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6LoWPAN                                                  P. Thubert, Ed.
Internet-Draft                                                     Cisco
Intended status: Standards Track                                  J. Hui
Expires: December 6, 2010                          Arch Rock Corporation
                                                            June 4, 2010


                LoWPAN fragment Forwarding and Recovery
           draft-thubert-6lowpan-simple-fragment-recovery-07

Abstract

   Considering that the IPv6 minimum MTU is 1280 bytes and that an an
   802.15.4 frame can have a payload limited to 74 bytes in the worst
   case, a packet might end up fragmented into as many as 18 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 forward and
   recover individual fragments that might be lost over multiple hops
   between 6LoWPAN endpoints.

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 December 6, 2010.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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



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


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Rationale  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  New Dispatch types and headers . . . . . . . . . . . . . . . .  7
     6.1.  Recoverable Fragment Dispatch type and Header  . . . . . .  8
     6.2.  Fragment Acknowledgement Dispatch type and Header  . . . .  8
   7.  Fragments Recovery . . . . . . . . . . . . . . . . . . . . . . 10
   8.  Forwarding Fragments . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  Upon the first fragment  . . . . . . . . . . . . . . . . . 12
     8.2.  Upon the next fragments  . . . . . . . . . . . . . . . . . 13
     8.3.  Upon the fragment acknowledgements . . . . . . . . . . . . 14
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 14
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     12.2. Informative References . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15






















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1.  Introduction

   In many 6LoWPAN applications, the majority of traffic is spent
   sending small chunks of data (order few bytes to few tens of bytes)
   per packet.  Given that an 802.15.4 frame can carry 74 bytes or more
   in all cases, fragmentation is often not needed for most application
   traffic.  However, many applications also require occasional bulk
   data transfer capabilities to support firmware upgrades of 6LoWPAN
   devices or extraction of logs from 6LoWPAN devices.  In the former
   case, bulk data is transferred to the 6LoWPAN device, and in the
   latter, bulk data flows away from the 6LoWPAN device.  In both cases,
   the bulk data size is often on the order of 10K bytes or more and
   end-to-end reliable transport is required.

   Mechanisms such as TCP or application-layer segmentation will be used
   to support end-to-end reliable transport.  One option to support bulk
   data transfer over 6LoWPAN links is to set the Maximum Segment Size
   to fit within the 802.15.4 MTU.  Doing so, however, can add
   significant header overhead to each 802.15.4 frame.  This causes the
   end-to-end transport to be aware of the delivery properties of
   6LoWPAN networks, which is a layer violation.

   An alternative mechanism combines the use of 6LoWPAN fragmentation in
   addition to transport or application-layer segmentation.  Increasing
   the Maximum Segment Size reduces header overhead by the end-to-end
   transport protocol.  It also encourages the transport protocol to
   reduce the number of outstanding datagrams, ideally to a single
   datagram, thus reducing the need to support out-of-order delivery
   common to 6LoWPAN networks.

   [RFC4944] defines a datagram fragmentation mechanism for 6LoWPAN
   networks.  However, because [RFC4944] does not define a mechanism for
   recovering fragments that are lost, datagram forwarding fails if even
   one fragment is not delivered properly to the next IP hop.  End-to-
   end transport mechanisms will require retransmission of all
   fragments, wasting resources in an already resource-constrained
   network.

   Past experience with fragmentation has shown that missassociated or
   lost fragments can lead to poor network behavior and, eventually,
   trouble at application layer.  The reader is encouraged to read
   [RFC4963] and follow the references for more information.  That
   experience led to the definition of the Path MTU discovery [RFC1191]
   protocol that limits fragmentation over the Internet.

   For one-hop communications, a number of media propose a local
   acknowledgement mechanism that is enough to protect the fragments.
   In a multihop environment, an end-to-end fragment recovery mechanism



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   might be a good complement to a hop-by-hop MAC level recovery.  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 [RFC5405].


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.


3.  Rationale

   There are a number of uses 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.




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      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.

      Data logs:  6LoWPAN nodes may generate large logs of sampled data
         for later extraction. 6LoWPAN nodes may also generate system
         logs to assist in diagnosing problems on the node or network.

      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 queuing
   and memory congestion may result while waiting for a low power next
   hop to emerge from its sleeping state.

