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rfc:rfc5508

Network Working Group P. Srisuresh Request for Comments: 5508 Kazeon Systems BCP: 148 B. Ford Category: Best Current Practice MPI-SWS

                                                          S. Sivakumar
                                                         Cisco Systems
                                                               S. Guha
                                                            Cornell U.
                                                            April 2009
               NAT Behavioral Requirements for ICMP

Status of This Memo

 This document specifies an Internet Best Current Practices for the
 Internet Community, and requests discussion and suggestions for
 improvements.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (c) 2009 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents in effect on the date of
 publication of this document (http://trustee.ietf.org/license-info).
 Please review these documents carefully, as they describe your rights
 and restrictions with respect to this document.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Srisuresh, et al. Best Current Practice [Page 1] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

Abstract

 This document specifies the behavioral properties required of the
 Network Address Translator (NAT) devices in conjunction with the
 Internet Control Message Protocol (ICMP).  The objective of this memo
 is to make NAT devices more predictable and compatible with diverse
 application protocols that traverse the devices.  Companion documents
 provide behavioral recommendations specific to TCP, UDP, and other
 protocols.

Table of Contents

 1. Introduction and Scope ..........................................3
 2. Terminology .....................................................4
 3. ICMP Query Handling .............................................6
    3.1. ICMP Query Mapping .........................................6
    3.2. ICMP Query Session Timeouts ................................7
 4. ICMP Error Forwarding ...........................................8
    4.1. ICMP Error Payload Validation ..............................8
    4.2. ICMP Error Packet Translation .............................10
         4.2.1. ICMP Error Packet Received from the External Realm .11
         4.2.2. ICMP Error Packet Received from the Private Realm ..13
    4.3. NAT Sessions Pertaining to ICMP Error Payload .............15
 5. Hairpinning Support for ICMP Packets ...........................16
 6. Rejection of Outbound Flows Disallowed by NAT ..................17
 7. Conformance to RFC 1812 ........................................17
    7.1. IP Packet Fragmentation ...................................19
         7.1.1.  Generating "Packet Too Big" ICMP Error Message ....19
         7.1.2.  Forwarding "Packet Too Big" ICMP Error Message ....20
    7.2. Time Exceeded Message .....................................20
    7.3. Source Route Options ......................................20
    7.4. Address Mask Request/Reply Messages .......................20
    7.5. Parameter Problem Message .................................21
    7.6. Router Advertisement and Solicitations ....................21
    7.7. DS Field Usage ............................................21
 8. Non-QueryError ICMP Messages ...................................22
 9. Summary of Requirements ........................................22
 10. Security Considerations .......................................25
 11. Acknowledgements ..............................................26
 12. References ....................................................27
    12.1. Normative References .....................................27
    12.2. Informative References ...................................27

Srisuresh, et al. Best Current Practice [Page 2] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

1. Introduction and Scope

 As pointed out in RFC 3424 [UNSAF], NAT implementations vary widely
 in terms of how they handle different traffic.  The purpose of this
 document is to define a specific set of requirements for NAT behavior
 with regard to ICMP messages.  The objective is to reduce the
 unpredictability and brittleness the NAT devices (NATs) introduce.
 This document is an adjunct to [BEH-UDP], [BEH-TCP], and other
 protocol-specific BEHAVE document(s) in the future that define
 requirements for NATs when handling protocol-specific traffic.
 The requirements of this specification apply to traditional NATs as
 described in [NAT-TRAD].  A traditional NAT has two variations,
 namely Basic NAT and Network Address Port Translator (NAPT).  Of
 these, NAPT is by far the most commonly deployed NAT device.  NAPT
 allows multiple private hosts to share a single public IP address
 simultaneously.
 This document only covers the ICMP aspects of NAT traversal,
 specifically the traversal of ICMP Query messages and ICMP Error
 messages.  Traditional NAT inherently mandates firewall-like
 filtering behavior [BEH-UDP].  However, firewall functionality in
 general or any other middlebox functionality is out of the scope of
 this document.
 In some cases, ICMP message traversal behavior on a NAT device may be
 overridden by local administrative policies.  Some administrators may
 choose to entirely prohibit forwarding of ICMP Error messages across
 a NAT device.  Some others may choose to prohibit ICMP-Query-based
 applications across a NAT device.  These are local policies and not
 within the scope of this document.  For this reason, some of the ICMP
 requirements listed in the document are preceded with a constraint of
 local policy permitting.
 This document focuses strictly on the behavior of the NAT device, and
 not on the behavior of applications that traverse NATs.  Application
 designers may refer to [BEH-APP] and [ICE] for recommendations and
 guidelines on how to make applications work robustly over NATs that
 follow the requirements specified here and the adjunct protocol-
 specific BEHAVE documents.
 Per [RFC1812], ICMP is a control protocol that is considered to be an
 integral part of IP, although it is architecturally layered upon IP
 -- it uses IP to carry its data end-to-end.  As such, many of the
 ICMP behavioral requirements discussed in this document apply to all
 IP protocols.

Srisuresh, et al. Best Current Practice [Page 3] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

 In case a requirement in this document conflicts with protocol-
 specific BEHAVE requirement(s), protocol-specific BEHAVE documents
 will take precedence.  The authors are not aware of any conflicts
 between this and any other IETF document at the time of this writing.
 Section 2 describes the terminology used throughout the document.
 Section 3 is focused on requirements concerning ICMP-Query-based
 applications traversing a NAT device.  Sections 4 and 5 describe
 requirements concerning ICMP Error messages traversing a NAT device.
 Sections 6 describes requirements concerning ICMP Error messages
 generated by a NAT device.  Section 7 reviews RFC 1812 conformance
 requirements and applicability to NATs when handling ICMP messages.
 Section 8 reviews a requirement for ICMP messages that are neither
 ICMP Query nor ICMP Error kind.  Section 9 summarizes all the
 requirements in one place.  Section 10 has a discussion on security
 considerations.

