OSPF Hello Packet Format


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Version #   |       1       |         Packet length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Router ID                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Area ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Checksum            |             AuType            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Network Mask                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         HelloInterval         |    Options    |    Rtr Pri    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     RouterDeadInterval                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Designated Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Backup Designated Router                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Neighbor                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Messages from client to server will have only upto the Backup designated
router.  Server will send its neighbors to the client.

Version    = 2 from client to server and vice versa
Type       = Hello(1) from client to server
             Link State Update (4) from server to client
Packet len = length of the packet
Router ID  = Local IP address
Area ID    = 0 for all
Checksum   = 0
AuType     = 0 if no auth, 1 for MD5, 2 for SHA-1
Auth       = 0 if no auth 
Netmask    = local subnet mask
Options    = 0
RtrPri     = 0
RouterDeadInterval = 0
Designated Router = server IP addr
Backup Desig router = 0
Neighbor = 0

In this, create client and server processes that will exchange the messages
with the format given above. Client sets type to 1 whereas server sets it 
to 4. The total length of the packet is calculated and set in packet len.
Group 5 will use MD5 authentication whereas Group 13 will use SHA-1
authentication. The authentication data is calculated using the following
functions:

FOR SHA-1
===========

        #include <openssl/sha.h>

        unsigned char *SHA1(const unsigned char *d, unsigned long n,
                         unsigned char *md);

FOR MD5
========
        #include <openssl/md5.h>

        unsigned char *MD5(const unsigned char *d, unsigned long n,
                         unsigned char *md);

SHA-1 returns a 20 byte character string and MD5 returns a 16-byte string. 
Truncate it to 8 bytes and copy those 8 bytes to the authentication field 
in the message format. Apply this on a string accepted from the client. 
Server will copy that in the reply sent back to the client. Verify that it 
is same as that sent.


Group 5 :
Implement as a TCP concurrent server with processes

Group 13 :
Implement as a TCP concurrent server with threads