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Versions: 00 01 02 03 04 05 06 07                                       
6LoWPAN                                                       P. Thubert
Internet-Draft                                                     Cisco
Intended status: Standards Track                            May 23, 2008
Expires: November 24, 2008


                    LoWPAN simple fragment Recovery
           draft-thubert-6lowpan-simple-fragment-recovery-01

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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   This Internet-Draft will expire on November 24, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   Considering that 6LoWPAN packets can be as large as 2K bytes and that
   an 802.15.4 frame with security will carry in the order of 80 bytes
   of effective payload, a packet might end up fragmented into as many
   as 25 fragments at the 6LoWPAN shim layer.  If a single one of those
   fragments is lost in transmission, all fragments must be resent,
   further contributing to the congestion that might have caused the
   initial packet loss.  This draft introduces a simple protocol to
   recover individual fragments between 6LoWPAN endpoints.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Rationale  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   6.  New Dispatch types and headers . . . . . . . . . . . . . . . .  7
     6.1.  Recoverable Fragment Dispatch type and Header  . . . . . .  7
     6.2.  Fragment Acknowledgement Dispatch type and Header  . . . .  8
   7.  Outstanding Fragments Control  . . . . . . . . . . . . . . . .  8
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 10
     11.2. Informative References . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
   Intellectual Property and Copyright Statements . . . . . . . . . . 12
































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1.  Introduction

   Considering that 6LoWPAN packets can be as large as 2K bytes and that
   a 802.15.4 frame with security will carry in the order of 80 bytes of
   effective payload, a packet might be fragmented into about 25
   fragments at the 6LoWPAN shim layer.  This level of fragmentation is
   much higher than that traditionally experienced over the Internet
   with IPv4 fragments.  At the same time, the use of radios increases
   the probability of transmission loss and Mesh-Under techniques
   compound that risk over multiple hops.

   Past experience with fragmentation has shown that missassociated or
   lost fragments can lead to poor network behaviour and, eventually,
   trouble at application layer.  The reader might start his research
   from [I-D.mathis-frag-harmful] and follow the references.  That
   experience led to the definition of the Path MTU discovery [RFC1191]
   protocol that avoids fragmentation over the Internet.

   An end-to-end fragment recovery mechanism might be a good complement
   to a hop-by-hop MAC level recovery with a limited number of retries.
   This draft introduces a simple protocol to recover individual
   fragments between 6LoWPAN endpoints.  Specifically in the case of
   UDP, valuable additional information can be found in UDP Usage
   Guidelines for Application Designers [I-D.ietf-tsvwg-udp-guidelines].


2.  Terminology

   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].

   Readers are expected to be familiar with all the terms and concepts
   that are discussed in "IPv6 over Low-Power Wireless Personal Area
   Networks (6LoWPANs): Overview, Assumptions, Problem Statement, and
   Goals" [RFC4919] and "Transmission of IPv6 Packets over IEEE 802.15.4
   Networks" [RFC4944].

   ERP

      Error Recovery Procedure.

   LoWPAN endpoints

      The LoWPAN nodes in charge of generating or expanding a 6LoWPAN
      header from/to a full IPv6 packet.  The LoWPAN endpoints are the
      points where fragmentation and reassembly take place.




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3.  Rationale

   There are a number of usages for large packets in Wireless Sensor
   Networks.  Such usages may not be the most typical or represent the
   largest amount of traffic over the LoWPAN; however, the associated
   functionality can be critical enough to justify extra care for
   ensuring effective transport of large packets across the LoWPAN.

   The list of those usages includes:

   Towards the LoWPAN node:

      Packages of Commands:  A number of commands or a full
         configuration can by packaged as a single message to ensure
         consistency and enable atomic execution or complete roll back.
         Until such commands are fully received and interpreted, the
         intended operation will not take effect.

      Firmware update:  For example, a new version of the LoWPAN node
         software is downloaded from a system manager over unicast or
         multicast services.  Such a reflashing operation typically
         involves updating a large number of similar 6LoWPAN nodes over
         a relatively short period of time.

   From the LoWPAN node:

      Waveform captures:  A number of consecutive samples are measured
         at a high rate for a short time and then transferred from a
         sensor to a gateway or an edge server as a single large report.

      Large data packets:  Rich data types might require more than one
         fragment.

   Uncontrolled firmware download or waveform upload can easily result
   in a massive increase of the traffic and saturate the network.

   When a fragment is lost in transmission, all fragments are resent,
   further contributing to the congestion that caused the initial loss,
   and potentially leading to congestion collapse.

   This saturation may lead to excessive radio interference, or random
   early discard (leaky bucket) in relaying nodes.  Additional queueing
   and memory congestion may result while waiting for a low power next
   hop to emerge from its sleeping state.







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4.  Requirements

   This paper proposes a method to recover individual fragments between
   LoWPAN endpoints.  The method is designed to fit the following
   requirements of a LoWPAN (with or without a Mesh-Under routing
   protocol):

   Number of fragments

      The recovery mechanism must support highly fragmented packets,
      with a maximum of 32 fragments per packet.

