DHCP, ARP, RARP, PXE, ICMP, OSI, TMA

Ben Scott dragonhawk at gmail.com
Sun Jan 13 22:11:22 EST 2008


On 12/30/07, Paul Lussier <p.lussier at comcast.net> wrote:
>>   RARP is a completely a different design vs DHCP/BOOTP.  RARP has the
>> game goal as DHCP, in that RARP exists to let nodes without an IP
>> address discover what their IP address should be.
>
> Huh? Is that right?  I thought RARP allowed an IP addressless client
> to broadcast it's MAC address to the network, at which point something
> like a DHCP server would answer it...

  Um... isn't that what I just wrote?  :)  Or do you specifically mean
to imply that RARP is somehow connected to DHCP?  That's wrong, I'm
pretty sure.  RARP is just like what it sounds like, ARP in reverse,
as an Ethernet frame outside of the IP layer.  Now, maybe there's a
DHCP server implementation that's also a RARP server implementation.
I could see that being useful.  But there's nothing in the protocol
design to connect them.  Or maybe you're thinking of BOOTP.  DHCP *is*
an extension to BOOTP, operating on the same ports and using a lot of
the same design.

>> ICMP is implemented on top of the IP layer, along side the TCP and
> > UDP layers.  All ICMP datagrams are IP datagrams, same as UDP or TCP.
>
> This seems wrong to me.  I was fairly certain that ICMP was more a
> peer with IP, and not build on top of it.

  I'm as sure as I can be that ICMP messages are normal IP datagrams.
RFC-792, page 2, states "ICMP messages are sent using the basic IP
header."

http://www.rfc-editor.org/rfc/rfc792.txt

  ICMP messages are often generated by the kernel router in response
to problematic network packets; maybe that's what you're thinking of?

> Do you have a decent reference for ICMP (other than the actual RFC) ?

  I kinda like the RFC's, but:

http://en.wikipedia.org/wiki/Internet_Control_Message_Protocol

W. Richard Stevens's classic _TCP/IP Illustrated_ devotes an entire
chapter to ICMP.  Volume 1, chapter 6, page 69-83 (AWL, 1994).

  Google also finds some good ones:

http://www.erg.abdn.ac.uk/users/gorry/course/inet-pages/icmp.html

http://www.vijaymukhi.com/vmis/icmp.htm

-- Ben


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