VIDEOS 》 FreeBSD Networking Sub-system

Refer:
GitHub main FreeBSD src tree (read-only mirror) http://www.FreeBSD.org/ - https://github.com/freebsd/freebsd
struct mbuf (and struct pkthdr) - packet buffer - /sys/sys/mbuf.h - https://github.com/freebsd/freebsd/blob/master/sys/sys/mbuf.h

FreeBSD Networking Subsystem vs Linux Kernel Networking The FreeBSD Channel

The source code of struct mbuf data-structure (/sys/sys/mbuf.h) for quick reference:

/*
 * The core of the mbuf object along with some shortcut defines for practical
 * purposes.
 */
struct mbuf {
	/*
	 * Header present at the beginning of every mbuf.
	 * Size ILP32: 24
	 *      LP64: 32
	 * Compile-time assertions in uipc_mbuf.c test these values to ensure
	 * that they are correct.
	 */
	union {	/* next buffer in chain */
		struct mbuf		*m_next;
		SLIST_ENTRY(mbuf)	m_slist;
		STAILQ_ENTRY(mbuf)	m_stailq;
	};
	union {	/* next chain in queue/record */
		struct mbuf		*m_nextpkt;
		SLIST_ENTRY(mbuf)	m_slistpkt;
		STAILQ_ENTRY(mbuf)	m_stailqpkt;
	};
	caddr_t		 m_data;	/* location of data */
	int32_t		 m_len;		/* amount of data in this mbuf */
	uint32_t	 m_type:8,	/* type of data in this mbuf */
			 m_flags:24;	/* flags; see below */
#if !defined(__LP64__)
	uint32_t	 m_pad;		/* pad for 64bit alignment */
#endif

	/*
	 * A set of optional headers (packet header, external storage header)
	 * and internal data storage.  Historically, these arrays were sized
	 * to MHLEN (space left after a packet header) and MLEN (space left
	 * after only a regular mbuf header); they are now variable size in
	 * order to support future work on variable-size mbufs.
	 */
	union {
		struct {
			struct pkthdr	m_pkthdr;	/* M_PKTHDR set */
			union {
				struct m_ext	m_ext;	/* M_EXT set */
				char		m_pktdat[0];
			};
		};
		char	m_dat[0];			/* !M_PKTHDR, !M_EXT */
	};
};

The source code of struct pkthdr data-structure (/sys/sys/mbuf.h) for quick reference:

/*
 * Record/packet header in first mbuf of chain; valid only if M_PKTHDR is set.
 * Size ILP32: 48
 *	 LP64: 56
 * Compile-time assertions in uipc_mbuf.c test these values to ensure that
 * they are correct.
 */
struct pkthdr {
	union {
		struct m_snd_tag *snd_tag;	/* send tag, if any */
		struct ifnet	*rcvif;		/* rcv interface */
	};
	SLIST_HEAD(packet_tags, m_tag) tags; /* list of packet tags */
	int32_t		 len;		/* total packet length */

	/* Layer crossing persistent information. */
	uint32_t	 flowid;	/* packet's 4-tuple system */
	uint32_t	 csum_flags;	/* checksum and offload features */
	uint16_t	 fibnum;	/* this packet should use this fib */
	uint8_t		 cosqos;	/* class/quality of service */
	uint8_t		 rsstype;	/* hash type */
	union {
		uint64_t	rcv_tstmp;	/* timestamp in ns */
		struct {
			uint8_t		 l2hlen;	/* layer 2 hdr len */
			uint8_t		 l3hlen;	/* layer 3 hdr len */
			uint8_t		 l4hlen;	/* layer 4 hdr len */
			uint8_t		 l5hlen;	/* layer 5 hdr len */
			uint32_t	 spare;
		};
	};
	union {
		uint8_t  eight[8];
		uint16_t sixteen[4];
		uint32_t thirtytwo[2];
		uint64_t sixtyfour[1];
		uintptr_t unintptr[1];
		void	*ptr;
	} PH_per;

	/* Layer specific non-persistent local storage for reassembly, etc. */
	union {
		uint8_t  eight[8];
		uint16_t sixteen[4];
		uint32_t thirtytwo[2];
		uint64_t sixtyfour[1];
		uintptr_t unintptr[1];
		void 	*ptr;
	} PH_loc;
};

The source code of struct ifnet data-structure (/sys/net/if_var.h) for quick reference:

/*
 * Structure defining a network interface.
 */
struct ifnet {
	/* General book keeping of interface lists. */
	TAILQ_ENTRY(ifnet) if_link; 	/* all struct ifnets are chained */
	LIST_ENTRY(ifnet) if_clones;	/* interfaces of a cloner */
	TAILQ_HEAD(, ifg_list) if_groups; /* linked list of groups per if */
					/* protected by if_addr_lock */
	u_char	if_alloctype;		/* if_type at time of allocation */

	/* Driver and protocol specific information that remains stable. */
	void	*if_softc;		/* pointer to driver state */
	void	*if_llsoftc;		/* link layer softc */
	void	*if_l2com;		/* pointer to protocol bits */
	const char *if_dname;		/* driver name */
	int	if_dunit;		/* unit or IF_DUNIT_NONE */
	u_short	if_index;		/* numeric abbreviation for this if  */
	short	if_index_reserved;	/* spare space to grow if_index */
	char	if_xname[IFNAMSIZ];	/* external name (name + unit) */
	char	*if_description;	/* interface description */

