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Arp basics (arp picture book 1 from visual land animations)
- 3. Overview ： ARP Basics Animation Link 05/16/11 www.visualland.net Goal: Visualize ARP activities of MAC lookup and interactions of ARP request, ARP reply, ARP cache 、 Ping command line. Topology: Two hosts H1, H2 are directly connected. There IP-MAC addresses are (IP.H1, MAC.H1), (IP.H2, MAC.H2) respectively. Steps: 1) H1 ping H2 fails due to ARP miss. H1 sends ARP Request to find out H2's MAC. 2) H2 responds ARP Reply with its MAC to H1. 3) H1 ping H2 again and succeeds.
- 5. H1 sends ARP request 05/16/11 www.visualland.net To find H2's MAC, H1 sends ARP Request and adds an entry (IP.H2, Incomplete) in ARP cache. It says: I'm looking for H2's MAC. ARP Request header includes a few parameters: sender IP is H1's IP, sender mac is H1's MAC, target IP is H2's IP, target mac is 0. (You may click ARP Request to see its headers).
- 6. H2 responds ARP reply 05/16/11 www.visualland.net After receiving ARP request, H2 adds H1's address to its ARP cache as (IP.H1, MAC.H2). Then H2 checks ARP Request's protocol header and learns that this query is for itself. H2 responds and sends ARP Reply to H1. (Click ARP Reply to see its protocol header)
- 7. H1 recevies ARP reply 05/16/11 www.visualland.net When receiving ARP Reply, H1 updates its ARP cache and changes the entry (IP.H2, Incomplete) to (IP.H2, Mac.H2).
- 8. H1 ping H2 again 05/16/11 www.visualland.net Now the first ping timeout. Ping prints a "." in the H1 command window to indicate a timeout. Then H1 sends next ping. No ARP miss this time. Ping does find H2's MAC in ARP cache.
- 9. H2 echo H1 05/16/11 www.visualland.net When receiving ping, H2 sends an Echo to respond. H2 is able to find H1's MAC in ARP cache. No ARP miss for sending Echo.
- 10. H1 receives echo 05/16/11 www.visualland.net After receiving Echo, H1 prints a "!" to indicate ping success.
Editor's Notes
- 1. What is ARP? ARP stands for Address Resolution Protocol. In Ethernet environment, when an application/protocol wants to send a packet, it needs to use the target node's MAC address to encapsulate its link header. But the application only knows the target's IP address, not its MAC address. So it uses ARP's service to get the target MAC address. 2. Why ARP? OSI model modularize network tasks into 7 layers and defines inter-layer interfaces. This way, a layer can implement several technologies. In IP network, IP address is in layer 3 (network layer), Ethernet MAC address is in layer 2 (link layer). However, when transmitting a packet, the application/protocol needs to encapsulate network header and link header first. Applications usually know about target node's IP address, not its MAC address. Therefore, we need a way to find layer 2 address without violating OSI layered structure? ARP is the answer: It maps a network address to link address. In the case of IP-Ethernet network, ARP maps an IP address to a MAC address... 3. What is ARP cache? ARP protocol stores discovered (IP, MAC) dresses in a local cache. When an application asks ARP protocol for the MAC address of an IP address, ARP lookup its ARP cache first. If found, return the MAC. Otherwise, send an ARP Request to broadcast an inquiry in the LAN. When ARP reply is received, ARP stores the newly discovered MAC in its ARP cache. Next time, if another application is asking for the MAC of this IP again, ASRP can return the MAC immediately. 4. What are ARP commands? It varies across operating systems or router vendors. But they do similar tasks. For example: - Display ARP cache content: show arp; arp -a - Clear ARP cache: arp -d 5. How is ARP related to encapsulation? When creating a packet, the application/protocol needs to encapsulate the packet's link header. The application needs to set destination MAC to the target node's MAC. Since the application does not know the MAC, It asks for ARP. To know more about encapsulation, see Encapsulation > 0. Encapsulation Overview.
- ARP 动画特色 可视化。观察 ARP 包在不同的拓扑上如何被洪泛、转发， ARP 表怎么被更新，数据包怎么用 ARP 表封装报头。 说白。把协议的要点、关键细节相结合，用白话说明 . 用漫画气泡表达节点间的互动性、协议的因果关系。 辅助教材。一般的网络书籍用 3-5 页严谨精确说明 ARP 知识点，这套动画用实例要点说明 ARP 及其它设备、协议、应用的关系。可以作为辅助教材，配合教科书。 Visualland.ney ( 北冰洋软件 ) 有两种动画、 理论动画的数据是作者编辑的，动画的数据、状态以示意为主，适合初学者。 实验动画的数据是从路由器采集，可以用 pcap 检查原始数据包。