6lo T. Watteyne, Ed.
Internet-Draft Analog Devices
Intended status: Informational C. Bormann
Expires: March 2, 2020 Universitaet Bremen TZI
P. Thubert
Cisco
August 30, 2019
6LoWPAN Fragment Forwarding
draft-ietf-6lo-minimal-fragment-04
Abstract
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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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 . . . . . . . . . . . . . . . . . . . 7
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
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.
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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.
The reassembly buffer for 6LoWPAN is indexed in node B by:
o a unique Identifier of Node A (e.g., Node A's link-layer address)
o the datagram_tag chosen by node A for this fragmented datagram
Because it may be hard for node B to correlate all possible link-
layer addresses that node A may use (e.g., short vs. long addresses),
node A must use the same link-layer address to send all the fragments
of a same datagram to node B.
Conceptually, the reassembly buffer in node B contains, assuming that
node B is neither the source nor the final destination:
o a datagram_tag as received in the incoming fragments, associated
to link-layer address of node A for which the received
datagram_tag is unique,
o the link-layer address that node B uses to forward the fragments
o the link-layer address of the next hop that is resolved on the
first fragment
o a datagram_tag that node B uniquely allocated for this datagram
and that is used when forwarding the fragments of the datagram
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 datagram_size,
o a buffer for the remainder of a previous fragment left to be sent,
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
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"datagram_tag", used to identify which packet each fragment belongs
to. Each datagram can be uniquely identified by the sender link-
layer addresses of the frame that carries it and the datagram_tag
that the sender allocated for this datagram. 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.
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
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
buffer, typical nodes only have enough memory for 1-3 reassembly
buffers.
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.
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+---+ +---+
... --->| 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.
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.
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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 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.
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.
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7. Informative References
[ARTICLE] Tanaka, Y., Minet, P., and T. Watteyne, "6LoWPAN Fragment
Forwarding", IEEE Communications Standards Magazine ,
2019.
[I-D.ietf-6lo-fragment-recovery]
Thubert, P., "6LoWPAN Selective Fragment Recovery", draft-
ietf-6lo-fragment-recovery-05 (work in progress), July
2019.
[I-D.ietf-lwig-6lowpan-virtual-reassembly]
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,
<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: thomas.watteyne@analog.com
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Email: cabo@tzi.org
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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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