ee3414 multimedia communication systems â€“ part i spring 2003

EE3414Multimedia Communication Systems –

Part ISpring 2003

Internet Technology

Shivendra PanwarElectrical and Computer Engineering

Polytechnic UniversityBased on the lecture note prepared by Jorg Liebeherr

Outline

The Internet– Internet protocol (IP) overview

- Addressing- IP Datagram format- Fragmentation

– ICMP (Internet Control Message Protocol)Transport layer– Overview– Flow control– Connection management

UDP vs TCPDNS

Polytechnic University

EL 536: Introduction to Com

munications N

etworks

© Jorg Liebeherr, 1998 3

History of the InternetMid 1960: Papers on “Packet Switching” emergeEnd 1969s: ARPA sponsors the development of a packet-

switching network, called the ARPANET. First four nodes are UCLA, SRI, U. Utah, UCSB

1974: The TCP/IP protocols and model are being proposed by Cerf/Kahn

1983: ARPANET adopts TCP/IP. At this time, the ARPANET has 200 routers.

1984: NSF funds a TCP/IP based backbone network. This backbone grows into the NSFNET, which becomes the successor of the ARPANET.

1995: NSF stops funding of NSFNET. The Internet is completely commercial.



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Main Applications of the Internet

Traditional core applications:EmailNewsRemote Login (Telnet to super computers)File Transfer

The killer application:World-Wide Web (WWW)

Future applications:Videoconferencing and TelephonyMultimedia ServicesInternet BroadcastAddressed by IPv6 and ISA in IPv4



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Growth of the Internet

• Latest data:Jul 98 : 36,739,000 Hosts Jan 98: 29,670,000 Hosts

Source: Network Wizards, http://www.nw.com/

Doubling every 12 month



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IP (Internet Protocol) is a Network Layer Protocol

IP’s current version is Version 4 (IPv4)

NetworkLayer

Link Layer

IP

ARP NetworkAccess RARP

Media

ICMP IGMP

TransportLayer

TCP UDP

NetworkLayer

Link Layer

IP

ARP NetworkAccess RARP

Media

ICMP IGMP

TransportLayer

TCP UDP

IP Protocol Stack



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Application

TCP

IP

NetworkAccess

Application

TCP

IP

NetworkAccess

Application protocol

TCP protocol

IP protocol IP protocol

DataLink

NetworkAccess

IP

NetworkAccess

NetworkAccess

IP

NetworkAccess

DataLink

DataLink

IP protocol

RouterRouter HostHost

Application

TCP

IP

NetworkAccess

Application

TCP

IP

NetworkAccess

Application protocol

TCP protocol

IP protocol IP protocol

DataLink

NetworkAccess

IP

NetworkAccess

NetworkAccess

IP

NetworkAccess

DataLink

DataLink

IP protocol

RouterRouter HostHost

IP Overview

IP is the highest layer protocol that is implemented at both routers and hosts:

Analogy between the transport of a datagram with the delivery of a letter: each datagram has a source and destination address, and each intermediate router forwards an entering datagram to the next intermediate router or the final host based on the destination address.



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IP Service

IP provides an unreliable and connectionless service (“datagram service”).– Unreliable: IP does not guarantee that a

transmitted packet will be delivered– Connectionless: Each packet (“datagram”) is

handled independently. IP is not aware that packets between hosts may be sent in a logical sequence

Consequences of an unreliable, connectionless service

– Lost Packets– Packets are delivered out-of-sequence– Duplicated Packets



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Internet Addresses

Each network interface on the Internet has a unique global address, called the IP addressAn IP address:

- is 32 bits long.- encodes a network number and a host number

IP addresses are written in a dotted decimal notation:128.238.42.112 means

10000000 in 1st Byte11101110 in 2nd Byte00101010 in 3rd Byte 01110000 in 4th Byte



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Internet Address Classes

IP distinguishes 5 classes of addresses.

