RFC 619 (rfc619) - Page 2 of 14

Mean round-trip times in the ARPANET

RFC 619           Mean Round-Trip Times in the ARPANET        March 1974


  T(6): The RFNM arrives at the source IMP

  T(7): The RFNM is accepted by the source HOST

The time intervals T(i)-T(i-1) are mainly due to the following delays
and waiting times:

  T(2)-T(1): -HOST processing delay
             -HOST-IMP transmission delay for the 32-bit leader
             -Waiting time for a message number to become free (only
              four messages can simultaneously be transmitted between
              any pair of source IMP - destination IMP)
             -Waiting time for a buffer to become free (there must be
              more than three buffers on the "free buffer list")
             -HOST-IMP transmission delay for the first packet
             -Waiting time for an entry in the PPT or PLT to become
              available (there are eight entries in the PPT and twelve
              in the PLT table)

  T(3)-T(2): -Waiting time for a store-and-forward (S/F) buffer to
              become free (the maximum number of S/F-buffers is 20).
             -Waiting time for a logical ACK-channel to become free
              (there are 8 logical ACK-channels for each physical
              channel).
             -For multiple packet messages, waiting time until the
              ALLOCATE is received (unless an allocation from a previous
              multiple-packet message still exists; such an allocation
              is returned in the RFNM and expires after 125 msec)

  T(4)-T(3): -Queuing delay, transmission delay, and propagation delay
              in all the IMPs and lines in the path from source IMP to
              destination IMP
             -Possibly retransmission delay due to transmission errors
              or lack of buffer space (for multiple packet messages the
              delays for the individual packets overlap)

  T(5)-T(4): -Queuing delay in the destination IMP
             -IMP-HOST transmission delay for the first packet
             -For multiple-packet messages, waiting time for reassembly
              buffers to become free to piggy-back an ALLOCATE on the
              RFNM (if this waiting time exceeds one second then the
              RFNM is sent without the ALLOCATE)

  T(6)-T(5): -Queuing delay, transmission delay, and propagation delay
              for the RFNM in all the IMPs and lines in the path from
              destination IMP to source IMP





Naylor & Opderbeck