	/* Variable fields that are touched by the stack and drivers. */
	int	if_flags;		/* up/down, broadcast, etc. */
	int	if_drv_flags;		/* driver-managed status flags */
	int	if_capabilities;	/* interface features & capabilities */
	int	if_capenable;		/* enabled features & capabilities */
	void	*if_linkmib;		/* link-type-specific MIB data */
	size_t	if_linkmiblen;		/* length of above data */
	u_int	if_refcount;		/* reference count */

	/* These fields are shared with struct if_data. */
	uint8_t		if_type;	/* ethernet, tokenring, etc */
	uint8_t		if_addrlen;	/* media address length */
	uint8_t		if_hdrlen;	/* media header length */
	uint8_t		if_link_state;	/* current link state */
	uint32_t	if_mtu;		/* maximum transmission unit */
	uint32_t	if_metric;	/* routing metric (external only) */
	uint64_t	if_baudrate;	/* linespeed */
	uint64_t	if_hwassist;	/* HW offload capabilities, see IFCAP */
	time_t		if_epoch;	/* uptime at attach or stat reset */
	struct timeval	if_lastchange;	/* time of last administrative change */

	struct  ifaltq if_snd;		/* output queue (includes altq) */
	struct	task if_linktask;	/* task for link change events */

	/* Addresses of different protocol families assigned to this if. */
	struct	rwlock if_addr_lock;	/* lock to protect address lists */
		/*
		 * if_addrhead is the list of all addresses associated to
		 * an interface.
		 * Some code in the kernel assumes that first element
		 * of the list has type AF_LINK, and contains sockaddr_dl
		 * addresses which store the link-level address and the name
		 * of the interface.
		 * However, access to the AF_LINK address through this
		 * field is deprecated. Use if_addr or ifaddr_byindex() instead.
		 */
	struct	ifaddrhead if_addrhead;	/* linked list of addresses per if */
	struct	ifmultihead if_multiaddrs; /* multicast addresses configured */
	int	if_amcount;		/* number of all-multicast requests */
	struct	ifaddr	*if_addr;	/* pointer to link-level address */
	void	*if_hw_addr;		/* hardware link-level address */
	const u_int8_t *if_broadcastaddr; /* linklevel broadcast bytestring */
	struct	rwlock if_afdata_lock;
	void	*if_afdata[AF_MAX];
	int	if_afdata_initialized;

	/* Additional features hung off the interface. */
	u_int	if_fib;			/* interface FIB */
	struct	vnet *if_vnet;		/* pointer to network stack instance */
	struct	vnet *if_home_vnet;	/* where this ifnet originates from */
	struct  ifvlantrunk *if_vlantrunk; /* pointer to 802.1q data */
	struct	bpf_if *if_bpf;		/* packet filter structure */
	int	if_pcount;		/* number of promiscuous listeners */
	void	*if_bridge;		/* bridge glue */
	void	*if_lagg;		/* lagg glue */
	void	*if_pf_kif;		/* pf glue */
	struct	carp_if *if_carp;	/* carp interface structure */
	struct	label *if_label;	/* interface MAC label */
	struct	netmap_adapter *if_netmap; /* netmap(4) softc */

	/* Various procedures of the layer2 encapsulation and drivers. */
	int	(*if_output)		/* output routine (enqueue) */
		(struct ifnet *, struct mbuf *, const struct sockaddr *,
		     struct route *);
	void	(*if_input)		/* input routine (from h/w driver) */
		(struct ifnet *, struct mbuf *);
	if_start_fn_t	if_start;	/* initiate output routine */
	if_ioctl_fn_t	if_ioctl;	/* ioctl routine */
	if_init_fn_t	if_init;	/* Init routine */
	int	(*if_resolvemulti)	/* validate/resolve multicast */
		(struct ifnet *, struct sockaddr **, struct sockaddr *);
	if_qflush_fn_t	if_qflush;	/* flush any queue */	
	if_transmit_fn_t if_transmit;   /* initiate output routine */

	void	(*if_reassign)		/* reassign to vnet routine */
		(struct ifnet *, struct vnet *, char *);
	if_get_counter_t if_get_counter; /* get counter values */
	int	(*if_requestencap)	/* make link header from request */
		(struct ifnet *, struct if_encap_req *);

	/* Statistics. */
	counter_u64_t	if_counters[IFCOUNTERS];

	/* Stuff that's only temporary and doesn't belong here. */

	/*
	 * Network adapter TSO limits:
	 * ===========================
	 *
	 * If the "if_hw_tsomax" field is zero the maximum segment
	 * length limit does not apply. If the "if_hw_tsomaxsegcount"
	 * or the "if_hw_tsomaxsegsize" field is zero the TSO segment
	 * count limit does not apply. If all three fields are zero,
	 * there is no TSO limit.
	 *
	 * NOTE: The TSO limits should reflect the values used in the
	 * BUSDMA tag a network adapter is using to load a mbuf chain
	 * for transmission. The TCP/IP network stack will subtract
	 * space for all linklevel and protocol level headers and
	 * ensure that the full mbuf chain passed to the network
	 * adapter fits within the given limits.
	 */
	u_int	if_hw_tsomax;		/* TSO maximum size in bytes */
	u_int	if_hw_tsomaxsegcount;	/* TSO maximum segment count */
	u_int	if_hw_tsomaxsegsize;	/* TSO maximum segment size in bytes */

	/*
	 * Network adapter send tag support:
	 */
	if_snd_tag_alloc_t *if_snd_tag_alloc;
	if_snd_tag_modify_t *if_snd_tag_modify;
	if_snd_tag_query_t *if_snd_tag_query;
	if_snd_tag_free_t *if_snd_tag_free;

	/*
	 * Spare fields to be added before branching a stable branch, so
	 * that structure can be enhanced without changing the kernel
	 * binary interface.
	 */
	int	if_ispare[4];		/* general use */
};