IP VPN Scaling Considerations
draft-gs-vpn-scaling-00
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| Last updated | 2011-10-24 | ||
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draft-gs-vpn-scaling-00
Internet Engineering Task Force W. George
Internet-Draft Time Warner Cable
Intended status: Informational R. Shakir
Expires: April 26, 2012 Cable and Wireless Worldwide
October 24, 2011
IP VPN Scaling Considerations
draft-gs-vpn-scaling-00
Abstract
This document discusses scaling considerations unique to
implementation of Layer 3 (IP) Virtual Private Networks, discusses a
few best practices, and identifies gaps in the current tools and
techniques which are making it more difficult for operators to cost-
effectively scale and manage their L3VPN deployments.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 26, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Contributing factors to L3VPN scale . . . . . . . . . . . . . 4
2.1. PE-CE routing protocols . . . . . . . . . . . . . . . . . 4
2.2. Multicast . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Network Events . . . . . . . . . . . . . . . . . . . . . . 5
2.4. General Route Scale . . . . . . . . . . . . . . . . . . . 6
3. Best common practices . . . . . . . . . . . . . . . . . . . . 8
3.1. PE-CE routing protocols . . . . . . . . . . . . . . . . . 8
3.2. Multicast . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3. Network Events . . . . . . . . . . . . . . . . . . . . . . 9
3.4. General Route Scale . . . . . . . . . . . . . . . . . . . 10
4. Common problems at scale limits . . . . . . . . . . . . . . . 11
4.1. PE-CE routing protocols . . . . . . . . . . . . . . . . . 11
4.2. Multicast . . . . . . . . . . . . . . . . . . . . . . . . 12
4.3. Network Events . . . . . . . . . . . . . . . . . . . . . . 12
4.4. General Route Scale . . . . . . . . . . . . . . . . . . . 13
5. Known issues and gaps . . . . . . . . . . . . . . . . . . . . 13
5.1. PE-CE routing protocols . . . . . . . . . . . . . . . . . 13
5.2. Multicast . . . . . . . . . . . . . . . . . . . . . . . . 14
5.3. Network Events . . . . . . . . . . . . . . . . . . . . . . 14
5.4. General Route Scale . . . . . . . . . . . . . . . . . . . 14
5.5. Modeling and Capacity planning . . . . . . . . . . . . . . 15
5.6. Performance issues . . . . . . . . . . . . . . . . . . . . 16
6. To-Do list . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
9. Security Considerations . . . . . . . . . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . . 18
10.2. Informative References . . . . . . . . . . . . . . . . . . 18
Appendix A. Additional Stuff . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
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1. Introduction
As IP networking has become more ubitquitous and mature, many
enterprises have begun migration away from legacy point to point or
layer 2 virtual private network (VPN) implementations towards layer 3
VPNs. The VPN implementation as defined by RFC 4364 [RFC4364]
enables flexible and robust implementations of IP VPNs. However, in
practice, it has become clear that it suffers from significant
scaling considerations beyond those discussed in RFC4364. In many
cases, the limits of scale for a given platform are not in sync with
the maximum physical and logical interface density supported by the
platform, such that a platform may be considered "full" long before
the physical slots and ports have all been filled with equipment and
connections. This represents an inefficient use of space and power,
as well as stranded capital assets, which increase the operator's
cost to provide the service as well as the complexity of managing the
platform to ensure proper service levels in a wide variety of
circumstances. While these scaling considerations are somewhat
similar to the scaling concerns experienced in the Global Internet,
those are at best a subset of the overall problem, and may not have a
great deal of overlap between solutions and best practices. The
added complexity and feature set required to support today's
enterprise IP networks drives additional scaling considerations for
large deployments. A common response to concerns about control plane
scale is simply to "throw hardware at the problem" in the form of
ever-increasing amounts of memory and CPU resources. In some cases,
this may be the only solution, but similarly to the concerns
identified in RFC 4984 [RFC4984], there are limits to the growth
curve that can be supported and cost-effectively deployed by a VPN
provider such that their service remains profitable, and therefore it
is necessary to explore the potential for optimization to make the
existing resources stretch further. This document discusses the most
commonly experienced scaling problems, notes best practices to
minimize their impacts on the carrier network and end customers, and
identifies gaps in the current tools and practices.
Generally, router scale can be considered in one of three areas:
forwarding capacity, interface density, and control plane capacity.
