# Parser

The parser must parse the following packet:

EthernetII | IPv4 | UDP | WireGuard

From this packet type, we must parse the Following:

1. Dest Mac address
2. Source Mac Address
3. Ethertype
4. Source IP address
5. Destination IP address
6. Protocol
7. UDP Source port
8. UDP Destination Port
9. Wireguard Message Type
10. Wireguard Receiver Index
11. Wireguard Counter

These fields all go to metadata lookup to determine the destination and
operations to perform on the packet.

The parser works by analyzing the packet 32 bits at a time as it flows in.
It uses a state machine which keeps track of the position in the packet,
and based on what it sees it updates the fields. Here are the states:

| State | Desciption |
| -------- | -------- |
| ETH_1 | Upper 32 bits of Dest Mac |
| ETH_2 | Lower 16 bits of Dest Mac, Upper 16 bits of source mac |
| ETH_3 | Lower 32 bits of Source Mac |
| ETH_4_IP_1 | Ethertype and  first 16 bits of IP. Version, IHL, DCSP, ECN |
| IP_2 | Total Length, Identification |
| IP_3 | Flags, Fragment Offset, TTL, Protocol |
| IP_4 | Header Checksum, upper 16 bits of source ip |
| IP_5 | lower 16 bits of source ip, upper 16 bits of dest ip |
| IP_6_UDP_1 | lower 16 bits of dest ip, UDP Source port |
| UDP_2 | Destination Port, Length |
| UDP_3_WG_1 | Checksum, Message Type |
| WG_2 | lower 16 bits of receiver index |
| WG_3 | upper 16 bits of receiver index, lower 16 bits of counter |
| WG_4 | middle 32 bits of counter |
| WG_5 | upper 16 bits of counter |
| IDLE | Do nothing until the packet ends |

As we parse, if we detect that the packet no longer follows the pattern, for example
if the protocol is not UDP or the Ethertype is not IPv4, then we set as many fields
as we parsed, for example if the protocol is TCP we would set the ethernet mac addresses,
the ethertype, ip addresses, and protcol, but the udp ports and wireguard ports would all
be 0. The outputs will either match the metadata rules or not, so we don't care in this
block.

The pattern specifically is IPv4 Ethertype (No VLAN), IHL 5 (No options), UDP protocol,
Wireguard dest port, and wireguard message type 4.