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rfc:rfc5320

Independent Submission F. Templin, Ed. Request for Comments: 5320 Boeing Research & Technology Category: Experimental February 2010 ISSN: 2070-1721

      The Subnetwork Encapsulation and Adaptation Layer (SEAL)

Abstract

 For the purpose of this document, subnetworks are defined as virtual
 topologies that span connected network regions bounded by
 encapsulating border nodes.  These virtual topologies may span
 multiple IP and/or sub-IP layer forwarding hops, and can introduce
 failure modes due to packet duplication and/or 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
 technologies.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for examination, experimental implementation, and
 evaluation.
 This document defines an Experimental Protocol for the Internet
 community.  This is a contribution to the RFC Series, independently
 of any other RFC stream.  The RFC Editor has chosen to publish this
 document at its discretion and makes no statement about its value for
 implementation or deployment.  Documents approved for publication by
 the RFC Editor are not a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc5320.

IESG Note

 This RFC is not a candidate for any level of Internet Standard.  The
 IETF disclaims any knowledge of the fitness of this RFC for any
 purpose and in particular notes that the decision to publish is not
 based on IETF review for such things as security, congestion control,
 or inappropriate interaction with deployed protocols.  The RFC Editor
 has chosen to publish this document at its discretion.  Readers of
 this document should exercise caution in evaluating its value for
 implementation and deployment.  See RFC 3932 for more information.

Templin Experimental [Page 1] RFC 5320 SEAL February 2010

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
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.

Templin Experimental [Page 2] RFC 5320 SEAL February 2010

Table of Contents

 1. Introduction ....................................................4
    1.1. Motivation .................................................4
    1.2. Approach ...................................................6
 2. Terminology and Requirements ....................................6
 3. Applicability Statement .........................................7
 4. SEAL Protocol Specification - Tunnel Mode .......................8
    4.1. Model of Operation .........................................8
    4.2. ITE Specification .........................................10
         4.2.1. Tunnel Interface MTU ...............................10
         4.2.2. Accounting for Headers .............................11
         4.2.3. Segmentation and Encapsulation .....................12
         4.2.4. Sending Probes .....................................14
         4.2.5. Packet Identification ..............................15
         4.2.6. Sending SEAL Protocol Packets ......................15
         4.2.7. Processing Raw ICMPv4 Messages .....................15
         4.2.8. Processing SEAL-Encapsulated ICMPv4 Messages .......16
    4.3. ETE Specification .........................................17
         4.3.1. Reassembly Buffer Requirements .....................17
         4.3.2. IPv4-Layer Reassembly ..............................17
         4.3.3. Generating SEAL-Encapsulated ICMPv4
                Fragmentation Needed Messages ......................18
         4.3.4. SEAL-Layer Reassembly ..............................19
         4.3.5. Delivering Packets to Upper Layers .................20
 5. SEAL Protocol Specification - Transport Mode ...................20
 6. Link Requirements ..............................................21
 7. End System Requirements ........................................21
 8. Router Requirements ............................................21
 9. IANA Considerations ............................................21
 10. Security Considerations .......................................21
 11. Related Work ..................................................22
 12. SEAL Advantages over Classical Methods ........................22
 13. Acknowledgments ...............................................24
 14. References ....................................................24
    14.1. Normative References .....................................24
    14.2. Informative References ...................................24
 Appendix A. Historic Evolution of PMTUD ...........................27
 Appendix B. Reliability Extensions ................................29

Templin Experimental [Page 3] RFC 5320 SEAL February 2010

1. Introduction

 As Internet technology and communication has grown and matured, many
 techniques have developed that use virtual topologies (including
 tunnels of one form or another) over an actual network that supports
 the Internet Protocol (IP) [RFC0791][RFC2460].  Those virtual
 topologies have elements that 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 that are often not
 even visible to the endpoints of the virtual hop.  This introduces
 failure modes that are not dealt with well in current approaches.
 The use of IP encapsulation has long been considered as the means 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 found to be harmful through operational
 experience and studies conducted over the course of many years
 [FRAG][FOLK][RFC4963].  Additionally, classical path MTU discovery
 [RFC1191] has known operational issues that are exacerbated by in-
 the-network tunnels [RFC2923][RFC4459].  In the following
 subsections, we present further details on the motivation and
 approach for addressing these issues.

1.1. Motivation

 Before discussing the approach, it is necessary to first understand
 the problems.  In both the Internet and private-use networks today,
 IPv4 is ubiquitously deployed as the Layer 3 protocol.  The two
 primary functions of IPv4 are to provide for 1) addressing, and 2) a
 fragmentation and reassembly capability used to accommodate links
 with diverse MTUs.  While it is well known that the addressing
 properties of IPv4 are limited (hence, the larger address space
 provided by IPv6), there is a lesser-known but growing consensus that
 other limitations may be unable to sustain continued growth.
 First, the IPv4 header Identification field is only 16 bits in
 length, meaning that at most 2^16 packets pertaining to the same
 (source, destination, protocol, Identification)-tuple may be active
 in the Internet at a given time.  Due to the escalating deployment of
 high-speed links (e.g., 1Gbps Ethernet), however, this number may
 soon become too small by several orders of magnitude.  Furthermore,
 there are many well-known limitations pertaining to IPv4
 fragmentation and reassembly -- even to the point that it has been
 deemed "harmful" in both classic and modern-day studies (cited
 above).  In particular, IPv4 fragmentation raises issues ranging from

