An MPLS SR OAM option reducing the number of end-to-end path validations

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Table of Contents

1. Introduction

Commodity MPLS isn't topology aware and it doesn't support standardized source routing methods. It is reasonable to validate connectivity and locate faults of MPLS LSPs by detecting and testing all existing LSP combinations between a source and a destination node. The source node originates all MPLS echo requests and evaluates all MPLS echo replies. Operational MPLS OAM implementations were present, when SR MPLS entered standardisation. They continue to work reliably in many cases. MPLS domains with a high number of equal cost paths between source and destination nodes push the detection capabilities of commodity MPLS OAM to the limit. So far, modes of MPLS OAM operation adding Segment Routing functionality to deal with limitations of commodity MPLS OAM have not been published within IETF.¶

This draft assumes readers to be aware of MPLS OAM functionality as specified by RFC 8029 [RFC8029] and RFC 8287 [RFC8287]. The function described in the following works for Shortest Path First Paths or Label stacks based on MPLS Node-SID and MPLS Adj-SIDs (if the latter are distributed by Interior Gateway Protocols).¶

Networks supporting a high number of equivalent cost paths between source and destination nodes require a high number of completed MPLS path validations. Consider a network with Multiple equal cost paths, as shown in figure 1.¶

          +-R120-+
         /        \
        8          12
       /            \
   R110--8--R121--4--R130
    /                  \
   4  numbers indicate  4
  /   parallel links     \
RS                        RD
  \    symmetric to      /
   4...upper network ...4

Figure 1: Multiple equal cost path example network.¶

The total number of MPLS LSP combinations between nodes RS and RD is multiplicative by the number of (equal cost, so to say) links per hop. That results in a maximum of 4096=2*4*(8*12+8*4)*4 path combinations which a commodity MPLS may try to validate. Assume node RS to start an MPLS traceroute to node RD, containing a Multipath Data Sub-TLV requesting Multipath information for 32 IP-addresses. By Equal Cost Multipath routing (ECMP, [RFC2991]) traffic of likely 16 of these IP-addresses is forwarded via R110 as next hop (the other 16 addresses are assumed to be forwarded along the symmetric and equal cost paths in the lower half, which are omitted in the figure for brevity). R110 can be expected to respond by an MPLS echo reply indicating prefixes to address each of the 4 equal cost (sub-)paths between RS and R110.¶

R110 is able to forward traffic addressed by these 16 IP addresses via 16 equal cost paths. There's a fairly high probability that this will not be possible, as some of R110's availble paths to forward traffic to RD will receive traffic of two or even three MPLS echo request destination IP addresses resulting in an MPLS Echo request being sent from RS to R110 and ahead, while other equal cost paths of R110 receive no traffic at all. The MPLS Echo Reply returned to RS will indicate that. A commodity solution is, to start an additional MPLS traceroute from RS with another 32 destination IP-addresses. This may help to then enable forwarding of MPLS Echo requests along all of R110's paths to RD via R120 and R121, respectively. With bad luck, R110 will forward only 14 or 15 addresses via R120. R120 forwards MPLS Echo requests along 12 equal cost paths to RD. Then again, there's a fair chance that more destination IP-addresses are required to forward at least one MPLS echo request along all of R120 equal cost paths to RD. Each new MPLS Echo Request containing additional IP destination addresses requires completion of the MPLS Echo-Request / Reply dialogue starting from RS to at least all routers along the path to R120.¶

In the example, roughly only a fourth of the addresses whose forwarding is validated starting from node RS will be routed via R120. ECMP load balancing "filters away" 75% of MPLS Echo requests carrying the destination IP-addresses whose forwarding is to be determined. If MPLS Echo requests carrying a full set of 32 destination IP-addresses were reaching R120, the probability of being unable to forward at least one MPLS Echo request to each outgoing interface (or path, respectively) at R110 destined to node RD was rather small.¶

The reason for completing all MPLS Echo Request / Reply dialogues along the path between RS and R120 is figuring out, which destination IP-addresses are routed from R110 to R120 to be available at the latter for local traffic forwarding along paths to RD which can't be addressed otherwise. RFC 8029 section 4.1 'Dealing with Equal-Cost Multipath (ECMP)' concludes, that 'full coverage may not be possible' [RFC8029].¶

