eec-484/584 computer networks
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EEC-484/584 Computer Networks. Lecture 16 Wenbing Zhao [email protected] (Part of the slides are based on Drs. Kurose & Ross ’ s slides for their Computer Networking book, and on materials supplied by Dr. Louise Moser at UCSB and Prentice-Hall). Outline. TCP Connection management - PowerPoint PPT PresentationTRANSCRIPT
EEC-484/584EEC-484/584Computer NetworksComputer Networks
Lecture 16Lecture 16
Wenbing ZhaoWenbing Zhao
[email protected]@ieee.org(Part of the slides are based on Drs. Kurose & Ross’s slides (Part of the slides are based on Drs. Kurose & Ross’s slides
for their for their Computer Networking Computer Networking book, and book, and on materials supplied by on materials supplied by Dr. Louise Moser at UCSB and Prentice-Hall)Dr. Louise Moser at UCSB and Prentice-Hall)
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OutlineOutline
• TCP– Connection management– Reliable data transfer– Flow control– TCP transmission policy– Congestion control
• Reminder: Quiz 4– MW session: 11/27 Monday 2-4pm– TTh session: 11/28 Tuesday 4-6pm
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TCP Connection ManagementTCP Connection Management
TCP sender, receiver establish “connection” before exchanging data segments
• Initialize TCP variables:– Sequence numbers– Buffers, flow control info (e.g. RcvWindow)
• Client: connection initiator Socket clientSocket = new
Socket("hostname","port number"); • Server: contacted by client Socket connectionSocket = welcomeSocket.accept();
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TCP Connection ManagementTCP Connection Management
Three way handshake:
Step 1: client host sends TCP SYN segment to server
– specifies initial sequence number
– no data
Step 2: server host receives SYN, replies with SYN/ACK segment
– server allocates buffers
– specifies server initial sequence number
Step 3: client receives SYN/ACK, replies with ACK segment, which may contain data
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TCP Connection ManagementTCP Connection Management
Three way handshake:• SYN segment is considered
as 1 byte• SYN/ACK segment is also
considered as 1 byte
client
SYN (seq=x)
server
SYN/ACK (seq=y, ACK=x+1)
ACK (seq=x+1, ACK=y+1)
connect accept
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TCP Connection ManagementTCP Connection Management
Closing a connection:
client closes socket: clientSocket.close();
Step 1: client end system sends TCP FIN control segment to
server
Step 2: server receives FIN, replies with ACK. Closes connection, sends FIN.
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
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TCP Connection ManagementTCP Connection Management
Step 3: client receives FIN, replies with ACK.
– Enters “timed wait” - will respond with ACK to received FINs
Step 4: server, receives ACK. Connection closed.
Note: with small modification, can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
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TCP Reliable Data TransferTCP Reliable Data Transfer• TCP creates rdt
service on top of IP’s unreliable service
• Pipelined segments• Cumulative acks• TCP uses single
retransmission timer
• Retransmissions are triggered by:– timeout events– duplicate acks
• Initially consider simplified TCP sender:– ignore duplicate acks– ignore flow control,
congestion control
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TCP Sender Events:TCP Sender Events:Data rcvd from app:• Create segment with
sequence number• seq # is byte-stream
number of first data byte in segment
• start timer if not already running (think of timer as for oldest unacked segment)
• expiration interval: TimeOutInterval
Timeout:• retransmit segment that
caused timeout• restart timer
Ack rcvd:• If acknowledges
previously unacked segments– update what is known to
be acked– start timer if there are
outstanding segment
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TCP: Retransmission ScenariosTCP: Retransmission ScenariosHost A
Seq=100, 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92, 8 bytes data
ACK=120
Seq=92, 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92, 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92, 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
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TCP Retransmission ScenariosTCP Retransmission Scenarios
Host A
Seq=92, 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100, 20 bytes data
ACK=120
time
SendBase= 120
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TCP ACK GenerationTCP ACK Generation
Event at Receiver
Arrival of in-order segment withexpected seq #. All data up toexpected seq # already ACKed
Arrival of in-order segment withexpected seq #. One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq. # .Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK. Wait up to 500msfor 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 thatsegment starts at lower end of gap
