Protocol Stack Layers
Hello. So, we’ll be continuing our discussion on Computer Network and InternetProtocols series of lectures. Today we will have a quick overview of the differentservices at the different Protocol Stack. So, if you recollect in our previous talks ordiscussion what we tried to look at it that how this inter-networking is possible andprimarily the concept is a layered architecture so that intermediate networking devicesare enabled up to the layer which is upon which is it supposed to look at. Like what wesee that if it is a simple hub or a repeater. So, it can look up to the physical layer, all otherlayer things will be a payload to this physical layer, or if we look at a, on the other handif we look at a router, so it has a network layer. So, it can look at the packets at the levelof the network layer or sometimes called IPDR or inter-networking layer and type ofthings right. So, these protocol stacks allows us to design different devices and alsoallows to inter-operate between heterogeneous devices and services. So, what today’stalk what we’ll be looking at, is that what are the different typical services provided atdifferent level layers of per say TCP/IP protocol stack or which is which will besomewhat true for OSI stack only. We will just look at it. So, that in our subsequentlectures we will go on looking at this stack individually like that how they behave.(Refer Slide Time: 02:19)So, just to have a quick recap; so, again if when we talk about network protocols; sowhat a protocol says? Protocol defines the interface between the layers in the samesystem and with the layers of peer system.So, it is a set of rules, set of guidelines or what that by which the different, the differentapplications or the different services in the same system or with the peer systems thingscan interchange. So, protocol basically allows us to inter-operate right.So buildings blocks of any network architecture that we have already seen.Each protocol object has two different interface. So, broadly two different interface oneis service interface - operations on this particular protocol, that is it keeps some interfacethat which explores the services which can operate on this protocols or peer-to-peerinterface - message exchange with the peer. So, one is allowing it some interface theservices can be exploited or executed or can be done or another is that how I can say inmessage pass between the peers of the two protocol, to particular protocol say.So, it includes specification of peer-to-peer interfaces if I want to communicate withpeers, then I should include the specification for peer-to-peer, that what way the messagewill be there, what will be the particular size of the message, what will be the structure ofthe message and what you expect when whether you expecting an acknowledgement; ifat all how things will be there, how will be and so on so forth right. So, other things isthat the module it implements this interfaces. So, there should be so, modules whichimplement this particular interfaces right at the things.So, typical features are like that can there are there are prose, pseudo-code, statetransition diagram and it allows interoperability when two or more protocol thatimplement the specification accurately, and overall it is guided by IETF right, InternetEngineering Task Force. Otherwise the standardization will be a major challenge. So, ifyou see these like any other systems, here also what we see this protocols provides aguideline how to inter-operate between two peers and how to basically provide ainterface with the services which are provided by that particular protocol right.So, this makes or this appropriate definition or appropriate guideline helps us to intercorrect this whole networking or what we say that making this inter-networking possibleright.(Refer Slide Time: 05:28)So, if we again little bit look at the key elements of the protocol, when we when we lookat individual protocol stack like when we look at the application layers or the protocolsin the application layer, protocols in the transport layer, protocol in IP or network layer ordata link layer though physical layer protocols are there, but physical layer based on thecommunication network in some of the cases we segregate we will try to give a overviewof the physical layer protocol. But if you look at that that this link layer, network layer,transport layer, application layer, these are the major contribution of this internetworkingthings, definitely there should be a physical layer and there are physical layerconstraint things that we’ll discuss.So, when we discuss at the individual protocol level, we’ll see that what is their differentwhat we say specifications right. So, if you like to like to look at the key elements of aprotocol, of any event protocol, so one is the syntax right or what we say syntacticalspecification that is data formats, signal levels and type of things right. So, this keeps theoverall syntax at the protocol. If we come to the other side or the next level is thesemantics right; what are the control informations or error handling capability, whetherthe at all the protocol has this error handling capability or not, what sort of the controlinformation or control structure is there in the protocol that comes in the things which ismore of the semantics.So, given a syntactical framework how this my how this protocol still works? Like I saythat when I keeping apart the networking, say I my protocol that I send a message in aparticular things, when I when I send a