LLN Minimal Fragment Forwarding
draft-ietf-6lo-minimal-fragment-00

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 is a companion document to [I-D.ietf-lwig-6lowpan-virtual-reassembly], which details the virtual Reassembly Buffer (VRB) implementation technique which reduces the latency and increases end-to-end reliability in route-over forwarding.

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Table of Contents

• 1. Overview of 6LoWPAN Fragmentation
• 2. Limits of Per-Hop Fragmentation and Reassembly
• 2.1. Latency
• 2.2. Memory Management and Reliability
• 3. Virtual Reassembly Buffer (VRB) Implementation
• 4. Security Considerations
• 5. IANA Considerations
• 6. Acknowledgments
• 7. Informative References
• Authors' Addresses

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 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:

• datagram_size,
• datagram_tag and link-layer sender and receiver addresses (to which the datagram_tag is local),
• 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 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.

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.

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 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 limits. One example is [I-D.thubert-6lo-fragment-recovery] which defines a protocol which allows fragment recovery.

4. 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.

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.

7. Informative References

Authors' Addresses