Internet Engineering Task Force (IETF) T. Watteyne, Ed.

Internet Engineering Task Force (IETF) T. Watteyne, Ed.
Request for Comments: 8930 Analog Devices
Category: Standards Track P. Thubert, Ed.
ISSN: 2070-1721 Cisco Systems
C. Bormann
Universität Bremen TZI
November 2020

On Forwarding 6LoWPAN Fragments over a Multi-Hop IPv6 Network

Abstract

This document provides generic rules to enable the forwarding of an
IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) fragment
over a route-over network. Forwarding fragments can improve both
end-to-end latency and reliability as well as reduce the buffer
requirements in intermediate nodes; it may be implemented using RFC
4944 and Virtual Reassembly Buffers (VRBs).

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8930.

Copyright Notice

Copyright (c) 2020 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction
2. Terminology
2.1. Requirements Language
2.2. Background
2.3. New Terms
3. Overview of 6LoWPAN Fragmentation
4. Limitations of Per-Hop Fragmentation and Reassembly
4.1. Latency
4.2. Memory Management and Reliability
5. Forwarding Fragments
6. Virtual Reassembly Buffer (VRB) Implementation
7. Security Considerations
8. IANA Considerations
9. References
9.1. Normative References
9.2. Informative References
Acknowledgments
Authors' Addresses

1. Introduction

The original 6LoWPAN fragmentation is defined in [RFC4944] for use
over a single Layer 3 hop, though multiple Layer 2 hops in a mesh-
under network is also possible, and was not modified by the update in
[RFC6282]. 6LoWPAN operations including fragmentation depend on a
link-layer security that prevents any rogue access to the network.

In a route-over 6LoWPAN network, an IP packet is expected to be
reassembled at each intermediate hop, uncompressed, pushed to Layer 3
to be routed, and then compressed and fragmented again. This
document introduces an alternate approach called 6LoWPAN Fragment
Forwarding (6LFF) whereby an intermediate node forwards a fragment
(or the bulk thereof, MTU permitting) without reassembling if the
next hop is a similar 6LoWPAN link. The routing decision is made on
the first fragment of the datagram, which has the IPv6 routing
information. The first fragment is forwarded immediately, and a
state is stored to enable forwarding the next fragments along the
same path.

Done right, 6LoWPAN Fragment Forwarding techniques lead to more
streamlined operations, less buffer bloat, and lower latency. But it
may be wasteful when fragments are missing, leading to locked
resources and low throughput, and it may be misused to the point that
the end-to-end latency of one packet falls behind that of per-hop
reassembly.

This specification provides a generic overview of 6LFF, discusses
advantages and caveats, and introduces a particular 6LFF technique
called "Virtual Reassembly Buffer" (VRB) that can be used while
retaining the message formats defined in [RFC4944]. Basic
recommendations such as the insertion of an inter-frame gap between
fragments are provided to avoid the most typical caveats.

2. Terminology

2.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

2.2. Background

Past experience with fragmentation, e.g., as described in "IPv4
Reassembly Errors at High Data Rates" [RFC4963] and references
therein, has shown that misassociated or lost fragments can lead to
poor network behavior and, occasionally, trouble at the application
layer. That experience led to the definition of the "Path MTU
Discovery for IP version 6" [RFC8201] protocol that limits
fragmentation over the Internet.

"IP Fragmentation Considered Fragile" [RFC8900] discusses security
threats that are linked to using IP fragmentation. The 6LoWPAN
fragmentation takes place underneath the IP Layer, but some issues
described there may still apply to 6LoWPAN fragments (as discussed in
further details in Section 7).

Readers are expected to be familiar with all the terms and concepts
that are discussed in "IPv6 over Low-Power Wireless Personal Area
Networks (6LoWPANs): Overview, Assumptions, Problem Statement, and
Goals" [RFC4919] and "Transmission of IPv6 Packets over IEEE 802.15.4
Networks" [RFC4944].

"Multiprotocol Label Switching Architecture" [RFC3031] states that
with MPLS,

| packets are "labeled" before they are forwarded. At subsequent
| hops, there is no further analysis of the packet's network layer
| header. Rather, the label is used as an index into a table which
| specifies the next hop, and a new label.

The MPLS technique is leveraged in the present specification to
forward fragments that actually do not have a network-layer header,
since the fragmentation occurs below IP.

