multiplexed link L1 B1 - MIT OpenCourseWare · Networks encounter a vast range of Data rates...

Boston Switch

Los AngelesSwitch

multiplexed link

shared switches

B1

B2

B3

L1

L2

L3 L4

Principles of Computer System Design © Saltzer & Kaashoek 2009

5,624 bit times Time

8-bit frame 8-bit frame 8-bit frame


D

Personal Computer service A

B multiplexed

link data crosses thislink in bursts andcan tolerate variable delay

C


frame

Time B D

Guidance 4000 bits 750 bits information


A packet

Workstationat network attachment

Packet Switch Packet

Switch

Packet Switch

PacketSwitch

1

2 3

B

Service at networkpoint A attachment point B

B


------------maximum

tolerable delayaverage 1queuing 1 – ρdelay

1

100%0 Utilization, r rmax


send request,set timer

receive response,reset timer

send request,set timer

timer expires,resend request,set new timer

receive response,reset timer

A B

request 1 time

response 1

X

request 2 overloadedX forwarder discards requestpacket.

request 2’

response 2’ X


A send request,

set timer

timer expires,resend request,set new timer

receive response, Xreset timer

request 3

X request 3

response 3’

’

B

overloaded forwarderdiscards response 3

duplicate arrives at BB sends response 3’


A send request,

set timer

timer expires,resend receive Xresponse,

reset timer

receiveduplicateresponse

request 4

request 4’

response 4

response 4’

B

packet containing responsegets delayed

duplicate arrives at BB sends response 4’


Application characteristics

isochronous (e.g., telephone

network)

NetworkType

asynchronous(e.g., Internet)

Continuous stream

(e.g., interactivevoice)

(hard-edged)wastesgood match either acceptscapacity or blocks call

(gradual)variable latency 1 variable delay

good matchupsets 2 discards data application 3 rate adaptation

Bursts of data (most

computer-to-computer data)

Responseto load

variations


Networks encounter a vast range of

Data rates Propagation, transmission, queuing, and processing delays. Loads Numbers of users

Networks traverse hostile environments

Noise damages dataLinks stop working

Best-effort networks have

Variable delaysVariable transmission ratesDiscarded packetsDuplicate packetsMaximum packet lengthReordered delivery


result ← FIRE (#, target, action) procedure FIRE (nmiss, where, react)... return result

Client stub Service stub

proc: FIRE

args: 3

type: integer value: 2

type: string value: “Lucifer”

type: procedure value: EVADE

Preparerequestmessage.Send to service

Receive requestmessage.Call requestedprocedure.Prepare responsemessage.Send to client.

response:

acknowledgment

type: string value: “destroyed”

request:

Wait for response.


Main program

RPC client stub

application protocol

presentation protocol

called procedure

RPC service stub


Main program application protocol

fire (return)(return) fire

presentation protocol

called procedure

RPC service stub

Client networkpackage

RPC client stub

Service networkpackage

send_send_ receive_ receive_ messagemessage message message

transport protocol


Layer One A B C D

Layer Two J K L

Layer Three X Y Z


DATA

LINK_SEND (pkt, link2) NETWORK_HANDLE

A B

DATA LH LT LinkLayer

link 2link 1LinkLayer link link

protocolprotocol

C

LinkLayer


NETWORK_SEND (segment

DATA

, “IP”, nap_1197)

NetworkLayer

network

protocolNetworkLayer

LINK_SEND (packet, link5) lINK_SEND (packet, link2

DATANT NH

) NETWORK_HANDLE

LinkLayer

link5DATANT NH LHLT

link 2 linkprotocol

LinkLayer

LinkLayer


FIRE (7, “Lucifer”, evade) DATA

end-to-end

protocol

Network Layer

End-to-End Layer

DATAET EH

(RPC)

Network Layer

DATANT NHET EH

LinkLayer

DATANT NH LHLT

LinkLayer

LinkLayer

ET EH

FIRE (7, “Lucifer”, evade)

End-to-End Layer(RPC)

NetworkLayer

LinkLayer


The end-to-end argument

The application knows best.


