Networks: Access Management Class Notes # 3 Protocols and Layers (part 1) September 19, 2003 Functions A small set of functions form the basis of all protocols. Not all protocols have all functions; this would involve a significant duplication of effort. However, there are many instances of the same type of function being present in protocols at different levels. We can group protocol functions into the following categories: Segmentation and reassembly Encapsulation Connection control Ordered delivery Flow control Error control Synchronization Addressing Multiplexing Transmission services Segmentation and reassembly Whether the application entity sends data in messages or in a continuous stream, lower level protocols may need to break the data up into blocks of some smaller bounded size. This process is called segmentation or fragmentation. The counterpart of segmentation is reassembly. Eventually, the segmented parts must be reassembled into messages appropriate to the application level. Encapsulation Each segment of data contains not only data but control information. The control information falls into three general categories: Address: the address of the sender and/or receiver may be indicated. Error-detection code: Some sort of frame check sequence (FCS) is often included for error detection. Protocol control: Additional information is included to implement the protocol functions. The addition of control information to data is referred to as encapsulation. Connection control Usually, data are sent via connectionless data transfer. However; connection-oriented data transfer is preferred if a lengthy exchange of data is anticipated and/or certain details of their protocol must be worked out dynamically. Three phases occur for connectionoriented data: Connection establishment Data transfer Connection termination 1
Ordered Delivery The packets may traverse different paths through the network and may not arrive in order in which they were sent. Packets are given sequence numbers to ensure the packets get reassembled in the right order. Flow Control In essence, flow control is a function performed by a receiving station to limit the amount of rate of data that is sent by a transmitting station. There are many different types of flow control. For example, a stop-and-wait procedure requires that each packet must be acknowledged before the next can be sent. Typically, flow control is implemented in several protocols (at different levels). Error Control Techniques are needed to guard against loss or damage of data and control information. Most techniques involve error detection, based on a frame check sequence, and retransmission. Error control is a function that must be performed at various levels of protocol. Addressing For two stations to communicate, other than over a point-to-point link, they must somehow be able to identify each other. A distinction is generally made among: Names: specifies what an object is Addresses: where it is Routes: indicates how to get there. Also, an address is a unique value that identifies the source and destination OSI Model The view of the OSI (Open System Interconnection) model is: a way of systematically describing and implementing the communication function by viewing the communication task in terms of a column of layers, each of which contains protocols. The communication functions are partitioned into a vertical set of layers. Each layer performs a related subset of the functions required to communicate with another system. It relies on the next lower layer to perform more primitive functions and to conceal the details of those functions. It provides services to the next higher layer. Ideally, the layers should be defined so that changes in one layer do not require changes in the other layers. Therefore, we have decomposed one problem into a number of more manageable subproblems. The ISO standard has two effects: Vendors feel encouraged to implement the standards because of an expectation that, because of wide usage of the standards, their products would be less marketable without them. Customers are in a position to demand that the standards be implemented by any vendor wishing to propose equipment (hardware and/or software) to them. 2
The OSI model consists of seven layers of services and protocols. The seven layers are: Layer 7: Application layer Layer 6: Presentation layer Layer 5: Session layer Layer 4: Transportation layer Layer 3: Network layer Layer 2: Data link layer Layer 1: Physical layer The seven layers can be grouped into two parts: Layer 1 4 concerned with transmitting data between computers, and Layer 5 7 concerned with communicating between computers with their applications. Physical Layer The physical layer covers the physical interface between computers and the rules by which bits are passed from one to another. The physical layer has four important characteristics: Mechanical Electrical Functional Procedural Mechanical pertain to the actual physical connection. The electrical characteristics have to do with the voltage levels and timing of voltage changes. Functional characteristics specify the functions that are performed, by assigning meaning to the various interchange circuits. Functions can be classified into the broad categories of data, control, timing, and ground. Procedural characteristics specify the sequence of events for transmitting data, based on the functional characteristics of the interface. For example, Mechanical: eight pin modular plug, Electrical: voltage levels of the signal, Functional: full-duplex, unidirectional data, Procedural: pin1 = Tx+, pin2=tx-, pin3=rx+, pin6=rx- Data Link Layer (DLL) While the physical is raw bit stream service, the data link layer attempts to make the physical link reliable and provides the means to activate, maintain, and deactivate the link. The principal service provided by the link layer to the higher layers is that of error detection and control. The data link layer performs the following services on the transmitting station: Network layer passes packets to the data link layer for transmission. Transmits the frames using the physical layer s services. Handshakes with its peer (data link layer on the receiving side) to ensure that the packet(s) is successfully received. Adds sequence information to each frame for the receiving peer process for reassembling the packets. Adds handshaking information for the receiving peer process to correct problems if they occur (e.g. lost frames) 3
Controls access to the physical layer medium. (e.g. MAC protocol) Adds error detection and correction codes to the frames. Adds flow control to the frames. The data link layer performs the following services on the receiving station: Accepts bits passed up from the physical layer and interprets groups of bits as frames. Checks each frame for errors. Detects problems through handshaking with its peer (transmitting DLL). Puts the frames back in the correct sequence to reconstruct the packets. Decapsulates the frames into packets and passes it to the network layer. There are many different DLL protocols. The most common is given below: HDLC (High-level Data Link Control) MAC (Medium Access Control) o FDDI o CSMA/CD o Token Ring o Token Bus LAP-B (Link Access Procedure, Balanced) SDLC (Synchronous Data Link Control) For example, HDLC frame has the following fields: Flag: 8 bits (pattern = 01111110) Address: one or more octets Control: 8 to 16 bits Information: variable FCS: 16 or 32 bits Flag: 8 bits (pattern = 01111110) Network Layer The network layer is responsible for transmitting data packets between stations (or nodes) that may not be connected by the same physical link (e.g. stations are connected on different LANs). In a transmitting cycle, the network layer takes data from the transport layer and encapsulates the data by adding a network layer header to the data (i.e. The header contains protocol information used by peer network layer processes as they forward the packet to its final destination). The network layer then passes the packet down to the DLL. The network layer services are: Maintains a unified addressing scheme that assigns each station (node) in the network a unique logical address. Handles packets to and from node types that may use different DLL protocols. In circuit switching networks: establishes and maintains the virtual circuit. 4
In connectionless-orientated networks: the network layer of an intermediate node independently forwards packets form one neighbor to the next. In connection-orientated networks: the network layer of an intermediate node routes a packet to the next node in the established connection. The common network layer protocols are: X.25 is a connection-orientated, packet switching protocol. IP (Internet Protocol) used across the internet. IPX (Internetwork Packet exchange) is Novell s Netware s network layer protocol. Network layer and data link layer addressing The network address is different from a DLL address. A DLL address (MAC address) identifies a particular piece of hardware and is unique from any other hardware. In contrast, the network layer addresses are logical addresses assigned by software. The network layer assigned address may be assigned to different computers over time (e.g. DHCP). 5