IP address provided by the 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 ISP. There is no inherent guarantee in this case that the ISP's
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addresses" to avoid confusing internal node I with its own external addresses" to avoid confusing internal node I with its own external
interface. In general, the NAT needs to allow all internal nodes interface. In general, the NAT needs to allow all internal nodes
(including I) to communicate with all external nodes having public (including I) to communicate with all external nodes having public
(non-RFC 1918) IP addresses or having private IP addresses that do (non-RFC 1918) IP addresses or having private IP addresses that do
not conflict with the addresses used by its internal network. not conflict with the addresses used by its internal network.
REQ-7: A NAT device whose external IP interface can be configured REQ-7: A NAT device whose external IP interface can be configured
dynamically MUST either (1) automatically ensure that its internal dynamically MUST either (1) automatically ensure that its internal
network uses IP addresses that do not conflict with its external network uses IP addresses that do not conflict with its external
network, or (2) be able to translate and forward traffic between network, or (2) be able to translate and forward traffic between
all internal nodes and all external nodes whose IP addresses do all internal nodes and all external nodes whose IP addresses
not numerically conflict with the internal network. numerically conflicts with the internal network.
Justification: If a NAT's external and internal interfaces are Justification: If a NAT's external and internal interfaces are
configured with overlapping IP subnets, then there is of course no 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 way for an internal host with RFC 1918 IP address Q to initiate a
direct communication session to an external node having the same direct communication session to an external node having the same
RFC 1918 address Q, or to other external nodes with IP addresses RFC 1918 address Q, or to other external nodes with IP addresses
that numerically conflict with the internal subnet. Such nodes that numerically conflict with the internal subnet. Such nodes
can still open communication sessions indirectly via NAT traversal can still open communication sessions indirectly via NAT traversal
techniques, however, with the help of a third-party server such as 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 a STUN server having a public, non-RFC 1918 IP address. In this
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on the IP address (because the external endpoint could in reality on the IP address (because the external endpoint could in reality
be two endpoints behind another NAT, where one of the two be two endpoints behind another NAT, where one of the two
endpoints is an attacker): however, such a policy could interfere endpoints is an attacker): however, such a policy could interfere
with applications that expect to receive UDP packets on more than with applications that expect to receive UDP packets on more than
one UDP port. Using Endpoint Independent Filtering or Address one UDP port. Using Endpoint Independent Filtering or Address
Dependent Filtering instead of Address and Port Dependent 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) that endpoint is behind a non-BEHAVE compliant NAT (say NAT-B) that
does not 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 UDP relay. However, if NAT-A uses Endpoint Independent Filtering
Filtering or Address Dependent Filtering, ICE will ultimately find or Address Dependent Filtering, ICE will ultimately find
connectivity without requiring a Media Relay. Having the connectivity without requiring a UDP relay. Having the filtering
filtering behavior being an option configurable by the behavior being an option configurable by the administrator of the
administrator of the NAT ensures that a NAT can be used in the NAT ensures that a NAT can be used in the widest variety of
widest variety of deployment scenarios. deployment scenarios.
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
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communicate even if they only use each other's external IP addresses communicate even if they only use each other's external IP addresses
and ports. and ports.
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 (X1:x1) or an external (X1':x1') source IP address and port.
NAT behavior can therefore be either "External source IP address and The hairpinning NAT behavior can therefore be either "External source
port" or "Internal source IP address and port". "Internal source IP IP address and port" or "Internal source IP address and port".
address and port" may cause problems by confusing implementations "Internal source IP address and port" may cause problems by confusing
that expect an external IP address and port. implementations that expect an external IP address and port.
REQ-9: 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: This requirement is to allow communications between Justification: This requirement is to allow communications between
two endpoints behind the same NAT when they are trying each two endpoints behind the same NAT when they are trying 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 source IP address is necessary for
with a restrictive policy of not accepting packets from IP applications with a restrictive policy of not accepting packets
addresses that differ from what is expected. from IP 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-10: If a NAT includes ALGs that affect UDP, it is RECOMMENDED REQ-10: To eliminate interference with UNSAF NAT traversal mechanisms
that all of those ALGs be disabled by default. and allow integrity protection of UDP communications, NAT ALGs for
UDP-based protocols SHOULD be turned off. Future standards track
specifications that define an ALG can update this to recommend
that the ALGs they define default on.
