    Internet Engineering Task Force (IETF)                          J. Touch
    Request for Comments: 6864                                       USC/ISI
    Updates: 791, 1122, 2003                                   February 2013
    Category: Standards Track
    ISSN: 2070-1721

                   Updated Specification of the IPv4 ID Field

    Abstract

       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
       address/destination address/protocol tuple.  If enforced, this
       uniqueness requirement would limit all connections to 6.4 Mbps for
       typical datagram sizes.  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 RFCs 791, 1122, and 2003 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.

    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/rfc6864.

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    Copyright Notice

       Copyright (c) 2013 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.

    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. Encouraging 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
               5.3.2. Filtering Middleboxes ..............................12
          5.4. Impact on Header Compression ..............................12
          5.5. Impact of Network Reordering and Loss .....................13
               5.5.1. Atomic Datagrams Experiencing Reordering or Loss ...13
               5.5.2. Non-atomic Datagrams Experiencing
                      Reordering or Loss .................................14
       6. Updates to Existing Standards ..................................14
          6.1. Updates to RFC 791 ........................................14
          6.2. Updates to RFC 1122 .......................................15
          6.3. Updates to RFC 2003 .......................................16
       7. Security Considerations ........................................16
       8. References .....................................................17
          8.1. Normative References ......................................17
          8.2. Informative References ....................................17
       9. Acknowledgments ................................................19

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    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
       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
       (assuming a 2-minute MDL, using the analysis presented in [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 RObust Header Compression
       (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",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
       document are to be interpreted as described in RFC 2119 [RFC2119].

       In this document, the characters ">>" preceding one or more indented
       lines indicate a requirement using the key words listed above.  This
       convention aids reviewers in quickly identifying or finding this
       document's explicit requirements.

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    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"
          (DF) flag, and a "More Fragments" (MF) flag [RFC791].

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

       IPv6 fragmentation differs from IPv4 fragmentation 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; thus, it 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 address/destination address/protocol tuple.

       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 be unique within the
       source address/destination address/protocol tuple when fragmentation
       is possible (e.g., DF=0) or when it has already occurred (e.g.,
       frag_offset>0 or MF=1).

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       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 Section 3.2.1.5 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 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 the MDL, within the source
       address/destination address/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 the MDL, within the source
       address/destination address/protocol tuple) in those cases.

    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 64 KB, 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 or not a datagram is fragmented.

       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.

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       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 Time
       to Live (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 Commercial-Off-The-Shelf
       (COTS) systems over wide-area paths have exhibited 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  Using the IPv4 ID field only for fragmentation

       o  Encouraging safe operation when the IPv4 ID field is used

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

       There are two kinds of datagrams, which are defined below and used in
       the following discussion:

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

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

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

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

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

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       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 RFC 1122 suggests that 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 reassembly:

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

       Datagram de-duplication can still be 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; thus, it can
       be set to an arbitrary value, i.e., the requirement for non-repeating
       IDs within the source address/destination address/protocol tuple 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
       field of atomic datagrams:

       >> 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 --
       either those datagrams that have 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;
       thus, this document 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 and
       from some sources that do not vary the ID at all, and no adverse
       impact has been observed.  Duplicate suppression was suggested

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       [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 purposes; this
       scenario has not been observed.  Besides, 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; for similar reasons, if the latter is used, this should have
       been noticed [RFC2003].  These and other legacy issues are discussed
       further in Section 5.1.

    4.2.  Encouraging Safe IPv4 ID Use

       This document also changes the specification of the IPv4 ID field to
       encourage its safe use.

       As discussed in RFC 1122, 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.  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 for receivers 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, that is:

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

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

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

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

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

       Specifically, 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 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 cellphones.  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 address/destination
       address/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 standards, and it is not feasible to update
       the standards to allow them as compliant.

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       The ID field has been envisioned for use in duplicate detection, as
       discussed in Section 4.1.  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.

       Thus, as a result of previous requirements, this document recommends
       that IPv4 duplicate detection and diagnostic mechanisms apply
       IPv6-compatible methods, i.e., methods 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 errors (the UDP and TCP checksums are weak in this
          regard, but better than nothing).

       Additional integrity checks can be employed using tunnels, as
       supported by the Subnetwork Encapsulation and Adaptation Layer (SEAL)
       [RFC5320], IPsec [RFC4301], or the Stream Control Transmission
       Protocol (SCTP) [RFC4960].  Such checks can avoid the reassembly

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       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 intervals that are smaller than typical
          MDL values.

       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 such as network address
       translators (NATs), network address/port translators (NAPTs), and
       other address-sharing mechanisms (ASMs).  They also include devices
       that inspect and filter datagrams but 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; all are therefore
       considered datagram sources, as are any devices that rewrite any
       portion of the source address/destination address/protocol/ID tuple
       for any datagrams [RFC3022].  This is also true for other ASMs,
       including IPv4 Residual Deployment (4rd) [De11], IVI [RFC6219], and
       others in the "A+P" (address plus port) family [Bo11].  It is equally
       true for any other datagram-rewriting mechanism.  As a result, they
       are subject to all the requirements of any datagram 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
       NAT/ASM/rewriter needs to ensure that the ID field is generated
       appropriately, rather than simply copied from the incoming datagram.

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       Specifically:

       >> Address-sharing or rewriting devices MUST ensure that the IPv4 ID
          field of datagrams whose addresses or protocols are translated
          comply with these requirements as if the datagram were sourced by
          that device.

       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, translated
       fragments already violate that requirement, as they repeat an IPv4 ID
       within the MDL for a given source address/destination
       address/protocol tuple.

