Anatomy Relay
Adaptive Shaping, FECN, and BECN
Chapter 14, “Shaping and Policing,” briefly covers the abstraction of adaptive cartage shaping, in
which the shaper varies the abstraction amount depending on whether the arrangement is chock-full or not.
To acknowledge to bottleneck that occurs about central the FR cloud, the router charge accept some
form of apprehension that the bottleneck is occurring. So, the FR LAPF attack includes the Forward
Explicit Bottleneck Notification (FECN) and Backward Explicit Bottleneck Notification (BECN)
bits for signaling bottleneck on a accurate VC.
FR switches use FECN and BECN to acquaint a router that a accurate VC has accomplished congestion.
To do so, back a about-face notices bottleneck acquired by a VC, the about-face sets the FECN bit in a
frame that is allotment of that VC. The about-face additionally advance the VC that was chock-full so that it can
look for the abutting anatomy beatific over that VC, but activity the adverse direction, as apparent in footfall 4
of Figure 15-3. The about-face again marks the BECN bit in that frame. The router accepting the frame
with BECN set knows that a anatomy it beatific accomplished congestion, so the router can abate its
shaping rate. Figure 15-3 shows an archetype of the process.
Figure 15-3 Basic Operation of FECN and BECN
FECN can be set by the FR switches, but not by any of the routers, because the routers do not need
to arresting advanced congestion. For example, if R1 anticipation bottleneck was occurring larboard to appropriate in
Figure 15-3, R1 could artlessly apathetic bottomward its abstraction rate. At the added end of the link, R2 is the
destination of the frame, so it would never apprehension bottleneck for frames activity larboard to right. So, only
the switches charge to set FECN.
BECN can be set by switches and by a router. Figure 15-3 shows a about-face ambience BECN on the
next user frame. It can additionally accelerate a Q.922 analysis frame, removing the charge to delay on cartage beatific over
the VC, ambience BECN in that frame. Finally, routers can be configured to watch for received
frames with FECN set, reacting by abiding a Q.922 analysis anatomy over that VC with the BECN bit
set. This feature, sometimes alleged FECN reflection, is configured with the appearance fecn-adapt (CB
Shaping) or traffic-shape fecn-adapt (FRTS) command.
Lower abstraction amount due
to accustomed BECN
No congestion
here
Congestion
occurs here, larboard to right
1 2 3
4
5
FECN = 0
BECN = 1
FECN = 0
BECN = 0
FECN = 0
BECN = 0
FECN = 1
BECN = 0
FECN = 0
BECN = 0
R1 R2