Internet DRAFT - draft-ietf-nat-protocol-complications


NAT Working Group                                          Matt Holdrege
INTERNET-DRAFT                              	                 ipVerse
Category: Informational                                   Pyda Srisuresh
Expires as of April 28, 2001                	              Consultant
                                                           October, 2000

     Protocol Complications with the IP Network Address Translator

Status of this Memo

   This document is an Internet-Draft and is in full conformance
   with all provisions of Section 10 of RFC2026.

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Copyright Notice

Copyright  (C) The Internet Society (2000).  All Rights Reserved.


   Many internet applications can be adversely affected when end nodes
   are not in the same address realm and seek the assistance of NAT 
   enroute to bridge the realms. NAT device alone cannot provide the 
   necessary application/protocol transparency in all cases and seeks
   the assistance of Application Level Gateways (ALGs) where possible, 
   to provide transparency. The purpose of this document is to identify
   the protocols and applications that break with NAT enroute. The 
   document also attempts to identify any known workarounds. It is not
   possible to capture all applications that break with NAT in a single
   document. This document attempts to capture as much information as 
   possible, but is by no means a comprehensive coverage. We hope the 
   coverage provides sufficient clues for applications not covered.

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Table of Contents

   1.0 Introduction

   2.0 Common Characteristics of Protocols broken by NAT

   3.0 Protocols that cannot work with NAT enroute

   4.0 Protocols that can work with the aid of an ALG
   5.0 Protocols designed explicitly to work with NAT enroute 

   6.0 Acknowledgements

   7.0 Security Considerations
   8.0 References

   9.0 Authors Addresses

1.0 Introduction

This document requires the reader to be familiar with the 
terminology and function of NAT devices as described in [NAT-TERM].
In a nutshell, NAT attempts to provide a transparent routing solution
to end hosts requiring communication to disparate address realms. NAT
modifies end node addresses (within the IP header of a packet) 
en-route and maintains state for these updates so that datagrams
pertaining to a session are transparently routed to the right 
end-node in either realm. Where possible, application specific 
ALGs may be used in conjunction with NAT to provide application level 
transparency. Unlike NAT, the function of ALG is application specific 
and would likely require examination and recomposition of IP payload.

The following sections attempt to list applications that are known
to have been impacted by NAT devices enroute. However, this is by no 
means a comprehensive list of all known protocols and applications
that have complications with NAT - rather just a subset of the list
gathered by the authors. It is also important to note that this 
document is not intended to advocate NAT, but rather to point out
the complications with protocols and applications when NAT devices
are enroute.

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2.0 Common Characteristics of Protocols broken by NAT

[NAT-TERM] and [NAT-TRAD] have sections listing the specific nature 
of problems and limitations to NAT devices. Some of these limitations 
are being restated in this section to summarize characteristics of 
protocols that are broken by NAT.

2.1 Realm-specific IP address information in payload

A wide range of applications fail with NAT enroute when IP packets
contain realm-specific IP address or port information in payload.
An ALG may be able to provide work around in some cases. But, 
if the packet payload is IPsec secured (or secure by a transport 
or application level security mechanisms), the application is 
bound to fail.

2.2 Bundled session applications

Bundled session applications such as FTP, H.323, SIP and RTSP, which 
use a control connection to establish a data flow are also usually 
broken by NAT devices enroute. This is because these applications 
exchange address and port parameters within control session to 
establish data sessions and session orientations. NAT cannot know 
the inter-dependency of the bundled sessions and would treat each 
session to be unrelated to one another. Applications in this case 
can fail for a variety of reasons. Two most likely reasons for 
failures are:  (a) addressing information in control payload is 
realm-specific and is not valid once packet crosses the originating
realm, (b) control session permits data session(s) to originate in 
a direction that NAT might not permit.
When DNS names are used in control payload, NAT device in conjunction 
with a DNS-ALG might be able to offer the necessary application level 
transparency, if NAT has no contention with data session orientation. 
However, using DNS names in place of realm-specific IP addresses may 
not be an option to many of these applications (e.g.: FTP). 

