Application layer

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OSI MODEL BLOCK DIAGRAM
3
Application-Layer Protocols

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., checksum 7

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. 9

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 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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Application layer protocols as follows:
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)
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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 GET
o POST
o PUT
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 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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 DNS Hierarchy
edu com org jp
rpi albany
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 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.
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 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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 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:
the user to
• nslookup is an interactive resolver that allows
communicate directly with a DNS server.
• nslookup is usually available on Unix workstations.
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 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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 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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 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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 The TELNET Protocol
o TCP connection
o data and control over the same connection.
o Network Virtual Terminal
o negotiated options
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 Network Virtual Terminal
o intermediate representation of a generic terminal.
o provides a standard language for communication of terminal control
functions.
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 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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 Control Functions:
o TELNET includes support for a series of control functions commonly
supported by servers.
for communication of (the
o This provides a uniform mechanism
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. 27

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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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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 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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 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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 Connection establishment:
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 Message Progress:
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 Connection Termination:
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TCP Connection Termination

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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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Questions
 Explain OSI model with 7 layers.
 Explain application layer protocol in detail.
Write note on:
o HTTP(Hypertext Transfer Protocol)
o DNS(Domain Name System)
o FTP(File Transfer Protocol)
o TELNET
o DHCP(Dynamic Host Configuration Protocol)
o SMTP(Simple Mail Transfer Protocol)
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Application layer

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