omahony@swift.cs.tcd.ie (11/23/89)
I have a (fairly basic) question for implementors of OSI protocol stacks: In order to establish an associaion with a remote application layer, one specifies an Application Context + a PSAP address ultimately this must expand to a SSAP, TSAP, NSAP, DSAP etc. How does one choose, for example, what DSAP should be used on the remote node? - in advance of any communication taking place. X.25 has a similar situation with logical channel identifiers, but these are purely a local matter between user and network and guidelines exist on how they should be allocated.
ak2@nvuxr.cc.bellcore.com (Arthur Knapp) (11/29/89)
In OSI naming & addressing & registration authorities, there is no such
thing as a simple question!
You are correct about needing an application-context and
presentation-address to establish an association between application-entities.
Selectors are chosen by the local systems administrator. The called party tells the calling party what the called systems address is. When provided with the (OSI X.500 Directory) distinguished name of an application-entity (aka application-entity-title), the OSI directory will provide the
presentation-address as a tuple { presentation-selector,
session-selector, transport-selector, list of Network-Addresses }. The
DLSAP and subnetwork point of attachment (SNPA) addresses are of no
concern to OSI applications, i.e., let the network, data link and physical
layers do whatever they have to in order to get the job done.
Connection endpoint identifiers (CEIs) are local and are not addresses,
are not part of an address, and are not part of the directory. CEIs are
dynamically assigned.
If you don't use the Directory, then you have to use magic. You write
these addresses down on the back of an envelope and have them entered into
a table.
Arthur Knapp ak2@nvuxr.cc.bellcore.com
Tel: 201-758-2198 Fax: 201-530-6875
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