Chapter 16 Protocols and Layering

Network Communication Protocol an agreement that specifies the format and meaning of messages computers exchange Network applications do not interact directly with network hardware; they interact with protocol software which adheres to protocol.

Protocol Suites Aka protocol family or protocol stack a set of protocols that works well together. Eg. TCP/IP, NetBuei, Novell IPX, AppleTalk, DecNet, SNA. Protocols from one stack cannot interact with protocols from another stack. A computer can run more than one protocol stack. Type field in each frame identifies which stack should handle the message

ISO 7-layer Reference Model network communication divided into 7 logical categories (fig 16.1)(fig 16.1)

Layer 1: Physical Layer – deals with the basic network hardware eg. Fiber, twisted-pair copper, coax Layer 2: Data Link layer – specifies how to organize data into frames and to transmit the frames eg. CSMA/CD, token passing, checksum, CRC Layer 3:Network layer protocols – specify how address are assigned and how packets are forwarded from one end of the network to the other end (eg. IP, ARP, RARP) Layer 4:Transport layer protocols – specify how to reliably transfer data from one end to the other (eg. TCP, UDP, RIP)

Layer 5:Session layer protocols – specify how to establish a communication session with a remote computer (eg. Password authentication, RPC) Layer 6:Presentation layer protocols – specify how to represent data. Needed because different computers may represent data differently (eg. XDR). Layer 7:Application layer protocols – eg. Lpr, rcp, rlogin,rsh, ftp, telnet, smtp, DNS, NFS, NIS, SNMP, bootp, ntp,tftp

3-layer network & 2 interface model Bottom Layer – the NIC board driver which is the software the controls the NIC board. This software is usually NDIS or ODI compliant driver Network Binding Interface – an interface that allows a transport protocol stack to bind (attach, or communicate) to a board driver eg. (Network Driver Interface Specification (NDIS) for NetBEUI, TCP/IP, and SPX/IPX). Middle Layer – transport protocol eg. TCP/IP, IPX/SPX, NetBEUI Network API interface – allows network applications in top layer to be written without having to know details of transport protocols. Top Layer – network services eg. application

Stacks and Layered Software software in each layer talks to the layers immediately adjacent to it (fig 16.2).(fig 16.2) Nested protocol headers – data is prepended with additional header information as it go down various layers prior to network transmission (fig 16.4)(fig 16.4)

Layering Principle Layer N software on the destination computer must receive exactly the message sent by layer N software on the sending computer Whatever transformation(adding headers or encrypting data) a protocol at a given layer applies before sending a frame must be completely reversed when the frame is received by the same layer software at the destination (fig 16.5).(fig 16.5) This simplifies protocol design and testing since the protocol and be designed and tested independent of the other layers

Sequencing needed to reorder out-of-order packet deliveries The receiving side stores both the sequence number of the last packet received in order as well as a list of packets that were received out of order When a new packet arrives, the receiver checks the sequence number of the packet to determine whether it should be sent to the next protocol layer or to appended it to the list of out-of-ordered packets. Other packets on this list is checked whether they can be delivered. Duplicate packets (same sequence number) are discarded

Retransmission of Lost Packets To guarantee reliable transfer (no data loss), protocols use positive acknowledgment with retransmission. Receiver sends an acknowledgement whenever data is received successfully. If sender does not receive an acknowledgement within a specified time (timeout), it retransmit the packet and restarts the timer.

Flow Control needed because transmitting computer may be faster than receiving computer, causing dropped packets

Flow Control Techniques stop-and-go method – transmitter waits for acknowledgement after a packet is sent and before sending another packet – prevents data overrun but is inefficient sliding window – sender and receiver uses a fixed window size, which is the maximum amount of data that can be sent before an acknowledgement arrives (fig. 16.6).(fig. 16.6) – Whenever an acknowledgement arrives, the transmitter adjusts (slides) the window appropriately and continues to send new data within the new window(fig. 16.7).(fig. 16.7)

Throughput can be increased dramatically using sliding window. T w = min (B, T g x W) – Where T w is throughput using sliding window protocol – Tg is throughput using stop-and-go protocol – B is underlying network bandwidth

Congestion Control needed because network bottlenecks can occur packet switch(router) queues grow until packets start to drop(discarded) due to limited buffer memory inside routers. Congestion control techniques – have packet switches (routers) inform senders to slow down (source quench) via using smaller window size when congestion occurs – sender automatically slow down when packet transmission timeout occurs and retransmission is needed.