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CSCI 233Internet Protocols

Class 3

Dave Roberts

First…a little review

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Internet Protocol Principles

Good Citizen Principle

Scarcest Internet Resource

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Tonight Internet addresses

Mapping IP addresses to physical addresses

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Internet addressing

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The Internet Is it a physical or virtual network?

It’s a virtual network, defined by protocols that run on hosts and routers.

Internet protocols make the Internet look like a world-wide uniform network, although it encompasses many networks that are very different from each other.

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Internet Addresses Each host connection on the Internet has a unique Internet address

The addresses are designed to make forwarding of Internet packets simple

An IP address has two parts: a prefix that identifies a network and a suffix that identifies a host on the network

Question: How many IP addresses can a router have?

Question: How many IP address can a host that is not a router have?

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IPv6 Addressing Each address is 128 bits

Enough addresses for every person on earth to have an internet with three times the addresses of the present Internet!

1024 addresses per square meter of the earth’s surface

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Functions of Internet Addresses

Provide a unique identification for a particular interface between a device and the network so that a datagram can be delivered to the correct recipient

Enable a path to be found across the Internet to reach the recipient, a process called routing

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IPv4 Address 32-bit integer, unique for each host on the network, used in all communication with the host

<IP address> ::= <netid> <hostid>◦ Netid: identifier of a network◦ Hostid: identifier of a host on the network

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Address Class Characteristics

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Host Capacities

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Advantages of Classful Addressing

Simplicity and clarity—addresses and their setup are very easy to understand

Flexibility to accommodate different sizes of networks

Ease of separating host address for routing Allows for reservation of some addresses for special purposes

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Classless IPv4 Addressing

Temporary addressing scheme that does away with class A, B, C addresses

Network prefix can be any specified length

Forwarding techniques expanded to account for this: called Classless Inter-Domain Routing (CIDR)

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Address Resolution Protocol (ARP)

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IP and MAC Addresses IP address is used to send datagrams across the Internet—that is, between networks, through routers

MAC address is used to deliver a frame of data within a single network

We send a datagram across the Internet with only an IP address

To deliver to a device at the destination network, a MAC address must be used

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Local Delivery The router at the destination network has the job of delivering the packet to the appropriate host

The router uses the local physical network to deliver to the local host

The local physical (MAC) address must be used, not the IP address

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Address Resolution The process of determining the physical address that corresponds to an IP address is called address resolution

Address resolution must occur at every network the packet encounters in its journey across the Internet

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Resolution by Direct Mapping

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HA = f(IA)

Direct Mapping Resolution

Choose a numbering scheme that makes address resolution efficient

PA = f(IA)

If either P’s or I’s can be chosen, a correspondence can be established

Alternatively, lists of P-A pairs can be stored

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Direct Mapping Sadly, NICs usually come with MAC addresses built in, so we can’t establish any function to determine IP addresses

We must do something else to track the correspondence of IP addresses to MAC addresses

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Resolution by Dynamic Binding

ARP broadcasts a request packet

Host who has IP address in packet replies with physical address

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IPv4 ARP Message Format

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Hardware type: 1 for EthernetProtocol type: 0800 for IP addressesXlen—length of physical and high-level addressesARP exchanges involve filling in missing addresses

ARP Caching Broadcasting an inquiry is expensive

Every host must have a cache of recently acquired bindings

Results of ARP requests are cached

Before sending request, the cache is checked

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ARP Cache Timeout Responsibility for cache correctness is with the host maintaining the cache

Timeout value is set, and addresses from cache are not used if timeout value is exceeded

ARP performance is sensitive to the value of the timeout

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Soft State ARP cache is an example of “soft state”

Cache owner keeps record of acquired results, avoids cost of future inquiries

Cache is usually timed out to automatically remove stale values

Soft state is another central concept of Internet protocols

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Central Concepts1. Good Citizen Principle

2. Conservation of Router Time

3. Soft State

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ARP Refinements Every ARP request has binding of source IP and MAC addresses

Since request is broadcast, all machines can extract sender’s IP to MAC address mapping and cache it

Most computers broadcast a gratuitous ARP request when they start up in case their mapping has changed

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IPv6 ARP Describe IPv6 ARP

There isn’t any!!!

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IPv6 Neighbor Discovery Neighbor is another computer on the same network

NDP allows an IPv6 host to discover all neighbors and routers upon startup

Early binding avoids delays when packets are transmitted

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Conventions 1’s refer to “all”

0’s refer to “this”

Hostid=0 address refers to this network

Hostid=1’s broadcast to all hosts◦ Directed broadcast—netid of a specific network◦ Local broadcast—32 1’s—used at startup

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Address Resolution Physical addresses are used by hardware devices that can communicate directly (ie, MAC addresses on a LAN)

IP addresses create a massive virtual network Network layer sends datagrams across the virtual network

Data link layer sends frames between physical devices

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Need for Address Resolution

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ARP Each host has an Internet address, Ia

Each host also has a physical address, Pa

How to route packet to physical address, given its Internet address? Two instances

◦ Sending packets to routers, which have physical addresses◦ Sending packets to hosts, which have physical addresses

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Dynamic Binding Resolution

ARP—Address Resolution Protocol

host A broadcasts packet with address IB

Asks host B to respond with PB

B recognizes the packet, responds with PB

A receives response, uses PB to send to B

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ARP Cache Cache of recently-acquired physical addresses is kept

ARP is used for packets after the first in a transmission

ARP cache times out after an interval

Example of “soft state”

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ARP Refinements Sender includes its own I to P binding in every ARP broadcast, so that receiving site, and others, can update ARP caches

Receivers update I to P binding in ARP cache before processing ARP packet

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ARP Encapsulation

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Internet Protocol Principles

Use the Good Citizen Principle to limit the impact of resource shortages

Conserve the scarcest Internet resource

Use caching to avoid repeated inquiries

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Summary IP Addressing

IP Address Resolution

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