Skip to main content

Updated Specification of the IPv4 ID Field

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 6864.
Author Dr. Joseph D. Touch
Last updated 2012-05-07 (Latest revision 2011-09-16)
Replaces draft-touch-intarea-ipv4-unique-id
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state WG Document
Awaiting Expert Review/Resolution of Issues Raised, Revised I-D Needed - Issue raised by WGLC
Document shepherd Julien Laganier
Shepherd write-up Show Last changed 2012-05-03
IESG IESG state Became RFC 6864 (Proposed Standard)
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Brian Haberman
IESG note
Send notices to,
Internet Area WG                                               J. Touch
Internet Draft                                                  USC/ISI
Updates: 791,1122,2003                               September 16, 2011
Intended status: Proposed Standard
Expires: March 2012

                Updated Specification of the IPv4 ID Field

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on March 16, 2012.

Touch                   Expires March 16, 2012                 [Page 1]
Internet-Draft    Updated Spec. of the IPv4 ID Field     September 2011

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
   ( 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.


   The IPv4 Identification (ID) field enables fragmentation and
   reassembly, and as currently specified is required to be unique
   within the maximum lifetime for all datagrams with a given
   source/destination/protocol tuple. If enforced, this uniqueness
   requirement would limit all connections to 6.4 Mbps. Because
   individual connections commonly exceed this speed, it is clear that
   existing systems violate the current specification. This document
   updates the specification of the IPv4 ID field in RFC791, RFC1122,
   and RFC2003 to more closely reflect current practice and to more
   closely match IPv6 so that the field's value is defined only when a
   datagram is actually fragmented. It also discusses the impact of
   these changes on how datagrams are used.

Table of Contents

   1. Introduction...................................................3
   2. Conventions used in this document..............................3
   3. The IPv4 ID Field..............................................3
   4. Uses of the IPv4 ID Field......................................4
   5. Background on IPv4 ID Reassembly Issues........................5
   6. Updates to the IPv4 ID Specification...........................6
      6.1. IPv4 ID Used Only for Fragmentation.......................7
      6.2. Encourage Safe IPv4 ID Use................................8
      6.3. IPv4 ID Requirements That Persist.........................8
   7. Impact on Datagram Use.........................................9
   8. Updates to Existing Standards..................................9
      8.1. Updates to RFC 791.......................................10
      8.2. Updates to RFC 1122......................................10
      8.3. Updates to RFC 2003......................................11

Touch                   Expires March 16, 2012                 [Page 2]
Internet-Draft    Updated Spec. of the IPv4 ID Field     September 2011

   9. Impact on NATs/ASMs, Rewriting Devices, and Tunnel Ingresses..11
   10. Impact on Header Compression.................................12
   11. Security Considerations......................................13
   12. IANA Considerations..........................................13
   13. References...................................................13
      13.1. Normative References....................................13
      13.2. Informative References..................................14
   14. Acknowledgments..............................................15

1. Introduction

   In IPv4, the Identification (ID) field is a 16-bit value that is
   unique for every datagram for a given source address, destination
   address, and protocol, such that it does not repeat within the
   Maximum Segment Lifetime (MSL) [RFC791][RFC1122]. As currently
   specified, all datagrams between a source and destination of a given
   protocol must have unique IPv4 ID values over a period of this MSL,
   which is typically interpreted as two minutes (120 seconds). This
   uniqueness is currently specified as for all datagrams, regardless of
   fragmentation settings.

   Uniqueness of the IPv4 ID is commonly violated by high speed devices;
   if strictly enforced, it would limit the speed of a single protocol
   between two IP endpoints to 6.4 Mbps for typical MTUs of 1500 bytes
   [RFC4963]. It is common for a single connection to operate far in
   excess of these rates, which strongly indicates that the uniqueness
   of the IPv4 ID as specified is already moot.

   This document updates the specification of the IPv4 ID field to more
   closely reflect current practice, and to include considerations taken
   into account during the specification of the similar field in IPv6.

2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC-2119 [RFC2119].

   In this document, the characters ">>" proceeding an indented line(s)
   indicates a requirement using the key words listed above. This
   convention aids reviewers in quickly identifying or finding this
   document's explicit requirements.

