Deterministic Upstream Neighbor Selection for PIM Joins

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Table of Contents

1. Introduction

In a densely interconnected network, there may be many equal-cost paths to a given source or RP. RFC7761 is silent on the issue of how to choose among these, just indicating that RPF_interface(S) and RPF_interface(RP) have a single answer. If different leaf routers make different choices, then traffic can flow over extra paths.¶

This document introduces two mechanisms: one for two-tier networks and one for arbitrary multi-tier networks, to allow routers to make the same decision of which neighbor to use in an ECMP scenario. This eliminates undesired redundant traffic flow.¶

2. Conventions and Definitions

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.¶

3. Hash Algorithm

In this document, the hash algorithm used is Bob Jenkins' one-at-a-time hash. This is a very high quality, but fast hash function. A suite of related hash functions was originally published in an article in Dr. Dobbs' Journal [Jenkins-DDJ]. The "one at a time" hash used in this document [Jenkins-Hash] is an update to that journal article that Jenkins later posted on his web site. This hash function is defined on sequences of octets; it is performed across all of the addresses given in network byte order.¶

Pseudocode like hash( address1, address2, address3 ) conceptually lays out these addresses adjacent to each other in memory in network byte order, and performs a single hash operation across all 12 octets.¶

+---+---+---+---+---+---+---+---+---+---+---+---+
|    address1   |    address2   |    address3   |
+---+---+---+---+---+---+---+---+---+---+---+---+

Similarly, when there are two IPv6 addresses and a router ID, we conceptually lay these out identically, simply with larger addresses, and perform the hash operation across all 36 octets.¶

+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|                            address1                           |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|                            address2                           |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   router ID   |
+---+---+---+---+

When the hash involves a color, that 32-bit value in network byte order takes the place of the 32-bit router ID.¶

4. Deterministic Selection by Router-ID

We use the [RFC6395] Hello Option to allow multiple routers to hash a given (S,G) join to the same RPF neighbor. The procedure consists of two phases: first, we compute hash( S, G, routerId ) for each eligible RPF neighbor, and select the highest hash value among this list. If there are multiple entries with the highest hash value, we re-hash among this sub-list using hash( S, G, local-information ), and use the highest single resulting hash value. If multiple hops still hash to the same value, we simply take the first one in the list. This results in no coordination between nodes, since each node may have a different order for the list.¶

The local-information is a value local to the router that can be influenced by the deployment to have the same result between multiple peers - e.g., it could be an interface name, and the deployment on multiple routers uses the same interface to connect to the same peer. It could also be a locally-configured value on each interface, which results in higher configuration overhead but more deployment flexibility.¶

def viaMultipathRouterId( source, group, vias ):
    bestHash = 0
    bestVias = []
    for via in vias:
        routerId = getRouterId( via )
        curHash = calculateHash( source, group, routerId )
        if curHash > bestHash:
            bestVias = [ via ]
            bestHash = curHash
        elif curHash == bestHash:
            bestVias.append( via )
    if len( bestVias ) == 1:
        return bestVias[ 0 ]
    bestHash = 0
    bestVia = None
    for via in bestVias:
        local_information = getLocalInformation( via )
        curHash = calculateHash( source, group, local_information )
        if curHash > bestHash:
            bestVia = via
            bestHash = curHash

    return bestVia

5. Hello Option to Exchange Color

We describe a Hello Option to exchange "Color", an abstract notion of grouping of nodes. For example, in a 3-tier network, the routers in the middle tier could be colored by the spine to which they connect in the top tier. In this way, the color value presented to the leaf routers by the middle tier is a proxy for the routers in the top tier.¶

This Hello option should only be advertised "downwards" towards the lower levels of the tree.¶

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Type = TBD          |           Length = 4          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             Color                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

6. Deterministic Selection by Color

We use the above Hello Option to add an initial round of hashing, falling back to the algorithm in Figure 1 to break ties. The eligible RPF neighbors are reduced to those that have advertised a color using the above Hello option. The first round is to calculate hash( S, G, color ) for each eligible RPF neighbor, and select the highest hash value among this list. If there are multiple entries with the highest hash value, we re-hash among this sub-list with viaMultipathRouterId defined above.¶

def viaMultipathColor( source, group, vias ):
    bestHash = 0
    bestVias = []
    for via in vias:
        color = getNeighborColor( via )
        if color is None:
            continue
        curHash = calculateHash( source, group, color )
        if curHash > bestHash:
            bestVias = [ via ]
            bestHash = curHash
        elif curHash == bestHash:
            bestVias.append( via )
    if len( bestVias ) == 1:
        return bestVias[ 0 ]
    return viaMultipathRouterId( source, group, bestVias )

