L3RTT(9)                XR32 REFERENCE MANUAL                 7/2/2013

NAME
        L3RTT -- Layer 3 Round Trip Time.

DESCRIPTION
        L3RTT is an acronym for "Layer 3 Round Trip Time". L3RTT is a
        protocol for measuring the round trip time (RTT) of links.

        Round trip time is used to estimate the performance of a link.
        The smoothed one-way trip time is used in making routing
        decisions, and during the automatic estimation of route
        QUALITY (Autoqual).

        Whilst the layer *2* RTT for a link can be estimated by timing
        how long it takes to get an acknowledgement to a given AX25L2
        frame, this doesn't tell the full story.  The RTT may be good,
        yet the link may be experiencing delays simply because there
        is too much traffic queued on the link.

        L3RTT uses a special type of NetRom layer 3 packet which is
        immediately reflected back by the neighbour if it understands
        the protocol.  This packet is queued with other link traffic,
        so it gives a more accurate estimate of the layer 3 RTT at
        that moment.  The interval between queueing the L3RTT packet
        and receiving the returned packet is the layer 3 RTT.

        XR32 sends L3RTT packets on already-open neighbour node links
        at 5 minute intervals.  L3RTT does not open closed links.


        L3RTT frame format:
        ~~~~~~~~~~~~~~~~~~

        Bytes: 7       7       1     1   1   1   1   1            1
           .-------.-------.-------.---.---.---.---.---.--------.----.
           | l3src | L3dst | l3ttl | 0 | 0 | 0 | 0 | 5 | <text> | CR |
           '-------'-------'-------'---'---'---'---'---'--------'----'
           <----- L3 Header ------><- Dummy L4header -><- Payload -->

           <l3src> is the sender's NODECALL
           <l3dst> is L3RTT-0
           <L3ttl> is 2 (decremeted to 1 by recipient)

        The dummy header simulates L4 <INFO S0 R0>, and the <text>
        field is as follows (one space between each field):

          6    10    10    10    10  6     11    n      n     n
        <fid> <ts> <srtt> <rtt> <pid> <alias> <id> <vers> <maxt> <flg>


            <fid>   Frame ID "L3RTT:"
            <ts>    Time stamp (10ms units)
            <srtt>  Smoothed Round Trip Time
            <rtt>   Last round trip time.
            <pid>   Unique ID for this packet.
            <alias> Nodealias of sender.
            <id>    INP3 version identifier: "LEVEL3_V2.1"
            <vers>  Software name and version e.g. "XRouter201c"
            <maxt>  Max trip time (10ms units) e.g. "$M60000"
            <flg>   Flags field: "$N" indicates INP3 capable

SEE ALSO
        AUTOQUAL(9) -- Automatic Route Quality
        QUALITY(1) -- Netrom Route Quality

L3RTT(9)              END OF DOCUMENT