The SKB structure buffer for data is manipulated using various support APIs. Some of the APIs are described below with a pictorial representation of how the operation takes place. 

• alloc_skb followed by skb_reserve and later followed by skb_put

• From the above picture, after alloc_skb function is invoked, the head, data and tail pointers are all pointing to the start of the data buffer.
• When skb_reserve API call is invoked to reserve headroom for various headers, the tailroom pointer moves down along with the data pointer
• when skb_put API call is invoked, the tailroom shrinks further giving space to the data region.
  • Now, the head pointer points to the start of the buffer(start of headroom)
  • The data pointer points to the start of the data region
  • The tail pointer points to the start of the tailroom region
  • The end pointer points to the absolute end of the allocated SKB buffer
  • The length of the data region is also saved in the length element of the structure

• skb_push API operation

• When some header is to be appended at the start of the Data packet, the skb_push API is utilised to achieve that result.
  • when skb_push API is used, the headroom shrinks and the data pointer is moved up to include the new header as part of the data region
  • The data packet length is updated to correspond to the new length of the data packet

• skb_put API operation

• When some information has to be appended at the bottom of the data packet (for e.g. CRC), the skb_put operation is used.
• The skb_put operation, provides space from the top of the tailroom
  • The tailroom shrinks and the tail pointer is moved down.
  • data packet length is updated to correspond to the new length of the data packet

• skb_trim API operation

• When some information has to be removed from the bottom of the data packet (for e.g. CRC), the skb_trim operation is used.
• The skb_trim operation
  • The tailroom grows and the tail pointer is moved up.
  • data packet length is updated to correspond to the new length of the data packet

A few other SKB APIs