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4. Detailed Operation of L-LSPs

4.1. L-LSP Definition​

L-LSPs are defined in section 1.3.

4.2. Populating the `Encaps-->PHB mapping' for an incoming L-LSP​

This section defines how the `Encaps-->PHB mapping' of the Diff-Serv Context is populated at label setup for an incoming L-LSP in order to allow Incoming PHB determination.

4.2.1. `EXP-->PHB mapping'​

If the LSR terminates the MPLS Shim Layer over this incoming L-LSP and the L-LSP ingresses on an interface which is not ATM nor Frame Relay, then the `Encaps-->PHB mapping' is populated in the following way:

  • it is actually a `EXP-->PHB mapping'

  • this mapping is a function of the PSC which is carried on this LSP, and must use the relevant mapping entries for this PSC from the Mandatory `EXP/PSC-->PHB mapping' defined in Section 4.2.1.1.

For example if the incoming label corresponds to an L-LSP supporting the AF1 PSC, then the `Encaps-->PHB mapping' will be populated with:

      EXP Field              PHB

        001        ---->    AF11
        010        ---->    AF12
        011        ---->    AF13

An LSR, supporting L-LSPs over PPP interfaces and LAN interfaces, is an example of an LSR terminating the Shim layer over ingress interfaces which are not ATM nor Frame Relay.

If the LSR terminates the MPLS Shim Layer over this incoming L-LSP and the L-LSP ingresses on an ATM or Frame Relay interface, then the `Encaps-->PHB mapping' is populated in the following way:

  • it should actually be a EXP-->PHB mapping'. Alternative optional ways of populating the Encaps-->PHB mapping' might be defined in the future (e.g., using a 'CLP/EXP--> PHB mapping' or a 'DE/EXP-->PHB mapping') but are outside the scope of this document.

  • when the Encaps-->PHB mapping' is an EXP-->PHB mapping', this EXP-->PHB mapping' mapping is a function of the PSC which is carried on the L-LSP, and must use the relevant mapping entries for this PSC from the Mandatory EXP/PSC-->PHB mapping' defined in Section 4.2.1.1.

An Edge-LSR of an ATM-MPLS domain or of a FR-MPLS domain is an example of an LSR terminating the shim layer over an ingress ATM/FR interface.

4.2.1.1. Mandatory `EXP/PSC --> PHB mapping'​

      EXP Field      PSC             PHB

        000          DF    ---->    DF
        000          CSn   ---->    CSn
        001          AFn   ---->    AFn1
        010          AFn   ---->    AFn2
        011          AFn   ---->    AFn3
        000          EF    ---->    EF

4.2.2. `CLP-->PHB mapping'​

If the LSR does not terminate an MPLS Shim Layer over this incoming label and uses ATM encapsulation (i.e., it is an ATM-LSR), then the `Encaps-->PHB mapping' for this incoming L-LSP is populated in the following way:

  • it is actually a `CLP-->PHB mapping'

  • the mapping is a function of the PSC, which is carried on this LSP, and should use the relevant mapping entries for this PSC from the Default `CLP/PSC-->PHB mapping' defined in Section 4.2.2.1.

For example if the incoming label corresponds to an L-LSP supporting the AF1 PSC, then the `Encaps-->PHB mapping' should be populated with:

      CLP Field              PHB

        0          ---->    AF11
        1          ---->    AF12

4.2.2.1. Default `CLP/PSC --> PHB mapping'​

      CLP Bit      PSC             PHB

         0          DF    ---->    DF
         0          CSn   ---->    CSn
         0          AFn   ---->    AFn1
         1          AFn   ---->    AFn2
         0          EF    ---->    EF

4.2.3. `DE-->PHB mapping'​

If the LSR does not terminate an MPLS Shim Layer over this incoming label and uses Frame Relay encapsulation (i.e., it is a FR-LSR), then the `Encaps-->PHB mapping' for this incoming L-LSP is populated in the following way:

  • it is actually a `DE-->PHB mapping'

  • the mapping is a function of the PSC which is carried on this LSP, and should use the relevant mapping entries for this PSC from the Default `DE/PSC-->PHB mapping' defined in Section 4.2.3.1.

4.2.3.1. Default `DE/PSC --> PHB mapping'​

      DE Bit      PSC             PHB

         0          DF    ---->    DF
         0          CSn   ---->    CSn
         0          AFn   ---->    AFn1
         1          AFn   ---->    AFn2
         0          EF    ---->    EF

4.3. Incoming PHB Determination On Incoming L-LSP​

This section defines how Incoming PHB determination is carried out when the considered label entry in the received label stack corresponds to an L-LSP. This requires that the `Encaps-->PHB mapping' is populated as defined in section 4.2.

When considering a label entry corresponding to an incoming L-LSP for Incoming PHB Determination, the LSR first determines the `Encaps-->PHB mapping' associated with the corresponding label.

4.3.1. `EXP-->PHB mapping'​

If the Encaps-->PHB mapping' is of the form EXP-->PHB mapping', then the LSR:

  • determines the incoming PHB by looking at the EXP field of the considered label entry and using the `EXP-->PHB mapping'.

4.3.2. `CLP-->PHB mapping'​

If the Encaps-->PHB mapping' is of the form CLP-->PHB mapping', then the LSR:

  • determines the incoming PHB by looking at the CLP field of the ATM Layer encapsulation and using the `CLP-->PHB mapping'.

