Example: Configuring Dynamic Shaping Parameters to Account for Overhead in Downstream Traffic Rates

Managing Traffic with Different Encapsulations

In this example, the MX Series router shown in Figure 1 sends stacked VLAN frames to the DSLAM, and the DSLAM sends single-tagged VLAN frames to the residential gateway.

To accurately shape traffic at the residential gateway, the MX Series router must account for the different frame sizes. The difference between the stacked VLAN (S-VLAN) frames sent by the router and the single-tagged VLAN frames received at the residential gateway is a 4-byte VLAN tag. The residential gateway receives frames that are 4 bytes less.

To account for the different frame sizes, you configure the frame shaping mode with -4 byte adjustment:

1. Configure the traffic shaping parameters in the dynamic profile and attach them to the interface.

   Enabling the overhead accounting feature affects the resulting shaping rate, guaranteed rate, and excess rate parameters, if they are configured.

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   [edit]
   dynamic-profiles {
       ethernet-downstream-network {
       interfaces {
           $junos-interface-ifd-name {
               unit $junos-underlying-interface-unit {
                   family inet;
               }
           }
       }
       class-of-service {
           traffic-control-profiles {
               tcp-example-overhead-accounting-frame-mode {
                   excess-rate percent $junos-cos-excess-rate
                   guaranteed-rate $junos-cos-guaranteed-rate
                   overhead-accounting $junos-cos-shaping-mode bytes $junos-cos-byte-adjust
                   shaping-rate $junos-cos-shaping-rate;
               }
           }
           interfaces {
               $junos-interface-ifd-name {
                   unit "$junos-underlying-interface-unit" {
                   output-traffic-control-profile tcp1;
                   }
               }
           }
       }
       }
   }
   

   Table 1 lists the initial values defined by the RADIUS administrator for the shaping rates.

   Table 1: Initial Shaping Values at Subscriber Login For Traffic With Different Encapsulations

   Predefined Variable	RADIUS Tag	Value
   $junos-cos-shaping-rate	T02	10m
   $junos-cos-guaranteed-rate	T03	2m
   $junos-cos-excess-rate	T05	50
   $junos-cos-shaping-mode	T07	frame-mode
   $junos-cos-byte-adjust	T08	–4

2. Verify the adjusted rates.

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   user@host#show class-of-service traffic-control-profile
   Traffic control profile: tcp-example-overhead-accounting-frame-mode, Index: 61785
   Excess rate 50
   Shaping rate: 10000000
   Guaranteed rate: 2000000
   Overhead accounting mode: Frame Mode
   Overhead bytes: —4

Managing Downstream Cell-Based Traffic

In this example, the DSLAM and residential gateway shown in Figure 1 are connected through an ATM cell-based network. The MX Series router sends Ethernet frames to the DSLAM, and the DSLAM sends ATM cells to the residential gateway.

To accurately shape traffic at the residential gateway, the MX Series router must account for the different physical network characteristics.

The administrator does not need to configure a byte adjustment value to account for the downstream ATM network, but has the option of configuring a byte adjustment value to account for different encapsulations or decapsulations.

To account for the different frame sizes, configure cell shaping mode:

1. Configure the traffic shaping parameters in the dynamic profile and attach them to the interface.

   Enabling the overhead accounting feature affects the resulting shaping rate, guaranteed rate, and excess rate parameters, if they are configured.

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   [edit]
   dynamic-profiles {
       atm-downstream-network {
       interfaces {
           $junos-interface-ifd-name {
               unit $junos-underlying-interface-unit {
                   family inet;
               }
           }
       }
       class-of-service {
           traffic-control-profiles {
               tcp-example-overhead-accounting-cell-mode {
                   excess-rate percent $junos-cos-excess-rate
                   guaranteed-rate $junos-cos-guaranteed-rate
                   overhead-accounting $junos-cos-shaping-mode
                   shaping-rate $junos-cos-shaping-rate
               }
           }
           interfaces {
               $junos-interface-ifd-name {
                   unit "$junos-underlying-interface-unit" {
                   output-traffic-control-profile tcp1;
                   }
               }
           }
       }
       }
   }
   

   Table 2 lists the initial values defined by the RADIUS administrator for the shaping rates.

   Table 2: Initial Shaping Values at Subscriber Login For Downstream Cell-Based Traffic

   Predefined Variable	RADIUS Tag	Value
   $junos-cos-shaping-rate	T02	10m
   $junos-cos-guaranteed-rate	T03	2m
   $junos-cos-excess-rate	T05	50
   $junos-cos-shaping-mode	T07	cell-mode

2. Verify the adjusted rates.

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   user@host#show class-of-service traffic-control-profile
   Traffic control profile: tcp-example-overhead-accounting-cell-mode, Index: 61785
   Shaping rate: 10000000
   Excess rate 50
   Guaranteed rate: 2000000
   Overhead accounting Cell Mode
   Overhead bytes: 0
   

   To account for ATM segmentation, the MX Series router adjusts all of the rates by 48/53 to account for ATM AAL5 encapsulation. In addition, the router accounts for cell padding, and internally adjusts each frame by 8 bytes to account for the ATM trailer.