This draft will focus almost exclusively on control plane capacity,
because while the others are important considerations for most
operators, they are less affected by the details of how L3VPN is
implemented either by the router vendor or the operator. Interface
density is usually a factor of the forwarding capacity of a given
module or slot as well as physical packaging. In this application,
interface density is interesting from the perspective of its impact
to the control plane - more interfaces means more of all of the
different factors that contribute to control plane load, and the
operator wants to be able to strike a balance between interface
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density and control plane capacity such that neither grows out of
pace with the other.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Contributing factors to L3VPN scale
2.1. PE-CE routing protocols
One of the things that makes IP VPNs so flexible and robust is their
ability to participate in the encapsulated network's routing
protocols, where the customer edge (CE) router has a direct neighbor
relationship with its upstream provider edge (PE) router in order to
exchange routing information about the Virtual Route Forwarding (VRF)
instance that represents the VPN. In many cases, this is managed
through a combination of static routes and BGP neighbors, but IGPs
such as OSPF RFC 4577 [RFC4577] are often supported, because it
enables a more complete integration into an existing enterprise
network design and topology. In some single-vendor implementations,
carriers sometimes support proprietary routing protocols such as
EIGRP [EIGRP]. IGPs may also be chosen due to a belief that they
will respond more rapidly during a failure than BGP will. In
reality, this may not be true due to the fact that VRF routing
information is still carried in MP-BGP from PE to PE, and the PE-CE
routing protocol's characteristics are only locally significant. In
fact, the increased overhead may lead to slower convergence times
than a more standard BGP implementation.
IGPs often translate to a significant increase in overhead due to
their inherent characteristics as link-state routing protocols
requiring full topology databases and flooding of updates to all
participants, and the fact that they invoke additional processes on
the router when compared to simply using BGP (which is already going
to be running on a router using MP-BGP for VPNs). While a router may
be able to scale almost effortlessly with a few thousand routes in a
single IGP plus hundreds of thousands of routes and many neighbors in
BGP, it may be quickly challenged if it is also required to run
multiple instances of an IGP each with a certain number of routes
that must be moved into MP-BGP to be passed to the rest of the VPN
infrastructure. The advent of support for IPv6 within a VPN (6VPE)
[RFC4659] has the potential to make this problem worse, especially in
the case of OSPF, where it now requires both OSPFv2 [RFC4577] and v3
[I-D.ietf-l3vpn-ospfv3-pece] to run as separate instances for the two
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address families.
Another consideration in PE-CE routing protocols is the timers used
for each session. These will be discussed in greater detail in the
best practices section.
2.2. Multicast
Multicast support within a VPN [I-D.ietf-l3vpn-2547bis-mcast] has
become an increasingly popular feature, but comes with its own
scaling considerations. Depending on the application, the frequency
at which multicast state changes within a given VPN (e.g. PIM joins
and prunes) will contribute to the CPU load on the router, and any
instability in the network can potentially increase these as remote
sites flap. In extreme cases, PIM neighborships can be lost during
events, disrupting the flow of multicast traffic.
It should be noted that, in some cases, dynamic action is required by
a PE device to support the transition of flooding of multicast data
from a non-optimal distribution tree (the default MDT in [RFC6037],
or the I-PMSI) onto a more optimal one (a data MDT or S-PMSI). Where
such a transition is required, consideration is required of the
nature of the traffic sourced by an end user of the L3VPN service.
The net result of this consideration is that it becomes increasingly
difficult to reliably gauge the scaling impact of specific end-site
deployments.
*** Author's note, remove before publication: Multicast scaling
considerations are weak throughout this document. We're looking for
contributors who can assist in fleshing this out ***
2.3. Network Events
Network events are an important scaling consideration because they
can have wide-ranging impacts far beyond the individual VRF or even
PE router that experiences the event. At high scale, a seemingly
innocuous event on one router or VRF can trigger secondary impacts
and outages on remote routers elsewhere in the network. Correlating
these events for root cause analysis can be challenging by itself,
and trying to characterize the impacts as they relate to scale in a
way that informs the provider's decisions is even more difficult.
Different types of Network Events that can contribute are: Interface
flaps, hardware and software outages (both planned and unplanned),
externally driven route-churn events (such as those that originate on
an NNI partner's network) and configuration changes.
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2.4. General Route Scale
PE routers in a carrier network can have many different
implementation scenarios. Some carriers implement a dedicated PE
router that is only responsible for carrying VPN routes and therefore
may only carry IGP routes in its global routing table, rather than a
full internet routing table. Others use combined edge routers that
carry full routes plus a complement of customer VPN routes, and some
even place the full internet routing table into one or more VRF
instances. The issue here is that the weight of all of these routes
and paths must be combined when considering the maximum scale of the
router, both in terms of memory footprint and in terms of convergence
times. The addition of an 8-byte RD appended to the IP address to
ensure uniqueness means that each VPN prefix takes up incrementally
more physical space in memory than an equivalent non-VPN route.