Templin Experimental [Page 4] RFC 5320 SEAL February 2010

 minor annoyances (e.g., slow-path processing in routers) to the
 potential for major integrity issues (e.g., mis-association of the
 fragments of multiple IP packets during reassembly).
 As a result of these perceived limitations, a fragmentation-avoiding
 technique for discovering the MTU of the forward path from a source
 to a destination node was devised through the deliberations of the
 Path MTU Discovery Working Group (PMTUDWG) during the late 1980's
 through early 1990's (see Appendix A).  In this method, the source
 node provides explicit instructions to routers in the path to discard
 the packet and return an ICMP error message if an MTU restriction is
 encountered.  However, this approach has several serious shortcomings
 that lead to an overall "brittleness".
 In particular, site border routers in the Internet are being
 configured more and more to discard ICMP error messages coming from
 the outside world.  This is due in large part to the fact that
 malicious spoofing of error messages in the Internet is made simple
 since there is no way to authenticate the source of the messages.
 Furthermore, when a source node that requires ICMP error message
 feedback when a packet is dropped due to an MTU restriction does not
 receive the messages, a path MTU-related black hole occurs.  This
 means that the source will continue to send packets that are too
 large and never receive an indication from the network that they are
 being discarded.
 The issues with both IPv4 fragmentation and this "classical" method
 of path MTU discovery are exacerbated further when IP-in-IP tunneling
 is used.  For example, site border routers that are configured as
 ingress tunnel endpoints may be required to forward packets into the
 subnetwork on behalf of hundreds, thousands, or even more original
 sources located within the site.  If IPv4 fragmentation were used,
 this would quickly wrap the 16-bit Identification field and could
 lead to undetected data corruption.  If classical IPv4 path MTU
 discovery were used instead, the site border router may be bombarded
 by ICMP error messages coming from the subnetwork that may be either
 untrustworthy or insufficiently provisioned to allow translation into
 error message to be returned to the original sources.
 The situation is exacerbated further still by IPsec tunnels, since
 only the first IPv4 fragment of a fragmented packet contains the
 transport protocol selectors (e.g., the source and destination ports)
 required for identifying the correct security association rendering
 fragmentation useless under certain circumstances.  Even worse, there
 may be no way for a site border router that configures an IPsec
 tunnel to transcribe the encrypted packet fragment contained in an

Templin Experimental [Page 5] RFC 5320 SEAL February 2010

 ICMP error message into a suitable ICMP error message to return to
 the original source.  Due to these many limitations, a new approach
 to accommodate links with diverse MTUs is necessary.

1.2. Approach

 For the purpose of this document, subnetworks are defined as virtual
 topologies that span connected network regions bounded by
 encapsulating border nodes.  Examples include the global Internet
 interdomain routing core, Mobile Ad hoc Networks (MANETs) and
 enterprise networks.  Subnetwork border nodes forward unicast and
 multicast IP packets over the virtual topology across multiple IP
 and/or sub-IP layer forwarding hops that may introduce packet
 duplication and/or traverse links with diverse Maximum Transmission
 Units (MTUs).
 This document introduces a Subnetwork Encapsulation and Adaptation
 Layer (SEAL) for tunnel-mode operation of IP over subnetworks that
 connect Ingress and Egress Tunnel Endpoints (ITEs/ETEs) of border
 nodes.  Operation in transport mode is also supported when subnetwork
 border node upper-layer protocols negotiate the use of SEAL during
 connection establishment.  SEAL accommodates links with diverse MTUs
 and supports efficient duplicate packet detection by introducing a
 minimal mid-layer encapsulation.
 The SEAL encapsulation introduces an extended Identification field
 for packet identification and a mid-layer segmentation and reassembly
 capability that allows simplified cutting and pasting of packets.
 Moreover, SEAL senses in-the-network IPv4 fragmentation as a "noise"
 indication that packet sizing parameters are "out of tune" with
 respect to the network path.  As a result, SEAL can naturally tune
 its packet sizing parameters to eliminate the in-the-network
 fragmentation.
 The SEAL encapsulation layer and protocol are specified in the
 following sections.

2. Terminology and Requirements

 The terms "inner", "mid-layer", and "outer", respectively, refer to
 the innermost IP (layer, protocol, header, packet, etc.) before any
 encapsulation, the mid-layer IP (protocol, header, packet, etc.)
 after any mid-layer '*' encapsulation, and the outermost IP (layer,
 protocol, header, packet etc.) after SEAL/*/IPv4 encapsulation.
 The term "IP" used throughout the document refers to either Internet
 Protocol version (IPv4 or IPv6).  Additionally, the notation
 IPvX/*/SEAL/*/IPvY refers to an inner IPvX packet encapsulated in any

Templin Experimental [Page 6] RFC 5320 SEAL February 2010

 mid-layer '*' encapsulations, followed by the SEAL header, followed
 by any outer '*' encapsulations, followed by an outer IPvY header,
 where the notation "IPvX" means either IP protocol version (IPv4 or
 IPv6).
 The following abbreviations correspond to terms used within this
 document and elsewhere in common Internetworking nomenclature:
    ITE - Ingress Tunnel Endpoint
    ETE - Egress Tunnel Endpoint
    PTB - an ICMPv6 "Packet Too Big" or an ICMPv4 "Fragmentation
          Needed" message
    DF - the IPv4 header "Don't Fragment" flag
    MHLEN - the length of any mid-layer '*' headers and trailers
    OHLEN - the length of the outer encapsulating SEAL/*/IPv4 headers
    HLEN - the sum of MHLEN and OHLEN
    S_MRU - the per-ETE SEAL Maximum Reassembly Unit
    S_MSS - the SEAL Maximum Segment Size
    SEAL_ID - a 32-bit Identification value, randomly initialized and
              monotonically incremented for each SEAL protocol packet
    SEAL_PROTO - an IPv4 protocol number used for SEAL
    SEAL_PORT - a TCP/UDP service port number used for SEAL
    SEAL_OPTION - a TCP option number used for (transport-mode) SEAL
 The key words MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
 SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
 document, are to be interpreted as described in [RFC2119].

3. Applicability Statement

 SEAL was motivated by the specific case of subnetwork abstraction for
 Mobile Ad hoc Networks (MANETs); however, the domain of applicability
 also extends to subnetwork abstractions of enterprise networks, the
 interdomain routing core, etc.  The domain of application therefore

Templin Experimental [Page 7] RFC 5320 SEAL February 2010

 also includes the map-and-encaps architecture proposals in the IRTF
 Routing Research Group (RRG) (see http://www3.tools.ietf.org/group/
 irtf/trac/wiki/RoutingResearchGroup).
 SEAL introduces a minimal new sublayer for IPvX in IPvY encapsulation
 (e.g., as IPv6/SEAL/IPv4), and appears as a subnetwork encapsulation
 as seen by the inner IP layer.  SEAL can also be used as a sublayer
 for encapsulating inner IP packets within outer UDP/IPv4 headers
 (e.g., as IPv6/SEAL/UDP/IPv4) such as for the Teredo domain of
 applicability [RFC4380].  When it appears immediately after the outer
 IPv4 header, the SEAL header is processed exactly as for IPv6
 extension headers.
 SEAL can also be used in "transport-mode", e.g., when the inner layer
 includes upper-layer protocol data rather than an encapsulated IP
 packet.  For instance, TCP peers can negotiate the use of SEAL for
 the carriage of protocol data encapsulated as TCP/SEAL/IPv4.  In this
 sense, the "subnetwork" becomes the entire end-to-end path between
 the TCP peers and may potentially span the entire Internet.
 The current document version is specific to the use of IPv4 as the
 outer encapsulation layer; however, the same principles apply when
 IPv6 is used as the outer layer.

