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CS 580 Client-Server Programming
Fall Semester, 2000
Types of Servers
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San Diego State University -- This page last updated 30-Oct-00
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Contents of Doc 17, Types of Servers
References
Internetworking
with TCP/IP, BSD Socket Version Vol. 3, Comer, Stevens, Prentice-Hall, 1993
RFC
1939 Post Office Protocol - Version 3, Myers, Rose, 1996,
http://info.internet.isi.edu:80/in-notes/rfc/files/rfc1939.txt
Types
Of Servers
- Connectionless(UDP)
verse Connection-Oriented (TCP)
- Iterative
verses Concurrent
Iterative
verses Concurrent
Iterative
- Single
process
- Handles
requests one at a time
- Good
for low volume & requests that are easy to satisfy
Concurrent
- Handle
multiple requests concurrently
- Normally
uses thread/processes
- Needed
for high volume & complex requests
- Harder
to implement than iterative
- Must
deal with currency
- Create
threads/processes
- Handle
deadlock/livelock issues
- How
to detect a "bad" thread/process
Single
Process/Thread Concurrent Server
One
can implement a concurrent server using one thread/process
while (true) {
check if any new connects (non-block accept)
if new connection accept
process a little on each current request
}
Stateless
verses Stateful
Servers
State
information
- Information
maintained by server about ongoing interactions with clients
Stateless
server
- Server
that does not maintain state information
Stateful
server
- Server
that does maintain state information
State
information cause problems
- Consumes
resources
- How
long does one maintain the state?
Modes
of Operation
Stateful
servers sometimes have different modes of operation
Each
mode has a set of legal commands
In
Login mode only the commands password & username are accepable
After
successful login client-server connection in transaction mode
In
transaction mode command X, Y Z are legal
These
modes are also called server states or just states
POP3
Protocol
Purpose:
Allow PC's, Macs, etc. to download mail from server
Port
number 110
Protocol
uses ASCII only
Command
to server
- keyword
blank argument1 [ blank argumentk ] 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 "-ERR"
- 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
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 saturn times out in 2 minutes
States
AUTHORIZATION
- Must
log in with password before entering the transaction state
TRANSACTION
- Client
can request actions of server, get mail for example
UPDATE
- Updates
mail box to reflect actions taken in the transaction state
AUTHORIZATION
Server
acknowledges connection from client with
- +OK
"message"
- +OK
UCB Pop server (version 2.1.2-R3) at saturn starting.
Commands:
USER, PASS, APOP, QUIT
USER
PASS
Combination
of commands are used to progress to the transaction state
USER
must come first
PASS
or QUIT must come after USER
Example
- rohan
14->
telnet
saturn 110
- Trying
130.191.229.1...
- Connected
to saturn.sdsu.edu.
- Escape
character is '^]'.
- +OK
UCB Pop server (version 2.1.2-R3) at saturn starting.
- USER
whitney
- +OK
Password required for whitney.
- PASS
don'tyouwish
- +OK
whitney has 21 message(s) (44285 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
22 messages (45595 octets)
- 1
2980
- 2
3064
- (
message 3 - 21 deleted to save space )
- 22
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 saturn.sdsu.ed
- u
(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 Mathematical Science Department
- whitney@cs.sdsu.edu
San Diego State University
- http://cs.sdsu.edu:8080/~whitney
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
Returns:
Top 10 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
- 4
826510843.001
- 5
826510843.002
- 6
826576073.000
- 7
826594928.004
- 8
826603475.000
- 9
826648617.001
- 10
826648617.002
- .
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.
2000 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
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