Data Communication IData Communication ILecture 5Lecture 5

presented by

Werner Wild, CEO EvolutionInnsbruck, San Francisco, Zurich

Contact: info@evolution.at

Today’s Lecture PlanToday’s Lecture Plan

TCP

TCPTCP

Connection oriented byte stream– All bytes arrive in order– No “boundaries”

Not a virtual circuit– “Two end” management

Full duplex connectionAlways point-to-point

TCP TCP RFCs: 793, 1122, 1323, 2018, 2581RFCs: 793, 1122, 1323, 2018, 2581

full duplex data:– bi-directional data flow in

same connection

– MSS: maximum segment size

connection-oriented: – handshaking (exchange of

control msgs) init’s sender, receiver state before data exchange

flow controlled:– sender will not overwhelm

receiver

point-to-point:– one sender, one receiver

reliable, in-order byte steam:– no “message boundaries”

pipelined:– TCP congestion and flow

control set window size

send & receive buffers

socketdoor

T C Psend buffer

T C Preceive buffer

socketdoor

segm ent

applicationwrites data

applicationreads data

The TCP HeaderThe TCP Header

Byte stream sent as sequence of segments– Segment may be 0 to 64k bytessource port destination port

sequence number

acknowledgment number

window size

urgent pointerchecksum

options

data

headerlength unused flags

Establishing a ConnectionEstablishing a Connection

Normal connection Host 1 initiates and

chooses its initial sequence number.

Host 2 replies with its own initial sequence number and acknowledges host 1.

Host 1 begins data.

host 1 host 2

CR (seq=x)

ACK (seq=y,ack=x)

DATA (seq=x’,ack=y)

Duplicate Request (I)Duplicate Request (I)

A previous connection request appears from nowhere.

Host 2 replies with its own initial sequence number and acknowledges host 1.

Host 1 rejects the connection request.

host 1 host 2

CR (seq=x)

ACK (seq=y,ack=x)

REJECT (ack=y)

Duplicate Connection (II)Duplicate Connection (II)

A previous connection request appears from nowhere .

Host 2 replies. The first data packet of

the previous connection also appears.

Host 1 rejects the connection request.

host 1 host 2

CR (seq=x)

ACK (seq=y,ack=x)

REJECT (ack=y)

DATA (seq=x’,ack=z)

Initial Sequence NumbersInitial Sequence Numbers

Ensure no sequence number is reused before all packets and acks have expired.

Includes restart after a crash.

Large sequence space.

Based on bits from a “clock”.

timese

qu

ence

nu

mb

ers

initialsequencenumbers

sequencenumbersused

forbiddenrange

TCP seq. #’s and ACKsTCP seq. #’s and ACKsSeq. #’s:

– byte stream “number” of first byte in segment’s data

ACKs:– seq # of next byte

expected from other side

– cumulative ACKQ: how receiver handles

out-of-order segments– A: TCP spec doesn’t

say, - up to implementor

Host A Host B

Seq=42, ACK=79, data = ‘C’

Seq=79, ACK=43, data = ‘C’

Seq=43, ACK=80

Usertypes

‘C’

host ACKsreceipt

of echoed‘C’

host ACKsreceipt of

‘C’, echoesback ‘C’

timesimple telnet scenario

TCP ACK generationTCP ACK generationEvent

in-order segment arrival, no gaps,everything else already ACKed

in-order segment arrival, no gaps,one delayed ACK pending

out-of-order segment arrivalhigher-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 singlecumulative ACK

send duplicate ACK, indicating seq. #of next expected byte

immediate ACK if segment startsat lower end of gap

TCP SequencingTCP Sequencing

seq=92, 8 bytes data

ack=100

seq=92, 8 bytes data

ack=100tim

eou

t

A B

Retransmission due to lost acknowledgment.

Note sequence number is bytes transmitted.

TCP Sequencing cont.TCP Sequencing cont.

seq=92, 8 bytes data

ack=100

A B

Cumulative acknowledgment avoids retransmission.

seq=100, 20 bytes data

ack=120tim

eou

t

TCP Sequencing cont.TCP Sequencing cont.

seq=92, 8 bytes data

seq=92, 8 bytes data

ack=100time

out

A B

Delayed acknowledgment causes retransmission

Subsequent acknowledgment prevents retransmission.

Not go-back-n Not selective repeat

seq=100, 20 bytes data

ack=120ack=120

TCP Flow ControlTCP Flow Control

receiver: explicitly informs sender of (dynamically changing) amount of free buffer space – RcvWindow field in TCP segment

sender: keeps the amount of transmitted, unACKed data less than most recently received RcvWindow

sender won’t overrun

receiver’s buffers bytransmitting too

much, too fast

flow control

receiver buffering

RcvBuffer = size or TCP Receive Buffer

RcvWindow = amount of spare room in Buffer

Receive WindowReceive Window (Example) (Example)

applicationwrites 2k

sender receiver

empty 4k2k, seq=0

2kack=2048, win=2048

applicationwrites 3k

2k, seq=20484k full

ack=4096, win=0

ack=4096, win=2048

2k

applicationreads 2k

1k, seq=40962k1k

TCP RTT and TimeoutTCP RTT and Timeout

Q: how to set TCP timeout value?

longer than RTT– note: RTT will

vary too short: premature

timeout– unnecessary

retransmissions too long: slow

reaction to segment loss

Q: how to estimate RTT? SampleRTT: measured time

from segment transmission until ACK receipt

– ignore retransmissions, cumulatively ACKed segments

SampleRTT will vary, want estimated RTT “smoother”

– use several recent measurements, not just current SampleRTT

TimeoutsTimeouts

What is the appropriate value for a timeout?

est_rtt = (1-p)(est_rtt) + p (sample_rtt)dev = (1-p) dev + p |sample_rtt - est_rtt|

The EndThe End

Thank you for your attention !

SourcesSources

For the preparation of this lectures a lot of sources where used – my special thanks go to :– Univ. Auckland– Univ. California – San Diego (UCSD)– Univ. Hongkong– … many others …