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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Obsoletes: rfc5320 (if approved)                        January 03, 2014
Updates: rfc2460 (if approved)
Intended status: Standards Track
Expires: July 7, 2014


        The Subnetwork Encapsulation and Adaptation Layer (SEAL)
                   draft-templin-intarea-seal-68.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 July 7, 2014.

Copyright Notice

   Copyright (c) 2014 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



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













































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Motivation . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Approach . . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.3.  Differences with RFC5320 . . . . . . . . . . . . . . . . .  7
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  9
   4.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  9
   5.  SEAL Specification . . . . . . . . . . . . . . . . . . . . . . 10
     5.1.  SEAL Tunnel Model  . . . . . . . . . . . . . . . . . . . . 10
     5.2.  SEAL Model of Operation  . . . . . . . . . . . . . . . . . 11
     5.3.  SEAL Encapsulation Format  . . . . . . . . . . . . . . . . 12
     5.4.  ITE Specification  . . . . . . . . . . . . . . . . . . . . 13
       5.4.1.  Tunnel MTU . . . . . . . . . . . . . . . . . . . . . . 13
       5.4.2.  Tunnel Neighbor Soft State . . . . . . . . . . . . . . 14
       5.4.3.  SEAL Layer Pre-Processing  . . . . . . . . . . . . . . 15
       5.4.4.  SEAL Encapsulation and Fragmentation . . . . . . . . . 16
       5.4.5.  Outer Encapsulation  . . . . . . . . . . . . . . . . . 16
       5.4.6.  Path MTU Probing and ETE Reachability Verification . . 17
       5.4.7.  Processing ICMP Messages . . . . . . . . . . . . . . . 18
       5.4.8.  Detecting Path MTU Changes . . . . . . . . . . . . . . 19
     5.5.  ETE Specification  . . . . . . . . . . . . . . . . . . . . 19
       5.5.1.  Reassembly Buffer Requirements . . . . . . . . . . . . 19
       5.5.2.  Tunnel Neighbor Soft State . . . . . . . . . . . . . . 19
       5.5.3.  IPv4-Layer Reassembly  . . . . . . . . . . . . . . . . 19
       5.5.4.  Decapsulation, SEAL-Layer Reassembly, and
               Re-Encapsulation . . . . . . . . . . . . . . . . . . . 20
   6.  Link Requirements  . . . . . . . . . . . . . . . . . . . . . . 21
   7.  End System Requirements  . . . . . . . . . . . . . . . . . . . 21
   8.  Router Requirements  . . . . . . . . . . . . . . . . . . . . . 21
   9.  Multicast/Anycast Considerations . . . . . . . . . . . . . . . 21
   10. Compatibility Considerations . . . . . . . . . . . . . . . . . 22
   11. Nested Encapsulation Considerations  . . . . . . . . . . . . . 22
   12. Reliability Considerations . . . . . . . . . . . . . . . . . . 23
   13. Integrity Considerations . . . . . . . . . . . . . . . . . . . 23
   14. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   15. Security Considerations  . . . . . . . . . . . . . . . . . . . 24
   16. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 24
   17. Implementation Status  . . . . . . . . . . . . . . . . . . . . 24
   18. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 25
   19. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     19.1. Normative References . . . . . . . . . . . . . . . . . . . 25
     19.2. Informative References . . . . . . . . . . . . . . . . . . 26
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 29






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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 which comprise the
   "subnetwork" over which the tunnel operates.

   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 Maximum Transmission Unit (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
   deemed harmful through operational experience and studies conducted
   over the course of many years [FRAG][FOLK][RFC4963].  Additionally,
   classical IPv4 PMTUD [RFC1191] has known operational issues that are
   exacerbated by in-the-network tunnels [RFC2923][RFC4459].

   The following subsections present further details on the motivation
   and approach for addressing these issues.

1.1.  Motivation

   Before discussing the approach, it is necessary to first understand
   the problems.  In both the Internet and private-use networks today,
   IP is ubiquitously deployed as the Layer 3 protocol.  The primary
   functions of IP are to provide for routing, addressing, and a
   fragmentation and reassembly capability used to accommodate links
   with diverse MTUs.  While it is well known that the IP address space
   is rapidly becoming depleted, there is also a growing awareness that
   other IP protocol limitations have already or may soon become



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   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
   encountered.  However, this approach has several serious shortcomings
   that lead to an overall "brittleness" [RFC2923].



