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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Obsoletes: rfc5320 (if approved)                          August 2, 2013
Intended status: Informational
Expires: February 3, 2014


        The Subnetwork Encapsulation and Adaptation Layer (SEAL)
                   draft-templin-intarea-seal-61.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 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 February 3, 2014.

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.


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 Tunnel Mode 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  . . . . . . . . . . . . . . 18
       5.4.4.  SEAL Encapsulation and Segmentation  . . . . . . . . . 19
       5.4.5.  Outer Encapsulation  . . . . . . . . . . . . . . . . . 20
       5.4.6.  Path Probing and ETE Reachability Verification . . . . 21
       5.4.7.  Processing ICMP Messages . . . . . . . . . . . . . . . 22
       5.4.8.  IPv4 Middlebox Reassembly Testing  . . . . . . . . . . 23
       5.4.9.  Stateful MTU Determination . . . . . . . . . . . . . . 24
       5.4.10. Detecting Path MTU Changes . . . . . . . . . . . . . . 25
     5.5.  ETE Specification  . . . . . . . . . . . . . . . . . . . . 25
       5.5.1.  Reassembly Buffer Requirements . . . . . . . . . . . . 25
       5.5.2.  Tunnel Neighbor Soft State . . . . . . . . . . . . . . 25
       5.5.3.  IP-Layer Reassembly  . . . . . . . . . . . . . . . . . 26
       5.5.4.  Decapsulation, SEAL-Layer Reassembly, and
               Re-Encapsulation . . . . . . . . . . . . . . . . . . . 26
     5.6.  The SEAL Control Message Protocol (SCMP) . . . . . . . . . 28
       5.6.1.  Generating SCMP Error Messages . . . . . . . . . . . . 29
       5.6.2.  Processing SCMP Error Messages . . . . . . . . . . . . 30
   6.  SEAL Transport Mode Specification  . . . . . . . . . . . . . . 32
   7.  Link Requirements  . . . . . . . . . . . . . . . . . . . . . . 33
   8.  End System Requirements  . . . . . . . . . . . . . . . . . . . 33
   9.  Router Requirements  . . . . . . . . . . . . . . . . . . . . . 33
   10. Nested Encapsulation Considerations  . . . . . . . . . . . . . 34
   11. Reliability Considerations . . . . . . . . . . . . . . . . . . 34
   12. Integrity Considerations . . . . . . . . . . . . . . . . . . . 34
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 35



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   14. Security Considerations  . . . . . . . . . . . . . . . . . . . 35
   15. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 36
   16. Implementation Status  . . . . . . . . . . . . . . . . . . . . 36
   17. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 36
   18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 37
     18.1. Normative References . . . . . . . . . . . . . . . . . . . 37
     18.2. Informative References . . . . . . . . . . . . . . . . . . 38
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 41











































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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 network layer hop, 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]).  Tunnels serve a wide variety of purposes,
   including mobility, security, routing control, traffic engineering,
   multihoming, etc., and will remain an integral part of the
   architecture moving forward.  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



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   is rapidly becoming depleted, there is also a growing awareness that
   other IP protocol limitations have already or may soon become
   problematic.

   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 (MTUDWG) 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



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   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
   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



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   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).

   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 specified in Section 6.

   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 (including limited segmentation and reassembly) 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.

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



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   found in the inner or outer IP headers, and is therefore compatible
   with any inner and outer IP protocol version combinations.

   Additionally, the SEAL segmentation and reassembly procedures defined
   in [RFC5320] differ significantly from those found in this
   specification.  In particular, this specification defines an 8-bit
   Offset field that allows for smaller segment sizes when SEAL
   segmentation is necessary.  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' because
      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, SO/HO 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 limited segmentation which the Egress Tunnel Endpoint
   (ETE) reassembles.  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.

   Finally, SEAL can be used in conjunction with existing tunnel types
   such as IPsec and GRE.  In that case, the SEAL header appears as an
   outer layer of encapsulation while the IPsec, GRE, etc. headers are
   seen as inner headers that immediately follow the SEAL header.


5.  SEAL Tunnel Mode Specification

   The following sections specify the operation of SEAL:

5.1.  SEAL Tunnel Model

   SEAL is an encapsulation sublayer used within point-to-point, point-
   to-multipoint, 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
   bidirectional, partially unidirectional or fully unidirectional.