1

Class A

1 1 1 0

0 network id7 bits

host24 bits

Class B 1 network id14 bits

host16 bits

0

Class C 1 network id21 bits

01 host8 bits

Class D 1 multicast group id28 bits

01 1

Class E (reserved for future use)27 bits



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Trade-off of Address Classes

There are a total of 232 = 4,294,967,296 IP addresses.The network numbers are assigned by InterNIC(Network Information Center).

Class A: 7 bits for netid (only 128 Class A networks)each net can have 16 million (224) hosts.

Class B:14 bits for netid (about 16,000 Class B networks)About 65,000 (216) hosts per network

Class C:21 bits for netid (about 2 million Class C networks)Only 255 hosts per network



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20 bytes ≤ Header Size ≤ 24 * 32 bit-words = 60 bytes20 bytes ≤ Total Length ≤ 216 bytes = 65536 bytes

IP Datagram Format

version(4 bits)

headerlength

Type of Service/TOS(8 bits)

Total Length (in bytes)(16 bits)

Identification (16 bits) flags(3 bits) Fragment Offset (13 bits)

Source IP address (32 bits)

Destination IP address (32 bits)

Options (if any, <40 bytes)

DATA

>= five 32-bit words

32-bit word0 31

TTL Time-to-Live(8 bits)

Protocol(8 bits) Header Checksum (16 bits)



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Fields of the IP Header

Protocol: Specifies the higher-layer protocol.Used for demultiplexing to higher layers.

Header checksum: verifies correctness of header.– 16 bit ones complement addition of all 16-bit words in the header,

verified and recomputed at each router

IP

ICMP IGMP

TCP UDP

Application



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IP Fragmentation

FDDIRing

RouterHost A Host B

Ethernet

• Host A sends a large IP datagram to Host B.

• Any Problem with that?

MTUs: FDDI: 4352 Ethernet: 1500

• IP router splits the datagram into several datagrams (=Fragmentation)



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What’s involved in Fragmentation?

The following fields in the IP header are involved:version(4 bits)

headerlength

Type of Service/TOS Total Length (in bytes)

Identification flags Fragment Offset

......

TTL Time-to-Live(8 bits)

Protocol(8 bits) Header Checksum (16 bits)

Identification is the same in all fragments.Flags contains a “more fragments” bit

(There is also a “don’t fragment bit” that can be set)Fragment offset contains the offset of current fragment

in the original datagramTotal length is changed by fragmentation



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Type(8 bits)

Code(8 bits)

Checksum(16 bits)

(additional information dependent on Type and Code)

32-bit word0 31

ICMP

The Internet Control Message Protocol (ICMP) is the protocol used for error and control messages in the Internet ICMP provides an error reporting mechanism of routers to the sourcesAll ICMP packets are encapsulated as IP datagramsThe packet format is simple:



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Types of ICMP Packets

Many ICMP packet types exist, each with its own format.A Selection:

Type Field: Message Type:0 Echo Reply3 Destination Unreachable4 Source Quench5 Redirect (Change Route)8 Echo Request11 Time Exceeded12 Parameter Problem in Datagram14 Timestamp Request



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ICMP Message Types

ICMP messages are either query messages or errormessages. ICMP query messages:

- Echo request / Echo reply- Router advertisement / Router solicitation- Timestamp request / Timestamp reply- Address mask request / Address mask reply

ICMP error messages:- Host unreachable- Source quench- Time Exceeded - Parameter Problem



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The PING program

PING (=Packet IntetNet Groper) is a program that utilizes the ICMP echo request and echo reply messages

PING is used to verify if a certain host is up and running. It is used extensively for fault isolation in IP networks



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PING’s are handled directly by the kernel.Each Ping is translated into an ICMP Echo RequestThe Ping’ed host responds with an ICMP Echo Reply