3. The IPv4 ID Field

   IP supports datagram fragmentation, where large datagrams are split
   into smaller components to traverse links with limited maximum

Touch                   Expires March 16, 2012                 [Page 3]
Internet-Draft    Updated Spec. of the IPv4 ID Field     September 2011

   transmission units (MTUs). Fragments are indicated in different ways
   in IPv4 and IPv6:

   o  In IPv4, fragments are indicated using four fields of the basic
      header: Identification (ID), Fragment Offset, a "Don't Fragment"
      flag (DF), and a "More Fragments" flag (MF) [RFC791]

   o  In IPv6, fragments are indicated in an extension header that
      includes an ID, Fragment Offset, and M (more fragments) flag
      similar to their counterparts in IPv4 [RFC2460]

   IPv4 and IPv6 fragmentation differs in a few important ways. IPv6
   fragmentation occurs only at the source, so a DF bit is not needed to
   prevent downstream devices from initiating fragmentation (i.e., IPv6
   always acts as if DF=1). The IPv6 fragment header is present only
   when a datagram has been fragmented, so the ID field is not present
   for non-fragmented datagrams, and thus is meaningful only for
   fragments. Finally, the IPv6 ID field is 32 bits, and required unique
   per source/destination address pair for IPv6, whereas for IPv4 it is
   only 16 bits and required unique per source/destination/protocol

   This document focuses on the IPv4 ID field issues, because in IPv6
   the field is larger and present only in fragments.

4. Uses of the IPv4 ID Field

   The IPv4 ID field was originally intended for fragmentation and
   reassembly [RFC791]. Within a given source address, destination
   address, and protocol, fragments of an original datagram are matched
   based on their IPv4 ID. This requires that IDs are unique within the
   address/protocol triple when fragmentation is possible (e.g., DF=0)
   or when it has already occurred (e.g., frag_offset>0 or MF=1).

   The IPv4 ID field can be useful for other purposes. The field has
   been proposed as a way to detect and remove duplicate datagrams,
   e.g., at congested routers (noted in Sec. of [RFC1122],
   proposed experimentally in Simplified Multicast Forwarding [Ma11]).
   It can similarly be used at end hosts to reduce the impact of
   duplication on higher-layer protocols (e.g., additional processing in
   TCP, or the need for application-layer duplicate suppression in UDP).

   The IPv4 ID field is also used in some debugging tools to correlate
   datagrams measured at various locations along a network path. This is
   already insufficient in IPv6 because unfragmented datagrams lack an
   ID, so these tools are already being updated to avoid such reliance
   on the ID field.

Touch                   Expires March 16, 2012                 [Page 4]
Internet-Draft    Updated Spec. of the IPv4 ID Field     September 2011

   The ID clearly needs to be unique (within MSL, within the
   src/dst/protocol tuple) to support fragmentation and reassembly, but
   not all packets are fragmented or allow fragmentation. This document
   deprecates non-fragementation uses, allowing the ID to be repeated
   (within MSL, within the src/dst/protocol tuple) in those cases.

5. Background on IPv4 ID Reassembly Issues

   The following is a summary of issues with IPv4 fragment reassembly in
   high speed environments raised previously [RFC4963]. Readers are
   encouraged to consult RFC 4963 for a more detailed discussion of
   these issues.

   With the maximum IPv4 datagram size of 64KB, a 16-bit ID field that
   does not repeat within 120 seconds means that the aggregate of all
   TCP connections of a given protocol between two IP endpoints is
   limited to roughly 286 Mbps; at a more typical MTU of 1500 bytes,
   this speed drops to 6.4 Mbps [RFC4963]. This limit currently applies
   for all IPv4 datagrams within a single protocol (i.e., the IPv4
   protocol field) between two IP addresses, regardless of whether
   fragmentation is enabled or inhibited, and whether a datagram is
   fragmented or not.

   IPv6, even at typical MTUs, is capable of 18.7 Tbps with
   fragmentation between two IP endpoints as an aggregate across all
   protocols, due to the larger 32-bit ID field (and the fact that the
   IPv6 next-header field, the equivalent of the IPv4 protocol field, is
   not considered in differentiating fragments). When fragmentation is
   not used the field is absent, and in that case IPv6 speeds are not
   limited by the ID field uniqueness.

