Network services1 Chapter 1 Network Services Some of these slides are copyrighted by: Computer...

transcript

Network services 1

Chapter 1Network Services

Some of these slides are copyrighted by:

Computer Networking: A Top Down Approach 5th edition. Jim Kurose, Keith RossAddison-Wesley, April 2009.

Departamento deTecnología Electrónica

Network services 2

Chapter 1: Network Services

Chapter goals: understand most common data services

in networks: Domain Name Servers (DNS) Remote access services File transfer services e-mail services Streaming services

Network services 3

1.1 Introduction 1.2 Domain Name

Server (DNS) 1.3 Access remote

services Telnet SSH

1.4 File transfer services TFTP FTP

1.5 e-mail services SMTP POP3

1.6 Streaming services

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services Telnet SSH

Network services 5

Introduction

Network Services -> key in companies and organizations

Work based in shared network resources & distributed services

Great diversity of services

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Types of Network ServicesConfiguration and management

Management of hosts. e.g. DHCP.

Remote Access Remote hosts are allowed to access to a network. e.g. SSH

File management File transfer, storage and management e.g. FTP.

Print services Printer share.

Information Information sharing and querying e.g. WWW, video sharing, IPTV

Communication User communication by means of text, audio and/or video

messages e.g. e-mail, chat, videoconference, IP telephony, games online.

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Architecture of Network Services

Approaches Client-server Peer-to-peer (P2P) Hybrid of client-server and P2P

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Client/Server architecture

server: always-on host permanent IP

address server farms for

scalingclients:

communicate with server

may be intermittently connected

may have dynamic IP addresses

do not communicate directly with each other

client/server

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P2P architecture

no always-on server arbitrary end systems

directly communicate peers are

intermittently connected and change IP addresses

Highly scalable but difficult to manage

peer-peer

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Hybrid of client-server and P2PSkype

voice-over-IP P2P application centralized server: finding address of

remote party: client-client connection: direct (not through

server) Instant messaging

chatting between two users is P2P centralized service: client presence

detection/location• user registers its IP address with central

server when it comes online• user contacts central server to find IP

addresses of buddies

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services Telnet SSH

Problem: Web browsing

Resource holder’s host

User’s host

Internet

DNS: Domain Name System

Network services

Approach: From the user’s point of view

o Identifies resource holder by means of an address (www.dte.us.es)

Network services

Approach: May www.dte.us.es be used as an identifier of the

queried resource?

User’s host

Internet

www.dte.us.es

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Approach: From the network’s point of view

o IP addresses are used (routing & addressing)

Internet

Network services

User’s hostResource

holder’s host

Approach: A mechanism to translate names into IP addresses is

necessary

Internet

www.dte.us.es

Network services

User’s hostResource

holder’s host

Hosts, routers: IP address (32 bits) – used to address datagrama “name”, e.g.: www.google.es – used by human beings How is the name generated?

130.213.40.3

Myserver.dte.us.es

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Name systems: Plain

o Non-hierarchico No location infoo E.g: ID card

Hierarchico Structuredo Location infoo E.g: ZIP code

Network services

Name Systems: Plain: simple -> centralized management Hierarchic -> distributed management (easier) - DNS

pc1 pc1 pc1 pc1

Company X

Company Y

Company X

Company Y

PLAIN Hierarchic

Network services

Name space: Inverted tree structure Every node is labelled with a name (max 63 characters) Start of the tree -> root (empty label) Variable depth (max 127 levels) Structure is similar to OS directories. To build a name : (root -> leave) com.google.www Reading: www.google.com

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Name Space:Important

Root is not labelled Each domain is a subtree Domains organized in levels Top-level domains (TLD) The same label may be assigned to two different

hosts unless they are brothers (in the tree)

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Name space: my-pc.cont.fake.es.

Host name

domain

Network services

Space name: my-pc.cont.fake.es.

Host name

domain

Network services

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IP Address 4 bytes in decimal format (69.146.202.8) Hierarchical structure -> precise information about host

location

Name No host location information except maybe the country

Q: map between IP addresses and name ?