2. Terminology

 Definitions for the majority of the NAT terms used throughout the
 document may be found in [NAT-TERM] and [BEH-UDP].
 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 RFC 2119 [RFC2119].
 The term "Realm" is adapted from [NAT-TERM] and is defined as
 follows.  "Realm" is often interchanged for "network domain" or
 simply "network" throughout the document.
 Address realm or Realm - An address realm is a network domain in
 which the network addresses are uniquely assigned to entities such
 that datagrams can be routed to them.  Routing protocols used within
 the network domain are responsible for finding routes to entities
 given their network addresses.  Note that this document is limited to
 describing NAT in the IPv4 environment and does not address the use
 of NAT in other types of environments (e.g., the IPV6 environment).
 The term "NAT Session" is adapted from [NAT-MIB] and is defined as
 follows:
 NAT Session - A NAT session is an association between a session as
 seen in the private realm and a session as seen in the public realm,
 by virtue of NAT translation.  If a session in the private realm were
 to be represented as (PrivateSrcAddr, PrivateDstAddr,
 TransportProtocol, PrivateSrcPort, PrivateDstPort) and the same
 session in the public realm were to be represented as (PublicSrcAddr,
 PublicDstAddr, TransportProtocol, PublicSrcPort, PublicDstPort), the

Srisuresh, et al. Best Current Practice [Page 4] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

 NAT session would provide the translation glue between the two
 session representations.  NAT sessions in the document are restricted
 to sessions based on TCP, UDP, and ICMP.  In the future, NAT sessions
 may be extended to be based on other transport protocols such as
 Stream Control Transmission Protocol (SCTP), UDP-lite, and Datagram
 Congestion Control Protocol (DCCP).
 ICMP Message Classification - Section 3.2.2 of [RFC1122] and Section
 4.3.1 of [RFC1812] broadly group ICMP messages into two main
 categories, namely "ICMP Query" messages and "ICMP Error" messages.
 All ICMP Error messages listed in RFC 1122 and RFC 1812 contain part
 of the Internet datagram that elicited the ICMP error.  All the ICMP
 Query messages listed in RFC 1122 and RFC 1812 contain an
 "Identifier" field, which is referred to in this document as the
 "Query Identifier".  There are however ICMP messages that do not fall
 into either of these two categories.  We refer to them as "Non-
 QueryError ICMP Messages".  All three ICMP message classes are
 described as follows:
 o ICMP Query Messages - ICMP Query messages are characterized by an
   Identifier field in the ICMP header.  The Identifier field used by
   the ICMP Query messages is also referred to as "Query Identifier"
   or "Query Id", for short throughout the document.  A Query Id is
   used by Query senders and responders as the equivalent of a TCP/UDP
   port to identify an ICMP Query session.  ICMP Query messages
   include ICMP messages defined after RFC 1122 or RFC 1812 (for
   example, Domain Name Request/Reply ICMP messages defined in RFC
   1788), as they include request/response pairs and contain an
   "Identifier" field.
 o ICMP Error Messages - ICMP Error messages provide signaling for IP.
   All ICMP Error messages are characterized by the fact that they
   embed the original datagram that triggered the ICMP Error message.
   The original datagram embedded within the ICMP Error payload is
   also referred to as the "Embedded packet" throughout the document.
   Unlike ICMP Query messages, ICMP Error messages do not have a Query
   Id in the ICMP header.
 o Non-QueryError ICMP Messages - ICMP messages that do not fall under
   either of the above two classes are referred to as "Non-QueryError
   ICMP Messages" throughout the document.  For example, Router
   Discovery ICMP messages [RFC1256] are "request/response" type ICMP
   messages.  However, they are not characterized as ICMP Query
   messages in this document as they do not have an "Identifier" field
   within the messages.  Likewise, there are other ICMP messages
   defined in [RFC4065] that do not fall in either of the ICMP Query
   or ICMP Error message categories, but will be referred to as Non-
   QueryError ICMP messages.

Srisuresh, et al. Best Current Practice [Page 5] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

 The reason for categorizing ICMP messages for NAT behavioral
 properties is that each category has different characteristics used
 for mapping (i.e., the Query Id and the Embedded datagram), which
 leaves the Non-QueryError ICMP messages in a separate, distinctive
 group.

3. ICMP Query Handling

 This section lists the behavioral requirements for a NAT device when
 processing ICMP Query packets.  The following subsections discuss
 requirements specific to ICMP Query handling in detail.

3.1. ICMP Query Mapping

 Unless explicitly overridden by local policy, a NAT device MUST
 permit ICMP Queries and their associated responses, when the Query is
 initiated from a private host to the external hosts.  ICMP Query
 mapping by NAT devices is necessary for current ICMP-Query-based
 applications to work.  This entails a NAT device to transparently
 forward ICMP Query packets initiated from the nodes behind NAT, and
 the responses to these Query packets in the opposite direction.  As
 specified in [NAT-TRAD], this requires translating the IP header.  A
 NAPT device further translates the ICMP Query Id and the associated
 checksum in the ICMP header prior to forwarding.
 NAT mapping of ICMP Query Identifiers SHOULD be external-host
 independent.  Say, an internal host A sent an ICMP Query out to an
 external host B using Query Id X.  And, say, the NAT assigned this an
 external mapping of Query Id X' on the NAT's public address.  If host
 A reused the Query Id X to send ICMP Queries to the same or different
 external host, the NAT device SHOULD reuse the same Query Id mapping
 (i.e., map the private host's Query Id X to Query Id X' on NAT's
 public IP address) instead of assigning a different mapping.  This is
 similar to the "endpoint independent mapping" requirement specified
 in the TCP and UDP requirement documents [BEH-UDP], [BEH-TCP].
 Below is justification for making the endpoint-independent mapping
 for ICMP Query Id a SHOULD [RFC2119] requirement.  ICMP Ping
 [RFC1470] and ICMP traceroute [MS-TRCRT] are two most commonly known
 legacy applications built on top of ICMP Query messages.  Neither of
 these applications require the ICMP Query Id to be retained across
 different sessions with external hosts.  But, that may not be the
 case with future applications.  In the future, when an end host
 application reuses the same Query Identifier in sessions with
 different target hosts, the end host application might require that
 the endpoint identity (i.e., the tuple of IP address and Query
 Identifier) appears the same across all its target hosts.  In an IP
 network without NAT requirements, such a requirement will be valid.

Srisuresh, et al. Best Current Practice [Page 6] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

 In a world with NAT devices, the above assumption will be valid when
 NAT devices enforce endpoint mapping that is external-host
 independent.  Given the dichotomy between legacy applications not
 requiring endpoint-independent mapping and future applications that
 might require it, the requirement level is kept at SHOULD [RFC2119].
 REQ-1: Unless explicitly overridden by local policy, a NAT device
        MUST permit ICMP Queries and their associated responses, when
        the Query is initiated from a private host to the external
        hosts.
        a) NAT mapping of ICMP Query Identifiers SHOULD be external-
           host independent.

