Network Working Group                                    F. Templin, Ed.
Internet-Draft                                      Boeing Phantom Works
Intended status: Informational                         February 13, 2008
Expires: August 16, 2008

             Subnetwork Encapsulation and Adaptation Layer

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   Subnetworks connect routers within a bounded region, and may also
   connect to other networks including the Internet.  These routers
   forward unicast and multicast packets over paths that span multiple
   IP- and/or sub-IP layer forwarding hops which may cross links with
   diverse Maximum Transmission Units (MTUs) and introduce packet
   duplication.  This document specifies a Subnetwork Encapsulation and
   Adaptation Layer (SEAL) that supports simplified duplicate packet
   detection and accommodates links with diverse MTUs.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology and Requirements . . . . . . . . . . . . . . . . .  3
   3.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  4
   4.  SEAL Protocol Specification  . . . . . . . . . . . . . . . . .  5
     4.1.  Model of Operation . . . . . . . . . . . . . . . . . . . .  5
     4.2.  Packetization  . . . . . . . . . . . . . . . . . . . . . .  6
       4.2.1.  Packet Size Considerations . . . . . . . . . . . . . .  6
       4.2.2.  Inner IPv4 Fragmentation . . . . . . . . . . . . . . .  7
       4.2.3.  SEAL Segmentation and Encapsulation  . . . . . . . . .  7
       4.2.4.  Setting DF and Sending Packets . . . . . . . . . . . . 10
     4.3.  Reassembly . . . . . . . . . . . . . . . . . . . . . . . . 11
       4.3.1.  Reassembly Buffer Requirements . . . . . . . . . . . . 11
       4.3.2.  IPv4-Layer Reassembly  . . . . . . . . . . . . . . . . 11
       4.3.3.  SEAL-Layer Reassembly  . . . . . . . . . . . . . . . . 11
     4.4.  Generating Fragmentation Reports . . . . . . . . . . . . . 12
     4.5.  Receiving Fragmentation Reports  . . . . . . . . . . . . . 13
     4.6.  S-MSS Probing  . . . . . . . . . . . . . . . . . . . . . . 13
     4.7.  Processing ICMP PTBs . . . . . . . . . . . . . . . . . . . 14
   5.  Link Requirements  . . . . . . . . . . . . . . . . . . . . . . 14
   6.  End System Requirements  . . . . . . . . . . . . . . . . . . . 15
   7.  Router Requirements  . . . . . . . . . . . . . . . . . . . . . 15
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 15
   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 . . . . . . . . . . . . . . . . . . . . . . . . . 19
   Intellectual Property and Copyright Statements . . . . . . . . . . 21

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1.  Introduction

   Mobile Ad-hoc Networks (MANETs) and other subnetworks connect routers
   on links with asymmetric reachability characteristics, and may also
   connect to other networks including the Internet.  These routers
   forward unicast and multicast packets over paths that span multiple
   IP- and/or sub-IP layer forwarding hops, which may traverse links
   with diverse Maximum Transmission Units (MTUs) and may also introduce
   packet duplication due to temporal or persistent routing loops.  It
   is also expected that these routers will support operation of the
   Internet protocols [RFC0791][RFC2460].

   The use of IPv4 encapsulation has long been considered as an
   alternative for introducing a well-behaved identification field
   useful for duplicate packet detection, such as required for
   Simplified Multicast Forwarding [I-D.ietf-manet-smf].  However, the
   16-bit ID field in the outer IPv4 header supports only 2^16 distinct
   identification values and therefore does not provide sufficient space
   for robust duplicate packet detection over modern link technologies.

   Additionally, the insertion of an outer IPv4 header reduces the
   effective path MTU as-seen by the IP layer.  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][RFC2923][RFC4459][RFC4963].

