Expand Configuration and Management Manual (H06.21+, J06.10+)

Subsystem Description
Expand Configuration and Management Manual — 529522-013
17 - 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 on PING messages, see Calculating Route Time
Factors on page 17-27.
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 17-3 on page 17-17, 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 17-4 on page 17-18 shows
an example of NAK sequence numbering.
ENQ sequence numbers indicate how many p
ackets 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 17-5 on
page 17-19 shows an example of ENQ sequence numbering.
Example of End-to-End Protocol Packet Exchanges
Figure 17-3 on page 17-17, Figure 17-4 on page 17-18, Figure 17-5 on page 17-19,
and Figure
17-6 on page 17-20 illustrate four different End-to-End protocol packet
exchanges. Figure 17-3 on page 17-17 shows an error-free exchange of data; the
remaining figures illustrate how the protocol recovers from problem situations.