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
<draft-thaler-multipath-03.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with all
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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
solutions.
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
random:
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
throughputs.
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.
Debugging
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-
hop.
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
forwarding.
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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.
Hash-Threshold
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
1998.
[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
Microsoft
One Microsoft Way
Redmond, WA 98052
Phone: +1 425 703 8835
EMail: dthaler@microsoft.com
Christian E. Hopps
Merit Network
4251 Plymouth Road, Suite C.
Ann Arbor, MI 48105
Phone: +1 734 936 0291
EMail: chopps@merit.edu
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FITNESS FOR A PARTICULAR PURPOSE."
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