message like a voice message at some other partit interprets. So, that is a particular format right. So, I send a message in a particularformat and it respond in a particular format right, like I say, I in order to knowing theresult of a particular things I send a SMS number and some reference number and so onso forth. So, three, four fields I send the things. On that I expect a response on the thingsright. So, within that it will respond either by voice or send me back some other messageand type of things.And if there is a fault in that overall things, if there is a packet drop or there is a failure inthe transmission line; so what is the error handling? One is that there can there may betotally no error handling mechanism. If you don’t get a response with a stipulated time,you think that the message is lost, you resend or whatever things you want to do you do,otherwise if there is a error handling mechanism. If it is there a loss, there is a specificmechanism to indicate that if it does not say receive acknowledgement, it will resend orretransmit the signal in the things right.So, that all depends on that how this protocol is specified. And there is in order to happenall those things, we require a timing relationship right. So, there can be a speed matchingissue right like what we say that the interfaces you’re pushing in say, usually what we doin case of our say when we have push message along the network typically it follows thestore and forward: store and then forward the things.In order to do what is one of the things what it tries to do, it tries to match this incomingand outgoing stream right. So, there should be a synchronization, appropriatesynchronization of the thing, there is a speed matching thing, because there areindependent devices working at different speed or different frequency rate whereas Ineed to have a appropriate speed matching otherwise the communication may not bepossible. So that also need to be somewhere looked into a within that particular protocolsto specify somewhere or other.Another in some cases we require sequencing of the things right, I send message one,two, three and it should be reached as a one, two, three right. So that and as we have seenin a packet switched network where a particular message is broken down into a differentpackets or datagrams and they are sent independently over the network. So, it is notnecessarily that always the datagram will follow the same path and at the end thedatagram need to be appropriately sequenced right.So, the sequencing mechanisms need to be ensured in some way or other right. So, thateither the it follows the same path or there is a sequence number which allows it toreconstruct and so and so forth, but nevertheless there should be sequencing mechanismin place. So, that if you look at the protocol as a whole at any layer of this network layeror for that for that matter any systems which communicate with one another, I need tolook at this different aspects – syntax, semantics and this timing relationships right. So,this is this is pretty important for to have the in realization of the protocol. So, whenevera protocol is designed, so these aspects are looked into right, that how things will work.(Refer Slide Time: 11:16)Now, so if you look at the very broad view: interfaces. So, if you see there are somehigher level objects right, at the both ends. So, this is host 1, host 2; it can be directlyconnected or it can be at any level of the internet right. It can be machine to machinesdirectly connected or two machines connected through different are different networkand frame.Nevertheless, it has a higher level object at which is say messaging or something whichis going on, and it has a service interface to this protocol, right. I put a service interfaceand this protocol carries the things and go on at the other service interfaces, like if I varybroadly try to say I am writing a letter to somebody with our standard postal system orcourier systems or what we see that we write a particular letter or write a particular formfor a application for something. And then, I push it to envelop then write the addressetcetera and then I put it in somewhere, either I put it to that letter box or I register or putthrough a courier system to their desk etcetera.So, there is the interface to take that and there is a separate way of carrying those thingsto the other end right. So, that is a that two protocols, how to carry something from IITKharagpur to something say Kolkata is, there is a interface for the protocol end. Now oneday once it is there then the again there are interfaces to go to this higher level things. So,this is so what we try to see that every protocol this interfaces should be properlydefined, that how need to talk to this interfaces.(Refer Slide Time: 13:05)And if you look at it there is a inherent hierarchy in the protocol itself right, like if wesee these are at the upper application layer, then the next layer of protocols should bethere and then there is a another level of things and go on going to the things. So, if youlook at so there is a appropriate hierarchy of these protocol stack; if you look at thenetworking things.So, what we what we trying to discuss is that, these are the different features of aprotocol stack and we will we will be going deep into the things once we look at theindividual layers.