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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 2012-11-26 (Latest revision 2012-10-09)
Replaces draft-touch-intarea-ipv4-unique-id
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Julien Laganier
Shepherd write-up Show Last changed 2012-05-03
IESG IESG state Became RFC 6864 (Proposed Standard)
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Needs a YES. Needs 10 more YES or NO OBJECTION positions to pass.
Responsible AD Brian Haberman
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Send notices to,
Internet Area WG                                               J. Touch
Internet Draft                                                 USC/ISI
Updates: 791,1122,2003                                  October 9, 2012
Intended status: Proposed Standard
Expires: April 2013

                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
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Copyright Notice

   Copyright (c) 2012 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
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   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..............................................4
      3.1. Uses of the IPv4 ID Field.................................4
      3.2. Background on IPv4 ID Reassembly Issues...................5
   4. Updates to the IPv4 ID Specification...........................6
      4.1. IPv4 ID Used Only for Fragmentation.......................7
      4.2. Encourage Safe IPv4 ID Use................................8
      4.3. IPv4 ID Requirements That Persist.........................8
   5. Impact of Proposed Changes.....................................9
      5.1. Impact on Legacy Internet Devices.........................9
      5.2. Impact on Datagram Generation............................10
      5.3. Impact on Middleboxes....................................11
         5.3.1. Rewriting Middleboxes...............................11

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         5.3.2. Filtering Middleboxes...............................12
      5.4. Impact on Header Compression.............................13
   6. Updates to Existing Standards.................................13
      6.1. Updates to RFC 791.......................................13
      6.2. Updates to RFC 1122......................................14
      6.3. Updates to RFC 2003......................................15
   7. Security Considerations.......................................15
   8. IANA Considerations...........................................15
   9. References....................................................16
      9.1. Normative References.....................................16
      9.2. Informative References...................................16
   10. Acknowledgments..............................................18

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 datagram lifetime (MDL) [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 MDL,
   which is typically interpreted as two minutes, and is related to the
   recommended reassembly timeout [RFC1122]. This uniqueness is
   currently specified as for all datagrams, regardless of fragmentation

   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. Further, some sources
   have been generating non-varying IPv4 IDs for many years (e.g.,
   cellphones), which resulted in support for such in ROHC [RFC5225].

   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

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   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
   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, or when the source has received
   a "packet too big" ICMPv6 error message indicating that the path
   cannot support the required minimum 1280-byte IPv6 MTU and is thus
   subject to translation [RFC2460][RFC4443]. The latter case is
   relevant only for IPv6 datagrams sent to IPv4 destinations to support
   subsequent fragmentation after translation to IPv4.

   With the exception of these two cases, the ID field is not present
   for non-fragmented datagrams, and thus is meaningful only for
   datagrams that are already fragmented or datagrams intended to be
   fragmented as part of IPv4 translation. 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 triple.

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

3.1. 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

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   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).

   Other uses have been envisioned for the IPv4 ID field. The field has
   been proposed as a way to detect and remove duplicate datagrams,
   e.g., at congested routers (noted in Sec. of [RFC1122]) or in
   network accelerators. It has similarly been proposed for use 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). This is also discussed further
   in Section 5.1.

   The IPv4 ID field is used in some diagnostic 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. This is also discussed further in Section 5.1.

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

3.2. 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 [RFC791][RFC1122][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

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   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 MDL. It is
   related to the reassembly timeout as a lower bound and the TCP
   Maximum Segment Lifetime as an upper bound (both as noted in
   [RFC1122]). Network delays are incurred in other ways, e.g.,
   satellite links, which can add seconds of delay even though the TTL
   is 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 with COTS systems over wide-area
   paths exhibit these speeds for over a decade. This strongly suggests
   that IPv4 ID uniqueness has been moot for a long time.

4. 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 are datagrams not yet fragmented and for which
      further fragmentation has been inhibited.

   o  Non-atomic datagrams are datagrams that either already have been
      fragmented or for which fragmentation remains possible.

   This same definition can be expressed in pseudo code as using common
   logical operators (equals is ==, logical 'and' is &&, logical 'or' is
   ||, greater than is >, and parenthesis function typically) as:

   o  Atomic datagrams: (DF==1)&&(MF==0)&&(frag_offset==0)

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   o  Non-atomic datagrams: (DF==0)||(MF==1)||(frag_offset>0)

   The test for non-atomic datagrams is the logical negative of the test
   for atomic datagrams, thus all possibilities are considered.

