RTP priority configuration

Discussion in 'Cisco' started by Weiguang Shi, Nov 5, 2003.

  1. Weiguang Shi

    Weiguang Shi Guest


    To improve the jitter of RTP traffic (between two IP phones) that shares the
    same 128 Kbps serial line with a best-effort session (FTP), I did the following
    on the serial interface:

    Router(config)# class-map voice
    Router(config-cmap)# match ip rtp 16384 16383
    Router(config-cmap)# exit

    Router(config)# policy-map voice-queue
    Router(config-pmap)# class voice
    Router(config-pmap-c)# priority 56
    Router(config-pmap-c)# exit
    Router(config-pmap)# exit

    Router(config)# interface serial se0/0
    Router(config-if)# service-policy output voice-queue

    It didn't work as I expected: the average jitter of RTP datagrams lingered
    between 60-100 ms, regardless of the configuration.

    What went wrong?

    Thanks very much!
    Weiguang Shi, Nov 5, 2003
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  2. Weiguang Shi

    Andre Beck Guest

    FTP will involve packets at full MTU, let's say 1500Bytes. That are
    12000Bits. Regardless of queueing strategy, if such packet has started
    transmitting on the outgoing interface, even a higher prioritized
    packet has to wait for that transmission to be completed until it can
    get on the wire. At 128kbps this will take roughly 95ms. If during this
    time, more RTP packets arrive at the interface, they will get precedence
    over the FTP data packets queing up, but as these RTP packets will be
    sparse, at some time they will all have left the queue. Now the next
    FTP bulk packet will start transmitting, and the process starts over.

    I don't see how you can avoid this basic problem, even with the most
    advanced queueing strategy. At some time, you *have* to start sending
    a fat packet, and the time this one takes up the transmitter is fixed.
    The only way around this is to lower the MTU, either at L3 so the FTP
    data is fragmented by IP and the blocking time reduced this way, or
    using an L2 method that can segment and even interleave frames, like
    MP. IIRC there is special support for that in Ciscos MP implementation,
    I'm just not sure whether this also works with a single link in the
    MP "bundle".
    Andre Beck, Nov 9, 2003
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