Application Layer

APPLICATION LAYER

Shashikant V. AthawaleAssistant Professor

Department of Computer Engineering, AISSMS COE ,Pune

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AGENDA OSI Model Block Diagram OSI Encapsulation Process Application Protocol Design Application Layer Protocols

1. HTTP(Hypertext Transfer Protocol)

2. DNS(Domain Name System)

3. FTP(File Transfer Protocol)

4. TELNET

5. DHCP(Dynamic Host Configuration Protocol)

6. SMTP(Simple Mail Transfer Protocol)

Summary References

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OSI MODEL BLOCK DIAGRAM

Application-Layer Protocols

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4. TELNET



Summary References

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OSI ENCAPSULATION PROCESS


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4. TELNET



Summary References

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APPLICATION PROTOCOL DESIGN Think of different people/teams, working on the client and

server programs.– Different programming languages.

– Diverse hardware, operating systems. Be unambiguous, precise.

– Consider potential error conditions. Allow for future extensions.

– Leave room for additional data, meta-data. Do not replicate services provided by lower layer protocols

– e.g., checksumApplication-Layer

Protocols

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4. TELNET



Summary References

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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.


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CONTINUE.. 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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CONTINUE..

Application layer protocols as follows:

1. HTTP(Hypertext Transfer Protocol)2. DNS(Domain Name System)3. FTP(File Transfer Protocol)4. TELNET5. DHCP(Dynamic Host Configuration Protocol)6. SMTP(Simple Mail Transfer Protocol)


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1. HTTP Hypertext Transfer Protocol is used to transfer files that

make up the web pages of the World Wide Web. HTTP: TCP port 80. Was originally developed to publish and retrieve HTML pages. used for distributed, collaborative information systems. HTTP is used across the world wide web for data transfer and

is one of the most used application protocols. HTTP specifies a request/response protocol. When a client, typically a web browser, sends a request

message to a server, the HTTP protocol defines the message types the client uses to request the web page and the message types the server uses to respond.

The three common message types are:o GETo POSTo PUT


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CONTINUE.. GET is a client request for data. A web browser sends the

GET message to request pages from a web server. POST and PUT are used to send messages that upload data

to the web server. HTTP protocol using GET.


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2. DNS HOSTNAMES:

IP Addresses are great for computers– IP address includes information used for routing.

IP addresses are tough for humans to remember. IP addresses are impossible to guess.

– ever guessed at the name of a WWW site?

The Domain Name System: The domain name system is usually used to translate a

host name into an IP address . Domain names comprise a hierarchy so that names are

unique, yet easy to remember.


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CONTINUE.. DNS Hierarchy

edu com org jp

rpi albany


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CONTINUE.. Host name structure:• Each host name is made up of a sequence of labels

separated by periods.– Each label can be up to 63 characters– The total name can be at most 255characters.

• Examples:– whitehouse.gov– barney.the.purple.dinosaur.com– monica.cs.rpi.edu

Domain Name• The domain name for a host is the sequence of labels that

lead from the host (leaf node in the naming tree) to the top of the worldwide naming tree.

• A domain is a subtree of the world wide naming tree.Application-Layer

Protocols

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CONTINUE.. Top level domains:• edu, gov, com, net, org, mil, …• Countries each have a top level domain(2 letter domain

name).• New top level domains include:

.aero .biz .coop .info .name .pro DNS Organization• Distributed Database

– The organization that owns a domain name is responsible for running a DNS server that can provide the mapping between hostnames within the domain to IP addresses.– So - some machine run by RPI is responsible for everything within the rpi.edu domain.


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CONTINUE.. DNS Clients:• A DNS client is called a resolver.• A call to getByName(host)is handled by a resolver

(typically part of the client).• Most Unix workstations have the file /etc/resolv.conf that

contains the local domain and the addresses of DNS servers for that domain.

Nslookup:• nslookup is an interactive resolver that allows the user to

communicate directly with a DNS server.• nslookup is usually available on Unix workstations.


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CONTINUE.. DNS Servers:• Servers handle requests for their domain directly.• Servers handle requests for other domains by contacting

remote DNS server(s).• Servers cache external mappings.

o DNS Message Format:


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CONTINUE.. DNS Message Header:


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3. FTP promote sharing of files. encourage indirect use of remote computers. shield user from variations in file storage. transfer data reliably and efficiently. “FTP, although usable directly by a user at a terminal, is

designed mainly for use by programs”. To successfully transfer files, FTP requires two connections

between the client and the server: one for commands and replies, and the other for the actual file transfer.

