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802.1p queues

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  • 1.  802.1p queues

    Posted May 12, 2021 01:06 PM

    Hi there,

    I have noticed that when using the 802.1p queue scheme and the VLAN PCP priorization, everything works pretty well when using TCP ( if I have 2 queues, 20% of the bandwidth goes to Q1 and the 80% to Q2) but when using UDP, if a stream has its packets tagged with the PCP =7 (highest prioririty, being mapped to Q2) and if the throughput is close to the physical port speed, there is a high percentage of dropped packets in the Q2 and the stream has lower perfomance when comparing to when it goes to Q1.

    Can somebody explain me this? Is that because the way UDP works? Does anyone know the size of the queues? Is this some bug?

    Thanks in advance.

    Rita Martinho

  • 2.  RE: 802.1p queues

    Posted May 14, 2021 10:01 AM
    If Q2 is saturated with flows causing congestion, I would expect packet drop with UDP (TCP can retransmit and reorder).

    Steve Baker

  • 3.  RE: 802.1p queues

    Posted May 14, 2021 02:34 PM

    Thank you for your answer Steve!

    Yes, that absolutely makes sense. I was more surprised by the amount of packet loss I was observing. But some days ago I discovered that the packets were being fragmented and so one lost fragment would lead to a complete packet loss. What I do not understand yet is how the behavior of Q1 outperforms the behavior of Q2 in this aspect. My only guess is that the size of Q2 is smaller (I can not find info about that anywhere).

    Rita Martinho