Oct 21, 2004CS573: Network Protocols and Standards1 IP: Addressing, ARP, Routing Network Protocols...

Oct 21, 2004 CS573: Network Protocols and Standards

1

IP: Addressing, ARP, Routing

Network Protocols and Standards

Autumn 2004-2005

Oct 21, 2004 CS573: Network Protocols and Standards 2

IPv4 IP Datagram Format IPv4 Addressing ARP and RARP IP Routing Basics Subnetting and Supernetting ICMP Network Address Translation (NAT) Dynamic Addressing


Internet Addresses (IP Addresses) Defined when IP was standardized in 1981 IP addresses are 32-bit long and consist of:

a network address part – network identifier a host address part – host number within that

network IP addresses are grouped into classes

(A,B,C) depending on the size of the network identifier and the host part of the address

A fourth class (Class D) was defined later (1988) for Multicast addresses


Internet Address Classes

NETWORK

0 8 16 24 32 bits

NETWORK

NETWORK

IP MULTICAST ADDRESSES (28 BITS)

RESERVED FOR EXPERIMENTS

0

10

110

1110

11110

Class A

Class B

Class C

Class D

Class E

HOST (24 BITS)

HOST (16 BITS)

HOST (8 BITS)


Internet Address Classes Class A

126 networks (0 and 127 reserved) Assigned to very large size networks where 65K <

number of hosts < 16M Class B

16384 networks Assigned to Intermediate size networks where 256 <

number of hosts < 65K Class C

2097152 networks Assigned to smaller networks where #hosts < 256


Dotted Decimal Notation Internet addresses are represented

in text by the dotted decimal notation each byte is written in decimal values

(from 0 to 255) example:

10000000 00001010 00000010 00011110is written as 128. 10. 2. 30


Internet Address Classes

Class Lowest Network Identifier Address

Highest Network Identifier Address

A 1.0.0.0 126.0.0.0

B 128.0.0.0 191.255.0.0

C 192.0.0.0 223.255.255.0

D 224.0.0.0 239.255.255.255

E 240.0.0.0 247.255.255.255


Uniqueness of IP Addresses Network numbers are assigned by a central

authority The Internet Network Information Center (InterNIC) Another authority, the IANA – Internet Assigned

Numbers Authority sets policy Network numbers are unique worldwide Host numbers are assigned by network

managers They must be unique within a given network

Thus, IP addresses are unique worldwide.


Special Purpose IP Addresses 0.0.0.0

Means this host, used by machines as source address when they boot up (if they don’t know their IP address, and need to get it from a boot server)

255.255.255.255 Means limited broadcast. Used as a destination address to send

packets to all hosts on the local network where the source is. Packets sent to this address are never relayed

Network part all zeros Means the host on this network

Host part all ones Broadcast address on the network specified in the network

identifier; routers typically do not forward these datagrams Host part all zeros

Broadcast address on the network specified in the network identifier (it was an implementation error in some networks)


Special Purpose IP Addresses 127.x.x.x

Means loopback (datagrams are looped back in software; they are not sent on any physical interface)

224.0.0.1 Multicast address for “All systems on this

subnetwork” 224.0.0.2

Multicast address for “All routers on this subnetwork”


Internet Addresses

This Host

Host on this network

Limited broadcast (local net)

Directed broadcast (for NET)

Loopback

ALL 0’s

ALL 0’s HOST

ALL 1’s

NET ALL 1’s

ANYTHING127


Translating Between IP and MAC Addresses (ARP and RARP) Each interface has an IP address at Layer

3, and a MAC address at Layer 2 Assume that host A wants to send a packet

to host B (A and B on the same network) Host A knows the IP address of host B;

however, in order to transmit the packet, host A must somehow know or find out what the MAC (layer 2) address of host B is!

Solution: the Address Resolution Protocol (ARP), RFC826


Address Resolution Protocol Used to find the physical address of a target device on

the local physical network, given only the target’s IP address

Mechanism: The source broadcasts a special packet asking the device

with target IP address to respond with a message carrying the (IP address, physical address) mapping

All devices on the local physical network receive the broadcast, but only the target recognizes its IP address and responds to the request

When the source receives the reply, it sends the packet to the target using the target’s physical address and places the mapping in its cache (a cache is used to prevent repeated broadcasts for the same destination)


More on ARP ARP refinements

Source includes its <IP address, physical address> mapping in the ARP request anticipating the target’s need for it in the near future. This avoids extra network traffic

When all machines receive the ARP request broadcast, they can store the address mapping in their cache. Do they?

