6lo                                                      P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Updates: 4944 (if approved)                                       J. Hui
Intended status: Standards Track                               Nest Labs
Expires: October 8, 2017                                   April 6, 2017

                  LLN Fragment Forwarding and Recovery


   Considering that an LLN link-layer frame can have a payload below 100
   bytes, an IPv6 packet might be fragmented more than 10 fragments at
   the 6LoWPAN layer.  In a 6LoWPAN mesh-under mesh network, the
   fragments can be forwarded individually across the mesh, whereas a
   route-over mesh network, a fragmented 6LoWPAN packet must be
   reassembled at every hop, which causes latency and congestion.  This
   draft introduces a simple protocol to forward individual fragments
   across a route-over mesh network, and, regardless of the type of
   mesh, recover the loss of individual fragments across the mesh and
   protect the network against bloat with a minimal flow control.

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 October 8, 2017.

Copyright Notice

   Copyright (c) 2017 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Updating RFC 4944 . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology and Referenced Work . . . . . . . . . . . . . . .   4
   4.  New Dispatch types and headers  . . . . . . . . . . . . . . .   5
     4.1.  Recoverable Fragment Dispatch type and Header . . . . . .   5
     4.2.  RFRAG Acknowledgment Dispatch type and Header . . . . . .   6
   5.  Fragments Recovery  . . . . . . . . . . . . . . . . . . . . .   8
   6.  Forwarding Fragments  . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Upon the first fragment . . . . . . . . . . . . . . . . .  10
     6.2.  Upon the next fragments . . . . . . . . . . . . . . . . .  11
     6.3.  Upon the RFRAG Acknowledgments  . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     10.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Appendix A.  Rationale  . . . . . . . . . . . . . . . . . . . . .  14
   Appendix B.  Requirements . . . . . . . . . . . . . . . . . . . .  16
   Appendix C.  Considerations On Flow Control . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   In most Low Power and Lossy Network (LLN) applications, the bulk of
   the traffic consists of small chunks of data (in the order few bytes
   to a few tens of bytes) at a time.  Given that an IEEE Std. 802.15.4
   [IEEE.802.15.4] frame can carry 74 bytes or more in all cases,
   fragmentation is usually not required.  However, and though this
   happens only occasionally, a number of mission critical applications
   do require the capability to transfer larger chunks of data, for
   instance to support a firmware upgrades of the LLN nodes or an
   extraction of logs from LLN nodes.  In the former case, the large
   chunk of data is transferred to the LLN node, whereas in the latter,
   the large chunk flows away from the LLN node.  In both cases, the
   size can be on the order of 10K bytes or more and an end-to-end
   reliable transport is required.

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   "Transmission of IPv6 Packets over IEEE 802.15.4 Networks" [RFC4944]
   defines the original 6LoWPAN datagram fragmentation mechanism for
   LLNs.  One critical issue with this original design is that routing
   an IPv6 packet across a route-over mesh requires to reassemble the
   full packet at each hop, which may cause latency along a path and an
   overall buffer bloat in the network.  Those undesirable effects can
   be alleviated by a hop-by-hop fragment forwarding technique such as
   the one proposed in this specification, and arguably this could be
   achieved without the need to define a new protocol.  However, adding
   that capability alone to the local implementation of the original
   6LoWPAN fragmentation would not address the bulk of the issues raised
   against it, and may create new issues like uncontrolled state in the

   Another issue against RFC 4944 [RFC4944] is that it does not define a
   mechanism to first discover the loss of a fragment along a multi-hop
   path (e.g. having exhausted the link-layer retries at some hop on the
   way), and then to recover that loss.  With RFC 4944, the forwarding
   of a whole datagram fails when one fragment is not delivered properly
   to the destination 6LoWPAN endpoint.  End-to-end transport or
   application-level mechanisms may require a full retransmission of the
   datagram, wasting resources in an already constrained network.

   In that situation, the source 6LoWPAN endpoint will not be aware that
   a loss occurred and will continue sending all fragments for a
   datagram that is already doomed.  The original support is missing
   signaling to abort a multi-fragment transmission at any time and from
   either end, and, if the capability to forward fragments is
   implemented, clean up the related state in the network.  It is also
   lacking flow control capabilities to avoid participating to a
   congestion that may in turn cause the loss of a fragment and trigger
   the retransmission of the full datagram.

