Transmission Control Protocol

1

• Transmission Control Protocol
• TCP
• RFC 793
• http//www.faqs.org/rfcs/rfc793.html

2
TCP Control Flags

• URG Urgent Pointer field significant
• ACK Acknowledgement field significant
• PSH Push Function (more later)?
• RST Reset the connection (more later)?
• SYN Synchronize sequence numbers
• FIN No more data from sender

Image Source Transmission Control Protocol,
Carey Williamson, http//pages.cpsc.ucalgary.ca/c
arey/CPSC441/lectures/tcp.ppt
3
RESET Generation

• RESET indicates receipt of spurious segments
• Not intended for current connection
• Three cases
• Connection does not exist on current port
• While in non - synchronized state
• ACK received for older connection
• While in synchronized state
• Out of window segment received

4
What happens next?

• First validate RESET packet
• Is sequence number within the window?
• Next step depends on current state
• LISTEN gt Ignore the reset
• SYN-RECEIVED
• previous state was LISTENgt return to LISTEN
• else abort
• All other states gt abort connection

5
The PUSH bit

• When sending
• Tells transport layer to send all pending data
  immediately
• No relationship between data boundary and push
  bit
• When receiving
• Tells transport layer to send all data received
  thus far to application layer, though buffer may
  not be full yet

6
Retransmission

• Retransmission timeout dynamically computed
• compute Round Trip Time (RTT)?
• Smoothed Round Trip Time (SRTT)?
• SRTT ALPHA SRTT (1 ALPHA) RTT
• Retransmission Timeout (RTO)?
• RTO min (UBOUND, max(LBOUND, BETA SRTT))?
• UBOUND upper bound (e.g. 1 minute)?
• LBOUND lower bound (e.g. 1 second)?
• ALPHA smoothing factor, between 0 and 1
• BETA delay variance (e.g. between 1.3 and 2)?

7
Delayed ACKs

• Silly window syndrome
• If sender sends data too slowly
• If receiver consumes data too slowly
• Data is sent in small segments - inefficient!

8
Silly Window Syndrome Illustrated
Image Source http//www.tcpipguide.com/free/t_TCP
SillyWindowSyndromeandChangesTotheSlidingWindow.ht
m
9
Solution?

• Delayed ACKs
• Obtain as much data to send while piggybacking
  ack
• Delay as much as 200ms
• Cumulative nature of acks helps increase
  throughput with delayed acks

10
Delayed ACKs

• What is the problem with delayed ACKs?
• Waiting too long causes retransmissions
• TCP uses ACKs to compute RTT
• Recommendations
• Delay up to 200ms at most
• Acknowledge at least every other received packet