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Versions: 00 01 02                                                      
BEHAVE                                                      S. Sivakumar
Internet-Draft                                                 K. Biswas
Expires: August 1, 2005                                    Cisco Systems
                                                                 B. Ford
                                                        January 28, 2005

                  NAT Behavioral Requirements for TCP

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at

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   This Internet-Draft will expire on August 1, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   Inconsistent behavior by NATs makes it difficult for the application
   developers and network administrators to predict the operation of
   NATs.  This document describes the behavior required by NATs when
   handling TCP traffic.  It also specifies the address and port binding

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   behavioral requirement, timeout aspects and adjusting the sequence
   numbers and the acknowledgement numbers when changing the payload

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  TCP requirements . . . . . . . . . . . . . . . . . . . . . . .  3
     3.1   State Machine  . . . . . . . . . . . . . . . . . . . . . .  3
     3.2   NAT address and port binding . . . . . . . . . . . . . . .  4
     3.3   Timeouts . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.4   Port reservation.  . . . . . . . . . . . . . . . . . . . .  5
     3.5   Processing IP fragments and TCP segments . . . . . . . . .  6
       3.5.1   IP Fragments handling  . . . . . . . . . . . . . . . .  6
       3.5.2   TCP Segments handling  . . . . . . . . . . . . . . . .  7
     3.6   Adjusting Sequence Acknowledgement Numbers . . . . . . . .  8
     3.7   Handling of ICMP Error messages  . . . . . . . . . . . . . 10
   4.  Security considerations  . . . . . . . . . . . . . . . . . . . 11
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   6.  IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 11
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.1   Normative References . . . . . . . . . . . . . . . . . . . 11
     8.2   Informative References . . . . . . . . . . . . . . . . . . 11
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
       Intellectual Property and Copyright Statements . . . . . . . . 14

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1.  Introduction

   A lot of issues caused by inconsistent NAT behavior are documented in
   [UDP-REQ].  Different NAT implementations behave differently when
   handling the TCP traffic streams.  This document defines the required
   behavior of NATs when handling TCP traffic.

   NATs maintain various pieces of session information to translate the
   TCP streams correctly.  NATs would have to run a state machine for
   TCP to keep track of the state changes.  The state machines are
   predominantly used for controlling the different timers that are
   maintained by NAT.  NATs should also keep track of the payload
   changes by upper layer ALGs in order to adjust the sequence numbers
   and acknowledgement numbers properly.

2.  Scope

   This document will focus specifically on issues that relates to TCP.
   This document will refer to [UDP-REQ] for all the common NAT
   behavioral issues and requirements.  Application Layer Gateways
   (ALGs) are out of scope for this document.  This document will not
   propose any solution but will define only the requirements of NAT
   when handling TCP.

3.  TCP requirements

   The behavioral requirements of NAT when processing TCP packets are
   described in this section.

3.1  State Machine

   NATs maintain a database of active TCP sessions flowing across the
   NAT.  Each session in the NAT's database has an associated state
   machine that dynamically tracks the state of the TCP session from the
   perspective of the NAT.  The NAT creates a database entry for a new
   session, and starts the TCP state machine for that session, when it
   forwards the first SYN packet for that session across the NAT.  The
   NAT's TCP state machine transitions from the "active" to the "closed"
   state when the NAT observes a FIN/FIN ACK sequence, representing
   graceful shutdown reached cooperatively by both endpoints, or when
   the NAT observes a RST from either endpoint, representing a
   non-graceful connection reset forced by one endpoint.  Finally, the
   NAT deletes its database entry for the session some time after the
   state machine enters the "closed" state, depending on the timeouts
   described below.

   In addition to this basic state information, many NATs also record
   information about the TCP sequence numbers and the acknowledgment

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   numbers they observe in the TCP packets flowing across the NAT.  If
   the NAT contains built-in ALGs that can change the payload length of
   TCP packets, effectively inserting or removing bytes from the TCP
   stream in one or both directions, then the NAT MUST adjust the
   sequence numbers in all subsequent packets exchanged in either
   direction to reflect these inserted or removed bytes.

