eec-484/584 computer networks lecture 7 wenbing zhao [email protected] (part of the slides are...

EEC-484/584EEC-484/584Computer NetworksComputer Networks

Lecture 7Lecture 7

Wenbing ZhaoWenbing Zhao

[email protected] [email protected] (Part of the slides are based on Drs. Kurose & (Part of the slides are based on Drs. Kurose &

RossRoss’’s slides for their s slides for their Computer Networking Computer Networking book)book)

22

04/18/2304/18/23 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks

OutlineOutline

• Reliable data tranfer (part II)

• UDP

• TCP– Segment header structure– Connection management


33

rdt3.0: channels with errors rdt3.0: channels with errors andand loss loss

New assumption: underlying channel can also lose packets (data or acks)– Checksum, seq. #, Acks,

retransmissions will be of help, but not enough

Approach: sender waits “reasonable” amount of time for ACK

• Retransmits if no ACK received in this time

• If pkt (or ACK) just delayed (not lost):– Retransmission will be

duplicate, but use of seq. #’s already handles this

– Receiver must specify seq # of pkt being acked

• Requires countdown timer

04/18/2304/18/23 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao

44

rdt3.0 senderrdt3.0 sender

sndpkt = make_pkt(0, data, checksum)udt_send(sndpkt)start_timer

rdt_send(data)

Wait for

ACK0

rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) ||isACK(rcvpkt,1) )

Wait for call 1 from

above

sndpkt = make_pkt(1, data, checksum)udt_send(sndpkt)start_timer

rdt_send(data)

rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,0)

rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) ||isACK(rcvpkt,0) )

rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,1)

stop_timer

stop_timer

udt_send(sndpkt)start_timer

timeout

udt_send(sndpkt)start_timer

timeout

rdt_rcv(rcvpkt)

Wait for call 0from

above

Wait for

ACK1

rdt_rcv(rcvpkt)


55

rdt3.0 in actionrdt3.0 in action


66

rdt3.0 in actionrdt3.0 in action


77

Performance of rdt3.0Performance of rdt3.0

• rdt3.0 works, but performance stinks• ex: 1 Gbps link, 15 ms prop. delay, 8000-bit packet:

U sender: utilization – fraction of time sender busy sending

U sender

= .008

30.008 = 0.00027

microseconds

L / R

RTT + L / R =

1KB pkt every 30 msec -> 33kB/sec thruput over 1 Gbps link network protocol limits use of physical resources!

dsmicrosecon8bps10

bits80009

R

Ldtrans


88

rdt3.0: stop-and-wait operationrdt3.0: stop-and-wait operation

first packet bit transmitted, t = 0

sender receiver

RTT

last packet bit transmitted, t = L / R

first packet bit arriveslast packet bit arrives, send ACK

ACK arrives, send next packet, t = RTT + L / R

U sender

= .008

30.008 = 0.00027

microseconds

L / R

RTT + L / R =


99

EEC-484/584: Computer NetworksEEC-484/584: Computer Networks

Pipelined ProtocolsPipelined ProtocolsPipelining: sender allows multiple, “in-flight”, yet-to-be-

acknowledged pkts– range of sequence numbers must be increased– buffering at sender and/or receiver

• Two generic forms of pipelined protocols: go-back-N, selective repeat

04/18/2304/18/23 Wenbing ZhaoWenbing Zhao

1010


Pipelining: Increased UtilizationPipelining: Increased Utilization

first packet bit transmitted, t = 0

sender receiver

RTT

last bit transmitted, t = L / R

first packet bit arriveslast packet bit arrives, send ACK

ACK arrives, send next packet, t = RTT + L / R

last bit of 2nd packet arrives, send ACKlast bit of 3rd packet arrives, send ACK

U sender

= .024

30.008 = 0.0008

microseconds

3 * L / R

RTT + L / R =

Increase utilizationby a factor of 3!


