Internet Engineering Task Force Dave Thaler
INTERNET-DRAFT Merit
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Multipath Issues in Unicast and Multicast
<draft-ietf-thaler-multipath-00.txt>
Status of this Memo
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1. Introduction
Various routing protocols, including OSPF [1] and ISIS, allow ''Equal-
Cost Multipath'' routing. At least two vendors also allow equal-cost
multipath usage with RIP [2]. Using equal-cost multipath means that if
multiple equal-cost routes to the same destination exist, they can all
be discovered and used to provide load balancing among redundant paths.
The effects 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. In this memo, we describe current practice, problems with
this, and potential solutions.
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2. Concerns
At least two deployed router implementations allow multipath forwarding.
This is typically done via round-robin, where each packet matching a
given destination route is forwarded using the subsequent next-hop in a
round-robin fashion. This approach does provide a form of load
balancing, but there are several problems with this approach:
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 bandwidth
available, this 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 this 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
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 loops and
duplicates are prevented by constructing a single tree to all receivers
of the same group address. Multicast routing protocols available today
construct use shortest-path trees rooted at some point (either the
source address, or the address of another router known as a Core or
Rendezvous Point) [2]. Hence, the way they insure that duplicates will
not arise is that a given tree must use only a single next-hop towards
the root of the tree.
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3. Requirements
All of the problems outlined in the previous section arise when packets
in the same (unicast or multicast) session are split among multiple
paths. The natural solution is therefore to insure that packets for the
same session (or 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
sessions affected by the addition or deletion of another path.
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 two possible methods for improving the performance of
multipath and then discuss their applicability to unicast and multicast
forwarding.
Modulo-N Hash
To select a next-hop from the list of N next-hops, the router
performs a modulo-N hash over the IP header fields that identify a
session. This has the advantage of being fast, at the expense of
(N-1)/N of all sessions changing paths whenever a next-hop is added
or removed.
Highest Random Weight (HRW)
The router uses a simple pseudo-random number function seeded with
the IP header fields that identify a session, as well as a next-hop
identifier (address or index) to assign a weight to each of the N
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next hops. An analysis of various deterministic weight functions
can be found in [3]. The next-hop receiving the highest weight is
chosen as the next hop. This has the advantage of minimizing the
number of sessions affected by a next-hop addition or deletion, but
is approximately N times as expensive as a modulo-N hash.
The applicability of these two alternatives depends on (at least) two
factors: whether the forwarder maintains per-flow state (or any finer
classification than "all unicast traffic" and "all multicast traffic"),
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, or HRW. Hash functions
such as modulo-N and HRW are better if the forwarder state may be
deleted for any reason during the lifetime of a session 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.), we specifically recommend the use of
HRW.
If no state finer than "all packets" is maintained in 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
session paths, a simple modulo-N hash may be used.
4.1. Unicast Forwarding
Depending on the implementation, unicast forwarding may or may 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 next-hop, and that forwarders without per-flow
state use a modulo-N hash over the source and destination addresses.
4.2. Multicast Forwarding
Multicast forwarding uses a cache of forwarding entries indexed by group
(or group prefix) and source (or source prefix). This means that,
logically, a multicast forwarder always keeps per-"session" state,
although a "session" may be fairly coarse (e.g., traffic from all
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sources to the same destination), depending on the multicast routing
protocol in use. Since per-session 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 [4] 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. References
[1] Moy, J., "OSPF Version 2", RFC 2178, July 1997.
[2] Semeria, C., and T. Maufer, "Introduction to IP Multicast Routing",
draft-ietf-mboned-intro-multicast-03.txt, October 1997.
[3] Thaler, D., and C.V. Ravishankar, "Using Name-Based Mappings to
Increase Hit Rates", IEEE/ACM Transactions on Networking, February
1998.
[4] 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 2117, June 1997.
6. Security Considerations
Security issues are not discussed in this memo.
7. Author's Address
Dave Thaler
Merit Network, Inc
4251 Plymouth Rd., Suite C
Ann Arbor, MI 48105-2785
Phone: +1 313 647 4813
EMail: thalerd@merit.net
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