[Click] extending simple_action() to multiple ports

[Ian Rose]

Howdy -

A few months ago there was an email to the list concerning (in part) the 
fact that simple_action always operates on input port 0 (when pulling) 
and output port 0 (when pushing):

http://pdos.csail.mit.edu/pipermail/click/2010-March/008810.html

Eddie pointed out that there were very few existing N-to-N elements 
where packets entering on port i exit only on port i, and since these 
are essentially the only kinds of elements that would benefit from the 
proposed change to simple action, the code would be left as-is.

At the time this logic seemed reasonable to me, but now I am beginning 
to think that maybe simple_action *should* be changed as proposed 
(namely, input and output ports should match).  AND a number of existing 
elements could be changed to N-to-N in order to make use of this.

For example:

all of the Counter elements can be used to count the sum of multiple 
independent packet flows

many of the Timestamp and TCP/IP Measurement elements can be used to 
monitor multiple independent packet flows

many modification elements (e.g. RandomBitErrors, EtherEncap, 
IPsecEncap, Paint, etc.) can be used to modify multiple packet flows - 
although you can of course do this with 1 element per flow, the 
advantage to using a single element is that it can simplify code, 
especially handlers (e.g. just 1 write handler call to change some 
property, rather than N write handlers - 1 per flow)


Its certainly possible to accomplish all of these currently, but its not 
always very clean.  For example, Paint and PaintSwitch can be used to 
mux and demux multiple streams, but that assumes that there is a free 
annotation byte somewhere that you can use on ALL input packets.  And 
this combo only works in a push context - in pull contexts you'd have to 
use N CheckPaint elements in a chain (one per packet stream - yuck).

The one downside that I can think of is that things will appear slightly 
more complex for new users.  Currently the fact that Counter (for 
example is a 1-to-1 make sense to a new user - if instead Counter is an 
N-to-N, the reason for that takes a bit longer to understand.

Thoughts?
- Ian