Internet Area WG                                               J. Touch
Internet Draft                                                  USC/ISI
Updates: 791,1122,2003                                    March 5, 2010
Intended status: Proposed Standard
Expires: September 2010

                Updated Specification of the IPv4 ID Field

Status of this Memo

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Copyright Notice

   Copyright (c) 2010 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
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   ( in effect on the date of
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   The IPv4 Identification (ID) field enables fragmentation and
   reassembly, and as currently specified is required to be unique
   within the maximum lifetime on all IP packets. If enforced, this
   uniqueness requirement would limit all connections to 6.4 Mbps.
   Because this is obviously not the case, it is clear that existing
   systems violate the current specification. This document updates the
   specification of the IP ID field to more closely reflect current
   practice and to more closely match IPv6, so that the field is defined
   only when a packet is actually fragmented and that fragmentation
   occurs only at originating hosts or their equivalent. When
   fragmentation occurs, this document recommends that the ID field be
   unique within the reordering context, rather than an arbitrary,
   unenforced upper bound on packet lifetime.

Table of Contents

   1. Introduction...................................................3
   2. Conventions used in this document..............................3
   3. The IPv4 ID Field..............................................4
   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.......................6
      6.2. Avoiding IPv4 ID Repetition and Its Impacts...............7
      6.3. Encourage Safe ID Use.....................................8
   7. Updates to Existing Standards..................................9
      7.1. Updates to RFC 791........................................9
      7.2. Updates to RFC 1122......................................10
      7.3. Updates to RFC 1812......................................11

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      7.4. Updates to RFC 2003......................................11
   8. Impacts on NATs and Tunnel Ingresses..........................11
   9. Impact on Header Compression..................................12
   10. Transitioning to This Update.................................12
   11. Security Considerations......................................13
   12. IANA Considerations..........................................13
   13. References...................................................14
      13.1. Normative References....................................14
      13.2. Informative References..................................14
   14. Acknowledgments..............................................15

1. Introduction

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

   The uniqueness of the IP ID is a known problem for high speed
   devices, because it limits the speed of a single protocol between two
   endpoints to 6.4 Mbps for typical MTUs of 1500 bytes [RFC4963]. This
   strongly indicates that the uniqueness of the IPv4 ID is moot, as has
   already been noted.

   This document updates the specification of the IP ID field to more
   closely reflect current practice, and to more closely match IPv6, in
   which the field is defined only when a packet is actually fragmented
   and in which fragmentation occurs only at the source. It also updates
   the recommended uniqueness interval to support the impact of
   reordering on reassembly, rather than using an arbitrary and
   unenforceable packet lifetime.

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.

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3. The IPv4 ID Field

   IP supports packet fragmentation, where large packets 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, the header contains four fields: 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 MF 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. The IPv6
   fragment header is present only when a packet has been fragmented, so
   the ID field is not present for non-fragmented packets, and thus is
   meaningful only for fragments. Finally, the ID field is 32 bits, and
   unique per source/destination address pair for IPv6, whereas for IPv4
   it is only 16 bits and 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.

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 packet are matched
   based on their IP ID. This requires that IDs are unique within the
   address/protocol triple when fragmentation is possible (e.g., DF=0).

   The ID field has been discussed as useful in other ways. It can be
   used to detect and discard duplicate packets, e.g., at congested
   routers (see Sec. of [RFC1122]).

   The ID field can also be useful for duplicate avoidance and ICMP
   validation. The field can be used at routers or receiving hosts to
   remove duplicate packets. The IP ID field can be used to validate
   payloads of ICMP responses as matching the originally transmitted
   packet at a host [RFC4963]. At a tunnel ingress, the ID enables
   returning ICMP messages to be matched to a cache of recently
   transmitted packets, to support ICMP relaying [RFC2003].

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   These latter uses require that the IP ID be unique across all
   packets, not only when fragmentation is enabled. This document
   deprecates all such non-fragmentation uses.