   Note also that 120 seconds is only an estimate on the maximum
   datagram lifetime. It is loosely based on half maximum value of the
   IP TTL field (255), measured in seconds, because the TTL is
   decremented not only for each hop, but also for each second a
   datagram is held at a router (as implied in [RFC791]). Network delays
   are incurred in other ways, e.g., satellite links, which can add
   seconds of delay even though the TTL is often not decremented by a
   corresponding amount. There is thus no enforcement mechanism to
   ensure that datagrams older than 120 seconds are discarded.

   Wireless Internet devices are frequently connected at speeds over 54
   Mbps, and wired links of 1 Gbps have been the default for several
   years. Although many end-to-end transport paths are congestion
   limited, these devices easily achieve 100+ Mbps application-layer
   throughput over LANs (e.g., disk-to-disk file transfer rates), and
   numerous throughput demonstrations have been performed with COTS

Touch                   Expires March 16, 2012                 [Page 5]
Internet-Draft    Updated Spec. of the IPv4 ID Field     September 2011

   systems over wide-area paths at these speeds for over a decade. This
   strongly suggests that IPv4 ID uniqueness has been moot for a long

6. Updates to the IPv4 ID Specification

   This document updates the specification of the IPv4 ID field in three
   distinct ways, as discussed in subsequent subsections:

   o  Use the IPv4 ID field only for fragmentation

   o  Avoiding a performance impact when the IPv4 ID field is used

   o  Encourage safe operation when the IPv4 ID field is used

   There are two kinds of datagrams used in the following discussion,
   named as follows:

   o  Atomic datagrams: datagrams not yet fragmented (MF=0 and fragment
      offset=0) and for which further fragmentation has been inhibited
      (DF=1), i.e., as a mathematical expression (equals is ==, logical
      'and' is &&, logical 'or' is ||, greater than is >, logical 'not'
      is ~, and parenthesis function typically):


   o  Non-atomic datagrams: datagrams which have either already been
      fragmented, i.e.:


      or for which fragmentation remains possible:


      I.e., non-atomic datagrams can be expressed in two equivalent


      which can also be expressed as follows, using DeMorgan's Law and
      other identities:


      Note that this final expression is the same as "not(atomic)".

Touch                   Expires March 16, 2012                 [Page 6]
Internet-Draft    Updated Spec. of the IPv4 ID Field     September 2011

6.1. IPv4 ID Used Only for Fragmentation

   Although RFC1122 suggests the IPv4 ID field has other uses, and it is
   currently being considered for the experimental Simplfied Mulitcast
   Forwarding (SMF) protocol [Ma11], this document asserts that this
   field's value is defined only for fragmentation and reassembly:

   o  >> IPv4 ID field MUST NOT be used for purposes other than
      fragmentation and reassembly.

   SMF includes non-ID hash-based duplicate packet detection for cases
   where the ID field is absent (IPv6), and already defines these for
   IPv4, where it should be preferred to ID-based duplicate detection.

   In atomic datagrams, the IPv4 ID field has no meaning, and thus can
   be set to an arbitrary value, i.e., the requirement for non-repeating
   IDs within the address/protocol triple is no longer required for
   atomic datagrams:

   o  >> Originating sources MAY set the IPv4 ID field of atomic
      datagrams to any value.

   Second, all network nodes, whether at intermediate routers,
   destination hosts, or other devices (e.g., NATs and other address
   sharing mechanisms, firewalls, tunnel egresses), cannot rely on the

   o  >> All devices that examine IPv4 headers MUST ignore the IPv4 ID
      field of atomic datagrams.

   The IPv4 ID field is thus meaningful only for non-atomic datagrams -
   datagrams that have either already been fragmented, or those for
   which fragmentation remains permitted. Atomic datagrams are detected
   by their DF, MF, and fragmentation offset fields as explained in
   Section 6, because such a test is completely backward compatible;
   this document thus does not reserve any IPv4 ID values, including 0,
   as distinguished.

