#Nitu Nicolae-Iulian-- Router functionality implementation
The project tries to implement a simple router precedure with its functions
and responsibilities (receiving, forwarding, requesting packets). The code
consists in one infinite loop in which packets are received, read, modified and
eventually forwarded or replied.
Data structures use and parsing explication:
- C++ stl vector for storing route table entries --- prefix, next-hop, mask,
interface;
- C++ stl unordered map for storing ARP (address resolution protocol) table
entries --- IP (protocol) address, MAC (hardware) address;
- C++ stl queue for storing unsent (on stand-by) packets with their next-hops
, saved for sending later when the net destination is well-known;
- route table entries are read at the beginning from rtable.txt file which
contains one entry per line;
- entries are kept sorted ascending by prefix and ascending by mask in vector
and the search is made in logarithmic time (binary): search by prefix, find
a matching entry and go down until the biggest mask;
- ARP table entries are stored dinamicaly, updating the table at every ARP
reply if needed;
Loop functionalities explication:
- assume that all packets contain the ethernet header alongside other eventual
headers
- after receiving a packet the router must know what type is --- ARP or IP;
- IP-types:
~ contains additional IP and ICMP (internt control message protocol) header;
~ check if packets are corrupted by recalculating the checksum value and
comparing with the old one, if the answer is positive, drop the packet;
~ check the ttl (time to leave) value, if the time is up (ttl <= 1) drop
the packet and send suitable message back (TIMXCEED) --- <resolve_timeout()>
function;
~ check if the packet is a ICMP_ECHO one for router and reply back if
necessary with ICMP_ECHOREPLY message --- <resolve_icmp_echo()> function;
~ check if the router can forward the packet (if the IP destination address
match at least one entry in route table), if not, drop the packet and send
ICMP_DEST_UNREACH message back --- <resolve_destination_unreachable()>
function;
~ check if the MAC address coresponding to IP next-hop address is known
(exists in ARP table), if not, save the packet in queue and send an ARP
request on next-hop interface --- <resolve_no_arp_entry()> function;
~ if the IP address is in route table and the next-hop entry returned
exists in ARP table send packet forward --- <resolve_forwarding()> function;
- ARP-types:
~ contains additional ARP header;
~ check if packet is an ARP request or an ARP reply message;
~ if ARPOP_REQUEST, the router send a message back with his MAC address
completed --- <resolve_arp_request()> function;
~ if ARPOP_REPLY, that means router has sent a request before and has
packets on stand-by, so it updates the ARP table with the MAC address
received and send packets that are unsent on the ARP message interface ---
<resolve_arp_reply()> function;
Headers used for router implementation are the Linux native ones found in
- net/ethernet.h (~ether_header struct~ for ethernet header)
- netinet/ip.h (~iphdr struct~ for ip header)
- netinet/ip_icmp.h (~icmphdr struct~ for icmp header)
- netinet/if_ehter.h (~ether_arp struct~ for arp header)