On Forwarding 6LoWPAN Fragments over a Multihop IPv6 Network

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Table of Contents

1. Introduction

The original 6LoWPAN fragmentation is defined in [RFC4944] and it is implicitly defined for use over a single IP hop through possibly multiple Layer-2 (mesh-under) hops in a meshed 6LoWPAN Network. Although [RFC6282] updates [RFC4944], it does not redefine 6LoWPAN fragmentation.¶

This means that over a Layer-3 (route-over) network, an IP packet is expected to be reassembled at every hop at the 6LoWPAN sublayer, pushed to Layer-3 to be routed, and then fragmented again if the next hop is another similar 6LoWPAN link. This draft introduces an alternate approach called 6LoWPAN Fragment Forwarding (FF) whereby an intermediate node forwards a fragment as soon as it is received if the next hop is a similar 6LoWPAN link. The routing decision is made on the first fragment, which has all 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. It may be wasteful if some fragments are missing after the first one since the first fragment will still continue till the 6LoWPAN endpoint that will attempt to perform the reassembly, and may be misused to the point that performances fall behind that of per-hop recomposition. This specification provides a generic overview of FF, discusses advantages and caveats, and introduces a particular 6LoWPAN Fragment Forwarding technique called Virtual Reassembly Buffer that can be used while conserving the message formats defined in [RFC4944].¶

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 IPv6 source and destination nodes which are neither A nor B.¶

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

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

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 as follows:¶

• 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:¶

• 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 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 datagram_tag as received in the incoming fragments, associated to 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,¶
• 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 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 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 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. 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.¶

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

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 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 datagram_tag as received in the incoming fragments, associated to Link-Layer address of node A for which the received datagram_tag is unique,¶
• the Link-Layer address that node B uses as source to forward the fragments¶
• 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 FF state after some timeout. The duration of the timer should be longer than that which covers the reassembly at the receiving end point.¶

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. In order to keep the operation simple, it makes sense to be consistent with [RFC4944] and enforce that the first fragment is always 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 this as an error and refrain 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.¶

One benefit of Fragment Forwarding is that the memory that is used to store the packet is now distributed along the path, which limits the buffer bloat effect. Multiple fragments may progress in parallel 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 the next fragment. 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 at B whereby the transmission from C interferes with that from A unbeknownst of node A. It results that consecutive fragments must be reasonably spaced in order to avoid the 2 forms of collision described above. A node that has multiple packets or fragments to send via different next-hop routers may interleave the messages in order to alleviate those effects.¶

6. Virtual Reassembly Buffer (VRB) Implementation

Virtual Reassembly Buffer (VRB) is the implementation technique described in [LWIG-VRB] in which a forwarder does not reassemble each packet in its entirety before forwarding it.¶

VRB overcomes the limits 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, however, limits:¶

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 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 [FRAG-RECOV] which defines a protocol which allows fragment recovery.¶

7. Security Considerations

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 that are linked to using IP fragmentation. The 6LoWPAN fragmentation takes place underneath, 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 recompose the IP packet, check it in the uncompressed form, and then forward it again as fragments if necessary.¶
• Resource exhaustion attacks are certainly possible and a sensitive issue in a constrained network. An attacker can perform a Denial-of-Service (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 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 pseudo-randomly in order to defeat such attacks.¶
• Evasion of Network Intrusion Detection Systems (NIDS) leverages ambiguity in the reassembly of the fragment. This sounds difficult and mostly useless in a 6LoWPAN network since the fragmentation is not end-to-end.¶

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 Georgies Papadopoulos and Dominique Barthel for their own reviews, and to Joerg Ott who helped through the IESG steps.¶

Authors' Addresses