Specifications

Ethernet Loop Testing Page E-10
In the interests of
simplicity and efficiency, loopback operation and
message formats are designed to meet the following requirements:
1. All fields begin on 16-bit boundaries.
2. Progressive operations on the same message (e.9. looping it
back) do not change the message size.
The general form of operation
is that different loopback message types
are encapsulated within one another. The first field, called the skip
count, in all messages indicates how many octets to skip after the
skip count to find the message type. When
a message is processed, the
processing system updates the skip count so that the next system will
process the next encapsulated message. Note that in order to meet the
16-bit boundary requirement, the skip count is always an even number.
The Loop Test protocol uses protocol type 90-00.
The operational descriptions assume the following Loop Test protocol
messages.
1. Reply - a message identifiable as a response to some request.
2. Forward Data - A message whose data portion is forwarded to
another station.
E.4.1 Loop Server
The ~oop Server always keeps a receive pending while the data link
is
turned on. Whenever a receive completes, it
is processed and another
receive posted. For purposes of not missing messages,
it may be
necessary to
keep more than one receive posted, although this is not
required.
In order to help bound the time a test can take, the Loop Server must
respond to the data link within one second of the time it receives a
valid message.
The received message is processed according to function code:
1. Forward Data message.
The skip count is increased by the length of the function code
and forward address. If the forward address
is a multicast
address, the message
is ignored. Otherwise, the message is
transmitted to the forward address.