When realm-specific addressing is specified in payload, and the payload 
is not encrypted, an ALG may in some cases be able to provide the work 
around necessary to make the applications run transparently across 
realms. The complexity of ALG depends on the application level 
knowledge required to process payload and maintain state.

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2.3 Peer-to-peer applications

Peer-to-peer applications more than client-server based applications
are likely to break with NAT enroute. Unlike Client-server 
applications, Peer-to-peer applications can be originated by any of 
the peers. When peers are distributed across private and public 
realms, a session originated from an external realm is just as 
likely as the session from  a host in private realm. External 
peers will be able to locate their peers in private realm only 
when they know the externally assigned IP address or the FQDN 
ahead of time. FQDN name to assigned address mapping can happen 
only so long as the enroute NAT device supports DNS-ALG. Examples
of Peer-to-peer applications include interactive games, Internet 
telephony and event-based protocols (such as Instant-Messaging). 

This is particularly a problem with traditional NAT and may be less 
of an issue with bi-directional NAT, where sessions are permitted 
in both directions. 

A possible work-around for this type of problem with traditional-NAT
is for private hosts to maintain an outbound connection with a 
server acting as their representative to the globally routed 

2.4 IP fragmentation with NAPT enroute

IP fragmentation with NAPT enroute is not an issue with any single 
application, but pervades across all TCP/UDP applications. The
problem is described in detail in [NAT-TRAD]. Briefly, the problem 
goes as follows. Say, two private hosts originated fragmented 
TCP/UDP packets to the same destination host.  And, they happened to
use the same fragmentation identifier. When the target host receives 
the two unrelated datagrams, carrying same fragmentation id, and 
from the same assigned host address, the target host is unable to
determine which of the two sessions the datagrams belong to. 
Consequently, both sessions will be corrupted. 

2.5 Applications requiring retention of address mapping

NAT will most likely break applications that require address 
mapping to be retained across contiguous sessions. These 
applications require the private-to-external address mapping 
to be retained between sessions so the same external address 
may be reused for subsequent session interactions. NAT cannot
know this requirement and may reassign external address to 
different hosts between sessions. 

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Trying to keep NAT from discarding an address mapping would 
require a NAT extention protocol to the application that would
allow the application to inform the NAT device to retain the
mappings. Alternately, an ALG may be required to interact with
NAT to keep the address mapping from being discarded by NAT.

2.6 Applications requiring more public addresses than available

This is a problem when the number of private hosts is larger than
the external addresses available to map the private addresses 
into. Take for example the rlogin service initiated from a host
in private realm supported by NAPT. Rlogin service clients 
use well-known rlogin port 512 as their TCP port ID. No more than
one host in private realm can initiate the service. This is a 
case of trying to use a service that fundamentally requires more 
public addresses than are available. NAT devices can conserve 
addresses, but they cannot create more addresses.

3.0 Protocols that cannot work with NAT enroute

3.1 IPsec and IKE

NAT fundamentally operates by modifying end node addresses (within the 
IP header) en-route. The IPsec AH standard [RFC 2402] on the other 
hand is explicitly designed to detect alterations to IP packet 
header. So when NAT alters the address information enroute in IP 
header, the destination host receiving the altered packet will 
invalidate the packet since the contents of the headers have been 
altered. The IPsec AH secure packet traversing NAT will simply not 
reach the target application, as a result.

IPsec ESP encrypted packets may be altered by NAT device enroute only 
in a limited number of cases. In the case of TCP/UDP packets, NAT would 
need to update the checksum in TCP/UDP headers, when an address in
IP header is changed. However, as the TCP/UDP header is encrypted by 
the ESP, NAT would not be able to make this checksum update. As a 
result, TCP/UDP packets encyrpted in transport mode ESP, traversing a 
NAT device will fail the TCP/UDP checksum validation on the receiving
end and will simply not reach the target application.

Internet Key Exchange Protocol IKE can potentially pass IP addresses 
as node identifiers during Main, Aggressive and Quick Modes. In order 
for an IKE negotiation to correctly pass through a NAT, these 
payloads would need to be modified.  However, these payloads are
often protected by hash or obscured by encryption. Even in the 

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case where IP addresses are not used in IKE payloads and an IKE 
negotiation could occur uninterrupted, there is difficulty with
retaining the private-to-external address mapping on NAT from the
time IKE completed negotiation to the time IPsec uses the key on
an application. In the end, the use of end-to-end IPsec is severely
hampered anyway, as described earlier. 

