Expand Configuration and Management Manual (G06.24+)

Subsystem Description
Expand Configuration and Management Manual—523347-008
18-18
Example of End-to-End Protocol Packet Exchanges
Data Exchange With Lost Data
Figure 18-4 shows a data exchange in which a packet is not received. This problem is
usually caused by network congestion and/or line failures and is indicated by a large
number of NAKs on the SCF STATS display.
Node \A sends three LRQs to node \B. Node \B receives LRQ sequence number 0 and
LRQ sequence number 2 but does not receive LRQ sequence number 1. Node \B
starts its out-of-sequence (OOS) timer as soon as LRQ sequence number 2 is received
out of order. When the OOS timeout period has been reached, node \B sends NAK
sequence number 1 to notify node \A that it has not received LRQ sequence number 1.
Node \A then resends its message starting at LRQ sequence number 1.
Node \B sends ACK sequence number 3 to positively acknowledge LRQ sequence
number 0, LRQ sequence number 1, and LRQ sequence number 2; and it then
responds to each LRQ with an LCMP. Node \A acknowledges the receipt of node \B’s
LCMPs by sending ACK sequence number 3.
Figure 18-4. Lost Data
013CDT .CDD
LRQ (0)
LRQ (1)
LRQ (2)
NAK (1)
NODE \A NODE \B
Lost
packet
ACK (3)
LCMP (0)
LCMP (1)
LCMP (2)
ACK (3)
LRQ (1)
LRQ (2)
Out-of-sequence (OOS) timeout period