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 packet size of 64KB, a 16-bit ID field that
   does not repeat within 120 seconds means that the sum of all TCP
   connections of a given protocol between two 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 packets, regardless of whether fragmentation is enabled or
   inhibited, and whether a packet is fragmented or not.

   IPv6, even at typical MTUs, is capable of 18.7 Tbps when fragments
   are present, due to the larger 32-bit ID field. When fragmentation is
   not used the field is absent, and so IPv6 speeds are not limited by
   the ID field uniqueness.

   Note also that 120 seconds is only an estimate on the maximum packet
   lifetime. It is loosely based on half maximum value of the IP TTL
   field, which is represents 0-255 seconds, although it must be
   decremented by 1 second for each router on a path even when held for
   less than a second [RFC791]. Network delays are incurred in other
   ways, e.g., satellite links, which can add seconds of delay even
   though the TTL is not affected. There is no enforcement mechanism to
   ensure that packets 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
   systems at these speeds for over a decade. This strongly suggests
   that IPv4 ID uniqueness has been moot for a long time.

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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 ID field only for fragmentation

   o  Avoid ID repetition and its impacts

   o  Encourage more safe use of the ID field

   There are two kinds of packets used in the following discussion:

   Atomic packets: packets not yet having been fragmented   (MF=0 and
   fragment offset=0) and for which further fragmentation has been
   inhibited (DF=1), i.e., as a C-code expression:


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


      or for which fragmentation remains possible (DF=0), i.e.:


      or (equivalently):


6.1. IPv4 ID Used Only for Fragmentation

   Although at least one document suggests the ID field has other uses,
   we assert here that the ID field is defined only for fragmentation
   and reassembly.

   o  >> The IPv4 ID field of MUST be ignored except for packet

   Such devices typically include receiving hosts and tunnel egresses,
   but may include any intermediate device that reassembles a packet,
   such as a firewall or NAT. The ID field is thus meaningful only for
   non-atomic packets that have actually been fragmented, either at the
   source or elsewhere along the path, and have not been reassembled
   before being examined. In atomic packets, the ID field has no

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   meaning, and thus its values are always to be ignored. Atomic packets
   are detected by their DF, MF, and fragmentation offset fields as
   defined in Section 6, because such a test is completely backward
   compatible; this document thus does not reserve any ID values,
   including 0, as distinguished.

   Note that this excludes some current practices that use the ID field
   and the remainder of the IP header as a unique tag. This tag has been
   suggested as a way to detect and remove duplicate packets, e.g., at
   congested routers, although this has been noted and no current
   deployments are known [RFC1122]. Some hosts use this tag to validate
   received ICMPs, in which the ICMP payload - an IP packet prefix - is
   matched against a cache of recently transmitted IP headers. This
   ensures that the received ICMP reflects a transmitted packet, though
   it does not prevent spoofing of ICMPs for attackers that can see
   those packets, and like ID reuse will cause problems at high packet
   rates. A similar sort of matching can be used in tunnels, to enable
   ICMP relaying at the tunnel ingress, with similar challenges

   Deprecating the use of the IPv4 ID field for these 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, and no impacts of ID reuse
   have been noted. Routers are not required to issue ICMPs on any
   particular timescale, and so 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
   specified to use soft state rather than a packet cache, and should
   have been noted if the latter for similar reasons [RFC2003].

6.2. Avoiding IPv4 ID Repetition and Its Impacts

   This document specifies that IPv4 be modified to more closely match
   IPv6's fragmentation constraints, to permit fragmentation only at
   devices that control the uniqueness of the IP ID field, e.g.,
   sources, tunnel ingresses (for the outer header), and packets emitted
   from a NAT to its public side (see Section 8).

   o  >> Sources SHOULD set DF=1.

   o  >> IPv4 fragmentation SHOULD be limited to the originating source,
      even when the DF field allows it.

   Keep in mind that a source is any device that uses one of its
   assigned IP addresses as a source IP address in emitted packets. This

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   includes hosts, routers when originating packets, packets emitted
   from NATs (see Section 8), and tunnel ingresses.