For all practical purposes, end-to-end IPsec is impossible to 
accomplish with NAT enroute. 

3.2 Kerberos 4

Kerberos 4 tickets are encrypted.  Therefore, an ALG cannot be
written. When the KDC receives a ticket request, it includes the
source IP address in the returned ticket. Not all Kerberos 4 
services actually check source IP addresses.  AFS is a good 
example of a Kerberos 4 service which does not. Services which 
don't check are not picky about NAT devices enroute. Kerberos 
tickets are tied to the IP address that requested the ticket and
the service with which to use the ticket.

The K4 ticket (response) contains a single IP address describing the 
interface used by the  client to retrieve the ticket from the TGT 
from the perspective of KDC. This works fine if the KDC is across a 
NAT gateway and as long as all of the Kerberos services are also 
across a NAT gateway. The end user on private network will not notice 
any problems.  

There is also the caveat that NAT uses the same address mapping for
the private host for the connection between the client and the KDC 
as for the connection between the client and the application server.
A work around this problem would be to keep an arbitrary connection 
open to remote server during throughout the ticket lifetime, so as 
not to let NAT drop the address binding. Alternately, an ALG will 
need to be deployed to ensure that NAT would not change address 
bindings during the lifetime of a ticket and between the time a 
ticket is issued to private host and the time the ticket is used 
by private host.

But, the ticket will be valid from any host within the private realm
of NAPT.  Without NAPT, an attacker needs to be able to 
spoof the source IP addresses of a connection that is being made in
order to use a stolen ticket on a different host. With NAPT, all 
the attacker needs to do from the private realm of NAPT is to simply 
gain possession of a ticket. Of course, this assumes, NAPT private
domain is not a trusted network - not surprisingly, since many attacks 
occur from inside the organization. 

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3.3 Kerberos 5

Just as with Kerberos 4, Kerberos 5 tickets are encrypted.  Therefore, 
an ALG cannot be written.  

In Kerberos 5, the client specifies a list of IP addresses which the
ticket should be valid for, or it can ask for a ticket valid for all
IP addresses.  By asking for an all-IP-addresses ticket or a ticket
containing the NAPT device address, you can get krb5 to work with an
NAPT device, although it isn't very transparent (it requires the
clients to behave differently than they otherwise would).  The MIT
krb5 1.0 implementation didn't have any configurability for what IP
addresses the client asked for (it always asked for the set of its
interface addresses) and did not interact well with NAT.  The MIT krb5
1.1 implementation allows you to put "noaddresses" somewhere in
krb5.conf to request all-IP-addresses-valid tickets.

The K5 ticket (response) contains IP addresses, as requested by the
client node, from which the ticket is to be considered valid. If the 
services being accessed with Kerberos authentication are on the 
public side of the NAT, then the Kerberos authentication will fail 
because the IP address used by the NAT (basic NAT or NAPT) is not in 
the list of acceptable addresses.

There are two workarounds in Kerberos 5 both of which reduce the 
security of the tickets.  The first is to have the clients in NAPT 
private realm specify the public IP address of the NAPT in the 
ticket's IP list.  But this leads to the same security problem 
detailed for K4.  Plus, it is not obvious for the client in the 
private domain to find out the public IP Address of the NAPT. That 
would be a change of application behavior on end-host.

The second method is to remove all IP addresses from the K5 tickets
but this now makes theft of the ticket even worse since the tickets 
can be used from anywhere.  Not just from within the private network.

3.4 The X Windowing System and X-term/Telnet

The X Windowing system is TCP based. However, the client-server 
relationship with these applications is reverse compared to 
most other applications. The X server or Open-windows server is the
display/mouse/keyboard unit (i.e., the one that controls the actual
Windows interface). The clients are the application programs driving
the Windows interface.

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Some machines run multiple X Windows servers on the same machine. The
first X Windows server is at TCP port 6000. The first Open Windows
server can be at port 6000 or port 2000 (more flexible).  We will 
mainly refer X windowing system for illustration purposes here.

