Types of Service

Types of service (1) A network architecture may have multiple protocols at the same layer in order to provide different types of service Or a single protocol may be able to provide different types of service if the higher layer entity chooses a different service primitive to use at the service interface (or different parameters) There are some basic service categories that arise throughout networking regardless of the layer where they are implemented 2

Types of service (2) Perhaps the most important service distinction is between Connection-oriented vs. Connectionless service models Connection-oriented service –All data on the connection follows the same path –Received data is in the same order it was sent –A connection is created from the source to the destination prior to the exchange of any data –The characteristics of the connection may be negotiated prior to the data exchange –The connection is “torn down” when no longer needed 3

Types of service (3) Connection-oriented service (cont.) –The phone system is the classic example Initially when the phone system was entirely analog (and not mechanized) the operators would literally plug wires into patch panels to create the connections (human switches!) When the phone company digitized most of their system the connections became virtual connections on a shared physical medium –Sometimes called a virtual-circuit service model –If the connection is broken then data flow stops and the connection must be re-established –Here statistical multiplexing means re-using the virtual connection for some other user at a later time 4

Types of service (4) Connectionless service –Messages from the same source to the same destination are unrelated from the perspective of a connectionless service –Messages can take different pathways –Messages may not arrive in the same order as sent –Data flows immediately, but the sender is not sure that the message can be delivered –More robust to failures of a single link in a path – route around failed link 5

Types of service (5) Reliable vs. unreliable service Reliable service –The service makes a significant effort to deliver all of the data and in the correct order Not a guaranteed service – a single point of failure in the hardware along the path can still stop all communication –Uses acknowledgements (ACKS) and retransmissions ACKS are small messages from a receiver that indicate successful receipt of some data –Also uses sequence numbers to maintain order –ACKS introduce additional delay and complexity Reliability has a cost! 6

Types of service (6) Unreliable service –This type of service provides only best effort –Used for applications that can’t afford the overhead associated with ACKS in a reliable service Also good for applications that can just ask again if a request fails (called idempotent request behavior) –Best effort is often very good and therefore often adequate A LAN has low error rates, potentially large capacity Chances of failure are low –A common mistake of application designers is to: Use an unreliable service on the LAN with good results Fail to design for poorer WAN behavior of the same service 7

Types of service (7) Applications provide many types of services to users but they are built on a small number of services at the lower layers –We can therefore easily add a new application using the application programming interface of the transport layer called the socket interface We will study two basic types of service that the transport layer provides to applications –A basic message or datagram service –A byte-stream service 8

Types of service (8) Datagram service (UDP in the TCP/IP stack) –Is a connectionless, unreliable service Messages may be lost, duplicated, or delivered out-of-order –The network accepts individual messages of limited size Similar to regular mail –The network delivers exactly the bytes handed to it by the application It doesn’t join messages together nor does it deliver a part of a message –The datagram name is derived from the telegram model –Is unidirectional It supports unicast, broadcast, or multicast recipients 9

Types of service (9) Byte-Stream service (TCP in the TCP/IP stack) –A connection-oriented, ordered, reliable service –An arbitrary-length sequence of individual bytes –Is bi-directional between two peer applications Offers only one-to-one communications, i.e., unicast The network accepts input from either application, and delivers the data to the other application –Chooses the number of bytes to deliver at any time A sending application can choose to generate one byte at a time, or it can generate blocks of bytes The byte-stream service can choose to combine smaller blocks into one large block or can divide a large block into smaller blocks (as it hands the data to the lower layers) The service treats the bytes as part of a larger stream 10

Types of service (10) A programmer who uses the datagram service must insure that the application operates correctly –Even if packets are lost or reordered Many applications require delivery guarantees –Programmers tend to use the byte-stream service except in special situations such as broadcast New combinations of reliable vs. unreliable, connection-oriented vs. connectionless, and datagram versus stream are emerging –For example: video, where a stream is multicast and the application can deal with packet reordering and loss 11

Types of service (11) ServiceExample Reliable message streamSequence of pages Reliable byte streamRemote login Unreliable connectionDigitized voice Unreliable datagramElectronic junk mail Acknowledged datagramRegistered mail Request-replyDatabase query 12 Connection - oriented Connectionless