NAT Working Group                                          Matt Holdrege
INTERNET-DRAFT                                     Ascend Communications
Category: Informational                                   Pyda Srisuresh
                                                     Lucent Technologies
Expires in six months                                        August                                      November 1998

          IP Network Address Translator (NAT) Protocol Issues.

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its
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   To view the entire list of current Internet-Drafts, please check
   the "1id-abstracts.txt" listing contained in the Internet-Drafts
   Shadow Directories on (Africa),
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Copyright Notice

Copyright  (C) The Internet Society (1998).  All Rights Reserved.

    Many common internet applications can be adversely affected when the
    end-to-end significance of an IP packet is disrupted. Network
    Address Translation (NAT) can cause such disruptions both at the
    protocol level and with application data. This draft covers issues with standard
protocols when transiting a NAT device.

It is worth noting that many non-standard protocols used on the Internet
also have issues with NAT devices. These include interactive games and
certain audio/video applications. However since such
applications/protocols are constantly coming and going, we will only
document long-living standard protocols in the main body. Please read
Appendix A for a current snapshot of non-standard protocols which are
known to have issues with NAT.

    While there are ready solutions for NATing each protocol listed,
    this document is only concerned with defining the native
    limitations. Future versions of this document may discuss

    *NOTE* the authors wish to make it clear that this work is editorial

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    order to better cover the range of applications that can be affected
    by NAT. This is a work in progress.


    NAT provides a transparent routing solution to end hosts trying to
    communicate from disparate routing realms. This transparent routing

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    is achieved by modifying end node addresses en-route and maintaining
    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, NAT must also be able to recompose valid data in
    the control streams, 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 thus populates messages
    with addressing information that is not 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


1.      PROTOCOL:

FTP     REFERENCE:      RFC 959

    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. NAT 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.

    Another issue can arise when applications when addresses and port
    numbers are encoded with ASCII. Changing these numbers can change
    the size of the overall packet. In rare cases, increasing the size
    of the

I-D                       NAT Protocol Issues                August 1998 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.      PROTOCOL

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H.323V1 REFERENCE       ITU-T SG16 H.323, Intel white paper, H.323 and Firewalls.
    Firewalls Dave Chouinard, John Richardson, Milind Khare (with
    further assistance from Jamie Jason).

    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 Internet, as well as from inside the
    NAT device. For conferencing to be useful, external users need to be
    able to establish calls directly with internal users' desktop

    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 NAT devices, which must
    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).

    Most of the control information is encoded in ASN.1 (only the User-User 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, we would have to go into the packet, decode
    the ASN.1, and translate the various H.323 control IP addresses.

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

          Q.931 Setup caller address =

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

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          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
                    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,
    any of the various gateway discovery schemes being discussed for use
    would have difficulty working through NAT. 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.      PROTOCOL

RSVP         REFERENCE    Reference RFC 2205

    RSVP is positioned in the protocol stack at the transport layer,

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    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

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    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 end-system communication. UDP-encapsulated 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 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
    NAT 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.


    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 true Internet names and addresses.

    Also DNS Zone Transfers are not translated by NAT and should not be
    sent through a NAT device.


    A DNS resolver is a UDP based application, initiated by accessing a
    well-known UDP port number 53 on a DNS server.


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Routing updates from

    A. UDP based protocol.

    B. Inverse name lookup queries embed the Internet will not be translated through NAT and
have ni significance to routers behind IP address in ASCII
      format. For example, a NAT device. Conversely routing
updates from behind NAT should not be forwarded resolver that wanted to find the Internet.


Another class
      hostname of problems with NAT is end-to-end security an address (externally assigned
      address of packets.
The IPsec AH standard [RFC 1826] a private realm host) would pursue a query of
      the form:


    An ALG is explicitly intended required to prevent what
NAT is good at. That is altering translate the header of query while forwarding to a DNS
    server within private realm, so that the packet. So query will appear as
    follows (replacing the externally assigned address with its private


      Clearly, payload length could change when NAT
does what it payload is supposed to do by altering the

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

    ADDITIONAL INFO: Refer RFC 1034, RFC 1035, DNS-ALG draft

EMAIL:  E-Mail programs - sendmail, Eudora, and others.

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    DESCRIPTION:    All e-mail programs listed above operate based on a
    TCP based SMTP protocol and use a well-known port number 25 to send
    messages and to listen on incoming messages.


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

    B. Mail messages are not expected to contain reference to
      private IP addresses or links to content data via names
      that are not visible to outside.


    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, instead of including individual hosts (such as The central address must have an MX
    record specified in the DNS name server accessible by external

    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.

    NAT would not perform any payload translation on SMTP messages.
    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: When the mail message contains reference to private IP
    addresses or links to content data via names that are not visible to



    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

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    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", where n>= 0, is used to tell clients to contact X
    server on <hostname> on TCP port (6000+n). It is very unlikely to be
    running more than a few (say 4) X-servers at a time on a host.

    Most 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-

    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.



    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

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    control messages during playback (ex:  pausing or stopping the audio

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


    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(?)


    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



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

    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


    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.

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



    Routing updates from the Internet will not be translated through NAT
    and have no significance to routers behind a NAT device. Conversely
    routing updates from behind NAT should not be forwarded to the


    Another class of problems with NAT is end-to-end security of
    packets. The IPsec AH standard [RFC 1826] is explicitly intended to
    prevent what NAT is good at. That is altering the header of the
    packet. So when NAT does what it is supposed to do by altering 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

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    dropped, or higher layer security activities could be started.

    Other IPsec protocols with NAT issues:

    Protocol        Issues

    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

    Authors Addresses:

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

       Pyda Srisuresh
       Lucent technologies Technologies
       4464 Willow Road
       Pleasanton, CA 94588-8519
       Voice: (925) 737-2153

Appendix A:

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