Frame-Relay Traffic Shaping

Frame-Relay Traffic Shaping (FRTS) can be hard to understand.
Traffic shaping vs policing
The most important difference between these two is the fact that shaping is designed to delay excess traffic, whereas policing drops the excess traffic.
Shaping uses credits. Before a packet can be sent, the amount of credits equaling the packet size in bits have to be available. Policing is not designed to guarantee bandwidth, it’s designed to limit bandwidth.
Terminology:

 Available rate (AR)
Also known as port speed. The actual physical speed on the serial interface (configed with the clock rate at DCE, at a DTE devices it’s learned with the “received” clock rate)
Committed Information Rate (CIR)
Also known as target rate. The rate (in bps) that the router will send at. The default CIR with traffic shaping enabled is 56Kbps.
The CIR configured at the router has not to be the same as the CIR of the provider.
Minimum CIR (minCIR)
The rate (in bps) the provider guarantees to accept. The provider sets the “DE” bit for all traffic above this rate.
minCIR is half the configed CIR, by default. It’s only used in conjunction with adaptive shaping
Committed Burst (Bc)
The number of committted bits allowed to be sent during a static time interval. (the provider agreed to accept this, but only if the configured CIR equals the configed CIR of the provider)
By default 1/8 of the configured CIR below 650kpbs, 1/16 of the configured CIR above 650kbps
Excess Burst (Be)
The number of non-committed bits allowed to be sent above the Bc during the first time interval. The credits used for this, is derived from the unused Bc credits in the previous time intervals. (There is no limit on keeping the Bc credits: it can be stored for multiple time intervals).
There is no default value
Committed Rate Maesurement Interval (Tc)
The time interval over which Bc or Bc+Be can be sent.
Maximum of 125ms (1/8 sec), minimum 10ms (1/100 sec)
Tc = Bc / CIR
Configuring, Commands
frame-relay traffic-shaping
Interface command to enable shaping. By default 56kbps shaping! (CIR of 56kbps)
map-class frame-relay <class-name>
Global conf command for shaping parameters
frame-relay cir <bps>
map-class command to define the CIR
frame-relay mincr <bps>
map-class command to define the minCIR
frame-relay bc <bits>
map-class command to define the Bc
frame-relay Be <bits>
map-class command to define the Be
frame-relay adaptive-shapig {becn | foresight | interface-congestion [queue-depth]}
map-class command used to throttle back in case of congestion. Router will throttle back 25% per Tc when a BECN is received. The router throttles back another 25% at the next Tc untill no more BECNs received or minCIR reached.
frame-relay fecn-adapt
map-class command used to reflect BECNs and FECNs. Only used in 1-way traffic connections (for example, videostreaming)
frame-relay class <name>
interface command used to bind a map-class to the interface
class <name>
used to bind a map-class to a DLCI. Only used with this command: frame-relay interface-dlci <dlci>
Token Bucket
This bucket is used to manage a traffic flow. It’s related to 3 parameters:

  • Bc
  • CIR
  • Tc

The relations are:

  • Bc = CIR * Tc
  • CIR = Bc / Tc
  • Tc = Bc / CIR

For every Tc, 8000 credits are filled into the bucket. For each packet that is sent, the amount of credits equals the packetsize in bits is removed from the bucket. If there are not enough credits left, the packet will get delayed untill the next Tc.  This is called, shaping.
If there are packets left over from the previous Tc, the bucket will be filled again untill 8000 credits. Extra credits will be deleted.
There is also a dual bucket configuration:
If the first bucket (Bc) is empty, the second bucket will be used (Be). These are 2 seperate buckets. There is no overlap in the buckets. If a packet is larger then the remaining credits in the Bc bucket, the Be bucket will be used for that entire packet.
At a new Tc, the Bc bucket will be filled again with 8000 credits. When there are credits left from the previous Tc, these are copied into the Be bucket (untill a max of 8000).

Leave a Reply

Your email address will not be published. Required fields are marked *