   This document proposes a Subnetwork Encapsulation and Adaptation
   Layer (SEAL) for the operation of IP over subnetworks that connect
   Ingress- and Egress Tunnel Endpoints (ITEs/ETEs).  SEAL supports
   simple and robust duplicate packet detection, and accommodates links
   with diverse MTUs.  SEAL additionally supports multiprotocol
   operation and provides extended quality of service for the protocols
   that use it.  The SEAL protocol is specified in the following

2.  Terminology and Requirements

   The terminology of [RFC3819][RFC2501][I-D.ietf-autoconf-manetarch] is
   used in this document.  The following abbreviations correspond to
   terms used within this document and elsewhere in common
   Internetworking nomenclature:

      MANET - Mobile Ad-hoc Network

      Subnetwork - a MANET or other network that connects (and is
      bounded by) ITEs and ETEs

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      SEAL - Subnetwork Encapsulation and Adaptation Layer

      VET - Virtual EThernet

      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

      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 packet encapsulated in outer
      SEAL/*/IPv4 headers

      SEAL ID - a 32-bit Identification value that is randomly
      initialized and monotonically incremented for each SEAL packet
      sent to an ETE

      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.

   While the SEAL approach was motivated by the specific use case of
   duplicate packet detection in MANETs, the domain of applicability is
   not limited to the MANET problem space and extends to other
   subnetwork uses such as tunneling across enterprise networks, the
   interdomain routing core, etc.

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   For further study, SEAL may also be useful for "transport-mode"
   applications, e.g., when the inner packet encapsulates ordinary
   protocol data rather than an 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 ITR/ETR subnetwork entry/exit points, respectively.  SEAL
   defines a new IP protocol type and a new mid-layer encapsulation for
   both unicast and multicast inner 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.

   o  For tunnel-mode IPsec/ESP encapsulations over IPv4,
      [RFC4301][RFC4303], the SEAL header is inserted between the ESP

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      and outer IPv4 headers as: IP/*/ESP/SEAL/IPv4.

   o  For IP encapsulations over transports such as UDP (e.g.,
      [RFC4380][I-D.farinacci-lisp]), the SEAL header is embedded in any
      middle- and outer-'*' encapsulations within the transport layer,
      e.g., as IP/*/SEAL/*/UDP/IPv4.

   Encapsulation and tunneling establishes a virtual point-to-multipoint
   interface abstraction of the subnetwork.  From a logical viewpoint,
   this interface appears as a Virtual EThernet (VET)
   [I-D.templin-autoconf-dhcp] that connects the ITE to all ETEs in the
   subnetwork as single-hop neighbors.  From a physical perspective,
   however, packets sent over the VET interface may be forwarded across
   many IPv4 and/or sub-IPv4 layer subnetwork 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 both duplicate packet detection within the subnetwork and also
   for multi-level segmentation and reassembly of large packets.

   SEAL enables a multi-level segmentation and reassembly capability.
   First, the ITE can use inner 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 packets
   without incurring IPv4 fragmentation on the outer packet.  Finally,
   ordinary IPv4 fragmentation for the outer IPv4 packet after SEAL
   encapsulation is also 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 are not delivered reliably, and any PTBs coming
   from within the subnetwork could be erroneous or maliciously
   fabricated.  The ITE therefore requires a means for conveying 1500
   byte (or smaller) original packets over the VET interface without
   loss due to link MTU restrictions and/or triggering PTB messages from
   within the subnetwork.

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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.
   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 over the VET interface even though there
   may be subnetwork links 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, set to
   1KB by default and configurable within the range of [128 - 2KB].  The
   ITE also institutes a [S-MSS - 2KB] segmentation region such that all
   inner packets within this size range are segmented into multiple SEAL
   packets.  For 1500 byte and smaller inner packets/fragments, the 2KB
   upper bound allows for ~500 bytes of additional subnetwork
   encapsulation overhead on the path from the original source to the
   ITE.  Similarly, the default 1KB lower bound allows ~500 bytes of
   additional encapsulation on the path between the ITE and ETE to
   accommodate each SEAL packet while avoiding IPv4 fragmentation along
   most paths within subnetwork that deploy 1500 byte links.

