6lo T. Watteyne, Ed.
Internet-Draft Analog Devices
Intended status: Informational C. Bormann
Expires: September 7, 2018 Universitaet Bremen TZI
P. Thubert
Cisco
March 06, 2018
LLN Minimal Fragment Forwarding
draft-watteyne-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 been always possible
with the original fragmentation design of RFC4944. This document
details the Virtual Reassembly Buffer (VRB) implementation technique
which reduces the latency and increases end-to-end reliability in
route-over forwarding, and discusses its limits.
Status of This Memo
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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. Critique of VRB . . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
8. Informative References . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
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
reassembly:
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
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
One implementation of 6LoWPAN fragmentation overcomes the limits
listed in Section 2. The idea is for a node to immediately
retransmit a fragment it receives, without fully reassembling the
packet. This idea was introduced in Section 2.5.2 of [BOOK]. That
is, a node 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.
We use Figure 3 to illustrate VRB, and focus on the behavior of node
E. With VRB, node E maintains a VRB table which functions similarly
to a switching table: when receiving a fragment from node B with
datagram_tag=2, forward it to node F with datagram_tag=8.
+---+ +---+
| A |----->| B | +-------------+-------------+
+---+ #(5) +---+\ #(2) | incoming | outgoing |
\ +-------+-----+-------+-----+
+---+ #(8) +---+ |L2 src | tag |L2 dest| tag |
| E |----->| F | +=======+=====+=======+=====+
+---+ %(5) +---+ | B | 2 | F | 8 |
/ | D | 2 | F | 5 |
/ %(2) | empty |
+---+ %(1) +---+ | empty |
| C |----->| D | +-------+-----+-------+-----+
+---+ +---+ Node E's VRB table.
Figure 3: Illustrating VRB. #(5) and %(1) are fragments from packets
coming from nodes A and C, with datagram_tag set to 5 and 1,
respectively.
The VRB table is initially empty. An implementation might have for
example pre-allocate memory for a VRB table with 4 entries (as in
Figure 3), initially cleared.
When node E receives fragment 1 from node B with datagram_tag=2, it
inspects the contents of the fragment and reads out the destination
IPv6 address. When it is not destined to it, node E identifies the
next hop to send this fragment to. It then creates an entry in the
VRB table which contains 4 fields: (1) the link-layer address of the
sender of the fragment it received, (2) the datagram_tag of the
fragment it received, (3) the link-layer address of the next hop, (4)
a datagram_tag for the fragments it will send. The latter
datagram_tag must be locally unique.
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Note that, if node E had multiple interfaces, the VRB table would
also need additional column to identify the incoming and outgoing
interface.
Any subsequent fragment that matches the "incoming" columns in the
node's VRB table are immediately forwarded using the information in
the "outgoing" columns. Note that, while this results in a behavior
similar to link-layer switching, what is really happening is that the
node has a virtual reassembly buffer. That is, it operates as if the
packet were reassembled and fragmented, without ever actually holding
a fully reassembled packet in memory.
Upon forwarding the last fragment of a packet, the VRB table entry
can be cleared, and reused for a future packet. If the last fragment
of a packet is dropped, the VRB table entry can be invalidated by
timeout. Its timeout value is set to a maximum of 60 seconds as the
reassembly timeout defined in [RFC4944].
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).
In case fragments can come out of order (a rare case, as all
fragments of a packet are sent between the same neighbors), an
implementation can use multiple the following two techniques. In
case fragment 1 isn't received first, it can temporarily buffer
fragments 2, 3, etc., until fragment 1 is received, and a next hop
neighbor can be identified. Similarly, as the final fragment of the
packet isn't necessarily received last, an implementation can
maintain a bitmap of already forwarded fragments to know when all
fragments have been forwarded (and the corresponding VRB entry can be
cleared).
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).
It is possible for a network to be composed of some nodes that
implement VRB, and others that don't. Nodes that do not implement
VRB reassemble the packet.
[RFC6282] defines the header compression format for 6LoWPAN. One
important impact of header compression is that the header is no
longer of a fixed length. 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.
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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.
4. Critique of VRB
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
packet.
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
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limits. One example is [I-D.thubert-6lo-fragment-recovery] which
defines a protocol which allows fragment recovery.
5. 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.
6. IANA Considerations
No requests to IANA are made by this document.
7. Acknowledgments
The authors would like to thank Yasuyuki Tanaka for his in-depth
review of this document.
8. Informative References
[BOOK] Shelby, Z. and C. Bormann, "6LoWPAN", John Wiley & Sons,
Ltd monograph, DOI 10.1002/9780470686218, November 2009.
[I-D.thubert-6lo-fragment-recovery]
Thubert, P., "6LoWPAN Selective Fragment Recovery", draft-
thubert-6lo-fragment-recovery-00 (work in progress),
February 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
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Thomas Watteyne (editor)
Analog Devices
32990 Alvarado-Niles Road, Suite 910
Union City, CA 94587
USA
Email: thomas.watteyne@analog.com
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Email: cabo@tzi.org
Pascal Thubert
Cisco Systems, Inc
Building D
45 Allee des Ormes - BP1200
MOUGINS - Sophia Antipolis 06254
France
Email: pthubert@cisco.com
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