Transport Layer - Local and Wide Area Networks - Lecture Slides, Slides of Social Work

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Intro to LAN/WAN

Transport Layer

What’s Left in this Class?

Transport Layer

Application Layer + Exam review

• DNS, Email, etc

)

Final exam

Introduction

)

Efficient, reliable and cost-effective service to users(application layer)

• despite limitations of network layer–

Example:

routers can drop packets (congestion) but

transport must guarantee reliable delivery

Analogy

: physical layer could corrupt bits but data link

layer guaranteed reliable delivery

)

Features (a lot like the Network layer?)

Connection oriented

vs.

Connectionless

Addressing

)

If similar, then why replicate functions acrosslayers?

• Transport layer completely runs on on host– Network layer distributed over hosts and routers

Introduction

Similar to data link layer

• Mechanisms: (ACKs), sliding window, seq.

numbers, etc

• Services: error control, sequencing, flow

control…

Difference?

• Data Link is point-to-point,– Transport is end-to-end

Transport Entity

Logical location of transport entity

The network, transport, and application layers.^ Physical: OS, separate process, network card

Application 1^ Socket

socketinterface

user kernel

Application 2

user kernel

Underlying communication

Protocols

Underlying communication

Protocols

Communications

network

Socket

socketinterface^ Figure 2.

Copyright ©2000 The McGraw Hill Companies

Leon-Garcia & Widjaja:

Communication Networks

TCP/UDP

IP Ethernet Physical Layer

TCP/UDPIP EthernetPhysical Layer

Generic Transport Layer Primitives

)

Already saw these in sockets earlier

)

LISTEN:

• block until a process connects

)

CONNECT:

• attempt to establish connection

)

SEND:

• send information

)

RECEIVE:

• receive information

)

DISCONNECT:

• this side wants to release connection

Transport Layer Topics

Introduction (6.1)

Elements of Transport Protocols (6.2)

Internet Transport Protocols: UDP (6.4)

Internet Transport Protocols: TCP (6.5)

Finding a Server

)

“Connect to a Server” is a Transport level service

)

How do you find it?

service mapper

• names to transport layer address

name server

)

Analogy

• how do you find phone number?

)

Standard servers wait at well-known port

ftp 21/tcp

telnet 23/tcp

finger 79/tcp

snmp 161/udp

Establishing a Connection

)

Imagine severely congested subnet

• Network can delay, lose, duplicate packets– Example: connection to bank to transfer big money!!

)

Consequences

• Connection can happen twice!

)

Solution(s)

• One time only transport address (problem: server

mapper cannot work)

• Use

unique sequence numbers

for each connection

• When establish connection, exchange seq. numbers–

three-way handshake

• prevents establishment of unwanted connection

Three-Way Handshake Handles

Problems

Case 1: delayed duplicateKey: host 1 knows it hasseen ACK before

Case 2: delayedconnection request, ACKKey: Wrong data seq. No.

Releasing a Connection

Asymmetricrelease canresult in dataloss

Symmetricrelease easy?

• “I’m done”– “Me, too”

Elements of Transport Protocols

)

Flow control and buffering

• Sliding window can still be used (flow control, etc)– Host may have several connections, so buffering may be

tough

• Example: protocol 6 with 64 connections and 4-bit sequence

numbers require 1024 buffers

)

Multiplexing

• Several applications (email, web, etc) on same host may

share one network address

• Multiplexing: how to combine

)

Crash recovery

• How to recover when host crashes