   The ITE additionally admits all inner packets larger than 2KB into
   the VET interface as single-segment SEAL packets under the assumption
   that original sources that send packets larger than 1500 bytes are
   using an end-to-end MTU determination capability such as specified in

4.2.2.  Inner IPv4 Fragmentation

   The IP layer 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 2KB before any mid-layer '*' encapsulations.  (It is also
   recommended that the fragment size be chosen small enough so as to
   avoid any SEAL segmentation and/or outer IPv4 fragmentation if
   possible).  The IP 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 inner IPv4 fragmentation, the ITE encapsulates the IPv4 packet/
   fragment in any mid-layer '*' headers, then performs segmentation on
   this inner packet based on a segment size that is likely to avoid

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   IPv4 fragmentation within the subnetwork.  The ITE maintains a SEAL
   Maximum Segment Size (S-MSS) variable for each ETR as per-ETR IPv4
   destination cache soft state, including IPv4 multicast destinations.
   S-MSS SHOULD be initialized to 1KB by default, and MAY be changed to
   different values in the range [128, 2KB] based on static
   configuration and/or dynamic segment size probing.

   The ITE MUST NOT break unfragmentable inner packets larger than 2KB
   into smaller segments, but rather MUST encapsulate them as a single
   segment SEAL packet.  The ITE breaks inner 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.  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 in
   either minimal- or extended- formats according to the following

       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 A |

                   Figure 2: Minimal SEAL Header Format

       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|       0       |
      |  RSVD |               Flow Label              | Next Header B |

                   Figure 3: Extended 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)
      Reserved, and must be zero.

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

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

      '00' - an ordinary SEAL packet.

      '01' - a Fragmentation Report (FRAGREP).

      '10' - an implicit probe.

      '11' - an explicit probe.

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

   Next Header A (8)  an 8-bit field that encodes either an IP protocol
      number the same as for the IPv4 protocol and IPv6 next header
      fields, or the value zero.  When Next Header A is non-zero, the
      SEAL header is in minimal format; otherwise, the SEAL header is in
      extended format.

   RSVD  a 4-bit Reserved field, present only in extended format.  Must
      be zero.

   Flow Label (20)  a 20-bit flow label field, present only in extended
      format.  Contains a 20-bit value corresponding to the inner packet
      during SEAL encapsulation.

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

   For N-segment inner packets (N <= 16), the ITE selects a SEAL header
   format (minimal or extended) and 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; Segment=N-1).  Note that single-segment
   inner packets instead set (M=0; Segment=0).

   During encapsulation, the ITE also sets CTL='00' in the SEAL header
   of each segment if this segment is not to be used as an explicit or
   implicit probe.  Otherwise, the ITE sets CTL='10' or '11' according

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   to the type of probe desired (see: Section 4.6).

   The ITE next writes either the IP protocol number corresponding to
   the inner packet (minimal format) or the value zero (extended format)
   in 'Next Header A' in the SEAL header of each segment.  When extended
   format is used, the ITE also writes a 20-bit flow label value
   corresponding to the inner packet into the Flow Label field and
   writes the IP protocol number corresponding to the inner packet in
   'Next Header B'.  The ITE then encapsulates the segment in the
   requisite */IPv4 outer headers.

   The ITE maintains a 32-bit SEAL-ID value as per-ETE soft state in the
   IPv4 destination cache.  The value is randomly-initialized when the
   soft state is created and monotonically incremented (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 header.

   The ITE finally sets other fields in the outer */IPv4 headers
   according to the specific encapsulation format (e.g., [RFC2003],
   [RFC4213], etc.).

4.2.4.  Setting DF and Sending Packets

   For inner packets larger than 2KB, the ITE determines whether the
   size of the packet plus the size of the SEAL/*/IPv4 encapsulation
   headers is larger than the MTU of the underlying interface over which
   the tunnel is configured.  If the packet is too large, the ITE
   discards it and sends an ICMP PTB message back to the original source
   with an MTU value taken from the underlying interface minus the size
   of the encapsulation headers.  Otherwise, the ITE sets the Don't
   Fragment (DF) bit in the outer IPv4 header to DF=1.

