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Versions: 00 01 02 draft-ietf-behave-nat-icmp

BEHAVE WG                                                   P. Srisuresh
Internet Draft                                                Consultant
Expires: April 20,  2006                                         B. Ford
                                                                  M.I.T.
                                                            S. Sivakumar
                                                           Cisco Systems
                                                            October 2005


                ICMP Protocol Behavioral Requirements
                  for Network Address Translators
                <draft-srisuresh-behave-nat-icmp-00.txt>


Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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Abstract

   This document identifies the behavioral properties required of the
   Network Address Translator devices (NATs) in conjunction with ICMP
   protocol. The objective of this memo is to make NAT middleboxes
   more predictable and compatible with diverse application protocols
   that traverse the device. Companion documents provide behavioral
   recommendations specific to TCP and UDP.





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Table of Contents

   1. Introduction & Scope ..........................................
   2. ICMP Behavioral Requirements ..................................
       2.1. ICMP query packet handling .....,,.......................
       2.2. ICMP Error packet handling .....,,.......................
       2.3. Rejection of IP packets not permitted by NAT ............
       2.4. Path MTU Discovery ......................................
   3. Summary of Requirements .......................................
   4. Security Considerations .......................................


1. Introduction & Scope

   As pointed out in RFC 3424 [UNSAF], "From observations of deployed
   networks, it is clear that different NAT boxes' implementation vary
   widely in terms of how they handle different traffic and addressing
   cases."  This wide degree of variability is one part of what
   contributes to the overall brittleness introduced by NATs and makes
   it extremely difficult to predict how any given protocol will behave
   on a network traversing NATs. This document defines a specific set
   of requirements for NAT behavior that will reduce unpredictability
   and enable applications to traverse them reliably.

   The requirements specified here apply  to all  NAT variations
   described in RFC 2663 [NAT-TERM], including Traditional
   NAT (i.e., Basic NAT and NAPT), Bi-directional NAT, and Twice NAT.

   Traditional NAT inherently mandates a certain level of firewall like
   functionality. However, firewall functionality in general is out of
   the scope of this specification.

   NAT traversal strategies that involve explicit signalling between the
   application and the NAT [SOCKS, RSIP, MIDCOM, UPNP] are out of the
   scope of this document.

   This document focusses strictly on the behavior of the NAT device,
   and not on the behavior of applications that traverse NATs.
   A separate companion document [BEH-APP] provides recommendations for
   application designers on how to make applications work robustly over
   NATs that follow the behavioral requirements specified here and the
   companion Behave documents.

   The following section lists the behavioral recommendations to the
   NAT device vendors in conjunction with handling ICMP protocol.
   Section 3 summarizes all the requirements in one place.





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2. ICMP Behavioral Requirements

   This section lists the behavioral requirements for a NAT device
   when processing ICMP packets. Even though ICMP is a transport
   protocol on top of IP, ICMP packet processing is often considered
   an integral of IP and is independent of other transport
   protocols. In the following sub sections, requirements are
   discussed in detail along with the rationale behind them.

2.1. ICMP query packet handling

   A NAT device MUST transparantly forward ICMP query packets between
   endnodes. This involves translating the IP header. The NAPT device
   MUST additionally modify the query ID and the associated checksum
   in the ICMP header. The ICMP NAT Session timeout must be set to
   30 seconds or longer. Setting the ICMP NAT Session timeout to a
   very large duration (say, much larger than 30 secs) longer could
   potentially tie up NAT resources. Caution is warranted. This is
   because applications (such as ping and traceroute) that are built
   on top of ICMP query messages complete within a few seconds.
   By setting the timeout to a large value, the NAT device could
   be holding up precious NAT resources such as Bindings and
   the NAT Sessions for the whole duration.

   REQ-1: A NAT device MUST transparantly forward all ICMP query
   packets. The ICMP NAT Session timeout MUST be 30 seconds or
   longer.

2.2. ICMP Error packet handling

   A NAT device MUST transparently forward ICMP error messages ([ICMP])
   it receives from intermediate or end nodes in either realm to the
   intended endnode. Unlike other IP packets, the basis for translation
   of an ICMP error packet is the NAT Session to which the packet
   embedded within the ICMP error message payload belongs to, not the IP
   and ICMP headers in the outer layer.

   Consider the following scenario in figure 1. Say, NAT-xy is a
   traditional NAT device connecting hosts in private and external
   networks. Router-x and Host-x are in the external network. Router-y
   and Host-y are in the private network. The subnets in the external
   network are routable from the private as well as the external
   domains. Whereas, the subnets in the private network are only
   routable within the private domain. When Host-y initiated a session
   to Host-x, let us say that the NAT device assigned an IP address of
   Host-y' to associate with Host-y in the external network.





