6lo                                                      P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Updates: 4944 (if approved)                                June 27, 2018
Intended status: Standards Track
Expires: December 29, 2018

                  6LoWPAN Selective Fragment Recovery


   This draft updates RFC 4944 with a simple protocol to recover
   individual fragments across a route-over mesh network, with a minimal
   flow control to protect the network against bloat.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on December 29, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Updating RFC 4944 . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Updating draft-watteyne-6lo-minimal-fragment  . . . . . . . .   4
   4.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  BCP 14  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  References  . . . . . . . . . . . . . . . . . . . . . . .   4
     4.3.  6LoWPAN Acronyms  . . . . . . . . . . . . . . . . . . . .   4
     4.4.  Referenced Work . . . . . . . . . . . . . . . . . . . . .   5
     4.5.  New Terms . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  New Dispatch types and headers  . . . . . . . . . . . . . . .   6
     5.1.  Recoverable Fragment Dispatch type and Header . . . . . .   7
     5.2.  RFRAG Acknowledgment Dispatch type and Header . . . . . .   9
   6.  Fragments Recovery  . . . . . . . . . . . . . . . . . . . . .  10
   7.  Forwarding Fragments  . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Upon the first fragment . . . . . . . . . . . . . . . . .  12
     7.2.  Upon the next fragments . . . . . . . . . . . . . . . . .  13
     7.3.  Upon the RFRAG Acknowledgments  . . . . . . . . . . . . .  13
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     11.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  Rationale  . . . . . . . . . . . . . . . . . . . . .  17
   Appendix B.  Requirements . . . . . . . . . . . . . . . . . . . .  18
   Appendix C.  Considerations On Flow Control . . . . . . . . . . .  19
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  20

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 10Kbytes or more and an end-to-end
   reliable transport is required.

   "Transmission of IPv6 Packets over IEEE 802.15.4 Networks" [RFC4944]
   defines the original 6LoWPAN datagram fragmentation mechanism for

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   LLNs.  One critical issue with this original design is that routing
   an IPv6 [RFC8200] 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.  The "6TiSCH
   Architecture" [I-D.ietf-6tisch-architecture] recommends to use a hop-
   by-hop fragment forwarding technique to alleviate those undesirable
   effects.  "LLN Minimal Fragment Forwarding"
   [I-D.watteyne-6lo-minimal-fragment] proposes such a technique, in a
   fashion that is compatible with [RFC4944] 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 remnant state in the network.

   Another issue against [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 updates the fragmentation mechanism that is
   specified in "Transmission of IPv6 Packets over IEEE 802.15.4
   Networks" [RFC4944] for use in route-over LLNs by providing a model
   where fragments can be forwarded end-to-end across a 6LoWPAN LLN, and
   where fragments that are lost on the way can be recovered
   individually.  A new format for fragment is introduces and new
   dispatch types are defined in Section 5.

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3.  Updating draft-watteyne-6lo-minimal-fragment

   This specification updates the fragment forwarding mechanism
   specified in "LLN Minimal Fragment Forwarding"
   [I-D.watteyne-6lo-minimal-fragment] by providing additional events to
   iprove the management of the Virtual Reassembly Buffer (VRB).

   At the time of this writing, [I-D.watteyne-6lo-minimal-fragment]
   allows for refragmenting in intermediate nodes, meaning that some
   bytes from a given fragment may be left in the VRB to be added to the
   next fragment.  The reason for this to happen would be the need for
   space in the outgoing fragment that was not needed in the incoming
   fragment, for instance because the 6LoWPAN Header Compression is not
   as efficient on the outgoing link, e.g., if the IID of the source
   IPv6 address is elided on the first hop because it matches the MAC
   address, but cannot be on the next hops.  This specification does not
   allow this since fragments are recovered end-to-end.  This means that
   the fragments that contain 6LoWPAN-compressed data must have enough
   slack in them to enable a lesser efficient compression in the next
   hops to still fit in one frame.

4.  Terminology

4.1.  BCP 14

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119][RFC8174] when, and only when, they appear in all
   capitals, as shown here.

