In the protocol stack, the transport layer protocol provides logical communication for processes running on different hosts, while the network layer protocol provides logical communication for different hosts. This difference is subtle, but it is very important. Below we use the family as an analogy to illustrate this difference:
Suppose you want to have two houses, one on the east coast and the other on the west coast, with 10 children in each house. The children in the house on the east coast and the children in the West Coast house are cousins. The children in the two houses like to communicate with each other - each child writes a letter to each cousin every week, and each letter is sent by an old post office in an envelope.
In this way, each family will have 100 letters sent to another every week (these children can save a lot of money if they can use e-mail!) in every home. The mail collection and distribution was carried out by a child, Ann in the West Coast house and BilI in the East Coast house. Every week, Ann collects letters from her brothers and sisters and sends them to the postal attendants who come every day. When the letter arrived at the house on the West Coast, Ann distributed the letters to her brothers and sisters. BilI has the same job on the East Coast.
In this example, the postal service provides a logical communication between the two houses—the postal service delivers the mail between the two houses, not the service for each person. On the other hand, Ann and BilI provide logical communication between cousins—Ann and BilI collect mail from their siblings and deliver the mail to them. Note that from the perspective of these cousins, Ann and BilI are the servants of the mail, although they are only part of the end-to-end delivery service (the terminal system part).
This example is an image metaphor of the relationship between the transport layer and the network layer: Host (also known as terminal system) = house process = cousin
Application message = letter in envelope
Network layer protocol = postal service (including postman) Transport layer protocol = Ann and Bill
Continuing with this example, Ann and Bill each do all the work in their home:
They are not responsible for mail sorting at individual postal centers and for sending mail from one center to another. This is the same as the transport layer protocol in the end system. In an end system, the transport layer protocol passes the message of the application process to the edge of the network (that is, the network layer) and vice versa: but it does not involve how the message is transmitted between network layers. In fact, the intermediate router cannot identify and process the information that the transport layer adds to the application message.
Continuing with this example, let's say that Ann and Bill are on vacation, and another cousin, Susan and Harvey, instead, they provide home mail collection and distribution. Unfortunately, the collection and distribution work provided by Susan and Harvey is not exactly the same as that provided by Ann and Bill. For younger Susan and Harvey, they collect and distribute mail less frequently, and occasionally lost mail.
In this way, the cousins ​​Susan and Harvey provide a different set of services than Ann and Bill (that is, the service model is different). For example, just as a computer network can accept different transport layer protocols, each protocol provides a different service model for the application.
The services that Ann and Bill can provide are significantly limited by the services provided by the postal service. For example, if the postal service does not provide the maximum time limit (eg, 3 days) required to deliver mail between the two houses, then Ann and Bill cannot guarantee the maximum delay for mail between the cousins.
Similarly, the services provided by the transport layer protocol are also typically limited by the network layer protocol located below it. If the network layer protocol cannot provide delay and bandwidth guarantee for PDUs transmitted between hosts, then the transport layer protocol cannot provide delay and bandwidth guarantee for messages transmitted between processes.
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