Brief History of the Internet ARPA (Advanced Research Project Agency) –

agency of the department of Defense. In the 1960s funded universities and organizations to

research the development of communication systems.

Let to the development of ARPANET – an experimental network that connected computer using packet switching.

Evolved in the Internet (capital I). http://www.computerhistory.org/internet_history

Section 19.1 – Logical addressing

IP address is a 32-bit number usually written in the form w.x.y.z. For example, 143.200.139.98.

There are 128-bit address (IPv6) but we’ll defer those until later.

nslookup can be used to determine the address. Also dig, host, named on Linux

Example: nslookup www.uwgb.edu or nslookup www.google.com

Devices have a physical address (Ethernet) and an IP address (logical address).

Command ipconfig /all (PC command prompt) Your IP address is given to you by your ISP and can

change; Network card determines the physical address.

Won’t change unless you install a new card.

An IP address consists of a Netid and Hostid Ex: Each campus computer has IP address

143.200.x.y 143.200 is the network number. x.y determined the device. Advantage:

Routers outside the campus network need only know in which direction 143.200 is located rather than tracking every possible machine.

Once on campus, then the specific machine is identified.

Address classes for the early Internet x’s define the Netid y’s define the Hostid

Class A: 0xxxxxxx.yyyyyyyy.yyyyyyyy.yyyyyyyy Class B: 10xxxxxx.xxxxxxxx.yyyyyyyy.yyyyyyyy Class C: 110xxxxx.xxxxxxxx.xxxxxxxx.yyyyyyyy Class D: 1110……multicast address……………..

Class determined by the first few bits Multicast (class D) identifies a group of hosts Unicast identifies one (Class A, B, C) 143.200 is a class B address since 14310 =1000 11112

Table 19.1 Number of blocks and block size in classful IPv4 addressingNOTE: Block means number of networks (globally)

Block size is the number of hosts (devices) in a network

Classless addressing Classful addressing too coarse for today’s needs. Need more flexibility than just class A, B, or C

addresses. An organization needing 5000 addresses (way too

large for a class C network) would be a class B network with ~65000 addresses.

Most would go unused.

Internet uses Classless Interdomain Routing (CIDR) Left most n bits define the Netid, rightmost n-32 bits

define the hostid. Question: how does a router extract the Netid for

forwarding?

Address mask Collection of contiguous 1s followed by contiguous 0s 1’s identify bits in the Netid; 0s the hostid Alternative way to identify the Netid

Table 19.2 Default masks for classful addressing

In general the notation x.y.z.t/n defines an IP address in which the leftmost n bits specify the Netid.

See ipconfig /all Subnet mask = 255.255.192.0 =

1111 1111-1111 1111-1100 0000- 0000 0000 Netid = logical AND of the IP address and mask HostID = logical AND of the IP address and mask

complement

Note that a 16-address block means an address mask of /28.

Host addresses differ ONLY in the rightmost 4 bits.

Supernetting Combining smaller physical networks into a single

larger one. Could combine several class C networks into a

single network.

Example Class C Network Bit Representation Address Range211.195.8.0 11010011-11000011-00001000-xxxxxxxx 211.195.8.0 to 211.195.8.255211.195.9.0 11010011-11000011-00001001-xxxxxxxx 211.195.9.0 to 211.195.9.255211.195.10.0 11010011-11000011-00001010-xxxxxxxx 211.195.10.0 to 211.195.10.255211.195.11.0 11010011-11000011-00001011-xxxxxxxx 211.195.11.0 to 211.195.11.255

Address mask is 255.255.252.0 (11111111.11111111.11111100.00000000)

All bits the same

Subnetting (reverse of supernetting): Dividing a network into smaller networks All hosts in a single subnet share the same subnet

number. Hosts and NetIDs are addressed consecutively Number of addresses in a subnet is a power of 2.