   To demonstrate the severity of the problem, consider a fairly
   reliable 802.15.4 frame delivery rate of 99.9% over a single 802.15.4
   hop.  The expected delivery rate of a 5-fragment datagram would be
   about 99.5% over a single 802.15.4 hop.  However, the expected
   delivery rate would drop to 95.1% over 10 hops, a reasonable network
   diameter for 6LoWPAN applications.  The expected delivery rate for a
   1280-byte datagram is 98.4% over a single hop and 85.2% over 10 hops.

   Considering that the IPv6 minimum MTU is 1280 bytes and that a
   802.15.4 frame can limit the MAC payload to as little as 74 bytes, a
   packet might be fragmented into at least 18 fragments at the 6LoWPAM
   shim layer.  Taking into account the worst-case header overhead for
   6LoWPAN Fragmentation and Mesh Addressing headers will increase the
   number of required fragments to around 32.  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



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   techniques compound that risk over multiple hops.


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.

   Minimum acknowledgement overhead

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

      The recovery mechanism should be able to acknowledge multiple
      fragments in a single message and not require an acknowledgement
      at all if fragments are already protected at a lower layer.

   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 congestion 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.





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5.  Overview

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

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

   Because a meshed LoWPAN might deliver frames out of order, it is
   virtually impossible to differentiate these situations.  In other
   words, from the sender standpoint, all outstanding fragments might
   still be in the network and contribute to its congestion.  There is
   an assumption, though, that after a certain amount of time, a frame
   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.


6.  New Dispatch types and headers

   This specification extends "Transmission of IPv6 Packets over IEEE
   802.15.4 Networks" [RFC4944] with 4 new dispatch types, for
   Recoverable Fragments (RFRAG) headers with or without Acknowledgment
   Request, and for the Acknowledgment back.

            Pattern    Header Type
          +------------+-----------------------------------------------+
          | 11  101000 | RFRAG      - Recoverable Fragment             |
          | 11  101001 | RFRAG-AR   - RFRAG with Ack Request           |
          | 11  10101x | RFRAG-ACK  - RFRAG Acknowledgment             |
          +------------+-----------------------------------------------+

             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
   changed from [RFC4944] Section 5.3.  "Fragmentation Type and Header."
   The size and offset are expressed on the compressed packet as opposed
   to the uncompressed form.




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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 Acknowledgment from the receiver

   Sequence

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

6.2.  Fragment Acknowledgement Dispatch type and Header

   The specification also defines an acknowledgement bitmap that is used
   to carry selective acknowlegements for the received fragments.  A
   given offset in the bitmap maps one to one with a given sequence
   number.

   The bitmap is compressed as a variable length field formed by control
   bits and acknowledgement bits.  The leftmost bits of the compressed
   form are control bits up to the first 0.  The rest is ack bits
   encoded right to left:

            Pattern                                   Size      Ack
          +--------------------------------------+----------+----------+
          | 0XXXXXXX                             | 1 octet  |  1 -> 7  |
          | 10XXXXXX XXXXXXXX                    | 2 octets |  1 -> 14 |
          | 110XXXXX XXXXXXXX XXXXXXXX           | 3 octets |  1 -> 21 |
          | 1110XXXX XXXXXXXX XXXXXXXX XXXXXXXX  | 4 octets |  1 -> 28 |
          +------------+-----------------------------------------------+

           Figure 3: Compressed acknowledgement bitmap encoding

   The highest sequence number to be acknowledged determines the pattern
   to be used.  The format can be extended for more fragments in the
   future but this specification only requires the support of up to 4
   octets encoding, which enables to acknowledge up to 28 fragments.



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   A 32 bits uncompressed bitmap is obtained by prepending zeroes to the
   XXX in the pattern above.  For instance:

        0 1 2 3 4 5 6 7
       +-+-+-+-+-+-+-+-+
       |0|1|1|0|1|1|1|1|  is expanded as:
       +-+-+-+-+-+-+-+-+
                            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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|1|1|0|1|1|1|1|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 4: Expanding 1 octet encoding

   and
                                     1                   2
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |1|1|0|1|1|1|1|0|1|1|1|1|1|1|1|1|1|1|1|1|1|0|0|1| is expanded as:
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                            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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0|0|0|0|0|0|0|0|0|0|0|1|1|1|1|0|1|1|1|1|1|1|1|1|1|1|1|1|1|0|0|1|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 5: Expanding 3 octets encoding

   whereas the 4 octets encoding is expanded by simply setting the first
   3 bits to 0.  The 32 bits uncompressed bitmap is written and read as
   follows:

                            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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Acknowledgment Bitmap                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                  ^                   ^
            bitmap indicating whether:            |                   |
         Fragment with sequence 10 was received --+                   |
         Fragment with sequence 00 was received ----------------------+

                    Figure 6: Expanded bitmap encoding

   So in the example in Figure 5 it appears that all fragments from
   sequence 0 to 20 were received but for sequence 1, 2 and 16 that were
   either lost or are still in the network over a slower path.



