Functions of the Network Layer Functions of the Network Layer Routing- Router (or gateways) connect independent networks gether, network layer routes the packet to their final destination To data link layer From data link layer The network layer is responsible for the source-to-destination systems are connected to the same link, no need for a network layer. =: The above figure shows the ca and 79 are the local physical Functions of the Transport Layer Functions of the Transport Layer ↓1rNK Segmentation and rs and this layer also ensures the whole message arrive liftable segments (can be handled by ontains a sequence number (for correct re-assembling at the ort layer is responsible for This layer has more control over sequencing, flow and error c nning several programs)by including a service-point address(port address)in its header Functions of the e sessio on Layer Functions of the Transport Layer Connection control- The transport layer can be either conne ter all dat To transport layer transferred without error the connection is termina is th dialog Flow control- Transport layer perform end to end flow control which maintains, and synchronizes the differs from the data link layer which is le link. log control- The session layer allows two systems to enter into a Error either half-duplex (one way loss or duplication). Error correction checkpoints(synchronization points)into a stream of data, when da loss during transmission occurred, more efficient re-transmission can be 55 哈工大计算机学院 李全龙 Network Application Development 1: Introduction 25 Functions of the Network Layer N-PDU NH T- PDU Network layer packet To data link layer From transport layer N-PDU NH T- PDU Network layer packet From data link layer To transport layer • Logical addressing - the network layer adds a header to the incoming packet from the upper layer with a logical address of the sender. If a packet passes the network boundary, this universal (logical) address helps to distinguish whether the packet belong to the source or destination systems The network layer is responsible for the source-to-destination delivery of a packet possibly across multiple networks, if two systems are connected to the same link, no need for a network layer. The functions of a network layer are: 哈工大计算机学院 李全龙 Network Application Development 1: Introduction 26 Functions of the Network Layer • Routing - Router (or gateways) connect independent networks together, network layer routes the packet to their final destination by router (gateway). F R K L 15 D 78 79 DT Data S D 02 46 DT Data S D 23 09 DT Data S D 29 79 The above figure shows the case for a computer "S" communicating with a remote computer "D". S and D are the logical addresses of the source and destination computers respectively, where 02 and 79 are the local physical addresses of the source and destination computers respectively. T Ring Router Router U X 46 23 61 09 29 S A 02 37 C 哈工大计算机学院 李全龙 Network Application Development 1: Introduction 27 Functions of the Transport Layer T-PDU S- PDU Transport layer To Network layer From session layer TH TH TH T-PDU T-PDU T-PDU S- PDU Transport layer From Network layer To session layer TH TH TH T-PDU T-PDU segments segments • responsible for source-to-destination (end-to-end) delivery of the entire message • segmentation occurs and this layer also ensures the whole message arrives intact and in order • a logical path can be set up (fixed pathway) between the source and destination for all packets in a message for additional security. • This layer has more control over sequencing, flow and error detection and correction 哈工大计算机学院 李全龙 Network Application Development 1: Introduction 28 Functions of the Transport Layer Data x y DT Data-1 x y S D DH DT Data-2 x y S D DH Data-1 x y S D Data-2 x y S D Transport layer Network layer Data link layer S Data x y DT Data-1 x y S D DH DT Data-2 x y S D DH Data-1 x y S D Data-2 x y S D Transport layer Network layer Data link layer D Internet • Segmentation and re-assembly - A message is divided into transmittable segments (can be handled by network layer) each contains a sequence number (for correct re-assembling at the destination). • Service-point addressing - the transport layer is responsible for the delivery of the entire message to the correct process on the computer ( that may be running several programs) by including a service-point address (port address) in its header. 哈工大计算机学院 李全龙 Network Application Development 1: Introduction 29 Functions of the Transport Layer • Connection control - The transport layer can be either connectionless or connection-oriented. A connectionless transport layer deliver each segments as an independent packet to the destination. A connection￾oriented transport layer firstly makes a connection with the transport layer at the destination, then deliver the packets and after all data are transferred without error, the connection is terminated. • Flow control - Transport layer perform end to end flow control which differs from the data link layer which is responsible for flow control in single link. • Error control - The transport layer performs end to end error control which make sure the entire message arrives at the receiving transport layer without error (damage, loss or duplication). Error correction is done by re-transmission. 哈工大计算机学院 李全龙 Network Application Development 1: Introduction 30 Functions of the Session Layer S-PDU P- PDU To transport layer From Presentation layer SH Session layer syn syn syn S-PDU P- PDU From transport layer To Presentation layer SH Session layer syn syn syn • Dialog control - The session layer allows two systems to enter into a dialog. The communication between two processes can be controlled in either half-duplex (one way at a time) or full-duplex (two ways at a time). The session layer is the network dialog controlling. It establishes, maintains, and synchronizes the interaction between communication systems. • Synchronization - The session layer allows a process to add checkpoints (synchronization points) into a stream of data, when data loss during transmission occurred, more efficient re-transmission can be performed