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TCP Flow ControlTCP Flow Control• Receive side of TCP
connection has a receive buffer:
• Speed-matching service: matching the send rate to the receiving app’s drain rate
• App process may be slow at reading from buffer
Flow control:sender won’t overflow
receiver’s buffer bytransmitting too much,
too fast
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TCP Flow ControlTCP Flow Control
(Suppose TCP receiver discards out-of-order segments)
Spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
• Rcvr advertises spare room by including value of RcvWindow in segments
• Sender limits unACKed data to RcvWindow– guarantees receive
buffer doesn’t overflow
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TCP Transmission PolicyTCP Transmission Policy
• Window management not directly tied to ACKs– The sender can send new segments only if the
receiver has room to receive them
• What if the receiver’s window drops to 0 ?– Sender may not normally send segments with two
exceptions– Exception 1: urgent data may be sent, e.g., to allow
user to kill process running on the remote machine– Exception 2: sender may send a 1-byte segment to
make the receiver re-announce the next byte expected and window size
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TCP Transmission PolicyTCP Transmission Policy
• Window management not directly tied to ACKs
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TCP Transmission PolicyTCP Transmission Policy
• Nagle’s algorithm– To address the 1-byte-at-a-time sender
problem
• Clark’s algorithm– To address the 1-byte-at-a-time receiver
problem
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1-byte-at-a-time Sender Problem1-byte-at-a-time Sender Problem• Sender sends 1 byte (e.g., typed one character in an editor)• A segment of 1 byte is sent to the remote machine (41-byte
IP packet)• Remote machine acks immediately (40-byte IP packet)• Editor (in remote machine) program reads the received 1
byte, a windows update segment is sent to user (40-byte IP packet)
• Editor program echoes the 1 byte received to the user terminal (41-byte IP packet)
• In all, 162 bytes of bandwidth used, 4 segments are sent for each character typed
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Nagle’s AlgorithmNagle’s Algorithm• When sender application passes data to TCP one
byte at a time– Send first byte – Buffer the rest until first byte ACKed– Then send all buffered bytes in one TCP segment– Start buffering again until all ACKed
• Implemented widely in TCP, can be disabled/enabled by using socket options
• For some application, it is necessary to disable the Nagle’s algorithm, e.g., X Windows program over Internet, to avoid erratic mouse movement, etc.
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Silly Window SyndromeSilly Window Syndrome• When receiver application accepts data from TCP 1 byte at
a time
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Clark’s AlgorithmClark’s Algorithm
• Receiver should not send window update until – It can handle max segment size it advertised when
connection established, or, – Its buffer is half empty, whichever is smaller
• Sender should wait until – It has accumulated enough space in window to send full
segment, or, – One containing at least half of receiver’s buffer size
• Nagle’s algorithm and Clark’s algorithm are complementary
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Principles of Congestion ControlPrinciples of Congestion Control
Congestion:• Informally: “too many sources sending too much
data too fast for network to handle”• Different from flow control!• Manifestations:
– lost packets (buffer overflow at routers)– long delays (queueing in router buffers)
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Approaches towards Congestion ControlApproaches towards Congestion Control
End-end congestion control:
• no explicit feedback from network
• congestion inferred from end-system observed loss, delay
• approach taken by TCP
Network-assisted congestion control:
• routers provide feedback to end systems– single bit indicating
congestion (SNA, DECbit, TCP/IP ECN, ATM)
– explicit rate sender should send at
Two broad approaches towards congestion control
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TCP Congestion Control: TCP Congestion Control: Additive Increase, Multiplicative DecreaseAdditive Increase, Multiplicative Decrease
• Approach: increase transmission rate (window size), probing for usable bandwidth, until loss occurs– Additive increase: increase cwnd by 1 MSS every RTT until loss detected– Multiplicative decrease: cut cwnd in half after loss
Saw toothbehavior: probing
for bandwidth
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TCP Congestion ControlTCP Congestion Control
• Sender limits transmission: LastByteSent-LastByteAcked cwnd
• Roughly,
• cwnd is dynamic, function of perceived network congestion
How does sender perceive congestion?
