Lecture 4 Communication Network Protocols

Outline Protocols and Layering TCP/IP Protocols Connections in TCP Interlayer Communication

Protocols and Layering The concept of Layered Architecture

Transferring a Text File (Cont.)

Transferring a Text File (Cont.)

The OSI Reference Model The International Organization for Standardization (ISO) has published a reference model for open systems interconnection called OSI reference model. It has seven layers as shown below.

The OSI Reference Model (Cont.)

The OSI Reference Model (Cont.) As shown, the top layer is concerned with application programs running in host computers The bottom layer is concerned with accessing various physical communication media.

Layered Structures (Cont.)

Abstraction Abstraction is a way to hide details of a system. The layers of the OSI model might be regarded as levels of abstraction.

TCP/IP Family TCP/IP (transmission control protocol/internet protocol) derives from the central two members of a whole family of protocols. TCP and IP layers form the core of the family as shown below.

TCP/IP Family (Cont.)

TCP/IP Family (Cont.) In OSI terms, IP is a network layer protocol, so it is concerned with finding a route and getting messages across the network. IP provides no guarantee, the service is often described as a best-effort delivery service. The main aim of IP is to have a system that is Flexible (usable with many different types of networks) Robust (should work as best it can even in the face of breakdown and damage),

TCP/IP Family (Cont.) TCP is the transport layer protocol which sits on top of IP and is concerned with end-to- end issues. TCP provides a reliable service, i.e., it guarantees error-free transportation of messages between source and destination application layers. TCP is flexible, i.e., it can offer service to a wide variety of applications.

TCP/IP Family (Cont.) Application layer protocols that run on top of TCP are Electronic mail (e-mail) using simple mail transfer protocol (SMTP) Accessing world-wide web pages using hypertext transfer protocol (HTTP)

TCP/IP Family (Cont.) The protocol for the exchange of e-mail messages between hosts is SMTP. A small set of commands and responses are used between a mail client and a mail server

Packets: Data format Many factors influence the choice of packet size: Performance Efficiency If packages are too small, communication channel will not be efficiently used because of the overheads incurred by each packet. If packets are too large, interactive performance may suffer because of long delays and re- transmission in case of failures. The application layer of TCP/IP  files, email message, etc The transport layer  TCP segment

TCP Segment Structure

Connections in TCP Internetwork

Internetwork (Cont.) Figure above shows a computer A on a LAN sending a message to another computer B. Every IP packet will contain the destination address of B. Because the data has to cross the LAN to reach the edge of the internet at router X, the IP packet has to be encapsulated within an Ethernet frame. Routers decide which port to send packets out on after reading the IP address. The router at X may not know exactly where B can be found, it will at least know which region it is in, so can send packets off in the direction of router Z.

Connection-Oriented and Connectionless Services During a file transfer between two computers using FTP protocol, the first thing to do is to open a connection to the FTP host computer. When finished with the transfer, the connection is closed. Both the telephone and FTP are connection- oriented services. In contrast, the idea of a connectionless service is illustrated by the postal service. In the internet layer, IP is the only protocol available and it provides no connections.

Opening Connections in TCP To establish a connection, TCP uses a three- way handshake. For example, if a client attempts to connect with a server, 1. The client sends a SYN to the server. 2. In response, the server replies with a SYN- ACK. 3. Finally the client sends an ACK (usually called SYN-ACK-ACK) back to the server.

Opening Connections in TCP TCP uses sequence numbers to match up acknowledgements with the octets being acknowledged. This is needed because IP is unreliable in the sense that there might be delays, omissions, and duplications of datagrams. Without sequence numbers it would not be clear which octet is being acknowledged.

Opening Connections in TCP (Cont.)

Example: 1. The initiating host (client) sends a synchronization (SYN flag set) packet to initiate a connection. Any SYN packet holds a Sequence Number. The Sequence Number is a 32-bit field in TCP segment header. For example let the Sequence Number value for this session be x. 2. The other host receives the packet, records the Sequence Number of x from the client, and replies with an acknowledgment and synchronization (SYN-ACK). The Acknowledgment Number is a 32-bit field in TCP segment header. It contains the next sequence number that this host is expecting to receive (x + 1). The host also initiates a return session. This includes a TCP segment with its own initial Sequence Number value of y. 3. The initiating host responds with the next Sequence Number (x+1) and a simple Acknowledgment Number value of y + 1, which is the Sequence Number value of the other host + 1

The Sliding Window Algorithm in TCP A technique known as sliding-window algorithm has been developed for alleviating the capacity wastage in the simple octet-by- octet acknowledgement. It permits more than one octets to be sent before stopping and waiting for an acknowledgement and therefore improves channel usage. The number of octets that can be sent before waiting for an ACK is the width of the window as shown below.

The Sliding Window Algorithm in TCP (Cont.)

Interlayer Communication

A Networked PC To connect a PC to a LAN, two major components must be installed. The network interface card NIC, which provides the hardware connection between a PC and a LAN. The interface includes the address of the PC in the LAN. The netBIOS (network BIOS) which is part of the operating system software that allows the Windows operating system to communicate through the NIC to access the LAN. The netBIOS provides an application program interface (API) for application programs to use the TCP/IP protocol.

A Networked PC (Cont.)

A Networked PC (Cont.) The role of the redirector layer is to determine whether a request from the PC’s operating system is for a local or remote resource. If a request is for a local resource, the request is passed (i.e. directed) to the BIOS layer. If a request is for a remote resource, the redirector layer passes the instruction to the server message block (SMB) layer.

A Networked PC (Cont.) The SMB protocol is a peer-to-peer protocol that allows one computer (the client) to request resources such as files from another computer (the server) over a LAN, a WAN or the Internet.

A Networked PC (Cont.)

A Networked PC (Cont.) The SMB peer-to-peer protocol defines four categories of messages which pass requests and acknowledgements: Session control File Printer Message. The SMB session control messages create and close a Redirector connection to a shared resource at a server. The SMB file messages are used by the Redirector to gain access to files at a server. The SMB printer messages are used by the Redirector to send data to a print queue at a server and to get status information about the print queue. The SMB message category allows an application to send messages to or receive messages from a server.

A Networked PC (Cont.) Through an exchange of messages defined by the SMB protocol, a client can read and write files, create and delete files and directories, and search directories on a server. For example, an SMB ‘read file’ request received by a server will be passed up to its operating system layer which will recognize it as a request for a local resource of the server. The green arrow shows the path taken by the data in the SMB request and response messaes.

A Networked PC (Cont.)

Problem: Data is being sent from Computer A to Computer B using a sliding window algorithm and having a window size of 24 octets. Computer A has already sent 26 octets of which 18 have been acknowledged. How many more octets can Computer A send without waiting for an acknowledgement? Answer Total size = 24 octets First message A sent 26 octets 18 octets of them acknowledged, means 26 – 18 = 8 octets Free size left = 24 – 8 = 16 octets 16 octets can be sent without waiting for acknowledgement.