6LoWPAN Selective Fragment Recovery

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6lo                                                      P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Updates: 4944 (if approved)                             18 February 2020
Intended status: Standards Track                                        
Expires: 21 August 2020

                  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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://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 21 August 2020.

Copyright Notice

   Copyright (c) 2020 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 (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  BCP 14  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  References  . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  New Terms . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Updating RFC 4944 . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Extending draft-ietf-6lo-minimal-fragment . . . . . . . . . .   6
     4.1.  Slack in the First Fragment . . . . . . . . . . . . . . .   7
     4.2.  Gap between frames  . . . . . . . . . . . . . . . . . . .   7
     4.3.  Modifying the First Fragment  . . . . . . . . . . . . . .   7
   5.  New Dispatch types and headers  . . . . . . . . . . . . . . .   8
     5.1.  Recoverable Fragment Dispatch type and Header . . . . . .   8
     5.2.  RFRAG Acknowledgment Dispatch type and Header . . . . . .  11
   6.  Fragment Recovery . . . . . . . . . . . . . . . . . . . . . .  12
     6.1.  Forwarding Fragments  . . . . . . . . . . . . . . . . . .  14
       6.1.1.  Receiving the first fragment  . . . . . . . . . . . .  15
       6.1.2.  Receiving the next fragments  . . . . . . . . . . . .  15
     6.2.  Receiving RFRAG Acknowledgments . . . . . . . . . . . . .  16
     6.3.  Aborting the Transmission of a Fragmented Packet  . . . .  16
     6.4.  Applying Recoverable Fragmentation along a Diverse
           Path  . . . . . . . . . . . . . . . . . . . . . . . . . .  17
   7.  Management Considerations . . . . . . . . . . . . . . . . . .  18
     7.1.  Protocol Parameters . . . . . . . . . . . . . . . . . . .  18
     7.2.  Observing the network . . . . . . . . . . . . . . . . . .  20
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  21
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  22
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  22
   12. Informative References  . . . . . . . . . . . . . . . . . . .  23
   Appendix A.  Rationale  . . . . . . . . . . . . . . . . . . . . .  26
   Appendix B.  Requirements . . . . . . . . . . . . . . . . . . . .  27
   Appendix C.  Considerations on Flow Control . . . . . . . . . . .  28
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Introduction

   In most Low Power and Lossy Network (LLN) applications, the bulk of
   the traffic consists of small chunks of data (on the order of a 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 a payload of 74 bytes or
   more, 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 the firmware upgrade of the LLN nodes
   or the extraction of logs from LLN nodes.  In the former case, the
   large chunk of data is transferred to the LLN node, whereas in the

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   latter, the large chunk flows away from the LLN node.  In both cases,
   the size can be on the order of 10 kilobytes 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
   LLNs.  One critical issue with this original design is that routing
   an IPv6 [RFC8200] packet across a route-over mesh requires
   reassembling 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 using a
   fragment forwarding (FF) technique to alleviate those undesirable
   effects.  "LLN Minimal Fragment Forwarding"
   [I-D.ietf-6lo-minimal-fragment] specifies the general behavior that
   all FF techniques including this specification follow, and presents
   the associated caveats.  In particular, the routing information is
   fully indicated in the first fragment, which is always forwarded
   first.  A state is formed and used to forward all the next fragments
   along the same path.  The Datagram_Tag is locally significant to the
   Layer-2 source of the packet and is swapped at each hop.

   "Virtual reassembly buffers in 6LoWPAN"
   [I-D.ietf-lwig-6lowpan-virtual-reassembly] (VRB) proposes a FF
   technique 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 inherent fragility of fragmentation (see
   [I-D.ietf-intarea-frag-fragile]) in particular the issues of
   resources locked on the receiver and the wasted transmissions due to
   the loss of a single fragment in a whole datagram.  [Kent] compares
   the unreliable delivery of fragments with a mechanism it calls
   "selective acknowledgements" that recovers the loss of a fragment
   individually.  The paper illustrates the benefits that can be derived
   from such a method in figures 1, 2 and 3, on pages 6 and 7.
   [RFC4944] has no selective recovery and the whole datagram fails when
   one fragment is not delivered to the destination 6LoWPAN endpoint.
   Constrained memory resources are blocked on the receiver until the
   receiver times out, possibly causing the loss of subsequent packets
   that cannot be received for the lack of buffers.

   That problem is exacerbated when forwarding fragments over multiple
   hops since a loss at an intermediate hop will not be discovered by
   either the source or the destination, and the source will keep on
   sending fragments, wasting even more resources in the network and
   possibly contributing to the condition that caused the loss to no
   avail since the datagram cannot arrive in its entirety.  RFC 4944 is
   also missing signaling to abort a multi-fragment transmission at any
   time and from either end, and, if the capability to forward fragments

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   is implemented, clean up the related state in the network.  It is
   also lacking flow control capabilities to avoid participating in
   congestion that may in turn cause the loss of a fragment and
   potentially the retransmission of the full datagram.