2.3. New Terms

This specification uses the following terms:

6LoWPAN Fragment Forwarding Endpoints: The 6LFF endpoints are the
first and last nodes in an unbroken string of 6LFF nodes. They
are also the only points where the fragmentation and reassembly
operations take place.

Compressed Form: This specification uses the generic term
"compressed form" to refer to the format of a datagram after the
action of [RFC6282] and possibly [RFC8138] for Routing Protocol
for Low-Power and Lossy Network (RPL) [RFC6550] artifacts.

Datagram_Size: The size of the datagram in its compressed form
before it is fragmented.

Datagram_Tag: An identifier of a datagram that is locally unique to
the Layer 2 sender. Associated with the link-layer address of the
sender, this becomes a globally unique identifier for the datagram
within the duration of its transmission.

Fragment_Offset: The offset of a fragment of a datagram in its
compressed form.

3. Overview of 6LoWPAN Fragmentation

Figure 1 illustrates 6LoWPAN fragmentation. We assume node A
forwards a packet to node B, possibly as part of a multi-hop route
between 6LoWPAN Fragment Forwarding endpoints, which may be neither A
nor B, though 6LoWPAN may compress the IP header better when they are
both the 6LFF and the 6LoWPAN compression endpoints.

+---+ +---+
... ---| A |-------------------->| B |--- ...
+---+ +---+
# (frag. 5)

123456789 123456789
+---------+ +---------+
| # ###| |### # |
+---------+ +---------+
outgoing incoming
fragmentation reassembly
buffer buffer

Figure 1: Fragmentation at Node A, and Reassembly at Node B

Typically, node A starts with an uncompressed packet and compacts 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 sub-layer cuts it into
multiple 6LoWPAN fragments, which it transmits as separate link-layer
frames to node B. Node B's 6LoWPAN sub-layer 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 as follows:

* Each fragment is represented by the '#' symbol.

* Node A has sent fragments 1, 2, 3, 5, and 6 to node B.

* Node B has received fragments 1, 2, 3, and 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:

* a unique identifier of node A (e.g., node A's link-layer address).

* 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 versus long
addresses), node A must use the same link-layer address to send all
the fragments of the same datagram to node B.

Conceptually, the reassembly buffer in node B contains:

* a Datagram_Tag as received in the incoming fragments, associated
with the interface and the link-layer address of node A for which
the received Datagram_Tag is unique,

* 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,

* a state indicating the fragments already received,

* a Datagram_Size, and

* 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 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. [RFC4944] also mandates
that the first fragment is sent first and with a particular format
that is different than that of the next fragments. Each fragment
except for the first one 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 fragment it receives first is not the first fragment. 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,
which performs the route lookup. This behavior typically results in
per-hop fragmentation and reassembly. That is, the packet is fully
reassembled, then (re-)fragmented, at every hop.

4. Limitations of Per-Hop Fragmentation and Reassembly

There are at least two limitations to doing per-hop fragmentation and
reassembly. See [ARTICLE] for detailed simulation results on both
limitations.

4.1. Latency

When reassembling, a node needs to wait for all the fragments to be
received before being able to re-form the IPv6 packet and possibly
forwarding 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.

4.2. Memory Management and Reliability

Constrained nodes have limited memory. Assuming a reassembly buffer
for a 6LoWPAN MTU of 1280 bytes as defined in Section 4 of [RFC4944],
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.

+---+ +---+
... --->| A |------>| B |
+---+ +---+\
\
+---+ +---+
| E |--->| F | ...
+---+ +---+
/
/
+---+ +---+
... --->| C |------>| D |
+---+ +---+

Figure 2: Illustrating the Memory Management Issue

When nodes A, B, and C concurrently send fragmented packets, all
three 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.

5. Forwarding Fragments

A 6LoWPAN Fragment Forwarding technique makes the routing decision on
the first fragment, which is always the one with the IPv6 address of
the destination. Upon receiving a first fragment, a forwarding node
(e.g., node B in an A->B->C sequence) that does fragment forwarding
MUST attempt to create a state and forward the fragment. This is an
atomic operation, and if the first fragment cannot be forwarded, then
the state MUST be removed.

Since the Datagram_Tag is uniquely associated with the source link-
layer address of the fragment, the forwarding node MUST assign a new
Datagram_Tag from its own namespace for the next hop and rewrite the
fragment header of each fragment with that Datagram_Tag.

