Re: fragment reassembly

[Guy Harris <guy () alum mit edu>]

On Jun 24, 2016, at 1:14 PM, John Thacker <johnthacker () gmail com> wrote:

> I am implementing fragment reassembly of PPP Multilink (RFC 1990) and also implementing multiclass extension (RFC 
> 2686). (https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=12548 https://code.wireshark.org/review/#/c/16044/)
> The protocol is unlike other protocols that do reassembly. Most protocols contain a sequence number that corresponds 
> to the reassembled PDU, plus generally fragment numbers that indicate the position of the fragment within the 
> reassembly and a flag that indicates when there are no more fragments expected. (There are some exceptions without 
> fragment numbers; those have to assume that fragments are received in order and use fragment_add_seq_next().)
>
> PPP Multilink only has a single sequence number, plus flags that indicate whether something is the first or last (or 
> both or neither) fragment of a PDU. There's no indicate of what the fragment number is within the sequence, other 
> than "First" or "not first."

I.e., it's more like TCP, where the sequence number is a sequence number within a "connection".

I seem to remember that there are some other protocols that work that way.  There should probably be an additional set 
of reassembly APIs to handle that case.

> A more thorough rewrite of how reassembly works could introduce new API calls that are designed to work for sequence 
> numbers as implemented here, "front end" sequence numbers that increment with each fragment instead of end result PDU 
> sequence numbers.

Exactly.
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