   This specification provides a method to forward fragments across a
   multi-hop route-over mesh, and a selective acknowledgment to recover
   individual fragments between 6LoWPAN endpoints.  The method is
   designed to limit congestion loss in the network and addresses the
   requirements that are detailed in Appendix B.

2.  Terminology

2.1.  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.

2.2.  References

   In this document, readers will encounter terms and concepts that are
   discussed in "Problem Statement and Requirements for IPv6 over
   Low-Power Wireless Personal Area Network (6LoWPAN) Routing" [RFC6606]

   "LLN Minimal Fragment Forwarding" [I-D.ietf-6lo-minimal-fragment]
   introduces the generic concept of a Virtual Reassembly Buffer (VRB)
   and specifies behaviours and caveats that are common to a large
   family of FF techniques including this, which fully inherits from
   that specification.

   Past experience with fragmentation has shown that misassociated or
   lost fragments can lead to poor network behavior and, occasionally,
   trouble at the 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"

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   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' along a Label Switched Path (LSP).  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.

2.3.  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.

   Compressed Form:  This specification uses the generic term Compressed
      Form to refer to the format of a datagram after the action of
      [RFC6282] and possibly [RFC8138] for RPL [RFC6550] artifacts.

   Datagram_Size:  The size of the datagram in its Compressed Form
      before it is fragmented.  The Datagram_Size is expressed in a unit
      that depends on the MAC layer technology, by default a byte.

   Datagram_Tag:  An identifier of a datagram that is locally unique to
      the Layer-2 sender.  Associated with the MAC address of the
      sender, this becomes a globally unique identifier for the

   Fragment_Offset:  The offset of a particular fragment of a datagram
      in its Compressed Form.  The Fragment_Offset is expressed in a
      unit that depends on the MAC layer technology and is by default a

   RFRAG:  Recoverable Fragment

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   RFRAG-ACK:  Recoverable Fragment Acknowledgement

   RFRAG Acknowledgment Request:  An RFRAG with the Acknowledgement
      Request flag ('X' flag) set.

   NULL bitmap:  Refers to a bitmap with all bits set to zero.

   FULL bitmap:  Refers to a bitmap with all bits set to one.

   Forward:  The direction of a LSP path, followed by the RFRAG.

   Reverse:  The reverse direction of a LSP path, taken by the RFRAG-

3.  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 fragments is introduced and new
   dispatch types are defined in Section 5.

   [RFC8138] allows modifying the size of a packet en route by removing
   the consumed hops in a compressed Routing Header.  This requires that
   Fragment_Offset and Datagram_Size (see Section 2.3) are also modified
   en route, which is difficult to do in the uncompressed form.  This
   specification expresses those fields in the Compressed Form and
   allows modifying them en route (see Section 4.3) easily.

   Note that consistent with Section 2 of [RFC6282], for the
   fragmentation mechanism described in Section 5.3 of [RFC4944], any
   header that cannot fit within the first fragment MUST NOT be
   compressed when using the fragmentation mechanism described in this

4.  Extending draft-ietf-6lo-minimal-fragment

   This specification implements the generic FF technique specified in
   "LLN Minimal Fragment Forwarding" [I-D.ietf-6lo-minimal-fragment] in
   a fashion that enables end-to-end recovery of fragments and some
   degree of flow control.

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4.1.  Slack in the First Fragment

   [I-D.ietf-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 happening 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 Interface ID (IID) of the source IPv6 address is
   elided by the originator on the first hop because it matches the
   source MAC address, but cannot be on the next hops because the source
   MAC address changes.

   This specification cannot allow this operation since fragments are
   recovered end-to-end based on a sequence number.  This means that the
   fragments that contain a 6LoWPAN-compressed header MUST have enough
   slack to enable a less efficient compression in the next hops that
   still fits in one MAC frame.  For instance, if the IID of the source
   IPv6 address is elided by the originator, then it MUST compute the
   Fragment_Size as if the MTU was 8 bytes less.  This way, the next hop
   can restore the source IID to the first fragment without impacting
   the second fragment.

4.2.  Gap between frames

   This specification introduces a concept of an inter-frame gap, which
   is a configurable interval of time between transmissions to the same
   next hop.  In the case of half duplex interfaces, this inter-frame
   gap ensures that the next hop has completed processing of the
   previous frame and is capable of receiving the next one.

