basic.p4

// SPDX-License-Identifier: Apache-2.0
/* -*- P4_16 -*- */
#include <core.p4>
#include <v1model.p4>

const bit<16> TYPE_IPV4 = 0x800;

/*************************************************************************
*********************** H E A D E R S  ***********************************
* This program skeleton defines minimal Ethernet and IPv4 headers and    *
* a simple LPM (Longest-Prefix Match) IPv4 forwarding pipeline.          *
* The exercise intentionally leaves TODOs for learners to implement.     *
*************************************************************************/

typedef bit<9>  egressSpec_t;   // Standard BMv2 uses 9 bits for egress_spec
typedef bit<48> macAddr_t;      // Ethernet MAC address
typedef bit<32> ip4Addr_t;      // IPv4 address

header ethernet_t {
    macAddr_t dstAddr;
    macAddr_t srcAddr;
    bit<16>   etherType;
}

header ipv4_t {
    bit<4>    version;
    bit<4>    ihl;
    bit<8>    diffserv;
    bit<16>   totalLen;
    bit<16>   identification;
    bit<3>    flags;
    bit<13>   fragOffset;
    bit<8>    ttl;
    bit<8>    protocol;
    bit<16>   hdrChecksum;
    ip4Addr_t srcAddr;
    ip4Addr_t dstAddr;
}

struct metadata {
    /* empty */
}

struct headers {
    ethernet_t   ethernet;
    ipv4_t       ipv4;
}

/*************************************************************************
*********************** P A R S E R  *************************************
* New to P4? A typical parser does this:
*   start -> parse_ethernet
*   parse_ethernet:
*       if etherType == TYPE_IPV4 -> parse_ipv4
*       else accept
*   parse_ipv4 -> accept
* This skeleton leaves the actual states as a TODO to implement later.   *
*************************************************************************/

parser MyParser(packet_in packet,
                out headers hdr,
                inout metadata meta,
                inout standard_metadata_t standard_metadata) {

    state start {
        /* TODO: add parser logic
         * Suggested outline:
         *   1) Extract Ethernet: packet.extract(hdr.ethernet);
         *   2) If hdr.ethernet.etherType == TYPE_IPV4 -> parse IPv4
         *   3) Otherwise -> transition accept
         */
        transition accept;
    }
}


/*************************************************************************
************   C H E C K S U M    V E R I F I C A T I O N   *************
*************************************************************************/

control MyVerifyChecksum(inout headers hdr, inout metadata meta) {
    apply {  }
}


/*************************************************************************
**************  I N G R E S S   P R O C E S S I N G   *******************
* High-level intent:
*   - Do an LPM lookup on IPv4 dstAddr
*   - On hit, call ipv4_forward(next-hop MAC, output port)
*   - Otherwise, drop or NoAction (as configured)                         *
*************************************************************************/

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

    action drop() {
        mark_to_drop(standard_metadata);
    }

    /*********************************************************************
     * NOTE FOR NEW READERS:
     * 'ipv4_forward(dstAddr, port)' is invoked by table 'ipv4_lpm'.
     *
     * The values for 'dstAddr' and 'port' are *action data* supplied by
     * the control plane when it installs entries in 'ipv4_lpm'.
     *
     * They mean:
     *   - dstAddr  => Ethernet destination MAC for the next hop
     *   - port     => output port (ultimately written to standard_metadata.egress_spec)
     *
     * Example (BMv2 simple_switch_CLI):
     *   table_add ipv4_lpm ipv4_forward 10.0.1.1/32 => 00:00:00:00:01:00 1
     * which passes MAC=00:00:00:00:01:00 and PORT=1 as action parameters
     * into ipv4_forward(dstAddr, port).
     *********************************************************************/
    action ipv4_forward(macAddr_t dstAddr, egressSpec_t port) {
        /*
            Action function for forwarding IPv4 packets.

            TODO: Implement the forwarding steps, for example:
              - standard_metadata.egress_spec = port;
              - hdr.ethernet.dstAddr = dstAddr;
              - (optionally) set hdr.ethernet.srcAddr to the switch MAC for 'port'
              - adjust IPv4 TTL and checksums as needed
        */
    }

    /*********************************************************************
     * LPM table for IPv4:
     *   - Matches on hdr.ipv4.dstAddr using longest-prefix match (lpm)
     *   - On hit, calls ipv4_forward with *action data* populated by the
     *     control plane when it installs the table entry.
     *********************************************************************/
    table ipv4_lpm {
        key = {
            hdr.ipv4.dstAddr: lpm;
        }
        actions = {
            ipv4_forward;
            drop;
            NoAction;
        }
        size = 1024;
        default_action = NoAction();
    }

    apply {
        /* TODO: fix ingress control logic
         *  - Good practice: apply ipv4_lpm only when the IPv4 header is valid, e.g.:
         *      if (hdr.ipv4.isValid()) { ipv4_lpm.apply(); }
         *    This skeleton currently applies unconditionally for the exercise.
         */
        ipv4_lpm.apply();
    }
}

/*************************************************************************
****************  E G R E S S   P R O C E S S I N G   *******************
* Often used for queue marks, mirroring, or post-routing edits.          *
*************************************************************************/

control MyEgress(inout headers hdr,
                 inout metadata meta,
                 inout standard_metadata_t standard_metadata) {
    apply {  }
}

/*************************************************************************
*************   C H E C K S U M    C O M P U T A T I O N   **************
* This block shows how to compute IPv4 header checksum when needed.      *
*************************************************************************/

control MyComputeChecksum(inout headers hdr, inout metadata meta) {
     apply {
        update_checksum(
            hdr.ipv4.isValid(),
            { hdr.ipv4.version,
              hdr.ipv4.ihl,
              hdr.ipv4.diffserv,
              hdr.ipv4.totalLen,
              hdr.ipv4.identification,
              hdr.ipv4.flags,
              hdr.ipv4.fragOffset,
              hdr.ipv4.ttl,
              hdr.ipv4.protocol,
              hdr.ipv4.srcAddr,
              hdr.ipv4.dstAddr },
            hdr.ipv4.hdrChecksum,
            HashAlgorithm.csum16);
    }
}


/*************************************************************************
***********************  D E P A R S E R  *******************************
* The deparser serializes headers back onto the packet in order.         *
*************************************************************************/

control MyDeparser(packet_out packet, in headers hdr) {
    apply {
        /*
        Typical implementation (left as a TODO for learners):
            packet.emit(hdr.ethernet);
            packet.emit(hdr.ipv4);   // per P4_16 spec, emit appends a header
                                     // only if it is valid; no 'if' needed.
        */
    }
}

/*************************************************************************
***********************  S W I T C H  ***********************************
*************************************************************************/

V1Switch(
MyParser(),
MyVerifyChecksum(),
MyIngress(),
MyEgress(),
MyComputeChecksum(),
MyDeparser()
) main;