   A bidirectional tunnel neighbor relationship is one over which both
   TEs can exchange both data and control messages.  A partially
   unidirectional tunnel neighbor relationship allows the near end ITE
   to send data packets forward to the far end ETE, while the far end
   only returns control messages when necessary.  Finally, a fully
   unidirectional mode of operation is one in which the near end ITE can
   receive neither data nor control messages from the far end ETE.



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

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.

   For SEAL encapsulations that involve other tunnel types (e.g., GRE,
   IPsec, etc.) the ITE inserts the SEAL header as a leading extension
   to the other tunnel headers, i.e., the SEAL encapsulation appears as
   part of the same tunnel and not a separate tunnel.  For example, for
   IPsec/ESP the ITE inserts the SEAL header as IP/SEAL/ESP-header/
   {inner packet}/ESP-trailer.  In that case, SEAL considers the length
   of the inner packet only (i.e., and not the other tunnel headers and
   trailers) when performing its packet size calculations.  Also, the
   other tunnel headers appear in only the first segment of a segmented
   SEAL packet (i.e., they do not appear in non-initial segments) and
   the other tunnel trailers (if any) appear only in the final segment.




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   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] as well as in
   Section 10 of this document.

   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
   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 segmentation and reassembly are necessary.  This is to
   avoid path MTU "black holes" for the minimum MTU configured by the
   vast majority of links in the Internet.  Note that in some scenarios,
   however, reassembly may place a heavy burden on the ETE.  In that
   case, the ITE can avoid invoking segmentation and instead report an
   MTU smaller than 1500 bytes to the original source.

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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    NEXTHDR    |VER|C|P|I|V|R|M|     Offset    | LINK_ID |LEVEL|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Identification (optional)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Integrity Check Vector (optional)            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   ...

                       Figure 2: SEAL Header Format



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   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.

   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.

   P (1)
      the "Probe" bit.  Set to 1 by the ITE in SEAL probe packets for
      which it wishes to receive an explicit acknowledgement from the
      ETE.

   I (1)
      the "Identification Included" bit.

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

   R (1)
      the "Redirects Permitted" bit when used by VET (see:
      [I-D.templin-intarea-vet]); reserved for future use in other
      contexts.

   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 (8)  an 8-bit Offset field.  Set to 0 in the first segment of
      a segmented SEAL packet.  Set to an integer multiple of 256 byte
      blocks in subsequent segments (e.g., an Offset of 4 indicates a
      block that begins at the 1024th byte in the packet).

   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.






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   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 randomly-initialized 32-bit value 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.

5.4.  ITE Specification

5.4.1.  Tunnel Interface MTU

   The tunnel interface must present a stable 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 and provide accommodations to
   ensure that packets up to that size are successfully conveyed to the
   ETE.

   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) bit 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.



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   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 (theoretical maximums are 64KB for IPv4 and
   4GB for IPv6 [RFC2675]).  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
   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
   the following sections.  The ITE MAY instead set a smaller MTU on
   tunnel *host* interfaces; in that case, the RECOMMENDED MTU is the
   maximum of 1500 bytes and the smallest MTU among all of the
   underlying links minus the size of the encapsulation headers.

5.4.2.  Tunnel Neighbor Soft State

   The ITE maintains a number of soft state variables for each ETE and
   for each SEAL path.

   The ITE maintains a per ETE window of Identification values for the
   packets it has recently sent to this ETE as welll as a per ETE window
   of Identification values for the packets it has recently received
   from 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:







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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |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
   (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.)  When SEAL is used in conjunction with
   another tunnel type such as GRE or IPsec, the length of the headers
   associated with those tunnels is also included in the HLEN
   calculation for the first segment only and the length of the
   associated trailers is included in the HLEN calculation for the final
   segment only.

   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.  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 per [RFC2460].  For IPv4 paths,
   the ITE sets MINMTU=576 based on practical interpretation of
   [RFC1122] even though the theoretical MINMTU for IPv4 is only 68
   bytes [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 the MTU is smaller, e.g., if 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



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   or from a router on the path and will be able to reduce its MINMTU to
   a smaller value.  (Note that since IPv4 links with MTUs smaller than
   1280 are almost certainly low data rate, the ITE can instead set
   MINMTU to 1280 the same as for IPv6.  If this value proves too large,
   standard IPv4 fragmentation and reassembly will provide short term
   accommodation for the sizing constraints.)