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   The compressed form of the acknowledgement bitmap is carried in a
   Fragment Acknowledgement as follows:

                            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          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Compressed Acknowledgment Bitmap (8 to 32 bits)
       +-+-+-+-+-+-+-+-+-+ ....

        Figure 7: Fragment Acknowledgement Dispatch type and Header

   Y bit

      Reserved for Explicit Congestion Notification (ECN) signalling

   Compressed Acknowledgement Bitmap

      An encoded form of an acknowledgement bitmap.


7.  Fragments Recovery

   The Recoverable Fragments header RFRAG and RFRAG-AR deprecate the
   original fragment headers from [RFC4944] and replace them in the
   fragmented packets.  The Fragment Acknowledgement RFRAG-ACK is
   introduced as a standalone header in message that is sent back to the
   fragment source endpoint as known by its MAC address.  This assumes
   that the source MAC address in the fragment (is any) and datagram_tag
   are enough information to send the Fragment Acknowledgement back to
   the source fragmentation endpoint.

   The node that fragments the packets at 6LoWPAN level (the sender)
   controls the Fragment Acknowledgements.  If may do that at any
   fragment to implement its own policy or perform congestion control
   which is out of scope for this document.  When the sender of the
   fragment knows that an underlying mechanism protects the Fragments
   already it MAY refrain from using the Acknowledgement mechanism, and
   never set the Ack Requested bit.  The node that recomposes the
   packets at 6LoWPAN level (the receiver) MUST acknowledge the
   fragments it has received when asked to, and MAY slightly defer that
   acknowledgement.

   The sender transfers a controlled number of fragments and MAY flag
   the last fragment of a series with an acknowledgment request.  The
   received MUST acknowledge a fragment with the acknowledgment request
   bit set.  If any fragment immediately preceding an acknowledgment



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   request is still missing, the receiver MAY intentionally delay its
   acknowledgment to allow in-transit fragments to arrive. delaying the
   acknowledgement might defeat the round trip delay computation so it
   should be configurable and not enabled by default.

   The receiver interacts with the sender using an Acknowledgment
   message with a bitmap that indicates which fragments were actually
   received.  The bitmap is a 32bit SWORD, 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.  All
   zeroes is a NULL bitmap that indicates that the fragmentation process
   was cancelled by the receiver for that datagram.

   The receiver MAY issue unsolicited acknowledgments.  An unsolicited
   acknowledgment enables the sender endpoint to resume sending if it
   had reached its maximum number of outstanding fragments or indicate
   that the receiver has cancelled the process of an individual
   datagram.  Note that acknowledgments might consume precious resources
   so the use of unsolicited acknowledgments should be configurable and
   not enabled by default.

   The sender arms a retry timer to cover the fragment that carries the
   Acknowledgment request.  Upon time out, the sender assumes that all
   the fragments on the way are received or lost.  The process must have
   completed within an acceptable time that is within the boundaries of
   upper layer retries.  The method detailed in [RFC2988] is recommended
   for the computation of the retry timer.  It is expected that the
   upper layer retries obey the same or friendly rules in which case a
   single round of fragment recovery should fit within the upper layer
   recovery timers.

   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.

   When the sender decides that a packet should be dropped and the
   fragmentation process canceled, 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.  If an
   acknowledgement is requested, the receiver responds with a NULL
   bitmap.

   The receiver might need to cancel the process of a fragmented packet
   for internal reasons, for instance if it is out of recomposition



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   buffers, or considers that this packet is already fully recomposed
   and passed to the upper layer.  In that case, the receiver SHOULD
   indicate so to the sender with a NULL bitmap.  Upon an
   acknowledgement with a NULL bitmap, the sender MUST drop the
   datagram.


8.  Forwarding Fragments

   This specification enables intermediate routers to forward fragments
   with no intermediate reconstruction of the entire packet.  Upon the
   first fragment, the routers lay an label along the path that is
   followed by that fragment (that is IP routed), and all further
   fragments are label switched along that path.  As a consequence,
   alternate routes not possible for individual fragments.  The datagram
   tag is used to carry the label, that is swapped at each hop.