   For inner packets that were no larger than 2KB before segmentation,
   the ITE sets DF=0 or DF=1 in the outer IPv4 header of each SEAL
   packet according to the desired behavior as follows:

   o  if the ITE is probing the path to the ETE, it MUST set DF=0 to
      allow the ETE to sense and report fragmentation.

   o  if S-MSS=128, the ITE MUST set DF=0 in case any unavoidable in-
      the-network IPv4 fragmentation is required.

   o  if the ITE has recently probed the path to the ETE, it MAY set
      DF=1 in subsequent SEAL packets until the next probing cycle.

   After setting the DF bits, the ITE SHOULD send all SEAL packets that

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   encapsulate segments of the same inner packet into the VET interface
   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 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 as is common for widely deployed
   implementations.  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 mark.

   Note that in the limiting case the ETE may choose to discard all
   reassemblies for packets that set CTL='1X' in the SEAL header and
   only perform reassembly for packets that set CTL='0X' in the SEAL
   header (see; Section 4.4).

4.3.3.  SEAL-Layer Reassembly

   After any IPv4-layer reassembly, the ETE performs SEAL-layer
   reassembly for N-segment inner 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 inner packet, inner packet
   reassembly entails the concatenation of the segments from 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)).  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.

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   Rather than set an absolute hold time, the ETE must 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 nature.

4.4.  Generating Fragmentation Reports

   When the ETE has received at least the leading 128 bytes (or up to
   the end) 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 over the VET
   interface to the original source.  The ETE also generates a FRAGREP
   for any SEAL packet with CTL='11' in the SEAL header (see: Section
   4.6), i.e. even if the packet was not fragmented.

   The ETE prepares the FRAGREP message by encapsulating the leading 128
   bytes of the fragmented SEAL packet in an outer SEAL/*/IPv4 header.
   The ETE sets the IPv4 length field in the encapsulated packet to the
   length of the largest IPv4 fragment received, i.e., even if the
   largest fragment received was not the first fragment.

   The ETE next sets CTL='01' and Segment=0 in the SEAL header and sets
   the fields of the IPv4 header set 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
   IPv4 first fragment, and sets the source address of the FRAGREP to
   the destination address that was included in the IPv4 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:

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   |                         |
   ~   Outer */IPv4 headers  ~
   |                         |
   |       SEAL Header       |
   |  (CTL='01'; Segment=0)  |
   |                         |
   ~ Up to 128 bytes of pkt, ~
   ~  with IPv4 len set to   ~
   | len of largest fragment |
   |                         |

             Figure 4: 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 corresponds to one of the SEAL packets
   that it actually sent to the ETE by examining the encapsulated IPv4

   For a valid FRAGREP, if the length field in the encapsulated IPv4
   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) .  This limited halving procedure accounts
   for the possibility that the ETE received the leading 128 bytes of
   the fragmented SEAL packet in IPv4 fragments that were significantly
   smaller than the path MTU.  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 path MTU
   feedback coming from the ETE and not a network middlebox.  But, the
   limited halving procedure ensures that convergence will occur quickly
   even in extreme cases, while the correct MTU will normally be
   determined in a single iteration since routers that use IPv4
   fragmentation are recommended to produce the minimum number of
   fragments [RFC1812].

4.6.  S-MSS Probing

   When S-MSS is larger than 128, the ITE MUST probe the path to the ETE
   periodically to detect and dampen any in-the-network IPv4
   fragmentation.  The ITE performs implicit probing of the path by
   setting CLT='10' in the SEAL header and DF=0 in the IPv4 header of

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   all SEAL packets containing segments of the same inner packet used
   for probing.  If any in-the-network fragmentation occurs, the ITE
   will receive verifiable FRAGREP messages from the ETE.