4.1. IPv4 ID Used Only for Fragmentation

   Although RFC1122 suggests the IPv4 ID field has other uses, including
   datagram de-duplication, such uses are already not interoperable with
   known implementations of sources that do not vary their ID. This
   document thus defines this field's value only for fragmentation and

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

   Datagram de-duplication is accomplished using hash-based duplicate
   detection for cases where the ID field is absent (IPv6 unfragmented
   datagrams), which can also be applied to IPv4 atomic datagrams
   without utilizing the ID field [RFC6621].

   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:

   >> 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

   >> 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 4, 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

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   repeated, e.g., over even moderately fast connections and from some
   sources that do not vary the ID at all, and no adverse impact has
   been observed. Duplicate suppression was suggested [RFC1122] and has
   been implemented in some protocol accelerators, but no impacts of
   IPv4 ID reuse have been noted to date. Routers are not required to
   issue ICMPs on any particular timescale, and so IPv4 ID repetition
   should not have been used for validation and has not been observed,
   and again repetition already occurs and would have been noticed
   [RFC1812]. ICMP relaying at tunnel ingresses is specified to use soft
   state rather than a datagram cache, and should have been noted if the
   latter for similar reasons [RFC2003]. These and other legacy issues
   are discussed further in Section 5.1.

4.2. 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 if TCP retransmits a segment it may be
   possible to reuse the IPv4 ID (see Section 6.2). 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:

   >> 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 with 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 datagram duplication, which can still occur. As
   a result this document does not change the need to support
   overlapping fragments.

4.3. IPv4 ID Requirements That Persist

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

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

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   Such sources include originating hosts, tunnel ingresses, and NATs
   (including other address sharing mechanisms) (see Section 5.3).

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

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

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

5. Impact of Proposed Changes

   This section discusses the impact of the proposed changes on legacy
   devices, datagram generation in updated devices, middleboxes, and
   header compression.

5.1. Impact on Legacy Internet Devices

   Legacy uses of the IPv4 ID field consist of fragment generation,
   fragment reassembly, duplicate datagram detection, and "other" uses.

   Current devices already generate ID values that are reused within the
   source address, destination address, protocol, and ID tuple in less
   than the current estimated Internet MDL of two minutes. They assume
   that the MDL over their end-to-end path is much lower.

   Existing devices have been known to generate non-varying IDs for
   atomic datagrams for nearly a decade, notably some cell phones. Such
   constant ID values are the reason for their support as an
   optimization of ROHC [RFC5225]. This is discussed further in Section
   5.4. Generation of IPv4 datagrams with constant (zero) IDs is also
   described as part of the IP/ICMP translation standard [RFC6145].

   Many current devices support fragmentation that ignores the IPv4
   Don't Fragment (DF) bit. Such devices already transit traffic from
   sources that reuse the ID. If fragments of different datagrams
   reusing the same ID (within the source/destination/protocol tuple)
   arrive at the destination interleaved, fragmentation would fail and
   traffic would be dropped. Either such interleaving is uncommon, or
   traffic from such devices is not widely traversing these DF-ignoring
   devices, because significant occurrence of reassembly errors has not
   been reported. DF-ignoring devices do not comply with existing

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   standards, and it is not feasible to update the standards to allow
   them as compliant.

   The ID field has been envisioned for use in duplicate detection, as
   discussed in Section 4.1 [RFC1122]. Although this document now allows
   IPv4 ID reuse for atomic datagrams, such reuse is already common (as
   noted above). Protocol accelerators are known to implement IPv4
   duplicate detection, but such devices are also known to violate other
   Internet standards to achieve higher end-to-end performance. These
   devices would already exhibit erroneous drops for this current
   traffic, and this has not been reported.

   There are other potential uses of the ID field, such as for
   diagnostic purposes. Such uses already need to accommodate atomic
   datagrams with reused ID fields. There are no reports of such uses
   having problems with current datagrams that reuse IDs. These and any
   other uses of the ID field are encouraged to apply IPv6-compatible
   methods for IPv4 as well.

   Thus, as a result of previous requirements, this document recommends
   that IPv4 duplicate detection and diagnostic mechanisms apply IPv6-
   compatible methods, i.e., that do not rely on the ID field (e.g., as
   suggested in [RFC6621]). This is a consequence of using the ID field
   only for reassembly, as well as the known hazard of existing devices
   already reusing the ID field.

