Checkpoint 5 Writeup
该lab较简单，没什么好说的
有两点是route函数for循环时必须用引用，还是ttl递减后要重新计算checksum，写的时候被坑到了。
至于找到最长前缀匹配，遍历路由表即可，注意prefix为0时要特判；为了方便判断是否在路由表中找到符合项，max_prefix设置为int8_t。
void Router::add_route( const uint32_t route_prefix,
const uint8_t prefix_length,
const optional<Address> next_hop,
const size_t interface_num )
{
cerr << "DEBUG: adding route " << Address::from_ipv4_numeric( route_prefix ).ip() << "/"
<< static_cast<int>( prefix_length ) << " => " << ( next_hop.has_value() ? next_hop->ip() : "(direct)" )
<< " on interface " << interface_num << "\n";
(void)route_prefix;
(void)prefix_length;
(void)next_hop;
(void)interface_num;
if ( next_hop.has_value() ) {
router_table.push_back( { route_prefix, prefix_length, next_hop.value().ipv4_numeric(), interface_num } );
} else {
router_table.push_back( { route_prefix, prefix_length, std::nullopt, interface_num } );
}
}
void Router::route()
{
// !!! &
for ( auto& inter : interfaces_ ) {
std::optional<InternetDatagram> maybe_dgram = inter.maybe_receive();
if ( maybe_dgram.has_value() ) {
InternetDatagram dgram = maybe_dgram.value();
if ( dgram.header.ttl <= 1 ) {
continue;
}
dgram.header.ttl--;
// !!!!
dgram.header.compute_checksum();
uint32_t dst_ip = dgram.header.dst;
int8_t max_prefix = -1;
size_t max_prefix_index = 0;
for ( size_t i = 0; i < router_table.size(); i++ ) {
if ( !router_table[i].prefix_length || (( dst_ip >> ( 32 - router_table[i].prefix_length ) )
== ( router_table[i].route_prefix >> ( 32 - router_table[i].prefix_length ) )) ) {
if(router_table[i].prefix_length > max_prefix)
{
max_prefix = router_table[i].prefix_length;
max_prefix_index = i;
}
}
}
if(max_prefix == -1)
continue;
interfaces_[router_table[max_prefix_index].interface_num].send_datagram(
dgram, Address::from_ipv4_numeric( router_table[max_prefix_index].next_hop.value_or( dst_ip ) ) );
}
}