Further, the greater number of Address-families running
simultaneously on the same router, the more sensitive it will be to
event-induced churn since each address-family (and VRF) often has its
own independent computation/SPF run. The addition of IPv6 support
within both the global routing table and within a VPN adds yet
another source for routing table bloat. A PE router can be running a
combination of any of the following address-families:
o Global IPv4 unicast
o Global IPv4 Multicast
o VPN IPv4 unicast
o VPN IPv4 multicast
o Global IPv6 unicast
o Global IPv6 multicast
o VPN IPv6 unicast
o VPN IPv6 multicast
On high-scale PE routers, the VPN routing tables are often as large
as or larger than the equivalent global routing table in both number
of routes and number of paths, i.e. if the IPv4 unicast table is
350,000 routes and 1M paths, the IPv4 VPN unicast table may be
400,000 routes and 750,000 paths (**** verify numbers ****). This is
at least partially due to the fact that there are no constraints on
the customer addressing plan within a VPN other than they cannot
conflict within a given VRF, or with any extranet with which the VRF
interconnects. As such, they may not necessarily adhere to any best
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practices to control the deaggregation of the routing table such as
heirarchical addressing, aggregation and summarization of
announcements, and minimum prefix lengths. It's also quite likely
that connected interfaces will be redistributed, and little or no
route filtering may take place. Most PE routers use the absence of a
given VRF instance (or RD/RT filtering) to limit the number of routes
that they must actually carry, but this is sometimes of limited
utility for a couple of reasons. First, it leads to an inconsistent
routing table footprint from one PE router to the next, and it can
change with every new customer turned up on the router. This leads
to non-deterministic performance and scale. Second, many customer
VPNs are so large and have such stringent diversity requirements that
they have a presence on nearly every PE router in a provider's
network, meaning that one cannot rely heavily on statistical
multiplexing to reduce the percentage of VRFs that must be installed
on a specific PE router. In addition, customers may request the use
of BGP multipath *** REFERENCE?*** for faster failover or better load
balancing, which has the net effect of installing more active routes
into the table, rather than simply selecting the single best path.
In addition to such intended behaviour, within many L3VPN networks, a
balance must be struck between complexity in OSS such as provisioning
and inventory systems, and complexity in network deployments. One
such example of this is the assignment of route distinguisher (RD)
attributes. Where it may be possible to assign a single RD per L3VPN
instance, and hence achieve some level of route aggregation on BGP
speakers within the solution, this has some consequences for both
convergence in the VPN (due to BGP convergence being relied upon) and
in its potential to exacerbate geographic distance between PE and
Route-reflector and is therefore undesirable in some circumstances.
In order to avoid this, multiple RDs are then required, which
requires OSS and inventory support to control the namespace. As
such, due to this requirement, often each VRF instance is deployed
with a specific RD - which, whilst achieving the desired convergence
effect, places load on all BGP control-plane elements of the provider
network.
Total supportable route scale on a given PE router will be driven by
multiple different variables, which have a roughly inverse
relationship to one another: Number of VRFs per router, number of
routes per VRF, number of neighbors per VRF. For example, a router
can support a low number of VRFs per router if each VRF has a large
number of routes per VRF and/or a large number of neighbors per VRF.
Conversely, a router can support a relatively high number of VRFs if
each VRF is kept to a much lower number of routes per VRF, and/or
lower numbers of neighbors per VRF. This provides a baseline that
then must be reduced based on the expected level of event-driven
churn, the type of protocol chosen, etc. In short, this is a
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difficult problem from a modeling and capacity planning perspective.
3. Best common practices
3.1. PE-CE routing protocols
***Do we make a recommendation to avoid IGPs altogether, avoid...
unless..., or use, but consider "blah blah blah"?***
Often, those designing VPN solutions attempt to use extremely
aggressive routing protocol timer and keepalive values as a means of
rapid failure detection and reconvergence. This tends to make PE-CE
routing protocols more fragile and increase the load on the PE router
with questionable benefit. This is especially common in scenarios
where the network designer is attempting to replicate native IGP-like
failure detection and reroute capabilities using BGP. In order to
avoid this, the preferred values should be set to something that is
appropriate for large-scale implementations (*** do we want to make a
specific recommendation? ***). Further, because timer and keepalive
values are often negotiated based on the more aggressive neighbor, it
is a good idea to set a minimum acceptable value, so that instead of
being forced to support negotiated timer values that are too
aggressive for the scale that a given PE router is expected to
support, the neighbor session will simply stay down until the remote
end timers are reconfigured to a more acceptable value. This acts as
a safety valve against abuse that can destabilize a router used by
multiple customers. Because aggressive timers may be unavoidable in
certain situations, it may be advisable to track the number of
sessions which are provisioned with aggressive timers vs how many are
using more conservative timers on a per-router basis, so that effort
can be made to balance aggressive and conservative timers on each
router. This will help to prevent "hot-spots" where given a similar
port and VRF density, some routers have significantly higher CPU
usage in steady-state than others.