3.2. ICMP Query Session Timeouts

 NATs maintain a mapping timeout for the ICMP Queries that traverse
 them.  The mapping timeout is the time a mapping will stay active
 without packets traversing the NAT.  There is great variation in the
 values used by different NATs.  The ICMP Query session timeout
 requirement is necessary for current ICMP Query applications to work.
 Query response times can vary.  ICMP-Query-based applications are
 primarily request/response driven.
 Ideally, the timeout should be set to Maximum Round Trip Time
 (Maximum RTT).  For the purposes of constraining the maximum RTT, the
 Maximum Segment Lifetime (MSL), defined in [RFC793], could be
 considered a guideline to set packet lifetime.  Per [RFC793], MSL is
 the maximum amount of time a TCP segment can exist in a network
 before being delivered to the intended recipient.  This is the
 maximum duration an IP packet can be assumed to take to reach the
 intended destination node before declaring that the packet will no
 longer be delivered.  For an application initiating an ICMP Query
 message and waiting for a response for the Query, the Maximum RTT
 could in practice be constrained to be the sum total of MSL for the
 Query message and MSL for the response message.  In other words,
 Maximum RTT could be constrained to no more than 2x MSL.  The
 recommended value for MSL in [RFC793] is 120 seconds, even though
 several implementations set this to 60 seconds or 30 seconds.  When
 MSL is 120 seconds, the Maximum RTT (2x MSL) would be 240 seconds.
 In practice, ICMP Ping [RFC1470] and ICMP traceroute [MS-TRCRT], the
 two most commonly known legacy applications built on top of ICMP
 Query messages, take less than 10 seconds to complete a round trip
 when the target node is operational on the network.

Srisuresh, et al. Best Current Practice [Page 7] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

 Setting the ICMP NAT session timeout to a very large duration (say,
 240 seconds) could potentially tie up precious NAT resources such as
 Query mappings and NAT Sessions for the whole duration.  On the other
 hand, setting the timeout very low can result in premature freeing of
 NAT resources and applications failing to complete gracefully.  The
 ICMP Query session timeout needs to be a balance between the two
 extremes.  A 60-second timeout is a balance between the two extremes.
 An ICMP Query session timer MUST NOT expire in less than 60 seconds.
 It is RECOMMENDED that the ICMP Query session timer be made
 configurable.
 REQ-2: An ICMP Query session timer MUST NOT expire in less than 60
        seconds.
        a) It is RECOMMENDED that the ICMP Query session timer be made
           configurable.

4. ICMP Error Forwarding

 Many applications make use of ICMP Error messages from end hosts and
 intermediate devices to shorten application timeouts.  Some
 applications will not operate correctly without the receipt of ICMP
 Error messages.  The following sub-sections discuss the requirements
 a NAT device must conform to in order to ensure reliable forwarding.

4.1. ICMP Error Payload Validation

 An ICMP Error message checksum covers the entire ICMP message,
 including the payload.  When an ICMP Error packet is received, if the
 ICMP checksum fails to validate, the NAT SHOULD silently drop the
 ICMP Error packet.  This is because NAT uses the embedded IP and
 transport headers for forwarding and translating the ICMP Error
 message (described in Section 4.2).  When the ICMP checksum is
 invalid, the embedded IP and transport headers, which are covered by
 the ICMP checksum, are also suspect.
 [RFC1812] and [RFC1122] require a router or an end host that receives
 an IP packet with an invalid IP header checksum to silently drop the
 IP packet.  As such, end hosts and routers do not generate an ICMP
 Error message in response to IP packets with invalid IP header
 checksums.  For this reason, if the IP checksum of the embedded
 packet within an ICMP Error message fails to validate, the NAT SHOULD
 silently drop the Error packet.
 When the IP packet embedded within the ICMP Error message includes IP
 options, the NAT device must not assume that the transport header of
 the embedded packet is at a fixed offset (as would be the case when
 there are no IP options associated with the packet) from the start of

Srisuresh, et al. Best Current Practice [Page 8] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

 the embedded packet.  Specifically, if the embedded packet includes
 IP options, the NAT device MUST traverse past the IP options to
 locate the start of transport header for the embedded packet.
 It is possible to compute the transport checksum of the embedded
 packet within an ICMP Error message when the ICMP Error message
 contains the entire transport segment.  However, ICMP Error messages
 do not contain the entire transport segment in many cases.  This is
 because [ICMP] stipulates that an ICMP Error message should embed an
 IP header and only a minimum of 64 bits of the IP payload.  Even
 though Section 4.3.2.3 of [RFC1812] recommends an ICMP Error
 originator include as much of the original packet as possible in the
 payload, the length of the resulting ICMP datagram cannot exceed 576
 bytes.  ICMP Error originators truncate IP packets that do not fit
 within the stipulations.
 A NAT device SHOULD NOT validate the transport checksum of the
 embedded packet within an ICMP Error message, even when it is
 possible to do so.  This is because a NAT dropping an ICMP Error
 message due to an invalid transport checksum will make it harder for
 end hosts to receive error reporting for certain types of corruption.
 End-to-end validation of ICMP Error messages is best left to end
 hosts.  Many newer revision end host TCP/IP stacks implement the
 improvements in [TCP-SOFT] and do not accept ICMP Error messages with
 a mismatched IP or TCP checksum in the embedded packet, if the
 embedded datagram contains a full IP packet and the TCP checksum can
 be calculated.
 In the case that the ICMP Error payload includes ICMP extensions
 [ICMP-EXT], the NAT device MUST exclude the optional zero-padding and
 the ICMP extensions when evaluating transport checksum for the
 embedded packet.  Readers are urged to refer to [ICMP-EXT] for
 information on identifying the presence of ICMP extensions in an ICMP
 message.
 REQ-3: When an ICMP Error packet is received, if the ICMP checksum
        fails to validate, the NAT SHOULD silently drop the ICMP Error
        packet.  If the ICMP checksum is valid, do the following:
        a) If the IP checksum of the embedded packet fails to
           validate, the NAT SHOULD silently drop the Error packet;
           and
        b) If the embedded packet includes IP options, the NAT device
           MUST traverse past the IP options to locate the start of
           the transport header for the embedded packet; and

Srisuresh, et al. Best Current Practice [Page 9] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

        c) The NAT device SHOULD NOT validate the transport checksum
           of the embedded packet within an ICMP Error message, even
           when it is possible to do so; and
        d) If the ICMP Error payload contains ICMP extensions
           [ICMP-EXT], the NAT device MUST exclude the optional zero-
           padding and the ICMP extensions when evaluating transport
           checksum for the embedded packet.

