2/15/2011Harvard Bits1. 2/15/2011Harvard Bits2 US Telegraph “Network” in 1856.

2/15/2011 Harvard Bits 1


US Telegraph “Network” in 1856



• Size of switch grows as square of number of telephones

• Impractical to centralize switching as number of telephones grows

Historically a telephone call is completed by setting switches so there is a continuous electric circuit from telephone to telephone

Pros: Dedicated line means uninterrupted service once circuit is completed


Number of calls limited by size and number of switches

Long telephone lines are “seized” by the call even if no one is talking, one party hangs up, etc.

Hard to utilize alternative routes if a switch along the principal path is overloaded


Alternative, used by the Internet Break message into data packets of 1-

2KB Address the packets to their destination

and serial number them so they can be reassembled at the other end

Let network figure out how to deliver them

Different packets of same message may take different routes


Extremely efficient use of network lines - whenever a link is not transporting a packet it is available for completely unrelated messages

Size of switch depends on actual data traffic, not on the number of simultaneous communications that might be happening


No performance guarantees possible, so hard to be sure real-time communications will work

Routing algorithms are not obvious, though if they can be made adaptive, the network could heal itself in case of localized catastrophes

Seems to require more intelligence at the edge of the network, while circuit switching requires intelligence in the core and can tolerate dumb devices at the edge



2/15/2011 Harvard Bits 11Client Computers

Web Server

www.harvard.edu

e-mail Server

pop.fas.harvard.edu

e-mail Server

smtp.fas.harvard.edu

download uploadTHE INTERNET

Router in network core receives incoming packets and stores them in “buffer” (temporary storage)

Routes packets on outgoing links May throw packets away if buffer is full


Routing Table

Routers are relatively dumb and rely on intelligence at the edge to compensate


• Packetize

• Add serial #s

• Add fingerprint

• Add destination address

• Insert into network

BEST EFFORT

• Reassemble packets

• (Maybe) report missing packets

• (Maybe) report damaged packets

• Deliver to application

Client application: email, web browser, iTunes Server application

IPv4: 32 bits written as 4 decimal numerals less than 256, e.g. 141.211.125.22 (UMich)

4 billion not enough IPv6: 128 bits written as 8 blocks of 4 hex digits each, e.g.

AF43:23BC:CAA1:0045:A5B2:90AC:FFEE:8080 At edge, translate URLs --> IP addresses, e.g. umich.edu

--> 141.211.125.22 Authoritative sites for address translation = “Domain

Name Server” (DNS) In the network core, IP addresses are used to route

packets using routing tables



We treat IP addresses as Non-Personal Information

We reserve the right to share Non-Personal Information with affiliates and other third parties.


ICANN = Internet Corporation for Assigned Names and Numbers

A US nonprofit … but it’s a long story.


Routers do not know what the bits in the packets represent

Do not know if they are email, streaming video, html web pages

Do not know if they are encrypted or unencrypted

You can invent your own new service adhering to IP standards

Gain Internet’s best-effort service and possibility of undelivered packets


Packet size (1.5 KB max) a compromise Small enough that they can be “handled”

quickly and with relatively low odds of being damaged

Large enough that packaging does not outweigh the contents or “payload”


Smallish packets also make better use of the network since later packets can leave before earlier packets arrive


1 2 3 4



1

2 3 4



12

3 4



123

4



1234



1234



1234



1

234



1 2

34



1 2 3

4



1 2 3 4

Store and Forward delays would add up if entire message had to be buffered at every router


1 2 3 4



1 2 3 4



1 2 3 4



1 2 3 4



1 2 3 4



1 2 3 4

Creates logical connection between two machines on the edge of the network

Connected machines seem to have a circuit connecting them even though they do not tie up the network

Provide reliable, perfect transport of messages, even though IP may drop packets

Regulates the rate at which packets are inserted into the network




1 2


1 2

“3-Way Handshaking”


1 2



1 2



1 2



1 2



1 2



1 2



1 2



1 2



1 2



1 2



1 2



1 2



1 2



1 2



1 2



1 2



1 2



1 2

“Virtual Circuit” now established between two hosts though the routers in between are not aware of it and the same path need not be followed by all packets


11 2


11 2


1 2 12


11 2 2


1 2 12


11 2 2


ACK1

1 2 12


ACK1

1 2 12


ACK1

1 2 1 2

ACK2


ACK1

11 2

ACK2


ACK1

1 2 1

ACK2

2


12 2

ACK2


2 21

ACK2


2 21

ACK2


1 2


1 2


11 2


11 2


11 2 2


1 2 2


1 2 2


1 2 2


1 2 2


1 2 2


1 2 12

TIMEOUT

Used for real-time applications (e.g. streaming audio and video) where timing is essential but perfect delivery is not



2 3 1


3 2 1


3 12


3 12


3 1


3 1


3 1


3 1


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Both TCP (guaranteed delivery) and UDP (fast delivery, no guarantees) use the lower-level Internet Protocol in the “link layer”

But TCP and UDP know nothing about links, routing, etc. All that knowledge is embedded in IP



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2/15/2011Harvard Bits1. 2/15/2011Harvard Bits2 US Telegraph “Network” in 1856.

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