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Network Working Group                                    F. Templin, Ed.
Internet-Draft                                      Boeing Phantom Works
Intended status: Informational                         November 14, 2007
Expires: May 17, 2008


  Simple Protocol for Robust IP/*/IP Tunnel Endpoint MTU Determination
                              (sprite-mtu)
                      draft-templin-inetmtu-06.txt

Status of this Memo

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   This Internet-Draft will expire on May 17, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   The nominal Maximum Transmission Unit (MTU) of today's Internet has
   become 1500 bytes, but IP/*/IP tunneling mechanisms impose an
   encapsulation overhead that can reduce the effective path MTU to
   smaller values.  Additionally, existing tunneling mechanisms are
   limited in their ability to support larger MTUs.  This document
   specifies a simple protocol for robust IP/*/IP tunnel endpoint MTU
   determination (sprite-mtu).



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology and Requirements . . . . . . . . . . . . . . . . .  3
   3.  Concept of Operation . . . . . . . . . . . . . . . . . . . . .  4
   4.  "sprite-udp" UDP Service . . . . . . . . . . . . . . . . . . .  5
   5.  "sprite-mtu" Protocol Specification  . . . . . . . . . . . . .  6
     5.1.  Interactions with End-to-End MTU Determination . . . . . .  6
     5.2.  Tunnel Virtual Interface MTU and linkMTU . . . . . . . . .  6
     5.3.  Sprite Addresses . . . . . . . . . . . . . . . . . . . . .  7
     5.4.  Per-Tunnel Tunnel Soft State . . . . . . . . . . . . . . .  7
       5.4.1.  TNE Soft State . . . . . . . . . . . . . . . . . . . .  7
       5.4.2.  TFE Soft State . . . . . . . . . . . . . . . . . . . .  8
     5.5.  Soft State Maintenance . . . . . . . . . . . . . . . . . .  9
       5.5.1.  TNE Soft State Maintenance . . . . . . . . . . . . . .  9
       5.5.2.  TFE Soft State Maintenance . . . . . . . . . . . . . .  9
     5.6.  Sending Packets  . . . . . . . . . . . . . . . . . . . . . 10
       5.6.1.  Conceptual Sending Algorithm . . . . . . . . . . . . . 10
       5.6.2.  Inner Packet Fragmentation . . . . . . . . . . . . . . 11
       5.6.3.  Encapsulation and Trailers . . . . . . . . . . . . . . 11
       5.6.4.  Outer Packet Fragmentation and Setting DF  . . . . . . 12
       5.6.5.  Admitting Packets into the Tunnel  . . . . . . . . . . 13
     5.7.  Receiving Packets  . . . . . . . . . . . . . . . . . . . . 13
       5.7.1.  IPv4 Reassembly Cache Management . . . . . . . . . . . 13
       5.7.2.  Decapsulation  . . . . . . . . . . . . . . . . . . . . 13
       5.7.3.  Receiving Packet Too Big (PTB) Errors  . . . . . . . . 14
       5.7.4.  Receiving Other ICMP Errors  . . . . . . . . . . . . . 14
     5.8.  MTU Probing and Black Hole Detection . . . . . . . . . . . 14
     5.9.  Congestion Control . . . . . . . . . . . . . . . . . . . . 14
     5.10. sprite-mtu Checksum Calculation  . . . . . . . . . . . . . 15
   6.  Updated Specifications . . . . . . . . . . . . . . . . . . . . 16
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 17
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     10.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19
   Intellectual Property and Copyright Statements . . . . . . . . . . 20












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1.  Introduction

   The nominal Maximum Transmission Unit (MTU) of today's Internet has
   become 1500 bytes due to the preponderance of networking gear that
   configures an MTU of that size.  Since not all links in the Internet
   configure a 1500 byte MTU, however, packets can be dropped due to an
   MTU restriction on the path.

   Upper layers see IP/*/IP tunnels as ordinary links, but even for
   small packets these links are susceptible to silent loss (e.g., due
   to path MTU restrictions, lost error messages, layered
   encapsulations, reassembly buffer limitations, etc.) resulting in
   poor performance and/or communications failures
   [RFC2923][RFC4459][RFC4821][RFC4963].

   This document specifies a simple protocol for robust IP/*/IP tunnel
   endpoint MTU determination (sprite-mtu), and updates the functional
   specifications for Tunnel Endpoints (TEs) found in existing tunneling
   mechanisms (see: Section 6).

