ADDRESS RESOLUTION PROTOCOL
“DATA NETWORKS” FOR JTOs PH-II – Address Resolution Protocol
ADDRESS RESOLUTION PROTOCOL What is Address Resolution Protocol? All Interfaces on the Network are identified by an unique 32-bit IP address. Every IPdatagram carries in its header the Source/Destination IP address for Routing the Packet. However, for actual transmission, these IP datagrams are encapsulated in Data Link Layer Frames. The Data Link layer Frame needs Hardware Addresses as part of their framing (See Fig. 1). The Protocols required to create the association between Hardware Addresses (Physical Addresses) and IP Addresses are called ADDRESS RESOLUTION PROTOCOL. Name: A IP Address : 144.12.12.06 144.12.12.26
Name: B IP Address:
HA: (080010C2A102) Hexa Hexa
HA: (080010310596)
??? Dest. H/W Addr
‘080010310596’ ‘0800’ IP datagram Source H/W Addr. Ethernet Type Fig.1
CRC
Need for Address Resolution Protocol The host system knows the IP address of the Destination by using DNS (Domain Name System) or a Table Look-up. But the IP datagram cannot be transmitted without Destination Hardware Address in a MAC (Media Access Control) Frame. (See Fig. 2). IP Datagram 144.12.12.26 144.12.12.06 Data Source IP Addr. Dest. IP Addr. ??? ‘080010310596’ Hexa ‘0800’ Hexa IP datagram CRC Dest. H/W Addr Source H/W Addr. Ethernet Type Data Link Frame (MAC Frame) Fig. 2 One Solution for this problem is manually configuring the TCP/IP system, the relation between IP Address and MAC Address of all Nodes in that Network Segment. The problem with this approach is if the Network Interface Card (NIC) is replaced on a host the MAC Address changes and the table is to be updated on all Nodes. Hence there is a need for a Dynamic Mechanism to determine the Destination Hardware Address knowing the its IP Address. This Dynamic Mechanism is BRBRAITT : Nov-2006
2
“DATA NETWORKS” FOR JTOs PH-II – Address Resolution Protocol implemented as a separate Protocol called the ADDRESS RESOLUTION PROTOCOL.
ARP Format Fig 3 shows the format of ARP-Request and ARP- Reply packets and its encapsulation in the Data Link Frame (for e.g. MAC Frame). Ehernet type value ‘0806’ Hexadecimal is reserved for ARP frames. 2 Octets 4 Octets
2 Octets
Hardware Type
Protocol Type
1 Octet
1 Octet
Hlen Add. Length
Plen Add. Length
2 Octets
Operatio n Field
= 1 for ARP-Request
“ Note 1” Dest. H/W Addr .
6 Octets
Sender H/W Addr.
4 Octets
Sender IP Addr.
Target H/W Addr.
6 Octets
Target IP Addr.
