BEHAVE F. AUDET
Internet-Draft Nortel Networks
Expires: January 9, 2005 C. Jennings
Cisco Systems
July 11, 2004
NAT/Firewall Behavioral Requirements
draft-audet-nat-behave-00
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Abstract
This document defines basic terminology for describing different
types of behavior for NATs and firewalls. It also defines a set of
requirements for NATs and firewalls that would allow many
applications, such as multimedia communications or online gaming, to
work consistently. Developing NATs and firewalls 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
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
3. UDP NAT Behavior . . . . . . . . . . . . . . . . . . . . . . 5
3.1 Address and port binding . . . . . . . . . . . . . . . . . 5
3.2 Port assignment . . . . . . . . . . . . . . . . . . . . . 7
3.3 Bind Refresh Direction . . . . . . . . . . . . . . . . . . 8
3.4 Bind Refresh Scope . . . . . . . . . . . . . . . . . . . . 8
4. UDP Firewall Behavior (Filtering) . . . . . . . . . . . . . 9
4.1 Filtering of unsolicited packets . . . . . . . . . . . . . 9
4.2 Firewall Filter Refresh . . . . . . . . . . . . . . . . . 10
5. Hairpinning Behavior . . . . . . . . . . . . . . . . . . . . 10
6. Deterministic Properties . . . . . . . . . . . . . . . . . . 11
7. ICMP Behavior . . . . . . . . . . . . . . . . . . . . . . . 12
8. Fragmentation Behavior . . . . . . . . . . . . . . . . . . . 12
9. TCP Behavior . . . . . . . . . . . . . . . . . . . . . . . . 13
10. Multicast and IGMP Behavior . . . . . . . . . . . . . . . . 13
11. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 14
11.1 Requirement Discussion . . . . . . . . . . . . . . . . . 15
12. Security Considerations . . . . . . . . . . . . . . . . . . 17
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . 17
14. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 18
15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 18
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
16.1 Normative References . . . . . . . . . . . . . . . . . . . 19
16.2 Informational References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . 21
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1. Introduction
Network Address Translators (NAT) and firewalls are well known to
cause very significant problems with applications that carry IP
addresses in the payload [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
and firewalls. Application Level Gateways [3] are one such
mechanism. STUN [7] describes a UNilateral Self-Address Translation
(UNSAF) mechanism[2]. Media Relays have also been used to enable
applications across NATs and firewalls, 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 Media Relay unless the
type of NAT/firewall is such that it forces the use of such a Media
Relay.
This specification defines requirements for NATs and firewalls aimed
at ensuring that a NAT or firewall that satisfies these requirements
will avoid forcing the use of a Media Relay for supporting
applications. "Peer-to-Peer (P2P) communication across middle boxes"
[10] made several recommendations regarding NATs and firewalls for
Peer-to-Peer media; this specification derives a lot 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 situation is not as problematic for firewalls but still exists:
there is no good common terminology even to describe the behavior of
firewalls.
The goals of this document are to define a set of common terminology
for describing the behavior of NATs and firewalls and to produce a
set of requirements on a specific set of behaviors for NATs and
firewalls. The requirements represent what many vendors are already
doing, and it is not expected that it should be any more difficult to
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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,
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.
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 also out of
scope. Media Relays are out of the scope of this document as well.
This document only covers the UDP aspects of NAT/firewall traversal
and does not cover TCP, ICMP, IPSEC, or other protocols.
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
[3] and RFC 3022 [4]: "Address binding is the phase in which a local
IP address is associated with an external address, or vice versa, for
purpose of translation."
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
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particular, it uses the terms "Full Cone", "Restricted Cone", "Port
Restricted Cone" and "Symmetric" to refer to different variations of
NATs/firewalls. Unfortunately, this terminology has been the source
of much confusion. This terminology does not distinguish between the
NAT and the firewalling behavior of NAT/firewall devices. It was
found that many devices' behaviors do not exactly fit into the
described variations. For example, a device could be symmetric from
a firewall point of view and Cone from a NAT point of view. Other
aspects of NAT/firewall 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/Firewall behaviors
instead of using the Cone/Symmetric terminology.
Note: RFC 3489 [7] 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 firewall
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 NATCECK [12] which uses condition (1), it 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.
3. UDP NAT 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
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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 tuple. Let's assume that 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 X2':x2' on
the NAT. The relationship between X1':x1' to X2':x2' for various
combinations of the relationship between Y1:y1 to 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 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.
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(From a RFC 3489 NAT perspective, this is a "Cone NAT" where the
sub-type is really based on the firewall 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
firewall 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 firewall 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 firewall behavior
in the firewall portions of the NAT/FW.
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".
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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.
3.3 Bind Refresh Direction
NAT UDP binding timeouts 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, but do not take into account packets from the
external side of the NAT to the internal side of the NAT. This is
referred to as having a NAT refresh direction behavior of "Outbound".
