| Internet-Draft | PCEP for Entropy Label Positions | March 2023 |
| Xiong, et al. | Expires 6 September 2023 | [Page] |
Entropy label (EL) can be used in the SR-MPLS data plane to improve load-balancing and multiple Entropy Label Indicator (ELI)/EL pairs SHOULD be inserted in the SR-MPLS label stack.¶
This document proposes a set of extensions for Path Computation Element Protocol (PCEP) to configure the entropy label positions for SR-MPLS networks.¶
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[RFC5440] describes the Path Computation Element Protocol (PCEP) which is used between a Path Computation Element (PCE) and a Path Computation Client (PCC) (or other PCE) to enable computation of Multi-protocol Label Switching (MPLS) for Traffic Engineering Label Switched Path (TE LSP). PCEP Extensions for the Stateful PCE Model [RFC8231] describes a set of extensions to PCEP to enable active control of MPLS-TE and Generalized MPLS (GMPLS) tunnels. [RFC8281] describes the setup and teardown of PCE-initiated LSPs under the active stateful PCE model, without the need for local configuration on the PCC, thus allowing for dynamic centralized control of a network.¶
Segment Routing (SR) leverages the source routing paradigm. Segment Routing can be instantiated on MPLS data plane which is referred to as SR-MPLS [RFC8660]. SR-MPLS leverages the MPLS label stack to construct the SR path. PCEP Extensions for Segment Routing [RFC8664] specifies extensions to the PCEP that allow a stateful PCE to compute and initiate TE paths, as well as a PCC to request a path subject to certain constraint(s) and optimization criteria in SR networks.¶
Entropy label (EL) [RFC6790] is a technique used in the MPLS data plane to improve load-balancing. Entropy Label Indicator (ELI) can be immediately preceding an EL in the MPLS label stack. The idea behind the EL is that the ingress router computes a hash based on several fields from a given packet and places the result in an additional label, named "entropy label". Then, this entropy label can be used as part of the hash keys used by an LSR. Using the entropy label as part of the hash keys reduces the need for deep packet inspection in the LSR while keeping a good level of entropy in the load-balancing. When the entropy label is used, the keys used in the hashing functions are still a local configuration matter and an LSR may use solely the entropy label or a combination of multiple fields from the incoming packet.¶
[RFC8662] proposes to use entropy labels for SR-MPLS networks and multiple <ELI, EL> pairs SHOULD be inserted in the SR-MPLS label stack. The ingress node may decide the number and place of the ELI/ELs which need to be inserted into the label stack. The extensions for Border Gateway Protocol (BGP) to indicate the entropy label positions in the SR-MPLS label stack has been proposed in [I-D.zhou-idr-bgp-srmpls-elp].¶
In some cases, the the controller(e.g. PCE) could be used to perform the TE path computation as well as the Entropy Label Position (ELP) which is useful for inter-domain scenarios. This document proposes a set of extensions for PCEP to configure the ELP information for SR-MPLS networks.¶
The terminology is defined as [RFC5440], [RFC6790], [RFC8664] and [RFC8662].¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
[RFC8662] proposes to use entropy labels for SR-MPLS networks. The Entropy Readable Label Depth (ERLD) is defined as the number of labels which means that the router will perform load-balancing using the ELI/EL.¶
As described in [RFC8662] section 7, the ERLD value is important for inserting ELI/EL and the ingress node need to evaluate the minimum ERLD value along the node segment path. But it will add complexity in the ELI/EL insertion process. Moreover, the ingress node cannot find the minimum ERLD along the path and does not support the computation of the minimum ERLD especially in inter-domain scenarios. As the Figure 1 shown, in SR-MPLS inter-domain scenario, the ingress node of the first domain could not get the ERLD information of other nodes of other domains.¶
+-----+ +-----+ +-----+
|PCE-1| |PCE-2| |PCE-3|
+--+--+ +--+--+ +--+--+
| | |
.........+.......... .........+.......... .........+...........
. . . . . .
.+---+ +---+ . . +---+ +---+ . .+---+ +----+ .
.| A |-------| B |------ | C |------| X |-------| Y |------| Z | .
.+---+ +---+ . . +---+ +---+ . .+---+ +----+ .
. SR-AS 1 . . SR-AS 2 . . SR-AS 3 .
