Re: QOS 802.1p implementation

Discussion in 'UK VOIP' started by Stephen, Aug 3, 2013.

  1. Stephen

    Stephen Guest

    On Mon, 29 Jul 2013 16:51:06 +0100, "www.GymRatZ.co.uk"
    <> wrote:

    >Hey folks hope all is good.
    >:¬)
    >Due to a major hardware catastrophe (router, switch, modem death) on my
    >main line I've now got some more up-to date kit in the shop.
    >
    >Namely a TP-Link TL-SG1016DE smart switch and a Draytek 2920n router
    >both of which are 802.1p compatible.
    >
    >The question really is how is 802.1p "implemented"?
    >The switch can be set as either port priority or 802.1p so which part of
    >the chain adds the priority tag. The Gigaset S685IP box doesn't have
    >anything to set up with regards to QOS so does the switch implement the
    >priority tagging automatically?


    the convention with IP telephony is that the IP packets for the real
    time audio stream should be marked as DSCP EF (expedited forwarding,
    46).

    Most kit seems to also mark the 802.1p bits as CoS 5 is those packets
    are 802.1Q VLAN tagged.

    When it comes to the switch then a lot depends on exactly what the
    hardware can do and how it gets configured.

    There are 3 more or less standard ways to mark "priority" in a packet,
    and many packets will carry more than 1 marking, so in reality a
    switch which understands QoS will need to ignore all of them, or use 1
    inbound "mark" and work on that.

    On packets sent out by the switch you can either
    - wipe out all CoS, DSCP or some subset of those
    - apply a port level marking
    - send out the markings that came in (transparent)
    - act on 1 inbound type of marking and manipulate that, other gets
    zeroed or flow through
    - act on 1 such as DSCP, propgate that and synthesize the 802.1p
    markings

    all of these are useful for some setups, so difficult to tell what
    your stuff is doing (and this level of detail doesnt seem to get into
    the docs)

    >
    >And... if a packet is tagged(?) with 802.1p would this then carry
    >through 21CN network and beyond?


    802.1p bits are "local significance" within an Ethernet bridged domain
    - they get stripped at a router or other layer 3 device
    - so unless the device has a mechanism to propagate the CoS level to
    the WAN or IP packet info then the priority is left behind.
    >
    >Trying to get my head around whether the benefits extend WAN side and
    >how VOIP data gets it 802.1p tag in the first place.


    802.1p is Ethernet specific

    devices are free to ignore it either because the hardware is blind to
    it, or because they are not configured to pay attention, so DSLAMs et
    al downstream are unlikely to do much

    however - LANs often see little benefit from CoS / QoS unless they are
    heavily loaded.
    - CoS /Qos is all about choosing which packet to operate on next and
    or what to delay or throw away, and a lightly loaded LAN working
    properly should not need to do much around that.
    >
    >Cheers
    >Pete

    --
    Regards

    - replace xyz with ntl
    Stephen, Aug 3, 2013
    #1
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