ip_router_protocol.cpp

#include "ip_router_protocol.h"

void ip_router_protocol::dinternal(double t) {

  if (!lower_layer_ctrl_in.empty()) {
    link::Control c = lower_layer_ctrl_in.front();
    this->processLinkControl(c);
    lower_layer_ctrl_in.pop();
    next_internal = process_link_control_time;
    return;
  }

  if (!lower_layer_data_in.empty()) {
  	logger.debug("Process ip packet input.");
    ip::Datagram p = lower_layer_data_in.front();
    this->processIPDatagram(p);
    lower_layer_data_in.pop();
    next_internal = process_ip_datagram_time;
    return;
  }

  next_internal = infinity;
  output = Event(0,5);
}

/***********************************/
/********* HELPER METHODS **********/
/***********************************/

void ip_router_protocol::processIPDatagram(ip::Datagram p) {
  ip::Routing_entry route;
  IPv4 dest_ip = p.header.dest_ip; 

  logger.info("Process Ip Datagram");
  // The used algorithm is the microsoft routing process that specifies 7 steps
  // Step 1
  if (!this->verifychecksum(p.header)) {
    // silent discard
    logger.info("Discard packet: checksum verification faild for packet with dest_ip: "
                + dest_ip.as_string());
    return;
  }

  // Step 2 ignored, no upper layer in a router. This step was made for 
  // routers running in a host

  // Step 3  
  if (this->TTLisZero(p.header.ttlp)) {
    // silent discard. Some IP implementations send 
    // ICMP destination Unreachable-Network message to sender
    logger.info("Discard packet: TTL zero for packet with dest_ip: " 
                + dest_ip.as_string());
    return; 
  }

  // Step 4  
  p.header.ttlp = this->decreaseTTL(p.header.ttlp);
  // Recalculates checksum because the TTL field has being updated
  p.header.header_checksum = this->calculateChecksum(p.header);

  // Step 5-7
  this->routeIPDatagram(p);
}

bool ip_router_protocol::TTLisZero(ushort ttlp) const {
  ushort ttl = ttlp >> 8;
  std::stringstream stream;
  stream << std::hex << ttl;
  logger.debug("TTL: " + stream.str());
  return ttl == 0;
}

ushort ip_router_protocol::decreaseTTL(ushort ttlp) const {
  std::stringstream stream;
  stream << std::hex << ttlp;
  logger.debug("Before decrease TTL: " + stream.str());
  ushort ttl = ttlp >> 8;
  
  --ttl;
  ttl = ttl << 8;
  ttlp = ttlp & 0x00FF;
  ttlp = ttl | ttlp;
  
  stream.str("");
  stream << std::hex << ttlp;
  logger.debug("After decrease TTL: " + stream.str());
  return ttlp;
}