Re: [lwip-devel] Updated default TCP_WND_UPDATE_THRESHOLD

[Simon Goldschmidt]

Joel Cunningham wrote:
> The fix for bug #45559 makes the following change (commit 
> 06d8dba4a0f04161f9899010982d297cf1b82c54) :
>  
> -#define TCP_WND_UPDATE_THRESHOLD   (TCP_WND / 4)
> +#define TCP_WND_UPDATE_THRESHOLD   LWIP_MIN((TCP_WND / 4), (TCP_MSS * 4))
> 
> The bug appears to be about handling problems with window scaling and didn't 
> seem to include any information on why
> TCP_WND_UPDATE_THRESHOLD was changed.  Was TCP_WND / 4 determined to perform 
> too poorly?

Yes. Too poorly. With window scaling, you can have window sizes of megabytes 
and we would send a window update only after accepting e.g. 256 KByte of data. 
The reason for having an update threshold is to limit the number of window 
updates sent to the remote host. 4*MSS is a good compromise for both small and 
large window sizes. Waking up the remote host to send 4*MSS doesn't seem too 
bad.

You're right on that it wasn't discussed in that bug, but I remember it being 
discussed *somewhere* (although not sure when and where, maybe there was 
another bug for that, or we discussed it during initial integration of wubdiw 
scaling?)


Simon