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Domain Name System: distributed database

implemented in hierarchy of many name servers

application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation) DNS uses UDP services

DNS services hostname to IP

address translation host aliasing

Canonical, alias names

mail server aliasing load distribution

replicated Web servers: set of IP addresses for one canonical name

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Basic foundations

1. App needs to know a remote IP address associated to a name

2. App requests IP address to DNS client

3. DNS client sends a request to the network

4. DNS client rcvs a reply including IP addr

5. DNS client gives IP address to app

Why not centralize DNS? single point of failure traffic volume distant centralized

database maintenance

doesn’t scale!

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Distributed, Hierarchical Database Big number of DNS servers hierarchically

organised and distributed all around the world

Database is also distributed by those servers

Three types of servers: o Root name serverso Top-Level Domain (TLD) serverso Authoritative servers

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Root DNS Servers

com DNS servers org DNS servers edu DNS servers

poly.eduDNS servers

umass.eduDNS servers

yahoo.comDNS servers

amazon.comDNS servers

pbs.orgDNS servers

Distributed, Hierarchical Database

Client wants IP for www.amazon.com; 1st approx: client queries a root server to find com DNS

server client queries com DNS server to get

amazon.com DNS server client queries amazon.com DNS server to get IP

address for www.amazon.com

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DNS: Root name servers

contacted by local name server that can not resolve name root name server:

contacts authoritative name server if name mapping not known

gets mapping returns mapping to local name server

13 root name servers worldwide

b USC-ISI Marina del Rey, CAl ICANN Los Angeles, CA

e NASA Mt View, CAf Internet Software C. Palo Alto, CA (and 36 other locations)

i Autonomica, Stockholm (plus 28 other locations)

k RIPE London (also 16 other locations)

m WIDE Tokyo (also Seoul, Paris, SF)

a Verisign, Dulles, VAc Cogent, Herndon, VA (also LA)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MDj Verisign, ( 21 locations)

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TLD and Authoritative Servers Top-level domain (TLD) servers:

responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, es, jp.

Network Solutions maintains servers for com TLD

Educause for edu TLD Types

Generic (gTLD) ≥ 3 characters Sponsored Unsponsored

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TLD and Authoritative Servers (Top-level domain, TLD) :

o Typeso Geographic

2 charactersRepresent countries (managed by the

country authorities) ICANN -> IANA

o .arpao Reserved

.test -> DNS tests .example -> documents .invalid -> installation & DNS tests for new

servers .localhost -> loopback

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TLD and Authoritative Servers Authoritative DNS servers:

organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web, mail).

can be maintained by organization or service provider

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Local Name Server

does not strictly belong to hierarchy each ISP (residential ISP, company,

university) has one. also called “default name server”

when host makes DNS query, query is sent to its local DNS server acts as proxy, forwards query into hierarchy

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requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

authoritative DNS serverdns.cs.umass.edu

TLD DNS server

DNS name resolution example

Host at cis.poly.edu wants IP address for gaia.cs.umass.edu

iterated query:

contacted server replies with name of server to contact

“I don’t know this name, but ask this server”

recursive query:

puts burden of name resolution on contacted name server

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DNS name resolution example

recursive queryiterative query

gaia.cs.umass.edu

root DNS server

TLD DNS server

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

TLD DNS server

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DNS: caching and updating records once (any) name server learns mapping, it

caches mapping cache entries timeout (disappear) after

some time TLD servers typically cached in local name

servers• Thus root name servers not often visited

update/notify mechanisms under design by IETF RFC 2136 http://www.ietf.org/html.charters/dnsind-charter.html

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DNS records

DNS: distributed db storing resource records (RR)

Type=NS name is domain (e.g.

foo.com) value is hostname of

authoritative name server for this domain

RR format: (name, value, type, ttl)

