Application Layer Protocols

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Application Layer Protocols

•

• Network Applications Requirements

Network Applications Requirements

•

• Application Layer Protocol Functions.

Application Layer Protocol Functions.

•

• Sample Internet Applications & Protocols:

Sample Internet Applications & Protocols:

–

File Transfer Protocol (FTP).

–

Sending E-Mail: SMTP.

–

HyperText Transfer Protocol (HTTP).

•

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Network Applications &

Application Layer Protocols

• The development of numerous network applications and

the associated application-layer protocols has been a major

driving force for computer network advancements over the

past 30 years.

• The range of such diverse applications include:

–

Text-based applications such as telnet, electronic mail, file

transfer, newsgroups, most popular in the 70’s-80’s.

–

More recent graphics- and multimedia-based applications

such as the World Wide Web (

the Internet’s killer-app

),

Internet telephony, video conferencing, and streaming

audio/video on demand, and interactive games.

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Common Network Applications Requirements

Application Type Data Loss Bandwidth Requirements Latency sensitivity File transfer No loss Variable none

Web documents No loss Variable none Real-time audio/video Loss-tolerant Audio: few Kbps to 1Mbpsyes 100's of msec

Video: 10's Kbps to 5 Mbps

Stored audio/video Loss-tolerant Same as interactive audio/video few seconds Interactive games Loss-tolerant Few Kbps to 10's Kbps 100's msecs Financial applications No loss Variable Application-dependent

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Application Layer Protocols

• An application layer protocol defines how an application processes (clients and servers) , running on different end systems, pass messages to each

other.

• In particular, an application layer protocol defines:

– The types of messages, e.g., request messages and response messages.

– The syntax of the various message types, i.e., the fields in the message and how the fields are delineated.

– The semantics of the fields, i.e., the meaning of the information that the field is supposed to contain;

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

• Many Internet application-layer protocols are fully specified in Request For Comments documents (RFCs) and are therefore in the public domain.

– For example, the HTTP 1.1 specification is included in RFC 2068, which was finalized and made public January 1997.

– If a browser (HTTP client) developer follows the rules of the HTTP 1.1 RFC, the browser will be able to retrieve Web pages from any Web server that has also has followed the rules of the HTTP 1.1 RFC.

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Structure of Internet Applications

Using TCP & The Sockets API

One or more TCP connections

Client

Server

Request or Command

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Sample Internet Applications

Application Type Application-layer protocol Transport Protocol Used/Port

Send: Simple Mail Transfer Protocol TCP 25 Electronic mail SMTP [RFC 821]

Receive: Post Office Protocol v3 TCP 110 POP3 [RCF 1939]

Remote terminal access Telnet [RFC 854] TCP 23 World Wide Web (WWW) HyperText Transfer Protocol 1.1 TCP 80

HTTP 1.1 [RFC 2068]

File Transfer Protocol TCP 21 File Transfer FTP [RFC 959]

Trivial File Transfer Protocol UDP 69 TFTP [RFC 1350]

Remote file server NFS [McKusik 1996] UDP or TCP Streaming multimedia Proprietary (e.g., Real Networks) UDP or TCP Internet telephony Proprietary (e.g., Vocaltec) Usually UDP

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Common Network/System Services & Ports

Port Number Process Name Description

1 TCPMUX TCP Port Service Multiplexer 5 RJE Remote Job Entry

7 ECHO Echo

9 DISCARD Discard 11 USERS Active Users 13 DAYTIME Daytime

17 Quote Quotation of the Day 19 CHARGEN Character generator

20 FTP-DATA File Transfer Protocol - Data 21 FTP File Transfer Protocol - Control 23 TELNET Telnet

25 SMTP Simple Mail Transfer Protocol 27 NSW-FE NSW User System Front End 29 MSG-ICP MSG-ICP

31 MSG-AUTH MSG Authentication 33 DSP Display Support Protocol 35 Private Print Servers 37 TIME Time

39 RLP Resource Location Protocol 41 GRAPHICS Graphics

42 NAMESERV Host Name Server 43 NICNAME Who Is

49 LOGIN Login Host Protocol 53 DOMAIN Domain Name Server 67 BOOTPS Bootstrap Protocol Server 68 BOOTPC Bootstrap Protocol Client

