6lo                                                     T. Watteyne, Ed.
Internet-Draft                                            Analog Devices
Intended status: Informational                                C. Bormann
Expires: January 22, 2020                        Universitaet Bremen TZI
                                                              P. Thubert
                                                           July 21, 2019

                      6LoWPAN Fragment Forwarding


   This document provides a simple method to forwarding 6LoWPAN
   fragments.  When employing adaptation layer fragmentation in 6LoWPAN,
   it may be beneficial for a forwarder not to have to reassemble each
   packet in its entirety before forwarding it.  This has always been
   possible with the original fragmentation design of RFC4944.  This
   method reduces the latency and increases end-to-end reliability in
   route-over forwarding.  It is the companion to the virtual Reassembly
   Buffer which is a pure implementation technique.

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   publication of this document.  Please review these documents
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Table of Contents

   1.  Overview of 6LoWPAN Fragmentation . . . . . . . . . . . . . .   2
   2.  Limits of Per-Hop Fragmentation and Reassembly  . . . . . . .   4
     2.1.  Latency . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Memory Management and Reliability . . . . . . . . . . . .   4
   3.  Virtual Reassembly Buffer (VRB) Implementation  . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Informative References  . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Overview of 6LoWPAN Fragmentation

   The original 6LoWPAN fragmentation is defined in [RFC4944] and it is
   implicitly defined for use over a single IP hop though possibly
   multiple Layer-2 hops in a meshed 6LoWPAN Network.  Although
   [RFC6282] updates [RFC4944], it does not redefine 6LoWPAN

   We use Figure 1 to illustrate 6LoWPAN fragmentation.  We assume node
   A forwards a packet to node B, possibly as part of a multi-hop route
   between IPv6 source and destination nodes which are neither A nor B.

                  +---+                     +---+
           ... ---| A |-------------------->| B |--- ...
                  +---+                     +---+
                                 # (frag. 5)

                123456789                 123456789
               +---------+               +---------+
               |   #  ###|               |###  #   |
               +---------+               +---------+
                  outgoing                incoming
             fragmentation                reassembly
                    buffer                buffer

         Figure 1: Fragmentation at node A, reassembly at node B.

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   Node A starts by compacting the IPv6 packet using the header
   compression mechanism defined in [RFC6282].  If the resulting 6LoWPAN
   packet does not fit into a single link-layer frame, node A's 6LoWPAN
   sublayer cuts it into multiple 6LoWPAN fragments, which it transmits
   as separate link-layer frames to node B.  Node B's 6LoWPAN sublayer
   reassembles these fragments, inflates the compressed header fields
   back to the original IPv6 header, and hands over the full IPv6 packet
   to its IPv6 layer.

   In Figure 1, a packet forwarded by node A to node B is cut into nine
   fragments, numbered 1 to 9.  Each fragment is represented by the '#'
   symbol.  Node A has sent fragments 1, 2, 3, 5, 6 to node B.  Node B
   has received fragments 1, 2, 3, 6 from node A.  Fragment 5 is still
   being transmitted at the link layer from node A to node B.

   Conceptually, a reassembly buffer for 6LoWPAN contains:

   o  a datagram_size,
   o  a datagram_tag, associated to the link-layer sender and receiver
      addresses to which the datagram_tag is local,
   o  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,
   o  a timer that allows discarding a partially reassembled packet
      after some timeout.

   A fragmentation header is added to each fragment; it indicates what
   portion of the packet that fragment corresponds to.  Section 5.3 of
   [RFC4944] defines the format of the header for the first and
   subsequent fragments.  All fragments are tagged with a 16-bit
   "datagram_tag", used to identify which packet each fragment belongs
   to.  Each datagram can be uniquely identified by the source and final
   destination link-layer addresses of the frame that carries it, the
   fragment size and the datagram_tag.  Each fragment can be identified
   within its datagram by the datagram-offset.

   Node B's typical behavior, per [RFC4944], is as follows.  Upon
   receiving a fragment from node A with a datagram_tag previously
   unseen from node A, node B allocates a buffer large enough to hold
   the entire packet.  The length of the packet is indicated in each
   fragment (the datagram_size field), so node B can allocate the buffer
   even if the first fragment it receives is not fragment 1.  As
   fragments come in, node B fills the buffer.  When all fragments have
   been received, node B inflates the compressed header fields into an
   IPv6 header, and hands the resulting IPv6 packet to the IPv6 layer.

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   This behavior typically results in per-hop fragmentation and
   reassembly.  That is, the packet is fully reassembled, then
   (re)fragmented, at every hop.

2.  Limits of Per-Hop Fragmentation and Reassembly

   There are at least 2 limits to doing per-hop fragmentation and
   reassembly.  See [ARTICLE] for detailed simulation results on both

2.1.  Latency

   When reassembling, a node needs to wait for all the fragments to be
   received before being able to generate the IPv6 packet, and possibly
   forward it to the next hop.  This repeats at every hop.