=2 for ARP-Reply
‘080010310596’ ‘0806’ ARP Data Source H/W Addr. Ethernet Type
CRC
Data Link Frame (MAC Frame) Note 1: For ARP Request Pkt : Dest. H/W Addr. is ‘FFFF FFFF FFFF’ ( Broadcast Addr.) For ARP Reply Pkt : Dest. H/W Addr. is H/W of the sender who has generated the ARP-Request Packet (Point-to-Point) Fig.3
ARP Request Format Hardware Type:- 2 Octets Value ‘1’ in Hardware type fields indicates it is Ethernet Network. Other values are listed in Table 1. Table 1 ARP Hardware Type Values Hardware Type value Description of Network 1 Ethernet (10 Mbps) 6 !EEE 802 Networks 7 ARCNET
BRBRAITT : Nov-2006
3
“DATA NETWORKS” FOR JTOs PH-II – Address Resolution Protocol Protocol Type:- 2 Octets Value ‘0800’ Hexadecimal indicates it is DoD IP Protocol. For other Protocol types see Table 2. Table 2 Frequently used Protocol Type(Ethernet Type) Values Ethernet Type (Hexa) Ethernet type Field Assignment 0800 DoD IP 0806 ARP (Address Resolution Protocol) 8035 RARP (Reverse Address Resolution Protocol) Hlen :- 1 Octet Hardware Address Length value is '6 Octets' in Ethernet Plen :- 1 Octet Protocol Address Length value is '4 Octets' in DoD IP Protocol. Operation:- 2 Octets For ARP Request operation field value is '1'. For ARP Reply the value is '2' Refer Table 3. for other values. Table 3 Operation Values for ARP Packet Operation Field Type of Operation Value 1 ARP-Request 2 ARP-Reply 3 RARP-Request 4 RARP-Reply 5 DRARP-Request 6 DRARP-Reply 7 DRARP-Error 8 InARP-Request 9 InARP-Reply 10 ARP-NAK Sender Hardware Address:- 6 Octets The Sender Hardware Address contains the Hardware Address of the Sender Sender IP Address :- 4 Octets Sender IP Address contains the IP Address of the Node sending the ARP Request. Target Hardware Address:- 4 Octets The Target Hardware Address is to be determined by ARP Protocol. It is either set to all '0's or all '1's. (all '1's in case of Ethernet).
BRBRAITT : Nov-2006
4
“DATA NETWORKS” FOR JTOs PH-II – Address Resolution Protocol Target IP Address:This is the IP Address of the Target Node . The Target node responds with Hardware address in ARP-Reply Packet after identifying this IP Address. Encapsulation of ARP-Request Packet at the Data Link Level:Data Link Source Hardware Address is Hardware address of the ARP Request sender. Data Link Destination Hardware Address is Ethernet Broadcast Address usually all '1's. (FFFF FFFF FFFF) Hexadecimal (See Fig.4) Note: ARP Protocol operates on the Physical Network which supports Broadcast capability viz. Ethernet, Token Ring, FDDI, ARCnet etc. Ethernet Type Value is '0806' Hexadecimal which indicates that the ARP Data is carried in the Frame. Name: A IP Address : 144.12.12.06 144.12.12.26 HA: (080010C2A102) Hexa Hexa
Name: B IP Address: HA: (080010310596)
ARP Broadcast ARP Reply
ARP Broadcast ARP Reply
Broadcast ‘FFFFFFFFFFFF’ ‘080010310596’ ‘0806’ ARP Request Pkt Dest. H/W Addr Source H/W Addr. Ethernet Type
CRC
Pont-to-Point ‘080010310596’ ‘080010C2A102’ ‘0806’ ARP Reply Pkt Dest. H/W Addr Source H/W Addr. Ethernet Type
CRC
Fig. 4
BRBRAITT : Nov-2006
5
“DATA NETWORKS” FOR JTOs PH-II – Address Resolution Protocol
ARP - Reply Format The ARP- Reply Packet uses the same format as ARP-Request, but the Operation field value is set to '2' to indicate it is ARP-Reply. Sender Hardware Address:- 6 Octets This contains the Target node's Hardware Address. (This is the Answer) Sender IP Address:- 4 Octets This contains the Target Node's IP address. Target Hardware Address:- 6 Octets This contains the Hardware Address of the Node which generated the ARPRequest Packet. Target IP Address:- 4 Octets This contains the IP Address of the Node which generated the ARP-Request Packet. Encapsulation of ARP-Reply Packet at the Data Link Level:Data Link Source Hardware Address is Hardware address of the Node generating ARP-Reply Packet. Data Link Destination Hardware Address is the Hardware Address of the Node which generated the ARP-Reply Packet. ARP-Reply is not Broadcast. It is Point- to-Point (See Fig. 4). Ethernet Type Value is '0806' Hexadecimal which indicates that the ARP Data is carried in the Frame.