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Common Network/System Services & Ports

Port Number Process Name Description

69 TFTP Trivial File Transfer Protocol 79 FINGER Finger

80 HTTP HyperText Transfer Protocol 101 HOSTNAME NIC Host Name Server

102 ISO-TSAP ISO TSAP 103 X400 X.400 104 X400SND X.400 SND

105 CSNET-NS CSNET Mailbox Name Server 109 POP2 Post Office Protocol v2

110 POP3 Post Office Protocol v3 111 RPC Sun RPC Portmap

119 NNTP Network News Transfer Protocol 137 NETBIOS-NS NETBIOS Name Server

138 NETBIOS-DG NETBIOS Datagram Service 139 NETBIOS-SS NETBIOS Session Service 146 ISO-TP0 ISO TP0 147 ISO-IP ISO IP 150 SQL-NET SQL NET 153 SGMP SGMP 156 SQLSRV SQL Service 160 SGMP-TRAPS SGMP TRAPS

161 SNMP Simple Network Management Protocol 162 SNMPTRAP SNMPTRAP

163 CMIP-MANAGE CMIP/TCP Manager 164 CMIP-AGENT CMIP/TCP Agent 165 XNS-Courier Xerox

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File Transfer Protocol (FTP), RFC 595

• A protocol dating back to 1971 used for transferring files between hosts.

• In a typical FTP session:

– The user first provides the hostname of the remote host.

– The FTP client process in the local host establishes a control TCP connection with the FTP server process in the remote host on port 21.

– The user then provides the user identification and password, which get sent over this TCP connection as part of the FTP commands.

– Once the server has authorized the user, for each file to be transferred, FTP opens a data TCP connection on server port 20 which is used to transfer the file and is closed once the transfer is completed.

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Common FTP Commands and Replies

• FTP commands, from client to server, and replies, from server to client, are sent across the control TCP connection encoded in 7-bit ASCII.

• In order to delineate successive commands, a carriage return and a line feed end each command (and reply).

• Commands consist of four uppercase ASCII characters, some with optional arguments. • Some of the more common commands are given below (with options in italics):

– USER username : Used to send the user identification to server. – PASS password : Used to send the user password to the server.

– LIST : Used to ask the server to send back a list of all the files in the current remote directory. The list of files is sent over a (new and non-persistent) data TCP connection and not over the control TCP connection.

– RETR filename : Used to get a file from the current directory of the remote host. – STOR filename : Used to store a file into the current directory of the remote host. • There is typically a one-to-one correspondence between the commands the user issues and

the FTP command sent across the TCP control connection.

• Each command is followed by a reply, sent from server to client. Replies are three-digit numbers, with an optional message following the number. message;

• Typical replies along with possible messages are as follows: – 331 Username OK, password required

– 125 Data connection already open; transfer starting – 425 Can't open data connection

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Sending Electronic Mail:

Simple Mail Transfer Protocol (SMTP), RFC 821

• SMTP transfers messages from senders' mail servers to the recipients' mail

servers using TCP connections.

• SMPT existed long before it was fully specified by RFC 821 in 1982. • Following the client/server model:

– SMTP has two sides: a client side which executes on a sender's mail server, and server side which executes on recipient's mail server.

– Both the client and server sides of SMTP run on every mail server.

– When a mail server sends mail (to other mail servers), it acts as an SMTP client. When a mail server receives mail (from other mail servers) it acts as an SMTP server.

• The process of sending a message:

– A user agent (mail reader) is used to create a message to be sent.

– The user agent directs the message to the outgoing message queue in the user's local mail server (acting as an SMTP client).

– The local mail server (SMTP client) opens a TCP connection directly to the remote destination SMTP mail server.

– After initial SMTP handshaking, the SMTP client sends the 7-bit ASCII encoded message into the TCP connection.

– The remote SMTP server receives the message over the TCP connection, closes the connection and places the message in the receipt's mailbox.

