1 Transport Layer Flow control Connection management TCP, UDP

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Transport Layer

Flow controlConnection management

TCP, UDP

Copyright Jorg Liebeherr 98, Modified with permission, Abdelzaher

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Announcements I hope you had a very happy

Thanksgiving break! My sincere apologies for the

Monday emergency cancellation!! SM5 is out. Due in one week.


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IntroductionTransport layer protocols are end-to-end protocols Transport layer is only implemented at the hosts

Application

Transport

Network

HOST

Data Link Data Link Data Link

Network

Application

Transport

Network

HOST

Data Link


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Functions of the Transport Layer

Reliability Connection Establishment Connection Termination Flow control Congestion control


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Reliability How to achieve it?


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Send data in chunks (called packets) Number the packets sequentially at the

sender Make sure all numbers are received without

gaps


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Send data in chunks (called packets) Number the packets sequentially at the sender Make sure all numbers are received without gaps

Acknowledge each packet number If sender sees an ack gap sender retransmits


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Send data in chunks (called packets) Number the packets sequentially at the sender Make sure all numbers are received without

gaps Acknowledge each packet number If sender sees an ack gap sender retransmits

Optimization: Cumulative acks (ACK N: acknowledge that all packets up to but not including packet N have been received).


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Send data in chunks (called packets) Number the packets sequentially at the sender Make sure all numbers are received without

gaps Acknowledge each packet number If sender sees an ack gap sender retransmits

Optimization: Cumulative acks (ACK N: acknowledge that all packets up to but not including packet N have been received).

What about the number of the first packet?


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Connection Establishment

Connection establishment is asymmetric: one side puts itself in a LISTEN state

(server) one side issues a request for connection

(client)


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Simple Solution (which has problems)

CR (SeqNo = x) Connection Request, A wants to start with SeqNo = x

ACK (SeqNo = y) Acknowledge request, B will wants to start with SeqNo = y

DATA (SeqNo = x)Data transmission with SeqNo x

Host A Host BHost A Host B

Two Way Handshake


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Problems with Two-Way Handshake

B responds to CR(SeqNo = z), an old duplicate connection requests from A

In the shown sce-nario, A believes that the ACK is for the connection request CR(SeqNo = y)


Result: A starts to send data with Sequence x. B will throw the data away since it expects SeqNo = z


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Three-Way Handshake

Note: A and B acknowledge the sequence number from the other side

This solution provides protection from old duplicate connection requests



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Scenario 1

Host A rejects the invalid connection request in the REJ(ACK=y) packet

Note: The connection request CR(SeqNo=x) is completed successfully


Duplicate connection request (CR) appears


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Scenario 2

Host A rejects the invalid ACK by sending REJ(ACK=y)

Note: The connection request CR(SeqNo=x) is completed successfully


A duplicate acknowledgement (ACK) appears


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Connection Termination A connection release should involve both sides

of the connection (otherwise data is lost)


Here: B should wait after Disconnection Request (DR) is sent until all data has arrived


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Connection Termination in 4 steps

An elegant way to terminate connections is to have each end shut down independently (“half-close”)

If one end wants to shut down, it sends a DR message

Four steps involved:(1) A sends a DR to B (active close)(2) B ACKs the DR,

(at this time: B can still send data to A)

(3) and B sends a DR to A (passive close) (4) A ACKs the DR


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Connection Termination in 4 steps


To account for packet losses, a timer is needed to limit the waiting time of a side


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TCP Segment Format

IP header TCP header TCP data

Sequence number (32 bits)

DATA

20 bytes 20 bytes

0 15 16 31

Source Port Number Destination Port Number

Acknowledgement number (32 bits)

window sizeheaderlength

0 Flags

Options (if any)

TCP checksum urgent pointer

20 bytes• TCP segments have a 20 byte header with >= 0 bytes of data.


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Flow Control Notion of window-based flow control:

Have a window of packets ready for transmission

Can send only those packets that are in the window up to the end of the window

Every time an acknowledgement is received, window slides (and its size may be increased).


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Flow Control Flow Control is a technique for speed-matching

of transmitter and receiver. Flow control ensures that a transmitting station does not overflow a receiving station with data

We will discuss two protocols for flow control: Stop-and-Wait On-Off Sliding Window

For the time being, we assume that we have a perfect channel (no errors)


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Stop-and-Wait Flow Control Simplest form of flow control

In Stop-and-Wait flow control, the receiver indicates its readiness to receive data for each frame

Operations:

1. Sender: Transmit a single frame

2. Receiver: Transmit acknowledgment (ACK)

3. Goto 1.


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Analysis of Stop-and-Wait

Frame ACK

Frame ACK

Receiver

Sender

propagationdelay

Last bit received End of Transmit ofACK

Start Transmit End Transmit Last bit of ACK received

transmissiondelay

Efficiency = ?


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Sliding Window Flow Control

Sliding Window Flow Control Allows transmission of multiple frames Assigns each frame a k-bit sequence

number Range of sequence number is [0..2k-1],

i.e., frames are counted modulo 2k


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Operation of Sliding Window

Sending Window: At any instant, the sender is permitted to

send frames with sequence numbers in a certain range (the sending window)

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6

Frames alreadytransmitted

Window of frames that maybe transmitted

Framesequence

number

Last frametransmitted

Window shrinks asframes are sent

Window expands asacknowledgements are

received


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Operation of Sliding Window

Receiving Window: The receiver maintains a receiving

window corresponding to the sequence numbers of frames that are accepted

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6

Frames already receivedWindow of frames that are

accepted by receiver

Last frameacknowledged

Window shrinks asframes are received

Window expands asacknowledgements are

sent


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Operation of Sliding Window How is “flow control” achieved?

