Linux Device Drivers, Second Edition

The Socket Buffers

We’ve now discussed most of the issues related to network interfaces. What’s still missing is some more detailed discussion of the sk_buff structure. The structure is at the core of the network subsystem of the Linux kernel, and we now introduce both the main fields of the structure and the functions used to act on it.

Although there is no strict need to understand the internals of sk_buff, the ability to look at its contents can be helpful when you are tracking down problems and when you are trying to optimize the code. For example, if you look in loopback.c, you’ll find an optimization based on knowledge of the sk_buff internals. The usual warning applies here: if you write code that takes advantage of knowledge of the sk_buff structure, you should be prepared to see it break with future kernel releases. Still, sometimes the performance advantages justify the additional maintenance cost.

We are not going to describe the whole structure here, just the fields that might be used from within a driver. If you want to see more, you can look at <linux/skbuff.h>, where the structure is defined and the functions are prototyped. Additional details about how the fields and functions are used can be easily retrieved by grepping in the kernel sources.

The Important Fields

The fields introduced here are the ones a driver might need to access. They are listed in no particular order.

struct net_device *rx_dev; , struct net_device *dev;

The devices receiving and sending ...