   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 smallest MTU 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.

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.



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   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 PTB message appropriate to the
   inner protocol version (subject to rate limiting) 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; P=0), 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



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   than 1500, the ITE sets (M=0; Offset=0).  Otherwise, the ITE breaks
   the inner packet into N approximately equal length non-overlapping
   segments (where N is minimized and each segment is significantly
   smaller than (MINMTU-HLEN) to allow for additional encapsulations in
   the path).  In this process, the ITE MUST ensure that each segment
   except the final segment contains an integer multiple of 256 byte
   blocks, and that the inner packet's network and transport layer
   headers are included in the first segment.  The ITE then appends a
   clone of the SEAL header from the first segment onto the head of each
   additional segment.  The ITE MUST also include an Identification
   field and set USE_ID=TRUE for each segment.  The ITE then sets (M=1;
   Offset=0) in the first segment, sets (M=0/1; Offset=O(1)) in the
   second segment, sets (M=0/1; Offset=O(2)) in the third segment (if
   needed), etc., then finally sets (M=0; Offset=O(n)) in the final
   segment (where O(i) is the number of 256 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 a randomly-initialized
   value 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
   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



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   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 [RFC6935][RFC6936].

   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 absent but implicitly set to 1).

   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
   that detect in-the-network fragmentation, while explicit probe
   packets can be constructed to probe the path MTU and/or verify ETE
   reachability.  These probes will elicit an SCMP message from the ETE
   if it needs to send an acknowledgement and/or report an error
   condition.  The probe 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.

   To generate an explicit probe packet, the ITE creates a duplicate of
   an actual data packet and uses the duplicate as a probe.
   (Alternatively, the ITE can create a packet buffer beginning with the



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   same outer headers, SEAL header and inner network layer headers that
   would appear in an ordinary data packet, then pad the packet with
   random data.)  The ITE then sets (C=0; P=1) in the SEAL header of the
   probe packet, and also sets DF=1 in the outer IP header when IPv4 is
   used.

   The ITE sends periodic explicit probes to determine whether SEAL
   segmentation is still necessary (see Section 5.4.4).  In particular,
   if a probe packet of 1500 bytes (i.e., a packet that becomes (1500+
   HLEN) bytes after encapsulation) succeeds the ITE is assured that the
   path MTU is large enough so that the segmentation/reassembly process
   can be suspended.  This probing discipline can therefore be
   considered as Packetization Layer Path MTU Discovery (PLPMTUD)
   [RFC4821] applied to tunnels, which operates independently of any
   application of PLPMTUD between end systems.

   Note that the explicit probe size of 1500 bytes is chosen since probe
   packets smaller than this size may be fragmented by another ITE
   further down the path.  For example, a successful probe for a packet
   size of 1400 bytes does not guarantee that fragmentation is not
   occurring at another ITE.

   While probing, the ITE processes ICMP messages as specified in
   Section 5.4.7 and 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 a router on the path to the ETE.  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").  Note that the ITE may
   receive an ICMP 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 the message the same
   as if it originated from an ordinary router.

   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 ETE does not implement SEAL.
   The ITE can optionally ignore other ICMP messages that do not include
   sufficient information in the packet-in-error, or process them as a
   hint that the SEAL path to the ETE may be failing.  The ITE then
   discards these types of messages.

   For other ICMP messages, the ITE first examines the SEAL data packet
   within the packet-in-error field.  If the IP source and/or



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   destination addresses are invalid, or if the value in the SEAL header
   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
   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).

   The ITE then transcribes the ICMP message into a message appropriate
   for the SEAL data packet within the packet-in-error.  If the previous
   hop toward the inner source address within the SEAL data packet is
   reached via the same SEAL interface, the ITE transcribes the message
   into an SCMP message the same as described for ETE generation of SCMP
   messages in Section 5.6.1, i.e., it copies the SEAL data packet
   within the packet-in-error into the packet-in-error field of the new
   message.  Otherwise, the ITE seeks beyond the SEAL header within the
   packet-in-error and transcribes the inner packet into a message
   appropriate for the inner protocol version (e.g., ICMPv4 for IPv4,
   ICMPv6 for IPv6, etc.).