4.2. ICMP Error Packet Translation

 Section 4.3 of [NAT-TRAD] describes the fields of an ICMP Error
 message that a NAT device translates.  In this section, we describe
 the requirements a NAT device must conform to while performing the
 translations.  Requirements identified in this section are necessary
 for the current applications to work correctly.
 Consider the following scenario in Figure 1.  Say, NAT-xy is a 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.  By
 contrast, 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 mapped the endpoint on Host-y into Host-y'
 in the external network.  The following subsections describe the
 processing of ICMP Error messages on the NAT device(NAT-xy) when the
 NAT device receives an ICMP Error message in response to a packet
 pertaining to this session.

Srisuresh, et al. Best Current Practice [Page 10] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

                             Host-x
                                |
                        ---------------+-------------------
                                       |
                                +-------------+
                                |  Router-x   |
                                +-------------+
          External Network             |
          --------------------+--------+-------------------
                              |   ^
                              |   | (Host-y', Host-x)
                              |   |
                        +-------------+
                        |    NAT-xy   |
                        +-------------+
                              |
  Private Network             |
 ----------------+------------+----------------
                 |
          +-------------+
          | Router-y    |
          +-------------+
                 |
 ----------------+-------+--------
                         | ^
                         | | (Host-y, Host-x)
                         | |
                       Host-y
   Figure 1.  A Session from a Private Host Traversing a NAT Device

4.2.1. ICMP Error Packet Received from the External Realm

 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 as described in Figure 2 below.

Srisuresh, et al. Best Current Practice [Page 11] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

                             Host-x
                                |
                        ---------------+-------------------
                                       |
                                +-------------+
                                |  Router-x   |
                                +-------------+
          External Network             |
          --------------------+--------+-------------------
                              |
                              |  | ICMP Error Packet to Host-y'
                              |  v
                        +-------------+
                        |    NAT-xy   |
                        +-------------+
  Private Network             |
 ----------------+------------+----------------
                 |
          +-------------+
          | Router-y    |
          +-------------+
                 |
 ----------------+-------+--------
                         |
                       Host-y
      Figure 2.  ICMP Error Packet Received from External Network
 When the NAT device receives the ICMP Error packet, the NAT device
 uses 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 to which
 the embedded packet belongs.  If the NAT device does not have an
 active mapping for the embedded packet, the NAT SHOULD silently drop
 the ICMP Error packet.  Otherwise, the NAT device MUST use the
 matching NAT Session to translate the embedded packet; that is,
 translate the source IP address of the embedded packet (e.g., Host-y'
 -> Host-y) and transport headers.
 The ICMP Error payload may contain ICMP extension objects [ICMP-EXT].
 NATs are encouraged to support ICMP extension objects.  At the time
 of this writing, the authors are not aware of any standard ICMP
 extension objects containing realm-specific information.
 The NAT device MUST also use the matching NAT Session to translate
 the destination IP address in 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 an
 external domain and is routable from the private domain.

Srisuresh, et al. Best Current Practice [Page 12] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

 REQ-4: If a NAT device receives an ICMP Error packet from an external
        realm, and the NAT device does not have an active mapping for
        the embedded payload, the NAT SHOULD silently drop the ICMP
        Error packet.  If the NAT has active mapping for the embedded
        payload, then the NAT MUST do the following prior to
        forwarding the packet, unless explicitly overridden by local
        policy:
        a) Revert the IP and transport headers of the embedded IP
           packet to their original form, using the matching mapping;
           and
        b) Leave the ICMP Error type and code unchanged; and
        c) Modify the destination IP address of the outer IP header to
           be the same as the source IP address of the embedded packet
           after translation.

4.2.2. ICMP Error Packet Received from the Private Realm

 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 as specified in Figure 3 below.

Srisuresh, et al. Best Current Practice [Page 13] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

                             Host-x
                                |
                        ---------------+-------------------
                                       |
                                +-------------+
                                |  Router-x   |
                                +-------------+
          External Network             |
          --------------------+--------+-------------------
                              |
                              |
                        +-------------+
                        |    NAT-xy   |
                        +-------------+
                              |  ^
                              |  | ICMP Error Packet to Host-x
  Private Network             |
 ----------------+------------+----------------
                 |
          +-------------+
          | Router-y    |
          +-------------+
                 |
 ----------------+-------+--------
                         |
                       Host-y
      Figure 3.  ICMP Error Packet Received from Private Network
 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 to which
 the embedded packet belongs.  If the NAT device does not have an
 active mapping for the embedded packet, the NAT SHOULD silently drop
 the ICMP Error packet.  Otherwise, the NAT device MUST use the
 matching NAT Session to translate the embedded packet.
 The ICMP Error payload may contain ICMP extension objects [ICMP-EXT].
 NATs are encouraged to support ICMP extension objects.  At the time
 of this writing, the authors are not aware of any standard ICMP
 extension objects containing realm-specific information.
 In the outer header, the destination IP address will remain
 unchanged, as the IP address 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'.  If the ICMP Error message is originated
 by the intermediate node Router-y, translation of the source IP

Srisuresh, et al. Best Current Practice [Page 14] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

 address varies depending on whether the Basic NAT or NAPT function
 [NAT-TRAD] is enforced by the NAT device.  A NAT device enforcing the
 Basic NAT function has a pool of public IP addresses and enforces
 address mapping (which is different from the endpoint mapping
 enforced by NAPT) when a private node initiates an outgoing session
 via the NAT device.  So, if the NAT device has active mapping for the
 IP address of the intermediate node Router-y, the NAT device MUST
 translate the source IP address of the ICMP Error packet with the
 public IP address in the mapping.  In all other cases, the NAT device
 MUST simply use its own IP address in the external domain to
 translate the source IP address.
 REQ-5: If a NAT device receives an ICMP Error packet from the private
        realm, and the NAT does not have an active mapping for the
        embedded payload, the NAT SHOULD silently drop the ICMP Error
        packet.  If the NAT has active mapping for the embedded
        payload, then the NAT MUST do the following prior to
        forwarding the packet, unless explicitly overridden by local
        policy:
        a) Revert the IP and transport headers of the embedded IP
           packet to their original form, using the matching mapping;
           and
        b) Leave the ICMP Error type and code unchanged; and
        c) If the NAT enforces Basic NAT function ([NAT-TRAD]), and
           the NAT has active mapping for the IP address that sent the
           ICMP Error, translate the source IP address of the ICMP
           Error packet with the public IP address in the mapping.  In
           all other cases, translate the source IP address of the
           ICMP Error packet with its own public IP address.

4.3. NAT Sessions Pertaining to ICMP Error Payload

 While processing an ICMP Error packet pertaining to an ICMP Query or
 Query response message, 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.  This ensures that
 the NAT Session will not be modified if someone is able to spoof ICMP
 Error messages for the session.  [ICMP-ATK] lists a number of
 potential ICMP attacks that may be attempted by malicious users on
 the network.  This requirement is necessary for current applications
 to work correctly.