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The Internet Mail System

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Example Transaction Between

An SMTP Client & Server

Server: 220 receive.edu Client: HELO send.edu

Server: 250 Hello send.edu, pleased to meet you Client: MAIL FROM: <sender-user@send.edu> Server: 250 sender-user@send.edu... Sender ok Client: RCPT TO: <receiver-user@receive.edu>

Server: 250 receiver-user@receive.edu ... Recipient ok Client: DATA

Server: 354 Enter mail, end with "." on a line by itself Client: First line of message in seven-bit ASCII

Client: Second and last line of example message. Client: .

Server : 250 Message accepted for delivery Client: QUIT

Server: 221 receive.edu closing connection

Once the SMTP Once the SMTP client (sender) client (sender) established a TCP established a TCP connection to connection to

the remote receiving

SMTP server,

the following is sent

into the connection:

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Multipurpose Internet Mail Extensions (MIME)

• SMTP can only handle ASCII-encoded messages, hence binary data is first encoded to ASCII in such a way that certain ASCII characters (including ".") are not used, using base64 encoding for example.

• If binary data is included in a message MIME headers are used to inform the receiving mail agent:

– Content-Transfer-Encoding: header. Alerts the receiving user agent that the message body has been ASCII encoded and the type of

encoding used.

– Content-Type: header. Informs the receiving mail agent about the type of data included in the message.

• Example message header when

a base64-encoded JPEG image is

included in the body of

the message:

From: sender-user@send.edu To: receiver-user@receive.edu Subject: A picture. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg

base64 encoded data ... ...

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MIME Document Types/Subtypes

Examples

The content type of

a document is described in the

MIME

Content-Type:

header

using a standard list of

document types and subtypes.

A few are shown here.

File

html

Wav

MPEG

pdf

VRML

text

Jpeg

MIME Type/Sub-type

text/plain

text/html

image/jpeg

audio/x-wav

video/mpeg

application/pdf

x-world/x-vrml

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The World Wide Web (WWW):

HyperText

Transfer Protocol (HTTP)

• The WWW was practically invented at CERN by Tim Berners-Lee and

associates in 1989-1991 when initial versions of HTML, HTTP, a Web server and a crude text-based browser (Web client) were developed.

• Marc Andreeseen, who helped to develop the popular GUI browser Mosaic for X at The National Center for Supercomputing Applications (NCSA), released an alpha version of his browser in 1993, and in 1994 formed Mosaic

Communications which later became Netscape.

• The Hypertext Transfer Protocol (HTTP) the Web's main application-layer protocol although current browsers can access other types of servers.

• As are most application-layer protocols, HTTP is implemented in two programs: a client program: Web browser and server program: Web server that talk to each other by exchanging HTTP messages.

• HTTP defines the structure of these messages and how the client and server exchange the messages.

• HTTP utilizes TCP connections to send client requests and server replies. • Current HTTP version: HTTP 1.1 (RFC 2068) January 97, adds among other

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Components

of The Web

Model

e.g. jasmine.isc.rit.edu:8000 Web server running on

Web

Clients

Web

Server

HTTP

HTML

Web Servers

Web Clients

HTML Document

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Common

HTML

Uniform Resource Locators (URLs)

• Web browsers may utilize several high level protocols to request

data from a variety of servers in addition to HTTP servers.

• A uniform source locator (URL) specifies the following:

–

Protocol used.

–

Host name, alias, or IP address.

–

Port number (if different from the default protocol port).

–

Path to data requested.

–

Resource requested (usually a file name).

• A few types of URLs:

http http://hostname:port/path/resource

e.g. http://jasmine.isc.rit.edu:8000/eecc694-spring2000/694-5-2-2000.ppt FTP ftp://hostname:port/path/file e.g. ftp://beast.isc.rit.edu:25 Local file file:///path/file e.g. file:///my_files/file1.txt Telnet telnet://hostname:port e.g. telnet:// jasmine.isc.rit.edu gopher gopher://hostname/path/

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Steps of Transferring A Web Page From Server To

Client Using Non-Persistent TCP Connections

http://www.rit.edu/~meseec/eecc694-spring2000/index.html

1. The HTTP client initiates a TCP connection to the server www.rit.edu.

2. The HTTP client sends a HTTP request message into the TCP connection. The request message either includes the entire URL or simply the path name

/~meseec/eecc694-spring2000/index.html .