4.2.  References

   In this document, readers will encounter terms and concepts that are
   discussed in the following documents:

   o  "Problem Statement and Requirements for IPv6 over Low-Power
      Wireless Personal Area Network (6LoWPAN) Routing" [RFC6606]

4.3.  6LoWPAN Acronyms

   This document uses the following acronyms:

   6BBR: 6LoWPAN Backbone Router

   6LBR: 6LoWPAN Border Router

   6LN:  6LoWPAN Node

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   6LR:  6LoWPAN Router

   LLN:  Low-Power and Lossy Network

4.4.  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 "IPv4
   Reassembly Errors at High Data Rates" [RFC4963] and follow the
   references for more information.

   That experience led to the definition of "Path MTU discovery"
   [RFC8201] (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"
   [RFC3031]: 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".  The
   MPLS technique is leveraged in the present specification to forward
   fragments that actually do not have a network layer header, since the
   fragmentation occurs below IP.

   "LLN Minimal Fragment Forwarding" [I-D.watteyne-6lo-minimal-fragment]
   introduces the concept of a Virtual Reassembly Buffer (VRB) and an
   associated technique to forward fragments as they come, using the
   datagram_tag as a label in a fashion similar to MLPS.  This
   specification reuses that technique with slightly modified controls.

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4.5.  New Terms

   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.

5.  New Dispatch types and headers

   This specification enables the 6LoWPAN fragmentation sublayer to
   provide an MTU up to 2048 bytes to the upper layer, which can be the
   6LoWPAN Header Compression sublayer 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 in
   order to forward individual fragments across a 6LoWPAN route-over
   mesh.  The datagram_tag is used as a label; it is locally unique to
   the node that is the source MAC address of the fragment, so together
   the MAC address and the label can identify the fragment globally.  A
   node may build the datagram_tag in its own locally-significant way,
   as long as the selected tag stays unique to the particular datagram
   for the lifetime of that datagram.  It results that the label does
   not need to be globally unique but also that it must be swapped at
   each hop as the source MAC address changes.

   This specification extends RFC 4944 [RFC4944] with 4 new Dispatch
   types, for Recoverable Fragment (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 from page 0 [RFC8025], as follows:

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              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-ECHO  - RFRAG Ack with ECN Echo    |

             Figure 1: Additional Dispatch Value Bit Patterns

   In the following sections, the semantics of "datagram_tag" are
   unchanged from [RFC4944] Section 5.3.  "Fragmentation Type and
   Header." and is compatible with the fragment forwarding operation
   described in [I-D.watteyne-6lo-minimal-fragment].

5.1.  Recoverable Fragment Dispatch type and Header

   In this specification, the size and offset of the fragments are
   expressed on the compressed packet form 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.

   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|E|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.

   E: 1 bit; Explicit Congestion Notification; the "E" flag is reset by
      the source of the fragment and set by intermediate routers to
      signal that this fragment experienced congestion along its path.

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   Fragment_size:  7 bit unsigned integer; the size of this fragment in
      a unit that depends on the MAC layer technology.  For IEEE Std.
      802.15.4, the unit is octet, and the maximum fragment size, which
      is constrained by the maximum frame size of 128 octet minus the
      overheads of the MAC and Fragment Headers, is not limited by this

   Sequence:  5 bit unsigned integer; the sequence number of the
      fragment.  Fragments are sequence numbered [0..N] where N is in
      [0..31].  A sequence of 0 indicates the first fragment in a
      datagram.  For IEEE Std. 802.15.4, as long as the overheads enable
      a fragment size of 64 octets or more, this enables to fragment a
      packet of 2047 octets.

   Fragment_offset:  11 bit unsigned integer;

      *  When set to a non-0 value, the semantics of the Fragment_offset
         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.  The fragment should
            be forwarded based on an existing VRB as described in
            Section 7.2, or silently dropped if none is found.

         +  For a first fragment (i.e. with a sequence of 0), 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.  This format limits
            the maximum MTU on a 6LoWPAN link to 2047 bytes, but 1280
            bytes is the recommended value to avoid issues with IPV6
            Path MTU Discovery [RFC8201].  The fragment should be routed
            based on the destination IPv6 address, and an VRB state
            should be installed as described in Section 7.1.