   This document seeks to achieve an appropriate balance between
   function in the network and function in the end systems [RFC1958],
   and further observes the tunnel management specifications in
   [RFC2003][RFC2473][RFC4213].


2.  Terminology and Requirements

   The following abbreviations and terms are used in this document:

      ICMP - ICMPv4 [RFC0793] or ICMPv6 [RFC4443].

      IP - IPv4 [RFC0791] or IPv6 [RFC2460].

      IP/*/IP - an inner IP packet encapsulated in outer */IP headers
      (e.g. for "*" = NULL, UDP, TCP, AH, ESP, etc.)

      inner packet/fragment/header - an IP packet/fragment/header before
      */IP encapsulation.

      outer packet/fragment/header - a */IP packet/fragment/header after
      encapsulation.

      AQM - Active Queue Management

      DF - the IPv4 header "Don't Fragment" flag





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      ENCAPS - the size of the encapsulating */IP headers plus trailers

      EMTU_R - Effective Maximum Transmission Unit to Receive [RFC1122]
      for the TFE

      MTU - Maximum Transmission Unit

      linkMTU - MTU assigned to a link over which the tunnel is
      configured

      pathMTU - the minimum path MTU for the tunnel

      PTB - an ICMPv4 "Destination Unreachable - fragmentation needed"
      [RFC1191] or an ICMPv6 "Packet Too Big" [RFC1981] message.

      sprite-mtu - the sprite-mtu protocol, specified in this document

      sprite-udp - the sprite-udp UDP messaging service, also specified
      in this document

      sprite - a message of the "sprite-udp" service

      TE - Tunnel Endpoint

      TFE - Tunnel Far End

      TNE - Tunnel Near End

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119].


3.  Concept of Operation

   TEs use the tunnel management specifications in
   [RFC2003][RFC2473][RFC4213] and also participate in the "sprite-mtu"
   protocol to confirm the participation of the TFE, to determine per-
   tunnel MTU values, to detect path MTU-related black-holes, and to
   detect congestion.  The protocol is supported through the exchange of
   messages between TEs using the "sprite-udp" UDP service.  The
   mechanisms provide robust MTU determination and congestion control
   when both TEs support the protocol, and support the legacy behavior
   otherwise.







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4.  "sprite-udp" UDP Service

   The "sprite-udp" service is a simple UDP service based on "sprite"
   messages.  Sprite requests set the UDP destination port to the
   sprite-udp service port (see: Section 7), and set the source port to
   either the sprite-udp service port or a dynamic port number chosen by
   the source.  Sprite replys set the UDP source port to the sprite-udp
   service port and set the UDP destination port to the value included
   in the UDP source port in the soliciting sprite request.

   All sprite requests and replys are formatted as shown in Figure 1
   (format for other sprite messages may be specified in future
   documents):

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Vers |  Type |      TTL      |           Checksum            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Identification         |        Sequence Number        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Data ...
      +-+-+-+-+-

                      Figure 1: Sprite Message Format

   where the fields of the message body are defined as follows:

   Vers
      the Version field indicates the sprite-udp protocol version.  This
      document describes version 1.

   Type
      the message type.  Currently defined values are:

      0 - request

      1 - reply

      2 - 15 reserved for future use

   TTL
      in sprite requests, set to 0; in sprite replies, set to the TTL/
      Hop Limit in the IP header of the request to which this packet is
      a reply.






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   Checksum
      the 16-bit one's complement of the one's complement sum of the
      message body, starting with the version field and ending at the
      end of the data field.  For computing the checksum, the checksum
      field is first set to zero.  An all zero transmitted checksum
      value means that the transmitter generated no checksum.

   Identification, Sequence Number
      Two 16-bit fields, used exactly as specified for the corresponding
      fields in ICMP echo request and reply messages.

   Data
      Zero or more octets of arbitrary data, included in the sprite
      request and echoed in the reply.


5.  "sprite-mtu" Protocol Specification

   TEs that implement the sprite-udp service MUST also participate in
   the "sprite-mtu" protocol to: 1) determine whether the TFE implements
   the scheme, 2) detect path MTU-related black holes, 3) provide timely
   aging of stale path MTU information, 4) determine the length of the
   forward path through the tunnel, 5) determine accurate round trip
   times, and 6) detect and report congestion.  The following sections
   specify the protocol details:

5.1.  Interactions with End-to-End MTU Determination

   The sprite-mtu protocol operates independently of any end-to-end MTU
   determination, however it offers improved convergence time and
   efficiency when end-to-end mechanisms such as [RFC4821] are also
   used.