Echo Request and Reply

AIDAAIDA

ICMP ECHO REQUEST

MNG MNG

ICMP ECH

O REPLY

Transport Layer Outline

Transport layer functionality overviewAddressingFlow control

– Credit allocationError controlConnection management

– Initiating and terminating connectionsExamples: TCP, UDP

– Overview of TCP vs. UDP– TCP flow control and error control

Yao Wang EL536 --- Data and Computer Communications



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Overview of Transport Layer• Transport layer protocols are end-to-end protocols • Transport layer is only implemented at the hosts• Session and presentation layer may not be implemented, most application layer protocols simply open a TCP/UDP connection

Application

Transport

Network

HOST

Data Link Data Link Data Link

Network

Application

Transport

Network

HOST

Data Link



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Protocol Mechanisms

AddressingMultiplexing Flow ControlError ControlConnection Management

Note: The mechanisms needed to implement a transport service are largely dependent on the existing network layer service



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Addressing

An address at the transport layer is typically a tuple (Station, Port) where – Station is the network address of the host, and– Port identifies the application

Recall: The <IP address, port number> tuples used in the Unix assignments are in fact transport layer addresses

Problem with Addressing: How to find the address of a service?



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Flow Control

Why do we need flow control at the transport layer? 1. User of receiving transport entity cannot keep up

with the data flow.2. Receiving transport entity itself cannot keep up

with flow of incoming packet.

Result: Buffer overflows in the receiving transport entity.



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Need for Flow Control

transportentitity

transportentitity

Site A Site B

networklayer

transportuser

transportlayer



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Flow Control at the Transport Layer

Flow Control at the transport layer is more complex than flow control at the data link layer:– Delays are variable and are longer– Flow control involves the transport users, the

transport entities, and the network service

Approaches to Flow Control

Do Nothing– TPDUs that overflow the buffer are discarded– May be appropriate for delay sensitive but loss insensitive

services (e.g. audio-visual conferences)Refuse to accept TPDUs from the network layer

– Requires a backpressure mechanism that pushes flow control to the network layer

(Fixed) Sliding-window Protocol– Problems of fixed:

- Withholding acknowledgments in an unreliable network results inretransmission

- Sliding window flow control not effective– Solution:

- separate acknowledgement from flow control – credit allocation- Adapt window size

Yao Wang EL536 --- Data and Computer Communications



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Credit Allocation Flow Control

Credit Allocation Flow Control is an extension of the sliding window flow control.Main Idea:– Enhance the sliding window protocol by a

mechanism that decouples acknowledgments from flow control.

Then:– Packets can be acknowledged without granting

permission for new transmissions– Used in many existing transport protocols,

including TCP



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Credit Allocation Flow Control

Initialization during connection setup: – Set initial window size of receiver– Receiver both acknowledges TPDUs and grants

credit by sending a message:(ACK N, CREDIT M)

– ACK N: Acknowledges all sequence numbers through N-1

– CREDIT M: Sets the number of credits to MCredit is the maximum window size (=window size that can be allocated at

the sender for this connection)



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Example

ReceiverTransmitter

0 1 2 3 4 5 6 7 0 1 2 3 0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3 0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3

T0T1T2

ACK2, Credit 6

0 1 2 3 4 5 6 7 0 1 2 3

T4T5

0 1 2 3 4 5 6 7 0 1 2 3

Initial Setting: Credit = 7

Fig. 17.5 in text

T3 T4

ACK3, Credit 5



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Example (cnt’d)

ReceiverTransmitter

0 1 2 3 4 5 6 7 0 1 2 3 0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3 0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3

D5D6D7

ACK0, Credit 4

0 1 2 3 4 5 6 7 0 1 2 3



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Connection Management

Connection establishment and connection termination are essential for a connection-oriented service

Connection establishment is asymmetric:– one side puts itself in a LISTEN state (server)– one side issues a request for connection or RFC

(client)– One connection is established, data flow is full

duplex in both directions



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Simple Solution (which has problems)

