The OSI Protocol Model. Functions of Protocols. Segmentation (Fragmentation) Chapter 2. Protocols & Architecture

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1 hapter 2 The OSI Protocol Model Protocols & rchitecture Protocol rchitecture is the layered structure of hardware & software that supports the exchange of data between systems t 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 Functions of Protocols Segmentation and reassembly Encapsulation onnection control Ordered delivery Flow control Error control ddressing Multiplexing Transmission services Segmentation (Fragmentation) Data blocks are of bounded size at each layer pplication layer messages may be large Network packets may be smaller Splitting larger blocks into smaller ones is segmentation (or fragmentation in TP/IP) TM blocks (cells) are 53 octets long Ethernet blocks (frames) are up to 1526 octets long Why Fragment? dvantages: 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 1

2 Encapsulation ddition of control information to data ddress information Error-detecting code Protocol control: data + control information -> PDU onnection ontrol onnection Establishment Data transfer onnection termination Ordered Delivery & Flow ontrol Error ontrol Ordered Delivery PDUs may traverse different paths through network PDUs may arrive out of order Sequentially number PDUs to allow for ordering Flow ontrol Done by receiving entity Limit amount or rate of data Stop and wait redit systems: Sliding window Needed at application as well as network layers 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 2

3 ddressing level ddressing scope ddressing Globally unique : Global address identifies unique system Locally Unique: Multiple simultaneous applications ddressing mode Unicast address: Sent to one machine or person roadcast: Sent to all machines or users Multicast: Sent to some machines or a group of users ddressing 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 (TP/IP) Network service access point or NSP (OSI) Process within the system Port number (TP/IP) Service access point or SP (OSI) ddress oncepts in TP/IP rchitecture Multiplexing Supporting multiple connections on one machine Mapping of multiple connections at one level to a single connection at another: e.g. arrying a number of connections on one fiber optic cable ggregating or bonding low speed lines to gain bandwidth (a) (b) MUX Trunk group MUX 3

4 Transmission Services The OSI Protocol Model Some common service example: Different Priorities for Different messages Quality of Service Guarantee Minimum acceptable throughput Maximum acceptable delay Security: ccess Restrictions OSI - 7 Layers Model OSI Layers Layer 1-4 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 hanges in one layer should not require changes in other layers Provide fundamental network functionalities/services Physical: overs 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 4

5 OSI Layers Layer 5-7 Use of a Relay Provide finer network service (more software orientated) Session:ontrol 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 pplication: provide means for application program to access OSI environment management functions; ; FTP TP/IP Protocol rchitecture pplication Layer ommunication between processes or applications End to end or transport layer (TP/UDP/ ) End to end transfer of data May include reliability mechanism (TP) Hides detail of underlying network Internet Layer (IP): Routing of data Network ccess Layer Logical interface between end system and network Physical Layer Transmission medium Signal rate and encoding PDUs in TP/IP rchitecture 5

6 Some Protocols in TP/IP Suite Further Reading Reference Stallings chapter 2 omer,d. Internetworking with TP/IP volume I omer,d. and Stevens,D. Internetworking with TP/IP volume II and volume III, Prentice Hall Halsall, F> Data ommunications, omputer Networks and Open Systems, ddison Wesley RFs 6