A server creates a port which it uses to receive requests from clients. Discuss the design issues concerning the relationship between the name of this port and the names used by clients.

Question

What will be an ideal response?

Answer 1

The main design issues for locating server ports are:

(i) How does a client know what port and IP address to use to reach a service? The options are:
• use a name server/binder to map the textual name of each service to its port;

• each service uses well-known location-independent port id, which avoids a lookup at a name server. The operating system still has to look up the whereabouts of the server, but the answer may be cached locally. (ii) How can different servers offer the service at different times?
Location-independent port identifiers allow the service to have the same port at different locations. If a binder is used, the client needs to reconsult the client to find the new location.

(iii) Efficiency of access to ports and local identifiers.

Sometimes operating systems allow processes to use efficient local names to refer to ports. This becomes an issue when a server creates a non-public port for a particular client to send messages to, because the local name is meaningless to the client and must be translated to a global identifier for use by the client.