man/man9/mbuf.9

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.\" $FreeBSD$
.\"
.Dd February 26, 2007
.Dt MBUF 9
.Os
.\"
.Sh NAME
.Nm mbuf
.Nd "memory management in the kernel IPC subsystem"
.\"
.Sh SYNOPSIS
.In sys/param.h
.In sys/systm.h
.In sys/mbuf.h
.\"
.Ss Mbuf allocation macros
.Fn MGET "struct mbuf *mbuf" "int how" "short type"
.Fn MGETHDR "struct mbuf *mbuf" "int how" "short type"
.Fn MCLGET "struct mbuf *mbuf" "int how"
.Fo MEXTADD
.Fa "struct mbuf *mbuf"
.Fa "caddr_t buf"
.Fa "u_int size"
.Fa "void (*free)(void *opt_arg1, void *opt_arg2)"
.Fa "void *opt_arg1"
.Fa "void *opt_arg2"
.Fa "short flags"
.Fa "int type"
.Fc
.Fn MEXTFREE "struct mbuf *mbuf"
.Fn MFREE "struct mbuf *mbuf" "struct mbuf *successor"
.\"
.Ss Mbuf utility macros
.Fn mtod "struct mbuf *mbuf" "type"
.Fn M_ALIGN "struct mbuf *mbuf" "u_int len"
.Fn MH_ALIGN "struct mbuf *mbuf" "u_int len"
.Ft int
.Fn M_LEADINGSPACE "struct mbuf *mbuf"
.Ft int
.Fn M_TRAILINGSPACE "struct mbuf *mbuf"
.Fn M_MOVE_PKTHDR "struct mbuf *to" "struct mbuf *from"
.Fn M_PREPEND "struct mbuf *mbuf" "int len" "int how"
.Fn MCHTYPE "struct mbuf *mbuf" "u_int type"
.Ft int
.Fn M_WRITABLE "struct mbuf *mbuf"
.\"
.Ss Mbuf allocation functions
.Ft struct mbuf *
.Fn m_get "int how" "int type"
.Ft struct mbuf *
.Fn m_getm "struct mbuf *orig" "int len" "int how" "int type"
.Ft struct mbuf *
.Fn m_getcl "int how" "short type" "int flags"
.Ft struct mbuf *
.Fn m_getclr "int how" "int type"
.Ft struct mbuf *
.Fn m_gethdr "int how" "int type"
.Ft struct mbuf *
.Fn m_free "struct mbuf *mbuf"
.Ft void
.Fn m_freem "struct mbuf *mbuf"
.\"
.Ss Mbuf utility functions
.Ft void
.Fn m_adj "struct mbuf *mbuf" "int len"
.Ft void
.Fn m_align "struct mbuf *mbuf" "int len"
.Ft int
.Fn m_append "struct mbuf *mbuf" "int len" "c_caddr_t cp"
.Ft struct mbuf *
.Fn m_prepend "struct mbuf *mbuf" "int len" "int how"
.Ft struct mbuf *
.Fn m_copyup "struct mbuf *mbuf" "int len" "int dstoff"
.Ft struct mbuf *
.Fn m_pullup "struct mbuf *mbuf" "int len"
.Ft struct mbuf *
.Fn m_pulldown "struct mbuf *mbuf" "int offset" "int len" "int *offsetp"
.Ft struct mbuf *
.Fn m_copym "struct mbuf *mbuf" "int offset" "int len" "int how"
.Ft struct mbuf *
.Fn m_copypacket "struct mbuf *mbuf" "int how"
.Ft struct mbuf *
.Fn m_dup "struct mbuf *mbuf" "int how"
.Ft void
.Fn m_copydata "const struct mbuf *mbuf" "int offset" "int len" "caddr_t buf"
.Ft void
.Fn m_copyback "struct mbuf *mbuf" "int offset" "int len" "caddr_t buf"
.Ft struct mbuf *
.Fo m_devget
.Fa "char *buf"
.Fa "int len"
.Fa "int offset"
.Fa "struct ifnet *ifp"
.Fa "void (*copy)(char *from, caddr_t to, u_int len)"
.Fc
.Ft void
.Fn m_cat "struct mbuf *m" "struct mbuf *n"
.Ft u_int
.Fn m_fixhdr "struct mbuf *mbuf"
.Ft void
.Fn m_dup_pkthdr "struct mbuf *to" "struct mbuf *from"
.Ft void
.Fn m_move_pkthdr "struct mbuf *to" "struct mbuf *from"
.Ft u_int
.Fn m_length "struct mbuf *mbuf" "struct mbuf **last"
.Ft struct mbuf *
.Fn m_split "struct mbuf *mbuf" "int len" "int how"