Gnutella (network layer)

Transport Protocol (end-to-end layer) Internet Protocol (network layer)

dialed connection (end-to-end layer) telephone switch (network layer)

(link

physical wire (link layer)

File Transfer Program (end-to-end layer)

layer)

layer)(link

File transfer system

Internet

dial-uptelephonenetwork


data

A Bready

acknowledge


V 1 0 1 0 1 0 1 0 1

time


A B


procedure FRAME_TO_BIT (frame_data, length) ones_in_a_row = 0 for i from 1 to length do // First send frame contents

SEND_BIT (frame_data[i]); if frame_data[i] = 1 then

ones_in_a_row ← ones_in_a_row + 1;if ones_in_a_row = 6 then

SEND_BIT (0); // Stuff a zero so that data doesn’t ones_in_a_row ← 0; // look like a framing marker

else ones_in_a_row ← 0;

for i from 1 to 7 do // Now send framing marker. SEND_BIT (1)


procedure BIT_TO_FRAME (rcvd_bit) ones_in_a_row integer initially 0 if ones_in_a_row < 6 then bits_in_frame ← bits_in_frame + 1frame_data[bits_in_frame] ← rcvd_bitif rcvd_bit = 1 then ones_in_a_row ← ones_in_a_row + 1else ones_in_a_row ← 0

else // This may be a seventh one-bit in a row, check it out. if rcvd_bit = 0 then ones_in_a_row ← 0 // Stuffed bit, don't use it.

else // This is the end-of-frame marker LINK_RECEIVE (frame_data, (bits_in_frame - 6), link_id) bits_in_frame ← 0 ones_in_a_row ← 0


Network protocol

Standard Highrobustness

protocolprotocol Experimental

protocol

Network Layer

Link Layer


Network Layer

Link Layer

Internet Protocol

AppletalkProtocol

Path Vector

ExchangeProtocol

Address Resolution

Protocol

Standard Highrobustness

protocolprotocol Experimental

protocol


structure frame structure checked_contents

bit_string net_protocol // multiplexing parameter bit_string payload // payload data

bit_string checksum

procedure LINK_SEND (data_buffer, link_identifier, link_protocol, network_protocol) frame instance outgoing_frame outgoing_frame.checked_contents.payload ← data_buffer outgoing_frame.checked_contents.net_protocol ← data_buffer.network_protocol frame_length ← LENGTH (data_buffer) + header_length outgoing_frame.checksum ← CHECKSUM (frame.checked_contents, frame_length) sendproc ← link_protocol[that_link.protocol] // Select link protocol. sendproc (outgoing_frame, frame_length, link_identifier) // Send frame.


procedure LINK_RECEIVE (received_frame, length, link_id) frame instance received_frame if CHECKSUM (received_frame.checked_contents, length) =

received_frame.checksum then // Pass good packets up to next layer. good_frame_count ← good_frame_count + 1; GIVE_TO_NETWORK_HANDLER (received_frame.checked_contents.payload,

received_frame.checked_contents.net_protocol); else bad_frame_count ← bad_frame_count + 1 // Just count damaged frame.

// Each network layer protocol handler must call SET_HANDLER before the first packet // for that protocol arrives…

procedure SET_HANDLER (handler_procedure, handler_protocol) net_handler[handler_protocol] ← handler_procedure

procedure GIVE_TO_NETWORK_HANDLER (received_packet, network_protocol) handler ← net_handler[network_protocol] if (handler ≠ NULL) call handler(received_packet, network_protocol) else unexpected_protocol_count ← unexpected_protocol_count + 1


networkattachment

Network

0107

24

16

11

39

33

35

40

41

42 network

point

address


structure packetbit_string sourcebit_string destinationbit_string end_protocolbit_string payload

procedure NETWORK_SEND (segment_buffer, destination, network_protocol, end_protocol)

packet instance outgoing_packet outgoing_packet.payload ← segment_buffer outgoing_packet.end_protocol ← end_protocol outgoing_packet.source ← MY_NETWORK_ADDRESS

outgoing_packet.destination ← destination NETWORK_HANDLE (outgoing_packet, net_protocol)


procedure NETWORK_HANDLE (net_packet, net_protocol) packet instance net_packet if net_packet.destination ≠ MY_NETWORK_ADDRESS then

next_hop ← LOOKUP (net_packet.destination, forwarding_table)LINK_SEND (net_packet, next_hop, link_protocol, net_protocol)

else GIVE_TO_END_LAYER (net_packet.payload,

net_packet.end_protocol, net_packet.source)


