[Codel] About Packet Drop in Codel

[Rick Jones]

On 03/03/2015 10:12 AM, divya singla wrote:
> But Sir i heard that UDP does not respond to congestion.Even though its
> packets are lost, it keeps sending packets at the same rate(unlike tcp)

UDP does not and never has.  All it has ever done is send what the 
application has told it to send as the application tells it to. The 
*application* using UDP is expected to respond to congestion.

rick jones

>
> -- please answer this too:
>   Does codel implement concept of marking(ECN)  in ns2?
>
>
> On Tue, Mar 3, 2015 at 12:37 AM, Jonathan Morton <chromatix99 at gmail.com
> <mailto:chromatix99 at gmail.com>> wrote:
>
>     With UDP, you're at the mercy of the application using it. With TCP,
>     you're merely at the mercy of the operating system.
>
>     AQM acts on UDP packets in the same way as TCP packets - in fact it
>     can't tell them apart. So any application which detects and responds
>     to UDP packet loss in the same way as TCP does, will back off just
>     the same.
>
>     In practice, UDP is used for several different types of application:
>
>     - simple request response, such as DNS and NTP, where eliminating
>     TCP's connection setup overhead is important. In any case, TCP's
>     congestion control doesn't get a chance to do any good on such s
>     short-lived connection. Packet loss in this situation is tolerated
>     by retry, with exponential backoff as an alternative congestion
>     control measure.
>
>     - latency sensitive and often isochronous (inelastic) flows like
>     VoIP. Packet loss may lead to a loss of quality, but there is little
>     the application can do to reduce its loss except dropping the call
>     completely.
>
>     - as a way to implement delay sensitive and pacific congestion
>     control algorithms, as in uTP.
>
>     A flow isolation system, such as that in fq_codel, will often leave
>     UDP flows alone completely, because they tend not to be the ones
>     using the bulk of the bandwidth. Conversely, if a single UDP flow
>     was responsible for the congestion, it would let the other traffic
>     bypass it. This is why fq_codel is better than just plain codel, if
>     you can get it.
>
>     - Jonathan Morton
>
>
>
>
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