.Ft int
.Fn m_apply "struct mbuf *mbuf" "int off" "int len" "int (*f)(void *arg, void *data, u_int len)" "void *arg"
.Ft struct mbuf *
.Fn m_getptr "struct mbuf *mbuf" "int loc" "int *off"
.Ft struct mbuf *
.Fn m_defrag "struct mbuf *m0" "int how"
.Ft struct mbuf *
.Fn m_unshare "struct mbuf *m0" "int how"
.\"
.Sh DESCRIPTION
An
.Vt mbuf
is a basic unit of memory management in the kernel IPC subsystem.
Network packets and socket buffers are stored in
.Vt mbufs .
A network packet may span multiple
.Vt mbufs
arranged into a
.Vt mbuf chain
(linked list),
which allows adding or trimming
network headers with little overhead.
.Pp
While a developer should not bother with
.Vt mbuf
internals without serious
reason in order to avoid incompatibilities with future changes, it
is useful to understand the general structure of an
.Vt mbuf .
.Pp
An
.Vt mbuf
consists of a variable-sized header and a small internal
buffer for data.
The total size of an
.Vt mbuf ,
.Dv MSIZE ,
is a constant defined in
.In sys/param.h .
The
.Vt mbuf
header includes:
.Pp
.Bl -tag -width "m_nextpkt" -offset indent
.It Va m_next
.Pq Vt struct mbuf *
A pointer to the next
.Vt mbuf
in the
.Vt mbuf chain .
.It Va m_nextpkt
.Pq Vt struct mbuf *
A pointer to the next
.Vt mbuf chain
in the queue.
.It Va m_data
.Pq Vt caddr_t
A pointer to data attached to this
.Vt mbuf .
.It Va m_len
.Pq Vt int
The length of the data.
.It Va m_type
.Pq Vt short
The type of the data.
.It Va m_flags
.Pq Vt int
The
.Vt mbuf
flags.
.El
.Pp
The
.Vt mbuf
flag bits are defined as follows:
.Bd -literal
/* mbuf flags */
#define	M_EXT		0x0001	/* has associated external storage */
#define	M_PKTHDR	0x0002	/* start of record */
#define	M_EOR		0x0004	/* end of record */
#define	M_RDONLY	0x0008	/* associated data marked read-only */
#define	M_PROTO1	0x0010	/* protocol-specific */
#define	M_PROTO2	0x0020 	/* protocol-specific */
#define	M_PROTO3	0x0040	/* protocol-specific */
#define	M_PROTO4	0x0080	/* protocol-specific */
#define	M_PROTO5	0x0100	/* protocol-specific */
#define	M_PROTO6	0x4000	/* protocol-specific (avoid M_BCAST conflict) */
#define	M_FREELIST	0x8000	/* mbuf is on the free list */

/* mbuf pkthdr flags (also stored in m_flags) */
#define	M_BCAST		0x0200	/* send/received as link-level broadcast */
#define	M_MCAST		0x0400	/* send/received as link-level multicast */
#define	M_FRAG		0x0800	/* packet is fragment of larger packet */
#define	M_FIRSTFRAG	0x1000	/* packet is first fragment */
#define	M_LASTFRAG	0x2000	/* packet is last fragment */
.Ed
.Pp
The available
.Vt mbuf
types are defined as follows:
.Bd -literal
/* mbuf types */
#define	MT_DATA		1	/* dynamic (data) allocation */
#define	MT_HEADER	MT_DATA	/* packet header */
#define	MT_SONAME	8	/* socket name */
#define	MT_CONTROL	14	/* extra-data protocol message */
#define	MT_OOBDATA	15	/* expedited data */
.Ed
.Pp
If the
.Dv M_PKTHDR
flag is set, a
.Vt struct pkthdr Va m_pkthdr
is added to the
.Vt mbuf
header.
