From: Scott Morris (swm@emanon.com)
Date: Mon Jun 19 2006 - 11:19:41 ART
Sorry for adding into the confusion for you!
Quite simply, the ECN information was supposed to come from the frame-relay
network (SP's FR switches). That was the original implementation where we
could count on the fact that it was end-to-end frame-relay in design!
Today, things are a little different (additions). The FECN will still
travel with the traffic through the Frame-relay network, but another
possibility introduced is the "fecn-adapt" feature where the actual router
(A Frame-Relay endpoint) can understand there is congestion and can assume
responsibility for setting the BECN bit in traffic going back towards the
source.
Per the DocCD:
"(Optional) Configure a Frame Relay interface to reflect received forward
explicit congestion notification (FECN) bits as BECN bits in Q.922 "Test
Response" messages."
The initial warning though, will always come from the switch network itself.
We just have to choose what to do with it. Options are to reflect BECN back
to the source, or to ignore. On BECN receipt, we can either do
adaptive-shaping (backoff our transmission) or ignore it.
This is for IP. :)
HTH,
Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713, JNCIE
#153, CISSP, et al.
CCSI/JNCI
IPExpert CCIE Program Manager
IPExpert Sr. Technical Instructor
smorris@ipexpert.com
http://www.ipexpert.com
-----Original Message-----
From: Arun Arumuganainar [mailto:aarumuga@hotmail.com]
Sent: Monday, June 19, 2006 8:41 AM
To: Scott Morris; 'Faryar Zabihi (fzabihi)'; 'Brian Dennis'; 'Petr
Lapukhov'; 'Ken'
Cc: 'Cisco certification'
Subject: Re: What does FECN do?
mmmm ...Now I am terribly confused !!! I thought I had a fair idea of FRTS
based on FECN/BECN mechanism !!! I think I overestimated myself .
Can somebody summarize what we have been discussing. For getting maximum
clarity I would request experts to restrict the discussion in the context of
IP .
Thanks very much in advance.
Thanks and Regards
Arun
----- Original Message -----
From: "Scott Morris" <swm@emanon.com>
To: "'Faryar Zabihi (fzabihi)'" <fzabihi@cisco.com>; "'Brian Dennis'"
<bdennis@internetworkexpert.com>; "'Petr Lapukhov'"
<petr@internetworkexpert.com>; "'Ken'" <hpnkpn103@yahoo.co.jp>
Cc: "'Cisco certification'" <ccielab@groupstudy.com>
Sent: Monday, June 19, 2006 8:33 AM
Subject: RE: What does FECN do?
> Does that cover ALL words in Texas? ;)
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
> Of Faryar Zabihi (fzabihi)
> Sent: Sunday, June 18, 2006 12:56 PM
> To: Scott Morris; Brian Dennis; Petr Lapukhov; Ken
> Cc: Cisco certification
> Subject: RE: What does FECN do?
>
> Them's fightin words!!!
> At least it is in TX
>
> Faryar
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
> Of Scott Morris
> Sent: Sunday, June 18, 2006 9:26 AM
> To: 'Brian Dennis'; 'Petr Lapukhov'; 'Ken'
> Cc: 'Cisco certification'
> Subject: RE: What does FECN do?
>
> Wow... I'm sorry, I didn't know these were still current lab topics.
>
> I'll jump right up and test that out. And while I'm at it, I think
> I'll worry about Appletalk and IPX as well in case they come back.
> What about
> LLC2 or RSRB? I hear token ring is on the rebound.
>
> DecNet and CLNS have built in mechanisms for ECN and are much more
> closely related with Layer 2 framing. IP on the other hand has had
> minor
mechanisms
> in the past, but more enhancements lately than for "a very long time".
> Hence the terminology of passing to "upper layer protocols".
>
> So... For the record, if we look at EVERY possibility that you ever
> may
run
> across, then my statement may at times be incorrect. If we have some
> semblance of paying attention to our lab discussions around here,
> which revolve in the IP world, then I think you'll find the statement
> to make perfect sense.
>
> And the DocCD writers have had no comment either way on that part.
> Since you know more than they do, you may want to point that out so
> then
at
> least we can be sure everyone is as politically correct...
>
> Scott
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
> Of Brian Dennis
> Sent: Sunday, June 18, 2006 2:36 AM
> To: swm@emanon.com; Petr Lapukhov; Ken
> Cc: Cisco certification
> Subject: RE: What does FECN do?
>
> Scott,
> Can you post an example configuration of how to configure DECnet
> support
for
> FECN since you say it's not the default? The reason I ask is because
> the DocCD states that "no configuration is required" which normally
> means "default".
>
> <DocCD>
> Cisco's Implementation of Frame Relay
>
> The Frame Relay software provides the following capabilities:
>
> * Transmission of congestion information from Frame Relay to DECnet
> Phase
IV
> and CLNS. This mechanism promotes Forward Explicit Congestion
> Notification
> (FECN) bits from the Frame Relay layer to upper-layer protocols after
> checking for the FECN bit on the incoming DLCI. Use this Frame Relay
> congestion information to adjust the sending rates of end hosts.
> FECN-bit promotion is enabled by default on any interface using Frame
Relay
> encapsulation. No configuration is required.
