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TCP Connection

351 words, 2 min read
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Xinyang YU

Abstract

Resource intense

More resource is required on both Client and Server because TCP is a Stateful Network Protocol.

TCP connection in browser

Browser usually keep multiple TCP Connection to the same Server. Chrome & Firefox default have 6 to have a balance between performance & resource usage

TCP Connection Termination

Graceful TCP Connection Termination

  • Four-way closure with FIN flags

4 events

  1. A -> B: A sends a FIN packet to B, with its final Sequence Number of x.
  2. A <- B: B ACK and responds to A with an acknowledgment number of x+1, indicating that it has received A’s FIN packet.
  3. A <- B: Since TCP is bidirectional, B also sends its own FIN packet to A, with its final sequence number of y.
  4. A -> B: A ACK and responds B with an acknowledgment number of y+1, indicating that it has received B’s FIN packet.

For events two and three, both occur from B to A, and the fields used in the TCP packet do not overlap, meaning both events can happen in the same packet.

However, these events are usually separated. For example, A may initiate the connection termination (step one) after finishing sending its data, but B may still have data to send. In this case, when B receives the FIN packet, it cannot immediately proceed to step three. B must finish sending its data before sending its own FIN. Meanwhile, A can still receive data from B even after sending its FIN packet.

Ungraceful TCP Connection Termination

  • One-way closure with RST flags. A RST flag is typically used in scenarios where the connection cannot be continued, either due to unexpected behaviour, errors, or forceful termination by the application

1 event

A <-> B: Either party can send an RST. Once sent, the party will forget everything about that TCP Connection, and the party that receives it will also forget everything.

The RST does not need to be acknowledged!

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