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Versions: 00 01 02                                                      
Behave Working Group                                        P. Srisuresh
Internet-Draft                                      Caymas Systems, Inc.
Expires: September 28, 2005                                 S. Sivakumar
                                                               K. Biswas
                                                     Cisco Systems, Inc.
                                                                 B. Ford
                                                                  M.I.T.
                                                          March 28, 2005


                  NAT Behavioral Requirements for TCP
                 <draft-sivakumar-behave-nat-tcp-req-01.txt>

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
   other groups may also distribute working documents as
   Internet-Drafts.

   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
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

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

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   NAT devices are available from a number of vendors and are in use by
   several residential and enterprise users. Yet, there is much



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   variation in how the NAT devices work. Application developers,
   network administrators and users of NAT devices seek some level of
   uniformity and predictability in how various of the NAT devices
   operate. The objective of this document is to specify the
   operational and behavioral requirements on the NAT devices while
   processing TCP packets. A NAT device that conforms to the
   requirements listed in the document will bring predictability in
   how NATs operate with regard to TCP packet processing. A NAT device
   is said to be IETF behave compliant when it complies with the
   requirements outlined in this document and two other companion
   documents ([BEH-GEN], [BEH-UDP]) which outline the requirements
   for processing IP, ICMP & UDP.


Table of Contents

   1.  Introduction & Scope . . . . . . . . . . . . . . . . . . . . .  2
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  TCP requirements discussion  . . . . . . . . . . . . . . . . .  3
     3.1   Address Binding and/or TCP Port Binding  . . . . . . . . .  3
     3.2   Timeouts for TCP Sessions  . . . . . . . . . . . . . . . .  4
     3.3   SYN packets during Connecting and Closing phases . . . . .  5
     3.4   Denial of Service (DoS) attacks  . . . . . . . . . . . . .  5
     3.5   NAT initiated TCP keep-alives  . . . . . . . . . . . . . .  6
     3.6   NAT initiated RST packets  . . .  . . . .  . . . . . . . .  7
   4.  Hints to implementers  . . . . . . . . . . . . . . . . . . . .  7
     4.1   Light weight TCP state machine is a common practice  . . .  7
     4.2   TCP segment processing in NATs supporting ALGs . . . . . .  8
     4.3   Adjusting Sequence and Acknowledgement Numbers . . . . . .  9
   5.  TCP behavioral requirements summary  . . . . . . . . . . . . . 10
   6.  Security considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.1   Normative References . . . . . . . . . . . . . . . . . . . 11
     8.2   Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
   Intellectual Property and Copyright Statements . . . . . . . . . . 13


1.  Introduction & Scope

   NAT implementations vary amongst vendors in how they handle TCP
   packets. This document defines the operational and behavioral
   requirements that the NAT devices should comply with while
   processing TCP packets. In addition, a section is devoted to
   describing hints to implementers in deciphering some of the
   requirements.




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   The requirements outlined here are applicable across all NAT types
   identified in [RFC2663], most importantly the Traditional Nat, as
   described in [RFC3022]. This document does not mandate a specific
   implementation choice. However, this does require NAT devices to
   adhere to the basic design principles and general behavioral
   requirements outlined in [BEH-GEN]. Behavioral requirements for UDP
   are covered in [BEH-UDP].

   Application Layer Gateways (ALGs) are out of scope for this
   document. However, hints on how a NAT could be extended to support
   ALGs are discussed under the hints section.

2. Terminology

   Definitions for the NAT terms used throughout the document may be
   found in [BEH-GEN] and/or [RFC2663]. TCP terms used in the document
   are as per the definitions given in [TCP].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


3.  TCP requirements discussion

   This section lists the behavioral requirements of a NAT device
   when processing TCP packets. Associated with each requirement, the
   rationale behind the requirement is discussed in detail.