.................... .................... .....................
The PCEs could get the information of all nodes such as Maximum SID Depth (MSD) and ERLD through Interior Gateway Protocol (IGP) and can compute the minimum ERLD along the end-to-end path. For example, the ERLD value can be collected via IS-IS [RFC9088], and OSPF [RFC9089]. IS-IS [RFC8491] and OSPF [RFC8476] provide examples of advertisement of the MSD. Moreover, the PCEs also can compute the Entropy Label Position (ELP) including the number and the places of the ELI/ELs. Then the ingress nodes MAY be required to support the capabilities of inserting multiple ELI/ELs and need to advertise the capabilities to the PCEs.¶
This document proposes the extensions for PCE to perform the computation of the end-to-end path as well as the positions of entropy labels in SR-MPLS networks. The ingress nodes can directly insert the ELI/ELs based on the positions.¶
As defined in [RFC8664], PCEP speakers use SR PCE Capability sub-TLV to exchange information about their SR capability when PST=1 in the PST List of the PATH-SETUP-TYPE-CAPABILITY TLV carried in Open object. This document defined a new flag (E-flag) for SR PCE Capability sub-TLV.¶
E (Entropy Label Configuration is supported) : A PCE sets this flag bit to 1 carried in Open message to indicate that it supports the computation of SR path with ELP information. A PCC sets this flag to 1 to indicate that it supports the capability of inserting multiple ELI/EL pairs and and supports the results of SR path with ELP from PCE.¶
The LSP Object is defined in Section 7.3 of [RFC8231]. This document defined a new flag (E-flag) for the LSP-EXTENDED-FLAG TLV carried in LSP Object as defined in [RFC9357].¶
E (Request for ELP Configuration) : If the bit is set to 1, it indicates that the PCC requests PCE to compute the SR path with ELP information. A PCE would also set this bit to 1 to indicate that the ELP information is included by PCE and encoded in the PCRep, PCUpd or PCInitiate message.¶
SR-ERO subobject is used for SR-TE path which consists of one or more SIDs as defined in [RFC8664]. This document defined a new flag (E-flag) for the SR-ERO subobject.¶
E (ELP Configuration) : If this flag is set, it means that the position after this SR-ERO subobject is the position to insert <ELI, EL>, otherwise it cannot insert <ELI, EL> after this segment.¶
The SR path is initiated by PCE or PCC with PCReq, PCInitiated or PCUpd messages and the E bit is set to 1 in LSP object to request the ELP configuration. The SR-TE path being received by PCC with SR-ERO segment list, for example, <S1, S2, S3, S4, S5, S6>, especially S3 and S6 with E-flag set. It indicates that two <ELI, EL> pairs MUST be inserted into the label stack of the SR-TE forwarding entry, respectively after the label for S3 and label for S6. With EL information, the label stack for SR-MPLS would be <label1, label2, label3, ELI, EL, label4, label5, label6, ELI, EL>.¶
This document defines a new E bit for entropy label, which do not introduce any new security considerations beyond those already listed in [RFC9357], [RFC8662] and [RFC8664].¶
Note that this specification enables a network controller to insert ELI/ELs pairs and an attacker may set the E bit in SR-ERO subobject incorrectly and exceed the length of stack.¶
SR PCE Capability TLV is defined in [RFC8664], and the registry to manage the Flag field of the SR PCE Capability TLV is requested in [RFC8664]. IANA is requested to make allocations from the registry, as follows:¶
| Value | Name | Reference |
|---|---|---|
| TBD11 | Entropy Label Configuration is supported (E) | [this document] |
[RFC9357] defines the LSP-EXTENDED-FLAG TLV. IANA is requested to make allocations from the Flag field registry, as follows:¶
| Value | Name | Reference |
|---|---|---|
| TBD | Request for ELP Configuration (E) | [this document] |
SR-ERO subobject is defined in [RFC8664], and the registry to manage the Flag field of SR-ERO is requested in [RFC8664]. IANA is requested to make allocations from the registry, as follows:¶
| Value | Name | Reference |
|---|---|---|
| 36 | ELP Configuration (E) | [this document] |
The authors would like to thank Stephane Litkowski, Dhruv Dhody, Tarek Saad, Zhenbin Li and Jeff Tantsura for their review, suggestions and comments to this document.¶