It contains a pointer to the interface
the packet has been received from
.Pq Vt struct ifnet Va *rcvif ,
and the total packet length
.Pq Vt int Va len .
Optionally, it may also contain an attached list of packet tags
.Pq Vt "struct m_tag" .
See
.Xr mbuf_tags 9
for details.
Fields used in offloading checksum calculation to the hardware are kept in
.Va m_pkthdr
as well.
See
.Sx HARDWARE-ASSISTED CHECKSUM CALCULATION
for details.
.Pp
If small enough, data is stored in the internal data buffer of an
.Vt mbuf .
If the data is sufficiently large, another
.Vt mbuf
may be added to the
.Vt mbuf chain ,
or external storage may be associated with the
.Vt mbuf .
.Dv MHLEN
bytes of data can fit into an
.Vt mbuf
with the
.Dv M_PKTHDR
flag set,
.Dv MLEN
bytes can otherwise.
.Pp
If external storage is being associated with an
.Vt mbuf ,
the
.Va 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
.Vt mbuf
using external storage has the
.Dv M_EXT
flag set.
.Pp
The system supplies a macro for allocating the desired external storage
buffer,
.Dv MEXTADD .
.Pp
The allocation and management of the reference counter is handled by the
subsystem.
.Pp
The system also supplies a default type of external storage buffer called an
.Vt mbuf cluster .
.Vt Mbuf clusters
can be allocated and configured with the use of the
.Dv MCLGET
macro.
Each
.Vt mbuf cluster
is
.Dv MCLBYTES
in size, where MCLBYTES is a machine-dependent constant.
The system defines an advisory macro
.Dv MINCLSIZE ,
which is the smallest amount of data to put into an
.Vt mbuf cluster .
It is equal to the sum of
.Dv MLEN
and
.Dv MHLEN .
It is typically preferable to store data into the data region of an
.Vt mbuf ,
if size permits, as opposed to allocating a separate
.Vt mbuf cluster
to hold the same data.
.\"
.Ss Macros and Functions
There are numerous predefined macros and functions that provide the
developer with common utilities.
.\"
.Bl -ohang -offset indent
.It Fn mtod mbuf type
Convert an
.Fa mbuf
pointer to a data pointer.
The macro expands to the data pointer cast to the pointer of the specified
.Fa type .
.Sy Note :
It is advisable to ensure that there is enough contiguous data in
.Fa mbuf .
See
.Fn m_pullup
for details.
.It Fn MGET mbuf how type
Allocate an
.Vt mbuf
and initialize it to contain internal data.
.Fa mbuf
will point to the allocated
.Vt mbuf
on success, or be set to
.Dv NULL
on failure.
The
.Fa how
argument is to be set to
.Dv M_TRYWAIT
or
.Dv M_DONTWAIT .
It specifies whether the caller is willing to block if necessary.
If
.Fa how
is set to
.Dv M_TRYWAIT ,
a failed allocation will result in the caller being put
to sleep for a designated
kern.ipc.mbuf_wait
.Xr ( sysctl 8
tunable)
number of ticks.
A number of other functions and macros related to
.Vt mbufs
have the same argument because they may
at some point need to allocate new
.Vt mbufs .