1111


Pipelining ProtocolsPipelining Protocols

Go-back-N: big picture:• Sender can have up to N

unacked packets in pipeline

• Rcvr only sends cumulative acks– Doesn’t ack packet if

there’s a gap

• Sender has timer for oldest unacked packet– If timer expires, retransmit

all unacked packets

Selective Repeat: big pic• Sender can have up to N

unacked packets in pipeline

• Rcvr acks individual packets

• Sender maintains timer for each unacked packet– When timer expires,

retransmit only unack packet


1212


Go-Back-NGo-Back-NSender:• k-bit seq # in pkt header• “window” of up to N consecutive unack’ed pkts allowed

ACK(n): ACKs all pkts up to, including seq # n - “cumulative ACK” may receive duplicate ACKs (see receiver)

timer for oldest in-flight pkt timeout(n): retransmit pkt n and all higher seq # pkts in window


1313


GBN: Sender Extended FSMGBN: Sender Extended FSM

Waitstart_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])…udt_send(sndpkt[nextseqnum-1])

timeout

rdt_send(data)

if (nextseqnum < base+N) { sndpkt[nextseqnum] = make_pkt(nextseqnum,data,chksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) // start timer if first unacked pkt start_timer nextseqnum++ }else refuse_data(data)

base = getacknum(rcvpkt)+1If (base == nextseqnum) // no more unacked pkts stop_timer else start_timer

rdt_rcv(rcvpkt) && notcorrupt(rcvpkt)

base=1nextseqnum=1

rdt_rcv(rcvpkt) && corrupt(rcvpkt)


1414


GBN: Receiver Extended FSMGBN: Receiver Extended FSM

ACK-only: always send ACK for correctly-received pkt with highest in-order seq #– may generate duplicate ACKs– need only remember expectedseqnum

• out-of-order pkt: – discard (don’t buffer) -> no receiver buffering!– Re-ACK pkt with highest in-order seq #

Wait

udt_send(sndpkt)

default

rdt_rcv(rcvpkt) && notcurrupt(rcvpkt) && hasseqnum(rcvpkt,expectedseqnum)

extract(rcvpkt,data)deliver_data(data)sndpkt = make_pkt(expectedseqnum,ACK,chksum)udt_send(sndpkt)expectedseqnum++

expectedseqnum=1sndpkt = make_pkt(expectedseqnum,ACK,chksum)


1515


GBN inGBN inactionaction


1616


Selective RepeatSelective Repeat

• Receiver individually acknowledges all correctly received pkts– Buffers pkts, as needed, for eventual in-order delivery to

upper layer

• Sender only resends pkts for which ACK not received– Sender timer for each unacked pkt

• Sender window– N consecutive seq #’s– Again limits seq #s of sent, unacked pkts


1717


Selective Repeat: Sender, Receiver WindowsSelective Repeat: Sender, Receiver Windows


1818


Selective RepeatSelective Repeat

data from above :• if next available seq # in

window, send pkt

timeout(n):• resend pkt n, restart timer

ACK(n) in [sendbase,sendbase+N-1]:

• mark pkt n as received• if n smallest unACKed pkt,

advance window base to next unACKed seq #

Senderpkt n in [rcvbase, rcvbase+N-1]

send ACK(n) out-of-order: buffer in-order: deliver (also deliver

buffered, in-order pkts), advance window to next not-yet-received pkt

pkt n in [rcvbase-N,rcvbase-1]

ACK(n)

otherwise: ignore

Receiver


1919


Selective Repeat In ActionSelective Repeat In Action


2020


Selective Repeat:Selective Repeat: Dilemma Dilemma

Example: • seq #’s: 0, 1, 2, 3• window size=3

• receiver sees no difference in two scenarios!

• incorrectly passes duplicate data as new in (a)

Q: what relationship between seq # size and window size?


2121


Non-Sequential Receive ProblemNon-Sequential Receive Problem

• The problem is caused by the overlap of sequence number between the new receiving window and the old receiving window

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

0 1 2 3 4 5 6

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

Overlap Overlap


2222


Non-Sequential Receive ProblemNon-Sequential Receive Problem

• Solution: – make sure no overlap when receiver advances its

window– Make window size w =1/2 range of seq numbers

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

0 1 2 3

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

No Overlap


2323


UDP: User Datagram ProtocolUDP: User Datagram Protocol

• “No frills,” “bare bones” Internet transport protocol• “Best effort” service, UDP segments may be:

– Lost– Delivered out of order to app

• Connectionless:– No handshaking between UDP sender, receiver– Each UDP segment handled independently of

others


2424


Why is There a UDP?Why is There a UDP?