BRBRAITT : Nov-2006
6
“DATA NETWORKS” FOR JTOs PH-II – Address Resolution Protocol
ARP Operation:When IP Datagram is ready for transmission the Routing Component in the Network Layer (IP Layer) determines whether the Destination IP address is in Local Network or Remote Network. If it is in Local Network the sender host needs to find out the Hardware Address of the Target Node. If it is in the Remote Network the sender host needs to find out the Hardware Address of the Router Port to which the IP Datagram is to be forwarded (See Fig 5). Datagram from Upper Layers Network/ IP
Routing Component
Data Link Physical
To External Network
Destination on Local Network IP Router
Destination on Remote Network
Fig 5 ARP Protocol cannot be routed. That is it cannot cross the Router boundary. Before sending the ARP request the ARP module tries to find the Target Address in the ARP Cache table . The ARP cache table keeps pairs of entries of IP addresses and the corresponding Hardware Addresses (See Table 4) Table 4 ARP Cache Table Protocol Type (IP) 0800 -------
Protocol Address (IP Address) 144.12.12.06 ---------------------
Hardware Address (MAC Address) 080010C2A102 ----------------------
Time Stamp (Minutes) 15 ---
If the Target IP address is found in the ARP Cache Table it returns the corresponding Hardware Address and the IP datagram is transmitted to the destination in a MAC Frame. If the Target IP Address is not found in the ARP Cache Table ARP Request is broadcast at the Data Link Layer and on receipt of the ARP-Reply and the ARP Cache Table is updated. Usually the Age of the ARP Cache entry is for 15 minutes. After time out ARP Request is again needed to find the Hardware Address of the Target. Procedure involved in Routing an IP Packet from ‘Node A1’ to ‘Node B1’:- (See Fig. 6)
BRBRAITT : Nov-2006
7
“DATA NETWORKS” FOR JTOs PH-II – Address Resolution Protocol Procedure involved at Node A1:1. Since the Destination IP Address is not in the Local Network the IP Datagram is to be forwarded to Router-A which is connected to the Remote Network. 2. Node A1 looks into ARP Cache Table to find the H/W Address of Router-A. 3. If found the IP datagram is forwarded to Destination H/W Router-A.
Address of
4. If not found the Node A1 generates ARP-Request packet to find the H/W Address of Router-A and Broadcasts the MAC Frame Containing the ARP-Request Packet. 5. Router-A responds with its H/W Address in the ARP-Reply Packet encapsulated in a MAC Frame addressed to Node A1. 6. Node A1 updates the ARP-Cache Table and sets the Time stamp value to 15 Minutes. 7. Node A1 sends the IP Datagram encapsulated in a MAC Frame to Router-A . Procedure involved at Router-A:1. Router-A analyses the Destination IP Address in IP Datagram and Routes the Packet to Router-B Procedure involved at Router-B:1. Since the Destination IP Address belongs to the Local Network the IP Datagram is to be forwarded to Node B1 which is directly connected to the Ethernet LAN. 2. Router-B looks into ARP Cache Table to find the H/W Address of the Node B1. 3. If found the IP datagram is forwarded to Destination H/W Address of Node B1. 4. If not found the Router-B generates ARP-Request packet to find the H/W Address of Node B1 and Broadcasts the MAC Frame Containing the ARP-Request Packet 5. Node B1 responds with its H/W Address in the ARP-Reply Packet encapsulated in a MAC Frame addressed to Router-B. 6. Router-B updates the ARP-Cache Table and sets the Time stamp value to 15 Minutes. 7. Router-B sends the IP Datagram encapsulated in a MAC Frame to Node B1.
BRBRAITT : Nov-2006
8
“DATA NETWORKS” FOR JTOs PH-II – Address Resolution Protocol Procedure involved at Node B1:1. Node B1 receives the IP Datagram sent by Node A1. Node
A1
Node B1
Router Network Router A
Router B
Node A2
Node A1 Node-B1
Node B2
Router -A
Router-B
ARP-Request ARP-Reply IP Datagram IP Datagram ARP-Request ARP-Reply IP Datagram
Flow of ARP Packets/ IP Datagrams Fig. 6
BRBRAITT : Nov-2006
9