Network Working Group                                    F. Templin, Ed.
Internet-Draft                                      Boeing Phantom Works
Intended status: Informational                             April 6, 2008
Expires: October 8, 2008

           The Subnetwork Encapsulation and Adaptation Layer

Status of this Memo

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   Subnetworks are connected network regions bounded by border routers
   that forward unicast and multicast packets over a virtual topology
   manifested by tunneling.  This virtual topology resembles a "virtual
   ethernet" link, but may span multiple IP- and/or sub-IP layer
   forwarding hops that can introduce packet duplication and/or traverse
   links with diverse Maximum Transmission Units (MTUs).  This document
   specifies a Subnetwork Encapsulation and Adaptation Layer (SEAL) that
   accommodates such virtual topologies over diverse underlying link

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology and Requirements . . . . . . . . . . . . . . . . .  4
   3.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  5
   4.  SEAL Protocol Specification  . . . . . . . . . . . . . . . . .  6
     4.1.  Model of Operation . . . . . . . . . . . . . . . . . . . .  6
     4.2.  Packetization  . . . . . . . . . . . . . . . . . . . . . .  7
       4.2.1.  Packet Size Considerations . . . . . . . . . . . . . .  7
       4.2.2.  Inner Fragmentation  . . . . . . . . . . . . . . . . .  8
       4.2.3.  SEAL Segmentation and Encapsulation  . . . . . . . . .  8
       4.2.4.  Sending Packets  . . . . . . . . . . . . . . . . . . . 11
     4.3.  Reassembly . . . . . . . . . . . . . . . . . . . . . . . . 11
       4.3.1.  Reassembly Buffer Requirements . . . . . . . . . . . . 11
       4.3.2.  IPv4-Layer Reassembly  . . . . . . . . . . . . . . . . 11
       4.3.3.  SEAL-Layer Reassembly  . . . . . . . . . . . . . . . . 12
       4.3.4.  Reassembly Integrity Checks  . . . . . . . . . . . . . 12
     4.4.  Generating Fragmentation Reports . . . . . . . . . . . . . 13
     4.5.  Receiving Fragmentation Reports  . . . . . . . . . . . . . 13
     4.6.  S-MSS Probing  . . . . . . . . . . . . . . . . . . . . . . 14
     4.7.  Processing ICMP PTBs . . . . . . . . . . . . . . . . . . . 15
   5.  Link Requirements  . . . . . . . . . . . . . . . . . . . . . . 15
   6.  End System Requirements  . . . . . . . . . . . . . . . . . . . 15
   7.  Router Requirements  . . . . . . . . . . . . . . . . . . . . . 15
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 16
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     11.2. Informative References . . . . . . . . . . . . . . . . . . 16
   Appendix A.  Historic Evolution of PMTUD (written 10/30/2002)  . . 18
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 20
   Intellectual Property and Copyright Statements . . . . . . . . . . 21

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1.  Introduction

   For the purpose of this document, subnetworks are defined as
   connected network regions bounded by border routers.  Examples
   include the global Internet interdomain routing core, Mobile Ad Hoc
   Networks (MANETs) and enterprise networks.  These subnetworks are
   manifested as a virtual topology that may span many underlying
   networks and traditional IP subnets, e.g., in the internal
   organization of an enterprise network.

   Subnetwork border routers forward unicast and multicast packets over
   the virtual topology across multiple IP- and/or sub-IP layer
   forwarding hops which may introduce packet duplication and/or
   traverse links with diverse Maximum Transmission Units (MTUs).  It is
   also expected that these subnetwork border routers will support
   operation of the Internet protocols [RFC0791][RFC2460].

   As internet technology and communication has grown and matured, many
   techniques have developed that use virtual topologies (frequently
   tunnels of one form or another) over an actual IP network.  Those
   virtual topologies have elements which appear as one hop in the
   virtual topology, but are actually multiple IP or sub-IP layer hops.
   These multiple hops often have quite diverse properties which are
   often not even visible to the end-points of the virtual hop.  This
   introduces many failure modes that are not dealt with well in current

   The use of IP encapsulation has long been considered as an
   alternative for creating such virtual topologies.  However, the
   insertion of an outer IP header reduces the effective path MTU as-
   seen by the IP layer.  When IPv4 is used, this reduced MTU can be
   accommodated through the use of IPv4 fragmentation, but unmitigated
   in-the-network fragmentation has been shown to be harmful through
   operational experience and studies conducted over the course of many
   years [FRAG][FOLK][RFC2923][RFC4459][RFC4963].

