HP X.25/9000 Programmer's Guide

18 Chapter2
X.25 Addressing
Addressing Options for Servers
Call-matching by Protocol ID
Call-matching by protocol ID is a flexible way to allow multiple servers to
service incoming calls over the same interface. First the incoming call’s
X.121 address and subaddress is tested and then the protocol ID. If the
x25pidlen field is 0, the protocol ID is not used.
The protocol ID field is at the beginning of the call user data field of the
CALL REQUEST packet. The server specifies the protocol ID in the
x25pid field of the x25addrstr structure. The protocol ID can be from 1
to 9 bytes long. Refer to the CCITT X.244 Recommendations for more
information on protocol ID addressing.
Client and server programmers must agree on how many bits to specify
for the protocol ID, but the length is not defined by the X.244 (1984) and
X.25 (1980) Recommendations. HP suggests that you use protocol IDs to
match incoming calls to sockets because a single listen socket can be
used for any number of X.25 interfaces (independent ports) and
subaddresses are not always supported over PSNs.
You can also set a bit mask to specify a range of protocol IDs. The bit
mask is described in “Using Wildcard Addressing” in this chapter.
Matching by protocol ID can identify higher-level protocols, such as those
specified by PAD support.
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