7. Comparison with RFC6754

[RFC6754], "ECMP Redirect", defines a mechanism that allows an upstream router to give an explicit RPF neighbor to a downstream, and suggest that the downstream start sending its Join messages to that different upstream. We compare the mechanism in this document with that one.¶

Operation

In ECMP Redirect, the upstream router responds to a Join or notices traffic flowing and makes a decision that the traffic should be taking a different path. With this mechanism, each downstream router makes a consistent decision so no correction is necessary.¶

Message Exchange

In ECMP Redirect, an explicit Redirect message is sent, up to one message for each (S,G) for each peer. With this mechanism, the messages exchanged are limited to options in the Hello exchange.¶

Dynamic Behavior

In ECMP Redirect, traffic flow can be disrupted by the change from one upstream neighbor to another. With this mechanism, the initial choice is stable and does not change.¶

Complete Information

In ECMP Redirect, each upstream router must have complete information about what upstream router is appropriate for what stream. With this mechanism, color information is distributed in Hello messages and does not require complete knowledge. # Security Considerations¶

TODO Security¶

8. IANA Considerations

IANA is requested to allocate a value from the PIM-Hello Options registry as shown:¶

9. References

Appendix A. Arista Networks Compatibility

This non-normative appendix describes the Arista Proprietary Color option, for the benefit of compatibility with the deployed base. A standards-compliant implementation SHOULD NOT emit or parse these options by default, but MAY have a configuration option to emit and parse these options on a given interface for interoperability.¶

A pair of PIM Hello options is required for compatibility with the deployed base of Arista EOS. Both types are allocated from the "Private Use" reserved range. The first option, with type 65001, only serves to indicate with a "magic number" that the type 65002 option is indeed the Arista Proprietary Color option (as opposed to some other Private Use).¶

These option formats are shown below.¶

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Type = 65001         |           Length = 4          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           4028514875                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

By including this PIM Hello option, with type 65001 and 32-bit value 4028514875, you indicate that the rest of the PIM Hello options that you are including are Arista-proprietary.¶

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Type = 65002         |           Length = 4          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             Color                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Arista Proprietary Color option is identical to the option described in figure Figure 2, except for the Type field. It is only recognized if the Arista Proprietary Hello options are enabled by the option above.¶

+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
|        address1       |        address2       |  color(little-endian) |
+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+

Appendix B. Cisco Systems Compatibility

Cisco has, independently of this draft, implemented hashing based on Router-ID as discussed in Section 4, except for a few differences listed here.¶

The hash algorithm used is the RP hash defined in [RFC7761] section 4.7.2. Using this hash, the hash value is calculated by doing hash(router-id, 32, hash(group, 32, source)). In case multiple entries hash to the same value, the re-hash is using the interface ID announced in the [RFC6395] Hello Option rather than local information.¶

Hashing based on color is not implemented.¶

Appendix C. Sample Hash Values

Hashing with Router-ID:¶

hash( 192.0.0.2, 224.1.1.1, 10.0.0.1 ) = 361722995
hash( 192.0.0.2, 224.1.1.1, 10.0.0.2 ) = 4027394415
hash( 192.0.0.2, 224.1.1.1, 10.0.0.3 ) = 670832976

In this case, the neighbor with Router-ID 10.0.0.2 would be chosen.¶

Hashing with Color, Arista-compatible:¶

hash( 192.0.0.2, 224.1.1.1, 10 ) = 1271947512
hash( 192.0.0.2, 224.1.1.1, 20 ) = 3140394629
hash( 192.0.0.2, 224.1.1.1, 30 ) = 3675908571

In this case, the neighbor with color 30 would be chosen.¶

Hashing with Color, RFC-compatible:¶

hash( 192.0.0.2, 224.1.1.1, 10 ) = 3358313248
hash( 192.0.0.2, 224.1.1.1, 20 ) = 2756903791
hash( 192.0.0.2, 224.1.1.1, 30 ) = 2580115048

In this case, the neighbor with color 10 would be chosen.¶

Appendix D. Change history

This section is to be removed before publishing as an RFC.¶

Acknowledgments

Thanks to Kalpesh Suthar of Juniper Networks for pointing out that the sample hash values in draft-fenner-pim-deterministic-ecmp-00 were calculated incorrectly.¶

Authors' Addresses