   This specification proposes a method to forward fragments across a
   multi-hop route over mesh, and to recover individual fragments
   between LLN endpoints.  The method is designed to limit congestion
   loss in the network and addresses the requirements that are detailed
   in Appendix B.

2.  Updating RFC 4944

   This specification deprecates the fragmentation mechanism that is
   specified in RFC 4944 [RFC4944] and replaces it with a model where
   fragments can be forwarded end-to-end across a 6LoWPAN mesh network
   of any type, and where fragments that are lost on the way can be
   recovered individually.  New dispatch types are defined in Section 4.

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3.  Terminology and Referenced Work

   Past experience with fragmentation has shown that miss-associated or
   lost fragments can lead to poor network behavior and, occasionally,
   trouble at application layer.  The reader is encouraged to read RFC
   4963 [RFC4963] and follow the references for more information.

   That experience led to the definition of "Path MTU discovery"
   [RFC1191] (PMTUD) protocol that limits fragmentation over the

   Specifically in the case of UDP, valuable additional information can
   be found in "UDP Usage Guidelines for Application Designers"

   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].

   "The Benefits of Using Explicit Congestion Notification (ECN)"
   [RFC8087] provides useful information on the potential benefits and
   pitfalls of using ECN.

   Quoting the "Multiprotocol Label Switching (MPLS) Architecture"
   [RFC8087]: with MPLS, "packets are "labeled" before they are
   forwarded.  At subsequent hops, there is no further analysis of the
   packet's network layer header.  Rather, the label is used as an index
   into a table which specifies the next hop, and a new label".  That
   technique is leveraged in this specification to forward fragments
   that actually do not have a network layer header, since the
   fragmentation occurs below IP.

   This specification uses the following terms:

   6LoWPAN endpoints

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

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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4.  New Dispatch types and headers

   This specification aims at enabling to provide an MTU that is
   equivalent to 2048 bytes to the upper layer, which can be the 6LoWPAN
   Header Compression that is defined in the "Compression Format for
   IPv6 Datagrams" [RFC6282] specification.  In order to achieve this,
   this specification enables the fragmentation and the reliable
   transmission of fragments over a multihop 6LoWPAN mesh network.

   This specification provides a technique that is derived from MPLS and
   allows to forward fragments across a 6LoWPAN route-over mesh, but is
   not needed in the mesh-under case.  The datagram_tag is used as the
   label and is locally unique to the node that is the MAC-layer source
   of the fragment.  There is thus no need for a global registry of
   datagram_tags and a node may build the datagram_tag in its own
   locally-significant way, as long as the resulting tag stays unique to
   the particular datagram for the lifetime of that datagram.

   This specification extends RFC 4944 [RFC4944] with 4 new Dispatch
   types, for Recoverable Fragments (RFRAG) headers with or without
   Acknowledgment Request (RFRAG vs. RFRAG-ARQ), and for the RFRAG
   Acknowledgment back, with or without ECN Echo (RFRAG-ACK vs. RFRAG-

   (to be confirmed by IANA) The new 6LoWPAN Dispatch types use the
   Value Bit Pattern of 11 1010xx, as follows:

              Pattern    Header Type
            | 11  101000 | RFRAG      - Recoverable Fragment        |
            | 11  101001 | RFRAG-ARQ  - RFRAG with Ack Request      |
            | 11  101010 | RFRAG-ACK  - RFRAG Acknowledgment        |
            | 11  101011 | RFRAG-ECN  - RFRAG Ack with ECN Echo     |

             Figure 1: Additional Dispatch Value Bit Patterns

4.1.  Recoverable Fragment Dispatch type and Header

   In this specification, the size and offset of the fragments are
   expressed on the compressed packet per as opposed to the uncompressed
   - native packet - form.

   The first fragment is recognized by a sequence of 0; it carries its
   fragment_size and the datagram_size of the compressed packet, whereas
   the other fragments carry their fragment_size and fragment_offset.
   The last fragment for a datagram is recognized when its
   fragment_offset and its fragment_size add up to the datagram_size.