5.2.  Tunnel Virtual Interface MTU and linkMTU

   TEs SHOULD configure an MTU on the tunnel virtual interface (i.e.,
   the MTU that is seen by upper layers) that is at least as large as
   the largest linkMTU for all underlying interfaces over which the
   tunnel virtual interface is configured.  For IPv6/*/IP tunnels, the
   tunnel virtual interface MUST configure an MTU no smaller than 1280
   bytes, and for IPv4/*/IP tunnels it SHOULD configure an MTU no
   smaller than 576 bytes.

   Additionally, operators SHOULD observe the recommendations in
   [RFC3819], Section 2, i.e., they should avoid setting a too-small
   linkMTU on any of the underlying interfaces over which the tunnel
   virtual interface is configured.




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5.3.  Sprite Addresses

   TEs must configure a time-varying link-local address (e.g., they
   regenerate a new link-local address every 30 seconds) for the tunnel
   interface known as the "sprite address".  The sprite address is
   associated with the tunnel interface (i.e., not assigned to the
   interface) hence it need not be checked for uniqueness and will only
   be used for the purpose of sprite exchanges for soft state management
   (see below).

   For IPv6/*/IPv4 tunnels, link-local IPv6 privacy addresses [RFC4941]
   are used.  For IPv4/*/IP tunnels, random assignments from the IPv4
   link local address range [RFC3927] are used, however the number of
   bits for randomness is significantly smaller than for IPv6.

   This addressing scheme is not available for unidirectional tunnels,
   since link local addresses would not be routable on the (non-
   tunneled) return path from the TFE to the TNE.

5.4.  Per-Tunnel Tunnel Soft State

   TEs maintain per-tunnel soft state information (e.g., in a conceptual
   neighbor cache) that is initialized when there is evidence that a
   continuous flow of data will traverse the tunnel and is scheduled for
   deletion based on idle timers, resource limitations, etc. thereafter.
   The TNE maintains state regarding the forward path to the TFE.  When
   necessary (e.g., when the tunnel is fragmenting), the TFE also
   maintains state regarding the number of packets received, packets in
   error, etc.  The minimum state kept by the TNE and TFE is given in
   the following sections:

5.4.1.  TNE Soft State

   The TNE keeps the following minimum per-tunnel soft state for active
   tunnels, and MAY keep additional soft state (e.g., packets/bytes
   sent, collisions, etc.):

   isQualified
      boolean indicating whether the TFE implements the protocol.

      Initial value: FALSE

   TFEAddr
      the inner IP address of the TFE.







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   pathMTU
      the current minimum path MTU across the tunnel to the TFE,
      determined through sprite probing to determine the largest size
      outer packet that can traverse the tunnel.

      Initial value: 0.

   TTL
      the current path length across the tunnel to the TFE, determined
      through sprite probing.

      Initial value: 0.

   RTT
      the round trip time for the TFE.

      Initial value: none

   spriteList
      a list of sprite requests that have been sent into the tunnel but
      not yet acknowledged by the TFE.

      Initial list: NULL

5.4.2.  TFE Soft State

   When requested by the TNE, the TFE keeps the following minimum per-
   tunnel soft state information, and MAY keep additional soft state
   (e.g., congestion, error rate, etc.):

   rxTime
      the system time at which the soft state for this TNE was
      initialized.

   rxPackets
      number of packets received.

   rxBytes
      number of bytes received.

   rxDropped
      number of packets dropped due to incorrect checksums, congestion,
      etc.








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5.5.  Soft State Maintenance

5.5.1.  TNE Soft State Maintenance

   When a TNE has data to send to a TFE, it obeys the specification in
   the normative IP/*/IP reference.  When there is evidence that a
   persistent flow of data will traverse the tunnel, the TNE also
   creates soft state per Section 5.3.1 (unless the soft state already
   exists) and sends sprite requests into the tunnel.  The TNE includes
   in the inner IP header of each sprite request its current sprite
   address as the source address and a destination address that is
   either the current sprite address of the TFE or a different link
   local address.  The TNE then sets 'Vers' to '1', sets 'Type' to '0',
   sets 'TTL' to '0', sets 'Identification' and 'Sequence Number' to
   identifying values, and includes a randomly-chosen nonce value (8
   bytes recommended) in the 'Data' field along with any other data to
   be echoed.  The TNE finally calculates the checksum and writes its
   value in the 'Checksum' field (or, writes the value '0'), then sends
   the sprite request into the tunnel.

