sk_buff Structure

The structure has changed many times in the history of the kernel,both to add new options and to reorganize existing fields into a cleaner layout.Its fields can be classified roughly into the following categories:

• Layout
• General
• Feature-specific
• Management functions

Layout Fields

Interesting fields of sk_buff:

• struct sock *sk:This is a pointer to a sock data structure of the socket that owns this buffer.
• unsigned int len:This is the size of the block of data in the buffer.This length include both the data in the main buffer and the data in the fragments.
• unsigned int data_len:Unlike len,data_len accounts only for the size of the data in the fragments.
• unsigned int mac_len:This is the size of the MAC header
• atomic_t users:This is the reference count.
• unsigned int truesize:This field represent the total size of the buffer,including the sk_buff structure itself.
• unsigned char head,end,data,tail:head and end point to the beginning and end of the space allocated to the buffer,and data and tail point to the beginning and end of the actual data.

General Fields

• struct timeval stamp:This is usually meaningful only for a received packet.It is a timestamp that represents when a packet was received or when one is scheduled for transmission.
• struce net_device * dev:describes a network device.
• struct dst_entry dst:This is used by the routing subsystem.
• char cb[40]:This is a "control buffer",or storage for private information,maintained by each layer for internal use
• unsigned char clone:indicates that this structure is a clone of another sk_buff buffer.
• unsigned char pkt_type:This field classifies the type of frame based on its L2 destination address.
• unsigned short protocol:This is the protocol used at the next-higher layer from the perspective of the device driver at L2.

Feature-Specific Fields

The Linux kernel is modular.allowing you to select what to include and waht to leave out.Thus some fields are included in the sk_buff data structure only if the kernel is compiled with support for particular features such as firewalling or QoS

• struct nf_conntrack *nfct
• struct nf_bridge_info *nf_bridge;
• u8 nfctinfo

• u8 nf_trace

  These parameters are used by Netfilter

• u16 tc_index

• u16 tc_verd

  These parameters are used by the Traffic Control

• struct sec_path *sp

  This is used by the IPsec protocol suite to keep track of transformations

Management Functions

Before and After:(a)skb_put,(b)skb_push,(c)skb_pull,(d)skb_reserve

Allocating memory:alloc_skb and dev_alloc_skb

creating a single buffer involves two allocations of memory(one for the buffer and one for the sk_buff struct)

The skb_shared_info structure and the skb_shinfo function

skb_shared_info at the end of the data buffer that keeps additional information about the data block(Fragment).

Note that there is no field inside sk_buff structure pointing at the skb_shared_info data structure.To access that structure,functions need to use the skb_shinfo macro,which simply returns the end pointer.

List management functions

skb_queue_head_init: initializes an sk_buff_head with an empty queue of elements.

skb_queue_head,skb_queue_tail:Adds one buffer to the head or to the tail of a queue,respectively

skb_dequeue,skb_dequeue_tail:Dequeues an element from the head or from the tail.

skb_queue_purge:Empties a queue.

skb_queue_walk:Runs a loop on each element of a queue in turn.