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NAT Working Group                                          Matt Holdrege
INTERNET-DRAFT                                     Ascend Communications
Category: Informational                                   Pyda Srisuresh
                                                     Lucent Technologies
Expires in six months                                          June 1999

  Protocol Complications with the IP Network Address Translator (NAT)

Status of this Memo

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

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other
   documents at any time.  It is inappropriate to use Internet-
   Drafts as reference material or to cite them other than as
   "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

Copyright Notice

Copyright  (C) The Internet Society (1999).  All Rights Reserved.


Many common internet applications can be adversely affected when the
communicating end  nodes are not in the same address realm and seek the
assistance of NAT (enroute) to bridge the realms. NAT by itself cannot
provide the necessary application/protocol transparency in all cases.
Often, a NAT device seeks the assistance of Application Level Gateways
(ALGs) to provide the transparency necessary for each application. The
purpose of this document is to identify the protocols and applications
that cannot function with NAT enroute. The document attempts to identify
the problem cause and describe known work-arounds and the requirements
on the part of ALGs to make the protocols/applications transparent with
NAT enroute. It is impossible to capture all the applications and their
issues with NAT in a single document.  This document attempts to capture
as much information as possible. We hope, the coverage provides
necessary clues for applications not covered by the document.

Table of Contents

1.0     Introduction

2.0     Protocols which require an ALG

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3.0     Protocols which do not require an ALG

4.0     Routing Updates

5.0     Protocols which cannot work with NAT enroute

6.0     References

7.0     Protocols which may have complications with NAT


NAT attempts to provide a transparent routing solution to end hosts that
need to communicate to disparate address realms. NAT modifies end node
addresses 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. NAT's fundamental role is to alter the
addresses in the IP header of a packet.

NAT can use much of the same solution set as a Stateful Inspection
firewall. However, the ALG's that complement NAT must also be able to
recompose valid data in the payload, since it must change the address
(and perhaps port) information. This is because the application running
on a host machine is typically unaware of NAT and may populate messages
with addressing information as required by the application protocol and
the addressing information may not be valid on the opposite side of the
NAT device.

One problem area is when a packet contains significant IP address or
port information in the payload of the packet rather than the header.
Network applications which use protocols that exhibit this behavior will
have problems when a NAT device is in mid-stream. In the next section we
will attempt to document standard protocols which have significant
address information in the payload of the packet.

Where this document mentions NAT, it is referring to Traditional NAT
rather than other NAT techniques.

*NOTE* the authors wish to make it clear that this work is editorial in
nature. Input from the Internet society is requested in order to better
cover the range of applications that can be affected by NAT. This is a
work in progress.

2.0     Protocols which require ALG's

2.1     Single Session based protocols

2.1.1   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

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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).

Destination Port - a generalized destination port which 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
teh IP address of the internal receiver. However, when the RSVP Path
message reaches the NAT device, the RSVP session must be chaned 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.

2.1.2   DNS

Domain Names are an issue for hosts which use local DNS servers behind a
NAT device. Such servers return site specific information which may
conflict with external domain addresses.

Zone transfers from private address realms to an external realm must be
avoided for address assignments that are not static. If primary and
backup name servers in the same private domain, zone transfers do not
cross the realm and DNS_ALG support for zone transfer is not an issue.


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A. TCP/UDP based protocol.

B. Inverse name lookup queries embed the IP address in ASCII
  format. For example, a resolver that wanted to find the
  hostname of an address (externally assigned
  address of a private realm host) would pursue a query of
  the form:


An ALG is required to translate the query while forwarding to a DNS
server within private realm, so that the query will appear as follows
(replacing the externally assigned address with its private address).


  Clearly, payload length could change when payload is

C. Serial reuse of an address assignment between independent
  sessions. This requires that the ALG keep the address
  assignment (between private and external addresses) valid
  for a pre-configured period of time, past the DNS query.

  Ex: DNS queries assume that the address assigned in response
      to a name lookup is serially reusable by a follow-on

D. A single DNS query payload could contain multiple queries at the
  same time, requiring translation of multiple addresses within
  a private domain.


DNS name to address mapping for hosts in private domain should be
configured on an authorititive  name server within the 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.

Alternately, if there isnt 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.