   If the TNE receives a sprite reply message that is apparently from
   the TFE and also includes an 'Identification', 'Sequence Number', and
   'Data' that matches a request in its 'spriteList', it sets
   'isQualified' to TRUE; otherwise, it discards the reply.  The TNE
   also records the round-trip time (RTT) relative to the time at which
   it sent the soliciting sprite request, as well as the difference
   between the TTL of the soliciting request and the TTL encoded in the
   reply.  The TNE finally records the source address in the inner IP
   header of the sprite reply in 'TFEAddr'.

   When the TNE requires the TFE to maintain state (e.g., when the
   tunnel is fragmenting), it sends continuous sprite requests at a rate
   of no more than 1 request per second and sets the inner destination
   address of each request to 'TFEAddr'.  When the flow of data ceases,
   the TNE stops sending sprite requests and schedules the soft state
   entry for deletion.  When the flow of data resumes, the TNE resumes
   sending sprite requests.

   The TNE removes sprite requests from its 'spriteList' when either a
   matching reply is received, or after a timeout period during which no
   matching reply is received.  (Timeouts on the order of IPv6 neighbor
   discovery [RFC4861] are recommended.)

5.5.2.  TFE Soft State Maintenance

   When a TFE receives a sprite request, it prepares a reply that
   includes the inner IP source address of the request in the inner IP
   destination address and its current sprite address in the inner IP



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   source address.  The TFE then sets 'Vers' to '1', sets 'Type to '1',
   copies the TTL/Hop Limit from the sprite request into the 'TTL'
   field, and copies 'Identification', 'Sequence Number' and 'Data' from
   the request message body into the corresponding fields of the reply.
   The TFE finally calculates the checksum and writes its value in the
   'Checksum' field (or, writes the value '0'), then sends the sprite
   reply back to the TNE.

   If the inner IP destination address of the sprite request was the
   same as the TFE's current sprite address, the TFE creates soft state
   per Section 5.3.2 if possible.  The TFE maintains the soft state as
   long as it continues to receive sprite requests from the TNE that
   include an inner destination address that matches its current sprite
   address.  When the flow of requests ceases, the TFE schedules the
   soft state entry for deletion.

   If the TFE is unable to maintain soft state, e.g., due to resource
   limitations, it sets the inner IP destination address in its sprite
   replys to a different link local IP address than the one included in
   the inner IP source address of the sprite request.  The TNE must
   accept this as an indication that the TFE is not currently
   maintaining soft state.

5.6.  Sending Packets

   Inner IP packets forwarded by upper layers that are larger than the
   tunnel virtual interface MTU are dropped with an ICMP Packet Too Big
   (PTB) sent back to the original source, as for any IP interface.
   Other inner IP packets are forwarded into the tunnel interface, which
   will encapsulate and send them on the underlying tunnel and/or return
   an internally-generated PTB when necessary.

   TEs that implement the sprite-mtu protocol use the specifications for
   sending packets found in the following sections:

5.6.1.  Conceptual Sending Algorithm

   TEs use the conceptual sending algorithm in Figure 2 for sending
   packets that are forwarded into the tunnel virtual interface by upper
   layers:











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           Fragment inner packet if necessary (Section 5.5.2)
           foreach inner packet/fragment
               Encapsulate as an outer packet (Section 5.5.3).
               Fragment outer packet if necessary (Sect. 5.5.4).
               foreach outer packet/fragment
                   Send packet/fragment (Section 5.5.5).
               endforeach
               Send PTB appropriate to the inner protocol if
               necessary (Section 5.5.6).
           end foreach

                  Figure 2: Conceptual Sending Algorithm

5.6.2.  Inner Packet Fragmentation

   The TE considers an inner IPv4 packet as fragmentable IFF the DF bit
   is set to 0 in the inner IPv4 header, and assumes that the original
   source has learned through some end-to-end means that the final
   destination is able to reassemble a packet of this size.  The TE uses
   IPv4 fragmentation to break fragmentable inner packets into fragments
   no larger than (576 - ENCAPS) before encapsulation; these inner
   fragments will ultimately be reassembled by the final destination.