   It may not be possible for sources to know whether all of the above
   specifications are satisfied. As a result, we recommend that:

   o  >> Sources unable to meet the non-repeating IP ID requirement
      above MUST NOT emit non-atomic packets.

   In other words, such sources can emit only non-fragmented packets
   where DF has been set. Such sources can repeat the ID field for
   atomic packets, as it is intended to be ignored.

   Sources emitting non-atomic IPv4 packets need to set the ID field
   sufficient to support reassembly, and encourages the use of stronger
   transport layer validation where possible. Uniqueness over a two
   minute interval may be excessive to support reassembly in some
   environments, and is clearly already being ignored.

   o  >> Sources emitting non-atomic IPv4 packets SHOULD NOT repeat ID
      field values within a given source IP, destination IP, and
      protocol tuple over the period that fragment reordering would
      affect reassembly.

   It is impractical to assert "MUST NOT" here, because there is no
   strict enforcement on packet lifetime and because sources may not be
   able to determine the reordering period.

   o  >> Sources that cannot ensure safe IPv4 ID generation and that
      allow DF=0 SHOULD employ integrity checks that would detect mis-
      reassembled fragments, e.g, as in SEAL [RFC5320]. Applications
      SHOULD NOT use UDP without checksums [RFC793], and SHOULD be very
      careful in their use of UDP-Lite [RFC3828] in such environments.

   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].

6.3. Encourage Safe ID Use

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

   RFC 1122 discusses that TCP retransmits a segment it may be possible
   to reuse the IP ID (see Section 7.2). This can make it difficult for
   a source to avoid ID repetition for received fragments. RFC 1122

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   concludes that this behavior "is not useful"; this document
   formalizes that conclusion as follows:

   o  >> The IP ID MUST NOT be reused when sending a copy of an earlier
      non-ATOMIC packet.

   RFC 1122 also suggests that fragments can overlap [RFC1122]. Such
   overlap can occur if successive retransmissions use different
   packetizing but the same reassembly Id.

   This overlap is noted as the result of reusing IDs when
   retransmitting packets, which this document deprecates. Overlapping
   fragments are themselves a hazard [RFC4963]. As a result:

   o  >> Overlapping packets MUST be silently ignored during reassembly.

7. Updates to Existing Standards

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

7.1. 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 packet 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

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      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  >> The IP ID is not defined if the packet (datagram) is atomic. IP
      packet sources MAY use any value as ID; all such values MUST BE
      ignored on examination at intermediate nodes and destinations.

   o  >> The IP ID of non-atomic packets MUST BE unique for the time
      where fragments are expected to overlap.

   o  >> Hosts SHOULD emit only atomic packets (i.e., not fragmented at
      the source, and with DF=1).

   We do not expect that it will be useful to involve higher-level
   protocols in determining ID values.

7.2. 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 IP ID field MUST NOT be used for duplicate detection or

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   o  >> IP ID values MUST NOT be repeated when packets are

   o  >> IP packet fragments MUST NOT overlap.

7.3. Updates to RFC 1812

   There are no updates to RFC1812.

7.4. Updates to RFC 2003

   RFC 2003 states that:

         Identification, Flags, Fragment Offset

            These three fields are set as specified in [RFC791].
            However, if the "Don't Fragment" bit is set in the inner IP
            header, it MUST be set in the outer IP header; if the "Don't
            Fragment" bit is not set in the inner IP header, it MAY be
            set in the outer IP header, as described in Section 5.1.

   This document changes RFC 2003 as follows:

   o  >> IP-in-IP tunnels SHOULD emit only atomic packets.

   Note that this recommendation applies to all tunnels, but the focus
   of this document is IPv4 requirements, so its explicit requirements
   focus on IPv4 cases.

8. Impacts on NATs and Tunnel Ingresses

   Network address translators (NATs) and address/port translators
   (NAPTs) rewrite IP fields, and tunnel ingresses (using IP
   encapsulation) copy and modify some IP fields, so all are considered
   sources, as do any devices that rewrite any portion of the IP source,
   IP destination, IP protocol, and IP ID tuple for non-atomic packets
   [RFC3022]. As a result, they are subject to all the requirements of
   any source, as has been noted.

