Client-Server Programming Spring Semester, 2005 Protocol
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Protocol

Well defined

Complete

Parsable

Available

Protocol Types

Protocol Design Issues

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Protocol

Communication between client and server

Good protocols are hard to design

Requirements for a "good protocol":

• Well defined

• Complete

• Parsable

• Extendable

• Available protocol document

Well defined

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

Protocol is a 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 must cover all possible situations.

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

Parsable

Both clients and servers are computer programs.

A computer program's IQ is generally 0.

Design goals:

• Distinct information packets or messages

• Allow parsing independent of semantics

• Consistency

• Allow for code reuse

• Flexibility

• For example name-value pairs

Available

Different 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

Protocol Types

Two basic types

• Synchronous
• Asynchronous

Typical synchronous

• Client sends request to server
• 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

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

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