BEHAVE F. Audet
Internet-Draft Nortel Networks
Expires: April 15, 2005 C. Jennings
Cisco Systems
October 15, 2004
NAT Behavioral Requirements for Unicast UDP
draft-ietf-behave-nat-00
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Abstract
This document defines basic terminology for describing different
types of behavior for NATs when handling unicast UDP. It also
defines a set of requirements for NATs that would allow many
applications, such as multimedia communications or online gaming, to
work consistently. Developing NATs that meet this set of
requirements will greatly increase the likelihood that applications
will function properly.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Network Address and Port Translation Behavior . . . . . . . 6
3.1 Address and Port Binding . . . . . . . . . . . . . . . . . 6
3.2 Port Assignment . . . . . . . . . . . . . . . . . . . . . 8
3.3 Bind Refresh Direction . . . . . . . . . . . . . . . . . . 9
3.4 Bind Refresh Scope . . . . . . . . . . . . . . . . . . . . 10
4. Filtering Behavior . . . . . . . . . . . . . . . . . . . . . 10
4.1 Filtering of Unsolicited Packets . . . . . . . . . . . . . 10
4.2 NAT Filter Refresh . . . . . . . . . . . . . . . . . . . . 11
5. Hairpinning Behavior . . . . . . . . . . . . . . . . . . . . 11
6. Application Level Gateways . . . . . . . . . . . . . . . . . 12
7. Deterministic Properties . . . . . . . . . . . . . . . . . . 12
8. ICMP Behavior . . . . . . . . . . . . . . . . . . . . . . . 13
9. Fragmentation of Packets . . . . . . . . . . . . . . . . . . 14
9.1 Smaller Adjacent MTU . . . . . . . . . . . . . . . . . . . 14
9.2 Smaller Network MTU . . . . . . . . . . . . . . . . . . . 14
10. Receiving Fragmented Packets . . . . . . . . . . . . . . . . 14
11. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 15
11.1 Requirement Discussion . . . . . . . . . . . . . . . . . 17
12. Security Considerations . . . . . . . . . . . . . . . . . . 19
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . 20
14. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 20
15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 21
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
16.1 Normative References . . . . . . . . . . . . . . . . . . . 21
16.2 Informational References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 23
Intellectual Property and Copyright Statements . . . . . . . 24
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1. 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
[11]), 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 [9]
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.
Several recommendations regarding NATs for Peer-to-Peer media were
made in [10] and this specification derives some of its requirements
from that draft.
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.
The authors strongly believe that if there were a common set of
requirements that were simple and useful for voice, video, and games,
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the bulk of the NAT vendors would choose to meet those requirements.
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 requirements for NATs.
1.1 Scope
This specification only covers Traditional NATs [4]. Bi-directional,
Twice NAT, and Multihomed NAT [3] are outside the scope of this
document.
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, ICMP, IPSEC, or other protocols. Since the
document is on UDP unicast only, packet inspection below the UDP
layer (including RTP) is also out-of-scope.
This document does not cover Firewalls. However, it does cover the
inherent filtering aspects of NATs.
2. 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].
It is assumed that the reader is familiar with the terminology
described in RFC 2663 [3] and RFC 3022 [4]. This specification
attempts to preserve the terminology used in those RFCs.
This document uses the term "session" as defined in RFC 2663 [3]:
"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 binding" as defined in RFC 2663
RFC 3022: "Address binding is the phase in which a local IP address
is associated with an external address, or vice versa, for purpose of
translation."
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The term NAT is used to refer to both traditional address translation
and address port translation. The authors understand that there was
a time when these were considered different, but terminology has
changed over time, and the term NAT has subsumed port translation as
part of it.
RFC 3489 [7] defines a terminology for different NAT variations. In
particular, it uses the terms "Full Cone", "Restricted Cone", "Port
Restricted Cone" and "Symmetric" to refer to different variations of
NATs. Unfortunately, this terminology has been the source of much
confusion. This terminology does not distinguish between the NAT
behavior and the filtering behavior of the device. It was found that
many devices' behaviors do not exactly fit into the described
variations. For example, a device could be symmetric from a
filtering point of view and Cone from a NAT point of view. Other
aspects of NATs are not covered by this terminology: for example,
many NATs will switch over from basic NAT (preserving ports) to NAPT
(mapping ports) in order to preserve ports when possible.