X-term Transmits IP addresses from the client to the server for the
purpose of setting the DISPLAY variable.  When set the DISPLAY 
variable is used for subsequent connections from X clients on the 
host to an X server on the workstation. The DISPLAY variable is 
sent inline during the TELNET negotiations as


where the <local-ip-addr> is retrieved by looking at the local ip
address associated with the socket used to connect to <server>.  The
<server> determines which port (6000 + <server>) should be used
to make the connection.  <display> is used to indicate which monitor
attached to the X server should be used but is not important to this

The <local-ip-addr> used is not a DNS name because:

 . The is no ability for the local machine to know its DNS name
   without performing a reverse DNS lookup on the local-ip-addr
 . There is no guarantee that the name returned by a reverse
   DNS lookup actually maps back to the local IP address.

 . Lastly, without DNSSEC, it may not be safe to use DNS addresses
   because they can easily be spoofed. NAT and DNS-ALG cannot work
   unless DNSSEC is disabled.
A common use of this application is people dialing in to corporate
offices from their X terminals at home. Say, the X client is running
on a host on the public side of the NAT and X server is running on a
host on the private side of the NAT. The DISPLAY variable is 
transmitted inline to the host the X client is running in some way. 
The process transmitting the contents of the DISPLAY variable does
not know the address of the NAT.

If the channel transmitting the DISPLAY variable is not encrypted,
NAT device might solicit the help of an ALG to replace the 
IP address and configure a port in the valid display range (ports 
6000 and higher) to act as a gateway. Alternately, NAT may be 
configured to listen for incoming connections to provide access
to the X Server(s), without requiring an ALG. But, this approach
increases the security risk by providing access to the X server that
would not otherwise be available. As the ALG tampers with the

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IP addresses it will also not be possible for X Authorization methods
other than MIT-MAGIC-COOKIE-1 to be used.  MIT-MAGIC-COOKIE-1 is the
least secure of all the documented X Authorization methods.

When START_TLS is used there may be client certificate verification
problems caused by the NAT depending on the information provided in
the certificate.


RSH uses multiple sessions to support separate streams for stdout and 
stderr. A random port number is transmitted inline from the client to
the server for use as stderr port. The stderr socket is a connection 
back from the server to the client. And unlike FTP, there is no 
equivalent to PASV mode. For traditional NAT, this is a problem 
as traditional NAT would not permit incoming sessions.

RLOGIN does not use multiple sessions. But the Kerberos protected 
versions of RSH and RLOGIN will not work in a NAT environment due 
to the ticket problems and the use of multiple sessions.

4.0 Protocols that can work with the aid of an ALG

This document predominantly addresses problems associated with 
Traditional NAT, especially NAPT. 

4.1 FTP

FTP is a TCP based application, used to reliably transfer files between
two hosts. FTP uses bundled session approach to accomplish this.

FTP is initiated by a client accessing a well-known port number 21 on
the FTP server.  This is called the FTP control session. Often, an
additional data session accompanies the control session. By default, the
data session would be from TCP port 20 on server to the TCP port client
used to initiate control session. However, the data session ports may be
altered within the FTP control sessions using ASCII encoded PORT and
PASV commands and responses.

Say, an FTP client is in a NAT supported private network. An FTP ALG
will be required to monitor the FTP control session (for both PORT and
PASV modes) to identify the FTP data session port numbers and modify the
private address and port number with the externally valid address and
port number.  In addition, the sequence and acknowledgement numbers, TCP
checksum, IP packet length and checksum have to be updated. Consequently
the sequence numbers in all subsequent packets for that stream must be

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adjusted as well as TCP ACK fields and checksums.

In rare cases, increasing the size of the packet could cause it to 
exceed the MTU of a given transport link. The packet would then have 
to be fragmented which could affect performance. Or, if the packet has 
the DF bit set, it would be ICMP rejected and the originating host 
would then have to perform Path MTU Discovery. This could have an 
adverse effect on performance.

Note however, if the control command channel is secured, it will be 
impossible for an ALG to update the IP addresses in the command 

When AUTH is used with Kerberos 4, Kerberos 5, and TLS, the same
problems that occur with X-Term/Telnet occur with FTP.

