Geekdocs
Toggle Dark/Light/Auto mode Toggle Dark/Light/Auto mode Toggle Dark/Light/Auto mode Back to homepage
  1. Geekdocs
  2. /
  3. IT Courses & Notes
  4. /
  5. Complete CCNA Course
  6. /
  7. Section 5

Section 5

Table of contents

  • The Transport layer provides transparent transfer of data between hosts and is responsible for the end-to-end recovery and flow control.
  • The flow control is the process of adjusting the flow of data to ensure that the receiver can handle all of it.
  • It also supports session multiplexing, meaning its a process by which the host is able to support multiple sessions simultaneously and manage the individual traffic streams over a single link. For this, it uses port numbers. Normally, HTTP uses 80, SSH 22, etc. The host will also add a random source port number, where the receiver is going to respond to.

TCP

  • TCP is connection oriented, meaning once a connection is established, data can be sent bidirectionally over that connection.
  • TCP carries out sequencing to ensure segments are processed in the correct order and none are missing.
  • TCP is reliable, meaning that the receiving host sends acknowledgments back to the sender, and if it doesn’t receive one, the packet is going to be resent.
  • TCP performs flow control.

This is done by a three-way handshake, where the sender first sends a Synchronized Message (SYN) over to the receiver, which he’ll answer with an Synchronized Acknowledgments (SYN-ACK). Finally, the sender is going to send an acknowledgments (ACK).

TCP Header

The TCP header contains important information about the packet sent. This includes the source port, destination port, sequence number, checksum and other information.

%% TODO: Add image of TCP header

UPD

  • The User Datagram Protocol sends traffic best effort.
  • It’s not connection oriented, meaning there is no handshake connection setup.
  • It doesn’t carry out sequencing to ensure all segments are processed in the correct order.
  • It’s not reliable, as the receiver doesn’t send back acknowledgments.
  • It doesn’t perform flow control.
  • All error detection is up to the upper layers.

It’s header is way more simple, having just four fields: source and destination port, length and UDP checksum.

%% TODO: Add image of UDP header

TCP vs UDP

  • Application developers will typically choose to use TCP for traffic which requieres reliability.
  • Real-time applications like VOIP can’t afford the extra overhead.
  • Some applications use both TCP and UDP, like a DNS application.