Use of Multipath with MPLS-TP and MPLS
draft-villamizar-mpls-multipath-use-00

Abstract

Many MPLS implementations have supported multipath techniques and many MPLS deployments have used multipath techniques, particularly in very high bandwidth applications, such as provider IP/MPLS core networks. MPLS-TP has strongly discouraged the use of multipath techniques. Some degradation of MPLS-TP OAM performance cannot be avoided when operating over many types of multipath implementations.

Using MPLS Entropy label, MPLS can LSP can be carried over multipath links while also providing a fully MPLS-TP compliant server layer for MPLS-TP LSP. This document describes the means of supporting MPLS as a server layer for MPLS-TP. The use of MPLS-TP LSP as a server layer for MPLS LSP is also discussed.

Status of This Memo

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Table of Contents

• 1. Introduction
• 2. Definitions
• 3. MPLS as a Server Layer for MPLS-TP
• 4. MPLS-TP as a Server Layer for MPLS
• 5. IANA Considerations
• 6. Security Considerations
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Author's Address

1. Introduction

Today the requirement to handle large aggregations of traffic, can be handled by a number of techniques which we will collectively call multipath. Multipath applied to parallel links between the same set of nodes includes Ethernet Link Aggregation [IEEE-802.1AX], link bundling [RFC4201], or other aggregation techniques some of which may be vendor specific. Multipath applied to diverse paths rather than parallel links includes Equal Cost MultiPath (ECMP) as applied to OSPF, ISIS, or BGP, and equal cost LSP. Some vendors support load split across equal cost MPLS LSP where the load is split proportionally to the reserved bandwidth of the set of LSP.

RFC 5654 requirement 33 requires the capability to carry a client MPLS-TP or MPLS layer over a server MPLS-TP or MPLS layer [RFC5654]. This is possible in all cases with one exception. When an MPLS LSP exceeds the capacity of any single component link it may be carried by a network using multipath techniques, but may not be carried by an MPLS-TP LSP due to the inherent MPLS-TP capacity limitation imposed by MPLS-TP OAM packet ordering constraints.

The term composite link is more general than terms such as link aggregation (which is specific to Ethernet) or ECMP (which implies equal cost paths within a routing protocol). The use of the term composite link here is consistent with the broad definition in [ITU-T.G.800]. Multipath is very similar to composite link as defined by ITU, but specifically excludes inverse multiplexing.

2. Definitions

Multipath

The term multipath includes all techniques in which

1. Traffic can take more than one path from one node to a destination.
2. Individual packets take one path only. Packets are not subdivided and reassembled at the receiving end.
3. Packets are not resequenced at the receiving end.
4. The paths may be:
   1. parallel links between two nodes, or
   2. may be specific paths across a network to a destination node, or
   3. may be links or paths to an intermediate node used to reach a common destination.

Link Bundle

Link bundling is a multipath technique specific to MPLS [RFC4201]. Link bundling supports two modes of operations. Either an LSP can be placed on one component link of a link bundle, or an LSP can be load split across all members of the bundle. There is no signaling defined which allows a per LSP preference regarding load split, therefore whether to load split is generally configured per bundle and applied to all LSP across the bundle.

Link Aggregation

The term "link aggregation" generally refers to Ethernet Link Aggregation [IEEE-802.1AX] as defined by the IEEE. Ethernet Link Aggregation defines a Link Aggregation Control Protocol (LACP) which coordinates inclusion of LAG members in the LAG.

Link Aggregation Group (LAG)

A group of physical Ethernet interfaces that are treated as a logical link when using Ethernet Link Aggregation is referred to as a Link Aggregation Group (LAG).

Equal Cost Multipath (ECMP)

Equal Cost Multipath (ECMP) is a specific form of multipath in which the costs of the links or paths must be equal in a given routing protocol. The load may be split equally across all available links (or available paths), or the load may be split proportionally to the capacity of each link (or path).

Loop Free Alternate Paths

"Loop-free alternate paths" (LFA) are defined in RFC 5714, Section 5.2 [RFC5714] as follows. "Such a path exists when a direct neighbor of the router adjacent to the failure has a path to the destination that can be guaranteed not to traverse the failure." Further detail can be found in [RFC5286]. LFA as defined for IPFRR can be used to load balance by relaxing the equal cost criteria of ECMP, though IPFRR defined LFA for use in selecting protection paths. When used with IP, proportional split is generally not used. LFA use in load balancing is implemented by some vendors though it may be rare or non-existent in deployments.

Composite Link

The term Composite Link had been a registered trademark of Avici Systems, but was abandoned in 2007. The term composite link is now defined by the ITU in [ITU-T.G.800]. The ITU definition includes multipath as defined here, plus inverse multiplexing which is explicitly excluded from the definition of multipath.

Inverse Multiplexing

Inverse multiplexing either transmits whole packets and resequences the packets at the receiving end or subdivides packets and reassembles the packets at the receiving end. Inverse multiplexing requires that all packets be handled by a common egress packet processing element and is therefore not useful for very high bandwidth applications.

Component Link

The ITU definition of composite link in [ITU-T.G.800] and the IETF definition of link bundling in [RFC4201] both refer to an individual link in the composite link or link bundle as a component link. The term component link is applicable to all multipath.

LAG Member

Ethernet Link Aggregation as defined in [IEEE-802.1AX] refers to an individual link in a LAG as a LAG member. A LAG member is a component link. An Ethernet LAG is a composite link. IEEE does not use the terms composite link or component link.

load split

Load split, load balance, or load distribution refers to subdividing traffic over a set of component links such that load is fairly evenly distributed over the set of component links and certain packet ordering requirements are met. Some existing techniques better acheive these objectives than others.

A small set of requirements are discussed. These requirements make use of keywords such as MUST and SHOULD as described in [RFC2119].