   This document proposes a Subnetwork Encapsulation and Adaptation
   Layer (SEAL) for the operation of IP over subnetworks that connect
   routers via Ingress- and Egress Tunnel Endpoints (ITEs/ETEs).  SEAL
   supports simple and robust duplicate packet detection, and
   accommodates links with diverse MTUs by introducing a new
   encapsulation format.  The SEAL encapsulation introduces an extended
   Identification field for packet identification and enables a mid-
   layer segmentation and reassembly capability that allows an in-the-
   network cutting and pasting of packets without invoking IP
   fragmentation.  The SEAL protocol is specified in the following

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2.  Terminology and Requirements

   The term "subnetwork" in this document refers to a connected network
   region bounded by border routers that connect over a virtual topology
   manifested through tunneling that appears as a fully-connected shared
   link, i.e., a "Virtual Ethernet (VET)" [I-D.templin-autoconf-dhcp].

   The terms "inner" and "outer" are used extensively throughout this
   document to respectively refer to the innermost IP {layer, protocol,
   header, packet, etc.} *before* any encapsulation, and the outermost
   IP {layer, protocol, header, packet etc.} *after* any encapsulation.
   Between these inner and outer layers, there may also be mid-layer
   encapsulations, including the SEAL encapsulation.  These mid-layer
   encapsulations are denoted as '*' (where '*' may signify NULL, a
   single mid-layer encapsulation, or multiple mid-layer

   The notation IPvX/*/IPvY refers to an inner IPvX packet encapsulated
   in any '*' mid-layer headers followed by an outer IPvY header.

   The notation "IP" means either IP protocol version (IPv4 or IPv6).

   The following abbreviations correspond to terms used within this
   document and elsewhere in common Internetworking nomenclature:

      Subnetwork - a connected network region that is bounded by border

      SEAL - Subnetwork Encapsulation and Adaptation Layer

      VET - Virtual EThernet

      MANET - Mobile Ad-hoc Network

      ITE - Ingress Tunnel Endpoint

      ETE - Egress Tunnel Endpoint

      MTU - Maximum Transmission Unit

      S-MSS - SEAL Maximum Segment Size

      EMTU_R - Effective MTU to Receive

      PTB - an ICMPv6 "Packet Too Big" or an ICMPv4 "fragmentation
      needed" message

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      DF - the IPv4 header Don't Fragment flag

      ENCAPS - the size of the outer encapsulating SEAL/*/IPv4 headers

      FRAGREP - a Fragmentation Report message

      SEAL packet - a segment of an inner IP packet encapsulated in
      outer SEAL/*/IPv4 headers

      SEAL-ID - a 32-bit Identification value; randomly initialized and
      monotonically incremented for each SEAL packet

      Unfragmentable - an IPv4 packet with DF=1, or an IPv6 packet

   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119].

3.  Applicability Statement

   SEAL inserts an additional mid-layer encapsulation when IP/*/IPv4
   encapsulation is used, and appears as a subnetwork encapsulation as
   seen by inner layers.  SEAL was motivated by the specific use case of
   subnetwork abstraction for MANETs, however the domain of
   applicability also extends to subnetwork abstractions of enterprise
   networks, the interdomain routing core, etc.

   SEAL can be used as a mid-layer encapsulation above an outer UDP/IPv4
   encapsulation, however the technique of concatenating the SEAL 16-bit
   ID Extension and the IPv4 ID (i.e., co-mingling the two identifier
   spaces) will not work when there are network address translators
   (NATs) in the path that may re-write the IPv4 ID, e.g., such as for
   the Teredo domain of applicability [RFC4380].  A variation of this
   proposal that maintains separate ID spaces for the SEAL-ID and IPv4
   ID and that operates in the presence of NATs and firewalls will be
   specified in a future version of this document.

   The current document version speaks exclusively to the use of IPv4 as
   the outer encapsulation layer, however the same principles apply when
   IPv6 is the outer layer.  In-the-network fragmentation is not
   permitted for encapsulations over IPv6, however, so the "implicit"
   probing capabilities specified for IPv4 in this document are not
   available.  Still, encapsulations over IPv6 can use "explicit"
   probing as well as the same architectural concepts as specified for
   IPv4 herein.  A future version of this document will address the case
   of IPv6 as the outer encapsulation layer in more detail.

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   For further study, SEAL may also be useful for "transport-mode"
   applications, e.g., when the inner layer includes ordinary protocol
   data rather than an encapsulated IP packet.