Reasons to subnet Separate different media (e.g. cable from optical fiber) Separate devices that provide different functions such as

various types of servers. Security concerns Better reflect the structure of an organization Better manage network traffic

example An organization is given a block of 64 addresses

defined by 17.12.14.0/26. This means it has 26=64 IP addresses. It wants 3 subnets of size 16, 16, and 32. Subnet mask for the larger subnet has twenty seven

1s followed by five 0s. The smaller ones have a mask with twenty eight 1’s

followed by four 0s A possible arrangement is

19.17

Figure 19.7 Configuration and addresses in a subnetted network

Last 8 bits of the IP addresses, Net IDs underlined 0000-0000 thru 0011-1111 (64 addresses) Subnet 1: 0000-0000 thru 0001-1111 (32 addresses) Subnet 2: 0010-0000 thru 0010-1111 (16 addresses) Subnet 3: 0011-0000 thru 0011-1111 (16 addresses)

Example 19.10 on page 561.

NAT (Network address translation) based router: If you all buy the same router from Best Buy, chances are

your computers will ALL have the same IP address given to it by the router.

For example:

192.168.x.x is a private address space.

internet

LAN

NAT-based router

192.168.0.2

192.168.0.4

192.168.0.3

A

C

B

24.164.37.109

Assigned by ISP Addresses assigned by router

Book covers a couple of designs; we’ll cover just their last one

Router has IP address Each device behind the router has an IP address,

BUT router hides them from the Internet world. A packet sent from a device to the router contains a

source IP address (w) and port # (x) Router replaces them both with a fixed IP address

(y) and another port # (z) and forwards packet to the internet.

Returning responses will be sent to y

Router maintains a table that relates (w, x) and (y, z) Packet from Internet arrives at router; router looks

up address in the NAT table It substitutes and forwards the packet.

Advantages: Hides IP addresses from Internet world allows IP addresses to be reused eliminates some tasks associated with managing

subnets (NAT-based router does it) useful for home networks where consumer does not want to manage IP addresses

NAT-based router looks like a single device to the Internet world

Disadvantages: Purists object to using port numbers to identify

addresses (when they were designed to identify applications). Some see it as a kludge (pronounced klooj – nonstandard technique) to solve a problem that should be solved via IPv6

other

IPv6 – section 19.2 but just the highlights There are not enough IPv4 addresses IPv6 uses a 128-bit address

19.26

Figure 19.14 IPv6 address in binary and hexadecimal colon notation

19.27

Figure 19.15 Abbreviated IPv6 addresses

Can specify Registry: which agency registered the address (

INTERNIC for north America, RIPNIC for Europe, APNIC for Asia and Pacific countries)

Provider: e.g. your ISP Subscriber: e.g. a provider’s customer Subnet: if the subscriber is an organization, it may have

multiple subnets. Node: the device.

IPv6 also provides Security Streaming support Streamlined packets and more flexible packet

headers for quicker routing Authentication It has been in the process of being phased in for

years.

Section 20.1 Internetworking Not a lot here, mostly setting the context and we’ve

seen this before.

20.31

Figure 20.2 Network layer in an internetwork

Section 20.2 IPv4

20.33

Figure 20.4 Position of IPv4 in TCP/IP protocol suite

20.34

Figure 20.5 IPv4 datagram format

IP Packet (also a datagram) contents See the book for most details but a couple of

relevant things follow. Source & destination addresses. Time-To-Live (TTL) field – decremented by one

each time a router forwards the packet. When it is 0, it is discarded.

Checksum (on header only) – for error detection. Needs to be recalculated at each router since the header

can change. Checksumming the header only is quicker Higher level protocols will error check the data if needed.

Fragmentation bits. The IP protocol allows for the possibility that an IP packet

might travel a network that forces an IP packet to divided into smaller pieces.

You can skip this section. Priority bits – could allow a router to prioritize the

packets it has in case of congestion . It was never really used.

Type of service (TOS) bits allow an app to request a type of handling.

20.38

Table 20.2 Default types of service

That same field also allows differentiated services – the ability of a router to examine this field and to determine the quality of service (QoS) expected of the higher layer. E.g. a file transfer or streaming real-time data.

Bits to define the protocol above IP using its services.

Allows the specification of a route to follow or to record the route taken.

Sections 20.3 and 20.4 deal with IPv6 and the transition from IPv4 to IPv6.

It’s not difficult reading but I won’t cover it. Be aware of the issues however.

Section 21.1 Address mapping Will cover ARP (address resolution protocol)

only – and only a general description of it.

The problem

Sender sends an IP packet across the Internet to a remote device.

