TCP: Software for Reliable Communication

Spring 2002Computer Networks

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Internet: a Collection of Disparate Networks Different goals:

Speed, cost, distance; Different standards for:

Expected carrier; Coding bits; Detecting and recovering from errors; Protocols for transmitting messages: bus,

token ring,… Packets sizes, and encoding for the start/end

of packets,… Types of computer addresses

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Solution: Routers

RouterRouter

Router

High speed connection

Routers: computers design to interconnect different networks

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Solution: Internet Protocol (IP) Divide a message in small blocks, called

packets; IP protocol for transmitting packets; IP hides the details of physical networks; Every computer connected to the Internet

must run IP software IP specifies:

Packet format; How routers should forward packets Define address format

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IP can be overrun Consider the following scenario:

Computers X and Y send simultaneously messages to a computer Z across the same network, d;

Both X and Y send 5000 packets/s Network d may transmit 5000 packets/s; There is an excess of 5000 packets each

second which are discarded.

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IP can be overrun (cont.)

TCP (Transmission Control Protocol) designed to handle this problem.

X Y Z

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TCP Checks for lost datagrams; Routers may fail TCP chooses new paths; Because datagrams may travel different

paths, they can arrive at the destination in a different order TCP assembles them in the correct order

Network failure may result in multiple copies of the same datagram TCP checks for duplicate datagrams, and accepts only the first copy

Spring 2002Computer Networks

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TCP- a connection oriented protocol IP: a connectionless protocol, i.e. there is no

connection between sender and destination;

In order to achieve its goals TCP establishes a connection between two computers;

A data transmission proceeds much like a telephone call:

The sender program contact the destination The destination accepts the incoming call; The sender and destination may exchange arbitrary amounts of

data; Any of the party may terminate the communication

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Recovering lost datagrams Problem: datagrams may be lost by a

router far away from the sender or the destination;

Solution: TCP includes a unique ID in each datagram; Whenever data arrives at the destination, it

sends an acknowledgment (ack) back to the source, containing the ID of the datagram;

If an ack is not received in a certain time the sender retransmits the message.

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Example of retransmission

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Retransmission is adaptive Different values for waiting times:

Short, if source and destination are “close” to each other;

Longer, if they are far apart. Adjusted if delays occur

TCP measures the delay in sending a message and adjusts the timer

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Retransmission examples

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Detecting duplicate datagram The receiver maintains a table with

all datagrams received so far; When the destination, receives a

datagram, checks the ID of incoming with the IDs of the datagrams received so far;

If it is a duplicate, the datagram is discarded.

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TCP and IP work together IP specifies how a packet should be

transmitted from the sender to the destination;

TCP specifies ways for making packet transmission reliable;

TCP/IP often used and sold as a single software packet.

Spring 2002Computer Networks

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Designing Protocols Two possible solutions:

A single, giant protocol that specifies all details;

Cons: difficult to design, and to update Divide the problem into subpieces, and

design a special protocol for each piece; Each piece is called a layer; The result: a suite of protocols; Requires that different parts share

information.

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TCP/IP---a layered protocol Has seven layers; Lowest layer

refers to hardware;

Top layers refer to the software;

The level of abstraction increases bottom to top.

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TCP/IP layers: Layer 1 (Physical layer):

Corresponds to network hardware; defines the carrier, the way to encode

bits, etc..

Example of a carrier

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TCP/IP layers (cont.) the carrier is reduced to 2/3 full strength to encode a 1

bit, and 1/3 strength to encode a 0 bit; Two successive pulses needed to encode one bit; an

unchanged pulse separates two bits.

Digital signal

The resulting wave encodingThe signal above.

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TCP/IP layers (cont.) Layer 2: data link

Specifies how to organize data into packets, and how to transmit packets over a network

For ex: maximum packet size, format packet header, checksum computation are defined at this layer.

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TCP/IP layers (cont.) Layer 3: Network

Specifies how addresses are formed Ex: IP addresses

How packets are forwarded: Ex: store and forward technique

Layer 4: Transport Handles details of reliable transfer; EX: format of acks, retransmission times,

rules for changing it

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TCP/IP layers (cont.) Layer5: Session

Specifies how to establish a communication with a remote system; ex: telnet

Authentication details; ex: passwords Layer 6: Presentation

Specifies how to represent data; Different computers use different internal

representation (Ex: ASCII, EBDIC) for integers and characters;

How to translate from one representation to another

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TCP/IP layers (cont.) Layer 7: Application

Specifies how one particular application uses a network;

Ex: FTP Specifies request format (how to

name a file) and how the application on another machine responds.

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How layered software works Each layer solves one part of the problem; To do so, each layer on the sending computer

adds information to the outgoing data; The same layer in the receiving computer uses

the additional information to process data; Ex: checksums in data layer;

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How layered software works (cont.)

Layering Principle: Layer N software on the destination

computer, must receive the exact message sent by layer N software on the sending computer.

Ex: if one layer adds a header, the corresponding layer has to remove it.

If one layer encrypts data, the receiving computer layer has to decrypt it.

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Why layering? Each layer can be:

Designed, Implemented Tested

independently of other layers.creating and evolving protocols,

much easier task.