JUNOSe 9.0.x Quality of Service Configuration Guide > Configuring Byte Adjustment for Shapng Rates with QoS Parameters > Byte Adjustment for Shaping Overview

Byte Adjustment for Shaping Overview

You can associate a parameter definition with the byte adjustment application (qos-byte-adjustment) to adjust the shaping rates for Ethernet interfaces configured on E-series routers.

Managing the bandwidth of downstream ATM traffic to Ethernet interfaces is difficult because of the different layer 2 encapsulations. To reduce the number of packet drops in the Ethernet network, you can use the byte adjustment application to account for the different encapsulations.

You can apply a parameter with the byte adjustment application to interfaces with either frame or cell shaping mode. You can configure the shaping mode by issuing the qos-shaping-mode command or by specifying the qos-cell-mode application with a parameter definition.

The byte adjustment differs for interfaces with frame shaping mode and cell shaping mode. Frame shaping mode is the default for all Ethernet interfaces. When you apply a parameter with the byte adjustment application on an interface with frame shaping mode, you adjust shaping rates to account for different layer 2 encapsulations only.

When you apply a parameter with the byte adjustment application on an interface with cell shaping mode, you adjust shaping rates to account for different layer 2 encapsulations as well as the ATM cell pad, header, and trailer.

The byte adjustment can have both a positive and negative value. The system performs the byte adjustment calculation before the shaping mode calculation.

Byte Adjustment Calculation and Example

The system counts the bytes transmitted to track the shaping rate. Instead of counting the actual packet size, the system uses the CPE packet size. You can configure the byte adjustment so that the shaping rate matches the CPE bandwidth.

By default, the byte adjustment is set to 0. If the overhead between the access node and CPE is 0, you do not need to configure the byte adjustment value.

Figure 63 displays an example of an Ethernet encapsulation and an ATM encapsulation.

Figure 63: Byte Adjustment Calculation for Ethernet and ATM Encapsulations

Table 28 lists the header lengths for the Ethernet encapsulation, which represents the CPE protocol overhead. The hierarchy is PPPoE over S-VLAN over Ethernet.

Table 28: Header Lengths for Ethernet Encapsulation 

Header	Number of Bytes
EnetHeader	14 bytes (6-SA, 6-DA, 2-ethertype)
Vstack	8 bytes (2-vmanTci, 2-ethertype, 2-vlanTci, 2-ethertype)
PppoeHeader	6 bytes (1-version/type, 1-code, 2-session id, 2-length)
Ppp	2 bytes (2-protocol id)
FCS	4 bytes
Total	34 bytes

Table 29 lists the header lengths for the ATM encapsulation, which represents the B-RAS protocol overhead. The interface stack is PPPoA over ATM 1483 with LLC Mux. The ATM AAL5 trailer is considered cell tax and is not part of the byte adjustment calculation.

Table 29: Header Lengths for ATM Encapsulation 

Header	Number of Bytes
ATM AAL5 LLC	4 bytes
PPP	2 bytes (2-protocol id)
Total	6 bytes

The byte adjustment calculation for these encapsulations is:

Related Topics

• Guidelines for Configuring Byte Adjustment of Shaping Using QoS Parameters
• Configuring a Parameter Definition for Byte Adjustment
• Example: QoS Parameter Configuration for QoS Cell Mode and Byte Adjustment
• For more information about configuring shaping modes for Ethernet, see QoS Shaping Mode for Ethernet Interfaces Overview and Cell Shaping Mode Using QoS Parameters Overview