What makes BOOTP routable, but RARP isn't?

Hi all,

I am not sure if this is the right place to ask this, but I couldn't
find better match so I was hoping some of you all might be able to help
me understand something.

I am reading about the TCP/IP protocol suite and how RARP (and maybe
ARP, I'm not sure, maybe someone can clarify for me) are not routable,
but BOOTP is. My question is why? They both come from clients that do
not know their IP address and they both send out broadcast messages
correct? So, what is the difference between them?

Any help in this area is greatly appreciated!

(E-Mail Removed) wrote:
> Hi all,
>
> I am not sure if this is the right place to ask this, but I couldn't
> find better match so I was hoping some of you all might be able to help
> me understand something.
>
> I am reading about the TCP/IP protocol suite and how RARP (and maybe
> ARP, I'm not sure, maybe someone can clarify for me) are not routable,
> but BOOTP is. My question is why? They both come from clients that do
> not know their IP address and they both send out broadcast messages
> correct? So, what is the difference between them?
>
> Any help in this area is greatly appreciated!
>

ARP and RARP are link-level protocols, and as such
their forwarding does not understand anything about
IP routing, which is one level above the link layer.

BOOTP runs on top of UDP which runs on top of IP,
and this is subject to normal IP routing rules.

--

Tauno Voipio
tauno voipio (at) iki fi

Hi,

Bootp runs above the UDP Layer i.e it is an application layer protocol,
which has IP header and IP address info in the Bootp packet, so a
router /layer3 device which has bootp relay finctionality will router
it to the dhcp server.

But RARP is a layer below IP so it does not have any IP header, and if
you see routiing is a layer 3 functionality.

let me know if you have any more questions.

Thanks,
Sunil
http://geocities.com/sunil3112000

Tauno Voipio wrote:
> (E-Mail Removed) wrote:
> > Hi all,
> >
> > I am not sure if this is the right place to ask this, but I couldn't
> > find better match so I was hoping some of you all might be able to help
> > me understand something.
> >
> > I am reading about the TCP/IP protocol suite and how RARP (and maybe
> > ARP, I'm not sure, maybe someone can clarify for me) are not routable,
> > but BOOTP is. My question is why? They both come from clients that do
> > not know their IP address and they both send out broadcast messages
> > correct? So, what is the difference between them?
> >
> > Any help in this area is greatly appreciated!
> >
>
> ARP and RARP are link-level protocols, and as such
> their forwarding does not understand anything about
> IP routing, which is one level above the link layer.
>
> BOOTP runs on top of UDP which runs on top of IP,
> and this is subject to normal IP routing rules.
>
> --
>
> Tauno Voipio
> tauno voipio (at) iki fi

ipnwsec wrote:
> Hi,
>
> Bootp runs above the UDP Layer i.e it is an application layer protocol,
> which has IP header and IP address info in the Bootp packet, so a
> router /layer3 device which has bootp relay finctionality will router
> it to the dhcp server.
>
> But RARP is a layer below IP so it does not have any IP header, and if
> you see routiing is a layer 3 functionality.
>
> let me know if you have any more questions.
>
> Thanks,
> Sunil
> http://geocities.com/sunil3112000
>
> Tauno Voipio wrote:
> > (E-Mail Removed) wrote:
> > > Hi all,
> > >
> > > I am not sure if this is the right place to ask this, but I couldn't
> > > find better match so I was hoping some of you all might be able to help
> > > me understand something.
> > >
> > > I am reading about the TCP/IP protocol suite and how RARP (and maybe
> > > ARP, I'm not sure, maybe someone can clarify for me) are not routable,
> > > but BOOTP is. My question is why? They both come from clients that do
> > > not know their IP address and they both send out broadcast messages
> > > correct? So, what is the difference between them?
> > >
> > > Any help in this area is greatly appreciated!
> > >
> >
> > ARP and RARP are link-level protocols, and as such
> > their forwarding does not understand anything about
> > IP routing, which is one level above the link layer.
> >
> > BOOTP runs on top of UDP which runs on top of IP,
> > and this is subject to normal IP routing rules.
> >
> > --
> >
> > Tauno Voipio
> > tauno voipio (at) iki fi


Thanks for the info guys. Just one other question. I understand now
that it is because BOOTP is wrapped in IP that makes it capable for
routers to route it. But, what I don't understand is that the
Destination IP is a broadcast address (255.255.255.255). I thought
broadcast addresses were not routed? So how does a workstation find a
DHCP server if it is not on the same segment (or does the DHCP server
HAVE to be on the same segment, which I thought that not having to have
a DHCP server on every segment was one of that advantages of using DHCP
in the first place)?

