Transcript csci5211: Computer Networks and Data Communications

Internet Engineering Course
Application Layer Protocols
Applications and application-layer protocols
Application: communicating, distributed
processes
◦ running in network hosts in “user
space”
◦ exchange messages
◦ e.g., email, file transfer, the Web
Application-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
application
transport
network
data link
physical
application
transport
network
data link
physical
application
transport
network
data link
physical
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Client-server paradigm
Typical network app has two
pieces: client and server
Client:
r initiates contact with server
(“speaks first”)
r typically requests service from
server,
r for Web, client is implemented
in browser; for e-mail, in mail
reader
Server:
r provides requested service to
client
r e.g., Web server sends
requested Web page, mail
server delivers e-mail
application
transport
network
data link
physical
request
reply
application
transport
network
data link
physical
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Auxiliary terms ++
Q: how does a process
socket: Internet application
“identify” the other
programming interface
process with which it
◦ 2 processes communicate by
wants to communicate?
sending data into socket,
reading data out of socket
(like sending out, receiving in
via doors)
◦ 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 to 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
r
r
r
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 common apps
Application
file transfer
e-mail
Web documents
real-time audio/
video
stored audio/video
interactive games
financial apps
Data loss
Bandwidth
Time Sensitive
no loss
no loss
No-loss
loss-tolerant
elastic
elastic
elastic
audio: 5Kb-1Mb
video:10Kb-5Mb
same as above
few Kbps up
elastic
no
no
no
yes, 100’s msec
loss-tolerant
loss-tolerant
no loss
yes, few secs
yes, 100’s msec
yes and no
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Internet apps: their protocols
Application
e-mail
remote terminal access
Web
file transfer
streaming multimedia
remote file server
Internet telephony
nslookup and many
others
Application
layer protocol
Underlying
transport protocol
» smtp [RFC 821]
» telnet [RFC 854]
» http [RFC 2068]
» ftp [RFC 959]
proprietary
(e.g. RealNetworks)
NSF
SIP, RTP, proprietary
(e.g., Skype)
» DNS
TCP
TCP
TCP
TCP
TCP or UDP
TCP or UDP
typically UDP, TCP
also possible
UDP
[RFC 882, 883,1034,1035]
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The TELNET Protocol
TCP connection
 data and control over the same
connection.
 Network Virtual Terminal
 negotiated options

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Network Virtual Terminal
intermediate representation of a generic
terminal.
 provides a standard language for
communication of terminal control
functions.

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Network Virtual Terminal
Server
Process
NVT
NVT
TCP
TCP
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Negotiated Options
All 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 examples

Line mode vs. character mode

echo modes

character set (EBCDIC vs. ASCII)
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Control Functions

TELNET 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 Functions

Interrupt 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 Functions

Erase 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 Structure
All 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 Commands
Each 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 TELNET
You 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 play
with

Many Unix systems have these servers
running (by default):
◦
◦
◦
◦
echo
discard
daytime
chargen
port 7
port 9
port 13
port 19
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telnet hostname port
> telnet rcs.rpi.edu 7
Trying 128.113.113.33...
Connected to cortez.sss.rpi.edu
(128.113.113.33).
Escape character is '^]'.
Hi dave
Hi dave
stop it
stop it
^]
telnet> quit
Connection closed.
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ftp: the file transfer protocol
user
at host




file transfer
FTP
FTP
user
client
interface
local file
system
FTP
server
remote file
system
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
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ftp: separate control, data connections



ftp client contacts ftp server at
port 21, specifying TCP as
transport protocol
two parallel TCP connections
opened:
◦ control: exchange commands,
FTP
responses between client,
client
server.
◦ data: file data to/from server
ftp server maintains “state”:
current directory, earlier
authentication
TCP control connection
port 21
TCP data connection
port 20
FTP
server
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ftp commands, responses
Sample commands:
Sample return codes
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





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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Sockets as means for inter-process communication (IPC)
application layer
application layer
Process
Internet
Socket
transport
layer (TCP/UDP)
OS network
network layer (IP)
stack
Socket
Internet
transport
(TCP/UDP)
OS layer
network
network layer (IP)
stack
link layer (e.g. ethernet)
link layer (e.g. ethernet)
physical layer
Process
Internet
physical layer
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Addressing server

Address the machine on the network
◦ By IP address (127.0.0.1 – localhost)

Address the process
◦ By the “port”-number

The pair of IP-address + port – makes up a “socketaddress”
Host#1
Process#1
Port#1
…
Host#n
Process#m
Process#1
Port#m
Port#1
OS network
stack
…
IP-address#1
…
Process#k
Port#k
OS network
stack
IP-address#n
Network
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Socket types

Datagram socket – using UDP
◦
◦
◦
◦

Stream socket – using TCP
◦
◦
◦
◦

Not sequenced
Not reliable
Not unduplicated
Connectionless
Sequenced
Reliable
Unduplicated
Connection-oriented
Raw and others
(extracurricular)
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Usage of port-numbers

Standard 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 primitives

SOCKET
BIND
LISTEN

ACCEPT

CONNECT
SEND
RECEIVE
CLOSE





create a new socket
attach a local address to a socket
announce a willingness to accept
connections
Accept an incoming connection, Create a
socket and a child process for the client
actively attempt to establish a connection
send some data over the connection
receive some data from the connection
release the connection (the port)
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Client+server: connectionless
CREATE
BIND
SEND
RECEIVE
SEND
CLOSE
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Client+server: connection-oriented
BIND
SOCKET
LISTEN
CONNECT
ACCEPT
TCP three-way
handshake
SEND
RECEIVE
SEND
RECEIVE
CLOSE
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Application Layer: Summary
application service
requirements:
◦ reliability, bandwidth,
delay
 client-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
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