   NATs also use the state machine information associated with a TCP
   session in order to filter packets arriving from the external realm
   toward the internal realm.  Unless specifically configured to do
   otherwise, any SYN packets originating from the external realm will
   be filtered out by the NAT if the NAT's database contains no entry
   for that session.  This behavior reflects the standard firewall
   policy of rejecting all "unsolicited incoming connection attempts" by
   default.  NATs that implement a state machine for keeping track of
   the TCP streams are referred to as State Aware NAT (abbreviated SM =


3.2  NAT address and port binding

   The address and port binding requirements remain the same as the
   requirements described in [UDP-REQ].

3.3  Timeouts

   NATs maintain different types of timeouts for the TCP sessions.
   These timeouts apply to the different states of the TCP state

   * The NAT's SYN timer has a relatively short timeout, and helps to
   protect the NAT (and, potentially, the hosts behind the NAT) from SYN
   flood attacks.  The SYN timeout starts when the NAT observes the
   first SYN on a new session, and is cancelled when the NAT receives an
   ACK for that SYN from the opposite endpoint, indicating that
   legitimate two-way communication is taking place.

   * The NAT's session timer is a relatively long timeout, and ensures
   that the NAT is eventually able to delete database entries for
   formerly-active TCP sessions on which both endpoints have silently
   ceased communication without either closing or resetting the
   connection.  The NAT's session timer starts when the TCP session
   enters the active, "fully-open" state (typically at the same time its
   SYN timer is cancelled), and the session timer MUST be reset whenever
   the NAT observes an outbound packet from the internal realm to the
   external realm.  Inbound packets SHOULD NOT cause the NAT to reset
   its session timer.

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   When the NAT's session timer expires, it SHOULD NOT immediately
   delete its database entry for the session, since TCP sessions are
   legitimately allowed to go idle for arbitrary lengths of time with no
   exchanged traffic.  Instead, the NAT SHOULD enter a special "probe"
   state, in which it sends TCP keep-alive packets to the internal
   endpoint, using the technique described in section of [HOST],
   in order to detect whether the internal endpoint still considers the
   connection to be open.  If the NAT receives an ACK or other traffic
   from the internal endpoint, it resets the session timer and re-enters
   the "active" state.  If the NAT receives a RST from the internal
   endpoint, it enters the "closed" state and starts the close timer as
   described below.  If the NAT receives no response from the internal
   endpoint after sending several keep-alive packets, the NAT assumes
   that the internal endpoint is dead and again enters the "closed"

   * The NAT's close timer is a relatively short timeout that ensures
   that the ACKs for the final FINs on a gracefully-closed TCP session
   have a chance to propagate in both directions, and also to allow time
   for either endpoint to re-open a recently closed or reset TCP session
   if desired.  The NAT starts the close timer after it observes a FIN
   packet in each direction, or after it observes an RST from either
   endpoint.  If a new SYN packet arrives from either endpoint before
   the close timer expires, the NAT re-enters the active, "half-open"
   state & re-starts the SYN timer as described above.  Otherwise, once
   the close timer expires the NAT is free to delete its database entry
   and release all resources allocated to the session.

   The following requirements apply to the NAT's timeouts:

   A NAT MUST have a SYN timer so that the box is not prone to SYN
   attacks.  The SYN timeout value MUST be configurable, and SHOULD
   default to at least 30 seconds and no more than 60 seconds.

   A NAT MUST have a session timeout.  The NAT's session timeout MUST be
   configurable.  The session timeout MUST by default be at least 60
   minutes if the NAT uses TCP keep-alives to probe the session after
   the session timeout expires, and the session timeout MUST by default
   be atleast 120 minutes if the NAT just silently deletes the database
   entry for the session after the session timeout expires.

   A NAT MUST have a close timeout.  NAT MAY have separate timeouts for
   session close due to FIN versus RST.  NAT's close timeouts MUST be at
   least 2xMSL, or 60 seconds.

3.4  Port reservation.

   The TCP port space associated with a NAT's own IP addresses,

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   particularly its IP addresses on the external network side, are
   commonly shared between two separate functions:

   1.  TCP endpoints for address and port translated sessions

   2.  TCP endpoints for applications residing on the NAT device

   The first function results in port reservation for address-translated
   client sessions.  The second function results in port reservation due
   to applications running on the NAT device itself.  Examples of such
   applications include: a web-based externally accessible management
   interface, or a port forwarding application that has a predefined
   binding.  Many NATs are actually general-purpose network hosts that
   also run ordinary TCP-based applications as well.  Even dedicated NAT
   middleboxes often provide remote management functionality, requiring
   communication between external hosts and applications on the NAT
   itself.  The resulting sharing of the NAT's TCP port space between
   address-translation sessions and local applications creates a
   potential for resource contention.

