| Internet-Draft | Slice Identification in MPLS EL | March 2021 |
| Decraene, et al. | Expires 23 September 2021 | [Page] |
This document defines a solution to encode a slice identifier in MPLS in order to distinguish packets that belong to different slices, to allow enforcing per network slice policies (.e.g, Qos).¶
The slice identification is independent of the topology. It allows for QoS/DiffServ policy on a per slice basis in addition to the per packet QoS/DiffServ policy provided by the MPLS Traffic Class field.¶
In order to minimize the size of the MPLS stack and to ease incremental deployment the slice identifier is encoded as part of the Entropy Label.¶
This document also extends the use of the TTL field of the Entropy Label in order to provide a flexible set of flags called the Entropy Label Control field.¶
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Segment Routing (SR) [RFC8402] leverages the source-routing paradigm. A node steers a packet through a controlled set of instructions, called segments, by prepending the packet with an SR header. In the SR-MPLS data plane [RFC8660], the SR header is instantiated through a label stack.¶
This document defines a solution to encode a slice identifier in MPLS in order to provide QoS on a per slice basis. It allows for QoS/DiffServ policy on a per slice basis in addition to the per packet QoS/DiffServ policy provided by the MPLS Traffic Class field. The slice identification is independent of the topology and the QoS of the network, thus enabling scalable network slicing.¶
This document encodes the slice identifier in a portion of the MPLS Entropy Label (EL) defined in [RFC6790]. This has advantages in SR-MPLS networks as it avoids the use of additional label which would increase the size of the label stack. This also reuses the data plane processing of the Entropy Label on the egress LSR, the signaling of the Entropy Label capability from the egress to the ingress [I-D.ietf-isis-mpls-elc] [I-D.ietf-ospf-mpls-elc], and the signaling capability of transit routers to read this label [RFC8491] which allows for an easier and faster incremental deployment.¶
[RFC6790] defines the MPLS Entropy Label. [RFC6790] section 4.2 defines the use of the Entropy Label Indicator (ELI) followed by the Entropy Label (EL) and the MPLS header fields (Label, TC, S, TTL) in each. [RFC6790] also specifies that the TTL field of the EL must be set to zero by the ingress LSR.¶
Following the procedures of [RFC6790] EL is never used for forwarding and its TTL is never looked at nor decremented:¶
Hence essentially the TTL field of the EL behaves as a reserved field which must be set to zero when sent and ignored when received.¶
This documents extends the TTL field of the EL and calls it the Entropy Label Control (ELC) field. The ELC is a set of eight flags: ELC0 for bit 0, ELC1 for bit 1,..., ELC7 for bit 7.¶
Given that the MPLS header is very compact (32 bits) with no reserved bits and that MPLS is used within a trusted administrative domain, the semantic of these bits is not standardized but defined on a per administrative domain basis. This allows for increased re-use and flexibility of this scarce resource. As a consequence, an application using one of those buts MUST allow the choice of the bit by configuration by the network operator.¶
Each network slice in an MPLS domain is uniquely identified by a Slice Identifier (SLID). This section proposes to encode the SLID in a portion of the MPLS Entropy Label defined in [RFC6790].¶
The number of bits to be used for encoding the SLID in the EL is governed by a local policy and uniform within a network slice policy domain.¶
When an ingress LSR classifies that a packet belongs to the slice and that the egress has indicated via signaling that it can process EL for the tunnel, the ingress LSR pushes an Entropy Label with the:¶
The choice of the ELC field used for SPI, and the number of bits to be used for encoding the SLID MUST be configurable by the network operator.¶
The slice classification method is outside the scope of this document.¶
Any router within the SR domain that forwards a packet with the SPI bit set MUST use the SLID to select a slice and apply per-slice policies.¶
There are many different policies that could define a slice for a particular application or service. The most basic of these is bandwidth-allocation, an implementation complying with this specification SHOULD support the bandwidth-allocation slice as defined in the next section.¶
A per-slice policy is configured at each interface of each router in the SR domain, with one traffic shaper per SLID. The bit rate of each shaper is configured to reflect the bandwidth allocation of the per-slice policy.¶
If shapers are not available, or desirable, an implementation MAY configure one scheduling queue per SLID with a guaranteed bandwidth equal to the bandwidth-allocation for the slice. This option allows a slice to consume more bandwidth than its allocation when available.¶
Per-slice shapers or queues effectively provides a virtual port per slice. This solution MAY be complemented with a per-virtual-port hierarchical DiffServ policy. Within the context of one specific slice, packets are further classified into children DiffServ queues which hang from the virtual port. The Traffic Class value in the MPLS header SHOULD be used for queue selection.¶
The Entropy Label usage described in this document is consistent with [RFC6790] as ingress LSRs freely chooses the EL of a given flow, and transit LSRs treat the EL as an opaque set of bits.¶
As per [RFC6790] an ingress LSR that does not support this extension has the SPI bit cleared, and thus does not enable the SLID semantic of the Entropy bits. Hence, SLID-aware transit LSRs will not classify these packets into a slice.¶
From a Segment Routing architecture perspective, this network slice identifier for SR-MPLS is inline with the network slice identifier for SRv6 proposed in [I-D.filsfils-spring-srv6-stateless-slice-id].¶
From an SR-MPLS perspective, using the EL to carry the network slice identifier has multiple benefits:¶
This section describes the usage of a ELC flag to enable packet loss measurements, as described in section 3.1 of [RFC8321], for SR-MPLS networks.¶
TBD¶
This section describes the usage of a ELC flag to detect end to end packet loss.¶
TBD¶