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Sublayers Under the Network Layer: BOOTP & DHCP

Sublayers Under the Network Layer: BOOTP & DHCP. How does a host get allocated an IP address? allocated manually by an administrator: OK as far as it goes but does not scale to large networks and some networks (e.g., home networks) don't have (competent) administrators

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Sublayers Under the Network Layer: BOOTP & DHCP

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  1. Sublayers Under the Network Layer: BOOTP & DHCP • How does a host get allocated an IP address? • allocated manually by an administrator: OK as far as it goes but does not scale to large networks and some networks (e.g., home networks) don't have (competent) administrators • Use the Dynamic Host Configuration Protocol

  2. Sublayers Under the Network Layer: BOOTP & DHCP • DHCP, and its predecessor BOOTP, exist to allocate IP addresses to hosts • We start with BOOTP then describe DHCP which is similar but massively extends BOOTP • Both extend RARP

  3. Sublayers Under the Network Layer: BOOTP & DHCP BOOTSTRAPPING PROTOCOL: BOOTP • BOOTP also supplies extra information, e.g., where to download an operating system: for bootstrapping diskless hosts • The source IP address in a request is 0.0.0.0: this is what we are trying to find • Destination is usually 255.255.255.255: a broadcast • “Can anyone tell me my IP address?”

  4. Sublayers Under the Network Layer: BOOTP & DHCP BOOTP Header

  5. Sublayers Under the Network Layer: BOOTP & DHCP BOOTP Header • IP addresses as described, UDP header explained later • Opcode: 1 for a request, 2 for a reply

  6. Sublayers Under the Network Layer: BOOTP & DHCP BOOTP Header • Hardware type: 1 for an Ethernet address • Length: 6 for an Ethernet address • Hop count: starts at 0 and used by BOOTP servers that pass requests on to other servers

  7. Sublayers Under the Network Layer: BOOTP & DHCP BOOTP Header • Transaction ID: 32 bits of random value generated by the client and returned by the server. This allows the host to identify the reply directed to itself as there might be many BOOTP replies floating on the network

  8. Sublayers Under the Network Layer: BOOTP & DHCP BOOTP Header • Seconds: number of seconds elapsed since the the client started to boot. If this gets large it allows a backup BOOTP server to realise the main server might have crashed

  9. Sublayers Under the Network Layer: BOOTP & DHCP BOOTP Header • Client address: client fills in its IP address, if known, else 0

  10. Sublayers Under the Network Layer: BOOTP & DHCP BOOTP Header • Your address: in the reply the allocated IP address • Server address: which server replied

  11. Sublayers Under the Network Layer: BOOTP & DHCP BOOTP Header • Gateway address: a gateway for the local network

  12. Sublayers Under the Network Layer: BOOTP & DHCP BOOTP Header • Client hardware address: can guide the server in choosing an IP address

  13. Sublayers Under the Network Layer: BOOTP & DHCP BOOTP Header • Server hostname: optionally, the name of the server

  14. Sublayers Under the Network Layer: BOOTP & DHCP BOOTP Header • Boot filename: if the client wants to download a boot file

  15. Sublayers Under the Network Layer: BOOTP & DHCP BOOTP Header • Other: used by DHCP

  16. Sublayers Under the Network Layer: BOOTP & DHCP Dynamic Host Configuration Protocol :DHCP • The Dynamic Host Configuration Protocol extends BOOTP considerably, but is backwardly compatible (a DHCP server can reply to a BOOTP request)‏ • DHCP is designed for general configuration of a host: address, subnet mask, gateway, name servers, and more • Usually for hosts with non-permanent connections, e.g., laptops

  17. Sublayers Under the Network Layer: BOOTP & DHCP • DHCP has a pool of available addresses that it can assign to hosts as they need • When a host leaves, its address can be reused • DHCP gives a lease time on an address • If the lease expires the host can renew via DHCP • A well-behaved host will signal (via DHCP) when it no longer needs an address

  18. Sublayers Under the Network Layer: BOOTP & DHCP • DHCP supplies • IP address • Subnet mask • Gateway • name servers • lease times • print servers • Gateway • boot servers • mail servers • host name • web servers

  19. Sublayers Under the Network Layer: BOOTP & DHCP DHCP • The data is encoded in a DHCP packet as • a tag byte to indicate the type of data • a length byte • the data itself • Tag 0 to pad data for alignment • Tag 255 to indicate end of data

  20. Sublayers Under the Network Layer: BOOTP & DHCP DHCP HEADER

  21. Sublayers Under the Network Layer: BOOTP & DHCP • Before supplying an address, DHCP should ping the network for that address: this is to check that no host is accidentally using that address already

  22. Sublayers Under the Network Layer: BOOTP & DHCP • Before supplying an address, DHCP should ping the network for that address: this is to check that no host is accidentally using that address already • After getting an address, a host can send an ARP reply containing its own address. This gratuitous ARP informs other hosts on the network of the new address association so they can update their ARP caches

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