5.2. Impact on Datagram Generation

   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:

   >> 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 MDL, reassembly hazards
   exist regardless of the IPv4 ID reuse interval or the reassembly
   timeout. As a result:

   >> Higher layer protocols SHOULD verify the integrity of IPv4
   datagrams, e.g., using a checksum or hash that can detect reassembly

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   errors (the UDP checksum is weak in this regard, but better than

   Additional integrity checks can be employed using tunnels, as
   supported by 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:

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

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

5.3. Impact on Middleboxes

   Middleboxes include rewriting devices that include network address
   translators (NATs), address/port translators (NAPTs), and other
   address sharing mechanisms (ASMs). They also include devices that
   inspect and filter datagrams that are not routers, such as
   accelerators and firewalls.

   The changes proposed in this document may not be implemented by
   middleboxes, however these changes are more likely to make current
   middlebox behavior compliant than to affect the service provided by
   those devices.

5.3.1. Rewriting Middleboxes

   NATs and 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 ASMs, including 4rd,
   IVI, and others in the "A+P" (address plus port) family [Bo11] [De11]
   [RFC6219]. It is equally true for any other datagram rewriting
   mechanism. As a result, they are subject to all the requirements of
   any source, as has been noted.

   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

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   NAT/ASM/rewriter needs to ensure that the ID field is generated
   appropriately, rather than simply copied from the incoming datagram.
   In specific:

   >> Address sharing or rewriting devices MUST ensure that the IPv4 ID
   field of datagrams whose address or protocol are translated comply
   with these 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 MDL 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.

5.3.2. Filtering Middleboxes

   Middleboxes also include devices that filter datagrams, including
   network accelerators and firewalls. Some such devices reportedly
   feature datagram de-duplication that relies on IP ID uniqueness to
   identify duplicates, which has been discussed in Section 5.1.

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5.4. 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 non-changing 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. Such non-changing
   IDs have been described in various RFCs (e.g., footnote 21 of
   [RFC1144] and cRTP [RFC2508]). When compression can assume a non-
   changing IPv4 ID - as with ROHC and ECRTP - efficiency can be

6. Updates to Existing Standards

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

6.1. 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.

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      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.

6.2. 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.


            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 to both atomic and non-atomic datagrams.

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

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6.3. 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. In specific, RFC 2003 states the
   following, where ref. [10] is RFC 791:

         Identification, Flags, Fragment Offset

            These three fields are set as specified in [10]...

   This document changes RFC 2003 as follows:

   o  The IPv4 ID field is set as permitted by RFCXXXX.

7. 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 it is now
   possible, and may be desirable, to rewrite the IPv4 ID field to avoid
   its use as such a channel. Rewriting would be prohibited for
   datagrams protected by IPsec Authentication Header (AH), although we
   do not recommend use of AH to achieve this result [RFC4302].

   The IPv4 ID also now adds much less to the entropy of the header of a
   datagram. Such entropy might be used as input to cryptographic
   algorithms or pseudorandom generators, although IDs have never been
   assured sufficient entropy for such purposes. The IPv4 ID had
   previously been unique (for a given source/address pair, and protocol
   field) within one MDL, 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,

8. IANA Considerations

   There are no IANA considerations in this document.

   The RFC Editor should remove this section prior to publication

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9. References

9.1. 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.

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

9.2. Informative References

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

   [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, Mar. 2011.

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

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

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   [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,
             Aug. 1999.

   [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
             Address Translator (Traditional NAT)", RFC 3022, Jan. 2001.

   [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, Jul. 2003.

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

   [RFC4443] Conta, A., S. Deering, M. Gupta (Ed.), "Internet Control
             Message Protocol (ICMPv6) for the Internet Protocol Version
             6 (IPv6) Specification", RFC 4443, March. 2006.

   [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, Jul. 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.

   [RFC6145] Li, X., C. Bao, F. Baker, "IP/ICMP Translation Algorithm,"
             RFC 6145, Apr. 2011.

   [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.

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   [RFC6621] Macker, J. (Ed.), "Simplified Multicast Forwarding," RFC
             6621, May 2012.

10. 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, Brian Haberman,
   Ted Hardie, 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

   This document was prepared using

Author's Address

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

   Phone: +1 (310) 448-9151

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