[linuxkernelnewbies] skb - Linux network buffers

[Peter Teoh]

http://ftp.gnumonks.org/pub/doc/skb-doc.html skb - Linux network buffers Harald Welte [EMAIL PROTECTED] 1.3, 2000/10/14 21:27:02 Short description about the linux network buffers (skb's) 1. Introduction 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 2. skbuff's 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.