Dynamic Host Configuration Protocol (DHCP)

Dynamic Host Configuration Protocol (DHCP)
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Motivation

Information the Host Needs:
- Its IP address
- The IP addresses of local caching DNS name servers
Two Ways to Provide this Information:
- Static (e.g., in a configuration file)
- At startup (e.g., using DHCP -- RFC2131, BOOTP RFC951, NIP)

The Underlying Transport Layer Protocol

Bootstrap Process

Client broadcasts a DHCP discovery message
One or more servers send (usually broadacsts though there can be exceptions) a DHCP offer message
The client accepts one of the offers and broadcasts a DHCP request message
The server that sent the accepted offer sends (usually broacasts though there can be exceptions) a DHCP acknowledgement; the other servers mark the IPs as available (thereby withdrawing the offers)

Orderly Shutdown Process

Leases

Rationale:
- Since a client might "crash", the server needs to be able to reclaim IP addresses
Tradeoff:
- Short leases allow IP addresses to be reclaimed quickly
- Long leases avoid the overhead of frequent renewals

Important Parts of the Acknowledgement Message

Message Format

Op Code/Message Type (1 octet)
Hardware/physical address type (1 octet)
Hardware/physical address length (1 octet)
Hops (1 octet)
- Client sets to 0; optionally used by relay agents
Transaction ID (4 octets)
- Chosen randomly by the client
Seconds elapsed (2 octets)
Flags (2 octets)
Client IP Address (4 octets)
- Used when in the BOUND, RENEW, or REBINDING states
"Your" IP Address (4 octets)
- The IP address being given to the client
Server's IP Address (4 octets)
- In an offer message
Relay agent's IP address (4 octets)
- If a relay agent is being used
Client's hardware/physical Address (16 octets)
Server host name (64 octets)
- Optional; terminated by a null character
Boot file name (128 octets)
- Terminated by a null character; generic/null for discovery, path name for offers
Options (variable)

Allocating IP Addresses

Using a Table:
- The DHCP server uses a configuration file to associate IP addresses with MAC addresses
Using a Pool:
- Server assigns addresses "on demand" (hence can use them more efficiently)

Things to Know

Clients:
- Can request their last-known IP address
- Typically requests a "new" IP address when the lease is half complete
Servers:
- As hinted at above, can relay the request to another agent
Security:
- DHCP does not include an authentication mechanism (though RFC 3118 does)
Layer:
- DHCP is an application layer protocol since it uses UDP

Tradeoffs: Some Real-World Data

Number of Active Leases at Different Times of Day (over Four Days) at Georgia Tech (for Four Different Lease Times)

Source: Khadilkar et. al (2007)

Dynamically Adjusting Lease Times

Single Adaptation:
- Set a long initial lease time (e.g., 90 minutes to correspond to the length of a course) and reduce the time for subsequent requests (e.g., to 30 minutes)
- At Georgia Tech, compared to fixed 90 minute leases, this resulted in 5% more addresses being used but reduced expirations by 11% and renewals by 29%
Exponential Adaptation:
- Issue short leases to initial requests and double the time for each renewal
- At Georgia Tech, compared to fixed 90 minute leases, did not increase peak address usage and reduced expirations by 37% and renewals by 78%