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   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, a tunnel ingress 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 tunnel egress.  If the ingress
   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, aviation networks and the global public Internet itself.
   Subnetwork border nodes forward unicast and multicast packets over
   the virtual topology across multiple IP and/or sub-IP layer
   forwarding hops that may introduce packet duplication and/or traverse



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   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 possible but out of scope for
   this document.)

   SEAL treats tunnels that traverse the subnetwork as ordinary links
   that must support network layer services.  Moreover, SEAL provides
   dynamic mechanisms (including limited fragmentation 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 fundamental differences
   from the earlier publication.  This specification therefore obsoletes
   (i.e., and does not update) [RFC5320].

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

   Additionally, the SEAL fragmentation and reassembly procedures
   defined in [RFC5320] differ significantly from those found in this
   specification.  In particular, this specification defines an 13-bit
   Offset field that allows for finer-grained fragment sizes when SEAL
   fragmentation and reassembly is necessary.  In contrast, [RFC5320]
   includes only a 3-bit Segment field and performs reassembly through
   concatenation of consecutive segments.

   Finally, SEAL no longer uses the IPv4 fragmentation sensing method
   specified in [RFC5320] as well as in earlier versions of this



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   document.  This departure is based on the fact that there is no way
   for the ITE or ETE to control the way in which middleboxes perform
   IPv4 fragmentation (e.g., largest fragment first, smallest fragment
   first, all fragments the same size, etc.).  Moreover, there may be
   middleboxes in the path that reassemble IPv4 fragmented packets
   before delivering them to the ETE as the final destination.  Use of
   IPv4 fragmentation sensing in the ETE also greatly complicated the
   specification and proved difficult to implement.  Therefore, although
   the IPv4 fragmentation sensing method is conceptually elegant and
   natural, it is no longer included.


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 portal over which an encapsulating border node (host or router)
      sends encapsulated packets into the subnetwork.

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

   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.







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   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.


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, mobile networks, aviation networks, ISP
   networks, SO/HO networks, the global public Internet itself, and any
   other connected network routing region.

   SEAL provides a network sublayer used during 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 the same manner as for IPv6 extension headers, i.e.,



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   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
   need to perform limited fragmentation which the Egress Tunnel
   Endpoint (ETE) reassembles.  The ITE and ETE further engage in
   minimal path probing to determine when the path can be traversed
   without fragmentation.  This allows the ITE to send whole packets
   instead of fragmented packets whenever possible.

   In practice, SEAL is typically used as an encapsulation sublayer in
   conjunction with existing tunnel types such as IPsec [RFC4301] ,
   GRE[RFC1701], IP-in-IPv6 [RFC2473], IP-in-IPv4 [RFC4213][RFC2003],
   etc.  When used with existing tunnel types that insert mid-layer
   headers between the inner and outer IP headers (e.g., IPsec, GRE,
   etc.), the SEAL header is inserted between the mid-layer headers and
   outer IP header.


5.  SEAL Specification

   The following sections specify the operation of SEAL:

5.1.  SEAL Tunnel Model

   SEAL is an encapsulation sublayer used within point-to-point, point-
   to-multipoint, and non-broadcast, multiple access (NBMA) tunnels.
   SEAL can also be used with multicast-capable tunnels, but the path
   probing mechanisms specified in the following sections may not always
   be sufficient to determine an optimal MTU for a multicast group.

   Each tunnel is configured over one or more underlying interfaces
   attached to subnetwork links, where each link represents a different
   subnetwork path.  The tunnel connects an ITE to one or more ETE
   "neighbors" via encapsulation across an underlying subnetwork, where
   each tunnel neighbor relationship is maintained over one or more
   subnetwork paths.  The tunnel neighbor relationship may be
   bidirectional, partially unidirectional or fully unidirectional.