.Pp
Programmers should be careful not to confuse the
.Vt mbuf
allocation flag
.Dv M_DONTWAIT
with the
.Xr malloc 9
allocation flag,
.Dv M_NOWAIT .
They are not the same.
.It Fn MGETHDR mbuf how type
Allocate an
.Vt mbuf
and initialize it to contain a packet header
and internal data.
See
.Fn MGET
for details.
.It Fn MCLGET mbuf how
Allocate and attach an
.Vt mbuf cluster
to
.Fa mbuf .
If the macro fails, the
.Dv M_EXT
flag will not be set in
.Fa mbuf .
.It Fn M_ALIGN mbuf len
Set the pointer
.Fa mbuf->m_data
to place an object of the size
.Fa len
at the end of the internal data area of
.Fa mbuf ,
long word aligned.
Applicable only if
.Fa mbuf
is newly allocated with
.Fn MGET
or
.Fn m_get .
.It Fn MH_ALIGN mbuf len
Serves the same purpose as
.Fn M_ALIGN
does, but only for
.Fa mbuf
newly allocated with
.Fn MGETHDR
or
.Fn m_gethdr ,
or initialized by
.Fn m_dup_pkthdr
or
.Fn m_move_pkthdr .
.It Fn m_align mbuf len
Services the same purpose as
.Fn M_ALIGN
but handles any type of mbuf.
.It Fn M_LEADINGSPACE mbuf
Returns the number of bytes available before the beginning
of data in
.Fa mbuf .
.It Fn M_TRAILINGSPACE mbuf
Returns the number of bytes available after the end of data in
.Fa mbuf .
.It Fn M_PREPEND mbuf len how
This macro operates on an
.Vt mbuf chain .
It is an optimized wrapper for
.Fn m_prepend
that can make use of possible empty space before data
(e.g.\& left after trimming of a link-layer header).
The new
.Vt mbuf chain
pointer or
.Dv NULL
is in
.Fa mbuf
after the call.
.It Fn M_MOVE_PKTHDR to from
Using this macro is equivalent to calling
.Fn m_move_pkthdr to from .
.It Fn M_WRITABLE mbuf
This macro will evaluate true if
.Fa mbuf
is not marked
.Dv M_RDONLY
and if either
.Fa mbuf
does not contain external storage or,
if it does,
then if the reference count of the storage is not greater than 1.
The
.Dv M_RDONLY
flag can be set in
.Fa mbuf->m_flags .
This can be achieved during setup of the external storage,
by passing the
.Dv M_RDONLY
bit as a
.Fa flags
argument to the
.Fn MEXTADD
macro, or can be directly set in individual
.Vt mbufs .
.It Fn MCHTYPE mbuf type
Change the type of
.Fa mbuf
to
.Fa type .
This is a relatively expensive operation and should be avoided.
.El
.Pp
The functions are:
.Bl -ohang -offset indent
.It Fn m_get how type
A function version of
.Fn MGET
for non-critical paths.
.It Fn m_getm orig len how type
Allocate
.Fa len
bytes worth of
.Vt mbufs
and
.Vt mbuf clusters
if necessary and append the resulting allocated
.Vt mbuf chain
to the
.Vt mbuf chain
.Fa orig ,
if it is
.No non- Ns Dv NULL .
If the allocation fails at any point,
free whatever was allocated and return
.Dv NULL .
If
.Fa orig
is
.No non- Ns Dv NULL ,
it will not be freed.
It is possible to use
.Fn m_getm
to either append
.Fa len
bytes to an existing
.Vt mbuf
or
.Vt mbuf chain
(for example, one which may be sitting in a pre-allocated ring)
or to simply perform an all-or-nothing
.Vt mbuf
and
.Vt mbuf cluster
allocation.
.It Fn m_gethdr how type
A function version of
.Fn MGETHDR
for non-critical paths.
