Download Chapter3 Transport Layer4

Transport Layer
CS 381
3/7/2017
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 3-2
Principles of Congestion Control
Congestion:
• 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
3-3
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
Host A
Host B
lin : original data
lout
unlimited shared output
link buffers
Transport Layer
3-4
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
3-5
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
3-6
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
lin : original data
lout
l'in : original data, plus
retransmitted data
Host B
finite shared output
link buffers
Transport Layer
3-7
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
R/2
• Realistic scenario?
•
NO!
lout
•
• Why?
lin
R/2
Transport Layer
3-8
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
lout
• Second cost of a congested network:
• Sender must retransmit to compensate for lost packets due to
router buffer overflow R/2
R/3
lin
R/2
Transport Layer
3-9
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 3-10
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
lin : original data
lout
Host B
l'in : original data, plus
retransmitted data
finite shared
output link
buffers
Host D
Host C
Transport Layer
3-11
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 3-12
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 3-13
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 3-14
Approaches towards congestion control
Two broad approaches
Transport Layer 3-15
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 3-16
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 3-17
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 3-18
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 3-19
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 3-20
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 3-21
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
sending rate
•
X
X loss, so decrease rate
X
X
TCP’s
“sawtooth”
behavior
X
time
• Q: How fast to increase/decrease?
• details to follow
Transport Layer 3-22
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:
Sending rate =
cwnd
RTT
bytes/sec
• Cwnd is dynamic, function of perceived network
congestion
Transport Layer 3-23
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 3-24
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 3-25
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 3-26
TCP Slow Start
• When connection begins, cwnd = 1
Host A
Host B
RTT
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
time
Transport Layer 3-27
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 3-28
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 3-29
TCP congestion control FSM: overview
cwnd > ssthresh
slow
start
loss:
timeout
congestion
avoidance
loss:
timeout
loss:
timeout
loss:
3dupACK
new ACK
loss:
3dupACK
fast
recovery
Transport Layer 3-30
TCP: Fast Recovery
• TCP Tahoe: Earlier version of TCP
•
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:
•
Reaction to loss events
• Timeout: Same as TCP Tahoe
• 3 dup ACK’s: set cwnd = cwnd/2 = ssthresh and enter
congestion avoidance
3-31
cwnd window size (in segments)
Versions of TCP
TCP Reno
ssthresh
ssthresh
TCP Tahoe
Transmission round
Transport Layer 3-32
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 3-33
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 3-34
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 3-35
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 3-36