Network Working Group C. Bormann
Internet-Draft Universitaet Bremen TZI
Intended status: Informational T. Watteyne
Expires: September 10, 2020 Analog Devices
March 09, 2020
Virtual reassembly buffers in 6LoWPAN
draft-ietf-lwig-6lowpan-virtual-reassembly-02
Abstract
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 RFC 4944. Apart from a brief mention of the way to do this in
Section 2.5.2 of the 6LoWPAN book, this has not been extensively
described in the literature. The present document attempts to fill
that gap.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Reassembly buffers . . . . . . . . . . . . . . . . . . . . . 3
3. Virtual reassembly . . . . . . . . . . . . . . . . . . . . . 3
4. Header compression . . . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. Security considerations . . . . . . . . . . . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
6LoWPAN [RFC4944] is the seminal standard for the transmission of
IPv6 packets over IEEE 802.15.4 networks and has served as a
blueprint for a number of related standards addressing low-power
radios and other IoT connectivity solutions (the "6Lo suite").
One of the problems that need to be solved to enable sending IPv6
packets over low-power radios is that some of these (including IEEE
802.15.4) do not support frames that are large enough to hold IPv6
packets of the minimum MTU (Maximum Transmission Unit) defined for
IPv6, 1280 bytes. This necessitates providing a fragmentation or
segmentation scheme in the IP adaptation layer for the radio.
When employing adaptation layer fragmentation on constrained-node
networks [RFC7228], 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 RFC 4944. Apart from a brief mention of the way to do this in
Section 2.5.2 of the 6LoWPAN book [BOOK], this has not been
extensively described in the literature. The present document
attempts to fill that gap.
[I-D.ietf-6lo-minimal-fragment] provides additional context and
discussion about handling fragment forwarding in the 6Lo standards
suite.
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2. Reassembly buffers
An adaptation layer implementation for 6LoWPAN needs to perform
reassembly of every fragmented packet received in order to be able to
forward the packet (re-fragmenting it in the process).
A reassembly buffer for 6LoWPAN contains:
o datagram_size,
o datagram_tag and L2 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.
This requires a reassembly buffer for each fragmented packet the
reception of which is in progress. Since the forwarder may be
receiving fragments for multiple packets concurrently (e.g., from
different senders), this means that multiple reassembly buffers are
needed, easily dominating the memory requirements in a 6LoWPAN
implementation. Worse, as this space may still be limited, any lack
of reassembly buffers may lead to an increased loss rate for
fragmented packets (which already have to cope with a higher compound
loss rate).
3. Virtual reassembly
To reduce the memory requirement for reassembly buffers, the
implementation may opt to not keep the actual packet data in the
reassembly buffer. Instead, it may attempt to send out the data for
a fragment in the form of a forwarded fragment, as soon as all
necessary information for that is available. Obviously, all
fragments need to be sent with the same outgoing address (otherwise a
full reassembly implementation would discard the fragments) and the
same datagram_tag.
To this end, the reassembly buffer now also stores, as soon as enough
of the packet is available to make a forwarding decision (i.e., as
soon as the first fragment has been received):
o L2 destination address used for forwarding,
o outgoing datagram_tag chosen for this packet.
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A simple implementation may do away with any attempt to keep packet
data in the virtual reassembly buffer. It then has to discard all
non-first fragments for which a reassembly buffer is not already
available (penalizing reordering, which however may be rare).
Note that the decision to do local processing of a packet needs to be
taken with the first fragment - such packets of course do need to be
fully reassembled (unless transport and application also can cope
with fragments, which they rarely can in the presence of security).
4. Header compression
[RFC6282] defines the header compression format for 6LoWPAN. One
important impact of header compression is that the header is no
longer of a length that stays fixed during forwarding. In
particular, changes made by a forwarder may gain or lose the ability
to use a more highly compressed variant, changing the length of the
header in the packet. If the change increases the size, the maximum
frame size may be exceeded, leading to the need to re-fragment in the
forwarder. This is less of a problem with full reassembly, but with
virtual reassembly can lead to the need for sending an additional
frame for each packet.
The well-known approach to minimize the probability of this need is
for the original sender to put all slack in the frame sizes into the
_first_ packet, making this the smallest fragment and not the last
one as would be done in a naive implementation. (This also has other
consequences related to delivery probability, which are not discussed
here.) This makes sure an additional fragment only needs to be sent
if the header expansion during forwarding would have created an
additional fragment with full reassembly as well.
5. IANA Considerations
This document makes no requests of IANA.
6. Security considerations
There are many security considerations with using fragmentation in
the first place, even with adaptation layer fragmentation (which is
not accessible outside the range of that adaptation layer). Some of
the more specific ones are documented in
[I-D.ietf-6lo-minimal-fragment] and will not be duplicated here.
In general, sending on fragments early from a node will relieve the
node that is doing the forwarding, but put additional onus on the
next node. This may or may not be in favor of an attacker.
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7. References
7.1. Normative References
[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>.
7.2. Informative References
[BOOK] Shelby, Z. and C. Bormann, "6LoWPAN", John Wiley & Sons,
Ltd monograph, DOI 10.1002/9780470686218, November 2009.
[I-D.ietf-6lo-minimal-fragment]
Watteyne, T., Thubert, P., and C. Bormann, "On Forwarding
6LoWPAN Fragments over a Multihop IPv6 Network", draft-
ietf-6lo-minimal-fragment-14 (work in progress), March
2020.
[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>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
Acknowledgements
Many people have mentioned that it would be good to have a
description of virtual reassembly in 6LoWPAN. Finally, Thomas
Watteyne assembled a design team that intends to work on 6Lo
fragmentation. Writing up the present document has been motivated by
that work.
Authors' Addresses
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: [email protected]
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Thomas Watteyne
Analog Devices
32990 Alvarado-Niles Road, Suite 910
Union City, CA 94587
USA
Email: [email protected]
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