CR (SeqNo = x) Connection Request, A wants to start with SeqNo = xACK (SeqNo = y) Acknowledge request, B will wants to start with SeqNo = yDATA (SeqNo = x)Data transmission with SeqNo x

Host A Host B

CR (SeqNo = x)

ACK (SeqNo = y)

DATA(SeqNo = x)

Two Way Handshake



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Problems with Two-Way Handshake

B responds toCR(SeqNo = z),an old duplicate connection requests from A In the shown sce-nario, A believes that the ACK is for the connection request CR(SeqNo = y)

Host A Host B

CR (SeqNo = x)

ACK (SeqNo = y)

DATA(SeqNo = x)

CR (SeqNo = z)

Result: A starts to send data with Sequence x. B will throw the data away since it expects SeqNo = z



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Three-Way Handshake

Note: A and B acknowledge the sequence number from the other side

This solution provides protection from old duplicate connection requests

Host A Host B

CR (SeqNo = x)

ACK (SeqNo = y, ACK = x)

DATA(SeqNo = x, ACK = y)



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Connection Termination

A connection release should involve both sides of the connection (otherwise data is lost)

CR ()

ACK ()

DATA()DATA()DATA()DATA()

DR ()

Host A Host B

Here: B should wait after Disconnection Request (DR) is sent until all data has arrived



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Connection Termination in 4 steps (TCP)

An elegant way to terminate connections is to have each end shut down independently (“half-close”)If one end wants to shut down, it sends a DR messageFour steps involved:

(1) A sends a DR to B (active close)(2) B ACKs the DR, (completing half-close)(at this time: B can still send data to A)

(3) After certain time, B sends a DR to A (passive close)(4) A ACKs the DR (completing full-close)



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Transport Protocols in the Internet

The Internet uses two transport protocols

– Transmission Control Protocol (TCP)

– User Datagram protocol (UDP)



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ApplicationLayer

NetworkLayer

Link Layer

IP

ARP HardwareInterface RARP

Media

ICMP IGMP

TransportLayer

TCP UDP

UserProcess

UserProcess

UserProcess

UserProcess



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UDP UDP -- User Datagram User Datagram ProtocolProtocol

datagram orientedUnreliable (best-effort), connectionlesssimpleunicast and multicastLow-delay, hence good for multimedia applicationsused a lot for services

– network management (SNMP), routing (RIP), naming (DNS), etc.

TCP - Transmission Control Protocol

stream oriented, in sequencereliable, connection-orientedcomplexonly unicastused for most Internet applications:

– web (http), email (smtp), file transfer (ftp), terminal (telnet), etc.



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UDP - User Datagram Protocol

UDP is supports unreliable transmissions of datagramsUDP merely extends the host-to-to-host delivery service of IP datagram to an application-to-application serviceThe only thing that UDP adds is multiplexing and demultiplexing (encaptulation)

UDP

IP IPIP IP IP

UDP

Applications Applications



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UDP Format

IP header UDP header UDP data

UDP message length Checksum

DATA

20 bytes 8 bytes

0 15 16 31

Source Port Number Destination Port Number

•Port numbers identify sending and receiving applications (processes). Maximum port number is 216-1= 65,535• Message Length is at least 8 bytes (I.e., Data field can be empty) and at most 65,535• Checksum is for header (of UDP and some of the IP header fields)



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Port Numbers

UDP (and TCP) use port numbers to identify applicationsA globally unique address at the transport layer (for both UDP and TCP) is a tuple <IP address, port number>There are 65,535 UDP ports per host

IP

TCP UDP

UserProcess

Demultiplexbased on

Protocol field in IPheader

UserProcess

UserProcess

UserProcess

UserProcess

UserProcess

Demultiplexbased on

port number



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TCP

TCP = Transmission Control ProtocolConnection-oriented protocolProvides a reliable unicast end-to-end byte stream over an unreliable internetwork.