   In the case of a mesh operating at a single frequency with
   omnidirectional antennas, a larger inter-frame gap is required to
   protect the frame against hidden terminal collisions with the
   previous frame of the same flow that is still progressing along a
   common path.

   The inter-frame gap is useful even for unfragmented datagrams, but it
   becomes a necessity for fragments that are typically generated in a
   fast sequence and are all sent over the exact same path.

4.3.  Modifying the First Fragment

   The compression of the Hop Limit, of the source and destination
   addresses in the IPv6 Header, and of the Routing Header may change en
   route in a Route-Over mesh LLN.  If the size of the first fragment is
   modified, then the intermediate node MUST adapt the Datagram_Size to
   reflect that difference.

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   The intermediate node MUST also save the difference of Datagram_Size
   of the first fragment in the VRB and add it to the Datagram_Size and
   to the Fragment_Offset of all the subsequent fragments for that

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 to
   forward individual fragments across a 6LoWPAN route-over mesh without
   reassembly at each hop.  The Datagram_Tag is used as a label; it is
   locally unique to the node that owns 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 chosen Datagram_Tag stays
   unique to the particular datagram for the lifetime of that datagram.
   The result is 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

   This specification extends RFC 4944 [RFC4944] with 2 new Dispatch
   types, for Recoverable Fragment (RFRAG) and for the RFRAG
   Acknowledgment back.  The new 6LoWPAN Dispatch types are taken from
   Page 0 [RFC8025] as indicated in Table 1 in Section 9.

   In the following sections, a "Datagram_Tag" extends the semantics
   defined in [RFC4944] Section 5.3."Fragmentation Type and Header".
   The Datagram_Tag is a locally unique identifier for the datagram from
   the perspective of the sender.  This means that the Datagram_Tag
   identifies a datagram uniquely in the network when associated with
   the source of the datagram.  As the datagram gets forwarded, the
   source changes and the Datagram_Tag must be swapped as detailed in

5.1.  Recoverable Fragment Dispatch type and Header

   In this specification, if the packet is compressed then the size and
   offset of the fragments are expressed with respect to the Compressed
   Form of the packet form as opposed to the uncompressed (native)
   packet form.

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   The format of the fragment header is shown in Figure 1.  It is the
   same for all fragments.  The format has a length and an offset, as
   well as a Sequence field.  This would be redundant if the offset was
   computed as the product of the Sequence by the length, but this is
   not the case.  The position of a fragment in the reassembly buffer is
   neither correlated with the value of the Sequence field nor with the
   order in which the fragments are received.  This enables out-of-
   sequence subfragmenting, e.g., a fragment seq. 5 that is retried end-
   to-end as smaller fragments seq. 5, 13 and 14 due to a change of MTU
   along the path between the 6LoWPAN endpoints.

                             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|E|  Datagram_Tag |
       |X| Sequence|   Fragment_Size   |       Fragment_Offset         |

                                                X set == Ack-Request

                  Figure 1: RFRAG Dispatch type and Header

   There is no requirement on the receiver to check for contiguity of
   the received fragments.  The sender knows that the datagram is fully
   received when the acknowledged fragments cover the whole datagram.
   This may be useful in particular in the case where the MTU changes
   and a fragment Sequence is retried with a smaller Fragment_Size, the
   remainder of the original fragment being retried with new Sequence

   The first fragment is recognized by a Sequence of 0; it carries its
   Fragment_Size and the Datagram_Size of the compressed packet before
   it is fragmented, 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.

   X:  1 bit; Ack-Request: 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:  10-bit unsigned integer; the size of this fragment in
      a unit that depends on the MAC layer technology.  Unless
      overridden by a more specific specification, that unit is the
      octet, which allows fragments up to 1024 bytes.

   Datagram_Tag:  8 bits; an identifier of the datagram that is locally
      unique to the sender.

   Sequence:  5-bit unsigned integer; the sequence number of the
      fragment in the acknowledgement bitmap.  Fragments are numbered
      [0..N] where N is in [0..31].  A Sequence of 0 indicates the first
      fragment in a datagram, but non-zero values are not indicative of
      the position in the reassembly buffer.

   Fragment_Offset:  16-bit unsigned integer.

      When the Fragment_Offset is set to a non-0 value, its semantics
      depend on the value of the Sequence field as follows:

      *  For a first fragment (i.e., with a Sequence of 0), this field
         indicates the Datagram_Size of the compressed datagram, to help
         the receiver allocate an adapted buffer for the reception and
         reassembly operations.  The fragment may be stored for local
         reassembly.  Alternatively, it may be routed based on the
         destination IPv6 address.  In that case, a VRB state must be
         installed as described in Section 6.1.1.
      *  When the Sequence is not 0, this field indicates the offset of
         the fragment in the Compressed Form of the datagram.  The
         fragment may be added to a local reassembly buffer or forwarded
         based on an existing VRB as described in Section 6.1.2.