   This may result in increased end-to-end latency compared to a case
   where each fragment is forwarded without per-hop reassembly.

2.2.  Memory Management and Reliability

   Constrained nodes have limited memory.  Assuming 1 kB reassembly
   buffer, typical nodes only have enough memory for 1-3 reassembly

   To illustrate this we use the topology from Figure 2, where nodes A,
   B, C and D all send packets through node E.  We further assume that
   node E's memory can only hold 3 reassembly buffers.

                  +---+       +---+
          ... --->| A |------>| B |
                  +---+       +---+\
                                    +---+    +---+
                                    | E |--->| F | ...
                                    +---+    +---+
                  +---+       +---+
          ... --->| C |------>| D |
                  +---+       +---+

            Figure 2: Illustrating the Memory Management Issue.

   When nodes A, B and C concurrently send fragmented packets, all 3
   reassembly buffers in node E are occupied.  If, at that moment, node
   D also sends a fragmented packet, node E has no option but to drop
   one of the packets, lowering end-to-end reliability.

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3.  Virtual Reassembly Buffer (VRB) Implementation

   Virtual Reassembly Buffer (VRB) is the implementation technique
   described in [I-D.ietf-lwig-6lowpan-virtual-reassembly] in which a
   forwarder does not reassemble each packet in its entirety before
   forwarding it.

   VRB overcomes the limits listed in Section 2.  Nodes do not wait for
   the last fragment before forwarding, reducing end-to-end latency.
   Similarly, the memory footprint of VRB is just the VRB table,
   reducing the packet drop probability significantly.

   There are, however, limits:

   Non-zero Packet Drop Probability:  The abstract data in a VRB table
       entry contains at a minimum the MAC address of the predecessor
       and that of the successor, the datagram_tag used by the
       predecessor and the local datagram_tag that this node will swap
       with it.  The VRB may need to store a few octets from the last
       fragment that may not have fit within MTU and that will be
       prepended to the next fragment.  This yields a small footprint
       that is 2 orders of magnitude smaller compared to needing a
       1280-byte reassembly buffer for each packet.  Yet, the size of
       the VRB table necessarily remains finite.  In the extreme case
       where a node is required to concurrently forward more packets
       that it has entries in its VRB table, packets are dropped.
   No Fragment Recovery:  There is no mechanism in VRB for the node that
       reassembles a packet to request a single missing fragment.
       Dropping a fragment requires the whole packet to be resent.  This
       causes unnecessary traffic, as fragments are forwarded even when
       the destination node can never construct the original IPv6
   No Per-Fragment Routing:  All subsequent fragments follow the same
       sequence of hops from the source to the destination node as the
       first fragment, because the IP header is required to route the
       fragment and is only present in the first fragment.  A side
       effect is that the first fragment must always be forwarded first.

   The severity and occurrence of these limits depends on the link-layer
   used.  Whether these limits are acceptable depends entirely on the
   requirements the application places on the network.

   If the limits are present and not acceptable for the application,
   future specifications may define new protocols to overcome these
   limits.  One example is [I-D.ietf-6lo-fragment-recovery] which
   defines a protocol which allows fragment recovery.

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4.  Security Considerations

   An attacker can perform a Denial-of-Service (DoS) attack on a node
   implementing VRB by generating a large number of bogus "fragment 1"
   fragments without sending subsequent fragments.  This causes the VRB
   table to fill up.  Note that the VRB does not need to remember the
   full datagram as received so far but only possibly a few octets from
   the last fragment that could not fit in it.  It is expected that an
   implementation protects itself to keep the number of VRBs within
   capacity, and that old VRBs are protected by a timer of a reasonable
   duration for the technology and destroyed upon timeout.

   Secure joining and the link-layer security that it sets up protects
   against those attacks from network outsiders.

5.  IANA Considerations

   No requests to IANA are made by this document.

6.  Acknowledgments

   The authors would like to thank Yasuyuki Tanaka, for his in-depth
   review of this document.  Also many thanks to Georgies Papadopoulos
   and Dominique Barthel for their own reviews.

7.  Informative References

   [ARTICLE]  Tanaka, Y., Minet, P., and T. Watteyne, "6LoWPAN Fragment
              Forwarding", IEEE Communications Standards Magazine ,

              Thubert, P., "6LoWPAN Selective Fragment Recovery", draft-
              ietf-6lo-fragment-recovery-04 (work in progress), June

              Bormann, C. and T. Watteyne, "Virtual reassembly buffers
              in 6LoWPAN", draft-ietf-lwig-6lowpan-virtual-reassembly-01
              (work in progress), March 2019.

   [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,

Authors' Addresses

   Thomas Watteyne (editor)
   Analog Devices
   32990 Alvarado-Niles Road, Suite 910
   Union City, CA  94587

   Email: thomas.watteyne@analog.com

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359

   Email: cabo@tzi.org

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

   Email: pthubert@cisco.com

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