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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Obsoletes: rfc5320 (if approved)                            May 03, 2013
Intended status: Informational
Expires: November 4, 2013


        The Subnetwork Encapsulation and Adaptation Layer (SEAL)
                   draft-templin-intarea-seal-55.txt

Abstract

   This document specifies a Subnetwork Encapsulation and Adaptation
   Layer (SEAL).  SEAL operates over virtual topologies configured over
   connected IP network routing regions bounded by encapsulating border
   nodes.  These virtual topologies are manifested by tunnels that may
   span multiple IP and/or sub-IP layer forwarding hops, where they may
   incur packet duplication, packet reordering, source address spoofing
   and traversal of links with diverse Maximum Transmission Units
   (MTUs).  SEAL uniquely addresses these issues through the
   encapsulation and messaging mechanisms specified in this document.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 4, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.














































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Motivation . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Approach . . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.3.  Differences with RFC5320 . . . . . . . . . . . . . . . . .  7
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.  Applicability Statement  . . . . . . . . . . . . . . . . . . . 10
   5.  SEAL Specification . . . . . . . . . . . . . . . . . . . . . . 11
     5.1.  SEAL Tunnel Model  . . . . . . . . . . . . . . . . . . . . 11
     5.2.  SEAL Model of Operation  . . . . . . . . . . . . . . . . . 12
     5.3.  SEAL Header and Trailer Format . . . . . . . . . . . . . . 13
     5.4.  ITE Specification  . . . . . . . . . . . . . . . . . . . . 15
       5.4.1.  Tunnel Interface MTU . . . . . . . . . . . . . . . . . 15
       5.4.2.  Tunnel Neighbor Soft State . . . . . . . . . . . . . . 16
       5.4.3.  SEAL Layer Pre-Processing  . . . . . . . . . . . . . . 17
       5.4.4.  SEAL Encapsulation and Segmentation  . . . . . . . . . 18
       5.4.5.  Outer Encapsulation  . . . . . . . . . . . . . . . . . 20
       5.4.6.  Path Probing and ETE Reachability Verification . . . . 21
       5.4.7.  Processing ICMP Messages . . . . . . . . . . . . . . . 21
       5.4.8.  IPv4 Middlebox Reassembly Testing  . . . . . . . . . . 22
       5.4.9.  Stateful MTU Determination . . . . . . . . . . . . . . 23
       5.4.10. Detecting Path MTU Changes . . . . . . . . . . . . . . 24
     5.5.  ETE Specification  . . . . . . . . . . . . . . . . . . . . 24
       5.5.1.  Minimum Reassembly Buffer Requirements . . . . . . . . 24
       5.5.2.  Tunnel Neighbor Soft State . . . . . . . . . . . . . . 24
       5.5.3.  IP-Layer Reassembly  . . . . . . . . . . . . . . . . . 25
       5.5.4.  Decapsulation and Re-Encapsulation . . . . . . . . . . 25
     5.6.  The SEAL Control Message Protocol (SCMP) . . . . . . . . . 27
       5.6.1.  Generating SCMP Error Messages . . . . . . . . . . . . 27
       5.6.2.  Processing SCMP Error Messages . . . . . . . . . . . . 29
   6.  Link Requirements  . . . . . . . . . . . . . . . . . . . . . . 31
   7.  End System Requirements  . . . . . . . . . . . . . . . . . . . 32
   8.  Router Requirements  . . . . . . . . . . . . . . . . . . . . . 32
   9.  Nested Encapsulation Considerations  . . . . . . . . . . . . . 32
   10. Reliability Considerations . . . . . . . . . . . . . . . . . . 33
   11. Integrity Considerations . . . . . . . . . . . . . . . . . . . 33
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 34
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 34
   14. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 34
   15. Implementation Status  . . . . . . . . . . . . . . . . . . . . 35
   16. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 35
   17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
     17.1. Normative References . . . . . . . . . . . . . . . . . . . 36
     17.2. Informative References . . . . . . . . . . . . . . . . . . 36
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 40




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1.  Introduction

   As Internet technology and communication has grown and matured, many
   techniques have developed that use virtual topologies (manifested by
   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 (also known as "tunneling") has long been
   considered as the means for creating such virtual topologies (e.g.,
   see [RFC2003][RFC2473]).  However, the encapsulation headers often
   include insufficiently provisioned per-packet identification values.
   IP encapsulation also allows an attacker to produce encapsulated
   packets with spoofed source addresses even if the source address in
   the encapsulating header cannot be spoofed.  A denial-of-service
   vector that is not possible in non-tunneled subnetworks is therefore
   presented.

   Additionally, the insertion of an outer IP header reduces the
   effective path MTU visible to the inner network layer.  When IPv6 is
   used as the encapsulation protocol, original sources expect to be
   informed of the MTU limitation through IPv6 Path MTU discovery
   (PMTUD) [RFC1981].  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 IPv4 PMTUD
   [RFC1191] has known operational issues that are exacerbated by in-
   the-network tunnels [RFC2923][RFC4459].

   The following subsections 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,
   IP is ubiquitously deployed as the Layer 3 protocol.  The primary
   functions of IP are to provide for routing, addressing, and a
   fragmentation and reassembly capability used to accommodate links
   with diverse MTUs.  While it is well known that the IP address space
   is rapidly becoming depleted, there is a lesser-known but growing
   consensus that other IP protocol limitations have already or may soon
   become problematic.



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   First, the Internet historically provided no means for discerning
   whether the source addresses of IP packets are authentic.  This
   shortcoming is being addressed more and more through the deployment
   of site border router ingress filters [RFC2827], however the use of
   encapsulation provides a vector for an attacker to circumvent
   filtering for the encapsulated packet even if filtering is correctly
   applied to the encapsulation header.  Secondly, the IP header does
   not include a well-behaved identification value unless the source has
   included a fragment header for IPv6 or unless the source permits
   fragmentation for IPv4.  These limitations preclude an efficient
   means for routers to detect duplicate packets and packets that have
   been re-ordered within the subnetwork.  Additionally, recent studies
   have shown that the arrival of fragments at high data rates can cause
   denial-of-service (DoS) attacks on performance-sensitive networking
   gear, prompting some administrators to configure their equipment to
   drop fragments unconditionally [I-D.taylor-v6ops-fragdrop].

   For IPv4 encapsulation, when fragmentation is permitted the header
   includes a 16-bit Identification field, meaning that at most 2^16
   unique packets with the same (source, destination, protocol)-tuple
   can be active in the network at the same time [RFC6864].  (When
   middleboxes such as Network Address Translators (NATs) re-write the
   Identification field to random values, the number of unique packets
   is even further reduced.)  Due to the escalating deployment of high-
   speed links, however, these numbers have become too small by several
   orders of magnitude for high data rate packet sources such as tunnel
   endpoints [RFC4963].

   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 (see above).
   In particular, IPv4 fragmentation raises issues ranging from minor
   annoyances (e.g., in-the-network router fragmentation [RFC1981]) to
   the potential for major integrity issues (e.g., mis-association of
   the fragments of multiple IP packets during reassembly [RFC4963]).

   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 which resulted in the publication of [RFC1191].
   In this negative feedback-based 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" [RFC2923].

   In particular, site border routers in the Internet have been known to



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   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 trivial since there is no way to
   authenticate the source of the messages [RFC5927].  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.  This
   behavior has been confirmed through documented studies showing clear
   evidence of PMTUD failures for both IPv4 and IPv6 in the Internet
   today [TBIT][WAND][SIGCOMM][RIPE].

   The issues with both IP fragmentation and this "classical" PMTUD
   method are exacerbated further when IP tunneling is used [RFC4459].
   For example, an ingress tunnel endpoint (ITE) may be required to
   forward encapsulated packets into the subnetwork on behalf of
   hundreds, thousands, or even more original sources.  If the ITE
   allows IP fragmentation on the encapsulated packets, persistent
   fragmentation could lead to undetected data corruption due to
   Identification field wrapping and/or reassembly congestion at the
   ETE.  If the ITE instead uses classical IP PMTUD it must rely on ICMP
   error messages coming from the subnetwork that may be suspect,
   subject to loss due to filtering middleboxes, or insufficiently
   provisioned for translation into error messages to be returned to the
   original sources.

   Although recent works have led to the development of a positive
   feedback-based end-to-end MTU determination scheme [RFC4821], they do
   not excuse tunnels from accounting for the encapsulation overhead
   they add to packets.  Moreover, in current practice existing
   tunneling protocols mask the MTU issues by selecting a "lowest common
   denominator" MTU that may be much smaller than necessary for most
   paths and difficult to change at a later date.  Therefore, a new
   approach to accommodate tunnels over links with diverse MTUs is
   necessary.

1.2.  Approach

   This document concerns subnetworks manifested through a virtual
   topology configured over a connected network routing region and
   bounded by encapsulating border nodes.  Example connected network
   routing regions include Mobile Ad hoc Networks (MANETs), enterprise
   networks and the global public Internet itself.  Subnetwork border
   nodes forward unicast and multicast 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).



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   This document introduces a Subnetwork Encapsulation and Adaptation
   Layer (SEAL) for tunneling inner network layer protocol packets over
   IP subnetworks that connect Ingress and Egress Tunnel Endpoints
   (ITEs/ETEs) of border nodes.  It provides a modular specification
   designed to be tailored to specific associated tunneling protocols.
   (A transport-mode of operation is also possible, but out of scope for
   this document.)

