BEHAVE F. Audet, Ed.
Internet-Draft Nortel Networks
Expires: January 16, 2006 C. Jennings
Cisco Systems
July 15, 2005
NAT Behavioral Requirements for Unicast UDP
draft-ietf-behave-nat-udp-03
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document defines basic terminology for describing different
types of NAT behavior when handling Unicast UDP and also defines a
set of requirements that would allow many applications, such as
multimedia communications or on-line gaming, to work consistently.
Developing NATs that meet this set of requirements will greatly
increase the likelihood that these applications will function
properly.
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Table of Contents
1. Applicability Statement . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Network Address and Port Translation Behavior . . . . . . . . 5
4.1 Address and Port Mapping . . . . . . . . . . . . . . . . . 5
4.2 Port Assignment . . . . . . . . . . . . . . . . . . . . . 8
4.2.1 Port Assignment Behavior . . . . . . . . . . . . . . . 8
4.2.2 Port Parity . . . . . . . . . . . . . . . . . . . . . 10
4.2.3 Port Contiguity . . . . . . . . . . . . . . . . . . . 10
4.3 Mapping Refresh . . . . . . . . . . . . . . . . . . . . . 11
5. Filtering Behavior . . . . . . . . . . . . . . . . . . . . . . 12
5.1 Filtering of Unsolicited Packets . . . . . . . . . . . . . 12
5.2 NAT Filter Refresh . . . . . . . . . . . . . . . . . . . . 14
6. Hairpinning Behavior . . . . . . . . . . . . . . . . . . . . . 14
7. Application Level Gateways . . . . . . . . . . . . . . . . . . 15
8. Deterministic Properties . . . . . . . . . . . . . . . . . . . 15
9. ICMP Destination Unreachable Behavior . . . . . . . . . . . . 16
10. Fragmentation of Outgoing Packets . . . . . . . . . . . . . 17
10.1 Smaller Adjacent MTU . . . . . . . . . . . . . . . . . . . 17
10.2 Smaller Network MTU . . . . . . . . . . . . . . . . . . . 18
11. Receiving Fragmented Packets . . . . . . . . . . . . . . . . 18
12. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 19
13. Security Considerations . . . . . . . . . . . . . . . . . . 20
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . 21
15. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 21
16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 22
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
17.1 Normative References . . . . . . . . . . . . . . . . . . . 23
17.2 Informational References . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . 25
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1. Applicability Statement
The purpose of this specification is to define a set of requirements
for NATs that would allow many applications, such as multimedia
communications or on-line gaming, to work consistently. Developing
NATs that meet this set of requirements will greatly increase the
likelihood that these applications will function properly.
The requirements of this specification apply to Traditional NATs as
described in RFC 2663 [3].
This document is meant to cover NATs of any size, from small
residential NATs to large Enterprise NATs. However, it should be
understood that Enterprise NATs normally provide much more than just
NAT capabilities: for example, they typically provide firewall
functionalities. Firewalls are specifically out-of-scope for this
specification; however, this specification does cover the inherent
filtering aspects of NATs.
Approaches using directly signaled control of middle boxes such as
Midcom, UPnP, or in-path signaling are out of scope.
UDP Relays are out-of-scope.
Application aspects are out-of-scope, as the focus here is strictly
on the NAT itself.
This document only covers the UDP Unicast aspects of NAT traversal
and does not cover TCP, IPSEC, or other protocols. Since the
document is for UDP only, packet inspection above the UDP layer
(including RTP) is also out-of-scope.
2. Introduction
Network Address Translators (NATs) are well known to cause very
significant problems with applications that carry IP addresses in the
payload RFC 3027 [5]. Applications that suffer from this problem
include Voice Over IP and Multimedia Over IP (e.g., SIP [6] and H.323
[20]), as well as online gaming.
Many techniques are used to attempt to make realtime multimedia
applications, online games, and other applications work across NATs.
Application Level Gateways [3] are one such mechanism. STUN [7]
describes a UNilateral Self-Address Translation (UNSAF) mechanism
[2]. UDP Relays have also been used to enable applications across
NATs, but these are generally seen as a solution of last resort. ICE
[16] describes a methodology for using many of these techniques and
avoiding a UDP Relay unless the type of NAT is such that it forces
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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 [2], "From observations of deployed networks,
it is clear that different NAT boxes' implementation vary widely in
terms of how they handle different traffic and addressing cases."
This wide degree of variability is one factor in the overall
brittleness introduced by NATs and makes it extremely difficult to
predict how any given protocol will behave on a network traversing
NAT. Discussions with many of the major NAT vendors have made it
clear that they would prefer to deploy NATs that were deterministic
and caused the least harm to applications while still meeting the
requirements that caused their customers to deploy NATs in the first
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.
