Internet Architecture

Reading 

Make sure you read all of chapter 2 & 3

© MMII JW Ryder

CS 428 Computer Networking

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Internet Architecture 

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How are networks interconnected to form an internetwork?  We’ve seen how hosts on a single network interconnect Physically  2 networks attached by a computer that attaches to both of them  Doesn’t guarantee that the computer will cooperate with other hosts Need computers willing to shuffle packets from one network to another  They are internet gateways (internet routers)

© MMII JW Ryder

CS 428 Computer Networking

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IP Routers  

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See figure 3.1 on page 56 (draw) Need to know about topology of internet beyond connected networks See figure 3.2 on page 57 

R 1 must transfer from network 1 to network 2 all packets destined for both network 2 and 3

© MMII JW Ryder

CS 428 Computer Networking

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IP Routers 

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In a TCP/IP internet, computers called routers or gateways provide all interconnections among physical networks. Routers are usually small computers with limited memory and disk storage Trick: Routers use the destination network, not the destination host when routing a packet

© MMII JW Ryder

CS 428 Computer Networking

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IP Routers 

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Amount of information needed to keep is proportional to the number of networks in the internet not the number of computers User’s view of internet is single virtual network to which all machines connect See figure 3.3 on page 59 Advantage of network level interconnection (over app level) is that application level programs need not be concerned with network details

© MMII JW Ryder

CS 428 Computer Networking

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All Networks are Equal 

Any communication system capable of transferring packets counts as a single network   

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Ethernet LAN WAN - ANSNET backbone Point to Point link

How do IP addresses work

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CS 428 Computer Networking

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Internet Addresses 

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TCP/IP internet, a virtual network built by interconnecting physical networks with routers Now we discuss addressing Hides physical network details

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CS 428 Computer Networking

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Universal Comm. Service   

Any to any host Needs globally accepted method of computer id Names - What an object is 

Addresses - Where it is 

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Route - How to get there

IP addresses - 32 bit integers  

Chosen to make routing efficient Encodes id of network to which the host attaches as well as host id itself

© MMII JW Ryder

CS 428 Computer Networking

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IP Addresses  

Unique 32 bit address used for ALL communication with that host Simplest view   

Each host has unique 32 bit addr Bits of all hosts on a common network share same prefix Each addr. is a pair (netid, hostid)  

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netid = network id hostid = host id CS 428 Computer Networking

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IP Address Classes - 2 bits can distinguish between 3 common classes Class A (000)

0

Class B (128)

1

Class C (192)

1

Class D (224)

1

Class E (240)

1

© MMII JW Ryder

netid

hostid

0

netid

1 0 1 1

1

hostid

netid 0

1 1

hostid multicast address

0

reserved for future use CS 428 Computer Networking

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Address Classes 

Class A - Handful of networks that each have more than 216 (65,536) hosts 

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Class B - Intermediate size networks between 28 and 216 hosts 

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7 bits to netid, 24 bits to hostid

14 bits to netid, 16 bits to hostid

Class C - Less than 28 hosts 

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21 bits to the netid, 8 to hostid CS 428 Computer Networking

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IP Addresses     

Dealt with as 32 bit addresses Assume following IP Addr. 137.141.152.35 (Decimal) 89.8D.98.23 (Hexadecimal) 1000 1001.1000 1101.1001 1000.0010 0011 (Binary)

Class B (128)

0

1

1

0

2 3 4...8

...

16

...

netid

‘898D’x

24

...

31

hostid

‘9823’x

1 0 0 0 1 0 0 1 -1 0 0 0 1 1 0 1 - 1 0 0 1 1 0 0 0 - 0 01 0 0 0 1 1

2445 © MMII JW Ryder

CS 428 Computer Networking

38947 decimal 12

Multi-Homed Hosts 

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A router connects 2 or more physical networks How is one IP address assigned to the router??? IP Addr. encode a network and a host A router is a multi-homed host

© MMII JW Ryder

CS 428 Computer Networking

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Network Connections   

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Routers have 2 or more IP Addresses One for each network on which it resides IP addresses do not specify and individual computer, but a connection to a network A router connecting n networks has n distinct IP addresses, one for each network connection

© MMII JW Ryder

CS 428 Computer Networking

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Network Addresses     

By convention, hostid 0 is never assigned to an individual host IP addr. with hostid = 0 refers to the network itself 137.141.0.0 (hostid = 0.0 right??) This allows IP addresses to refer to networks as well as hosts All bits of hostid = 0 ? 