   NATs MUST NOT use a single TCP port for both address-translated
   sessions and local application sessions at the same time.  NATs MAY
   dynamically re-assign a TCP port from one function to the other on
   demand, but only after any previous TCP sessions involving that port
   have become inactive.

3.5  Processing IP fragments and TCP segments

   This section describes how a NAT is recommended to behave with
   regards to handling IP fragments and TCP segments.  There are 2
   scenarios which can cause fragmentation/segmentation:

   1.  The MTU of the link can be as such as to cause the IP packets to
   be fragmented

   2.  The TCP stack at the either end-point can have the TCP MSS set to
   value lower than the application payload size which will cause the
   packets to be TCP segmented.

3.5.1  IP Fragments handling

   A NAT may receive a fragmented TCP packet.  The following section is
   provided from [UDP-REQ] for completeness.

   The IP packet containing the TCP header could arrive first or last
   depending of various conditions.  NAT that is capable only of
   receiving TCP fragments in order (that is, with the TCP header in the
   first packet) and forwarding each of the fragments to the internal

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   host is described as Received Fragments Ordered (abbreviated RF=O).

   NAT that is capable of receiving TCP fragments in or out of order and
   forwarding the individual packets (or a reassembled packet) to the
   internal host is referred to as Receive Fragments Out of Order
   (abbreviated RF=OO).

   A NAT that is neither of these is referred to as Receive Fragments
   None (abbreviated RF=N).

   NAT MUST support RF=O or RF=OO.  NAT MAY support RF=OO.

3.5.2  TCP Segments handling

   A NAT may receive a TCP segmented packet.  The following diagram
   explains this:

   +-------------------+                         +-------------------+
   | Application-Layer |                         | Application-Layer |
   +-------------------+                         +-------------------+
   | TCP [MSS = 536]   |                         | TCP [MSS = 536]   |
   +-------------------+                         +-------------------+
   | IP                |                         | IP                |
   +-------------------+                         +-------------------+
   | Lower-Layer       |                         | Lower-Layer       |
   | (MTU = 1500       |                         | (MTU = 1500       |
   +-------------------+                         +-------------------+
        End-host -1                                   End-host -2
            |                   +--------+                |
            +-------------------| NAT    |----------------+

   Say the application layer is sending data with size = 600.  Since the
   MTU is 1500 this should not be an issue with respect to IP
   fragmentations, but this packet will still be TCP segmented.  The
   following 2 TCP segments might appear on the wire:

   TCP Segment 1:
   |IP[Frag-offset=0,More-flags =0|TCP hdr[Payload-Len=536]|Appl-data1|

   TCP Segment 2:
   |IP[Frag-offset=0,More-flags =0|TCP hdr[Payload-Len=64] |Appl-data2|

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   There are 2 scenarios :

   1.  The TCP segments are received by NAT, in-order

   2.  The TCP segments are received by NAT, out-of-order

   For out-of-order segments, if the NAT has the intelligence to process
   the application-payload, it should be able to figure out the
   out-of-order TCP segments and enforce some queueing mechanism such
   that when the prior segment is received it SHOULD reassemble the TCP
   segments and proceed with the application-payload processing.  In
   addition it SHOULD send a TCP ACK for getting subsequent TCP segments
   from the endpoint.

   For in-order segments, the application has to provide the length of
   data.  In such cases the NAT has to enforce the same queuening
   mechanism as mentioned above and reassemble the TCP segments and
   proceed for its processing.  This applies only for application which
   has the support for providing the length in the application-data.

   NATs that behave as described in this section are refered to as
   "Support Segmentation Yes" (abbreviated SS=Y).  NATs that do not
   support any of the above are called "Support Segmentation none"
   (abbreviated SS=N).