   NATs present a particularly challenging situation for fragmentation.
   Because NATs overwrite portions of the reassembly tuple in both
   directions, they can destroy tuple uniqueness and result in a
   reassembly hazard. Not only do NATs need to behave as a source for
   the purposes of this document, but also:

   o  >> NATs MUST either silently drop fragments or reassemble them
      before translating and emitting them.

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   Problems with transmitting fragments through NATs 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 is required
   for IP ID uniqueness.

   Note that NATs/NAPTs already need to exercise special care when
   emitting packets on their public side, because merging packets from
   many sources onto a single outgoing source IP address can result in
   IP ID collisions. This situation precedes this document, and is not
   affected by it. It is exacerbated in large-scale, so-called "carrier
   grade" NATs [Ni09].

   Tunnel ingresses act as sources for the outermost header, but tunnels
   act as routers for the inner headers (i.e., the packet as arriving at
   the tunnel ingress). Ingresses can fragment as originating sources of
   the outer header, because they control the uniqueness of that IP ID
   field. They need to avoid fragmenting the packet at the inner header,
   for the same reasons as any intermediate device, as noted elsewhere
   in this document.

9. Impact on Header Compression

   Header compression algorithms already accommodate various ways in
   which the IP ID changes between sequential packets. Such algorithms
   already need to preserve the IP ID. This document relaxes that
   constraint, making preservation optional for most atomic packets as a

   >> Header compression MAY preserve the IP ID of atomic packets that
   are not protected by IPsec AH [RFC4302]. The IP ID of non-atomic
   packets, and those of packets protected by IPsec AH MUST be

   Note that this can impact the efficiency of header compression in
   various ways. When compression can assume a nonchanging ID,
   efficiency can be increased. However, when compression assumes a
   changing ID as a default, having a non-changing ID can make
   compression less efficient (see footnote 21 of [RFC1144], which is
   optimized for non-atomic packets).

10. Transitioning to This Update

   ?? Do we need this transition?

   ?? Do we want to say when to stop the transition?

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   During the transition period, there may continue to be tunnel
   ingresses and NATs that fragment even when the DF bit is set, or that
   validate ICMP payloads based on cached packets. It may be useful to
   use a small ID space to help detect such behaviors without causing
   full disruption, as might occur by using a single value when the DF
   flag is set (e.g., ID=0).

   As a result, during the transition period, this document recommends

   >> During the transition period, a small ID space SHOULD be used to
   assist with debugging and detection; such a space SHOULD use the
   lower bits (i.e., lower 4 bits) of the ID field and clear (i.e.,
   zero) the remaining high order bits.

11. Security Considerations

   This document attempts to address the security considerations
   associated with fragmentation in IPv4 [RFC4459].

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

   The IP ID also now adds much less entropy of the header of an IP
   packet. The ID had previously been unique (for a given IP
   source/address pair, and protocol field) within 2MSL, although this
   requirement was not enforced and clearly is typically ignored. IDs of
   non-atomic packets are now required unique only within the expected
   reordering of fragments, which could substantially reduce the amount
   of entropy in that field. The IP ID of atomic packets is not required
   unique, and so contributes no entropy to the header.

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

12. IANA Considerations

   There are no IANA considerations in this document.

   The RFC Editor should remove this section prior to publication

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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.

   [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.

   [Ni09]    Nishitani, T., I. Yamagata, S. Miyakawa, A. Nakagawa, H.
             Ashida, "Common Functions of Large Scale NAT (LSN) ", (work
             in progress), draft-nishitani-cgn-03, Nov. 2009.

   [RFC793]  Postel, J., "User Datagram Protocol", RFC 793 / STD 6,
             August 1980.

   [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.

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

   [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.

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   [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.

   [RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the-
             Network Tunneling", RFC 4459, April 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, July 2007.

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

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 Carlos Pignataro. 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

Author's Address

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

   Phone: +1 (310) 448-9151

Touch                 Expires September 1, 2010               [Page 15]