3.1 Address Binding and/or TCP Port Binding

   NAT provides transparent routing between address realms by
   assigning realm-specific endpoint locator(s), as packets pertaining
   to a session cross realm boundaries. Several applications use the
   same endpoint within a realm to establish multiple simultaneous
   sessions. Many peer-to-peer applications use the public endpoint
   registration of peering hosts to initiate sessions into.

   In order to support peer-to-peer applications and applications
   that entertain multiple simultaneous session using the same TCP
   endpoint, NAT MUST retain the association it assigned to an endpoint
   between realms and reuse the same endpoint association when
   multiple sessions using the same endpoint are routed through the
   NAT device. Such a binding between endpoints can occur when a NAT
   device maintains Address Bindings and/or TCP Port Bindings.

   REQ-1: A NAT device MUST maintain Address and/or TCP Port
   Bindings.



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3.2  Timeouts for TCP NAT Sessions

   As may be noted from [TCP], an end-to-end TCP session in its
   lifetime goes through three phases, namely Connecting, Established,
   and Closing. Each end-to-end TCP session is managed through a
   separate NAT Session within NAT. The NAT Session must be capable of
   identifying the current phase of the end-to-end TCP session it
   represents and use an idle timeout period that is appropriate for
   the current phase.

   Connecting Phase: An end-to-end TCP session is said to enter the
   Connecting Phase when either of the endpoints sends the first SYN
   for the TCP session and exit the phase upon completion of 3-way SYN
   handshake. The idle timeout used by the NAT Session during this
   phase is called the SYN timeout. SYN timeout needs to be relatively
   short, so NAT can protect itself (and, potentially, the hosts behind
   it) from SYN flood attacks. A NAT session is freed when the SYN
   timeout expires.

   Established Phase: An end-to-end TCP session is said to enter the
   Established Phase upon completion of 3-way handshake and exit the
   phase upon seeing the first FIN or RST for the session. The idle
   timeout used by the NAT during this phase of the end-to-end TCP
   session is called the Session timeout. Session timeout needs to be
   relatively long, so the NAT Session can retain state of the
   end-to-end TCP Session within itself even after long periods of
   inactivity in the session. Long periods of inactivity is not
   uncommon with applications such as telnet and ftp. When Session
   timer expires, the corresponding NAT Session may be freed (or) the
   NAT Session may assume the TCP connection to have transitioned
   into the Closing phase.

   Closing Phase: An end-to-end TCP session is said to enter the
   Closing Phase when either of the endpoitns sends the first FIN
   or RST for the session. Alternately, the NAT Session may deem the
   TCP session to have entered this phase when the TCP Session timer
   expires. The idle timeout used by the NAT Session during this phase
   is called the Close timeout. Close timeout is relatively short to
   ensure that the ACKs for the final FINs on a gracefully-closed TCP
   session had a chance to propagate in both directions, and to allow
   time for either endpoint to re-open a  recently closed or reset TCP
   session if desired. A NAT device MAY opt to have different Close
   timeouts depending upon whether the Closing phase is triggered by
   FIN or not. Once the Close timer expires, the NAT Session will be
   freed.




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   The following requirements apply to the NAT's timeouts:

   REQ-2: A NAT device MUST be capable of identifying the current phase
   of an end-to-end TCP session and use different idle timeout periods
   for each phase of the TCP Session. The timeouts used for each phase
   SHOULD be admin configurable. The recommended value for SYN timeout
   is 30 seconds. The recommended value for TCP session timeout is 30
   minutes. Lastly, the recommended value for close timeout is 2 x MSL
   (Maximum Segment Lifetime) or 4 minutes.


3.3 SYN packets during Connecting and Closing phases

   A NAT device might allow sessions to be initiated in just one
   direction and not the other. However, once a NAT session is created
   for a permitted TCP session, and the TCP session is in Connecting
   phase, the NAT device MUST let the SYN packets through in either
   direction. Likewise, if the TCP session is in Closing phase and a
   new SYN packet arrives from either endpoint before the close
   timer expires, the NAT device should assume that the TCP session
   has re-entered the Connecting phase and initiate  SYN timer as
   described above.

