Re: [RFC PATCHv5 5/5] [TCP]: non-FACK SACK follows conservative SACK loss recovery (RFC3517)

[David Miller <davem@davemloft.net>]

From: "Ilpo_Järvinen" <ilpo.jarvinen@helsinki.fi>
Date: Fri, 16 Mar 2007 20:41:06 +0200

> Many assumptions that are true when no reordering or other
> strange events happen are not a part of the RFC3517. FACK
> implementation is based on such assumptions. Previously (before
> the rewrite) the non-FACK SACK was basically doing fast rexmit
> and then it times out all skbs when first cumulative ACK arrives,
> which cannot really be called SACK based recovery :-).
> 
> RFC3517 SACK disables these things:
> - Per SKB timeouts & head timeout entry to recovery
> - Marking at least one skb while in recovery (RFC3517 does this
>   only for the fast retransmission but not for the other skbs
>   when cumulative ACKs arrive in the recovery)
> - B & C flavors of loss detection (see comment before
>   tcp_sacktag_write_queue)
> 
> This does not implement the "last resort" rule 3 of NextSeg, which
> allows retransmissions also when not enough SACK blocks have yet
> arrived above a segment for IsLost to return true [RFC3517].
> 
> The implementation differs from RFC3517 in two points:
> - Rate-halving is used instead of FlightSize / 2
> - Instead of using dupACKs to trigger the recovery, the number
>   of SACK blocks is used as FACK does with SACK blocks+holes
>   (which provides more accurate number). It seems that the
>   difference can affect negatively only if the receiver does not
>   generate SACK blocks at all even though it claimed to be
>   SACK-capable.
> 
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>

I'm OK with this one too, applied, thanks a lot!