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Dynamic Host Configuration Protocol (DHCP)

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1. Overview and Architecture

The Dynamic Host Configuration Protocol (DHCP) (RFC 2131) is an application-layer network management protocol that automates the process of configuring devices on IP networks.

DHCP extends the BOOTP architecture and message format, maintaining backward compatibility while introducing Dynamic IP Address Pools and Temporary Leases:

• UDP Port 67: DHCP Server listens on Port 67.
• UDP Port 68: DHCP Client listens on Port 68.

Three Address Allocation Mechanisms in DHCP:

1. Dynamic Allocation: DHCP leases an IP address to a client from an available pool for a limited duration of time (a Lease). When the lease expires, the IP is automatically returned to the pool for reuse.
2. Automatic Allocation: DHCP assigns a permanent IP address to a client from a pool on a first-come, first-served basis (no lease expiration).
3. Manual / Static Reservation: The administrator binds a specific IP address permanently to a specific MAC address (used for web servers, default printers, and routers).

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2. The DHCP "DORA" Four-Way Handshake

When a new client joins a network, it obtains its network configuration via the DORA sequence:

1. Discover (DHCPDISCOVER)

• The client boots with no IP address ($0.0.0.0$) and broadcasts a DHCPDISCOVER packet to destination 255.255.255.255 on UDP Port 67 to locate any available DHCP servers on the subnet.

2. Offer (DHCPOFFER)

• Each DHCP server that receives the broadcast checks its pool, reserves an available IP, and transmits a DHCPOFFER containing the proposed IP address, subnet mask, lease duration, and the server's own IP address.

3. Request (DHCPREQUEST)

• The client may receive multiple offers from different servers. It selects one offer (typically the first received) and broadcasts a DHCPREQUEST.
• Why Broadcast? It informs the selected server of its acceptance while simultaneously alerting all other competing servers that their offers were rejected, allowing them to release their reserved IPs back into the pool!

4. Acknowledgment (DHCPACK)

• The chosen server sends a DHCPACK finalizing the transaction and delivering complete network parameters:
  • Assigned Client IP Address.
  • Subnet Mask (e.g., 255.255.255.0).
  • Default Gateway Router IP (e.g., 192.168.1.1).
  • DNS Server IP Addresses (e.g., 8.8.8.8).
  • Lease Time duration (e.g., $86{,}400\text{ seconds} = 24\text{ hours}$).

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3. DHCP Lease Renewal Lifecycle

To prevent long-disconnected devices from permanently occupying IP addresses, DHCP uses timed leases:

1. At 50% of Lease ($T_1$ Renewal Timer):
   • Client sends a unicast DHCPREQUEST directly to the original leasing server. If the server is online, it replies with DHCPACK, resetting the lease timer back to $100\%$.
2. At 87.5% of Lease ($T_2$ Rebinding Timer):
   • If the original server failed to respond at $T_1$, the client broadcasts a DHCPREQUEST to any active DHCP server on the network to extend its lease.
3. At 100% (Lease Expiration):
   • If no server responds, the client must immediately stop using the IP address, release it, and restart the full DORA process from DHCPDISCOVER.

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4. DHCP Relay Agent across Subnets

Like BOOTP, DHCP relies on the DHCP Relay Agent (RFC 3046 / Cisco ip helper-address) running on default routers. When the router intercepts a local broadcast DHCPDISCOVER on UDP 67, it converts it into a unicast IP datagram, inserts its incoming interface IP into the Gateway IP (giaddr) field, and forwards it to the central enterprise DHCP server across the WAN.

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5. Comprehensive Comparison: ARP vs. RARP vs. BOOTP vs. DHCP

Feature	ARP	RARP	BOOTP	DHCP
Layer	Data Link / Network	Data Link	Transport / App (UDP 67/68)	Transport / App (UDP 67/68)
Resolves	IP $\rightarrow$ MAC	MAC $\rightarrow$ IP	MAC $\rightarrow$ IP + Boot info	Dynamic IP + Full Configuration
Crosses Routers?	No (L2 broadcast)	No (L2 broadcast)	Yes (via Relay Agent)	Yes (via Relay Agent)
Allocation	N/A (Address lookup)	Static table	Static manual table	Dynamic Leases, Auto, Manual
Delivered Info	Hardware MAC only	Single IP only	IP, Mask, Gateway, TFTP	IP, Mask, Gateway, DNS, Lease, NTP
Status	Active (IPv4)	Obsolete	Legacy	Universal Standard

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