BEHAVE F. Audet, Ed.
Internet-Draft Nortel Networks
Expires: October 13, 2005 C. Jennings
Cisco Systems
April 11, 2005
NAT Behavioral Requirements for Unicast UDP
draft-ietf-behave-nat-udp-01
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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 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
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likelihood that these applications will function properly.
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 . . . . . . . . . . . . . . . . . . . . . 7
4.2.1 Port Assignment Behavior . . . . . . . . . . . . . . . 7
4.2.2 Port Parity . . . . . . . . . . . . . . . . . . . . . 9
4.2.3 Port Contiguity . . . . . . . . . . . . . . . . . . . 9
4.3 Mapping Refresh Direction . . . . . . . . . . . . . . . . 10
4.4 Mapping Refresh Scope . . . . . . . . . . . . . . . . . . 10
5. Filtering Behavior . . . . . . . . . . . . . . . . . . . . . . 11
5.1 Filtering of Unsolicited Packets . . . . . . . . . . . . . 11
5.2 NAT Filter Refresh . . . . . . . . . . . . . . . . . . . . 11
6. Hairpinning Behavior . . . . . . . . . . . . . . . . . . . . . 12
7. Application Level Gateways . . . . . . . . . . . . . . . . . . 12
8. Deterministic Properties . . . . . . . . . . . . . . . . . . . 13
9. ICMP Behavior . . . . . . . . . . . . . . . . . . . . . . . . 14
10. Fragmentation of Packets . . . . . . . . . . . . . . . . . . 14
10.1 Smaller Adjacent MTU . . . . . . . . . . . . . . . . . . . 14
10.2 Smaller Network MTU . . . . . . . . . . . . . . . . . . . 15
11. Receiving Fragmented Packets . . . . . . . . . . . . . . . . 15
12. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 15
12.1 Requirement Discussion . . . . . . . . . . . . . . . . . . 17
13. Security Considerations . . . . . . . . . . . . . . . . . . 19
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . 20
15. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 20
16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 21
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
17.1 Normative References . . . . . . . . . . . . . . . . . . . 21
17.2 Informational References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 23
Intellectual Property and Copyright Statements . . . . . . . . 24
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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 generally to all NAT
variations, including the ones described in RFC 2663 [3] (Traditional
NAT, Basic NAT, NAPT, Bi-directional NAT, Twice NAT, and Multihomed
NATs). However, it is not within the scope of this specification to
address all issues specific to all possible NAT variations.
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
capabilities. Firewalls is specifically out-of-scope of this
specification: however, this specification does cover the inherent
filtering aspects of NAT.
Approaches using directly signaled control off the middle boxes such
as Midcom, UPnP, or in-path signaling are out of scope.
UDP Relays are out of the scope of this document.
Application aspects are out of scope as the focus 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 below the UDP layer
(including RTP) is also out-of-scope.
2. Introduction
Network Address Translators (NAT) 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
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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
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 part of what contributes to
the overall brittleness introduced by NATs and makes it extremely
difficult to predict how any given protocol will behave on a network
traversing NATs. 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 the NAT vendors face is 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).
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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
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.
Earlier documents 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 proved
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 NAT.
4.1 Address and Port Mapping
When an internal endpoint opens an outgoing UDP 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
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the NAT when there are multiple simultaneous sessions established to
different external endpoints.
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:
External NAT mapping is endpoint independent:
The NAT reuses the port mapping for subsequent sessions
initiated 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.
External NAT mapping is endpoint address dependent:
The NAT reuses the port mapping for subsequent sessions
initiated from the same internal IP address and port (X:x) only
for sessions to the same external IP address, regardless of the
external port. Specifically, X1':x1' equals X2':x2' if, and
only if, Y2 equals Y1.
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External NAT mapping is endpoint address and port dependent:
The NAT reuses the port mapping for subsequent sessions
initiated from the same internal IP address and port (X:x) only
for sessions to the same external and port. 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.
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).
4.2 Port Assignment
4.2.1 Port Assignment Behavior
This section uses the following diagram for reference.
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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 a 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
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
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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 dynamics 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.
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. 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 rule that RTP use even ports and RTCP use odd ports when
the application takes a single port number as a parameter and derives
the RTP and RTCP port pair from that number. 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.
4.2.3 Port Contiguity
Some NATs attempt to preserve the port contiguity rule of RTCP=RTP+1.