4.  SEAL Protocol Specification

4.1.  Model of Operation

   Ingres Tunnel Endpoints (ITEs) insert a SEAL header in the IP/*/
   IPv4-encapsulated packets they inject into a subnetwork, where the
   outermost IPv4 header contains the source and destination addresses
   of the subnetwork entry/exit points (i.e., the ITE/ETE),
   respectively.  SEAL defines a new IP protocol type and a new mid-
   layer encapsulation for both unicast and multicast inner IP packets.
   The ITE inserts a SEAL header during encapsulation as shown in
   Figure 1:

                                      |                         |
                                      ~   Outer */IPv4 headers  ~
                                      |                         |
                                      |       SEAL Header       |
   +-------------------------+        +-------------------------+
   |                         |        |                         |
   ~ Any mid-layer * headers ~        ~ Any mid-layer * headers ~
   |                         |        |                         |
   +-------------------------+        +-------------------------+
   |                         |        |                         |
   ~        Inner IP         ~  --->  ~        Inner IP         ~
   ~         Packet          ~  --->  ~         Packet          ~
   |                         |        |                         |
   +-------------------------+        +-------------------------+
   |  Any mid-layer trailers |        |  Any mid-layer trailers |
   +-------------------------+        +-------------------------+
                                      |    Any outer trailers   |

                       Figure 1: SEAL Encapsulation

   where the SEAL header is inserted as follows:

   o  For simple IP/IPv4 encapsulations (e.g.,
      [RFC2003][RFC2004][RFC4213]), the SEAL header is inserted between
      the inner IP and outer IPv4 headers as: IP/SEAL/IPv4.

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   o  For tunnel-mode IPsec encapsulations over IPv4, [RFC4301], the
      SEAL header is inserted between the {AH,ESP} header and outer IPv4
      headers as: IP/*/{AH,ESP}/SEAL/IPv4.

   o  For IP encapsulations over transports such as UDP (e.g.,
      [I-D.farinacci-lisp]), the SEAL header is inserted immediately
      after the outer transport layer header, e.g., as IP/*/SEAL/UDP/

   Encapsulation and tunneling establishes an abstraction of the
   subnetwork that connects all ITEs and ETEs as single-hop neighbors as
   though they were attached to a virtual ethernet (VET).  From a
   physical perspective, however, packets sent over the subnetwork may
   be forwarded across many IP and/or sub-IP layer hops.

   SEAL-encapsulated packets include a 32-bit SEAL-ID formed from the
   concatenation of the 16-bit ID Extension field in the SEAL header as
   the most-significant bits and with the 16-bit ID value in the outer
   IPv4 header as the least-significant bits.  Routers use the SEAL-ID
   for duplicate packet detection within the subnetwork as well as for
   SEAL segmentation and reassembly.

   SEAL enables a multi-level segmentation and reassembly capability.
   First, the ITE can use IPv4 fragmentation for fragmentable inner IPv4
   packets before encapsulation to avoid lower-level segmentation and
   reassembly.  Secondly, the SEAL layer itself provides a simple mid-
   layer cutting-and-pasting of inner IP packets to avoid IPv4
   fragmentation on the outer packet.  Finally, ordinary IPv4
   fragmentation for the outer IPv4 packet after SEAL encapsulation is
   permitted under certain limited and carefully managed circumstances.

4.2.  Packetization

4.2.1.  Packet Size Considerations

   Due to the ubiquitous deployment of standard Ethernet and similar
   networking gear, the nominal Internet cell size has become 1500
   bytes; this is the de facto size that end systems have come to expect
   will be delivered by the network without loss due to an MTU
   restriction on the path, or a suitable ICMP PTB message returned.
   However, PTB messages can be dropped in the network, and any PTBs
   received could be erroneous or maliciously fabricated.  (Indeed, in
   the case of treating the global Internet interdomain routing core as
   a subnetwork, the PTB messages could come from anywhere in the
   Internet.)  The ITE therefore requires a means for conveying 1500
   byte (or smaller) original packets to the ETE without loss due to
   link MTU restrictions and/or triggering PTB messages from within the

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   In common deployments, there may be many forwarding hops between the
   source and the ITE.  Within those hops, there may be additional
   encapsulations (IPSec, L2TP, etc.) such that a 1500 byte original
   packet might grow to a larger size by the time it reaches the ITE.
   Similarly, additional encapsulations on the path from the ITE to the
   ETE could cause the packet to become larger still and trigger in-the-
   network fragmentation.  In order to preserve the end system
   expectation of delivery for 1500 byte and smaller packets, the ITE
   therefore requires a means for conveying this larger packet to the
   ETE even though there may be links within the subnetwork that
   configure a smaller MTU.