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   Recoverable Fragments are sequenced and a bitmap is used in the RFRAG
   Acknowledgment to indicate the received fragments by setting the
   individual bits that correspond to their sequence.

                            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|R|fragment_size|         datagram_tag          |
       |sequence |  fragment_offset    |
                                                  X set == Ack Requested

                 Figure 2: RFRAG Dispatch type and Header

   X: 1 bit; Ack Requested: when set, the sender requires an RFRAG
      Acknowledgment from the receiver.

   R: 1 bit; Reserved, MUST be set to 0 by the source and ignored by all

   Fragment_size:  7 bits unsigned integer.  The size of this fragment
      in units that depend on the MAC layer technology.  For IEEE Std.
      802.15.4, the unit is octet.

   Sequence:  5 bits unsigned integer; the sequence number of the
      fragment.  Fragments are sequence numbered [0..N] where N is in

   Fragment_offset:  10 bits unsigned integer; when set to 0, this field
      indicates an abort condition; else, its value depends on the value
      of the Sequence.  When the sequence is not 0, this field indicates
      the offset of the fragment in the compressed form.  When the
      sequence is 0, denoting the first fragment of a datagram, this
      field is overloaded to indicate the total_size of the compressed
      packet, to help the receiver allocate an adapted buffer for the
      reception and reassembly operations.

4.2.  RFRAG Acknowledgment Dispatch type and Header

   The specification also defines a 4-octet RFRAG Acknowledgment bitmap
   that is used to confirm selectively the reception of individual
   fragments.  A given offset in the bitmap maps one to one with a given
   sequence number.

   The offset of the bit in the bitmap indicates which fragment is
   acknowledged as follows:

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

              Figure 3: RFRAG Acknowledgment bitmap encoding

   Figure 4 shows an example Acknowledgment bitmap which indicates that
   all fragments from sequence 0 to 20 were received, except for
   fragments 1, 2 and 16 that were either lost or are still in the
   network over a slower path.

                            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

                   Figure 4: Expanding 3 octets encoding

   The RFRAG Acknowledgment Bitmap is included in a RFRAG Acknowledgment
   header, 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          |
       |          RFRAG Acknowledgment Bitmap (32 bits)                |

          Figure 5: RFRAG Acknowledgment Dispatch type and Header

   Y: 1 bit; Explicit Congestion Notification

      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.

   RFRAG Acknowledgment Bitmap

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      An RFRAG Acknowledgment Bitmap, whereby but at offset x indicates
      that fragment x was received.

5.  Fragments Recovery

   The Recoverable Fragment headers RFRAG and RFRAG-ARQ are used to
   transport a fragment and optionally request an RFRAG Acknowledgment
   that will confirm the good reception of a one or more fragments.  An
   RFRAG Acknowledgment can optionally carry an ECN indication; it is
   carried as a standalone header in a message that is sent back to the
   6LoWPAN endpoint that was the source of the fragments, as known by
   its MAC address.  The process ensures that at every hop, the source
   MAC address and the datagram_tag in the received fragment are enough
   information to send the RFRAG Acknowledgment back towards the source
   6LoWPAN endpoint.

   The 6LoWPAN endpoint that fragments the packets at 6LoWPAN level (the
   sender) also controls the RFRAG Acknowledgments by setting the Ack
   Requested flag in the RFRAG packets.  It may set the Ack Requested
   flag on any fragment so as to implement its own policy or perform
   congestion control by limiting the number of fragments in the air,
   IOW fragments that have been sent but for which reception or loss was
   not positively confirmed by the other 6LoWPAN endpoint.  When the
   sender of the fragment knows that an underlying link-layer mechanism
   protects the Fragments already it may refrain from using the RFRAG
   Acknowledgment mechanism, and never set the Ack Requested bit.  When
   it receives a fragment with the ACK Request flag set, the 6LoWPAN
   endpoint that reassembles the packets at 6LoWPAN level (the receiver)
   sends back an RFRAG Acknowledgment to confirm reception of all the
   fragments it has received so far, though it may slightly defer it to
   let additional packets in.