This specification will therefore not use the Cone/Symmetric
terminology. Furthermore, many other important behaviors are not
fully described by the Cone/Symmetric terminology. This
specification refers to specific individual NAT behaviors instead of
using the Cone/Symmetric terminology.
Note: RFC 3489 defines a "Symmetric NAT" in effectively two parts:
1. All requests from the same internal IP address and port to a
specific destination IP address and port are mapped to the same
external IP address and port. If a host sends a packet with the
same source address and port to different destination addresses
or ports, a different mapping is used for each.
2. Furthermore, only the external host that receives a packet can
send a UDP packet back to the internal host.
Condition 1 is the NAT behavior and condition 2 is the filtering
behavior. However, they are not necessarily dependent: we have
observed NATs that will conform to condition (1) but not to (2).
Using RFC 3489, this type of NAT would be detected as a "Cone NAT"
since it uses condition (2). Using a different algorithm such as the
one described in NATCHECK [12] which uses condition (1), the same NAT
would be detected as a "Symmetric NAT". If the endpoint receiving
the media has a permissive policy on accepting media, condition (2)
is more appropriate, but if it has a restrictive policy, condition
(1) is more appropriate.
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3. Network Address and Port Translation Behavior
This section describes the various NAT behaviors applicable to
dynamic NAT; static NAT is outside the scope of this document.
3.1 Address and Port Binding
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 binding 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.
The key behavior to describe is the criteria for re-use of a binding
for new sessions to external endpoints, after establishing a first
binding 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
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l
The following address and port binding behavior are defined:
External NAT binding is endpoint independent: The NAT reuses the port
binding 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.
(From an RFC 3489 NAT perspective, this is a "Cone NAT" where the
sub-type is really based on the filtering behavior.)
External NAT binding is endpoint address dependent: The NAT reuses
the port binding 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. (From an RFC 3489 NAT perspective, but not necessarily a
filtering perspective, this is a "Symmetric NAT".)
External NAT binding is endpoint address and port dependent: The NAT
reuses the port binding 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. (From an RFC 3489 NAT
perspective, but not necessarily a filtering perspective, this is
a "Symmetric NAT".)
The three possibilities are abbreviated as NB=I, NB=AD, and NB=APD,
respectively. NB stands for Nat Binding, I for independent, AD for
Address Dependent, and APD for Address Port Dependent.
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 binding in an arbitrary fashion
(i.e. randomly): one internal IP address could have multiple
external IP address bindings active at the same time for different
sessions. These NATs have an "IP address pooling" behavior of
"arbitrary". Other NATs use the same external IP address binding for
all sessions associated with the same internal IP address in any
circumstance, even if ports are running out, in which case they would
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fail to establish a session. These NATs have an "IP address pooling"
behavior of "Paired, strict." Yet other NATs use the same external IP
address binding for all sessions associated with the same IP internal
IP address only when possible. These NATs have an "IP address
pooling" behavior of "Paired, best effort." NATs that use an "IP
address pooling" behavior of "arbitrary" can cause issues for
applications that use multiple ports from the same endpoint but have
no way to negotiate IP addresses individually (e.g., RTP and RTCP
ports).
3.2 Port Assignment
Some NATs attempt to preserve the port number used internally when
assigning a binding to an external IP address and port (e.g., 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., X1:x to X':x
and X2:x to X':x'). This 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".
Tools such as network sniffers identify traffic based on the
destination port, not the source port, so port preservation does not
help these tools.
Some particularly nasty NATs use port overloading, i.e. they always
use port preservation even in the case of collision (i.e., X1:x to
X':x, and X2:x to X':x). These NATs rely on the source of the
response from the external endpoint (Y:y, Z:z) 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. This is referred to as "port overloaded".
Most applications fail in some cases with "Port overloaded". It is
clear that "port overloaded" behavior will result in many problems.
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
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even ports, and RTCP use odd ports. There is some very unfortunate
wording in RFC 3550 section 11, which can cause some problematic
behavior in RTP clients. The wording could be interpreted as saying
that if a client receives an odd port for sending RTP, it SHOULD
change it to the next lower even number. This would obviously result
in the loss of RTP/UDP. In the event that a NAT supporting port
parity preservation is running out of available ports for a specific
parity, it may either fail to assign a port, or it could decide not
respect port parity. We refer to these properties as "port parity
preservation" of "No", "Yes, strict" and "Yes, best effort".
