RFC2991 - Multipath Issues in Unicast and Multicast Next-Hop Selection

Network Working Group D. Thaler

Request for Comments: 2991 Microsoft

Category: Informational C. Hopps

NextHop Technologies

November 2000

Multipath Issues in Unicast and Multicast Next-Hop Selection

Status of this Memo

This memo provides information for the Internet community. It does

not specify an Internet standard of any kind. Distribution of this

memo is unlimited.

Abstract

Various routing protocols, including Open Shortest Path First (OSPF)

and Intermediate System to Intermediate System (ISIS), eXPlicitly

allow "Equal-Cost Multipath" (ECMP) routing. Some router

implementations also allow equal-cost multipath usage with RIP and

other routing protocols. 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.

1. IntrodUCtion

Various routing protocols, including OSPF 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 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 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.

Packet reordering causes TCP to believe that loss has taken

place when packets with higher sequence numbers arrive before

an earlier one. When three or more packets are received before

a "late" packet, TCP enters a mode called "fast-retransmit" [6]

which consumes extra bandwidth (which could potentially cause

more loss, decreasing throughput) as it attempts to

unnecessarily retransmit the delayed packet(s). Hence,

reordering can be detrimental to network performance.

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

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

In the remainder of this document, we will use the term "flow" to

represent the granularity at which the router keeps state (if at all)

for classes of traffic. The exact definition of a flow may depend on

the actual implementation. For example, a flow might be identified

solely by destination address, or it might be identified by (source

address, destination address, protocol id) triplet. Hence "flow" is

not necessarily synonymous with the term "microflow" as used in RFC

2474 [7], which also includes port numbers. Indeed, including

transport-layer information in the next-hop selection process can

actually be problematic. For example, if packets are fragmented, the

transport-layer information may not be available in every packet.

Furthermore, having the choice of path depend on transport-layer

fields may negate the benefit of caching information such as MTU for

use in subsequent connections between the same endpoints.

All of the problems outlined in the previous section arise when

packets in the same unicast or multicast "flow" 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). Since a higher number of routes

may actually be used for forwarding when multipath is in use,

the potential for packet reordering and packet loss due to

route flaps can be much greater than when not using multipath.

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 should be small. 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.

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 over the

packet header fields that identify the flow. The N next-hops

have been assigned unique regions in the hash function's output

space. By comparing the hash value against region boundaries

the router can determine which region the hash value 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. For ECMP

hash-threshold's lookup can be done with a simple division

(hash_value / fixed_region_size). 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 computes a key for EACH next-hop by performing a

hash over the packet header fields that identify the flow, as

well as over the address of the next-hop. The router then

chooses the next-hop with the highest resulting key value [4].

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.

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.

Some routers may maintain per-flow state for reasons other than for

supporting multipath. For example, routers typically keep per-flow

state for multicast flows so that they can maintain the list of

interfaces to which packets in the flow should be copied.

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 compared with normal forwarding to a single next-hop,

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 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 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.

One issue that is worth mentioning, however, is that when next-hop

selection is predictable, an attacker can synthesize traffic that

will all hash the same, making it possible to launch a denial-of-

service attack that overloads a particular path. Since a special

case of this is when the same (single) next-hop is always selected,

such an attack is easiest when multipath is not being used.

Introducing multipath routing can make such an attack more difficult;

the more unpredictable the hash is, the harder it becomes to conduct

a denial-of-service attack against any single link.

8. References

[1] Moy, J., "OSPF Version 2", STD 54, RFC2328, April 1998.

[2] Maufer, T., "Deploying IP Multicast in the Enterprise",

Prentice-Hall, 1998.

[3] Hopps, C., "Analysis of an Equal-Cost Multi-Path Algorithm", RFC

2992, November 2000.

[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", RFC2362, June 1998.

[6] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion Control",

RFC2581, April 1999.

[7] Nichols, K., Blake, S., Baker, F. and D. Black., "Definition of

the Differentiated Services Field (DS Field) in the IPv4 and

IPv6 Headers", RFC2474, December 1998.

9. Authors' Addresses

Dave Thaler

Microsoft

One Microsoft Way

Redmond, WA 98052

Phone: +1 425 703 8835

EMail: dthaler@dthaler.microsoft.com

Christian E. Hopps

NextHop Technologies, Inc.

517 W. William Street

Ann Arbor, MI 48103-4943

U.S.A

Phone: +1 734 936 0291

EMail: chopps@nexthop.com

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