   SEAL provides a mid-layer encapsulation that accommodates links with
   diverse MTUs, and allows routers in the subnetwork to perform
   efficient duplicate packet and packet reordering detection.  The
   encapsulation further ensures message origin authentication, packet
   header integrity and anti-replay in environments in which these
   functions are necessary.

   SEAL treats tunnels that traverse the subnetwork as ordinary links
   that must support network layer services.  Moreover, SEAL provides
   dynamic mechanisms to ensure a maximal path MTU over the tunnel.
   This is in contrast to static approaches which avoid MTU issues by
   selecting a lowest common denominator MTU value that may be overly
   conservative for the vast majority of tunnel paths and difficult to
   change even when larger MTUs become available.

   The following sections provide the SEAL normative specifications,
   while the appendices present non-normative additional considerations.

1.3.  Differences with RFC5320

   This specification of SEAL is descended from an experimental
   independent RFC publication of the same name [RFC5320].  However,
   this specification introduces a number of important differences from
   the earlier publication.

   First, this specification includes a protocol version field in the
   SEAL header whereas [RFC5320] does not, and therefore cannot be
   updated by future revisions.  This specification therefore obsoletes
   (i.e., and does not update) [RFC5320].

   Secondly, [RFC5320] forms a 32-bit Identification value by
   concatenating the 16-bit IPv4 Identification field with a 16-bit
   Identification "extension" field in the SEAL header.  This means that
   [RFC5320] can only operate over IPv4 networks (since IPv6 headers do
   not include a 16-bit version number) and that the SEAL Identification
   value can be corrupted if the Identification in the outer IPv4 header
   is rewritten.  In contrast, this specification includes a 32-bit
   Identification value that is independent of any identification fields
   found in the inner or outer IP headers, and is therefore compatible
   with any inner and outer IP protocol version combinations.



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   Additionally, the SEAL segmentation and reassembly procedures defined
   in [RFC5320] differ significantly from those found in this
   specification.  In particular, this specification defines a 6-bit
   Offset field that allows for smaller segment sizes when SEAL
   segmentation is necessary (e.g., in order to observe the IPv4 minimum
   MTU of 68 bytes).  In contrast, [RFC5320] includes a 3-bit Segment
   field and performs reassembly through concatenation of consecutive
   segments.

   The SEAL header in this specification also includes an optional
   Integrity Check Vector (ICV) that can be used to digitally sign the
   SEAL header and the leading portion of the encapsulated inner packet.
   This allows for a lightweight integrity check and a loose message
   origin authentication capability.  The header further includes new
   control bits as well as a link identification and encapsulation level
   field for additional control capabilities.

   Finally, this version of SEAL includes a new messaging protocol known
   as the SEAL Control Message Protocol (SCMP), whereas [RFC5320]
   performs signalling through the use of SEAL-encapsulated ICMP
   messages.  The use of SCMP allows SEAL-specific departures from ICMP,
   as well as a control messaging capability that extends to other
   specifications, including Virtual Enterprise Traversal (VET)
   [I-D.templin-intarea-vet].


2.  Terminology

   The following terms are defined within the scope of this document:

   subnetwork
      a virtual topology configured over a connected network routing
      region and bounded by encapsulating border nodes.

   IP
      used to generically refer to either Internet Protocol (IP)
      version, i.e., IPv4 or IPv6.

   Ingress Tunnel Endpoint (ITE)
      a virtual interface over which an encapsulating border node (host
      or router) sends encapsulated packets into the subnetwork.

   Egress Tunnel Endpoint (ETE)
      a virtual interface over which an encapsulating border node (host
      or router) receives encapsulated packets from the subnetwork.






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   SEAL Path
      a subnetwork path from an ITE to an ETE beginning with an
      underlying link of the ITE as the first hop.  Note that, if the
      ITE's interface connection to the underlying link assigns multiple
      IP addresses, each address represents a separate SEAL path.

   inner packet
      an unencapsulated network layer protocol packet (e.g., IPv4
      [RFC0791], OSI/CLNP [RFC0994], IPv6 [RFC2460], etc.) before any
      outer encapsulations are added.  Internet protocol numbers that
      identify inner packets are found in the IANA Internet Protocol
      registry [RFC3232].  SEAL protocol packets that incur an
      additional layer of SEAL encapsulation are also considered inner
      packets.

   outer IP packet
      a packet resulting from adding an outer IP header (and possibly
      other outer headers) to a SEAL-encapsulated inner packet.

   packet-in-error
      the leading portion of an invoking data packet encapsulated in the
      body of an error control message (e.g., an ICMPv4 [RFC0792] error
      message, an ICMPv6 [RFC4443] error message, etc.).

   Packet Too Big (PTB) message
      a control plane message indicating an MTU restriction (e.g., an
      ICMPv6 "Packet Too Big" message [RFC4443], an ICMPv4
      "Fragmentation Needed" message [RFC0792], etc.).

   Don't Fragment (DF) bit
      a bit that indicates whether the packet may be fragmented by the
      network.  The DF bit is explicitly included in the IPv4 header
      [RFC0791] and may be set to '0' to allow fragmentation or '1' to
      disallow further in-network fragmentation.  The bit is absent from
      the IPv6 header [RFC2460], but implicitly set to '1' becauuse
      fragmentation can occur only at IPv6 sources.

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

      HLEN - the length of the SEAL header plus outer headers

      ICV - Integrity Check Vector

      MAC - Message Authentication Code

      MTU - Maximum Transmission Unit




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      SCMP - the SEAL Control Message Protocol

      SDU - SCMP Destination Unreachable message

      SPP - SCMP Parameter Problem message

      SPTB - SCMP Packet Too Big message

      SEAL - Subnetwork Encapsulation and Adaptation Layer

      TE - Tunnel Endpoint (i.e., either ingress or egress)

      VET - Virtual Enterprise Traversal


3.  Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].  When used
   in lower case (e.g., must, must not, etc.), these words MUST NOT be
   interpreted as described in [RFC2119], but are rather interpreted as
   they would be in common English.


4.  Applicability Statement

   SEAL was originally motivated by the specific case of subnetwork
   abstraction for Mobile Ad hoc Networks (MANETs), however the domain
   of applicability also extends to subnetwork abstractions over
   enterprise networks, ISP networks, SOHO networks, the global public
   Internet itself, and any other connected network routing region.

   SEAL provides a network sublayer for encapsulation of an inner
   network layer packet within outer encapsulating headers.  SEAL can
   also be used as a sublayer within a transport layer protocol data
   payload, where transport layer encapsulation is typically used for
   Network Address Translator (NAT) traversal as well as operation over
   subnetworks that give preferential treatment to certain "core"
   Internet protocols, e.g., TCP, UDP, etc..  (However, note that TCP
   encapsulation may not be appropriate for all use cases; particularly
   those that require low delay and/or delay variance.)  The SEAL header
   is processed in a similar manner as for IPv6 extension headers, i.e.,
   it is not part of the outer IP header but rather allows for the
   creation of an arbitrarily extensible chain of headers in the same
   way that IPv6 does.

   To accommodate MTU diversity, the Ingress Tunnel Endpoint (ITE) may



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   need to perform any necessary segmentation which the Egress Tunnel
   Endpoint (ETE) must reassemble.  The ETE further acts as a passive
   observer that informs the ITE of any packet size limitations.  This
   allows the ITE to return appropriate PMTUD feedback even if the
   network path between the ITE and ETE filters ICMP messages.

   SEAL further provides mechanisms to ensure message origin
   authentication, packet header integrity, and anti-replay.  The SEAL
   framework is therefore similar to the IP Security (IPsec)
   Authentication Header (AH) [RFC4301][RFC4302], however it provides
   only minimal hop-by-hop authenticating services while leaving full
   data integrity, authentication and confidentiality services as an
   end-to-end consideration.

   In many aspects, SEAL also very closely resembles the Generic Routing
   Encapsulation (GRE) framework [RFC1701].  SEAL can therefore be
   applied in the same use cases that are traditionally addressed by
   GRE, but goes beyond GRE to also provide additional capabilities
   (e.,g., path MTU accommodation, message origin authentication, etc.)
   as described in this document.


5.  SEAL Specification

   The following sections specify the operation of SEAL:

5.1.  SEAL Tunnel Model

   SEAL is an encapsulation sublayer used within point-to-point and non-
   broadcast, multiple access (NBMA) tunnels.  Each SEAL path is
   configured over one or more underlying interfaces attached to
   subnetwork links.  The SEAL tunnel connects an ITE to one or more ETE
   "neighbors" via encapsulation across an underlying subnetwork, where
   the tunnel neighbor relationship may be either unidirectional or
   bidirectional.

   A unidirectional tunnel neighbor relationship allows the near end ITE
   to send data packets forward to the far end ETE, while the ETE only
   returns control messages when necessary.  A bidirectional tunnel
   neighbor relationship is one over which both TEs can exchange both
   data and control messages.

   Implications of the SEAL unidirectional and bidirectional models are
   the same as discussed in [I-D.templin-intarea-vet].