4. SEAL Protocol Specification - Tunnel Mode

4.1. Model of Operation

 SEAL supports the encapsulation of inner IP packets in mid-layer and
 outer encapsulating headers/trailers.  For example, an inner IPv6
 packet would appear as IPv6/*/SEAL/*/IPv4 after mid-layer and outer
 encapsulations, where '*' denotes zero or more additional
 encapsulation sublayers.  Ingres Tunnel Endpoints (ITEs) add mid-
 layer 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 uses a new
 Internet Protocol type and a new encapsulation sublayer for both
 unicast and multicast.  The ITE encapsulates an inner IP packet in
 mid-layer and outer encapsulations as shown in Figure 1:

Templin Experimental [Page 8] RFC 5320 SEAL February 2010

                                          +-------------------------+
                                          |                         |
                                          ~   Outer */IPv4 headers  ~
                                          |                         |
 I                                        +-------------------------+
 n                                        |       SEAL Header       |
 n      +-------------------------+       +-------------------------+
 e      ~ Any mid-layer * headers ~       ~ Any mid-layer * headers ~
 r      +-------------------------+       +-------------------------+
        |                         |       |                         |
 I -->  ~         Inner IP        ~  -->  ~         Inner IP        ~
 P -->  ~         Packet          ~  -->  ~         Packet          ~
        |                         |       |                         |
 P      +-------------------------+       +-------------------------+
 a      ~  Any mid-layer trailers ~       ~  Any mid-layer trailers ~
 c      +-------------------------+       +-------------------------+
 k                                        ~    Any outer trailers   ~
 e                                        +-------------------------+
 t
         (After mid-layer encaps.)        (After SEAL/*/IPv4 encaps.)
                     Figure 1: SEAL Encapsulation
 where the SEAL header is inserted as follows:
 o  For simple IPvX/IPv4 encapsulations (e.g.,
    [RFC2003][RFC2004][RFC4213]), the SEAL header is inserted between
    the inner IP and outer IPv4 headers as: IPvX/SEAL/IPv4.
 o  For tunnel-mode IPsec encapsulations over IPv4, [RFC4301], the
    SEAL header is inserted between the {AH,ESP} header and outer IPv4
    headers as: IPvX/*/{AH,ESP}/SEAL/IPv4.
 o  For IP encapsulations over transports such as UDP, the SEAL header
    is inserted immediately after the outer transport layer header,
    e.g., as IPvX/*/SEAL/UDP/IPv4.
 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 Identification value
 in the outer IPv4 header as the least-significant bits.  (For tunnels
 that traverse IPv4 Network Address Translators, the SEAL_ID is
 instead maintained only within the 16-bit ID Extension field in the
 SEAL header.)  Routers within the subnetwork use the SEAL_ID for
 duplicate packet detection, and ITEs/ETEs use the SEAL_ID for SEAL
 segmentation and reassembly.
 SEAL enables a multi-level segmentation and reassembly capability.

Templin Experimental [Page 9] RFC 5320 SEAL February 2010

 First, the ITE can use IPv4 fragmentation to fragment inner IPv4
 packets with DF=0 before SEAL encapsulation to avoid lower-layer
 segmentation and reassembly.  Secondly, the SEAL layer itself
 provides a simple cutting-and-pasting capability for mid-layer
 packets to avoid IPv4 fragmentation on the outer packet.  Finally,
 ordinary IPv4 fragmentation is permitted on the outer packet after
 SEAL encapsulation and used to detect and dampen any in-the-network
 fragmentation as quickly as possible.
 The following sections specify the SEAL-related operations of the ITE
 and ETE, respectively:

4.2. ITE Specification

4.2.1. Tunnel Interface MTU

 The ITE configures a tunnel virtual interface over one or more
 underlying links that connect the border node to the subnetwork.  The
 tunnel interface must present a fixed MTU to the inner IP layer
 (i.e., Layer 3) as the size for admission of inner IP packets into
 the tunnel.  Since the tunnel interface may support a potentially
 large set of ETEs, however, care must be taken in setting a greatest-
 common-denominator MTU for all ETEs while still upholding end system
 expectations.
 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 either be delivered by the network without loss due to an MTU
 restriction on the path or a suitable PTB message returned.  However,
 the network may not always deliver the necessary PTBs, leading to
 MTU-related black holes [RFC2923].  The ITE therefore requires a
 means for conveying 1500 byte (or smaller) packets to the ETE without
 loss due to MTU restrictions and without dependence on PTB messages
 from within the subnetwork.
 In common deployments, there may be many forwarding hops between the
 original source and the ITE.  Within those hops, there may be
 additional encapsulations (IPSec, L2TP, etc.) such that a 1500 byte
 packet sent by the original source might grow to a larger size by the
 time it reaches the ITE for encapsulation as an inner IP packet.
 Similarly, additional encapsulations on the path from the ITE to the
 ETE could cause the encapsulated packet to become larger still and
 trigger in-the-network fragmentation.  In order to preserve the end
 system expectations, the ITE therefore requires a means for conveying
 these larger packets to the ETE even though there may be links within
 the subnetwork that configure a smaller MTU.

Templin Experimental [Page 10] RFC 5320 SEAL February 2010

 The ITE should therefore set a tunnel virtual interface MTU of 1500
 bytes plus extra room to accommodate any additional encapsulations
 that may occur on the path from the original source (i.e., even if
 the path to the ETE does not support an MTU of this size).  The ITE
 can set larger MTU values still, but should select a value that is
 not so large as to cause excessive PTBs coming from within the tunnel
 interface (see Sections 4.2.2 and 4.2.6).  The ITE can also set
 smaller MTU values; however, care must be taken not to set so small a
 value that original sources would experience an MTU underflow.  In
 particular, IPv6 sources must see a minimum path MTU of 1280 bytes,
 and IPv4 sources should see a minimum path MTU of 576 bytes.
 The inner IP layer consults the tunnel interface MTU when admitting a
 packet into the interface.  For inner IPv4 packets larger than the
 tunnel interface MTU and with the IPv4 Don't Fragment (DF) bit set to
 0, the inner IPv4 layer uses IPv4 fragmentation to break the packet
 into fragments no larger than the tunnel interface MTU (but, see also
 Section 4.2.3), then admits each fragment into the tunnel as an
 independent packet.  For all other inner packets (IPv4 or IPv6), the
 ITE admits the packet if it is no larger than the tunnel interface
 MTU; otherwise, it drops the packet and sends an ICMP PTB message
 with an MTU value of the tunnel interface MTU to the source.