Srisuresh, et al. Best Current Practice [Page 15] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

 REQ-6: While processing an ICMP Error packet pertaining to an ICMP
        Query or Query response message, a NAT device MUST NOT refresh
        or delete the NAT Session that pertains to the embedded
        payload within the ICMP Error packet.

5. Hairpinning Support for ICMP Packets

 [BEH-UDP] and [BEH-TCP] mandate support for hairpinning for UDP and
 TCP sessions, respectively, on NAT devices.  A NAT device needs to
 support hairpinning for ICMP Query sessions as well.  Specifically,
 NAT devices enforcing Basic NAT [NAT-TRAD] MUST support the traversal
 of hairpinned ICMP Query sessions.  Say, for example, individual
 private hosts register their NAT assigned external IP address with a
 rendezvous server.  Other hosts that wish to initiate ICMP Query
 sessions to the registered hosts might do so using the public address
 registered with the rendezvous server.  For this reason, Basic NAT
 devices are required to support the traversal of hairpinned ICMP
 Query sessions.  This requirement is necessary for current
 applications to work correctly.
 Packets belonging to any of the hairpinned sessions could, in turn,
 trigger ICMP Error messages directed to the source of hairpinned IP
 packets.  Such hairpinned ICMP Error messages will traverse the NAT
 devices en route.  All NAT devices (i.e., Basic NAT as well as NAPT
 devices) MUST support the traversal of hairpinned ICMP Error
 messages.  Specifically, the NAT device must translate not only the
 embedded hairpinned packet, but also the outer IP header that is
 hairpinned.  This requirement is necessary for current applications
 to work correctly.
 A hairpinned ICMP Error message is received from a node in a private
 network.  As such, the ICMP Error processing requirement specified in
 Req-5 is applicable in its entirety in processing the ICMP Error
 message.  In addition, the NAT device MUST translate the destination
 IP address of the outer IP header to be same as the source IP address
 of the embedded IP packet after the translation.
 REQ-7: NAT devices enforcing Basic NAT [NAT-TRAD] MUST support the
        traversal of hairpinned ICMP Query sessions.  All NAT devices
        (i.e., Basic NAT as well as NAPT devices) MUST support the
        traversal of hairpinned ICMP Error messages:
        a) When forwarding a hairpinned ICMP Error message, the NAT
           device MUST translate the destination IP address of the
           outer IP header to be same as the source IP address of the
           embedded IP packet after the translation.

Srisuresh, et al. Best Current Practice [Page 16] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

6. Rejection of Outbound Flows Disallowed by NAT

 A NAT device typically permits all outbound sessions.  However, a NAT
 device may disallow some outbound sessions due to resource
 constraints or administration considerations.  For example, a NAT
 device may not permit the first packet of a new outbound session if
 the NAT device is out of resources (out of addresses or TCP/UDP
 ports, or NAT Session resources) to set up a state for the session,
 or, if the specific session is administratively restricted by the NAT
 device.
 When a NAT device is unable to establish a NAT Session for a new
 transport-layer (TCP, UDP, ICMP, etc.) flow due to resource
 constraints or administrative restrictions, the NAT device SHOULD
 send an ICMP destination unreachable message, with a code of 13
 (Communication administratively prohibited) to the sender, and drop
 the original packet.  This requirement is meant primarily for future
 use.  Current applications do not require this for them to work
 correctly.  The justification for using ICMP code 13 in the ICMP
 Error message is as follows: Section 5.2.7.1 of [RFC1812] recommends
 routers use ICMP code 13 (Communication administratively prohibited)
 when they administratively filter packets.  ICMP code 13 is a soft
 error and is on par with other soft error codes generated in response
 to transient events such as "network unreachable" (ICMP type=3,
 code=0).
 Some NAT designers opt to never reject an outbound flow.  When a NAT
 runs short of resources, they prefer to steal a resource from an
 existing NAT Session rather than reject the outbound flow.  Such a
 design choice may appear conformant to REQ-8 below.  However, the
 design choice is in violation of the spirit of both REQ-8 and REQ-2.
 Such a design choice is strongly discouraged.
 REQ-8: When a NAT device is unable to establish a NAT Session for a
 new transport-layer (TCP, UDP, ICMP, etc.) flow due to resource
 constraints or administrative restrictions, the NAT device SHOULD
 send an ICMP destination unreachable message, with a code of 13
 (Communication administratively prohibited) to the sender, and drop
 the original packet.

7. Conformance to RFC 1812

 This document specifies NATs to have a behavior that is consistent
 with the way routers handle ICMP messages, as specified in Section
 4.3 of [RFC1812].  However, since the publication of [RFC1812], some
 of its requirements are no longer best current practices.  Thus, the
 following requirements are derived from [RFC1812] and apply to NATs
 compliant with this specification:

Srisuresh, et al. Best Current Practice [Page 17] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

 REQ-9: A NAT device MAY implement a policy control that prevents ICMP
        messages being generated toward certain interface(s).
        Implementation of such a policy control overrides the MUSTs
        and SHOULDs in REQ-10.
 REQ-10: Unless overridden by REQ-9's policy, a NAT device needs to
         support ICMP messages as below, some conforming to Section
         4.3 of [RFC1812] and some superseding the requirements of
         Section 4.3 of [RFC1812]:
        a. MUST support:
           1. Destination Unreachable Message, as described in Section
              7.1 of this document.
           2. Time Exceeded Message, as described in Section 7.2 of
              this document.
           3. Echo Request/Reply Messages, as described in REQ-1.
        b. MAY support:
           1. Redirect Message, as described in Section 4.3.3.2 of
              [RFC1812].
           2. Timestamp and Timestamp Reply Messages, as described in
              Section 4.3.3.8 of [RFC1812].
           3. Source Route Options, as described in Section 7.3 of
              this document.
           4. Address Mask Request/Reply Message, as described in
              Section 7.4 of this document.
           5. Parameter Problem Message, as described in Section 7.5
              of this document.
           6. Router Advertisement and Solicitations, as described in
              Section 7.6 of this document.
        c. SHOULD NOT support:
           1. Source Quench Message, as described in Section 4.3.3.3
              of [RFC1812].
           2. Information Request/reply, as described in Section
              4.3.3.7 of [RFC1812].

