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MBUF(9)			 BSD Kernel Developer's	Manual		       MBUF(9)

     mbuf -- memory management in the kernel IPC subsystem

     #include <sys/param.h>
     #include <sys/systm.h>
     #include <sys/mbuf.h>

   Mbuf	allocation macros
     MGET(struct mbuf *mbuf, int how, short type);

     MGETHDR(struct mbuf *mbuf,	int how, short type);

     MCLGET(struct mbuf	*mbuf, int how);

     MEXTADD(struct mbuf *mbuf,	char *buf, u_int size,
	 void (*free)(struct mbuf *), void *opt_arg1, void *opt_arg2,
	 int flags, int	type);

   Mbuf	utility	macros
     mtod(struct mbuf *mbuf, type);

     M_ALIGN(struct mbuf *mbuf,	u_int len);

     MH_ALIGN(struct mbuf *mbuf, u_int len);

     M_LEADINGSPACE(struct mbuf	*mbuf);

     M_TRAILINGSPACE(struct mbuf *mbuf);

     M_MOVE_PKTHDR(struct mbuf *to, struct mbuf	*from);

     M_PREPEND(struct mbuf *mbuf, int len, int how);

     MCHTYPE(struct mbuf *mbuf,	short type);

     M_WRITABLE(struct mbuf *mbuf);

   Mbuf	allocation functions
     struct mbuf *
     m_get(int how, short type);

     struct mbuf *
     m_get2(int	size, int how, short type, int flags);

     struct mbuf *
     m_getm(struct mbuf	*orig, int len,	int how, short type);

     struct mbuf *
     m_getjcl(int how, short type, int flags, int size);

     struct mbuf *
     m_getcl(int how, short type, int flags);

     struct mbuf *
     m_gethdr(int how, short type);

     struct mbuf *
     m_free(struct mbuf	*mbuf);

     m_freem(struct mbuf *mbuf);

   Mbuf	utility	functions
     m_adj(struct mbuf *mbuf, int len);

     m_align(struct mbuf *mbuf,	int len);

     m_append(struct mbuf *mbuf, int len, c_caddr_t cp);

     struct mbuf *
     m_prepend(struct mbuf *mbuf, int len, int how);

     struct mbuf *
     m_copyup(struct mbuf *mbuf, int len, int dstoff);

     struct mbuf *
     m_pullup(struct mbuf *mbuf, int len);

     struct mbuf *
     m_pulldown(struct mbuf *mbuf, int offset, int len,	int *offsetp);

     struct mbuf *
     m_copym(struct mbuf *mbuf,	int offset, int	len, int how);

     struct mbuf *
     m_copypacket(struct mbuf *mbuf, int how);

     struct mbuf *
     m_dup(const struct	mbuf *mbuf, int	how);

     m_copydata(const struct mbuf *mbuf, int offset, int len, caddr_t buf);

     m_copyback(struct mbuf *mbuf, int offset, int len,	caddr_t	buf);

     struct mbuf *
     m_devget(char *buf, int len, int offset, struct ifnet *ifp,
	 void (*copy)(char *from, caddr_t to, u_int len));

     m_cat(struct mbuf *m, struct mbuf *n);

     m_catpkt(struct mbuf *m, struct mbuf *n);

     m_fixhdr(struct mbuf *mbuf);

     m_dup_pkthdr(struct mbuf *to, const struct	mbuf *from, int	how);

     m_move_pkthdr(struct mbuf *to, struct mbuf	*from);

     m_length(struct mbuf *mbuf, struct	mbuf **last);

     struct mbuf *
     m_split(struct mbuf *mbuf,	int len, int how);

     m_apply(struct mbuf *mbuf,	int off, int len,
	 int (*f)(void *arg, void *data, u_int len), void *arg);

     struct mbuf *
     m_getptr(struct mbuf *mbuf, int loc, int *off);

     struct mbuf *
     m_defrag(struct mbuf *m0, int how);

     struct mbuf *
     m_collapse(struct mbuf *m0, int how, int maxfrags);

     struct mbuf *
     m_unshare(struct mbuf *m0,	int how);

     An	mbuf is	a basic	unit of	memory management in the kernel	IPC subsystem.
     Network packets and socket	buffers	are stored in mbufs.  A	network	packet
     may span multiple mbufs arranged into a mbuf chain	(linked	list), which
     allows adding or trimming network headers with little overhead.