4.2.2. Accounting for Headers

 As for any transport layer protocol, ITEs use the MTU of the
 underlying IPv4 interface, the length of any mid-layer '*' headers
 and trailers, and the length of the outer SEAL/*/IPv4 headers to
 determine the maximum size for a SEAL segment (see Section 4.2.3).
 For example, when the underlying IPv4 interface advertises an MTU of
 1500 bytes and the ITE inserts a minimum-length (i.e., 20-byte) IPv4
 header, the ITE sees a maximum segment size of 1480 bytes.  When the
 ITE inserts IPv4 header options, the size is further reduced by as
 many as 40 additional bytes (the maximum length for IPv4 options)
 such that as few as 1440 bytes may be available for the upper-layer
 payload.  When the ITE inserts additional '*' encapsulations, the
 maximum segment size is reduced further still.
 The ITE must additionally account for the length of the SEAL header
 itself as an extra encapsulation that further reduces the maximum
 segment size.  The length of the SEAL header is not incorporated in
 the IPv4 header length; therefore, the network does not observe the
 SEAL header as an IPv4 option.  In this way, the SEAL header is
 inserted after the IPv4 options but before the upper-layer payload in
 exactly the same manner as for IPv6 extension headers.

Templin Experimental [Page 11] RFC 5320 SEAL February 2010

4.2.3. Segmentation and Encapsulation

 For each ETE, the ITE maintains the length of any mid-layer '*'
 encapsulation headers and trailers (e.g., for '*' = AH, ESP, NULL,
 etc.) in a variable 'MHLEN' and maintains the length of the outer
 SEAL/*/IPv4 encapsulation headers in a variable 'OHLEN'.  The ITE
 further maintains a variable 'HLEN' set to MHLEN plus OHLEN.  The ITE
 maintains a SEAL Maximum Reassembly Unit (S_MRU) value for each ETE
 as soft state within the tunnel interface (e.g., in the IPv4
 destination cache).  The ITE initializes S_MRU to a value no larger
 than 2KB and uses this value to determine the maximum-sized packet it
 will require the ETE to reassemble.  The ITE additionally maintains a
 SEAL Maximum Segment Size (S_MSS) value for each ETE.  The ITE
 initializes S_MSS to the maximum of (the underlying IPv4 interface
 MTU minus OHLEN) and S_MRU/8 bytes, and decreases or increases S_MSS
 based on any ICMPv4 Fragmentation Needed messages received (see
 Section 4.2.6).
 The ITE performs segmentation and encapsulation on inner packets that
 have been admitted into the tunnel interface.  For inner IPv4 packets
 with the DF bit set to 0, if the length of the inner packet is larger
 than (S_MRU - HLEN), the ITE uses IPv4 fragmentation to break the
 packet into IPv4 fragments no larger than (S_MRU - HLEN).  For
 unfragmentable inner packets (e.g., IPv6 packets, IPv4 packets with
 DF=1, etc.), if the length of the inner packet is larger than
 (MAX(S_MRU, S_MSS) - HLEN), the ITE drops the packet and sends an
 ICMP PTB message with an MTU value of (MAX(S_MRU, S_MSS) - HLEN) back
 to the original source.
 The ITE then encapsulates each inner packet/fragment in the MHLEN
 bytes of mid-layer '*' headers and trailers.  For each such resulting
 mid-layer packet of length 'M', if (S_MRU >= (M + OHLEN) > S_MSS),
 the ITE must perform SEAL segmentation.  To do so, it breaks the mid-
 layer packet into N segments (N <= 8) that are no larger than
 (MIN(1KB, S_MSS) - OHLEN) bytes each.  Each segment, except the final
 one, MUST be of equal length, while the final segment 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 should generate the smallest
 number of segments possible, e.g., it should not generate 6 smaller
 segments when the packet could be accommodated with 4 larger
 segments.
 Note that this SEAL segmentation ignores the fact that the mid-layer
 packet may be unfragmentable.  This segmentation process is a mid-
 layer (not an IP layer) operation employed by the ITE to adapt the
 mid-layer packet to the subnetwork path characteristics, and the ETE
 will restore the packet to its original form during reassembly.

Templin Experimental [Page 12] RFC 5320 SEAL February 2010

 Therefore, the fact that the packet may have been segmented within
 the subnetwork is not observable outside of the subnetwork.
 The ITE next encapsulates each segment in a SEAL header formatted as
 follows:
  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         |A|R|M|RSV| SEG |  Next Header  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     Figure 2: SEAL Header Format
 where the header fields are defined as follows:
 ID Extension (16)
    a 16-bit extension of the Identification field in the outer IPv4
    header; encodes the most-significant 16 bits of a 32 bit SEAL_ID
    value.
 A (1)
    the "Acknowledgement Requested" bit.  Set to 1 if the ITE wishes
    to receive an explicit acknowledgement from the ETE.
 R (1)
    the "Report Fragmentation" bit.  Set to 1 if the ITE wishes to
    receive a report from the ETE if any IPv4 fragmentation occurs.
 M (1)
    the "More Segments" bit.  Set to 1 if this SEAL protocol packet
    contains a non-final segment of a multi-segment mid-layer packet.
 RSV (2)
    a 2-bit field reserved for future use.  Must be set to 0 for the
    purpose of this specification.
 SEG (3)
    a 3-bit segment number.  Encodes a segment number between 0 - 7.
 Next Header (8)
    an 8-bit field that encodes an Internet Protocol number the same
    as for the IPv4 protocol and IPv6 next header fields.

Templin Experimental [Page 13] RFC 5320 SEAL February 2010

 For single-segment mid-layer packets, the ITE encapsulates the
 segment in a SEAL header with (M=0; SEG=0).  For N-segment mid-layer
 packets (N <= 8), the ITE encapsulates each segment in a SEAL header
 with (M=1; SEG=0) for the first segment, (M=1; SEG=1) for the second
 segment, etc., with the final segment setting (M=0; SEG=N-1).
 The ITE next sets RSV='00' and sets the A and R bits in the SEAL
 header of the first segment according to whether the packet is to be
 used as an explicit/implicit probe as specified in Section 4.2.4.
 The ITE then writes the Internet Protocol number corresponding to the
 mid-layer packet in the SEAL 'Next Header' field and encapsulates
 each segment in the requisite */IPv4 outer headers according to the
 specific encapsulation format (e.g., [RFC2003], [RFC4213], [RFC4380],
 etc.), except that it writes 'SEAL_PROTO' in the protocol field of
 the outer IPv4 header (when simple IPv4 encapsulation is used) or
 writes 'SEAL_PORT' in the outer destination service port field (e.g.,
 when UDP/IPv4 encapsulation is used).  The ITE finally sets packet
 identification values as specified in Section 4.2.5 and sends the
 packets as specified in Section 4.2.6.

