pbuf_type enum

pbuf data is stored in RAM, used for TX mostly, struct pbuf and its payload are allocated in one piece of contiguous memory (so the first payload byte can be calculated from struct pbuf). pbuf_alloc() allocates PBUF_RAM pbufs as unchained pbufs (although that might change in future versions). This should be used for all OUTGOING packets (TX).

pbuf data is stored in ROM, i.e. struct pbuf and its payload are located in totally different memory areas. Since it points to ROM, payload does not have to be copied when queued for transmission.

pbuf comes from the pbuf pool. Much like PBUF_ROM but payload might change so it has to be duplicated when queued before transmitting, depending on who has a 'ref' to it.

pbuf payload refers to RAM. This one comes from a pool and should be used for RX. Payload can be chained (scatter-gather RX) but like PBUF_RAM, struct pbuf and its payload are allocated in one piece of contiguous memory (so the first payload byte can be calculated from struct pbuf). Don't use this for TX, if the pool becomes empty e.g. because of TCP queuing, you are unable to receive TCP acks!.