Network Class 4

1.  Client-server architecture

 

• server

     - always-on host

     - permanent IP address

     - data centers for scaling

 

• clients

     - communicate with server

     - may be intermittently connected

     - may have dynamic IP addresses

 

 

2.  P2P architecture

 

• no always-on server

• peers request service from other peers, provide service in return to other peers

     - self scalability (new peers bring new service capacity, as well as new service demands)

• peers are intermittently connected and change IP addresses

     - complex management

 

 

3.  Processes communicating

 

• process : program running within a host

     - client process : process that initiates communication

     - server process : process that waits to be contacted

 

     - aside : applications with P2P architectures have client processes & server processes

 

 

4.  Sockets

 

• process sends/receives messages to/from its socket

• socket analogous to door

 

 

 

5.  Addressing processes

 

• to receive messages, process must have identifier

• host device has unique 32-bit IP address

• identifier includes both IP address and port numbers associated with process on host

        ‣ fixed port numbers

                  - HTTP server : 80

                  - mail server : 25

 

 

6.  What transport service does an app need?

 

• data integrity (file transfer)

 

• timing (interactive games)

 

• throughput (multimedia)

 

• security

 

 

7.  Internet transport protocols services

 

• TCP service:

     - reliable transport

     - flow control

     - connection-oriented

     - congestion control

     - does not provide : timing, minimum throughput guarantee, security

 

•UDP service :

     - unreliable data transfer

     - does not provide : reliablility, flow control, congestion control, timing ...

 

 

❓ why bother? why is there a UDP?

 

1. Because TCP set up a connection, the protocol overhead may be large

2. A certain amount of delivery per unit time can be prevented at the transport layer

3. It Can be repeated with checks in the application layer

 

 

8.  App-layer protocol defines

 

• types of messages exchanged

     - e.g) request, response

• message syntax

• message semantics

     - meaning of information in fields

• rules

 

• kinds of

       ‣ open protocols

       ‣ proprietary protocols

 

 

9.  Web and HTTP

 

• web page consists of objects

• object can be HTML file, JPEG image, Java applet, audio file...

• web page consists of base HTML-file which includes several referenced objects

• each object is addressable by a URL

 

HTTP overview

HTTP : hypertext transfer protocol

• Web's application layer protocol

• client/server model

 

uses TCP :

• client initiates TCP connection to server, port 80

• server accepts TCP connection from client

 

HTTP is "stateless"

• server maintains no information about past client requests

 

HTTP connections

1. non-persistent HTTP

     - at most one object sent over TCP connection, connection then closed

     - downloading multiple objects requires multiple connections

 

2. persistent HTTP

     - multiple objects can be sent over single TCP connection between client, server