EE 4272 Spring, 2003

Protocols & Architecture • A Protocol Architecture is the layered structure of hardware

& software that supports the exchange of data between systems

• At each layer, one or more common protocols are implemented in communication systems. Each protocol provides a set of rules for the exchanges of data between systems

• Key Functions of a protocol: encapsulation, segmentation & reassembly, connection control, ordered delivery, flow control, error control, addressing, and multiplexing

Chapter 2

EE 4272 Spring, 2003

The OSI Protocol Model

EE 4272 Spring, 2003

Functions of Protocols

• Segmentation and reassembly• Encapsulation• Connection control• Ordered delivery• Flow control• Error control• Addressing• Multiplexing• Transmission services

EE 4272 Spring, 2003

Segmentation (Fragmentation)

• Data blocks are of bounded size at each layer• Application layer messages may be large• Network packets may be smaller• Splitting larger blocks into smaller ones is

segmentation (or fragmentation in TCP/IP) ATM blocks (cells) are 53 octets long Ethernet blocks (frames) are up to 1526 octets long

Why Fragment?• Advantages: 1) .More efficient error control; 2) More

equitable access to network facilities; 3) Shorter delays; 4). Smaller buffers needed

• Disadvantages: 1). Overheads; 2). Increased interrupts at receiver; 3). More processing time

EE 4272 Spring, 2003

Encapsulation

• Addition of control information to data Address information Error-detecting code Protocol control: data + control information -> PDU

EE 4272 Spring, 2003

Connection Control• Connection Establishment• Data transfer• Connection termination

EE 4272 Spring, 2003

Ordered Delivery & Flow Control

• Ordered Delivery PDUs may traverse different paths through network PDUs may arrive out of order Sequentially number PDUs to allow for ordering

• Flow Control Done by receiving entity Limit amount or rate of data Stop and wait Credit systems: Sliding window Needed at application as well as network layers

EE 4272 Spring, 2003

Error Control

• Guard against loss or damage• Error detection

Sender inserts error detecting bits Receiver checks these bits If OK, acknowledge If error, discard packet

• Retransmission If no acknowledge in given time, re-transmit

• Performed at various levels

EE 4272 Spring, 2003

Addressing

• Addressing level• Addressing scope

Globally unique : Global address identifies unique system Locally Unique: Multiple simultaneous applications

• Addressing mode Unicast address: Sent to one machine or person Broadcast: Sent to all machines or users Multicast: Sent to some machines or a group of users

EE 4272 Spring, 2003

Addressing level

• Level in architecture at which entity is named

• Unique address for each end system (computer) and router

• Network level address IP or internet address (TCP/IP) Network service access point or NSAP (OSI)

• Process within the system Port number (TCP/IP) Service access point or SAP (OSI)

EE 4272 Spring, 2003

Address Concepts – in TCP/IP Architecture

EE 4272 Spring, 2003

Multiplexing

• Supporting multiple connections on one machine• Mapping of multiple connections at one level to a single

connection at another: e.g. Carrying a number of connections on one fiber optic cable Aggregating or bonding low speed lines to gain bandwidth

B B

C C

A A

B

C

A

B

C

A

MUXMUX

(a) (b)

Trunkgroup

EE 4272 Spring, 2003

Transmission Services

Some common service example:• Different Priorities for Different messages• Quality of Service Guarantee

Minimum acceptable throughput Maximum acceptable delay

• Security: Access Restrictions

EE 4272 Spring, 2003

The OSI Protocol Model

EE 4272 Spring, 2003

OSI - 7 Layers Model

• A layer model• Each layer performs a subset of the required

communication functions• Each layer relies on the next lower layer to perform more

primitive functions• Each layer provides services to the next higher layer• Changes in one layer should not require changes in other

layers

EE 4272 Spring, 2003

OSI Layers – Layer 1 - 4

Provide fundamental network functionalities/services• Physical: Covers the physical interface between devices & the

rules by which bits are passed from one to another• Data Link

Provide a reliable link by Error detection and control Higher layers may assume error free transmission

• Network: routing/switching; establish/maintain/terminate connections

• Transport: Exchange of data between end-to-end systems Error free; In sequence; No losses; No duplicates; QoS

EE 4272 Spring, 2003

OSI Layers – Layer 5 - 7

Provide finer network service (more software orientated)• Session:Control of dialogues between applications in end system

Dialogue discipline: full-duplex or half-duplex Recovery: provide checkpoint mechanism.

• Presentation: Defines data formats and coding. e.g., Data compression Encryption

• Application: provide means for application program to access OSI environment management functions; email; FTP

EE 4272 Spring, 2003

Use of a Relay

EE 4272 Spring, 2003

TCP/IP Protocol Architecture• Application Layer

Communication between processes or applications

• End to end or transport layer (TCP/UDP/…) End to end transfer of data May include reliability mechanism (TCP) Hides detail of underlying network

• Internet Layer (IP): Routing of data• Network Access Layer

Logical interface between end system and network

• Physical Layer Transmission medium Signal rate and encoding

EE 4272 Spring, 2003

PDUs in TCP/IP Architecture

EE 4272 Spring, 2003

Some Protocols in TCP/IP Suite

EE 4272 Spring, 2003

Further Reading Reference

• Stallings chapter 2• Comer,D. Internetworking with TCP/IP volume

I• Comer,D. and Stevens,D. Internetworking

with TCP/IP volume II and volume III, Prentice Hall

• Halsall, F> Data Communications, Computer Networks and Open Systems, Addison Wesley

• RFCs