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The fourth and “middle” layer of the OSI Reference Model protocol stack is the transport layer. I consider the transport layer in some ways to be part of both the lower and upper “groups” of layers in the OSI model. It is more often associated with the lower layers, because it concerns itself with the transport of data, but its functions are also somewhat high-level, resulting in the layer having a fair bit in common with layers 5 through 7 as well.
Recall that layers 1, 2 and 3 are concerned with the actual packaging, addressing, routing and delivery of data; the physical layer handles the bits; the data link layer deals with local networks and the network layer handles routing between networks. The transport layer, in contrast, is sufficiently conceptual that it no longer concerns itself with these “nuts and bolts” matters. It relies on the lower layers to handle the process of moving data between devices.
The transport layer really acts as a “liaison” of sorts between the abstract world of applications at the higher layers, and the concrete functions of layers one to three. Due to this role, the transport layer’s overall job is to provide the necessary functions to enable communication between software application processes on different computers. This encompasses a number of different but related duties
Modern computers are multitasking, and at any given time may have many different software applications all trying to send and receive data. The transport layer is charged with providing a means by which these applications can all send and receive data using the same lower-layer protocol implementation. Thus, the transport layer is sometimes said to be responsible for end-to-end or host-to-host transport (in fact, the equivalent layer in the TCP/IP model is called the “host-to-host transport layer”).
The third-lowest layer of the OSI Reference Model is the network layer. If the data link layer is the one that basically defines the boundaries of what is considered a network, the network layer is the one that defines how internetworks (interconnected networks) function. The network layer is the lowest one in the OSI model that is concerned with actually getting data from one computer to another even if it is on a remote network; in contrast, the data link layer only deals with devices that are local to each other.
While all of layers 2 through 6 in the OSI Reference Model serve to act as “fences” between the layers below them and the layers above them, the network layer is particularly important in this regard. It is at this layer that the transition really begins from the more abstract functions of the higher layers—which don't concern themselves as much with data delivery—into the specific tasks required to get data to its destination. The transport layer, which is related to the network layer in a number of ways, continues this “abstraction transition” as you go up the OSI protocol stack. Network Layer Functions
Some of the specific jobs normally performed by the network layer include:
The basic difference between network layer and transport layer is that transport layer protocol provides logical communication between processes running on different hosts , whereas network layer protocol provides logical communication between hosts.
Let’s try to understand this difference with the help of an analogy.
Consider two houses, one on the East Coast and the other on the West Coast, with each house being home to a dozen kids. The kids in the East Coast household are cousins of the kids on the West Coast household. The kids in the two households love to write to each other – each kid writes each cousin every week, with each letter delivered by the traditional postal service in a separate envelope.
In each of the households there is one kid – Ann in the West Coast house and Bill in the East Coast house – responsible for mail collection and mail distribution. Each week Ann visits all her brothers and sisters, collects the mail, and gives the mail to a postal-service mail career, who makes daily visits to the house. When letters arrive at the West Coast house, Ann also has the job of distributing the mail to her brothers and sisters. Bill has a similar job on the East Coast.
In this example, the postal service provides logical communication between the two houses – the postal service provides moves mail from house to house, not from person to person. On the other hand, Ann and Bill provide logical communication among the cousins – Ann and Bill pick up mail from, and deliver mail to, their brothers and sisters. Note that from the cousin’s perspective, Ann and Bill are the mail service, even though Ann and Bill are only a part (the end-system part) of the end-to-end delivery process. This household example, servers as a nice analogy for explaining how the transport layer relates to the network layer.application messages = letters in envelopes processes = cousin hosts (also called end systems) = houses transport layer protocol = Ann and Bill network layer protocol = postal service (including mail carriers)
Continuing with this analogy, note that Ann and Bill do all their work within their respective homes; they are not involved , for example, in sorting mail in any intermediate mail centre or in moving mail from one mail centre to another. Similarly, transport layer protocols live in the end systems. Within an end system, a transport protocol moves messages from application processes to the network edge (that is, the network layer) and vice versa, but it doesn’t have any say about how the messages are moved within the network core. In fact, intermediate routers neither act on, nor recognize, any information that the transport layer may have added to the application messages.
Continuing with our family saga, suppose now that when Ann and Bill go on vacation, another cousin pair – say, Susan and Harvey – substitute for them and provide the household-internal collection and delivery of mail. Unfortunately for the two families, Susan and Harvey do not do the collection and delivery in exactly the same way as Ann and Bill. Being younger kids, Susan and Harvey pick up and drop off the mail less frequently and occasionally lose letters (which are sometimes chewed up by the family dog). Thus, the cousin-pair Susan and Harvey do not provide the same set of services (that is, the same service model) as Ann and Bill. In an analogous manner, a computer network may make available multiple transport protocols, with each protocol offering a different service model to applications.
The possible services that Ann and Bill can provide are clearly constrained by the possible services that the postal services provides. For example, if the postal service doesn’t provide a maximum bound on how long it can take to deliver mail between the two houses (for example, three days), then there is no way that Ann and Bill can guarantee a maximum delay of mail delivery between any of the cousin pairs. In a similar manner, the services that a transport protocol can provide are often constrained by the service model of the underlying network-layer protocol. If the network layer protocol cannot provide delay or bandwidth guarantees for transport layer segments sent between hosts, then the transport layer protocol cannot provide delay or bandwidth guarantee for application messages sent between processes.
Nevertheless, certain services can be offered by a transport protocol even when the underlying network protocol doesn’t offer the corresponding service at the network layer. For example, a transport protocol can offer reliable data transfer service to an application even when the underlying network protocol is unreliable, that is, even when the network protocol loses, garbles, or duplicates packets. As another example a transport protocol can use encryption to guarantee that application messages are not read by intruders, even when the network layer cannot guarantee the confidentiality of transport layer segments.