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5.3 Fragmentation

5.3 Fragmentation​

ECN-capable packets MAY have the DF (Don't Fragment) bit set. Reassembly of fragmented packets MUST NOT lose congestion indications. In other words, if any of the fragments of an IP packet to be reassembled has the CE codepoint set, then one of the following two actions MUST occur:

  • The CE codepoint is set in the reassembled packet. However, this MUST NOT occur if any of the other fragments contributing to this reassembly carries the Not-ECT codepoint.

  • The packet is dropped for any other reason than reassembly.

Either of the two actions may be chosen if both apply. Reassembly of a fragmented packet MUST NOT change the ECN codepoint when all fragments carry the same codepoint.

We note that, because RFC 2481 did not specify reassembly behavior, older ECN implementations conforming to that Experimental RFC might not perform reassembly correctly in terms of preserving CE codepoints in fragments. Senders can avoid the consequences of such behavior by setting the DF bit in ECN-capable packets.

There could be situations where the reassembly specifications above might not be precise enough. For example, if there is a malicious or corrupted entity in the path at or after the fragmentation point, packet fragments could carry a mixture of ECT(0), ECT(1), and/or Not-ECT codepoints. The reassembly specifications above do not place requirements on reassembly of fragments in such a case. In situations where more precise reassembly behavior is required, protocol specifications should instead specify that the DF MUST be set in all ECN-capable packets sent by that protocol.

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