.It Fn m_getcl how type flags
Fetch an
.Vt mbuf
with a
.Vt 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
.Vt mbuf
and
.Vt mbuf cluster
together, as it avoids having to unlock/relock between allocations.
Returns
.Dv NULL
on failure.
.It Fn m_getclr how type
Allocate an
.Vt mbuf
and zero out the data region.
.It Fn m_free mbuf
Frees
.Vt mbuf .
Returns
.Va m_next
of the freed
.Vt mbuf .
.El
.Pp
The functions below operate on
.Vt mbuf chains .
.Bl -ohang -offset indent
.It Fn m_freem mbuf
Free an entire
.Vt mbuf chain ,
including any external storage.
.\"
.It Fn m_adj mbuf len
Trim
.Fa len
bytes from the head of an
.Vt mbuf chain
if
.Fa len
is positive, from the tail otherwise.
.\"
.It Fn m_append mbuf len cp
Append
.Vt len
bytes of data
.Vt cp
to the
.Vt mbuf chain .
Extend the mbuf chain if the new data does not fit in
existing space.
.\"
.It Fn m_prepend mbuf len how
Allocate a new
.Vt mbuf
and prepend it to the
.Vt mbuf chain ,
handle
.Dv M_PKTHDR
properly.
.Sy Note :
It does not allocate any
.Vt mbuf clusters ,
so
.Fa len
must be less than
.Dv MLEN
or
.Dv MHLEN ,
depending on the
.Dv M_PKTHDR
flag setting.
.\"
.It Fn m_copyup mbuf len dstoff
Similar to
.Fn m_pullup
but copies
.Fa len
bytes of data into a new mbuf at
.Fa dstoff
bytes into the mbuf.
The
.Fa dstoff
argument aligns the data and leaves room for a link layer header.
Returns the new
.Vt mbuf chain
on success,
and frees the
.Vt mbuf chain
and returns
.Dv NULL
on failure.
.Sy Note :
The function does not allocate
.Vt mbuf clusters ,
so
.Fa len + dstoff
must be less than
.Dv MHLEN .
.\"
.It Fn m_pullup mbuf len
Arrange that the first
.Fa len
bytes of an
.Vt mbuf chain
are contiguous and lay in the data area of
.Fa mbuf ,
so they are accessible with
.Fn mtod mbuf type .
It is important to remember that this may involve
reallocating some mbufs and moving data so all pointers
referencing data within the old mbuf chain
must be recalculated or made invalid.
Return the new
.Vt mbuf chain
on success,
.Dv NULL
on failure
(the
.Vt mbuf chain
is freed in this case).
.Sy Note :
It does not allocate any
.Vt mbuf clusters ,
so
.Fa len
must be less than
.Dv MHLEN .
.\"
.It Fn m_pulldown mbuf offset len offsetp
Arrange that
.Fa len
bytes between
.Fa offset
and
.Fa offset + len
in the
.Vt mbuf chain
are contiguous and lay in the data area of
.Fa mbuf ,
so they are accessible with
.Fn mtod mbuf type .
.Fa len must be smaller than, or equal to, the size of an
.Vt mbuf cluster .
Return a pointer to an intermediate
.Vt mbuf
in the chain containing the requested region;
the offset in the data region of the
.Vt mbuf chain
to the data contained in the returned mbuf is stored in
.Fa *offsetp .
If
.Fa offp
is NULL, the region may be accessed using
.Fn mtod mbuf type .
If
.Fa offp
is non-NULL, the region may be accessed using
.Fn mtod mbuf uint8_t + *offsetp .
The region of the mbuf chain between its beginning and
.Fa off
is not modified, therefore it is safe to hold pointers to data within
this region before calling
.Fn m_pulldown .
.\"
.It Fn m_copym mbuf offset len how
Make a copy of an
.Vt mbuf chain
starting
.Fa offset
bytes from the beginning, continuing for
.Fa len
bytes.
