1. TCP
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2. TCP Adaptive Retransmission Algorithm - Original
Theory
Estimate RTT
Multiply by 2 to allow for variations
Practice
Use exponential moving average (A = 0.1 to 0.2)
Estimate = (A) * measurement + (1- A) * estimate
Problem
Did not handle variations well
Ambiguity for retransmitted packets
Was ACK in response to first, second, etc transmission?
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3. TCP Adaptive Retransmission Algorithm – Karn-Partridge
Exclude retransmitted packets from RTT estimate
For each retransmission
Double RTT estimate
Exponential backoff from congestion
Problem
Still did not handle variations well
Did not solve network congestion problems as well as desired
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4. TCP Adaptive Retransmission Algorithm – Jacobson
Estimate variance of RTT
Calculate mean interpacket RTT deviation to approximate variance
Use second exponential moving average
Deviation = (B) * |RTT_Estimate – Measurement| + (1–B) * deviation
B = 0.25, A = for RTT_estimate
Use variance estimate as component of RTT estimate
Next_RTT = RTT_Estimate + 4 * Deviation
Protects against high jitter
Notes
Algorithm is only as good as the granularity of the clock
Accurate timeout mechanism is important for congestion control
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5. TCP Connection Establishment
3-Way Handshake
Sequence Numbers
J,K
Message Types
Synchronize (SYN)
Acknowledge (ACK)
Passive Open
Server listens for connection from client
Active Open
Client initiates connection to server
Client
Server
Time flows down
listen
Synchronize (SYN) J
acknowledge (ACK) J+1
SYN K,
ACK K+1
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6. TCP Connection Termination
Message Types
Finished (FIN)
Acknowledge (ACK)
Active Close
Sends no more data
Passive close
Accepts no more data
Client
Server
Time flows down
Finished (FIN) J
ACK J+1
FIN K
ACK K+1
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7. TCP Connection Management (cont)
TCP server
lifecycle
TCP client
lifecycle
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8. TCP State Descriptions
CLOSED
Disconnected
LISTEN
Waiting for incoming connection
SYN_RCVD
Connection request received
SYN_SENT
Connection request sent
ESTABLISHED
Connection ready for data transport
CLOSE_WAIT
Connection closed by peer
LAST_ACK
Connection closed by peer, closed locally, await ACK
FIN_WAIT_1
Connection closed locally
FIN_WAIT_2
Connection closed locally and ACK’d
CLOSING
Connection closed by both sides simultaneously
TIME_WAIT
Wait for network to discard related packets
CLOSED
Disconnected
LISTEN
Waiting for incoming connection
SYN_RCVD
Connection request received
SYN_SENT
Connection request sent
ESTABLISHED
Connection ready for data transport
CLOSE_WAIT
Connection closed by peer
LAST_ACK
Connection closed by peer, closed locally, await ACK
FIN_WAIT_1
Connection closed locally
FIN_WAIT_2
Connection closed locally and ACK’d
CLOSING
Connection closed by both sides simultaneously
TIME_WAIT
Wait for network to discard related packets
CLOSED
Disconnected
LISTEN
Waiting for incoming connection
SYN_RCVD
Connection request received
SYN_SENT
Connection request sent
ESTABLISHED
Connection ready for data transport
CLOSE_WAIT
Connection closed by peer
LAST_ACK
Connection closed by peer, closed locally, await ACK
FIN_WAIT_1
Connection closed locally
FIN_WAIT_2
Connection closed locally and ACK’d
CLOSING
Connection closed by both sides simultaneously
TIME_WAIT
Wait for network to discard related packets
CLOSED
Disconnected
LISTEN
Waiting for incoming connection
SYN_RCVD
Connection request received
SYN_SENT
Connection request sent
ESTABLISHED
Connection ready for data transport
CLOSE_WAIT
Connection closed by peer
LAST_ACK
Connection closed by peer, closed locally, await ACK
FIN_WAIT_1
Connection closed locally
FIN_WAIT_2
Connection closed locally and ACK’d
CLOSING
Connection closed by both sides simultaneously
TIME_WAIT
Wait for network to discard related packets
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9. TCP State Transition Diagram
Passive open
Close/FIN
ACK
SYN/SYN + ACK
Active
open/SYN
Close
Timeout
FIN +
ACK/ACK
SYN + ACK/ACK
Send/SYN
FIN/ACK
Close/ACK
CLOSED
LISTEN
SYN_RCVD
SYN_SENT
ESTABLISHED
FIN_WAIT_1
FIN_WAIT_2
CLOSING
TIME_WAIT
CLOSE_WAIT
LAST_ACK
CLOSED
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10. TCP State Transition Diagram
Questions
State transitions
Describe the path taken by a server under normal conditions
Describe the path taken by a client under normal conditions
Describe the path taken assuming the client closes the connection first