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. There has been some suggestion that some IDS systems with
deep packet inspection devices may end up looking at the ports. This
is clearly true for destination ports but less understood for source
ports. 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.
3.3 Bind Refresh Direction
NAT UDP binding timeout implementations vary but include the timer's
value and the way the binding timer is refreshed to keep the binding
alive.
The binding timer is defined as the time a binding will stay active
without packets traversing the NAT. There is great variation in the
values used by different NATs.
Some NATs keep the binding 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 an Outbound NAT
refresh behavior of "True".
Some NATs keep the binding 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 an Inbound NAT refresh direction behavior of
"True".
Some NATs keep the binding active on both, in which case both
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properties are "True".
3.4 Bind Refresh Scope
If the binding is refreshed for all sessions on that bind by any
outbound traffic, the NAT is said to have a NAT refresh method
behavior of "Per binding". If the binding is refreshed only on a
specific session on that particular bind by any outbound traffic, the
NAT is said to have a "Per session" NAT refresh method behavior.
4. Filtering Behavior
This section describes various filtering behaviors observed in NATs.
4.1 Filtering of Unsolicited Packets
When an internal endpoint opens an outgoing UDP session through a
NAT, the NAT assigns a filtering rule for the binding 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 open: The NAT does not filter any packets.
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. (From an RFC 3489 filtering perspective, this is a
"Full Cone NAT".)
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. (From an RFC 3489
filtering perspective, this is a "Restricted Cone NAT".)
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
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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. (From an RFC 3489 filtering
perspective, this is both a "Port Restricted Cone NAT" and a
"Symmetric NAT" as they have the same filtering behavior.)
These are abbreviated as EF=O, EF=I, EF=AD, EF=APD respectively. In
the case of a NAT, "open" cannot forward a packet unless there is a
NAT binding, so for all practical purposes, a NAT will never be
"open" but will be one of the others.
4.2 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 EF=AD or EF=APD NATs, the scope of the refresh could
include the filters for just the particular port and destination or
for all the ports and destinations sharing the same external address
and port on the NAT.
5. 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 X2':x2' to X1:x1 | 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
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originating from an internal address, X1:x1, destined for an external
address X2':x2' that has an active binding to an internal address
X2:x2, back to that internal address X2:x2. (Note that typically,
X1'=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.
6. Application Level Gateways
Certain NATs have implemented Application Level Gateways (ALGs) for
various protocols, including protocols for negotiating peer-to-peer
UDP sessions.
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 and must therefore be used
with extreme caution.
7. Deterministic Properties
The diagnosis 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 Binding
characteristics and the External Filter characteristics change
between the X1:x and the X2:x binding. To make matters worse, there
are NATs where the behavior may be the same on the X1:x and X2:x
binds but different on the third X3:x binding.
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
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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 Binding 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
Bindings 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 Binding or the External Filtering at any
point in time or 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 bind. The NAT binding 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
additional testing may be required.
8. ICMP Behavior
There are cases in which a host inside the NAT sends a packet to the
NAT that gets relayed towards a host on the external side of the NAT
that results in an ICMP Destination Unreachable message being
returned to the NAT. Most NATs will send an appropriate ICMP
Destination Unreachable message to the internal host that sent the
original packet. NATs that do not filter out this ICMP Destination
Unreachable message when it is in reply to a IP packet sent are
referred to as "Support Destination Unreachable" (abbreviated SU).
Incoming Destination Unreachable messages can be ignored after some
period of time after the packet which elicited the Destination
Unreachable message. This IMCP timeout needs to be greater than the
RTT for any destination the NAT may attempt to send IP packets to.
Keep in mind satellite links when setting this timeout.
Applications use the destination unreachable message to decide that
they can stop trying to retransmit to a particular IP address and can
fail over to a secondary address. If a destination unreachable
message is not received, the fail over will take too long for many
applications. Another key use of this message is for MTU discovery
(described in RFC 1191 [14]). MTU discovery is important for
allowing applications to avoid the fragmentation problems discussed
in the next section. The arrival of a destination unreachable packet
cannot destroy the NAT binding, as the occasional arrival of these
messages is normal for black-hole discovery RFC 2923 [17].