It is important to realize that while use of aggressive routing
protocol timers is not a scalable way to do fast failure detection,
fast failure detection is still a requirement for many customers.
Because this is becoming such a table-stakes requirement, the
provider must consider other alternatives such as Bidirectional
Forwarding Detection ([RFC5880]), Ethernet OAM 802.1ag [IEEE802.1],
ITU-T &.1731 [Y.1731] LACP 802.3ad [IEEE802.3] and the like. These
extensions often come with their own scaling considerations, but more
and more they are implemented in a distributed fashion so that
instead of affecting the main router CPU like a routing protocol
might, they offload that processing to the linecard CPU, and
therefore can support more aggressive scale. The general philosophy
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is that these lower-layer detection mechanisms should serve as the
primary detection and failure point, with the upper layer routing
protocols only serving as a backstop if the failure is not detected
by the lower level protocols for some period of time.
3.2. Multicast
Multicast BCPs???
3.3. Network Events
While this document suggests that lower layer failure detection
protocols like BFD and Ethernet OAM be more aggressive so that
routing protocol timers can be more conservative, it is still
important to remember that this can generate false positives or
excessive churn that will cascade into a scaling problem at other
parts of the system, so the timers should not automatically be
configured to their minimum supported values. Rather, each
application may be slightly different, and the timers should only be
set as aggressively as necessary to ensure acceptable performance of
the applications in question. It may be appropriate to set limits as
to the number of interfaces per router and per VRF that can use
aggressive, moderate, and conservative interface timers.
Even with timers set as conservatively as the application will allow,
churn is unavoidable. For this reason, it is also a good idea to use
interface-level dampening such as hold-down timers or event dampening
in order to ensure that interfaces that flap too rapidly will not
telegraph that churn into the upper-layer routing protocols any more
than necessary. This helps to ensure that problems are localized to
a single PE or even a single interface, rather than causing
instability and routing churn throughout the VRF and the provider
network.
In addition to interface dampening, it may be advisable to consider
implementing some manner of route flap dampening to assist in
reducing the impact that route churn may have on the SP's network
infrastructure. This is currently fairly uncommon within VPN
environments, and is not without controversy. While it may help with
scaling, it also requires each PE to maintain more state to store and
compute the per-prefix penalty values, which may reduce the benefits
gained by implementing RFD. Further, customers typically expect a
fair amount of transparency in the provider's participation in their
routing instances. Many providers and customers view a VPN or VRF as
a part of the customer's internal network and therefore
compartmentalized so that the customer can only affect their own
routing if they have a problem with excessive route flaps. Further,
if routes are dampened it requires intervention from the SP to clear
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the dampening, which can potentially add to the outage time that a
customer experiences once the issue that triggered the dampening is
resolved. Implementing RFD may even drive the need for a customer-
accessible looking glass, which is far more complex in the VPN space
owing to the requirement to prevent one customer from looking at
another's VRF routes on a common platform.
3.4. General Route Scale
A number of things can be done to improve the general route scaling.
Most BGP sessions can be configured with a similar set of protections
as they would be if they were global Internet eBGP sessions, such as
maximum prefix limits, inbound and outbound prefix filtering, etc.
Prefix filtering is less common within VPNs because it is treated
more like iBGP, where filtering is typically not recommended
(***REF?***), or as noted above, it's part of the customer's network
and therefore not the SP's business/problem to do filtering in an
application that can only break that customer's network. What is
often more important in the case of individual VRFs is to configure
an acceptable maximum number of routes that the VRF is permitted to
carry. This allows the SP to control their exposure to sudden
increases in the memory footprint of the routing table, especially if
a misconfiguration on the CE side leads to significant amounts of
route leakage, such as to suddenly leak a significant amount of the
Global Internet Routing Table into their VRF. However, it can also
be used to enforce the assumptions on number of routes per VRF that
the SP has used to determine what the other max scaling values such
as number of VRFs per router, number of sessions per router, etc.