The client establishes the first connection to the server on TCP port 21. This connection is used for control traffic, consisting of client commands and server replies.

The client establishes the second connection to the server over TCP port 20. This connection is for the actual file transfer and is created every time a file is transferred.


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CONTINUE.. The client can download (pull) a file from the server or

upload (push) a file to the server. FTP Process


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4. TELNET TELNET is a protocol that provides “a general, bi-directional,

eight-bit byte oriented communications facility”. telnet is a program that supports the TELNET protocol over

TCP. Many application protocols are built upon the TELNET

protocol. TELNET service:


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CONTINUE..

The TELNET Protocolo TCP connectiono data and control over the same connection.o Network Virtual Terminalo negotiated options


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CONTINUE.. Network Virtual Terminalo intermediate representation of a generic terminal.o provides a standard language for communication of terminal

control functions.


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CONTINUE.. Negotiated Options:o All NVTs support a minimal set of capabilities.o Some terminals have more capabilities than the minimal set.o The 2 endpoints negotiate a set of mutually acceptable

options (character set, echo mode, etc).o The protocol for requesting optional features is well defined

and includes rules for eliminating possible negotiation “loops”.

o The set of options is not part of the TELNET protocol, so that new terminal features can be incorporated without changing the TELNET protocol.


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CONTINUE.. Control Functions:o TELNET includes support for a series of control functions commonly

supported by servers.o This provides a uniform mechanism for communication of (the

supported) control functions.o Interrupt Process (IP)

– suspend/abort process.o Abort Output (AO)

– process can complete, but send no more output to user’s terminal.o Are You There (AYT)

– check to see if system is still running.o Erase Character (EC)

– delete last character sent– typically used to edit keyboard input.

o Erase Line (EL)– delete all input in current line.Application-Layer

Protocols

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5. DHCP DHCP enables clients on a network to obtain IP addresses

and other information from a DHCP server. DHCP allows a host to obtain an IP address dynamically

when it connects to the network. The DHCP server is contacted by sending a request, and an

IP address is requested. The DHCP server chooses an address from a configured

range of addresses called a pool and assigns it to the host client for a set period.

different ways of having DHCP servers arranged in next diagram


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CONTINUE..


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7. SMTP Protocol originated in 1982 (RFC821, Jon Postel) Standard message format (RFC822,2822, D. Crocker) Goal: To transfer mail reliably and efficiently


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CONTINUE.. SMTP clients and servers have

two main components: User Agents – Prepares the

message, encloses it in an envelope. (ex. Thunderbird, Eudora)

Mail Transfer Agent – Transfers the mail across the internet (ex. Sendmail, Exim)

Analogous to the postal system in many ways


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CONTINUE.. SMTP also allows the use

of Relays allowing other MTAs to relay the mail.

Mail Gateways are used to relay mail prepared by a protocol other than SMTP and convert it to SMTP.


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CONTINUE.. Connection establishment:


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CONTINUE.. Message Progress:


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CONTINUE.. Connection Termination:

TCP Connection Termination


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CONTINUE..Limitations in SMTP:

Only uses NVT 7 bit ASCII format How to represent other data types?

No authentication mechanisms Messages are sent un-encrypted Susceptible to misuse (Spamming,

faking sender address)


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SUMMARY OSI model has 7 layers. In particular, an application layer protocol defines:

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

Application layer has 6 protocols:1. HTTP(Hypertext Transfer Protocol)



4. TELNET



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REFERENCES Web Sites:

http://www.faqs.org/rfcs/rfc821.html (RFC 821). http://www.faqs.org/rfcs/rfc2821.html (RFC 2821) http://www.tcpipguide.com/free/

t_SMTPSpecialFeaturesCapabilitiesandExtensions.html http://cns.utoronto.ca/usg/technotes/smtp-intro.html http://computer.howstuffworks.com/email5.htm

Books: Programmer's Guide to Internet Mail by John Rhoton TCP/IP Illustrated Volume 1 by Richard Stevens

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THANK YOU…!!!

Application Layer

Engineering

Transcript of Application Layer