      A Fragment_Offset that is set to a value of 0 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

      *  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 6.1.2, 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 6.1.1, but no state is conserved
         afterwards.  In that case, the session if it exists is aborted
         and the packet is also forwarded in an attempt to clean up the
         next hops along the path indicated by the IPv6 header (possibly
         including a routing header).

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      If the fragment cannot be forwarded or routed, then an abort
      RFRAG-ACK is sent back to the source as described in
      Section 6.1.2.

5.2.  RFRAG Acknowledgment Dispatch type and Header

   This specification also defines a 4-octet RFRAG Acknowledgment bitmap
   that is used by the reassembling endpoint to confirm selectively the
   reception of individual fragments.  A given offset in the bitmap maps
   one-to-one with a given sequence number and 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 9 was received
        +----------------------- Fragment with Sequence 0 was received

               Figure 2: RFRAG Acknowledgment Bitmap Encoding

   Figure 3 shows an example Acknowledgment bitmap which indicates that
   all fragments from Sequence 0 to 20 were received, except for
   fragments 1, 2 and 16 were lost and must be retried.

                            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 3: Example RFRAG Acknowledgment Bitmap

   The RFRAG Acknowledgment Bitmap is included in an 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|E|  Datagram_Tag |
       |          RFRAG Acknowledgment Bitmap (32 bits)                |

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          Figure 4: RFRAG Acknowledgment Dispatch type and Header

   E:  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.  More in Appendix C.

   RFRAG Acknowledgment Bitmap:  An RFRAG Acknowledgment Bitmap, whereby
      setting the bit at offset x indicates that fragment x was
      received, as shown in Figure 2.  A NULL bitmap indicates that the
      fragmentation process is aborted.  A FULL bitmap indicates that
      the fragmentation process is complete; all fragments were received
      at the reassembly endpoint.

6.  Fragment Recovery

   The Recoverable Fragment header RFRAG is used to transport a fragment
   and optionally request an RFRAG Acknowledgment that will confirm the
   good reception of one or more fragments.  An RFRAG Acknowledgment is
   carried as a standalone fragment header (i.e., with no 6LoWPAN
   payload) in a message that is propagated back to the 6LoWPAN endpoint
   that was the originator of the fragments.  To achieve this, each hop
   that performed an MPLS-like operation on fragments reverses that
   operation for the RFRAG_ACK by sending a frame from the next hop to
   the previous hop as known by its MAC address in the VRB.  The
   Datagram_Tag in the RFRAG_ACK is unique to the receiver and is enough
   information for an intermediate hop to locate the VRB that contains
   the Datagram_Tag used by the previous hop and the Layer-2 information
   associated with it (interface and MAC address).

   The 6LoWPAN endpoint that fragments the packets at the 6LoWPAN level
   (the sender) also controls the number of acknowledgments by setting
   the Ack-Request flag in the RFRAG packets.  The sender may set the
   Ack-Request 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.  The maximum
   number of outstanding fragments is controlled by the Window-Size.  It
   is configurable and may vary in case of ECN notification.  When the
   6LoWPAN endpoint that reassembles the packets at the 6LoWPAN level
   (the receiver) receives a fragment with the Ack-Request flag set, it
   MUST send an RFRAG Acknowledgment back to the originator to confirm
   reception of all the fragments it has received so far.

   The Ack-Request ('X') set in an RFRAG marks the end of a window.
   This flag MUST be set on the last fragment if the sender wishes to

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   protect the datagram, and it MAY be set in any intermediate fragment
   for the purpose of flow control.

   This automatic repeat request (ARQ) process MUST be protected by a
   Retransmission Time Out (RTO) timer, and the fragment that carries
   the 'X' flag MAY be retried upon a time out for a configurable number
   of times (see Section 7.1).  Upon exhaustion of the retries the
   sender may either abort the transmission of the datagram or retry the
   datagram from the first fragment with an 'X' flag set in order to
   reestablish a path and discover which fragments were received over
   the old path in the acknowledgment bitmap.  When the sender of the
   fragment knows that an underlying link-layer mechanism protects the
   fragments, it may refrain from using the RFRAG Acknowledgment
   mechanism, and never set the Ack-Request bit.

   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 the sender that the reassembling
   endpoint aborted the processing of an individual datagram.

   The RFRAG Acknowledgment can optionally carry an ECN indication for
   flow control (see Appendix C).  The receiver of a fragment with the
   'E' (ECN) flag set MUST echo that information by setting the 'E'
   (ECN) flag in the next RFRAG Acknowledgment.