   The ITE upholds the 1500-byte-and-smaller packet delivery expectation
   by instituting a SEAL Maximum Segment Size (S-MSS) variable,
   configurable within the range of [128 - 2KB].  The ITE also
   institutes a segmentation region for packet sizes [S-MSS - 2KB] such
   that all inner IP packets within this size range are segmented into
   multiple SEAL packets to avoid in-the-network IPv4 fragmentation.

   The ITE must be configured to either drop unfragmentable inner IP
   packets larger than 2KB (and return a suitable ICMP PTB message), or
   admit them into the tunnel as single-segment SEAL packets.  If the
   ITE is configured to admit such packets, it MUST maintain sufficient
   state for caching the MTU values reported in PTB messages received
   from within the tunnel.  Configuration can be either on a per-
   interface or per-ETE basis.

4.2.2.  Inner Fragmentation

   The IPv4 layer of a subnetwork border router that configures an ITE
   fragments inner IPv4 packets larger than 2KB and with the IPv4 Don't
   Fragment (DF) bit set to 0 into IPv4 fragments no larger than
   MIN(2KB, S-MSS).  The IPv4 layer then submits each inner IPv4
   fragment to the ITE as an independent IP packet for encapsulation.
   Note that inner fragmentation may not be available for certain ITE
   types, e.g., for tunnel-mode IPsec.  Any inner IPv4 fragments created
   in this fashion will be reassembled by the final destination.

4.2.3.  SEAL Segmentation and Encapsulation

   After any inner fragmentation, the ITE encapsulates each inner IP
   packet/fragment according to its size.

   When the ITE is configured to admit unfragmentable inner IP packets
   larger than 2KB into the tunnel, it MUST NOT break them into smaller
   segments but rather MUST encapsulate each inner packet as a single
   segment SEAL packet.  When the ITE is configured to discard
   unfragmentable inner packets larger than 2KB, it drops the packet and

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   sends a suitable ICMP PTB message back to the original source.

   For inner IP packets no larger than 2KB, the ITE encapsulates the
   packet in any mid-layer '*' headers, then performs SEAL segmentation
   on this inner packet based on a segment size (S-MSS) that will avoid
   IPv4 fragmentation within the subnetwork.  The ITE maintains S-MSS
   for each ETR (including IPv4 multicast destinations) as per-ETR soft
   state, where S-MSS is configured to a value within the [128 - 2KB]
   range based on static configuration and/or dynamic segment size

   Note that this SEAL segmentation ignores the DF bit in the inner IPv4
   header or (in the case of IPv6) ignores the fact that the network is
   not permitted to perform IPv6 fragmentation.  This segmentation
   process is a mid-layer (not an IP layer) operation employed by the
   ITE to adapt the inner IP packet to the subnetwork path
   characteristics, and the ETE will restore the inner packet to its
   original form when it removed the packet from the subnetwork.
   Therefore, the fact that the packet may have been segmented within
   the subnetwork is not observable by the final destination.

   The ITE breaks inner IP packets no larger than 2KB into N segments (N
   <= 16) that are no larger than S-MSS bytes each, i.e., even if the
   inner packet is unfragmentable.  Each segment except the final one
   MUST be of equal length, while the final segment MAY be of different
   length and MUST be no larger than the initial segment.  The first
   byte of each segment MUST begin immediately after the final byte of
   the previous segment, i.e., the segments MUST NOT overlap.

   The ITE encapsulates each segment in a SEAL header formatted as

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |          ID Extension         |R|M|CTL|Segment|  Next Header  |

                       Figure 2: SEAL Header Format

   where the header fields are defined as follows:

   ID Extension (16)
      a 16-bit extension of the 16-bit ID field in the outer IPv4
      header; encodes the most-significant 16 bits of a 32 bit SEAL-ID

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   R (1)

   M (1)
      the "More Segments" bit.  Set to 1 if this SEAL packet contains a
      non-final segment of a multi-segment inner IP packet.

   CTL (2)
      a 2-bit "Control" field that identifies the type of SEAL packet as

      '00' - a Fragmentation Report (FRAGREP).

      '01' - a non-probe SEAL packet.

      '10' - an implicit probe.

      '11' - an explicit probe.

   Segment (4)
      a 4-bit Segment number.  Encodes a segment number between 0 - 15.

   Next Header (8)  an 8-bit field that encodes an IP protocol number
      the same as for the IPv4 protocol and IPv6 next header fields.

