Re: [lwip-devel] [bug #24493] Dropped incoming packets cause a burst of duplicate ACKs

[Jonathan Larmour]

Kieran Mansley wrote:
>>> On Thu, 2008-10-09 at 09:25 +0100, David Woodhouse wrote:
>>>> Has anyone looked at implementing SACK?
> 
> As an option it would be possible, and if someone wanted to implement it
> in that way I wouldn't stop them.  I am wary however of ending up with
> too many optional blocks of code, particularly those that aren't used by
> active developers, as they tend to rot.
> 
> But yes, SACK is pretty complex compared to TCP (another way of putting
> this is that TCP as a base protocol is pretty simple) and it would add a
> fair bit of extra code to both send and receive SACK options and deal
> with them appropriately.  At the moment the level of option we support
> for loss is should we even buffer out of order packets at all.  SACK is
> quite a level of sophistication above that rather crude approach.

I've seen, I think, two attempts at implementing SACK support out there.
Neither were properly contributed IIRC (so it would not be wise, legally,
to just take them) and there were rough edges. A web search could probably
dig out the links. In neither case was the implementation that invasive
really (although it should indeed be optional regardless).

SACK is now a widely used "option", and it is beneficial to small memory
targets as it allows you to free up packet buffers sooner! As such I think
it would be a good fit with lwIP.

Jifl
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