Internet Protocols

Address Resolution

IP Addresses are not recognized by hardware.

If we know the IP address of a host, how do we find out the hardware address ?

The process of finding the hardware address of a host given the IP address is called Address Resolution.Address Resolution.

Address Resolution Protocol (ARP) The Address Resolution Protocol is used by

a sending host when it knows the IP address of the destination but needs the Ethernet address.

ARP is a broadcast protocol - every host on the network receives the request.

Each host checks the request against it’s IP address - the right one responds.

Address Resolution Protocol (ARP) ARP does not need to be done every time an

IP datagram is sent - hosts remember the hardware addresses of each other.

Part of the ARP protocol specifies that the receiving host should also remember the IP and hardware addresses of the sending host.

ARP

ARPARP

Map IP MAC Local ARP

172.16.3.1

IP: 172.16.3.2 Ethernet: 0800.0020.1111 IP: 172.16.3.2 Ethernet: 0800.0020.1111

172.16.3.2

IP: 172.16.3.2 = ???IP: 172.16.3.2 = ???

I heard that broadcast. The message is for me. Here is my Ethernet address.

I need the Ethernet address of 176.16.3.2.

Reverse ARPReverse ARP

Map MAC IP

Ethernet: 0800.0020.1111IP: 172.16.3.25

Ethernet: 0800.0020.1111IP: 172.16.3.25

Ethernet: 0800.0020.1111 IP = ???Ethernet: 0800.0020.1111 IP = ???

What is my IP address?

I heard that broadcast. Your IP address is 172.16.3.25.

Internet Control Message Protocol ICMP is a protocol used for exchanging

control messages. ICMP uses IP to deliver messages. ICMP messages are usually generated and

processed by the IP software, not the user process.

ICMP Features Used by IP to send error and control messages Uses IP to send its messages Does not report errors on ICMP messages. ICMP message are not required on datagram

checksum errors. ICMP reports error only on the first fragment

ICMP Header ICMP Data

IP Header IP DataDatalink Header Datalink Data

ICMP Message Format

IP HeaderType of Message

Error CodeChecksum

Parameters, if anyInformation

8b8b16bVarVar

Sample ICMP Messages Source Quench: Please slow down! I just

dropped one of your datagrams. Time Exceeded: Time to live field in one of

your packets became zero.” or “Reassembly timer expired at the destination.

Fragmentation Required: Datagram was longer than and “No Fragment bit” was set.

Sample ICMP Messages (Continued) Address Mask Request/Reply: What is the

subnet mask on this net? Replied by “Address mask agent”

Redirect: Send to router X instead of me. Time Stamp Request/Reply: used to find

current time . ICMP error messages normally include the IP

header of the datagram that generated the error, plus at least 8 bytes following the IP header => ICMP message sizes = 70 bytes

ICMP: Message Types Summary

Type Message0 Echo reply3 Destination unreachable4 Source quench5 Redirect

8 Echo request11 Time exceeded12 Parameter unintelligible13 Time-stamp request14 Time-stamp reply15 Information request16 Information reply17 Address mask request18 Address mask reply

Ping Ping: Used to test

destination reachability, compute round trip time count the # of hops to destination may provide record route option. Sample output:

Reply from 164.107.144.3: 48 bytes in 47 msec. TTL: 253

Traceroute

Traceroute: Exploit TTL and ICMP Send the packet with time-to-live = 1 (hop) The first router discards the packet and sends an

ICMP “time-to-live exceeded message” Send the packet with time-to-live = 2 (hops) etc… Does not use optional features like record route

Path MTU Discovery

Send a large IP datagram with “Don’t fragment” bit set. Failure to fragment at a link will result in ICMP

message.

Summary

ICMP is the control sibling of IP ICMP is used by IP and uses IP as network

layer protocol ICMP is used for ping, traceroute, and path

MTU discovery.