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
requirements on a specific set of behaviors for NATs. The
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
useful for voice, video, and games, which can be implemented by NAT
vendors. This document will simplify the analysis of protocols for
deciding whether or not they work in this environment and will allow
providers of services that have NAT traversal issues to make
statements about where their applications will work and where they
will not, as well as to specify their own NAT requirements.
3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
Readers are urged to refer to RFC 2263 [3] for information on NAT
taxonomy and terminology. Traditional NAT is the most common type of
NAT device deployed. Readers may refer to RFC 3022 [4] for detailed
information on traditional NAT. Traditional NAT has two main
varieties - Basic NAT and Network Address/Port Translator (NAPT).
NAPT is by far the most commonly deployed NAT device. NAPT allows
multiple internal hosts to share a single public IP address
simultaneously. When an internal host opens an outgoing TCP or UDP
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session through a NAPT, the NAPT assigns the session a public IP
address and port number so that subsequent response packets from the
external endpoint can be received by the NAPT, translated, and
forwarded to the internal host. The effect is that the NAPT
establishes a NAT session to translate the (private IP address,
private port number) tuple to (public IP address, public port number)
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
internal host initiates multiple simultaneous sessions from a single
(private IP, private port) endpoint to multiple distinct endpoints on
the external network. In this specification, the term "NAT" refers
to both "Basic NAT" and "Network Address/Port Translator (NAPT)".
This document uses the term "session" as defined in RFC 2663: "TCP/
UDP sessions are uniquely identified by the tuple of (source IP
address, source TCP/UDP ports, target IP address, target TCP/UDP
Port)."
This document uses the term "address and port mapping" as the
translation between an external address and port and an internal
address and port. Note that this is not the same as an "address
binding" as defined in RFC 2663.
RFC 3489 used the terms "Full Cone", "Restricted Cone", "Port
Restricted Cone" and "Symmetric" to refer to different variations of
NATs applicable to UDP only. Unfortunately, this terminology has
been the source of much confusion as it has proven inadequate at
describing real-life NAT behavior. This specification therefore
refers to specific individual NAT behaviors instead of using the
Cone/Symmetric terminology.
4. Network Address and Port Translation Behavior
This section describes the various NAT behaviors applicable to NATs.
4.1 Address and Port Mapping
When an internal endpoint opens an outgoing session through a NAT,
the NAT assigns the session an external IP address and port number so
that subsequent response packets from the external endpoint can be
received by the NAT, translated, and forwarded to the internal
endpoint. This is a mapping between an internal IP address and port
IP:port and external IP:port tuple. It establishes the translation
that will be performed by the NAT for the duration of the session.
For many applications, it is important to distinguish the behavior of
the NAT when there are multiple simultaneous sessions established to
different external endpoints.
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The key behavior to describe is the criteria for re-use of a mapping
for new sessions to external endpoints, after establishing a first
mapping between an internal X:x address and port and an external
Y1:y1 address tuple. Let's assume that the internal IP address and
port X:x is mapped to X1':x1' for this first session. The endpoint
then sends from X:x to an external address Y2:y2 and gets a mapping
of X2':x2' on the NAT. The relationship between X1':x1' and X2':x2'
for various combinations of the relationship between Y1:y1 and Y2:y2
is critical for describing the NAT behavior. This arrangement is
illustrated in the following diagram:
E
+------+ +------+ x
| Y1 | | Y2 | t
+--+---+ +---+--+ e
| Y1:y1 Y2:y2 | r
+----------+ +----------+ n
| | a
X1':x1' | | X2':x2' l
+--+---+-+
...........| NAT |...............
+--+---+-+ I
| | n
X:x | | X:x t
++---++ e
| X | r
+-----+ n
a
l
The following address and port mapping behavior are defined:
Endpoint Independent Mapping:
The NAT reuses the port mapping for subsequent packets sent
from the same internal IP address and port (X:x) to any
external IP address and port. Specifically, X1':x1' equals
X2':x2' for all values of Y2:y2.
Address Dependent Mapping:
The NAT reuses the port mapping for subsequent packets sent
from the same internal IP address and port (X:x) to the same
external IP address, regardless of the external port.
Specifically, X1':x1' equals X2':x2' if, and only if, Y2 equals
Y1.
Address and Port Dependent Mapping:
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The NAT reuses the port mapping for subsequent packets sent
from the same internal IP address and port (X:x) to the same
external and port while the mapping is still active.
Specifically, X1':x1' equals X2':x2' if, and only if, Y2:y2
equals Y1:y1.
It is important to note that these three possible choices make no
difference to the security properties of the NAT. The security
properties are fully determined by which packets the NAT allows in
and which it does not. This is determined by the filtering behavior
in the filtering portions of the NAT.
