Internet Engineering Task Force                               D. Thaler
INTERNET-DRAFT                                                Microsoft
Expires September 1999                                         C. Hopps
                                                          Merit Network
                                                          14 April 1999

               Multipath Issues in Unicast and Multicast

Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

This document is an Internet Draft.  Internet Drafts are working
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Copyright Notice

Copyright (C) The Internet Society (1999).  All Rights Reserved.

1.  Introduction

Various routing protocols, including OSPF [1] and ISIS, explicitly allow
"Equal-Cost Multipath" routing.  Some router implementations also allow
equal-cost multipath usage with RIP and other routing protocols.  Using
equal-cost multipath means that if multiple equal-cost routes to the

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same destination exist, they can be discovered and used to provide load
balancing among redundant paths.

The effect of multipath routing on a forwarder is that the forwarder
potentially has several next-hops for any given destination and must use
some method to choose which next-hop should be used for a given data
packet.  This memo summarizes current practices, problems, and

2.  Concerns

Several router implementations allow multipath forwarding.  This is
sometimes done naively via round-robin, where each packet matching a
given destination route is forwarded using the subsequent next-hop, in a
round-robin fashion.  This does provide a form of load balancing, but
there are several problems with approaches such as round-robin or

Variable Path MTU
     Since each of the redundant paths may have a different MTU, this
     means that the overall path MTU can change on a packet-by-packet
     basis, negating the usefulness of path MTU discovery.

Variable Bandwidth
     Since each of the redundant paths may have a different amount of
     bandwidth available, bandwidth may also change on a packet-by-
     packet basis.  Rate-adaptive protocols such as TCP are designed to
     optimize their performance to adapt to the available bandwidth.
     Varying the bandwidth on a packet-by-packet basis causes problems
     with TCP's congestion control mechanisms, resulting in much lower

Variable Latencies
     Since each of the redundant paths may have a different latency
     involved, having packets take separate paths can cause packets to
     always arrive out of order, increasing delivery latency and
     buffering requirements.

     Common debugging utilities such as ping and traceroute are much

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     less reliable in the presence of multiple paths and may even
     present completely wrong results.

In multicast routing, the problem with multiple paths is that multicast
routing protocols prevent loops and duplicates by constructing a single
tree to all receivers of the same group address.  Multicast routing
protocols deployed today (DVMRP, PIM-DM, PIM-SM) [2] construct shortest-
path trees rooted at either the source, or another router known as a
Core or Rendezvous Point.  Hence, the way they ensure that duplicates
will not arise is that a given tree must use only a single next-hop
towards the root of the tree.

3.  Requirements

All of the problems outlined in the previous section arise when packets
in the same unicast or multicast "flow" (or session) are split among
multiple paths.  The natural solution is therefore to ensure that
packets for the same flow always use the same path.

Two additional features are desirable:

Minimal disruption
     When multipath is used, meaning that multiple routes contribute
     valid next-hops, the chances are higher of routes being added and
     deleted from consideration than when only the "best" route is used
     (in which case metric changes in alternate routes have no effect on
     traffic paths).  Hence, it is desirable to minimize the number of
     active flows affected by the addition or deletion of another next-

Fast implementation
     The amount of additional computation required to forward a packet
     must be as small as possible.  For example, when doing round-robin,
     this computation might consist of incrementing (modulo the number
     of next-hops) a next-hop index.

4.  Solutions

We now provide three possible methods for improving the performance of
multipath and then discuss their applicability to unicast and multicast

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Modulo-N Hash
     To select a next-hop from the list of N next-hops, the router
     performs a modulo-N hash over the packet header fields that
     identify a flow.  This has the advantage of being fast, at the
     expense of (N-1)/N of all flows changing paths whenever a next-hop
     is added or removed.

     The router first selects a key by performing a hash (e.g., modulo-K
     where K is large, or CRC16) over the packet header fields that
     identify the flow.  The N next-hops have been assigned unique
     regions in the key space. By comparing the key against region
     boundaries the router can determine which region the key belongs to
     and thus which next-hop to use.  This method has the advantage of
     only affecting flows near the region boundaries (or thresholds)
     when next-hops are added or removed.  Hash-threshold's lookup can
     be done in software using a binary search yielding O(logN), or in
     hardware in parallel for O(1).  When a next-hop is added or
     removed, between 1/4 and 1/2 of all flows change paths. An analysis
     of this method can be found in [3].

Highest Random Weight (HRW)
     The router uses a simple pseudo-random number function seeded with
     the packet header fields that identify a flow, as well as a next-
     hop identifier (address or index), to assign a weight to each of
     the N next-hops.  The next-hop receiving the highest weight is
     chosen as the next-hop.  This has the advantage of minimizing the
     number of flows affected by a next-hop addition or deletion (only
     1/N of them), but is approximately N times as expensive as a
     modulo-N hash.  An analysis of various deterministic weight
     functions can be found in [4].