Type=A name is hostname value is IP address

Type=CNAME name is alias name for

some “canonical” (the real) name

www.ibm.com is really servereast.backup2.ibm.com value is canonical name

Type=MX value is name of

mailserver associated with name

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DNS protocol, messages

Name, type fields for a query

RRs in responseto query

records forauthoritative servers

additional “helpful”info that may be used

Network services 39

DNS protocol, messages

Header, 6 fields, 2 bytes each

Network services 40

Inserting records into DNS

example: new startup “Network Utopia” register name networkuptopia.com at DNS

registrar (e.g., Network Solutions) provide names, IP addresses of authoritative name

server (primary and secondary) registrar inserts two RRs into com TLD server:

(networkutopia.com, dns1.networkutopia.com, NS)(dns1.networkutopia.com, 212.212.212.1, A)

create authoritative server Type A record for www.networkuptopia.com; Type MX record for networkutopia.com

How do people get IP address of your Web site?

Network services 41Network services 41

services Telnet SSH

Access remote services

Virtual connection to a remote terminal “Substitutes” a direct cable Access remote services

Telnet SSH

Telnet

Basically -> TCP connection (port 23) Some other features negotiated in handshake

TCP/ IPInternet

Telnetclient

Operative system

Telnetserver

Operative system

Client readsfrom terminal

Client sends to server

Server receives from client

Server sends to pseudo-terminal

No security –> even passwds are visible Solution: encryptation -> SSH Telnet is now quite obsolete

SSH (Secure Shell): RFC 4251 Aims similar to telnet -> provides security! TCP connection (port 22) Other additional functions

o Secure FTPo support any insecure protocol ->

tunnelingo secure connections for X Windows

servers -> graphic apps

SSH. Features

Reduces security menaces:o Man in the middle -> sniffer o Spoofing

Cyphered data -> 128-bit encryptation Security scheme -> public/private key

(RSA) Client-server arquitecture. Two versions

o SSHv1 (1995) -> vulnerabilities

o SSHv2 (1997) Improvements over SSHv1 Current standard

services Telnet SSH

File Transfer Services

File transfer between remote hosts Aims:

Remote hosts may share files Client & server file systems are independent Efficient data transfer

Two main protocols FTP (File Transfer Protocol): uses TCP -> reliable

• RFC 959 TFTP (Trivial File Transfer Protocol): uses UDP ->

more simple• RFC 1350

Trivial File Transfer Protocol File transfer Very simple protocol Non reliable -> UDP (port 69) No folders; no encryptation For transfering small files

TFTP TFTP messages

Opcode(2 bytes)

Variable length data

Opcode: type of message• 01: RRQ (Read Request) • 02: WRQ (Write Request)• 03: DATA• 04: ACK• 05: Error message

Variable length data: depend on the opcode

TFTP TFTP messages -> RRQ & WRQ

RRQ is the first msg the client sends to server’s port 69 when getting a file

After RRQ -> DATA or Error Transmission mode: ‘netascii’ or ‘octet’ (binary files) N possible options with N values (one for each option) WRQ message format is the same as RRQ -> but opcode

value = 02 After WRQ -> ACK (server must give permission) or Error

Opcode = 01

File name

00Transmission

mode00

OptionN

00Value

N options

TFTP TFTP messages -> DATA & ACK

# block -> 1-65535 (guarantees order of data -> UDP can’t!)

Last block is recognised as its size < 512 bytes (what happens if total file lenght is a multiple of 512 bytes?)

Problem: long files -> a lost msg means a complete retx.

