TCP seq. #’s and ACKs

Seq. #’s:

• byte stream “number” of first byte in segment’s data

ACKs:

• seq # of next byte expected from other side
• cumulative ACK

Q: how receiver handles out-of-order segments

• A: TCP spec doesn’t say, - up to implementor

Host A

Host B

Seq=42, ACK=79, data = ‘C’

Seq=79, ACK=43, data = ‘C’

Seq=43, ACK=80

User

types

‘C’

host ACKs

receipt

of echoed

‘C’

host ACKs

receipt of

‘C’, echoes

back ‘C’

simple telnet scenario

Previous slide

Next slide

Back to first slide

View graphic version