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rfc:rfc6145

Internet Engineering Task Force (IETF) X. Li Request for Comments: 6145 C. Bao Obsoletes: 2765 CERNET Center/Tsinghua Category: Standards Track University ISSN: 2070-1721 F. Baker

                                                         Cisco Systems
                                                            April 2011
                   IP/ICMP Translation Algorithm

Abstract

 This document describes the Stateless IP/ICMP Translation Algorithm
 (SIIT), which translates between IPv4 and IPv6 packet headers
 (including ICMP headers).  This document obsoletes RFC 2765.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6145.

Copyright Notice

 Copyright (c) 2011 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
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Li, et al. Standards Track [Page 1] RFC 6145 IPv4/IPv6 Translation April 2011

 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.

Table of Contents

 1.  Introduction and Motivation  . . . . . . . . . . . . . . . . .  3
   1.1.  IPv4-IPv6 Translation Model  . . . . . . . . . . . . . . .  3
   1.2.  Applicability and Limitations  . . . . . . . . . . . . . .  3
   1.3.  Stateless vs. Stateful Mode  . . . . . . . . . . . . . . .  4
   1.4.  Path MTU Discovery and Fragmentation . . . . . . . . . . .  5
 2.  Changes from RFC 2765  . . . . . . . . . . . . . . . . . . . .  5
 3.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  6
 4.  Translating from IPv4 to IPv6  . . . . . . . . . . . . . . . .  6
   4.1.  Translating IPv4 Headers into IPv6 Headers . . . . . . . .  7
   4.2.  Translating ICMPv4 Headers into ICMPv6 Headers . . . . . . 10
   4.3.  Translating ICMPv4 Error Messages into ICMPv6  . . . . . . 13
   4.4.  Generation of ICMPv4 Error Message . . . . . . . . . . . . 14
   4.5.  Transport-Layer Header Translation . . . . . . . . . . . . 14
   4.6.  Knowing When to Translate  . . . . . . . . . . . . . . . . 15
 5.  Translating from IPv6 to IPv4  . . . . . . . . . . . . . . . . 15
   5.1.  Translating IPv6 Headers into IPv4 Headers . . . . . . . . 17
     5.1.1.  IPv6 Fragment Processing . . . . . . . . . . . . . . . 19
   5.2.  Translating ICMPv6 Headers into ICMPv4 Headers . . . . . . 20
   5.3.  Translating ICMPv6 Error Messages into ICMPv4  . . . . . . 22
   5.4.  Generation of ICMPv6 Error Messages  . . . . . . . . . . . 23
   5.5.  Transport-Layer Header Translation . . . . . . . . . . . . 24
   5.6.  Knowing When to Translate  . . . . . . . . . . . . . . . . 24
 6.  Special Considerations for ICMPv6 Packet Too Big . . . . . . . 24
 7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
 8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
 9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
   9.1.  Normative References . . . . . . . . . . . . . . . . . . . 26
   9.2.  Informative References . . . . . . . . . . . . . . . . . . 28
 Appendix A.  Stateless Translation Workflow Example  . . . . . . . 30
   A.1.  H6 Establishes Communication with H4 . . . . . . . . . . . 30
   A.2.  H4 Establishes Communication with H6 . . . . . . . . . . . 32

Li, et al. Standards Track [Page 2] RFC 6145 IPv4/IPv6 Translation April 2011

1. Introduction and Motivation

 This document is a product of the 2008-2010 effort to define a
 replacement for NAT-PT [RFC2766] (which was changed to Historic
 status when [RFC4966] was published in 2007).  It is directly derived
 from Erik Nordmark's "Stateless IP/ICMP Translation Algorithm (SIIT)"
 [RFC2765], which provides stateless translation between IPv4
 [RFC0791] and IPv6 [RFC2460], and between ICMPv4 [RFC0792] and ICMPv6
 [RFC4443].  This document obsoletes RFC 2765 [RFC2765].  The changes
 from RFC 2765 [RFC2765] are listed in Section 2.
 Readers of this document are expected to have read and understood the
 framework described in [RFC6144].  Implementations of this IPv4/IPv6
 translation specification MUST also support the address translation
 algorithms in [RFC6052].  Implementations MAY also support stateful
 translation [RFC6146].

1.1. IPv4-IPv6 Translation Model

 The translation model consists of two or more network domains
 connected by one or more IP/ICMP translators (XLATs) as shown in
 Figure 1.
  1. ——– ———



          /             +----+              \
         |              |XLAT|               | XLAT: IP/ICMP
         |   IPv4       +----+   IPv6        |       Translator
         |   Domain     |    |   Domain      |
         |              |    |               |
          \             |    |              /
           \\         //      \\          //
              --------          ---------
                 Figure 1: IPv4-IPv6 Translation Model
 The scenarios of the translation model are discussed in [RFC6144].

1.2. Applicability and Limitations

 This document specifies the translation algorithms between IPv4
 packets and IPv6 packets.
 As with [RFC2765], the translating function specified in this
 document does not translate any IPv4 options, and it does not
 translate IPv6 extension headers except the Fragment Header.

Li, et al. Standards Track [Page 3] RFC 6145 IPv4/IPv6 Translation April 2011

 The issues and algorithms in the translation of datagrams containing
 TCP segments are described in [RFC5382].
 Fragmented IPv4 UDP packets that do not contain a UDP checksum (i.e.,
 the UDP checksum field is zero) are not of significant use in the
 Internet, and in general will not be translated by the IP/ICMP
 translator.  However, when the translator is configured to forward
 the packet without a UDP checksum, the fragmented IPv4 UDP packets
 will be translated.
 Fragmented ICMP/ICMPv6 packets will not be translated by the IP/ICMP
 translator.
 The IP/ICMP header translation specified in this document is
 consistent with requirements of multicast IP/ICMP headers.  However,
 IPv4 multicast addresses [RFC5771] cannot be mapped to IPv6 multicast
 addresses [RFC3307] based on the unicast mapping rule [RFC6052].

1.3. Stateless vs. Stateful Mode

 An IP/ICMP translator has two possible modes of operation: stateless
 and stateful [RFC6144].  In both cases, we assume that a system (a
 node or an application) that has an IPv4 address but not an IPv6
 address is communicating with a system that has an IPv6 address but
 no IPv4 address, or that the two systems do not have contiguous
 routing connectivity and hence are forced to have their
 communications translated.
 In the stateless mode, a specific IPv6 address range will represent
 IPv4 systems (IPv4-converted addresses), and the IPv6 systems have
 addresses (IPv4-translatable addresses) that can be algorithmically
 mapped to a subset of the service provider's IPv4 addresses.  Note
 that IPv4-translatable addresses are a subset of IPv4-converted
 addresses.  In general, there is no need to concern oneself with
 translation tables, as the IPv4 and IPv6 counterparts are
 algorithmically related.
 In the stateful mode, a specific IPv6 address range will represent
 IPv4 systems (IPv4-converted addresses), but the IPv6 systems may use
 any IPv6 addresses [RFC4291] except in that range.  In this case, a
 translation table is required to bind the IPv6 systems' addresses to
 the IPv4 addresses maintained in the translator.
 The address translation mechanisms for the stateless and the stateful
 translations are defined in [RFC6052].

Li, et al. Standards Track [Page 4] RFC 6145 IPv4/IPv6 Translation April 2011

1.4. Path MTU Discovery and Fragmentation

 Due to the different sizes of the IPv4 and IPv6 header, which are 20+
 octets and 40 octets respectively, handling the maximum packet size
 is critical for the operation of the IPv4/IPv6 translator.  There are
 three mechanisms to handle this issue: path MTU discovery (PMTUD),
 fragmentation, and transport-layer negotiation such as the TCP
 Maximum Segment Size (MSS) option [RFC0879].  Note that the
 translator MUST behave as a router, i.e., the translator MUST send a
 Packet Too Big error message or fragment the packet when the packet
 size exceeds the MTU of the next-hop interface.
 Don't Fragment, ICMP Packet Too Big, and packet fragmentation are
 discussed in Sections 4 and 5 of this document.  The reassembling of
 fragmented packets in the stateful translator is discussed in
 [RFC6146], since it requires state maintenance in the translator.

