http://ftp.gnumonks.org/pub/doc/skb-doc.html
skb - Linux network
buffers
1.3, 2000/10/14 21:27:02
Short
description about the linux network buffers (skb's)
At the time I wanted
to know more about the Linux network stack, I
always wanted a document like this to exist. But unfortunately I never
found one. After I gained some basic knowledge about the Linux network
stack internals, I wrote one.
I'm happy if this
document is of any use for other people trying to learn about the Linux
kernel.
Please let me know of
any bugs in this document. It should resemble kernel revision
2.4.0-test4
skbuffs are the
buffers in which the linux kernel handles network
packets. The packet is received by the network card, put into a skbuff
and then passed to the network stack, which uses the skbuff all the
time.
2.1 struct sk_buff
The struct sk_buff is
defined in <linux/skbuff.h> as follows:
- next
-
next buffer in
list
- prev
-
previous buffer
in list
- list
-
list we are on
- sk
-
socket we belong
to
- stamp
-
timeval we
arrived at
- dev
-
device we are
leaving by
- rx_dev
-
device we arrived
at
- h
-
transport layer
header (tcp,udp,icmp,igmp,spx,raw)
- nh
-
network layer
header (ip,ipv6,arp,ipx,raw)
- mac
-
link layer header
- dst
-
FIXME:
- cb
-
control buffer,
used internally
- len
-
length of actual
data
- csum
-
checksum
- used
-
FIXME: data moved
to user and not MSG_PEEK
- is_clone
-
we are a clone
- cloned
-
head may be
cloned
- pkt_type
-
packet class
- ip_summed
-
driver fed us ip
checksum
- priority
-
packet queuing
priority
- users
-
user count
- protocol
-
packet protocol
from driver
- security
-
security level of
packet
- truesize
-
real size of the
buffer
- head
-
pointer to head
of buffer
- data
-
data head pointer
- tail
-
tail pointer
- end
-
end pointer
- destructor
-
destructor
function
- nfmark
-
netfilter mark
- nfcache
-
netfilter
internal caching info
- nfct
-
associated
connection, if any
- tc_index
-
traffic control
index
2.2 skb support
functions
There are a bunch of
skb support functions provided by the sk_buff
layer. I briefly describe the most important ones in this section.
allocation / free /
copy / clone and expansion functions
- struct sk_buff
*alloc_skb(unsigned int size, int gfp_mask)
-
This function
allocates a new skb. This is provided by the skb layer to initialize
some privat data and do memory statistics. The returned buffer has no
headroom and a tailroom of /size/ bytes.
- void
kfree_skb(struct sk_buff *skb)
-
Decrement the
skb's usage count by one and free the skb if no references left.
- struct sk_buff
*skb_get(struct sk_buff *skb)
-
Increments the
skb's usage count by one and returns a pointer to it.
- struct sk_buff
*skb_clone(struct sk_buff *skb, int gfp_mask)
-
This
function clones a skb. Both copies share the packet data but have their
own struct sk_buff. The new copy is not owned by any socket, reference
count is 1.
- struct sk_buff
*skb_copy(const struct sk_buff *skb, int gfp_mask)
-
Makes
a real copy of the skb, including packet data. This is needed, if You
wish to modify the packet data. Reference count of the new skb is 1.
- struct
skb_copy_expand(const struct sk_buff *skb, int new_headroom, int
new_tailroom, int gfp_mask)
-
Make
a copy of the skb, including packet data. Additionally the new skb has
a haedroom of /new_headroom/ bytes size and a tailroom of
/new_tailroom/ bytes.
anciliary functions
- int
skb_cloned(struct sk_buff *skb)
-
Is the skb a
clone?
- int
skb_shared(struct sk_Buff *skb)
-
Is this skb
shared? (is the reference count > 1)?
operations on lists
of skb's
- struct sk_buff
*skb_peek(struct sk_buff_head *list_)
-
peek a skb from
front of the list; does not remove skb from the list
- struct sk_buff
*skb_peek_tail(struct sk_buff_head *list_)
-
peek a skb from
tail of the list; does not remove sk from the list
- __u32
skb_queue_len(sk_buff_head *list_)
-
return the length
of the given skb list
- void
skb_queue_head(struct sk_buff_head *list_, struct sk_buff *newsk)
-
enqueue a skb at
the head of a given list
- void
skb_queue_tail(struct sk_buff_head *list_, struct sk_buff *newsk)
-
enqueue a skb at
the end of a given list.
struct sk_buff
*skb_dequeue(struct sk_buff_head *list_)
dequeue a skb
from the head of the given list.
struct sk_buff
*sbk_dequeue_tail(struct sk_buff_head *list_)
dequeue a skb
from the tail of the given list
operations on skb
data
- unsigned char
*skb_put(struct sk_buff *sbk, int len)
-
extends
the data area of the skb. if the total size exceeds the size of the
skb, the kernel will panic. A pointer to the first byte of new data is
returned.
- unsigned char
*skb_push(struct sk_buff *skb, int len)
-
extends
the data area of the skb. if the total size exceeds the size of the
skb, the kernel will panic. A pointer to the first byte of new data is
returned.
- unsigned char
*skb_pull(struct sk_buff *skb, int len)
-
remove data from
the start of a buffer, returning the bytes to headroom. A pointr to the
next data in the buffer is returned.
- int
skb_headroom(struct sk_buff *skb)
-
return the amount
of bytes of free space at the head of skb
- int
skb_tailroom(struct sk_buff *skb)
-
return the amount
of bytes of free space at the end of skb
- struct sk_buff
*skb_cow(struct sk_buff *skb, int headroom)
-
if the buffer
passed lacks sufficient headroom or is a clone it is copied and
additional headroom made available.
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