     While a developer should not bother with mbuf internals without serious
     reason in order to	avoid incompatibilities	with future changes, it	is
     useful to understand the general structure	of an mbuf.

     An	mbuf consists of a variable-sized header and a small internal buffer
     for data.	The total size of an mbuf, MSIZE, is a constant	defined	in
     <sys/param.h>.  The mbuf header includes:

	   m_next     (struct mbuf *) A	pointer	to the next mbuf in the	mbuf

	   m_nextpkt  (struct mbuf *) A	pointer	to the next mbuf chain in the

	   m_data     (caddr_t)	A pointer to data attached to this mbuf.

	   m_len      (int) The	length of the data.

	   m_type     (short) The type of the data.

	   m_flags    (int) The	mbuf flags.

     The mbuf flag bits	are defined as follows:

     #define M_EXT	     0x00000001	/* has associated external storage */
     #define M_PKTHDR	     0x00000002	/* start of record */
     #define M_EOR	     0x00000004	/* end of record */
     #define M_RDONLY	     0x00000008	/* associated data marked read-only */
     #define M_NOMAP	     0x00000100	/* mbuf	data is	unmapped */
     #define M_NOFREE	     0x00000200	/* do not free mbuf, embedded in cluster */
     #define M_BCAST	     0x00000010	/* send/received as link-level broadcast */
     #define M_MCAST	     0x00000020	/* send/received as link-level multicast */
     #define M_PROMISC	     0x00000040	/* packet was not for us */
     #define M_VLANTAG	     0x00000080	/* ether_vtag is valid */
     #define M_TSTMP	     0x00000400	/* rcv_tstmp field is valid */
     #define M_TSTMP_HPREC   0x00000800	/* rcv_tstmp is	high-prec, typically
					   hw-stamped on port (useful for IEEE 1588
					   and 802.1AS)	*/

     #define M_PROTO1	     0x00001000	/* protocol-specific */
     #define M_PROTO2	     0x00002000	/* protocol-specific */
     #define M_PROTO3	     0x00004000	/* protocol-specific */
     #define M_PROTO4	     0x00008000	/* protocol-specific */
     #define M_PROTO5	     0x00010000	/* protocol-specific */
     #define M_PROTO6	     0x00020000	/* protocol-specific */
     #define M_PROTO7	     0x00040000	/* protocol-specific */
     #define M_PROTO8	     0x00080000	/* protocol-specific */
     #define M_PROTO9	     0x00100000	/* protocol-specific */
     #define M_PROTO10	     0x00200000	/* protocol-specific */
     #define M_PROTO11	     0x00400000	/* protocol-specific */
     #define M_PROTO12	     0x00800000	/* protocol-specific */

     The available mbuf	types are defined as follows:

     #define MT_DATA	     1	     /*	dynamic	(data) allocation */
     #define MT_HEADER	     MT_DATA /*	packet header */

     #define MT_VENDOR1	     4	     /*	for vendor-internal use	*/
     #define MT_VENDOR2	     5	     /*	for vendor-internal use	*/
     #define MT_VENDOR3	     6	     /*	for vendor-internal use	*/
     #define MT_VENDOR4	     7	     /*	for vendor-internal use	*/

     #define MT_SONAME	     8	     /*	socket name */

     #define MT_EXP1	     9	     /*	for experimental use */
     #define MT_EXP2	     10	     /*	for experimental use */
     #define MT_EXP3	     11	     /*	for experimental use */
     #define MT_EXP4	     12	     /*	for experimental use */

     #define MT_CONTROL	     14	     /*	extra-data protocol message */
     #define MT_EXTCONTROL   15	     /*	control	message	with externalized contents */
     #define MT_OOBDATA	     16	     /*	expedited data	*/

     The available external buffer types are defined as	follows:

     #define EXT_CLUSTER     1	     /*	mbuf cluster */
     #define EXT_SFBUF	     2	     /*	sendfile(2)'s sf_bufs */
     #define EXT_JUMBOP	     3	     /*	jumbo cluster 4096 bytes */
     #define EXT_JUMBO9	     4	     /*	jumbo cluster 9216 bytes */
     #define EXT_JUMBO16     5	     /*	jumbo cluster 16184 bytes */
     #define EXT_PACKET	     6	     /*	mbuf+cluster from packet zone */
     #define EXT_MBUF	     7	     /*	external mbuf reference	*/
     #define EXT_RXRING	     8	     /*	data in	NIC receive ring */
     #define EXT_PGS	     9	     /*	array of unmapped pages	*/

