EC-11.5 End to End: TCP, UDP and the Price of Reliability
You can explain what a transport protocol adds above a network that may lose packets, describe how a reliable stream is built from an unreliable one, say what congestion control is doing and why it exists, and choose between a reliable and an unreliable transport for a given application.
This is the topic where a reader stops seeing reliability as a property a network has and starts seeing it as something built at the ends, at a measurable cost. It is also where the difference between delay and throughput becomes concrete, which matters for anyone who will later be asked why a link with plenty of capacity still feels slow.
Work through these
Ports, and delivering to a program rather than a machine
The network layer gets a packet to a machine and stops there. A port number is what identifies which program on that machine the data belongs to, and it is the transport layer's first job.
NPTEL: Computer Networks and Internet Protocol (IIT Kharagpur) · CourseThe unreliable transport, and when it is the right one
Sending a message with no acknowledgement, no ordering and no retransmission is the right choice when a late message is worthless. Voice, video and measurement streams are all of that kind.
NPTEL: Computer Networks (IIT Indore) · CourseBuilding a reliable stream on an unreliable network
Numbering every byte, acknowledging what arrived, and resending what was not acknowledged turns a lossy packet service into an ordered stream. Everything else in this protocol exists to make that efficient.
NPTEL: Computer Networks and Internet Protocol (IIT Kharagpur) · CourseSetting up and tearing down a connection
Both ends must agree on starting numbers before data flows, which takes an exchange of three messages. Closing takes more, because each direction is closed separately and a delay is needed at the end.
NPTEL: Computer Networks and Internet Protocol (IIT Kharagpur) · CourseFlow control against congestion control
Flow control stops a fast sender overwhelming a slow receiver. Congestion control stops many senders overwhelming the network between them. They are different problems solved by different mechanisms.
NPTEL: Computer Networks and Internet Protocol (IIT Kharagpur) · CourseThroughput, delay, and why a fast link can feel slow
Throughput is limited by the window divided by the round-trip time, so a long path needs a large window to fill a fast link. Capacity that cannot be kept busy is capacity you do not get.
NPTEL: Computer Networks (IIT Indore) · Course
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