Internet-Draft fragment forwarding March 2020
Watteyne, et al. Expires 24 September 2020 [Page]
Intended Status:
Standards Track
T. Watteyne, Ed.
Analog Devices
P. Thubert, Ed.
Cisco Systems
C. Bormann
Universitaet Bremen TZI

On Forwarding 6LoWPAN Fragments over a Multihop IPv6 Network


This document provides generic rules to enable the forwarding of 6LoWPAN fragment over a route-over network. Forwarding fragments can improve both the end-to-end latency and reliability, and reduce the buffer requirements in intermediate nodes; it may be implemented using RFC 4944 and virtual reassembly buffers.

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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 24 September 2020.

Table of Contents

1. Introduction

The original 6LoWPAN fragmentation is defined in [RFC4944] for use over a single Layer 3 hop, though possibly multiple Layer 2 hops in a mesh-under network, and was not modified by the [RFC6282] update. 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 draft introduces an alternate approach called 6LoWPAN Fragment Forwarding (6FF) 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 6FF, discusses advantages and caveats, and introduces a particular 6LoWPAN Fragment Forwarding technique called Virtual Reassembly Buffer 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. BCP 14

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. Referenced Work

Past experience with fragmentation, e.g., as described in "IPv4 Reassembly Errors at High Data Rates" [RFC4963] and references therein, has shown that mis-associated 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" [RFC8201] (PMTUD) protocol that limits fragmentation over the Internet.

"IP Fragmentation Considered Fragile" [FRAG-ILE] 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 (MPLS) Architecture" [RFC3031] says that with MPLS, 'packets are "labeled" before they are forwarded.' It goes on to say, "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 6FF endpoints are the first and last nodes in an unbroken string of 6LoWPAN Fragment Forwarding 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 RPL [RFC6550] artifacts.
The size of the datagram in its Compressed Form before it is fragmented.
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.
The offset of a fragment of a datagram in its Compressed Form.

3. Overview of 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 6LoWPAN Fragment Forwarding endpoints which may be neither A nor B, though 6LoWPAN may compress the IP header better when they are both the 6FF and the 6LoWPAN compression endpoints.

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

             123456789                 123456789
            +---------+               +---------+
            |   #  ###|               |###  #   |
            +---------+               +---------+
               outgoing                incoming
          fragmentation                reassembly
                 buffer                buffer
Figure 1: Fragmentation at node A, 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 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 as follows:

The reassembly buffer for 6LoWPAN is indexed in node B by:

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 the same datagram to node B.

Conceptually, the reassembly buffer in node B contains:

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 but 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 2 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 reform 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.

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 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.

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 a 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 to 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, assuming that node B is neither the source nor the final destination:

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 a node C down the path then node B will miss it. If node C forwards the previous fragment to a 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 C interferes at node B with that from A unbeknownst of 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 Virtual Reassembly Buffer (VRB) [LWIG-VRB] is a particular incarnation of a 6LoWPAN Fragment Forwarding 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.

There are other caveats, however:

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 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. 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 depends 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 [FRAG-RECOV] which specifies a 6LoWPAN Fragment Forwarding technique that allows the end-to-end fragment recovery between the 6LoWPAN FF 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" [FRAG-ILE] 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.

8. IANA Considerations

No requests to IANA are made by this document.

9. 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 improvement suggestions. Also many thanks to Georgios Papadopoulos and Dominique Barthel for their own reviews, and to Roman Danyliw, Barry Leiba, Murray Kucherawy, Derrell Piper, Sarah Banks, Joerg Ott, Francesca Palombini, Mirja Kuhlewind, Eric Vyncke, and especially Benjamin Kaduk for their constructive reviews through the IETF last call and IESG process.

10. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
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, , <>.
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, , <>.

11. Informative References

Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly Errors at High Data Rates", RFC 4963, DOI 10.17487/RFC4963, , <>.
Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label Switching Architecture", RFC 3031, DOI 10.17487/RFC3031, , <>.
Hui, J., Ed. and P. Thubert, "Compression Format for IPv6 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, DOI 10.17487/RFC6282, , <>.
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, , <>.
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, , <>.
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, , <>.
Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O., and F. Gont, "IP Fragmentation Considered Fragile", Work in Progress, Internet-Draft, draft-ietf-intarea-frag-fragile-17, , <>.
Bormann, C. and T. Watteyne, "Virtual reassembly buffers in 6LoWPAN", Work in Progress, Internet-Draft, draft-ietf-lwig-6lowpan-virtual-reassembly-02, , <>.
Thubert, P., "6LoWPAN Selective Fragment Recovery", Work in Progress, Internet-Draft, draft-ietf-6lo-fragment-recovery-20, , <>.
Tanaka, Y., Minet, P., and T. Watteyne, "6LoWPAN Fragment Forwarding", IEEE Communications Standards Magazine , .

Authors' Addresses

Thomas Watteyne (editor)
Analog Devices
32990 Alvarado-Niles Road, Suite 910
Union City, CA 94587
United States of America
Pascal Thubert (editor)
Cisco Systems, Inc
Building D
45 Allee des Ormes - BP1200
06254 Mougins - Sophia Antipolis
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
D-28359 Bremen