1. Transport Layer
CS 381
3/7/2017

2. Chapter 3 outline 3.1 transport-layer services
3.2 multiplexing and demultiplexing
3.3 connectionless transport: UDP
3.4 principles of reliable data transfer
3.5 connection-oriented transport: TCP
segment structure
reliable data transfer
flow control
connection management
3.6 principles of congestion control
3.7 TCP congestion control
Transport Layer

3. Principles of Congestion Control
Informally: “too many sources sending too much data too fast for network to handle”
Different from flow control!
Flow Control: Sender decreases packet transmission to accommodate receiver.
Manifestations:
Lost packets (buffer overflow at routers)
Long delays (queueing in router buffers)
a top-10 problem!
Transport Layer

4. Causes/costs of congestion: scenario 1
Two senders, two receivers
One router, infinite buffers
No errors/retransmissions
No flow control
No congestion control
Both hosts transmitting continuously at the same time
unlimited shared output link buffers
Host A
lin : original data
Host B
lout
Transport Layer

5. Causes/costs of congestion: scenario 1
Maximum throughput for two senders: R/2
Link can’t deliver packets to receiver at a rate that exceeds R/2
As the sending rate approaches R/2, the average package delay increases (as the number of buffered packets increase).
When the sending rate exceeds R/2, packet delay becomes infinite.
Transport Layer

6. Causes/costs of congestion: scenario 1
First cost of a congested network:
Large queuing delays are experienced as the packet arrival rate nears the link capacity
Why?
Processing packet header takes time!
Transport Layer

7. Causes/costs of congestion: scenario 2
Two senders, two receivers
one router, finite buffers
Packets dropped when arriving to router with full buffer
sender retransmission of lost packet
Host A
lout
lin : original data
l'in : original data, plus retransmitted data
Host B
finite shared output link buffers
Transport Layer

8. Causes/costs of congestion: scenario 2
Case 1: Hosts only transmits data when it knows buffer space in the router is free.
No packet loss
Throughput up to R/2
Sending rate cannot exceed R/2 since packet loss is assumed never to occur
Realistic scenario?
NO!
Why?
R/2
lin
lout
Transport Layer

9. Causes/costs of congestion: scenario 2
Case 2: Sender retransmits only when a packet is know for certain to be lost.
How does the sender know for sure a packet is lost?
Performance is decreased as the sender resends lost packets
Assume for each 3 packets transmitted, 1 packet is duplicated
33% performance decrease due to packet retransmissions in this case
Second cost of a congested network:
Sender must retransmit to compensate for lost packets due to router buffer overflow
R/2
lin
lout
R/3
Transport Layer

10. Causes/costs of congestion: scenario 2
Case 3: Sender times out prematurely, retransmits a dup packet that has been delayed in the router buffer, but not lost
Destination receives duplicate data
Work done by router to forward duplicate data is wasted
Assume each packet has to be forwarded twice by the router
Due to large queueing delays
50% performance decrease due to packet retransmissions in this case
Third cost of a congested network:
Duplicate transmissions from sender due to large delays may cause routers to forward unneeded copies of data
Transport Layer

11. Causes/costs of congestion: scenario 3
Four senders, four receivers
Four routers, finite buffers
Packets dropped when arriving to router with full buffer
Sender retransmission of lost packet
Host A
lout
lin : original data
Host B
l'in : original data, plus retransmitted data
finite shared output link buffers
Host D
Host C
Transport Layer

12. Causes/costs of congestion: scenario 3
Fourth cost of a congested network:
When a packet is dropped along a path, any “upstream” transmission capacity used for that packet is wasted!
Transport Layer

13. Approaches towards congestion control
Two broad approaches
Network-assisted congestion control:
Routers provide feedback to end systems about current buffer state.
Two ways:
“Choke packet”: Examine sender packet header to obtain IP, notify about state of buffers.
Router can update a field in the sender’s packet header that indicates state of buffers. Note: This can take up to 1 RTT.
Complexity? Router explicit communication with end host processes
Flooding, loss of choke packets, etc.
Transport Layer