ARP is used when an IP to physical address mapping changes to notify hosts on the network of the change

ARP messages are encapsulated in MAC frames. A special value in the type field of the frame is used to indicate that it is carrying an ARP message (0806 hex is used for ARP)

Entries in the local ARP cache for each host time out after a certain period


ARP Message Format

PROTOCOL TYPE

TARGET IP (octets 0-3)

160 8 24

HLEN

HARDWARE TYPE

SENDER HA (octets 0-3)

TARGET HA (octets 2-5)

31

PLEN OPERATION

SENDER IP (octetS 0-1)SENDER HA (octetS 4-5)

TARGET HA (octets 0-1)SENDER IP (octetS 2-3)


ARP Message HARDWARE TYPE: specifies type of hardware interface

for which the request is made (e.g., 1 for Ethernet) PROTOCOL TYPE: specifies high level protocol address

supplied in message (e.g. 0800 hex for IP) HLEN and PLEN: specify length of fields for hardware

address and protocol address respectively OPERATION: specifies if this is an ARP request or reply

message (1 for ARP request, 2 for ARP response, 3 for RARP request and 4 for RARP response)

HA and IP: hardware and IP addresses respectively


Reverse ARP Usually, a machine’s IP address is kept on its

secondary storage (OS finds it at start up) Issue : Diskless Workstations!

files are stored on a remote server need IP address to use TCP/IP to obtain initial boot image

Solution : Use physical address to identify machine Given a physical network address, find the

corresponding Internet address Reverse Address Resolution Protocol (RARP),

RFC903


RARP Mechanism

Sender broadcasts a RARP request, supplying its physical network address in the Target HA field

Only machines authorized to supply the RARP service (RARP servers) process the request and send a reply filling in the target internet address

Mechanism allows a host to ask about an arbitrary target

thus sender HA is separate from target HA address RARP server replies to sender’s HA

Ethernet frame Protocol Type for RARP is 8035 hex

Oct 21, 2004 CS573: Network Protocols and Standards

19

IP Routing

Network Protocols and Standards

Autumn 2002-2003


Routing IP Datagram Direct Delivery (i.e., not involving routers):

Transmission of an IP datagram between two machines on a single physical network does not involve routers

The sender encapsulates the datagram in a physical frame, binds the destination IP address to a physical hardware address (using ARP), and sends the resulting frame directly to the destination

The two machines are known to be on the same network because they have the same network identifier

Example: A sends IP Datagram to B

Router

A B C


Routing IP Datagram Indirect delivery (i.e. through

intermediate routers) Host performs routing decisions based on

routing table indicating “next hop” “Next hop” refers to next router IP address on

this network, via which the destination is reached

Routing decisions are made based on network prefixes (not full IP address)

The sender encapsulates the datagram in a frame with the router’s physical destination address (which is found by means of ARP).


Direct and Indirect Routing

Host A204.240.18.10

Host B204.240.18.20

Router

204.240.18.1

Internet

Host C36.14.0.200

Direct Routing: Packets sent directly usingMAC address of A

Indirect Routing: Packets sent to the MAC addressof the router. At the IP level, B isThe source and C is the destination

B wants to send packets to A and C!


IP Routing Decisions

Network10.0.0.0

Network40.0.0.0

Network20.0.0.0

Network30.0.0.0

R1R2

R3

10.0.0.5

20.0.0.520.0.0.6 30.0.0.6

30.0.0.7

40.0.0.7

Routing Table of R2

To Reach Hosts on Network

Next Hop Address

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


IP Routing Algorithm Router receives an IP datagram with network portion N

and destination D If N is directly connected

Transmit on that network Else If host specific entry for D exists

Use next hop in that entry Else If route entry for N exists

Use next hop in that entry Else If default route for next hop exists

Use default route for next hop Else

Declare error


Routing Within Same Network Consider a small company with a single LAN

to which a class C network address has been assigned

The company is interested in adding another small physical network (connected to old network through a router) with a few hosts

Question: Could this company assign these hosts IP addresses from the same C class network? i.e., could the two LANs share the same class C network address?


Proxy ARP

Used to allow two physical networks to share the same IP network prefix

Router R’s table is configured manually to route between these two networks

Router R answers ARP requests on each network for hosts on the other network, giving its own hardware address as the target address

Main Router

A B C

Main NetworkTo Internet

Hidden NetworkED

Router R


Proxy ARP Advantage of Proxy ARP Router

Can be added without disturbing the routing table in other hosts or routers on that network

Disadvantages: Does not generalize to complex network topologies

(does not scale) Does not support a reasonable form of routing.

(relies on network managers to maintain tables of machines and addresses manually)

Issues: Several IP addresses map to the same physical

address. How to distinguish between a legitimate Proxy ARP router and spoofing?

Oct 21, 2004CS573: Network Protocols and Standards1 IP: Addressing, ARP, Routing Network Protocols...

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