2. Changes from RFC 2765

 The changes from RFC 2765 are the following:
 1.  Redescribing the network model to map to present and projected
     usage.  The scenarios, applicability, and limitations originally
     presented in RFC 2765 [RFC2765] are moved to the framework
     document [RFC6144].
 2.  Moving the address format to the address format document
     [RFC6052], to coordinate with other documents on the topic.
 3.  Describing the header translation for the stateless and stateful
     operations.  The details of the session database and mapping
     table handling of the stateful translation is in the stateful
     translation document [RFC6146].
 4.  Having refined the header translation, fragmentation handling,
     ICMP translation and ICMP error translation in the IPv4-to-IPv6
     direction, as well as the IPv6-to-IPv4 direction.
 5.  Adding more discussion on transport-layer header translation.
 6.  Adding Section 5.1.1 for "IPv6 Fragment Processing".
 7.  Adding Section 6 for "Special Considerations for ICMPv6 Packet
     Too Big".
 8.  Updating Section 7 for "Security Considerations".
 9.  Adding Appendix A "Stateless translation workflow example".

Li, et al. Standards Track [Page 5] RFC 6145 IPv4/IPv6 Translation April 2011

3. Conventions

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

4. Translating from IPv4 to IPv6

 When an IP/ICMP translator receives an IPv4 datagram addressed to a
 destination towards the IPv6 domain, it translates the IPv4 header of
 that packet into an IPv6 header.  The original IPv4 header on the
 packet is removed and replaced by an IPv6 header, and the transport
 checksum is updated as needed, if that transport is supported by the
 translator.  The data portion of the packet is left unchanged.  The
 IP/ICMP translator then forwards the packet based on the IPv6
 destination address.
            +-------------+                 +-------------+
            |    IPv4     |                 |    IPv6     |
            |   Header    |                 |   Header    |
            +-------------+                 +-------------+
            |  Transport- |                 |  Fragment   |
            |   Layer     |      ===>       |   Header    |
            |   Header    |                 | (if needed) |
            +-------------+                 +-------------+
            |             |                 |  Transport- |
            ~    Data     ~                 |   Layer     |
            |             |                 |   Header    |
            +-------------+                 +-------------+
                                            |             |
                                            ~    Data     ~
                                            |             |
                                            +-------------+
                  Figure 2: IPv4-to-IPv6 Translation
 Path MTU discovery is mandatory in IPv6, but it is optional in IPv4.
 IPv6 routers never fragment a packet -- only the sender can do
 fragmentation.
 When an IPv4 node performs path MTU discovery (by setting the Don't
 Fragment (DF) bit in the header), path MTU discovery can operate end-
 to-end, i.e., across the translator.  In this case, either IPv4 or
 IPv6 routers (including the translator) might send back ICMP Packet
 Too Big messages to the sender.  When the IPv6 routers send these
 ICMPv6 errors, they will pass through a translator that will

Li, et al. Standards Track [Page 6] RFC 6145 IPv4/IPv6 Translation April 2011

 translate the ICMPv6 error to a form that the IPv4 sender can
 understand.  As a result, an IPv6 Fragment Header is only included if
 the IPv4 packet is already fragmented.
 However, when the IPv4 sender does not set the DF bit, the translator
 MUST ensure that the packet does not exceed the path MTU on the IPv6
 side.  This is done by fragmenting the IPv4 packet (with Fragment
 Headers) so that it fits in 1280-byte IPv6 packets, since that is the
 minimum IPv6 MTU.  The IPv6 Fragment Header has been shown to cause
 operational difficulties in practice due to limited firewall
 fragmentation support, etc.  In an environment where the network
 owned/operated by the same entity that owns/operates the translator,
 the translator MAY provide a configuration function for the network
 administrator to adjust the threshold of the minimum IPv6 MTU to a
 value that reflects the real value of the minimum IPv6 MTU in the
 network (greater than 1280 bytes).  This will help reduce the chance
 of including the Fragment Header in the packets.
 When the IPv4 sender does not set the DF bit, the translator SHOULD
 always include an IPv6 Fragment Header to indicate that the sender
 allows fragmentation.  The translator MAY provide a configuration
 function that allows the translator not to include the Fragment
 Header for the non-fragmented IPv6 packets.
 The rules in Section 4.1 ensure that when packets are fragmented,
 either by the sender or by IPv4 routers, the low-order 16 bits of the
 fragment identification are carried end-to-end, ensuring that packets
 are correctly reassembled.  In addition, the rules in Section 4.1 use
 the presence of an IPv6 Fragment Header to indicate that the sender
 might not be using path MTU discovery (i.e., the packet should not
 have the DF flag set should it later be translated back to IPv4).
 Other than the special rules for handling fragments and path MTU
 discovery, the actual translation of the packet header consists of a
 simple translation as defined below.  Note that ICMPv4 packets
 require special handling in order to translate the content of ICMPv4
 error messages and also to add the ICMPv6 pseudo-header checksum.
 The translator SHOULD make sure that the packets belonging to the
 same flow leave the translator in the same order in which they
 arrived.

4.1. Translating IPv4 Headers into IPv6 Headers

 If the DF flag is not set and the IPv4 packet will result in an IPv6
 packet larger than 1280 bytes, the packet SHOULD be fragmented so the
 resulting IPv6 packet (with Fragment Header added to each fragment)
 will be less than or equal to 1280 bytes.  For example, if the packet

Li, et al. Standards Track [Page 7] RFC 6145 IPv4/IPv6 Translation April 2011

 is fragmented prior to the translation, the IPv4 packets should be
 fragmented so that their length, excluding the IPv4 header, is at
 most 1232 bytes (1280 minus 40 for the IPv6 header and 8 for the
 Fragment Header).  The translator MAY provide a configuration
 function for the network administrator to adjust the threshold of the
 minimum IPv6 MTU to a value greater than 1280-byte if the real value
 of the minimum IPv6 MTU in the network is known to the administrator.
 The resulting fragments are then translated independently using the
 logic described below.
 If the DF bit is set and the MTU of the next-hop interface is less
 than the total length value of the IPv4 packet plus 20, the
 translator MUST send an ICMPv4 "Fragmentation Needed" error message
 to the IPv4 source address.
 If the DF bit is set and the packet is not a fragment (i.e., the More
 Fragments (MF) flag is not set and the Fragment Offset is equal to
 zero), then the translator SHOULD NOT add a Fragment Header to the
 resulting packet.  The IPv6 header fields are set as follows:
 Version:  6
 Traffic Class:  By default, copied from the IP Type Of Service (TOS)
    octet.  According to [RFC2474], the semantics of the bits are
    identical in IPv4 and IPv6.  However, in some IPv4 environments
    these fields might be used with the old semantics of "Type Of
    Service and Precedence".  An implementation of a translator SHOULD
    support an administratively configurable option to ignore the IPv4
    TOS and always set the IPv6 traffic class (TC) to zero.  In
    addition, if the translator is at an administrative boundary, the
    filtering and update considerations of [RFC2475] may be
    applicable.
 Flow Label:  0 (all zero bits)
 Payload Length:  Total length value from the IPv4 header, minus the
    size of the IPv4 header and IPv4 options, if present.
 Next Header:  For ICMPv4 (1), it is changed to ICMPv6 (58);
    otherwise, the protocol field MUST be copied from the IPv4 header.
 Hop Limit:  The hop limit is derived from the TTL value in the IPv4
    header.  Since the translator is a router, as part of forwarding
    the packet it needs to decrement either the IPv4 TTL (before the
    translation) or the IPv6 Hop Limit (after the translation).  As
    part of decrementing the TTL or Hop Limit, the translator (as any
    router) MUST check for zero and send the ICMPv4 "TTL Exceeded" or
    ICMPv6 "Hop Limit Exceeded" error.