REQ-1: A NAT MUST have an "External NAT mapping is endpoint
independent" behavior.
Justification for REQ-1: 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 which is very often impractical.
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
single IP address. This is especially common with larger NATs. Some
NATs use the external IP address mapping in an arbitrary fashion
(i.e. randomly): one internal IP address could have multiple external
IP address mappings active at the same time for different sessions.
These NATs have an "IP address pooling" behavior of "Arbitrary".
Some large Enterprise NATs use an IP address pooling behavior of
"Arbitrary" as a means of hiding the IP address assigned to specific
endpoints by making their assignment less predictable. Other NATs
use the same external IP address mapping for all sessions associated
with the same internal IP address. These NATs have an "IP address
pooling" behavior of "Paired." NATs that use an "IP address pooling"
behavior of "arbitrary" can cause issues for applications that use
multiple ports from the same endpoint but do not negotiate IP
addresses individually (e.g., some applications using RTP and RTCP).
REQ-2: It is RECOMMENDED that a NAT have an "IP address pooling"
behavior of "Paired". Note that this requirement is not
applicable to NATs that do not support IP address pooling.
Justification for REQ-2: This will allow applications that use
multiple ports originating from the same internal IP address to
also have the same external IP address. This is to avoid breaking
peer-to-peer applications which are not capable of negotiating the
IP address for RTP and the IP address for RTCP separately. As
such it is envisioned that this requirement will become less
important as applications become NAT-friendlier with time. The
main reason why this requirement is here is that in a peer-to-peer
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application, you are subject to the other peer's mistake. In
particular, in the context of SIP, if my application supports the
extensions defined in RFC 3605 [9] for indicating RTP and RTCP
addresses and ports separately, but the other peer does not, there
may still be breakage in the form of letting the stream loose the
RTP packets. This requirement will avoid the loss of RTP in this
context, although the loss of RTCP may be inevitable in this
particular example. It is also worth noting that RFC 3605 is
unfortunately not a mandatory part of SIP (RFC 3261). This
requirement will therefore address a particularly nasty problem
that will prevail for a significant amount of time.
4.2 Port Assignment
4.2.1 Port Assignment Behavior
This section uses the following diagram for reference.
E
+-------+ +-------+ x
| Y1 | | Y2 | t
+---+---+ +---+---+ e
| Y1:y1 Y2:y2 | r
+---------+ +---------+ n
| | a
X1':x1' | | X2':x2' l
+--+---+--+
...........| NAT |...............
+--+---+--+ I
| | n
+---------+ +---------+ t
| X1:x1 X2':x2 | e
+---+---+ +---+---+ r
| X1 | | X2 | n
+-------+ +-------+ a
l
Some NATs attempt to preserve the port number used internally when
assigning a mapping to an external IP address and port (e.g.,
x=x1=x2=x1'=x2', or more succinctly, a mapping of X:x to X':x). A
basic NAT, for example, will preserve the same port and will assign a
different IP address from a pool of external IP addresses in case of
port collision (e.g. X1:x to X1':x and X2:x to X2':x). This is only
possible as long as the NAT has enough external IP addresses. If the
port x is already in use on all available external IP addresses, then
the NAT needs to switch from Basic NAT to Network Address and Port
Translator (NAPT) mode (i.e., X'=X1'=X2' and x=x1=x2 but x1'!=x2', or
a mapping of X1:x to X':x1' and X2:x to X':x2'). This port
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assignment behavior is referred to as "port preservation". It does
not guarantee that the external port x' will always be the same as
the internal port x but only that the NAT will preserve the port if
possible.
A NAT that does not attempt to make the external port numbers match
the internal port numbers in any case (i.e., X1:x to X':x1', X2:x to
X':x2') is referred to as "No port preservation".
Some NATs use "Port overloading", i.e. they always use port
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).
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 (X1 or X2). Port overloading fails if the two internal
endpoints are establishing sessions to the same external destination.
Most applications fail in some cases with "Port Overloading". It is
clear that "Port Overloading" behavior will result in many problems.
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
as a SIP proxy, or a web server, etc.)
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"
from 0 to 1023, "registered" from 1024 to 49151, and "dynamic/
private" from 49152 through 65535. For most protocols, these are
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
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
number of ports available. Other NAT devices may use everything but
the well-known range and may prefer to use the dynamic range first or
possibly avoid the actual registered ports in the registered 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.
REQ-3: A NAT MUST NOT have a "Port assignment" behavior of "Port
overloading".
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
host's source port was in the range 1024-65535, it is
RECOMMENDED that the NAT's source port be in that range.
