Request-reply protocols

Question 1

What is the typical pattern for client-server interaction?

Answer 1

Client sends request
Server receives request
Server handles request
Server handles response
Client receives response and continues

Question 2

What are some data requirements for request-reply APIs?

Answer 2

The server needs to be identified by a remote reference (host IP and port number)
Choice of operation as the server may support several different operations (explicit operation ID)
Client and server must agree on how arguments are encoded - external data representation

Question 3

Name some things needed for request-reply implementation?

Answer 3

Message identifiers, failure model, timeouts, duplicate request messages, history

Question 4

Describe message identifiers

Answer 4

Typically each request is given a unique ID

Question 5

Describe failure model

Answer 5

Can suffer from communication omission faults and process omission faults

Question 6

Describe timeouts

Answer 6

Client typically uses a timeout to detect a missing reply - the client then retransmits the request

Question 7

Describe duplicate request messages

Answer 7

Can be detected in the server from the duplicate message identifier

Question 8

Describe history

Answer 8

With idempotent operations performing the operation twice makes no difference (e.g. setting a variable) but with other operations the server should not execute a duplicate request twice
Results are held in history and re-sent back to the client if a duplicate request is received

Question 9

Compare and contrast TCP and UDP