   This is because TCP protocol fundamentally permits simultaneous TCP
   Open from either end. A number of TCP based Peer-to-peer
   applications utilize the simultaneous TCP open technique to
   establish peer to peer connections.

   The following requirement applies to SYN packets arriving during
   Connecting and Closing phases of a TCP connection.

   REQ-3: A NAT device MUST let the SYN packets through when the SYN
   Packets are received on a TCP connection which is in one of
   Connecting or Closing phase.


3.4 Denial of Service (DoS) attacks

   Since NAT devices are Internet hosts, they can be the target of a
   number of different DoS attacks, such as SYN floods and RST attacks.
   NAT devices SHOULD employ the same sort of protection techniques as
   Internet-based servers do.

   Let us examine two types of Dos attacks that are well known with
   regard to TCP connections. A SYN flood attack is a DoS attack in
   which one or more external entities initiate a number of
   simultaneous TCP connections using a SYN packet, but donot complete
   the 3-way handshake. Naturally, a NAT device is prone to this type



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   of attack when the NAT device is in the traversal path of the SYN
   attacks. One technique to defend against this type of attack is to
   ensure that the NAT device employs a short SYN timeout and reduce
   the timeout even further when it determines it is under SYN flood
   attack.

   RST attack is another well known DoS attack. An attacker could
   simply forge a number of RST packets for a variety of Established
   TCP connections and cause the NAT sessions to be reset and freed.
   One technique to defend against this type of attack is to validate
   the RST packet and not let the packet through unless the sequence
   number used in the RST packet is within the expected TCP window
   size of the TCP Session.

   REQ-4: A NAT device SHOULD employ necessary techniques to defend
   against well known DoS attacks.


3.5  NAT initiated TCP keep-alives

   When session timer expires for a NAT session, that indicates that the
   associated TCP session has been idle with no activity for the period
   matching the TCP Session timeout. Sensing no activity, NAT could free
   up the NAT Session and remove the state associated with the TCP
   connection within the NAT device. However, doing so would violate the
   end-to-end reliability of the IP network. Ideally speaking, IP
   network is not supposed to retain any hard state. Unfortunately, a
   NAT device retains session state within itself (via the NAT Sessions)
   and this information should not be dropped without confirming that
   one or both halfs of the TCP session are alive.

   A NAT device may validate the liveness of a TCP client by sending
   keep-alive packets to the TCP client using the technique described
   in section 4.2.3.6 of [HOST]. If the NAT device receives an ACK or
   other traffic from the internal endpoint, it resets the session
   timer and assumes the connection to be in ôEstablished" phase. If
   the NAT device receives a RST from the TCP client, the NAT device
   transitions the TCP connection into the "closing" phase and
   initiates Close timeout for the session. If the NAT device receives
   no response from the internal endpoint after sending several
   keep-alive packets, the NAT assumes that the internal endpoint is
   dead and again assumes that the TCP connection has entered the
   "closing" phase.

   Below is the keep-alive requirement on NAT devices

   REQ-5: When session timer expires for an established TCP connection,
   the NAT device MAY initiate sending TCP keep-alives to the clients



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   prior to freeing up the Session state within NAT.

3.6.  NAT initiated RST packets

   When session timer expires for a NAT session, it is an indication
   that the associated TCP connection has been idle with no activity
   for the duration matching the TCP Session timeout. Sensing no
   activity, NAT could free up the NAT Session and remove the state
   associated with the TCP connection. When this happens, the two TCP
   endpoints in the network, which might potentially be alive, may be
   unable to resume activity on the connection because the NAT device
   enroute no longer has the state information pertaining to the
   end-to-end TCP connection. This can be problematic for application
   servers that impose limits on the number of connections a user
   might be allowed to setup in a given period of time. After a few
   zombie sessions, the server might deny access to its clients, when
   the connection count on the server exceeds the set limit. The Server
   has no way to know that some of the client sessions it retains are
   zombies and shouldnt be counted as real. In such a situation, the
   NAT device sending a RST packet to both parties will alert the two
   parties of the connection going away. And, the application servers
   are not fooled with zombie sessions. Note, a NAT device may choose
   to send RST packets after it probed the TCP client with TCP
   Keep-alive packets.