   For single-segment inner IP packets, the ITE encapsulates the segment
   in a SEAL header with (M=0; Segment=0).  For N-segment inner packets
   (N <= 16), the ITE encapsulates each segment in a header of the same
   format with (M=1; Segment=0) for the first segment, (M=1; Segment=1)
   for the second segment, etc., with the final segment setting (M=0;

   The ITE next sets CTL in the SEAL header of each segment according to
   the SEAL packet type (see: Section 4.6), writes the IP protocol
   number corresponding to the inner payload in the 'Next Header' field,
   and encapsulates the segment in the requisite */IPv4 outer headers.

   The ITE maintains a 32-bit SEAL-ID value as per-ETE soft state, e.g.
   in the IPv4 destination cache.  The ITE randomly-initializes SEAL-ID
   when the soft state is created and monotonically increments it
   (modulo 2^32) for each successive SEAL packet sent to the ETE.  For
   each SEAL packet, the ITE writes the least-significant 16 bits of the
   SEAL-ID value in the ID field in the outer IPv4 header, and writes
   the most-significant 16 bits in the ID Extension field in the SEAL

   The ITE finally sets other fields in the outer */IPv4 headers
   according to the specific encapsulation format (e.g., [RFC2003],

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   [RFC4213], etc.).

4.2.4.  Sending Packets

   For unfragmentable inner IP packets larger than 2KB, if the ITE is
   configured to drop the packet it sends an ICMP PTB message back to
   the original source with an MTU value of 2KB.  Otherwise, it
   determines whether the size of the packet plus the size of the SEAL/
   */IPv4 encapsulation headers is larger than the IPv4 path MTU for the
   ETE.  If the packet is too large, the ITE discards it and sends a PTB
   message back to the original source with an MTU value set to the IPv4
   path MTU minus the size of the encapsulating headers.  Otherwise, the
   ITE sets the Don't Fragment (DF) bit in the outer IPv4 header to DF=1
   and admits the packet into the tunnel.

   For inner IP packets that were no larger than 2KB before
   segmentation, the ITE sets DF=0 in the outer IPv4 header of each
   segment and sends them into the tunnel in canonical order, i.e.,
   Segment 0 first, then Segment 1, etc.

4.3.  Reassembly

4.3.1.  Reassembly Buffer Requirements

   ETEs MUST be capable of using IPv4-layer reassembly to reassemble
   SEAL packets of at least (2KB+ENCAPS) bytes, i.e., ETEs MUST
   configure an IPv4 Effective MTU to Receive (EMTU_R) of at least (2KB+

   ETEs MUST also be capable of using SEAL-layer reassembly to
   reassemble inner IP packets of at least 2KB, i.e., ETEs MUST
   configure a SEAL EMTU_R of at least 2KB.

4.3.2.  IPv4-Layer Reassembly

   The ETE performs IPv4 reassembly as-normal, and maintains a
   conservative high- and low-water mark for the number of outstanding
   reassemblies pending for each ITE per common operational practices.
   When the size of the reassembly buffer exceeds this high-water mark,
   the ETE actively discards incomplete reassemblies (e.g., using an
   Active Queue Management (AQM) strategy such as drop-eldest, Random
   Early Drop (RED), etc.) until the size falls below the low-water

   After reassembly, the ETE either accepts or discards the reassembled
   SEAL packet based on the current status of the IPv4 reassembly cache
   (congested vs uncongested).  The choice of accepting/discarding a
   reassembly may also depend on the strength of the upper-layer

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   integrity check if known (e.g., IPSec/ESP provides a strong upper-
   layer integrity check) and/or the corruption tolerance of the data
   (e.g., multicast streaming audio/video may be more corruption-
   tolerant than file transfer, etc.).

   The 32-bit SEAL-ID included in the IPv4 first-fragment provides an
   additional level of reassembly assurance, since it can record a
   distinct arrival timestamp useful for associating the first fragment
   with its corresponding non-initial fragments.

4.3.3.  SEAL-Layer Reassembly

   After any IPv4-layer reassembly, the ETE performs SEAL-layer
   reassembly for N-segment inner IP packets through simple in-order
   concatenation of the encapsulated segments from N consecutive SEAL
   packets.  These packets contain Segment numbers 0 through N-1, and
   with consecutive SEAL-ID values encoded in the 32-bit concatenation
   of the ID Extension field in the SEAL header and the ID field in the
   IPv4 header.  That is, for an N-segment packet, reassembly of the
   inner packet entails the concatenation of the encapsulated segments
   of SEAL packets with (Segment 0, SEAL-ID i), followed by (Segment 1,
   SEAL-ID ((i + 1) mod 2^32)), etc. up to (Segment N-1, SEAL-ID ((i +
   N-1) mod 2^32)).  (The SEAL header and outer */IPv4 headers are
   discarded during this process.)  This requires the ETE to maintain a
   cache of recently received SEAL packets for a hold time that would
   allow for reasonable inter-segment delays.