Lastly, it is of interest to note section 4 of RFC 2428 (FTP extensions
for IPv6 and NATs) which describes how a new FTP port command (EPSV ALL)
can be used to allow NAT devices to fast-track the FTP protocol, 
eliminating further processing through ALG, if the remote server 
accepts "EPSV ALL".

4.2 RSVP

RSVP is positioned in the protocol stack at the transport layer,
operating on top of IP (either IPv4 or IPv6). However, unlike other
transport protocols, RSVP does not transport application data but
instead acts like other Internet control protocols (for example, ICMP,
IGMP, routing protocols).  RSVP messages are sent hop-by-hop between
RSVP-capable routers as raw IP datagrams using protocol number 46. It is
intended that raw IP datagrams should be used between the end systems
and the first (or last) hop router.  However, this may not always be
possible as not all systems can do raw network I/O. Because of this, it
is possible to encapsulate RSVP messages within UDP datagrams for end-
system communication. UDP-encapsulated RSVP messages are sent to either
port 1698 (if sent by an end system) or port 1699 (if sent by an RSVP-
enabled router). For more information concerning UDP encapsulation of
RSVP messages; consult Appendix C of RFC 2205.

An RSVP session, a data flow with a particular destination and
transport-layer protocol, is defined by:

Destination Address - the destination IP address for the data packets.
This may be either a unicast or a multicast address.

Protocol ID - the IP protocol ID (for example, UDP or TCP).

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Destination Port - a generalized destination port that is used for
demultiplexing at a layer above the IP layer.

NAT devices are presented with unique problems when it comes to
supporting RSVP. Two issues are:

1. RSVP message objects may contain IP addresses. The result is that an
RSVP-ALG must be able to replace the IP addresses based upon the
direction and type of the message. For example, if an external sender
were to send an RSVP Path message to an internal receiver, the RSVP
session will specify the IP address that the external sender believes is
the IP address of the internal receiver. However, when the RSVP Path
message reaches the NAT device, the RSVP session must be changed to
reflect the IP address that is used internally for the receiver. Similar
actions must be taken for all message objects that contain IP addresses.

2. RSVP provides a means, the RSVP Integrity object, to guarantee the
integrity of RSVP messages. The problem is that because of the first
point, a NAT device must be able to change IP addresses within the RSVP
messages.  However, when this is done, the RSVP Integrity object is no
longer valid as the RSVP message has been changed. Therefore an RSVP-ALG
will not work when RSVP Integrity Object is used.

4.3 DNS

DNS is a TCP/UDP based protocol. Domain Names are an issue for hosts 
which use local DNS servers in NAT private realm. DNS name to address 
mapping for hosts in private domain should be configured on an 
authoritative name server within private domain. This server would 
be accessed by external and internal hosts alike for name 
resolutions. A DNS-ALG would be required to perform address to name
conversions on DNS queries and responses. RFC 2694 describes DNS-ALG
in detail. If DNS packets are encrypted/authenticated per DNSSEC, 
then DNS_ALG will fail because it won't be able to perform payload 

Applications using DNS resolver to resolve a DNS name into an 
IP address, assume availability of address assignment for reuse 
by the application specific session. As a result, DNS-ALG will 
be required to keep the address assignment (between private and 
external addresses) valid for a pre-configured period of time, 
past the DNS query.

Alternately, if there isn't a need for a name server within private
domain, private domain hosts could simply point to an external name
server for external name lookup.  No ALG is required when the name
server is located in external domain.

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4.4 SMTP

SMTP is a TCP based protocol (Refer RFC 821), used by Internet 
email programs such as sendmail to send TCP-based email messages 
to well-known port 25. The mail server may be located within or 
outside private domain. But, the server must be assigned a global 
name and address, accessible by external hosts. When mail server 
is located within private domain, inbound SMTP sessions must be
redirected to the private host from its externally assigned 
address. No special mapping is required when Mail server is 
located in external domain.