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5.2.  SEAL Model of Operation

   SEAL-enabled ITEs encapsulate each inner packet in a SEAL header and
   any outer header encapsulations as shown in Figure 1:

                                +--------------------+
                                ~   outer IP header  ~
                                +--------------------+
                                ~  other outer hdrs  ~
                                +--------------------+
                                ~    SEAL Header     ~
   +--------------------+       +--------------------+
   |                    |  -->  |                    |
   ~        Inner       ~  -->  ~        Inner       ~
   ~       Packet       ~  -->  ~       Packet       ~
   |                    |  -->  |                    |
   +--------------------+       +----------+---------+

                       Figure 1: SEAL Encapsulation

   The ITE inserts the SEAL header according to the specific tunneling
   protocol.  For simple encapsulation of an inner network layer packet
   within an outer IP header, the ITE inserts the SEAL header following
   the outer IP header and before the inner packet as: IP/SEAL/{inner
   packet}.

   For encapsulations over transports such as UDP, the ITE inserts the
   SEAL header following the outer transport layer header and before the
   inner packet, e.g., as IP/UDP/SEAL/{inner packet}.  In that case, the
   UDP header is seen as an "other outer header" as depicted in Figure 1
   and the outer IP and transport layer headers are together seen as the
   outer encapsulation headers.

   SEAL supports both "nested" tunneling and "re-encapsulating"
   tunneling.  Nested tunneling occurs when a first tunnel is
   encapsulated within a second tunnel, which may then further be
   encapsulated within additional tunnels.  Nested tunneling can be
   useful, and stands in contrast to "recursive" tunneling which is an
   anomalous condition incurred due to misconfiguration or a routing
   loop.  Considerations for nested tunneling and avoiding recursive
   tunneling are discussed in Section 4 of [RFC2473].

   Re-encapsulating tunneling occurs when a packet arrives at a first
   ETE, which then acts as an ITE to re-encapsulate and forward the
   packet to a second ETE connected to the same subnetwork.  In that
   case each ITE/ETE transition represents a segment of a bridged path
   between the ITE nearest the source and the ETE nearest the
   destination.  Considerations for re-encapsulating tunneling are



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   discussed in[I-D.templin-ironbis].  Combinations of nested and re-
   encapsulating tunneling are also naturally supported by SEAL.

   The SEAL ITE considers each underlying interface as the ingress
   attachment point to a SEAL path to the ETE.  The ITE therefore may
   experience different path MTUs on different SEAL paths.

   Finally, the SEAL ITE ensures that the inner network layer protocol
   will see a minimum MTU of 1500 bytes over each SEAL path regardless
   of the outer network layer protocol version, i.e., even if a small
   amount of fragmentation and reassembly are necessary.  This is
   necessary to avoid path MTU "black holes" for the minimum MTU
   configured by the vast majority of links in the Internet.

5.3.  SEAL Header and Trailer Format

   The SEAL header is 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |VER|C|A|I|V|R|RES|M|   Offset  |    NEXTHDR    | LINK_ID |LEVEL|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Identification (optional)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Integrity Check Vector (optional)            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   ...

                       Figure 2: SEAL Header Format

   VER (2)
      a 2-bit version field.  This document specifies Version 0 of the
      SEAL protocol, i.e., the VER field encodes the value 0.

   C (1)
      the "Control/Data" bit.  Set to 1 by the ITE in SEAL Control
      Message Protocol (SCMP) control messages, and set to 0 in ordinary
      data packets.

   A (1)
      the "Acknowledgement Requested" bit.  Set to 1 by the ITE in SEAL
      data packets for which it wishes to receive an explicit
      acknowledgement from the ETE.

   I (1)
      the "Identification Included" bit.





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   V (1)
      the "Integrity Check Vector included" bit.

   R (1)
      the "Redirects Permitted" bit (reserved for use by VET:
      [I-D.templin-intarea-vet]).

   RES (2)  a 2-bit reserved field.

   M (1)  the "More Segments" bit.  Set to 1 in a non-final segment and
      set to 0 in the final segment of the SEAL packet.

   Offset (6)  a 6-bit Offset field.  Set to 0 in the first segment of a
      segmented SEAL packet.  Set to an integral number of 32 byte
      blocks in subsequent segments (e.g., an Offset of 10 indicates a
      block that begins at the 320th byte in the packet).

   NEXTHDR (8)  an 8-bit field that encodes the next header Internet
      Protocol number the same as for the IPv4 protocol and IPv6 next
      header fields.

   LINK_ID (5)
      a 5-bit link identification value, set to a unique value by the
      ITE for each SEAL path over which it will send encapsulated
      packets to the ETE (up to 32 SEAL paths per ETE are therefore
      supported).  Note that, if the ITE's interface connection to the
      underlying link assigns multiple IP addresses, each address
      represents a separate SEAL path that must be assigned a separate
      LINK_ID.

   LEVEL (3)
      a 3-bit nesting level; use to limit the number of tunnel nesting
      levels.  Set to an integer value up to 7 in the innermost SEAL
      encapsulation, and decremented by 1 for each successive additional
      SEAL encapsulation nesting level.  Up to 8 levels of nesting are
      therefore supported.

   Identification (32)
      an optional 32-bit per-packet identification field; present when
      I==1.  Set to a 32-bit value (beginning with 0) that is
      monotonically-incremented for each SEAL packet transmitted to this
      ETE.

   Integrity Check Vector (ICV) (variable)
      an optional variable-length integrity check vector field; present
      when V==1.





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5.4.  ITE Specification

5.4.1.  Tunnel Interface MTU

   The tunnel interface must present a constant MTU value to the inner
   network layer as the size for admission of inner packets into the
   interface.  Since NBMA tunnel virtual interfaces may support a large
   set of SEAL paths that accept widely varying maximum packet sizes,
   however, a number of factors should be taken into consideration when
   selecting a tunnel interface MTU.

   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 ICMP Packet Too Big (PTB)
   message returned.  When large packets sent by end systems incur
   additional encapsulation at an ITE, however, they may be dropped
   silently within the tunnel since the network may not always deliver
   the necessary PTBs [RFC2923].  The ITE SHOULD therefore set a tunnel
   interface MTU of at least 1500 bytes.

   The inner network layer protocol consults the tunnel interface MTU
   when admitting a packet into the interface.  For non-SEAL inner IPv4
   packets with the IPv4 Don't Fragment (DF) cleared (i.e, DF==0), if
   the packet is larger than the tunnel interface MTU the inner IPv4
   layer uses IPv4 fragmentation to break the packet into fragments no
   larger than the tunnel interface MTU.  The ITE then admits each
   fragment into the interface as an independent packet.

   For all other inner packets, the inner network layer admits the
   packet if it is no larger than the tunnel interface MTU; otherwise,
   it drops the packet and sends a PTB error message to the source with
   the MTU value set to the tunnel interface MTU.  The message contains
   as much of the invoking packet as possible without the entire message
   exceeding the network layer minimum MTU size.

   The ITE can alternatively set an indefinite MTU on the tunnel
   interface such that all inner packets are admitted into the interface
   regardless of their size.  For ITEs that host applications that use
   the tunnel interface directly, this option must be carefully
   coordinated with protocol stack upper layers since some upper layer
   protocols (e.g., TCP) derive their packet sizing parameters from the
   MTU of the outgoing interface and as such may select too large an
   initial size.  This is not a problem for upper layers that use
   conservative initial maximum segment size estimates and/or when the
   tunnel interface can reduce the upper layer's maximum segment size,
   e.g., by reducing the size advertised in the MSS option of outgoing



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   TCP messages (sometimes known as "MSS clamping").

   In light of the above considerations, the ITE should configure an
   indefinite MTU on tunnel *router* interfaces so that SEAL performs
   all subnetwork adaptation from within the interface as specified in
   Section 5.4.3.  The ITE can instead set a smaller MTU on tunnel
   *host* interfaces (e.g., the smallest MTU among all of the underlying
   links minus the size of the encapsulation headers) but SHOULD NOT set
   an MTU smaller than 1500 bytes.

5.4.2.  Tunnel Neighbor Soft State

   The tunnel virtual interface maintains a number of soft state
   variables for each ETE and for each SEAL path.

   When per-packet identification is required, the ITE maintains a per
   ETE window of Identification values for the packets it has recently
   sent to this ETE.  The ITE then sets a variable "USE_ID" to TRUE, and
   includes an Identification in each packet it sends to this ETE;
   otherwise, it sets USE_ID to FALSE.

   When message origin authentication and integrity checking is
   required, the ITE also includes an ICV in the packets it sends to the
   ETE.  The ICV format is shown in Figure 3:

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |F|Key|Algorithm|       Message Authentication Code (MAC)       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   ...

               Figure 3: Integrity Check Vector (ICV) Format

   As shown in the figure, the ICV begins with a 1-octet control field
   with a 1-bit (F)lag, a 2-bit Key identifier and a 5-bit Algorithm
   identifier.  The control octet is followed by a variable-length
   Message Authentication Code (MAC).  The ITE maintains a per ETE
   algorithm and secret key to calculate the MAC in each packet it will
   send to this ETE.  (By default, the ITE sets the F bit and Algorithm
   fields to 0 to indicate use of the HMAC-SHA-1 algorithm with a 160
   bit shared secret key to calculate an 80 bit MAC per [RFC2104] over
   the leading 128 bytes of the packet.  Other values for F and
   Algorithm are out of scope.)  The ITE then sets a variable "USE_ICV"
   to TRUE, and includes an ICV in each packet it sends to this ETE;
   otherwise, it sets USE_ICV to FALSE.

