Copyright 1996 Erik Lindsley

Tour overview (draft)

An SKB can be dissected into several parts, and a couple of these parts are used only with a specific protocol. Please load figure 1 in a separate window and follow along. The parts are as follows:

GENERAL

This part is concerned primarily with the maitainence of the doublely-linked list (next,prev), a reference counter (count), a pointer to the socket that the SKB is going to/from (sk), a pointer to the device it is going to/from, a pointer to the list header (list) so that the length of the queue (qlen) can be updated properly, and a destructor function to call once the PDU is no longer needed (destructor).

PDU HEADER

This is a union structure (a.k.a variant record) of pointers to several popular types of link-layer transport headers, such as Ethernet, TCP, UDP, File handles, Raw (unkown) headers, etc. It can only point to a single header at a time, however.

MEMORY

This part is concerned with the actual kernel-space memory reserved for the SKB's used. It has pointer to the beginning (head) and end (tail) of the reserved space, as well as its size (truesize).

BUFFER

This part indicates where the actual PDU data is located within the MEMORY space reserved for the SKB. It too has a beginning (data), an end (end), and length of valid data (len).

IP

This part originaly stored all of the fields of an IP datagram, as well as all of the state information for the transmission protocol. It is very messy. It does, however, have the field which indicates which higher-layer protocol is being used with this SKB.

ATM

This part adds some additional information that Werner's ATM driver will occationally use. It currently store such information as the size of the interface's PDU (size), some adapter-specific data (pos), a pointer to the ATM drivers VC data structure (VCC), a state information for I/O vectoring (iovcnt), and an additional timestamp field (timestamp) -- the IP part has one as well, but it is used for round trip time calculations.
In addition to the SKB structure, a structure is used to hold a list of SKBs. This structure is called sk_buff_head, and is made up of the links for the doublely-linked list (QUEUE), and a counter for the number of SKBs in the que (qlen). Note that while all of the (list) fields of the SKBs placed in the que will point to the same header, the (dev) and (sk) pointers commonly do not.

The standard kernel source provides many library functions to manipulate and use these structions.