Re: [6lo] WG Last Call: draft-ietf-6lo-minimal-fragment-01

[Georgios PAPADOPOULOS <[e-mail]>]

Hello Carles, Thomas and co-authors, 

As promised during the last IETF meeting in Prague, here is my review.

In overall, the document seems solid, well structured and well written.
It is easy to understand and to follow.

I have one main question/comment:
Considering node A that fragments its packet to three fragments, and intend to transmit to B.
B, when employing VRB technique, what would do when it will receive first the fragment 2
instead of fragment 1? 

Bellow is the draft contents with my annotations, starting with “[GP]".

Cheers,
Georgios

— — — 

6lo                                                     T. Watteyne, Ed.
Internet-Draft                                            Analog Devices
Intended status: Informational                                C. Bormann
Expires: September 12, 2019                      Universitaet Bremen TZI
                                                              P. Thubert
                                                                   Cisco
                                                          March 11, 2019


                    LLN Minimal Fragment Forwarding
                   draft-ietf-6lo-minimal-fragment-01

Abstract

   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 always been 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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 12, 2019.

Copyright Notice

   Copyright (c) 2019 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



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   (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 and restrictions with respect
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   described in the Simplified BSD License.

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 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



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   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.


[GP] Isn’t it more straightforward to consider 1, 2, 3 and 4 been sent, 
5 is still being transmitted, while, 6, 7, 8 and 9 to be transmitted?
Otherwise, there are concerns, why fragments 5 and 6 before 4?
Fragment 4 was transmitted earlier and was not successful?


   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 fragment can be uniquely identified by the source and
   destination link-layer addresses of the frame that carries it, and
   the datagram_tag.

[GP] This is not entirely true, right? This is yet, another limitation 
of RFC 4944. There is no guarantee that two different source nodes 
(A and B) may pick up the same “datagram_tag”, and send their 
fragments to C, and then C when sending to D, will have two fragments
with same tags, and the same source and destination link-layer addresses.

	  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
   reassembly.  See [ARTICLE] for detailed simulation results on both
   limits.

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
   buffers, typical nodes only have enough memory for 1-3 reassembly
   buffers.

   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 
   
[GP] don’t —> 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:  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
       packet.
   No Per-Fragment Routing:  All subsequent fragments follow the same
       sequence of hops from the source to the destination node as
       fragment 1.

[GP] Maybe a clarification could be added in the last limit, i.e., that 
only the fragment 1 has the sources and destination addresses.

   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.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

[GP] DoS is not defined earlier

   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

   [ARTICLE]  Tanaka, Y., Minet, P., and T. Watteyne, "6LoWPAN Fragment
              Forwarding", IEEE Communications Standards Magazine ,
              2009.

[GP] I assume the year is 2019 and not 2009.


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

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

   [I-D.thubert-6lo-fragment-recovery]
              Thubert, P., "6LoWPAN Selective Fragment Recovery", draft-
              thubert-6lo-fragment-recovery-01 (work in progress), June
              2018.

   [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,
              <https://www.rfc-editor.org/info/rfc4944>.

   [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,
              <https://www.rfc-editor.org/info/rfc6282>.

Authors' Addresses

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

   Email: [e-mail]




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   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Email: [e-mail]


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

   Email: [e-mail]

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----- Original Message -----
From: "Carles Gomez Montenegro" <[e-mail]>
To: [e-mail]
Sent: Wednesday, April 24, 2019 10:54:39 AM
Subject: [6lo] WG Last Call: draft-ietf-6lo-minimal-fragment-01

Dear 6lo WG,

Authors of draft-ietf-6lo-minimal-fragment-01 expressed in IETF 104 that
they believe that the document is ready for WGLC. No comments or
objections were made in this regard.

Therefore, this email starts a WGLC for draft-ietf-6lo-minimal-fragment-01:
https://tools.ietf.org/html/draft-ietf-6lo-minimal-fragment-01

The WGLC will end on Wednesday, 8th of May.

Please provide your feedback on the document on the mailing list, even if
it is just a short "looks good" statement.

Thank you in advance.

Cheers,

Shwetha and Carles

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