6.1   Network protocols

• On Unix systems, mainly two protocols used:
  • TCP
  • UDP
• Protocols use sockets: points of contact to a process
• Three possible implementations of communications:
  1. Point to point TCP
  2. Dynamic TCP sockets
  3. Daemon mediated communications

Point to point TCP

TCP sockets are established between every pair of nodes when the program is initiated

Referred to as a TCP socket crossbar

Its run into trouble when scaling up to large number of nodes

No used in PVM

Dynamic TCP sockets

Establish a socket between two communicating nodes at run time when they first communicate with each other

Scales better on large set of nodes

But first communication is significantly slower than subsequent ones.

This startup cost must be amortized by following communications

Used in PVM if the programmer specifies

pvm_setopt (PvmRoute, PvmRouteDirect) ;
        

Daemon mediated communications

Use UDP communication

A socket is attached to each daemon (pvmd)

The processes communicate to their local daemons which forward the communication to another daemon (or to the local process)

That is the default communication mode in PVM