   For each SEAL path, the ITE must also account for encapsulation
   header lengths.  The ITE therefore maintains the per SEAL path
   constant values "SHLEN" set to the length of the SEAL header, "THLEN"
   set to the length of the outer encapsulating transport layer headers



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   (or 0 if outer transport layer encapsulation is not used), "IHLEN"
   set to the length of the outer IP layer header, and "HLEN" set to
   (SHLEN+THLEN+IHLEN).  (The ITE must include the length of the
   uncompressed headers even if header compression is enabled when
   calculating these lengths.)  In addition, the ITE maintains a per
   SEAL path variable "MAXMTU" initialized to the maximum of 1500 bytes
   and the MTU of the underlying link minus HLEN.  (Thereafter, the ITE
   must not reduce MAXMTU to a value smaller than 1500 bytes.)

   The ITE further sets a variable 'MINMTU' to the minimum MTU for the
   SEAL path over which encapsulated packets will travel.  For IPv6
   paths the ITE sets MINMTU=1280 (see: [RFC2460]) and for IPv4 paths
   the ITE sets MINMTU=576 even though the true MINMTU for IPv4 is only
   68 bytes (see: [RFC0791]).

   The ITE can also set MINMTU to a larger value if there is reason to
   believe that the minimum path MTU is larger, or to a smaller value if
   there is reason to believe there may be additional encapsulations on
   the path.  If this value proves too large, the ITE will receive PTB
   message feedback either from the ETE or from a router on the path and
   will be able to reduce its MINMTU to a smaller value.

   The ITE may instead maintain the packet sizing variables and
   constants as per ETE (rather than per SEAL path) values.  In that
   case, the values reflect the lowest-common-denominator size across
   all of the SEAL paths associated with this ETE.

5.4.3.  SEAL Layer Pre-Processing

   The SEAL layer is logically positioned between the inner and outer
   network protocol layers, where the inner layer is seen as the (true)
   network layer and the outer layer is seen as the (virtual) data link
   layer.  Each packet to be processed by the SEAL layer is either
   admitted into the tunnel interface by the inner network layer
   protocol as described in Section 5.4.1 or is undergoing re-
   encapsulation from within the tunnel interface.  The SEAL layer sees
   the former class of packets as inner packets that include inner
   network and transport layer headers, and sees the latter class of
   packets as transitional SEAL packets that include the outer and SEAL
   layer headers that were inserted by the previous hop SEAL ITE.  For
   these transitional packets, the SEAL layer re-encapsulates the packet
   with new outer and SEAL layer headers when it forwards the packet to
   the next hop SEAL ITE.

   We now discuss the SEAL layer pre-processing actions for these two
   classes of packets.





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5.4.3.1.  Inner Packet Pre-Processing

   For each inner packet admitted into the tunnel interface, if the
   packet is itself a SEAL packet (i.e., one with the port number for
   SEAL in the transport layer header or one with the protocol number
   for SEAL in the IP layer header) and the LEVEL field of the SEAL
   header contains the value 0, the ITE silently discards the packet.

   Otherwise, for non-SEAL IPv4 inner packets with DF==0 in the IP
   header and IPv6 inner packets with a fragment header and with (MF=0;
   Offset=0), if the packet is larger than (MINMTU-HLEN) the ITE uses IP
   fragmentation to fragment the packet into N roughly equal-length
   pieces, where N is minimized and each fragment is significantly
   smaller than (MINMTU-HLEN) to allow for additional encapsulations in
   the path.  The ITE then submits each fragment for SEAL encapsulation
   as specified in Section 5.4.4.

   For all other inner packets, if the packet is no larger than MAXMTU
   for the corresponding SEAL path the ITE submits it for SEAL
   encapsulation as specified in Section 5.4.4.  Otherwise, the ITE
   drops the packet and sends an ordinary ICMP PTB message appropriate
   to the inner protocol version with the MTU field set to MAXMTU.  (For
   IPv4 SEAL packets with DF==0, the ITE should set DF=1 and re-
   calculate the IPv4 header checksum before generating the PTB message
   in order to avoid bogon filters.)  After sending the PTB message, the
   ITE discards the inner packet.

5.4.3.2.  Transitional SEAL Packet Pre-Processing

   For each transitional packet that is to be processed by the SEAL
   layer from within the tunnel interface, the ITE sets aside the SEAL
   encapsulation headers that were received from the previous hop.
   Next, if the packet is no larger than MAXMTU for the next hop SEAL
   path the ITE submits it for SEAL encapsulation as specified in
   Section 5.4.4.  Otherwise, the ITE drops the packet and sends an SCMP
   Packet Too Big (SPTB) message to the previous hop subject to rate
   limiting (see: Section 5.6.1.1) with the MTU field set to MAXMTU.
   After sending the SPTB message, the ITE discards the packet.

5.4.4.  SEAL Encapsulation and Segmentation

   For each inner packet/fragment submitted for SEAL encapsulation, the
   ITE next encapsulates the packet in a SEAL header formatted as
   specified in Section 5.3.  The SEAL header includes an Identification
   field when USE_ID is TRUE, followed by an ICV field when USE_ICV is
   TRUE.

   The ITE next sets C=0 and RES=0 in the SEAL header.  The ITE also



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   sets A=1 if necessary for ETE reachability determination (see:
   Section 5.4.6) or for stateful MTU determination (see Section 5.4.9).
   Otherwise, the ITE sets A=0.

   The ITE then sets LINK_ID to the value assigned to the underlying
   SEAL path, and sets NEXTHDR to the protocol number corresponding to
   the address family of the encapsulated inner packet.  For example,
   the ITE sets NEXTHDR to the value '4' for encapsulated IPv4 packets
   [RFC2003], '41' for encapsulated IPv6 packets [RFC2473][RFC4213],
   '80' for encapsulated OSI/CLNP packets [RFC1070], etc.

   Next, if the inner packet is not itself a SEAL packet the ITE sets
   LEVEL to an integer value between 0 and 7 as a specification of the
   number of additional layers of nested SEAL encapsulations permitted.
   If the inner packet is a SEAL packet that is undergoing nested
   encapsulation, the ITE instead sets LEVEL to the value that appears
   in the inner packet's SEAL header minus 1.  If the inner packet is
   undergoing SEAL re-encapsulation, the ITE instead copies the LEVEL
   value from the SEAL header of the packet to be re-encapsulated.

   Next, if the inner packet is no larger than (MINMTU-HLEN) or larger
   than 1500, the ITE sets (M=0; Offset=0).  Otherwise, the ITE breaks
   the inner packet into a N roughly equal-length non-overlapping
   segments (where N is minimized and each fragment is significantly
   smaller than (MINMTU-HLEN) to allow for additional encapsulations in
   the path) then appends a clone of the SEAL header from the first
   segment onto the head of each additional segment.  The ITE then sets
   (M=1; Offset=0) in the first segment, sets (M=0/1; Offset=i) in the
   second segment, sets (M=0/1; Offset=j) in the third segment (if
   needed), etc., then finally sets (M=0; Offset=k) in the final segment
   (where i, j, k, etc. are the number of 32 byte blocks that preceded
   this segment).

   When USE_ID is FALSE, the ITE next sets I=0.  Otherwise, the ITE sets
   I=1 and writes a monotonically-incrementing integer value for this
   ETE in the Identification field beginning with 0 in the first packet
   transmitted.  (For SEAL packets that have been split into multiple
   pieces, the ITE writes the same Identification value in each piece.)
   The monotonically-incrementing requirement is to satisfy ETEs that
   use this value for anti-replay purposes.  The value is incremented
   modulo 2^32, i.e., it wraps back to 0 when the previous value was
   (2^32 - 1).

   When USE_ICV is FALSE, the ITE next sets V=0.  Otherwise, the ITE
   sets V=1, includes an ICV and calculates the MAC using HMAC-SHA-1
   with a 160 bit secret key and 80 bit MAC field.  Beginning with the
   SEAL header, the ITE sets the ICV field to 0, calculates the MAC over
   the leading 128 bytes of the packet (or up to the end of the packet



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   if there are fewer than 128 bytes) and places the result in the MAC
   field.  (For SEAL packets that have been split into multiple pieces,
   each piece calculates its own MAC.)  The ITE then writes the value 0
   in the F flag and 0x00 in the Algorithm field of the ICV control
   octet (other values for these fields, and other MAC calculation
   disciplines, are outside the scope of this document and may be
   specified in future documents.)

   The ITE then adds the outer encapsulating headers as specified in
   Section 5.4.5.

5.4.5.  Outer Encapsulation

   Following SEAL encapsulation, the ITE next encapsulates each segment
   in the requisite outer transport (when necessary) and IP layer
   headers.  When a transport layer header such as UDP or TCP is
   included, the ITE writes the port number for SEAL in the transport
   destination service port field.

   When UDP encapsulation is used, the ITE sets the UDP checksum field
   to zero for IPv4 packets and also sets the UDP checksum field to zero
   for IPv6 packets even though IPv6 generally requires UDP checksums.
   Further considerations for setting the UDP checksum field for IPv6
   packets are discussed in
   [I-D.ietf-6man-udpzero][I-D.ietf-6man-udpchecksums].