   In order to protect the datagram, the sender transfers a controlled
   number of fragments and flags the last fragment of a window with an
   RFRAG Acknowledgment Request.  The receiver 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.  Because it might defeat the round-trip
   delay computation, delaying the acknowledgment should be configurable
   and not enabled by default.

   When enough fragments are received to cover the whole datagram, the
   receiving endpoint reconstructs the packet, passes it to the upper
   layer, sends an RFRAG Acknowledgment on the reverse path with a FULL
   bitmap, and arms a short timer, e.g., on the order of an average
   round-trip delay in the network.  As the timer runs, the receiving
   endpoint absorbs the fragments that were still in flight for that
   datagram without creating a new state.  The receiving endpoint aborts
   the communication if it keeps going on beyond the duration of the

   Note that acknowledgments might consume precious resources so the use

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   of unsolicited acknowledgments should be configurable and not enabled
   by default.

   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
   [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 retried.

   When a single frequency is used by contiguous hops, the sender should
   insert a delay between the frames (e.g., carrying fragments) that are
   sent to the same next hop.  The delay should cover multiple
   transmissions so as to let a frame 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], where nodes that communicate at Layer-2 are scheduled to
   send and receive respectively, and different hops operate on
   different channels.

6.1.  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.  It inherits operations like Datagram_Tag
   switching and using a timer to clean the VRB once the traffic ceases.
   The first fragment carries the IP header and it is routed all the way
   from the fragmenting endpoint to the reassembling endpoint.  Upon
   receiving 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, the next fragments can
   only follow the path that was set up by the first fragment and cannot
   follow an alternate route.  The Datagram_Tag is used to carry the
   label, which is swapped in each hop.  All fragments follow the same
   path and fragments are delivered in the order in which they are sent.

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6.1.1.  Receiving the first fragment

   In Route-Over mode, the source and destination MAC addresses in a
   frame change at each hop.  The label that is formed and placed in the
   Datagram_Tag is associated with the source MAC address and only valid
   (and unique) for that source MAC address.  Upon receiving the first
   fragment (i.e., with a Sequence of zero), an intermediate router
   creates a VRB and the associated LSP state 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.ietf-6lo-minimal-fragment], where the receiving
   endpoint allocates a reassembly buffer.

   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 hop and the swapped
   Datagram_Tag.  This reverse LSP state also points at the VRB and
   enables matching 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.

   The first fragment may be received a second time, indicating that it
   did not reach the destination and was retried.  In that case, it
   SHOULD follow the same path as the first occurrence.  It is up to
   sending endpoint to determine whether to abort a transmission and
   then retry it from scratch, which may build an entirely new path.

6.1.2.  Receiving the next fragments

   Upon receiving the next fragment (i.e., with a non-zero Sequence), an
   intermediate 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:

   *  The source and destination MAC addresses are swapped from those
      found in the fragment
   *  The Datagram_Tag is set to the Datagram_Tag found in the fragment
   *  A NULL bitmap is used to signal the abort condition

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   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.

   [I-D.ietf-6lo-minimal-fragment] indicates that the receiving endpoint
   stores "the actual packet data from the fragments received so far, in
   a form that makes it possible to detect when the whole packet has
   been received and can be processed or forwarded".  How this is
   computed is implementation specific but relies on receiving all the
   bytes up to the Datagram_Size indicated in the first fragment.  An
   implementation may receive overlapping fragments as the result of
   retries after an MTU change.

6.2.  Receiving RFRAG Acknowledgments

   Upon receipt of 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.ietf-6lo-minimal-fragment], but using the reverse LSP so that
   the RFRAG-ACK flows back to the sender endpoint.

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

   Either way, if the RFRAG-ACK indicates that the fragment was entirely
   received (FULL bitmap), it arms a short timer, and upon timeout, the
   VRB and all the associated state are destroyed.  Until the timer
   elapses, fragments of that datagram may still be received, e.g. if
   the RFRAG-ACK was lost on the way back and the source retried the
   last fragment.  In that case, the router forwards the fragment
   according to the state in the VRB.

   This specification does not provide a method to discover the number
   of hops or the minimal value of MTU along those hops.  But should the
   minimal MTU decrease, it is possible to retry a long fragment (say
   Sequence of 5) with first a shorter fragment of the same Sequence (5
   again) and then one or more other fragments with a Sequence that was
   not used before (e.g., 13 and 14).  Note that Path MTU Discovery is
   out of scope for this document.

6.3.  Aborting the Transmission of a Fragmented Packet

   A reset is signaled on the forward path with a pseudo fragment that
   has the Fragment_Offset, Sequence, and Fragment_Size all set to 0,
   and no data.

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   When the sender or a router on the way decides that a packet should
   be dropped and the fragmentation process aborted, it generates a
   reset pseudo fragment and forwards it down the fragment path.