(Refer Slide Time: 13:48)And then another things what we have already discussed in some form is thatencapsulation right, like if I have say application layer, transport layer, network layer,data link layer and physical layer, then things are encapsulated. That means, the datagenerated at the application layer becomes a payload for this next layer, becomes thatalong with its own header and other information, it becomes a payload for the nextheader. So, what we see, that a data generated at a higher level get encapsulated in someform of other and at the end, it is carried through this physical media to the other endwhere it is again been exploded or extracted out from this particular bundle at thedifferent level.So, this there is a distinct beauty of the thing, like intermediate layer which is looking atit opens up the packet up to the layer it needs to look at right, like as we are saying that anetwork interface, a router interface will open up the things at the, up to the networklayer. It is not opening up the so, at the higher layer. That’s why we say these are layer 1devices, layer 2 devices, layer 3 devices, layer 4 devices and so and so forth, right up tolayer 7 devices; if we follow that protocol OSI or we say layer 5 device and type ofthings; that means it opens up the packet up to that layer right.So, layer 1 device will open up to the data link layer and so on and so forth right. So, thatkeeps a beauty that I need to open up to look at take it is enough to up that level, it is restof the things is a payload to me. So, open system interconnect as if we if we quickly tryto revisit, so that is the OSI model says there are 7 layers like physical, data link,network, transport, session, presentation, application and intermediate hops there thelayers are may be like if there are intermediate routers, so they are up to the networklayer.(Refer Slide Time: 16:00)So, what we say this peers are in some sort of a virtual connection or virtually connectedI should not say virtual connection. So, this network layer whatever is there, can be openup by this network layer and this network layer and go on doing that right, and rest of thethings at the up to this is become a payload for the network layer. So, it carries that dataas a payload for this network layer right. So, this goes on. So, if there is a layer 2 devicein between, then it will can it could have seen only up to this data link layer. If there is ahub or simple repeater, then it could have been the physical thing which is onlyregenerating the signal and transmitting the signal right.So, this if we so if there can be one or more nodes within the networks which allows it tohop to the destination. So, if it is the two end host or the remote host, so it will go onlooking at it.
Protocol Stack Layers- Part 2
So, if we look at that protocol layers typical functionalities: so we have physical layerswhich handles transmission of raw bits over a communication link right. So, physicallayer is concerned to the things. If it is a, if it receives a data how this bits can betransmitted to the through the communication link. As one of the predominant thing isthat these are transmitted as most in analog signal, there are provision for detailtransmissions the analog signals so that the data gets converted, transmitted andregenerated at the things right.So, as long as the physical layer is concerned, it is not bothered about what is going onthe upper layer things right. So, this is the one there is the functionality of the physicallayer is to transmit to the other end in a error free manner, so that it can be reconstructedin the things right, and does it individually without bothering that where the upper layerthings are doing. If we look at the data link layer or the layer 2, it collects a stream of bitsinto a larger aggregate called frame. So it collects a stream of bits called frame and so,what we say that if we say raw bits at the physical layer then, what we say if sayaggregation of this bits into frames.Network adapter along with device driver in the OS implement in the protocol is in thelayer right, like all of you know all of us know that any of our devices like laptop ordesktop or even our mobile devices with Wi-Fi connectivity, it needs a network interfacecard or it needs a interface card to connect it to the what we say to the transmission lines,so to say right, like it can be wired, it can be wireless and type of things. So, those whoare using wired connection like RJ-45, so there is a interface card which takes the signalsor the data from the systems and interface to the RJ-45 to this our copper cable right.If you have a fiber connect, then it converts to that particular things. So, that the interfacecard is responsible to convert this data to that appropriate signal level. So, that it can betransmitted right. So, that is the basic property or one of the major functionalities of thesedata link layer and data link layer also what we what we what we see that it mentionmaintains a hop to hop connectivity like a what will be the next hop see this hop to hopconnectivity has to be at the data link layer level. Data link layer also has a property of ahaving a hardware address or MAC address what we popularly known as. So that isaddress which is which comes along with this interface card right. So, it goes to the hopto hop.So, even there is a network connectivity like or between several hops of routers thingsare going on, it has to has to come to that particular hop, and then find out that particularwhat is the data link layer, then the hop to hop connectivity is established; data istransmitted to this particular signal right, a particular transmission line whateverunderlying transmission line is