   As for IPv6 reassembly [RFC2460], SEAL reassembly uses a maximum
   segment lifetime of 60 seconds, i.e., the time after which an
   incomplete reassembly is discarded.  However, the ETE must also
   actively discard any pending reassemblies that appear to have no
   opportunity for completion, e.g., when a considerable number of SEAL
   packets have been received before a packet that completes the pending
   reassembly has arrived.  This assumes that any packet reordering
   within the subnetwork will be on the order of a small number of
   positions and that any gross reordering will be short-lived in

4.3.4.  Reassembly Integrity Checks

   TBD - a future version of this draft may specify an integrity check
   vector, inserted by the ITE during encapsulation and used by the ETE
   to detect packet splicing errors during IPv4 reassembly.  Such an
   integrity check capability is specified in [I-D.templin-inetmtu].

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4.4.  Generating Fragmentation Reports

   When the ETE receives the first fragment of a SEAL packet that was
   delivered as multiple IPv4 fragments and with CTL='1X' in the SEAL
   header, it generates a Fragmentation Report (FRAGREP) message to send
   back to the ITE.  The ETE also generates a FRAGREP for any SEAL
   packet with CTL='11' even if the packet was not fragmented.

   The ETE prepares the FRAGREP message by encapsulating the leading 128
   bytes (or up to the end) of the first fragment in outer SEAL/*/IPv4
   headers.  The ETE next sets CTL='00' in the SEAL header and sets the
   fields of the outer */IPv4 headers according to the specific
   encapsulation type.  In particular, the ETE sets the destination
   address of the FRAGREP to the source address that was included in the
   first fragment, and sets the source address of the FRAGREP to the
   destination address that was included in the first fragment.  If the
   destination address in the first fragment was multicast, the ETE
   instead sets the source address of the FRAGREP to an address assigned
   to the underlying IPv4 interface.

   The FRAGREP message has the following format:

   |                         |
   ~   Outer */IPv4 headers  ~
   |                         |
   |       SEAL Header       |
   |  (CTL='00', Segment=0)  |
   |                         |
   ~   First 128 bytes of    ~
   ~  IPv4 first fragment    ~
   |                         |

             Figure 3: Fragmentation Report (FRAGREP) Message

4.5.  Receiving Fragmentation Reports

   When the ITE receives a potential FRAGREP message, it first verifies
   that the message was formatted correctly by the ETE (per Section 4.4)
   and confirms that the FRAGREP matches one of the implicit/explicit
   probes that it actually sent to the ETE, e.g., by examining the
   SEAL-ID embedded in the encapsulated IPv4 first fragment.  If the
   FRAGREP matches one of its probes, the ITE advances its window of
   outstanding probes (see: Section 4.6).

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   For each FRAGREP that contains the leading portion of a whole IPv4
   packet, if the length field in the whole packet contains a value
   larger than S-MSS the ITE sets S-MSS for this ETE to this length
   minus ENCAPS.  For each FRAGREP that contains the leading portion of
   an IPv4 fragment, if the length field in the fragment contains a
   value larger than (128+ENCAPS), the ITE sets S-MSS for this ETE to
   this length minus ENCAPS; otherwise, it sets S-MSS = MIN(S-MSS/2,
   128) .

   The above "limited halving" procedure accounts for the possibility
   that the ETE receives IPv4 first fragments that were created as the
   smallest fragment (rather than the largest).  In that case,
   convergence to an acceptable S-MSS size may require multiple
   iterations of sending SEAL packets and receiving FRAGREP messages in
   a manner that parallels classical path MTU discovery [RFC1191],
   albeit with all feedback coming from the ETE and not a network
   middlebox.  This limited halving procedure ensures that convergence
   will occur quickly even in extreme cases and without packet loss,
   while the correct MTU will normally be determined in a single
   iteration since routers typically produce the first fragment as the
   largest [RFC1812].