• No connection establishment (which can add delay)

• Simple: no connection state at sender and receiver

• Small segment header

• No congestion control: UDP can blast away as fast as desired


2525


UDPUDP• Often used for streaming

multimedia apps– Loss tolerant– Rate sensitive

• Other UDP uses– DNS– SNMP

• Reliable transfer over UDP: add reliability at application layer

source port # dest port #

32 bits

Applicationdata

(message)

UDP segment format

length checksumLength, in

bytes of UDPsegment,including

header


2626


UDP ChecksumUDP Checksum

Sender:• treat segment contents as

sequence of 16-bit integers• checksum: addition (1’s

complement sum) of segment contents

• sender puts checksum value into UDP checksum field

Receiver:• compute checksum of

received segment• check if computed checksum

equals checksum field value:– NO - error detected– YES - no error detected. But

maybe errors nonetheless?

Goal: detect “errors” (e.g., flipped bits) in transmitted segment


2727


TCP: OverviewTCP: Overview• 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


2828


TCP: OverviewTCP: Overview

• TCP connection is byte stream, not message stream, no message boundaries

• TCP may send immediately or buffer before sending• Receiver stores the received bytes in a buffer


2929


TCP Segment StructureTCP Segment Structure

source port # dest port #

32 bits

applicationdata

(variable length)

sequence number

acknowledgement numberReceive window

Urg data pnterchecksum

FSRPAUheadlen

notused

Options (variable length)

URG: urgent data (generally not used)

ACK: ACK #valid

PSH: push data now(generally not used)

RST, SYN, FIN:connection estab(setup, teardown

commands)

# bytes rcvr willingto accept

countingby bytes of data(not segments!)

Internetchecksum

(as in UDP)

A TCP segment must fit into an IP datagram!


3030


The TCP Segment HeaderThe TCP Segment Header

• Source port and destination port: identify local end points of the connection– Source and destination end points together identify the connection

• Sequence number: identify the byte in the stream of data that the first byte of data in this segment represents

• Acknowledgement number: the next sequence number that the sender of the ack expects to receive– Ack # = Last received seq num + 1– Ack is cumulative: an ack of 5 means 0-4 bytes have been

received

• TCP header length – number of 32-bit words in header


3131



• URG – indicates urgent pointer field is set• Urgent pointer – points to the seq num of the last byte in a

sequence of urgent data• ACK – acknowledgement number is valid• SYN – used to establish a connection

– Connection request: ACK = 0, SYN = 1– Connection confirm: ACK=1, SYN = 1

• FIN – release a connection, sender has no more data• RST – reset a connection that is confused• PSH – sender asked to send data immediately


3232



• Receiver window size –number of bytes that may be sent beyond the byte acked

• Checksum – add the header, the data, and the conceptual pseudoheader as 16-bit words, take 1’s complement of sum– For more info: http://www.netfor2.com/tcpsum.htm

http://www.netfor2.com/checksum.html

• Options – provides a way to add extra facilities not covered by the regular header– E.g., communicate buffer sizes during set up


3333


TCP Sequence Numbers and ACKsTCP Sequence Numbers and ACKs

Sequence numbers:– byte stream “number” of

first byte in segment’s data

ACKs:– seq # of next byte

expected from other side

– cumulative ACK

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/ssh scenario


3434


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();


3535



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


3636



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


3737



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


3838



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


3939

ExerciseExercise

• A process at host A wants to establish a TCP connection with another process at host B. Assuming that host A chooses to use 1628 as the initial sequence number, and host B chooses to use 3217 as the initial sequence number for this connection, show the segments involved with the connection establishment process. You must include the following information for each such segment: (1) sequence number, (2) acknowledgement number (if applicable), (3) the SYN flag bit status, and (4) the ACK flag bit status.


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