      *  When set to 0, this field indicates an abort condition and all
         state regarding the datagram should be cleaned up once the
         processing of the fragment is complete; the processing of the
         fragment depends on whether there is a VRB already established
         for this datagram, and the next hop is still reachable:

         +  if a VRB already exists and is not broken, the fragment is
            to be forwarded along the associated Label Switched Path
            (LSP) as described in Section 7.2, but regardless of the
            value of the Sequence field;

         +  else, if the Sequence is 0, then the fragment is to be
            routed as described in Section 7.1 but no state is conserved

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         If the fragment cannot be forwarded or routed, then an abort
         RFRAG-ACK is sent back to the source.

5.2.  RFRAG Acknowledgment Dispatch type and Header

   This specification also defines a 4-octet RFRAG Acknowledgment bitmap
   that is used by the reassembling end point 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:

                            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:

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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
                      |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 Echo

      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

      An RFRAG Acknowledgment Bitmap, whereby setting the bit at offset
      x indicates that fragment x was received, as shown in Figure 3.
      All 0's is a NULL bitmap that indicates that the fragmentation
      process is aborted.  All 1's is a FULL bitmap that indicates that
      the fragmentation process is complete, all fragments were received
      at the reassembly end point.

6.  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 by reversing the MPLS operation.

   The 6LoWPAN endpoint that fragments the packets at 6LoWPAN level (the
   sender) also controls when the reassembling end point sends the RFRAG
   Acknowledgments by setting the Ack Requested flag in the RFRAG
   packets.  It may set the Ack Requested flag on any fragment to
   perform congestion control by limiting the number of outstanding
   fragments, which are the fragments that have been sent but for which
   reception or loss was not positively confirmed by the reassembling
   endpoint.  When the sender of the fragment knows that an underlying
   link-layer mechanism protects the Fragments, it may refrain from

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   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.

   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 MAY issue unsolicited acknowledgments.  An unsolicited
   acknowledgment signals to the sender endpoint that it can resume
   sending if it had reached its maximum number of outstanding
   fragments.  Another use is to inform that the reassembling endpoint
   has canceled 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

   An observation is that streamlining forwarding of fragments generally
   reduces the latency over the LLN mesh, providing room for retries
   within existing upper-layer reliability mechanisms.  The sender
   protects the transmission over the LLN mesh with a retry timer that
   is computed according to the method detailed in [RFC6298].  It is
   expected that the upper layer retries obey the recommendations in
   "UDP Usage Guidelines" [RFC8085], 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 a single frequency is used by contiguous hops, the sender should
   wait a reasonable amount of time between fragments so as to let a
   fragment progress a few hops and avoid hidden terminal issues.  This
   precaution is not required on channel hopping technologies such as
   Time Slotted CHannel Hopping (TSCH) [RFC6554]

   When the sender decides that a packet should be dropped and the
   fragmentation process canceled, it sends a pseudo fragment with the

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   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 in a RFRAG
   Acknowledgment.  Upon an acknowledgment with a NULL bitmap, the
   sender endpoint MUST abort the transmission of the fragmented

7.  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.

   This specification extends the Virtual Reassembly Buffer (VRB)
   technique to forward fragments with no intermediate reconstruction of
   the entire packet.  The first fragment carries the IP header and it
   is routed all the way from the fragmenting end point to the
   reassembling end point.  Upon the first fragment, the routers along
   the path install a label-switched path (LSP), and the following
   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.
   All fragments follow the same path and fragments are delivered in the
   order at which they are sent.

7.1.  Upon the first fragment

   In Route-Over mode, the source and destination MAC addressed in a
   frame change 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.  Upon a first fragment (i.e. with a
   sequence of zero), a VRB and the associated LSP state are created for
   the tuple (source MAC address, datagram_tag) and the fragment is
   forwarded along the IPv6 route that matches the destination IPv6
   address in the IPv6 header as prescribed by
   [I-D.watteyne-6lo-minimal-fragment].  The LSP state enables to match
   the (previous MAC address, datagram_tag) in an incoming fragment to
   the tuple (next MAC address, swapped datagram_tag) used in the
   forwarded fragment and points at the VRB.  In addition, the router
   also forms a Reverse LSP state indexed by the MAC address of the next

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   hop and the swapped datagram_tag.  This reverse LSP state also points
   at the VRB and enables to match the (next MAC address,
   swapped_datagram_tag) found in an RFRAG Acknowledgment to the tuple
   (previous MAC address, datagram_tag) used when forwarding a Fragment
   Acknowledgment (RFRAG-ACK) back to the sender endpoint.