Refers to a network

© MMII JW Ryder

CS 428 Computer Networking

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Broadcast Address 

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Any hostid consisting of strictly 1’s is reserved for broadcast Broadcast not supported on all networks Early release of Berkeley Unix used all 0’s, still around  Sometime option of specifying 0’s - not recommended Directed broadcast still correctly identifies the network but broadcasts to all hosts on a given network

© MMII JW Ryder

CS 428 Computer Networking

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Limited Broadcast   

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32 bits of 1’s Broadcast only to local network Host may use this prior to knowing its IP addr. at start up Field of 1’s = ‘All’ Field of 0’s = ‘This’ IP addr. with hostid = 0 refers to ‘this’ host Network id = 0 refers to ‘this’ network

© MMII JW Ryder

CS 428 Computer Networking

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This 

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Receive a packet with netid of dest. addr = 0 and hostid matches its address Interpret to mean ‘this’ network Addressing scheme actually slightly more intricate 

Subnetting

© MMII JW Ryder

CS 428 Computer Networking

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Internet Addresses   

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Disadvantages Address refers to network connections Host moves to a different network, it must change its IP addr. If IP addr referred to host itself the could move without changing IP addr.

© MMII JW Ryder

CS 428 Computer Networking

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Traveling PC Example 

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Disconnect your computer from your local network. Take it with you on a trip Reconnect to an internet at destination PC cannot ‘own’ an IP address because the IP addr. is associated with the physical network not the host Do 137.141.152.35 example in Lons le Saunier

© MMII JW Ryder

CS 428 Computer Networking

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Another Disadvantage 

Any class C network that grows beyond 255 hosts must change its class to Class B  

Class C (192)

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Big pain to change One less Class B network address available 0

1

2 3 4...8

1

1

0

...

16

... netid

CS 428 Computer Networking

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...

31

hostid

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Largest Flaw  

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Not entirely apparent until we do routing Routing is based on internet addresses (IP addr) with netid portion used to make routing decisions Multi-homed host 

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Because routing uses the netid portion of IP addr. the path taken to a multi-homed host will depend entirely upon which IP addr. was supplied CS 428 Computer Networking

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Multiple Routes  

Multiple names for same host yield packets for same host behaving differently It may be impossible to reach a dest. even though you know a valid IP addr. Network 1 I1 I2 I3 R

A I4

I = IP Addr. © MMII JW Ryder

Network 2

R = Router

B I5

B known by two IP addresses

CS 428 Computer Networking

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Dotted Decimals 

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IP Addresses written as 4 decimal integers separated by decimal points 10000000 00001010 00000010 00011110 is 128 . 10 . 2 . 30 Unix netstat command - displays current routing telnet, ftp - application programs all use dotted decimal notation

© MMII JW Ryder

Class

Lowest Addr.

A B C D E

0.1.0.0 128.0.0.0 192.0.1.0 224.0.0.0 240.0.0.0

Highest Addr.

CS 428 Computer Networking

126.0.0.0 191.255.0.0 223.255.255.0 239.255.255.255 247.255.255.255 24

Loopback Address 

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Not all possible addresses are assigned to classes 127.0.0.0 not used - Class A Used for loopback - usual addr used is 127.0.0.1 Intended for TCP/IP testing and IPC on local machine

© MMII JW Ryder

CS 428 Computer Networking

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Special Addresses All 0’s All 0’s

This host

Host

Host on this net Limited Broadcast (local net)

All 1’s Net

All 1’s

Directed Broadcast for specific network

127

Anything,often 1

Loopback

Limited and Direct Broadcast addresses are never valid source addresses. Length of netid portion of directed broadcast depends on network class. All 0’s only allowed at system start up. © MMII JW Ryder

CS 428 Computer Networking

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Internet Addressing Authority  

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Internet Assigned Number Authority (IANA) Ultimate control over numbers assigned and sets policy An organization can obtain network addresses from the Internet Network Information Center (INTERNIC) Only needed to assign network portion of IP Address. Why ???

© MMII JW Ryder

CS 428 Computer Networking

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More IP Addr Stuff 

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Only necessary to assign IP Addresses to networks which will participate in the Internet 9.0.0.0 = IBM; 12.0.0.0 = AT&T; 10.0.0.0 = ARPANET See figure 4.5 on page 73 

3 networks, what are their classes? How do you know?