   The TE is not permitted to fragment inner IPv6 packets, therefore an
   inner IPv6 packet is never fragmentable.

5.6.3.  Encapsulation and Trailers

   TEs encapsulate inner IP packets according to the specific IP/*/IP
   document.  When 'isQualified' is TRUE, the TE includes a trailer with
   a correct non-zero checksum in all packets that may incur outer
   fragmentation (see: Section 5.5.4).  For other packets, the TE can
   either: 1) include a trailer with a correct non-zero checksum, 2)
   include a trailer with a zero checksum, or 3) omit the trailer.

   For outer packets that will include a trailer during encapsulation,
   the TE includes zero or more padding bytes plus a 4-byte trailing
   checksum immediately following the inner IP packet.  The TE
   increments the innermost '*' header length field by the number of
   trailer bytes added before applying the outermost */IP
   encapsulation(s).  For example, it increments the UDP length field
   for IP/UDP/IP tunnels ('*' = UDP), the IPv4 length field for IP/IPv4
   tunnels ('*' = NULL), etc.  The encapsulation is shown in Figure 3:








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                          +---------------------------------+
                          |         Outer IP Header         |
                          |                                 |
                          +---------------------------------+
                          |            * Headers            |
                          |                                 |
   +-------------+        +---------------------------------+
   |   Inner IP  |        |             Inner IP            |
   ~   packet    ~  ===>  ~             packet              ~
   |             |        |                                 |
   +-------------+        +---------------------------------+ -\  T
    Inner Packet          |                                 |  |  r
                          ~     Padding (0 or more bytes)   ~  |  a
                          |                                 |   > i
                          +----------------+----------------+  |  l
                          |  cksum-A (16b) |  cksum-B (16b) |  |  e
                          +----------------+----------------+ -/  r
                          |  Any */IP protocol trailers ...
                          +------------------------------
                                      Outer Packet

                Figure 3: Encapsulation Format with Trailer

   For all packets that will include a trailer, the TE appends any
   padding bytes as necessary to extend the packet to a specific length
   then calculates the checksum as specified in Section 5.10.  It then
   appends the results in the A and B fields of the trailing checksum.
   (The TE instead writes the value 0 in these fields if the trailer is
   to include a zero checksum).  The checksum is byte-aligned only,
   i.e., it need not be aligned on an even word/longword/etc. boundary.

   The TE SHOULD NOT include a trailer during encapsulation when
   'isQualified' is FALSE.

5.6.4.  Outer Packet Fragmentation and Setting DF

   The TE is not permitted to fragment outer */IPv6 packets using IPv6
   fragmentation.

   The TE considers an outer */IPv4 packet as fragmentable IFF:

   o  for IPv6/*/IPv4 tunnels, 'pathMTU' is less than (1280 bytes +
      ENCAPS) and the inner IPv6 packet is no larger than 1280 bytes,
      or:

   o  for IPv4/*/IPv4 tunnels, 'pathMTU' is less than MIN(EMTU_R, 1280)
      bytes and the inner IPv4 packet is no larger than MIN(EMTU_R,
      1280) minus ENCAPS.  (When EMTU_R for the TFE is not known, 576



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      bytes must be assumed.)

   The TE's */IPv4 encapsulation layer(s) MAY fragment a fragmentable
   outer packet before admitting it into the tunnel.  The TE SHOULD set
   DF=1 in the outer IPv4 header of each fragment if 'pathMTU' is known;
   otherwise, the TE MAY set DF=0 if there is assurance that the TFE can
   receive non-initial IPv4 fragments.  These outer fragments will be
   reassembled by the TFE.

   The TE MUST set DF=1 in the outer IPv4 header of all unfragmentable
   outer packets.

5.6.5.  Admitting Packets into the Tunnel

   For IP/*/IPv4 tunnels, when 'pathMTU' is smaller than the minimum
   values listed in Section 5.5.4 and the TFE is not maintaining soft
   state, the TE MUST institute pacing and AQM to minimize IPv4
   reassembly misassociations and/or congestion at the TFE.  The TE MAY
   relax this pacing when the TFE indicates that it is maintaining soft
   state, but MUST resume pacing if it subsequently detects congestion
   at the TFE (see: Section 5.8).

