Re: [lwip-users] MSS maximum

[address@hidden]

Hi Art,

Can you tell us which version of lwIP you are using? There has been a 
change in the MSS handling in 1.3.0.

Apart from that, TCP_MSS is the mss advertised to the remote host (the 
remote host will use this to calculate the mss used to send to lwIP), 
while pcb->mss contains the mss as advertised by the remote host. lwIP 
will use this for sending.

So if you see lwIP sending with 536, it might be the other side 
advertises this.

In my tests, I'm sure to have seen mss's of 1460, so your problem might 
probably have another reason. The value of 536 is a historical value as 
described in on of the TCP RFCs and has probably nothing to do with PPP.

Simon


Art R. schrieb:
> The lwIP code seems to enforce an absolute maximum value for MSS of 536. If
> you set the TCP_MSS tag larger, the lwIP code limits the actual mss to 536.
> This value is hard-coded in tcp.c. I do not see any way for pcb->mss to
> become larger than 536, implying that the largest MSS that can be advertised
> by lwIP to be larger than 536.
>
> The result is that a tcp write of more than 536 bytes creates multiple
> outbound TCP packets.
>
> I would like to send 'full size' packets of 1460 bytes. Is there a way to do
> so using lwIP?
>
> (I realize that the 536 value is probably in place to limit packets destined
> for PPP or etc. I assume there is nothng in lwIP currently to discover
> packet delivery failures and reduce MSS in response? Is this the reason why
> lwIP clamps MSS to 536? Or ...?)
>
> Thanks,
> Art R.
>
>
>
>