   Each router next along the path the way forwards the pseudo fragment
   based on the VRB state.  If an acknowledgment is not requested, the
   VRB and all associated state are destroyed.

   Upon reception of the pseudo fragment, the receiver cleans up all
   resources for the packet associated with the Datagram_Tag.  If an
   acknowledgment is requested, the receiver responds with a NULL

   The other way around, the receiver might need to abort the process of
   a fragmented packet for internal reasons, for instance if it is out
   of reassembly buffers, already uses all 256 possible values of the
   Datagram_Tag, or if it keeps receiving fragments beyond a reasonable
   time while it considers that this packet is already fully reassembled
   and was passed to the upper layer.  In that case, the receiver SHOULD
   indicate so to the sender with a NULL bitmap in an RFRAG
   Acknowledgment.  The RFRAG Acknowledgment is forwarded all the way
   back to the source of the packet and cleans up all resources on the
   way.  Upon an acknowledgment with a NULL bitmap, the sender endpoint
   MUST abort the transmission of the fragmented datagram with one
   exception: In the particular case of the first fragment, it MAY
   decide to retry via an alternate next hop instead.

6.4.  Applying Recoverable Fragmentation along a Diverse Path

   The text above can be read with the assumption of a serial path
   between a source and a destination.  Section 4.5.3 of the "6TiSCH
   Architecture" [I-D.ietf-6tisch-architecture] defines the concept of a
   Track that can be a complex path between a source and a destination
   with Packet ARQ, Replication, Elimination and Overhearing (PAREO)
   along the Track.  This specification can be used along any subset of
   the complex Track where the first fragment is flooded.  The last
   RFRAG Acknowledgment is flooded on that same subset in the reverse
   direction.  Intermediate RFRAG Acknowledgments can be flooded on any
   sub-subset of that reverse subset that reach back to the source.

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7.  Management Considerations

   This specification extends "On Forwarding 6LoWPAN Fragments over a
   Multihop IPv6 Network" [I-D.ietf-6lo-minimal-fragment] and requires
   the same parameters in the receiver and on intermediate nodes.  There
   is no new parameter as echoing ECN is always on.  These parameters
   typically include the reassembly time-out at the receiver and an
   inactivity clean-up timer on the intermediate nodes, and the number
   of messages that can be processed in parallel in all nodes.

   The configuration settings introduced by this specification only
   apply to the sender, which is in full control of the transmission.
   LLNs vary a lot in size (there can be thousands of nodes in a mesh),
   in speed (from 10 Kbps to several Mbps at the PHY layer), in traffic
   density, and in optimizations that are desired (e.g., the selection
   of a RPL [RFC6550] Objective Function [RFC6552] impacts the shape of
   the routing graph).

   For that reason, only a very generic guidance can be given on the
   settings of the sender and on whether complex algorithms are needed
   to perform flow control or estimate the round-trip time.  To cover
   the most complex use cases, this specification enables the sender to
   vary the fragment size, the window size, and the inter-frame gap,
   based on the number of losses, the observed variations of the round-
   trip time and the setting of the ECN bit.

7.1.  Protocol Parameters

   The management system SHOULD be capable of providing the parameters
   listed in this section.

   An implementation must control the rate at which it sends packets
   over the same path to allow the next hop to forward a packet before
   it gets the next.  In a wireless network that uses the same frequency
   along a path, more time must be inserted to avoid hidden terminal
   issues between fragments (more in Section 4.2).

   This is controlled by the following parameter:

   inter-frame gap:  Indicates the minimum amount of time between
      transmissions.  The inter-frame gap protects the propagation of
      one transmission before the next one is triggered and creates a
      duty cycle that controls the ratio of air time and memory in
      intermediate nodes that a particular datagram will use.

   An implementation should consider the generic recommendations from
   the IETF in the matter of flow control and rate management in
   [RFC5033].  To control the flow, an implementation may use a dynamic

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   value of the window size (Window_Size), adapt the fragment size
   (Fragment_Size), and insert an inter-frame gap that is longer than
   necessary.  In a large network where nodes contend for the bandwidth,
   a larger Fragment_Size consumes less bandwidth but also reduces
   fluidity and incurs higher chances of loss in transmission.  This is
   controlled by the following parameters:

   MinFragmentSize:  The MinFragmentSize is the minimum value for the

   OptFragmentSize:  The OptFragmentSize is the value for the
      Fragment_Size that the sender should use to start with.  It is
      greater than or equal to MinFragmentSize.  It is less than or
      equal to MaxFragmentSize.  For the first fragment, it must account
      for the expansion of the IPv6 addresses and of the Hop Limit field
      within MTU.  For all fragments, it is a balance between the
      expected fluidity and the overhead of MAC and 6LoWPAN headers.
      For a small MTU, the idea is to keep it close to the maximum,
      whereas for larger MTUs, it might makes sense to keep it short
      enough, so that the duty cycle of the transmitter is bounded,
      e.g., to transmit at least 10 frames per second.