> </DocCD>
>
> Also if this is against Cisco's long-standing idea as you say someone
better
> let the people who write the DocCD know ;-)
>
> Thanks!
>
> Brian Dennis, CCIE #2210 (R&S/ISP-Dial/Security)
> bdennis@internetworkexpert.com
>
> Internetwork Expert, Inc.
> http://www.InternetworkExpert.com
> Toll Free: 877-224-8987
> Direct: 775-745-6404 (Outside the US and Canada)
>
> ________________________________
>
> From: Scott Morris [mailto:swm@emanon.com]
> Sent: Sat 6/17/2006 1:22 PM
> To: Brian Dennis; Petr Lapukhov; 'Ken'
> Cc: 'Cisco certification'
> Subject: RE: What does FECN do?
>
>
> However, unless specifically configured on Cisco devices, received
> BECN's
or
> FECNs have been routinely ignored regardless of the upper layer
> protocol support. Cisco is not alone in this long-standing idea though.
>
> Just because you CAN do it, doesn't mean you will. ;)
>
> scott
>
> ________________________________
>
> From: Brian Dennis [mailto:bdennis@internetworkexpert.com]
> Sent: Saturday, June 17, 2006 12:37 PM
> To: Scott Morris; Petr Lapukhov; Ken
> Cc: Cisco certification
> Subject: RE: What does FECN do?
>
>
> It really depends on the upper layer protocols in regards to Cisco's
support
> and there has been support for FECN and BECN for protocols that
> understood the concept of an ECN (Explicit Congestion Notification).
> The IOS has supported DECnet and OSI for a very long time. Also SNA
> using direct encapsulation LLC2 encapsulation supports BECN.
>
> Brian Dennis, CCIE #2210 (R&S/ISP-Dial/Security)
> bdennis@internetworkexpert.com
>
> Internetwork Expert, Inc.
> http://www.InternetworkExpert.com
> Toll Free: 877-224-8987
> Direct: 775-745-6404 (Outside the US and Canada)
>
>
>
>
> ________________________________
>
> From: nobody@groupstudy.com on behalf of Scott Morris
> Sent: Sat 6/17/2006 9:27 AM
> To: Petr Lapukhov; 'Ken'
> Cc: 'Cisco certification'
> Subject: RE: What does FECN do?
>
>
>
> While very true, this gives an example of us being concerned with our
> endpoints only. (Which very well may be the design of the network
> these
> days!)
>
> The history of FECN and BECN was revolving around a true end-to-end
> frame-relay network, and a Frame Switch in the middle would actually
> generate both types of ECNs (each a separate bit within the frame
> relay frame format).
>
> The FECN would follow with the traffic telling receiving stations
> (upper layer protocols, if informed) to expect some delays in incoming
> traffic
due
> to congestion. The BECN would go on the back path telling the
> receiver to slow down in transmission. If, of course, it were paying
attention to
> those! Cisco's default behavior is to ignore these notifications. Go
> figure.
>
> In today's frame-relay networks though, where SPs often re-encapsulate
> the frames into IP-IP tunnels, or MPLS clouds or whatever their choice
> is, the functionality is a bit warped.
>
> http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/frame.htm#wp1
> 02
> 0620
>
> There are capabilities for ECN marking within MPLS clouds (MPLS-VPN),
> but
I
> don't have any data suggesting which service providers do or do not
actually
> support this.... Bottom line, in real life you should know your
> network's capabilities. In the lab, you should do whatever the lab asks
for!
>
> Ahhhhh..... Evolution. ;)
>
>
> Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713,
> JNCIE #153, CISSP, et al.
> CCSI/JNCI
> IPExpert CCIE Program Manager
> IPExpert Sr. Technical Instructor
> smorris@ipexpert.com
> http://www.ipexpert.com
>
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
> Of
Petr
> Lapukhov
> Sent: Saturday, June 17, 2006 8:27 AM
> To: Ken
> Cc: Cisco certification
> Subject: Re: What does FECN do?
>
> Ken,
>
> Most of the time FECN is an informative bit, that may be used simply
> for statistical purposes.
>
> On the over hand, BECN frames actually signal source to slow down, but
that
> would work okay only if data exchange is bidirectional.
>
> Now, for a good example of FECN usefulness consider "frame-relay
> fecn-adapt" command wihin map-class.
> (Or it's equivalent "shape fecn-adapt" with MQC).
>
> Imagine that you have unidirectional stream of packets (e.g. video
> feed) that overloads your FR network. You will have a lot of FECN bit
> set
packets
> in direction from sender to receivers, but not a single packet
> backwards,
so
> no BECN frames will arrive to sender.
>
> Here you may use fecn-adapt, that enables reflection of FECN bit in
special
> FR frames back to sender with BECN bit set. In this way, a FECN
> signalling is converted to BECNs, that actually signal sender to slow
> down sending rate.
>
> http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgc
> r/
> fqos
> _r/qrfcmd9.htm#wp1103558
>
> HTH
>
> --
> Petr Lapukhov, CCIE #16379
> petr@internetworkexpert.com
>
> Internetwork Expert, Inc.
> http://www.InternetworkExpert.com
> Toll Free: 877-224-8987
> Outside US: 775-826-4344
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