   A bidirectional tunnel neighbor relationship is one over which both
   tunnel endpoints 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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5.2.  SEAL Model of Operation

   SEAL-enabled ITEs encapsulate each inner packet in any ancillary
   tunnel protocol headers and trailers, a SEAL header, and any outer
   header encapsulations as shown in Figure 1:

                                +--------------------+
                                ~   outer IP header  ~
                                +--------------------+
                                ~  other outer hdrs  ~
                                +--------------------+
                                ~    SEAL header     ~
                                +--------------------+
                                ~   tunnel headers   ~
   +--------------------+       +--------------------+
   |                    |  -->  |                    |
   ~        Inner       ~  -->  ~        Inner       ~
   ~       Packet       ~  -->  ~       Packet       ~
   |                    |  -->  |                    |
   +--------------------+       +--------------------+
                                ~   tunnel trailers  ~
                                +--------------------+

                       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.  (Note that outer transport layer
   headers such as UDP must sometimes be included to ensure that SEAL
   packets will traverse the path to the ETE without loss due filtering
   middleboxes.  The ETE MUST accept both IP/SEAL and IP/UDP/SEAL as
   equivalent packets so that the ITE can discontinue outer transport
   layer encapsulation if the path supports raw IP/SEAL encapsulation.)

   For SEAL encapsulations that involve tunnel types that include
   ancillary tunnel headers (e.g., GRE, IPsec, etc.) the ITE inserts the
   SEAL header as a leading extension to the tunnel headers, i.e., the
   SEAL encapsulation appears as part of the same tunnel and not a
   separate tunnel.  For example, for GRE the ITE inserts the SEAL



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   header as IP/SEAL/GRE/{inner packet}, and for IPsec the ITE inserts
   the SEAL header as IP/SEAL/IPsec-header/{inner packet}/IPsec-trailer.
   In such cases, 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.

   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 9 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.  Uses for re-encapsulating tunneling are discussed in
   [I-D.templin-aerolink].  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 separate subnetwork path to the ETE.  The ITE
   therefore may experience different path MTUs on different subnetwork
   paths.

   Finally, the SEAL ITE ensures that the inner network layer protocol
   will see a minimum MTU of 1500 bytes over each subnetwork path
   regardless of the outer network layer protocol version, i.e., even if
   a small amount of fragmentation 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.

5.3.  SEAL Encapsulation Format

   The SEAL header shares the same format and IP protocol number ('44')
   as the IPv6 Fragment Header specified in Section 4.5 of [RFC2460].
   The SEAL header is differentiated from the IPv6 Fragment Header by
   defining bit number 30 as the "SEAL (S)" bit which is set to 1 when
   SEAL encapsulation is used and set to 0 for ordinary IPv6
   fragmentation.  SEAL therefore updates the IPv6 Fragment Header
   specification as shown in Figure 2:





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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Next Header  |    Reserved   |      Fragment Offset    |R|S|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Identification                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 2: SEAL Encapsulation Format

5.4.  ITE Specification

5.4.1.  Tunnel MTU

   The tunnel must present a stable MTU value to the inner network layer
   as the size for admission of inner packets into the tunnel.  Since
   tunnels may support a large set of subnetwork paths that accept
   widely varying maximum packet sizes, however, a number of factors
   should be taken into consideration when selecting a tunnel 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
   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 MTU when
   admitting a packet into the tunnel.  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 MTU the inner IPv4 layer uses IPv4
   fragmentation to break the packet into fragments no larger than the
   MTU.  The ITE then admits each fragment into the tunnel as an
   independent packet.

   For all other inner packets, the inner network layer admits the
   packet if it is no larger than the tunnel MTU; otherwise, it drops
   the packet and sends a PTB error message to the source with the MTU
   value set to the 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 tunnel MTU such that all



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   inner packets are admitted into the tunnel regardless of their size
   (practical maximums are 64KB for IPv4 and 4GB for IPv6 [RFC2675]).
   For ITEs that host applications that use the tunnel 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 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 *router* tunnels so that SEAL performs all
   subnetwork adaptation from within the tunnel as specified in the
   following sections.  The ITE MAY instead set a smaller MTU on *host*
   tunnels; 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 and constants.

   The ITE maintains a per-ETE window of Identification values for the
   packets it sends to the ETE.  The ITE increments the current
   Identification value monotonically (modulo 2^32) for each packet it
   sends.

   For each subnetwork path, the ITE must also account for encapsulation
   header lengths.  The ITE therefore maintains the per subnetwork 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).  When calculating these lengths, the ITE must
   include the length of the uncompressed headers even if header
   compression is enabled.  When SEAL is used in conjunction with tunnel
   types that insert additional headers/trailers such as GRE or IPsec,
   the length of the additional headers and trailers is also included in
   the HLEN calculation.

