6lo                                                     T. Watteyne, Ed.
Internet-Draft                                            Analog Devices
Intended status: Informational                                C. Bormann
Expires: January 17, 2019                        Universitaet Bremen TZI
                                                              P. Thubert
                                                           July 16, 2018

                    LLN Minimal Fragment Forwarding


   This document gives an overview of LLN Minimal Fragment Forwarding.
   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 been always possible
   with the original fragmentation design of RFC4944.  This document is
   a companion document to [I-D.ietf-lwig-6lowpan-virtual-reassembly],
   which details the virtual Reassembly Buffer (VRB) implementation
   technique which reduces the latency and increases end-to-end
   reliability in route-over forwarding.

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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 . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Informative References  . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Overview of 6LoWPAN Fragmentation

   6LoWPAN fragmentation is defined in [RFC4944].  Although [RFC6282]
   updates [RFC4944], it does not redefine 6LoWPAN fragmentation.

   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.

   Node A starts by compacting the IPv6 packet using header compression
   defined in [RFC6282].  If the resulting 6LoWPAN packet does not fit
   into a single link-layer frame, node A's 6LoWPAN sublayer cuts it

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   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.

   A reassembly buffer for 6LoWPAN contains:

   o  datagram_size,
   o  datagram_tag and link-layer sender and receiver addresses (to
      which the datagram_tag is local),
   o  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 the partial packet after a 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 fragment can be uniquely identified by the source and
   destination link-layer addresses of the frame that carries it, and
   the datagram_tag.  The value of the datagram_tag only needs to be
   locally unique to nodes A and B.

   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.

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

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2.  Limits of Per-Hop Fragmentation and Reassembly

   There are at least 2 limits to doing per-hop fragmentation and

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 the case
   where each fragment would be forwarded without per-hop reassembly.

2.2.  Memory Management and Reliability

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

   Assuming 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 don't 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:  Each VRB table entry can be 12 B
       (assuming 16-bit link-layer addresses).  This is a footprint 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
       fragment 1.

   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 both present and not accepted by the application,
   future specifications may define new protocols to overcome these
   limits.  One example is [I-D.thubert-6lo-fragment-recovery] which
   defines a protocol which allows fragment recovery.

4.  Security Considerations

   An attacker can perform a 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.

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

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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.

7.  Informative References

   [BOOK]     Shelby, Z. and C. Bormann, "6LoWPAN", John Wiley & Sons,
              Ltd monograph, DOI 10.1002/9780470686218, November 2009.

              Bormann, C. and T. Watteyne, "Virtual reassembly buffers
              in 6LoWPAN", draft-ietf-lwig-6lowpan-virtual-reassembly-00
              (work in progress), July 2018.

              Thubert, P., "6LoWPAN Selective Fragment Recovery", draft-
              thubert-6lo-fragment-recovery-01 (work in progress), June

   [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,

Authors' Addresses

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

   Email: thomas.watteyne@analog.com

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   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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