© MMII JW Ryder

CS 428 Computer Networking

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IP Address Assignment  

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See figure 4.6 on page 73 Host computers now attached and IP addresses assigned 4 hosts  Arthur, Merlin, Guenevere, Lancelot 2 routers  Taliesyn, Glatisant Multi-homed host Note network IP addresses, remember classes

© MMII JW Ryder

CS 428 Computer Networking

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Network Byte Order 

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32 bit binary integer; What does it look like in memory ??? Let’s take the hexadecimal number ‘6789ABCD’x Which one does it look like ?     

a) 6789ABCD b) ABCD6789 c) CDAB8967 d) 8967CDAB e) None of the above

© MMII JW Ryder

CS 428 Computer Networking

30

Network Byte Order Let’s take the hexadecimal number ‘6789ABCD’x  Which one does it look like ?  a) 6789ABCD (Big Endian)  b) ABCD6789 (Little Endian)  c) CDAB8967 (LE&ByteSwap)  d) 8967CDAB (Just odd)  e) None of the above  Little Endian stores LOB portion of the integer at the lowest address  Big Endian stores HOB portion of the integer at the lowest address © MMII JW Ryder CS 428 Computer Networking 

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Network Byte Order 

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With TCP/IP, Network Standard Byte Order is defined to be Big Endian  Most Significant Byte is sent first and down the line If the architecture of the machine does not match this pattern then it is the responsibility of the TCP/IP developers for that machine to convert data on the way in and out

© MMII JW Ryder

CS 428 Computer Networking

32

Internet Routing  

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Read Chapters 7 & 8 Routing - Process of choosing a path over which to send packets Router - A computer making such a choice Goal: “… provide a virtual network that encompasses multiple physical networks and offers a connectionless datagram delivery service.” - Comer

© MMII JW Ryder

CS 428 Computer Networking

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Routers 

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An internet composed of multiple physical networks interconnected by computers called routers Router - direct connections to 2 or more physical networks Host computer - a single, direct connection

© MMII JW Ryder

CS 428 Computer Networking

34

Routers     

Multi-homed hosts - direct connections to 2 or more physical networks Both hosts and routers route IP datagrams Hosts?? Two or more routers on network? Which to send to? Host must make decision Any computer with multiple network connections can act as a router

© MMII JW Ryder

CS 428 Computer Networking

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Delivery 

Direct - Local   

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Sender encapsulates datagram in a physical frame Binds dest IP addr to a physical hardware addr Sends frame to destination

Known local because netid matches host’s netid Indirect - Between physical networks

© MMII JW Ryder

CS 428 Computer Networking

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Indirect Delivery   

Sender must identify a router to send datagram to Router then forwards it toward its destination When frame reaches the router  Software extract the encapsulated datagram  IP software selects next router along the path  Datagram again placed in a frame  Sent over physical network to next router

© MMII JW Ryder

CS 428 Computer Networking

37

Table Driven Routing 

IP Routing Table on each machine 

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 

netstat -nr

Want to contain most minimal information at all times Next hop routing done by using the routing tables (N, R) pairs  

N = netid part of IP addr R = Next router along path to N

© MMII JW Ryder

CS 428 Computer Networking

38

Next Hop Routing 

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Important - all routers identified in machine M’s routing table are physically located on the same network as M When a datagram ready to leave M   

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IP software locates dest IP addr extracts the network portion to make routing decision M will select a router which M can directly reach

Hosts have minimal information, rely on routers

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CS 428 Computer Networking

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10.0.0.0 10.0.0.5 Q

R

20.0.0.5 20.0.0.0

IP Addr Gateway

20.0.0.6

20.0.0.0

Direct Delivery

30.0.0.0

Direct Delivery

10.0.0.0

20.0.0.5

40.0.0.0

30.0.0.7

30.0.0.6 30.0.0.0 S

30.0.0.7 40.0.0.7 40.0.0.0

© MMII JW Ryder

Routing Example CS 428 Computer Networking

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Consequences     

All traffic destined for given network use same path Even when multiple paths exist, won’t be used Only final router can determine if dest. host exists or is operational Need way for routers to return delivery problem errors Datagrams travelling from A to B may follow a different path from B to A

© MMII JW Ryder

CS 428 Computer Networking

41

Default Routes 

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Especially useful when local network has small set of hosts and 1 connection to internet 2 tests  

One for local network Default that points to default router

© MMII JW Ryder

CS 428 Computer Networking

42

Routing Algorithm RouteDatagram (Datagram, RoutingTable) Extract destination IP addr, D, from the datagram and compute the network prefix, N; If N matches any directly connected network address, deliver datagram to destination D over that network (This involves resolving D to a physical address, encapsulating the datagram, and sending the frame) else if the table contains a host-specific route for D send the datagram to next hop specified in the table else if the table contains a route for network N, send datagram to next hop specified in the table else if the table contains a default route, send datagram to the default router specified in the table else declare a routing error © MMII JW Ryder

CS 428 Computer Networking

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