Other NATs keep the binding active when packets go in any direction.
This is referred to as "Bidirectional" NAT refresh direction
behavior.
Yet other NATs keep the binding active when a packet goes from the
external side of the NAT to the internal side of the NAT but do not
take into account packets from the internal side of the NAT to the
external side of the NAT. This is referred to as having a NAT
refresh direction behavior of "Inbound".
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.
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4. UDP Firewall Behavior (Filtering)
This section describes various firewall behaviors.
4.1 Filtering of unsolicited packets
When an internal endpoint opens an outgoing UDP session through a
firewall, the firewall 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
firewall to filter packets originating from specific external
endpoints.
External filtering is open: The firewall does not filter any packets.
External filtering is endpoint independent: The firewall 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
firewall forwards any packets destined to X:x. In other words,
sending packets from the internal side of the firewall to any
external IP address is sufficient to allow any packets back to the
internal endpoint. (From an RFC 3489 Firewall perspective, this
is a "Full Cone Firewall".)
External filtering is endpoint address dependent: The firewall
filters out packets not destined to the internal address X:x.
Additionally, the firewall 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
Firewall perspective, this is a "Restricted Cone Firewall".)
External filtering is endpoint address and port dependent: This is
similar to the previous behavior, except that the external port is
also relevant. The firewall filters out packets not destined for
the internal address X:x. Additionally, the firewall 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. (From an RFC 3489
Firewall perspective, this is both a "Port Restricted Cone
Firewall" and a "Symmetric Firewall" as they have the same
firewall behavior.)
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These are abbreviated at 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 Firewall Filter Refresh
The time for which a firewall filter is valid can be refreshed based
on packets that are inbound, outbound, or going either direction. In
the case of a EF=AD or EF=APD firewall, 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 firewall.
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/firewall that supports hairpinning forwards
packets 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
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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. 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
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.
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7. 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 and firewalls will send an
appropriate ICMP Destination Unreachable message to the internal host
that sent the original packet. NATs and firewalls 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.
There is no significant security advantage to blocking these ICMP
Destination Unreachable packets.
8. Fragmentation Behavior
When a fragmented packet is received on the external side, some NATs
forward the packet to the same location as a recent initial fragment
packet with the same identifier in the IP header. Other NATs
reassemble fragmented packets and forward them after reassembly.
NATs that do either of these are referred to as "Support
Fragmentation" (abbreviated SF).
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 opens up a DOS
opportunity.
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Fragmentation has been a tool used in many attacks, some involving
passing fragmented packets through firewalls and others involving DOS
attacks based on the state needed to reassemble the fragments.
Firewall implementers should be aware of RFC 3128 [17] and RFC 1858
[16].
NATs that do not remap the identification field in the IP header run
the risk that two hosts behind the NAT will choose the same value,
and a host receiving packets from them will not be able to correctly
reassemble the packets. It seems unlikely that this will happen
often in practice.
9. TCP Behavior
TCP connections are often long lived with long periods of no traffic.
The timeouts for the NAT bindings and firewall filters need to be set
appropriately. In the initial stage where a SYN has been sent but
there is no ACK, the timeouts can be fairly short. Typically they
are set at around one minute. After an ACK is received the session
is connected and needs to have a very long timer, typically hours.
This time is called the TCP timeout. After a RST or FIN packet is
seen, the timeout can be reduced to a short time such as one minute.
10. Multicast and IGMP Behavior
This section is weak and requires more discussion, thought, and
experimentation with existing systems. Take it with a grain of salt
and expect it to change significantly as this document matures.
Some NATs support multicast while others block it. In general to
support multicast, the NAT needs to process the source address as it
would processes other UDP packets but not modify the destination
address. It also needs to process IGMP packets as a normal router
would. Multicast is used by various applications including some that
deliver video to residences.
The simplest implementation would forward packets that are addressed
to a multicast destination and would proxy IGMP messages in the same
way that a NAT can proxy ICMP messages. A more complex
implementation would fully process the IGMPv3 RFC 3376 [15] messages
and only forward multicast packets based on the information IGMP has
provided.
If a device inside the NAT can receive multicast traffic from a
sender outside the NAT after the device inside sends an appropriate
IGMP message, the NAT is said to "Support Multicast" (abbreviated
SM).
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11. Requirements
The requirements in this section are aimed at minimizing the damage
caused by NATs and firewalls 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 or firewall 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 a "No port preservation"
behavior.
REQ-2a: A NAT MAY use a "Port preservation" behavior.
REQ-2b: A NAT MUST NOT have a "Port overloaded" behavior.
REQ-3: A dynamic NAT UDP binding timer MUST NOT expire in less
than 2 minutes.
REQ-3a: The value of the NAT UDP binding timer MAY be
configurable.
REQ-3b: A default value of 5 minutes for the NAT UDP binding
timer of 5 minutes is RECOMMENDED.
