Re: [multipathtcp] High-level design decisions /architecture

[Joe Touch <[e-mail]>]

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Hi, Bill,

Sorry; this appears to have found a dusty corner of my mailbox. Here's a
late reply...

William Herrin wrote:
> On Tue, Nov 3, 2009 at 5:20 PM, Joe Touch <[e-mail]> wrote:
>> TCP negotiates all options during the SYN - that's the only 'handshake'
>> there is. TCP does not define option negotiation after connection
>> establishment.
>>
>> I.e., you can decide to start the separate connections later, but you
>> need to negotiate support for the option during the SYN.
> 
> Hi Joe,
> 
> I assume that's a convention rather than a hard protocol requirement.
> Something previous folks working in the space have deemed a "wise
> idea" rather than a "all these things will break if we don't" kind of
> thing, yes?

It's an artifact of how TCP works.

> Is there a good discussion on this somewhere so I can read up on the
> rationale that brought it into being?

Since it's an artifact, I don't think there is. The issue is that
unknown options are ignored (as per RFC1122). The only time when you can
find out if an option is supported in a way that you know will preceded
all other segments is to put it in the SYN.

If you put it in a later segment, you don't know when the segment will
be received. Here's the problem, when A sends to B (assuming the
connection is already open):

	A sends segment #223 with option X
		presuming this is the first segment A sends
		with option X

Does A stop sending segments beyond 223 until option X is ACK'd? This
violates window processing. If A sends segment 224 with option X, and B
receives segment 224 before 223, it MUST ignore option X.

So you can't put in a new option that has a required effect or is
confirmed; there's no way to 'stop the stream' until the endpoints sync.
The only times the endpoints sync is during the SYN.

Joe
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