   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".
   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 SEAL data
   packet for packet analysis purposes but then forward the fragments on
   to the final destination rather than forwarding the reassembled
   packet.  (This process is often referred to as "Virtual Fragmentation
   Reassembly" (VFR)).




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   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 sending explicit
   probe packets.  The ITE should only send probe packets that are no
   larger than (1500-HLEN) before encapsulation since the most an
   ordinary node can be expected to reassemble is 1500 bytes.  To
   generate a probe, the ITE either creates a clone of an ordinary data
   packet or creates a packet buffer beginning with the same outer
   headers, SEAL header and inner network layer header that would appear
   in an ordinary data packet.  The ITE then pads the probe packet with
   random data to a length that is at least 128 bytes but no longer than
   (1500-HLEN) bytes.

   The ITE then sets (C=0; P=1) in the SEAL header of the probe packet
   and sets the NEXTHDR field to the inner network layer protocol type.
   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 IP 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.

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 in violation of the specs
   (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.



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   To determine a static MTU value, the ITE sends a series of probe
   packets of various sizes to the ETE with (C=0; P=1) in the SEAL
   header and DF=1 in the outer IP header.  The ITE then caches the size
   'S' of 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.  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 also MUST 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
   further MUST configure a minimum SEAL reassembly buffer size of (1500
   + HLEN) bytes for the reassembly of segmented SEAL packets (see:
   Section 5.5.4).

   Note that the value "HLEN" may be variable and initially unknown to
   the ETE, and would typically range from a few bytes to a few tens of
   bytes or even more.  It is therefore RECOMMENDED that the ETE
   configure slightly larger minimum IP/SEAL reassembly buffer sizes of
   2048 bytes (2KB).

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.  The ETE also maintains a
   window of Identification values for the packets it has recently
   received from this ITE as well as a window of Identification values



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   for the packets it has recently sent to 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.

5.5.3.  IP-Layer Reassembly

   The ETE reassembles fragmented IP packets that are explicitly
   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.)

5.5.4.  Decapsulation, SEAL-Layer Reassembly, 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).




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   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
   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 all other
   segments have already arrived, the ETE discards the SEAL header and
   other outer headers from the non-initial segments and appends the
   segments onto the end of the first segment according to their offset
   value.  Otherwise, the ETE caches the new 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, and also
   discards any segments that have M==1 in the SEAL header but do not
   contain an integer multiple of 256 byte blocks.  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 P==1, the
   ETE drops the packet unconditionally and sends an SPTB message back
   to the ITE with MTU=0 (see: Section 5.6.1.1) if it has not already
   sent an SPTB message based on IP fragmentation.  (Note that the ETE
   therefore sends only a single SPTB message for a probe packet that
   also experienced IP fragmentation, i.e., it does not send multiple
   SPTB messages.)

   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 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.




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   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.

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].  The SCMP messaging
   protocol operates over bidirectional and partially unidirectional
   tunnels.  (For fully unidirectional tunnels, SEAL must operate
   without the benefit of SCMP meaning that steady-state fragmentation
   and reassembly may be necessary in extreme cases.  In that case, the
   ITE must select a conservative MINMTU to ensure that IPv4
   fragmentation is avoided in order to avoid reassembly errors at high
   data rates [RFC4963].)

   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.





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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:

       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; P=0; M=0; Offset=0) in the SEAL header, then
   sets I, V, NEXTHDR, LINK_ID and LEVEL to the same values that
   appeared in the SEAL header of the data packet.

   When I==1, the ETE next sets the Identification field to the next
   Identification value scheduled for this ITE, then increments the next



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   Identification value.

   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 probe
   packet (i.e., one with P==1 in the SEAL header) that arrived as an
   unfragmented outer IP packet.  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 conditions 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
   next verifies the Checksum value in the SCMP message header.  If any
   of these values are incorrect, the ITE silently discards the message;



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   otherwise, it processes the message as follows:

5.6.2.1.  Processing SCMP PTB Messages

   After an ITE sends a SEAL 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 SPTB messages with MTU==0, the
   ITE processes the message as confirmation that the ETE received an
   unfragmented SEAL probe packet with P==1 in the SEAL header.  Based
   on the size of the probe, the ITE then suspends the segmentation/
   reassembly process and/or caches the size of the probe as a stateful
   MTU limitation for this ETE.  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 and discards the message.  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 a small MTU
   and inner packet length 4KB, it can rewrite the MTU to 2KB.  If the
   ITE subsequently receives an IPv4 SPTB message with a small MTU 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 transcribes the message into a control message format
   appropriate for the packet-in-error (i.e., the same as described for
   transcribing ICMP messages in Section 5.4.7) and sends the resulting
   transcribed message to the previous hop.  When the previous hop is
   reached via the same tunnel interface as the SCMP message arrived on,
   the ITE performs a "reverse re-encapsulation" and replaces the outer
   headers up to and including the SEAL header with headers that will be
   recognized and accepted by the previous hop.  This means that any
   Identification and/or ICV values within the SEAL header within the
   packet-in-error will be meaningless to the previous hop and should
   therefore be ignored.

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



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   Destination Unreachable message.  The ITE transcribes the message and
   forwards it 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
   different settings 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.  SEAL Transport Mode Specification

   SEAL also provides a transport-mode of operation.  Transport mode
   refers to a SEAL encapsulation in which a layer-4 header appears
   immediately following the SEAL header.  The type of layer-4 header is
   indicated in the "NEXTHDR" field the same as for tunnel mode.  The
   SEAL header is identical to the version used for tunnel mode, except
   that the "LINK_ID" and "LEVEL" fields are omitted, and the transport
   layer port numbers are included in each non-initial segment (see:
   Figure 6).

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    NEXTHDR    |VER|C|P|I|V|R|M|     Offset    |    Reserved   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Source Port (when Offset!=0)  |   Dest Port (when Offset!=0)  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Identification (optional)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Integrity Check Vector (optional)            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   ...

                       Figure 6: SEAL Header Format

   The "Source Port" and "Dest Port" are taken from the corresponding
   fields in the transport layer next header that appears immediately
   following the SEAL header in the initial segment.  For example, for
   UDP [RFC0768] the transport layer source/dest ports are 16 bits in
   length and are copied from the transport layer header.  All
   segmentation/reassembly is performed the same as specified for tunnel



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   mode, and the SEAL Control Message Protocol (SCMP) operates the same
   as for tunnel mode.  For example, the source node can probe the path
   MTU to the destination by setting the P bit in a probe packet and
   wating for an SCMP acknowledgement message from the destination.

   SEAL transport mode implementations SHOULD configure reassembly
   buffers that are large enough to accommodate a maximum-sized
   segmented SEAL packet, i.e., it is RECOMMENDED that they configure a
   64KB reassembly buffer size.

   SEAL transport mode is useful for transport layer protocols that have
   no way to segment the large packets they send.  It is a universal
   format that can be applied to any such transport.


7.  Link Requirements

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


8.  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 PLPMTUD
   mechanism will therefore be able to function as designed in order to
   discover and utilize larger MTUs.


9.  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.




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10.  Nested Encapsulation Considerations

   SEAL supports nested tunneling for up to 8 layers of encapsulation.
   An example would be a recursive nesting of 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.

   In such nested arrangements, 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 TEs 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.

   (Note that the SCMP protocol could instead be extended to allow an
   outer nesting level ITE 'A' to return an SCMP message to an inner
   nesting level 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.)


11.  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.


12.  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.



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   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 detected early and tuned
   out through proper application of SEAL segmentation and reassembly.


13.  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'.


14.  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].




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15.  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
   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].


16.  Implementation Status

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


17.  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



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   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
   comments and suggestions are gratefully acknowledged.  Discussions on
   the IETF IPv6 and Intarea mailing lists in the summer 2013 timeframe
   also stimulated several useful ideas.

   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.


18.  References

18.1.  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



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              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.

18.2.  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.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-01 (work in
              progress), June 2013.

   [I-D.templin-intarea-vet]
              Templin, F., "Virtual Enterprise Traversal (VET)",
              draft-templin-intarea-vet-40 (work in progress), May 2013.

   [I-D.templin-ironbis]
              Templin, F., "The Interior Routing Overlay Network
              (IRON)", draft-templin-ironbis-15 (work in progress),
              May 2013.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [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.



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   [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.

   [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,



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              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.

   [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.




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   [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
              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.

   [RFC6935]  Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets", RFC 6935, April 2013.

   [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the Use of IPv6 UDP Datagrams with Zero Checksums",
              RFC 6936, April 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.







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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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