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Chapter 23: ARP, ICMP, DHCP
IS333
Spring 2016
Role of ARP
Q: What role does ARP play in the TCP/IP
protocol stack?
A: See Figure 23.5 on p 389. ARP bridges the
Layer 2 / Layer 3 addressing boundary, allowing
IP to be agnostic about layer 2’s addressing, and
yet still use layer 2 to deliver packets.
Note that ARP is designed to work with not just
IP and Ethernet, but any pair of protocols.
Who does a machine ARP for?
Q: Does a machine ARP for a machine that is not on
its local LAN?
A: No! It is trying to find the MAC address – which
means layer 2. So, it is trying to get the packet to its
next-hop (which might be the ultimate destination).
If the destination address is not on the local
network, then the packet is sent to the router,
which is on the local network. So, if it needs to, it
will ARP for the router’s IP address on the local
network.
ARP Request/Response
Q: How does a machine send a request to another
machine to get its MAC if it doesn’t know the MAC
of the machine?
A: The requesting machine broadcasts the request
at layer 2, asking for a certain IP address. All
machines on the LAN accept that packet, but only
the machine with that IP address responds, using
layer 2 to deliver the packet back directly to the
requester.
(Note: ARP is not carried in an IP packet. It is
carried directly on layer 2.)
ARP question
Q: Would it be possible that if computer A sends an
ARP request for computer B out to the network,
that an entity other than another host, such as a
switch or a router, or a DHCP server, could send
back an ARP response?
A: Yes, it is possible! It could be a real security
problem. If some malevolent machine responds to
ARPs saying its own MAC is the MAC for every IP
address, then it will receive all packets and be able
to snoop them.
Cache?
Q: What is a “cache”?
A: A cache is a small “database” of information.
The information in the database can be
retrieved elsewhere, but much more slowly. The
cache stores the information for much faster
retrieval. Entries in the cache are typically timed
out because the information found remotely
might change.
ARP cache size
Q: Storing IP addresses accessed from ARP
requests take memory. Does the usage of this
memory slow down switches/routers as the
number of hosts grows?
A: I suppose theoretically it could. A cache,
however, typically has a max size, and when that
size is hit, old entries are removed. So, the
memory usage won’t grow too large.
What if…
Q: A machine does not answer ARP requests?
A: No one talks to it.
Q: Two machines answer an ARP request, with
their own different MAC addresses?
A: Houston, we have a problem!
Probably the second response will stay in the
cache and the first machine won’t be talked to.
ICMP
Q: What layer is ICMP at? It is carried in IP
packets, so does that make it layer 4?
A: The protocol definition says that ICMP must
be implemented if IPv4 is implemented. So, it is
a part of IP at layer 3. Also, it is not a transport
protocol, which is what is at layer 4.
DHCP review
Q: What kind of information does a DHCP
response carry?
A: Usually at least 4 things:
• IP address
• Subnet mask
• Default router IP address
• DNS server IP address
DHCP Servers
Q: Do you always need a dedicated DHCP server
if you want to use DHCP? It seems like it would
not take a lot of processing power to hand out
addresses. Could this functionality be rolled up
with other services on one server?
A: DHCP servers typically run on routers or some
machine that runs lots of services.
Q: Do you need a DHCP server on every LAN?
A: Yes, unless you use a relay agent.
Boot file name
Q: In the DHCP message format, if the BOOT FILE
NAME field is filled with a Linux OS, is it ever able to
boot on a windows or different OS?
A: The boot file name is typically used for systems
which boot into a very primitive OS that is only
capable to sending a DHCP request and getting a
response. When the response comes in, the tiny OS
uses TFTP (trivial ftp) to download the actual OS
from a TFTP server, and boot into it.
BOOTP vs DHCP
Q: I’m confused about the relationship between
BOOTP and DHCP.
A: BOOTP came first and was OK. Then DHCP
came along and was specified as being
backward compatible with BOOTP. But, it has
more capabilities, like offering subnet masks,
default routes, etc. So, DHCP is really a superset
of the BOOTP functionality.
BOOTP is not used anymore.
DHCP back-off
Q: On page 394, the author says that in order to
avoid synchronized flooding, DHCP requires each
host to delay a random time before transmitting or
retransmitting a request. Is this the same or similar
to binary exponential backoff used in ethernet?
A: It is a little similar. If a whole network goes down
(power outage) and comes back up, and all
machines do a DHCP request at the same time,
there will be collisions, etc. So, it is a way to reduce
collisions.
DHCP relay agent
Q: How does a DHCP relay agent work?
A: A relay agent listens on a LAN for a DHCP
request. When it receives that request, it
encapsulates the information in the request in a
special message it sends to the (remote) DHCP
server. The server responds to it, and then the
agent sends out a DHCP reply, as if it were the
actual server.
(There are similar servers out there called proxy
ARP servers…)
Old Slides
IP vs MAC addresses
Q: What is the point of IP addresses if they are
just resolved to the computers MAC address?
A: An IP address is unique across the entire
universe, and so uniquely identifies a
destination. A MAC address only identifies a
machine on the local network (so it only has to
be unique on the local network, although MAC
addresses are universally unique).
2 ARP replies?
Q: If a computer receives two ARP replies for a
single request, and the first reply claims that the
MAC address is M1, and the second reply claims
that the MAC address is M2, how does ARP
handle the replies?
A: I think it treats each reply independently and
updates the cache to have M2 <--> IP address.
ARP Caching
Q: Can you explain ARP Caching a bit more? Does it
do this mainly because it expects to send the same
datagram over again or is it more for when the
datagram gets split up? I guess I’m still a bit fuzzy
on why caching improves performance.
A: ARP caching is remembering the results of
previous ARPs – so, remembering the binding
between MAC address and IP address. This is done
purely for efficiency. If you send a packet to one IP
address on your network, you will probably send
one again.