   Deprecating the use of the IPv4 ID field for non-reassembly uses
   should have little - if any - impact. IPv4 IDs are already frequently
   repeated, e.g., over even moderately fast connections. Duplicate
   suppression was only suggested [RFC1122], and no impacts of IPv4 ID
   reuse have been noted. Routers are not required to issue ICMPs on any
   particular timescale, and so IPv4 ID repetition should not have been
   used for validation, and again repetition occurs and probably could
   have been noticed [RFC1812]. ICMP relaying at tunnel ingresses is

Touch                   Expires March 16, 2012                 [Page 7]
Internet-Draft    Updated Spec. of the IPv4 ID Field     September 2011

   specified to use soft state rather than a datagram cache, and should
   have been noted if the latter for similar reasons [RFC2003].

6.2. Encourage Safe IPv4 ID Use

   This document makes further changes to the specification of the IPv4
   ID field and its use to encourage its safe use as corollary
   requirements changes as follows.

   RFC 1122 discusses that TCP retransmits a segment it may be possible
   to reuse the IPv4 ID (see Section 8.2). This can make it difficult
   for a source to avoid IPv4 ID repetition for received fragments. RFC
   1122 concludes that this behavior "is not useful"; this document
   formalizes that conclusion as follows:

   o  >> The IPv4 ID of non-atomic datagrams MUST NOT be reused when
      sending a copy of an earlier non-atomic datagram.

   RFC 1122 also suggests that fragments can overlap [RFC1122]. Such
   overlap can occur if successive retransmissions are fragmented in
   different ways but the same reassembly IPv4 ID. This overlap is noted
   as the result of reusing IPv4 IDs when retransmitting datagrams,
   which this document deprecates. However, it is also the result of in-
   network packet duplication, which can still occur. As a result this
   document does not change the need to support overlapping fragments.

6.3. IPv4 ID Requirements That Persist

   This document does not relax the IPv4 ID field uniqueness
   requirements of [RFC791] for non-atomic datagrams, i.e.:

   o  >> Sources emitting non-atomic datagrams MUST NOT repeat IPv4 ID
      values within one MSL for a given source address/destination
      address/protocol triple.

   Such sources include originating hosts, tunnel ingresses, and NATs
   (including other address sharing mechanisms) (see Section 9).

   This document does not relax the requirement that all network devices
   honor the DF bit, i.e.:

   o  >> IPv4 datagrams whose DF=1 MUST NOT be fragmented.

   o  >> IPv4 datagram transit devices MUST NOT clear the DF bit.

   In specific, DF=1 prevents fragmenting atomic datagrams. DF=1 also
   prevents further fragmenting received fragments. In-network

Touch                   Expires March 16, 2012                 [Page 8]
Internet-Draft    Updated Spec. of the IPv4 ID Field     September 2011

   fragmentation is permitted only when DF=0; this document does not
   change that requirement.

7. Impact on Datagram Use

   The following is a summary of the recommendations that are the result
   of the previous changes to the IPv4 ID field specification.

   Because atomic datagrams can use arbitrary IPv4 ID values, the ID
   field no longer imposes a performance impact in those cases. However,
   the performance impact remains for non-atomic datagrams. As a result:

   o  >> Sources of non-atomic IPv4 datagrams MUST rate-limit their
      output to comply with the ID uniqueness requirements.

   Such sources include, in particular, DNS over UDP [RFC2671].

   Because there is no strict definition of the MSL, reassembly hazards
   exist regardless of the IPv4 ID reuse interval or the reassembly
   timeout. As a result:

   o  >> Higher layer protocols SHOULD verify the integrity of IPv4
      datagrams, e.g., using a checksum or hash that can detect
      reassembly errors (the UDP checksum is weak in this regard, but
      better than nothing), as in SEAL [RFC5320].

   Additional integrity checks can be employed using tunnels, as in
   SEAL, IPsec, or SCTP [RFC4301][RFC4960][RFC5320]. Such checks can
   avoid the reassembly hazards that can occur when using UDP and TCP
   checksums [RFC4963], or when using partial checksums as in UDP-Lite
   [RFC3828]. Because such integrity checks can avoid the impact of
   reassembly errors:

   o  >> Sources of non-atomic IPv4 datagrams using strong integrity
      checks MAY reuse the ID within MSL values smaller than is typical.