     #define EXT_VENDOR1     224     /*	for vendor-internal use	*/
     #define EXT_VENDOR2     225     /*	for vendor-internal use	*/
     #define EXT_VENDOR3     226     /*	for vendor-internal use	*/
     #define EXT_VENDOR4     227     /*	for vendor-internal use	*/

     #define EXT_EXP1	     244     /*	for experimental use */
     #define EXT_EXP2	     245     /*	for experimental use */
     #define EXT_EXP3	     246     /*	for experimental use */
     #define EXT_EXP4	     247     /*	for experimental use */

     #define EXT_NET_DRV     252     /*	custom ext_buf provided	by net driver(s) */
     #define EXT_MOD_TYPE    253     /*	custom module's	ext_buf	type */
     #define EXT_DISPOSABLE  254     /*	can throw this buffer away w/page flipping */
     #define EXT_EXTREF	     255     /*	has externally maintained ref_cnt ptr */

     If	the M_PKTHDR flag is set, a struct pkthdr m_pkthdr is added to the
     mbuf header.  It contains a pointer to the	interface the packet has been
     received from (struct ifnet *rcvif), and the total	packet length (int
     len).  Optionally,	it may also contain an attached	list of	packet tags
     (struct m_tag).  See mbuf_tags(9) for details.  Fields used in offloading
     checksum calculation to the hardware are kept in m_pkthdr as well.	 See

     If	small enough, data is stored in	the internal data buffer of an mbuf.
     If	the data is sufficiently large,	another	mbuf may be added to the mbuf
     chain, or external	storage	may be associated with the mbuf.  MHLEN	bytes
     of	data can fit into an mbuf with the M_PKTHDR flag set, MLEN bytes can

     If	external storage is being associated with an mbuf, the m_ext header is
     added at the cost of losing the internal data buffer.  It includes	a
     pointer to	external storage, the size of the storage, a pointer to	a
     function used for freeing the storage, a pointer to an optional argument
     that can be passed	to the function, and a pointer to a reference counter.
     An	mbuf using external storage has	the M_EXT flag set.

     The system	supplies a macro for allocating	the desired external storage
     buffer, MEXTADD.

     The allocation and	management of the reference counter is handled by the

     The system	also supplies a	default	type of	external storage buffer	called
     an	mbuf cluster.  Mbuf clusters can be allocated and configured with the
     use of the	MCLGET macro.  Each mbuf cluster is MCLBYTES in	size, where
     MCLBYTES is a machine-dependent constant.	The system defines an advisory
     macro MINCLSIZE, which is the smallest amount of data to put into an mbuf
     cluster.  It is equal to MHLEN plus one.  It is typically preferable to
     store data	into the data region of	an mbuf, if size permits, as opposed
     to	allocating a separate mbuf cluster to hold the same data.

   Macros and Functions
     There are numerous	predefined macros and functions	that provide the de-
     veloper with common utilities.

	   mtod(mbuf, type)
	   Convert an mbuf pointer to a	data pointer.  The macro expands to
	   the data pointer cast to the	specified type.	 Note: It is advisable
	   to ensure that there	is enough contiguous data in mbuf.  See
	   m_pullup() for details.

	   MGET(mbuf, how, type)
	   Allocate an mbuf and	initialize it to contain internal data.	 mbuf
	   will	point to the allocated mbuf on success,	or be set to NULL on
	   failure.  The how argument is to be set to M_WAITOK or M_NOWAIT.
	   It specifies	whether	the caller is willing to block if necessary.
	   A number of other functions and macros related to mbufs have	the
	   same	argument because they may at some point	need to	allocate new

	   MGETHDR(mbuf, how, type)
	   Allocate an mbuf and	initialize it to contain a packet header and
	   internal data.  See MGET() for details.

	   MEXTADD(mbuf, buf, size, free, opt_arg1, opt_arg2, flags, type)
	   Associate externally	managed	data with mbuf.	 Any internal data
	   contained in	the mbuf will be discarded, and	the M_EXT flag will be
	   set.	 The buf and size arguments are	the address and	length,	re-
	   spectively, of the data.  The free argument points to a function
	   which will be called	to free	the data when the mbuf is freed; it is
	   only	used if	type is	EXT_EXTREF.  The opt_arg1 and opt_arg2 argu-
	   ments will be saved in ext_arg1 and ext_arg2	fields of the struct
	   m_ext of the	mbuf.  The flags argument specifies additional mbuf
	   flags; it is	not necessary to specify M_EXT.	 Finally, the type ar-
	   gument specifies the	type of	external data, which controls how it
	   will	be disposed of when the	mbuf is	freed.	In most	cases, the
	   correct value is EXT_EXTREF.