   Below is the requirement on sending TCP RST packet.

   REQ-6: When Session timer expires on an Established TCP connection,
   the NAT device SHOULD send a RST packet to both halfs of a TCP
   connection and enter Closing state on the connection prior to
   freeing up the NAT Session.


4. Hints to implementers

4.1 Light weight TCP state machine is a common practice

   Unlike UDP, TCP sessions are fundamentally unicast in nature and
   multiple NAT Sessions cannot be aggregated. NAT devices maintain a
   separate NAT Session to track each end-to-end TCP connection that
   traverses the NAT device. A NAT device needs to be able to track the
   current phase of a TCP session at any given time so an idle timer
   for a duration appropriate for the phase is initiated. Further, a
   NAT device defending against even the most trivial type of DoS
   attack will require the knowledge of TCP sequence number and window
   Size to defend itself against such attacks. As such, many vendors
   use a light-weight state machine within the NAT Session to
   track the current state of a TCP connection. Items tracked



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   within the state machine would include the last acknowledged
   sequence number from either half of the TCP session, TCP window
   size, and the TCP connection phase.

   The State machine within a NAT Session enters the Connecting state
   when NAT sees the first SYN packet for that session. The state
   machine transitions from the Connecting to Established state once
   the 3-way handshake is completed. The state machine transitions from
   the Established state to the Closing 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. The NAT device deletes the NAT session after
   the Close timer expires while the TCP connection is in the Closing
   state.

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


4.2  TCP segment processing in NATs supporting ALGs

   The following discussion on TCP segment processing is relevant only
   when a NAT device includes support for one or more embedded ALGs.
   Many NAT devices have the ALG for FTP enabled by default.
   A NAT device may receive payload relevant to an ALG in multiple TCP
   segments. Consider the following diagram where the MSS is set to
   536 bytes in each endpoint of the TCP connection.


   +-------------------+                         +-------------------+
   | 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
            |                   +--------+                |



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            +-------------------| NAT    |----------------+
                                +--------+

   Say the application layer on end-host-1 is sending a payload of size
   600 bytes. Even though the MTU is 1500 bytes, the payload is sent to
   the recipient in 2 TCP segments as follows, because the MSS is set
   to 536 bytes.

   TCP Segment 1:
   +-------+------------------------+----------+
   |IP hdr |TCP hdr[Payload-Len=536]|Appl-data1|
   +-------+------------------------+----------+

   TCP Segment 2:
   +-------+------------------------+----------+
   |IP hdr |TCP hdr[Payload-Len=64] |Appl-data2|
   +-------+------------------------+----------+


   A NAT device enroute may receive the TCP segments either in order or
   out of order. In either case, the NAT device needs to assemble the
   individual segments into a contiguous payload and make the complete
   payload available for the ALG to process prior to forwarding the
   segments transparently to another realm.

   In order to do this, a NAT device is required to enforce some type
   of queuing mechanism such that when all relevant segments of a
   payload are received, it is able to reassemble the TCP segments and
   make the contiguous payload available for ALG processing.

   For in-order segments, the NAT device needs to send a TCP ACK for
   the initial segments it received, but didnt forward to the recipient
   enpoint. This is done so the NAT device can prompt the sending
   endpoint to continue to send the remaining TCP segments.


4.3  Adjusting Sequence and Acknowledgement Numbers

   The following discussion on adjusting Sequence and Acknowledgement
   numbers is relevant only when a NAT device includes support for one
   or more embedded ALGs.