TIME_WAIT state
What purpose does this state serve
Prove that at least one side of a connection enters this state
Explain how both sides might enter this state
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11. TCP State Transition Diagram
Active
open/SYN
Active
open/SYN
CLOSED
Passive open
Passive open
Close
Close
LISTEN
SYN_RCVD
SYN_SENT
SYN/SYN + ACK
SYN/SYN + ACK
Send/SYN
Send/SYN
ACK
ACK
SYN/SYN + ACK
SYN/SYN + ACK
SYN + ACK/ACK
SYN + ACK/ACK
ESTABLISHED
TCP A TCP B
1. CLOSED LISTEN
2. SYN-SENT > <SEQ=100><CTL=SYN> > SYN-RECEIVED
3. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
4. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK> > ESTABLISHED
5. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED
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12. TCP State Transition Diagram
Active
open/SYN
CLOSED
Passive open
Close
Close
LISTEN
SYN_RCVD
SYN_SENT
SYN/SYN + ACK
Send/SYN
ACK
SYN/SYN + ACK
SYN + ACK/ACK
Close/FIN
Close/FIN
Close/FIN
Close/FIN
Close/FIN
ESTABLISHED
FIN/ACK
FIN/ACK
FIN/ACK
FIN/ACK
FIN_WAIT_1
FIN_WAIT_2
CLOSING
TIME_WAIT
CLOSE_WAIT
LAST_ACK
ACK
ACK
FIN +
ACK/ACK
Close/FIN
Close/FIN
FIN +
ACK/ACK
ACK
ACK
FIN/ACK
ACK
ACK
FIN/ACK
Timeout
Timeout
Timeout
Timeout
CLOSED
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13. TCP Sliding Window Protocol
Sequence numbers
Indices into byte stream
ACK sequence number
Actually next byte expected as opposed to last byte received
Advertised window
Enables dynamic receive window size
Receive buffers
Data ready for delivery to application until requested
Out-of-order data out to maximum buffer capacity
Sender buffers
Unacknowledged data
Unsent data out to maximum buffer capacity
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14. TCP Sliding Window Protocol – Sender Side
LastByteAcked <= LastByteSent
LastByteSent <= LastByteWritten
Buffer bytes between LastByteAcked and LastByteWritten
Maximum buffer size
Advertised window
First unacknowledged byte
Last byte sent
Data available, but outside window
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15. TCP Sliding Window Protocol – Receiver Side
LastByteRead < NextByteExpected
NextByteExpected <= LastByteRcvd + 1
Buffer bytes between NextByteRead and LastByteRcvd
Maximum buffer size
Advertised window
Buffered, out-of-order data
Next byte to be read by application
Next byte expected (ACK value)
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16. TCP ACK generation - 1
Maximum buffer size
Available buffer size
Next byte to be read by application
Next byte expected (ACK value)
Arrival of in-order segment with expected seq #. All data up to expected seq # already ACKed
Delayed ACK. Wait up to 500ms for next segment.
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17. TCP ACK generation - 2
Maximum buffer size
Available buffer size
Next byte to be read by application
Next byte expected (ACK value)
Arrival of in-order segment with expected seq #. One other segment has ACK pending
Immediately send single cumulative ACK, ACKing both in-order segments
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18. TCP ACK generation - 3
Maximum buffer size
Available buffer size
Next byte to be read by application
Next byte expected (ACK value)
Arrival of out-of-order segment higher-than-expect seq. # Gap detected
Immediately send duplicate ACK, indicating seq. # of next expected byte
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19. TCP ACK generation - 4
Maximum buffer size
Available buffer size
Next byte to be read by application
Next byte expected (ACK value)
Arrival of segment that partially or completely fills gap
Immediate send ACK, provided that segment starts at lower end of gap
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20. TCP ACK generation [RFC 1122, RFC 2581]
Event at Receiver
Arrival of in-order segment with
expected seq #. All data up to
expected seq # already ACKed
expected seq #. One other
segment has ACK pending
Arrival of out-of-order segment
higher-than-expect seq. # .
Gap detected
Arrival of segment that
partially or completely fills gap
TCP Receiver action
Delayed ACK. Wait up to 500ms
for next segment. If no next segment,
send ACK
Immediately send single cumulative
ACK, ACKing both in-order segments
Immediately send duplicate ACK,
indicating seq. # of next expected byte
Immediate send ACK, provided that
segment starts at lower end of gap
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21. Fast Retransmit
If sender receives 3 ACKs for the same data, it supposes that segment after ACKed data was lost:
fast retransmit: resend segment before timer expires
Why 3?