Srisuresh, et al. Best Current Practice [Page 18] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

        In addition, a NAT device is RECOMMENDED to conform to the
        following implementation considerations:
        d. DS Field Usage, as described in Section 7.7 of this
           document.
        e. When Not to Send ICMP Errors, as described in Section
           4.3.2.7 of [RFC1812].
        f. Rate Limiting, as described in Section 4.3.2.8 of
           [RFC1812].

7.1. IP Packet Fragmentation

 Many networking applications (which include TCP- as well as UDP-based
 applications) depend on ICMP Error messages from the network to
 perform end-to-end path MTU discovery [PMTU].  Once the path MTU is
 discovered, an application that chooses to avoid fragmentation may do
 so by originating IP packets that fit within the path MTU en route
 and setting the DF (Don't Fragment) bit in the IP header, so the
 intermediate nodes en route do not fragment the IP packets.  The
 following sub-sections discuss the need for NAT devices to honor the
 DF bit in the IP header and be able to generate "Packet Too Big" ICMP
 Error message when they cannot forward the IP packet without
 fragmentation.  Also discussed is the need to seamlessly forward ICMP
 Error messages generated by other intermediate devices.

7.1.1. Generating "Packet Too Big" ICMP Error Message

 When a router is unable to forward a datagram because it exceeds the
 MTU of the next-hop network and its Don't Fragment (DF) bit is set,
 the router is required by [RFC1812] to return an ICMP Destination
 Unreachable message to the source of the datagram, with the code
 indicating "fragmentation needed and DF set".  Further, [PMTU] states
 that the router MUST include the MTU of that next-hop network in the
 low-order 16 bits of the ICMP header field that is labeled "unused"
 in the ICMP specification [ICMP].
 A NAT device MUST honor the DF bit in the IP header of the packets
 that transit the device.  The NAT device may not be able to forward
 an IP packet without fragmentation if the MTU on the forwarding
 interface of the NAT device is not adequate for the IP packet.  If
 the 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 the next-hop
 MTU back to the sender and drop the original IP packet.  The sender
 will usually resend after taking the appropriate corrective action.

Srisuresh, et al. Best Current Practice [Page 19] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

 If the DF bit is not set and the MTU on the forwarding interface of
 the NAT device mandates fragmentation, the NAT device MUST fragment
 the packet and forward the fragments [RFC1812].

7.1.2. Forwarding "Packet Too Big" ICMP Error Message

 This is the 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" messages.
 When the NAT device is the recipient of a "Packet Too Big" ICMP
 message from the network, the NAT device MUST forward the ICMP
 message back to the intended recipient, pursuant to the previously
 stated requirements (REQ-3, REQ-4, and REQ-5).

7.2. Time Exceeded Message

 A NAT device MUST generate a "Time Exceeded" ICMP Error message when
 it discards a packet due to an expired Time to Live (TTL) field.  A
 NAT device MAY have a per-interface option to disable origination of
 these messages on that interface, but that option MUST default to
 allowing the messages to be originated.
 When a NAT device conforms to the above requirement, it ensures that
 legacy applications such as Traceroute [RFC1470], [MS-TRCRT], which
 depend upon the "Time Exceeded" ICMP Error message, will continue to
 operate even as NAT devices are en route.

7.3. Source Route Options

 A NAT device MAY support modifying IP addresses in the source route
 option so the IP addresses in the source route option are realm
 relevant.  If a NAT device does not support forwarding packets with
 the source route option, the NAT device SHOULD NOT forward outbound
 ICMP messages that contain the source route option in the outer or
 inner IP header.  This is because such messages could reveal private
 IP addresses to the external realm.

7.4. Address Mask Request/Reply Messages

 Section 4.3.3.9 of [RFC1812] says an IP router MUST implement support
 for receiving ICMP Address Mask Request messages and responding with
 ICMP Address Mask Reply messages.  However, several years (more than
 13 years at the time of this document) have elapsed since the text in
 RFC 1812 was written.  In the intervening time, DHCP [DHCP] has
 replaced the use of address mask request/reply.  At the current time,

Srisuresh, et al. Best Current Practice [Page 20] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

 there is rarely any host that does not meet host requirements
 [RFC1122] and needs a NAT device to support address mask
 request/reply.
 For this reason, a NAT device is not required to support this ICMP
 message.
 A NAT device MAY support address mask request/reply messages.

7.5. Parameter Problem Message

 Section 4.3.3.5 of [RFC1812] says an IP router MUST generate a
 Parameter Problem message for any error not specifically covered by
 another ICMP message.  However, this message is rarely used in
 practice in networks where IPv4 NATs are deployed.
 For this reason, a NAT device is not required to support this ICMP
 message.
 A NAT device MAY support parameter problem messages.

7.6. Router Advertisement and Solicitations

 Section 4.3.3.10 of [RFC1812] says an IP router MUST support the
 router part of the ICMP Router Discovery Protocol on all connected
 networks on which the router supports either IP multicast or IP
 broadcast addressing.  However, this message is rarely used in
 practice in networks where IPv4 NATs are deployed.
 For this reason, a NAT device is not required to support this ICMP
 message.
 A NAT device MAY support Router Advertisement and Solicitations.

7.7. DS Field Usage

 [RFC1812] refers to the Type of Service (TOS) octet in the IP header,
 which contains the TOS and IP precedence fields.  However, the TOS
 and IP precedence fields are no longer in use today.  [RFC2474]
 renamed the TOS octet as the DS field and defined diffserv classes
 within the DS field.
 When generating an ICMP message, a NAT device SHOULD copy the
 diffserv class of the message that causes the sending of the ICMP
 error message.  A NAT device MAY allow configuration of the diffserv
 class to be used for the different types of ICMP messages.

Srisuresh, et al. Best Current Practice [Page 21] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

8. Non-QueryError ICMP Messages

 In the preceding sections, ICMP requirements were identified for NAT
 devices, with a primary focus on ICMP Query and ICMP Error messages,
 as defined in the Terminology Section (see Section 2).  This document
 provides no guidance on the handling of Non-QueryError ICMP messages
 by the NAT devices.  A NAT MAY drop or appropriately handle Non-
 QueryError ICMP messages.
     REQ-11: A NAT MAY drop or appropriately handle Non-QueryError
         ICMP messages.  The semantics of Non-QueryError ICMP messages
         is defined in Section 2.

