Internet Engineering Internet Engineering CourseCourse

Application Layer Protocols

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Applications and application-layer protocolsApplications and application-layer protocols

Application: communicating, distributed processes◦ running in network hosts in

“user space”◦ exchange messages◦ e.g., email, file transfer, the

WebApplication-layer protocols

◦ one “piece” of an application -others are e.g. user agents. Web:browser E-mail: mail reader streaming audio/video: media

player

◦ define messages exchanged by apps and actions taken

◦ use services provided by lower layer protocols

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

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Client-server paradigmClient-server paradigm

Typical network app has two pieces: client and server

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

Client: initiates contact with server

(“speaks first”) typically requests service

from server, for Web, client is

implemented in browser; for e-mail, in mail reader

Server: provides requested service

to client e.g., Web server sends

requested Web page, mail server delivers e-mail

request

reply

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Auxiliary terms ++Auxiliary terms ++

socket: Internet application programming interface◦ 2 processes

communicate by sending data into socket, reading data out of socket (like sending out, receiving in via doors)

Q: how does a process “identify” the other process with which it wants to communicate?◦ IP address of host

running other process◦ “port number” - allows

receiving host to determine to which local process the message should be delivered

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Properties of transport service of interest Properties of transport service of interest

to the appto the app Data loss some apps (e.g., audio)

can tolerate some loss other apps (e.g., file

transfer, telnet) require 100% reliable data transfer

Connection-oriented vs connectionless services

Bandwidth, Timing some apps (e.g.,

multimedia) require at least a minimum amount of bandwidth

some apps (e.g., Internet telephony, interactive games) require low delay and/or low jitter

other apps (elastic apps, e.g. file transfer) make use of whatever bandwidth, timing they get

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Transport service requirements of Transport service requirements of common appscommon apps

Application

file transfere-mail

Web documentsreal-time audio/

videostored audio/videointeractive games

financial apps

Data loss

no lossno lossNo-lossloss-tolerant

loss-tolerantloss-tolerantno loss

Bandwidth

elasticelasticelasticaudio: 5Kb-1Mbvideo:10Kb-5Mbsame as above few Kbps upelastic

Time Sensitive

nononoyes, 100’s msec

yes, few secsyes, 100’s msecyes and no

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Internet apps: their protocolsInternet apps: their protocols

Application

e-mail remote terminal access Web file transferstreaming multimedia

remote file serverInternet telephony

nslookup and many others

Applicationlayer protocol

» smtp [RFC 821]» telnet [RFC 854]» http [RFC 2068]» ftp [RFC 959]proprietary(e.g. RealNetworks)NSFSIP, RTP, proprietary (e.g., Skype)» DNS [RFC 882, 883,1034,1035]

Underlyingtransport protocol

TCPTCPTCPTCPTCP or UDP

TCP or UDPtypically UDP, TCP also possibleUDP

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The TELNET ProtocolThe TELNET ProtocolTCP connectiondata and control over the same

connection.Network Virtual Terminalnegotiated options

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Network Virtual TerminalNetwork Virtual Terminalintermediate representation of a

generic terminal.provides a standard language for

communication of terminal control functions.

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Network Virtual TerminalNetwork Virtual Terminal

NVTNVT NVTNVT

ServerServerProcessProcess

TCPTCP TCPTCP

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Negotiated OptionsNegotiated OptionsAll NVTs support a minimal set of

capabilities.Some terminals have more

capabilities than the minimal set.The 2 endpoints negotiate a set

of mutually acceptable options (character set, echo mode, etc).

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Option examplesOption examples

Line mode vs. character mode

echo modes

character set (EBCDIC vs. ASCII)

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Control FunctionsControl FunctionsTELNET includes support for a

series of control functions commonly supported by servers.

This provides a uniform mechanism for communication of (the supported) control functions.

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Control FunctionsControl FunctionsInterrupt Process (IP)

◦suspend/abort process.Abort Output (AO)

◦process can complete, but send no more output to user’s terminal.

Are You There (AYT)◦check to see if system is still

running.

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More Control FunctionsMore Control FunctionsErase Character (EC)

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

Erase Line (EL)◦delete all input in current line.

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Command StructureCommand StructureAll TELNET commands and data

flow through the same TCP connection.

Commands start with a special character called the Interpret as Command escape character (IAC).

The IAC code is 255.If a 255 is sent as data - it must

be followed by another 255.

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Looking for CommandsLooking for CommandsEach receiver must look at each

byte that arrives and look for IAC.If IAC is found and the next byte

is IAC - a single byte is presented to the application/terminal (a 255).