Srisuresh, Ford, Sivakumar                                      [Page 3]


Internet=Draft    NAT Behavioral Requirements for ICMP      October 2005


                                  Host-x
                                     |
                             ---------------+-------------------
                                            |
                                     +-------------+
                                     |  Router-x   |
                                     +-------------+
               External Network             |
               --------------------+--------+-------------------
                                   |   ^                   |
                                   |   | (Host-y', Host-x) |
                                   |   |                   v
                             +-------------+
                             |    NAT-xy   |
                             +-------------+
                                   |
                                   | Private Network
      ----------------+------------+----------------
                      |
               +-------------+
               | Router-y    |
               +-------------+
                      |
      ----------------+-------+--------
                              | ^                  |
                              | | (Host-y, Host-x) |
                              | |                  v
                            Host-y

   Figure 1. NAT topology with routers in private & external realms


   Say, a packet from Host-y to Host-x triggered an ICMP error message
   from one of Router-x or Host-x (both of which are in the external
   domain). Such an ICMP error packet will have one of Router-x or
   Host-x as the source IP address and Host-y' as the destination IP
   address. When the NAT device receives the ICMP error packet, the
   NAT device must use the packet embedded within the ICMP error
   message (i.e., the IP packet from Host-y to Host-x) to look up the
   NAT Session the embedded packet belongs to and use the NAT Session
   to translate the embedded payload. The NAT device must also use the
   NAT Session to translate the outer IP header. In the outer header,
   the source IP address will remain unchanged because the originator
   of the ICMP error message (Host-x or Router-x) is in external
   domain and routable from the private domain. The destination IP
   address Host-y' must however be translated to Host-y using the NAT
   Session parameters.




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   Now, say, a packet from Host-x to Host-y triggered an ICMP error
   message from one of Router-y or Host-y (both of which are in the
   private domain). Such an ICMP error packet will have one of
   Router-y or Host-y as the source IP address and Host-x as the
   destination IP address. When the NAT device receives the ICMP error
   packet, the NAT device must use the packet embedded within the ICMP
   error message (i.e., the IP packet from Host-x to Host-y) to look up
   the NAT Session the embedded packet belongs to and use the NAT
   Session to translate the embedded payload. The NAT device must also
   use the NAT Session to translate the outer IP header. In the outer
   header, the destination IP address will remain unchanged, as the IP
   addresses for Host-x is already in the external domain. If the ICMP
   error message is generated by Host-y, the NAT device must simply
   use the NAT Session to translate the source IP address Host-y to
   Host-y'. However, if the ICMP error message is generated by the
   intermediate node Router-y, the NAT device will not have had a
   translation entry for Router-y within the NAT Session. The NAT may
   also not have an Address Binding in place for Router-y. In such a
   case, the NAT device must simply use its own IP address in the
   external domain to translate the source IP address.

   Changes to ICMP error message ([ICMP]) MUST include changes to IP and
   ICMP headers on the outer layer as well as changes to the relevant
   IP and transport headers of the packet embedded within the ICMP-error
   message payload. Section 4.3 of the RFC 3022 describes the various
   items within the ICMP error message that MUST be translated by the
   NAT device.

   REQ-2: A NAT device MUST transparently forward ICMP error packets
   to the target end node. The NAT device MUST translate not only the
   outer IP header, but also the embedded payload within the ICMP error
   packet. In the case the ICMP error packet is originated by a node
   for which the NAT device has no Binding, the NAT device MUST use
   its own IP address in the realm of the target node to translate
   the originating node IP address.

2.2.1. NAT Sessions pertaining to ICMP error packets

   While processing an ICMP error packet, a NAT device MUST not
   refresh or delete the NAT Session that pertains to the embedded
   payload within the ICMP error packet. This is in spite of the
   fact that the NAT device uses the NAT Session to translate the
   embedded payload. By not effecting the NAT Sessions, the NAT
   device is able to retain them, even as someone spoofs ICMP error
   messages pertaining to the NAT Sessions.

   REQ-3: While processing an ICMP error packet, a NAT device MUST not
   refresh or delete the NAT Session that pertains to the embedded



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   payload within the ICMP error packet.

2.3. Rejection of IP packets not permitted by NAT

   Unlike a router, a NAT device is session oriented and is restrictive
   in the packes it permits. A NAT device may also be restrictive due
   to NAT specific resource constraints. For example, a NAT device may
   not permit a packet across, if the packet happens to be the first
   packet of a new session and the NAT device is out of resources (out
   of addresses or TCP/UDP ports or a NAT Session resource) to set up
   a state for the session.

   In the case where a packet is prohibited by a NAT device to
   traverse through it due to resource/administration considerations,
   the NAT device SHOULD send ICMP destination unreachable message,
   with a code of 10 (Communication with destination host
   administratively prohibited) to the sender prior to dropping the
   packet. Unfortunately, there is not another ICMP code
   currently defined to indicate  "Communication with destination host
   port administratively prohibited". So, the same code should be used
   for host as well as port filtering.

   REQ-4: When an inbound packet is prohibited by a NAT device due to
   resource/authorization consideration, the NAT device SHOULD send
   ICMP destination unreachable message, with a code of 10
   (Communication with destination host administratively prohibited)
   to the sender prior to dropping the packet.