9. Summary of Requirements

 Below is a summary of all the requirements.
 REQ-1: Unless explicitly overridden by local policy, a NAT device
        MUST permit ICMP Queries and their associated responses, when
        the Query is initiated from a private host to the external
        hosts.
        a) NAT mapping of ICMP Query Identifiers SHOULD be external
           host independent.
 REQ-2: An ICMP Query session timer MUST NOT expire in less than 60
        seconds.
        a) It is RECOMMENDED that the ICMP Query session timer be made
           configurable.
 REQ-3: When an ICMP Error packet is received, if the ICMP checksum
        fails to validate, the NAT SHOULD silently drop the ICMP Error
        packet.  If the ICMP checksum is valid, do the following:
        a) If the IP checksum of the embedded packet fails to
           validate, the NAT SHOULD silently drop the Error packet;
           and
        b) If the embedded packet includes IP options, the NAT device
           MUST traverse past the IP options to locate the start of
           the transport header for the embedded packet; and
        c) The NAT device SHOULD NOT validate the transport checksum
           of the embedded packet within an ICMP Error message, even
           when it is possible to do so; and

Srisuresh, et al. Best Current Practice [Page 22] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

        d) If the ICMP Error payload contains ICMP extensions
           [ICMP-EXT], the NAT device MUST exclude the optional zero-
           padding and the ICMP extensions when evaluating transport
           checksum for the embedded packet.
 REQ-4: If a NAT device receives an ICMP Error packet from an external
        realm, and the NAT device does not have an active mapping for
        the embedded payload, the NAT SHOULD silently drop the ICMP
        Error packet.  If the NAT has active mapping for the embedded
        payload, then the NAT MUST do the following prior to
        forwarding the packet, unless explicitly overridden by local
        policy:
        a) Revert the IP and transport headers of the embedded IP
           packet to their original form, using the matching mapping;
           and
        b) Leave the ICMP Error type and code unchanged; and
        c) Modify the destination IP address of the outer IP header to
           be same as the source IP address of the embedded packet
           after translation.
 REQ-5: If a NAT device receives an ICMP Error packet from the private
        realm, and the NAT does not have an active mapping for the
        embedded payload, the NAT SHOULD silently drop the ICMP Error
        packet.  If the NAT has active mapping for the embedded
        payload, then the NAT MUST do the following prior to
        forwarding the packet, unless explicitly overridden by local
        policy.
        a) Revert the IP and transport headers of the embedded IP
           packet to their original form, using the matching mapping;
           and
        b) Leave the ICMP Error type and code unchanged; and
        c) If the NAT enforces Basic NAT function [NAT-TRAD], and the
           NAT has active mapping for the IP address that sent the
           ICMP Error, translate the source IP address of the ICMP
           Error packet with the public IP address in the mapping.  In
           all other cases, translate the source IP address of the
           ICMP Error packet with its own public IP address.
 REQ-6: While processing an ICMP Error packet pertaining to an ICMP
        Query or Query response message, a NAT device MUST NOT refresh
        or delete the NAT Session that pertains to the embedded
        payload within the ICMP Error packet.

Srisuresh, et al. Best Current Practice [Page 23] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

 REQ-7: NAT devices enforcing Basic NAT ([NAT-TRAD]) MUST support the
        traversal of hairpinned ICMP Query sessions.  All NAT devices
        (i.e., Basic NAT as well as NAPT devices) MUST support the
        traversal of hairpinned ICMP Error messages.
        a) When forwarding a hairpinned ICMP Error message, the NAT
           device MUST translate the destination IP address of the
           outer IP header to be same as the source IP address of the
           embedded IP packet after the translation.
 REQ-8: When a NAT device is unable to establish a NAT Session for a
        new transport-layer (TCP, UDP, ICMP, etc.) flow due to
        resource constraints or administrative restrictions, the NAT
        device SHOULD send an ICMP destination unreachable message,
        with a code of 13 (Communication administratively prohibited)
        to the sender, and drop the original packet.
 REQ-9: A NAT device MAY implement a policy control that prevents ICMP
        messages being generated toward certain interface(s).
        Implementation of such a policy control overrides the MUSTs
        and SHOULDs in REQ-10.
 REQ-10: Unless overridden by REQ-9's policy, a NAT device needs to
         support ICMP messages as below, some conforming to Section
         4.3 of [RFC1812] and some superseding the requirements of
         Section 4.3 of [RFC1812]:
        a. MUST support:
           1. Destination Unreachable Message, as described in Section
              7.1 of this document.
           2. Time Exceeded Message, as described in Section 7.2 of
              this document.
           3. Echo Request/Reply Messages, as described in REQ-1.
        b. MAY support:
           1. Redirect Message, as described in Section 4.3.3.2 of
              [RFC1812].
           2. Timestamp and Timestamp Reply Messages, as described in
              Section 4.3.3.8 of [RFC1812].
           3. Source Route Options, as described in Section 7.3 of
              this document.

Srisuresh, et al. Best Current Practice [Page 24] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

           4. Address Mask Request/Reply Message, as described in
              Section 7.4 of this document.
           5. Parameter Problem Message, as described in Section 7.5
              of this document.
           6. Router Advertisement and Solicitations, as described in
              Section 7.6 of this document.
        c. SHOULD NOT support:
           1. Source Quench Message, as described in Section 4.3.3.3
              of [RFC1812].
           2. Information Request/reply, as described in Section
              4.3.3.7 of [RFC1812].
        In addition, a NAT device is RECOMMENDED to conform to the
        following implementation considerations:
        d. DS Field Usage, as described in Section 7.7 of this
           document.
        e. When Not to Send ICMP Errors, as described in Section
           4.3.2.7 of [RFC1812].
        f. Rate Limiting, as described in Section 4.3.2.8 of
           [RFC1812].
 REQ-11: A NAT MAY drop or appropriately handle Non-QueryError ICMP
         messages.  The semantics of Non-QueryError ICMP messages is
         defined in Section 2.