	   MCLGET(mbuf,	how)
	   Allocate and	attach an mbuf cluster to mbuf.	 On success, a non-
	   zero	value returned;	otherwise, 0.  Historically, consumers would
	   check for success by	testing	the M_EXT flag on the mbuf, but	this
	   is now discouraged to avoid unnecessary awareness of	the implemen-
	   tation of external storage in protocol stacks and device drivers.

	   M_ALIGN(mbuf, len)
	   Set the pointer mbuf-_m_data	to place an object of the size len at
	   the end of the internal data	area of	mbuf, long word	aligned.  Ap-
	   plicable only if mbuf is newly allocated with MGET()	or m_get().

	   MH_ALIGN(mbuf, len)
	   Serves the same purpose as M_ALIGN()	does, but only for mbuf	newly
	   allocated with MGETHDR() or m_gethdr(), or initialized by
	   m_dup_pkthdr() or m_move_pkthdr().

	   m_align(mbuf, len)
	   Services the	same purpose as	M_ALIGN() but handles any type of

	   Returns the number of bytes available before	the beginning of data
	   in mbuf.

	   Returns the number of bytes available after the end of data in

	   M_PREPEND(mbuf, len,	how)
	   This	macro operates on an mbuf chain.  It is	an optimized wrapper
	   for m_prepend() that	can make use of	possible empty space before
	   data	(e.g. left after trimming of a link-layer header).  The	new
	   mbuf	chain pointer or NULL is in mbuf after the call.

	   M_MOVE_PKTHDR(to, from)
	   Using this macro is equivalent to calling m_move_pkthdr(to, from).

	   This	macro will evaluate true if mbuf is not	marked M_RDONLY	and if
	   either mbuf does not	contain	external storage or, if	it does, then
	   if the reference count of the storage is not	greater	than 1.	 The
	   M_RDONLY flag can be	set in mbuf-_m_flags.  This can	be achieved
	   during setup	of the external	storage, by passing the	M_RDONLY bit
	   as a	flags argument to the MEXTADD()	macro, or can be directly set
	   in individual mbufs.

	   MCHTYPE(mbuf, type)
	   Change the type of mbuf to type.  This is a relatively expensive
	   operation and should	be avoided.

     The functions are:

	   m_get(how, type)
	   A function version of MGET()	for non-critical paths.

	   m_get2(size,	how, type, flags)
	   Allocate an mbuf with enough	space to hold specified	amount of
	   data.  If the size is is larger than	MJUMPAGESIZE, NULL will	be re-

	   m_getm(orig,	len, how, type)
	   Allocate len	bytes worth of mbufs and mbuf clusters if necessary
	   and append the resulting allocated mbuf chain to the	mbuf chain
	   orig, if it is non-NULL.  If	the allocation fails at	any point,
	   free	whatever was allocated and return NULL.	 If orig is non-NULL,
	   it will not be freed.  It is	possible to use	m_getm() to either ap-
	   pend	len bytes to an	existing mbuf or mbuf chain (for example, one
	   which may be	sitting	in a pre-allocated ring) or to simply perform
	   an all-or-nothing mbuf and mbuf cluster allocation.

	   m_gethdr(how, type)
	   A function version of MGETHDR() for non-critical paths.

	   m_getcl(how,	type, flags)
	   Fetch an mbuf with a	mbuf cluster attached to it.  If one of	the
	   allocations fails, the entire allocation fails.  This routine is
	   the preferred way of	fetching both the mbuf and mbuf	cluster	to-
	   gether, as it avoids	having to unlock/relock	between	allocations.
	   Returns NULL	on failure.

	   m_getjcl(how, type, flags, size)
	   This	is like	m_getcl() but the specified size of the	cluster	to be
	   allocated must be one of MCLBYTES, MJUMPAGESIZE, MJUM9BYTES,	or

	   Frees mbuf.	Returns	m_next of the freed mbuf.

     The functions below operate on mbuf chains.

