Computer Networks. Chapter 1 Reference models (OSI and TCP/IP)

Transcription

1 Computer Networks Chapter 1 Reference models (OSI and TCP/IP)

2 Network structure Reduce network design complexity organize networks as stacks of layers (levels)

3 Network layers Use of layers data hiding, encapsulation Offers only the service not details about implementation through interface Communication between 2 layers on the same level common protocol Network architecture a set of layers and protocols Offer information for HW/SW implementation Protocol stack a list of protocols, one per layer used by a system

4 Protocol stack Header control information specific for each layer/protocol

5 Layer functions Addressing Error control Flow control Multiplexing Routing Segmentation and reassembly

6 Reference models OSI (Open System interconnection) Rarely used protocols General valid model for system interconnection TCP/IP Protocols widely used Network model never used

7 OSI model ISO OSI proposed in 1983, revised in 1995 first step toward international standardization of the protocols used in the various layers

8 Physical layer Transmitting medium Mechanical and Electrical considerations

9 Data link layer Present the physical transmission line and a error free medium to the next layer Break the message in small pieces Transmit them sequentially Perform error correction/detection Handle retransmission (acknowledgement frame) Contains a sub-layer for concurrent access to shared medium MAC Medium Access Control

10 Network layer Control the operation of the subnet Route packets from source to destination Support different types of routing strategies

11 Transport layer Handle transmission of large messages from the upper levels Split the message into smaller packets Pass to the next layers Ensure that packets arrive in the same order at the other end It can be connection oriented or connectionless

12 Session layer Allow users on different machines to establish communication sessions by implementing following services Dialog control Resource management Synchronization

13 Presentation layer Allow exchange of high-level data abstraction in different formats Banking records etc

14 Application layer Contains protocols commonly needed by user HTTP FTP SMTP etc

15 TCP/IP Defined in 1974, revised in 1985 Offer a flexible way to connect networks Able to maintain the transmission if part of the network is damaged

16 Host-to-Network layer Is not well defined It must allow the upper layer to send data into the network

17 Internet layer Is an connectionless inter-network layer that allows host to connect to a packet switching network Key aspect for its wide and rapid spread It allows to access the network, inject data in small packages that travel independently to the destination

18 Transport layer Its function is to allow two peer entities to establish and carry a conversation Defines 2 end-to-end protocols TCP (Transport Control Protocol) UDP (User Datagram Protocol)

19 TCP Connection oriented protocol Allow a stream of bytes to be send from source to destination without errors Fragmenting at source Re-assembling at destination Implements flow-control to avoid situation when a slow host is flooded with data by a fast transmitter

20 UDP Connectionless protocol No flow control Not reliable like TCP Very fast Used by application where accuracy is not important Video, voice Used in one-shot, client-server-type requestreply queries

21 Application layer Contains the high-level protocols HTTP FTP SMTP DNS NNTP

22 TCP/IP model prtocols

23 Comparison Both based on stack of independent protocols OSI defines 3 concepts: Services what layer does, not how it does Interfaces how to access a layer Protocols can be independently used by layers Main idea is data encapsulation General valid model TCP/IP model lacks the data encapsulation Model suited only for the networks with TCP and IP protocols