10. Security Considerations

 This document does not introduce any new security concerns related to
 ICMP message handling in the NAT devices.  However, the requirements
 in the document do mitigate some security concerns known to exist
 with ICMP messages.
 [ICMP-ATK] lists a number of ICMP attacks that can be directed
 against end host TCP stacks.  For example, a rogue entity could
 bombard the NAT device with a large number of ICMP Errors.  If the
 NAT device did not validate the legitimacy of the ICMP Error packets,
 the ICMP Errors would be forwarded directly to the end nodes.  End
 hosts not capable of defending themselves against such bogus ICMP
 Error attacks could be adversely impacted by such attacks.  Req-3
 recommends validating the ICMP checksum and the IP checksum of the

Srisuresh, et al. Best Current Practice [Page 25] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

 embedded payload prior to forwarding.  These checksum validations by
 themselves do not protect end hosts from attacks.  However, checksum
 validation mitigates end hosts from malformed ICMP Error attacks.
 Req-4 and Req-5 further mandate that when a NAT device does not find
 a mapping selection for the embedded payload, the NAT should drop the
 ICMP Error packets, without forwarding.
 A rogue source could also try to send bogus ICMP Error messages for
 the active NAT sessions, with intent to destroy the sessions.  Req-6
 averts such an attack by ensuring that an ICMP Error message does not
 affect the state of a session on the NAT device.
 Req-8 recommends a NAT device sending an ICMP Error message when the
 NAT device is unable to create a NAT session due to lack of
 resources.  Some administrators may choose not to have the NAT device
 send an ICMP Error message, as doing so could confirm to a malicious
 attacker that the attack has succeeded.  For this reason, sending of
 the specific ICMP Error message stated in REQ-8 is left to the
 discretion of the NAT device administrator.
 Unfortunately, ICMP messages are sometimes blocked at network
 boundaries due to local security policy.  Thus, some of the
 requirements in this document allow local policy to override the
 recommendations of this document.  Blocking such ICMP messages is
 known to break some protocol features (most notably path MTU
 Discovery) and some applications (e.g., ping, traceroute), and such
 blocking is NOT RECOMMENDED.

11. Acknowledgements

 The authors wish to thank Fernando Gont, Dan Wing, Carlos Pignataro,
 Philip Matthews, and members of the BEHAVE working group for doing a
 thorough review of early versions of the document and providing
 valuable input and offering generous amounts of their time in shaping
 the ICMP requirements.  Their valuable feedback made this document a
 better read.  Dan Wing and Fernando Gont were a steady source of
 encouragement.  Fernando Gont spent many hours preparing slides and
 presenting the document in an IETF meeting on behalf of the authors.
 The authors wish to thank Carlos Pignataro and Dan Tappan, authors of
 the [ICMP-EXT] document, for their feedback concerning ICMP
 extensions.  The authors wish to thank Philip Matthews for agreeing
 to be a technical reviewer for the document.  Lastly, the authors
 highly appreciate the rigorous feedback from the IESG members.

Srisuresh, et al. Best Current Practice [Page 26] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

12. References

12.1. Normative References

 [BEH-UDP]  Audet, F., Ed., and C. Jennings, "Network Address
            Translation (NAT) Behavioral Requirements for Unicast
            UDP", BCP 127, RFC 4787, January 2007.
 [ICMP]     Postel, J., "Internet Control Message Protocol", STD 5,
            RFC 792, September 1981.
 [ICMP-EXT] Bonica, R., Gan, D., Tappan, D., and C. Pignataro,
            "Extended ICMP to Support Multi-Part Messages", RFC 4884,
            April 2007.
 [NAT-TRAD] Srisuresh, P. and K. Egevang, "Traditional IP Network
            Address Translator (Traditional NAT)", RFC 3022, January
            2001.
 [RFC793]   Postel, J., "Transmission Control Protocol", STD 7, RFC
            793, September 1981.
 [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
            RFC 1812, June 1995.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.

12.2. Informative References

 [BEH-APP]  Ford, B., Srisuresh, P., and D. Kegel, "Application Design
            Guidelines for Traversal through Network Address
            Translators", Work in Progress, March 2007.
 [BEH-TCP]  Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
            Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
            RFC 5382, October 2008.
 [DHCP]     Droms, R., "Dynamic Host Configuration Protocol", RFC
            2131, March 1997.
 [ICE]      Rosenberg, J., "Interactive Connectivity Establishment
            (ICE): A Protocol for Network Address Translator (NAT)
            Traversal for Offer/Answer Protocols", Work in Progress,
            October 2007.
 [ICMP-ATK] Gont, F., "ICMP Attacks against TCP", Work in Progress,
            October 2008.

Srisuresh, et al. Best Current Practice [Page 27] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

 [MS-TRCRT] Microsoft Support, "How to use the Tracert command-line
            utility to troubleshoot TCP/IP problems in Windows",
            http://support.microsoft.com/kb/162326, October, 2006.
 [NAT-MIB]  Rohit, R., Srisuresh, P., Raghunarayan, R., Pai, N., and
            C. Wang, "Definitions of Managed Objects for Network
            Address Translators (NAT)", RFC 4008, March 2005.
 [NAT-TERM] Srisuresh, P. and M. Holdrege, "IP Network Address
            Translator (NAT) Terminology and Considerations", RFC
            2663, August 1999.
 [PMTU]     Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
            November 1990.
 [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
            Communication Layers", STD 3, RFC 1122, October 1989.
 [RFC1256]  Deering, S., Ed., "ICMP Router Discovery Messages", RFC
            1256, September 1991.
 [RFC1470]  Enger, R. and J. Reynolds, "FYI on a Network Management
            Tool Catalog: Tools for Monitoring and Debugging TCP/IP
            Internets and Interconnected Devices", FYI 2, RFC 1470,
            June 1993.
 [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
            "Definition of the Differentiated Services Field (DS
            Field) in the IPv4 and IPv6 Headers", RFC 2474, December
            1998.
 [RFC4065]  Kempf, J., "Instructions for Seamoby and Experimental
            Mobility Protocol IANA Allocations", RFC 4065, July 2005.
 [TCP-SOFT] Gont, F., "TCP's Reaction to Soft Errors", RFC 5461,
            February 2009.
 [UNSAF]    Daigle, L., Ed., and IAB, "IAB Considerations for
            UNilateral Self-Address Fixing (UNSAF) Across Network
            Address Translation", RFC 3424, November 2002.

Srisuresh, et al. Best Current Practice [Page 28] RFC 5508 NAT Behavioral Requirements for ICMP April 2009

Authors' Addresses

 Pyda Srisuresh
 Kazeon Systems, Inc.
 1161 San Antonio Rd.
 Mountain View, CA 94043
 U.S.A.
 Phone: +1 408 836 4773
 EMail: srisuresh@yahoo.com
 Bryan Ford
 Max Planck Institute for Software Systems
 Campus Building E1 4
 D-66123 Saarbruecken
 Germany
 Phone: +49-681-9325657
 EMail: baford@mpi-sws.org
 Senthil Sivakumar
 Cisco Systems, Inc.
 7100-8 Kit Creek Road
 PO Box 14987
 Research Triangle Park, NC  27709-4987
 U.S.A.
 Phone: +1 919 392 5158
 EMail: ssenthil@cisco.com
 Saikat Guha
 Cornell University
 331 Upson Hall
 Ithaca, NY  14853
 U.S.A.
 Phone: +1 607 255 1008
 EMail: saikat@cs.cornell.edu

Srisuresh, et al. Best Current Practice [Page 29]

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