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ARP and ICMP
Outline
à Address resolution protocol
à Proxy ARP à Reverse ARP à Internet Control Message Protocol
ADDRESS RESOLUTION PROTOCOL (ARP)
In the Protocol Model
TCP UDP
RPC
XDR
NFS
RTP SNMP
Telnet FTP SMTP HTTP
IP
Data link
Network
Trasport
Session
Presentation
Application
ICMP ARP
Rou6ng protocols
IGMP
General Features
à Solicitation protocol based on broadcast
à Mapping between a layer 3 address and a layer 2 address
à Any layer 2 and 3 protocols à Specified in each message
Operating Principles
à Ethernet encapsultion à Ethertype OxO8O6
à Address mapping <MAC address> <IP address>
à Stored in a cache à APR cache
à When a layer 3 packet is to be sent à If mapping is in cache, send
à If not, an ARP Request is generated
Address Resolution
MAC broadcast h H (IP) h (MAC) G (IP) ??
Relevant fields of a MAC frame Relevant fields of an ARP message
ARP Req
h (MAC) g G (IP) g (MAC) H (IP) h ARP Reply
G
H
3
ARP Reply: from (G,g) to (H,h)
ARP Request: from (H,h) to (G,?)
h
R
r g Ethernet address
IP address
..... G → g .....
4
ARP Cache
..... H → h .....
..... H → h .....
ARP Cache 2
1
ARP Message Format
Hardware Type Protocol Type
Opera6on
Sender Hardware Address
Sender HW Address
Sender IP Address
Target Hardware Address
HLEN PLEN
Sender IP Address
Target HW Address
Target IP Address
0 8 16 31
PROXY ARP
Relaxing the Physical/Logical Network Correspondence
Same LIS (logical IP subnet) on two or more physical networks
H1 R
192.168.1.5
192.168.1.1
H2
192.168.1.130 192.168.1.129
Why one LIS on multiple physical networks?
à Unexpected growth and expansion
H1 R
192.168.1.5 192.168.1.1
H2
192.168.1.2
à An originally single physical network is split
H1 R
192.168.1.5
192.168.1.1
H2
192.168.1.130 192.168.1.2
Why one LIS on multiple physical networks?
à By mistake
H1 R
192.168.1.5/24
192.168.1.1/25
H2
192.168.1.130/25 192.168.1.129/25
Operating Principle
H1 R
192.168.1.5
192.168.1.1
H2
192.168.1.130 192.168.1.129
à Proxy ARP (router) replies on behalf of host
REVERSE ARP (RARP)
General Features
à Knowing the layer 2 address of a host, discover layer 3 one
à Same packet format, different value for Operation field
à Solicitation protocol based on broadcast
à Once used by diskless hosts to find out their own address at boot
à RARP EtherType: Ox8O35 à Replaced by DHCP
à More flexible
Operating Principle
RARP Request A B
E D C
RARP Reply
A B
E D C
MAC A ?? MAC A ?? MAC A RARP Req
Relevant fields of a MAC frame Relevant fields of an ARP message
MAC broadcast
MAC E IP E MAC E IP A MAC A RARP Reply MAC A
INTERNET CONTROL MESSAGE PROTOCOL
(ICMP)
In the Protocol Model
TCP UDP
RPC
XDR
NFS
RTP SNMP
Telnet FTP SMTP HTTP
IP
ICMP ARP
Rou6ng Protocols
IGMP
Data link
Network
Transport
Session
Presentation
Application
General Features
à Service protocol à Encapsulated in IP
à Protocol type OxO1 à Error/condition reporting protocol à It does not specify (re)actions
à Message generation is not mandatory
à Messages can be ignored à Use cases
à Check network status à Report anomalies à Discover netmask à Improve routing
Message Format
0 7
Type-‐specific Data IP Header
+ First 64 bytes of datagram
Checksum
Type Code 15
Echo Message
à Used to check whether a host is reachable
à A Sequence Number is used to correlate Reply and Request messages
à Used by the PING application
Destination Unreachable Message 0 Network unreachable 1 Host unreachable 2 Protocol unreachable 3 Port unreachable 4 Fragmenta6on needed and DF set 6 Des6na6on network failed 7 Des6na6on host failed 8 Source host isolated 9 Comm. with dest. network administra6vely prohibited 10 Comm. with dest. host administra6vely prohibited 11 Network unreachable for type of service 12 Host unreachable for type of service
Code
à To suggest a better next hop to destination
à Not usable to notify a router (not the source)
Redirect Message
DG R
à TTL on an IP packet is zero à Used by the TRACEROUTE application
à Reassembly time out expires
Time Exceeded
à A problem with any of the fields in an IP header à E.g., invalid option
à Pointer field shows problematic field
Parameter Problem
Autoconfiguration Support
à Router Advertisement/ Solicitation à Default gateway autoconfiguration
à Address Mask à From host to know the netmask in use on subnet
à Directly to router or broadcasted
Source Quence
à Request to reduce packet rate