netdev - Re: tcp_mtu_probe implementation details

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Date:	Wed, 21 Dec 2011 13:50:30 +0400
From:	"Anatoly Sivov" <mm05@...l.ru>
To:	"Vijay Subramanian" <subramanian.vijay@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: tcp_mtu_probe implementation details

Hello Vijay,

Thank you for your response.

>> The other question is about size_needed variable.
>> It is assigned to value probe_size + (tp->reordering + 1) *  
>> tp->mss_cache
>> And that is not clear for me.
>> What is this "(tp->reordering + 1) * tp->mss_cache" addition?
>>
>
> I think the idea is that you want enough bytes in the write_queue so
> that even if the probe is lost, the sender will get an ack even if
> there is reordering in the network. Without sufficient bytes, the
> probe will not be sent. This is what I make of the code but I could be
> wrong.

I believe, I found the explanation of this addition in RFC 4821:
   "TCP Fast Retransmit is not robust unless there are
    sufficient segments following a probe; that is, the sender SHOULD
    have enough data queued and sufficient receiver window to send the
    probe plus at least Tcprexmtthresh [RFC2760] additional segments."

However, I'm still confused with magic number 11 in "tp->snd_cwnd < 11"  
check.

Thanks.
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