   The TE admits outer packets into the tunnel subject to pacing and TTL
   restrictions.  For unfragmentable outer packets that are larger than
   'pathMTU', the TE admits the packet but also sends a PTB message
   appropriate to the inner IP protocol with an MTU size of ('pathMTU' -
   ENCAPS).

5.7.  Receiving Packets

   TEs that implement the sprite-mtu protocol use the specifications for
   receiving packets found in the following sections:

5.7.1.  IPv4 Reassembly Cache Management

   IP/*/IPv4 TEs SHOULD perform AQM in their IPv4 reassembly cache.  In
   particular, they should actively discard "stale" reassemblies that
   have no apparent opportunity for successful completion, i.e., even
   before the packets have reached the normal reassembly timeout
   expiration recommended in [RFC1122], Section 3.3.2.

5.7.2.  Decapsulation

   When the TE receives (and, if necessary, reassembles) an encapsulated
   packet, and the packet includes a trailing checksum, the TE accepts
   the packet if the checksum is correct and drops the packet if the
   checksum is incorrect.  Otherwise, the TE decapsulates the packet
   exactly as specified in the appropriate IP/*/IP document and discards



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   the trailer.

   During decapsulation, it the TE is maintaining soft state it also
   records 'rx*' statistics in its soft state entry for the tunnel,
   i.e., it increments 'rxPackets' and 'rxBytes' for packets received
   with no errors, and increments 'rxDropped' for packets dropped due to
   checksum errors, congestion, etc.

5.7.3.  Receiving Packet Too Big (PTB) Errors

   TNEs may receive PTB errors in response to any packets they admit
   into the tunnel.  When the TNE receives a PTB with an MTU value
   smaller than 'pathMTU', it SHOULD record the new 'pathMTU' in its
   soft state entry for the tunnel.

5.7.4.  Receiving Other ICMP Errors

   TEs SHOULD observe the specifications in [RFC2003][RFC2473][RFC4213]
   when they receive other ICMP errors from within the tunnel, but are
   advised that ICMP denial-of-service attacks are possible.

5.8.  MTU Probing and Black Hole Detection

   When 'isQualified' is TRUE, the TE can send sprite requests to the
   TFE with trailing padding added during encapsulation to create an MTU
   probe of the desired length.  In particular, when the TE sends a data
   packet into the tunnel with a size that exceeds the current
   'pathMTU', it MAY also send a padded sprite request of the same size
   into the tunnel.  If the TE receives a matching sprite reply, it
   advances 'pathMTU' to the size of the request.

   The TE MAY send additional sprite requests into the tunnel to
   determine a maximum 'pathMTU' independent of any data packets sent
   into the tunnel, to detect a smaller 'pathMTU' due to routing changes
   and/or to detect MTU-related black holes.

   For IP/*/IPv4 tunnels, the TE MUST set DF=1 in any sprite request
   used for the purpose of 'pathMTU' probing.

5.9.  Congestion Control

   The TNE MUST set the ECN field in the inner IP header of sprite
   requests used for soft state maintenance to either ECT(0) or ECT(1)
   [RFC3168] and MUST also examine the ECN field in the inner IP headers
   of sprite replys.  When the TNE begins to observe the CE codepoint in
   the ECN field in the inner IP headers of successive sprite replys at
   a rate that exceeds a high water threshold, it institutes pacing per
   Section 5.5.5.  The TNE MAY relax pacing when the rate falls below a



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   low water threshold.

   When soft state management is requested by the TNE, the TFE MUST
   track the rate at which packets received from the TNE are dropped due
   to, e.g., checksum errors, congestion, etc.  When the rate exceeds a
   high water threshold, the TFE MUST begin setting the CE codepoint in
   the ECN field in the inner IP headers of sprite replys sent in
   response to requests with the ECN field set to other than not-ECT.
   The TFE MUST also set the CE codepoint in ordinary data packets with
   the ECN field set to other than not-ECT when it forwards them to the
   next IP hop.  When the rate of dropped packets falls below a low
   water threshold, the TFE MAY relax CE codepoint marking.

   When the TFE is temporarily unable to maintain soft state, it
   includes a link local address in the inner IP destination address of
   its sprite replies that is different than the inner IP source address
   that the TNE included in its sprite request.  The TNE must interpret
   this as an indication that pacing should resume.

