L3RTT

Name

L3RTT -- Layer 3 Round Trip Time Protocol.

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.

XRPi 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(2) -- Netrom Route Quality