As noted above, the number of VRFs per router, number of routes per
VRF, and number of sessions per router and per VRF are all inter-
related values in the way that they contribute to overall router
scale. The more of this information is known in advance based on the
design of the customer's network, the more it can be used as input to
the provisioning system to determine the best available PE router on
which to terminate the connections for consistent loading. Since
these values are usually estimates, and considerations like diverse
router terminations may drive a specific choice, this is not by any
means fool-proof, but is a valuable optimization to improve the
density of customers on a given router and maximize the return on
investment for the capacity deployed. It is worth noting, however,
that many SP VPN networks have a different geographic spread than do
their Internet service counterparts, where there will be more POPs
with fewer routers, as it is important to provide more local handoffs
to customers. This may limit the SP's flexibility in terms of homing
locations and router choices, and thus may be of limited value when
controlling scale impacts on individual PE routers.
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*** Discuss incremental SPF, next-hop tracking, SPF timer tuning (By
protocol and AF), prefix prioritization, etc? All of these are
generally thought of as convergence optimizations, and may be
applicable here as a way to both reduce the CPU load and ensure that
behavior is more deterministic, but I'm not sure how much depth we
want to get into here, especially since some are vendor-specific or
FIB-specific optimizations... ***
4. Common problems at scale limits
4.1. PE-CE routing protocols
Two common problems when working on a heavily-loaded system:
CPU cycle constraints, even before the system reaches the point of
scheduler thrashing often lead to one or more routing protocol
neighbor hello drops. If several consecutive drops occur, the remote
neighbor may declare the session dead, which triggers a restart of
the connection and a resync of the routing data. Because this
connection initialization requires dedicated CPU cycles to generate,
receive, acknowledge, and process the updates, it increases the CPU
utilization further, which may trigger additional hello failures and
neighbor resets, resulting in a snowball effect where a relatively
minor event rapidly becomes a major one due to interactions between
multiple scaling limitations. This problem is made worse by
extremely aggressive timer values, because they raise the baseline
CPU load with more frequent hellos and responses, and are more
sensitive to drops caused by increased CPU load. Further, because
failures brought on by loss of hello packets are unlikely to invoke
any graceful restart [RFC4781] machinery that the system may support,
it is unlikely that the session reset will be able to take advantage
of optimizations like only synching the changes that occured while
the session was dead, thus increasing the outage time and the CPU
cycles to get things back into sync.
Another potential issue during times of high-CPU operation is related
to process prioritization. This is applicable in different ways for
both multithreaded and interrupt-driven OS architectures. In each
case, the scheduling algorithm that the router uses to prioritize
different CPU cycle work items and manage the timeslices individual
tasks are given to complete may require significant tuning and
prioritization in order to ensure the desired behavior during high
CPU usage. Improperly tuned or prioritized processes may
significantly delay completion of routing table/update processing
such that it may take an excessive amount of time for the routing
table to converge properly. This issue is further exacerbated if the
VRF instance has a large amount of routes, or is prone to frequent
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event-driven route churn. In some cases, the routing table in a
given VRF may never fully converge, leading to routing loops, traffic
loss, inconsistent latency, and a generally adverse customer
experience.
It worth noting that these items also have a cascade effect on other
routers in the system that participate in a given VRF that is being
affected by this type of scaling issue. Not only is the local PE
router affected, but any upstream Route reflectors, as well as other
PEs, and even CEs participating in this VRF will see increased CPU
cycles in order to receive and process the increased flow of updates
driven by the local churn.
***specific items related to different PE-CE protocols?***
4.2. Multicast
Multicast tree interruptions
PIM neighbor adjacency drops
4.3. Network Events
Network events are both a cause and a symptom of a system running at
or near its scaling limits. As noted above, event-driven routing
table churn or routing protocol interactions can significantly drive
up CPU usage on the locally connected PE as well as on other PEs and
CEs participating in the VRF. If routes are constantly changing due
to a preferred path repeatedly being added and removed, latency and
jitter numbers can be affected in a way that adversely effects
applications sensitive to this sort of change. Network events can
also be triggered by routers with high CPU, because similarly to
systems which may have aggressive routing protocol timers for
enhanced failure detection, systems with centralized CPU-based
implementions for lower-layer protocols (such as HDLC [ISO13239] PPP
[RFC1661], LACP, BFD/EOAM) may start losing keepalives and declaring
outages that result in physical interfaces being torn down and
restored. Again, implementations that choose timer and multiplier
values or numbers of sessions at or near the maximum rated scaling
for the device put the operator in a position where there is very
little headroom to deal with an event that momentarily spikes CPU
usage, meaning that the liklihood of a cascade failure dramatically
increases.