Li, et al. Standards Track [Page 8] RFC 6145 IPv4/IPv6 Translation April 2011

 Source Address:  The IPv4-converted address derived from the IPv4
    source address per [RFC6052], Section 2.3.
    If the translator gets an illegal source address (e.g., 0.0.0.0,
    127.0.0.1, etc.), the translator SHOULD silently drop the packet
    (as discussed in Section 5.3.7 of [RFC1812]).
 Destination Address:  In the stateless mode, which is to say that if
    the IPv4 destination address is within a range of configured IPv4
    stateless translation prefix, the IPv6 destination address is the
    IPv4-translatable address derived from the IPv4 destination
    address per [RFC6052], Section 2.3.  A workflow example of
    stateless translation is shown in Appendix A of this document.
    In the stateful mode (which is to say that if the IPv4 destination
    address is not within the range of any configured IPv4 stateless
    translation prefix), the IPv6 destination address and
    corresponding transport-layer destination port are derived from
    the Binding Information Bases (BIBs) reflecting current session
    state in the translator as described in [RFC6146].
 If any IPv4 options are present in the IPv4 packet, they MUST be
 ignored and the packet translated normally; there is no attempt to
 translate the options.  However, if an unexpired source route option
 is present then the packet MUST instead be discarded, and an ICMPv4
 "Destination Unreachable, Source Route Failed" (Type 3, Code 5) error
 message SHOULD be returned to the sender.
 If there is a need to add a Fragment Header (the DF bit is not set or
 the packet is a fragment), the header fields are set as above with
 the following exceptions:
 IPv6 fields:
    Payload Length:  Total length value from the IPv4 header, plus 8
       for the Fragment Header, minus the size of the IPv4 header and
       IPv4 options, if present.
    Next Header:  Fragment Header (44).
 Fragment Header fields:
    Next Header:  For ICMPv4 (1), it is changed to ICMPv6 (58);
       otherwise, the protocol field MUST be copied from the IPv4
       header.
    Fragment Offset:  Fragment Offset copied from the IPv4 header.

Li, et al. Standards Track [Page 9] RFC 6145 IPv4/IPv6 Translation April 2011

    M flag:  More Fragments bit copied from the IPv4 header.
    Identification:  The low-order 16 bits copied from the
       Identification field in the IPv4 header.  The high-order 16
       bits set to zero.

4.2. Translating ICMPv4 Headers into ICMPv6 Headers

 All ICMPv4 messages that are to be translated require that the ICMPv6
 checksum field be calculated as part of the translation since ICMPv6,
 unlike ICMPv4, has a pseudo-header checksum just like UDP and TCP.
 In addition, all ICMPv4 packets MUST have the Type translated and,
 for ICMPv4 error messages, the included IP header also MUST be
 translated.
 The actions needed to translate various ICMPv4 messages are as
 follows:
 ICMPv4 query messages:
    Echo and Echo Reply (Type 8 and Type 0):  Adjust the Type values
       to 128 and 129, respectively, and adjust the ICMP checksum both
       to take the type change into account and to include the ICMPv6
       pseudo-header.
    Information Request/Reply (Type 15 and Type 16):  Obsoleted in
       ICMPv6.  Silently drop.
    Timestamp and Timestamp Reply (Type 13 and Type 14):  Obsoleted in
       ICMPv6.  Silently drop.
    Address Mask Request/Reply (Type 17 and Type 18):  Obsoleted in
       ICMPv6.  Silently drop.
    ICMP Router Advertisement (Type 9):  Single-hop message.  Silently
       drop.
    ICMP Router Solicitation (Type 10):  Single-hop message.  Silently
       drop.
    Unknown ICMPv4 types:  Silently drop.
    IGMP messages:  While the Multicast Listener Discovery (MLD)
       messages [RFC2710] [RFC3590] [RFC3810] are the logical IPv6
       counterparts for the IPv4 IGMP messages, all the "normal" IGMP
       messages are single-hop messages and SHOULD be silently dropped
       by the translator.  Other IGMP messages might be used by

Li, et al. Standards Track [Page 10] RFC 6145 IPv4/IPv6 Translation April 2011

       multicast routing protocols and, since it would be a
       configuration error to try to have router adjacencies across
       IP/ICMP translators, those packets SHOULD also be silently
       dropped.
     ICMPv4 error messages:
       Destination Unreachable (Type 3):  Translate the Code as
          described below, set the Type to 1, and adjust the ICMP
          checksum both to take the type/code change into account and
          to include the ICMPv6 pseudo-header.
          Translate the Code as follows:
          Code 0, 1 (Net Unreachable, Host Unreachable):  Set the Code
             to 0 (No route to destination).
          Code 2 (Protocol Unreachable):  Translate to an ICMPv6
             Parameter Problem (Type 4, Code 1) and make the Pointer
             point to the IPv6 Next Header field.
          Code 3 (Port Unreachable):  Set the Code to 4 (Port
             unreachable).
          Code 4 (Fragmentation Needed and DF was Set):  Translate to
             an ICMPv6 Packet Too Big message (Type 2) with Code set
             to 0.  The MTU field MUST be adjusted for the difference
             between the IPv4 and IPv6 header sizes, i.e.,
             minimum(advertised MTU+20, MTU_of_IPv6_nexthop,
             (MTU_of_IPv4_nexthop)+20).  Note that if the IPv4 router
             set the MTU field to zero, i.e., the router does not
             implement [RFC1191], then the translator MUST use the
             plateau values specified in [RFC1191] to determine a
             likely path MTU and include that path MTU in the ICMPv6
             packet.  (Use the greatest plateau value that is less
             than the returned Total Length field.)
             See also the requirements in Section 6.
          Code 5 (Source Route Failed):  Set the Code to 0 (No route
             to destination).  Note that this error is unlikely since
             source routes are not translated.
          Code 6, 7, 8:  Set the Code to 0 (No route to destination).

Li, et al. Standards Track [Page 11] RFC 6145 IPv4/IPv6 Translation April 2011

          Code 9, 10 (Communication with Destination Host
             Administratively Prohibited):  Set the Code to 1
             (Communication with destination administratively
             prohibited).
          Code 11, 12:  Set the Code to 0 (No route to destination).
          Code 13 (Communication Administratively Prohibited):  Set
             the Code to 1 (Communication with destination
             administratively prohibited).
          Code 14 (Host Precedence Violation):  Silently drop.
          Code 15 (Precedence cutoff in effect):  Set the Code to 1
             (Communication with destination administratively
             prohibited).
          Other Code values:  Silently drop.
       Redirect (Type 5):  Single-hop message.  Silently drop.
       Alternative Host Address (Type 6):  Silently drop.
       Source Quench (Type 4):  Obsoleted in ICMPv6.  Silently drop.
       Time Exceeded (Type 11):  Set the Type to 3, and adjust the
          ICMP checksum both to take the type change into account and
          to include the ICMPv6 pseudo-header.  The Code is unchanged.
       Parameter Problem (Type 12):  Set the Type to 4, and adjust the
          ICMP checksum both to take the type/code change into account
          and to include the ICMPv6 pseudo-header.
          Translate the Code as follows:
          Code 0 (Pointer indicates the error):  Set the Code to 0
             (Erroneous header field encountered) and update the
             pointer as defined in Figure 3.  (If the Original IPv4
             Pointer Value is not listed or the Translated IPv6
             Pointer Value is listed as "n/a", silently drop the
             packet.)
          Code 1 (Missing a required option):  Silently drop.