   The ITE also sets a global constant value "MINMTU" to 1500 bytes and
   sets a per subnetwork path constant value 'FRAGMTU' to (1280-HLEN)
   bytes (where 1280 is the minimum path MTU for IPv6 [RFC2460]).  The
   value 1280 is used regardless of the outer IP protocol version even
   though the practical minimum MTU for IPv4 is only 576 bytes [RFC1122]
   and the theoretical minimum MTU for IPv4 is only 68 bytes [RFC0791].



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   The value 1280 is applied also to IPv4 since IPv4 links with MTUs
   smaller than 1280 are presumably performance-constrained such that
   IPv4 fragmentation can be used to accommodate MTU underruns without
   risk of high data rate reassembly misassociations.

   The ITE also sets a per subnetwork path variable "MAXMTU" to the
   maximum of MINMTU and the MTU of the underlying interface minus HLEN.
   The ITE thereafter adjusts MAXMTU based on any PTB messages it
   receives from the subnetwork, but does not reduce MAXMTU below
   MINMTU.

   The ITE finally maintains a per subnetwork path boolean variable
   "DOFRAG", which is initially set to TRUE and may be reset to FALSE if
   the ITE discovers that the MTU on the path to the ETE is sufficient
   to accommodate packet sizes of MINMTU bytes or larger.

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 by the inner network layer protocol as
   described in Section 5.4.1 or is undergoing re-encapsulation from
   within the tunnel.  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 IPv4 inner packet with DF==0 in the IP header, if the packet
   is larger than MINMTU bytes the ITE first uses standard IPv4
   fragmentation to fragment the packet into N pieces of at most MINMTU
   bytes each.  In this process, the ITE MUST additionally ensure that N
   is minimized, the first fragment is the largest fragment and no
   fragments are overlapping.  The ITE then submits each fragment for
   SEAL encapsulation as specified in Section 5.4.4.

   For all other inner packets, if the packet is no larger than MAXMTU
   the ITE submits it for SEAL encapsulation as specified in Section



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   5.4.4.  Otherwise, the ITE discards the packet and sends a PTB
   message appropriate to the inner protocol version (subject to rate
   limiting) with the MTU field set to MAXMTU.

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, if the packet is larger than MAXMTU for
   the next hop subnetwork path the ITE discards the packet and sends a
   PTB message appropriate to the inner protocol version (subject to
   rate limiting) with the MTU field set to MAXMTU.  Otherwise, the ITE
   sets aside the encapsulating SEAL and outer headers for later
   reference (see Section 5.4.5) and submits the inner packet for SEAL
   re-encapsulation as discussed in the following sections.

5.4.4.  SEAL Encapsulation and Fragmentation

   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 ITE next sets S=1 and sets the Next
   Header field to the protocol number corresponding to the address
   family of the encapsulated inner packet.  For example, the ITE sets
   the Next Header field to the value '4' for encapsulated IPv4 packets
   [RFC2003], '41' for encapsulated IPv6 packets [RFC2473][RFC4213],
   '47' for GRE [RFC1701], '80' for encapsulated OSI/CLNP packets
   [RFC1070], etc.

   Next, if the inner packet is no larger than FRAGMTU, or if the inner
   packet is larger than MINMTU, or if the DOFRAG flag is FALSE, the ITE
   sets (M=0; Offset=0) and considers the packet an "atomic fragment"
   (see: [RFC6946]).  Otherwise, the ITE fragments the inner packet
   using the fragmentation procedures specified in Section 4.5 of
   [RFC2460].  In this process, the ITE breaks the inner packet into two
   non-overlapping fragments, where the encapsulated SEAL packet
   containing the first fragment MUST be as large as possible without
   exceeding 1280 bytes (i.e., the IPv6 minimum MTU) and the
   encapsulated SEAL packet containing the second fragment MUST include
   the remainder of the inner packet.  This ensures that the entire IP
   header (plus extensions) is likely to fit within the first fragment
   and that the number of fragments is minimized.  The ITE then adds the
   outer encapsulating headers as specified in Section 5.4.5.

5.4.5.  Outer Encapsulation

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



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   the transport destination service port field.

   When UDP encapsulation is used, the ITE sets the UDP checksum field
   to zero for both IPv4 and IPv6 packets (see: [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 original outer IP header of the
   transitional packet into the corresponding fields in the new 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,
   IPv6 Fragment Header, 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, if the encapsulated SEAL packet is no larger than
   1280 bytes the ITE sets DF=0 in the IPv4 header to allow the packet
   to be fragmented if it encounters a restricting link; otherwise, the
   ITE sets DF=1 (for IPv6 subnetwork paths, the DF bit is absent but
   implicitly set to 1).  The ITE finally sends each outer packet via
   the corresponding underlying subnetwork path.