   MaxFragmentSize:  The MaxFragmentSize is the maximum value for the
      Fragment_Size.  It MUST be lower than the minimum MTU along the
      path.  A large value augments the chances of buffer bloat and
      transmission loss.  The value MUST be less than 512 if the unit
      that is defined for the PHY layer is the octet.

   MinWindowSize:  The minimum value of Window_Size that the sender can

   OptWindowSize:  The OptWindowSize is the value for the Window_Size
      that the sender should use to start with.  It is greater than or
      equal to MinWindowSize.  It is less than or equal to
      MaxWindowSize.  The Window_Size should be maintained below the
      number of hops in the path of the fragment to avoid stacking
      fragments at the bottleneck on the path.  If an inter-frame gap is
      used to avoid interference between fragments then the Window_Size
      should be at most on the order of the estimation of the trip time
      divided by the inter-frame gap.

   MaxWindowSize:  The maximum value of Window_Size that the sender can
      use.  The value MUST be less than 32.

   An implementation may perform its estimate of the RTO or use a
   configured one.  The ARQ process is controlled by the following

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   MinARQTimeOut:  The maximum amount of time a node should wait for an
      RFRAG Acknowledgment before it takes the next action.

   OptARQTimeOut:  The initial value of the RTO, which is the amount of
      time that a sender should wait for an RFRAG Acknowledgment before
      it takes the next action.  It is greater than or equal to
      MinARQTimeOut.  It is less than or equal to MaxARQTimeOut.  See
      Appendix C for recommendations on computing the round-trip time.

   MaxARQTimeOut:  The maximum amount of time a node should wait for the
      RFRAG Acknowledgment before it takes the next action.  It must
      cover the longest expected round-trip time, and be several times
      less than the time-out that covers the recomposition buffer at the
      receiver, which is typically on the order of the minute.

   MaxFragRetries:  The maximum number of retries for a particular

   MaxDatagramRetries:  The maximum number of retries from scratch for a
      particular datagram.

   An implementation may be capable of performing flow control based on
   ECN; see in Appendix C.  This is controlled by the following

   UseECN:  Indicates whether the sender should react to ECN.  The
      sender may react to ECN by varying the Window_Size between
      MinWindowSize and MaxWindowSize, varying the Fragment_Size between
      MinFragmentSize and MaxFragmentSize, and/or by increasing the
      inter-frame gap.

7.2.  Observing the network

   The management system should monitor the number of retries and of ECN
   settings that can be observed from the perspective of both the sender
   and the receiver, and may tune the optimum size of Fragment_Size and
   of Window_Size, OptFragmentSize, and OptWindowSize, respectively, at
   the sender.  The values should be bounded by the expected number of
   hops and reduced beyond that when the number of datagrams that can
   traverse an intermediate point may exceed its capacity and cause a
   congestion loss.  The inter-frame gap is another tool that can be
   used to increase the spacing between fragments of the same datagram
   and reduce the ratio of time when a particular intermediate node
   holds a fragment of that datagram.

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8.  Security Considerations

   This document specifies an instantiation of a 6LoWPAN Fragment
   Forwarding technique.  [I-D.ietf-6lo-minimal-fragment] provides the
   generic description of Fragment Forwarding and this specification
   inherits from it.  The generic considerations in the Security
   sections of [I-D.ietf-6lo-minimal-fragment] apply equally to this

   This specification does not recommend a particular algorithm for the
   estimation of the duration of the RTO that covers the detection of
   the loss of a fragment with the 'X' flag set; regardless, an attacker
   on the path may slow down or discard packets, which in turn can
   affect the throughput of fragmented packets.

   Compared to "Transmission of IPv6 Packets over IEEE 802.15.4
   Networks" [RFC4944], this specification reduces the Datagram_Tag to 8
   bits and the tag wraps faster than with [RFC4944].  But for a
   constrained network where a node is expected to be able to hold only
   one or a few large packets in memory, 256 is still a large number.
   Also, the acknowledgement mechanism allows cleaning up the state
   rapidly once the packet is fully transmitted or aborted.