What’s the problem with time-out?
time-out period often relatively long
Detect lost segments via duplicate ACKs.
Sender often sends many segments back-to-back
If segment is lost, there will likely be many duplicate ACKs.
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22. Fast retransmit algorithm:
event: ACK received, with ACK field value of y
if (y > SendBase) {
SendBase = y
if (there are currently not-yet-acknowledged segments)
start timer }
else {
increment count of dup ACKs received for y
if (count of dup ACKs received for y = 3) {
resend segment with sequence number y
a duplicate ACK for
already ACKed segment
fast retransmit
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23. A 4th situation? Receiver side Maximum buffer size
Available buffer size
What if?
Buffered, out-of-order data
Next byte to be read by application
Next byte expected (ACK value)
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24. TCP Flow Control Sender Receiver Maximum buffer size Sliding window
First unacknowledged byte
Data available, but outside window
Last byte sent
Receiver
Maximum buffer size
Avoid?
Available buffer size
Buffered, out-of-order data
Next byte to be read by application
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Next byte expected (ACK value)

25. Flow Control vs. Congestion Control
Preventing senders from overrunning the capacity of the receivers
Congestion control
Preventing too much data from being injected into the network, causing switches or links to become overloaded
TCP provides both
flow control based on advertised window
congestion control discussed later in class
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26. TCP Flow Control Receiving side Sending side
Receive buffer size = MaxRcvBuffer
LastByteRcvd - LastByteRead < = MaxRcvBuffer
AdvertisedWindow = MaxRcvBuffer - (NextByteExpected NextByteRead)
Shrinks as data arrives and
Grows as the application consumes data
Sending side
Send buffer size = MaxSendBuffer
LastByteSent - LastByteAcked < = AdvertisedWindow
EffectiveWindow = AdvertisedWindow - (LastByteSent LastByteAcked)
EffectiveWindow > 0 to send data
LastByteWritten - LastByteAcked < = MaxSendBuffer
block sender if (LastByteWritten - LastByteAcked) + y > MaxSenderBuffer
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27. TCP Flow Control Problem: Slow receiver application Solution
Advertised window goes to 0
Sender cannot send more data
Non-data packets used to update window
Receiver may not spontaneously generate update or update may be lost
Solution
Sender periodically sends 1-byte segment, ignoring advertised window of 0
Eventually window opens
Sender learns of opening from next ACK of 1-byte segment
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28. TCP Flow Control
Problem: Application delivers tiny pieces of data to TCP
Example: telnet in character mode
Each piece sent as a segment, returned as ACK
Very inefficient
Solution
Delay transmission to accumulate more data
Nagle’s algorithm
Send first piece of data
Accumulate data until first piece ACK’d
Send accumulated data and restart accumulation
Not ideal for some traffic (e.g. mouse motion)
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29. TCP Flow Control Problem: Slow application reads data in tiny pieces
Receiver advertises tiny window
Sender fills tiny window
Known as silly window syndrome
Solution
Advertise window opening only when MSS or 1/2 of buffer is available
Sender delays sending until window is MSS or 1/2 of receiver’s buffer (estimated)
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30. TCP Bit Allocation Limitations
Sequence numbers vs. packet lifetime
Assumed that IP packets live less than 60 seconds
Can we send 232 bytes in 60 seconds?
Less than an STS-12 line
Advertised window vs. delay-bandwidth
Only 16 bits for advertised window
Cross-country RTT = 100 ms
Adequate for only 5.24 Mbps!
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31. TCP Sequence Numbers – 32-bit
Bandwidth
Speed
Time until wrap around T1
1.5 Mbps
6.4 hours
Ethernet
10 Mbps
57 minutes T3
45 Mbps
13 minutes
FDDI
100 Mbps
6 minutes
STS-3
155 Mbps
4 minutes
STS-12
622 Mbps
55 seconds
STS-24
1.2 Gbps
28 seconds
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32. TCP Advertised Window – 16-bit
Bandwidth
Speed
Max RTT T1
1.5 Mbps
350 ms
Ethernet
10 Mbps
52.4 ms T3
45 Mbps
11.6 ms
FDDI
100 Mbps
5.2 ms
STS-3
155 Mbps
3.4 ms
STS-12
622 Mbps
843 µs
STS-24
1.2 Gbps
437 µs
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