there.Network layer handles routing among nodes within the packet switched network. So,network layer handles this routing along the nodes. So, if there are if there are twodevices or two stations then this network layer takes care of the routing, it basicallyresponsible to finding out the path between the source to the destination, source node tothe destination node, source network to the destination network and so and so forth. So,it finds a path or route or it helps in routing right.So, even if finds the path going to one hop to hop by hop, it has to come back it comedown to this data link layer to go to the hop to hop. In order to come to the data linklayer again transmission the thing, it has to come to the physical layer to find out thatappropriate media to transmit the things right. So, data link network layer is muchhigher, it cares about this how this path will be there.So, unit of data exchanged between nodes in this layer are called packets. So, what wesay bits then the frames then the packets. So, these are different nomenclature which arecommonly looped into the things. So, this if we in some of the literature or some of thebooks we refer this 3 layers at the lower 3 layers at typically implemented in all network,mostly implemented in all network nodes. So, these layers are mostly available in allnetwork nodes which allows routing right. So, it is all what we are thinking that allintermediate network node are having at least routing things. So, it allows routing on thething.(Refer Slide Time: 22:33)So, protocol layers functionality continues, the transport layer implements process toprocess communication right. So, network layer this what is the path and transport layerprocess to process communication, unit of data exchange in this case what we saymessage right. So, this is typically called message. There are in case of OSI, we have twomore layers, like session layer that provides name space that is used to tie togetherpotentially different transport stream that are part of the single application. So, session isbasically maintaining a session which gives a some sort of a name for that session whichallows to have a transport stream of transport things which are at transport layer tohaving the same session right. So it is session to session communication.Similarly, then the present to presentation layer concerned about the format of exchangedata format, that format of the data exchange between the peers. So, what should be theexchange format and support; and application is basically the application where the enduser are interested. So, it is standardized common types of exchangers using this networkstack. So, I have some applications like it can be file transport application, remotelogging application and so and so forth. This layer allows us that how this applicationcan be transmitted or can be standardized so that it can goes over this network.So, transport layer and higher layers typically run on the end host right, and not in theintermediate switches or router. So, this is typically if we see this transport layerfunctionalities or higher layer like session presentation etcetera that are the end devicesright.(Refer Slide Time: 24:18)So, if we look at the network architecture to revisit the thing. One way that we haveupper layer things, then we have this transport layer, then IP which allows me to connectto the thing. Other way of looking at this there are applications which uses this transportlayer which instead IP and the sub network is rest of the network what we say that lastthree network and there can be application which directly talk with the IP, there are canbe application which directly talk to this sub network right. So, there are things which areother way of so, different way of looking at the things what it allows it a large scale interoperation.(Refer Slide Time: 24:52)And if we look at the internet architecture which is defined by IETF, what this say doesnot imply strict layering right. This application is free to bypass different transport layer,different transport layers and to strictly use IP and other thing. So what it says that IETFis not very stringent on this type of things right. It may be good to have a standardizethings so that you can receive you can inter-operate in better way. And if you see themodel is some way hour glass right you have very at the thin at the things and thenexplode at both the side right; wide on the top, narrow in the middle and wide of thethings right.If you look at the network, I have different networks and then, it boils down to the things.Even if you see the telecom type of structure you have telecom circles etcetera, then youhave the trunk line which connects them. So, it is some sort of a hour glass. So, in orderfor a new protocol to officially included in the architecture; there needs to be bothprotocol specification, at least one preferably two representation in the implementations.So, that how you can include the thing we’ll come to that thing that how a new protocolcan be included in the architecture when we go on different discussion on the network.