4.6.  S-MSS Probing

   For inner IP packets no larger than 2KB, when S-MSS is larger than
   128 the ITE uses each packet as an implicit probe to detect any in-
   the-network IPv4 fragmentation.  The ITE sets CTL='10' in the SEAL
   header and DF=0 in the outer IPv4 header of each SEAL packet, and
   will receive FRAGREP messages from the ETE if fragmentation occurs.
   When S-MSS=128, the ITE instead sets CTL='01' in the SEAL header to
   avoid generating FRAGREPs for unavoidable in-the-network

   The ITE should also send explicit probes periodically to manage a
   "window" of outstanding probes that allows the ITE to validate any
   FRAGREPs it receives (e.g., by examining the SEAL-ID).  The ITE sends
   explicit probes by setting CTL='11' in the SEAL header and DF=0 in
   the IPv4 header.  The ITE can also probe for larger S-MSS values by
   sending explicit probes with trailing padding added to create a probe
   packet of up to 2KB.  When the ETE receives an explicit probe, it
   will return a FRAGREP message whether or not any in-the-network
   fragmentation occured, which the ITE will process exactly as for any
   FRAGREP per Section 4.5.

   For inner IP packets larger than 2KB, the ITE set DF=1 in the outer
   IPv4 header and and may set CTL to any value other than '00', i.e.,
   the packets may be sent as either non-probes or implicit/explict
   probes but their use for probing may be of little value.

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4.7.  Processing ICMP PTBs

   The ITE may receive ICMP PTB messages in response to any packets that
   were admitted into the tunnel with DF=1.  The ITE SHOULD consult the
   SEAL 32-bit ID included in the packet-in-error to ensure that the PTB
   corresponds to a recently-sent packet.  The ITE then records the MTU
   value from the PTB message in the IPv4 path MTU cache.  If the PTB
   message includes enough information, the ITE then translates the
   message into a suitable PTB to send back to the original source;
   otherwise, it discards the message.  During translation, the ITE sets
   the MTU value in the PTB message to MAX(2KB, the MTU reported in the
   non-translated PTB).

5.  Link Requirements

   Subnetwork designers are strongly encouraged to follow the
   recommendations in [RFC3819] when configuring link MTUs.

6.  End System Requirements

   End systems that send unfragmentable IP packets larger than 1500
   bytes are strongly encouraged to use Packetization Layer Path MTU
   Discovery per [RFC4821], since the network may not always be able to
   return ICMP PTB messages in 1-to-1 correspondence with dropped

7.  Router Requirements

   IPv4 routers observe the requirements in [RFC1812].

8.  IANA Considerations

   A new IP protocol number for the SEAL protocol is requested.

9.  Security Considerations

   Unlike IPv4 fragmentation, overlapping fragment attacks are not
   possible due to the requirement that SEAL segments be non-

   An amplification/reflection attack is possible when an attacker sends
   spoofed IPv4 first fragments to an ETE, resulting in a stream of
   FRAGREP messages returned to a victim ITE.  The encapsulated segment

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   of the spoofed IPv4 first fragment provides mitigation for the ITE to
   detect and discard spurious FRAGREPs.

   The SEAL header is sent in-the-clear (outside of any IPsec/ESP
   encapsulations) the same as for the IPv4 header.  As for IPv6
   extension headers, the SEAL header is also protected only by L2
   integrity checks, and is not covered under any L3 integrity checks.

10.  Acknowledgments

   Path MTU determination through the report of fragmentation
   experienced by the final destination was first proposed by Charles
   Lynn of BBN on the TCP-IP mailing list in May 1987.  An historical
   analysis of the evolution of path MTU discovery appears in and is
   reproduced in Appendix A of this document.

   The following individuals are acknowledged for helpful comments and
   suggestions: Jari Arkko, Fred Baker, Teco Boot, Iljitsch van Beijnum,
   Brian Carpenter, Steve Casner, Ian Chakeres, Remi Denis-Courmont,
   Aurnaud Ebalard, Gorry Fairhurst, Joel Halpern, John Heffner, Bob
   Hinden, Christian Huitema, Joe Macker, Matt Mathis, Dan Romascanu,
   Dave Thaler, Joe Touch, Magnus Westerlund, Robin Whittle, and James

11.  References

11.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

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

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

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

11.2.  Informative References

   [FOLK]     C, C., D, D., and k. k, "Beyond Folklore: Observations on
              Fragmented Traffic", December 2002.

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   [FRAG]     Kent, C. and J. Mogul, "Fragmentation Considered Harmful",
              October 1987.

              Farinacci, D., "Locator/ID Separation Protocol (LISP)",
              draft-farinacci-lisp-06 (work in progress), February 2008.

              Macker, J. and S. Team, "Simplified Multicast Forwarding
              for MANET", draft-ietf-manet-smf-07 (work in progress),
              February 2008.

