Re: [lwip-devel] TCP MSS question

[Jonathan Larmour]

address@hidden wrote:
> Two questions about our TCP implementation:
>
> - RFC 1122 says TCP should send with an MSS of "536 if no MSS option is 
> received" (chapter 4.2.2.6). We send with our configured TCP_MSS 
> instead. Since even Windows sends with 536 (btw: what does linux do?), 
> we should maybe change our code to meet the RFC?

I think the intention of that is to limit it so that it does not exceed 
536, rather than be 536 exactly. This is almost certainly because the 
standard minimum MTU for the internet is 576. In practice of course, the 
MTU is usually more than 576, and the MSS therefore could be more than 
536, which is why the temptation would be to send more.

So we could probably change tcp_connect and tcp_alloc to set up the pcb with:
  pcb->mss = (TCP_MSS > 536) ? 536 : TCP_MSS;
and ensure the MSS option we build to send ourselves uses TCP_MSS, not 
pcb->mss.

It's true that people may want to configure a larger TCP_MSS in general, 
especially when using devices on a LAN that are not routeable on the wider 
net, and when they know the environment it's going to be used in (more 
likely in the contexts lwIP is used in than Windows/*nix). But we're only 
talking about the case when the remote end hasn't sent an MSS option 
(which wouldn't even include lwIP for example) so I don't think it's a big 
concern.

> - tcp_parseopt() doesn't allow a send MSS (advertised by the remote 
> side) to be larger than the configured TCP_MSS. Is this included in 
> order to configure memory usage or does it have other reasons?

That was my belief. It certainly seems reasonable that a system with a 
small receive MSS is not likely to have enough memory for an unlimited (up 
to the MTU) send MSS.

Jifl
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