   When the embedded ALG on a NAT device modifies the TCP payload, the
   corresponding payload may increase or decrease in size.  As a
   result, the NAT device is expected to remember the delta change and
   adjust sequence/acknowledgement numbers in all subsequent TCP
   packets within the session. Implementors of NAT devices often keep
   the delta changes in payload due to ALG processing within the NAT



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   Session as an extension of the state information the NAT device
   keeps.


5. TCP behavioral requirements summary

   Below is a summary of all the TCP behavioral requirements.

   REQ-1: A NAT device MUST maintain Address and/or TCP Port
   Bindings.

   REQ-2: A NAT device MUST be capable of identifying the current phase
   of an end-to-end TCP session and use different idle timeout periods
   for each phase of the TCP Session.

   The timeouts used for each phase SHOULD be admin configurable. The
   recommended value for SYN timeout is 30 seconds. The recommended
   value for TCP session timeout is 30 minutes. Lastly, the
   recommended value for close timeout is 2 x MSL (Maximum Segment
   Lifetime) or 4 minutes.

   REQ-3: A NAT device MUST let the SYN packets through when the SYN
   Packets are received on a TCP connection which is in one of
   Connecting or Closing phase.

   REQ-4: A NAT device SHOULD employ necessary techniques to defend
   against well known DoS attacks.

   REQ-5: When session timer expires for an Established TCP connection,
   the NAT device MAY initiate sending TCP keep-alives to the clients
   prior to freeing up the Session state within NAT.

   REQ-6: When session timer expires for an Established TCP connection,
   the NAT device SHOULD send a RST packet to both halfs of a TCP
   connection and enter Closing state on the connection prior to freeing
   Up the NAT Session.

6.  Security considerations

   The security considerations described in [RFC2663] for all
   variations of NATs are applicable here. The recommendations and
   requirements in this document do not effect the security
   properties of the NAT devices adversely.


7.  Acknowledgements

   The authors would like to thank Nagendra Modadugu and Cullen



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   Jennings for their feedback and comments.


















































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8.  References

8.1  Normative References

  [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", RFC 2119, BCP 14, March 1997.

  [TCP]     Postel, J., "Transmission Control Protocol (TCP)
            Specification", STD 7,  RFC 793, September 1981.

  [HOST]    Braden, R., "Requirements for Internet Hosts --
            Communication Layers", RFC 1122, October 1989.

  [RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
            Translator (NAT) Terminology and  Considerations",
            RFC 2663, August 1999.

  [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
            Address Translator (Traditional  NAT)", RFC 3022, January
            2001.

  [BEH-GEN] Ford, B., Srisuresh, P., and S. Sivakumar, ôDesign
            Principles and General Behavioral Requirements for
            NATsö, draft-ford-behave-gen-01.txt (work in progress),
            March 2005.


8.2  Informative References

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

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


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

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


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

  [UNSAF]    Daigle, L. and IAB, "IAB Considerations for Unilateral



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             Self-Address Fixing  (UNSAF) Across Network Address
             Translation", RFC 3424, November 2002.

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



Authors' Addresses:

   Pyda Srisuresh
   Caymas Systems, Inc.
   1179-A North McDowell Blvd.
   Petaluma, CA 94954
   USA

   Phone: (707) 283-5063
   E-mail: srisuresh@yahoo.com

   Senthil Sivakumar
   Cisco Systems, Inc.
   170 West Tasman Dr.
   San Jose, CA  95134
   USA
   Phone:
   Email: ssenthil@cisco.com


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

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


   Bryan Ford
   M.I.T.
   Laboratory for Computer Science
   77 Massachusetts Ave.
   Cambridge, MA  02139
   USA

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



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Copyright Statement

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Acknowledgment

   Funding for the RFC Editor function is currently provided by the



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   Internet Society.

















































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