   The ITE then sets the outer IP layer headers the same as specified
   for ordinary IP encapsulation (e.g., [RFC1070][RFC2003], [RFC2473],
   [RFC4213], etc.) except that for ordinary SEAL packets the ITE copies
   the "TTL/Hop Limit", "Type of Service/Traffic Class" and "Congestion
   Experienced" values in the inner network layer header into the
   corresponding fields in the outer IP header.  For transitional SEAL
   packets undergoing re-encapsulation, the ITE instead copies the "TTL/
   Hop Limit", "Type of Service/Traffic Class" and "Congestion
   Experienced" values in the outer IP header of the received packet
   into the corresponding fields in the outer IP header of the packet to
   be forwarded (i.e., the values are transferred between outer headers
   and *not* copied from the inner network layer header).

   The ITE also sets the IP protocol number to the appropriate value for
   the first protocol layer within the encapsulation (e.g., UDP, TCP,
   SEAL, etc.).  When IPv6 is used as the outer IP protocol, the ITE
   then sets the flow label value in the outer IPv6 header the same as
   described in [RFC6438].  When IPv4 is used as the outer IP protocol,
   the ITE instead sets DF=0 in the IPv4 header to allow the packet to
   be fragmented if it encounters a restricting link (for IPv6 SEAL
   paths, the DF bit is implicitly set to 1).




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   The ITE finally sends each outer packet via the underlying link
   corresponding to LINK_ID.

5.4.6.  Path Probing and ETE Reachability Verification

   All SEAL data packets sent by the ITE are considered implicit probes.
   SEAL data packets will elicit an SCMP message from the ETE if it
   needs to acknowledge a probe and/or report an error condition.  SEAL
   data packets may also be dropped by either the ETE or a router on the
   path, which may or may not result in an ICMP message being returned
   to the ITE.

   The ITE processes ICMP messages as specified in Section 5.4.7.

   The ITE processes SCMP messages as specified in Section 5.6.2.

5.4.7.  Processing ICMP Messages

   When the ITE sends SEAL packets, it may receive ICMP error messages
   [RFC0792][RFC4443] from an ordinary router within the subnetwork.
   Each ICMP message includes an outer IP header, followed by an ICMP
   header, followed by a portion of the SEAL data packet that generated
   the error (also known as the "packet-in-error") beginning with the
   outer IP header.

   The ITE should process ICMPv4 Protocol Unreachable messages and
   ICMPv6 Parameter Problem messages with Code "Unrecognized Next Header
   type encountered" as a hint that the IP destination address does not
   implement SEAL.  The ITE can optionally ignore ICMP messages that do
   not include sufficient information in the packet-in-error, or process
   them as a hint that the SEAL path may be failing.

   For other ICMP messages, the ITE should use any outer header
   information available as a first-pass authentication filter (e.g., to
   determine if the source of the message is within the same
   administrative domain as the ITE) and discards the message if first
   pass filtering fails.

   Next, the ITE examines the packet-in-error beginning with the SEAL
   header.  If the value in the Identification field (if present) is not
   within the window of packets the ITE has recently sent to this ETE,
   or if the MAC value in the SEAL header ICV field (if present) is
   incorrect, the ITE discards the message.

   Next, if the received ICMP message is a PTB the ITE sets the
   temporary variable "PMTU" for this SEAL path to the MTU value in the
   PTB message.  If PMTU==0, the ITE consults a plateau table (e.g., as
   described in [RFC1191]) to determine PMTU based on the length field



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   in the outer IP header of the packet-in-error.  For example, if the
   ITE receives a PTB message with MTU==0 and length 4KB, it can set
   PMTU=2KB.  If the ITE subsequently receives a PTB message with MTU==0
   and length 2KB, it can set PMTU=1792, etc. to a minimum value of
   PMTU=(1500+HLEN).  If the ITE is performing stateful MTU
   determination for this SEAL path (see Section 5.4.9), the ITE next
   sets MAXMTU=MAX((PMTU-HLEN), 1500).

   If the ICMP message was not discarded, the ITE then transcribes it
   into a message to return to the previous hop.  If the inner packet
   was a SEAL data packet, the ITE transcribes the ICMP message into an
   SCMP message.  Otherwise, the ITE transcribes the ICMP message into a
   message appropriate for the inner protocol version.

   To transcribe the message, the ITE extracts the inner packet from
   within the ICMP message packet-in-error field and uses it to generate
   a new message corresponding to the type of the received ICMP message.
   For SCMP messages, the ITE generates the message the same as
   described for ETE generation of SCMP messages in Section 5.6.1.  For
   (S)PTB messages, the ITE writes (PMTU-HLEN) in the MTU field.

   The ITE finally forwards the transcribed message to the previous hop
   toward the inner source address.

5.4.8.  IPv4 Middlebox Reassembly Testing

   The ITE can perform a qualification exchange to ensure that the
   subnetwork correctly delivers fragments to the ETE.  This procedure
   can be used, e.g., to determine whether there are middleboxes on the
   path that violate the [RFC1812], Section 5.2.6 requirement that: "A
   router MUST NOT reassemble any datagram before forwarding it".

   The ITE should use knowledge of its topological arrangement as an aid
   in determining when middlebox reassembly testing is necessary.  For
   example, if the ITE is aware that the ETE is located somewhere in the
   public Internet, middlebox reassembly testing should not be
   necessary.  If the ITE is aware that the ETE is located behind a NAT
   or a firewall, however, then reassembly testing can be used to detect
   middleboxes that do not conform to specifications.

   The ITE can perform a middlebox reassembly test by selecting a data
   packet to be used as a probe.  While performing the test with real
   data packets, the ITE should select only inner packets that are no
   larger than (1500-HLEN) bytes for testing purposes.  The ITE can also
   construct a dummy probe packet instead of using ordinary SEAL data
   packets.

   To generate a dummy probe packet, the ITE creates a packet buffer



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   beginning with the same outer headers, SEAL header and inner network
   layer header that would appear in an ordinary data packet, then pads
   the packet with random data to a length that is at least 128 bytes
   but no longer than (1500-HLEN) bytes.  The ITE then writes the value
   '0' in the inner network layer TTL (for IPv4) or Hop Limit (for IPv6)
   field.

   The ITE then sets C=0 in the SEAL header of the probe packet and sets
   the NEXTHDR field to the inner network layer protocol type.  (The ITE
   may also set A=1 if it requires a positive acknowledgement;
   otherwise, it sets A=0.)  Next, the ITE sets LINK_ID and LEVEL to the
   appropriate values for this SEAL path, sets Identification and I=1
   (when USE_ID is TRUE), then finally calculates the ICV and sets V=1
   (when USE_ICV is TRUE).

   The ITE then encapsulates the probe packet in the appropriate outer
   headers, splits it into two outer IPv4 fragments, then sends both
   fragments over the same SEAL path.

   The ITE should send a series of probe packets (e.g., 3-5 probes with
   1sec intervals between tests) instead of a single isolated probe in
   case of packet loss.  If the ETE returns an SCMP PTB message with MTU
   != 0, then the SEAL path correctly supports fragmentation; otherwise,
   the ITE enables stateful MTU determination for this SEAL path as
   specified in Section 5.4.9.

   (Examples of middleboxes that may perform reassembly include stateful
   NATs and firewalls.  Such devices could still allow for stateless MTU
   determination if they gather the fragments of a fragmented IPv4 SEAL
   data packet for packet analysis purposes but then forward the
   fragments on to the final destination rather than forwarding the
   reassembled packet.)

5.4.9.  Stateful MTU Determination

   SEAL supports a stateless MTU determination capability, however the
   ITE may in some instances wish to impose a stateful MTU limit on a
   particular SEAL path.  For example, when the ETE is situated behind a
   middlebox that performs IPv4 reassembly (see: Section 5.4.8) it is
   imperative that fragmentation be avoided.  In other instances (e.g.,
   when the SEAL path includes performance-constrained links), the ITE
   may deem it necessary to cache a conservative static MTU in order to
   avoid sending large packets that would only be dropped due to an MTU
   restriction somewhere on the path.

   To determine a static MTU value, the ITE sends a series of dummy
   probe packets of various sizes to the ETE with A=1 in the SEAL header
   and DF=1 in the outer IP header.  The ITE then caches the size 'S' of



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   the largest packet for which it receives a probe reply from the ETE
   by setting MAXMTU=MAX((S-HLEN), 1500) for this SEAL path.

   For example, the ITE could send probe packets of 4KB, followed by
   2KB, followed by 1792 bytes, etc.  While probing, the ITE processes
   any ICMP PTB message it receives as a potential indication of probe
   failure then discards the message.

5.4.10.  Detecting Path MTU Changes

   When stateful MTU determination is used, the ITE SHOULD periodically
   reset MAXMTU and/or re-probe the path to determine whether MAXMTU has
   increased.  If the path still has a too-small MTU, the ITE will
   receive a PTB message that reports a smaller size.

5.5.  ETE Specification

5.5.1.  Minimum Reassembly Buffer Requirements

   For IPv6, the ETE must configure a minimum reassembly buffer size of
   (1500 + HLEN) bytes for the reassembly of outer IPv6 packets, i.e.,
   even though the true minimum reassembly size for IPv6 is only 1500
   bytes [RFC2460].  For IPv4, the ETE must also configure a minimum
   reassembly buffer size of (1500 + HLEN) bytes for the reassembly of
   outer IPv4 packets, i.e., even though the true minimum reassembly
   size for IPv4 is only 576 bytes [RFC1122].

