Ch_15_Address Resolution Protocol (ARP)

8/8/2019 Ch_15_Address Resolution Protocol (ARP)

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Fall 2004 FSU CIS 5930 Internet Protocols 1

Address Resolution Protocol

(ARP)

Reading: Chapter 15


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ARP Mapping between layer 3 and layer 2

addresses

IP and MAC addresses

Given an IP address, what is the

corresponding MAC address?

Built on top of data-link layer Encapsulated and transmitted in data-link

layer data frames


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An example

Computer A:IP: 129.25.10.72

MAC: 49:72:16:08:64:14

Computer B:IP: 129.25.10.97

MAC: 49:72:16:08:80:70

Computer C:IP: 129.25.10.81

MAC: 49:17:92:96:96:96

IP: 129.1.11.72

MAC: 10:11:72:AB:02:01

IP: 129.25.10.11

MAC: 49:78:21:21:23:90

Router R

. . .

: : : : : : . . .

MAC address of 129.25.10.11 ?

Reply to: 49:72:16:08:64:14 (129.25.10.72)

ARP-Request to MAC FF:FF:FF:FF:FF:FF

: : . . . : : : : ::: . . . : : : : :

Reply:t: 129.25.10.11 (49:78:21:21:23:90)Requested by:: 129.25.10.72 (49:72:16:08:64:14)

ARP-Reply to MAC 49:72:16:08:64:14

1

2


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ARP packet format0 15 31

ARP Protocol Data Unit

Hardware type (layer2) Protocol type (layer3)

Address lengthLayer2 (n)

Address lengthLayer3 (m)

Operation

Source address (layer2): n bytes

Source address (layer3): m bytes

Destiniation address (layer2): n bytes

Destination address (layer3): m bytes

Layer-2 header Layer-2 payload Layer-3 trailer


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ARP packet format: example0 15 31

ARP-Request to FF:FF:FF:FF:FF:FF

0x00 01 (Ethernet) 0x80 00 (Internet Protocol)

6 4 0x00 01 (ARP-Request)

49 72 16 08

64 14 129 25

10 72 00 00

00 00 00 00

129 25 10 11

0 15 31

ARP-Reply to 49:72:16:08:64:14

0x00 01 (Ethernet) 0x80 00 (Internet Protocol)

6 4 0x00 02 (ARP-Reply)

49 72 16 08

64 14 129 25

10 72 49 78

21 21 23 90

129 25 10 11


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Receiving ARP packet and

replying Insert layer 2 address (MAC address)

Swapping source destination address

fields

Changing operation field (reply)

Sending ARP reply

Sources IP/MAC pair also inserted intoARP cache at receiver


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Implementation

Neighbors Computers that can be directly reachable

Data structures

arp_tbl neigh_table

neighbor

neigh_ops

Functions Sending/receiving ARP packets Managing ARP data structures


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neighbor

dev: Pointer to corresponding network device

timer:

Pointer to timer to initiate handling routine neigh_timer_handler()

ha:

Hardware address of the neighbor

hh: Hardware header

nud_state:

State concerning the neighboring computer

output:

Function to send data packet to the neighbor

arp_queue

Queue of packets waiting to be transmitted

opts:

Pointer to a neigh_ops structure


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neigh_ops Different neighbor characteristics

Generic, direct, hh, and broken

This structure defines the corresponding

functions for different devices

destructor, solicit, error_report, output,

connected_output, hh_output, queue_xmit


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States

NUD_NONE

NUD_NOARP

NUD_NONE

No

Emtru

NUD_

INCOMPLETE

arp_constructor:

initialize an entry,Send an ARP request

neigh_create:

Create an entry

NUD_

REACHABLE

Receive ARP-Reply

Permanent

Eintrag

NOARP-Device

Receive

packet

NUD_STALE

NUD_DELAY

NUD_PROBE

NUD_FAILED

receiveARP-Reply

No sign of life inreachable_time Set timer

Timeout

max_probes

Requests sent,

No reply

Garbage Collectioncompleted.

Delete entry


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ARP operation

. , .arp.c, neighbour.c

IPv4

Higher Layers

.dev.c

net_rx_action

IPv4

ip_finish_output2

arp.c, neighbour.c

.dev.c

ip_queue_xmit

dev_queue_xmit

neigh_resolve_output

arp_rcv arp_send

neigh_lookup

arp_tblneigh_update

ETH_P_ARP

ARP-Reply

ARP-Request


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Handling ARP PDUs arp_rcv()

Some sanity check

NF_ARP_IN arp_process()

arp_process() Some more sanity check

For both request/reply Update ARP cache (neigh_lookup())

For request arp_send() to send a reply


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Handling ARP packets arp_send()

Allocating socket buffer

Filling hardware header

Filling ARP data

NF_ARP_OUT

dev_queue_xmit()

neigh_update()

Updating neighbor entry state

Setting up corresponding output function, etc


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Handling unresolved IP

packets neigh_resolve_output()

Sending the packet if it is OK (e.g., REACHALBEstate)

Otherwise (e.g., INCOMPLETE state)

Storing the pkt in arp_queue queue byneigh_event_send()

neigh_event_send() Checking if the pkt can be sent

If not, storing IP packet and sending ARP request(arp_solicit())

arp_solicit() Send ARP request by arp_send()


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Some other neighbor

managing functions neigh_connect()

neigh_suspect()

neigh_destroy()

neigh_sync()

neigh_periodic_timer() neigh_timer_handler()

Ch_15_Address Resolution Protocol (ARP)

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