5.4.6.  Path MTU Probing and ETE Reachability Verification

   When the ITE is actively sending packets over a subnetwork path to an
   ETE, it also sends explicit probes subject to rate limiting to test
   the path MTU.  To generate a probe, the ITE creates an ICMPv6 Echo
   Request message [RFC4443] of length MINMTU bytes and encapsulates the
   message in a SEAL header and any other outer headers, i.e., with the
   length of the resulting SEAL packet being (MINMTU+HLEN) bytes.  It
   then sets (Offset=0; S=1; M=0) in the SEAL header, and also sets DF=1
   in the outer IP header when IPv4 is used.  It finally writes the
   value '58' in the Next Header field of the SEAL header to indicate
   that the message is a SEAL-encapsulated ICMPv6 message.

   The ITE sends such MINMTU probes to determine whether SEAL
   fragmentation is still necessary (see Section 5.4.4).  In particular,
   if the ITE sends a probe and receives a SEAL-encapsulated ICMPv6 Echo
   Reply message probe reply (see: section 5.5.4), it SHOULD set DOFRAG



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   for this subnetwork path to FALSE.  Note that the nominal probe size
   of MINMTU bytes is RECOMMENDED since probes slightly smaller than
   this size may be fragmented by the ITE of a nested tunnel further
   down the path.  For example, a successful probe size of 1400 bytes
   does not guarantee that fragmentation is not occurring at the ITE of
   another tunnel nesting level.  While this would not necessarily
   result in communication failure, it could yield poor performance not
   only for the other tunnel nesting levels but also for the ITE itself.

   The ITE can also send smaller probes to determine whether the ETE is
   still reachable over this subnetwork path.  The ITE prepares the
   probe as described above then sends the message to the ETE.  If the
   ITE receives a probe reply, its upper layers can consider the message
   as a reachability indication.  The ITE can also send larger probes to
   test for larger MTU sizes; however, SEAL considers probing for MTU
   sizes larger than MINMTU as an end-to-end consideration to be
   addressed by end systems (see: Section 7).

   Finally, the ITE can also send probes to detect whether an outer
   transport layer header is no longer necessary to reach this ETE.  For
   example, if the ITE sends its initial packets as IP/UDP/SEAL/*, it
   can send probes constructed as IP/SEAL/[probe] to determine whether
   the ETE is reachable without the use of UDP encapsulation.  If so,
   the ITE should also send a new MINMTU probe since switching to a new
   encapsulation format may result in a path change.

   While probing, the ITE processes ICMP messages as specified in
   Section 5.4.7.

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 packet that generated the error
   (also known as the "packet-in-error").  Note that the ITE may receive
   an ICMP message from either an ordinary router on the path or 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 ICMP Protocol/Port Unreachable messages 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 subnetwork
   path to the ETE may be failing.  The ITE then discards these types of
   messages.



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   For other ICMP messages, the ITE SHOULD examine the SEAL data packet
   within the packet-in-error field.  If the IP source and/or
   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, the ITE discards the message.

   Next, if the received ICMP message is a PTB the ITE sets MAXMTU to
   the maximum of MINMTU and the MTU value in the message minus HLEN.
   If the MTU value in the message is smaller than (MINMTU+HLEN), the
   ITE also resets DOFRAG to TRUE and discards the message.

   If the ICMP message was not discarded, the ITE transcribes it into a
   message appropriate for the inner protocol version (e.g., ICMPv4 for
   IPv4, ICMPv6 for IPv6, etc.) and forwards the transcribed message to
   the previous hop toward the inner source address.

5.4.8.  Detecting Path MTU Changes

   The ITE SHOULD periodically reset MAXMTU to the MTU of the underlying
   subnetwork interface to determine whether the subnetwork path MTU 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

   The ETE MUST configure a minimum SEAL reassembly buffer size of
   (MINMTU+HLEN) bytes for the reassembly of fragmented SEAL packets
   (see: Section 5.5.4).  Note that the value "HLEN" may be variable and
   initially unknown to the ETE.  It is therefore RECOMMENDED that the
   ETE configure a slightly larger SEAL reassembly buffer size of 2048
   bytes (2KB).