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: 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
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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 [3] and RFC1435 [4]), failover, UDP Path MTU Discovery (see [RFC1191] and [RFC1435]), and
and traceroute. Blocking any ICMP message is discouraged, and traceroute. Blocking any ICMP message is discouraged, and blocking
blocking ICMP Destination Unreachable is strongly discouraged. ICMP Destination Unreachable is strongly discouraged.
REQ-12: Receipt of any sort of ICMP message MUST NOT terminate the REQ-12: Receipt of any sort of ICMP message MUST NOT terminate the
NAT mapping. NAT 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: This is easy to do, is used for many things including Justification: This is easy to do, is used for many things including
MTU discovery and rapid detection of error conditions, and has no MTU discovery and rapid detection of error conditions, and has no
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when sending large packets and small higher-priority packets, or for when sending large packets and small higher-priority packets, or for
other reasons. other reasons.
It is worth nothing that many IP stacks do not use Path MTU Discovery It is worth nothing that many IP stacks do not use Path MTU Discovery
with UDP packets. with UDP packets.
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).
REQ-13: If the packet received on an internal IP address has DF=1, REQ-13: If the packet received on an internal IP address has DF=1,
the NAT SHOULD send back an ICMP message "fragmentation needed and the NAT MUST send back an ICMP message "fragmentation needed and
DF set" message to the host as described in RFC 792 [2]. DF set" message to the host as described in [RFC0792].
a) If the packet has DF=0, the NAT SHOULD fragment the packet and a) If the packet has DF=0, the NAT MUST fragment the packet and
send the fragments, in order. SHOULD send the fragments in order.
Justification: This is as per RFC 792. Justification: This is as per RFC 792.
a) This is the same function a router performs in a similar a) This is the same function a router performs in a similar
situation RFC 1812 [5]. situation [RFC1812].
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
fragments to the internal host is described as "Received Fragments fragments to the internal host is described as "Received Fragments
Ordered". Ordered".
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 fragments (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-14: A NAT MUST support receiving in order and out of order REQ-14: A NAT MUST support receiving in order and out of order
fragments, so it MUST have "Received Fragment Out of Order" fragments, so it MUST have "Received Fragment Out of Order"
behavior. behavior.
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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 [18]. normal procedures such as ICE [I-D.ietf-mmusic-ice].
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., including 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 "Endpoint Independent Mapping" behavior. REQ-1: A NAT MUST have an "Endpoint Independent Mapping" 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"
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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.
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, unless REQ-5a applies. minutes, unless REQ-5a applies.
a) A NAT MAY have UDP mapping timers that have much shorter a) For specific destination ports in the well-known port range
timers, but only for specific ports in the well-known port (ports 0-1023), a NAT MAY have shorter UDP mapping timers that
range (i.e., ports 0-1023) where the IANA- registered protocol are specific to the IANA-registered application running over
is strictly a request/response protocol, such as for example that specific destination port.
DNS over UDP/53.
b) The value of the NAT UDP mapping timer MAY be configurable. b) The value of the NAT UDP mapping timer MAY be configurable.
c) A default value of 5 minutes for the NAT UDP mapping timer is c) A default value of 5 minutes or more for the NAT UDP mapping
RECOMMENDED. timer is 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 A NAT device whose external IP interface can be configured REQ-7 A NAT device whose external IP interface can be configured
dynamically MUST either (1) Automatically ensure that its internal dynamically MUST either (1) Automatically ensure that its internal
network uses IP addresses that do not conflict with its external network uses IP addresses that do not conflict with its external
network, or (2) Be able to translate and forward traffic between network, or (2) Be able to translate and forward traffic between
all internal nodes and all external nodes whose IP addresses do all internal nodes and all external nodes whose IP addresses
not numerically conflict with the internal network. numerically conflicts with the internal network.
REQ-8: If application transparency is most important, it is 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.
REQ-9: 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".
REQ-10: If a NAT includes ALGs that affect UDP, it is RECOMMENDED REQ-10: To eliminate interference with UNSAF NAT traversal mechanisms
that all of those ALGs be disabled by default. and allow integrity protection of UDP communications, NAT ALGs for
UDP-based protocols SHOULD be turned off. Future standards track
specifications that define an ALG can update this to recommend
that the ALGs they define default on.
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-11: 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 translation (Section 4) or the Filtering change the NAT translation (Section 4) or the Filtering
(Section 5) Behavior at any point in time or under any particular (Section 5) Behavior at any point in time or under any particular
conditions. conditions.