Li, et al. Standards Track [Page 12] RFC 6145 IPv4/IPv6 Translation April 2011

          Code 2 (Bad length):  Set the Code to 0 (Erroneous header
             field encountered) and update the pointer as defined in
             Figure 3.  (If the Original IPv4 Pointer Value is not
             listed or the Translated IPv6 Pointer Value is listed as
             "n/a", silently drop the packet.)
          Other Code values:  Silently drop.
       Unknown ICMPv4 types:  Silently drop.
   +--------------------------------+--------------------------------+
   |   Original IPv4 Pointer Value  | Translated IPv6 Pointer Value  |
   +--------------------------------+--------------------------------+
   |  0  | Version/IHL              |  0  | Version/Traffic Class    |
   |  1  | Type Of Service          |  1  | Traffic Class/Flow Label |
   | 2,3 | Total Length             |  4  | Payload Length           |
   | 4,5 | Identification           | n/a |                          |
   |  6  | Flags/Fragment Offset    | n/a |                          |
   |  7  | Fragment Offset          | n/a |                          |
   |  8  | Time to Live             |  7  | Hop Limit                |
   |  9  | Protocol                 |  6  | Next Header              |
   |10,11| Header Checksum          | n/a |                          |
   |12-15| Source Address           |  8  | Source Address           |
   |16-19| Destination Address      | 24  | Destination Address      |
   +--------------------------------+--------------------------------+
           Figure 3: Pointer Value for Translating from IPv4 to IPv6
       ICMP Error Payload:  If the received ICMPv4 packet contains an
          ICMPv4 Extension [RFC4884], the translation of the ICMPv4
          packet will cause the ICMPv6 packet to change length.  When
          this occurs, the ICMPv6 Extension length attribute MUST be
          adjusted accordingly (e.g., longer due to the translation
          from IPv4 to IPv6).  If the ICMPv4 Extension exceeds the
          maximum size of an ICMPv6 message on the outgoing interface,
          the ICMPv4 extension SHOULD be simply truncated.  For
          extensions not defined in [RFC4884], the translator passes
          the extensions as opaque bit strings, and those containing
          IPv4 address literals will not have those addresses
          translated to IPv6 address literals; this may cause problems
          with processing of those ICMP extensions.

4.3. Translating ICMPv4 Error Messages into ICMPv6

 There are some differences between the ICMPv4 and the ICMPv6 error
 message formats as detailed above.  The ICMP error messages
 containing the packet in error MUST be translated just like a normal
 IP packet.  If the translation of this "packet in error" changes the

Li, et al. Standards Track [Page 13] RFC 6145 IPv4/IPv6 Translation April 2011

 length of the datagram, the Total Length field in the outer IPv6
 header MUST be updated.
            +-------------+                 +-------------+
            |    IPv4     |                 |    IPv6     |
            |   Header    |                 |   Header    |
            +-------------+                 +-------------+
            |   ICMPv4    |                 |   ICMPv6    |
            |   Header    |                 |   Header    |
            +-------------+                 +-------------+
            |    IPv4     |      ===>       |    IPv6     |
            |   Header    |                 |   Header    |
            +-------------+                 +-------------+
            |   Partial   |                 |   Partial   |
            |  Transport- |                 |  Transport- |
            |   Layer     |                 |   Layer     |
            |   Header    |                 |   Header    |
            +-------------+                 +-------------+
             Figure 4: IPv4-to-IPv6 ICMP Error Translation
 The translation of the inner IP header can be done by invoking the
 function that translated the outer IP headers.  This process MUST
 stop at the first embedded header and drop the packet if it contains
 more embedded headers.

4.4. Generation of ICMPv4 Error Message

 If the IPv4 packet is discarded, then the translator SHOULD be able
 to send back an ICMPv4 error message to the original sender of the
 packet, unless the discarded packet is itself an ICMPv4 message.  The
 ICMPv4 message, if sent, has a Type of 3 (Destination Unreachable)
 and a Code of 13 (Communication Administratively Prohibited), unless
 otherwise specified in this document or in [RFC6146].  The translator
 SHOULD allow an administrator to configure whether the ICMPv4 error
 messages are sent, rate-limited, or not sent.

4.5. Transport-Layer Header Translation

 If the address translation algorithm is not checksum neutral (see
 Section 4.1 of [RFC6052]), the recalculation and updating of the
 transport-layer headers that contain pseudo-headers need to be
 performed.  Translators MUST do this for TCP and ICMP packets and for
 UDP packets that contain a UDP checksum (i.e., the UDP checksum field
 is not zero).

Li, et al. Standards Track [Page 14] RFC 6145 IPv4/IPv6 Translation April 2011

 For UDP packets that do not contain a UDP checksum (i.e., the UDP
 checksum field is zero), the translator SHOULD provide a
 configuration function to allow:
 1.  Dropping the packet and generating a system management event that
     specifies at least the IP addresses and port numbers of the
     packet.
 2.  Calculating an IPv6 checksum and forwarding the packet (which has
     performance implications).
     A stateless translator cannot compute the UDP checksum of
     fragmented packets, so when a stateless translator receives the
     first fragment of a fragmented UDP IPv4 packet and the checksum
     field is zero, the translator SHOULD drop the packet and generate
     a system management event that specifies at least the IP
     addresses and port numbers in the packet.
     For a stateful translator, the handling of fragmented UDP IPv4
     packets with a zero checksum is discussed in [RFC6146]), Section
     3.1.
 Other transport protocols (e.g., DCCP) are OPTIONAL to support.  In
 order to ease debugging and troubleshooting, translators MUST forward
 all transport protocols as described in the "Next Header" step of
 Section 4.1.

4.6. Knowing When to Translate

 If the IP/ICMP translator also provides a normal forwarding function,
 and the destination IPv4 address is reachable by a more specific
 route without translation, the translator MUST forward it without
 translating it.  Otherwise, when an IP/ICMP translator receives an
 IPv4 datagram addressed to an IPv4 destination representing a host in
 the IPv6 domain, the packet MUST be translated to IPv6.

5. Translating from IPv6 to IPv4

 When an IP/ICMP translator receives an IPv6 datagram addressed to a
 destination towards the IPv4 domain, it translates the IPv6 header of
 the received IPv6 packet into an IPv4 header.  The original IPv6
 header on the packet is removed and replaced by an IPv4 header.
 Since the ICMPv6 [RFC4443], TCP [RFC0793], UDP [RFC0768], and DCCP
 [RFC4340] headers contain checksums that cover the IP header, if the
 address mapping algorithm is not checksum neutral, the checksum MUST
 be evaluated before translation and the ICMP and transport-layer

Li, et al. Standards Track [Page 15] RFC 6145 IPv4/IPv6 Translation April 2011

 headers MUST be updated.  The data portion of the packet is left
 unchanged.  The IP/ICMP translator then forwards the packet based on
 the IPv4 destination address.
            +-------------+                 +-------------+
            |    IPv6     |                 |    IPv4     |
            |   Header    |                 |   Header    |
            +-------------+                 +-------------+
            |  Fragment   |                 |  Transport  |
            |   Header    |      ===>       |   Layer     |
            |(if present) |                 |   Header    |
            +-------------+                 +-------------+
            |  Transport  |                 |             |
            |   Layer     |                 ~    Data     ~
            |   Header    |                 |             |
            +-------------+                 +-------------+
            |             |
            ~    Data     ~
            |             |
            +-------------+
                  Figure 5: IPv6-to-IPv4 Translation
 There are some differences between IPv6 and IPv4 (in the areas of
 fragmentation and the minimum link MTU) that affect the translation.
 An IPv6 link has to have an MTU of 1280 bytes or greater.  The
 corresponding limit for IPv4 is 68 bytes.  Path MTU discovery across
 a translator relies on ICMP Packet Too Big messages being received
 and processed by IPv6 hosts, including an ICMP Packet Too Big that
 indicates the MTU is less than the IPv6 minimum MTU.  This
 requirement is described in Section 5 of [RFC2460] (for IPv6's
 1280-octet minimum MTU) and Section 5 of [RFC1883] (for IPv6's
 previous 576-octet minimum MTU).
 In an environment where an ICMPv4 Packet Too Big message is
 translated to an ICMPv6 Packet Too Big message, and the ICMPv6 Packet
 Too Big message is successfully delivered to and correctly processed
 by the IPv6 hosts (e.g., a network owned/operated by the same entity
 that owns/operates the translator), the translator can rely on IPv6
 hosts sending subsequent packets to the same IPv6 destination with
 IPv6 Fragment Headers.  In such an environment, when the translator
 receives an IPv6 packet with a Fragment Header, the translator SHOULD
 generate the IPv4 packet with a cleared Don't Fragment bit, and with
 its identification value from the IPv6 Fragment Header, for all of
 the IPv6 fragments (MF=0 or MF=1).

