335
Figure 132 Typical XOT application
X.25 depends on its link layer protocol LAPB to provide reliable links. To ensure link reliability within
the IP network, XOT uses TCP, which offers reliable transmission and window flow control
mechanisms, to tunnel X.25 packets across the IP network as application payload. In this case, TCP
works at the link layer for X.25 and the IP network is analogous to a big X.25 switch.
See Figure 132. The followi
ng describes how XOT works to forward X.25 packets from Router A to
Router D (this example uses SVCs):
• Router A sends a call request to Router D for setting up a virtual circuit.
• Router B receives the request and decides that this is an XOT application. It sets up a TCP
connection with Router C, adds an XOT header to the X.25 call packet, and encapsulates the
packet in a TCP packet, and transmits the TCP packet to Router C.
• Router C receives the TCP packet, removes the TCP and XOT headers, and forwards the call
request to Router D by performing X.25 local switching.
• After receiving the X.25 call request, Router D sends back a call acknowledgement to Router A
through Router C and Router B.
• After receiving the call acknowledgement, Router A and Router D establishes a link for data
transmission.
During this process, the TCP connection establishment between Router B and Router C is
transparent to Router A and Router D, which do not care whether data is forwarded through an IP or
X.25 network.
Hewlett Packard Enterprise implements XOT in compliance with RFC 1613, delivering the following
features:
• Supports SVCs—Two devices can dynamically establish an SVC by sending call packets.
When no data is present, the SVC is cleared automatically.
• Supports PVCs—Once you configure a PVC between two devices, it enters the data
transmission state directly without going through the call establishment process. Even if no data
is present, the PVC remains.
• Supports the TCP keepalive function—If TCP keepalive is configured, TCP checks link
availability regularly. If TCP fails to receive responses from its peer for several times, it
considers the link has failed and disconnects the TCP connection automatically. If TCP
keepalive is not configured, the TCP connection remains for a long time after the link fails.
Configuration procedure
Configuring XOT
When you configure XOT, follow these guidelines:
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