Subject: [Roll] [roll] #32: Asymetric Link

From: "roll issue tracker" <[email-masked]>
Date: Thu, 01 Apr 2010 07:53:44 -0000

#32: Asymetric Link
--------------------------------+-------------------------------------------
 Reporter:  jpv@…               |       Owner:  richard.kelsey@…        
     Type:  defect              |      Status:  new                     
 Priority:  major               |   Milestone:                          
Component:  rpl                 |     Version:                          
 Severity:  Active WG Document  |    Keywords:                          
--------------------------------+-------------------------------------------
 In a network with asymmetric links, DODAGs can be formed to
 optimize upward, downward, or bidirectional routes.  There
 needs to be some way to indicate how asymmetric link metrics
 are to be used when forming a particular DODAG.  A DODAG
 meant for routing upwards would want to use the upward
 metric values, for example.  A DODAG meant for use in both
 directions might want the average of the upward and downward
 values, or the max or min, depending on the metric.

-- 
Ticket URL: <http://trac.tools.ietf.org/wg/roll/trac/ticket/32>
roll <http://tools.ietf.org/wg/roll/>

---

From: Yoav Ben-Yehezkel <[email-masked]>
Date: Thu, 1 Apr 2010 14:02:21 +0300

Hi,

Maybe occasional unicast DIS to a parent(s) from the parent set can be used
to solve this?
This coincides with the issue of DIS options.

Thanks,
Yoav

-----Original Message-----
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issue tracker
Sent: Thursday, April 01, 2010 10:54 AM
To: [email-masked]; [email-masked]
Cc: [email-masked]
Subject: [Roll] [roll] #32: Asymetric Link

#32: Asymetric Link

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From: Thomas Heide Clausen <[email-masked]>
Date: Thu, 1 Apr 2010 13:15:04 +0200

Just a related thought here....

If the L2 employs any kind of bi-directional signaling RPL may need to  
specifically exclude uni-directional links. This is not just  
hypothetic: if, for example, a L2 employs ACKs for unicast, but not  
for broadcast, the use of an uni-directional link should be excluded.  
This may be particularly relevant for NUD (which is suggested used for  
detecting when a preferred parent is unreachable), where absence of an  
L2 ack may be interpreted as such.

This, of course, leads to the question of below which  
"metric" (quality, ...) a link should be considered "uni-directional"  
rather than just "asymmetric"? To take an abstract example, assuming a  
L2 employing ack's for each unicast transmission: if the probability  
of getting a transmission from A to B is 100% and from B to A is 10%,  
is the link A->B "asymmetric" or "uni-directional"? Can the metric for  
A->B be considered without also considering the metric in the opposite  
direction B->A?

It's a rather common occurrence in the wireless networks I've seen --  
and presumably something we want RPL to handle correctly (other than  
by having NUD flag an error 9 times out of 10, and then hunt around  
for a new preferred parent, etc....)

I guess my point is: the metric A->B can not be universally considered  
independent of the metric B->A, especially considering the reliance on  
NUD.

Thomas

On Apr 1, 2010, at 13:02 PM, Yoav Ben-Yehezkel wrote:

> -------------------------------- 
> +-------------------------------------------
> -------------------------------- 
> +-------------------------------------------

---

From: Yoav Ben-Yehezkel <[email-masked]>
Date: Thu, 1 Apr 2010 15:46:18 +0300

Thomas,

I agree with you completely. In PLC networks, there's no way to ensure
that the metric is the same or even close in both directions. Noise
variations between sockets are *completely* different (consider a switch
with a modem trying to transmit to a modem near a working washing machine,
dryer and TV). The variation can reach more than 20dB.

This is why I was suggesting to use a DIS/DIO exchange between child and
parent to get the twofold advantage: i.e. exchange L3 metric between these
nodes, and also ensuring a bi-directional link exists (since the child
receives DIOs, the bad link well could be from child->parent, in which
case the DIS from the child will not be received by the parent).

RPL suggestion to use L2 ACK cannot provide any L3 metric exchange from
child to parent (up), which could be an important factor since the traffic
is going that way and not down from parent to child, though this is the
metric on which the OF relies in the child.

Thanks,
Yoav

-----Original Message-----
From: Thomas Heide Clausen [mailto:[email-masked]]
Sent: Thursday, April 01, 2010 2:15 PM
To: Yoav Ben-Yehezkel
Cc: [email-masked]; [email-masked]; [email-masked]
Subject: Re: [Roll] [roll] #32: Asymetric Link

Thomas

On Apr 1, 2010, at 13:02 PM, Yoav Ben-Yehezkel wrote:

> Reporter:  jpv@.               |       Owner:  richard.kelsey@.

---

From: Michael Richardson <[email-masked]>
Date: Thu, 01 Apr 2010 09:44:21 -0400

>>>>> "Thomas" == Thomas Heide Clausen <[email-masked]> writes:
    Thomas> Just a related thought here....

    Thomas> If the L2 employs any kind of bi-directional signaling RPL
    Thomas> may need to specifically exclude uni-directional links. This

This ticket/topic, is not about *uni*-directional links (no bandwidth in
one direction), it's about asymmetric links (some bandwidth only).

