MBUF(9) FreeBSD Kernel Developer's Manual MBUF(9)
NAME
mbuf - memory management in the kernel IPC subsystem
SYNOPSIS
#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);
int
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
type
mtod(struct mbuf *mbuf, type);
void *
mtodo(struct mbuf *mbuf, offset);
M_ALIGN(struct mbuf *mbuf, u_int len);
MH_ALIGN(struct mbuf *mbuf, u_int len);
int
M_LEADINGSPACE(struct mbuf *mbuf);
int
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);
int
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_get3(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);
void
m_freem(struct mbuf *mbuf);
Mbuf utility functions
void
m_adj(struct mbuf *mbuf, int len);
void
m_align(struct mbuf *mbuf, int len);
int
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);
void
m_copydata(const struct mbuf *mbuf, int offset, int len, caddr_t buf);
void
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));
void
m_cat(struct mbuf *m, struct mbuf *n);
void
m_catpkt(struct mbuf *m, struct mbuf *n);
u_int
m_fixhdr(struct mbuf *mbuf);
int
m_dup_pkthdr(struct mbuf *to, const struct mbuf *from, int how);
void
m_move_pkthdr(struct mbuf *to, struct mbuf *from);
u_int
m_length(struct mbuf *mbuf, struct mbuf **last);
struct mbuf *
m_split(struct mbuf *mbuf, int len, int how);
int
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);
DESCRIPTION
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
chain.
m_nextpkt (struct mbuf *) A pointer to the next mbuf chain in the
queue.
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_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_EXTPG 0x00000100 /* has array of unmapped pages and TLS */
#define M_NOFREE 0x00000200 /* do not free mbuf, embedded in cluster */
#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
HARDWARE-ASSISTED CHECKSUM CALCULATION for details.
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
otherwise.
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
subsystem.
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
developer 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.
mtodo(mbuf, offset)
Return a data pointer at an offset (in bytes) into the data
attached to mbuf. Returns a void * pointer . Note: The caller
must ensure that the offset is in bounds of the attached data.
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
mbufs.
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,
respectively, 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
arguments 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 argument 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
implementation 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.
Applicable 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
mbuf.
M_LEADINGSPACE(mbuf)
Returns the number of bytes available before the beginning of data
in mbuf.
M_TRAILINGSPACE(mbuf)
Returns the number of bytes available after the end of data in
mbuf.
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).
M_WRITABLE(mbuf)
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 larger than MJUMPAGESIZE, NULL will be
returned.
m_get3(size, how, type, flags)
Allocate an mbuf with enough space to hold specified amount of
data. If the size is larger than MJUM16BYTES, NULL will be
returned.
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
append 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
together, 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
MJUM16BYTES.
m_free(mbuf)
Frees mbuf. Returns m_next of the freed mbuf.
The functions below operate on mbuf chains.
m_freem(mbuf)
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
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 mbuf chain on success, NULL on
failure (the mbuf chain is freed in this case). Note: It does not
allocate any mbuf clusters, so len must be less than or equal to
MHLEN.
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() or mtodo(). len must be smaller than, or
equal to, the size of an mbuf cluster. Return a pointer to an
intermediate 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) or mtodo(mbuf,
0). If offsetp is non-NULL, the region may be accessed using
mtodo(mbuf, *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
beginning, 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
incremented.
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
incremented.
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
identical 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
attributes cleared.
m_fixhdr(mbuf)
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
attempts 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
argument 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
offset 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
completed, 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. how should be either M_WAITOK or
M_NOWAIT, depending on the caller's preference.
This function is especially useful in network drivers, where
certain 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_WAITOK or M_NOWAIT.
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
compacted.
This function is especially useful in the transmit path of network
code, when data must be encrypted or otherwise altered prior to
transmission.
HARDWARE-ASSISTED CHECKSUM CALCULATION
This section currently applies to SCTP, TCP, and UDP over 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
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.
When a packet is sent by SCTP, TCP, or UDP, the computation of the
checksum is delayed until the outgoing interface has been determined for
a packet. The interface-specific field ifnet.if_data.ifi_hwassist (see
ifnet(9)) is consulted by IP for the capabilities of the network
interface selected for output 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 interface are done in the software prior to passing the
packet down to the interface driver; such actions will never be requested
through csum_flags.
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_SCTP The SCTP checksum is to be computed. (See below.)
CSUM_TCP The TCP checksum is to be computed. (See below.)
CSUM_UDP The UDP checksum is to be computed. (See below.)
Should a SCTP, 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 the case of TCP or UDP, the
checksum field will be initially set by the TCP or UDP implementation to
the checksum of the pseudo header defined by the TCP and UDP
specifications. In the case of SCTP, the checksum field will be
initially set by the SCTP implementation to 0.
When a packet is received by an interface, it 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
indicate 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
validation purposes.
If a particular network interface just indicates success or failure of
SCTP, TCP, or UDP checksum validation without returning the exact value
of the checksum to the host CPU, its driver can mark CSUM_DATA_VALID in
csum_flags as well as, for TCP and UDP, CSUM_PSEUDO_HDR 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. Note that for SCTP the value of csum_data is not relevant and
CSUM_PSEUDO_HDR in csum_flags is not set, since SCTP does not use a
pseudo header checksum.
If IP delivers a packet with the flags CSUM_IP, CSUM_SCTP, CSUM_TCP, or
CSUM_UDP set in csum_flags to a local IP, SCTP, TCP, or UDP stack, the
packet will be processed without computing or validating the checksum,
since the packet has not been on the wire. This can happen if the packet
was handled by a virtual interface such as tap(4) or epair(4).
STRESS TESTING
When running a kernel compiled with the option MBUF_STRESS_TEST, the
following sysctl(8)-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 mbufs.
net.inet.ip.mbuf_frag_size
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 drivers.
kern.ipc.m_defragrandomfailures
Causes the function m_defrag() to randomly fail, returning NULL.
Any piece of code which uses m_defrag() should be tested with
this feature.
RETURN VALUES
See above.
SEE ALSO
ifnet(9), mbuf_tags(9)
S. J. Leffler, W. N. Joy, R. S. Fabry, and M. J. Karels, "Networking
Implementation Notes", 4.4BSD System Manager's Manual (SMM).
HISTORY
Mbufs appeared in an early version of BSD. 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 FreeBSD use of mbufs is almost entirely limited to
packet storage, with uma(9) zones being used directly to store other
network-related memory.
Historically, the 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 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.
AUTHORS
The original mbuf manual page was written by Yar Tikhiy. The uma(9) mbuf
allocator was written by
Bosko Milekic.
FreeBSD 15.1-STABLE-HBSD January 20, 2026 MBUF(9)