   Minimimum acknowledgement overhead

      Because the radio is half duplex, and because of silent time spent
      in the various medium access mechanisms, an acknowledgement
      consumes roughly as many resources as data fragment.

      The recovery mechanism should be able to acknowledge multiple
      fragments in a single message.

   Controlled latency

      The recovery mechanism must succeed or give up within the time
      boundary imposed by the recovery process of the Upper Layer
      Protocols.

   Support for out-of-order fragment delivery

      A Mesh-Under load balancing mechanism such as the ISA100 Data Link
      Layer can introduce out-of-sequence packets.  The recovery
      mechanism must account for packets that appear lost but are
      actually only delayed over a different path.

   Optional flow control

      The aggregation of multiple concurrent flows may lead to the
      saturation of the radio network and congestion collapse.

      The recovery mechanism should provide means for controlling the
      number of fragments in transit over the LoWPAN.

   Backward compatibility

      A node that implements this draft should be able to communicate
      with a node that implements [RFC4944].  This draft assumes that
      compatibility information about the remote LoWPAN endpoint is
      obtained by external means.



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5.  Overview

   Considering that a multi-hop LoWPAN can be a very sensitive
   environment due to the limited queueing capabilities of a large
   population of its nodes, this draft recommends a simple and
   conservative approach to flow control, based on TCP congestion
   avoidance.

   Congestion on the forward path is assumed in case of packet loss, and
   packet loss is assumed upon time out.

   Congestion on the forward path can also be indicated by an Explicit
   Congestion Notification (ECN) mechanism.  This draft provides a way
   for the destination LoWPAN endpoint to echo en ECN indication back to
   the source LoWPAN endpoint in an acknowledgement message as
   represented in Figure 3 in Section 6.2.

   From the standpoint of a source LoWPAN endpoint, an outstanding
   fragment is a fragment that was sent but for which no explicit
   acknowledgement was received yet.  This means that the packet might
   be on the way, received but not yet acknowledged, or the
   acknowledgement might be on the way back.  It is also possible that
   either the fragment or the acknowledgement was lost on the way.

   Because a meshed LoWPAN might deliver packets out of order, it is
   virtually impossible to differentiate these situations.  In other
   words, from the sender standpoint, all outstanding packets might
   still be in the network and contribute to its congestion.  There is
   an assumption, though, that after a certain amount of time, a packet
   is either received or lost, so it is not causing congestion anymore.
   This amount of time can be estimated based on the round trip delay
   between the LoWPAN endpoints.  The method detailed in [RFC2988] is
   recommended for that computation.

   The reader is encouraged to read through "Congestion Control
   Principles" [RFC2914].  Additionally [RFC2309] and [RFC2581] provide
   deeper information on why this mechanism is needed and how TCP
   handles Congestion Control.  Basically, the goal here is to manage
   the amount of fragments present in the network; this is achieved by
   to reducing the number of outstanding fragment over a congested path
   by throttling the sources.

   Whether and how ECN [RFC3168] is carried out over the LoWPAN is out
   of scope for this draft.  Section 7 describes how the sender decides
   how many fragments are (re)sent before an acknowledgement is
   required, and how the sender adapts that number to the network
   conditions.




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6.  New Dispatch types and headers

   This specification extends "Transmission of IPv6 Packets over IEEE
   802.15.4 Networks" [RFC4944] with 3 new dispatch types, for
   Recoverable Fragments (RFRAG) headers with or without Acknowledgement
   Request, and for the Acknowledgement back.



            Pattern    Header Type
          +------------+-----------------------------------------------+
          | 11  101000 | RFRAG      - Recoverable Fragment             |
          | 11  101001 | RFRAG-AR   - RFRAG with Acknowledgement Req   |
          | 11  101010 | RFRAG-ACK  - RFRAG Acknowledgement            |
          +------------+-----------------------------------------------+


             Figure 1: Additional Dispatch Value Bit Patterns

   In the following sections, the semantics of "datagram_tag,"
   "datagram_offset" and "datagram_size" and the reassembly process are
   unchanged from [RFC4944] Section 5.3.  "Fragmentation Type and
   Header."

6.1.  Recoverable Fragment Dispatch type and Header

                            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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |1 1 1 0 1 0 0 X|datagram_offset|         datagram_tag          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Sequence |    datagram_size    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     X set == Ack Requested

          Figure 2: Recoverable Fragment Dispatch type and Header

   X bit

      When set, the sender requires an Acknowledgement from the receiver

   Sequence

      The sequence number of the fragment.  Fragments are numbered
      [0..N] where N is in [0..31].







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6.2.  Fragment Acknowledgement Dispatch type and Header

                            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
                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                       |1 1 1 0 1 0 1 Y|         datagram_tag          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Acknowledgement Bitmap                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+
        ^                   ^
        |                   |      Y set == Backward Explicit Congestion
        |                   |                               Notification
        |                   |        bitmap indicating whether
        |                   +-----Fragment with sequence 10 was received
        +-------------------------Fragment with sequence 00 was received


        Figure 3: Fragment Acknowledgement Dispatch type and Header

   Y bit

      When set, the sender indicates that at least one of the
      acknowledged fragments was received with an Explicit Congestion
      Notification, indicating that the path followed by the fragments
      is subject to congestion.