   If a NAT is doing application-payload processing it SHOULD support

3.6  Adjusting Sequence Acknowledgement Numbers

   A NAT might modify the TCP data carrying the application-payload
   resulting in the TCP data to increase or decrease in size.  As a
   result of this the NAT is expected change/remember the sequence/
   acknowledgement number in the TCP header and save this information
   along with the delta of change, in the NAT entry associated with this
   particular packet flow so that subsequent TCP packets of this flow
   are adjusted accordingly.

   If a NAT receives a particular type of TCP payload for which it is
   capable/enabled to do a deep packet inspection it is expected to
   create a NAT entry with the full flow information irrespective of the
   type of NAT configuration, and save the [seq, ack, delta] associated
   with the TCP flow.  It may chose to store any other additional

   For eg:

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                                                  +-------| Host B|
                                                  |       +-------+
    +-------+         +------+                    |
    |Host A |---------| NAT  |--------------------+
    +-------+         +------+                    |
                                                  |       +-------+
                                                  +-------| Host C|

   Say the NAT is configured to do NAT and not PAT, meaning it is
   configured to do 1:1 translation say (A->A').  In such cases most
   NATs will create a simple NAT entry in the database to optimize on
   the memory or otherwise, and not create one with the full flow

   So when A is sending a TCP flow to B & to C and the NAT does not have
   the intelligence to do a deep packet inspection for this TCP payload,
   then the NAT can chose to create just the following entry in the NAT

   |Prt|L-Addr|L-Port|G-Addr|G-Port|Dest Addr| Dest Port|
   |-  |A     |-     |A     |-     |-        | -        |

   Where L-Addr/Port are the internal address/port of the Host, G-Addr/
   port are the NAT Translated address/port.

   But if A sends TCP traffic for which the NAT can do deep packet
   inspection and thereby possibly change the TCP payload-data-length,
   then the NAT SHOULD create entries with full flow information with
   the information about the [seq, ack,delta].

   |Prt|L-Addr|L-Port|G-Addr|G-Port|Dest Addr| Dest Port|       |
   |TCP|A     |p     |A     |p'    |B        | q        |[seq,  |
   |   |      |      |      |      |         |          | ack,  |
   |   |      |      |      |      |         |          | delta]|

   Say for example the NAT is not doing so and the TCP sessions from
   A->B and A->C are not differentiable then the subsequent TCP flows'

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   sequence/acknowledgement number-fixups will be incorrect.

   NATs that operate as described in this section are described as
   "Supports  Seq/Ack Delta Adjustment" (abbreviated SDA=S).  NATs that
   do not operate in this mode are described as "Supports Seq/Ack Delta
   Adjustment none" (abbreviated SDA=N).  NATs MAY chose to be SDA=N for
   TCP flows for which it does not do deep packet inspection but SHOULD
   do SDA=S for the ones it does.

   A NAT SHOULD support SDA=S.

3.7  Handling of ICMP Error messages

   In addition to what has been described in [UDP-REQ] with respect to
   handling ICMP Error-messages, a NAT is expected to do a fixup of the
   embedded TCP payload in the ICMP Error message.  As a result the ICMP
   Error message will correctly communicate the proper address/port
   information to the TCP stack on endpoint which can associate it with
   its internal data-structures and act accordingly.

   For eg:

   +-------+          +---------+          +--------+
   |Host A | ---------|NAT      |----------| Host B |
   +-------+          +---------+          +--------+

   Say A sends a TCP packet via NAT to Host B (A,p) -> (B,p).  NAT
   changes the source address/port of the TCP header to say (A',p').
   For some reason the B sends back an ICMP Error message back towards
   A'.  This ICMP Error message will contain part of the TCP header.
   The NAT is expected to not only modify the ICMP header but look into
   the TCP payload in the ICMP Error message and modify the TCP header
   accordingly so that when the ICMP Error is processed by the stack on
   A, it is able to correctly co-relate with its internal
   data-structures and act accordingly.

   NATs that operate as described in this section are described as
   "Supports ICMP Fixup Yes" (abbreviated SIF=Y).

   NATs that do not are described as "Supports ICMP Fixup No"
   (abbreviated SIF=N).

   A NAT SHOULD support SIF=Y.

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4.  Security considerations

   NATs are often deployed to achieve security goals.  Most of the
   recommendations and requirements in this document do not affect the
   security properties of these devices, but a few of them do have
   security implications and are discussed in this section.

