CS 596: Client-Server Programming
Spring Semester, 1997
Doc 11, Client Server Protocols

Contents of Doc 11, Client Server Protocols

Doc 11, Client Server Protocols Slide # 2

Protocol

Communication between client and server

Good protocols are hard to design

Requirements for a "good protocol":

• Well defined (Protocol cannot be vague)
• Complete (All possible situations must be addressed)
• Parsable (Both clients and servers need to parse the protocol)
• Extendable (Protocols rarely remain the same)
• Available protocol document (Clients and servers that use a protocol can be written by different people)

Doc 11, Client Server Protocols Slide # 3

"Well defined"

Every bit of data sent in either direction has to have its place in the protocol description.

Protocol == Language

Common formal description:

BNF and Augmented BNF

Format of the description language needs to be part of the protocol document.

Examples are important

"Complete"

The protocol document must cover all possible situations.

• Garbage data
• Old client or server (different protocol versions)
• Illegal requests
• Boundary conditions
• Etc.

Doc 11, Client Server Protocols Slide # 4

"Parsable"

Both clients and servers are computer programs.

A computer program's IQ is generally 0.

Important design goals: (Not always possible)

• Distinct information packets or messages (allows separation of syntax and semantics)
• Consistency (allows for code reuse)
• Flexibility (remember the name-value pairs?)

"Available"

For a given protocol, distinct groups may write clients and servers at different times.

Central registry for internet protocols

Self regulating:

• RFC == Request For Comment
• IETF == Internet Engineering Task Force

Official:

• ISO
• ANSI

Doc 11, Client Server Protocols Slide # 5

Protocol Types

Two basic types:

1. Synchronous
2. Asynchronous

Typical synchronous:

1. Client sends request to server
2. Server responds with a reply

Examples: HTTP, POP, SMTP, GOPHER, XMODEM

Typical asynchronous:

Client and server both send information to each other concurrently.

Examples: TELNET, RLOGIN, ZMODEM

A hybrid protocol is also possible

Doc 11, Client Server Protocols Slide # 6

Protocol Design Issues

Protocol design is difficult!

Learn from examples

Some issues:

• Protocol extendibility and versioning
• Byte order used for sending values
• ASCII vs. Binary protocol
• Easy of debugging
• Efficiency
• Synchronous vs. Asynchronous
• Protocol overhead
• Roundtrip delays
• State
• Who is writing, who is reading?
• Timeouts
• Timeouts vs. Synchronous protocols