HP X.25/9000 Programmer's Guide

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Chapter 2 17

X.25 Addressing

Addressing Options for Servers

with a called address ﬁeld equal to the X.121 address assigned to the

interface will be delivered to the interface. In this case, specifying an

X.121 address is equivalent to specifying an interface name.

In back-to-back conﬁgurations, a CALL REQUEST packet with any valid

X.121 address can be received by the interface. Any CALL REQUEST

packet, regardless of its X.121 address, is processed by the interface. If a

socket with a matching X.121 address is found, a connection is made.

When issuing a bind() system call, the

x25_host

ﬁeld of the

x25addrstr can contain an X.121 address or it can be empty. If the

x25_host

ﬁeld is not empty and wildcards are not used, then the

speciﬁed address must exactly match the called address ﬁeld of the

CALL REQUEST packet (that is, the called address ﬁeld must be the

same length and match digit-for-digit with the

x25_host[]

ﬁeld in the

x25addrstr structure speciﬁed in the bind() call). See “Wildcard

Addresses in the x25_host[] Field” in this chapter for more information

on using wildcards for this ﬁeld.

If the

x25_host

ﬁeld is empty, then the

x25hostlen

ﬁeld of the bind

address is zero (no X.121 address is speciﬁed) and will match any

incoming CALL REQUEST packet with no subaddress.

Call-matching by Subaddress

Call matching by subaddress is actually an extension of call matching by

X.121 address. The subaddress is appended to the called address ﬁeld in

the CALL REQUEST packet and the

x25_host[]

ﬁeld in the

x25addrstr structure of the bind(). The subaddress, like the X.121

address, is a string of decimal digits (0 – 9). Note that not all PSNs

support subaddresses and some support a varying number of subaddress

digits. Ask your node or network manager for information on conﬁguring

subaddresses.

Call-matching by subaddress is one method by which several servers can

service different calls over the same interface. The programmer of the

client process must know the subaddress as well as the X.121 address

before connection begins. The X.121 addresses and the subaddresses

must exactly match in order for an incoming request to be bound to a

socket.

The combined length of the X.121 address and the subaddress must be

less than sixteen digits. The

x25hostlen

ﬁeld must include the length of

the X.121 address and the length of the subaddress.

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