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$FreeBSD: stable/10/share/man/man9/mbuf.9 289963 2015-10-25 22:55:19Z ngie $
.Dd March 11, 2015 .Dt MBUF 9 .Os
.Sh NAME .Nm mbuf .Nd "memory management in the kernel IPC subsystem"
.Sh SYNOPSIS n sys/param.h n sys/systm.h n 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
.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_collapse "struct mbuf *m0" "int how" "int maxfrags" .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.
p 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 .
p 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 n sys/param.h . The .Vt mbuf header includes: l -tag -width "m_nextpkt" -offset indent t Va m_next
q Vt struct mbuf * A pointer to the next .Vt mbuf in the .Vt mbuf chain . t Va m_nextpkt
q Vt struct mbuf * A pointer to the next .Vt mbuf chain in the queue. t Va m_data
q Vt caddr_t A pointer to data attached to this .Vt mbuf . t Va m_len
q Vt int The length of the data. t Va m_type
q Vt short The type of the data. t Va m_flags
q Vt int The .Vt mbuf flags. .El
p The .Vt mbuf flag bits are defined as follows: d -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
p The available .Vt mbuf types are defined as follows: d -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
p The available external buffer types are defined as follows: d -literal /* external buffer types */ #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 (M_IOVEC) */ #define EXT_NET_DRV 100 /* custom ext_buf provided by net driver(s) */ #define EXT_MOD_TYPE 200 /* custom module's ext_buf type */ #define EXT_DISPOSABLE 300 /* can throw this buffer away w/page flipping */ #define EXT_EXTREF 400 /* has externally maintained ref_cnt ptr */ .Ed
p 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
q Vt struct ifnet Va *rcvif , and the total packet length
q Vt int Va len . Optionally, it may also contain an attached list of packet tags
q 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.
p 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.
p 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.
p The system supplies a macro for allocating the desired external storage buffer, .Dv MEXTADD .
p The allocation and management of the reference counter is handled by the subsystem.
p
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
.Dv MHLEN
plus one.
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.
l -ohang -offset indent t 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.
t 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_WAITOK
or
.Dv M_NOWAIT .
It specifies whether the caller is willing to block if necessary.
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 .
p Historical .Vt mbuf allocator (See .Sx HISTORY section) used allocation flags .Dv M_WAIT and .Dv M_DONTWAIT . These constants are kept for compatibility and their use in new code is discouraged. t 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. t Fn MEXTADD mbuf buf size free opt_arg1 opt_arg2 flags type Associate externally managed data with .Fa mbuf . Any internal data contained in the mbuf will be discarded, and the .Dv M_EXT flag will be set. The .Fa buf and .Fa size arguments are the address and length, respectively, of the data. The .Fa free argument points to a function which will be called to free the data when the mbuf is freed; it is only used if .Fa type is .Dv EXT_EXTREF . The .Fa opt_arg1 and .Fa opt_arg2 arguments will be passed unmodified to .Fa free . The .Fa flags argument specifies additional .Vt mbuf flags; it is not necessary to specify .Dv M_EXT . Finally, the .Fa type argument specifies the type of external data, which controls how it will be disposed of when the .Vt mbuf is freed. In most cases, the correct value is .Dv EXT_EXTREF . t 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 . t 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 . t 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 . t Fn m_align mbuf len Services the same purpose as .Fn M_ALIGN but handles any type of mbuf. t Fn M_LEADINGSPACE mbuf Returns the number of bytes available before the beginning of data in .Fa mbuf . t Fn M_TRAILINGSPACE mbuf Returns the number of bytes available after the end of data in .Fa mbuf . t 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. t Fn M_MOVE_PKTHDR to from Using this macro is equivalent to calling .Fn m_move_pkthdr to from . t 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 . t Fn MCHTYPE mbuf type Change the type of .Fa mbuf to .Fa type . This is a relatively expensive operation and should be avoided. .El
p The functions are: l -ohang -offset indent t Fn m_get how type A function version of .Fn MGET for non-critical paths. t 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. t Fn m_gethdr how type A function version of .Fn MGETHDR for non-critical paths. t 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. t Fn m_getclr how type Allocate an .Vt mbuf and zero out the data region. t Fn m_free mbuf Frees .Vt mbuf . Returns .Va m_next of the freed .Vt mbuf . .El
p
The functions below operate on
.Vt mbuf chains .
l -ohang -offset indent t Fn m_freem mbuf Free an entire
.Vt mbuf chain ,
including any external storage.
t 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.
t 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.
t 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.
t 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 .
t 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 or equal to
.Dv MHLEN .
t 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 .
t 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.
t 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.
t 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 .
t 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 .
t 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.
t Fn m_length mbuf last Return the length of the
.Vt mbuf chain ,
and optionally a pointer to the last
.Vt mbuf .
t 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.
t 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.
t Fn m_fixhdr mbuf Set the packet-header length to the length of the
.Vt mbuf chain .
t 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 .
t 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.
t 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.
t 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 .
p 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.
p
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 .
t 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 .
t 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_WAITOK
or
.Dv M_NOWAIT ,
depending on the caller's preference.
p This function is especially useful in network drivers, where certain long mbuf chains must be shortened before being added to TX descriptor lists. t Fn m_collapse m0 how maxfrags Defragment an mbuf chain, returning a chain of at most .Fa maxfrags mbufs and clusters. If allocation fails or the chain cannot be collapsed as requested, .Dv NULL will be returned, with the original chain possibly modified. As with .Fn m_defrag , .Fa how should be one of .Dv M_WAITOK or .Dv M_NOWAIT . t 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_WAITOK or .Dv M_NOWAIT , depending on the caller's preference. As a side-effect of this process the returned mbuf chain may be compacted.
p 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.
p 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 .
p The flags demanding a particular action from an interface are as follows: l -tag -width ".Dv CSUM_TCP" -offset indent t 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. t Dv CSUM_TCP The TCP checksum is to be computed. (See below.) t Dv CSUM_UDP The UDP checksum is to be computed. (See below.) .El
p 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.
p 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: l -tag -width ".Dv CSUM_IP_CHECKED" -offset indent t Dv CSUM_IP_CHECKED The IP header checksum has been computed. t Dv CSUM_IP_VALID The IP header has a valid checksum. This flag can appear only in combination with .Dv CSUM_IP_CHECKED . t 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. t 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
p
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.
.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 .
l -tag -width ident t 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.
t 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
x . 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.
p 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.