Communication Networks (12)

TCP reliable data transfer

TCP ACK generation

Arrival of in-order segment with expected seq #. One other segment has ACK pending | Immediately send single cumulative ACK, ACKing both in-order segments
Arrival of out-of-order segment higher-than-expect seq. #. Gap detected. | Immediately send duplicate ACK, indicating seq. # of next expected byte
Arrival of segment that partially or completely fills gap.|Immediate send ACK, provided that segment starts at lower end of gap

TCP fast retransmit

TCP fast retransmit

if sender receives 3 ACKs for same data (“triple duplicate ACKs”), resend unacked segment with smallest seq #

likely that unacked segment lost, so don’t wait for timeout

TCP flow control

flow control

receiver controls sender, so sender won’t overflow receiver’s buffer by transmitting too much, too fast

• receiver “advertises” free buffer space by including rwnd value in TCP header of receiver-to-sender segments
• sender limits amount of unacked (“in-flight”) data to receiver’s rwnd value
• guarantees receive buffer will not overflow

Connection Management

before exchanging data, sender/receiver “handshake”:

• agree to establish connection (each knowing the other willing to establish connection)
• agree on connection parameters

TCP 3-way handshake

TCP closing a connection

Congestion Control

Principles of congestion control

congestion

“too many sources sending too much data too fast for network to handle”

lost packets (buffer overflow at routers)

• congestion control: for network
• flow control: for reciever

one router, infinite buffers

$$T=\frac{1}{\mu-\lambda}$$

one router, finite buffers

TODO

TCP Congestion Control: CWND

cwnd

congestion window

dynamic, function of perceived network congestion

LastByteSent - LastByteAcked $\le$ cwnd

$$\text { rate } \approx \frac{\text { cwnd }}{\mathrm{RTT}} \text { bytes/sec }$$

TCP Slow Start