As above, these network events may be something that occurs elsewhere
in the network, and may trigger a failure on a completely different
PE or CE router. The danger with this is that it is extremely
difficult to troubleshoot and correlate root causes when the outage
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observed isn't caused by an event on the same router. Failures
become increasingly non-deterministic and difficult for operators to
manage and address.
4.4. General Route Scale
As mentioned above, systems that are carrying a large number of VRFs
and/or VRFs with large numbers of routes tend to be more sensitive
during events due to the increased amount of periodic and event-
driven processing that must be done to complete a walk of the routing
table to process updates. While optimization techniques may reduce
the overhead of (re)programming the FIB after an update, there are
less tricks to be employed in managing the RIB, and they are often
vendor-specific, which leads to a lowest-common-denominator threshold
in multivendor environments.
In addition to CPU constraints, it's common for route memory
footprint to be a consideration if there are large numbers of VRFs
with large numbers of routes. Similarly to the way that high scale
reduces the cushion of available CPU resources to absorb temporary
peaks, as memory use reaches its high threshold, allocation of the
remaining memory becomes less efficient and more fragmented, such
that memory allocations may begin to fail well before the available
memory is actually exhausted. Depending on the specific
implementation, the "largest free" may be more important than the
"total free" and it may be difficult or impossible to coalesce the
free memory to reduce fragmentation to an acceptable level. As with
other scaling problems, a failure of this type has the nasty habit of
causing a cascade of problems. Depending on how robust the system is
at recovering from memory allocation failures, it may trigger
restarts of critical routing processes or even the entire system.
These may or may not be graceful and hitless, and even if they are
locally a fairly low impact, these may trigger events on other
routers due to the ripple effect of the network event itself. It is
also worth noting that there are hardware and software limits to how
much memory a given system can use - if the router in question does
not use a 64-bit OS, then it is unable to address more than 4GB of
RAM, for example. This may make an otherwise robust system incapable
of scaling to the necessary level, and make memory usage an even more
significant consideration.
5. Known issues and gaps
5.1. PE-CE routing protocols
While support for route flap dampening in BGP as a PE-CE routing
protocol is equivalent to its support in non-VPN applications, the
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addition of IGP routing protocols such as OSPF creates a new problem,
in that there is not really a way to manage route dampening, either
by configuring it within the context of the IGP itself, or by
configuring it in the translation point where the IGP's routing
information is moved into the MP-BGP control plane infrastructure to
be exchanged between participating PEs across the VPN network. This
means that in the case where IGPs are used, which is often more CPU-
intensive and performance-conscious to start with, the route flaps
associated with an unstable network will make a bad problem even
worse. It may be advisable for the IETF to document updates to
standards managing use of IGPs as PE-CE routing protocols to
explicitly define the use of RFD in this application.
*** is this a cisco-only problem? ***
There are also not clear guidelines based on testing and real-world
experience for recommended timer values or appropriate use cases for
an IGP vs BGP as a PE-CE routing protocol. In other words, rather
than enterprises simply defaulting to whatever IGP is already in use
or they are most comfortable with, there may be certain cases where
use of an IGP is recommended, and those where it is not. Guidance in
this area may be very useful to both the SPs supporting these
networks and the engineers designing the corporate networks that make
use of them.
5.2. Multicast
Issues in multicast VPN scale?
5.3. Network Events
Guidance on interface event dampening values (research and testing),
correlation tools to help determine root cause in a cascade failure,
5.4. General Route Scale
Discuss Virtual Aggregation as a potential solution here?
Route flap dampening may potentially be a best practice, but it has a
number of shortcomings. First, there is no systematic way for end
customers to view and clear dampening without some sort of advanced-
functionality looking glass that allows them to view only the routes
in their authorized VRFs. Also, allowing customers to make
unattended clears of dampened routes may defeat the purpose of having
dampening enabled at all, since customers may clear the dampening
without addressing the underlying cause of the problem. In addition,
as noted in [I-D.ymbk-rfd-usable] and
[I-D.shishio-grow-isp-rfd-implement-survey] , Route flap Dampening is
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not widely used even within the Global Internet routing table, and
its values probably need to be tweaked. Due to the differences in
the characteristics of VPN routes compared with the global routing
table, additional study and recommendations as to appropriate RFD
values within a VPN are likely required. Additionally, it is not
possible to configure RFD on IGPs, either natively within the PE-CE
routing protocol or upstream where the learned routes are carried in
MP-BGP. This means that in some cases, there is no way to insulate
the SP network from the adverse impacts of rapid route churn.
