A Simulation of Adaptive Packet Size in TCP Congestion Control

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A Simulation of Adaptive Packet Size in TCP
Congestion Control

• Zohreh Jabbari

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Traffic management

• routing, congestion control and traffic
  engineering

Traffic Management
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Traffic Management Today
Operator Traffic Engineering
Routers Routing Protocols
User Congestion Control
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TCP

• Send a packet receive an Acknowledgement (ACK).
• Multiple packets. Congestion Window (CWND) and
  Advertised Window.

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TCP Congestion Control

• Possible events
• On time Acknowledgement.
• Timeout.
• Multiple out of order ACKs.

? No congestion Open cwnd
? Congestion (lost pkt) Slow down!
? Congestion (lost pkt) Slow down!
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TCP Congestion Control

• Congestion control algorithm
• On any timeout,
• set cwnd to half the current window size
  (multiplicative decrease).
• On each ack for new data,
• increase cwnd by 1/cwnd (additive increase).
• When sending,
• send the minimum of the receivers advertised
  window and cwnd.

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TCP Congestion Control

• Severe congestion
• Small cwnd.
• Large RTT.
• Exp backoff
• Starved sources
• TCP doesnt
• work here!

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Solution

• A more flexible algorithm
• Changes packet size.
• Smaller packets when network is congested.
• Larger packets otherwise.

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TCP VP Congestion Control

• Severe congestion
• Adaptive Packet size
• Less congestion
• Large packets
• What is Severe congestion?
• min_cwnd and max_cwnd
• min_pkt_size and max_pkt_size

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The simplified algorithm

• Timeout
• if (cwndltmin_cwnd pkt_sizegtmin_pkt_size)
• Pkt_size/ 2 (Multiplicative Decrease)
• Receiving an in-order ACK
• If (cwndgtmax_cwnd pkt_sizeltmax_pkt_size)
• Increase pkt_size by pkt_size/(cwnd1) (Additive
  Increase)

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Working Environment

• Working with Ns (Ns-2 or Network Simulator).
• Algorithm in C (added to Ns source code)
• A test network.
• Testing scenarios in TCL
• Simulating the scenarios.

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Network Topology

• Only study the left gateway.
• Different Queue Managements. FIFO and RED

Pckts drop here!
Senders
Receivers
100 M
100 M
10 M
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Simulation Scenarios
Scenario Num TCP senders TCP VP senders UDP senders Queuing Policy
1 100 0 0 FIFO
2 0 100 0 FIFO
3 50 50 0 FIFO
4 50 0 10 FIFO
5 0 50 10 FIFO
6 25 25 10 FIFO
7 100 0 0 RED
8 0 100 0 RED
9 50 50 0 RED
10 50 0 10 RED
11 0 50 10 RED
12 25 25 10 RED
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Queue management

• FIFO
• First in First out.
• Last ones are dropped.
• RED (in byte mode)
• Fair sharing!
• Per flow Queuing.
• Considers the size of the packets (byte mode)

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Simulation Analysis1

• 2 scenarios
• 10 UDP sources, 10 TCP sources and 10 TCPVP
  sources
• One with FIFO one with RED
• Sorting the sources
• by the data transferred by each source.
• Comparing median of TCP and TCPVP sources over
  time.

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Results
The same plot when the router is using RED in
its byte mode.
Number of bytes received vs. simulation time
for medians of TCP and TCPVP sources in a network
with 10 TCP 10 UDP and 10 TCPVP senders in FIFO
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Simulation analysis2

• Time independent results
• Long simulations.
• Sort sources by their total data transfer.
• Plot the CDF of the results for TCP and TCP VP.

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Results

• (a) TCP and VP comparison in scenario 3 (50 TCP
  50 TCPVP, FIFO)
• (b) scenario 9 (50 TCP 50 TCPVP, RED)

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Results

• (a) TCP and VP comparison in scenario 6 (25 TCP,
  25 TCPVP, 10 UDP, FIFO)
• (b) and scenario 12. (25 TCP, 25 TCPVP, 10 UDP,
  RED)

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Results

• A comparison between scenario 1 and 2 in (a)
• and between 7 and 8 in (b).
• Scenario 1 and 7 are all TCP scenarios, with FIFO
  and RED respectively. 2 and 8 are all VP
  scenarios with FIFO and RED respectively

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Hypothesis

• Why TCP is better?
• 40 bytes of header on each pckt!
• Decreased throughput for TCP VP.

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Summery
Scenario Description Scenario Description Scenario Description FIFO FIFO RED RED
scenario TCP Sources VP Sources UDP Sources TCP VP TCP VP
3 50 50 0
9 50 50 0
6 25 25 10
12 25 25 10
1 100 0 0
2 0 100 0
7 100 0 0
8 0 100 0
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Conclusion

• When using RED in byte mode
• TCP VP is usually much better than TCP.
• Otherwise
• Smaller packet sizes get penalized
• Better to use TCP.

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Future Work

• Testing the packet header overhead Hypothesis.

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Future Work

• What is happening here?

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Future Work

• Tuning the TCP VP parameters.
• min_pkt_size, min_cwnd max_cwnd
• Minimizing the variance of sent bytes over
  sources.

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Future Work

• A study of TCP VP and TCP when both FIFO and RED
  are present in the network!

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References

• 1 V. Jacobson. Congestion avoidance and
  control. SIGCOMM Comput. Commun. Rev. ,
  18(4)314-329, 1988.
• 2 Sally Floyd and Van Jacobson. Random early
  detection gateways for congestion avoidance.
  IEEE/ACM Transactions on Networking,
  1(4)397-413, 1993.
• 3 Frank Hutter, Holger Hoos, and Thomas
  Stützle. Automatic Algorithm Configuration based
  on Local Search. AAAI, 2007.
• 4 NS-2 network simulator webpage
  http//www.isi.edu/nsnam/ns/.
• 5 W. Stevens. TCP Slow Start, Congestion
  Avoidance, Fast Retransmit, and Fast Recovery
  Algorithms, January 1997, RFC2001
• 6 M. Allman, V. Paxson, and W. Stevens. TCP
  Congestion Control, April 1999, RFC 2581.
• 7 The official Nsnam Wiki http//nsnam.isi.edu/
  nsnam/index.php/Main_Page
• 8 R. Jain, K.K. Ramakrishnan, and D. Chiu.
  Congestion avoidance in computer networks with a
  connectionless network layer. Technical Report
  DEC-TR-506, Digital Equipment Corporation, 1987.

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Question?
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TCP ACK generation RFC 1122, RFC 2581
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
Event at Receiver Arrival of in-order segment
with expected seq . All data up to expected seq
already ACKed Arrival of in-order segment
with 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
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TCP RTT
EstimatedRTT (1- ?)EstimatedRTT ?SampleRTT
DevRTT (1-?)DevRTT
?SampleRTT-EstimatedRTT (typically, ? 0.25)
TimeoutInterval EstimatedRTT 4DevRTT
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• Question
• If TCP frequently timeouts even if it sends only
  1 segment per round, does it imply anything, and
  how does TCP handle it?

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Exponential back-off

• When?
• Consecutive timeouts, possibly due to severe
  congestion
• How? On each timeout
• Retransmit the smallest sequence number not yet
  acknowledged
• Double the timer
• End of exponential back-off
• If ACK is received

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• Question
• What is fast retransmit and how does it work?

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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

• Time-out period often relatively long
• long delay before resending lost packet
• 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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Fast retransmit algorithm
event ACK received, with ACK field value of y
if (y gt 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