2.4. Path MTU support

   IP fragmentation by intermediate nodes often results in
   degraded performance. In some cases, IP fragmentation by the
   intermediate nodes could even cause end-to-end communication
   to entirely fail. Many applications avoid fragmentation in
   the network by originating IP packets that fit within the
   maximum Path MTU enroute and setting the DF (Don't Fragment)
   bit so the intermediate nodes enroute do not fragment the
   packets. For example, a number of TCP connections have the
   DF bit set in the IP header of the TCP segments they
   transmit. Likewise, IP based VPN tunnels also often set the
   DF bit on the external IP encapsulation.

2.4.1. Honoring the DF bit in IP header

   A NAT device MUST honor the DF bit in the IP header of the
   packets that transit the device. If the DF bit is set and the
   MTU on the forwarding interface of the NAT device is such that
   the IP datagram cannot be forwarded without fragmentation, the



Srisuresh, Ford, Sivakumar                                      [Page 6]


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   NAT device MUST issue a "packet too big" ICMP message (ICMP
   type 3, Code 4) with a suggested MTU back to the sender and
   drop the original IP packet. The sender will resend after
   taking the appropriate corrective action. If the DF bit is
   not set and the MTU on the forwarding interface of the NAT
   device mandates fragmentation, the NAT device must simply
   send this fragmented, just as any router does [RFC1812]

   REQ-5: If DF bit is set on a transit IP packet and the NAT
   device cannot forward the packet without fragmentation, the
   NAT device MUST send a "packet too big" ICMP message (ICMP
   type 3, Code 4) with a suggested MTU back to the sender and
   drop the original IP packet.

2.4.2. Honoring the "Packet too big" ICMP message

   This is flip side of the argument for the above section. By
   virtue of the address translation NAT performs, NAT may end
   up being the recipient of "Packet too big" message.

   When NAT device is the recipient of "packet too big"
   ICMP message from the network, the NAT device must simply
   forward the ICMP message back to the intended recipient, as
   stated in REQ-2.

3. Summary of Requirements

   This section summarizes the requirements discussed at length in the
   preceding section.

   REQ-1: A NAT device MUST transparantly forward all ICMP query
   packets. The ICMP NAT Session timeout MUST be 30 seconds or
   longer.

   REQ-2: A NAT device MUST transparently forward ICMP error packets
   to the target end node. The NAT device MUST translate not only the
   outer IP header, but also the embedded payload within the ICMP error
   packet. In the case the ICMP error packet is originated by a node
   for which the NAT device has no Binding, the NAT device MUST use
   its own IP address in the realm of the target node to translate
   the originating node IP address.

   REQ-3: While processing an ICMP error packet, a NAT device MUST not
   refresh or delete the NAT Session that pertains to the embedded
   payload within the ICMP error packet.

   REQ-4: When an inbound packet is prohibited by a NAT device due to
   resource/authorization considerations, the NAT device SHOULD send



Srisuresh, Ford, Sivakumar                                      [Page 7]


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   ICMP destination unreachable message, with a code of 10 to the
   sender prior to dropping the packet.

   REQ-5: If DF bit is set on a transit IP packet and the NAT
   device cannot forward the packet without fragmentation, the
   NAT device MUST send a "packet too big" ICMP message (ICMP
   type 3, Code 4) with a suggested MTU back to the sender and
   drop the original IP packet.


4. Security Considerations

   None yet.


Normative References

[ICMP]      Postel, J., "Internet Control Message Protocol", STD 5,
            RFC 792, September 1981.

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

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

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

[PRIV-ADDR] Y. Rekhter, B. Moskowitz, D. Karrenberg, G. J. de Groot, and
            E. Lear, "Address Allocation for Private Internets", RFC
            1918, February 1996.


Informative References

[BEH-APP]   B. Ford, P. Srisuresh, and D. Kegel, "Application Design
            Guidelines for Traversal of Network Address Translators",
            draft-ford-behave-nat-app-01.txt (Work In Progress),
            October 2005.

[BEH-UDP]   F. Audet and C. Jennings, "NAT Behavioral Requirements for
            Unicast UDP", draft-ietf-behave-nat-udp-04.txt (Work In
            Progress), September 2005.

[BEH-TCP]   P. Srisuresh, S. Sivakumar, K. Biswas, and, B. Ford, "NAT
            Behavioral Requirements for TCP",



Srisuresh, Ford, Sivakumar                                      [Page 8]


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            draft-sivakumar-behave-nat-tcp-req-02.txt (Work In
            Progress), October 2005.

[PMTU]      Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
            November 1990.

[RFC1812]   Baker, F., "Requirements for IP Version 4 Routers",
            RFC 1812, June 1995.

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


Author's Address

   Bryan Ford
   Computer Science and Artificial Intelligence Laboratory
   Massachusetts Institute of Technology
   77 Massachusetts Ave.
   Cambridge, MA 02139
   U.S.A.
   Phone: (617) 253-5261
   E-mail: baford@mit.edu
   Web: http://www.brynosaurus.com/

   Pyda Srisuresh
   Consultant
   20072 Pacifica Dr.
   Cupertino, CA 95014
   U.S.A.
   Phone: (408)836-4773
   E-mail: srisuresh@yahoo.com

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


Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions



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   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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Srisuresh, Ford, Sivakumar                                     [Page 10]


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