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    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 1 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 1: 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 1), 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 2 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 2: 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 3 indicates the PHB-to-EXP mapping for outgoing traffic on L-LSPs.

        Table 3: 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.

    Tasks to perform static configuration and signaled mapping between the EXP bits and the PHB of traffic include the following sets of tasks:

    Configuring Static EXP-to-PHB Mapping

    You can configure static EXP-to-PHB mapping at the per-VR level, only for LSPs that do not have specific policies attached (by either per-LSP configured mapping or signaled mapping). The configured mapping applies regardless of label distribution protocol, BGP. LDP, or RSVP-TE.

    The PHB of incoming packets is determined from the EXP bits by the match values set with the mpls match exp-bits command.

    The EXP bits of outgoing packets are determined from the PHB by the match values set with the mpls match traffic-class command.

    To configure static EXP-to-PHB mapping:

    1. Set a combination of traffic class and color for incoming traffic that matches the specified EXP bits value in the shim header.
      host1(config)#mpls match exp-bits 1 set traffic-class bronze color red

      You can repeat the command to support the eight possible EXP bit values.

    2. Set the EXP bits in the shim header of outgoing traffic that matches a particular combination of traffic class and color.
      host1(config)#mpls match traffic-class gold color green set exp-bits 7

      You can repeat the command to support up to 24 combinations: eight traffic classes supported on the router times three colors.

    Signaled Mapping for RSVP-TE Tunnels

    For signaled mapping between EXP and PHB, policies apply the EXP bits matching and setting on a per-LSP basis rather than a per-VR basis. Signaled mapping applies only when RSVP-TE is the label distribution protocol.

    When traffic is mapped onto the ingress router of the LSP, the EXP bits are set according to a policy attached to the LSP. The policy corresponds to the EXP-to-PHB mapping defined for the LSP. Typically, the policy sets the EXP bits differently according to classifier lists that match on internal class/color information or on a user packet class associated with a packet.

    For transit routers and egress routers along the path of the LSP, the incoming EXP bits are matched to determine the traffic class and drop preference (color red, yellow, or green). This matching is accomplished by means of a policy corresponding to the signaled EXP-to-PHB mapping that is created and attached when the LSP is established.

    EXP bits are not normally changed on transit routers, but when traffic is sent out of an LSP on a transit router, the bits can be changed by the policy. Normally, however, the net effect is that the EXP-bits remain the same through the mapping sequence of EXP bits to an internal traffic class/color combination back to EXP bits, unless the traffic class/color combination is also modified by other factors.

    Because the policy (which maps the EXP bits to an internal traffic class/color combination and vice versa) attached to an LSP is created according to the PHB-ID–to–EXP mapping signaled by RSVP-TE, you must configure on each router a mapping association between PHB IDs and the internal traffic class/color combinations.

    The JunosE Software automatically generates and attaches policies when tunnels are established.

    Figure 1 shows the mapping associations between PHB IDs, EXP bits, and traffic class (TC)/color combination in an E-LSP case.

    • Mapping association between PHB ID and EXP bits is configured on ingress routers using the tunnel mpls diff-serv phb-id command.
    • Mapping association between PHB ID and traffic class/color combination is configured on all routers using the mpls diff-serv phb-id traffic-class command.
    • Mapping association between EXP bits and traffic class/color combination is done automatically by the JunosE Software at the appropriate routers along the path.

      Figure 1: Associations Between PHB ID, EXP Bits, and Traffic Classes/Colors

      Associations Between PHB ID, EXP Bits,
and Traffic Classes/Colors

    Figure 2 shows the operations performed at ingress, transit, and egress systems during signaled mapping sessions.

    Figure 2: Signaled Mapping

    Signaled Mapping

    To define a policy rule that sets the EXP bits in packets to which the policy is applied:

    • Issue the mark-exp command.
      host1(config-policy-list)#mark-exp 5 classifier-group claclEXP precedence 32

    To create or modify an MPLS classifier control list to match on traffic class/color combination or EXP bits:

    • Issue the mpls classifier-listcommand.
      host1(config)#mpls classifier-list be-green traffic-class best-effort color yellow

    To map the specified PHB ID to the internal traffic class/color combination:

    • Issue the mpls diff-serv phb-id traffic-class command.
      host1(config)#mpls diff-serv phb-id standard 45 traffic-class gold color green

    To create or modify an MPLS policy:

    • Issue the mpls policy-list command.
      host1(config)#mpls policy-list mpls-exp-setting

    To enable collection of policy statistics for a tunnel or LSP. Collection is disabled by default.

    • Issue the mpls policy-statistics command.
      host1#mpls policy-statistics boston2dc

    Policy statistics are displayed when you issue the show mpls forwarding or show mpls tunnel command, if a policy is attached and policy statistics are enabled.

    To specify the traffic class for which LSP-level queues are created and the scheduler profile to be used with the queues:

    • Issue the mpls traffic-class command.
      host1(config)#mpls traffic-class af1 scheduler-profile af1-scheduler-profile

    These classes originate from E-LSPs and L-LSPs (classes derived from the signaled PHB-ID) or regular LSPs (classes configured with the mpls traffic-class command)

    To specify the PHB supported by a signaled tunnel:

    • Issue the tunnel mpls diff-serv phb-id command.
      host1(config-if)#tunnel mpls diff-serv phb-id standard 35 exp-bits 5

    For E-LSPs, you also use this command to map the PHB to the specified exp-bits bitValue. You can repeat the command for up to eight PHB mappings.

    For L-LSPs, do not use the exp-bits keyword. If you repeat the command, the most recent command overwrites the previous command.

    Preference of per-VR Versus per-LSP Behavior

    MPLS always prefers the per-LSP method of matching and setting EXP bits by means of applied policies over the per-VR method.

    Per-VR matching of EXP bits is not performed on the LSP when an input policy (matching on incoming EXP bits) is attached to the ingress segment of the LSP.

    Similarly, per-VR setting of EXP bits is not performed on the LSP when an output policy (setting the outgoing EXP bits) is attached to the egress segment of the LSP.

    See the JunosE Policy Management Configuration Guide for more information about defining policies.

    Published: 2014-08-18