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About DSCP and 802.1p tag

    Question

  • For packet priority, I can use QWave to set DSCP tag, which is verifyed by capturing packet with ethereal. However, 802.1p tag is not showned in the captured packet. How to set 802.1p tag by qWAVE? Thanks for any help...

     

    Wednesday, September 6, 2006 9:11 AM

Answers

  • With Qos2 (qWAVE), you don't have to worry about DSCP and 802.1p. The underlying platform will always add the correct DSCP and will also add 802.1p if:

    1. The end-to-end path supports it
    2. The destination host your sending to is on the same IP subnet
    3. Your qWAVE flow is adaptive (as opposed to non-adaptive)

    Regarding (1), packets with 802.1p tags get erroneously dropped all the time. Myriad APs, NIC drivers, switches, etc. drop packets with 1p tags for different reasons. Because of this, qWAVE will do a behind-the-scenes experiment to determine whether the path between your sending and receiving machine has an element that drops packets with 1p tags. If qWAVE finds this to be true, we will not add the tag. This is in your favor because otherwise connectivity would be lost. (2) is obvious because 1p tags only make sense on a layer-2 domain. After the packet crosses a layer-3 boundary (router), all bets are off.

    If you are sure 1p tags aren't dropped on the end-to-end path, and you're still not seeing them; the problem is likely:

    1. Sending NIC either does not support adding 1p tags, or the feature is not turned on in the advanced properties of the NIC. (yes, the NIC has to support this for a tag to be added to outgoing frames)
    2. You don't see the tag in your sniffer because the NIC on your sniffer machine is by design stripping the 1p tag before passing the packet up the stack where the sniffer parses it. As I mentioned, this is by design and very important. If you *really* want to see the tag, the sniffer machine must be using a NIC that does not strip tags. I know for certain the in-box Realtek driver (in RC1 build 5600 anyway) does not strip tags on receive. This machine topologically could be the receiver, or attached to a hub that the sending machine is also attached to.

    Hope this helps.

    Gabe [MSFT]

    Wednesday, September 6, 2006 6:16 PM