Expand Configuration and Management Manual (G06.24+)

Subsystem Description
Expand Configuration and Management Manual—523347-008
18-13
Path Function of the Expand Subsystem
Path Function of the Expand Subsystem
This subsection describes the end-to-end (Layer 3) and packet routing (Layer 4)
messages that are generated by the End-to-End protocol. Layers 3 and 4 of the End-
to-End protocol provide the path function of the Expand subsystem.
This subsection describes the following topics:
•
Protocol Packet Types on page 18-13
•
Packet Synchronization on page 18-16
•
Example of End-to-End Protocol Packet Exchanges on page 18-16
•
Layer 4 Send Window on page 18-21
You must be familiar with the information in this subsection before you can effectively
tune or troubleshoot an Expand network. Layer 3 and Layer 4 protocol statistics are
reported by the Expand SCF STATS PATH command.
Protocol Packet Types
The End-to-End protocol defines the following types of packets:
Connection Request (CONN REQ)
A CONN REQ is a connection-establishment–request-setup packet. Before data can
be exchanged between two nodes over one or more physical lines, a logical
communications path must be opened between the nodes. $NCP selects the best path
to the destination node and directs the Expand line-handler process to send a CONN
REQ packet, which is the first packet to be sent by $NCP when a logical
communications path is to be opened.
Connection Response (CONN RSP)
A CONN RSP is a connection-establishment–response-setup packet. This packet is
sent by $NCP when it receives a CONN REQ packet, which indicates to the requesting
$NCP that the responding $NCP is available for connection establishment.
Connection Acknowledgment (CONN ACK)
A CONN ACK is a connection-establishment–acknowledgment-setup packet. This
packet is sent by $NCP when it receives a CONN RSP packet, which confirms to both
the requesting and the responding $NCPs that a logical connection has been
established.
Note. This subsection does not describe the protocols used by Expand line-handler processes
at the OSI Data Link Layer (Layer 2). For information regarding standards such as HDLC and
X.25, refer to the documentation provided for these standards.