TCP

IP Internetwork

Byte

Stre

am

Byte

Stre

am

TCP



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TCP is Connection-Oriented

Before any data transfer, TCP establishes a connection:

- One TCP entity is waiting for a connection (“server”)- The other TCP entity (“client”) contacts the server

The actual procedure for setting up connections is the three way handshakeEach connection is full duplex

CLIENT SERVER

waiting forconnection

request

Request a connection

Accept a connection

DisconnectData Transer



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Reliable Data Transfer

Byte stream is broken up into chunks which are called segments– Receiver sends acknowledgements (ACKs) for

segments– TCP maintains a timer. If an ACK is not received

in time, the segment is retransmitted

Detecting errors:– TCP has checksums for header and data.

Segments with invalid checksums are discarded– Each byte that is transmitted has a sequence

number



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TCP Segment Format

IP header TCP header TCP data

Sequence number (32 bits)

DATA

20 bytes 20 bytes

0 15 16 31

Source Port Number Destination Port Number

Acknowledgement number (32 bits)

window sizeheaderlength 0 Flags

Options (if any)

TCP checksum urgent pointer

20 bytes

• TCP segments have a 20 byte header with >= 0 bytes of data.



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Flow Control in TCP

Flow Control: – How to prevent that the sender overruns the

receiver with information?Flow Control in TCP– TCP implements sliding window flow control– Sending acknowledgements is separated from

setting the window size at sender. Acknowledgements do not automatically increase the window size (credit allocation flow control)

– Acknowledgements are cumulativeHow to determine window size adaptively?



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Slow Start/Congestion Avoidance

TCP has a mechanism for congestion control. The mechanism is implemented at the sender

The sender has two additional parameters:– Congestion Window (cwnd; Initial value is 1 MSS counted

as bytes) (For flow control)– SlowStartThreshhold (ssthresh; Initial value is 65536

bytes) (For congestion avoidance)The receiver tells the sender the flow control window (=credit in the ACK) sizeThe window size at the sender is set as follows:Allowed Window =

MIN (flow control window, congestion window)congestion window: flow control by the senderflow-control window (credit): flow control by the receiver

PC (web client)

DNS Server

eeweb server

128.238.37

.15

.3

.40a:11:23:45:0c:de

1b:3d:23:75:9c:a7

12:23:34:6f:1a:30

• PC web client, DNS server and eeweb server are all on the same ethernet LAN and same IP subnet.

• PC client has the IP address of the DNS server ethernet card, but it does not have the MAC address

ARP (128.238.37.3)

DNS query/UDP/IP(eeweb.poly.edu)

ARP reply (12:23:34:6f:1a:30)

DNS reply(128.238.37.4)

ARP(128.238.37.4)

ARP reply(1b:3d:23:75:9c:a7)

Http/tcp/ip/ethernet(src. IP address: 128.238.37.15dst. IP address: 128.238.37.4src. MAC address: 0a:11:23:45:0c:dedst. MAC address: 1b:3d:23:75:9c:a7)

Domain Name Server Illustration

Summary

Transport layer functions– Addressing: host address + port (process)– Connection management: establishment and termination– Flow control: sliding window with credit allocation – Error control: retransmission with adaptive timeout

Necessary transport layer protocols depend on the underlying network layer

– If the network layer provides connection-oriented, reliable service, then transport control is easy

- Ex. X.25, ATM, Ethernet– Otherwise, providing reliable transport is more complicated

- Ex. Internet

Summary (Cnt’d)

Transport protocols in IP networks – IP is a connectionless, unreliable network– UDP provides connectionless, unreliable service, merely adds the

port number and checksum header to the IP layer– TCP provides connection-oriented, reliable service

- Connection management: – 3 way handshake for open, – 4 step for close.

- Congestion control: sender window adapts to congestion status (slow start with congestion avoidance)

- Error control using retransmission: – Retransmission time out (RTO) based on smoothed RTT

measurements– Exponential backoff (doubling RTO) after timeout

ee3414 multimedia communication systems â€“ part i spring 2003

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