Segment presented tothe network layer DATA

Packet presented tothe link layer DATAend

protocol source &

destination

frame check frameDATAend protocol

Frame appearingon the link

source & destination sum

network protocolmark mark

1111111 97142 1111111“Fire”RPCExample 41 —> 24 55316IP


B

4

1

H

1

G

K

2 5A 1

C

113source

4 2 3

destination

F

1

2 4 51 5

J

E2

1

3 3 114

D

2


B

C

A 1 1 H 4

1

3

F 5 J 1

2 E

destination link

G

K

source 1

2

D

5

3 4 5

1

2

2

34

1

2

31

1

1

4

destination

A end-layerall other 1


destination link

A 1

C D E F

H J K

2

3

4

2 3 4

2

4

B

G end-layer

G

K

J

source 1

2 3

A

B

C

D

E

F

4

5

3 4 5

1

2

1 2

34

1

2

35

1

1

1 1

1

12

4

H

destination


C

B

A 11

4

1

H

1

G

K 2

2 5

1source 3

4 2 3

destination1

4 5F 51J

E

12

3 3 14

21

D

to path

G < >


B

A 1 C

1

source

destination

F

E

D

G

K

J

1

2 3

4

5

4 5

21

3 1

2

1 3

4

1

2

351 1

12

4

H

From A, From H, From J, From K,via link 1 via link 2: via link 3: via link 4:

to path to path to path to path A < > H < > J < > K < >


B

A 11

1 C

D

4

1

HG

K

2

342

2 5

1source 3

4 3

1 destination

5F 51 J

E

12

3 14

21

path vector forwarding table to AGHJK

path to <A> A < > G<H> H<J> J<K> K

link

1end-layer

234


G

K

J

source 1

2 3

D F

4

5

3 4 5

1

2

1 2

34

1

2

35

1

1

1

1

1

4

H

destination

B

C

1A

2 E

From A, From H, From J, From K,via link 1 via link 2: via link 3: via link 4: to path to path to path to path A < > B <D> E<B> D <E>G <G> C <C> E <E> F <F>G <G> G <G> G <G>H < > H <H> H <H>J <J> J < > <J>K <K> K <K> K

J< >


forwarding tablepath vector

G

K

J

source 1

2 3

A

B

C

D

E

F

4

5

3 4 5

1

2

1 2

34

1

2

35

1

1

1 1

1

12

4

H

destination

to

ABCDEFGHJK

path <A>

<H, B><H, C><J, D><J, E><K, F>

< ><H><J><K>

to

ABCDEFGHJK

link

122334

end-layer234


// Maintain routing and forwarding tables.

vector associative array // vector[d_addr] contains path to destination d_addr neighbor_vector instance of vector // A path vector received from some neighbor my_vector instance of vector // My current path vector. addr associative array // addr[j] is the address of the network attachment

// point at the other end of link j. // my_addr is address of my network attachment point. // A path is a parsable list of addresses, e.g. {a,b,c,d}

procedure main() // Initialize, then start advertising. SET_TYPE_HANDLER (HANDLE_ADVERTISEMENT, exchange_protocol) clear my_vector; // Listen for advertisements do occasionally // and advertise my paths

for each j in link_ids do // to all of my neighbors. status ← SEND_PATH_VECTOR (j, my_addr, my_vector, exch_protocol) if status ≠ 0 then // If the link was down,

clear new_vector // forget about any paths FLUSH_AND_REBUILD (j) // that start with that link.


procedure HANDLE_ADVERTISEMENT (advt, link_id) // Called when an advt arrives. addr[link_id] ← GET_SOURCE (advt) // Extract neighbor’s address neighbor_vector ← GET_PATH_VECTOR (advt) // and path vector. for each neighbor_vector.d_addr do // Look for better paths. new_path ←{addr[link_id], neighbor_vector[d_addr]} // Build potential path. if my_addr is not in new_path then // Skip it if I’m in it.

