From: Ken (hpnkpn103@yahoo.co.jp)
Date: Sat Jun 17 2006 - 19:03:54 ART
Hi,
Thank you all for your comments.
In short, when there is congestion in FR switch,
- FR switch set the FECN bit to 1 on frames going toward the destination. It
is up to a higher-layer protocol of the receiving DTE device what to do with
the FECN bit.
- FR switch set the BECN bit to 1 in frames traveling in the opposite
direction if there is any traffic on that direction.
- If the traffic is unidirectional and there is no frame to set the BECN,
then FECN-adaptive can be enabled on an end-node which enables the node to
send frames with BECN bit 1 as a reflection of frame with FECN bit 1.
- Whether the router that received BECN will lower the rate would depend on
the implementation.
Anyone has any more comments?
Thanks,
Ken
----- Original Message -----
From: "Scott Morris" <swm@emanon.com>
To: "'Brian Dennis'" <bdennis@internetworkexpert.com>; "'Petr Lapukhov'"
<petr@internetworkexpert.com>; "'Ken'" <hpnkpn103@yahoo.co.jp>
Cc: "'Cisco certification'" <ccielab@groupstudy.com>
Sent: Sunday, June 18, 2006 2:22 AM
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#wp1020620
>
> 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/ios122/122cgcr/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
> 24/7 Support: http://forum.internetworkexpert.com
> Live Chat: http://www.internetworkexpert.com/chat/
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
This archive was generated by hypermail 2.1.4 : Sat Jul 01 2006 - 07:57:33 ART