WHAT BREAKS: Authoritative name server for public domain access mUst not
contain hosts with private IP addresses.

2.1.3   SMTP

DESCRIPTION: SMTP is used by Internet email programs such as sendmail to
send TCP-based email messages to well known port 25.


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A. SMTP is a TCP based protocol, based on a well known TCP port
  number 25.

B. 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.

Some mail messages do contain IP addresses of teh MTA's 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: " field.


You need to specify a mail server, with a globally assigned IP address
to receive mail from external hosts.

Generally speaking, you would want to configure your mail system such
that all users specify a single centralized address (such as
fooboo@company.com), instead of including individual hosts (such as
fooboo@hostA.company.com). The central address must have an MX record
specified in the DNS name server accessible by external hosts.

The mail server may be located within or outside private domain. But,
the requirement is that the server be assigned a global name and
address, accessible by external hosts.

If one or more MTA's were to be located behind NAT in a private domain,
an SMTP-ALG will be required to translate the IP address information
registered by the MTA's. Typically, the MTA's will be expected to have a
static address mapping make the translation valid across realms for long
periods of time.

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.

WHAT BREAKS: You do not have an SMTP-ALG and yet the mail message or
headers con tains reference to private IP addresses or links to content
data via names that are not visible to the outside. The ability to trace
the mail route may also be hampered or prevented by NAT. This can
consequently cause problems when debuggin g mail problems or tracking
down abusive users of mail.


2.1.4   SIP

Description:  SIP can run on either TCP or UDP, but by default on the
same port;

When used with UDP, a response to a SIP request does not go to the

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source port t he request came from. Rather, the SIP message contains the
port number the repon se should be sent to. SIP makes use of ICMP port
unreachable errors in response to request transmissions. Request
messages are usually sent on the connected soc ket. If responses are
sent to the source port in the request, each thread handli ng 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 mes sage. Then each thread can listen for responses separately.
Since the port numbe r for a response may not go to the source port of
the request, SIP will not norm ally traverse a NAT and would require a

SIP messages carry arbitrary content which is defined by a MIME type.
For multim edia 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 lose significance when traversing a NAT. Thus a SIP-ALG would
need th e intelligence to decipher and translate realm-relevant

SIP carries URL's in its Contact, To and From fields that specify
signalling add resses. These URL's can contain IP addresses or domain
names in the host port po rtion 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-resi de with NAT and function on the globally significant NAT port.
Such a proxy woul d have to a locally specific configuration.

2.1.5   RealAudio

DESCRIPTION:    In its default mode, 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).

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


A. Real Audio has a TCP control session in one direction directed
  to a well-known port (7070) and the UDP based audio session in
  the opposite direction.

B. Audio session parameters are embedded in the TCP control
  session as byte stream(?)

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You could have an ALG examine the TCP traffic to determine the audio
session parameters and selectively enable inbound UDP sessions for the
ports agreed upon 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 simply perform IP and TCP/UDP header level translations.

2.2     Protocols with multiple sessions

2.2.1   FTP

FTP is a TCP based application, used to reliably transfer files between
two hosts.

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
adjusted as well as TCP ACK fields and checksums.

Note, the above issue with ASCII encoded address and port can occur with
other applications as well. Changing these numbers can change the size
of the overall packet. 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 perform Path MTU Discovery. This could also
have an adverse effect on performance.

2.2.2   H.323V1

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,

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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 those 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
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


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      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
                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.

3.0     Applications which do not require ALG's

3.1     X-Windows:

DESCRIPTION:    These applications are 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.

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 refer
X-windows mainly for illustration purposes here.

On a UNIX system, the csh DISPLAY command "setenv DISPLAY <hostname>:n",

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where n>= 0, is used to tell clients to contact X server on <hostname>
on TCP port (6000+n).

A common use of this application is people dialing in to corporate
offices from their X terminals at home.


A. X-Windows is a TCP based protocol, with the server
  servicing TCP ports in the range of 6000 - 6000+n.
  Open-Windows is also a TCP based protocol, with the server
  servicing TCP ports in the range of 6000 - 6000+n or
  2000 - 2000+n.

B. The X-Windows applications are not expected to contain
  reference to private IP addresses or links to content
  data via names that are not visible to the outside. All
  the information required for Client-Server communication
  is in the IP and TCP headers.