   In addition to this outer reassembly buffer requirement, the ETE must
   further configure a minimum SEAL reassembly buffer size of (1500 +
   HLEN) bytes for the reassembly of segmented SEAL packets (see:
   Section 5.5.4).

5.5.2.  Tunnel Neighbor Soft State

   When message origin authentication and integrity checking is
   required, the ETE maintains a per-ITE MAC calculation algorithm and a
   symmetric secret key to verify the MAC.  When per-packet
   identification is required, the ETE also maintains a window of
   Identification values for the packets it has recently received from
   this ITE.

   When the tunnel neighbor relationship is bidirectional, the ETE
   further maintains a per SEAL path mapping of outer IP and transport
   layer addresses to the LINK_ID that appears in packets received from
   the ITE.






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5.5.3.  IP-Layer Reassembly

   The ETE reassembles fragmented IP packets that are explcitly
   addressed to itself.  For IP fragments that are received via a SEAL
   tunnel, the ETE SHOULD maintain conservative reassembly cache high-
   and low-water marks.  When the size of the reassembly cache exceeds
   this high-water mark, the ETE SHOULD actively discard stale
   incomplete reassemblies (e.g., using an Active Queue Management (AQM)
   strategy) until the size falls below the low-water mark.  The ETE
   SHOULD also actively discard any pending reassemblies that clearly
   have no opportunity for completion, e.g., when a considerable number
   of new fragments have arrived before a fragment that completes a
   pending reassembly arrives.

   The ETE processes non-SEAL IP packets as specified in the normative
   references, i.e., it performs any necessary IP reassembly then
   discards the packet if it is larger than the reassembly buffer size
   or delivers the (fully-reassembled) packet to the appropriate upper
   layer protocol module.

   For SEAL packets, the ETE performs any necessary IP reassembly then
   submits the packet for SEAL decapsulation as specified in Section
   5.5.4.  (Note that if the packet is larger than the reassembly buffer
   size, the ETE still examines the leading portion of the (partially)
   reassembled packet during decapsulation as specified in the next
   section.)

5.5.4.  Decapsulation and Re-Encapsulation

   For each SEAL packet accepted for decapsulation, when I==1 the ETE
   first examines the Identification field.  If the Identification is
   not within the window of acceptable values for this ITE, the ETE
   silently discards the packet.

   Next, if V==1 the ETE SHOULD verify the MAC value (with the MAC field
   itself reset to 0) and silently discard the packet if the value is
   incorrect.

   Next, if the packet arrived as multiple IP fragments, the ETE sends
   an SPTB message back to the ITE with MTU set to the size of the
   largest fragment received minus HLEN (see: Section 5.6.1.1).

   Next, if the packet arrived as multiple IP fragments and the inner
   packet is larger than 1500 bytes, the ETE silently discards the
   packet; otherwise, it continues to process the packet.

   Next, if there is an incorrect value in a SEAL header field (e.g., an
   incorrect "VER" field value), the ETE discards the packet.  If the



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   SEAL header has C==0, the ETE also returns an SCMP "Parameter
   Problem" (SPP) message (see Section 5.6.1.2).

   Next, if the SEAL header has C==1, the ETE processes the packet as an
   SCMP packet as specified in Section 5.6.2.  Otherwise, the ETE
   continues to process the packet as a SEAL data packet.

   Next, if the SEAL header has (M==1 || Offset!==0) the ETE checks to
   see if the other segments of this already-segmented SEAL packet have
   arrived, i.e., by looking for additional segments that have the same
   outer IP source address, destination address, source transport port
   number (if present) and SEAL Identification value.  If the other
   segments have already arrived, the ETE discards the SEAL header and
   other outer headers from the non-initial segments and appends them
   onto the end of the first segment according to their offset value.
   Otherwise, the ETE caches the segment for at most 60 seconds while
   awaiting the arrival of its partners.  During this process, the ETE
   discards any segments that are overlapping with respect to segments
   that have already been received.  The ETE further SHOULD manage the
   SEAL reassembly cache the same as described for the IP-Layer
   Reassembly cache in Section 5.5.3, i.e., it SHOULD perform an early
   discard for any pending reassemblies that have low probability of
   completion.

   Next, if the SEAL header in the (reassembled) packet has A==1, the
   ETE sends an SPTB message back to the ITE with MTU=0 (see: Section
   5.6.1.1).

   Finally, the ETE discards the outer headers and processes the inner
   packet according to the header type indicated in the SEAL NEXTHDR
   field.  If the inner (TTL / Hop Limit) field encodes the value 0, the
   ETE silently discards the packet.  Otherwise, if the next hop toward
   the inner destination address is via a different interface than the
   SEAL packet arrived on, the ETE discards the SEAL header and delivers
   the inner packet either to the local host or to the next hop
   interface if the packet is not destined to the local host.

   If the next hop is on the same interface the SEAL packet arrived on,
   however, the ETE submits the packet for SEAL re-encapsulation
   beginning with the specification in Section 5.4.3 above and without
   decrementing the value in the inner (TTL / Hop Limit) field.  In this
   process, the packet remains within the tunnel (i.e., it does not exit
   and then re-enter the tunnel); hence, the packet is not discarded if
   the LEVEL field in the SEAL header contains the value 0.







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5.6.  The SEAL Control Message Protocol (SCMP)

   SEAL provides a companion SEAL Control Message Protocol (SCMP) that
   uses the same message types and formats as for the Internet Control
   Message Protocol for IPv6 (ICMPv6) [RFC4443].  As for ICMPv6, each
   SCMP message includes a 32-bit header and a variable-length body.
   The ITE encapsulates the SCMP message in a SEAL header and outer
   headers as shown in Figure 4:

                                       +--------------------+
                                       ~   outer IP header  ~
                                       +--------------------+
                                       ~  other outer hdrs  ~
                                       +--------------------+
                                       ~    SEAL Header     ~
          +--------------------+       +--------------------+
          | SCMP message header|  -->  | SCMP message header|
          +--------------------+       +--------------------+
          |                    |  -->  |                    |
          ~  SCMP message body ~  -->  ~  SCMP message body ~
          |                    |  -->  |                    |
          +--------------------+       +--------------------+

               SCMP Message                  SCMP Packet
           before encapsulation          after encapsulation

                   Figure 4: SCMP Message Encapsulation

   The following sections specify the generation, processing and
   relaying of SCMP messages.

5.6.1.  Generating SCMP Error Messages

   ETEs generate SCMP error messages in response to receiving certain
   SEAL data packets using the format shown in Figure 5:
















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       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |     Code      |           Checksum            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Type-Specific Data                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      As much of the invoking SEAL data packet as possible     |
      ~       (beginning with the SEAL header) without the SCMP       ~
      |             packet exceeding MINMTU bytes (*)                 |

      (*) also known as the "packet-in-error"

                    Figure 5: SCMP Error Message Format

   The error message includes the 32-bit SCMP message header, followed
   by a 32-bit Type-Specific Data field, followed by the leading portion
   of the invoking SEAL data packet beginning with the SEAL header as
   the "packet-in-error".  The packet-in-error includes as much of the
   invoking packet as possible extending to a length that would not
   cause the entire SCMP packet following outer encapsulation to exceed
   MINMTU bytes.

   When the ETE processes a SEAL data packet for which the
   Identification and ICV values are correct but an error must be
   returned, it prepares an SCMP error message as shown in Figure 5.
   The ETE sets the Type and Code fields to the same values that would
   appear in the corresponding ICMPv6 message [RFC4443], but calculates
   the Checksum beginning with the SCMP message header using the
   algorithm specified for ICMPv4 in [RFC0792].

   The ETE next encapsulates the SCMP message in the requisite SEAL and
   outer headers as shown in Figure 4.  During encapsulation, the ETE
   sets the outer destination address/port numbers of the SCMP packet to
   the values associated with the ITE and sets the outer source address/
   port numbers to its own outer address/port numbers.

   The ETE then sets (C=1; A=0; RES=0; M=0; Offset=0) in the SEAL
   header, then sets I, V, NEXTHDR and LEVEL to the same values that
   appeared in the SEAL header of the data packet.  If the neighbor
   relationship between the ITE and ETE is unidirectional, the ETE next
   sets the LINK_ID field to the same value that appeared in the SEAL
   header of the data packet.  Otherwise, the ETE sets the LINK_ID field
   to the value it would use in sending a SEAL packet to this ITE.

   When I==1, the ETE next sets the Identification field to an
   appropriate value for the ITE.  If the neighbor relationship between
   the ITE and ETE is unidirectional, the ETE sets the Identification



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   field to the same value that appeared in the SEAL header of the data
   packet.  Otherwise, the ETE sets the Identification field to the
   value it would use in sending the next SEAL packet to this ITE.

   When V==1, the ETE then prepares the ICV field the same as specified
   for SEAL data packet encapsulation in Section 5.4.4.

   Finally, the ETE sends the resulting SCMP packet to the ITE the same
   as specified for SEAL data packets in Section 5.4.5.