4.2.4. Sending Probes

 When S_MSS is larger than S_MRU/8 bytes, the ITE sends ordinary
 encapsulated data packets as implicit probes to detect in-the-network
 IPv4 fragmentation and to determine new values for S_MSS.  The ITE
 sets R=1 in the SEAL header of a packet with SEG=0 to be used as an
 implicit probe, and will receive ICMPv4 Fragmentation Needed messages
 from the ETE if any IPv4 fragmentation occurs.  When the ITE has
 already reduced S_MSS to the minimum value, it instead sets R=0 in
 the SEAL header to avoid generating fragmentation reports for
 unavoidable in-the-network fragmentation.
 The ITE should send explicit probes periodically to manage a window
 of SEAL_IDs of outstanding probes as a means to validate any ICMPv4
 messages it receives.  The ITE sets A=1 in the SEAL header of a
 packet with SEG=0 to be used as an explicit probe, where the probe
 can be either an ordinary data packet or a NULL packet created by
 setting the 'Next Header' field in the SEAL header to a value of "No
 Next Header" (see Section 4.7 of [RFC2460]).
 The ITE should further send explicit probes, periodically, to detect
 increases in S_MSS by resetting S_MSS to the maximum of (the
 underlying IPv4 interface MTU minus OHLEN) and S_MRU/8 bytes, and/or
 by sending explicit probes that are larger than the current S_MSS.
 Finally, the ITE MAY send "expendable" probe packets with DF=1 (see
 Section 4.2.6) in order to generate ICMPv4 Fragmentation Needed
 messages from routers on the path to the ETE.

Templin Experimental [Page 14] RFC 5320 SEAL February 2010

4.2.5. Packet Identification

 For the purpose of packet identification, 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 protocol packet it sends to the ETE.  For each
 packet, the ITE writes the least-significant 16 bits of the SEAL_ID
 value in the Identification field in the outer IPv4 header, and
 writes the most-significant 16 bits in the ID Extension field in the
 SEAL header.
 For SEAL encapsulations specifically designed for the traversal of
 IPv4 Network Address Translators (NATs), e.g., for encapsulations
 that insert a UDP header between the SEAL header and outer IPv4
 header such as IPv6/SEAL/UDP/IPv4, the ITE instead maintains SEAL_ID
 as a 16-bit value that it randomly initializes when the soft state is
 created and monotonically increments (modulo 2^16) for each
 successive packet.  For each packet, the ITE writes SEAL_ID in the ID
 extension field of the SEAL header and writes a random 16-bit value
 in the Identification field in the outer IPv4 header.  This is due to
 the fact that the ITE has no way to control IPv4 NATs in the path
 that could rewrite the Identification value in the outer IPv4 header.

4.2.6. Sending SEAL Protocol Packets

 Following SEAL segmentation and encapsulation, the ITE sets DF=0 in
 the outer IPv4 header of every outer packet it sends.  For
 "expendable" packets (e.g., for NULL packets used as probes -- see
 Section 4.2.4), the ITE may instead set DF=1.
 The ITE then sends each outer packet that encapsulates a segment of
 the same mid-layer packet into the tunnel in canonical order, i.e.,
 segment 0 first, followed by segment 1, etc. and finally segment N-1.

4.2.7. Processing Raw ICMPv4 Messages

 The ITE may receive "raw" ICMPv4 error messages from either the ETE
 or routers within the subnetwork that comprise an outer IPv4 header,
 followed by an ICMPv4 header, followed by a portion of the SEAL
 packet that generated the error (also known as the "packet-in-
 error").  For such messages, the ITE can use the 32-bit SEAL ID
 encoded in the packet-in-error as a nonce to confirm that the ICMP
 message came from either the ETE or an on-path router.  The ITE MAY
 process raw ICMPv4 messages as soft errors indicating that the path
 to the ETE may be failing.

Templin Experimental [Page 15] RFC 5320 SEAL February 2010

 The ITE should specifically process raw ICMPv4 Protocol Unreachable
 messages as a hint that the ETE does not implement the SEAL protocol.

4.2.8. Processing SEAL-Encapsulated ICMPv4 Messages

 In addition to any raw ICMPv4 messages, the ITE may receive SEAL-
 encapsulated ICMPv4 messages from the ETE that comprise outer ICMPv4/
 */SEAL/*/IPv4 headers followed by a portion of the SEAL-encapsulated
 packet-in-error.  The ITE can use the 32-bit SEAL ID encoded in the
 packet-in-error as well as information in the outer IPv4 and SEAL
 headers as nonces to confirm that the ICMP message came from a
 legitimate ETE.  The ITE then verifies that the SEAL_ID encoded in
 the packet-in-error is within the current window of transmitted
 SEAL_IDs for this ETE.  If the SEAL_ID is outside of the window, the
 ITE discards the message; otherwise, it advances the window and
 processes the message.
 The ITE processes SEAL-encapsulated ICMPv4 messages other than ICMPv4
 Fragmentation Needed exactly as specified in [RFC0792].
 For SEAL-encapsulated ICMPv4 Fragmentation Needed messages, the ITE
 sets a variable 'L' to the IPv4 length of the packet-in-error minus
 OHLEN.  If (L > S_MSS), or if the packet-in-error is an IPv4 first
 fragment (i.e., with MF=1; Offset=0) and (L >= (576 - OHLEN)), the
 ITE sets (S_MSS = L).
 Note that 576 in the above corresponds to the nominal minimum MTU for
 IPv4 links.  When an ITE instead receives an IPv4 first fragment
 packet-in-error with (L < (576 - OHLEN)), it discovers that IPv4
 fragmentation is occurring in the network but it cannot determine the
 true MTU of the restricting link due to a router on the path
 generating runt first fragments.  The ITE should therefore search for
 a reduced S_MSS value (to a minimum of S_MRU/8) through an iterative
 searching strategy that parallels (Section 5 of [RFC1191]).
 This searching strategy may require multiple iterations of sending
 SEAL packets with DF=0 using a reduced S_MSS and receiving additional
 Fragmentation Needed messages, but it will soon converge to a stable
 value.  During this process, it is essential that the ITE reduce
 S_MSS based on the first Fragmentation Needed message received, and
 refrain from further reducing S_MSS until ICMPv4 Fragmentation Needed
 messages pertaining to packets sent under the new S_MSS are received.
 As an optimization only, the ITE MAY transcribe SEAL-encapsulated
 Fragmentation Needed messages that contain sufficient information
 into corresponding PTB messages to return to the original source.

