Computer Networks (CS425)

Instructor: Dr. Dheeraj Sanghi

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Distributed Applications

We can use any of the following two approaches in designing a distributed application.

Semantics of Applications

Passing Arguments in Distributed Programs

Problem: Incompatibility in argument storage

For example, some machines may use 7 bit for storing characters while some others might use 8 bit, some machines may use Big-endian representation while others might use Small-endian representation.

Possible Solutions

XDR (External Data Representation)

XDR was the solution adopted by SUN RPC. RPC was mainly the outcome of the need for distributed filesystems(NFS).

Buffer Paradigm

The program allocates a buffer large enough to hold the external representation of a message and adds items one at a time. The library routine invoked to allocate space for the buffer is xdr_mem_create . After allocating space we may append data to this buffer using various conversion library routines like xdr_int (xdr_int coverts an integer to it's external representaion and appends it to the buffer) to convert native objects to external representaion and then append to the buffer. After all the data to be passed has been converted and appended we send the buffer.

ASN.1

First add the information related to the the data being sent to the buffer and then append the data to the buffer. For example, to send a character followed by an integer (if the sending machine uses one byte for char and two bytes for integers) we send the information as - one byte char, two byte integer ...

The routines for encoding and decoding are the same, depending on the type of the buffer which may be (specified at the time fo allocating space for the buffer) XDR_ENCODE or XDR_DECODE encoding or decoding are performed respectively.

For the routine xdr_int(xdrs, &i)

There are routines (like xdr_stdin_create) to write/read from sockets and file descriptors.
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