REQ-12: Receipt of any sort of ICMP message MUST NOT terminate the REQ-12: Receipt of any sort of ICMP message MUST NOT terminate the
NAT mapping. NAT 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-13 If the packet received on an internal IP address has DF=1, the REQ-13 If the packet received on an internal IP address has DF=1, the
NAT SHOULD send back an ICMP message "fragmentation needed and DF NAT MUST send back an ICMP message "fragmentation needed and DF
set" message to the host as described in RFC 792 [2]. set" message to the host as described in [RFC0792].
a) If the packet has DF=0, the NAT SHOULD fragment the packet and a) If the packet has DF=0, the NAT MUST fragment the packet and
send the fragments, in order. SHOULD send the fragments in order.
REQ-14: A NAT MUST support receiving in order and out of order REQ-14: A NAT MUST support receiving in order and out of order
fragments, so it MUST have "Received Fragment Out of Order" fragments, so it MUST have "Received Fragment Out of Order"
behavior. behavior.
a) A NAT's out of order fragment processing mechanism MUST be a) A NAT's out of order fragment processing mechanism MUST be
designed so that fragmentation-based DoS attacks do not designed so that fragmentation-based DoS attacks do not
compromise the NAT's ability to process in-order and compromise the NAT's ability to process in-order and
unfragmented IP packets. 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 document recommends that the timers for mapping be refreshed
outgoing packets and does not make recommendations about whether or only on outgoing packets and does not make recommendations about
not inbound packets should update the timers. If inbound packets whether or not inbound packets should update the timers. If inbound
update the timers, an external attacker can keep the mapping alive packets update the timers, an external attacker can keep the mapping
forever and attack future devices that may end up with the same alive forever and attack future devices that may end up with the same
internal address. A device that was also the DHCP server for the internal address. A device that was also the DHCP server for the
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 document recommends that the NAT filters be specific to the
IP only and not to the external IP and port. It can be argued that external IP address only and not to the external IP and port. It can
this is less secure than using the IP and port. Devices that wish to be argued that this is less secure than using the IP and port.
filter on IP and port do still comply with these requirements. Devices that wish to 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 This document requires that NATs have an "external NAT mapping is
independent" behavior. This does not reduce the security of devices. endpoint independent" behavior. This does not reduce the security of
Which packets are allowed to flow across the device is determined by devices. Which packets are allowed to flow across the device is
the external filtering behavior, which is independent of the mapping determined by the external filtering behavior, which is independent
behavior. of the mapping behavior.
When a fragmented packet is received from the external side and the When a fragmented packet is received from the external side and the
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 [11] and RFC 1858 [6]. of [RFC3128] and [RFC1858].
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 [15]. collaborative protocol reflection mechanism [RFC3424].
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
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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
NATs. In addition, UNSAF lists five architectural considerations. NATs. In addition, UNSAF lists five architectural considerations.
Although this is not an UNSAF proposal, it is interesting to consider Although this is not an UNSAF proposal, it is interesting to consider
the impact of this work on these architectural considerations. the impact of this work on these architectural considerations.
Arch-1: The scope of this is limited to UDP packets in NATs like the Arch-1: The scope of this is limited to UDP packets in NATs like the
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 [19] of various NATs represent the Arch-4: This work and the results [I-D.jennings-behave-test-results]
most comprehensive work at IETF on what the real issues are of various NATs represent the most comprehensive work at IETF
with NATs for applications like VoIP. This work and STUN on what the real issues are with NATs for applications like
have pointed out more than anything else the brittleness NATs VoIP. This work and STUN have pointed out more than anything
introduce and the difficulty of addressing these issues. else the brittleness NATs introduce and the difficulty of
Arch-5: This work and the test results [19] provide a reference model addressing these issues.
for what any UNSAF proposal might encounter in deployed NATs. Arch-5: This work and the test results [I-D.jennings-behave-test-
results] provide a reference model 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, Paul Hoffman, Lisa Dusseault, Pekka Savola and Jari Arkko 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 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
Levels", BCP 14, RFC 2119, March 1997. August 1980.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
17.2. Informational References 17.2. Informational References
[2] Postel, J., "Internet Control Message Protocol", STD 5, [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981. RFC 792, September 1981.
[3] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990. November 1990.
