The man behind the curtain Old friends - Wellesley...

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CS242 Computer Networks Department of Computer Science Wellesley College

Connection-oriented transport TCP

TCP 11-2

Once through the door ... ... TCP directs data to the connection's

send buffer.

Periodically TCP "grabs" chunks of data and ships it

through the network layer portal*

*TCP specifications are very laid back about when this should happen stating: "Send that data in segments at its own convenience."

TCP 11-3

The man behind the curtain o  TCP pairs each chunk of

client data* with a TCP header, forming a TCP segment.

o  Segments are passed down the network layer, where they are separately encapsulated within network-layer IP datagrams and sent out over the network.

*Maximum Segment Size (MSS) is determined by OS. Common values are 1,500, 536, and 512 bytes.

Old friends

TCP 11-4

We have already discussed source and

destination port numbers ...

... as well as the Internet checksum

Chunk of data being sent

2

More TCP fields

TCP 11-5

Specifies the length of TCP header in 32-bit words (4 bits)

TCP header length is variable due to option field. Typically,

options field is empty, so length is usually 20 bytes.

Two rarely used bit fields*

TCP 11-6

Indicates upper layer of sending-side has marked some

data in segment as urgent; location of last byte of urgent

data indicated by Urgent Data Point field

Receiver should pass the data to upper layer immediately

*Sequence, acknowledgement, receive window, and the remaining bit fields are coming attractions.

TCP 11-7

Sequence and acknowledgment numbers o  The sequence number for a segment is a byte-stream number

of the first byte in the segment. o  The acknowledgement number that host A puts in its segment

is the sequence number of the next byte host A is expecting from host B.

o  TCP only acknowledges bytes up to the first missing byte in the stream (cumulative acknowledgement).

TCP 11-8

Host B sends A a packet*

Seq # Ack # 0 5000 -------------------- Bytes 0 -- 999 of Host B’s file

Host A Host B

*Host A having already sent bytes 0 through 4999 of its data to host B.

Bytes 0 -- 4999

of Host A’s file

A’s Buffer B’s Buffer

We assume the initial sequence number was zero. In truth, both sides of a TCP connection randomly choose an initial sequence number.

3

TCP 11-9

Host A sends B next 1000 bytes + ACK

Seq # Ack # 5000 1000 --------------------- Bytes 5000 -- 5999 of Host A’s file

Bytes 0 -- 999

of Host B’s file

Host A Host B

A’s Buffer

Bytes 0 -- 4999

of Host A’s file

B’s Buffer

TCP 11-10

Host B sends A next 1000 bytes & ACK


Host A Host B

Bytes 0 -- 5999

of Host A’s file

B’s Buffer

Bytes 0 -- 999

of Host B’s file

A’s Buffer

TCP 11-11

B’s packet 1000 -- 1999 is lost

Host A Host B

Bytes 0 -- 5999

of Host A’s file

B’s Buffer

Bytes 0 -- 999

of Host B’s file

A’s Buffer


Seq

#

Ack

#

1000

600

0 --

----

----

----

----

--

Byte

s 10

00 -

- 19

99

of H

ost

B’s

file

TCP 11-12

Host A responds ACK 1000

Seq # Ack # 6000 1000 --------------------- Bytes 6000 -- 6999 of Host A’s file

Bytes 0 -- 999

of Host B’s file

Host A Host B

A’s Buffer

Bytes 0 -- 5999

of Host A’s file

B’s Buffer

Cumulative Ack: TCP only acknowledges

bytes up to the first missing byte

4

TCP 11-13

Host B will resend missing packet

Bytes 0 -- 999

of Host B’s file

Host A Host B

A’s Buffer

Bytes 0 -- 6999

of Host A’s file

B’s Buffer


TCP 11-14

What does A do with out-of-order packets?

Host A Host B

Bytes 0 -- 5999

of Host A’s file

B’s Buffer

Bytes 0 -- 999

of Host B’s file

A’s Buffer


Seq

#

Ack

#

1000

600

0 --

----

----

----

----

--

Byte

s 10

00 -

- 19

99

of H

ost

B’s

file

TCP RFCs are mum on this score Two Choices: 1.  The receiver immediately

discards out-of-order segments;

2.  The receiver keeps the out-of-order bytes and waits for the missing bytes to fill in the gaps.

TCP 11-15

*We've seen both in the previous slide set.

TCP 11-16

Telnet application over TCP

Acknowledgement for client-to-server is

"piggybacked" in a segment carrying server-to-client

data A segment with a sequence number, but no data; hum ...

5

TCP 11-17

Timeout o  Like our rdt protocol, TCP uses a timeout/retransmit

mechanism to recover from lost segments.

o  Timeout should be larger than the connection’s RTT, but

how much larger and how does TCP know the RRT in the first place.