3. MPLS as a Server Layer for MPLS-TP

MPLS LSP may be used as a server layer for MPLS-TP LSP as long as all MPLS-TP requirements are met, including the requirement that packets within an MPLS-TP LSP are not reordered, including both payload and OAM packets.

Supporting MPLS-TP LSP overa fully MPLS-TP conformant MPLS LSP server layer where the MPLS LSP are making use of multipath, requires special treatment of the MPLS-TP LSP such that those LSP only are not subject to the multipath load slitting. This implies the following brief set of requirements.

MP#1

It MUST be possible to identify MPLS-TP LSP.

MP#2

It SHOULD be possible to completely exclude MPLS-TP LSP from the multipath hash and load split.

MP#3

It SHOULD be possible to insure that an MPLS-TP LSP will not be moved to another component link as a result of a composite link load rebalancing operation.

MP#4

Where an RSVP-TE control plane is used, it MUST be possible for an ingress LSR which is setting up an MPLS-TP or MPLS LSP to determine at CSPF time whether a link or MPLS PSC LSP within the topology can support the MPLS-TP requirements of the LSP.

There is currently no signaling mechanism defined to support requirement MP#1. In the absense of a signaling extension, MPLS-TP can be identified through some form of configuration, such as configuration which provides an MPLS-TP compatible server layer to all LSP arriving on a specific interface or originating from a specific set of ingress LSR. Alternately an MPLS-TP LSP can be created with and Entropy Label Indicator (ELI) and entropy label (EL) below the MPLS-TP label [I-D.ietf-mpls-entropy-label].

Some hardware which exists today can support requirement MP#2. Signaling in the absense of MPLS Entropy Label can make use of link bundling with a specific component for MPLS-TP LSP and link bundling with the all-zeros component for MPLS LSP. This prevents MPLS-TP LSP from being carried within MPLS LSP but does allow the co-existance of MPLS-TP and very large MPLS LSP.

MPLS-TP LSP can be carried as client LSP within an MPLS server LSP if an Entropy Label Indicator (ELI) and entropy label (EL) is added after the server layer LSP label(s) in the label stack, just above the MPLS-TP LSP label entry [I-D.ietf-mpls-entropy-label]. This allows MPLS-TP LSP to be carried as client LSP within MPLS LSP and satisfies requirement MP#2 but requires that MPLS LSR be able to identify MPLS-TP LSP (requirement MP#1).

MPLS-TP traffic can be protected from an degraded performance due to an imperfect load split if the MPLS-TP traffic is given queuing priority (using strict priority and policing or shaping at ingress or locally or weighted queuing locally). This can be accomplished using the Traffic Class field and Diffserv treatment of traffic [RFC5462][RFC2475]. In the event of congestion due to load imbalance, other traffic will suffer as long as there is a minority of MPLS-TP traffic.

If MPLS-TP LSP are carried within MPLS LSP and ELI and EL are used, requirement MP#2 is satisfied, but without a signaling extension, requirement MP#3 is not satisfied if there is a need to rebalance the load on any composite link carrying the MPLS server LSP. Load rebalance is generally needed only when congestion occurs, therefore restricting MPLS-TP to be carried only over MPLS LSP that are known to traverse only links which are expected to be uncongested can satisfy requirement MP#3.

Requirement MP#4 can be supported using administrative attributes. Administrative attributes are defined in [RFC3209]. Some configuration is required to support this.

4. MPLS-TP as a Server Layer for MPLS

Carrying MPLS LSP which are larger than a component link over a MPLS-TP server layer requires that the large MPLS client layer LSP be accommodated by multiple MPLS-TP server layer LSPs. MPLS multipath can be used in the client layer MPLS.

Creating multiple MPLS-TP server layer LSP places a greater ILM scaling burden on the LSR. High bandwidth MPLS cores with a smaller amount of nodes have the greatest tendency to require LSP in excess of component links, therefore the reduction in number of nodes offsets the impact of increasing the number of server layer LSP in parallel. Today, only in cases where deployed LSR ILM are small would this be an issue.

The most significant disadvantage of MPLS-TP as a Server Layer for MPLS is that the use MPLS-TP server layer LSP reduces the efficiency of carrying the MPLS client layer. The service which provides by far the largest offered load in provider networks is Internet, for which the LSP capacity reservations are predictions of expected load. Many of these MPLS LSP may be smaller than component link capacity. Using MPLS-TP as a server layer results in bin packing problems for these smaller LSP. For those LSP that are larger than component link capacity, their capacity are not increments of convenient capacity increments such as 10Gb/s. Using MPLS-TP as an underlying server layer greatly reduces the ability of the client layer MPLS LSP to share capacity. For example, when one MPLS LSP is underutilizing its predicted capacity, the fixed allocation of MPLS-TP to component links may not allow another LSP to exceed its predicted capacity. Using MPLS-TP as a server layer may result in less efficient use of resources may result in a less cost effective network.

No additional requirements beyond MPLS-TP as it is now currently defined are required to support MPLS-TP as a Server Layer for MPLS. It is therefore viable but has some undesirable characteristics discussed above.

5. IANA Considerations

This memo includes no request to IANA.

6. Security Considerations

This document specifies requirements with discussion of framework for solutions using existing MPLS and MPLS-TP mechanisms. The requirements and framework are related to the coexistence of MPLS/GMPLS (without MPLS-TP) when used over a packet network, MPLS-TP, and multipath. The combination of MPLS, MPLS-TP, and multipath does not introduce any new security threats. The security considerations for MPLS/GMPLS and for MPLS-TP are documented in [RFC5920] and [I-D.ietf-mpls-tp-security-framework].

7. References

7.1. Normative References

7.2. Informative References

Author's Address