Templin Experimental [Page 16] RFC 5320 SEAL February 2010

4.3. ETE Specification

4.3.1. Reassembly Buffer Requirements

 ETEs MUST be capable of using IPv4-layer reassembly to reassemble
 SEAL protocol outer IPv4 packets up to 2KB in length, and MUST also
 be capable of using SEAL-layer reassembly to reassemble mid-layer
 packets up to (2KB - OHLEN).  Note that the ITE must retain the
 SEAL/*/IPv4 header during both IPv4-layer and SEAL-layer reassembly
 for the purpose of associating the fragments/segments of the same
 packet.

4.3.2. IPv4-Layer Reassembly

 The ETE performs IPv4 reassembly as normal, and should maintain a
 conservative high- and low-water mark for the number of outstanding
 reassemblies pending for each ITE.  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) until the size falls below the low-water mark.  The ETE
 should also use a reduced IPv4 maximum segment lifetime value (e.g.,
 15 seconds), i.e., the time after which it will discard an incomplete
 IPv4 reassembly for a SEAL protocol packet.  Finally, the ETE should
 also actively discard any pending reassemblies that clearly have no
 opportunity for completion, e.g., when a considerable number of new
 IPv4 fragments have been received before a fragment that completes a
 pending reassembly has arrived.
 After reassembly, the ETE either accepts or discards the reassembled
 packet based on the current status of the IPv4 reassembly cache
 (congested versus uncongested).  The 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.  The choice of accepting/discarding a reassembly may also
 depend on the strength of the upper-layer 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.).  In the limiting case, the ETE may choose to discard
 all IPv4 reassemblies and process only the IPv4 first fragment for
 SEAL-encapsulated error generation purposes (see the following
 sections).

Templin Experimental [Page 17] RFC 5320 SEAL February 2010

4.3.3. Generating SEAL-Encapsulated ICMPv4 Fragmentation Needed

      Messages
 During IPv4-layer reassembly, the ETE determines whether the packet
 belongs to the SEAL protocol by checking for SEAL_PROTO in the outer
 IPv4 header (i.e., for simple IPv4 encapsulation) or for SEAL_PORT in
 the outer */IPv4 header (e.g., for '*'=UDP).  When the ETE processes
 the IPv4 first fragment (i.e, one with DF=1 and Offset=0 in the IPv4
 header) of a SEAL protocol IPv4 packet with (R=1; SEG=0) in the SEAL
 header, it sends a SEAL-encapsulated ICMPv4 Fragmentation Needed
 message back to the ITE with the MTU field set to 0.  (Note that
 setting a non-zero value in the MTU field of the ICMPv4 Fragmentation
 Needed message would be redundant with the length value in the IPv4
 header of the first fragment, since this value is set to the correct
 path MTU through in-the-network fragmentation.  Setting the MTU field
 to 0 therefore avoids the ambiguous case in which the MTU field and
 the IPv4 length field of the first fragment would record different
 non-zero values.)
 When the ETE processes a SEAL protocol IPv4 packet with (A=1; SEG=0)
 for which no IPv4 reassembly was required, or for which IPv4
 reassembly was successful and the R bit was not set, it sends a SEAL-
 encapsulated ICMPv4 Fragmentation Needed message back to the ITE with
 the MTU value set to 0.  Note therefore that when both the A and R
 bits are set and fragmentation occurs, the ETE only sends a single
 ICMPv4 Fragmentation Needed message, i.e., it does not send two
 separate messages (one for the first fragment and a second for the
 reassembled whole SEAL packet).
 The ETE prepares the ICMPv4 Fragmentation Needed message by
 encapsulating as much of the first fragment (or the non-fragmented
 packet) as possible in outer */SEAL/*/IPv4 headers without the length
 of the message exceeding 576 bytes, as shown in Figure 3:

Templin Experimental [Page 18] RFC 5320 SEAL February 2010

    +-------------------------+ -
    |                         |   ~ Outer */SEAL/*/IPv4 hdrs~   |
    |                         |   |
    +-------------------------+   |
    |      ICMPv4 Header      |   |
    |(Dest Unreach; Frag Need)|   |
    +-------------------------+   |
    |                         |    > Up to 576 bytes
    ~    IP/*/SEAL/*/IPv4     ~   |
    ~ hdrs of packet/fragment ~   |
    |                         |   |
    +-------------------------+   |
    |                         |   |
    ~ Data of packet/fragment ~   |
    |                         |   /
    +-------------------------+ -
     Figure 3: SEAL-Encapsulated ICMPv4 Fragmentation Needed Message
 The ETE next sets A=0, R=0, and SEG=0 in the outer SEAL header, sets
 the SEAL_ID the same as for any SEAL packet, then sets the SEAL Next
 Header field and the fields of the outer */IPv4 headers the same as
 for ordinary SEAL encapsulation.  The ETE then sets the outer IPv4
 destination and source addresses to the source and destination
 addresses (respectively) of the packet/fragment.  If the destination
 address in the packet/fragment was multicast, the ETE instead sets
 the outer IPv4 source address to an address assigned to the
 underlying IPv4 interface.  The ETE finally sends the SEAL-
 encapsulated ICMPv4 message to the ITE the same as specified in
 Section 4.2.5, except that when the A bit in the packet/fragment is
 not set, the ETE sends the messages subject to rate limiting since it
 is not entirely critical that all fragmentation be reported to the
 ITE.