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There is no significant security advantage to blocking these ICMP
Destination Unreachable packets.
9. 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.
9.1 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 RFC 792 [18].
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 [19].
NATs that operate as described in this section are described as
"Supports Fragmentation" (abbreviated SF).
9.2 Smaller Network MTU
The second situation is when the MTU in the middle of the network is
too small. If DF=0, the router adjacent to the small-MTU segment
will fragment the packet and forward the fragments RFC 1812 [19].
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.
10. 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
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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 (abbreviated RF=O).
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
(abbreviated RF=OO). 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 (abbreviated RF=N).
11. Requirements
The requirements in this section are aimed at minimizing the damage
caused by NATs to applications such as realtime communications and
online gaming.
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 [9].
REQ-1: A NAT MUST have an "External NAT Binding is endpoint
independent" behavior (NB=I).
REQ-2: It is RECOMMENDED that a NAT have an "IP address pooling"
behavior of "Paired, best effort". Note that this requirement is
not applicable to NATs that do not support IP address pooling.
REQ-2a: A NAT MAY have an "IP address pooling" behavior of
"Yes, strict."
REQ-2b: A NAT MUST NOT have an "IP address pooling" behavior of
"No".
REQ-3: It is RECOMMENDED that a NAT have a "No port preservation"
behavior.
REQ-3a: A NAT MAY use a "Port preservation" behavior.
REQ-3b: A NAT MUST NOT have a "Port overloaded" behavior.
REQ-3c: If the NAT changes the source port, it MUST NOT
allocate the new port from within the range of 0-1023.
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REQ-4: It is RECOMMENDED that a NAT have a "Port parity
preservation" behavior of "Yes, best effort".
REQ-4a: A NAT MAY have a "Port parity preservation" behavior of
"Yes, strict."
REQ-4b: A NAT MUST NOT have a "Port parity preservation"
behavior of "No".
REQ-5: A dynamic NAT UDP binding timer MUST NOT expire in less
than 2 minutes.
REQ-5a: The value of the NAT UDP binding timer MAY be
configurable.
REQ-5b: A default value of 5 minutes for the NAT UDP binding
timer is RECOMMENDED.
REQ-6: The NAT UDP timeout binding MUST have a NAT Outbound
refresh behavior of "True".
REQ-6a: The NAT UDP timeout binding MAY have a NAT Inbound
refresh behavior of "True".
REQ-6b: The NAT UDP timeout binding MUST have a NAT refresh
method behavior of "Per binding" (i.e. refresh all sessions
active on a particular bind).
REQ-7: It is RECOMMENDED that a NAT have an "External filtering is
endpoint address dependent" behavior. (EF=AD)
REQ-7a: A NAT MAY have an "External filtering is endpoint
independent" behavior. (EF=I)
REQ-7b: A NAT MAY have an "External filtering is endpoint
address and port dependent" behavior. (EF=APD)
REQ-8: The NAT UDP filter timeout behavior MUST be the same as the
NAT UDP binding timeout.
REQ-9: A NAT MUST support "Hairpinning" behavior.
REQ-9a: A NAT Hairpinning behavior MUST be "External source IP
address and port".
REQ-10: A NAT MUST have the capability to turn off individually
all ALGs it supports, except for DNS and IPsec.
REQ-10a: Any NAT ALG for SIP MUST be turned off by default.
REQ-11: A NAT MUST have deterministic behavior.
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REQ-12: A NAT MUST support ICMP Destination Unreachable (SU).
REQ-12a: The ICMP timeout SHOULD be greater than 2 seconds.
REQ-13: A NAT MUST support fragmentation of packets larger than
link MTU (SF).
REQ-14: A NAT MUST support receiving in order fragments, so it
MUST be RF=O or RF=OO.
REQ-14a: A NAT MAY support receiving fragmented packets that
are out of order and be of type RF=OO.
11.1 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, but without running out of ports
unnecessarily.
REQ-3: NATs that implement port preservation have to deal with
conflicts on ports, and the multiple code paths this introduces
often result in nondeterministic behavior.
REQ-3a: Port preservation can work, but the NAT implementors
need to be very careful that it does not become a
nondeterministic NAT.
REQ-3b: REQ-2b 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.