3. The HTTP server receives the request message, retrieves the object

/~meseec/eecc694-spring2000/index.html from its storage, encapsulates the object in a HTTP response message, and sends the response message into the TCP

connection.

4. The HTTP server tells TCP to close the TCP connection. (TCP doesn't terminate the connection until the client has received the response message)

5. The HTTP client receives the response message. The message indicates that the encapsulated object is an HTML file. The client extracts the file from the

response message, parses the HTML file and finds references to a number of JPEG and GIF objects.

6. The first four steps are then repeated for each of the referenced JPEG and GIF objects.

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HTTP Message Examples

• Typical Request Message From A Client:

GET /eecc694-spring2000/index.html HTTP/1.0 Connection: close

User-agent:

Mozilla/4.72 [en] (Win98; I)

Accept: text/html, image/gif, image/jpeg Accept-language:en

(extra carriage return, line feed)

• Typical Response Message From A Server:

HTTP/1.0 200 OK Connection: close

Date: Wed, 05 April 2000 12:00:15 GMT Server: NCSA/1.5.2

Last-Modified: Tue, 25 April 2000 11:23:24 GMT Content-Length: 20419

Content-Type: text/html

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HTTP Message Formats:

General Format of A Request Message

Messages encoded in standard ASCII text.

Method: GET, POST, and HEAD. The great majority of HTTP request messages use the GET method. The GET method is used when the browser requests an object, with the object identified in the URL. POST is used when the client user fills out a form.

URL: No need to include server hostname since the TCP connection is already connected to the server. Version: HTTP version number used. (e.g. HTTP/1.0 or HTTP/1.1)

Entity Body: Not used in the GET method, form data included in POST method.

Space

Carriage Return

Line feed

Connection: close, to request non-persistent TCP connections. User-agent: Browser used.

Accept: type of objects the browser is prepared to accept

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HTTP Message Formats:

General Format of A Response Message

Version: HTTP version number used (e.g. HTTP/1.0 or HTTP/1.1).

Status code and associated phrase indicate the result of the request. Some example 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. 400 Bad Request: A generic error code indicating that the request could not be understood by the server. 404 Not Found: The requested document does not exist

505 HTTP Version Not Supported: The request HTTP protocol version is not supported by the server. Entity Body: The requested object if the response is successful.

Connection: close for non-persistent TCP connections.

Date: Current GMT date Server: Server type used Last-Modified: of object Content-Length: of object

Content-Type: MIME Type/sub-type of object

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Domain Name System (DNS)

• DNS is a hierarchical system, based on a distributed database, that uses a hierarchy of Name Servers to resolve Internet host names into the

corresponding IP addresses required for packet routing by issuing a DNS query to a name server.

• Name servers are usually Unix machines running the Berkeley Internet Name Domain (BIND) software.

• On many Unix-based machines using the sockets-API, gethostbyname() is the library routine that an application calls in order to issue a DNS query.

• Resource record: Associated with each host on the Internet, includes IP address, domain name, domain name server, etc.

• When resolving a host name, DNS returns the associated resource record of the host.

• Internet domain names are divided into generic top-level domains (edu, com, gov, mil) which include all US domains and country domains.

• The DNS space is divided into non-overlapping zones.

• Resource records of all hosts in a sub-domain are kept as a DNS database stored at the domain name server responsible for that sub-domain or zone.

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Principal DNS Resource

Record Types

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A Partial

DNS

Database

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Zone Division of DNS Name Space

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Recursive DNS Queries Example

A network application running on beast.isc.rit.edu issues a DNS query using gethostbyname()to resolve hostname halcyon.usc.edu Returns DNS Resource recordfor halcyon.usc.edu including IP address(s) Hostname to be resolved