When a forwarding node receives a fragment other than a first
fragment, it MUST look up state based on the source link-layer
address and the Datagram_Tag in the received fragment. If no such
state is found, the fragment MUST be dropped; otherwise, the fragment
MUST be forwarded using the information in the state found.

Compared to Section 3, the conceptual reassembly buffer in node B now
contains the following, assuming that node B is neither the source
nor the final destination:

* a Datagram_Tag as received in the incoming fragments, associated
with the interface and the link-layer address of node A for which
the received Datagram_Tag is unique.

* the link-layer address that node B uses as the source to forward
the fragments.

* the interface and the link-layer address of the next-hop C that is
resolved on the first fragment.

* a Datagram_Tag that node B uniquely allocated for this datagram
and that is used when forwarding the fragments of the datagram.

* a buffer for the remainder of a previous fragment left to be sent.

* a timer that allows discarding the stale 6LFF state after some
timeout. The duration of the timer should be longer than that
which covers the reassembly at the receiving endpoint.

A node that has not received the first fragment cannot forward the
next fragments. This means that if node B receives a fragment, node
A was in possession of the first fragment at some point. To keep the
operation simple and consistent with [RFC4944], the first fragment
MUST always be sent first. When that is done, if node B receives a
fragment that is not the first and for which it has no state, then
node B treats it as an error and refrains from creating a state or
attempting to forward. This also means that node A should perform
all its possible retries on the first fragment before it attempts to
send the next fragments, and that it should abort the datagram and
release its state if it fails to send the first fragment.

Fragment forwarding obviates some of the benefits of the 6LoWPAN
header compression [RFC6282] in intermediate hops. In return, the
memory used to store the packet is distributed along the path, which
limits the buffer-bloat effect. Multiple fragments may progress
simultaneously along the network as long as they do not interfere.
An associated caveat is that on a half-duplex radio, if node A sends
the next fragment at the same time as node B forwards the previous
fragment to node C down the path, then node B will miss it. If node
C forwards the previous fragment to node D at the same time and on
the same frequency as node A sends the next fragment to node B, this
may result in a hidden terminal problem. In that case, the
transmission from node C interferes at node B with that from node A,
unbeknownst to node A. Consecutive fragments of a same datagram MUST
be separated with an inter-frame gap that allows one fragment to
progress beyond the next hop and beyond the interference domain
before the next shows up. This can be achieved by interleaving
packets or fragments sent via different next-hop routers.

6. Virtual Reassembly Buffer (VRB) Implementation

The VRB [LWIG-VRB] is a particular incarnation of a 6LFF that can be
implemented without a change to [RFC4944].

VRB overcomes the limitations listed in Section 4. 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.

However, there are other caveats:

Non-zero Packet Drop Probability: The abstract data in a VRB table
entry contains at a minimum the link-layer address of the
predecessor and 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 than 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 the
first fragment, because the IP header is required in order 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 caveats depend on the link layer
used. Whether they are acceptable depends entirely on the
requirements the application places on the network.

If the caveats are present and not acceptable for the application,
alternative specifications may define new protocols to overcome them.
One example is [RFC8931], which specifies a 6LFF technique that
allows the end-to-end fragment recovery between the 6LFF endpoints.

7. 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.

"IP Fragmentation Considered Fragile" [RFC8900] discusses security
threats and other caveats that are linked to using IP fragmentation.
The 6LoWPAN fragmentation takes place underneath the IP Layer, but
some issues described there may still apply to 6LoWPAN fragments.

* Overlapping fragment attacks are possible with 6LoWPAN fragments,
but there is no known firewall operation that would work on
6LoWPAN fragments at the time of this writing, so the exposure is
limited. An implementation of a firewall SHOULD NOT forward
fragments but instead should recompose the IP packet, check it in
the uncompressed form, and then forward it again as fragments if
necessary. Overlapping fragments are acceptable as long as they
contain the same payload. The firewall MUST drop the whole packet
if overlapping fragments are encountered that result in different
data at the same offset.