   The following sections describe additional considerations for various
   SCMP error messages:

5.6.1.1.  Generating SCMP Packet Too Big (SPTB) Messages

   An ETE generates an SPTB message when it receives a SEAL data packet
   that arrived as multiple outer IP fragments.  The ETE prepares the
   SPTB message the same as for the corresponding ICMPv6 PTB message,
   and writes the length of the largest outer IP fragment received minus
   HLEN in the MTU field of the message.

   The ETE also generates an SPTB message when it accepts a SEAL
   protocol data packet with A==1 in the SEAL header.  The ETE prepares
   the SPTB message the same as above, except that it writes the value 0
   in the MTU field.

5.6.1.2.  Generating Other SCMP Error Messages

   An ETE generates an SCMP "Destination Unreachable" (SDU) message
   under the same circumstances that an IPv6 system would generate an
   ICMPv6 Destination Unreachable message.

   An ETE generates an SCMP "Parameter Problem" (SPP) message when it
   receives a SEAL packet with an incorrect value in the SEAL header.

   TEs generate other SCMP message types using methods and procedures
   specified in other documents.  For example, SCMP message types used
   for tunnel neighbor coordinations are specified in VET
   [I-D.templin-intarea-vet].

5.6.2.  Processing SCMP Error Messages

   An ITE may receive SCMP messages with C==1 in the SEAL header after
   sending packets to an ETE.  The ITE first verifies that the outer
   addresses of the SCMP packet are correct, and (when I==1) that the
   Identification field contains an acceptable value.  The ITE next
   verifies that the SEAL header fields are set correctly as specified
   in Section 5.6.1.  When V==1, the ITE then verifies the ICV.  The ITE



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   next verifies the Checksum value in the SCMP message header.  If any
   of these values are incorrect, the ITE silently discards the message;
   otherwise, it processes the message as follows:

5.6.2.1.  Processing SCMP PTB Messages

   After an ITE sends a SEAL data packet to an ETE, it may receive an
   SPTB message with a packet-in-error containing the leading portion of
   the packet (see: Section 5.6.1.1).  For IP SPTB messages with MTU==0,
   the ITE processes the message as confirmation that the ETE received a
   SEAL data packet with A==1 in the SEAL header.  The ITE then discards
   the message.

   For SPTB messages with MTU != 0, the ITE processes the message as an
   indication of a packet size limitation as follows.  If the inner
   packet is no larger than 1500 bytes, the ITE reduces its MINMTU value
   for this ITE.  If the inner packet length is larger than 1500 and the
   MTU value is not substantially less than MINMTU bytes, the value is
   likely to reflect the true MTU of the restricting link on the path to
   the ETE; otherwise, a router on the path may be generating runt
   fragments.

   In that case, the ITE can consult a plateau table (e.g., as described
   in [RFC1191]) to rewrite the MTU value to a reduced size.  For
   example, if the ITE receives an IPv4 SPTB message with MTU==256 and
   inner packet length 4KB, it can rewrite the MTU to 2KB.  If the ITE
   subsequently receives an IPv4 SPTB message with MTU==256 and inner
   packet length 2KB, it can rewrite the MTU to 1792, etc., to a minimum
   of 1500 bytes.  If the ITE is performing stateful MTU determination
   for this SEAL path, it then writes the new MTU value minus HLEN in
   MAXMTU.

   The ITE then checks its forwarding tables to discover the previous
   hop toward the source address of the inner packet.  If the previous
   hop is reached via the same tunnel interface the SPTB message arrived
   on, the ITE relays the message to the previous hop.  In order to
   relay the message, the first writes zero in the Identification and
   ICV fields of the SEAL header within the packet-in-error.  The ITE
   next rewrites the outer SEAL header fields with values corresponding
   to the previous hop and recalculates the MAC using the MAC
   calculation parameters associated with the previous hop.  Next, the
   ITE replaces the SPTB's outer headers with headers of the appropriate
   protocol version and fills in the header fields as specified in
   Section 5.4.5, where the destination address/port correspond to the
   previous hop and the source address/port correspond to the ITE.  The
   ITE then sends the message to the previous hop the same as if it were
   issuing a new SPTB message.  (Note that, in this process, the values
   within the SEAL header of the packet-in-error are meaningless to the



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   previous hop and therefore cannot be used by the previous hop for
   authentication purposes.)

   If the previous hop is not reached via the same tunnel interface, the
   ITE instead transcribes the message into a format appropriate for the
   inner packet (i.e., the same as described for transcribing ICMP
   messages in Section 5.4.7) and sends the resulting transcribed
   message to the original source.  (NB: if the inner packet within the
   SPTB message is an IPv4 SEAL packet with DF==0, the ITE should set
   DF=1 and re-calculate the IPv4 header checksum while transcribing the
   message in order to avoid bogon filters.)  The ITE then discards the
   SPTB message.

   Note that the ITE may receive an SPTB message from another ITE that
   is at the head end of a nested level of encapsulation.  The ITE has
   no security associations with this nested ITE, hence it should
   consider this SPTB message the same as if it had received an ICMP PTB
   message from an ordinary router on the path to the ETE.  That is, the
   ITE should examine the packet-in-error field of the SPTB message and
   only process the message if it is able to recognize the packet as one
   it had previously sent.

5.6.2.2.  Processing Other SCMP Error Messages

   An ITE may receive an SDU message with an appropriate code under the
   same circumstances that an IPv6 node would receive an ICMPv6
   Destination Unreachable message.  The ITE either transcribes or
   relays the message toward the source address of the inner packet
   within the packet-in-error the same as specified for SPTB messages in
   Section 5.6.2.1.

   An ITE may receive an SPP message when the ETE receives a SEAL packet
   with an incorrect value in the SEAL header.  The ITE should examine
   the SEAL header within the packet-in-error to determine whether a
   different setting should be used in subsequent packets, but does not
   relay the message further.

   TEs process other SCMP message types using methods and procedures
   specified in other documents.  For example, SCMP message types used
   for tunnel neighbor coordinations are specified in VET
   [I-D.templin-intarea-vet].


6.  Link Requirements

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




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7.  End System Requirements

   End systems are encouraged to implement end-to-end MTU assurance
   (e.g., using Packetization Layer Path MTU Discovery (PLPMTUD) per
   [RFC4821]) even if the subnetwork is using SEAL.

   When end systems use PLPMTUD, SEAL will ensure that the tunnel
   behaves as a link in the path that assures an MTU of at least 1500
   bytes while not precluding discovery of larger MTUs.  The PMPMTUD
   mechanism will therefore be able to function as designed in order to
   discover and utilize larger MTUs.


8.  Router Requirements

   Routers within the subnetwork are expected to observe the standard IP
   router requirements, including the implementation of IP fragmentation
   and reassembly as well as the generation of ICMP messages
   [RFC0792][RFC1122][RFC1812][RFC2460][RFC4443][RFC6434].

   Note that, even when routers support existing requirements for the
   generation of ICMP messages, these messages are often filtered and
   discarded by middleboxes on the path to the original source of the
   message that triggered the ICMP.  It is therefore not possible to
   assume delivery of ICMP messages even when routers are correctly
   implemented.


9.  Nested Encapsulation Considerations

   SEAL supports nested tunneling for up to 8 layers of encapsulation.
   In this model, the SEAL ITE has a tunnel neighbor relationship only
   with ETEs at its own nesting level, i.e., it does not have a tunnel
   neighbor relationship with other ITEs, nor with ETEs at other nesting
   levels.

   Therefore, when an ITE 'A' within an outer nesting level needs to
   return an error message to an ITE 'B' within an inner nesting level,
   it generates an ordinary ICMP error message the same as if it were an
   ordinary router within the subnetwork.  'B' can then perform message
   validation as specified in Section 5.4.7, but full message origin
   authentication is not possible.

   Since ordinary ICMP messages are used for coordinations between ITEs
   at different nesting levels, nested SEAL encapsulations should only
   be used when the ITEs are within a common administrative domain
   and/or when there is no ICMP filtering middlebox such as a firewall
   or NAT between them.  An example would be a recursive nesting of



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   mobile networks, where the first network receives service from an
   ISP, the second network receives service from the first network, the
   third network receives service from the second network, etc.

   NB: As an alternative, the SCMP protocol could be extended to allow
   ITE 'A' to return an SCMP message to ITE 'B' rather than return an
   ICMP message.  This would conceptually allow the control messages to
   pass through firewalls and NATs, however it would give no more
   message origin authentication assurance than for ordinary ICMP
   messages.  It was therefore determined that the complexity of
   extending the SCMP protocol was of little value within the context of
   the anticipated use cases for nested encapsulations.


10.  Reliability Considerations

   Although a SEAL tunnel may span an arbitrarily-large subnetwork
   expanse, the IP layer sees the tunnel as a simple link that supports
   the IP service model.  Links with high bit error rates (BERs) (e.g.,
   IEEE 802.11) use Automatic Repeat-ReQuest (ARQ) mechanisms [RFC3366]
   to increase packet delivery ratios, while links with much lower BERs
   typically omit such mechanisms.  Since SEAL tunnels may traverse
   arbitrarily-long paths over links of various types that are already
   either performing or omitting ARQ as appropriate, it would therefore
   be inefficient to require the tunnel endpoints to also perform ARQ.