	   Free	an entire mbuf chain, including	any external storage.

	   m_adj(mbuf, len)
	   Trim	len bytes from the head	of an mbuf chain if len	is positive,
	   from	the tail otherwise.

	   m_append(mbuf, len, cp)
	   Append len bytes of data cp to the mbuf chain.  Extend the mbuf
	   chain if the	new data does not fit in existing space.

	   m_prepend(mbuf, len,	how)
	   Allocate a new mbuf and prepend it to the mbuf chain, handle
	   M_PKTHDR properly.  Note: It	does not allocate any mbuf clusters,
	   so len must be less than MLEN or MHLEN, depending on	the M_PKTHDR
	   flag	setting.

	   m_copyup(mbuf, len, dstoff)
	   Similar to m_pullup() but copies len	bytes of data into a new mbuf
	   at dstoff bytes into	the mbuf.  The dstoff argument aligns the data
	   and leaves room for a link layer header.  Returns the new mbuf
	   chain on success, and frees the mbuf	chain and returns NULL on
	   failure.  Note: The function	does not allocate mbuf clusters, so
	   len + dstoff	must be	less than MHLEN.

	   m_pullup(mbuf, len)
	   Arrange that	the first len bytes of an mbuf chain are contiguous
	   and lay in the data area of mbuf, so	they are accessible with
	   mtod(mbuf, type).  It is important to remember that this may	in-
	   volve reallocating some mbufs and moving data so all	pointers ref-
	   erencing data within	the old	mbuf chain must	be recalculated	or
	   made	invalid.  Return the new mbuf chain on success,	NULL on	fail-
	   ure (the mbuf chain is freed	in this	case).	Note: It does not al-
	   locate any mbuf clusters, so	len must be less than or equal to

	   m_pulldown(mbuf, offset, len, offsetp)
	   Arrange that	len bytes between offset and offset + len in the mbuf
	   chain are contiguous	and lay	in the data area of mbuf, so they are
	   accessible with mtod(mbuf, type).  len must be smaller than,	or
	   equal to, the size of an mbuf cluster.  Return a pointer to an in-
	   termediate mbuf in the chain	containing the requested region; the
	   offset in the data region of	the mbuf chain to the data contained
	   in the returned mbuf	is stored in *offsetp.	If offsetp is NULL,
	   the region may be accessed using mtod(mbuf, type).  If offsetp is
	   non-NULL, the region	may be accessed	using mtod(mbuf, uint8_t) +
	   *offsetp.  The region of the	mbuf chain between its beginning and
	   offset is not modified, therefore it	is safe	to hold	pointers to
	   data	within this region before calling m_pulldown().

	   m_copym(mbuf, offset, len, how)
	   Make	a copy of an mbuf chain	starting offset	bytes from the begin-
	   ning, continuing for	len bytes.  If len is M_COPYALL, copy to the
	   end of the mbuf chain.  Note: The copy is read-only,	because	the
	   mbuf	clusters are not copied, only their reference counts are in-

	   m_copypacket(mbuf, how)
	   Copy	an entire packet including header, which must be present.
	   This	is an optimized	version	of the common case m_copym(mbuf, 0,
	   M_COPYALL, how).  Note: the copy is read-only, because the mbuf
	   clusters are	not copied, only their reference counts	are incre-

	   m_dup(mbuf, how)
	   Copy	a packet header	mbuf chain into	a completely new mbuf chain,
	   including copying any mbuf clusters.	 Use this instead of
	   m_copypacket() when you need	a writable copy	of an mbuf chain.

	   m_copydata(mbuf, offset, len, buf)
	   Copy	data from an mbuf chain	starting off bytes from	the beginning,
	   continuing for len bytes, into the indicated	buffer buf.

	   m_copyback(mbuf, offset, len, buf)
	   Copy	len bytes from the buffer buf back into	the indicated mbuf
	   chain, starting at offset bytes from	the beginning of the mbuf
	   chain, extending the	mbuf chain if necessary.  Note:	It does	not
	   allocate any	mbuf clusters, just adds mbufs to the mbuf chain.  It
	   is safe to set offset beyond	the current mbuf chain end: zeroed
	   mbufs will be allocated to fill the space.

	   m_length(mbuf, last)
	   Return the length of	the mbuf chain,	and optionally a pointer to
	   the last mbuf.

	   m_dup_pkthdr(to, from, how)
	   Upon	the function's completion, the mbuf to will contain an identi-
	   cal copy of from-_m_pkthdr and the per-packet attributes found in
	   the mbuf chain from.	 The mbuf from must have the flag M_PKTHDR
	   initially set, and to must be empty on entry.

	   m_move_pkthdr(to, from)
	   Move	m_pkthdr and the per-packet attributes from the	mbuf chain
	   from	to the mbuf to.	 The mbuf from must have the flag M_PKTHDR
	   initially set, and to must be empty on entry.  Upon the function's
	   completion, from will have the flag M_PKTHDR	and the	per-packet at-
	   tributes cleared.