* Resource-exhaustion attacks are certainly possible and a sensitive
issue in a constrained network. An attacker can perform a DoS
attack on a node implementing VRB by generating a large number of
bogus first fragments without sending subsequent fragments. This
causes the VRB table to fill up. When hop-by-hop reassembly is
used, the same attack can be more damaging if the node allocates a
full Datagram_Size for each bogus first fragment. With the VRB,
the attack can be performed remotely on all nodes along a path,
but each node suffers a lesser hit. This is because 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. An implementation MUST protect itself to keep the number of
VRBs within capacity and to ensure that old VRBs are protected by
a timer of a reasonable duration for the technology and destroyed
upon timeout.

* Attacks based on predictable fragment identification values are
also possible but can be avoided. The Datagram_Tag SHOULD be
assigned pseudorandomly in order to reduce the risk of such
attacks. A larger size of the Datagram_Tag makes the guessing
more difficult and reduces the chances of an accidental reuse
while the original packet is still in flight, at the expense of
more space in each frame. Nonetheless, some level of risk remains
because an attacker that is able to authenticate to and send
traffic on the network can guess a valid Datagram_Tag value, since
there are only a limited number of possible values.

* Evasion of Network Intrusion Detection Systems (NIDSs) leverages
ambiguity in the reassembly of the fragment. This attack makes
little sense in the context of this specification since the
fragmentation happens within the Low-Power and Lossy Network
(LLN), meaning that the intruder should already be inside to
perform the attack. NIDS systems would probably not be installed
within the LLN either but rather at a bottleneck at the exterior
edge of the network.

8. IANA Considerations

This document has no IANA actions.

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
over Low-Power Wireless Personal Area Networks (6LoWPANs):
Overview, Assumptions, Problem Statement, and Goals",
RFC 4919, DOI 10.17487/RFC4919, August 2007,
<https://www.rfc-editor.org/info/rfc4919>.

[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>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2. Informative References

[ARTICLE] Tanaka, Y., Minet, P., and T. Watteyne, "6LoWPAN Fragment
Forwarding", IEEE Communications Standards Magazine, Vol.
3, Issue 1, pp. 35-39, DOI 10.1109/MCOMSTD.2019.1800029,
March 2019,
<https://ieeexplore.ieee.org/abstract/document/8771317>.

[LWIG-VRB] Bormann, C. and T. Watteyne, "Virtual reassembly buffers
in 6LoWPAN", Work in Progress, Internet-Draft, draft-ietf-
lwig-6lowpan-virtual-reassembly-02, 9 March 2020,
<https://tools.ietf.org/html/draft-ietf-lwig-6lowpan-
virtual-reassembly-02>.

[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031,
DOI 10.17487/RFC3031, January 2001,
<https://www.rfc-editor.org/info/rfc3031>.

[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963,
DOI 10.17487/RFC4963, July 2007,
<https://www.rfc-editor.org/info/rfc4963>.

[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>.

[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550,
DOI 10.17487/RFC6550, March 2012,
<https://www.rfc-editor.org/info/rfc6550>.

[RFC8138] Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie,
"IPv6 over Low-Power Wireless Personal Area Network
(6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138,
April 2017, <https://www.rfc-editor.org/info/rfc8138>.

[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.

[RFC8900] Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
and F. Gont, "IP Fragmentation Considered Fragile",
BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020,
<https://www.rfc-editor.org/info/rfc8900>.

[RFC8931] Thubert, P., Ed., "IPv6 over Low-Power Wireless Personal
Area Network (6LoWPAN) Selective Fragment Recovery",
RFC 8931, DOI 10.17487/RFC8931, November 2020,
<https://www.rfc-editor.org/info/rfc8931>.

Acknowledgments

The authors would like to thank Carles Gomez Montenegro, Yasuyuki
Tanaka, Ines Robles, and Dave Thaler for their in-depth review of
this document and suggestions for improvement. Many thanks to
Georgios Papadopoulos and Dominique Barthel for their contributions
during the WG activities. And many thanks as well to Roman Danyliw,
Barry Leiba, Murray Kucherawy, Derrell Piper, Sarah Banks, Joerg Ott,
Francesca Palombini, Mirja Kühlewind, Éric Vyncke, and especially
Benjamin Kaduk for their constructive reviews through the IETF last
call and IESG process.

Authors' Addresses

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

Email: [email protected]

Pascal Thubert (editor)
Cisco Systems, Inc
Building D
45 Allee des Ormes - BP1200
06254 Mougins - Sophia Antipolis
France

Phone: +33 497 23 26 34
Email: [email protected]

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

Phone: +49-421-218-63921
Email: [email protected]