11.  Integrity Considerations

   The SEAL header includes an integrity check field that covers the
   SEAL header and at least the inner packet headers.  This provides for
   header integrity verification on a segment-by-segment basis for a
   segmented re-encapsulating tunnel path.

   Fragmentation and reassembly schemes must also consider packet-
   splicing errors, e.g., when two fragments from the same packet are
   concatenated incorrectly, when a fragment from packet X is
   reassembled with fragments from packet Y, etc.  The primary sources
   of such errors include implementation bugs and wrapping IPv4 ID
   fields.

   In particular, the IPv4 16-bit ID field can wrap with only 64K
   packets with the same (src, dst, protocol)-tuple alive in the system
   at a given time [RFC4963].  When the IPv4 ID field is re-written by a
   middlebox such as a NAT or Firewall, ID field wrapping can occur with
   even fewer packets alive in the system.  It is therefore essential
   that IPv4 fragmentation and reassembly be avoided.




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12.  IANA Considerations

   The IANA is requested to allocate a User Port number for "SEAL" in
   the 'port-numbers' registry.  The Service Name is "SEAL", and the
   Transport Protocols are TCP and UDP.  The Assignee is the IESG
   (iesg@ietf.org) and the Contact is the IETF Chair (chair@ietf.org).
   The Description is "Subnetwork Encapsulation and Adaptation Layer
   (SEAL)", and the Reference is the RFC-to-be currently known as
   'draft-templin-intarea.seal'.


13.  Security Considerations

   SEAL provides a segment-by-segment message origin authentication,
   integrity and anti-replay service.  The SEAL header is sent in-the-
   clear the same as for the outer IP and other outer headers.  In this
   respect, the threat model is no different than for IPv6 extension
   headers.  Unlike IPv6 extension headers, however, the SEAL header can
   be protected by an integrity check that also covers the inner packet
   headers.

   An amplification/reflection/buffer overflow attack is possible when
   an attacker sends IP fragments with spoofed source addresses to an
   ETE in an attempt to clog the ETE's reassembly buffer and/or cause
   the ETE to generate a stream of SCMP messages returned to a victim
   ITE.  The SCMP message ICV, Identification, as well as the inner
   headers of the packet-in-error, provide mitigation for the ETE to
   detect and discard SEAL segments with spoofed source addresses.

   Security issues that apply to tunneling in general are discussed in
   [RFC6169].


14.  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.

   Section 3 of [RFC4459] describes inner and outer fragmentation at the
   tunnel endpoints as alternatives for accommodating the tunnel MTU.

   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.

   IPsec/AH is [RFC4301][RFC4301] is used for full message integrity



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   verification between tunnel endpoints, whereas SEAL only ensures
   integrity for the inner packet headers.  The AYIYA proposal
   [I-D.massar-v6ops-ayiya] uses similar means for providing message
   authentication and integrity.

   SEAL, along with the Virtual Enterprise Traversal (VET)
   [I-D.templin-intarea-vet] tunnel virtual interface abstraction, are
   the functional building blocks for the Interior Routing Overlay
   Network (IRON) [I-D.templin-ironbis] and Routing and Addressing in
   Networks with Global Enterprise Recursion (RANGER) [RFC5720][RFC6139]
   architectures.

   The concepts of path MTU determination through the report of
   fragmentation and extending the IPv4 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.  An historical analysis of the evolution of these concepts,
   as well as the development of the eventual PMTUD mechanism, appears
   in [RFC5320].


15.  Implementation Status

   An early implementation of the first revision of SEAL [RFC5320] is
   available at: http://isatap.com/seal.


16.  Acknowledgments

   The following individuals are acknowledged for helpful comments and
   suggestions: Jari Arkko, Fred Baker, Iljitsch van Beijnum, Oliver
   Bonaventure, Teco Boot, Bob Braden, Brian Carpenter, Steve Casner,
   Ian Chakeres, Noel Chiappa, Remi Denis-Courmont, Remi Despres, Ralph
   Droms, Aurnaud Ebalard, Gorry Fairhurst, Washam Fan, Dino Farinacci,
   Joel Halpern, Sam Hartman, John Heffner, Thomas Henderson, Bob
   Hinden, Christian Huitema, Eliot Lear, Darrel Lewis, Joe Macker, Matt
   Mathis, Erik Nordmark, Dan Romascanu, Dave Thaler, Joe Touch, Mark
   Townsley, Ole Troan, Margaret Wasserman, Magnus Westerlund, Robin
   Whittle, James Woodyatt, and members of the Boeing Research &
   Technology NST DC&NT group.

   Discussions with colleagues following the publication of [RFC5320]
   have provided useful insights that have resulted in significant
   improvements to this, the Second Edition of SEAL.

   This document received substantial review input from the IESG and
   IETF area directorates in the February 2013 timeframe.  IESG members
   and IETF area directorate representatives who contributed helpful



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   comments and suggestions are gratefully acknowledged.

   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.


17.  References

17.1.  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.

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

   [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.

   [RFC3971]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

17.2.  Informative References

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

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

   [I-D.ietf-6man-udpchecksums]
              Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and



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              UDP Checksums for Tunneled Packets",
              draft-ietf-6man-udpchecksums-08 (work in progress),
              February 2013.

   [I-D.ietf-6man-udpzero]
              Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the use of IPv6 UDP Datagrams with Zero Checksums",
              draft-ietf-6man-udpzero-12 (work in progress),
              February 2013.

   [I-D.massar-v6ops-ayiya]
              Massar, J., "AYIYA: Anything In Anything",
              draft-massar-v6ops-ayiya-02 (work in progress), July 2004.

   [I-D.taylor-v6ops-fragdrop]
              Jaeggli, J., Colitti, L., Kumari, W., Vyncke, E., Kaeo,
              M., and T. Taylor, "Why Operators Filter Fragments and
              What It Implies", draft-taylor-v6ops-fragdrop-00 (work in
              progress), October 2012.

   [I-D.templin-intarea-vet]
              Templin, F., "Boeing's Virtual Enterprise Traversal (VET)
              Abstraction", draft-templin-intarea-vet-39 (work in
              progress), April 2013.

   [I-D.templin-ironbis]
              Templin, F., "Boeing's Interior Routing Overlay Network
              (IRON)", draft-templin-ironbis-14 (work in progress),
              April 2013.

   [RFC0994]  International Organization for Standardization (ISO) and
              American National Standards Institute (ANSI), "Final text
              of DIS 8473, Protocol for Providing the Connectionless-
              mode Network Service", RFC 994, March 1986.

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

   [RFC1070]  Hagens, R., Hall, N., and M. Rose, "Use of the Internet as
              a subnetwork for experimentation with the OSI network
              layer", RFC 1070, February 1989.

   [RFC1146]  Zweig, J. and C. Partridge, "TCP alternate checksum
              options", RFC 1146, March 1990.

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




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   [RFC1701]  Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic
              Routing Encapsulation (GRE)", RFC 1701, October 1994.

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

   [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.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, December 1998.

   [RFC2675]  Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
              RFC 2675, August 1999.

   [RFC2764]  Gleeson, B., Heinanen, J., Lin, A., Armitage, G., and A.
              Malis, "A Framework for IP Based Virtual Private
              Networks", RFC 2764, February 2000.

   [RFC2780]  Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
              Values In the Internet Protocol and Related Headers",
              BCP 37, RFC 2780, March 2000.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

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

   [RFC3232]  Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by
              an On-line Database", RFC 3232, January 2002.

   [RFC3366]  Fairhurst, G. and L. Wood, "Advice to link designers on
              link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366,
              August 2002.

   [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.



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   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, November 2005.

   [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.

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

   [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.

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, August 2007.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

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

   [RFC5445]  Watson, M., "Basic Forward Error Correction (FEC)
              Schemes", RFC 5445, March 2009.

   [RFC5720]  Templin, F., "Routing and Addressing in Networks with
              Global Enterprise Recursion (RANGER)", RFC 5720,
              February 2010.

   [RFC5927]  Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010.

   [RFC6139]  Russert, S., Fleischman, E., and F. Templin, "Routing and
              Addressing in Networks with Global Enterprise Recursion
              (RANGER) Scenarios", RFC 6139, February 2011.

   [RFC6169]  Krishnan, S., Thaler, D., and J. Hoagland, "Security



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              Concerns with IP Tunneling", RFC 6169, April 2011.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, August 2011.

   [RFC6434]  Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node
              Requirements", RFC 6434, December 2011.

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, November 2011.

   [RFC6864]  Touch, J., "Updated Specification of the IPv4 ID Field",
              RFC 6864, February 2013.

   [RIPE]     De Boer, M. and J. Bosma, "Discovering Path MTU Black
              Holes on the Internet using RIPE Atlas", July 2012.

   [SIGCOMM]  Luckie, M. and B. Stasiewicz, "Measuring Path MTU
              Discovery Behavior", November 2010.

   [TBIT]     Medina, A., Allman, M., and S. Floyd, "Measuring
              Interactions Between Transport Protocols and Middleboxes",
              October 2004.

   [WAND]     Luckie, M., Cho, K., and B. Owens, "Inferring and
              Debugging Path MTU Discovery Failures", October 2005.


Author's Address

   Fred L. Templin (editor)
   Boeing Research & Technology
   P.O. Box 3707
   Seattle, WA  98124
   USA

   Email: fltemplin@acm.org










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