hurenxu/router-implementation

NAME: Renxu Hu PID: A92067683 EMAIL: [email protected]

Unfortunately, I am not competing for the George Varghese Espresso Prize.

DESCRIPTION/OVERVIEW

This program is a simple router implementation. The user can use this

implementation to ping the router inferfaces and app servers from the client, to traceroute from the client to any of the router's interfaces and any of the app servers, and at the same time, the user can download a file using HTTP from one of the app servers with the wget command. With the help of vmware, I can create a vitural network, and with the help of wireshark, I can see the configuration of the packets that are transferred within the networks. The simple router implementation that I implemented is divided into two parts, IP and ARP. IP is using routing table as the base to forward the packet; ARP is used to check the MAC address of the specific host.

DESIGN DECISION AND TRADEOFFS

I realize that the design that I used to have is taking more time to 

send the arp packet because of the order of the insertion. Therefore, in order to solve that, I implement the stack with the recursion to send the arp packet at the end; therefore, it will be faster to send the arp packet.

When recived the raw ethernet frame, it will concern the packet with 

two consideration, and it will check whether it is an IP packet or an ARP packet. If it is a IP packet, we will check whether it is for me or other router. If it is for me and if it is a ICMP echo request, the router will send echo reply. Or if it is TCP/UDP, send ICMP port unreachable, and I will explain the ICMP implementation below. If the IP pakcet is not for the current interface, I will check the routing table, and perform the largest prefix matach to find the largest prefix interface. If there is a match, I will send ICMP net unreachable. If there is a match, I will check the ARP cache. If there is a miss, I will send ARP request, and if the resent is larger than 5 times, then I will send ICMP host unreachable. If there is a hit, I will send this frame to next hop. If it is a ARP packet, I will check whether it is a request or reply. If it is a request to the current interface, I will construct an ARP reply and send it back. If it is a reply to the current interface, I will cache it, and go through my request queue and send outstanding packets. The request queue will be constructed during IP forwarding.

ARP Packet: 

If it is a arp apcket, I will do the sanitation check (length of 

packet, and checksum, etc). I will then check the opcode to see what kind of ARP packet is being sent. If it is an ARP request, update all fields of the packet and use sr_send_packet() to send an ARP reply. If it is a reply, I will updae the ARP cache and ARP queue. Send all the packets in the queue to the destination.

IP Packet:

If it is an ip packet, I will do the sanitation check first(ip header

and checksum). Then I will check whether it is desintated for the current interface, if it is for the current interface, I will check the ip protocol, and if it is a TCP/UDP, I will send ICMP port unreachable. If the is an echo request, then genereate ICMP echo reply. If IP packet is not desinted to the router, then I will check the ip_ttl, if ttl <= 1 then send ICMP time exceeded. Look up next-hop address by doing a longest prefix match on the routing table using the packet's destination address. If it does not exist, send ICMP host unreachable. If it doe exist, then reduce ttl and update checksum. From next- hop address, determine outgoing interface and next-hop MAC addres. If necessary, send ARP request to determine MAC address. At last, forward packet to outgoing interface.

IP Forwarding how to check the address of next hop:

Longest prefix match, if packet not destined to router and ttl != 1,

check the routing table to see if a matching entry for the destination IP address exists. I can check the address with mask from the routing table. I will do a bitwise and between dest and mask to see if there is a matach. If mutiple matches, check to see which match has the longest mask.s_addr.

Creating ARP Requests:

If LPM entry (type sr_rt) is found, then reduce TTL and update

checksum for the IP header. Now, I will update the frame headers source and destination fields. Then I will do a sr_arpcache_lookup. gw.s_addr (next hop IP address) is one of the variables to be passed to the function. If it returns NULL, I will use sr_arpcache_queuereq function to add the ARP request to the ARP request queue. Then I will send the sr instance and the queue to handle_arpreq.

Handle_arpreq function in arpcache.c:

If the lookup returned an arp entry, then modify the Ethernet

source and destination values and use sr_send_packet to handle the arp request.

ICMP TYPE 11 handle:

Check if the destination IP address of the packet is not equal to the

IP addresses of the router interfaces. If not destined and TTL of the IP header packet == 1, create an ICMP type 11 (time exceeded) packet. Then, send the packet using sr_send_packet. The data field in the ICMP segment is 28 bytes starting from the IP header of the original packet which the router received.

ICMP handle:

Bascially, the ICMP packet will return different errors that the IP 

packet is encountered. There are mutiple errors that IP packet will encounter, host unreachable, port unreachable, live time exceeded, and network unreachable. I will send the ICMP packet with different error code and type.