- -- 
]       He who is tired of Weird Al is tired of life!           |  firewalls  [
]   Michael Richardson, Sandelman Software Works, Ottawa, ON    |net architect[
] [email-masked] http://www.sandelman.ottawa.on.ca/ |device driver[
   Kyoto Plus: watch the video <http://www.youtube.com/watch?v=kzx1ycLXQSE>
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---

From: Jonathan Hui <[email-masked]>
Date: Thu, 1 Apr 2010 08:34:44 -0700

The specific metric that led to the opening of this ticket was latency  
- I'm not sure where that falls in your definitions of asymmetric vs.  
uni-directional.  It would seem the latter if infinite latency is  
considered, but it's one example where the two directions may be  
viewed as independent.

Addressing your specific thought:  I think it's reasonable to say that  
any link metrics used should also be responsible for also considering  
specific properties of the link.  Extending that notion, if, at the  
link layer, communication from A -> B requires some from of  
communication from B -> A, then metrics used should consider the full  
process of communicating from A -> B -> A if that's important to the  
metric (e.g. reliability, margin, etc.).  I'm not sure I prefer to  
rely on L3 to communicate such metrics back and forth simply to be  
combined into some aggregate link-layer metric.

--
Jonathan Hui

On Apr 1, 2010, at 4:15 AM, Thomas Heide Clausen wrote:

> If the L2 employs any kind of bi-directional signaling RPL may need  
> to specifically exclude uni-directional links. This is not just  
> for broadcast, the use of an uni-directional link should be  
> excluded. This may be particularly relevant for NUD (which is  
> suggested used for detecting when a preferred parent is  
> unreachable), where absence of an L2 ack may be interpreted as such.
>
> "metric" (quality, ...) a link should be considered "uni- 
> directional" rather than just "asymmetric"? To take an abstract  
> example, assuming a L2 employing ack's for each unicast  
> transmission: if the probability of getting a transmission from A to  
> B is 100% and from B to A is 10%, is the link A->B "asymmetric" or  
> "uni-directional"? Can the metric for A->B be considered without  
> also considering the metric in the opposite direction B->A?
>
> It's a rather common occurrence in the wireless networks I've seen  
> -- and presumably something we want RPL to handle correctly (other  
> than by having NUD flag an error 9 times out of 10, and then hunt  
> around for a new preferred parent, etc....)
>
> I guess my point is: the metric A->B can not be universally  
> considered independent of the metric B->A, especially considering  
> the reliance on NUD.
>
> Thomas
>

---

From: Thomas Heide Clausen <[email-masked]>
Date: Thu, 1 Apr 2010 17:40:40 +0200

On Apr 1, 2010, at 17:34 PM, Jonathan Hui wrote:

>
> The specific metric that led to the opening of this ticket was  
> latency - I'm not sure where that falls in your definitions of  
> asymmetric vs. uni-directional.  It would seem the latter if  
> infinite latency is considered, but it's one example where the two  
> directions may be viewed as independent.

Presumably, this would depend on the value of infinity ..... or  
rather, the time after which a L2 "gives up waiting for an  
ack" (again, assuming that an L2 uses such) and NUD takes it to mean  
that "somethin's up"....

> Addressing your specific thought:  I think it's reasonable to say  
> that any link metrics used should also be responsible for also  
> considering specific properties of the link.  Extending that notion,  
> if, at the link layer, communication from A -> B requires some from  
> of communication from B -> A, then metrics used should consider the  
> full process of communicating from A -> B -> A if that's important  
> to the metric (e.g. reliability, margin, etc.).  I'm not sure I  
> prefer to rely on L3 to communicate such metrics back and forth  
> simply to be combined into some aggregate link-layer metric.

I do not have any specific opinions on this matter -- other than that  
my experience has been that expecting sensible L2 behavior in a  
wireless world, or even coherence between different L2  
implementations, usually has let me down..... I'd therefore suggest  
that we're able to "handle" this correctly, regardless of what  
caprices an L2 might have, on L3.

Thomas

>> transmission: if the probability of getting a transmission from A  
>> to B is 100% and from B to A is 10%, is the link A->B "asymmetric"  
>> or "uni-directional"? Can the metric for A->B be considered without

---

From: Philip Levis <[email-masked]>
Date: Thu, 1 Apr 2010 09:41:56 -0700

On Apr 1, 2010, at 8:34 AM, Jonathan Hui wrote:

> 
> The specific metric that led to the opening of this ticket was latency - I'm not sure where that falls in your definitions of asymmetric vs. uni-directional.  It would seem the latter if infinite latency is considered, but it's one example where the two directions may be viewed as independent.
> 
> Addressing your specific thought:  I think it's reasonable to say that any link metrics used should also be responsible for also considering specific properties of the link.  Extending that notion, if, at the link layer, communication from A -> B requires some from of communication from B -> A, then metrics used should consider the full process of communicating from A -> B -> A if that's important to the metric (e.g. reliability, margin, etc.).  I'm not sure I prefer to rely on L3 to communicate such metrics back and forth simply to be combined into some aggregate link-layer metric.

I agree -- this is really a link metric concern, which we want to separate from the core workings of RPL. 

Phil