Generally speaking, mail systems are configured such that all
users specify a single centralized address (such as, instead of including individual hosts (such
as The central address must have an MX 
record specified in the DNS name server accessible by external 

In the majority of cases, mail messages do not contain reference
to private IP addresses or links to content data via names that 
are not visible to outside. However, some mail messages do contain
IP addresses of the MTAs that relay the message in the "Received: " 
field. Some mail messages use IP addresses in place of FQDN for 
debug purposes or due to lack of a DNS record, in "Mail From: " 

If one or more MTAs were to be located behind NAT in a private 
domain, and the mail messages are not secured by signature or 
cryptographic keys, an SMTP-ALG may be used to translate the 
IP address information registered by the MTAs. If the MTAs 
have static address mapping, the translation would be valid 
across realms for long periods of time.

The ability to trace the mail route may be hampered or prevented
by NAT alone, without the ALG. This can cause problems when 
debugging mail problems or tracking down abusive users of mail.

4.5 SIP

SIP (Refer [SIP]) can run on either TCP or UDP, but by default on 
the same port 5060.

When used with UDP, a response to a SIP request does not go to the
source port the request came from. Rather the SIP message contains 

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the port number the response should be sent to. SIP makes use of 
ICMP port unreachable errors in the response to request 
transmissions. Request messages are usually sent on the connected 
socket. If responses are sent to the source port in the request, 
each thread handling a request would have to listen on the socket 
it sent the request on. However, by allowing responses to come to 
a single port, a single thread can be used for listening instead.

A server may prefer to place the source port of each connected 
socket in the message. Then each thread can listen for responses 
separately. Since the port number for a response may not go to 
the source port of the request, SIP will not normally traverse 
a NAT and would require a SIP-ALG.

SIP messages carry arbitrary content, which is defined by a MIME type.
For multimedia sessions, this is usually the Session Description
Protocol (SDP RFC 2327). SDP may specify IP addresses or ports to be
used for the exchange of multimedia. These may loose significance when
traversing a NAT. Thus a SIP-ALG would need the intelligence to 
decipher and translate realm-relevant information.

SIP carries URL's in its Contact, To and From fields that specify
signaling addresses. These URL's can contain IP addresses or domain
names in the host port portion of the URL. These may not be valid 
once they traverse a NAT.

As an alternative to an SIP-ALG, SIP supports a proxy server which
could co-reside with NAT and function on the globally significant
NAT port. Such a proxy would have a locally specific configuration.

4.6 RealAudio

In default mode, RealAudio clients (say, in a private domain)
access TCP port 7070 to initiate conversation with a real-audio server
(say, located an external domain) and to exchange control messages
during playback (ex: pausing or stopping the audio stream). Audio 
session parameters are embedded in the TCP control session as 
byte stream.

The actual audio traffic is carried in the opposite direction on 
incoming UDP based packets (originated from the server) directed 
to ports in the range of 6970-7170.

As a result, RealAudio is broken by default on a traditional NAT
device. A work around for this would be for the ALG to examine the
TCP traffic to determine the audio session parameters and 
selectively enable inbound UDP sessions for the ports agreed upon 

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in the TCP control session.  Alternately, the ALG could simply 
redirect all inbound UDP sessions directed to ports 6970-7170 to 
the client address in the private domain.

For bi-Directional NAT, you will not need an ALG.  Bi-directional NAT
could simply treat each of the TCP and UDP sessions as 2 unrelated
sessions and perform IP and TCP/UDP header level translations.

The readers may contact RealNetworks, Inc. for detailed guidelines
on how their applications can be made to work, traversing through NAT
and firewall devices.

4.7 H.323

H.323 is complex, uses dynamic ports, and includes multiple UDP streams.
Here is a summary of the relevant issues:

An H.323 call is made up of many different simultaneous connections. At
least two of the connections are TCP.  For an audio-only conference,
there may be up to 4 different UDP 'connections' made.

All connections except one are made to ephemeral (dynamic) ports.

Calls can be initiated from the private as well as the external domain.
For conferencing to be useful, external users need to be able to
establish calls directly with internal users' desktop systems.

The addresses and port numbers are exchanged within the data stream of
the 'next higher' connection. For example, the port number for the H.245
connection is established within the Q.931 data stream. (This makes it
particularly difficult for the ALG, which will be required to modify the
addresses inside these data streams.)  To make matters worse, it is
possible in Q.931, for example, to specify that the H.245 connection
should be secure (encrypted). If a session is encrypted, it is
impossible for the ALG to decipher the data stream, unless it has access
to the shared key.

