Ipv4_packet.MarshalSourcepseudoheader src dst proto len constructs a pseudoheader, suitable for inclusion in transport-layer checksum calculations, including the information supplied. len should be the total length of the transport-layer header and payload.
into_cstruct ~payload_len t buf attempts to write a header representing t (including t.options) into buf at offset 0. If there is insufficient space to represent t, an error will be returned.
make_cstruct ~payload_len t allocates, fills, and returns a buffer representing the IPV4 header corresponding to t. If t.options is non-empty, t.options will be concatenated onto the result. A variable amount of memory (at least 20 bytes for a zero-length options field) will be allocated. Note: no space is allocated for the payload.