If IAC is followed by any other code - the TELNET layer interprets this as a command.

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Playing with TELNETPlaying with TELNETYou can use the telnet program

to play with the TELNET protocol.telnet is a generic TCP client.

◦Sends whatever you type to the TCP socket.

◦Prints whatever comes back through the TCP socket.

◦Useful for testing TCP servers (ASCII based protocols).

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Some TCP Servers you can Some TCP Servers you can play withplay withMany Unix systems have these

servers running (by default):◦echo port 7◦discard port 9◦daytime port 13◦chargen port 19

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telnet hostname porttelnet hostname port> telnet rcs.rpi.edu 7Trying 128.113.113.33...Connected to cortez.sss.rpi.edu (128.113.113.33).

Escape character is '^]'.Hi daveHi davestop itstop it^]telnet> quitConnection closed.

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ftp: the file transfer protocolftp: the file transfer protocol

transfer file to/from remote host client/server model

◦ client: side that initiates transfer (either to/from remote)

◦ server: remote host ftp: RFC 959 ftp server: port 21

file transfer FTPserver

FTPuser

interface

FTPclient

local filesystem

remote filesystem

user at host

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ftp: separate control, data ftp: separate control, data connectionsconnections

ftp client contacts ftp server at port 21, specifying TCP as transport protocol

two parallel TCP connections opened:◦ control: exchange

commands, responses between client, server.

◦ data: file data to/from server

ftp server maintains “state”: current directory, earlier authentication

FTPclient

FTPserver

TCP control connectionport 21

TCP data connectionport 20

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ftp commands, responsesftp commands, responses

Sample commands: sent as ASCII text over

control channel USER username PASS password LIST return list of file in

current directory RETR filename

retrieves (gets) file STOR filename stores

(puts) file onto remote host

Sample return codes status code and phrase

(as in http) 331 Username OK, password required

125 data connection already open; transfer starting

425 Can’t open data connection

452 Error writing file

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application layer

transport layer (TCP/UDP)

network layer (IP)

link layer (e.g. ethernet)

physical layer

application layer

transport layer (TCP/UDP)

network layer (IP)

link layer (e.g. ethernet)

physical layer

OS network

stack

Sockets as means for inter-process Sockets as means for inter-process communication (IPC)communication (IPC)

Process ProcessSocket

OS network

stack

Socket

Internet

Internet

Internet

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Addressing serverAddressing serverAddress the machine on the network

◦ By IP address (127.0.0.1 – localhost)Address the process

◦ By the “port”-numberThe pair of IP-address + port – makes up

a “socket-address”

OS network

stack

Process#m

Port#m

Process#1

Port#1…

Host#1

OS network

stack

Process#k

Port#k

Process#1

Port#1…

Host#n

…

NetworkIP-address#1 IP-address#n

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Socket typesSocket types Datagram socket – using

UDP◦ Not sequenced◦ Not reliable◦ Not unduplicated◦ Connectionless

Stream socket – using TCP◦ Sequenced◦ Reliable◦ Unduplicated◦ Connection-oriented

Raw and others (extracurricular)

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Usage of port-numbersUsage of port-numbersStandard applications use predefined port-

numbers◦ 21 - ftp◦ 23 - telnet◦ 80 - http◦ 110 - pop3 (email)◦ …

Other applications should choose between 1024 and 65535◦ 4662 – eMule◦ …

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Socket primitivesSocket primitives SOCKET create a new socket BIND attach a local address to a socket LISTEN announce a willingness to accept

connections ACCEPT Accept an incoming connection,

Create a socket and a child process for the client

CONNECT actively attempt to establish a connection

SEND send some data over the connection RECEIVE receive some data from the

connection CLOSE release the connection (the port)

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Client+server: Client+server: connectionlessconnectionless

CREATE

BIND

SEND

SEND

CLOSE

RECEIVE

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Client+server: connection-Client+server: connection-orientedoriented

SOCKETBIND

LISTEN

CONNECT

ACCEPT

RECEIVE

RECEIVE

SEND

SEND

CLOSE

TCP three-way handshake

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Application Layer: SummaryApplication Layer: Summary

application service requirements:◦ reliability,

bandwidth, delayclient-server paradigm Internet transport

service model◦ connection-oriented,

reliable: TCP◦ unreliable,

datagrams: UDP

• specific protocols:– http– ftp– smtp, pop3– Dns– …

• sockets– client/server

implementation– using tcp, udp sockets