Segment Routing (SR) allows node RS to forward MPLS Echo Request packets with up to, e.g., 32 IP addresses to every node which RS detects on a path to node RD. Doing so reduces the number of local router path options to be checked to no more than the sum of the interfaces belonging to one of the ECMP routes between nodes RS and RD. In the case of the example network above, this sum is 2*(4+8+8+12+4+4)=80 different local router interfaces of routers RS, R110, R120, R121 and R130. That means, that around 2% of the messages and MPLS Label Switched Path checks required with commodity MPLS traceroute implementations are sufficient to validate all local forwarding options for paths from RS to RD (note that the calculation isn't exact, it rather indicates the order of magnitude). The commodity MPLS OAM implementations are neither broken nor not working. SR allows an additional router local MPLS OAM method to validate high numbers of ECMP routes reliably and fast. The method proposed here reduces the number of MPLS Echo-Request / -Reply dialogues to be stored and completed by the origing of the path validation and it reduces the number of MPLS Echo-Request / -Reply processing at intermediate nodes.¶

The functions specified by this document do not require changes in the MPLS OAM protocol as specified by [RFC8029] and [RFC8287].¶

2. MPLS OAM adding MPLS SR mechanisms

MPLS Segment Routing (SR) provides each node of an MPLS SR domain with this domain's MPLS Node-SID topology [RFC8402]. The SR source routing feature allows to forward packets to each individual node within a SR domain. Combining topology awareness and source routing allows complete validation of all local router shortest path forwarding options from an RS node to an RD node in a domain supporting ECMP.¶

Suppose SR to be deployed in the case of the example network and digits following the letter "R" to indicate the corresponding Node-SIDs. Assume "mixed operation" of commodity MPLS OAM and the option applying SR. RS starts a commodity MPLS Echo request to R110. After having received an MPLS Echo reply from R110 indicating local paths of R110 on which none of the packets with the remaing 16 IP addresses will be forwarded, RS creates an MPLS Echo Request which transports the original 32 IP addresses to R110. To do so, an additional top-Segment is pushed carrying the R110 Node-SID, 110. The message below this additional segment is coded as a standard RFC8287 MPLS Echo request. Two things are special: the TTL of the MPLS header containing the Node SID of RD is always set to 1. Further, a seperate sequence number series needs to be started to distinguish the starting point of this SR using MPLS OAM sequence. Coding space for MPLS OAM Sender's Handle and Sequence Number offer sufficient coding space [RFC8029]. If PHP is active, the R110 Node-SID is implicitly present only on the link to a neighboring node. Still packets with all 32 IP-destination addresses are forwarded to R110. The chances to address all of the 16 ECMP paths of R110 to RD with the originally configured 32 IP-addresses increase. The same method is repeated for R120. Now the top Segment picked by node RS is the Node-SID of R120, again with a separate Sender's Handle and Sequence Number combination. Note, that the MPLS Echo request destined to R120 doesn't require execution of MPLS OAM functions in R110. That latter node simply forwards the packet to R120. Also R120 receives 32 IP-addresses (which is a significant increase as compared to commodity MPLS OAM).¶

As a result, the MPLS Echo reply tables maintained by RS likely indicate several forwarding masks correlated to the same IP address range (discerned by the intermediate node receiving an MPLS Echo request with top Segment TTL=1). For every path at an indermediate node, to which the latter can't foward an MPLS Echo request due to the limited number of available IP-addresses, a suitable SR top segement is added for an additional next MPLS Echo request of node RS. This in the end allows to circumvent the IP-address filtering effect caused by ECMP.¶

Being able to forward a "complete" set of IP addresses to any interface along an end-to-end path is helpful in locating errors. Different MPLS OAM addressing options also offer more possibilities to test and unambiguosly locate a failed sub-path.¶

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Node-SID of the node whose forwarding information is requested |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                 Sender node MPLS Echo request                 +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Node-SID of node preceding the node whose fwd info is requested|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Adj-SID corresp. to inc-IF of node whose fwd info is requested |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                 Sender node MPLS Echo request                 +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3. IANA Considerations

This memo includes no request to IANA.¶

4. Security Considerations

This document does not introduce new functionality. It combines Segment Routing functions with those of MPLS OAM. The related security sections apply, see [RFC8029] and [RFC8402].¶

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