4.3.4. SEAL-Layer Reassembly

 Following IPv4 reassembly of a SEAL packet with (RSV!=0; SEG=0), if
 the packet is not a SEAL-encapsulated ICMPv4 message, the ETE
 generates a SEAL-encapsulated ICMPv4 Parameter Problem message with
 pointer set to the flags field in the SEAL header, sends the message
 back to the ITE in the same manner specified in Section 4.3.3, then
 drops the packet.  For all other SEAL packets, the ETE adds the
 packet to a SEAL-Layer pending-reassembly queue if either the M bit
 or the SEG field in the SEAL header is non-zero.
 The ETE performs SEAL-layer reassembly through simple in-order
 concatenation of the encapsulated segments from N consecutive SEAL
 protocol packets from the same mid-layer packet.  SEAL-layer

Templin Experimental [Page 19] RFC 5320 SEAL February 2010

 reassembly requires the ETE to maintain a cache of recently received
 segments for a hold time that would allow for reasonable inter-
 segment delays.  The ETE uses a SEAL maximum segment lifetime of 15
 seconds for this purpose, i.e., the time after which it will discard
 an incomplete reassembly.  However, the ETE should also actively
 discard any pending reassemblies that clearly have no opportunity for
 completion, e.g., when a considerable number of new SEAL packets have
 been received before a packet that completes a pending reassembly has
 arrived.
 The ETE reassembles the mid-layer packet segments in SEAL protocol
 packets that contain segment numbers 0 through N-1, with M=1/0 in
 non-final/final segments, respectively, and with consecutive SEAL_ID
 values.  That is, for an N-segment mid-layer packet, reassembly
 entails the concatenation of the SEAL-encapsulated segments 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)).  (For
 SEAL encapsulations specifically designed for traversal of IPv4 NATs,
 the ETE instead uses only a 16-bit SEAL_ID value, and uses mod 2^16
 arithmetic to associate the segments of the same packet.)

4.3.5. Delivering Packets to Upper Layers

 Following SEAL-layer reassembly, the ETE silently discards the
 reassembled packet if it was a NULL packet (see Section 4.2.4).  In
 the same manner, the ETE silently discards any reassembled mid-layer
 packet larger than (2KB - OHLEN) that either experienced IPv4
 fragmentation or did not arrive as a single SEAL segment.
 Next, if the ETE determines that the inner packet would cause an
 ICMPv4 error message to be generated, it generates a SEAL-
 encapsulated ICMPv4 error message, sends the message back to the ITE
 in the same manner specified in Section 4.3.3, then either accepts or
 drops the packet according to the type of error.  Otherwise, the ETE
 delivers the inner packet to the upper-layer protocol indicated in
 the Next Header field.

5. SEAL Protocol Specification - Transport Mode

 Section 4 specifies the operation of SEAL in "tunnel mode", i.e.,
 when there are both an inner and outer IP layer with a SEAL
 encapsulation layer between.  However, the SEAL protocol can also be
 used in a "transport mode" of operation within a subnetwork region in
 which the inner-layer corresponds to a transport layer protocol
 (e.g., UDP, TCP, etc.) instead of an inner IP layer.

Templin Experimental [Page 20] RFC 5320 SEAL February 2010

 For example, two TCP endpoints connected to the same subnetwork
 region can negotiate the use of transport-mode SEAL for a connection
 by inserting a 'SEAL_OPTION' TCP option during the connection
 establishment phase.  If both TCPs agree on the use of SEAL, their
 protocol messages will be carried as TCP/SEAL/IPv4 and the connection
 will be serviced by the SEAL protocol using TCP (instead of an
 encapsulating tunnel endpoint) as the transport layer protocol.  The
 SEAL protocol for transport mode otherwise observes the same
 specifications as for Section 4.

6. Link Requirements

 Subnetwork designers are expected to follow the recommendations in
 Section 2 of [RFC3819] when configuring link MTUs.

7. End System Requirements

 SEAL provides robust mechanisms for returning PTB messages; however,
 end systems that send unfragmentable IP packets larger than 1500
 bytes are strongly encouraged to use Packetization Layer Path MTU
 Discovery per [RFC4821].

8. Router Requirements

 IPv4 routers within the subnetwork are strongly encouraged to
 implement IPv4 fragmentation such that the first fragment is the
 largest and approximately the size of the underlying link MTU, i.e.,
 they should avoid generating runt first fragments.

9. IANA Considerations

 SEAL_PROTO, SEAL_PORT, and SEAL_OPTION are taken from their
 respective range of experimental values documented in [RFC3692] and
 [RFC4727].  These values are for experimentation purposes only, and
 not to be used for any kind of deployments (i.e., they are not to be
 shipped in any products).

10. Security Considerations

 Unlike IPv4 fragmentation, overlapping fragment attacks are not
 possible due to the requirement that SEAL segments be non-
 overlapping.
 An amplification/reflection attack is possible when an attacker sends
 IPv4 first fragments with spoofed source addresses to an ETE,
 resulting in a stream of ICMPv4 Fragmentation Needed messages

Templin Experimental [Page 21] RFC 5320 SEAL February 2010

 returned to a victim ITE.  The encapsulated segment of the spoofed
 IPv4 first fragment provides mitigation for the ITE to detect and
 discard spurious ICMPv4 Fragmentation Needed messages.
 The SEAL header is sent in-the-clear (outside of any IPsec/ESP
 encapsulations) the same as for the outer */IPv4 headers.  As for
 IPv6 extension headers, the SEAL header is protected only by L2
 integrity checks and is not covered under any L3 integrity checks.

11. Related Work

 Section 3.1.7 of [RFC2764] provides a high-level sketch for
 supporting large tunnel MTUs via a tunnel-level segmentation and
 reassembly capability to avoid IP level fragmentation, which is in
 part the same approach used by tunnel-mode SEAL.  SEAL could
 therefore be considered as a fully functioned manifestation of the
 method postulated by that informational reference; however, SEAL also
 supports other modes of operation including transport-mode and
 duplicate packet detection.
 Section 3 of [RFC4459] describes inner and outer fragmentation at the
 tunnel endpoints as alternatives for accommodating the tunnel MTU;
 however, the SEAL protocol specifies a mid-layer segmentation and
 reassembly capability that is distinct from both inner and outer
 fragmentation.
 Section 4 of [RFC2460] specifies a method for inserting and
 processing extension headers between the base IPv6 header and
 transport layer protocol data.  The SEAL header is inserted and
 processed in exactly the same manner.
 The concepts of path MTU determination through the report of
 fragmentation and extending the IP Identification field were first
 proposed in deliberations of the TCP-IP mailing list and the Path MTU
 Discovery Working Group (MTUDWG) during the late 1980's and early
 1990's.  SEAL supports a report fragmentation capability using bits
 in an extension header (the original proposal used a spare bit in the
 IP header) and supports ID extension through a 16-bit field in an
 extension header (the original proposal used a new IP option).  A
 historical analysis of the evolution of these concepts, as well as
 the development of the eventual path MTU discovery mechanism for IP,
 appears in Appendix A of this document.