Most of the control information is encoded in ASN.1 (only the User-User
Information within Q.931 Protocol Data Units, or PDUs, is ASN.1-encoded
(other parts of each Q.931 PDU are not encoded). For those unfamiliar
with ASN.1, suffice it to say that it is a complex encoding scheme,
which does not end up with fixed byte offsets for address information.
In fact, the same version of the same application connecting to the
same destination may negotiate to include different options, changing
the byte offsets.

Below is the protocol exchange for a typical H.323 call between User A

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and User B. A's IP address is and B's IP address is  Note that the Q.931 and H.245 messages are encoded in
ASN.1 in the payload of an RTP packet. So to accomplish a connection
through a NAT device, an H.323-ALG will be required to examine the
packet, decode the ASN.1, and translate the various H.323 control IP

User A                                                  User B
      A establishes connection to B on well-
      known Q.931 port (1720)

      Q.931 Setup caller address =
                  caller port    = 1120
                  callee address =
                  callee port    = 1720

      Q.931 Alerting
      Q.931 Connect H.245 address =
                    H.245 port    = 1092

      User A establishes connection to User B at, port 1092

      Several H.245 messages are exchanged (Terminal
      Capability Set, Master Slave Determination and
      their respective ACKs)

      H.245 Open Logical Channel, channel = 257
                RTCP address =
                RTCP port    = 1093
      H.245 Open Logical Channel Ack, channel = 257
                RTP address =
                RTP port    = 2002
                (This is where User A would like RTP
                 data sent to)
                RTCP address =
                RTCP port    = 2003
      H.245 Open Logical Channel, channel = 257

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                RTCP address =
                RTCP port    = 2003
      H.245 Open Logical Channel Ack, channel = 257
                RTP address =
                RTP port    = 1092
                (This is where User B would like RTP data
                 sent to)
                RTCP address =
                RTP port     = 1093

Also note that if an H.323 Gateway resided inside a NAT boundary, the
ALG would have to be cognizant of the various gateway discovery schemes
and adapt to those schemes as well. Or if just the H.323 host/terminal
was inside the NAT boundary and tried to register with a Gatekeeper, the
IP information in the registration messages would have to be translated
by NAT.

4.8 SNMP 

SNMP is a network management protocol based on UDP. SNMP payload may 
contain IP addresses or may refer IP addresses through an index 
into a table. As a result, when devices within a private network 
are managed by an external node, SNMP packets transiting a NAT 
device may contain information that is not relevant in external
domain. In some cases, as described in [SNMP-ALG], an SNMP ALG may
be used to transparently convert realm-specific addresses into 
globally unique addresses. Such an ALG assumes static address 
mapping and bi-directional NAT. It can only work for the set of data
types (textual conventions) understood by the SNMP-ALG implementation
and for a given set of MIB modules. Furthermore, replacing IP 
addresses in the SNMP payload may lead to communication failures due
to changes in message size or changes in the lexicographic ordering.
Making SNMP ALGs completely transparent to all management
applications is not an achievable task. The ALGs will run into
problems with SNMPv3 security features, when authentication (and 
optionally privacy) is enabled, unless the ALG has access to
security keys. [NAT-ARCH] also hints at potential issues with
SNMP management via NAT.

Alternately,  SNMP proxies, as defined in [SNMP-APPL], may be used
in conjunction with NAT to forward SNMP messages to external SNMP 
engines (and vice versa). SNMP proxies are tailored to the private 
domain context and can hence operate independent of the specific 
managed object types being accessed. The proxy solution will require 

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the external management application to be aware of the proxy 
forwarder and the individual nodes being managed will need to be 
configured to direct their SNMP traffic (notifications and requests)
to the proxy forwarder.

5.0 Protocols designed explicitly to work with NAT enroute 

5.1 Activision Games

Activision Games were designed to be NAT-friendly so as not to require 
an ALG for the games to work transparently through traditional NAT 
devices.  Game players within a private domain can play with other 
players in the same domain or external domain. Activision gaming 
protocol is proprietary and is based on UDP. The address server uses 
UDP port number 21157 and is expected to be located in the global
address realm. 

