Internet Transport Protocol

ENGG1015

1st Semester, 2010

Hayden Kwok-Hay So

Transport Layer   Two important Internet protocols on top of IP:

•  UDP and TCP

  Concerns with how processes (applications) running at different hosts communicate with each other •  More than 1 processes on each host want to use

the Internet

  Hide uncertainties of the IP network from applications •  Reliable communication •  In-order delivery

UDP - User Datagram Protocol   Use the notion of port to

multiplex among processes on a host •  In UNIX system, use socket to

bind an application with a port •  The <port, host> pair uniquely

identifies an instance of running application on the Internet.

  Low overhead •  Connectionless •  Unreliable delivery

  Used mostly for applications that don’t need reliable transmission •  E.g streaming applications or

non-essential apps

TCP - Transmission Control Protocol   Connection oriented

•  Point-to-point: one sender, one receiver •  Full duplex data: Each connection support bidirectional data

transmission

  Addresses end-to-end issues of application data transmission on unreliable Internet •  Reliable delivery •  In-order delivery •  Congestion control •  Flow control

Reliable Transmission   IP layer provides best effort delivery

•  Packets dropped •  Out-of-order delivery

  TCP uses a sliding window retransmission scheme •  Multiple data frame transmitted within a

transmission window •  Accumulated ACK

  Background: Stop-and-Wait retransmission •  Sender waits for Acknowledgement (ACK) from

receiver before sending the next packet. •  If no ACK within certain time frame (timeout

period), sender retransmit the previous packet.

Retransmission Scenarios   (a) ACK

received before timeout

  (b) data lost

  (c) ACK lost

  (d) ACK received after timeout •  Redundant data

frames received at receiver

TCP Retransmission: Seq#, ACK   Sequence #s

•  Byte stream number of the first byte in segment’s data

  ACKs •  Seq# of the next byte

expected from other side

  Note: Cumulative ACK •  ACK all the received

data

seq#: 49, ACK 75, data ‘a’ seq#: 50, ACK 75, data ‘b’ seq#: 51, ACK 75, data ‘c’

seq#: 75, ACK 52, data ‘1’

seq#: 52, ACK 76, data ‘d’

A B

 Example: • A sends “abcd” to B • B sends “123” to A

trans

mis

sion

win

dow

Congestion and Flow Control   Congestion Control

•  Avoid injecting too much data into the network that will overload the network

•  Interaction between a host and the network

  Flow Control •  Avoid sending too much data from the sender to the receiver

that will overload the receiver •  An end-to-end issue involving the two peers of a connection

  TCP contains state about the network and the receiver •  Transmission windows depending on the advertised window

of receiver and the observed congestion window •  Receiver window advertised in ACK packet from receiver •  Congestion window adjusted by the sender according to

TCP specific algorithm

Making Connections   TCP is connection

based

  Data is sent only after a connection is made

  Connections end •  when either end of the

connection closes them •  when one side do not

hear from the other side for a timeout period

Internet Summary   Internet was built upon layers of

abstractions on top of a mix of different physical networks

 3 key innovations •  Packet switching network •  Layering concept •  End-to-End design principle

 Network layer: IP  Transport Layer: TCP, UDP

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Project (1)  Next Wednesday Dec 1, 10am, LG205  Highlight of the day: •  10am – 1pm: Qualifying Round •  1pm – 2pm: Lunch + final tune-up •  2pm – 4pm: Final Round •  4pm – Refreshments, semester wrap up

  Judging Panel: •  Prof. Victor Li •  Dr. Kenneth Wong •  Dr. Hayden So

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Project (2)  3 Competition Criteria: •  Functionality •  Creativity •  Teamwork

 3 Bonuses •  Timeliness •  Most number of stages •  User’s Choice

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Project – Qualifying Round   Qualifying Round:

•  3 minutes each group •  Demonstrate the functionality/creativity of the

machine •  Discussion about team work

  Graded on •  Functionality •  Creativity •  Teamwork

  Cast vote on User’s Choice •  Vote for your favorite design

  8 teams selected to enter the final round

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Project – Final Round  10 minutes per group

 Explain in detail the design and the implementation process to the panel

  Impress the panel!

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