5.5. Modeling and Capacity planning
There is a significant lack of multidimensional scale guidance and
modeling for capacity planning and troubleshooting large-scale VPN
deployments. This has a number of contributing factors. First,
behavior at scale becomes increasingly non-deterministic the more
variables you're working with simultaneously, so this is classically
a difficult problem to model. Even worse, it's difficult to account
in a model for latent design/implementation flaws: things that work
well enough at moderate scale, but are not efficient enough for high
scale, or suffer some sort of secondary impact due to dependencies,
race conditions, etc. These problems are often only found through
extensive testing or even escape into production. Second, it is
difficult to characterize an "average" implementation in such a way
that it can be tested to failure in mulitple permutations to provide
a reasonably accurate multidimensional model. Consequently, the
guidance available normally takes the form of multiple uni-
dimensional scale thresholds plus some very conservative multi-
dimensional thresholds that avoid risk to both the vendor and the
implementer by catering to the lowest common denominator and leaving
a lot of capacity sitting idle. Some vendors make an effort to
characterize their customers' large scale implementations such that
they can better replicate real-world conditions, but gathering this
information and devising ways to replicate the behavior in a lab is
problematic and time-consuming.
This leads to a follow-on issue, which is that there is a lack of
instrumentation on critical scaling vectors. Some routers have very
limited abilities to provide useful data about critical scaling
vectors (routing updates per second, changes in multicast state,
sources of internal bottlenecks, etc), either for use in a model or
for use as additional capacity monitoring thresholds. While most
routers can provide information about CPU usage and memory
thresholds, and even which processes are consuming large amounts of
resources, it often takes special instrumented versions of the OS to
provide a window into what is actually causing some sort of failure
at scale. Because these are not routinely monitored, it means that
the provider may be blind to one or more early warning signs that the
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router is nearing its scaling limits and cannot take action to
prevent exceeding those limits before it causes customer impacts.
Additionally, even if this information is available, the provisioning
systems used by most providers do not currently have the intelligence
or visibility to make a decision regarding which PE to provision new
customers on to evenly load the available PE routers. The
provisioning system is often aware of the available physical or
logical port capacity on a given router or site, and uses this as a
key input to its port choice for newly provisioned customers.
Howvever, these additional capacity and scale vectors are based on
real-time statistics from the router (CPU, memory load, etc) and
there is no interaction or feedback loop between the provisioning
system and these types of real-time router scale stats. As a result,
manual intervention is often required to either remove busy routers
from the available capacity pool, move spare port capacity from a
busy router to a full one, or even to reprovision customers to move
them from one device to another to rebalance the load on each router.
5.6. Performance issues
In many ways, it's difficult to define a hard-and-fast scale limit,
because each provider and customer have a differing view on what is
an acceptable performance envelope both in steady state and during
recovery from outages, whether planned or unplanned. In the most
extreme sorts of network events, such as a heavily loaded PE router
undergoing a cold restart, the scale considerations may take
something like boot time and convergence from what the involved
parties consider acceptable and extend them to the point where they
significantly prolong the pain that to which an end customer is
exposed. They often have the added problem of making it difficult to
predict the duration of an outage, because individual customer VRFs
may be affected for differing amounts of time based on all of the
factors that contribute to scaling. For example, if a customer has
one critical route that happens to be among the last to converge,
they perceive the outage to be ongoing until that last route
converges, even if the entire rest of their network has been
functional for a significant amount of time prior to that point.
When dealing with scheduled outages, customers obviously prefer that
they never are impacted. Since this is not really possible, they
expect the provider to give them very clear and accurate guidance on
what the impacts will be, when they will occur, and for what
duration, so that they can set expectations for their customers.
VPNs are often carrying mission-critical services and data, so any
downtime is bad downtime. While a customer may be understanding of a
scheduled maintenance with a 15-30 minute traffic interruption while
a router reloads, they may be less so if the outage actually
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stretches for 60-90 minutes while the router runs at 100% CPU trying
to deal with this worst-case sort of load or suffers intermittent
cascade problems while any remaining cushion is used up dealing with
the results of the event. These impacts may be largely invisible to
the provider unless they have probes within each VRF or other means
to verify that traffic is no longer impacted for a given customer.
It's often difficult or impossible for a provider to tell the
difference between a router that is fully converged but running near
100% CPU after a reload from one that is thrashing and causing delays
in convergence and customer traffic impacts while it runs at 100% CPU
after a reload. Even worse, a scheduled or known outage on one
router may trigger unplanned outages on other high-CPU devices. Even
in unplanned outages, communication regarding impacts and duration is
key, and these sorts of scale issues make it difficult to predict the
impacts.