12. SEAL Advantages over Classical Methods

 The SEAL approach offers a number of distinct advantages over the
 classical path MTU discovery methods [RFC1191] [RFC1981]:

Templin Experimental [Page 22] RFC 5320 SEAL February 2010

 1.  Classical path MTU discovery *always* results in packet loss when
     an MTU restriction is encountered.  Using SEAL, IPv4
     fragmentation provides a short-term interim mechanism for
     ensuring that packets are delivered while SEAL adjusts its packet
     sizing parameters.
 2.  Classical path MTU discovery requires that routers generate an
     ICMP PTB message for *all* packets lost due to an MTU
     restriction; this situation is exacerbated at high data rates and
     becomes severe for in-the-network tunnels that service many
     communicating end systems.  Since SEAL ensures that packets no
     larger than S_MRU are delivered, however, it is sufficient for
     the ETE to return ICMPv4 Fragmentation Needed messages subject to
     rate limiting and not for every packet-in-error.
 3.  Classical path MTU may require several iterations of dropping
     packets and returning ICMP PTB messages until an acceptable path
     MTU value is determined.  Under normal circumstances, SEAL
     determines the correct packet sizing parameters in a single
     iteration.
 4.  Using SEAL, ordinary packets serve as implicit probes without
     exposing data to unnecessary loss.  SEAL also provides an
     explicit probing mode not available in the classic methods.
 5.  Using SEAL, ETEs encapsulate ICMP error messages in an outer SEAL
     header such that packet-filtering network middleboxes can
     distinguish them from "raw" ICMP messages that may be generated
     by an attacker.
 6.  Most importantly, all SEAL packets have a 32-bit Identification
     value that can be used for duplicate packet detection purposes
     and to match ICMP error messages with actual packets sent without
     requiring per-packet state.  Moreover, the SEAL ITE can be
     configured to accept ICMP feedback only from the legitimate ETE;
     hence, the packet spoofing-related denial-of-service attack
     vectors open to the classical methods are eliminated.
 In summary, the SEAL approach represents an architecturally superior
 method for ensuring that packets of various sizes are either
 delivered or deterministically dropped.  When end systems use their
 own end-to-end MTU determination mechanisms [RFC4821], the SEAL
 advantages are further enhanced.

Templin Experimental [Page 23] RFC 5320 SEAL February 2010

13. Acknowledgments

 The following individuals are acknowledged for helpful comments and
 suggestions: Jari Arkko, Fred Baker, Iljitsch van Beijnum, Teco Boot,
 Bob Braden, Brian Carpenter, Steve Casner, Ian Chakeres, Remi Denis-
 Courmont, Aurnaud Ebalard, Gorry Fairhurst, Joel Halpern, John
 Heffner, Thomas Henderson, Bob Hinden, Christian Huitema, Joe Macker,
 Matt Mathis, Erik Nordmark, Dan Romascanu, Dave Thaler, Joe Touch,
 Magnus Westerlund, Robin Whittle, James Woodyatt, and members of the
 Boeing PhantomWorks DC&NT group.
 Path MTU determination through the report of fragmentation was first
 proposed by Charles Lynn on the TCP-IP mailing list in 1987.
 Extending the IP identification field was first proposed by Steve
 Deering on the MTUDWG mailing list in 1989.

14. References

14.1. Normative References

 [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
            1981.
 [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
            RFC 792, September 1981.
 [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.

14.2. 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.
 [MTUDWG]   "IETF MTU Discovery Working Group mailing list,
             gatekeeper.dec.com/pub/DEC/WRL/mogul/mtudwg-log,
            November 1989 - February 1995.".
 [RFC1063]  Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP
            MTU discovery options", RFC 1063, July 1988.

Templin Experimental [Page 24] RFC 5320 SEAL February 2010

 [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.
 [RFC2764]  Gleeson, B., Lin, A., Heinanen, J., Armitage, G., and A.
            Malis, "A Framework for IP Based Virtual Private
            Networks", RFC 2764, February 2000.
 [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery", RFC
            2923, September 2000.
 [RFC3692]  Narten, T., "Assigning Experimental and Testing Numbers
            Considered Useful", BCP 82, RFC 3692, January 2004.
 [RFC3819]  Karn, P., Ed., 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.
 [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.
 [RFC4727]  Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
            ICMPv6, UDP, and TCP Headers", RFC 4727, November 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.

Templin Experimental [Page 25] RFC 5320 SEAL February 2010

 [TCP-IP]   "Archive/Hypermail of Early TCp-IP Mail List",
            http://www-mice.cs.ucl.ac.uk/multimedia/misc/tcp_ip/,
            May 1987 - May 1990.

Templin Experimental [Page 26] RFC 5320 SEAL February 2010

Appendix A. Historic Evolution of PMTUD

 (Taken from "Neighbor Affiliation Protocol for IPv6-over-(foo)-over-
 IPv4"; 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 (straw
     RFC draft by McCloughrie, Fox and Mogul on Jan 12, 1990)
 4.  Combination of the Lynn proposal with TCP (Fred Bohle, Jan 30,
     1990)
 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:

Templin Experimental [Page 27] RFC 5320 SEAL February 2010

    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.
 (Author's 2/11/08 note: this final point was based on a loose
 interpretation of [FOLK], and is more accurately addressed in
 [RFC4963].)

Templin Experimental [Page 28] RFC 5320 SEAL February 2010

Appendix B. Reliability Extensions

 The SEAL header includes a Reserved (RSV) field that is set to zero
 for the purpose of this specification.  This field may be used by
 future updates to this specification for the purpose of improved
 reliability in the face of loss due to congestion, signal
 intermittence, etc.  Automatic Repeat-ReQuest (ARQ) mechanisms are
 used to ensure reliable delivery between the endpoints of physical
 links (e.g., on-link neighbors in an IEEE 802.11 network) as well as
 between the endpoints of an end-to-end transport (e.g., the endpoints
 of a TCP connection).  However, ARQ mechanisms may be poorly suited
 to in-the-network elements such as the SEAL ITE and ETE, since
 retransmission of lost segments would require unacceptable state
 maintenance at the ITE and would result in packet reordering within
 the subnetwork.
 Instead, alternate reliability mechanisms such as Forward Error
 Correction (FEC) may be specified in future updates to this
 specification for the purpose of improved reliability.  Such
 mechanisms may entail the ITE performing proactive transmissions of
 redundant data, e.g., by sending multiple copies of the same data.
 Signaling from the ETE (e.g., by sending SEAL-encapsulated ICMPv4
 Source Quench messages) may be specified in a future document as a
 means for the ETE to dynamically inform the ITE of changing FEC
 conditions.

Author's Address

 Fred L. Templin, Editor
 Boeing Research & Technology
 P.O. Box 3707
 Seattle, WA  98124
 USA
 EMail: fltemplin@acm.org

Templin Experimental [Page 29]

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