Subject: [NSIS] yet another attempted summary of fragmentation discussion

From: "Hancock, Robert" <[email-masked]>
Date: Tue, 17 Jun 2003 23:32:25 +0100

Dear all,

Here is another attempt to summarise the fragmentation issues
(for those who are still following).

We have basically the same 4 options on the table:
1. Leave it up to the IP layer
2. Do it only in the NTLP
3. Do it only in the NSLP (NTLP provides information about how
big fragments are allowed to be)
4. Combination of (2) and (3) - how a message is treated depends
on a flag in it (for example).

Arguments basically revolve around on concerns about the complexity 
and processing burden on the NTLP at nodes which wouldn't otherwise
have to worry about fragmentation.

Some factoids (the most interesting/controversial ones only):
A) There are enough trivial/minor/annoying/insuperable concerns
about relying on IP level fragmentation to rule out (1).
B) For all the others, PMTU discovery has to be done at least
at the NTLP level. For (3) and (4) it has to be done at the
NSLP level as well (so there is some additional error handling
needed in the NTLP).
C) It's possible to implement the fragmentation so that reassembly
is only needed at the destination NSLP node (whichever layer it
actually takes place in). So, the processing cost we are moving around
relates only to the fragmentation part.
D) I believe that the processing cost tradeoff between (2) and (3)
is just as likely to favour (2) as (3) so far as the 'vulnerable'
NTLP node is concerned, and favours (2) for the other nodes on the
path (see separate emails), especially when interactions with bundling
are taken into account. I certainly don't believe the 'fragmentation
is obviously expensive' argument.
E) (4) is the most algorithmically functionally complex of all the 
solutions for both layers.
F) Fragmentation itself as functionality is not complex (it takes
about 5 pages to describe the BSD code for IP in its full glory in
Stevens).
G) Fragmentation amplifies message loss rates. If you think that
message loss is a problem, then fragmentation makes it a bigger problem;
however, if you think that message loss is a problem, you probably
believe in some kind of recovery (retransmission), which solves
the problem anyway.

As might come across, I'm still a fan of (2) as a resolution of the
argument; I certainly don't see many stones left un-turned in the
discussion so far.

I'd like to move on to the next open issue (congestion and flow control,
IMHO much more tricky and interesting). Any ideas on how to do that
sooner rather than later?

cheers,

robert h.

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From: "Georgios Karagiannis" <[email-masked]>
Date: Wed, 18 Jun 2003 10:03:39 +0200

Hi Robert

I do not see how you came to the conclusion in
(E) that option (4) is the most algorithmically functionally complex of all
the
solutions for both layers.

If a NSLP chooses to use the NTLP fragmentation then the complexity is
the same as (2) and if a NSLP does not need fragmentation then the
complexity
is actually reduced.

I am still in favour of option (4).

Best regards,
Georgios