TCP 11-18

o  TCP takes one SampleRTT measurement at a time EstimatedRTT = (1-α)•EstimatedRTT + α•SampleRTT

Recommended alpha is 1/8

EstimatedRTT: An exponential weighted moving average

SampleRTT only measured for segments that have been transmitted once

TCP 11-19

o  TCP takes one SampleRTT measurement at a time EstimatedRTT = (1-α)•EstimatedRTT + α•SampleRTT

EstimatedRTT: An exponential weighted moving average

The average puts more weight on recent samples than on older

samples

Weight given to past SampleRTT decay

exponentially over time

TCP 11-20

Should we use EstimatedRTT as our timeout value?

6

TCP 11-21

DevRTT o  DevRTT estimates of how much SampleRTT bounces around

DevRTT = (1-β)•DevRTT + β•|SampleRTT-EstimatedRTT|

Recommended beta is 1/4

TCP 11-22

Setting and managing timeout interval o  TCP’s method for determining the retransmission timeout

interval is !TimeoutInterval = EstimatedRTT + 4•DevRTT!

o  When a timeout event occurs, TCP retransmits the not yet ACKed segment with the smallest sequence number and sets the timeout interval to twice the previous value.*

*Why in the world would TCP do that?

TCP 11-23

TCP sender (simplified)

NextSeqNum = InitialSeqNum SendBase = InitialSeqNum loop (forever) { switch(event) event: data received from application above create TCP segment with sequence number NextSeqNum if (timer currently not running) start timer pass segment to IP NextSeqNum = NextSeqNum + length(data)

event: timer timeout retransmit not-yet-acknowledged segment with smallest sequence number start timer event: ACK received, with ACK field value of y if (y > SendBase) { SendBase = y if (there are currently not-yet-acknowledged segments) start timer } } /* end of loop forever */

SendBase-1 is last cumulatively

ACKed byte

TCP 11-24

Retransmission due to lost ACK

7

TCP 11-25

Segment 100 not retransmitted

TCP 11-26

Cumulative ACK avoids retransmission

TCP 11-27

TCP ACK generation [RFC 1122, RFC 2581] 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

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

TCP 11-28

Fast retransmittion o  Timeout periods are often

pretty long. o  If the sender receives

many duplicate ACKs for the same segment, even the dimmest watt bulb knows something is up.

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TCP 11-29

event: ACK received, with ACK field value of y if (y > 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 is 3) { resend segment with sequence number y }

Fast retransmit algorithm

a duplicate ACK for already ACKed segment

fast retransmit

TCP 11-30

Flow control o  TCP provides flow-control to eliminate the possibility of the

sender overflowing the receiver’s buffer LastByteRcvd - LastByteRead ≤ RcvBuffer

o  The amount of spare room in buffer is stored in RcvWindow = RcvBuffer - [LastByteRcvd -

! ! ! ! ! !LastByteRead]!

*Remember TCP is full duplex, both sides maintain distinct receive windows.

TCP 11-31

Receiver informs sender of spare room o  Recall TCP is full-duplex,

so while “sender” transmits data to “receiver”, the latter is posting its vacancy rate, RcvWindow, to “sender” in every segment.

o  The sending host makes sure LastByteSent - !

!LastByteAcked !

! !≤ RcvWindow.!

TCP 11-32

Avoiding gridlock o  There is one minor

technical problem with this scheme: Suppose the the B’s receive buffer fills, and RcvWindow = 0 is sent to “sender” A.

o  Suppose further, that the B has no additional data to the “sender”.*

o  So what's the problem?

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TCP 11-33

Mr. Ed will only speak when he has something to say ...

o  ... or when the other end's RevWindow is zero.

o  The solution is TCP continues to send segments with one data byte when B's receive window is zero.

o  These segments will be acknowledged by the receiver and eventually ...

TCP 11-34

Setting up a TCP connection

TCP 11-35

1. Client requests TCP connection: Hi

Client host sends TCP SYN segment to server specifying

initial sequence number ...

but sends no application-layer data.

...and setting the SYN bit,

TCP 11-36

Server host receives SYN, allocates the TCP buffers and variables to the

client TCP,

and replies with SYNACK by choosing its initial sequence

number,

setting its Acknowledgment field to the client_isn+1,

and setting the ACK and SYN bits (still no data)

2. The server replies: Hi!

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TCP 11-37

Client host receives ACKSYN, allocates its own TCP buffers and variables to

the server TCP,

replies with SYN bit set to zero since connection is

established,

and may include data

3. Client responds: Got the time?

TCP 11-38

TCP three-way handshake

seq=client_isn+1

ack=server_isn+1

TCP 11-39

Closing a connection 1.  Client end system sends

FIN control segment to server.

2.  Server receives FIN, replies with ACK, begins closing.

3.  Server sends its own FIN. 4.  Client receives FIN,

replies with ACK. Enters timed wait to respond to additional FINs.

5.  Server receives ACK. Connection closed.

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