   When IPv4 is used as the outer layer of encapsulation, the ETE MUST
   also configure a minimum IPv4 reassembly buffer size of 1280 bytes.

5.5.2.  Tunnel Neighbor Soft State

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

5.5.3.  IPv4-Layer Reassembly

   The ETE reassembles fragmented IPv4 packets that are explicitly
   addressed to itself.  For IPv4 fragments of SEAL packets, the ETE



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   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 IPv4 fragments as specified in the normative
   references, i.e., it performs any necessary IPv4 reassembly then
   submits the packet to the appropriate upper layer protocol module.
   For SEAL packets, the ETE then performs SEAL decapsulation as
   specified in Section 5.5.4.

5.5.4.  Decapsulation, SEAL-Layer Reassembly, and Re-Encapsulation

   For each SEAL packet accepted for decapsulation, 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 the SEAL header has (Offset!=0 || M=1) the ETE submits the
   packet for reassembly as specified for IPv6 reassembly in Section 4.5
   of [RFC2460].  During the reassembly process, the ETE discards any
   fragments that are overlapping with respect to fragments that have
   already been received (see: [RFC5722]), and also discards any
   fragments that have M=1 in the SEAL header but do not contain an
   integer multiple of 8 bytes.  The ETE further SHOULD manage the SEAL
   reassembly cache the same as described for the IPv4-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 (reassembled) packet is an ICMPv6 Echo Request probe
   message, the ETE prepares an ICMPv6 Echo Reply probe reply message to
   send back to the ITE.  The ETE then encapsulates the probe reply as
   specified in Section 5.4.4 and fragments the message if necessary
   according to the DOFRAG flag (i.e., to ensure that the probe reply is
   delivered to the ITE).  The ETE then sends the probe reply to the ITE
   and discards the probe.  When the ITE receives the probe reply, it
   reassembles the message if necessary and processes it as specified in
   Section 5.4.6.

   Finally, the ETE discards the outer headers of the (reassembled)
   packet and processes the inner packet according to the header type
   indicated in the SEAL Next Header field.  If the next hop toward the
   inner destination address is via a different interface than the SEAL



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   packet arrived on, the ETE discards the SEAL and outer headers and
   delivers the inner packet either to the local host or to the next hop
   if the packet is not destined to the local host.

   If the next hop is on the same tunnel 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.


6.  Link Requirements

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


7.  End System Requirements

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

   When end systems use PLPMTUD, SEAL will ensure that the tunnel
   behaves as a link in the path that assures an MTU of at least 1500
   bytes while still allowing end systems to discover larger MTUs.  The
   PLPMTUD mechanism will therefore be able to function as designed in
   order to discover and utilize larger MTUs.


8.  Router Requirements

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

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


9.  Multicast/Anycast Considerations

   On multicast-capable tunnels, encapsulated packets sent by an ITE may
   be received by potentially many ETEs.  In that case, the ITE can



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   still send unicast probe messages to receive probe replies from a
   specific ETE, or it can send multicast probe messages to receive
   replies from all ETEs in the multicast group that receive the probe.
   If the ITE were to send a multicast MINMTU probe message as described
   in Section 5.4.6, however, it would be unable to discern whether all
   ETEs received the probe unless it had some way of tracking the full
   constituency of the multicast group.  For multicast ETE addresses,
   the ITE would therefore ordinarily set MAXMTU=MINMTU and DOFRAG=TRUE.
   But, the setting of these values may be situation-dependent and based
   on whether the ITE can tolerate packet loss to ETEs that may be
   reached by subnetwork paths having small MTUs.

   For ETEs that configure an anycast address, if the ITE sends a MINMTU
   probe message it may receive a probe reply from a first ETE but then
   be re-routed to a second ETE.  It is therefore necessary for the ITE
   to continue to send periodic probes (subject to rate limiting) as
   described in Section 5.4.6 so that any path oscillations between ETEs
   that configure the same anycast address will not result in a
   sustained path MTU black hole.


10.  Compatibility Considerations

   Since SEAL is based on the standard IPv6 fragment header, the ITE can
   implement the scheme independently of any ETE implementations.
   Therefore, if the ITE uses SEAL but the ETE does not the ITE can
   still send a MINMTU probe as specified in Section 5.4.6 but may
   receive an ordinary (i.e., non SEAL-encapsulated) probe reply.  If
   so, it SHOULD reset DOFRAG to FALSE the same as if the ETE returned a
   SEAL-encapsulated probe reply.

