How to add and remove a protocol header in a packet？

1

I want to delete the IP header in a data packet and add a custom network layer header, how can I do it? For example, a data packet header contains mac, ipv4, tcp, and now I want to delete the ipv4 header and replace it with a custom header sip, and the final output header format is mac, sip, tcp.

2

Hi @Duang ,

Welcome to the forum.

Removing a header is as easy as:

hdr.ipv4.setInvalid();

If you want to add another header, you just setValid() that header, that you needed to define beforehand and then add the values to the fields. Make sure you also emit the new header as depicted below:


header new_l3_header_t {
    addr_t  dstAddr;
}

struct headers {
    ipv4_t ipv4;
    new_l3_header_t   new_l3_header;
}

//define yourself the parser

control Whatevergress (inout headers hdr,
                  inout metadata meta,
                  inout standard_metadata_t standard_metadata){

    apply(){

        hdr.ipv4.setInvalid();
        hdr.new_l3_header.setValid();
        hdr.new_l3_header.dstAddr = (addr_t) some_value;

    }
}


// Deparse the packet

control YourDeparser(packet_out packet, 
                                   in headers hdr) {
    apply {
        // Other headers
        packet.emit(hdr.ipv4);
        packet.emit(hdr.new_l3_header);
    }
}

You can learn these concepts if you try all the exercises from the P4 tutorial: GitHub - p4lang/tutorials: P4 language tutorials

Cheers,

3 Likes
3

One minor additional point, but not only must you call setValid() on a header in order to add it, but it must also appear in your deparser code with an emit call. Doing setValid() on a header that is NOT emit’ed in the deparser will NOT cause it to appear in the output packet.

1 Like
4

Modified my answer. Thanks @andyfingerhut :pray: :slightly_smiling_face:

5

Thank you for your help, it worked great for me.

6

Thanks a lot for your advice and help。