5.10.  sprite-mtu Checksum Calculation

   This specification uses a 16-bit variation of the Fletcher Checksum
   [RFC1146][STONE1][STONE2] called: the "sprite-mtu checksum" which
   provides a lightweight integrity check with different properties than
   those used by common link layers and upper layer protocols.

   The TE calculates the sprite-mtu checksum by summing every 10th byte
   of the packet beginning with the inner IP header up to the end of the
   inner packet including trailing padding, but not including the
   trailing checksum field itself.  The TE calculates the checksum
   according to the algorithm below, which represents a slight variation
   of that found in [RFC1146]:

         The sprite-mtu checksum is calculated over a sequence of
         unsigned data octets (call them D[0] through D[N-1]) by
         maintaining unsigned 1's-complement 16-bit accumulators
         A and B whose contents are initially zero, and performing
         the following loop where i ranges from 1 to N:

                A := A + D[i]
                B := B + A
                i := i + 10

         If, at the end of the loop, either or both of the A and B
         accumulators encode the value 0x0000, invert the value in the
         accumulator(s) to 0xffff.

   Note that faster algorithms are possible and may be used instead of



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   the algorithm above (see: [RFC1146]).  Note also that for '*/IP'
   encapsulations that include an additional checksum, the sprite-mtu
   checksum can be calculated in parallel.


6.  Updated Specifications

   This document updates the following specifications, and possibly
   others:

   o  RFC1853 (IP-in-IP)

   o  RFC2003 (IP-in-IP)

   o  RFC2473 (Generic packet tunneling in IPv6)

   o  RFC2529 (6over4)

   o  RFC2661 (L2TP)

   o  RFC2784 (GRE)

   o  RFC3056 (6to4)

   o  RFC3378 (ETHERIP)

   o  RFC3884 (IPSec Transport Mode for Dynamic Routing)

   o  RFC4023 (MPLS-in-IP)

   o  RFC4213 (Basic IPv6 Transition Mechanisms)

   o  RFC4214 (ISATAP)

   o  RFC4301 (IPSec)

   o  RFC4302 (AH)

   o  RFC4303 (ESP)

   o  RFC4380 (TEREDO)

   o  LISP

   o  IPAE

   o  others....




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7.  IANA Considerations

   A new UDP port number for the "sprite-udp" service is requested.


8.  Security Considerations

   A possible denial of service attack vector involves an off-path
   attacker sending sprite replys with spoofed source addresses, however
   the 8-byte nonce serves as an effective mitigation.

   A possible resource exhaustion attack vector exist when TEs use well-
   known and/or time-invariant addresses.  Sprite addresses serve as an
   effective mitigation for bidirectional tunnels, as specified in
   Section 5.9.

   Security considerations for specific IP/*/IP tunneling mechanisms are
   specified in their respective documents.


9.  Acknowledgments

   This work has benefited from discussions with Fred Baker, Iljitsch
   van Beijnum, Brian Carpenter, Steve Casner, Remi Denis-Courmont,
   Aurnaud Ebalard, Gorry Fairhurst, John Heffner, Bob Hinden, Christian
   Huitema, Joe Macker, Matt Mathis, Dave Thaler, Joe Touch, Magnus
   Westerlund, Robin Whittle, and James Woodyatt.

   This work is dedicated to the editor's family.


10.  References

10.1.  Normative References

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

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

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

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

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery



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              for IP version 6", RFC 1981, August 1996.

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
              October 1996.

   [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.

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

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, September 2001.

   [RFC3927]  Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
              Configuration of IPv4 Link-Local Addresses", RFC 3927,
              May 2005.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.

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

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, September 2007.

10.2.  Informative References

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

   [RFC1958]  Carpenter, B., "Architectural Principles of the Internet",
              RFC 1958, June 1996.

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

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



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   [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.

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

   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963, July 2007.

   [STONE1]   Stone, J., "Checksums in the Internet (Stanford Doctoral
              Dissertation)", August 2001.

   [STONE2]   Stone, J., Greenwald, M., Partridge, C., and J. Hughes,
              "Performance of Checksums and CRC's over Real Data, IEEE/
              ACM Transactions on Networking, Vol 6, No. 5",
              October 1998.


Author's Address

   Fred L. Templin (editor)
   Boeing Phantom Works
   P.O. Box 3707
   Seattle, WA  98124
   USA

   Email: fred.l.templin@boeing.com




















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Full Copyright Statement

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