Bandwidth Consumed by DHCP Message

[Aggarwal Vivek-Q4997C]

Hi Simon

Can you please help me with actual figures?

As per the RFC I guess DHCP Message size is 576 bytes. And lets take the
worst case scenario that all 150,000 clients require IP at the same
time. Then according to you how much bandwidth will be required.

Regards
Vivek Aggarwal

-----Original Message-----
From: dhcp-users-bounce at isc.org [mailto:dhcp-users-bounce at isc.org] On
Behalf Of Simon Hobson
Sent: Thursday, March 27, 2008 1:16 PM
To: dhcp-users at isc.org
Subject: Re: Bandwidth Consumed by DHCP Message

Aggarwal Vivek-Q4997C wrote:

>I v a network of 150,000 clients requiring an IP Address simultaneously
>from the ISC-DHCP Server (3.0.6). Can anyone please help me in guiding
>that how much bandwidth will be required?


When you say "... requiring an IP Address simultaneously ...", is 
that simply the total of all your clients, or do you mean that you 
might want to service them all obtaining a lease at the the same 
point in time ? It makes a big difference !

The packets themselves are small, perhaps 100 to 150 bytes - it 
depends on what options you use. Discover and Request are the 
smallest.

It takes a two packet handshake to renew an existing lease, four 
packets to obtain a lease from cold. So worst case, assume all 
clients need to do do Discover-Offer-Request-Ack - that's going to be 
about 1/2k total traffic. For 150,000 clients that's going to be 
around 75,000k, or 75MByte of traffic. It will be split at about 30MB 
client-server and 45MB server-client.

However, the server would not handle that number of requests in a 
short space of time, check the archives for a number of threads 
related to supporting large numbers of clients.

More normally, many of your clients will already have a lease, they 
just want to renew it - so that halves the traffic. Worst case for 
them in a business environment is the 'morning rush hour' as users 
come in and switch on their PCs. I would guess that with 150,000 
clients they are not all going to be switched on at once, and will 
more likely be spread over at least an hour, more likely several 
timezones.

If your devices are left on permanently, then the load will be spread 
over whatever lease time you use - or more correctly half the lease 
time as most clients will renew at somewhere around half the lease 
time. If your leasetime is (say) 15 days, then that's going to be 
20,000 clients/day renewing, or about one every 4 seconds on average.

So, there's your parameters, you'll need to do the sums for your 
particular setup and operating regime.