Receiver can control the size of the sending window

By limiting the size of the sending window data flow from sender to receiver can be limited

Interpretation of ACK N message: Receiver acknowledges all packets until (but

not including) sequence number N


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ExampleReceiverTransmitter

0 1 2 3 4 5 6 7 0 1 2 3 0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3 0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3

F0F1F2

ACK43


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Example ContinuedReceiverTransmitter

0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 3

0 1 2 3 4 5 6 7 0 1 2 30 1 2 3 4 5 6 7 0 1 2 3

F3

ACK3F4F5F6

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Slow Start Whenever starting traffic on a new

connection, or whenever increasing traffic after congestion was experienced:

Set cwnd = MSS bytes (=1 segment) Each time an ACK is received, slide the congestion

window and increase size by 1 segment (= MSS bytes). Does Slow Start increment slowly? Not

really. In fact, the increase of cwnd can be exponential


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Slow Start Example The congestion

window size grows very rapidly For every ACK,

we increase cwnd by 1 irrespective of the number of segments ACK’ed

TCP slows down the increase of cwnd when cwnd > ssthresh

cwnd =1xMSS

cwnd =2xMSS

cwnd =4xMSS

cwnd =7xMSS


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Slow Start “Slow Start” slows down if the congestion

window is larger than a threshold value If cwnd > ssthresh then each time an ACK

is received, increment cwnd as follows: cwnd = cwnd + MSS * MSS / cwnd

So cwnd is increased by one only if all segments have been acknowledged.

ssthresh is modified if there is congestion in the network


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Slow Start Example

Assume that ssthresh = 8

cwnd = 1

cwnd = 2

cwnd = 4

cwnd = 8

cwnd = 9

cwnd = 10

0

2

4

6

8

10

12

14

t=0

t=2

t=4

t=6

Roundtrip times

Cw

nd

(in

seg

men

ts)

ssthresh


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Slow Start / Congestion Avoidance Here we give a more accurate version

than in our earlier discussion of Slow Start: If cwnd <= ssthresh then

Each time an Ack is received:cwnd = cwnd + MSS

else /* cwnd > ssthresh */Each time an Ack is received :cwnd = cwnd + MSS * MSS / cwnd

endif


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Slow Start / Congestion Avoidance Each time when congestion occurs

(timeout or receipt of duplicate ACK), cwnd is reset to one:

cwnd = 1 ssthresh is set to half the current size

of the congestion window: ssthressh = cwnd / 2


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Slow Start / Congestion Avoidance

A typical plot of cwnd for a TCP connection (MSS = 1500 bytes) :


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Round-Trip Time Measurements

The retransmission mechanism of TCP is adaptive The retransmission timers are set based on round-trip

time (RTT) measurements that TCP performs

RTT #1

RTT #2

RTT #3

The RTT is based on time difference between segment transmission and receipt of ACKBut:

TCP does not ACK each segmentEach connection has only one timer


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Round-Trip Time Measurements

Retransmission timer is set to a Retransmission Timeout (RTO) value

RTO is calculated based on the RTT measurements The RTT measurements are smoothed by the following

estimators srtt and rttvar:srttn+1 = RTT + (1- ) srttn

rttvarn+1 = ( | RTT - srttn+1 | ) + (1- ) rttvarn

RTOn+1 = srttn+1 + 4 rttvarn+1

The gains are set to =1/4 and =1/8 srtt0 = 0 sec, rttvar0 = 3 sec, Also: RTO0 = srtt0 + 2 rttvar0


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TCP header fields Port Number:

A port number identifies the endpoint of a connection. A pair <IP address, port number> identifies one endpoint of

a connection. Two pairs <client IP address, server port number> and <server IP address, server port number> identify a TCP connection.

TCP

IP

Applications

23 10480Ports:

TCP

IP

Applications

7 1680 Ports:


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TCP header fields Sequence Number (SeqNo):

Sequence number is 32 bits long. So the range of SeqNo is

0 <= SeqNo <= 232 -1 4.3 Gbyte

Each sequence number identifies a byte in the byte stream

Initial Sequence Number (ISN) of a connection is set during connection establishment


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TCP header fields Acknowledgement Number (AckNo):

Acknowledgements are piggybacked A hosts uses the AckNo field to send

acknowledgements. (If a host sends an AckNo in a segment it sets the “ACK flag”)

The AckNo contains the next SeqNo that a hosts wants to receiveExample: The acknowledgement for a segment with sequence numbers 0-1500 is AckNo=1501


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TCP header fields Flag bits:

URG: Urgent pointer is valid If the bit is set, the following bytes contain an urgent

message in the sequence number range “SeqNo <= urgent message <= SeqNo+urgent pointer”

ACK: Segment carries a valid acknowledgement PSH: PUSH Flag

Notification from sender to the receiver that the receiver should pass all data that it has to the application.

Normally set by sender when the sender’s buffer is empty


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TCP header fields Flag bits:

RST: Reset the connection The flag causes the receiver to reset the connection Receiver of a RST terminates the connection and

indicates higher layer application about the reset SYN: Synchronize sequence numbers

Sent in the first packet when initiating a connection FIN: Sender is finished with sending

Used for closing a connection Both sides of a connection must send a FIN


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TCP States in Connection Lifetime

SYN_SENT(active open)

SYN_RCVD

ESTABLISHED

ESTABLISHED

FIN_WAIT_1(active close)

LISTEN(passive open)

CLOSE_WAIT(passive close)

LAST_ACK

FIN_WAIT_2

TIME_WAIT

CLOSED

Documents

1 Transport Layer Flow control Connection management TCP, UDP