The applicability of these three alternatives depends on (at least) two
factors: whether the forwarder maintains per-flow state, and how
precious CPU is to a multipath forwarder.

If per-flow state is maintained in a multipath forwarder, then
computation of the next-hop can be done by the router at state creation
time.  This entails no additional computations at packet forwarding
time, since the next-hop is precomputed.  In this case, any method can
be used, including round-robin, random, modulo-N, hash-threshold or HRW.
Hash functions such as modulo-N, hash-threshold and HRW are better if
the forwarder state may be deleted for any reason during the lifetime of

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a flow since subsequent next-hop computations by the router will always
select the same path.  This also improves the usefulness of debugging
utilities such as traceroute.  Finally, to maximize the stability of
paths (and hence the usefulness of traceroute, etc.), the use of HRW is
recommended over the other methods mentioned herein.

If per-flow state is not maintained by the forwarder, then using
multiple next-hops requires that the next-hop be calculated at packet
arrival time.  When CPU is more precious than stability of flow paths,
hash-threshold is recommended over the other methods mentioned herein.

4.1.  Unicast Forwarding

Depending on the implementation, unicast forwarding may or may not keep
per-flow state.  We recommend that where forwarder implementations keep
flow state, routers should use HRW at state creation time (and next-hop
deletion time) to select the next-hop, and that forwarders without per-
flow state use hash-threshold.

4.2.  Multicast Forwarding

Today's multicast forwarding engines use a cache of forwarding entries
indexed by group (or group prefix) and source (or source prefix).  This
means that today's multicast forwarder's always keep per-flow state,
although for some multicast routing protocols, the "flow" may be fairly
coarse (e.g., traffic from all sources to the same destination).  Since
per-flow state is kept by the forwarder, it is recommended that the
router always use HRW to select the next-hop.

Routers using explicit-joining protocols such as PIM-SM [5] should thus
use the multipath information when determining to which neighbor a join
message should be sent.  For example, when multiple next-hops exist for
a given Rendezvous Point (RP) toward which a (*,G) Join should be sent,
it is recommended that HRW be used to select the next-hop to use for
each group.

5.  Applicability

The algorithms discussed above (except round-robin) all rely on some
form of hash function.  Equal flow distribution is achieved when the
hash function is uniformly distributed.  Since the commonly used hash
functions only become uniformly distributed when the number of inputs is

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relatively large, these algorithms are more applicable to routers used
to route many flows, than in, for example, a small business setting.

6.  Redundant Parallel Links

A related problem occurs when multiple parallel links are used between
the same pair of routers.  A common solution is to bundle the two links
together into a "super"-link when is then used for routing.  For
multicast forwarding, this results in the two links being reduced to a
single next-hop (over the combined link) which can be used to prevent
duplicates.  When a unicast or multicast packet is queued to the
combined link, some method, such as those discussed earlier, is still
required to determine the physical link on which to transmit the packet.
If the parallel links are identical, then most of the concerns discussed
in this document are avoided with the combined link.  The exception is
packet reordering, which can still occur with round-robin, adversely
affecting TCP.

7.  Security Considerations

This document discusses issues with various methods of choosing a next-
hop from among multiple valid next-hops.  As such, it does not directly
impact the security of the Internet infrastructure or its applications.

8.  References

[1]  Moy, J., "OSPF Version 2", RFC 2178, July 1997.

[2]  Maufer, T., "Deploying IP Multicast in the Enterprise", Prentice-
     Hall, 1998.

[3]  Hopps, C., "Analysis of an Equal-Cost Multi-Path Algorithm",,
     draft-hopps-ecmp-algo-analysis-03.txt, April 1999.

[4]  Thaler, D., and C.V. Ravishankar, "Using Name-Based Mappings to
     Increase Hit Rates", IEEE/ACM Transactions on Networking, February

[5]  Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering, S.,
     Handley, M., Jacobson, V., Liu, C., Sharma, P., and L. Wei,
     "Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol
     Specification", RFC 2362, June 1998.

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9.  Authors' Addresses

    Dave Thaler
    One Microsoft Way
    Redmond, WA  98052
    Phone: +1 425 703 8835

    Christian E. Hopps
    Merit Network
    4251 Plymouth Road, Suite C.
    Ann Arbor, MI  48105
    Phone: +1 734 936 0291

10.  Full Copyright Statement

Copyright (C) The Internet Society (1999).  All Rights Reserved.

This document and translations of it may be copied and furnished to
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The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an "AS

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