7.2.  Upon the next fragments

   Upon a next fragment (i.e. with a non-zero sequence), the router
   looks up a LSP indexed by the tuple (MAC address, datagram_tag) found
   in the fragment.  If it is found, the router forwards the fragment
   using the associated VRB as prescribed by

   if the VRB for the tuple is not found, the router builds an RFRAG-ACK
   to abort the transmission of the packet.  The resulting message has
   the following information:

   o  The source and destination MAC addresses are swapped from those
      found in the fragment

   o  The datagram_tag set to the datagram_tag found in the fragment

   o  A null bitmap is used to signal the abort condition

   At this point the router is all set and can send the RFRAG-ACK back
   to the previous router.  The RFRAG-ACK should normally be forwarded
   all the way to the source using the reverse LSP state in the VRBs in
   the intermediate routers as described in the next section.

7.3.  Upon the RFRAG Acknowledgments

   Upon an RFRAG-ACK, the router looks up a Reverse LSP indexed by the
   tuple (MAC address, datagram_tag), which are respectively the source
   MAC address of the received frame and the received datagram_tag.  If
   it is found, the router forwards the fragment using the associated
   VRB as prescribed by [I-D.watteyne-6lo-minimal-fragment], but using
   the Reverse LSP so that the RFRAG-ACK flows back to the sender

   If the Reverse LSP is not found, the router MUST silently drop the
   RFRAG-ACK message.

   Either way, if the RFRAG-ACK indicates either an error (NULL bitmap)
   or that the fragment was entirely received (FULL bitmap), arms a
   short timer, and upon timeout, the VRB and all associate state are
   destroyed.  During that time, fragments of that datagram may still be
   received, e.g. if the RFRAG-ACK was lost on the way back and the

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   source retried the last fragment.  In that case, the router sends an
   abort RFRAG-ACK along the Reverse LSP to complete the clean up.

8.  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].

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 Thomas Watteyne and Michael Richardson for
   in-depth reviews and comments.  Also many thanks to Jonathan Hui, Jay
   Werb, Christos Polyzois, Soumitri Kolavennu, Pat Kinney, Margaret
   Wasserman, Richard Kelsey, Carsten Bormann and Harry Courtice for
   their various contributions.

11.  References

11.1.  Normative References

              Watteyne, T., Bormann, C., and P. Thubert, "LLN Minimal
              Fragment Forwarding", draft-watteyne-6lo-minimal-
              fragment-01 (work in progress), March 2018.

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

   [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,

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   [RFC6554]  Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
              Routing Header for Source Routes with the Routing Protocol
              for Low-Power and Lossy Networks (RPL)", RFC 6554,
              DOI 10.17487/RFC6554, March 2012,

   [RFC8025]  Thubert, P., Ed. and R. Cragie, "IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Paging Dispatch",
              RFC 8025, DOI 10.17487/RFC8025, November 2016,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

11.2.  Informative References

              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", draft-ietf-6tisch-architecture-14 (work
              in progress), April 2018.

              IEEE, "IEEE Standard for Low-Rate Wireless Networks",
              IEEE Standard 802.15.4, DOI 10.1109/IEEE

   [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,

   [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,

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

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

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

   [RFC6606]  Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
              Statement and Requirements for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Routing",
              RFC 6606, DOI 10.17487/RFC6606, May 2012,

   [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,

   [RFC7567]  Baker, F., Ed. and G. Fairhurst, Ed., "IETF
              Recommendations Regarding Active Queue Management",
              BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

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

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

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   [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 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:

      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.

      Packages of Commands:  A number of commands or a full
         configuration can be 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.

   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, the lack of recovery in the
   original fragmentation system of RFC 4944 implies that all fragments

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   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 RFC 4944 defines an MTU is 1280 bytes and that in
   most incarnations (but 802.15.4g) a IEEE Std. 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
   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

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      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

   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 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 5.2.

   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

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   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 makes full sense to transmit the next fragment
   as soon as the previous fragment has had its chance to be forwarded
   at the next hop.

   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
   acknowledgment might be on the way back.  It is also possible that
   either the fragment or the acknowledgment was lost on the way.

   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 [RFC7567] 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 6 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.

Author's Address

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   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

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