ARP problems
Q: Can a network have problems related to address
resolution? If so, would the problems mostly arise
from the protocol or can there be external factors
that cause those problems?
A: The only problems I can think of: 1) a machine
doesn’t answer ARP requests  no one can talk to
it; 2) multiple machines answer an ARP request 
some machines might talk to one machine and
some to another, thinking they are talking to the
one machine with the unique IP address.
ICMP
Q: If ICMP is used to report errors, why is it used
to handle pings and traceroutes?
A: ICMP is used for network debugging. That’s
what pings are for.
traceroute makes use of ICMP error messages by
setting the TTL too low in the messages it
generates. That way the routers will send errors
back and traceroute will learn the IP addresses
of the routers on the path to the destination.
“Destination unreachable”
Q: About the destination unreachable error on
page 390; would that be referring to a computer
on a remote network?
A: This is reported when either your host
machine has no route to the destination
network/machine, or, a remote router (a “nexthop” router) has no route to the destination
network/machine.
DHCP overview
Q: Could you explain DHCP a bit more (perhaps with a
drawing)?
A: Sure.
1) Machine boots up and finds it has no IP address.
2) It broadcasts a DHCP request.
3) The DHCP server allocates an IP address and sends it.
4) The machine receives it and installs the IP address.
- The response also typically includes a network mask, default
route, and DNS server IP hostname/address.
DHCP details
Q: What is the "recommended" or "normal"
time of a DHCP lease?
A: 24 hours, I think.
Q: From what I can gather, DHCP is not a
necessity. Is that correct?
A: Not necessary, but awfully useful. Otherwise,
the user has to set the IP address every time the
machine is rebooted.
Configuration
Q: Can DHCP be configured any way that the admin
wants? Can selective leasing be used to give a
specific computer a specific address when the
computer boots, even if another computer has that
IP address - can the DHCP server take the address
back to give to the new computer?
A: DHCP can be configured in many different ways.
We’ll do this in lab.
A server can force a machine to give up an IP
address, I think.
Fixed addresses?
Q: Although DHCP is used extensively, what are the
advantages of setting a fixed address assignment are there any?
A: I have used fixed IP addresses when I want some
network service running on a certain machine with
a certain IP address always. E.g., you typically hardcode a router’s interface to be x.y.z.1.
Or, maybe you have some license server software
and you want to configure all your machines to
always to go a certain IP address to get some
license for the software.
BOOTP vs DHCP
Q: Since BOOTP was extended to become DHCP,
can you boot a BOOTP client on a DHCP server
or vice-versa?
A: You can use a BOOTP server with a DHCP
client, but not vice versa, I think. The BOOTP
server will send out a message a client can
understand, but won’t contain all the data the
client could receive… I think…
“Parameterized” software
Q: Could you explain what is meant by "binary
image" in this statement:
In essence, protocol software consists of a
binary image that is parameterized, and
initialization fills in a set of parameters.
A: What this means is that the software
components (IP, ARP, DHCP, etc.) have to be
configured to work on each machine, somehow.
Old Slides
How does ARP work?
Q: How does ARP work?
A: IP hands a packet and a next-hop IP address
to layer 2 to forward for it. Layer 2 has to figure
out how to get the MAC address for this IP
address. It checks its ARP cache for a resolution.
If not found, it sends an ARP request and waits
for a response. Then, it caches the results and
sends to the MAC delivered in the response.
ARP Request/Response
Q: How does a machine send a request to another
machine to get its MAC if it doesn’t know the MAC
of the machine?
A: The requesting machine broadcasts the request
at layer 2, asking for a certain IP address. All
machines on the LAN accept that packet, but only
the machine with that IP address responds, using
layer 2 to deliver the packet back directly to the
requester.
(Note: ARP is not carried in an IP packet. It is
carried directly on layer 2.)
Details
• The ARP responder caches the MAC/IP of the
requester in its ARP cache.
• Although all machines on the LAN could
update their ARP cache from an ARP request,
they don’t. Why?
– Because they will fill up their cache with bindings
they may never need.
ICMP
• Internet Control Message Protocol
• Mostly not implemented these days.
• Only echo request/responses are
implemented/enabled often.
– Many others are security liabilities.
• Designed to handle reporting
errors/misconfigurations in an IP network.
• Layer 4 protocol (carried by IP).
DHCP
• Not necessary to implement to get a working
network, but awfully convenient.
• What is its role?
– Allows a computer to “automatically” get an IP
address.
– And, subnet mask.
– And, default route.
– And, DNS server IP address(es).
• All of these can be done by hand configuration.
• Initially was called BOOTP.
DHCP problem…
Q: How does a machine without an IP address
request an IP address over an IP network?
A: Sends a (layer 2 and 3) broadcast message, filling
in the src IP address as 0.0.0.0 (“this computer”).
Layer 4 is UDP, port 67, indicating a DHCP message.
All machines receive the broadcast and forward up
through IP to UDP. Only the machine listening on
port 67 accepts and responds – that’s the DHCP
server. The server responds directly to the MAC
address used in the request.
DHCP Server Configuration
Q: How do you configure a DHCP Server?
A: You typically edit a file that specifies a range
of addresses to be leased dynamically, and/or
specific MAC <--> IP address bindings. You also
configure the default route and DNS server IP
addresses to send out.
Note: only one DHCP server must be running per
LAN, or chaos ensues...
DHCP Relay Agents
Q: Do you have to run a DHCP server on each
LAN?
A: Without some special provisions, yes. DHCP
requests use limited bcast (all 1s), so don’t get
forwarded across routers.
But, you can implement a DHCP relay agent to
forward requests/responses to/from the DHCP
server. Still must be on relay agent on each LAN.