14. Approaches towards congestion control
two broad approaches
End-end congestion control:
No explicit feedback from network core
Congestion of network “inferred” by the end systems based on packet loss and delay
This is the approach taken by TCP
Segment loss (3 dup ACK, timeout) indicates congestion
TCP adjusts sender’s transmission rate based on congestion
Transport Layer

15. Approaches towards congestion control
Two broad approaches
Transport Layer

16. Chapter 3 outline 3.1 transport-layer services
3.2 multiplexing and demultiplexing
3.3 connectionless transport: UDP
3.4 principles of reliable data transfer
3.5 connection-oriented transport: TCP
segment structure
reliable data transfer
flow control
connection management
3.6 principles of congestion control
3.7 TCP congestion control
Transport Layer

17. TCP congestion control:
Goal:
TCP sender should transmit as fast as possible, but without congesting the network
Q: how to find rate just below congestion level?
Decentralized approach: each TCP sender sets its own rate, based on implicit feedback
ACK: segment received (a good thing!), network not congested
increase sending rate
Lost segment: assume loss due to congested network
decrease sending rate
Transport Layer

18. TCP congestion control:
TCP sender rate is dynamic based on current status of the network
Questions:
How does a TCP sender limit transmission rate?
How does a TCP sender perceive network congestion?
How does TCP react to network congestion?
Transport Layer

19. TCP congestion control:
How does a TCP sender limit transmission rate?
Ignoring flow control, a TCP sender keeps track of a variable, congestion window, or cwnd.
By adhering to the constraint:
LastByteSent – LastByteAcked <= min{cwnd, rwnd}
The TCP sender modifies the value of cwnd to adjust the rate at which it sends data into the network.
Sending rate: ~cwnd/RTT bps
Transport Layer

20. TCP congestion control:
How does a TCP sender perceive network congestion?
Excessive congestion in the network causes packets to be dropped or delayed, which creates a “loss” event at the TCP sender
TCP considers a loss event as:
Timeout for receiving ACK
Receiving 3 dup ACKs
The TCP sender perceives the above loss events as network congestion and adjusts its transmission rate to accommodate
Transport Layer

21. TCP congestion control:
How does a TCP sender react to network congestion?
A lost packet implies congestion
TCP sender rate should be decreased
Successfully ACK’d packets implies that the network is delivering packets to the destination
TCP sender rate should be increased
Transport Layer

22. TCP congestion control: bandwidth probing
“Probing for bandwidth”: increase transmission rate on receipt of ACK, until eventually loss occurs, then decrease transmission rate
Continue to increase on ACK, decrease on loss
Since available bandwidth is changing, depending on other connections in network
ACKs being received,
so increase rate X
loss, so decrease rate X X X
TCP’s
“sawtooth”
behavior
sending rate X
time
Q: How fast to increase/decrease?
details to follow
Transport Layer

23. TCP Congestion Control: details
Sender limits rate by limiting number of unACKed bytes “in pipeline”:
Cwnd: differs from rwnd (how, why?)
Cwnd set by TCP sender (congestion control window)
Rwnd set by TCP receiver (used for flow control)
Sender limited by min(cwnd,rwnd)
Roughly:
Cwnd is dynamic, function of perceived network congestion
Sending rate =
cwnd
RTT
bytes/sec
Transport Layer

24. TCP Congestion Control: more details
Segment loss event: reducing cwnd
Timeout: no response from receiver
Set cwnd to 1MSS
Aggressive response to congestion
Why reduce cwnd to 1?
3 duplicate ACKs: at least some segments getting through (recall fast retransmit)
Set cwnd in half, not as aggressive as timeout event
Why?
Transport Layer

25. TCP Congestion Control: more details
ACK received: increase cwnd
Slowstart phase:
Increase exponentially fast (despite the name) at connection start, or following timeout
Doubles cwnd for each successful ACK, until cwnd reaches predetermined threshold
Congestion avoidance:
Cwnd increase linearly
Generally happens after slowstart phase reaches threshold
Transport Layer