ACK message format is the same as DATA -> but opcode value = 04 & there are no data blocks

Opcode = 03

# block(2 bytes)

Data block of the transmitted file

0- 512 bytes

TFTP TFTP messages -> Error

Error code: cause of the error. Examples

• 0 -> Not defined. See error description• 1 -> File not found• 2 -> Access violation• 3 -> Disk full …• 6 -> File already exists

Opcode = 05

Errorcode

Errordescription

FTP: 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 Uses TCP: ports 20, 21 -> reliable transfer

file transfer FTPserver

FTPuser

interface

FTPclient

local filesystem

remote filesystem

user at host

FTP Client FTP Server

Data transfer

Control dialogue

Controldialogue

TCP protocol

Two TCP connections Data: transferred data (port 20) Control: allows the user moving through the

directory structure and downloading & uploading files (port 21)

FTP Client/Server model

Data transfer

Control dialogue

Controldialogue

TCP protocol

FTP Client initiates connection (server’s port 21) Connection parameters are negotiated in handshake

Data port Connection mode: active/pasive Transfer mode: ASCII/binary

Standard mode 2 TCP connections

Control: Client’s random port (>1024) to server’s port 21

Data: when server ACK -> from server’s port 20 to client’s port (indicated in first control command)

Data connection is initiated from the server

Data port (e.g. 1034)

Data port (20)

Control port (e.g.

Control port (21)

Command PORT 1034

Command ACK

Comando ACK

Active connection mode

Pasive connection mode 2 TCP connections

Control: Client’s random port (>1024) to server’s port 21 -> PASV command. Server indicates a random port for data connection (>1024)

Data: client establishes connection

Control port (e.g.

Control port (21)

PASVcommand

PORT 1820 command

ACK command

Connection establishment

Data port (20)

FTP Servers

Configuration parameters Control port (default: port 21) Max # of connections to server & max # of connections

per IP Connection timeout Welcome & goodbye msgs Passive mode port numbers

Users & groups Authenticated users: have login & passwd -> registered

in server Anonymous users Groups: share the same properties in FTP server

FTP Servers

Permissions Read, write, execution (rwx) Permissions for the owner, for groups & for the rest of

Bandwidth limit Server may limit transfer rate for the users

Logs Register data or any other info about user connections

& errors

FTP Clients

ftp <ip_addr> Commands

cd get put mkdir exit … Do not mistake FTP

commands typed by client for FTP control commands

FTP 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

services Telnet SSH

Network services 63

E-mail services

Main features: One of the most important services on the

Internet Allows users to exchange mails in an easy,

fast and cheap way Multiple receivers Client-server scheme Types of client apps

Graphic interfaces (Microsoft Outlook, Mozilla Thunderbird, Apple Mail)

Text (pine, elm, mail) Web (Gmail, Hotmail, SquirrelMail)

Network services 64

E-mail services

Related concepts: Mail account

Associated to user name & passwduser@domain.com

Mailbox Mail alias Mail list

Network services 65

E-mail services

Standards: SMTP (Simple Mail Transfer Protocol) IMF (Internet Mail Format) MIME (Multipurpose Internet Mail

Extensions) POP (Post Office Protocol) IMAP (Internet Message Access Protocol)

Network services 66

E-mail services

Components: Mail User Agent (MUA) Mail Transfer Agent (MTA) Mail Delivery Agent (MDA)

Mail User Agents (MUA) Mail client Compose, edit, read mail

messages Use two mail servers:

Outgoing mail server (SMTP)

Incoming mail server (POP or IMAP)

User mailbox

Outgoing mail

MUAMTA

POPIMAP

Network services 67

E-mail services

Mail Agent Transfer (MTA) Mail server Stores sender’s mails for

delivery (outgoing queue) Stores incoming mails for

their users

Mail Delivery Agent (MDA)

In charge of copying incoming messages to user mailbox

MUAMTA

POPIMAP

User mailbox

Outgoing mail

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Scenario: Alice sends message to Bob1) Alice uses UA to compose

message and “to” bob@someschool.edu

2) Alice’s UA sends message to her mail server; message placed in message queue

3) Client side of SMTP opens TCP connection with Bob’s mail server

4) SMTP client sends Alice’s message over the TCP connection

5) Bob’s mail server places the message in Bob’s mailbox

6) Bob invokes his user agent to read message

useragent

mailserver

mailserver user

2 3 4 56

Network services 69

Message format

IMF (RFC 5322) Headers

To: From: Subject: Date:

Body Simple text messages

(no extended ASCII ) up to 998 characters(no CRLF)

Headers

Blank line

Network services 70

Message format

MIME extensions: Add funcionality

Attached files Extended ASCII

New headers Mime-Version: Content-Type:

Default -> text/plainAttachments ->

Multipart Content-Description: Content-Transfer-

Encoding:

Types de encoding 7 bits 8 bits & binary quoted-printable &

base64.