(Refer Slide Time: 26:01)And there is concept of Application Program Interface. So interface exported by thenetworks in most network protocols and implemented those in the high protocol stack isin software right; what we see that at the higher level things are mostly on the softwareand nearly all computer system implement their network protocols a part of the operatingsystems itself, like if you see any operating system, they have that protocol stackimplemented. You have a interface card and rest of the things in your protocol, all thething are there available in to the thing.So, this is the interface this sort of interface is called network API or ApplicationProgram Interface. So, that is important. So, that it can allows you to talk application toapplication and at different level of the things. So, we will we will be again taking up thisAPIs at different context when we will talk about that different application layer things.(Refer Slide Time: 27:01)So, when you took at the TCP/IP protocols stack vis-a-vis OSI, so, is same more or lessthe same functionalities not more or less, the same functionalities. Here it has beenshown as a 4 layer, but it you can look it as a 5 layer in several references books andetcetera. So, application, transport, internetwork and there are data link and physical. Inthis case it has been network interface and hardware are clubbed together. So it is a somesort of 4 layer, but typically we will discuss the things as a interchangeably had 5 layerwhere the data link at physical things are separated out, as physical connectivity orcommunication have lot of complicacies and integrity we need to look at those type ofthings in a separate way. And if you have that at the top level application, thispredominant protocol here is TCP, UDP, then we have here the dominant protocol is IP,there are companion protocol which is ICMP. We will look at to those ARP RARP, ARPRARP again allows to interface with the lower layers right, like data link layer and soand so forth.(Refer Slide Time: 28:12)So, if we quickly try to visit these TCP/IP protocol stack. So, there are application layer,application layer of the TCP/IP provides by the program uses TCP/IP communicationand application is a user process right, cooperating with another process usually on thedifferent host right. So, it is a application like one popular thing is FTP file transferprotocol. So, I have FTP client somewhere FTP server is running. So I can basicallyconnect to the things. How do I identify a particular application? It is a typicallyidentified by the IP where the application is running and the port where it’s the interfacewhere you need to connect to.So, we come to a concept called socket. So, in this particular course we will have somesocket programming. We will be detailing little more on the socket when we talk aboutthis socket programming aspects. So socket is allows us to connect to applications.(Refer Slide Time: 29:08)Similarly, in case of a transport layer, so it is it identifies a, it is a process to it basicallyhelps in process to process communication so; that means, it looks at that port of thething. So, transport layer provide the end-to-end data transfer by delivering data from theapplication to its remotes peer right; so two application by at the port level. Multipleoperation can be supported simultaneously, There it concurrent application that we willsee. The predominant protocol is TCP or transmission control protocol which providesconnection-orientated reliable data recovery, duplicate data suppression, congestioncontrol, flow control and so on so forth. There is another protocol which is also verypopularly used - it’s UDP, User Datagram Protocol. So, it is connection less, unreliable,best-effort service right.So, in some cases what will see that so, if you require this other type of things, errorcontrol, flow control etcetera, then this has to be handled by at the upper layers right. Sousually UDP is used for application that need a fast transport mechanisms and cantolerate some loss of data right. We will see that where which type of applications whichwhere this UDP will be useful and where we use this type of thing.Then we have internet interface layer what we say data link layer and rest of the physicallayer type of things. So, this is this provides the data link layer. So, predominant protocolhere are IEEE 802.2, 2.3 which are that we have the Ethernet protocol there are X.25ATM, FDDI, SNA and different type of protocols. And what we assume there is aunderlying communication path which allows to which this particular frames is the datalink layer and pushed as a bits into that particular physical layer connectivity.And if we look at the same thing in a little different way, so there are peer-to-peerapplication understand application; FTP client understand FTP server, HTTP clientunderstand HTTP server. How things goes on? It is transparent to that application right.So, it goes to the transport if the TCP is the basic thing, then it connects to the TCP orUDP, it goes to the network access IP, IP routes the packets form one to another and itgoes to data link, then it goes to the physical and being communicated through thecommunication network right. So that goes in a ubiquitous way right.So, what we try to see in this particular talk is that how a communication between twohost anywhere across the internet is possible by the, or different type of services areprovided into the things. And in the subsequent lecture slowly what will start we willlook at different layers individually right, that how different layers individually what arethe different properties, what are the predominant protocols, how they work and so onand so forth right. And also, we will try to, as I was mentioning that we look at little bitof programming like socket programming; how you can how two things works andetcetera right.So, with this let us stop today’s discussion.Thank you.
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