Persist and Keepalive Timers

1
Persist and Keepalive Timers
2
Window Advertisement

• What happens when a window advertisement with
  recvwin gt 0 is lost?
• ACKs are not retransmitted
• TCP could stall
• Solution
• Send periodic probes to elicit ACKs from receiver

3
Persist Timer in Action

• Why 1 byte is sent?
• Persist timer probes are sent at intervals based
  on the RTO
• But it is at least 5 sec and at most 60 sec
• TCP never gives up sending persist timer probes
  unless
• Window opens up
• Application terminates

4
Silly Window Syndrome

• Window based flow control schemes may suffer from
  silly window syndrome
• Receiver side
• Receiver advertises small window and sender sends
  small chunk of data
• Sender side
• Sender sends small amount of data instead of
  waiting for more data

5
Receiver Side Avoidance of SWS

• Do not send a new window update unless it is
  larger than current window by
• 1 MSS
• Or, half of receivers buffer space, whichever is
  smaller (rare)

6
Receiver AvoidanceSWS

• Notice persist timer in action
• Although bytes are being received, win is still
  0. Why?

7
?

• Why 1022 bytes are not advertised?
• Why 509 bytes are advertised?

8
Sender Side Avoidance of SWS

• Do not transmit data unless
• A full-size segment can be sent
• Or, we can send at least half of the window that
  the other side has ever advertised (older,
  primitive hosts)
• Or, we can send everything we have and either we
  are not expecting an ACK or Nagles algorithm is
  disabled

9
Keepalive Timer

• Send packets every 2 hours to check if client is
  up
• If no activity from client, four possibilities
• Client is up
• Client is down
• Client has crashed and rebooted
• Client unreachable (similar to scenario 2)

10
Host has crashed

• If no response, send 10 probe packets 75 seconds
  apart, before declaring the connection dead

11
Host has crashed and rebooted

• TCP reset is useful

12
Host unreachable

• ICMP host unreachable messages are ignored by TCP
• TCP returns error of No route to host to the
  application