REQ-4: The NAT UDP timeout binding MUST have a NAT refresh
direction behavior of "Outbound" (i.e. based on outbound traffic
only).
REQ-4a: 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-5: It is RECOMMENDED that a firewall have an "External
filtering is endpoint address dependent" behavior. (EF=AD)
REQ-5a: A firewall MAY have an "External filtering is endpoint
independent" behavior. (EF=I)
REQ-5b: A firewall MAY have an "External filtering is endpoint
address and port dependent" behavior. (EF=APD)
REQ-6: The firewall UDP filter timeout behavior MUST be the same
as the NAT UDP binding timeout.
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REQ-7: A NAT/FW MUST support "Hairpinning" behavior.
REQ-7a: A NAT/FW Hairpinning NAT behavior MUST be "External
source IP address and port".
REQ-8: A NAT MUST have the capability to turn off individually all
ALGs it supports, except for DNS and IPsec.
REQ-8a: Any NAT ALG for SIP MUST be turned off by default.
REQ-9: A NAT/firewall MUST have deterministic behavior.
REQ-10: The TCP binding timeout for NATs and the filter rule
timeout for firewalls MUST be greater than 7800 seconds.
REQ-11: A NAT/firewall SHOULD support forwarding fragmented
packets (SF).
REQ-12: A NAT/FW MUST support ICMP Destination Unreachable (SU).
REQ-12a: The ICMP timeout SHOULD be greater than 2 seconds.
REQ-13: A NAT/FW SHOULD support forwarding multicast packets (SM).
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: 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-2a: Port preservation can work, but the NAT implementors need
to be very careful that it does not become a nondeterministic NAT.
REQ-2b: 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-3: This requirement is to ensure that the timeout is long
enough to avoid too frequent timer refresh packets.
REQ-3a: Configuration is desirable for adapting to specific
networks and troubleshooting.
REQ-3b: This default is to avoid too frequent timer refresh
packets.
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REQ-4: This requirement is a security concern: it is not secure to
let inbound traffic refresh the timer, as an outside party could
use it to keep a port open on the NAT/firewall.
REQ-4a: Using the refresh on a per binding basis avoids the need
for separate keep-alives for all the available sessions.
REQ-5: Filtering based on the IP address is felt to have the
maximum balance between security and usefulness. See below.
REQ-5a: 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-5b: 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-6: This is to avoid overly complex applications.
REQ-7: This requirement is to allow communications between two
endpoints behind the same NAT/firewall when they are trying each
other's external IP addresses.
REQ-7a: 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-8: NAT ALGs may interfere with UNSAF methods.
REQ-8a: A SIP NAT ALG will interfere with UNSAF methods.
REQ-9: 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-10: Most operating systems have a default TCP keep alive time
of 2 hours, plus it can take 10 minutes for the keep alive to
happen or fail with all the default timeouts. The sum of these
leads to the recommendation of 7800 seconds.
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Req-11: Fragmented packets become more common with large video
packets and should continue to work. Applications can use MTU
discovery to work around this.
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: Minimal support of multicast for NATs is simple and allows
interesting applications.
12. Security Considerations
Firewalls and 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 that inbound packets should not 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. Some devices today do update
the timers on inbound packets.
This work recommends that the firewall 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 and firewalls 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, it 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
behavior.
13. IANA Considerations
There are no IANA considerations.
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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 constitute in itself an UNSAF
application. It consist of a series of requirements for NATs and
firewalls 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 [2] lists five architectural
considerations. Though 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 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 combined with the test 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 solving 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
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
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in this specification is derived. Thanks to Rohan Mahy for many
discussions on this and much helpful text. Jonathan Rosenberg
provided key suggestions and corrections, and Mary Barnes provided
very helpful review.
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.
[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-00 (work in progress), February 2004.
[10] Ford, B., "Network Address Translation and Peer-to-Peer
Applications (NATP2P)", draft-ford-natp2p-00 (work in
progress), April 2003.
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[11] "Packet-based Multimedia Communications Systems (includes Annex
C - H.323 on ATM)", ITU-T Recommendation H.323v3, September
1999.
[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-00 (work in progress),
February 2004.
[14] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[15] Cain, B., Deering, S., Kouvelas, I., Fenner, B. and A.
Thyagarajan, "Internet Group Management Protocol, Version 3",
RFC 3376, October 2002.
[16] Ziemba, G., Reed, D. and P. Traina, "Security Considerations
for IP Fragment Filtering", RFC 1858, October 1995.
[17] Miller, I., "Protection Against a Variant of the Tiny Fragment
Attack (RFC 1858)", RFC 3128, June 2001.
Authors' Addresses
Francois AUDET
Nortel Networks
4655 Great America Parkway
Santa Clara, CA 95054
USA
Phone: +1 408 495 3756
EMail: audets@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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