   Acknowledgement Bitmap

      Each bit in the Bitmap refers to a particular fragment: bit n set
      indicates that fragment with sequence n was received, for n in
      [0..31].

      All zeroes means that the fragment was dropped because it
      corresponds to an obsolete datagram_tag.  This happens if the
      packet was already reassembled and passed to the network upper
      layer, or the packet expired and was dropped.


7.  Outstanding Fragments Control

   A mechanism based on TCP congestion avoidance dictates the maximum
   number of outstanding fragments.

   The maximum number of outstanding fragments for a given packet toward
   a given LoWPAN endpoint is initially set to a configured value,
   unless recent history indicates otherwise.  Each time that maximum
   number of fragments is fully acknowledged, that number can be
   incremented by 1.  ECN echo and packet loss cause the number to be



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   divided by 2.

   The sender transfers a controlled number of fragments and flags the
   last fragment of a series with an acknowledgement request.

   The sender arms a timer to cover the fragment that carries the
   Acknowledgement request.  Upon time out, the sender assumes that all
   the fragments on the way are received or lost.  It divides the
   maximum number of outstanding fragments by 2 and resets the number of
   outstanding fragments to 0.

   Upon receipt of an Acknowledgement request, the receiver responds
   with an Acknowledgement containing a bitmap that indicates which
   fragments were actually received.  The bitmap is a 32bit DWORD, which
   accommodates up to 32 fragments and is sufficient for the 6LoWPAN
   MTU.  For all n in [0..31], bit n is set to 1 in the bitmap to
   indicate that fragment with sequence n was received, otherwise the
   bit is set to 0.

   The receiver MAY issue unsolicited acknowledgements.  An unsolicited
   acknowledgement enables the sender endpoint to resume sending if it
   had reached its maximum number of outstanding fragments.  Note that
   acknowledgements might consume precious resources so the use of
   unsolicited acknowledgements should be configurable and not enabled
   by default.

   The received MUST acknowledge a fragment with the acknowledgement
   request bit set.  If any fragment immediately preceding an
   acknowledgement request is still missing, the receiver MAY
   intentionally delay its acknowledgement to allow in-transit fragments
   to arrive.  This mechanism might defeat the round trip delay
   computation so it should be configurable and not enabled by default.

   Fragments are sent in a round robin fashion: the sender sends all the
   fragments for a first time before it retries any lost fragment; lost
   fragments are retried in sequence, oldest first.  This mechanism
   enables the receiver to acknowledge fragments that were delayed in
   the network before they are actually retried.

   The process must complete within an acceptable time that is within
   the boundaries of upper layer retries.  Additional work is required
   to define how this is achieved.  When the source endpoint decides
   that a packet should be dropped and the fragmentation process
   cancelled, it sends a pseudo fragment with the datagram_offset,
   sequence and datagram_size all set to zero, and no data.  Upon
   reception of this message, the receiver should clean up all resources
   for the packet associated to the datagram_tag.




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8.  Security Considerations

   The process of recovering fragments does not appear to create any
   opening for new threat.


9.  IANA Considerations

   Need extensions for formats defined in "Transmission of IPv6 Packets
   over IEEE 802.15.4 Networks" [RFC4944].


10.  Acknowledgments

   The author wishes to thank Jay Werb, Christos Polyzois, Soumitri
   Kolavennu and Harry Courtice for their contribution and review.


11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2988]  Paxson, V. and M. Allman, "Computing TCP's Retransmission
              Timer", RFC 2988, November 2000.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, September 2007.

11.2.  Informative References

   [I-D.ietf-tsvwg-udp-guidelines]
              Eggert, L. and G. Fairhurst, "Guidelines for Application
              Designers on Using Unicast UDP",
              draft-ietf-tsvwg-udp-guidelines-07 (work in progress),
              May 2008.

   [I-D.mathis-frag-harmful]
              Mathis, M., "Fragmentation Considered Very Harmful",
              draft-mathis-frag-harmful-00 (work in progress),
              July 2004.

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




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   [RFC2309]  Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
              S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
              Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
              S., Wroclawski, J., and L. Zhang, "Recommendations on
              Queue Management and Congestion Avoidance in the
              Internet", RFC 2309, April 1998.

   [RFC2581]  Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
              Control", RFC 2581, April 1999.

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, September 2000.

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

   [RFC4919]  Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
              over Low-Power Wireless Personal Area Networks (6LoWPANs):
              Overview, Assumptions, Problem Statement, and Goals",
              RFC 4919, August 2007.


Author's Address

   Pascal Thubert
   Cisco Systems
   Village d'Entreprises Green Side
   400, Avenue de Roumanille
   Batiment T3
   Biot - Sophia Antipolis  06410
   FRANCE

   Phone: +33 4 97 23 26 34
   Email: pthubert@cisco.com
















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