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Date:	Thu, 29 Oct 2009 17:32:22 +0100
From:	Arnd Hannemann <>
To:	Andreas Petlund <>
Cc:	William Allen Simpson <>,
	"" <>,
	"" <>,
	"" <>,
	"" <>,
	"" <>
Subject: Re: [PATCH 1/3] net: TCP thin-stream detection

Andreas Petlund schrieb:
> Den 28. okt. 2009 kl. 04.09 skrev William Allen Simpson:
>> Andreas Petlund wrote:
>>> +/* Determines whether this is a thin stream (which may suffer from
>>> + * increased latency). Used to trigger latency-reducing mechanisms.
>>> + */
>>> +static inline unsigned int tcp_stream_is_thin(const struct  
>>> tcp_sock *tp)
>>> +{
>>> +	return tp->packets_out < 4;
>>> +}
>>> +
>> This bothers me a bit.  Having just looked at your Linux presentation,
>> and not (yet) read your papers, it seems much of your justification  
>> was
>> with 1 packet per RTT.  Here, you seem to be concentrating on 4,  
>> probably
>> because many implementations quickly ramp up to 4.
> The limit of 4 packets in flight is based on the fact that less than 4  
> packets in flight makes fast retransmissions impossible, thus limiting  
> the retransmit options to timeout-retransmissions. The criterion is  

There is Limited Transmit! So this is generally not true.

> therefore as conservative as possible while still serving its purpose.  
> If further losses occur, the exponential backoff will increase latency  
> further. The concept of using this limit is also discussed in the  
> Internet draft for Early Retransmit by Allman et al.:

This ID is covering exactly the cases which Limited Transmit does not
cover and works "automagically" without help of application. So why not
just implement this ID?

Best regards,
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