Re: [Bloat] emulating non-duplex media in linux qdiscs

[Luca Muscariello]

Dave,

If the objective is to run experiments with some emulated wifi I could
suggest
a workaround that we are using. It is not based on qdisc only.

It is based on ns3 emulation and linux bridges.

https://git.fd.io/cicn/tree/emu-radio/README.md?h=vicn/master

The radio emulation piece includes 802.11n and LTE.
For wifi you get the contention behaviour you're looking for and listen
before talk etc.
You also get the TX or RX behaviour. There is also the propagation models
from ns3 (fast/slow fading, etc. etc.).

The radio piece is a single Linux process so depending on the scaling
you're looking for this tool
might or might not be useful.

In the repo, there is some documentation, but if you plan to use this stuff
and need help let me know.
There is also a ML, in case it helps.

Luca




On Mon, Oct 9, 2017 at 3:54 AM, Dave Taht  wrote:

> I have been hacking away at netem for a while now in the hope that
> eventually - with a great deal more hacking - it could be used to more
> accurately emulate shared media like wifi and lte.
>
> (Some people try to describe these as simplex (which is not true
> because you can have multiple destinations), and they certainly are
> not duplex, so I tend to say non-duplex and still hope some better
> word emerges)
>
> So... one sticking point for me has been wanting to emulate the fact
> that on shared media, that you cannot transmit and receive at the same
> time; that these are "coupled" events, and what I'd like to be able to
> express might be something like:
>
> tc qdisc add dev eth0 root netem rate 100mbit coupled some_identifier
> ... some tc mirred magic for ifb here ...
> tc qdisc add dev ifb0 root netem rate 10mbit coupled the_same_identifier
>
> "some_identifier" would be a mutex of some sort, and I confess to
> not having much grip on the kernel outside of the net/sched directory.
>
> What facility would be best to try and leverage? It would be created
> (globally) on first use, ref-counted (thus destroyed when it goes to
> zero), atomically updated... posix shared memory seems too heavyweight
> to use
>
> --
>
> Dave Täht
> CEO, TekLibre, LLC
> http://www.teklibre.com
> Tel: 1-669-226-2619
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Re: [Bloat] emulating non-duplex media in linux qdiscs

[erik.taraldsen]

> wifi is not p2p, all data is broadcast to many potential recievers,
> only one can transmit at one time.

On the radio level that is true for the time being.  That may change with 
802.11ax, as MU-MIMO is set up to get an multiple sender version.  OFDMA I 
believe.


> Saying that is half duplex, doesn't work for me. In their example of
> "half duplex", (using push to talk), it still means that everybody on
> that channel hears who is talking. "half duplex" to me, given the
> definition of duplex, means more that there is a *p2p* channel (a wire),
> that you can ping pong data across.

Well, as Mikael stated as well, half duplex has been used on shared medium in 
the ethernet world since it's inception.  As you state it does not match 100% 
with the wikipedia definition, but the tradition is there.
 

> This conflation of ideas has always bugged me and I've longed to find
> another word that more accurately describes what happens, therefore
> I've been saying "non-duplex".

Given your distaste for the term half duplex, and the coming advances in .11ax 
which further confuses the issue, there may be a real need for a better term.  
How about starting with a description of the transmission properties and making 
a set of terms?  Shared medium, single sender, multiple recivers. (current 
mu-mimo wifi).  Shared medium multiple sender, multiple receivers. (OFDMA 
802.11ax if they get it working).  Shared medium entails coordinated action.  
Sender/receiver description also encapsulates that there is different behavior 
in the different direction in the air. 


> > Simplex is one way communication like traditional AM radio.
>
> wifi/lte are not very similar similar to AM radio.

Which was my point. :)   Simplex seems to be a non applicable term here.



-Erik
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Re: [Bloat] emulating non-duplex media in linux qdiscs

[Mikael Abrahamsson]

On Mon, 9 Oct 2017, Dave Taht wrote:

Saying that is half duplex, doesn't work for me. In their example of 
"half duplex", (using push to talk), it still means that everybody on 
that channel hears who is talking. "half duplex" to me, given the 
definition of duplex, means more that there is a *p2p* channel (a wire), 
that you can ping pong data across.


A 10base-T hub connected to a 10base-2 or 10base-5 segment, all in the 
same broadcast domain, is considered to be "half duplex" in ethernet port 
configuration term.


So it doesn't have to be p2p. And I do think this mimics a shared radio as 
well (because a coax wire with multiple nodes on it seems very similar to 
a radio channel over the air).


Now, radio has the difference that two stations might not hear each other, 
and that's of course a problem in CSMA/CD terms.


Back to your netem problem. What you need is to force all packets through 
the same queue, right? So I tried to dream up a complicated scheme with 4 
bridges and some kind of "forced forwarding", but I don't think it'd pan 
out.


So the best way is probably to have a shaper that feeds 
transmit-tokens/does scheduling into two different shapers (rx/tx on the 
same interface). So whatever scheduling they are fed in order to tell them 
the rate they're allowed to transmit, they get it from the same source. 
That way they have to compete for the same resources.


This will not perfectly mimic the exponential backoff of CSMA/CD, but it 
might be good enough for what you need? Also, I just realised I have no 
idea how wifi is scheduled. Is it even close to CSMA/CD?


--
Mikael Abrahamssonemail: swm...@swm.pp.se
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