Start work on parser

docs/core/parser.md

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+# 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.