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Chapter: Computer Networks : Transport Layer

Application Requirements

The characteristics used to categorize the applications are, 1. tolerance of loss of data 2. adaptability



Applications are divided into two classes. They are


·   real time  


·        non real time – they are called as traditional data applications. Since they have traditionally been the major applications found on data networks. Examples are, Telnet, FTP, email, web browsing etc.





The characteristics used to categorize the applications are,


1.     tolerance of loss of data


2.     adaptability





The approaches are divided into two broad categories. They are,


1.     Fine-grained approaches, which provide QoS to individual applications of flows.


2.     Coarse-grained approaches, which provides QoS to large class of data or aggregated traffic.



In the first category, integrated services are used and in the second category differentiated services are used.




The term “Integrated Services” refers to a body of work that was produced by the IETF around 1995-97.The IntServ working group developed the specifications of a number of service classes designed to meet the needs of some of the application types described above. It also defined how RSVP could be used to make reservations using these service classes.




One of the service classes is designed for intolerant applications. These applications require that a packet never arrive late. The network should guarantee that the maximum delay that any packet will experience has some specified value; the application can then set its playback point so that no packet will ever arrive after its playback time.


The aim of the controlled load service is to emulate a lightly loaded network for those applications that request service, even though the network as a whole may in fact be heavily loaded. The trick to this is to use a queuing mechanism such as WFQ to isolate


the controlled load traffic from the other traffic and some form of admission control to limit the total amount of controlled load traffic on a link such that the load is kept reasonably low.




The set of information that we provide to the network is referred to as a flow spec. When we ask the network to provide us with a particular service, the network needs


to decide if it can in fact provide that service.


The process of deciding when it says no is called admission control. We need a mechanism by which the users of the network and the components of the network itself exchange the information such requests for service, flow specs, and admission control decisions. This is called signaling in the ATM world, but since this word has several meanings, we refer to this process as resource reservation, and it is achieved using a Resource Reservation Protocol.


When flows and their requirements have been described, and admission control decisions have been made, the network switches and routers need to meet the requirements of flows. A key part of meeting these requirements is managing the way packets are queued and scheduled for transmission in the switches and routers. This last mechanism is packet scheduling.




There are two separable parts to the flow spec: the part that describes the flows traffic characteristics and the part that describes the service requested from the network. The RSpec is very service specific and relatively easy to describe.


The TSpec is a little more complicated.




When some new flow wants to receive a particular level of service, admission control looks at the TSpec and RSpec of the flow and tries to decide if the desired service can be provided to that amount of traffic, given the currently available resources, without causing any previously admitted flow to receive worse service it had requested. If it can


provide the service, the flow is admitted; if not then denied. The hard part is figuring out when to say yes and when to say no.



Admission control is very dependent on the type of requested service and on the queuing discipline employed in the routers; when discuss the latter topic later in this section. For a guaranteed service, you need to have a good algorithm to make a definitive yes/no decision.




While connection oriented networks have always needed some sort of setup protocol to establish the necessary virtual circuit state in the switches, connectionless networks like the internet have had no such protocols. While there have been a number of setup protocols p[proposed for the internet, the one on which most current attention is focused is called resource reservation protocol (RSVP).


The characteristics of RSVP are,


It tries to maintain the robustness by using the idea of soft state in the routers. It aims to support multicast flows just as effectively unicast flows.




Once we have described our traffic and our desired network service and have installed a suitable reservation at all the routers on the path, the only thing that remains is for the routers to actually deliver the requested service to the data packets. There are two things that need to be done:


Associate each packet with the appropriate reservation so that it can be handled correctly, a process known as classifying packets. It is done by examining five fields in the packet: the source address, the destination address, protocol number, source port, destination port.


o   Manage the packets in the queues so that they receive the service that has been requested, a process known as packet scheduling.


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Computer Networks : Transport Layer : Application Requirements |

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