   The sender transfers a controlled number of fragments and MAY flag
   the last fragment of a series with an RFRAG Acknowledgment Request.
   The received MUST acknowledge a fragment with the acknowledgment
   request bit set.  If any fragment immediately preceding an
   acknowledgment request is still missing, the receiver MAY
   intentionally delay its acknowledgment to allow in-transit fragments
   to arrive. delaying the acknowledgment might defeat the round trip
   delay computation so it should be configurable and not enabled by

   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 bitstring (a DWORD), which
   accommodates up to 32 fragments and is sufficient to transport 2028
   bytes over an IEEE Std. 802.15.4 MAC payload.  For all n in [0..31],
   bit n is set to 1 in the bitmap to indicate that fragment with

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   sequence n was received, otherwise the bit is set to 0.  All 0s is a
   NULL bitmap that indicates that the fragmentation process was
   canceled 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 [RFC6298] 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
   fragment_offset, sequence and fragment_size all set to 0, 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
   acknowledgment is requested, the receiver responds with a NULL

   The receiver might need to cancel the process of a fragmented packet
   for internal reasons, for instance if it is out of reassembly
   buffers, or considers that this packet is already fully reassembled
   and passed to the upper layer.  In that case, the receiver SHOULD
   indicate so to the sender with a NULL bitmap.  Upon an acknowledgment
   with a NULL bitmap, the sender MUST drop the datagram.

6.  Forwarding Fragments

   It is assumed that the first Fragment is large enough to carry the
   IPv6 header and make routing decisions.  If that is not so, then this
   specification MUST NOT be used.

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   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.

6.1.  Upon the first fragment

   In Route-Over mode, the MAC address changes at each hop.  The label
   that is formed and 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

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

      a MAC address 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,

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

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      Swaps the datagram_tag to DT_nxt

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

6.2.  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

   if the Label swap entry for (MAC_prv, DT_prv) is not found, the
   router builds an RFRAG-ACK to indicate the error.  The resulting
   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 0es to indicate the error

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

6.3.  Upon the RFRAG Acknowledgments

   Upon an RFRAG Acknowledgment, the router expects to have already
   Label swap structure indexed by (MAC_nxt, DT_nxt), which are
   respectively the source MAC address of the received frame and the
   received datagram_tag.  DT_nxt should have been computed by this
   router and this router should have assigned it to this particular
   datagram.  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;

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      Swaps the datagram_tag to DT_prv

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

   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.

7.  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].

8.  IANA Considerations

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

9.  Acknowledgments

   The author wishes to thank Jay Werb, Christos Polyzois, Soumitri
   Kolavennu, Pat Kinney, Margaret Wasserman, Richard Kelsey, Carsten
   Bormann and Harry Courtice for their contributions and review.

10.  References

10.1.  Normative References

              IEEE, "IEEE Standard for Low-Rate Wireless Networks",
              IEEE Standard 802.15.4,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,

   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,

10.2.  Informative References

              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", draft-ietf-6tisch-architecture-11 (work
              in progress), January 2017.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,

   [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, DOI 10.17487/RFC2309, April 1998,

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, DOI 10.17487/RFC2914, September 2000,

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,

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   [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, DOI 10.17487/RFC4919, August 2007,

   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963,
              DOI 10.17487/RFC4963, July 2007,

   [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
              for Application Designers", RFC 5405,
              DOI 10.17487/RFC5405, November 2008,

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,

   [RFC7554]  Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
              IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
              Internet of Things (IoT): Problem Statement", RFC 7554,
              DOI 10.17487/RFC7554, May 2015,

   [RFC8087]  Fairhurst, G. and M. Welzl, "The Benefits of Using
              Explicit Congestion Notification (ECN)", RFC 8087,
              DOI 10.17487/RFC8087, March 2017,

Appendix A.  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 LLN; however, the associated
   functionality can be critical enough to justify extra care for
   ensuring effective transport of large packets across the LLN.

   The list of those usages includes:

   Towards the LLN 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 LLN 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 LLN nodes over a
         relatively short period of time.

   From the LLN 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:  LLN nodes may generate large logs of sampled data for
         later extraction.  LLN 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

   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.

