IN2140: Introduction to Operating Systems and Data Communication Addressing in the TCP/IP model Layer 2 Address Resolution: ARP
Address resolution: Ethernet example Dest address (48 bits) Ethernet Header Dest addr (cont) Source addr (48 bits) Source address (cont) data length data Ethernet Trailer data checksum checksum § MAC address structure − Ethernet and WiFi are L2 layers using “EUI-48”: Extended Unique Identifier with 48 bits − 6 bytes, written like this: f2:18:98:3a:b8:97 − to recognize easily that the text is supposed to mean a MAC address § Ethernet MAC addresses should be globally unique University of Oslo IN2140 – Introduction to operating systems and data communication — 2
Addresses § my lab machine’s MAC addresses: Ethernet MAC WiFi MAC University of Oslo IN2140 – Introduction to operating systems and data communication — 3
Addresses § MAC addresses known to my lab machine DSL Modem’s Ethernet MAC § MAC addresses know to nordur, one of the login.ifi.uio.no machines (incomplete) University of Oslo IN2140 – Introduction to operating systems and data communication — 4
Address Resolution address resolution protocol end systems network intermediate system node with a packet to deliver: if a local cache contains IP address ⟷ MAC address send packet & update cache removal timeout else send broadcast to all stations “Who has IP address ?” if one node responds add IP address ⟷ MAC address mapping to cache set timeout for removal from cache to some minutes send packet else drop packet University of Oslo IN2140 – Introduction to operating systems and data communication — 5
Address Resolution Protocol (ARP) H H H H H H ARP Request source @IP: 9.228.50.8 @HW: 0xaa target @IP: 9.228.50.3 @IP: 9.228.50.3 @HW: @HW: 0xa3e ARP Response source @IP: 9.228.50.3 @HW: 0xa3e target @IP: 9.228.50.8 @HW: 0xaa University of Oslo IN2140 – Introduction to operating systems and data communication — 6
ARP Probe: verify a DHCP address H received address 9.228.50.3 from a DHCP server H H H H H H ARP Request source @IP: 0.0.0.0 sends ARP Probe @HW: 0xaa to ask if this address is target really available @IP: 9.228.50.3 @IP: 9.228.50.3 @HW: 0 @HW: 0xa3e ARP Response source @IP: 9.228.50.3 one host has this IP address: @HW: 0xa3e the network configuration is incorrect target @IP: 0.0.0.0 typical error when someone started a @HW: 0xaa second DHCP server in the same subnet University of Oslo IN2140 – Introduction to operating systems and data communication — 7
Address Resolution Protocol (ARP) ES 1 § End system not directly available 1GB Ethernet by broadcast 1GB Ethernet § Example: ES 1 to ES 2 UNINETT’s − ARP would not receive a response N x 10GB Ethernet • Ethernet broadcast is not rerouted over a router 1GB Ethernet § L2 solution: proxy ARP ES 2 − the local router knows all remote networks with their respective routers • responds to local ARP − local ES 1 sends data for ES 2 always to the local router, this router forwards the data (by interpreting the IP address contained in the data) § L3 solution: remote network address is known − local ES 1 sends data to the appropriate remote router − local router forwards packets University of Oslo IN2140 – Introduction to operating systems and data communication — 8
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