Layered Architecture

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Layered Architecture
Required reading: Kurose 1.5

CSE 3214, Winter 2011 Instructor: N. Vlajic

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OSI Layers

Internet Layers

Why Layering?!
Montreal Alice wants to send a mail to Bob and a parcel to John. London Paris

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Alice drops off both at the same Post Office.

Post Office performs final delivery based on the street and personal names.

Post Office looks atthe addresses ( city names!!! ) and arranges for proper transportation.

City A

City B

City C

Why Layering?! (cont.)
Layering
network technologies
Application Intermediate layer Transmission Media Coaxial cable Fiber optic Packet radio Telnet FTP NFS HTTP

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• intermediate layer(s) provide a unique abstraction for various

No Layering

• each new application has to bere-implemented for every

network technology!
Application Telnet FTP NFS HTTP

Transmission Media

Coaxial cable

Fiber optic

Packet radio

Why Layering?! (cont.)
Why Protocol Layering?
1) modularity – one problem is decomposed

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into a number of smaller more manageable subproblems ⇒ more flexibility in designing
2) troubleshooting – each layer has clearly

definedresponsibilities ⇒ easy to pinpoint the actual ‘place and cause” of a problem

3) functionality reuse – a common functionality

of a lower layer can be shared by many upper layers ⇒ easy to modify and evolve computer networks

A monolithic network design that A monolithic network design that uses a single large body of hardware and software to meet all network requirements uses a single large body ofhardware and software to meet all network requirements can quickly become obsolete can quickly become obsolete and also is extremely difficult and expensive to modify. and also is extremely difficult and expensive to modify.

Layered approach accommodates incremental changes much more rapidly. Layered approach accommodates incremental changes much more rapidly.

Layered Architecture
ProtocolLayering
–

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grouping of related communication functions into hierarchical set of layers
• each layer:

Layer 7

(1) performs a relates subset of functions required for communication with another system
service to Layer N+1 protocol with peer Layer N

. . .
Layer N

(2) relies on next lower layer to perform more primitive functions
(3) provides service to next higher layerLayer N

(4) implements a protocol for communication with peer layer in other systems
•

. . .
Layer 1

service from Layer N-1

vertical communication – communication between
adjacent layers – requires mutual understanding of what services and/or information lower layer must provide to layer above

•

horizontal communication – communication between
software or hardware elementsrunning at the same layer on different machines

Communication between peer processes is virtual, i.e. indirect. Communication between peer processes is virtual, i.e. indirect.

Layered Architecture (cont.)
Entity
–

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any device capable of sending and receiving information over the Internet (i.e. computer network)

Protocol

– set of rules that governs data comm. between peer entities• key elements: syntax, semantics, timing

(1) Protocol Syntax – format of protocol data units, i.e. number and size of fields in a packet and how these fields are delineated
• e.g. packet = 4 bytes header + 12 bytes data

(2) Protocol Semantics – meaning associated with bits carried in each control field (3) Protocol Timing – when data should be sent and how fast

Layered Architecture(cont.)
Example [ TCP-protocol syntax, semantics, timing ]
Protocol Syntax: Protocol Timing: connection establishment

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Protocol Semantics:

e.g. HLEN field indicates the length of the TCP header in 32-bit words (i.e. # of 4-byte segments in the header)

Given the importance of protocols to the Internet, it is crucial that Given the importance of protocols to the Internet, it is crucial...