Example quoted-printableF3 = ó & F1 = ñTransmisión de ñ

Transmisi=F3n de =F1

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SMTP [RFC 5321]

Features: Simple functioning: client – server Used in communication between MUA –> MTA & MTA -

> MTA Uses TCP connection -> port 25 Three phases

handshaking Message transfer (may be several ones) Connection closure

Messages encoded in 7-bit ASCII Binary -> ASCII (delivery) ASCII -> Binary (reception)

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SMTP [RFC 5321]

Features: command/response

response: free text & state code (3 figures):

First figure shows command success/fail

4xx -> Temporary error 5xx -> Permanent error

commands: ASCII text HELO: handshake after

TCP connection MAIL FROM: identifes

sender RCPT TO: identifies

receiver DATA: message init

End of line message -> ‘.’ QUIT: Closes SMTP session

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Sample SMTP interaction S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <alice@crepes.fr> S: 250 alice@crepes.fr... Sender ok C: RCPT TO: <bob@hamburger.edu> S: 250 bob@hamburger.edu ... Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C: . S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection

Network services 74

POP [RFC 1939]

Features: Very simple Allows the access to the incoming messages in

mailbox Default -> Delete accessed msgs, but allows saving

them Uses TCP connection -> port 110 Requires user authentication Three phases

authorization transaction Update

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POP [RFC 1939]

authorization phase client commands:

user: declare username pass: password

server responses +OK -ERR

transaction phase, client: list: list message numbers retr: retrieve message by

number dele: delete quit

C: list S: 1 498 S: 2 912 S: . C: retr 1 S: <message 1 contents> S: . C: dele 1 C: retr 2 S: <message 1 contents> S: . C: dele 2 C: quit S: +OK POP3 server signing off

S: +OK POP3 server ready C: user bob S: +OK C: pass hungry S: +OK user successfully logged on

Network services 76

IMAP [RFC 3501]

Features More complex than POP Allows the access to the incoming messages in

mailbox Allows msg organization in folders in the server When receiving a new mail ->INBOX folder in user

mailbox Allows the access to single components of a msg Keeps information about the state between IMAP

sessions

Network services 77

Web access

Features: A browser is used instead of a mail client MUA is integrated in a web page Host uses HTTP to communicate with web server Web server habitually uses IMPA for the access to

the incoming msgs in the mail server

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Problems

Main problems: Unencrypted msgs are transmitted

Use mechanisms of security (PGP, PEM, s/MIME) Bad uses

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Contact with many -> low cost Non-requested massive mail Types

Commercial Nigerian (fraud) Phishing Others

Origin Personal Computer Bad configured mail servers Bad configured proxy servers

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How are destination e-mail addresses obtained? guessing web page infected PC

Mail headers falsification (FROM) How to stop them

stop/make difficult to obtain mail addresses Identify them efficiently

Measures Do not disclose our mail address When publishing our mail address -> protected data use alternative addresses Care about our PC security

services Telnet SSH

MM Networking Applications

Fundamental characteristics:

typically delay sensitive end-to-end delay delay jitter

loss tolerant: infrequent losses cause minor glitches

antithesis of data, which are loss intolerant but delay tolerant.

Classes of MM applications:

1) stored streaming2) live streaming3) interactive, real-time

Streaming Stored Multimedia

Stored streaming: media stored at source transmitted to client streaming: client playout begins

before all data has arrived

o timing constraint for still-to-be transmitted data: in time for playout

Streaming Live Multimedia

Examples: Internet radio talk show live sporting event

Streaming (as with streaming stored multimedia) playback buffer playback can lag tens of seconds after

transmission still have timing constraint

Interactivity fast forward impossible rewind, pause possible!

end-end delay requirements: audio: < 150 msec good, < 400 msec OK

applications: IP telephony, video conference, new services not supported by traditional networks

Real-Time Interactive Multimedia

Streaming Stored Multimedia

browser GETs metafile browser launches player, passing metafile player contacts server server streams audio/video to player

audio or video stored in file

files transferred as HTTP object received in entirety

at client then passed to player

Streaming Multimedia: UDP or TCP?