Intermediate routers will route based solely on destination IP address.

The last router must deliver the IP packet directly to the device, most likely by embedding the IP packet into an Ethernet frame and sending it over the underlying LAN.

How does it determine the physical address?

ARP (Address Resolution Protocol). Router sends a broadcast (containing the IP address)

to all devices on a LAN. Device associated with that IP address responds by

sending its MAC address. Router stores that info and then embeds the IP

packet in a MAC frame and sends it. The following diagram illustrates but I will not go

into detail with regard to the ARP packet format or variations of this. It’s accessible to you based on what we’ve covered.

21.44

Figure 21.1 ARP operation

Chapter 22: Delivery, Forwarding, and Routing

Network Layer: Routing and IP Problem

A network may be visualized as a graph Find a route from S (source address) to D (destination

address)

Does it matter which you choose?

An edge may have costs Cost of a route = sum of edge costs

May just treat all edges the same (cost=1) Cost of route = number of edges (number of hops)

Delivery: Section 22.1 Direct delivery

Packet goes from one device to a destination located on the same physical network

Indirect delivery Packet goes through multiple devices on its way to its

destination. Devices are routers. Last router is on the same physical network as the

destination. From there, it’s direct delivery.

Forwarding: Section 22.2 A router will: receive a packet and send it to some

other router or to the destination. Route method:

Either the router or packet contains the complete route Can be used by some maintenance protocols to test

routes, but not common. Next Hop method

Router knows ONLY the next router (hop) in a path Analogies to the US mail

In this case, the next node is along a “cheapest path”. If all costs are 1, then cheapest is shortest. Other criteria might be used

Method of forwarding Host specific

Router has one table entry for every possible destination Not realistic

Network specific Router has one table entry for each physical network that

is reachable. It identifies the network number. One entry for all destinations on the same physical

network.

22.52

Figure 22.3 Host-specific versus network-specific method

Router actions Get packet and extract IP address If source route is specified, extract info and route,

otherwise Determine Netid from the IP packet and search the

routing table If Netid found and router attached to that network

determine physical address via ARP. Embed packet into an Ethernet frame and send. Otherwise

If Netid found and router not attached to that network send over link specified in the routing table

If Netid is not found send to default router.

22.55

Figure 22.6 Configuration for Example 22.1

22.56

Table 22.1 Routing table for router R1 in Figure 22.6

Skip the rest of 22.3 after the previous example

Routing

Discuss Dijkstra shortest path algorithm.[http://www.dgp.toronto.edu/people/JamesStewart/270/9798s/Laffra/DijkstraApplet.html]

Routing protocols Autonomous system (AS): collection of networks

and routers under a single administration. Intradomain routing: routing inside an AS Interdomain routing: routing between AS’s

Routing Information Protocol (RIP) An implementation of a distance vector

protocol. Route with minimum distance Minimum is shortest if all edge costs are 1. In

that case the cost is the hop count.

Bellman-Ford (also Distance vector). Based on the principle of optimality

Distance vector algorithm

Routing table contains possible destinations, costs to get there, and the next node in the route.

Get information from each neighbor’s routing table. Is it cheaper to get to a node by going through that

neighbor first? If so, update the entry in the current routing table.

Example:

Each row is a routing table for the node at the left end

Linux traceroute command DOS tracert command on australia.net, hawaii.net,

alaska.net See [

http://www.uwgb.edu/serverstatus/netmanagement/wiscnet.htm]

Distance Vector Routing has some problems when routers are connected in a loop but there are ways to deal with them.

That would be for a second class.

Link State Routing Each router shares its routing table with all others. Over time, each router learns the network topology Can apply algorithms such as Dijkstra’s algorithm to

find the cheapest path to any destination. Neither Link State nor distance vector routing scale

well to LARGE numbers of routers. Again – they are intradomain routing

Border Gateway Protocol Based on a path vector routing algorithm Interdomain routing Routes among speaker nodes (one that acts for an

entire AS); there is one for each AS Speaker nodes communicate, indicating accessibility

to nodes within their domain.

22.69

Figure 22.30 Initial routing tables in path vector routing

22.70

Figure 22.31 Stabilized tables for three autonomous systems