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CS 580 Client-Server Programming
Fall Semester, 2002
Client-Server Protocols & POP
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San Diego State University -- This page last updated 10-Oct-02
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Contents of Doc 14, Client-Server Protocols & POP
Reference
Reference
RFC 1939, http://www.ietf.org/rfc/rfc1939.txt
CS
580, Spring 1997 lecture notes,
http://www.eli.sdsu.edu/courses/spring97/cs596/notes/protocol/protocol.html
Protocol
Communication
between client and server
Good
protocols are hard to design
Requirements
for a "good protocol":
- 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
-
- Allow
for code reuse
-
- 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:
Protocol
Types
Two
basic types
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
-
- Who
is writing, who is reading?
-
- Timeouts
vs. Synchronous protocols
POP3
Protocol
Purpose:
Allow PC's, Macs, etc. to download mail from server
Port
number 110
Protocol
uses ASCII only
Command
Format
Format
of commands to server
keyword
blank argument
1
[ blank argument
k
] CRLF
|
keyword | = 3, 4 characters
|
argument | <= 40 characters
keyword
and arguments are separated by single space character
Server
Response
Status
keyword additionalInfo
Status
is either "+OK" or "-ERR0.3."
A
single line response ends in CRLF
If
response requires more than one line:
-
- Each
line ends in a CRLF
- The
response ends in CRLF.CRLF
- If
a line starts with a "." prepend a "." to it
Timeouts
A
POP3 server may have an autologout timer
A
server must wait at least 10 minutes before timing out a client
The
POP3 server on cs.sdsu.edu times out in 2 minutes
States
AUTHORIZATION
- Must
log in with password before entering transaction state
TRANSACTION
- Client
can request actions of server, get mail for example
UPDATE
- Updates
mail box to reflect actions taken in transaction state
Commands
AUTHORIZATION
Server
acknowledges connection from client with
- +OK
"message"
- +OK
UCB Pop server (version 2.1.2-R3) at sciences.sdsu.edu starting.
Commands:
USER, PASS, APOP, QUIT
USER
PASS
Combination
is used to progress to transaction state
USER
must come first
PASS
or QUIT must come after USER
Example
- Ti
38->
telnet
cs.sdsu.edu 110
- Trying
130.191.226.116...
- Connected
to cs.sdsu.edu.
- Escape
character is '^]'.
- +OK
QPOP (version 3.1.2) at sciences.sdsu.edu starting.
- USER
whitney
- +OK
Password required for whitney.
- PASS
typeYourPasswordHere
- +OK
whitney has 116 visible messages (0 hidden) in 640516 octets.
TRANSACTION
Commands:
STAT, LIST, RETR, RSET, QUIT
STAT
Arguments:
none
Returns
"+OK" numberOfMessages SizeOfMail
Example
- STAT
- +OK
22 45595
LIST
Arguments:
a message-number ( optional )
Returns:
size of message in octets
Examples:
- LIST
2
- +OK
2 3064
- LIST
- +OK
116 visible messages (640516 octets)
- 1
2980
- 2
3064
- (
message 3 - 116 deleted to save space )
- 116
1290
- .
RETR
Arguments:
a message-number
Returns:
the message
Example:
- RETR
21
- +OK
825 octets
- Received:
from [130.191.9.18] (ebb2p9.sdsu.edu [130.191.9.18]) by sciences.sdsu.edu
-
(4.1/8.6.10) with SMTP id UAA29486 for <whitney@saturn.sdsu.edu>; Mon, 11
Mar
- 1996
20:16:07 -0800 (PST)
- X-Sender:
whitney@cs.sdsu.edu (Unverified)
- Message-Id:
<v02110100ad6aaaf097b6@[130.191.9.70]>
- Mime-Version:
1.0
- Content-Type:
text/plain; charset="us-ascii"
- Date:
Mon, 11 Mar 1996 20:16:50 -0800
- To:
whitney@saturn.sdsu.edu
- From:
whitney@saturn.sdsu.edu (Roger Whitney)
- Subject:
Sample Mail
- X-UIDL:
826604201.000
- this
is a test
- ..
- the
end
- ---
- Roger
Whitney Math & Computer Science Dept.
- whitney@cs.sdsu.edu
San Diego State University
- http://www.eli.sdsu.edu
San Diego, CA 92182-7720
- (619)
594-3535
- (619)
594-6746 (fax)
- .
DELE
Arguments:
a message-number to delete
Returns:
a confirmation of deletion
Marks
a message to be deleted
NOOP
Arguments:
none
Returns:
a positive response
Does
nothing
QUIT
Arguments:
none
Returns:
a positive response
Send
POP3 server to UPDATE state
UPDATE
State
Updates
mail box to reflect transactions taken during the transaction state, then logs
user out
If
session ends by any method except the QUIT command during the transaction
state, the update state is not entered
Optional
POP3 Commands
TOP
Arguments:
a message-number and number of lines to return
Returns:
Requested lines of indicated message
State
allowed in: transaction
UIDL
Arguments:
a message-number ( optional )
Returns:
a unique-id listing for message
State
allowed in: transaction
Examples:
- UIDL
1
- +OK
1 826312760.001
- UIDL
- +OK
uidl command accepted.
- 1
826312760.001
- 2
826312760.006
- 3
826493796.004
- etc.
- .
APOP
Arguments:
a mailbox and a MD5 digest string
State
allowed in: authorization
Action:
If MD5 string is correct move to transaction state
Copyright ©, All rights reserved.
2002 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
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