Re: mbuf data is 1400 bytes but the most common (?) use, Ethernet II frames, allow for 1518 bytes

[Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>]

Hi Ken,

2023-10-04 06:52 (UTC+0000), Nicolson Ken (ニコルソン ケン): 
> Looking at sending Ethernet packets over a DPDK connection, I see that an
> Ethernet Type II frame has 1518 max, but mbuf's data block is 1400 bytes.

rte_mbuf has no fixed-length "data block", so there is no limitation of 1400.
Buffer size is usually configured when creating a packet mempool.
Typically (rte_mbuf_core.h):

/**
 * Some NICs need at least 2KB buffer to RX standard Ethernet frame without
 * splitting it into multiple segments.
 * So, for mbufs that planned to be involved into RX/TX, the recommended
 * minimal buffer length is 2KB + RTE_PKTMBUF_HEADROOM.
 */
#define	RTE_MBUF_DEFAULT_DATAROOM	2048
#define	RTE_MBUF_DEFAULT_BUF_SIZE	\
	(RTE_MBUF_DEFAULT_DATAROOM + RTE_PKTMBUF_HEADROOM)

> Is this what the headroom and tailroom are for?

It is for prepending headers/trailers without copying the packet into a new
buffer, that is, it is used to change the packet efficiently.
Sometimes some headers still need to be moved, but not the entire frame.

Documentation shows how headroom, dataroom, tailroom, and buffer are related:

	https://doc.dpdk.org/guides/prog_guide/mbuf_lib.html

> I see example code that uses the headroom for the MAC Header, but what is
> the preferred way of copying a 1518 Ethernet packet into an mbuf? Should I
> use rte_pktmbuf_append() to reserve the extra bytes, or is there a better
> way?

Is the question still relevant given the explanations above?