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These NATs do things like sequential assignment, port reservation and
so forth. 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.
Furthermore, there is a glare problem if many applications (or
endpoints) are trying to open mapping simultaneously. Port
reservation is also problematic since it is wasteful, especially
considering that a NAT can not 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 follows rules to negotiate both 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 Direction
NAT UDP 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.
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
properties are "True".
4.4 Mapping Refresh Scope
If the mapping is refreshed for all sessions on that mapping by any
outbound traffic, the NAT is said to have a NAT Mapping Refresh Scope
of "Per mapping". If the mapping is refreshed only on a specific
session on that particular mapping by any outbound traffic, the NAT
is said to have a "Per session" NAT mapping Refresh Scope.
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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 UDP 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.
External filtering is endpoint independent:
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.
External filtering is endpoint address dependent:
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 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.
External filtering is endpoint address and port dependent:
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.
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. In
the case of "External Filtering" of "Address dependent" or "Address
and port dependent" NATs, the scope of the refresh could include the
filters for just the particular port and destination or for all the
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ports and destinations sharing the same external address and port on
the NAT.
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.
7. Application Level Gateways
Certain NATs have implemented Application Level Gateways (ALGs) for
various protocols, including protocols for negotiating peer-to-peer
UDP 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
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them turned off by default but let them be turned on.
NAT ALGs may interfere with UNSAF methods and must therefore be used
with extreme caution.
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
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 in the case where 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 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
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certainly possible that they exist.
9. ICMP Behavior
When a NAT sends a UDP 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
the UDP mapping is active. Receipt of any sort of ICMP message MUST
NOT destroy the NAT binding. A NAT which performs the functions
described in the paragraph above is referred to as "UDP Support
Destination Unreachable".
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 with traceroute. Blocking any
ICMP message is discouraged, and blocking ICMP Destination
Unreachable is strongly discouraged.
10. Fragmentation of 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.
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
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the fragments, in order. This is the same function a router performs
in a similar situation RFC 1812 [14].
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 RFC 1812.
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
the ICMP section of this document.
11. Receiving Fragmented Packets
For a variety of reasons, a NAT may receive a fragmented UDP packet.
The IP packet containing the UDP header could arrive first or last
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 UDP fragments in order (that
is, with the UDP 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 UDP 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".
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.
It should be understood, however, that applications normally do not
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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 IA 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 also 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 also 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.
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".
b) The NAT mapping Refresh Direction MUST have a "NAT refresh
method behavior" of "Per mapping" (i.e. refresh all
sessions active on a particular mapping).
REQ-7 It is RECOMMENDED that a NAT have an "External filtering is
endpoint address dependent" behavior.
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.
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a) If a NAT includes ALGs, it is RECOMMENDED that the NAT
allow the user 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 It is RECOMMENDED that a NAT support ICMP Destination
Unreachable.
a) The ICMP timeout SHOULD be greater than 2 seconds.
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".
12.1 Requirement Discussion
This section describes why each of these requirements was chosen and
the consequences of violating any of them:
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.
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 because in a
peer-to-peer 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 lost of the RTP stream. This requirements 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 (i.e., RFC 3261). This requirement will therefore
address a particularly nasty problem that will prevail for a
significant amount of time.
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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 a NAT 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.
REQ-4 This is to avoid breaking peer-to-peer applications which do
not explicity 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.
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.
REQ-6 Outbound refresh is necessary for allowing the client to keep
the mapping alive.
a) Inbound refresh may be useful for applications where there
is no outgoing UDP traffic.
b) Using the refresh on a per mapping basis avoids the need
for separate keep alive packets for all the available
sessions.
REQ-7 Filtering based on the IP address is felt to have the maximum
balance between security and usefulness. Filtering
independently of the external IP address and port is not as
secure: an unauthorized packet could get at 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 restrictive policy could
interfere with certain applications that use more than one
port.
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.
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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.
REQ-9 NAT ALGs may interfere with UNSAF methods.
a) This requirement allows the user to enable ALGs which are
necessary to aid operation of some applications without
enabling ALGs which interfere with operation of other
applications.
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.
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.
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.
REQ-13 See Security Considerations.
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
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.
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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. 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",
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
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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 accross a NAT, from which a lot of the material in
this specification is derived.
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 and Spencer Dawkins for their important
contributions.
17. References
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.
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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.
[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)",
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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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