skb - Linux network buffers
Harald Welte laforge@gnumonks.org
1.3, 2000/10/14 =
21:27:02=20
Short description about the linux network buffers (skb's)=20
1. Introduction
At the time I wanted to know more about the Linux network stack, I = always=20 wanted a document like this to exist. But unfortunately I never found = one. After=20 I gained some basic knowledge about the Linux network stack internals, I = wrote=20 one.=20
I'm happy if this document is of any use for other people trying to = learn=20 about the Linux kernel.=20
Please let me know of any bugs in this document. It should resemble = kernel=20 revision 2.4.0-test4=20
2. skbuff's
skbuffs are the buffers in which the linux kernel handles network = packets.=20 The packet is received by the network card, put into a skbuff and then = passed to=20 the network stack, which uses the skbuff all the time.=20
2.1 struct sk_buff
The struct sk_buff is defined in <linux/skbuff.h> as follows:=20
- 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=20
- nfmark
-
netfilter mark
- nfcache
-
netfilter internal caching info
- nfct
-
associated connection, if any=20
- 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=20 briefly describe the most important ones in this section.=20
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=20 initialize some privat data and do memory statistics. The returned = buffer has=20 no headroom and a tailroom of /size/ bytes.=20
- void kfree_skb(struct sk_buff *skb)
-
Decrement the skb's usage count by one and free the skb if no = references=20 left.=20
- struct sk_buff *skb_get(struct sk_buff *skb)
-
Increments the skb's usage count by one and returns a pointer to = it.=20
- 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=20 their own struct sk_buff. The new copy is not owned by any socket, = reference=20 count is 1.=20
- 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=20 wish to modify the packet data. Reference count of the new skb is 1.=20
- struct skb_copy_expand(const struct sk_buff *skb, int = new_headroom, int=20 new_tailroom, int gfp_mask)
-
Make a copy of the skb, including packet data. Additionally the new = skb has=20 a haedroom of /new_headroom/ bytes size and a tailroom of = /new_tailroom/=20 bytes.=20
anciliary functions
- int skb_cloned(struct sk_buff *skb)
-
Is the skb a clone?=20
- int skb_shared(struct sk_Buff *skb)
-
Is this skb shared? (is the reference count > 1)?=20
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=20
- 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=20
- __u32 skb_queue_len(sk_buff_head *list_)
-
return the length of the given skb list=20
- void skb_queue_head(struct sk_buff_head *list_, struct sk_buff=20 *newsk)
-
enqueue a skb at the head of a given list=20
- void skb_queue_tail(struct sk_buff_head *list_, struct sk_buff=20 *newsk)
-
enqueue a skb at the end of a given list.=20
struct sk_buff *skb_dequeue(struct sk_buff_head *list_)=20
dequeue a skb from the head of the given list.=20
struct sk_buff *sbk_dequeue_tail(struct sk_buff_head *list_)=20
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=20 skb, the kernel will panic. A pointer to the first byte of new data is = returned.=20
- 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=20 skb, the kernel will panic. A pointer to the first byte of new data is = returned.=20
- unsigned char *skb_pull(struct sk_buff *skb, int len)
-
remove data from the start of a buffer, returning the bytes to = headroom. A=20 pointr to the next data in the buffer is returned.=20
- int skb_headroom(struct sk_buff *skb)
-
return the amount of bytes of free space at the head of skb=20
- int skb_tailroom(struct sk_buff *skb)
-
return the amount of bytes of free space at the end of skb=20
- 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=20 and additional headroom made available.=20
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