Li, et al. Standards Track [Page 16] RFC 6145 IPv4/IPv6 Translation April 2011

 In an environment where an ICMPv4 Packet Too Big message is filtered
 (by a network firewall or by the host itself) or not correctly
 processed by the IPv6 hosts, the IPv6 host will never generate an
 IPv6 packet with the IPv6 Fragment Header.  In such an environment,
 the translator SHOULD set the IPv4 Don't Fragment bit.  While setting
 the Don't Fragment bit may create PMTUD black holes [RFC2923] if
 there are IPv4 links smaller than 1260 octets, this is considered
 safer than causing IPv4 reassembly errors [RFC4963].
 Other than the special rules for handling fragments and path MTU
 discovery, the actual translation of the packet header consists of a
 simple translation as defined below.  Note that ICMPv6 packets
 require special handling in order to translate the contents of ICMPv6
 error messages and also to remove the ICMPv6 pseudo-header checksum.
 The translator SHOULD make sure that the packets belonging to the
 same flow leave the translator in the same order in which they
 arrived.

5.1. Translating IPv6 Headers into IPv4 Headers

 If there is no IPv6 Fragment Header, the IPv4 header fields are set
 as follows:
 Version:  4
 Internet Header Length:  5 (no IPv4 options)
 Type of Service (TOS) Octet:  By default, copied from the IPv6
    Traffic Class (all 8 bits).  According to [RFC2474], the semantics
    of the bits are identical in IPv4 and IPv6.  However, in some IPv4
    environments, these bits might be used with the old semantics of
    "Type Of Service and Precedence".  An implementation of a
    translator SHOULD provide the ability to ignore the IPv6 traffic
    class and always set the IPv4 TOS Octet to a specified value.  In
    addition, if the translator is at an administrative boundary, the
    filtering and update considerations of [RFC2475] may be
    applicable.
 Total Length:  Payload length value from the IPv6 header, plus the
    size of the IPv4 header.
 Identification:  All zero.  In order to avoid black holes caused by
    ICMPv4 filtering or non-[RFC2460]-compatible IPv6 hosts (a
    workaround is discussed in Section 6), the translator MAY provide
    a function to generate the identification value if the packet size
    is greater than 88 bytes and less than or equal to 1280 bytes.

Li, et al. Standards Track [Page 17] RFC 6145 IPv4/IPv6 Translation April 2011

    The translator SHOULD provide a method for operators to enable or
    disable this function.
 Flags:  The More Fragments flag is set to zero.  The Don't Fragment
    (DF) flag is set to one.  In order to avoid black holes caused by
    ICMPv4 filtering or non-[RFC2460]-compatible IPv6 hosts (a
    workaround is discussed in Section 6), the translator MAY provide
    a function as follows.  If the packet size is greater than 88
    bytes and less than or equal to 1280 bytes, it sets the DF flag to
    zero; otherwise, it sets the DF flag to one.  The translator
    SHOULD provide a method for operators to enable or disable this
    function.
 Fragment Offset:  All zeros.
 Time to Live:  Time to Live is derived from Hop Limit value in IPv6
    header.  Since the translator is a router, as part of forwarding
    the packet it needs to decrement either the IPv6 Hop Limit (before
    the translation) or the IPv4 TTL (after the translation).  As part
    of decrementing the TTL or Hop Limit the translator (as any
    router) MUST check for zero and send the ICMPv4 "TTL Exceeded" or
    ICMPv6 "Hop Limit Exceeded" error.
 Protocol:  The IPv6-Frag (44) header is handled as discussed in
    Section 5.1.1.  ICMPv6 (58) is changed to ICMPv4 (1), and the
    payload is translated as discussed in Section 5.2.  The IPv6
    headers HOPOPT (0), IPv6-Route (43), and IPv6-Opts (60) are
    skipped over during processing as they have no meaning in IPv4.
    For the first 'next header' that does not match one of the cases
    above, its Next Header value (which contains the transport
    protocol number) is copied to the protocol field in the IPv4
    header.  This means that all transport protocols are translated.
    Note:  Some translated protocols will fail at the receiver for
       various reasons: some are known to fail when translated (e.g.,
       IPsec Authentication Header (51)), and others will fail
       checksum validation if the address translation is not checksum
       neutral [RFC6052] and the translator does not update the
       transport protocol's checksum (because the translator doesn't
       support recalculating the checksum for that transport protocol;
       see Section 5.5).
 Header Checksum:  Computed once the IPv4 header has been created.
 Source Address:  In the stateless mode (which is to say that if the
    IPv6 source address is within the range of a configured IPv6
    translation prefix), the IPv4 source address is derived from the
    IPv6 source address per [RFC6052], Section 2.3.  Note that the

Li, et al. Standards Track [Page 18] RFC 6145 IPv4/IPv6 Translation April 2011

    original IPv6 source address is an IPv4-translatable address.  A
    workflow example of stateless translation is shown in Appendix A
    of this document.  If the translator only supports stateless mode
    and if the IPv6 source address is not within the range of
    configured IPv6 prefix(es), the translator SHOULD drop the packet
    and respond with an ICMPv6 "Destination Unreachable, Source
    address failed ingress/egress policy" (Type 1, Code 5).
    In the stateful mode, which is to say that if the IPv6 source
    address is not within the range of any configured IPv6 stateless
    translation prefix, the IPv4 source address and transport-layer
    source port corresponding to the IPv4-related IPv6 source address
    and source port are derived from the Binding Information Bases
    (BIBs) as described in [RFC6146].
    In stateless and stateful modes, if the translator gets an illegal
    source address (e.g., ::1, etc.), the translator SHOULD silently
    drop the packet.
 Destination Address:  The IPv4 destination address is derived from
    the IPv6 destination address of the datagram being translated per
    [RFC6052], Section 2.3.  Note that the original IPv6 destination
    address is an IPv4-converted address.
 If a Routing header with a non-zero Segments Left field is present,
 then the packet MUST NOT be translated, and an ICMPv6 "parameter
 problem/erroneous header field encountered" (Type 4, Code 0) error
 message, with the Pointer field indicating the first byte of the
 Segments Left field, SHOULD be returned to the sender.

5.1.1. IPv6 Fragment Processing

 If the IPv6 packet contains a Fragment Header, the header fields are
 set as above with the following exceptions:
 Total Length:  Payload length value from IPv6 header, minus 8 for the
    Fragment Header, plus the size of the IPv4 header.
 Identification:  Copied from the low-order 16 bits in the
    Identification field in the Fragment Header.
 Flags:  The IPv4 More Fragments (MF) flag is copied from the M flag
    in the IPv6 Fragment Header.  The IPv4 Don't Fragment (DF) flag is
    cleared (set to zero), allowing this packet to be further
    fragmented by IPv4 routers.
 Fragment Offset:  Copied from the Fragment Offset field of the IPv6
    Fragment Header.

Li, et al. Standards Track [Page 19] RFC 6145 IPv4/IPv6 Translation April 2011

 Protocol:  For ICMPv6 (58), it is changed to ICMPv4 (1); otherwise,
    extension headers are skipped, and the Next Header field is copied
    from the last IPv6 header.
 If a translated packet with DF set to 1 will be larger than the MTU
 of the next-hop interface, then the translator MUST drop the packet
 and send the ICMPv6 Packet Too Big (Type 2, Code 0) error message to
 the IPv6 host with an adjusted MTU in the ICMPv6 message.