Thanks for your help guys! I am slowly starting to understand

(E-Mail Removed) wrote:
>
> Thanks for the info guys. Just one other question. I understand now
> that it is because BOOTP is wrapped in IP that makes it capable for
> routers to route it. But, what I don't understand is that the
> Destination IP is a broadcast address (255.255.255.255). I thought
> broadcast addresses were not routed? So how does a workstation find a
> DHCP server if it is not on the same segment (or does the DHCP server
> HAVE to be on the same segment, which I thought that not having to have
> a DHCP server on every segment was one of that advantages of using DHCP
> in the first place)?

For a non-local DHCP server, you need a DHCP relay server
in the LAN. You're right that the broadcast does not reach
out of the local network, so there's a need for somebody in
the LAN to handle the DHCP request. The relay server can
then consult the real master of IP addresses for the DHCP
responses to the client.

After the initial dialog, the networking is up and running
and the rest of the startup can be served from farther away.

There may also be several DHCP servers (or relays) responding
to the potential client. It's then up to the client to pick
up the server for the rest of the DHCP dialog.

HTH

--

Tauno Voipio
tauno voipio (at) iki fi

On 21 Oct 2006, in the Usenet newsgroup comp.os.linux.networking, in article
<(E-Mail Removed). com>,
(E-Mail Removed) wrote:

>But, what I don't understand is that the Destination IP is a broadcast
>address (255.255.255.255). I thought broadcast addresses were not routed?

Not _forwarded_

RFC1122 section 3.2.1.3
RFC1812 section 4.2.2.11

Neither explicitly FORBID forwarding - and think about the difference in
the meaning of the word "network" verses "subnet". Most sanely configured
_perimeter_ routers drop packets addressed to broadcast addresses. See
RFC2827 and RFC3704. (For example,. we drop such broadcasts at the company
perimeter, BUT ALSO drop them at facility and/or division perimeters.)

>So how does a workstation find a DHCP server if it is not on the same
>segment (or does the DHCP server HAVE to be on the same segment, which I
>thought that not having to have a DHCP server on every segment was one of
>that advantages of using DHCP in the first place)?

RFC0951 section 8
RFC1541 section 1.4, section 3.1

RFC2131 doesn't change RFC1541 in this respect. Also notice that this
relay function is not forwarding _everything_ sent to the broadcast
address, but only DHCP (or BOOTP) requests (from 68/udp to 67/udp)..

Old guy

Moe Trin wrote:
> On 21 Oct 2006, in the Usenet newsgroup comp.os.linux.networking, in article
> <(E-Mail Removed). com>,
> (E-Mail Removed) wrote:
>
> >But, what I don't understand is that the Destination IP is a broadcast
> >address (255.255.255.255). I thought broadcast addresses were not routed?
>
> Not _forwarded_
>
> RFC1122 section 3.2.1.3
> RFC1812 section 4.2.2.11
>
> Neither explicitly FORBID forwarding - and think about the difference in
> the meaning of the word "network" verses "subnet". Most sanely configured
> _perimeter_ routers drop packets addressed to broadcast addresses. See
> RFC2827 and RFC3704. (For example,. we drop such broadcasts at the company
> perimeter, BUT ALSO drop them at facility and/or division perimeters.)
>
> >So how does a workstation find a DHCP server if it is not on the same
> >segment (or does the DHCP server HAVE to be on the same segment, which I
> >thought that not having to have a DHCP server on every segment was one of
> >that advantages of using DHCP in the first place)?
>
> RFC0951 section 8
> RFC1541 section 1.4, section 3.1
>
> RFC2131 doesn't change RFC1541 in this respect. Also notice that this
> relay function is not forwarding _everything_ sent to the broadcast
> address, but only DHCP (or BOOTP) requests (from 68/udp to 67/udp)..
>
> Old guy


Thanks for the info guys. This has cleared up a lot of confusion in my
head!