If
.Fa len
is
.Dv M_COPYALL ,
copy to the end of the
.Vt mbuf chain .
.Sy Note :
The copy is read-only, because the
.Vt mbuf clusters
are not copied, only their reference counts are incremented.
.\"
.It Fn m_copypacket mbuf how
Copy an entire packet including header, which must be present.
This is an optimized version of the common case
.Fn m_copym mbuf 0 M_COPYALL how .
.Sy Note :
the copy is read-only, because the
.Vt mbuf clusters
are not copied, only their reference counts are incremented.
.\"
.It Fn m_dup mbuf how
Copy a packet header
.Vt mbuf chain
into a completely new
.Vt mbuf chain ,
including copying any
.Vt mbuf clusters .
Use this instead of
.Fn m_copypacket
when you need a writable copy of an
.Vt mbuf chain .
.\"
.It Fn m_copydata mbuf offset len buf
Copy data from an
.Vt mbuf chain
starting
.Fa off
bytes from the beginning, continuing for
.Fa len
bytes, into the indicated buffer
.Fa buf .
.\"
.It Fn m_copyback mbuf offset len buf
Copy
.Fa len
bytes from the buffer
.Fa buf
back into the indicated
.Vt mbuf chain ,
starting at
.Fa offset
bytes from the beginning of the
.Vt mbuf chain ,
extending the
.Vt mbuf chain
if necessary.
.Sy Note :
It does not allocate any
.Vt mbuf clusters ,
just adds
.Vt mbufs
to the
.Vt mbuf chain .
It is safe to set
.Fa offset
beyond the current
.Vt mbuf chain
end: zeroed
.Vt mbufs
will be allocated to fill the space.
.\"
.It Fn m_length mbuf last
Return the length of the
.Vt mbuf chain ,
and optionally a pointer to the last
.Vt mbuf .
.\"
.It Fn m_dup_pkthdr to from how
Upon the function's completion, the
.Vt mbuf
.Fa to
will contain an identical copy of
.Fa from->m_pkthdr
and the per-packet attributes found in the
.Vt mbuf chain
.Fa from .
The
.Vt mbuf
.Fa from
must have the flag
.Dv M_PKTHDR
initially set, and
.Fa to
must be empty on entry.
.\"
.It Fn m_move_pkthdr to from
Move
.Va m_pkthdr
and the per-packet attributes from the
.Vt mbuf chain
.Fa from
to the
.Vt mbuf
.Fa to .
The
.Vt mbuf
.Fa from
must have the flag
.Dv M_PKTHDR
initially set, and
.Fa to
must be empty on entry.
Upon the function's completion,
.Fa from
will have the flag
.Dv M_PKTHDR
and the per-packet attributes cleared.
.\"
.It Fn m_fixhdr mbuf
Set the packet-header length to the length of the
.Vt mbuf chain .
.\"
.It Fn m_devget buf len offset ifp copy
Copy data from a device local memory pointed to by
.Fa buf
to an
.Vt mbuf chain .
The copy is done using a specified copy routine
.Fa copy ,
or
.Fn bcopy
if
.Fa copy
is
.Dv NULL .
.\"
.It Fn m_cat m n
Concatenate
.Fa n
to
.Fa m .
Both
.Vt mbuf chains
must be of the same type.
.Fa N
is still valid after the function returned.
.Sy Note :
It does not handle
.Dv M_PKTHDR
and friends.
.\"
.It Fn m_split mbuf len how
Partition an
.Vt mbuf chain
in two pieces, returning the tail:
all but the first
.Fa len
bytes.
In case of failure, it returns
.Dv NULL
and attempts to restore the
.Vt mbuf chain
to its original state.
.\"
.It Fn m_apply mbuf off len f arg
Apply a function to an
.Vt mbuf chain ,
at offset
.Fa off ,
for length
.Fa len
bytes.
Typically used to avoid calls to
.Fn m_pullup
which would otherwise be unnecessary or undesirable.
