Expand Configuration and Management Manual (G06.24+)

Subsystem Description
Expand Configuration and Management Manual—523347-008
18-16
Packet Synchronization
Trace Request (TRACE)
A TRACE is a data packet that is sent in response to an SCF PROBE command. It
contains the identifier of each node it encounters on its route from its sender to its
receiver.
PING Message
A PING message is sent by an Expand line-handler process to measure the round trip
time to a neighbor node. The information obtained by sending a PING message is
used to calculate the effective time factor (ETF) of a path that is a member of a multi-
CPU path. For more information about PING messages, refer to Calculating Route
Time Factors on page 18-27.
Packet Synchronization
Each End-to-End protocol packet includes a sequence number that is used for
synchronization.
LRQs and LCMPs are numbered sequentially. The first LRQ sent is sequence number
0, the second is sequence number 1, and so on.
ACK sequence numbers indicate the acknowledgment of specific LRQs and LCMPs.
For example, an ACK sequence number of 3 acknowledges the receipt of LRQs with
sequence numbers up to but not including 3. In Figure 18-3, the first ACK sent from
node \A acknowledges LRQ sequence numbers 0, 1, and 2.
NAK sequence numbers indicate the negative acknowledgment of specific requests.
For example, a NAK sequence number 1 indicates that LRQ sequence number 0 was
received but that LRQ sequence number 1 was not. Figure 18-4 shows an example of
NAK sequence numbering.
ENQ sequence numbers indicate how many packets have been sent. For example,
when an ENQ sequence number 3 is sent, the sender is telling the recipient that it has
sent packets with sequence numbers up to but not including 3. Figure 18-5 shows an
example of ENQ sequence numbering.
Example of End-to-End Protocol Packet Exchanges
Figure 18-3, Figure 18-4, Figure 18-5, and Figure 18-6 illustrate four different End-to-
End protocol packet exchanges. Figure 18-3 shows an error-free exchange of data; the
remaining figures illustrate how the protocol recovers from problem situations.