IST362: Lecture 2.

Application Layer.

Application Layer Basics

Processes running on different computers communicate with each other by exchanging message across the computer network. It's important to distinguish between network applications and network-layer protocols. For example, Web application consists of many components:

web browser

web server

standard document format (like HTML)

and an application-layer protocol (HTTP).

Application layer protocol defines:

The types of messages exchanged, for example, request messages and response messages.

The syntax of the various message types, such as the fields in the message and how the fields are delineated.

The semantics of the fields, that is, the meaning of the information in the fields.

Rules for determining when and how a process sends messages and responds to messages.

Typically a network application has two parts or "sides", a server side and a client side. Let's agree that the host that initiates the session is labeled the client.

Ideas of

socket as a door between the application layer and transport layer;

IP address

destination port number

User agent is an interface between the user and the network.

Application's needs and available services

Needs:

Reliable data transfer (loss-tolerant applications vs. no loss-tolerant ones)

Bandwidth. Some application must be able to transfer data at a certain rate in order to be effective. For example, Internet telephony requires at least 32 Kbps. (band-width sensitive vs. elastic applications.)

Timing. Some application require timing constraints on data delivery in order to be effective.

Available services from the Transport Layer:

TCP -- connection-oriented reliable transport service.

UDP -- no-frills, lightweight transport protocol

Telnet protocol

transport layer service used: TCP

port number: 23

HTTP protocol

transport layer service used: TCP

port number: 80

maintaining information about the client: No => stateless protocol

type of connection: persistent or non-persistent

/somedir/index.html

www.somesite.edu

The browser initiates a TCP connection to the server www.somesite.edu on port 80.
The browser sends an HTTP request to the server via socket established in step 1. The request message includes the path name /somedir/index.html
The HTTP server receives the message via the socket associated with the connection, retrieves the requested object from a storage, encapsulates the object in an HTTP response message, and sends the message to the client via the socket.
The HTTP server tells TCP to close the connection.
The HTTP client receives the response message. The connection terminates. The client extracts the file from the response message, examines the file, and finds references to other objects to request.

There are two versions of persistent connection: without pipelining and with pipelining.

Client side messages

Client side message format:

Possible protocols: HTTP/1.0 and HTTP/1.1
Possible methods: GET, POST, HEAD, PUT, DELETE
Some of the possible headers:
- Host
- Connection
- User-Agent
- Accept
- Accept-language
- Set-cookie
- Cookie
- If-modified-since
- Authorization

Server side messages

The server side message format is:

The status code and associated phrase indicate the result of the request. Some common status codes and associated phrases include:

200 OK: Request succeeded and the information is returned in the response.
301 Moved Permanently: Requested object has been permanently moved; new URL is specified in Location: header of the response message. The client software will automatically retrieve the new URL.
304 Not modified: The requested document was not modified after the date specified at If-modified-after: field of the client request.
400 Bad Request: A generic error code indicating that the request could not be understood by the server.
401 Authorization Required: The requested document is located in a directory that requires authorization. Field Authorization was expected from the client.
404 Not Found: The requested document does not exist on this server.
505 HTTP Version Not Supported: The requested HTTP protocol version is not supported by the server.

This page will help you to see server side messages.

Some useful links:

HTTP/1.0: http://www.rfc-editor.org/rfc/rfc1945.txt

HTTP/1.1: http://www.rfc-editor.org/rfc/rfc2068.txt

Cookies: http://home.netscape.com/newsref/std/cookie_spec.html