              Templin, F., Russert, S., and S. Yi, "The MANET Virtual
              Ethernet (VET) Abstraction",
              draft-templin-autoconf-dhcp-14 (work in progress),
              April 2008.

              Templin, F., "Simple Protocol for Robust IP/*/IP Tunnel
              Endpoint MTU Determination  (sprite-mtu)",
              draft-templin-inetmtu-06 (work in progress),
              November 2007.

   [MTUDWG]   "IETF MTU Discovery Working Group mailing list,
    , November
              1989 - February 1995.".

   [RFC1063]  Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP
              MTU discovery options", RFC 1063, July 1988.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              November 1990.

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, August 1996.

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

   [RFC2004]  Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
              October 1996.

   [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery",
              RFC 2923, September 2000.

   [RFC3819]  Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.

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              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, July 2004.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.

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

   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380,
              February 2006.

   [RFC4459]  Savola, P., "MTU and Fragmentation Issues with In-the-
              Network Tunneling", RFC 4459, April 2006.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, March 2007.

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

   [TCP-IP]   "TCP-IP mailing list archives,
    , May
              1987 - May 1990.".

Appendix A.  Historic Evolution of PMTUD (written 10/30/2002)

   The topic of Path MTU discovery (PMTUD) saw a flurry of discussion
   and numerous proposals in the late 1980's through early 1990.  The
   initial problem was posed by Art Berggreen on May 22, 1987 in a
   message to the TCP-IP discussion group [TCP-IP].  The discussion that
   followed provided significant reference material for [FRAG].  An IETF
   Path MTU Discovery Working Group [MTUDWG] was formed in late 1989
   with charter to produce an RFC.  Several variations on a very few
   basic proposals were entertained, including:

   1.  Routers record the PMTUD estimate in ICMP-like path probe
       messages (proposed in [FRAG] and later [RFC1063])

   2.  The destination reports any fragmentation that occurs for packets
       received with the "RF" (Report Fragmentation) bit set (Steve
       Deering's 1989 adaptation of Charles Lynn's Nov. 1987 proposal)

   3.  A hybrid combination of 1) and Charles Lynn's Nov. 1987 proposal
       (straw RFC draft by McCloughrie, Fox and Mogul on Jan 12, 1990)

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   4.  Combination of the Lynn proposal with TCP (Fred Bohle, Jan 30,

   5.  Fragmentation avoidance by setting "IP_DF" flag on all packets
       and retransmitting if ICMPv4 "fragmentation needed" messages
       occur (Geof Cooper's 1987 proposal; later adapted into [RFC1191]
       by Mogul and Deering).

   Option 1) seemed attractive to the group at the time, since it was
   believed that routers would migrate more quickly than hosts.  Option
   2) was a strong contender, but repeated attempts to secure an "RF"
   bit in the IPv4 header from the IESG failed and the proponents became
   discouraged. 3) was abandoned because it was perceived as too
   complicated, and 4) never received any apparent serious
   consideration.  Proposal 5) was a late entry into the discussion from
   Steve Deering on Feb. 24th, 1990.  The discussion group soon
   thereafter seemingly lost track of all other proposals and adopted
   5), which eventually evolved into [RFC1191] and later [RFC1981].

   In retrospect, the "RF" bit postulated in 2) is not needed if a
   "contract" is first established between the peers, as in proposal 4)
   and a message to the MTUDWG mailing list from jrd@PTT.LCS.MIT.EDU on
   Feb 19. 1990.  These proposals saw little discussion or rebuttal, and
   were dismissed based on the following the assertions:

   o  routers upgrade their software faster than hosts

   o  PCs could not reassemble fragmented packets

   o  Proteon and Wellfleet routers did not reproduce the "RF" bit
      properly in fragmented packets

   o  Ethernet-FDDI bridges would need to perform fragmentation (i.e.,
      "translucent" not "transparent" bridging)

   o  the 16-bit IP_ID field could wrap around and disrupt reassembly at
      high packet arrival rates

   The first four assertions, although perhaps valid at the time, have
   been overcome by historical events leaving only the final to
   consider.  But, [FOLK] has shown that IP_ID wraparound simply does
   not occur within several orders of magnitude the reassembly timeout
   window on high-bandwidth networks.

   (Authors 2/11/08 note: this final point was based on a loose
   interpretation of [FOLK], and is more accurately addressed in

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Author's Address

   Fred L. Templin (editor)
   Boeing Phantom Works
   P.O. Box 3707
   Seattle, WA  98124


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