	   Set the packet-header length	to the length of the mbuf chain.

	   m_devget(buf, len, offset, ifp, copy)
	   Copy	data from a device local memory	pointed	to by buf to an	mbuf
	   chain.  The copy is done using a specified copy routine copy, or
	   bcopy() if copy is NULL.

	   m_cat(m, n)
	   Concatenate n to m.	Both mbuf chains must be of the	same type.  n
	   is not guaranteed to	be valid after m_cat() returns.	 m_cat() does
	   not update any packet header	fields or free mbuf tags.

	   m_catpkt(m, n)
	   A variant of	m_cat()	that operates on packets.  Both	m and n	must
	   contain packet headers.  n is not guaranteed	to be valid after
	   m_catpkt() returns.

	   m_split(mbuf, len, how)
	   Partition an	mbuf chain in two pieces, returning the	tail: all but
	   the first len bytes.	 In case of failure, it	returns	NULL and at-
	   tempts to restore the mbuf chain to its original state.

	   m_apply(mbuf, off, len, f, arg)
	   Apply a function to an mbuf chain, at offset	off, for length	len
	   bytes.  Typically used to avoid calls to m_pullup() which would
	   otherwise be	unnecessary or undesirable.  arg is a convenience ar-
	   gument which	is passed to the callback function f.

	   Each	time f() is called, it will be passed arg, a pointer to	the
	   data	in the current mbuf, and the length len	of the data in this
	   mbuf	to which the function should be	applied.

	   The function	should return zero to indicate success;	otherwise, if
	   an error is indicated, then m_apply() will return the error and
	   stop	iterating through the mbuf chain.

	   m_getptr(mbuf, loc, off)
	   Return a pointer to the mbuf	containing the data located at loc
	   bytes from the beginning of the mbuf	chain.	The corresponding off-
	   set into the	mbuf will be stored in *off.

	   m_defrag(m0,	how)
	   Defragment an mbuf chain, returning the shortest possible chain of
	   mbufs and clusters.	If allocation fails and	this can not be	com-
	   pleted, NULL	will be	returned and the original chain	will be	un-
	   changed.  Upon success, the original	chain will be freed and	the
	   new chain will be returned.	how should be either M_WAITOK or
	   M_NOWAIT, depending on the caller's preference.

	   This	function is especially useful in network drivers, where	cer-
	   tain	long mbuf chains must be shortened before being	added to TX
	   descriptor lists.

	   m_collapse(m0, how, maxfrags)
	   Defragment an mbuf chain, returning a chain of at most maxfrags
	   mbufs and clusters.	If allocation fails or the chain cannot	be
	   collapsed as	requested, NULL	will be	returned, with the original
	   chain possibly modified.  As	with m_defrag(), how should be one of

	   m_unshare(m0, how)
	   Create a version of the specified mbuf chain	whose contents can be
	   safely modified without affecting other users.  If allocation fails
	   and this operation can not be completed, NULL will be returned.
	   The original	mbuf chain is always reclaimed and the reference count
	   of any shared mbuf clusters is decremented.	how should be either
	   M_WAITOK or M_NOWAIT, depending on the caller's preference.	As a
	   side-effect of this process the returned mbuf chain may be com-

	   This	function is especially useful in the transmit path of network
	   code, when data must	be encrypted or	otherwise altered prior	to

     This section currently applies to TCP/IP only.  In	order to save the host
     CPU resources, computing checksums	is offloaded to	the network interface
     hardware if possible.  The	m_pkthdr member	of the leading mbuf of a
     packet contains two fields	used for that purpose, int csum_flags and int
     csum_data.	 The meaning of	those fields depends on	the direction a	packet
     flows in, and on whether the packet is fragmented.	 Henceforth,
     csum_flags	or csum_data of	a packet will denote the corresponding field
     of	the m_pkthdr member of the leading mbuf	in the mbuf chain containing
     the packet.