.Fa arg
is a convenience argument which is passed to the callback function
.Fa f .
.Pp
Each time
.Fn f
is called, it will be passed
.Fa arg ,
a pointer to the
.Fa data
in the current mbuf, and the length
.Fa len
of the data in this mbuf to which the function should be applied.
.Pp
The function should return zero to indicate success;
otherwise, if an error is indicated, then
.Fn m_apply
will return the error and stop iterating through the
.Vt mbuf chain .
.\"
.It Fn m_getptr mbuf loc off
Return a pointer to the mbuf containing the data located at
.Fa loc
bytes from the beginning of the
.Vt mbuf chain .
The corresponding offset into the mbuf will be stored in
.Fa *off .
.It Fn 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 completed,
.Dv NULL
will be returned and the original chain will be unchanged.
Upon success, the original chain will be freed and the new
chain will be returned.
.Fa how
should be either
.Dv M_TRYWAIT
or
.Dv M_DONTWAIT ,
depending on the caller's preference.
.Pp
This function is especially useful in network drivers, where
certain long mbuf chains must be shortened before being added
to TX descriptor lists.
.It Fn 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,
.Dv NULL
will be returned.
The original mbuf chain is always reclaimed and the reference
count of any shared mbuf clusters is decremented.
.Fa how
should be either
.Dv M_TRYWAIT
or
.Dv M_DONTWAIT ,
depending on the caller's preference.
As a side-effect of this process the returned
mbuf chain may be compacted.
.Pp
This function is especially useful in the transmit path of
network code, when data must be encrypted or otherwise
altered prior to transmission.
.El
.Sh HARDWARE-ASSISTED CHECKSUM CALCULATION
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
.Va m_pkthdr
member of the leading
.Vt mbuf
of a packet contains two fields used for that purpose,
.Vt int Va csum_flags
and
.Vt int Va csum_data .
The meaning of those fields depends on the direction a packet flows in,
and on whether the packet is fragmented.
Henceforth,
.Va csum_flags
or
.Va csum_data
of a packet
will denote the corresponding field of the
.Va m_pkthdr
member of the leading
.Vt mbuf
in the
.Vt mbuf chain
containing the packet.
.Pp
On output, checksum offloading is attempted after the outgoing
interface has been determined for a packet.
The interface-specific field
.Va ifnet.if_data.ifi_hwassist
(see
.Xr ifnet 9 )
is consulted for the capabilities of the interface to assist in
computing checksums.
The
.Va 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 interface are done in the
software prior to passing the packet down to the interface driver;
such actions will never be requested through
.Va csum_flags .
.Pp
The flags demanding a particular action from an interface are as follows:
.Bl -tag -width ".Dv CSUM_TCP" -offset indent
.It Dv 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.
.It Dv CSUM_TCP
The TCP checksum is to be computed.
(See below.)
.It Dv CSUM_UDP
The UDP checksum is to be computed.
(See below.)
.El
.Pp
Should a TCP or UDP checksum be offloaded to the hardware,
the field
.Va 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 initially
set by the TCP/IP module to the checksum of the pseudo header
defined by the TCP and UDP specifications.
.Pp
For outbound packets which have been fragmented
by the host CPU, the following will also be true,
regardless of the checksum flag settings:
.Bl -bullet -offset indent
.It
all fragments will have the flag
.Dv M_FRAG
set in their
.Va m_flags
field;
.It
the first and the last fragments in the chain will have
.Dv M_FIRSTFRAG
or
.Dv M_LASTFRAG
set in their
.Va m_flags ,
correspondingly;
.It
the first fragment in the chain will have the total number
of fragments contained in its
.Va csum_data
field.
.El
.Pp
The last rule for fragmented packets takes precedence over the one
for a TCP or UDP checksum.