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Justification for REQ-3: This requirement must be met in order to
enable two applications on the internal side of the NAT both to
use the same port to try to communicate with the same destination.
NATs that implement port preservation have to deal with conflicts
on ports, and the multiple code paths this introduces often result
in nondeterministic behavior. However, it should be understood
that when a port is randomly assigned, it may just randomly happen
to be assigned the same port. Applications must therefore be able
to deal with both port preservation, and no port preservation.
a) Certain applications expect the source UDP port to be in the
well-known range. See RFC 2623 for an example.
4.2.2 Port Parity
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
odd port. This behavior respects the RFC 3550 [8] rule that RTP use
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
separately, for example using RFC 3605 [9]. However, some
implementations do not include RFC 3605 and do not recognize when the
peer has specified the RTCP port separately using RFC 3605. If such
an implementation receives an odd RTP port number from the peer
(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,
this would obviously result in the loss of RTP. NAT-friendly
application aspects are outside the scope of this document. It is
expected that this issue will fade away with time, as implementations
improve. Preserving the port parity allows for supporting
communication with peers that do not support explicit specification
of both RTP and RTCP port numbers.
REQ-4: It is RECOMMENDED that a NAT have a "Port parity preservation"
behavior of "Yes".
Justification for REQ-4: This is to avoid breaking peer-to-peer
applications which do not explicitly and separately specify RTP
and RTCP port numbers and which follow the RFC 3550 rule to
decrement an odd RTP port to make it even. The same
considerations as per the IP address pooling requirement apply.
4.2.3 Port Contiguity
Some NATs attempt to preserve the port contiguity rule of RTCP=RTP+1.
These NATs do things like sequential assignment or port reservation.
Sequential port assignment assumes that the application will open a
mapping for RTP first and then open a mapping for RTCP. It is not
practical to enforce this requirement on all applications.
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Furthermore, there is a glaring problem if many applications (or
endpoints) are trying to open mapping simultaneously. Port
preservation is also problematic since it is wasteful, especially
considering that a NAT cannot reliably distinguish between RTP over
UDP and other UDP packets where there is no contiguity rule. For
those reasons, it would be too complex to attempt to preserve the
contiguity rule by suggesting specific NAT behavior, and it would
certainly break the deterministic behavior rule.
In order to support both RTP and RTCP, it will therefore be necessary
that applications follow rules to negotiate RTP and RTCP separately,
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
of the scope of this document.
4.3 Mapping Refresh
NAT mapping timeout implementations vary but include the timer's
value and the way the mapping timer is refreshed to keep the mapping
alive.
The mapping timer is defined as the time a mapping will stay active
without packets traversing the NAT. There is great variation in the
values used by different NATs.
REQ-5: A NAT UDP mapping timer MUST NOT expire in less than 2
minutes.
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
RECOMMENDED.
Justification for REQ-5: This requirement is to ensure that the
timeout is long enough to avoid too frequent timer refresh
packets.
a) Configuration is desirable for adapting to specific networks
and troubleshooting.
b) This default is to avoid too frequent timer refresh packets.
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
side of the NAT. This is referred to as having a NAT Outbound
refresh behavior of "True".
Some NATs keep the mapping active when a packet goes from the
external side of the NAT to the internal side of the NAT. This is
referred to as having a NAT Inbound Refresh Behavior of "True".
Some NATs keep the mapping active on both, in which case both
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properties are "True".
REQ-6: The NAT mapping Refresh Direction MUST have a "NAT Outbound
refresh behavior" of "True".
a) The NAT mapping Refresh Direction MAY have a "NAT Inbound
refresh behavior" of "True".
Justification for REQ-6: Outbound refresh is necessary for allowing
the client to keep the mapping alive.
a) Inbound refresh may be useful for applications with no outgoing
UDP traffic. However, allowing inbound refresh may allow an
application to keep a mapping alive indefinitely. This may be
a security risk. Also, if the process is repeated with
different ports, over time, it could use up all the ports on
the NAT.
5. Filtering Behavior
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,
the NAT assigns a filtering rule for the mapping between an internal
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
filter packets originating from specific external endpoints.
Endpoint Independent Filtering:
The NAT filters out only packets not destined to the internal
address and port X:x, regardless of the external IP address and
port source (Z:z). The NAT forwards any packets destined to
X:x. In other words, sending packets from the internal side of
the NAT to any external IP address is sufficient to allow any
packets back to the internal endpoint.
Address Dependent Filtering:
The NAT filters out packets not destined to the internal
address X:x. Additionally, the NAT will filter out packets
from Y:y destined for the internal endpoint X:x if X:x has not
sent packets to Y:any previously (independently of the port
used by Y). In other words, for receiving packets from a
specific external endpoint, it is necessary for the internal
endpoint to send packets first to that specific external
endpoint's IP address.
