If an ip packet is fragmented, ip_ass_table[] (the ip assemble table) holds the fragments until they are reassembled by reassemble(). Each of the 3 elements (which is an oddly small number) of ip_ass_table[] corresponds to a packet that has been fragmented and is of type ip_ass_t (see below). find_ass_ent() searches through ip_ass_table[] for fragments of the same packet and adds the fragment to this packet if found and starts a new packet otherwise. If ip_ass_table[] is full, the oldest fragmented packet is dropped and replaced by the new fragmented packet and an icmp packet is sent to the source of the dropped packet.
typedef struct ip_ass
{
acc_t *ia_frags;
int ia_min_ttl;
ip_port_t *ia_port;
time_t ia_first_time;
ipaddr_t ia_srcaddr, ia_dstaddr;
int ia_proto, ia_id;
} ip_ass_t;
acc_t *ia_frags: The first fragment in the linked list of fragments. These accessors hold the data contained in the fragments and are linked by the accessors' acc_ext_link field.
int ia_min_ttl: Set to IP_MAX_TTL (#define'd as 255 in in.h). This value is in seconds and is the maximum time that a fragmented packet may be in ip_ass_table[] before the source is sent an icmp packet.
ip_port_t *ia_port: The ip port on which the packet arrived.
time_t ia_first_time: The time at which the first fragment of the packet is added.
ipaddr_t ia_srcaddr, ia_dstaddr: The source and destination ip address of the fragment.
int ia_proto: The protocol of the packet to which the fragment belongs. For example, if the packet is a udp packet, ia_proto will be 17. If the packet is a tcp packet, ia_proto will be 6.
ia_id: The value of the ih_id field for the ip header of the first packet sent out is determined by ip_init() and is equal to the number of clock ticks since reboot (i.e., the value returned by get_time) and is incremented for each packet sent out. This value is used to combine fragments at the receiving end if fragmentation has occurred. More specifically, if a packet is fragmented during transit, ia_id will be the same for all the fragments.
