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Versions: 00 01 02 03 04 draft-ietf-behave-tcp

Network Working Group                                         P. Hoffman
Internet-Draft                                 Cybersecurity Association
Expires: January 18, 2006                                        S. Guha
                                                              P. Francis
                                                      Cornell University
                                                           July 17, 2005


              NAT Behavioral Requirements for Unicast TCP
                    draft-hoffman-behave-tcp-02.txt

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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document defines a set of requirements for NATs that handle
   unicast TCP that would allow many applications, such as peer-to-peer
   reliable communications or on-line gaming, to work consistently.
   Developing NATs that meet this set of requirements will greatly
   increase the likelihood that these applications will function
   properly.



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1.  Introduction

   [BEHAVE-UDP] defines many terms relating to NATs, lays out general
   requirements for all NATs, and sets requirements for NATs that handle
   unicast UDP traffic.  This document is an adjunct to [BEHAVE-UDP] and
   sets requirements for NATs that handle unicast TCP traffic (that is,
   almost every NAT).  All definitions and requirements in [BEHAVE-UDP]
   are inherited here.

2.  Incoming SYN packets and outgoing SYNACK packets on an open TCP
    connection

   Some TCP-based protocols attempt to start multiple TCP connections
   for a single flow of traffic.  Such protocols can sometimes work
   through NATs.  In some cases, it may require a SYN packet to be
   received after a SYN packet is sent, or a SYNACK packet to be sent
   after a SYN packet it sent.  Normal TCP stacks correctly handle such
   cases, in accordance with [RFC-TCP].

   REQ-1: A NAT MUST allow all TCP packets in either direction on any
      open NAT mapping that is a TCP connection.

   Justification for REQ-1: This allows end-hosts to follow any valid
      transition defined in TCP.

3.  Incoming SYN packets to non-existent mappings

   NATs have two basic choices when they receive a SYN packet for a
   mapping that does not exist: ignore it or respond to it with a RST or
   ICMP packet.  A NAT that receives a SYN packet from the external side
   for an address and port mapping that does not exist must decide
   whether or not to respond.  Ignoring the SYN instead of responding
   with an RST can help some applications that start multiple TCP
   connections.  Sending an ICMP soft-error does not affect other NATs
   or OS stacks and allows the application to decide whether or not to
   abort the connection attempt.

   REQ-2: A NAT MUST NOT send a RST packet in response to a SYN packet
      sent from the external side to an address and port mapping that
      does not exist.
      a) A NAT MAY send an ICMP Host Unreachable (Type=3, Code=1) in
         response to such a SYN packet.

   Justification for REQ-2: Not sending an RST helps applications that
      open multiple TCP connections and attempt to trigger a TCP
      Simultaneous-open.  Sending an ICMP soft error allows applications
      to choose to abort a TCP attempt without having to wait for a TCP
      timeout.



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4.  Resource exhaustion and timers

   A NAT maintains state associated with the mappings that it maintains.
   Because of this, a NAT is susceptible to a resource-exhaustion attack
   whereby an attacker (or virus) on the internal side attempts to cause
   the NAT to create more state than it has resources.  To prevent such
   an attack, a NAT needs to abandon mappings in order to free the state
   resources.

   A common method that is applicable only to TCP connections is to look
   for partially-open or partially closed TCP connections and abandon
   those based on timers specific to those states.  The states of a TCP
   connection are defined in [RFC-TCP] and can be inferred by examining
   the TCP flags of incoming and outgoing packets for that mapping.  The
   mapping timer is defined as the time a mapping will stay active
   without packets traversing the NAT.  The timer values for various
   states are mentioned in [RFC-1122].

   REQ-3: A NAT TCP mapping timer SHOULD NOT expire in less than 4
      minutes for a TCP connection in SYN_SENT, SYN_RCVD, CLOSING,
      LAST_ACK or TIME_WAIT state.  A NAT TCP mapping timer SHOULD NOT
      expire in less than 2 hours for a TCP connection in any other
      state except CLOSED.
      a) When a NAT closes an open TCP mapping, it MUST send RST packets
         to each end of the connection.
      b) To maximize application transparency, a NAT MAY expire mappings
         only when it is low on resources.

   Justification for REQ-3: TCP Keep-alive packets, when enabled, are
      sent at 2 hour intervals on an idle connection by default.  During
      connection setup or teardown, TCP waits 2xMSL (4 minutes) for in-
      flight packets to be delivered and acknowledged.  This requirement
      can be easily implemented by examining the ACK flag in both
      directions during connection setup and examining the FIN flag in
      both directions during connection teardown.

5.  Requirements

   A NAT that supports all of the mandatory requirements of this
   specification (i.e., the "MUST"), is "compliant with this
   specification."  A NAT that supports all of the requirements of this
   specification (i.e., included the "RECOMMENDED") is "fully compliant
   with all the mandatory and recommended requirements of this
   specification."







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   REQ-1: A NAT MUST allow all TCP packets in either direction on any
      open NAT mapping that is a TCP connection.
   REQ-2: A NAT MUST NOT send a RST packet in response to a SYN packet
      sent from the external side to an address and port mapping that
      does not exist.
      a) A NAT MAY send an ICMP Host Unreachable (Type=3, Code=1) in
         response to such a SYN packet.
   REQ-3: A NAT TCP mapping timer SHOULD NOT expire in less than 4
      minutes for a TCP connection in SYN_SENT, SYN_RCVD, CLOSING,
      LAST_ACK or TIME_WAIT state.  A NAT TCP mapping timer SHOULD NOT
      expire in less than 2 hours for a TCP connection in any other
      state except CLOSED.
      a) When a NAT closes an open TCP mapping, it MUST send RST packets
         to each end of the connection.
      b) To maximize application transparency, a NAT MAY expire mappings
         only when it is low on resources.

   Requirements inherited from [BEHAVE-UDP]:

   [[Once finalized, BEHAVE-UDP requirement list with UDP changed to TCP
   where applicable.]]

6.  Security considerations

   The security considerations for this document are the same as for
   [BEHAVE-UDP].  The fact that this document covers unicast TCP does
   not change any of the security considerations there. [[ Is this
   statement actually true? ]]

   [[If the NAT is examining TCP flags, then it needs to track sequence
   numbers and the TCP timestamp option to correctly determine out-of-
   window packets.]]

7.  IANA considerations

   This document does not change or create any IANA-registered values.

8.  Normative References

   [BEHAVE-UDP]
              Audet, F. and C. Jennings, "NAT Behavioral Requirements
              for Unicast UDP", draft-ietf-behave-nat-udp (work in
              progress).

   [RFC-1122]
              Braden, R., "Requirements for Internet Hosts --
              Communication Layers", RFC 793.




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   [RFC-TCP]  Postel, J., "Transmission Control Protocol", RFC 793.


Authors' Addresses

   Paul Hoffman
   Cybersecurity Association
   127 Segre Place
   Santa Cruz, CA 95060
   USA

   Email: paul.hoffman@cybersecurity.org


   Saikat Guha
   Cornell University
   331 Upson Hall
   Ithaca, NY 14853
   USA

   Email: saikat@cs.cornell.edu


   Paul Francis
   Cornell University
   4108 Upson Hall
   Ithaca, NY 14853
   USA

   Email: francis@cs.cornell.edu





















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