26. TCP Congestion Control: MSS
MSS: Maximum Segment Size
TCP MSS is calculated by Link Layer frame MTU (maximum transmission unit) – IP header – TCP header.
Ethernet typical MTU size: 1500 bytes
So, TCP typical MSS size over Ethernet:
1500 – 20 – 20 = 1460 bytes
Transport Layer

27. TCP Slow Start
When connection begins, cwnd = 1 MSS (maximum segment size)
Ex: MSS = 500 bytes & RTT = 200 msec
Initial rate = 20 kbps
Available bandwidth may be >> MSS/RTT
Desirable to quickly ramp up to respectable rate
Increase rate exponentially until first loss event or when threshold reached
Double cwnd every RTT
Done by increasing cwnd for every ACK received
Host A
Host B
one segment
RTT
two segments
four segments
time
Transport Layer

28. TCP: congestion avoidance
Due to previous loss events, the TCP sender will probe for bandwidth less aggressively
ssthresh: cwnd/2 when loss occurs
When cwnd > ssthresh grow cwnd linearly
Increase cwnd by 1 MSS (1460 bytes) per RTT
Approach possible network congestion slower than in slowstart
Transport Layer

29. TCP: congestion avoidance
AIMD: Additive Increase Multiplicative Decrease
ACKs:
Increase cwnd by 1 MSS per RTT: additive increase
loss:
Set cwnd to half (non-timeout-detected loss ): multiplicative decrease
Remember:
The TCP sender responds to all timeout events by reducing cwnd to 1.
Transport Layer

30. TCP congestion control FSM: overview
slow
start
congestion
avoidance
cwnd > ssthresh
loss:
timeout
loss:
timeout
loss:
timeout
new ACK
loss:
3dupACK
fast
recovery
loss:
3dupACK
Transport Layer

31. TCP: Fast Recovery TCP Tahoe: Earlier version of TCP TCP Reno:
Reaction to loss events
Timeout: set ssthresh = cwnd/2, then cwnd to 1 and enter slowstart until cwnd >= ssthresh. Then enter congestion avoidance
3 dup ACK’s: set ssthresh = cwnd/2, then cwnd to 1 and enter slowstart until cwnd >= ssthresh. Then enter congestion avoidance
TCP Reno:
Timeout: Same as TCP Tahoe
3 dup ACK’s: set cwnd = cwnd/2 = ssthresh and enter congestion avoidance

32. Versions of TCP TCP Reno cwnd window size (in segments) ssthresh
TCP Tahoe
Transmission round
Transport Layer

33. Summary: TCP Congestion Control
When cwnd < ssthresh, the TCP sender is in slow- start phase, window grows exponentially.
When cwnd >= ssthresh, the TCP sender is in congestion-avoidance phase, window grows linearly.
When triple duplicate ACK occurs, ssthresh set to cwnd/2, cwnd set to ~ ssthresh
When timeout occurs, ssthresh set to cwnd/2, cwnd set to 1 MSS.
Transport Layer

34. Transport layer Protocol Fairness
Fairness and UDP
Multimedia apps often do not use TCP
Does not want rate throttled by congestion control
No flow control
Instead use UDP:
Pump audio/video at constant rate
Tolerates packet loss
Current research interests for developing congestion-control mechanisms for the Internet that prevent UDP traffic from unfairly transmitting data
Transport Layer

35. Transport layer Protocol Fairness
Fairness and parallel TCP connections
Nothing prevents app from opening parallel connections between 2 hosts.
Web browsers do this
Example: link of rate R supporting 9 connections;
New app asks for 1 TCP connection, gets rate R/10
New app instead asks for 11 TCP connections, gets R/2 !
Transport Layer

36. Chapter 3: Summary Principles behind transport layer services:
Multiplexing, demultiplexing
Reliable data transfer
Flow control
Congestion control
Instantiation and implementation in the Internet
UDP
TCP
Next:
Leaving the network “edge” (application, transport layers)
Into the network “core”
Transport Layer