   Note, however, that such more frequent reuse can still result in
   corrupted reassembly and poor throughput, although it would not
   propagate reassembly errors to higher layer protocols.

8. Updates to Existing Standards

   The following sections address the specific changes to existing
   protocols indicated by this document.

Touch                   Expires March 16, 2012                 [Page 9]
Internet-Draft    Updated Spec. of the IPv4 ID Field     September 2011

8.1. Updates to RFC 791

   RFC 791 states that:

      The originating protocol module of an internet datagram sets the
      identification field to a value that must be unique for that
      source-destination pair and protocol for the time the datagram
      will be active in the internet system.

   And later that:

      Thus, the sender must choose the Identifier to be unique for this
      source, destination pair and protocol for the time the datagram
      (or any fragment of it) could be alive in the internet.

      It seems then that a sending protocol module needs to keep a table
      of Identifiers, one entry for each destination it has communicated
      with in the last maximum datagram lifetime for the internet.

      However, since the Identifier field allows 65,536 different
      values, some host may be able to simply use unique identifiers
      independent of destination.

      It is appropriate for some higher level protocols to choose the
      identifier. For example, TCP protocol modules may retransmit an
      identical TCP segment, and the probability for correct reception
      would be enhanced if the retransmission carried the same
      identifier as the original transmission since fragments of either
      datagram could be used to construct a correct TCP segment.

   This document changes RFC 791 as follows:

   o  IPv4 ID uniqueness applies to only non-atomic datagrams.

   o  Retransmitted non-atomic IPv4 datagrams are no longer permitted to
      reuse the ID value.

8.2. Updates to RFC 1122

   RFC 1122 states that:  Identification: RFC-791 Section 3.2

            When sending an identical copy of an earlier datagram, a
            host MAY optionally retain the same Identification field in
            the copy.

Touch                   Expires March 16, 2012                [Page 10]
Internet-Draft    Updated Spec. of the IPv4 ID Field     September 2011


            Some Internet protocol experts have maintained that when a
            host sends an identical copy of an earlier datagram, the new
            copy should contain the same Identification value as the
            original.  There are two suggested advantages:  (1) if the
            datagrams are fragmented and some of the fragments are lost,
            the receiver may be able to reconstruct a complete datagram
            from fragments of the original and the copies; (2) a
            congested gateway might use the IP Identification field (and
            Fragment Offset) to discard duplicate datagrams from the

   This document changes RFC 1122 as follows:

   o  The IPv4 ID field is no longer permitted to be used for duplicate
      detection. This applies both atomic and non-atomic datagrams.

   o  Retransmitted non-atomic IPv4 datagrams are no longer permitted to
      reuse the ID value.

8.3. Updates to RFC 2003

   This document updates how IPv4-in-IPv4 tunnels create IPv4 ID values
   for the IPv4 outer header [RFC2003], but only in the same way as for
   any other IPv4 datagram source.

9. Impact on NATs/ASMs, Rewriting Devices, and Tunnel Ingresses

   Network address translators (NATs) and address/port translators
   (NAPTs) rewrite IP fields, and tunnel ingresses (using IPv4
   encapsulation) copy and modify some IPv4 fields, so all are
   considered sources, as do any devices that rewrite any portion of the
   source address, destination address, protocol, and ID tuple for any
   datagrams [RFC3022]. This is also true for other address sharing
   mechanisms (ASMs), including to include 4rd, IVI, and others in the
   "A+P" (address plus port) family [Bo11] [De11] [RFC6219]. It is
   equally true for any other packet rewriting mechanism. As a result,
   they are subject to all the requirements of any source, as has been

   NATs/ASMs/rewriters present a particularly challenging situation for
   fragmentation. Because they overwrite portions of the reassembly
   tuple in both directions, they can destroy tuple uniqueness and
   result in a reassembly hazard. Whenever IPv4 source address,
   destination address, or protocol fields are modified, a
   NAT/ASM/rewriter needs to ensure that the ID field is generated

