Re: Netlink connector

From: Stephen Hemminger
Date: Mon Jul 25 2005 - 23:56:46 EST

Evgeniy Polyakov wrote:

On Tue, Jul 26, 2005 at 01:46:04AM +0200, Patrick McHardy (kaber@xxxxxxxxx) wrote:

Evgeniy Polyakov wrote:

On Mon, Jul 25, 2005 at 04:32:32PM +0200, Patrick McHardy (kaber@xxxxxxxxx) wrote:

If I understand correctly it tries to workaround some netlink
limitations (limited number of netlink families and multicast groups)
by sending everything to userspace and demultiplexing it there.
Same in the other direction, an additional layer on top of netlink
does basically the same thing netlink already does. This looks like
a step in the wrong direction to me, netlink should instead be fixed
to support what is needed.

Not only it.
The main _first_ idea was to simplify userspace mesasge handling as much
as possible.
In first releases I called it ioctl-ng - any module that want ot
communicate with userspace in the way ioctl does,

Usually netlink is easily extendable by using nested TLVs. By hiding
this you basically remove this extensibility.

Current netlink is not extensible for _many_ different users.
It has only 32 sockets.

requires skb allocation/freeing/handling.
Does RTC driver writer need to know what is the difference between
shared and cloned skb? Should kernel user of such message bus
have to know about skb at all?

Netlink users don't have to care about shared or cloned skbs. I don't
think its a big issue to use alloc_skb and then the usual netlink
macros. Thomas added a number of macros that simplfiy use a lot.

Kernel user also must know about difference between unicast/broadcast,
how to dequeue the skb, how to free it and in what context.
ioctl users do not need to know how file_operations is bound to file.

But my main objection is that it sends everything to userspace even
if noone is listening. This can't be used for things that generate
lots of events, and also will get problematic is the number of users

It is a problem for existing netlink - either check in bind time, what could be done for connector, or in socket creation time.

Actually it is not even a problem, since checking is being done, but after allocation and message filling, such check can be moved into
cn_netlink_send() in connector, but different netlink users, who prefers to use different sockets, must perform it by itself in each
place, where skb is allocated...

Connector is a solution for current situation, it can be deployed with few casualties.
Creating a new netlink2 socket for device, which wants to replace ioctl
controlling or broadcast it's state is a wrong way.
Different sockets/flows does not allow easy flow control.

We have one pipe - ethernet, and many protocols inside this pipe
with different headers - it is the same here - netlink is such a pipe,
and with connector it allows to have different protocols in it.

With char device I only need to register my callback - with kernel
connector it is the same, but allows to use the whole power of netlink,
especially without nice ioctl features like different pointer size in userspace and kernelspace.

You still have to take care of mixed 64/32 bit environments, u64 fields
for example are differently alligned.

Connector has a size in it's header - ioctl does not.

And number of free netlink sockets is _very_ small, especially
if allocate new one for simple notifications, which can be easily done
using connector.

Then fix it so we can use more families and groups. I started some work
on this, but I'm not sure if I have time to complete it.

It does not "fix" the "problem" of skb management knowledge, which I
Netlink is a transport protocol, some general logic must be created on
top of it, like it is done in TCP/IP.

And netlink can be extended to support it - netlink is a transport
protocol, it should not care about higher layer message handling,
connector instead will deliver message to the end user in a very
convenient form.

You can still built this stuff on top, but the workarounds for netlink
limitations need to be fixed in netlink.

I could not call it workaround, I think it is a management layer,
which allows :
1. easy usage. Just register a callback and that is all. Callback will
be invoced each time new message arrives. No need to
2. easy usage. Call one function for message delivering, which can
care of nonexistent users, perform flow control, congestion control,
guarantee delivery and any other.
3. Easily deployable - current implementation is so simple, and it does
work with existing netlink.
4. It is logical level on top of transport protocol, it is UDP/IP over
ethernet :)

If it is a transport, then it should be in the kernel. Otherwise, it becomes painful
for applications with multiple input sources. Think of epoll/poll/select and threads,
doing the demultiplexing in user space would be a pain for applications and libraries.

The other way to go is to use something like dbus/hal and use a higher level
application oriented interface. The problem with that approach, is it assumes
every management app wants to drag in gnome..

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