When X-Windows server (i.e., the machine that displays the X-Windows on
its console) runs in a private domain, we need to allow inbound X-server
access for the X terminals at home. I.e., Users that need to provide X-
terminal access must have inbound access permissions.  This can be done
statically or dynamically for private hosts.

In case of a NAPT setup, the individual X-Windows ports namely, 6000,
6001, 6002, 6003 and so on till (6000+n) on the external address may be
statically redirected to different hosts running X-server.

For Example, you could redirect inbound TCP sessions to <External
address>:6000 to <private Host A>, sessions to <External Address>:6001
to <private Host B> and so on.

WHAT BREAKS:  Accessing more X-servers than are configured.

Activision Games

DESCRIPTION: The goal of Activision Games is to work transparently
through traditional NAT devices. As such, the protocol described is
intended to be NAT friendly so game players within a private domain can
play with other players in the same domain or external domain.

All peers are somehow informed of each others' public and private
addresses, and each client opens up symmetrical direct connections to
each other and use whichever address (private or external)  works first.

Now, the clients can have a session directly with other clients directly
(or) they can have session with other clients via the gaming server.

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A. Activision gaming protocol is proprietary and is based on UDP. The
server uses UDP port no. 21157.

B. The protocol is designed with keeping NAT and NAPT in mind.  The game
players can be within the same private domain, in a combination of
multiple private domains and external domain.

C. The key is to allow the reuse of the tuple of the same (global
address, assigned UDP port) for initial connection to the game server
(helper) and the subsequent connection to the client. A game player is
recognized by one of (private address, UDP port) or (Assigned global
address, assigned UDP port) by all other peer players. So, the binding
between tuples should remain unchanged 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

But, an NAPT configuration MUST allow multiple simultaneous UDP
connections on the same assigned global address/port.




Routing advertisement varies considerably based on the NAT flavor in
use. A traditional-NAT and bi-directional-NAT may advertise untranslated
external routes to the private realm. However, a Twice-NAT device must
translate external routes (into their private realm address blocks), if
it chooses to advertise those routes into private realm.

All flavors of NAT must refrain from advertising private realm routes
into external realms. Instead, every NAT device must advertise (or be
made apparent through static configuration of neighboring routers or
some other means) the external address block it uses for mapping private
realm addresses.

5.0     Applications which cannot work with NAT enroute

5.1     IPsec

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Another class of problems with NAT is end-to-end security of packets.
The IPsec AH standard [RFC 1826] is explicitly intended to detect what
NAT is good at. That is altering the header of the packet. So when NAT
alters the address information in the header of the packet, the
destination host receives the altered packet and begins digesting the AH
message. The AH routines at this host will invalidate the packet since
the contents of the headers have been altered. Depending on the
configuration of the end host, the packet could be simply dropped, or
higher layer security activities could be started.

Other IPsec protocols with NAT complications:

ESP: Protects/obscures the packet contents (which would
     need to be visible for NATing some protocols).

IKE: Potentially passes IP addresses during both Main, Aggressive and
Quick Modes. In order for a 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.

6.0     Protocols which are suspected to have complications (but further
study is required.)

Rlogin/rsh ONC/RPC/NFS Kerberos


Authors Addresses:

   Matt Holdrege
   Ascend Communications, Inc.
   One Ascend Plaza
   1701 Harbor Bay Parkway
   Alameda, CA 94502
   Voice: (510) 769-6001
   EMail: matt@ascend.com

   Pyda Srisuresh
   Lucent technologies
   4464 Willow Road
   Pleasanton, CA 94588-8519
   Voice: (925) 737-2153
   EMail: suresh@ra.lucent.com

8.0     References

NAT             RFC XXXX, NAT Terminology and Considerations

H.323           ITU-T SG16 H.323, Intel white paper, H.323 and
Firewalls; Dave C                houinard, John Richardson, Milind Khare
(with further assistancefrom Jamie Jason).

SMTP            RFC 821

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FTP             RFC 959

SIP             RFC 2543

X-Windows       RFC 1198

RSVP            RFC 2205

RealAudio       http://www.real.com/firewall/packetfil.html

DNS             RFC 1034, RFC 1035, DNS-ALG draft

IPsec           RFC 2411

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