   The ITE can also send explicit probes to periodically probe for
   larger S-MSS values (to a maximum of 2KB) by sending single-segment
   SEAL packets with CTL='11' in the SEAL header and DF=0 in the IPv4
   header.  The ETE will return a FRAGREP message whether or not any in-
   the-network fragmentation occurs, which the ITE will process exactly
   as for any FRAGREP per Section 4.5.  The ITE MAY pad the length of
   SEAL packets used for explicit probing (to a maximum size of 2KB+
   ENCAPS) if permitted by the specific */IPv4 encapsulation method.

   The ITE can optionally send intervening SEAL packets between probing
   intervals as passive probes by setting DF=0, or as non-probes by
   setting DF=1.

   When S-MSS=128, the ITE MUST set CTL='00' in the SEAL header of each
   SEAL packet that is not being used as an explicit probe such that the
   ETE will not generate FRAGREPs for unavoidable in-the-network

4.7.  Processing ICMP PTBs

   The ITE may receive ICMP PTB messages in response to any packets that
   were admitted into the VET interface with DF=1.  The ITE may
   optionally ignore, log, or honor the messages according to the
   subnetwork trust basis.  For example, ITEs connected to subnetworks
   managed under a single administrative domain may be configured to
   honor ICMP PTBs while ITEs connected to the global interdomain
   routing core may be configured to ignore/log them.

   When ICMP PTBs are honored, the ITE:

   o  SHOULD send translated ICMP PTB messages back to the original
      source (if possible) for ICMP PTBs that correspond to SEAL packets
      that encapsulate a segment larger than 2KB.

   o  SHOULD treat ICMP PTBs that correspond to SEAL packets that
      encapsulate segments no larger than 2KB as an indication to resume

5.  Link Requirements

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

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6.  End System Requirements

   SEAL is a router-to-router encapsulation protocol and therefore makes
   no requirements for end systems.  However, end systems that send
   unfragmentable IP packets of 1501 bytes or larger are strongly
   encouraged to use Packetization Layer Path MTU Discovery per
   [RFC4821], since the network may not always be able to return useful
   ICMP PTB messages.

7.  Router Requirements

   IPv4 routers observe the requirements in [RFC1812].

8.  IANA Considerations

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

   A new IPv4 site-scoped ALL_MANET_ROUTERS multicast group is

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 fragments to an ETE with CTL='1X' in the SEAL header,
   resulting in a stream of FRAGREP messages returned to a victim ITE.
   The encapsulated segment of the spoofed IPv4 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 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

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   reproduced in Appendix A of this document.

   This work was inspired in part by discussions on the IETF MANET and
   IRTF RRG mailing lists in the 12/07 - 01/08 timeframe, and the author
   acknowledges those who participated in the discussions.  The work
   also draws on the earlier investigations of [I-D.templin-inetmtu]
   which acknowledges many who contributed to the effort.

   The extended SEAL header format was inspired by recent discussions.

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.

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

              Farinacci, D., "Locator/ID Separation Protocol (LISP)",
              draft-farinacci-lisp-05 (work in progress), November 2007.

              Chakeres, I., Macker, J., and T. Clausen, "Mobile Ad hoc
              Network Architecture", draft-ietf-autoconf-manetarch-07
              (work in progress), November 2007.

              Macker, J. and S. Team, "Simplified Multicast Forwarding
              for MANET", draft-ietf-manet-smf-06 (work in progress),
              November 2007.

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              Templin, F., Russert, S., and S. Yi, "MANET
              Autoconfiguration", draft-templin-autoconf-dhcp-11 (work
              in progress), February 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.

   [RFC2501]  Corson, M. and J. Macker, "Mobile Ad hoc Networking
              (MANET): Routing Protocol Performance Issues and
              Evaluation Considerations", RFC 2501, January 1999.

   [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.
              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.

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   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, 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)

   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]

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       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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