TCP ‐ Transport Control Protocol - O-LavenderAshburn/Knowledgebase_NetworkAndConnectivity GitHub Wiki

TCP (Transmission Control Protocol) is a connection-oriented protocol used in the transport layer of the OSI and TCP/IP models which provides reliable, ordered, and error-checked delivery of data between applications. TCP 3-way handshake is used to establish a reliable connection between a client and server.

TCP 3-Way Handshake

Client                                               Server
  |                                                    |
  | ----------- SYN (Seq = x) ------------------------>|
  |                                                    |
  |<--------- SYN-ACK (Seq = y, Ack = x + 1) ----------|
  |                                                    |
  | ----------- ACK (Ack = y + 1) -------------------->|
  |                                                    |
Connection Established

Packet Structure

                              Packet Bits

0                                15|16                              31
|__________________________________|________________________________|
|      Source Port (0-15)          |    Destination Port (16-31)    |
|-------------------------------------------------------------------|
|                            Sequence Number                        |
|                                (0-31)                             |
|-------------------------------------------------------------------|
|                         Acknowlegment Number                      |
|                                (0-31)                             |
|-------------------------------------------------------------------|
|Data    | Resvd |N|C|E|U|A|P|R|S|F|                                |
| Offset |       |S|W|C|R|C|S|S|Y|I|         Window Size            |
|  (0-4) | (4-7) | |R|E|G|K|H|T|N|N|           (16-31)              |
|-------------------------------------------------------------------|
|         Checkusm (0-15)          |      Urgent Pointers (16-31)   |
--------------------------------------------------------------------|
|                                 Data                              |
|                                (0-31)                             |
---------------------------------------------------------------------

  1. Source port: Identifies the port number on the sender's device.

  2. Destination port: Identifies the port number on the receivening device.

  3. Sequence Number: If not initial sequence number (SYN Flag is set), then the accumulated sequence number of the first data byte of this segment for the current session.

  4. Acknowlegment Number: If the ACK flag is set, this value tells the sender which byte is expected next.

  5. Data Offset: Specifies the length of the TCP header in 32-bit words.

  6. Reserved: Reserved for future use.

  7. Flags: Determine behaviour.

  8. Window Size: Specifies the size of the receive window.

  9. Checksum: Used to verify integrity of header and data. Ensures the segment wasn’t corrupted in transit.

  10. Urgent Pointer: Points to the end of urgent data if the URG flag is set.

  11. Data: Contains the payload (e.g., part of an HTTP request/response).

TCP Congestion control

Are algorithms within TCP to control congestion over the internet.

TCP Slow Start