Classifying Traffic for Differentiated Services

In a differentiated services domain, traffic is classified into a behavior aggregate (BA), based on the type of diff-serv behavior for the traffic. At each node, traffic belonging to a particular BA is mapped to the corresponding per-hop behavior (PHB), which provides the scheduling behavior and drop probability required by the traffic.

MPLS uses the EXP bits in the shim header to support differentiated services. The JUNOSe software supports both statically configured and signaled mapping between the EXP bits and the PHB of traffic.

In a signaled environment, you can configure on the ingress node the set of PHBs that a tunnel supports, and then the set of PHBs is signaled end to end.

To support differentiated services, MPLS employs two types of LSPs: E-LSPs and L-LSPs. The two types differ in how their PHB is determined. In the JUNOSe software, the PHB is a combination of traffic class (also called per-hop scheduling class, or PSC) and drop precedence (color).

• E-LSPs (EXP-inferred-PSC LSP) can transport as many as eight BAs. For E-LSPs, the traffic’s PHB is learned from the MPLS shim header.
• L-LSPs (Label-only-inferred-PSC LSP) transport a single PSC. The PHB is determined from a combination of the packet’s label, which indicates the traffic class, and the EXP field of the shim header, which indicates the drop precedence.
  • Table 27 indicates how the PSC (column 1) is combined with the EXP field (column 2) to determine the PHB for incoming traffic on L-LSPs.

    Table 27: Incoming L-LSP PHB Determination

    PSC	+	EXP Field	=	PHB
    BE		000		BE
    CSn		000		CSn
    AFn		001		AFn1
    AFn		010		AFn2
    AFn		011		AFn3
    EF		000		EF

    For nonstandard PHBs (any that are not listed in Table 27), the JUNOSe software uses mapping similar to AFn mapping; EXP 001 is mapped to color green, EXP 010 is mapped to yellow, and EXP 011 is mapped to red.

    Table 28 presents three examples that indicate how the PSC and the EXP field are combined to determine the PHB for traffic on incoming L-LSPs.

    Table 28: Examples of Incoming L-LSP PHB Determination

    PSC	+	EXP Field	=	PHB
    AF2		010		AF22
    AF3		010		AF32
    AF3		011		AF33

  • For outgoing L-LSPs, the EXP is determined by the PHB. Table 29 indicates the PHB-to-EXP mapping for outgoing traffic on L-LSPs.

    Table 29: Outgoing L-LSP PHB Determination

    PHB	=	EXP Field
    BE		000
    CSn		000
    AFn1		001
    AFn2		010
    AFn3		011
    EF		000

For nonstandard PHBs, the mapping is similar to AFn mapping. Red color maps to 011, yellow maps to 010, and green maps to 001.

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