       Such problems with transmitting fragments through NATs/ASMs/rewriters
       are already known; translation is typically 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, such as
       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.

    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

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       the assumption that the ID does not change (e.g., ROHC [RFC5225]),
       where others include non-changing IDs via zero deltas (e.g., Enhanced
       Compressed RTP (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 increased.

    5.5.  Impact of Network Reordering and Loss

       Tolerance to network reordering and loss is a key feature of the
       Internet architecture.  Although most current IP networks avoid
       gratuitous such events, both reordering and loss can and do occur.
       Datagrams are already intended to be reordered or lost, and recovery
       from those errors (where supported) already occurs at the transport
       or higher protocol layers.

       Reordering is typically associated with routing transients or where
       flows are split across multiple paths.  Loss is typically associated
       with path congestion or link failure (partial or complete).  The
       impact of such events is different for atomic and non-atomic
       datagrams and is discussed below.  In summary, the recommendations of
       this document make the Internet more robust to reordering and loss by
       emphasizing the requirements of ID uniqueness for non-atomic
       datagrams and by more clearly indicating the impact of these
       requirements on both endpoints and datagram transit devices.

    5.5.1.  Atomic Datagrams Experiencing Reordering or Loss

       Reusing ID values does not affect atomic datagrams when the DF bit is
       correctly respected, because order restoration does not depend on the
       datagram header.  TCP uses a transport header sequence number; in
       some other protocols, sequence is indicated and restored at the
       application layer.

       When DF=1 is ignored, reordering or loss can cause fragments of
       different datagrams to be interleaved and thus incorrectly
       reassembled and discarded.  Reuse of ID values in atomic datagrams,
       as permitted by this document, can result in higher datagram loss in
       such cases.  Situations such as this already can exist because there
       are known devices that use a constant ID for atomic datagrams (some
       cellphones), and there are known devices that ignore DF=1, but high
       levels of corresponding loss have not been reported.  The lack of
       such reports indicates either a lack of reordering or a loss in such
       cases or a tolerance to the resulting losses.  If such issues are

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       reported, it would be more productive to address non-compliant
       devices (that ignore DF=1), because it is impractical to define
       Internet specifications to tolerate devices that ignore those
       specifications.  This is why this document emphasizes the need to
       honor DF=1, as well as that datagram transit devices need to retain
       the DF bit as received (i.e., rather than clear it).

    5.5.2.  Non-atomic Datagrams Experiencing Reordering or Loss

       Non-atomic datagrams rely on the uniqueness of the ID value to
       tolerate reordering of fragments, notably where fragments of
       different datagrams are interleaved as a result of such reordering.
       Fragment loss can result in reassembly of fragments from different
       origin datagrams, which is why ID reuse in non-atomic datagrams is
       based on datagram (fragment) maximum lifetime, not just expected
       reordering interleaving.

       This document does not change the requirements for uniqueness of IDs
       in non-atomic datagrams and thus does not affect their tolerance to
       such reordering or loss.  This document emphasizes the need for ID
       uniqueness for all datagram sources, including rewriting middleboxes;
       the need to rate-limit sources to ensure ID uniqueness; the need to
       not reuse the ID for retransmitted datagrams; and the need to use
       higher-layer integrity checks to prevent reassembly errors -- all of
       which result in a higher tolerance to reordering or loss events.

    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.

       It later states 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.

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          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.

    6.2.  Updates to RFC 1122

       RFC 1122 states in Section 3.2.1.5 ("Identification: RFC 791
       Section 3.2") that:

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

          DISCUSSION:
               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
               queue.

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       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.

    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.  Specifically, RFC 2003 states the
       following, where [10] refers to 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 RFC 6864.

    7.  Security Considerations

       When the IPv4 ID is ignored on receipt (e.g., for atomic datagrams),
       its value becomes unconstrained; therefore, that field 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 the IPsec Authentication Header (AH), although
       we do not recommend use of the 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,
       however.

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    8.  References

    8.1.  Normative References

       [RFC791]   Postel, J., "Internet Protocol", STD 5, RFC 791,
                  September 1981.

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

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

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

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

    8.2.  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., Touch, J., Levis, P., and R. Penno,
                  "Analysis of Solution Candidates to Reveal a Host
                  Identifier in Shared Address Deployments", Work in
                  Progress, September 2011.

       [De11]     Despres, R., Ed., Matsushima, S., Murakami, T., and O.
                  Troan, "IPv4 Residual Deployment across IPv6-Service
                  networks (4rd) ISP-NAT's made optional", Work in Progress,
                  March 2011.

       [Pe11]     Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa,
                  A., and H. Ashida, "Common requirements for Carrier Grade
                  NATs (CGNs)", Work in Progress, December 2012.

       [RFC1144]  Jacobson, V., "Compressing TCP/IP Headers for Low-Speed
                  Serial Links", RFC 1144, February 1990.

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

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       [RFC2508]  Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP
                  Headers for Low-Speed Serial Links", RFC 2508,
                  February 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 2001.

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

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

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

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

       [RFC4443]  Conta, A., Deering, S., and 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, September 2007.

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

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

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

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

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       [RFC6219]  Li, X., Bao, C., Chen, M., Zhang, H., and 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.

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

    9.  Acknowledgments

       This document was inspired by numerous discussions with the author by
       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 Submissions stream document
       co-authored by Matt Mathis, PSC, and his contributions are greatly
       appreciated.

       This document was initially prepared using 2-Word-v2.0.template.dot.

    Author's Address

       Joe Touch
       USC/ISI
       4676 Admiralty Way
       Marina del Rey, CA  90292-6695
       U.S.A.

       Phone: +1 (310) 448-9151
       EMail: touch@isi.edu

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