ccnet-lec-13-tcp-udp

8/6/2019 ccnet-lec-13-TCP-UDP

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Lecture

13Data Communication & Networks

Transport Layer

(TCP/UDP)

Muhammad Yousaf


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Where we are in protocol stack:

todays topic

application

end-to-end

physical

IP

data link }Until now

we have been discussing

the issues related to

lower three layer

}


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Users Perspective:

Users demand connectivity, reliability, functionalityThey dont concern of underlying technology

Our IP network is an unreliable network

IP provides the Best Effort Service

Packets may be discarded, duplicated, receive with error,etc.

Users want the reliable communication between

end applicationsConnectivity between end host applications iscalled End-to-End connectivity


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Scope of different layer:

Data Link Layer:Communicate between directly-connected nodesWithin a single network only

IP Layer:

Communicate between hosts residing anywhere in the world

Across multiple networks

Take end host as a single entity

Transport Layer:

We possess multi-tasking computers

For users, each application is different entity

Communicate among different applications on individual hosts acrossmultiple networks


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Transport Layer:

For communication among applications:we need another level of addressing

that uniquely distinguish different applications on host

Port numbers are used to achieve this level of

addressingThese are logical port numbers, in contrast to physicalports

Port number is a 16-bit positive integer value

Theoretically it means, a computer can supportthousands of simultaneous connections! Enough

Transport layer protocols e.g. TCP & UDP provide portnumbers


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Port Numbers:

Well-Known Ports:Permanently assigned to servers listening for requestsAssigned by IANA

Ranging from 0 to 1023

Registered Ports:Not permanently assigned

But registered with IANA

e.g. WINS uses port-1512

Ephemeral Ports:Temporarily assigned to clients

Can change for client applications in each session

Can range from 1024 to 65536


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Well-Known Ports:

For TCP:Port-21 = FTP (File Transfer Protocol)

Port-25 = SMTP (Simple Mail Transfer Protocol)

Port-53 = DNS (Domain Name System)

Port-80 = HTTP (Hyper Text Transfer Protocol), [www]

For UDP:

Port-69 = TFTP (Trivial FTP)

Port-123 = NTP (Network Time Protocol)Port-161 = SNMP (Simple Network Management Protocol)

Port-520 = RIP (Routing Information Protocol)


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Transport Layer Services:

Other than application multiplexing, there are twoother types of end-to-end services at transportlayer:

Connectionless Service

Connection-Oriented Service


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Connectionless Service:

Connectionless:Unreliable

No Acknowledgments

No Control over out-of-order delivery

No Flow ControlNo Congestion Control

Protocol = UDP

Helpful for applications that dont need guaranteed

delivery

e.g. multimedia, delay-sensitive applications


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Transport Layer Protocols:

Many transport layer protocols are supported byInternet

User Datagram Protocol (UDP)

Transmission Control Protocol (TCP)

Stream Control Transmission Protocol (SCTP)Datagram Congestion Control Protocol (DCCP)


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UDP:

UDP = User Datagram ProtocolDefined in RFC-768

There is no notion of connection

You have the data, you send it without checkingwhether destination is alive or not

Also, there is no acknowledgment of sent data

In case of error there is no retransmissions


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UDP header fields:

Source Port:Port number of sending application

Destination Port:

Port number of receiving application

Length:

Length of entire UDP packet including header & data

Checksum:

Optional, if not used then set equal to zero


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TCP:

Transmission Control ProtocolDefined in RFC-793

For sending data:

First establishes connection

Then transfer data

In the end, terminates connection

Provides reliable byte-stream for end-users


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TCP header:


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TCP header fields:

Source Port:Port number of sending application

Destination Port:

Port number of receiving application

Sequence Number:

Sequence number (byte count) of first byte of TCPsegment

Used for in-order delivery

Acknowledgment Number:

Sequence number of next byte expected by the receiver

Used for reliability & flow control


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TCP header fields: cont

Data Offset:

Number of 32-bit words in the header (header length)

Reserved:

Reserved for future use

TCP Header Flags:

CWR: Congestion Window Reduced indication (*RFC-3168)

ECE: ECN (Explicit Congestion Notification) indication

URG: urgent pointer field is significant

ACK: Acknowledgment field is significant

PSH: Push data from TCP send buffer

RST: Reset connection

SYN: Synchronize, set in connection-request message

FIN: Finish, set in connection close message


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TCP header fields: cont

Window:Indicate number of bytes that receiver can receive

Used for flow control

Checksum:

Same as IP & UDP checksum

Urgent Pointer:

Indicate to receiver that some urgent data is coming

Options + Padding:Optional information if any

e.g. negotiation of maximum segment size


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TCP connection:

Two applications talking to each other over TCPestablish a TCP-connection between each other

For this connection, TCP at each host reservessome resources for this connection

This is an agreement between end hosts only

Network is unaware of any such connection

Network doesnt know any thing above IP layer

This is in contrast to Circuit-Switched connectionWhere network reserves resources for end nodes


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TCP connection: cont

Over Internet each application is identified through2-parameters

Port Number of application & IP Address over which thatapplication is being executed

(port number, IP address)And each TCP connection between twoapplications is identified through 4-parameters

Two from source application, two from destination

application

(source port, source IP, destination port, destination IP)


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TCP connection setup:

A single TCP connection is full-duplexBoth client & server can send/receive simultaneously

Connection is established through a three stepprocess

3-way handshake

Client sends request (syn segment)

Server acknowledges it and also send its own synsegment

Client acknowledges servers syn

Recall the flags in TCP headerSYN, FIN, RESET, PUSH, URG,ACK


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TCP connection setup: cont

3-way HandshakeJ and K are (different) sequence numbers for messages

Sequence numbers need not start at zero

Active

participant

(client)

Passive

participant

(server)

SYN J

SYN K

ACK J+1

ACK K+1


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Data transfer over TCP:

After establishment of connection, in terms ofcommunication there is no discrimination betweenclient & server

They both are now sender & receiver

(NOTE: we are talking in terms of communication, not interms of services)

Sender sends with byte sequence number

Receiver replies with acknowledgment number &advertised window size


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Data transfer over TCP: cont

Reliability & Flow controlSequenceNum, ACK, AdvertisedWindow

Sender

Data(SequenceNum)

Acknowledgment +AdvertisedWindow

Receiver


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TCP connection termination:

Either client or server can initiate connection terminationTCP connection teardown in 4 steps

client server

FIN J

FIN K

ACK K+1

ACK J+1

active close

passive close

closes connection


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TCP connection: (concept)

client server

my-machinemail.yahoo.com

I am

mail.yahoo.com,

port b

I accept

connections

I will talk to

my-machine,

port a

Talk to

mail.yahoo.com,

port b

Resulting TCP connection identified by

(my-machine:a, mail.yahoo.com:b)


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TCP connection: (implementation)

I am

mail.yahoo.com,

port b

I accept

connections

I will talk to

my-machine,

port a

Talk to

mail.yahoo.com,

port b1. socket()

2. bind()

3. listen()

4. socket()

5. connect()

6. accept()

7. send() / sendto()

8. recv() / recvfrom()

9. close() / shutdown()

client server

my-machinemail.yahoo.com


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To be continued

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