| # Netstack3 Implementation Details |
| |
| This document describes various implementation details that are confusing enough |
| to be worth calling out explicitly, and cannot naturally be documented in inline |
| code comments. |
| |
| ## Ethernet Frame Padding |
| |
| Ethernet frames have a minimum payload length. In order to ensure that this |
| minimum is achieved, small payloads must sometimes be padded with zeroes. The |
| Ethernet header's EtherType field may sometimes be used to encode a length. If |
| this is the case, then the real length of the payload (not including the padding) |
| can be encoded in the EtherType, and so the receiver will know how many bytes of |
| the payload to pay attention to. |
| |
| However, the EtherType field is sometimes used to indicate the type of the |
| encapsulated packet (e.g., IPv4, ARP, etc). In this case, it does not encode the |
| payload length. When this happens, the receiver must a) be able to parse a |
| payload whose length is longer than the length of the real encapsulated packet |
| and, b) be able to infer from the payload's header (e.g., the IPv4 header, the |
| ARP header, etc) how many bytes of the payload to parse, and how many to treat |
| as padding, and thus to ignore. |
| |
| See for details: |
| - https://stackoverflow.com/a/35077070 |
| - https://serverfault.com/a/510688 |