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Address and Port Dependent Filtering:
This is similar to the previous behavior, except that the
external port is also relevant. The NAT filters out packets
not destined for the internal address X:x. Additionally, the
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
other words, for receiving packets from a specific external
endpoint, it is necessary for the internal endpoint to send
packets first to that external endpoint's IP address and port.
REQ-7: If application transparency is most important, it is
RECOMMENDED that a NAT have an "Endpoint independent filtering"
behavior. If a more stringent filtering behavior is most
important, it is RECOMMENDED that a NAT have an "Address dependent
filtering" behavior.
a) The filtering behavior MAY be an option configurable by the
administrator of the NAT.
OPEN ISSUE: Should REQ-7a be a SHOULD instead of a MAY?
Justification for REQ-7: The recommendation to use Endpoint
Independent Filtering is aimed at maximizing application
transparency, in particular for applications that receive media
simultaneously from multiple locations (e.g., gaming), or
applications that use rendezvous techniques. However, it is also
possible that in some circumstances, it may be preferable to have
a more stringent filtering behavior. Filtering independently of
the external endpoint is not as secure: an unauthorized packet
could get through a specific port while the port was kept open if
it was lucky enough to find the port open. In theory, filtering
based on both IP address and port is more secure than filtering
based only on the IP address (because the external endpoint could
in reality be two endpoints behind another NAT, where one of the
two endpoints is an attacker): however, such a policy could
interfere with applications that expect to receive UDP packets on
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
endpoint is behind a non-BEHAVE compliant NAT (say NAT-B) which
doesn't support REQ-1. When the endpoints use ICE, if NAT-A uses
Address and Port Dependent Filtering, connectivity will require a
Media Relay. However, if NAT-A uses Endpoint Independent
Filtering or Address Dependent Filtering, ICE will ultimately find
connectivity without requiring a Media Relay. Having the
filtering behavior being an option configurable by the
administrator of the NAT ensures that a NAT can be used in the
widest variety of deployment scenarios.
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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
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
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
referred to as hairpinning and is illustrated below.
NAT
+----+ from X1:x1 to X2':x2' +-----+ X1':x1'
| X1 |>>>>>>>>>>>>>>>>>>>>>>>>>>>>>--+---
+----+ | v |
| v |
| v |
| v |
+----+ from X1':x1' to X2:x2 | v | X2':x2'
| X2 |<<<<<<<<<<<<<<<<<<<<<<<<<<<<<--+---
+----+ +-----+
Hairpinning allows two endpoints on the internal side of the NAT to
communicate even if they only use each other's external IP addresses
and ports.
More formally, a NAT that supports hairpinning forwards packets
originating from an internal address, X1:x1, destined for an external
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'
is the same as X2'.
Furthermore, the NAT may present the hairpinned packet with either an
internal or an external source IP address and port. The hairpinning
NAT behavior can therefore be either "External source IP address and
port" or "Internal source IP address and port". "Internal source IP
address and port" may cause problems by confusing an implementation
that is expecting an external IP address and port.
REQ-8: A NAT MUST support "Hairpinning".
a) A NAT Hairpinning behavior MUST be "External source IP address
and port".
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Justification for REQ-8: This requirement is to allow communications
between two endpoints behind the same NAT when they are trying
each other's external IP addresses.
a) Using the external IP address is necessary for applications
with a restrictive policy of not accepting packets from IP
addresses that differ from what is expected.
7. Application Level Gateways
Certain NATs have implemented Application Level Gateways (ALGs) for
various protocols, including protocols for negotiating peer-to-peer
sessions such as SIP.
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
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-aware and must therefore be used with extreme caution.
REQ-9: If a NAT includes ALGs, it is RECOMMENDED that all of those
ALGs (except for DNS [19] and FTP [18]) be disabled by default.
a) If a NAT includes ALGs, it is RECOMMENDED that the NAT allow
the NAT administrator to enable or disable each ALG separately.
Justification for REQ-9: NAT ALGs may interfere with UNSAF methods.
a) This requirement allows the user to enable those ALGs that are
necessary to aid in the operation of some applications without
enabling ALGs which interfere with the operation of other
applications.
8. Deterministic Properties
The classification of NATs is further complicated by the fact that
under some conditions the same NAT will exhibit different behaviors.
This has been seen on NATs that preserve ports or have specific
algorithms for selecting a port other than a free one. If the
external port that the NAT wishes to use is already in use by another
session, the NAT must select a different port. This results in
different code paths for this conflict case, which results in
different behavior.