8.1.  Upon the first fragment

   In route over the L2 source changes at each hop.  The label that is
   formed adnd placed in the datagram tag is associated to the source
   MAC and only valid (and unique) for that source MAC.  Say the first
   fragment has:

      Source IPv6 address = IP_A (maybe hops away)

      Destination IPv6 address = IP_B (maybe hops away)

      Source MAC = MAC_prv (prv as previous)

      Datagram_tag= DT_prv

   The intermediate router that forwards individual fragments does the
   following:

      a route lookup to get Next hop IPv6 towards IP_B, which resolves
      as IP_nxt (nxt as next)

      a ND resolution to get the MAC address associated to IP_nxt, which
      resolves as MAC_nxt

   Since it is a first fragment of a packet from that source MAC address
   MAC_prv for that tag DT_prv, the router:

      cleans up any leftover resource associated to the tupple (MAC_prv,
      DT_prv)





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      allocates a new label for that flow, DT_nxt, from a Least Recently
      Used pool or some siumilar procedure.

      allocates a Label swap structure indexed by (MAC_prv, DT_prv) that
      contains (MAC_nxt, DT_nxt)

      allocates a Label swap structure indexed by (MAC_nxt, DT_nxt) that
      contains (MAC_prv, DT_prv)

      swaps the MAC info to from self to MAC_nxt

      Swaps the datagram_tag to DT_nxt

   At this point the router is all set and can forward the packet to
   nxt.

8.2.  Upon the next fragments

   Upon next fragments (that are not first fragment), the router expects
   to have already Label swap structure indexed by (MAC_prv, DT_prv).
   The router:

      lookups up the Label swap entry for (MAC_prv, DT_prv), which
      resolves as (MAC_nxt, DT_nxt)

      swaps the MAC info to from self to MAC_nxt;

      Swaps the datagram_tag to DT_nxt

   At this point the router is all set and can forward the packet to
   nxt.

   if the Label swap entry for (MAC_src, DT_src) is not found, the
   router builds an RFRAG-ACK to indicate the error.  The acknowledgment
   message has the following information:

      MAC info set to from self to MAC_prv as found in the fragment

      Swaps the datagram_tag set to DT_prv

      Bitmap of all zeroes to indicate the error

   At this point the router is all set and can send the RFRAG-ACK back
   ot the previous router.







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8.3.  Upon the fragment acknowledgements

   Upon fragment acknowledgements next fragments (that are not first
   fragment), the router expects to have already Label swap structure
   indexed by (MAC_nxt, DT_nxt).  The router:

      lookups up the Label swap entry for (MAC_nxt, DT_nxt), which
      resolves as (MAC_prv, DT_prv)

      swaps the MAC info to from self to MAC_prv;

      Swaps the datagram_tag to DT_prv

   At this point the router is all set and can forward the RFRAG-ACK to
   prv.

   if the Label swap entry for (MAC_nxt, DT_nxt) is not found, it simply
   drops the packet.

   if the RFRAG-ACK indicates either an error or that the fragment was
   fully receive, the router schedules the Label swap entries for
   recycling.  If the RFRAG-ACK is lost on the way back, the source may
   retry the last fragments, which will result as an error RFRAG-ACK
   from the first router on the way that has already cleaned up.


9.  Security Considerations

   The process of recovering fragments does not appear to create any
   opening for new threat compared to "Transmission of IPv6 Packets over
   IEEE 802.15.4 Networks" [RFC4944].


10.  IANA Considerations

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


11.  Acknowledgments

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


12.  References





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12.1.  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.

12.2.  Informative References

   [I-D.ietf-roll-rpl]
              Winter, T., Thubert, P., and R. Team, "RPL: IPv6 Routing
              Protocol for Low power and Lossy Networks",
              draft-ietf-roll-rpl-08 (work in progress), May 2010.

   [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.

   [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.

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

   [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
              for Application Designers", BCP 145, RFC 5405,
              November 2008.













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Authors' Addresses

   Pascal Thubert (editor)
   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


   Jonathan W. Hui
   Arch Rock Corporation
   501 2nd St. Ste. 410
   San Francisco, California  94107
   USA

   Phone: +415 692 0828
   Email: jhui@archrock.com





























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