[Click] [click]SetIPChecksum

[Pradeep Pradeep]

Hello,
      
  In the code for the element SetIPChecksum element  there is part which tests if a packet is IP packet or not. 

It uses 
---------------------
ip=p->ip_header();
if(!ip)

  When can a situation arise which satisfies   !ip?
-------------------------

 I am genarating  packets which have the following format
{IP Header}
 {QoS Data}
{UDP Header}


The code I used is 
_______________________________

void push(Packet* p)
{
  
 
WritablePacket *packet = Packet::make(0, 0, data_len+header_room, 0);
  
  if (packet == 0) {
      click_chatter( "cannot make packet!");
      packet = NULL;
      return;
    }
    
    // fill IP header
  
   click_ip *iph = (click_ip*) packet->data();
    memcpy(iph, p->ip_header(), sizeof(click_ip));
    iph->ip_tos |= 0x0c;  // 12 for QoS
    iph->ip_tos |= 0x01;  // Set ECN capability

    // fill QoS fields
    QoSSignalFormat* format = 
      (QoSSignalFormat*)(packet->data()+sizeof(click_ip));
   
   
    // fill UDP header
    click_udp *udph = 
      (click_udp*)(packet->data()+sizeof(click_ip)+sizeof(struct QoSSignalFormat));
    memcpy(udph, p->udp_header(), sizeof(click_udp));

       p->kill(); // destroy data packet
    output(0).push((Packet *)packet);
  }
  _______________________________

As you can see I am making some changes to IP header. So I want to recompue the checksum by passing the packets through SetIPChecksum element. When I forward the these packets SetIPChecksum (which is slightly modified that it  detects the condtion !ip ) the packets are being struck at this !ip condtion. Please help me with this.

Regards,
Pradeep.

 


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