UDP server sends at rate appropriate for client (oblivious to network congestion !)

often send rate = encoding rate = constant rate then, fill rate = constant rate - packet loss

short playout delay (2-5 seconds) to remove network jitter worse QoS due to packet loss

TCP send at maximum possible rate under TCP fill rate fluctuates due to TCP congestion control larger playout delay: congestion and flow control HTTP/TCP passes more easily through firewalls

User Control of Streaming Media: RTSP HTTP does not target

multimedia content no commands for fast

forward, etc.

RTSP: RFC 2326 Real-Time Streaming

Protocol: client-server application layer protocol

user control: rewind, fast forward, pause, resume, repositioning, etc…

What it doesn’t do: doesn’t define how

audio/video is encapsulated for streaming over network

doesn’t restrict how streamed media is transported (UDP or TCP possible)

doesn’t specify how media player buffers audio/video

User Control of Streaming Media: RTSP RTSP messages sent

out-of-band: RTSP control

messages use different port numbers than media stream: out-of-band. port 554 Similar functioning

to FTP (control & data)

media stream is considered “in-band”.

User Control of Streaming Media: RTSP C: SETUP rtsp://audio.example.com/twister/audio RTSP/1.0 Transport: rtp/udp; compression; port=3056; mode=PLAY

S: RTSP/1.0 200 1 OK Session 4231

C: PLAY rtsp://audio.example.com/twister/audio.en/lofi RTSP/1.0 Session: 4231 C: PAUSE rtsp://audio.example.com/twister/audio.en/lofi RTSP/1.0 Session: 4231 C: TEARDOWN rtsp://audio.example.com/twister/audio.en/lofi RTSP/1.0 Session: 4231

S: 200 3 OK

Real-Time Protocol (RTP) RTP specifies packet structure for packets

carrying audio, video data RFC 3550 RTP packet provides

payload type identification packet sequence numbering time stamping

interoperability: if two Internet phone applications run RTP, then they may be able to work together

Real-Time Protocol (RTP) RTP libraries provide transport-

layer interface that extends UDP: port numbers, IP addresses payload type identification packet sequence numbering time-stamping

RTP header

Payload Type (7 bits): Indicates type of encoding currently being used. If sender changes encoding in middle of conference, sender informs receiver via payload type field.

Payload type 0: PCM mu-law, 64 kbpsPayload type 3, GSM, 13 kbpsPayload type 26, Motion JPEGPayload type 33, MPEG2 video

Sequence Number (16 bits): Increments by one for each RTP packet sent, and may be used to detect packet loss and to restore packet sequence.

RTP header

Timestamp field (32 bytes long): sampling instant of first byte in this RTP data packet for audio, timestamp clock typically increments by one for each sampling period

SSRC field (32 bits long): identifies source of RTP stream. Each stream in RTP session should have distinct SSRC.

Real-Time Control Protocol (RTCP) works in conjunction with

RTP. each participant in RTP

session periodically transmits RTCP control packets to all other participants.

each RTCP packet contains sender and/or receiver reports report statistics useful to

application: # packets sent, # packets lost, interarrival jitter, etc.

feedback can be used to control performance sender may modify its

transmissions based on feedback

RTP, RTCP packets distinguished from each other via distinct port numbers.

to limit traffic, each participant reduces RTCP traffic as number of conference participants increases

Real-Time Control Protocol (RTCP)Receiver report packets: fraction of packets

lost, last sequence number, average interarrival jitter

Sender report packets: SSRC of RTP stream,

current time, number of packets sent, number of bytes sent

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