CSE 480 Lecture Notes
January 30, 1996

Interprocess Communication

The Client-Server Model
- Servers -- a group of cooerating processes that provide OS services
- Clients -- user processes

To avoid overhead of the connection-oriended protocols,
the client server model is based on a simple connectionless
request/reply protocol.

Shorter protocol stack for efficiency (Fig 2-7) -- no routing, no connection

E.g., Fig 2-9

+Addressing
1) Direct Communication

   send (P,msg) -- send a msg to process P
   receive (Q, msg) -- receive a msg from process Q
 or
   send (P, msg) -- send a msg to process P
   receive (id, msg) -- receive a msg from any process and set id to the sender

- limited modularity since name change requires changes to others
- if process id is of the form (machine-id, process-id) pair, then
  not location transparent

2) Indirect Communication
  
  send (A, msg) -- send a msg to mailbox A
  receive (A, msg) -- receive a msg from mailbox A

- mailboxes
- can be used for multicasting

+ Blocking versus Non-blocking Primivitives (Fig. 2-11)

1) blocking privimitives (synchronous primitives)
2) non-blocking primitives (asynchronous primitives)

- send
  blocking send
  nonblocking send with copy
  nonblocking send with interrupt
- receive
  blocking receive
  nonblocking receive

+ Buffering

- problem: what if a msg is sent before receiver is ready

1) zero capacity -- synchronous communication or rendezvous
2) bounded capacity -- queue with lenght n
   what happen of queue overflows
3) unbounded capacity

+ Reliable versus Unreliable Primitives

- how does a sender knows that its msg is delivered?

1) unreliable send -- up to the user, e.g., post office letter
2) ack by the kernel on the receiving machine (Fig 2-13(a))
3) ack by the reply (Fig 2-14(b))

+ Implementing the Client-Server Model

- Fig 2-14 possibilities

- Use of acknowledgement -- large msg divided into packets

1) every packet -- more traffice due to ack, faster
2) entire msg -- less traffice due to ack, slow

- how long a packet should be kept at the sender side?
  how many sent but not acked packets should be allowed?
  e.g., alternative bit protocol


- packet types (Fig 2-15)
- typical packet exchange senario (Fig 2-16)