Description
We assume that only the interpreter knows all details of the configuration,
i.e. that devices will not cheat and look into interpreter data structures.
Thus, before building an IP layer, it would be advantageous to support
ARP so that we can map ipaddrs to macaddrs for hosts/routers that are on
the same switch. We will print msgs showing the flow of the arp messages.
Recall that the ‘type’ field was 0 in p2.txt, indicating a simple test
packet. However, we will begin adding new protocols (types) now:
1 for ARP req
2 for ARP reply
3+ for ICMP and IP in later projects
NOTE: the test protocol (0) and ARP (1,2) are NOT routable;
ICMP and IP will be routable.
For this project, develop some new commands to be interpreted:
arptest to_vh_or_vr ipaddr # e.g. arptest h1 1.2 OR arptest r1 2.2
goes from intepreter to specified host/router causing that host to
generate an arpreq for the specified ipaddr; the result will
be placed into the host’s arp cache
requester generates simple ARP request packet which is a macsend
(with mac addrs etc) for a protocol 1 type of message; the payload
will contain:
2 bytes (payload is ipaddr that we need the mac for, e.g. 2.1)
replyer generates simple ARP reply packet which is also a correct
macsend where the payload contains the requested ipaddr and the
correct mac for that ipaddr:
3 bytes (payload is 2 bytes of ipaddr requested, and 1 byte mac)
NOTE that it should be clear from the network portion of the ipaddr
which switch to place the arpreq onto.
NOTE that an ipaddr (for another host) should default to unreachable via
the local switch until an arpreq is sent and an arprep is received.
Assume that about 1 sec is sufficient time for an arprep to arrive
before treating an ipaddr as not reachable.
Sending hosts/routers print:
from_hostname: arpxxxx to to_hostname on from_macaddr: payload
# where xxxx is either req or reply
If no arprep is received after an arpreq, print:
from_hostname: arpreq to to_hostname timed out
Otherwise:
Receiving hosts/routers print:
to_hostname: arpxxxx from from_hostname on to_macaddr: payload
# where xxxx is either req or reply
arpprt hostname
Cause the specified host to print its arp cache.
This is merely a debugging tool between the interpreter and the host/router,
and does not generate traffic among the virtual systems.
The host should print for each iPaddr in its cache:
my_hostname: ipaddr macaddr_for_ipaddr my_network_to_send_on
ARP caches will initially be empty.