   In some cases, a non-SEAL ETE may not be able to reassemble
   fragmented SEAL packets up to (MINMTU+HLEN) bytes, since [RFC2460]
   only requires IPv6 nodes to reassemble packets up to 1500 bytes in
   length.  To test for this condition, the ITE can create a MINMTU
   probe message, fragment the message into two pieces, then send both
   fragments to the ETE.  If the ETE returns a probe reply, the ITE has
   assurance that the ETE is capable of reassembly.  Otherwise, the ITE
   SHOULD reset MAXMTU for this subnetwork path to (MINMTU-HLEN) or even
   smaller if the ETE still cannot accept packets of this size.


11.  Nested Encapsulation Considerations

   SEAL supports nested tunneling - 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,



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   etc.  Since it is imperative that such nesting not extend
   indefinitely, tunnels that use SEAL SHOULD honor the Encapsulation
   Limit option defined in [RFC2473].


12.  Reliability Considerations

   Although a 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 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.


13.  Integrity Considerations

   Fragmentation and reassembly schemes must 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 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.

   Fortunately, SEAL includes a 32-bit ID field the same as for IPv6
   fragmentation and also only employs SEAL fragmentation for packets up
   to 1500 bytes in length.  SEAL also only allows IPv4 network
   fragmentation for packets up to 1280 bytes in length, but this size
   is small enough to fit within the MTU of modern high-speed IPv4 links
   without fragmentation.  IPv4 links with smaller MTUs certainly exist,
   but typically support data rates that are slow enough to preclude
   high data rate reassembly misassociations errors; hence, a small
   amount of IPv4 fragmentation is deemed acceptable.


14.  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



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   (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'.


15.  Security Considerations

   Neighbor relationships between the ITE and ETE should be secured in
   environments where authentication and/or confidentiality are a matter
   of concern.  Securing mechanisms such as Secure Neighbor Discovery
   (SeND) [RFC3971] and IPsec [RFC4301] can be used for this purpose,
   however the tunnel neighbor relationship is managed by the tunnel
   protocols that ride over SEAL (as an encapsulation sublayer) rather
   than by SEAL itself.

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


16.  Related Work

   Section 3.1.7 of [RFC2764] provides a high-level sketch for
   supporting large tunnel MTUs via a tunnel-layer fragmentation and
   reassembly capability to avoid IP layer 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.

   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].


17.  Implementation Status

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




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   An implementation of the current version of SEAL is available at:
   http://linkupnetworks.com/seal/sealv2-1.0.tgz.


18.  Acknowledgments

   The following individuals are acknowledged for helpful comments and
   suggestions: Jari Arkko, Fred Baker, Iljitsch van Beijnum, Oliver
   Bonaventure, Teco Boot, Bob Braden, Brian Carpenter, Steve Casner,
   Ian Chakeres, Noel Chiappa, Remi Denis-Courmont, Remi Despres, Ralph
   Droms, Aurnaud Ebalard, Gorry Fairhurst, Washam Fan, Dino Farinacci,
   Joel Halpern, Brian Haberman, 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.  In particular,
   this work has been encouraged and supported by Boeing colleagues
   including Balaguruna Chidambaram, Jeff Holland, Cam Brodie, Yueli
   Yang, Wen Fang, Ed King, Mike Slane, Kent Shuey, Gen MacLean, and
   other members of the BR&T and BIT mobile networking teams.

   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.  Steve Deering also
   proposed the IPv6 minimum MTU of 1280 bytes on the IPng mailing list
   in 1997.


19.  References

19.1.  Normative References

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




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   [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.

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

19.2.  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.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.

   [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.

   [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.

   [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.




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   [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.

   [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.

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

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

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

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

   [RFC3819]  Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, July 2004.

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

   [RFC4459]  Savola, P., "MTU and Fragmentation Issues with In-the-



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              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.

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

   [RFC5722]  Krishnan, S., "Handling of Overlapping IPv6 Fragments",
              RFC 5722, December 2009.

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

   [RFC6169]  Krishnan, S., Thaler, D., and J. Hoagland, "Security
              Concerns with IP Tunneling", RFC 6169, April 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.

   [RFC6946]  Gont, F., "Processing of IPv6 "Atomic" Fragments",
              RFC 6946, May 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.



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


Author's Address

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

   Email: fltemplin@acm.org






































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