For example, if three hosts X1, X2, and X3 all send from the same
port x, through a port preserving NAT with only one external IP
address, called X1', the first one to send (i.e., X1) will get an
external port of x but the next two will get x2' and x3' (where these
are not equal to x). There are NATs where the External NAT mapping
characteristics and the External Filter characteristics change
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between the X1:x and the X2:x mapping. To make matters worse, there
are NATs where the behavior may be the same on the X1:x and X2:x
mappings but different on the third X3:x mapping.
Some NATs that try to reuse external ports flow from two internal IP
addresses to two different external IP addresses. For example, X1:x
is going to Y1:y1 and X2:x is going to Y2:y2, where Y1:y1 does not
equal Y2:y2. Some NATs will map X1:x to X1':x and will also map X2:x
to X1':x. This works if the NAT mapping is address port dependent.
However some NATs change their behavior when this type of port reuse
is happening. The NAT may look like it has NAT mappings that are
independent when this type of reuse is not happening but may change
to Address Port Dependent when this reuse happens.
Any NAT that changes the NAT mapping or the External Filtering
without configuration changes, at any point in time under any
particular conditions is referred to as a "non-deterministic" NAT.
NATs that don't are called "deterministic".
Non-deterministic NATs generally change behavior when a conflict of
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
Filtering in the absence of conflict is referred to as the Primary
behavior. The behavior after the first conflict is referred to as
Secondary and after the second conflict is referred to as Tertiary.
No NATs have been observed that change on further conflicts but it is
certainly possible that they exist.
REQ-10: A NAT MUST have deterministic behavior, i.e., it MUST NOT
change the NAT mapping or the External Filtering Behavior at any
point in time or under any particular conditions.
Justification for REQ-10: Non-deterministic NATs are very difficult
to troubleshoot because they require more intensive testing. This
non-deterministic behavior is the root cause of much of the
uncertainty that NATs introduce about whether or not applications
will work.
9. ICMP Destination Unreachable Behavior
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
ICMP message may be sent by the destination host or by any router
along the network path. The NAT's default configuration SHOULD NOT
filter ICMP messages based on their source IP address. Such ICMP
messages SHOULD be rewritten by the NAT (specifically the IP headers
and the ICMP payload) and forwarded to the appropriate internal or
external host. The NAT needs to perform this function for as long as
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the UDP mapping is active. Receipt of any sort of ICMP message MUST
NOT destroy the NAT mapping. A NAT which performs the functions
described in the paragraph above is referred to as "support ICMP
Processing".
There is no significant security advantage to blocking ICMP
Destination Unreachable packets. Additionally, blocking ICMP
Destination Unreachable packets can interfere with application
failover, UDP Path MTU Discovery (see RFC1191 [10] and RFC1435 [15]),
and traceroute. Blocking any ICMP message is discouraged, and
blocking ICMP Destination Unreachable is strongly discouraged.
REQ-11: Receipt of any sort of ICMP message MUST NOT destroy the NAT
mapping.
a) The NAT's default configuration SHOULD NOT filter ICMP messages
based on their source IP address.
b) It is RECOMMENDED that a NAT support ICMP Destination
Unreachable messages.
Justification for REQ-11: This is easy to do, is used for many things
including MTU discovery and rapid detection of error conditions,
and has no negative consequences.
10. Fragmentation of Outgoing Packets
When sending a packet, there are two situations that may cause IP
fragmentation for packets from the inside to the outside. It is
worth noting that many IP stacks do not use Path MTU Discovery with
UDP packets.
10.1 Smaller Adjacent MTU
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
been configured with a small MTU to reduce serialization delay when
sending large packets and small higher-priority packets, or for other
reasons.
The packet could have its Don't Fragment bit set to 1 (DF=1) or 0
(DF=0).
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
RFC 792 [13].
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
in a similar situation RFC 1812 [14].
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NATs that operate as described in this section are described as
"Supports Fragmentation" (abbreviated SF).
10.2 Smaller Network MTU
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
router adjacent to the small-MTU segment will fragment the packet and
forward the fragments as specified in RFC 1812 [14].
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.
The NAT needs to forward this ICMP message to the inside host.
The classification of NATs that perform this behavior is covered in
Section 9.
REQ-12: A NAT MUST support fragmentation of packets larger than link
MTU.
Justification for REQ-12: Fragmented packets become more common with
large video packets and should continue to work. Applications can
use MTU discovery to work around this problem.
11. Receiving Fragmented Packets
For a variety of reasons, a NAT may receive a fragmented packet. The
IP packet containing the header could arrive in any fragment
depending on network conditions, packet ordering, and the
implementation of the IP stack that generated the fragments.
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
fragments to the internal host is described as "Received Fragments
Ordered".
A NAT that is capable of receiving fragments in or out of order and
forwarding the individual packets (or a reassembled packet) to the
internal host is referred to as "Receive Fragments Out of Order".