REQ-3c: This requirement is because some applications may not
be able to use ports in the well-known range because of
priviledge restrictions.
REQ-4: This will respect the RTP/RTCP parity convention when
possible, but without running out of ports unnecessarily.
REQ-5: This requirement is to ensure that the timeout is long
enough to avoid too frequent timer refresh packets.
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REQ-5a: Configuration is desirable for adapting to specific
networks and troubleshooting.
REQ-5b: This default is to avoid too frequent timer refresh
packets.
REQ-6: Outbound refresh is necessary for allowing the client to
keep the binding alive.
REQ-6a: Inbound refresh may be useful for applications where
there is no outgoing UDP traffic.
REQ-6b: Using the refresh on a per binding basis avoids the
need for separate keep alives for all the available sessions.
REQ-7: Filtering based on the IP address is felt to have the
maximum balance between security and usefulness. See below.
REQ-7a: 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.
REQ-7b: 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 is to avoid overly complex applications.
REQ-9: This requirement is to allow communications between two
endpoints behind the same NAT when they are trying each other's
external IP addresses.
REQ-9a: 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-10: NAT ALGs may interfere with UNSAF methods.
REQ-10a: A SIP NAT ALG will interfere with UNSAF methods.
REQ-11: 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.
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REQ-12: 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-13 and REQ-13: Fragmented packets become more common with
large video packets and should continue to work. Applications can
use MTU discovery to work around this.
12. 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 binding 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 binding 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 bindings
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 binding 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 binding
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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 [16] and RFC
1858 [15].
13. IANA Considerations
There are no IANA considerations.
14. 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
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.
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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 [13] 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 [13] provide a reference model
for what any UNSAF proposal might encounter in deployed NATs.
15. Acknowledgments
Dan Wing contributed substantial text on IP fragmentation.
The editor would like to acknowledge Bryan Ford, Pyda Srisuresh and
Dan Kegel for the NATP2P [10] draft, from which a lot of the material
in this specification is derived. Thanks to Rohan Mahy, Jonathan
Rosenberg, Mary Barnes, Dan Wing, Melinda Shore, Lyndsay Campbell,
Geoff Huston, Jiri Kuthan, Harald Welte and Spencer Dawkins for their
important contributions.
16. References
16.1 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.
16.2 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.
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[7] Rosenberg, J., Weinberger, J., Huitema, C. and R. Mahy, "STUN -
Simple Traversal of User Datagram Protocol (UDP) Through
Network Address Translators (NATs)", RFC 3489, March 2003.
[8] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC
3550, July 2003.
[9] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
Methodology for Network Address Translator (NAT) Traversal for
the Session Initiation Protocol (SIP)",
draft-ietf-mmusic-ice-02 (work in progress), July 2004.
[10] Ford, B., Srisuresh, P. and D. Kegel, "Peer-to-Peer(P2P)
communication across Network Address Translators(NATs)",
draft-ford-midcom-p2p-03 (work in progress), June 2004.
[11] "Packet-based Multimedia Communications Systems", ITU-T
Recommendation H.323, July 2003.
[12] Ford, B. and D. Andersen, "Nat Check Web Site:
http://midcom-p2p.sourceforge.net", June 2004.
[13] Jennings, C., "NAT Classification Results using STUN",
draft-jennings-midcom-stun-results-01 (work in progress), July
2004.
[14] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[15] Ziemba, G., Reed, D. and P. Traina, "Security Considerations
for IP Fragment Filtering", RFC 1858, October 1995.
[16] Miller, I., "Protection Against a Variant of the Tiny Fragment
Attack (RFC 1858)", RFC 3128, June 2001.
[17] Reynolds, J. and J. Postel, "Assigned numbers", RFC 923,
October 1984.
[18] Postel, J., "Internet Control Message Protocol", STD 5, RFC
792, September 1981.
[19] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812,
June 1995.
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Authors' Addresses
Francois Audet
Nortel Networks
4655 Great America Parkway
Santa Clara, CA 95054
USA
Phone: +1-408-495-3756
EMail: audet@nortelnetworks.com
Cullen Jennings
Cisco Systems
170 West Tasman Drive
MS: SJC-21/2
San Jose, CA 95134
USA
Phone: +1-408-902-3341
EMail: fluffy@cisco.com
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