   When a fragmented packet is received from the external side and the
   packets are out of order so that the initial fragment does not arrive
   first, many systems simply discard the out of order packets.

   Moreover, since some networks deliver small packets ahead of large
   ones, there can be many out of order fragments.  NATs that are
   capable of delivering these out of order packets are possible but
   they need to store the out of order fragments, which can open up a
   DoS opportunity.  Fragmentation has been a tool used in many attacks,
   some involving passing fragmented packets through NATs and others
   involving DoS attacks based on the state needed to reassemble the
   fragments.  NAT implementers SHOULD be aware of [TINY] and [FRAG]

5.  IANA Considerations

   There are no IANA considerations.

6.  IAB Considerations

   There are no IAB considerations.

7.  Acknowledgements

   The authors would like to thank Cullen Jennings & Nagendra Modadugu
   for their review comments.

8.  References

8.1  Normative References

   [KEYWRD]  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", RFC 2119, BCP 14, March 1997.

   [UNSAF]   Daigle, L. and IAB, "IAB Considerations for Unilateral
             Self-Address Fixing  (UNSAF) Across Network Address
             Translation", RFC 3424, November 2002.

8.2  Informative References

   [ASND]     Reynolds, J. and J. Postel, "Assigned numbers", RFC 923,
              October 1984.

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   [FRAG]     Ziemba, G. and D. Reed, "Security Considerations for IP
              Fragment Filtering", RFC 1858, October 1995.

   [H323]     "Packet-based Multimedia Communications Systems, ITU-T
              Recommendation H.323", July 2003.

   [HOST]     Braden, R., "Requirements for Internet-Hosts -
              Communication Layers", RFC 1122, October 1998.

   [ICE]      Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Methodology for Network Address Translator (NAT)
              Traversal for the Session Initiation Protocol (SIP)
              draft-ietf-mmusic-ice-02 (work in progress)", RFC , July

   [ICMP]     Postel, J., "Internet Control Message Protocol", RFC 792,
              September 1981.

   [NAT-1]    Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional  NAT)", RFC 3022, January

   [NAT-2]    Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and  Considerations",
              RFC 2663, August 1999.

   [NAT-3]    Holdrege, M. and P. Srisuresh, "Protocol Complications
              with the IP Network Address  Translator", RFC 3027,
              January 2001.

   [P2P-1]    Ford, B., Srisuresh, P. and D. Kegel, "Peer-to-Peer(P2P)
              communication across Network Address  Translators(NATs)
              draft-ford-midcom-p2p-03 (work in progress)", June 2004.

   [P2P-2]    Ford, B. and D. Andersen, "Nat Check Web Site:
              http://midcom-p2p.sourceforge.net", June 2004.

   [RTP]      Schulzrinne, H., Casner, S., Frederick, R. and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", RFC 3550, July 2003.

   [SIP]      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.

   [STUN-1]   Rosenbert, J., Weinberger, J., Huitema, C. and R. Mahy,
              "STUN - Simple Traversal of User Datagram Protocol (UDP)
              Through Network Address Translators (NATs)", RFC 3489,

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              March 2003.

   [STUN-2]   Jennings, C., "NAT Classification Results using STUN,
              draft-jennings-midcom-stun-results-01 (work in progress)",
              July 2004.

   [TINY]     Miller, I., "Protection Against a Variant of the Tiny
              Fragment Attack", RFC 3128, June 2001.

   [UDP-REQ]  Audet, F. and C. Jennings, "NAT Behavioral Requirements
              for Unicast UDP,  draft-ietf-behave-nat-00.txt
              (work-in-progress)", January 2005.

   [V4-REQ]   Baker, F., "Requirements for IP Version 4 Routers",
              RFC 1812, June 1995.

Authors' Addresses

   Senthil Sivakumar
   Cisco Systems, Inc.
   170 West Tasman Dr.
   San Jose, CA  95134

   Email: ssenthil@cisco.com

   Kaushik Biswas
   Cisco Systems, Inc.
   170 West Tasman Dr.
   San Jose, CA  95134

   Phone: +1 408 525 5134
   Email: kbiswas@cisco.com

   Bryan Ford
   Laboratory for Computer Science
   77 Massachusetts Ave.
   Cambridge, MA  02139

   Phone: 1-617-253-5261
   Email: baford@mit.edu

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