• loss event = timeout or 3 duplicate acks
• TCP sender reduces rate (cwnd) after loss event
three mechanisms:– AIMD– slow start– conservative after
timeout events
rate = cwnd
RTT Bytes/sec
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TCP Slow StartTCP Slow Start
• When connection begins, cwnd = 1 MSS– Example: MSS = 500 bytes
& RTT = 200 msec– Initial rate = 20 kbps
• Available bandwidth may be >> MSS/RTT– Desirable to quickly ramp
up to respectable rate
• When connection begins, increase rate exponentially fast until first loss event
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TCP Slow StartTCP Slow Start• When connection
begins, increase rate exponentially until first loss event:– Double cwnd every RTT– Done by incrementing cwnd for every ACK received
• Summary: initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
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Congestion AvoidanceCongestion AvoidanceQ: When should the
exponential increase switch to linear?
A: When cwnd gets to 1/2 of its value before timeout
Implementation:• Variable Threshold • At loss event, Threshold is
set to 1/2 of cwnd just before loss event
How to increase cwnd linearly:cwnd (new) = cwnd + mss*mss/cwnd
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Congestion ControlCongestion Control
• After 3 duplicated ACKs:– cwnd is cut in half– window then grows linearly
• But after timeout event:– cwnd instead set to 1 MSS– window then grows
exponentially– to a threshold, then grows
linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a “more alarming” congestion scenario
Philosophy:
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Summary: TCP Congestion ControlSummary: TCP Congestion Control
• When cwnd is below Threshold, sender in slow-start phase, window grows exponentially
• When cwnd is above Threshold, sender is in congestion-avoidance phase, window grows linearly
• When a triple duplicate ACK occurs, Threshold set to cwnd/2 and cwnd set to Threshold
• When timeout occurs, Threshold set to cwnd/2 and cwnd is set to 1 MSS
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TCP Sender Congestion ControlTCP Sender Congestion Control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS,
If (CongWin > Threshold) set state to “Congestion Avoidance”
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+ MSS * (MSS/CongWin)
Additive increase, resulting in increase of CongWin by 1 MSS every RTT
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TCP Sender Congestion ControlTCP Sender Congestion Control
State Event TCP Sender Action Commentary
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin/2, CongWin = Threshold,Set state to “Congestion Avoidance”
Fast recovery, implementing multiplicative decrease. CongWin will not drop below 1 MSS.
SS or CA Timeout Threshold = CongWin/2, CongWin = 1 MSS,Set state to “Slow Start”
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
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TCP Congestion ControlTCP Congestion Control
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ExerciseExercise
• Suppose that the TCP congestion window is set to 18 KB and a timeout occurs. How big will the window be if the next four transmission bursts are all successful? Assume that the maximum segment size is 1 KB.
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ExerciseExerciseSegment#
Action VariablesSend Receive Comment cwnd ssthresh
Timeout 18KB -
Retransmit 1024 9216
1 1:1025(1024) 1024 9216
2 ACK 1025 Slow start 2048 9216
3 1025:2049(1024) 2048 9216
4 2049:3073(1024) 2048 9216
5 ACK 2049 Slow start 3072 9216
6 ACK 3073 Slow start 4096 9216
7 3073:4097(1024) 4096 9216
8 4097:5121(1024) 4096 9216
9 5121:6145(1024) 4096 9216
10 6145:7169(1024) 4096 9216
11 ACK 4097 Slow start 5120 9216
12 ACK 5121 Slow start 6144 9216
13 ACK 6145 Slow start 7168 9216
14 ACK 7169 Slow start 8192 9216
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Exercise Problem #3Exercise Problem #315 7169:8193(1024) 8192 9216
16 8193:9217(1024) 8192 9216
17 9217:10241(1024) 8192 9216
18 10241:11265(1024) 8192 9216
19 11265:13313(1024) 8192 9216
20 12289:14337(1024) 8192 9216
21 14337:15361(1024) 8192 9216
22 15361:16385(1024) 8192 9216
23 ACK 8193 Slow start 9216 9216
24 ACK 9217 Slow start 10240 9216
25 ACK 10241Cong. Avoid.New cwnd =
cwnd + mss*mss/cwnd10342 9216
26 ACK 11265 Cong. Avoid. 10443 9216
27 ACK 13313 Cong. Avoid. 10543 9216
28 ACK 14337 Cong. Avoid. 10642 9216
29 ACK 15361 Cong. Avoid. 10740 9216
30 ACK 16385 Cong. Avoid. 10837 9216