What guarantees does TCP provide?

TCP provides a set of guarantees that make data transmission reliable and predictable for applications. More simply: TCP guarantees that data will arrive correctly, in the right order, and at a suitable speed.

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#Main TCP guarantees

#1. Delivery guarantee

TCP acknowledges receipt of data:

• every segment must be acknowledged (ACK)
• if an acknowledgment does not arrive, the segment is sent again

If a TCP connection ends successfully, the application can be sure that:

• the data was delivered to the receiver

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#2. Delivery order guarantee

TCP:

• numbers segments
• on the receiver's side assembles the data in the correct order

Even if segments arrive:

• out of order
• with different delays

the application receives an ordered stream of data.

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#3. No duplicates

TCP:

• detects duplicate segments
• does not pass duplicates to the application

The application sees the data exactly once.

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#4. Integrity control

TCP checks:

• whether the data was corrupted during transmission

Corrupted segments:

• are treated as undelivered
• are requested again

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#5. Flow control

TCP protects the receiver:

• it does not send more data than the receiver can accept
• it uses a window mechanism

This prevents:

• buffer overflow
• data loss on the receiver's side

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#6. Network congestion control

TCP monitors the state of the network:

• it reduces speed under congestion
• it increases speed when the network is free

This makes the transmission:

• stable
• fair with respect to other flows

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#7. End-to-end connection

TCP:

• establishes a connection before transmission
• correctly closes it after the exchange ends

This guarantees:

• a consistent state on the sender and the receiver

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#What TCP does NOT guarantee (important)

TCP does not guarantee:

• minimal latency
• a constant speed
• delivery "by a certain time"

If the network is poor:

• TCP can slow down significantly
• but it will not sacrifice reliability