   Considering that [RFC4944] defines an MTU is 1280 bytes and that in
   most incarnations (but 802.15.4G) a 802.15.4 frame can limit the MAC
   payload to as few as 74 bytes, a packet might be fragmented into at
   least 18 fragments at the 6LoWPAN 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 techniques compound that risk over
   multiple hops.

   Mechanisms such as TCP or application-layer segmentation could be
   used to support end-to-end reliable transport.  One option to support
   bulk data transfer over a frame-size-constrained LLN is to set the
   Maximum Segment Size to fit within the link maximum frame size.
   Doing so, however, can add significant header overhead to each

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   802.15.4 frame.  In addition, deploying such a mechanism requires
   that the end-to-end transport is aware of the delivery properties of
   the underlying LLN, which is a layer violation, and difficult to
   achieve from the far end of the IPv6 network.

Appendix B.  Requirements

   For one-hop communications, a number of Low Power and Lossy Network
   (LLN) link-layers propose a local acknowledgment mechanism that is
   enough to detect and recover the loss of fragments.  In a multihop
   environment, an end-to-end fragment recovery mechanism 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 that may be multiple hops away.  The method
   addresses the following requirements of a LLN:

   Number of fragments

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

   Minimum acknowledgment 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 new end-to-end fragment recovery mechanism should be able to
      acknowledge multiple fragments in a single message and not require
      an acknowledgment 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

   Support for out-of-order fragment delivery

      Forwarding over a mesh network with rerouting and load balancing
      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

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      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 LLN.

Appendix C.  Considerations On Flow Control

   Considering that a multi-hop LLN 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.

   Congestion on the forward path is assumed in case of packet loss, and
   packet loss is assumed upon time out.  The draft allows to control
   the number of outstanding fragments, that have been transmitted but
   for which an acknowledgment was not received yet.  It must be noted
   that the number of outstanding fragments should not exceed the number
   of hops in the network, but the way to figure the number of hops is
   out of scope for this document.

   Congestion on the forward path can also be indicated by an Explicit
   Congestion Notification (ECN) mechanism.  Though whether and how ECN
   [RFC3168] is carried out over the LoWPAN is out of scope, this draft
   provides a way for the destination endpoint to echo an ECN indication
   back to the source endpoint in an acknowledgment message as
   represented in Figure 5 in Section 4.2.

   It must be noted that congestion and collision are different topics.
   In particular, when a mesh operates on a same channel over multiple
   hops, then the forwarding of a fragment over a certain hop may
   collide with the forwarding of a next fragment that is following over
   a previous hop but in a same interference domain.  This draft enables
   an end-to-end flow control, but leaves it to the sender stack to pace
   individual fragments within a transmit window, so that a given
   fragment is sent only when the previous fragment has had a chance to
   progress beyond the interference domain of this hop.  In the case of
   6TiSCH [I-D.ietf-6tisch-architecture], which operates over the
   TimeSlotted Channel Hopping [RFC7554] (TSCH) mode of operation of
   IEEE802.14.5, a fragment is forwarded over a different channel at a
   different time and it make full sense to fire a next fragment as soon
   as the previous fragment has had its chance to be forwarded at the
   next hop, retry (ARQ) operations included.

   From the standpoint of a source 6LoWPAN 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

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   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 LLN 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 6LoWPAN endpoints.  The method detailed in [RFC6298] is
   recommended for that computation.

   The reader is encouraged to read through "Congestion Control
   Principles" [RFC2914].  Additionally [RFC2309] and [RFC5681] 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 fragments over a congested path
   by throttling the sources.

   Section 5 describes how the sender decides how many fragments are
   (re)sent before an acknowledgment is required, and how the sender
   adapts that number to the network conditions.

Authors' Addresses

   Pascal Thubert (editor)
   Cisco Systems, Inc
   Building D
   45 Allee des Ormes - BP1200
   MOUGINS - Sophia Antipolis  06254

   Phone: +33 497 23 26 34
   Email: pthubert@cisco.com

   Jonathan W. Hui
   Nest Labs
   3400 Hillview Ave
   Palo Alto, California  94304

   Email: jonhui@nestlabs.com

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