     On	output,	checksum offloading is attempted after the outgoing interface
     has been determined for a packet.	The interface-specific field
     ifnet.if_data.ifi_hwassist	(see ifnet(9)) is consulted for	the capabili-
     ties of the interface to assist in	computing checksums.  The csum_flags
     field of the packet header	is set to indicate which actions the interface
     is	supposed to perform on it.  The	actions	unsupported by the network in-
     terface are done in the software prior to passing the packet down to the
     interface driver; such actions will never be requested through

     The flags demanding a particular action from an interface are as follows:

	   CSUM_IP   The IP header checksum is to be computed and stored in
		     the corresponding field of	the packet.  The hardware is
		     expected to know the format of an IP header to determine
		     the offset	of the IP checksum field.

	   CSUM_TCP  The TCP checksum is to be computed.  (See below.)

	   CSUM_UDP  The UDP checksum is to be computed.  (See below.)

     Should a TCP or UDP checksum be offloaded to the hardware,	the field
     csum_data will contain the	byte offset of the checksum field relative to
     the end of	the IP header.	In this	case, the checksum field will be ini-
     tially set	by the TCP/IP module to	the checksum of	the pseudo header de-
     fined by the TCP and UDP specifications.

     On	input, an interface indicates the actions it has performed on a	packet
     by	setting	one or more of the following flags in csum_flags associated
     with the packet:

	   CSUM_IP_CHECKED  The	IP header checksum has been computed.

	   CSUM_IP_VALID    The	IP header has a	valid checksum.	 This flag can
			    appear only	in combination with CSUM_IP_CHECKED.

	   CSUM_DATA_VALID  The	checksum of the	data portion of	the IP packet
			    has	been computed and stored in the	field
			    csum_data in network byte order.

	   CSUM_PSEUDO_HDR  Can	be set only along with CSUM_DATA_VALID to in-
			    dicate that	the IP data checksum found in
			    csum_data allows for the pseudo header defined by
			    the	TCP and	UDP specifications.  Otherwise the
			    checksum of	the pseudo header must be calculated
			    by the host	CPU and	added to csum_data to obtain
			    the	final checksum to be used for TCP or UDP vali-
			    dation purposes.

     If	a particular network interface just indicates success or failure of
     TCP or UDP	checksum validation without returning the exact	value of the
     checksum to the host CPU, its driver can mark CSUM_DATA_VALID and
     CSUM_PSEUDO_HDR in	csum_flags, and	set csum_data to 0xFFFF	hexadecimal to
     indicate a	valid checksum.	 It is a peculiarity of	the algorithm used
     that the Internet checksum	calculated over	any valid packet will be
     0xFFFF as long as the original checksum field is included.

     When running a kernel compiled with the option MBUF_STRESS_TEST, the fol-
     lowing sysctl(8)-controlled options may be	used to	create various fail-
     ure/extreme cases for testing of network drivers and other	parts of the
     kernel that rely on mbufs.

	    Causes ip_output() to fragment outgoing mbuf chains	into fragments
	    of the specified size.  Setting this variable to 1 is an excellent
	    way	to test	the long mbuf chain handling ability of	network	driv-

	    Causes the function	m_defrag() to randomly fail, returning NULL.
	    Any	piece of code which uses m_defrag() should be tested with this

     See above.

     ifnet(9), mbuf_tags(9)

     Mbufs appeared in an early	version	of BSD.	 Besides being used for	net-
     work packets, they	were used to store various dynamic structures, such as
     routing table entries, interface addresses, protocol control blocks, etc.
     In	more recent FreeBSD use	of mbufs is almost entirely limited to packet
     storage, with uma(9) zones	being used directly to store other network-re-
     lated memory.

     Historically, the mbuf allocator has been a special-purpose memory	allo-
     cator able	to run in interrupt contexts and allocating from a special
     kernel address space map.	As of FreeBSD 5.3, the mbuf allocator is a
     wrapper around uma(9), allowing caching of	mbufs, clusters, and mbuf +
     cluster pairs in per-CPU caches, as well as bringing other	benefits of
     slab allocation.

     The original mbuf manual page was written by Yar Tikhiy.  The uma(9) mbuf
     allocator was written by
     Bosko Milekic.

BSD				 July 23, 2020				   BSD


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