Touch                   Expires March 16, 2012                [Page 11]
Internet-Draft    Updated Spec. of the IPv4 ID Field     September 2011

   appropriately, rather than simply copied from the incoming datagram.
   In specific:

   o  >> Address sharing or rewriting devices MUST ensure that the IPv4
      ID field of datagrams whose address or protocol are translated
      comply with requirements as if the datagram were sourced by that

   This compliance means that the IPv4 ID field of non-atomic datagrams
   translated at a NAT/ASM/rewriter needs to obey the uniqueness
   requirements of any IPv4 datagram source. Unfortunately, fragments
   already violate that requirement, as they repeat an IPv4 ID within
   the MSL for a given source address, destination address, and protocol

   Such problems with transmitting fragments through NATs/ASMs/rewriters
   are already known; translation is based on the transport port number,
   which is present in only the first fragment anyway [RFC3022]. This
   document underscores the point that not only is reassembly (and
   possibly subsequent fragmentation) required for translation, it can
   be used to avoid issues with IPv4 ID uniqueness.

   Note that NATs/ASMs already need to exercise special care when
   emitting datagrams on their public side, because merging datagrams
   from many sources onto a single outgoing source address can result in
   IPv4 ID collisions. This situation precedes this document, and is not
   affected by it. It is exacerbated in large-scale, so-called "carrier
   grade" NATs [Pe11].

   Tunnel ingresses act as sources for the outermost header, but tunnels
   act as routers for the inner headers (i.e., the datagram as arriving
   at the tunnel ingress). Ingresses can always fragment as originating
   sources of the outer header, because they control the uniqueness of
   that IPv4 ID field and the value of DF on the outer header
   independent of those values on the inner (arriving datagram) header.

10. Impact on Header Compression

   Header compression algorithms already accommodate various ways in
   which the IPv4 ID changes between sequential datagrams [RFC1144]
   [RFC2508] [RFC3545] [RFC5225]. Such algorithms currently assume that
   the IPv4 ID is preserved end-to-end. Some algorithms already allow
   assuming the ID does not change (e.g., ROHC [RFC5225]), where others
   include nonchanging IDs via zero deltas (e.g., ECRTP [RFC3545]).

   When compression assumes a changing ID as a default, having a non-
   changing ID can make compression less efficient (see footnote 21 of

Touch                   Expires March 16, 2012                [Page 12]
Internet-Draft    Updated Spec. of the IPv4 ID Field     September 2011

   [RFC1144] or cRTP [RFC2508]). When compression can assume a
   nonchanging IPv4 ID - as with ROHC and ECRTP - efficiency can be

11. Security Considerations

   When the IPv4 ID is ignored on receipt (e.g., for atomic datagrams),
   its value becomes unconstrained; that field then can more easily be
   used as a covert channel. For some atomic datagrams - notably those
   not protected by IPsec Authentication Header (AH) [RFC4302] - it is
   now possible, and may be desirable, to rewrite the IPv4 ID field to
   avoid its use as such a channel.

   The IPv4 ID also now adds much less entropy of the header of a
   datagram. The IPv4 ID had previously been unique (for a given
   source/address pair, and protocol field) within one MSL, although
   this requirement was not enforced and clearly is typically ignored.
   The IPv4 ID of atomic datagrams is not required unique, and so
   contributes no entropy to the header.

   The deprecation of the IPv4 ID field's uniqueness for atomic
   datagrams can defeat the ability to count devices behind a
   NAT/ASM/rewriter [Be02]. This is not intended as a security feature,

12. IANA Considerations

   There are no IANA considerations in this document.

   The RFC Editor should remove this section prior to publication

13. References

13.1. Normative References

   [RFC791]  Postel, J., "Internet Protocol", RFC 791 / STD 5, September

   [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
             Communication Layers", RFC 1122 / STD 3, October 1989.

   [RFC1812] Baker, F. (Ed.), "Requirements for IP Version 4 Routers",
             RFC 1812 / STD 4, Jun. 1995.

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

Touch                   Expires March 16, 2012                [Page 13]
Internet-Draft    Updated Spec. of the IPv4 ID Field     September 2011

   [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
             October 1996.

13.2. Informative References

   [Be02]    Bellovin, S., "A Technique for Counting NATted Hosts",
             Internet Measurement Conference, Proceedings of the 2nd ACM
             SIGCOMM Workshop on Internet Measurement, November 2002.