See the Security Considerations section of this document for a
discussion of this behavior.
A NAT that is neither of these is referred to as "Receive Fragments
None".
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REQ-13: A NAT MUST support receiving in order fragments, so it MUST
be "Received Fragment Ordered" or "Received Fragment Out of
Order".
a) A NAT MAY support receiving fragmented packets that are out of
order and be of type "Received Fragment Out of Order".
Justification for REQ-13: See Security Considerations.
12. Requirements
The requirements in this section are aimed at minimizing the
complications caused by NATs to applications such as realtime
communications and online gaming. The requirements listed earlier in
the document are consolidated here into a single section.
It should be understood, however, that applications normally do not
know in advance if the NAT conforms to the recommendations defined in
this section. Peer-to-peer media applications still need to use
normal procedures such as ICE [16].
A NAT that supports all of the mandatory requirements of this
specification (i.e., the "MUST"), is "compliant with this
specification." A NAT that supports all of the requirements of this
specification (i.e., included the "RECOMMENDED") is "fully compliant
with all the mandatory and recommended requirements of this
specification."
REQ-1: A NAT MUST have an "External NAT mapping is endpoint
independent" behavior.
REQ-2: It is RECOMMENDED that a NAT have an "IP address pooling"
behavior of "Paired". Note that this requirement is not
applicable to NATs that do not support IP address pooling.
REQ-3: A NAT MUST NOT have a "Port assignment" behavior of "Port
overloading".
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
host's source port was in the range 1024-65535, it is
RECOMMENDED that the NAT's source port be in that range.
REQ-4: It is RECOMMENDED that a NAT have a "Port parity preservation"
behavior of "Yes".
REQ-5: A NAT UDP mapping timer MUST NOT expire in less than 2
minutes.
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
RECOMMENDED.
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REQ-6: The NAT mapping Refresh Direction MUST have a "NAT Outbound
refresh behavior" of "True".
a) The NAT mapping Refresh Direction MAY have a "NAT Inbound
refresh behavior" of "True".
REQ-7: If application transparency is most important, it is
RECOMMENDED that a NAT have "Endpoint independent filtering"
behavior. If a more stringent filtering behavior is most
important, it is RECOMMENDED that a NAT have "Address dependent
filtering" behavior.
a) The filtering behavior MAY be an option configurable by the
administrator of the NAT.
OPEN ISSUE: Should REQ-7a be a SHOULD instead of a MAY?
REQ-8: A NAT MUST support "Hairpinning".
a) A NAT Hairpinning behavior MUST be "External source IP address
and port".
REQ-9: If a NAT includes ALGs, it is RECOMMENDED that all of those
ALGs (except for DNS [19] and FTP [18]) be disabled by default.
a) If a NAT includes ALGs, it is RECOMMENDED that the NAT allow
the NAT administrator to enable or disable each ALG separately.
REQ-10: A NAT MUST have deterministic behavior, i.e., it MUST NOT
change the NAT mapping or the External External Filtering Behavior
at any point in time or under any particular conditions.
REQ-11: Receipt of any sort of ICMP message MUST NOT destroy the NAT
mapping.
a) The NAT's default configuration SHOULD NOT filter ICMP messages
based on their source IP address.
b) It is RECOMMENDED that a NAT support ICMP Destination
Unreachable messages.
REQ-12: A NAT MUST support fragmentation of packets larger than link
MTU.
REQ-13: A NAT MUST support receiving in order fragments, so it MUST
be "Received Fragment Ordered" or "Received Fragment Out of
Order".
a) A NAT MAY support receiving fragmented packets that are out of
order and be of type "Received Fragment Out of Order".
13. Security Considerations
NATs are often deployed to achieve security goals. Most of the
recommendations and requirements in this document do not affect the
security properties of these devices, but a few of them do have
security implications and are discussed in this section.
This work recommends that the timers for mapping be refreshed only on
outgoing packets and does not make recommendations about whether or
not inbound packets should update the timers. If inbound packets
update the timers, an external attacker can keep the mapping alive
forever and attack future devices that may end up with the same
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internal address. A device that was also the DHCP server for the
private address space could mitigate this by cleaning any mappings
when a DHCP lease expired. For unicast UDP traffic (the scope of
this document), it may not seem relevant to support inbound timer
refresh; however, for multicast UDP, the question is harder. It is
expected that future documents discussing NAT behavior with multicast
traffic will refine the requirements around handling of the inbound
refresh timer. Some devices today do update the timers on inbound
packets.
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
is less secure than using the IP and port. 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
are very difficult to test because they are, well, non-deterministic.
Testing by a person configuring one may result in the person thinking
it is behaving as desired, yet under different conditions, which an
attacker can create, the NAT may behave differently. These
requirements recommend that devices be deterministic.
