1.1.f Explain UDP operations

1.1.f [ii] Latency

from rfc 1242 http://www.rfc-editor.org/rfc/rfc1242.txt
3.8   Latency

        Definition:
                For store and forward devices:
                The time interval starting when the last bit of the
                input frame reaches the input port and ending when
                the first bit of the output frame is seen on the
                output port.

                For bit forwarding devices:
                The time interval starting when the end of the first
                bit of the input frame reaches the input port and
                ending when the start of the first bit of the output
                frame is seen on the output port.

        Discussion:
                Variability of latency can be a problem.
                Some protocols are timing dependent (e.g., LAT and IPX).
                Future applications are likely to be sensitive to

Benchmarking Methodology Working Group                          [Page 5]

RFC 1242                Benchmarking Terminology               July 1991

                network latency.  Increased device delay can reduce
                the useful diameter of net.  It is desired to
                eliminate the effect of the data rate on the latency
                measurement.  This measurement should only reflect the
                actual within device latency.  Measurements should be
                taken for a spectrum of frame sizes without changing
                the device setup.

                Ideally, the measurements for all devices would be from
                the first actual bit of the frame after the preamble.
                Theoretically a vendor could design a device that
                normally would be considered a store and forward
                device, a bridge for example, that begins transmitting
                a frame before it is fully received.  This type of
                device is known as a "cut through" device.  The
                assumption is that the device would somehow invalidate
                the partially transmitted frame if in receiving the
                remainder of the input frame, something came up that
                the frame or this specific forwarding of it was in
                error.  For example, a bad checksum.  In this case,
                the device would still be considered a store and
                forward device and the latency would still be
                from last bit in to first bit out, even though the
                value would be negative.  The intent is to treat
                the device as a unit without regard to the internal
                structure.