   [Bo11]    Boucadair, M., J. Touch, P. Levis, R. Penno, "Analysis of
             Solution Candidates to Reveal a Host Identifier in Shared
             Address Deployments", (work in progress), draft-boucadair-
             intarea-nat-reveal-analysis, Sept. 2011.

   [De11]    Despres, R. (Ed.), S. Matsushima, T. Murakami, O. Troan,
             "IPv4 Residual Deployment across IPv6-Service networks
             (4rd)", (work in progress), draft-despres-intarea-4rd,
             March 2011.

   [Ma11]    Macker, J. (Ed.), "Simplified Multicast Forwarding," (work
             in progress), draft-ietf-manet-smf-12, Jul. 2011.

   [Pe11]    Perreault, S., (Ed.), I. Yamagata, S. Miyakawa, A.
             Nakagawa, H. Ashida, "Common requirements of IP address
             sharing schemes", (work in progress), draft-ietf-behave-
             lsn-requirements, March 2011.

   [RFC1144] Jacobson, V., "Compressing TCP/IP Headers", RFC 1144, Feb.

   [RFC2460] Deering, S., R. Hinden, "Internet Protocol, Version 6
             (IPv6) Specification", RFC 2460, December 1998.

   [RFC2508] Casner, S., V. Jacobson. "Compressing IP/UDP/RTP Headers
             for Low-Speed Serial Links", RFC 2508, Feb. 1999.

   [RFC2671] Vixie,P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671,
             August 1999.

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

   [RFC3545] Koren, T., S. Casner, J. Geevarghese, B. Thompson, P.
             Ruddy, "Enhanced Compressed RTP (CRTP) for Links with High
             Delay, Packet Loss and Reordering", RFC 3545, July 2003.

Touch                   Expires March 16, 2012                [Page 14]
Internet-Draft    Updated Spec. of the IPv4 ID Field     September 2011

   [RFC3828] Larzon, L-A., M. Degermark, S. Pink, L-E. Jonsson, Ed., G.
             Fairhurst, Ed., "The Lightweight User Datagram Protocol
             (UDP-Lite)", RFC 3828, July 2004.

   [RFC4301] Kent, S., K. Seo, "Security Architecture for the Internet
             Protocol", RFC 4301, Dec. 2005.

   [RFC4302] Kent, S., "IP Authentication Header", RFC 4302, Dec. 2005.

   [RFC4960] Stewart, R. (Ed.), "Stream Control Transmission Protocol",
             RFC 4960, Sep. 2007.

   [RFC4963] Heffner, J., M. Mathis, B. Chandler, "IPv4 Reassembly
             Errors at High Data Rates," RFC 4963, July 2007.

   [RFC5225] Pelletier, G., K. Sandlund, "RObust Header Compression
             Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP and UDP-
             Lite", RFC 5225, Apr. 2008.

   [RFC5320] Templin, F., Ed., "The Subnetwork Encapsulation and
             Adaptation Layer (SEAL)", RFC 5320, Feb. 2010.

   [RFC6219] Li, X., C. Bao, M. Chen, H. Zhang, J. Wu, "The China
             Education and Research Network (CERNET) IVI Translation
             Design and Deployment for the IPv4/IPv6 Coexistence and
             Transition", RFC 6219, May 2011.

14. Acknowledgments

   This document was inspired by of numerous discussions among the
   authors, Jari Arkko, Lars Eggert, Dino Farinacci, and Fred Templin,
   as well as members participating in the Internet Area Working Group.
   Detailed feedback was provided by Gorry Fairhurst, Mike Heard, Erik
   Nordmark, Carlos Pignataro, and Dan Wing. This document originated as
   an Independent Stream draft co-authored by Matt Mathis, PSC, and his
   contributions are greatly appreciated.

   This document was prepared using

Touch                   Expires March 16, 2012                [Page 15]
Internet-Draft    Updated Spec. of the IPv4 ID Field     September 2011

Author's Address

   Joe Touch
   4676 Admiralty Way
   Marina del Rey, CA 90292-6695

   Phone: +1 (310) 448-9151

Touch                   Expires March 16, 2012                [Page 16]