[4] Knowles, S., "IESG Advice from Experience with Path MTU [RFC1435] Knowles, S., "IESG Advice from Experience with Path MTU
Discovery", RFC 1435, March 1993. Discovery", RFC 1435, March 1993.
[5] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, [RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
June 1995. RFC 1812, June 1995.
[6] Ziemba, G., Reed, D., and P. Traina, "Security Considerations [RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security
for IP Fragment Filtering", RFC 1858, October 1995. Considerations for IP Fragment Filtering", RFC 1858,
October 1995.
[7] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. [RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
Lear, "Address Allocation for Private Internets", BCP 5, E. Lear, "Address Allocation for Private Internets",
RFC 1918, February 1996. BCP 5, RFC 1918, February 1996.
[8] Srisuresh, P. and M. Holdrege, "IP Network Address Translator [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(NAT) Terminology and Considerations", RFC 2663, August 1999. (IPv6) Specification", RFC 2460, December 1998.
[9] Srisuresh, P. and K. Egevang, "Traditional IP Network Address [RFC2623] Eisler, M., "NFS Version 2 and Version 3 Security Issues
Translator (Traditional NAT)", RFC 3022, January 2001. and the NFS Protocol's Use of RPCSEC_GSS and Kerberos V5",
RFC 2623, June 1999.
[10] Holdrege, M. and P. Srisuresh, "Protocol Complications with the [RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
IP Network Address Translator", RFC 3027, January 2001. Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.
[11] Miller, I., "Protection Against a Variant of the Tiny Fragment [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Attack (RFC 1858)", RFC 3128, June 2001. Address Translator (Traditional NAT)", RFC 3022,
January 2001.
[12] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., [RFC3027] Holdrege, M. and P. Srisuresh, "Protocol Complications
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: with the IP Network Address Translator", RFC 3027,
Session Initiation Protocol", RFC 3261, June 2002. January 2001.
[13] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN [RFC3128] Miller, I., "Protection Against a Variant of the Tiny
- Simple Traversal of User Datagram Protocol (UDP) Through Fragment Attack (RFC 1858)", RFC 3128, June 2001.
Network Address Translators (NATs)", RFC 3489, March 2003.
[14] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
"RTP: A Transport Protocol for Real-Time Applications", STD 64, A., Peterson, J., Sparks, R., Handley, M., and E.
RFC 3550, July 2003. Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[15] Daigle, L. and IAB, "IAB Considerations for UNilateral Self- [RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
Address Fixing (UNSAF) Across Network Address Translation", "STUN - Simple Traversal of User Datagram Protocol (UDP)
RFC 3424, November 2002. Through Network Address Translators (NATs)", RFC 3489,
March 2003.
[16] Huitema, C., "Real Time Control Protocol (RTCP) attribute in [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Session Description Protocol (SDP)", RFC 3605, October 2003. Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[17] Rosenberg, J., "Simple Traversal of UDP Through Network Address [RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-03 Self-Address Fixing (UNSAF) Across Network Address
(work in progress), March 2006. Translation", RFC 3424, November 2002.
[18] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A [RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute
Methodology for Network Address Translator (NAT) Traversal for in Session Description Protocol (SDP)", RFC 3605,
Offer/Answer Protocols", draft-ietf-mmusic-ice-08 (work in October 2003.
progress), March 2006.
[19] Jennings, C., "NAT Classification Test Results", [RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
draft-jennings-behave-test-results-01 (work in progress), Network Address Translations (NATs)", RFC 4380,
July 2005. February 2006.
[20] "Packet-based Multimedia Communications Systems", ITU- [I-D.ietf-behave-rfc3489bis]
T Recommendation H.323, July 2003. Rosenberg, J., "Simple Traversal of UDP Through Network
Address Translators (NAT) (STUN)",
draft-ietf-behave-rfc3489bis-03 (work in progress),
March 2006.
[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Methodology for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-08 (work in progress), March 2006.
[I-D.jennings-behave-test-results]
Jennings, C., "NAT Classification Test Results",
draft-jennings-behave-test-results-01 (work in progress),
July 2005.
[I-D.ietf-behave-turn]
Rosenberg, J., "Obtaining Relay Addresses from Simple
Traversal of UDP Through NAT (STUN)",
draft-ietf-behave-turn-00 (work in progress), March 2006.
[ITU.H323]
"Packet-based Multimedia Communications Systems", ITU-
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
US US
Phone: +1 408 495 3756 Phone: +1 408 495 3756
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