5.2. Translating ICMPv6 Headers into ICMPv4 Headers

 If a non-checksum-neutral translation address is being used, ICMPv6
 messages MUST have their ICMPv4 checksum field be updated as part of
 the translation since ICMPv6 (unlike ICMPv4) includes a pseudo-header
 in the checksum just like UDP and TCP.
 In addition, all ICMP packets MUST have the Type translated and, for
 ICMP error messages, the included IP header also MUST be translated.
 Note that the IPv6 addresses in the IPv6 header may not be IPv4-
 translatable addresses and there will be no corresponding IPv4
 addresses representing this IPv6 address.  In this case, the
 translator can do stateful translation.  A mechanism by which the
 translator can instead do stateless translation of this address is
 left for future work.
 The actions needed to translate various ICMPv6 messages are:
 ICMPv6 informational messages:
    Echo Request and Echo Reply (Type 128 and 129):  Adjust the Type
       values to 8 and 0, respectively, and adjust the ICMP checksum
       both to take the type change into account and to exclude the
       ICMPv6 pseudo-header.
    MLD Multicast Listener Query/Report/Done (Type 130, 131, 132):
       Single-hop message.  Silently drop.
    Neighbor Discover messages (Type 133 through 137):  Single-hop
       message.  Silently drop.
    Unknown informational messages:  Silently drop.
 ICMPv6 error messages:
    Destination Unreachable (Type 1)  Set the Type to 3, and adjust
       the ICMP checksum both to take the type/code change into
       account and to exclude the ICMPv6 pseudo-header.

Li, et al. Standards Track [Page 20] RFC 6145 IPv4/IPv6 Translation April 2011

       Translate the Code as follows:
       Code 0 (No route to destination):  Set the Code to 1 (Host
          unreachable).
       Code 1 (Communication with destination administratively
          prohibited):  Set the Code to 10 (Communication with
          destination host administratively prohibited).
       Code 2 (Beyond scope of source address):  Set the Code to 1
          (Host unreachable).  Note that this error is very unlikely
          since an IPv4-translatable source address is typically
          considered to have global scope.
       Code 3 (Address unreachable):  Set the Code to 1 (Host
          unreachable).
       Code 4 (Port unreachable):  Set the Code to 3 (Port
          unreachable).
       Other Code values:  Silently drop.
    Packet Too Big (Type 2):  Translate to an ICMPv4 Destination
       Unreachable (Type 3) with Code 4, and adjust the ICMPv4
       checksum both to take the type change into account and to
       exclude the ICMPv6 pseudo-header.  The MTU field MUST be
       adjusted for the difference between the IPv4 and IPv6 header
       sizes, taking into account whether or not the packet in error
       includes a Fragment Header, i.e., minimum(advertised MTU-20,
       MTU_of_IPv4_nexthop, (MTU_of_IPv6_nexthop)-20).
       See also the requirements in Section 6.
    Time Exceeded (Type 3):  Set the Type to 11, and adjust the ICMPv4
       checksum both to take the type change into account and to
       exclude the ICMPv6 pseudo-header.  The Code is unchanged.
    Parameter Problem (Type 4):  Translate the Type and Code as
       follows, and adjust the ICMPv4 checksum both to take the type/
       code change into account and to exclude the ICMPv6 pseudo-
       header.

Li, et al. Standards Track [Page 21] RFC 6145 IPv4/IPv6 Translation April 2011

       Translate the Code as follows:
       Code 0 (Erroneous header field encountered):  Set to Type 12,
          Code 0, and update the pointer as defined in Figure 6.  (If
          the Original IPv6 Pointer Value is not listed or the
          Translated IPv4 Pointer Value is listed as "n/a", silently
          drop the packet.)
       Code 1 (Unrecognized Next Header type encountered):  Translate
          this to an ICMPv4 protocol unreachable (Type 3, Code 2).
       Code 2 (Unrecognized IPv6 option encountered):  Silently drop.
    Unknown error messages:  Silently drop.
   +--------------------------------+--------------------------------+
   |   Original IPv6 Pointer Value  | Translated IPv4 Pointer Value  |
   +--------------------------------+--------------------------------+
   |  0  | Version/Traffic Class    |  0  | Version/IHL, Type Of Ser |
   |  1  | Traffic Class/Flow Label |  1  | Type Of Service          |
   | 2,3 | Flow Label               | n/a |                          |
   | 4,5 | Payload Length           |  2  | Total Length             |
   |  6  | Next Header              |  9  | Protocol                 |
   |  7  | Hop Limit                |  8  | Time to Live             |
   | 8-23| Source Address           | 12  | Source Address           |
   |24-39| Destination Address      | 16  | Destination Address      |
   +--------------------------------+--------------------------------+
          Figure 6: Pointer Value for Translating from IPv6 to IPv4
    ICMP Error Payload:  If the received ICMPv6 packet contains an
       ICMPv6 Extension [RFC4884], the translation of the ICMPv6
       packet will cause the ICMPv4 packet to change length.  When
       this occurs, the ICMPv6 Extension length attribute MUST be
       adjusted accordingly (e.g., shorter due to the translation from
       IPv6 to IPv4).  For extensions not defined in [RFC4884], the
       translator passes the extensions as opaque bit strings and any
       IPv6 address literals contained therein will not be translated
       to IPv4 address literals; this may cause problems with
       processing of those ICMP extensions.

5.3. Translating ICMPv6 Error Messages into ICMPv4

 There are some differences between the ICMPv4 and the ICMPv6 error
 message formats as detailed above.  The ICMP error messages
 containing the packet in error MUST be translated just like a normal
 IP packet.  The translation of this "packet in error" is likely to

Li, et al. Standards Track [Page 22] RFC 6145 IPv4/IPv6 Translation April 2011

 change the length of the datagram; thus, the Total Length field in
 the outer IPv4 header MUST be updated.
            +-------------+                 +-------------+
            |    IPv6     |                 |    IPv4     |
            |   Header    |                 |   Header    |
            +-------------+                 +-------------+
            |   ICMPv6    |                 |   ICMPv4    |
            |   Header    |                 |   Header    |
            +-------------+                 +-------------+
            |    IPv6     |      ===>       |    IPv4     |
            |   Header    |                 |   Header    |
            +-------------+                 +-------------+
            |   Partial   |                 |   Partial   |
            |  Transport- |                 |  Transport- |
            |   Layer     |                 |   Layer     |
            |   Header    |                 |   Header    |
            +-------------+                 +-------------+
             Figure 7: IPv6-to-IPv4 ICMP Error Translation
 The translation of the inner IP header can be done by invoking the
 function that translated the outer IP headers.  This process MUST
 stop at the first embedded header and drop the packet if it contains
 more embedded headers.  Note that the IPv6 addresses in the IPv6
 header may not be IPv4-translatable addresses, and there will be no
 corresponding IPv4 addresses.  In this case, the translator can do
 stateful translation.  A mechanism by which the translator can
 instead do stateless translation is left for future work.

5.4. Generation of ICMPv6 Error Messages

 If the IPv6 packet is discarded, then the translator SHOULD send back
 an ICMPv6 error message to the original sender of the packet, unless
 the discarded packet is itself an ICMPv6 message.
 If the ICMPv6 error message is being sent because the IPv6 source
 address is not an IPv4-translatable address and the translator is
 stateless, the ICMPv6 message (if sent) MUST have Type 1 and Code 5
 (Source address failed ingress/egress policy).  In other cases, the
 ICMPv6 message MUST have Type 1 (Destination Unreachable) and Code 1
 (Communication with destination administratively prohibited), unless
 otherwise specified in this document or [RFC6146].  The translator
 SHOULD allow an administrator to configure whether the ICMPv6 error
 messages are sent, rate-limited, or not sent.

Li, et al. Standards Track [Page 23] RFC 6145 IPv4/IPv6 Translation April 2011

5.5. Transport-Layer Header Translation

 If the address translation algorithm is not checksum neutral (see
 Section 4.1 of [RFC6052]), the recalculation and updating of the
 transport-layer headers that contain pseudo-headers need to be
 performed.  Translators MUST do this for TCP, UDP, and ICMP.
 Other transport protocols (e.g., DCCP) are OPTIONAL to support.  In
 order to ease debugging and troubleshooting, translators MUST forward
 all transport protocols as described in the "Protocol" step of
 Section 5.1.

5.6. Knowing When to Translate

 If the IP/ICMP translator also provides a normal forwarding function,
 and the destination address is reachable by a more specific route
 without translation, the router MUST forward it without translating
 it.  When an IP/ICMP translator receives an IPv6 datagram addressed
 to an IPv6 address representing a host in the IPv4 domain, the IPv6
 packet MUST be translated to IPv4.

