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Arp and DHCP Address Resolution The address resolution problem: remember IEEE 802.2 encapsulation... Dest Source CRC Length DSAP SSAP cntl code type Address Address Data ( 38 - 1492) (4) (6) (6) (2) (1) (1) (1) (3) (2) 1 Address Resolution Resolution through direct binding: Phys = f(IP) • Objective is to encode the IP address in Physical Dynamic binding protocol is required for Ethernet networks where: •Physical address larger than IP address (48 vs 32 bits) •Physical address can change •Potentially many hosts on the network Address Resolution Protocol (ARP) defined by RFC 826 2 Address Resolution •ARP is a protocol outside the set of TCP/IP protocols ARP R-ARP IP Link PHY 3 Address Resolution ARP Protocol: Case 1: Host A sends to Host D Specific example, host a issues ‘ping –c hostd’ Host B Host A Host D Host C ARP whois broadcast Arp ‘I am “ reply Unicast to Host Ping ICMP request Ping ICMP reply 4 Address Resolution Case 1 Issues •Why might there not be a response from D ? •What happens if D does not respond? •ARP cache entries timeout after 20 minutes •Why? •Why is the entry removed regardless of if the host uses the information during the 20 minutes? 5 Address Resolution Case 1 Issues •Why might there not be a response from D ? •Host D might not exist or might not be turned on •What happens if D does not respond? •Arp waits for a timeout amount of time and then generates an error….which then triggers IP to generate an ICMP error message to the offending host •ARP cache entries timeout after 20 minutes •Why? • To ensure unused IP’s are placed back in the pool •Why is the entry removed regardless of if the host uses the information during the 20 minutes? • To handle the case if the target has been assigned a new IP address. 6 Address Resolution ARP Protocol: Case 1: Host A sends to Host D BUT Host D does not exit Specific example, host a issues ‘ping –c hostd’ Host A Host B ping When IP Datagram reaches link layer, whois broadcast Ping timeouts, initiates next ping and next arp whois broadcast Arp does not RETRY!!!! Ping timesout and causes the next arp ‘whois’ 7 Address Resolution Case 2: Host D receives an ARP request message from Host A with target Host D. ….. Host D replies to Host A with a unicast ARP message providing Host D’s mac addr. Host D will cache The MAC/IP binding of Host A assuming it will need it in the near future. Case 3: Host C observes the request from A to D. It will use it only if A’s mapping is in its cache AND it is different. ARP topics: •Arp command: CLI to access the arp cache •Gratuitous arp: A Gratuitous ARP is an ARP Response that was not prompted by an ARP Request. The Gratuitous ARP is sent as a broadcast, as a way for a node to announce or update its IP to MAC mapping to the entire network. 8 Address Resolution Reverse Address Resolution Protocol •Used when a diskless host boots to learn its IP address •There are other ways a Host can learn its Address: Bootp and DHCP Host A R-ARP Server --------→ r arp req broadcast, supplies Host A’s mac addr ------------------- If R-arp server sees Host A’s mac in its table, it returns a R-ARP reply supplying Host A an IP addr to us Host B All other stations ignore the exchange9 DHCP •Reverse ARP: used by a host at boot to map its physical address to its IP address to use. •Three drawbacks with RARP as a dynamic IP assignment protocol: •Operates at too low a level •Protocol is inefficient •Does not support dynamically assigned hardware addresses. 10 DHCP •Two alternatives: •The Bootstrap protocol (BOOTP) •Dynamic Host Configuration Protocol (DHCP) •BOOTP: •client / server model •client finds bootstrap information •defined by RFC 951 (year 1985) •Uses UDP as the transport (with checksum for error detection) •BOOTP adds reliability (timeouts and retransmission) •Uses a limited broadcast to find server 11 DHCP •BOOTP is a two step process – Assume Host A boots with Boot P. Requires a Boot P server on the network. And optionally requires a TFTP server holding OS images. •Step 1: broadcast a request to the Bootp server asking for an IP and the IP addr of a TFTP server holding an image Host A will download and boot •Step 2: If necessary, Host A uses an tiny FTP client to download an OS image from the TFTP server Problems: •BOOTP is a static protocol •Requires a config file for each host •Can not dynamically change settings 12 DHCP •DHCP (RFC 2131 in year 1997) •Handles automated address assignment. •Allows a host to acquire all IP info needed (e.g., Subnet mask) in a single message. •DHCP server is supplied an address pool •Supports manual configuration or automatic DHCP supports 3 modes: •manual configuration – DHCP server must have an entry that maps a specific Host MAC addr with an IP •automatic configuration: the DHCP server assigns to the client a permanent addr from the pool. •dynamic configuration: The DHCP server leases the client an IP address for a certain amount of time. 13 DHCP Message format: DHCP Msg fields: OP: specifies req or reply Options fields holds all other info: the first three fields of all options are: Byte 1: code; Byte 2: length of option Byte 3: Type The Type specifies the type of DHCP msg: DHCPDISCOVER, …. Over 200 options can be appended in the options field in a reply – The code/length specify the type of option (starting in octet 4) and how many octets are 14 in the option DHCP client state machine DHCP client states: Init: Select: Bound: Renew: Rebind 15 DHCP Example: This is all that is in our ubuntu dhcp client conf file: send host-name = gethostname(); #send dhcp-lease-time 3600; request subnet-mask, broadcast-address, time-offset, routers, domain-name, domain-name-servers, domain-search, host- name, dhcp6.name-servers, dhcp6.domain-search, netbios-name-servers, netbios-scope, interface-mtu, rfc3442-classless-static-routes, ntp-servers, dhcp6.fqdn, dhcp6.sntp-servers; *It simply requests a number of configuration items…. 16.
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