4.3.3. `DE-->PHB mapping'​

If the Encaps-->PHB mapping' is of the form DE-->PHB mapping', then the LSR:

  • determines the incoming PHB by looking at the DE field of the Frame Relay encapsulation and by using the `DE-->PHB mapping'.

4.4. Populating the `Set of PHB-->Encaps mappings' for an outgoing L-LSP​

This section defines how the `Set of PHB-->Encaps mappings' of the Diff-Serv Context is populated at label setup for an outgoing L-LSP in order to allow Encoding of Diff-Serv Information.

4.4.1. `PHB-->EXP mapping'​

If the LSR uses an MPLS Shim Layer over this outgoing L-LSP, then one PHB-->EXP mapping' is added to the Set of PHB-->Encaps mappings' for this outgoing L-LSP. This `PHB-->EXP mapping' is populated in the following way:

  • it is a function of the PSC supported on this LSP, and must use the mapping entries relevant for this PSC from the Mandatory `PHB-->EXP mapping' defined in section 4.4.1.1.

For example, if the outgoing label corresponds to an L-LSP supporting the AF1 PSC, then the following PHB-->EXP mapping' is added into the Set of PHB-->Encaps mappings':

         PHB                EXP Field

         AF11       ---->      001
         AF12       ---->      010
         AF13       ---->      011

4.4.1.1. Mandatory `PHB-->EXP mapping'​

         PHB                EXP Field

         DF         ---->      000
         CSn        ---->      000
         AFn1       ---->      001
         AFn2       ---->      010
         AFn3       ---->      011
         EF         ---->      000

4.4.2. `PHB-->CLP mapping'​

If the L-LSP is egressing on an ATM interface (i.e., it is an ATM-LSR or it is a frame-based LSR sending packets on an LC-ATM interface or on an ATM interface which is not label switching controlled), then one PHB-->CLP mapping' is added to the Set of PHB-->Encaps mappings' for this outgoing L-LSP.

If the L-LSP is egressing over an ATM interface which is not label-controlled, the `PHB-->CLP mapping' is populated as per section 3.4.2.

If the L-LSP is egressing over an LC-ATM interface, the `PHB-->CLP mapping' is populated in the following way:

  • it is a function of the PSC supported on this LSP, and should use the relevant mapping entries for this PSC from the Default `PHB-->CLP mapping' defined in section 3.4.2.1.

Notice that if the LSR is a frame-based LSR supporting an L-LSP egressing over an ATM interface, then the Set of PHB-->Encaps mappings' contains both a PHB-->EXP mapping' and a PHB-->CLP mapping'. If the LSR is an ATM-LSR supporting an L-LSP, then the Set of PHB-->Encaps mappings' only contains a `PHB-->CLP mapping'.

4.4.3. `PHB-->DE mapping'​

If the L-LSP is egressing over a Frame Relay interface (i.e., it is an LSR sending packets on an LC-FR interface or on a Frame Relay interface which is not label switching controlled), one PHB-->DE mapping' is added to the Set of PHB-->Encaps mappings' for this outgoing L-LSP.

If the L-LSP is egressing over a FR interface which is not label switching controlled, the `PHB-->DE mapping' is populated as per section 3.4.3.

If the L-LSP is egressing over an LC-FR interface, the `PHB-->DE mapping' is populated in the following way:

  • it is a function of the PSC supported on this LSP, and should use the relevant mapping entries for this PSC from the Default `PHB-->DE mapping' defined in section 3.4.3.1.

Notice that if the LSR is an Edge-LSR supporting an L-LSP egressing over a LC-FR interface, then the Set of PHB-->Encaps mappings' contains both a PHB-->EXP mapping' and a PHB-->DE mapping'. If the LSR is a FR-LSR supporting an L-LSP, then the Set of PHB-->Encaps mappings' only contains a `PHB-->DE mapping'.

4.4.4. `PHB-->802.1 mapping'​

If the LSP is egressing over a LAN interface on which multiple 802.1 Traffic Classes are supported, as defined in [IEEE_802.1], then one `PHB-->802.1 mapping' is added as per section 3.4.4.

4.5. Encoding Diff-Serv Information into Encapsulation Layer on Outgoing L-LSP​

This section defines how to encode Diff-Serv information into the MPLS encapsulation Layer for a transmitted label entry corresponding to an outgoing L-LSP. This requires that the `Set of PHB-->Encaps mappings' is populated as defined in section 4.4.

The LSR first determines the `Set of PHB-->Encaps mappings' of the Diff-Serv Context associated with the corresponding label in the NHLFE and then performs corresponding encoding as specified in sections 3.5.1, 3.5.2, 3.5.3 and 3.5.4.

4.6. L-LSP Merging​

In an MPLS domain, two or more LSPs can be merged into one LSP at one LSR. L-LSPs are compatible with LSP Merging under the following condition:

L-LSPs can only be merged into one L-LSP if they support the same PSC.

The above merge condition MUST be enforced by LSRs, through explicit checking at label setup, that the same PSC is supported on the merged LSPs.

Note that when L-LSPs merge, the bandwidth that is available for the PSC downstream of the merge point must be sufficient to carry the sum of the merged traffic. This is particularly important in the case of EF traffic. This can be ensured in multiple ways (for instance via provisioning, or via bandwidth signaling and explicit admission control).