6. Special Considerations for ICMPv6 Packet Too Big

 Two recent studies analyzed the behavior of IPv6-capable web servers
 on the Internet and found that approximately 95% responded as
 expected to an IPv6 Packet Too Big that indicated MTU = 1280, but
 only 43% responded as expected to an IPv6 Packet Too Big that
 indicated an MTU < 1280.  It is believed that firewalls violating
 Section 4.3.1 of [RFC4890] are at fault.  Both failures (the 5% wrong
 response when MTU = 1280 and the 57% wrong response when MTU < 1280)
 will cause PMTUD black holes [RFC2923].  Unfortunately, the
 translator cannot improve the failure rate of the first case (MTU =
 1280), but the translator can improve the failure rate of the second
 case (MTU < 1280).  There are two approaches to resolving the problem
 with sending ICMPv6 messages indicating an MTU < 1280.  It SHOULD be
 possible to configure a translator for either of the two approaches.
 The first approach is to constrain the deployment of the IPv6/IPv4
 translator by observing that four of the scenarios intended for
 stateless IPv6/IPv4 translators do not have IPv6 hosts on the
 Internet (Scenarios 1, 2, 5, and 6 described in [RFC6144], which
 refer to "An IPv6 network").  In these scenarios, IPv6 hosts, IPv6-
 host-based firewalls, and IPv6 network firewalls can be administered
 in compliance with Section 4.3.1 of [RFC4890] and therefore avoid the
 problem witnessed with IPv6 hosts on the Internet.

Li, et al. Standards Track [Page 24] RFC 6145 IPv4/IPv6 Translation April 2011

 The second approach is necessary if the translator has IPv6 hosts,
 IPv6-host-based firewalls, or IPv6 network firewalls that do not (or
 cannot) comply with Section 5 of [RFC2460] -- such as IPv6 hosts on
 the Internet.  This approach requires the translator to do the
 following:
 1.  In the IPv4-to-IPv6 direction: if the MTU value of ICMPv4 Packet
     Too Big (PTB) messages is less than 1280, change it to 1280.
     This is intended to cause the IPv6 host and IPv6 firewall to
     process the ICMP PTB message and generate subsequent packets to
     this destination with an IPv6 Fragment Header.
     Note: Based on recent studies, this is effective for 95% of IPv6
     hosts on the Internet.
 2.  In the IPv6-to-IPv4 direction:
     A.  If there is a Fragment Header in the IPv6 packet, the last 16
         bits of its value MUST be used for the IPv4 identification
         value.
     B.  If there is no Fragment Header in the IPv6 packet:
         a.  If the packet is less than or equal to 1280 bytes:
  1. The translator SHOULD set DF to 0 and generate an IPv4

identification value.

  1. To avoid the problems described in [RFC4963], it is

RECOMMENDED that the translator maintain 3-tuple state

                for generating the IPv4 identification value.
         b.  If the packet is greater than 1280 bytes, the translator
             SHOULD set the IPv4 DF bit to 1.

7. Security Considerations

 The use of stateless IP/ICMP translators does not introduce any new
 security issues beyond the security issues that are already present
 in the IPv4 and IPv6 protocols and in the routing protocols that are
 used to make the packets reach the translator.
 There are potential issues that might arise by deriving an IPv4
 address from an IPv6 address -- particularly addresses like broadcast
 or loopback addresses and the non-IPv4-translatable IPv6 addresses,
 etc.  [RFC6052] addresses these issues.

Li, et al. Standards Track [Page 25] RFC 6145 IPv4/IPv6 Translation April 2011

 As with network address translation of IPv4 to IPv4, the IPsec
 Authentication Header [RFC4302] cannot be used across an IPv6-to-IPv4
 translator.
 As with network address translation of IPv4 to IPv4, packets with
 tunnel mode Encapsulating Security Payload (ESP) can be translated
 since tunnel mode ESP does not depend on header fields prior to the
 ESP header.  Similarly, transport mode ESP will fail with IPv6-to-
 IPv4 translation unless checksum-neutral addresses are used.  In both
 cases, the IPsec ESP endpoints will normally detect the presence of
 the translator and encapsulate ESP in UDP packets [RFC3948].

8. Acknowledgements

 This is under development by a large group of people.  Those who have
 posted to the list during the discussion include Alexey Melnikov,
 Andrew Sullivan, Andrew Yourtchenko, Brian Carpenter, Dan Wing, Dave
 Thaler, David Harrington, Ed Jankiewicz, Hiroshi Miyata, Iljitsch van
 Beijnum, Jari Arkko, Jerry Huang, John Schnizlein, Jouni Korhonen,
 Kentaro Ebisawa, Kevin Yin, Magnus Westerlund, Marcelo Bagnulo Braun,
 Margaret Wasserman, Masahito Endo, Phil Roberts, Philip Matthews,
 Reinaldo Penno, Remi Denis-Courmont, Remi Despres, Sean Turner,
 Senthil Sivakumar, Simon Perreault, Stewart Bryant, Tim Polk, Tero
 Kivinen, and Zen Cao.

9. References

9.1. Normative References

 [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
            August 1980.
 [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
            September 1981.
 [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
            RFC 792, September 1981.
 [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
            RFC 793, September 1981.
 [RFC1812]  Baker, F., "Requirements for IP Version 4 Routers",
            RFC 1812, June 1995.
 [RFC1883]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
            (IPv6) Specification", RFC 1883, December 1995.

Li, et al. Standards Track [Page 26] RFC 6145 IPv4/IPv6 Translation April 2011

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
            (IPv6) Specification", RFC 2460, December 1998.
 [RFC2765]  Nordmark, E., "Stateless IP/ICMP Translation Algorithm
            (SIIT)", RFC 2765, February 2000.
 [RFC3948]  Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
            Stenberg, "UDP Encapsulation of IPsec ESP Packets",
            RFC 3948, January 2005.
 [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
            Architecture", RFC 4291, February 2006.
 [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
            Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
 [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
            Message Protocol (ICMPv6) for the Internet Protocol
            Version 6 (IPv6) Specification", RFC 4443, March 2006.
 [RFC4884]  Bonica, R., Gan, D., Tappan, D., and C. Pignataro,
            "Extended ICMP to Support Multi-Part Messages", RFC 4884,
            April 2007.
 [RFC5382]  Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
            Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
            RFC 5382, October 2008.
 [RFC5771]  Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
            IPv4 Multicast Address Assignments", BCP 51, RFC 5771,
            March 2010.
 [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
            Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
            October 2010.
 [RFC6146]  Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful
            NAT64: Network Address and Protocol Translation from IPv6
            Clients to IPv4 Servers", RFC 6146, April 2011.

Li, et al. Standards Track [Page 27] RFC 6145 IPv4/IPv6 Translation April 2011

9.2. Informative References

 [RFC0879]  Postel, J., "TCP maximum segment size and related topics",
            RFC 879, November 1983.
 [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
            November 1990.
 [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.
 [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
            and W. Weiss, "An Architecture for Differentiated
            Services", RFC 2475, December 1998.
 [RFC2710]  Deering, S., Fenner, W., and B. Haberman, "Multicast
            Listener Discovery (MLD) for IPv6", RFC 2710,
            October 1999.
 [RFC2766]  Tsirtsis, G. and P. Srisuresh, "Network Address
            Translation - Protocol Translation (NAT-PT)", RFC 2766,
            February 2000.
 [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery",
            RFC 2923, September 2000.
 [RFC3307]  Haberman, B., "Allocation Guidelines for IPv6 Multicast
            Addresses", RFC 3307, August 2002.
 [RFC3590]  Haberman, B., "Source Address Selection for the Multicast
            Listener Discovery (MLD) Protocol", RFC 3590,
            September 2003.
 [RFC3810]  Vida, R. and L. Costa, "Multicast Listener Discovery
            Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
 [RFC3849]  Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix
            Reserved for Documentation", RFC 3849, July 2004.
 [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
            December 2005.
 [RFC4890]  Davies, E. and J. Mohacsi, "Recommendations for Filtering
            ICMPv6 Messages in Firewalls", RFC 4890, May 2007.