Game players connect to the address server first, and send their 
private IP address information (such as private IP address and
UDP port number) in the initial connect message. The server notes
private address information from the connect message and external
address information from the IP and UDP headers. The server then 
sends both the private and external address information of the 
game player to all the other peer players. At this point, each 
game player knows the private and public address information of 
every other peer. Subsequent to this, each client opens up 
symmetrical direct connection to each other and uses whichever
address (private or external) works first.

Now, the clients can have a session directly with other clients (or) 
they can have session with other clients via the gaming server.
The key is to allow reuse of the same (global address, assigned UDP
port) tuple used for initial connection to the game server for all
subsequent connections to the client. A game player is recognized by
one of (private address, UDP port) or (global address, assigned UDP 
port) by all other peer players. So, the binding between tuples 
should remain unchanged on NAT, so long as the gaming player is in
session with one or multiple other players.

Opening a connection to a game server in external realm from a private
host is no problem. All NAT would have to do is provide routing
transparency and retain the same private-to-external address binding
so long as there is a minimum of one gaming session with an external
node. But, an NAPT configuration must allow multiple simultaneous UDP
connections on the same assigned global address/port.

The above approach has some problems. For example, a client could

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try contacting a private address, but that private address could be in
use locally, when the private address at some other realm is meant. 
If the node that was contacted wrongfully has some other service 
or no service registered for the UDP port, the activision connect 
messages are expected to be simply dropped. In the unlikely event, a
registered application chooses to interpret the message, the results
can be unpredictable. 

The readers may refer Activision for the proprietary, detailed 
information on the function and design of this protocol.

6.0 Acknowledgements

The authors would like to express sincere thanks to Bernard Aboba,
Bill Sommerfield, Dave Cridland, Greg Hudson, Henning Schulzrine, 
Jeffrey Altman, Keith Moore, Thomas Narten, Vernon Shryver and 
others that had provided valuable input in preparing this 
document. Special thanks to Dan Kegel for sharing the Activision 
games design methodology.

7.0 Security considerations

The security considerations outlined in [NAT-TERM] are relevant to
all NAT devices. This document does not warrant additional security 

8.0 References

[NAT-TERM]   Srisuresh, P., Holdrege, M. "IP Network Address Translator 
             (NAT) Terminology and Considerations", RFC 2663

[NAT-TRAD]   Srisuresh, P., Egevang, K. "Traditional IP Network Address 
             Translator(Traditional NAT)", 
             <draft-ietf-nat-traditional-04.txt>, Work in progress.

[H.323]      ITU-T SG16 H.323, Intel white paper, "H.323 and 
             Firewalls", Dave Chouinard, John Richardson, Milind Khare 
             (with further assistance from Jamie Jason).

[SNMP-ALG]   Raz, D., Schoenwaelder, J., and Sugla, B. "An SNMP 
             Application Level Gateway for Payload Address Translation",
             RFC 2962

[SNMP-APPL]  Levi, D., Meyer, P. and B. Stewart, "SNMP Applications", 
             RFC 2573

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[NAT-ARCH]   Hain, T. "Architectural Implications of NAT", 
[SMTP]       RFC 821

[FTP]        RFC 959

[SIP]        RFC 2543

[X Windows]  RFC 1198

[RSVP]       RFC 2205

[DNS]        RFC 1034, RFC 1035, RFC 2694

[IPsec]      RFC 2401, RFC 2402, RFC 2406, RFC 2411, RFC 2709

[PRIV-ADDR]  Rekhter, Y., Moskowitz, B., Karrenberg, D., G. de Groot,
             and, Lear, E.  "Address Allocation for Private Internets", 
             RFC 1918 

[ADDR-BEHA]  Brian carpenter, Jon Crowcroft, Yakov Rekhter, "IPv4 
             Address Behaviour Today", RFC 2101

Authors Addresses:

   Matt Holdrege
   223 Ximeno Ave.
   Long Beach, CA 90803

   Pyda Srisuresh
   849 Erie Circle
   Milpitas, CA 95035
   Voice: (408) 263-7527

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