   The abstract Virtual Recovery Buffer inherited from
   [I-D.ietf-6lo-minimal-fragment] may be used to perform a Denial-of-
   Service (DoS) attack against the intermediate Routers since the
   routers need to maintain a state per flow.  The particular VRB
   implementation technique described in
   [I-D.ietf-lwig-6lowpan-virtual-reassembly] allows realigning which
   data goes in which fragment, which causes the intermediate node to
   store a portion of the data, which adds an attack vector that is not
   present with this specification.  With this specification, the data
   that is transported in each fragment is conserved and the state to
   keep does not include any data that would not fit in the previous

9.  IANA Considerations

   This document allocates 2 patterns for a total of 4 dispatch values
   in Page 0 for recoverable fragments from the "Dispatch Type Field"
   registry that was created by "Transmission of IPv6 Packets over IEEE
   802.15.4 Networks" [RFC4944] and reformatted by "6LoWPAN Paging
   Dispatch" [RFC8025].

   The suggested patterns (to be confirmed by IANA) are indicated in
   Table 1.

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   | Bit Pattern | Page |           Header Type            | Reference |
   | 11 10100x   | 0    | RFRAG - Recoverable Fragment     | THIS RFC  |
   | 11 10100x   | 1-14 | Unassigned                       |           |
   | 11 10100x   | 15   | Reserved for Experimental Use    | RFC 8025  |
   | 11 10101x   | 0    | RFRAG-ACK - RFRAG                | THIS RFC  |
   |             |      | Acknowledgment                   |           |
   | 11 10101x   | 1-14 | Unassigned                       |           |
   | 11 10101x   | 15   | Reserved for Experimental Use    | RFC 8025  |

              Table 1: Additional Dispatch Value Bit Patterns

10.  Acknowledgments

   The author wishes to thank Michel Veillette, Dario Tedeschi, Laurent
   Toutain, Carles Gomez Montenegro, Thomas Watteyne, and Michael
   Richardson for in-depth reviews and comments.  Also many thanks to
   Roman Danyliw, Peter Yee, Colin Perkins, Tirumaleswar Reddy Konda,
   and Erik Nordmark for their careful reviews and for helping through
   the IETF Last Call and IESG review process, and to Jonathan Hui, Jay
   Werb, Christos Polyzois, Soumitri Kolavennu, Pat Kinney, Margaret
   Wasserman, Richard Kelsey, Carsten Bormann, and Harry Courtice for
   their various contributions in the long process that lead to this

11.  Normative References

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

   [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,

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   [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,

   [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,

   [RFC8138]  Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie,
              "IPv6 over Low-Power Wireless Personal Area Network
              (6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138,
              April 2017, <https://www.rfc-editor.org/info/rfc8138>.

   [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>.

              Watteyne, T., Thubert, P., and C. Bormann, "On Forwarding
              6LoWPAN Fragments over a Multihop IPv6 Network", Work in
              Progress, Internet-Draft, draft-ietf-6lo-minimal-fragment-
              10, 1 February 2020, <https://tools.ietf.org/html/draft-

12.  Informative References

   [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,

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

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

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

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

   [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,

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

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,

   [RFC6552]  Thubert, P., Ed., "Objective Function Zero for the Routing
              Protocol for Low-Power and Lossy Networks (RPL)",
              RFC 6552, DOI 10.17487/RFC6552, March 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,

   [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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   [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,

   [RFC5033]  Floyd, S. and M. Allman, "Specifying New Congestion
              Control Algorithms", BCP 133, RFC 5033,
              DOI 10.17487/RFC5033, August 2007,

   [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,

              Bormann, C. and T. Watteyne, "Virtual reassembly buffers
              in 6LoWPAN", Work in Progress, Internet-Draft, draft-ietf-
              lwig-6lowpan-virtual-reassembly-01, 11 March 2019,

              Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
              and F. Gont, "IP Fragmentation Considered Fragile", Work
              in Progress, Internet-Draft, draft-ietf-intarea-frag-
              fragile-17, 30 September 2019,

              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", Work in Progress, Internet-Draft,
              draft-ietf-6tisch-architecture-28, 29 October 2019,

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

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   [Kent]     Kent, C. and J. Mogul, ""Fragmentation Considered
              Harmful", In Proc. SIGCOMM '87 Workshop on Frontiers in
              Computer Communications Technology",
              DOI 10.1145/55483.55524, August 1987,

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

                       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, the lack of recovery in the
   original fragmentation system of RFC 4944 implies that all fragments
   would need to be resent, further contributing to the congestion that

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   caused the initial loss, and potentially leading to congestion

   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 an 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 a data fragment.

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      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 Protocols.

   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 controlling
   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 4 in Section 5.2.

   It must be noted that congestion and collision are different topics.
   In particular, when a mesh operates on the same channel over multiple
   hops, then the forwarding of a fragment over a certain hop may
   collide with the forwarding of the next fragment that is following
   over a previous hop but in the same interference domain.  This draft
   enables 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

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   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 "Computing
   TCP's Retransmission Timer" [RFC6298] is recommended for that

   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 number 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

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

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

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