Li, et al. Standards Track [Page 28] RFC 6145 IPv4/IPv6 Translation April 2011

 [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
            Errors at High Data Rates", RFC 4963, July 2007.
 [RFC4966]  Aoun, C. and E. Davies, "Reasons to Move the Network
            Address Translator - Protocol Translator (NAT-PT) to
            Historic Status", RFC 4966, July 2007.
 [RFC5737]  Arkko, J., Cotton, M., and L. Vegoda, "IPv4 Address Blocks
            Reserved for Documentation", RFC 5737, January 2010.
 [RFC6144]  Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
            IPv4/IPv6 Translation", RFC 6144, April 2011.

Li, et al. Standards Track [Page 29] RFC 6145 IPv4/IPv6 Translation April 2011

Appendix A. Stateless Translation Workflow Example

 A stateless translation workflow example is depicted in the following
 figure.  The documentation address blocks 2001:db8::/32 [RFC3849],
 192.0.2.0/24, and 198.51.100.0/24 [RFC5737] are used in this example.
          +--------------+                   +--------------+
          | IPv4 network |                   | IPv6 network |
          |              |     +-------+     |              |
          |   +----+     |-----| XLAT  |---- |  +----+      |
          |   | H4 |-----|     +-------+     |--| H6 |      |
          |   +----+     |                   |  +----+      |
          +--------------+                   +--------------+
                               Figure 8
 A translator (XLAT) connects the IPv6 network to the IPv4 network.
 This XLAT uses the Network-Specific Prefix (NSP) 2001:db8:100::/40
 defined in [RFC6052] to represent IPv4 addresses in the IPv6 address
 space (IPv4-converted addresses) and to represent IPv6 addresses
 (IPv4-translatable addresses) in the IPv4 address space.  In this
 example, 192.0.2.0/24 is the IPv4 block of the corresponding IPv4-
 translatable addresses.
 Based on the address mapping rule, the IPv6 node H6 has an IPv4-
 translatable IPv6 address 2001:db8:1c0:2:21:: (address mapping from
 192.0.2.33).  The IPv4 node H4 has IPv4 address 198.51.100.2.
 The IPv6 routing is configured in such a way that the IPv6 packets
 addressed to a destination address in 2001:db8:100::/40 are routed to
 the IPv6 interface of the XLAT.
 The IPv4 routing is configured in such a way that the IPv4 packets
 addressed to a destination address in 192.0.2.0/24 are routed to the
 IPv4 interface of the XLAT.

A.1. H6 Establishes Communication with H4

 The steps by which H6 establishes communication with H4 are:
 1.  H6 performs the destination address mapping, so the IPv4-
     converted address 2001:db8:1c6:3364:2:: is formed from
     198.51.100.2 based on the address mapping algorithm [RFC6052].
 2.  H6 sends a packet to H4.  The packet is sent from a source
     address 2001:db8:1c0:2:21:: to a destination address
     2001:db8:1c6:3364:2::.

Li, et al. Standards Track [Page 30] RFC 6145 IPv4/IPv6 Translation April 2011

 3.  The packet is routed to the IPv6 interface of the XLAT (since
     IPv6 routing is configured that way).
 4.  The XLAT receives the packet and performs the following actions:
  • The XLAT translates the IPv6 header into an IPv4 header using

the IP/ICMP Translation Algorithm defined in this document.

  • The XLAT includes 192.0.2.33 as the source address in the

packet and 198.51.100.2 as the destination address in the

        packet.  Note that 192.0.2.33 and 198.51.100.2 are extracted
        directly from the source IPv6 address 2001:db8:1c0:2:21::
        (IPv4-translatable address) and destination IPv6 address
        2001:db8:1c6:3364:2:: (IPv4-converted address) of the received
        IPv6 packet that is being translated.
 5.  The XLAT sends the translated packet out of its IPv4 interface,
     and the packet arrives at H4.
 6.  H4 node responds by sending a packet with destination address
     192.0.2.33 and source address 198.51.100.2.
 7.  The packet is routed to the IPv4 interface of the XLAT (since
     IPv4 routing is configured that way).  The XLAT performs the
     following operations:
  • The XLAT translates the IPv4 header into an IPv6 header using

the IP/ICMP Translation Algorithm defined in this document.

  • The XLAT includes 2001:db8:1c0:2:21:: as the destination

address in the packet and 2001:db8:1c6:3364:2:: as the source

        address in the packet.  Note that 2001:db8:1c0:2:21:: and
        2001:db8:1c6:3364:2:: are formed directly from the destination
        IPv4 address 192.0.2.33 and the source IPv4 address
        198.51.100.2 of the received IPv4 packet that is being
        translated.
 8.  The translated packet is sent out of the IPv6 interface to H6.
 The packet exchange between H6 and H4 continues until the session is
 finished.

Li, et al. Standards Track [Page 31] RFC 6145 IPv4/IPv6 Translation April 2011

A.2. H4 Establishes Communication with H6

 The steps by which H4 establishes communication with H6 are:
 1.  H4 performs the destination address mapping, so 192.0.2.33 is
     formed from the IPv4-translatable address 2001:db8:1c0:2:21::
     based on the address mapping algorithm [RFC6052].
 2.  H4 sends a packet to H6.  The packet is sent from a source
     address 198.51.100.2 to a destination address 192.0.2.33.
 3.  The packet is routed to the IPv4 interface of the XLAT (since
     IPv4 routing is configured that way).
 4.  The XLAT receives the packet and performs the following actions:
  • The XLAT translates the IPv4 header into an IPv6 header using

the IP/ICMP Translation Algorithm defined in this document.

  • The XLAT includes 2001:db8:1c6:3364:2:: as the source address

in the packet and 2001:db8:1c0:2:21:: as the destination

        address in the packet.  Note that 2001:db8:1c6:3364:2::
        (IPv4-converted address) and 2001:db8:1c0:2:21::
        (IPv4-translatable address) are obtained directly from the
        source IPv4 address 198.51.100.2 and destination IPv4 address
        192.0.2.33 of the received IPv4 packet that is being
        translated.
 5.  The XLAT sends the translated packet out its IPv6 interface, and
     the packet arrives at H6.
 6.  H6 node responds by sending a packet with destination address
     2001:db8:1c6:3364:2:: and source address 2001:db8:1c0:2:21::.
 7.  The packet is routed to the IPv6 interface of the XLAT (since
     IPv6 routing is configured that way).  The XLAT performs the
     following operations:
  • The XLAT translates the IPv6 header into an IPv4 header using

the IP/ICMP Translation Algorithm defined in this document.

  • The XLAT includes 198.51.100.2 as the destination address in

the packet and 192.0.2.33 as the source address in the packet.

        Note that 198.51.100.2 and 192.0.2.33 are formed directly from
        the destination IPv6 address 2001:db8:1c6:3364:2:: and source
        IPv6 address 2001:db8:1c0:2:21:: of the received IPv6 packet
        that is being translated.

Li, et al. Standards Track [Page 32] RFC 6145 IPv4/IPv6 Translation April 2011

 8.  The translated packet is sent out the IPv4 interface to H4.
 The packet exchange between H4 and H6 continues until the session is
 finished.

Authors' Addresses

 Xing Li
 CERNET Center/Tsinghua University
 Room 225, Main Building, Tsinghua University
 Beijing,   100084
 China
 Phone: +86 10-62785983
 EMail: xing@cernet.edu.cn
 Congxiao Bao
 CERNET Center/Tsinghua University
 Room 225, Main Building, Tsinghua University
 Beijing,   100084
 China
 Phone: +86 10-62785983
 EMail: congxiao@cernet.edu.cn
 Fred Baker
 Cisco Systems
 Santa Barbara, California  93117
 USA
 Phone: +1-408-526-4257
 EMail: fred@cisco.com

Li, et al. Standards Track [Page 33]

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