The work requires that NATs have an "external NAT mapping is endpoint
independent" behavior. This does not reduce the security of devices.
Which packets are allowed to flow across the device is determined by
the external filtering behavior, which is independent of the mapping
behavior.
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
first, many systems simply discard the out of order packets.
Moreover, since some networks deliver small packets ahead of large
ones, there can be many out of order fragments. NATs that are
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
DoS opportunity if done incorrectly. Fragmentation has been a tool
used in many attacks, some involving passing fragmented packets
through NATs and others involving DoS attacks based on the state
needed to reassemble the fragments. NAT implementers should be aware
of RFC 3128 [12] and RFC 1858 [11].
14. IANA Considerations
There are no IANA considerations.
15. IAB Considerations
The IAB has studied the problem of "Unilateral Self Address Fixing",
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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
collaborative protocol reflection mechanism [2].
This specification does not in itself constitute an UNSAF
application. It consists of a series of requirements for NATs aimed
at minimizing the negative impact that those devices have on peer-to-
peer media applications, especially when those applications are using
UNSAF methods.
Section 3 of UNSAF lists several practical issues with solutions to
NAT problems. This document makes recommendations to reduce the
uncertainty and problems introduced by these practical issues with
NATs. In addition, UNSAF lists five architectural considerations.
Although this is not an UNSAF proposal, it is interesting to consider
the impact of this work on these architectural considerations.
Arch-1: The scope of this is limited to UDP packets in NATs like the
ones widely deployed today. The "fix" helps constrain the
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
imply any protocol machinery that would continue to live
after NATs were gone or make it more difficult to remove
them.
Arch-3: This does not reduce the overall brittleness of NATs but will
hopefully reduce some of the more outrageous NAT behaviors
and make it easer to discuss and predict NAT behavior in
given situations.
Arch-4: This work and the results [17] of various NATs represent the
most comprehensive work at IETF on what the real issues are
with NATs for applications like VoIP. This work and STUN
have pointed out more than anything else the brittleness NATs
introduce and the difficulty of addressing these issues.
Arch-5: This work and the test results [17] provide a reference model
for what any UNSAF proposal might encounter in deployed NATs.
16. Acknowledgments
The editor would like to acknowledge Bryan Ford, Pyda Srisuresh and
Dan Kegel for the their multiple contributions on peer-to-peer
communications across a NAT. Dan Wing contributed substantial text
on IP fragmentation and ICMP behavior. Thanks to Rohan Mahy,
Jonathan Rosenberg, Mary Barnes, Melinda Shore, Lyndsay Campbell,
Geoff Huston, Jiri Kuthan, Harald Welte, Steve Casner, Robert
Sanders, Spencer Dawkins, Saikat Guha, Christian Huitema, Yutaka
Takeda and Paul Hoffman for their contributions.
17. References
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17.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-
Address Fixing (UNSAF) Across Network Address Translation",
RFC 3424, November 2002.
17.2 Informational References
[3] Srisuresh, P. and M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations", RFC 2663, August 1999.
[4] Srisuresh, P. and K. Egevang, "Traditional IP Network Address
Translator (Traditional NAT)", RFC 3022, January 2001.
[5] Holdrege, M. and P. Srisuresh, "Protocol Complications with the
IP Network Address Translator", RFC 3027, January 2001.
[6] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[7] Rosenberg, J., Huitema, C., and R. Mahy, "STUN - Simple
Traversal of User Datagram Protocol (UDP) Through Network
Address Translators (NATs)", draft-ietf-behave-rfc3489bis (work
in progress), February 2003.
[8] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
RFC 3550, July 2003.
[9] Huitema, C., "Real Time Control Protocol (RTCP) attribute in
Session Description Protocol (SDP)", RFC 3605, October 2003.
[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.
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[14] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812,
June 1995.
[15] Knowles, S., "IESG Advice from Experience with Path MTU
Discovery", March 1993.
[16] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
Methodology for Network Address Translator (NAT) Traversal for
the Session Initiation Protocol (SIP)",
draft-ietf-mmusic-ice-04 (work in progress), February 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
SPECIFICATION", RFC 1035, November 1987.
[20] "Packet-based Multimedia Communications Systems", ITU-
T Recommendation H.323, July 2003.
Authors' Addresses
Francois Audet (editor)
Nortel Networks
4655 Great America Parkway
Santa Clara, CA 95054
US
Phone: +1 408 495 3756
Email: audet@nortel.com
Cullen Jennings
Cisco Systems
170 West Tasman Drive
MS: SJC-21/2
San Jose, CA 95134
US
Phone: +1 408 902 3341
Email: fluffy@cisco.com
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