Nevertheless, offloading a TCP or UDP checksum is possible for a
fragmented packet if the flag
.Dv CSUM_IP_FRAGS
is set in the field
.Va ifnet.if_data.ifi_hwassist
associated with the network interface.
However, in this case the interface is expected to figure out
the location of the checksum field within the sequence of fragments
by itself because
.Va csum_data
contains a fragment count instead of a checksum offset value.
.Pp
On input, an interface indicates the actions it has performed
on a packet by setting one or more of the following flags in
.Va csum_flags
associated with the packet:
.Bl -tag -width ".Dv CSUM_IP_CHECKED" -offset indent
.It Dv CSUM_IP_CHECKED
The IP header checksum has been computed.
.It Dv CSUM_IP_VALID
The IP header has a valid checksum.
This flag can appear only in combination with
.Dv CSUM_IP_CHECKED .
.It Dv CSUM_DATA_VALID
The checksum of the data portion of the IP packet has been computed
and stored in the field
.Va csum_data
in network byte order.
.It Dv CSUM_PSEUDO_HDR
Can be set only along with
.Dv CSUM_DATA_VALID
to indicate that the IP data checksum found in
.Va 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
.Va csum_data
to obtain the final checksum to be used for TCP or UDP validation purposes.
.El
.Pp
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
.Dv CSUM_DATA_VALID
and
.Dv CSUM_PSEUDO_HDR
in
.Va csum_flags ,
and set
.Va csum_data
to
.Li 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
.Li 0xFFFF
as long as the original checksum field is included.
.Pp
For inbound packets which are IP fragments, all
.Va csum_data
fields will be summed during reassembly to obtain the final checksum
value passed to an upper layer in the
.Va csum_data
field of the reassembled packet.
The
.Va csum_flags
fields of all fragments will be consolidated using logical AND
to obtain the final value for
.Va csum_flags .
Thus, in order to successfully
offload checksum computation for fragmented data,
all fragments should have the same value of
.Va csum_flags .
.Sh STRESS TESTING
When running a kernel compiled with the option
.Dv MBUF_STRESS_TEST ,
the following
.Xr sysctl 8 Ns
-controlled options may be used to create
various failure/extreme cases for testing of network drivers
and other parts of the kernel that rely on
.Vt mbufs .
.Bl -tag -width ident
.It Va net.inet.ip.mbuf_frag_size
Causes
.Fn ip_output
to fragment outgoing
.Vt mbuf chains
into fragments of the specified size.
Setting this variable to 1 is an excellent way to
test the long
.Vt mbuf chain
handling ability of network drivers.
.It Va kern.ipc.m_defragrandomfailures
Causes the function
.Fn m_defrag
to randomly fail, returning
.Dv NULL .
Any piece of code which uses
.Fn m_defrag
should be tested with this feature.
.El
.Sh RETURN VALUES
See above.
.Sh SEE ALSO
.Xr ifnet 9 ,
.Xr mbuf_tags 9
.Sh HISTORY
.\" Please correct me if I'm wrong
.Vt Mbufs
appeared in an early version of
.Bx .
Besides being used for network packets, they were used
to store various dynamic structures, such as routing table
entries, interface addresses, protocol control blocks, etc.
In more recent
.Fx
use of
.Vt mbufs
is almost entirely limited to packet storage, with
.Xr uma 9
zones being used directly to store other network-related memory.
.Pp
Historically, the
.Vt mbuf
allocator has been a special-purpose memory allocator able to run in
interrupt contexts and allocating from a special kernel address space map.
As of
.Fx 5.3 ,
the
.Vt mbuf
allocator is a wrapper around
.Xr uma 9 ,
allowing caching of
.Vt mbufs ,
clusters, and
.Vt mbuf
+ cluster pairs in per-CPU caches, as well as bringing other benefits of
slab allocation.
.Sh AUTHORS
The original
.Nm
manual page was written by Yar Tikhiy.
The
.Xr uma 9
.Vt mbuf
allocator was written by Bosko Milekic.