6. To-Do list
Still not discussed in the document:
Inter-AS VPN NNI scaling considerations (separate discussions on 10A,
10B/hybrid, 10C?) - include discussion on number of VRFs per NNI,
routes per VRF, NNIs per router
Label Exhaustion
BGP Fast External Fallover
Comparison/disussion about horizontal scaling- pros/cons, limits,
gaps
additional scaling considerations if using L2TPv3 or RSVP-TE
tunneling for PE-PE transport
Future scaling considerations (MPLS-TP at the edge, interworking with
L2 technologies, significant increases in density, etc)
7. Acknowledgements
The idea for this draft came from a presentation made by Ning So
during the CDNI working group meeting at IETF 81 in Quebec City where
some of these same scaling considerations are discussed.
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8. IANA Considerations
This draft makes no request to IANA..
9. Security Considerations
Security considerations for IP VPNs are covered in the protocol
definitions. This draft does not introduce any new security
considerations, but it is worth noting that attack vectors that
result in minor impacts in a low-scale environment may make the
problems observed in a high-scale or resource-constrained environment
worse, thereby magnifying the potential for impacts.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
10.2. Informative References
[EIGRP] Wikipedia.org, "Enhanced Interior Gateway Routing
Protocol", <http://en.wikipedia.org/wiki/
Enhanced_Interior_Gateway_Routing_Protocol>.
[I-D.ietf-l3vpn-2547bis-mcast]
Aggarwal, R., Bandi, S., Cai, Y., Morin, T., Rekhter, Y.,
Rosen, E., Wijnands, I., and S. Yasukawa, "Multicast in
MPLS/BGP IP VPNs", draft-ietf-l3vpn-2547bis-mcast-10 (work
in progress), January 2010.
[I-D.ietf-l3vpn-ospfv3-pece]
Pillay-Esnault, P., Moyer, P., Doyle, J., Ertekin, E., and
M. Lundberg, "OSPFv3 as a PE-CE routing protocol",
draft-ietf-l3vpn-ospfv3-pece-09 (work in progress),
September 2011.
[I-D.shishio-grow-isp-rfd-implement-survey]
Tsuchiya, S., Kawamura, S., Bush, R., and C. Pelsser,
"Route Flap Damping Deployment Status Survey",
draft-shishio-grow-isp-rfd-implement-survey-02 (work in
progress), June 2011.
[I-D.ymbk-rfd-usable]
Pelsser, C., Bush, R., Patel, K., Mohapatra, P., and O.
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Maennel, "Making Route Flap Damping Usable",
draft-ymbk-rfd-usable-01 (work in progress), June 2011.
[IEEE802.1]
IEEE, "Connectivity Fault Management", <http://
standards.ieee.org/getieee802/download/802.1ag-2007.pdf>.
[IEEE802.3]
IEEE, "Carrier Sense Multiple Access with Collision
Detection (CSMA/CD) Access Method and Physical Layer
Specifications",
<http://standards.ieee.org/about/get/802/802.3.html>.
[ISO13239]
ISO, "High-level Data Link Control protocol", <http://
read.pudn.com/downloads79/doc/comm/306220/
ISO%2013239.pdf>.
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[RFC4577] Rosen, E., Psenak, P., and P. Pillay-Esnault, "OSPF as the
Provider/Customer Edge Protocol for BGP/MPLS IP Virtual
Private Networks (VPNs)", RFC 4577, June 2006.
[RFC4659] De Clercq, J., Ooms, D., Carugi, M., and F. Le Faucheur,
"BGP-MPLS IP Virtual Private Network (VPN) Extension for
IPv6 VPN", RFC 4659, September 2006.
[RFC4781] Rekhter, Y. and R. Aggarwal, "Graceful Restart Mechanism
for BGP with MPLS", RFC 4781, January 2007.
[RFC4984] Meyer, D., Zhang, L., and K. Fall, "Report from the IAB
Workshop on Routing and Addressing", RFC 4984,
September 2007.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, June 2010.
[RFC6037] Rosen, E., Cai, Y., and IJ. Wijnands, "Cisco Systems'
Solution for Multicast in BGP/MPLS IP VPNs", RFC 6037,
October 2010.
[Y.1731] ITU-T, "OAM functions and mechanisms for Ethernet based
networks", <http://www.itu.int/rec/T-REC-Y.1731/en>.
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Appendix A. Additional Stuff
This becomes an Appendix.
Authors' Addresses
Wesley George
Time Warner Cable
13820 Sunrise Valley Drive
Herndon, VA 20171
US
Phone: +1 703-561-2540
Email: wesley.george@twcable.com
Rob Shakir
Cable and Wireless Worldwide
London,
UK
Phone: +
Email: rjs@cw.net
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