if my_vector[d_addr] = NULL) then // Is it a new destination? my_vector[d_addr] ← new_path // Yes, add this one.

else // Not new; if better, use it. my_vector[d_addr] ← SELECT_PATH (new_path, my_vector[d_addr])

FLUSH_AND_REBUILD (link_id)


procedure SELECT_PATH (new, old) // Decide if new path is better than old one. if first_hop(new) = first_hop(old) then return new // Update any path we were

// already using. else if length(new) ≥ length(old) then return old // We know a shorter path, keep else return new // OK, the new one looks better.

procedure FLUSH_AND_REBUILD (link_id) // Flush out stale paths from this neighbor. for each my_vector,d_addr

if first_hop(my_vector[d_addr]) = addr[link_id] and new_vector[d_addr] = NULL

then delete my_vector[d_addr] // Delete paths that are not still advertised.

REBUILD_FORWARDING_TABLE (my_vector, addr) // Pass info to forwarder.


region R1

to

region R2R1.B

R3.C

R1.C

32

R1.D

1 R1.A forwarding table in R1.B

region forwarding local forwardingsection section

R1R2R3R4

region R3 region R4

link to

local R1.A1 R1.B1 R1.C3 R1.D

link

1end-layer

23


sender send first segment

receive ACK,send second segment

receive ACK,send third segment

(repeat N times)

Done.

segment 1

segment 2 ACK 1

ACK 2

3 ••• N

ACK N

receiver time

accept segment 1

accept segment 2

accept segment N


sendersend segment 1send segment 2send segment 3

receive ACK 1receive ACK 2

(repeat N times)

receive ACK N, done.

segment 1

ack 1

2

ack N

ack 2

3

• •

N •

receiver

time

acknowledge segment 1acknowledge segment 2

acknowledge segment N


receive permission,send segment 1send segment 2send segment 3send segment 4

receive ACK 1receive ACK 2receive ACK 3receive ACK 4,

wait … receive permission,

send segment 5send segment 6

sender receiver

segment #1 #2

ack # 2

#3

may I send?

yes, 4 segments

send 4 more

segment #5 #6

• • •

ack # 1

ack # 3 ack # 4

#4

time receive request,open a 4-segmentwindow

buffer segment 1buffer segment 2buffer segment 3buffer segment 4

finished processingsegments 1–4, reopenthe window

buffer segment 5buffer segment 6


usefulworkdone

unlimited resource

limited resourcewith no waste

congestioncollapse

capacityof a limited resource

offered load


duplicate acknowledgmentreceived

decrease additive

increase

multiplicative

delayWindow

size

slow start,again

timerexpires,stop sending

slow start

Time


leader destination source type data checksum 64 bits 48 bits 48 bits 16 bits 368 to 12,000 bits 32 bits


Station Identifier(Ethernet

17 24 12 05 19 Address)


procedure ETHERNET_HANDLE (net_packet, length) destination ← net_packet.target_id if destination = my_station_id or destination = BROADCAST_ID then

GIVE_TO_END_LAYER (net_packet.data, net_packet.end_protocol, net_packet.source_id)

else ignore packet


upper-layer network address

Ethernet station identifier

link identifier

Ethernet

server routerwork station

work station

work station

work station

1 61111 17 18 14 22 1915

L M N P Q K 1

… 2 34 5

G

H J

E

F


upper-layer network address link identifier G

L M N P work

station

1

Q K H J

server work

station station station work work

1 111 E 17 15 18 14 22 19

FEthernetEthernet station identifier

router

6

1

… 2 34 5

internetaddress

M N P QK E

Ethernet/station

enet/15enet/18enet/14enet/22enet/19enet/19


upper-layer network address link identifier

workstation

1

L M N P Q

server work

station

11

work station

1

work station

1 17 15 18 14 22

Ethernet Ethernet station identifier

internet Ethernet/ internet address station address

M enet/15 M E

K

router

6

1

… 2 34 5

19

Ethernet/station

enet/15enet/19

G

H J

E

F


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