Internet Engineering Task Force W. George
Internet-Draft Time Warner Cable
Intended status: Informational R. Shakir
Expires: September 7, 2012 BT
March 6, 2012
IP VPN Scaling Considerations
draft-gs-vpn-scaling-01
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
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on September 7, 2012.
Copyright Notice
Copyright (c) 2012 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
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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. Intention of this Document . . . . . . . . . . . . . . . . 4
1.2. Horizontal vs. Vertical Scaling . . . . . . . . . . . . . 5
1.3. Developing Requirements for Scaled L3VPN Environments . . 6
1.4. Requirements Language . . . . . . . . . . . . . . . . . . 6
2. PE-CE routing protocols . . . . . . . . . . . . . . . . . . . 7
2.1. Best Common Practice . . . . . . . . . . . . . . . . . . . 7
2.2. Common Problems at Scale Limits . . . . . . . . . . . . . 9
3. Multicast . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1. Best Common Practices . . . . . . . . . . . . . . . . . . 10
3.2. Common Problems at Scale Limits . . . . . . . . . . . . . 10
4. Network Events . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Best Common Practices . . . . . . . . . . . . . . . . . . 11
4.2. Common Problems at Scale Limits . . . . . . . . . . . . . 12
5. General Route Scale . . . . . . . . . . . . . . . . . . . . . 13
5.1. Best Common Practices . . . . . . . . . . . . . . . . . . 15
5.2. Common problems at scale limits . . . . . . . . . . . . . 16
6. Known issues and gaps . . . . . . . . . . . . . . . . . . . . 17
6.1. PE-CE routing protocols . . . . . . . . . . . . . . . . . 17
6.2. Multicast . . . . . . . . . . . . . . . . . . . . . . . . 18
6.3. Network Events . . . . . . . . . . . . . . . . . . . . . . 18
6.4. General Route Scale . . . . . . . . . . . . . . . . . . . 18
6.5. Modeling and Capacity planning . . . . . . . . . . . . . . 18
6.6. Performance issues . . . . . . . . . . . . . . . . . . . . 20
6.7. High Availability and Network Resiliency . . . . . . . . . 21
7. To-Do list . . . . . . . . . . . . . . . . . . . . . . . . . . 21
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
10. Security Considerations . . . . . . . . . . . . . . . . . . . 22
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
11.1. Normative References . . . . . . . . . . . . . . . . . . . 22
11.2. Informative References . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
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1. Introduction
As IP networking has become more ubiquitous 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.
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
density and control plane capacity such that neither grows out of
pace with the other.
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1.1. Intention of this Document
This document is intended to provide a discussion of the challenges
that network operators face in deploying large-scale L3VPN
environments at the time of writing, with two key sets of
recommendations. As such, these outcomes can be divided into those
that apply to network operators regarding the deployment of
particular technologies, and those that apply to network protocol and
operating system implementors relating to allowing better
understanding of scaling characteristics in deployments of such
equipment.
The best practices defined in this document are intended to allow
more optimal scaling of L3VPN networks, whilst minimising the impact
on end-customer network behaviour. It is intended that such guidance
can be directly utilised by Service Providers to improve the
scaleability of network elements. However, the guidance in this
document should not be viewed as a panacea to the problems of scaling
network elements. It is the intention of the authors to document a
number of key problems experienced in such environments and provide
information to the SP that may result in more optimal deployment of
existing technologies to this audience. It is appreciated that there
is a point at which the limits of hardware will be reached, and hence
new network elements are required. The key intention of the
recommendations provided to Service Providers within this document
are intended to allow the resources that exist within existing
elements to be utilised in the most efficient manner. Clearly, the
optimal point in this balance is that the data-plane and control-
plane scale to support similar levels of service termination, so as
to result in minimal "over provisioning" of one element.
The scaling considerations presented in this document are intended to
provide both network operators and network equipment implementors
further guidance around the toolset, and information required to
provide accurate means of capacity planning in L3VPN environments.
Again, the authors consider that the scaling characteristics, and
toolsets required of L3VPN PE equipment diverge somewhat from those
required by Internet network equipment. In Internet deployments,
relatively standardised interconnects exist across all deployments -
typically utilising either static routing, or BGP-4. As such, each
connected port comes with a relatively standard overhead in terms of
the protocols required. Whilst there is some variance in how
"chatty" each customer connection may be, this is balanced by the
fact that the whole Internet routing table is typically held on such
edge equipment (and hence individual customer's instability tends to
be relatively small when compared to the instability of the Internet
DFZ). In addition, since such instability is limited to relatively
few impacts to a node (interface or BGP session flapping, and BGP
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UPDATE messages) routers can be optimised to cope with such
instability. Counter to this, the L3VPN environment does not have a
standardised connectivity model, and typically connects to much less
controlled environments. Further details of this are provided within
later sections of this document. The result of this difference is
that 'headline' scaling figures presented for particular equipment
tends to be of limited utility to a network operator. The
recommendations within this document outline some of the
considerations that must be made in considering the scaling of such
elements, and provide guidance as to the missing inputs and tools
that are required to provide information around the capacity of such
elements.
1.2. Horizontal vs. Vertical Scaling
Within this document, two forms of 'scaling' are referred to - the
"throw hardware at the problem" approach outlined previously involves
deploying additional network elements in order to provide further
network capacity. Throughout this document, this approach is
referred to a horizontal scaling - insofar as it requires parallel
deployment of numerous similar elements and balancing the load across
the combined capacity of all of the elements. The approach of
increasing the capacity of an individual node through allowing the
control plane capacity to support the maximum forwarding plane
capacity (be it data forwarded, or available ports) is referred to as
vertical scaling. It is obvious that at some point the approach of
horizontal scaling of elements is required - due to either exhausting
available port capacities, or available forwarding plane - however,
it should be noted that there are a number of motivations for
delaying such provisioning, some of which relate directly to the
characteristics of L3VPN environments.
Since a significant proportion of the customers who purchase L3VPN
services are Enterprise customers, typically, the service is utilised
as a WAN for their inter-location connectivity. Clearly, as such
customer base tends to be distributed based on differing factors,
this implies that such customers connect in numerous geographical
locations. The requirement to support service in these locations
therefore results in a requirement for the service provider network
architecture to support geographically distributed access into such
services. A balance must be struck between the extent to which
access networks are utilised to backhaul traffic to the service
layer, and the geographical distribution of the service layer itself.
Both scale and performance characteristics of such networks tend to
result in more geographical distribution of service layer elements
than in Internet deployments. This distribution results in two
particular changes - primarily that the idea of a "point-of-presence"
must be reconsidered - where an assumption in Internet environments
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may be that there are separated core and access elements within a
single location, within a distributed L3VPN environment, a point of
presence may be a single PE device. The result of these small scale
points of presence is that numerous core and edge functions must be
collapsed onto a single device. For this reason, the approach of
adding additional devices to the network may have an impact on a
further subset of devices within the network (particularly due to any
mesh-based protocols that are deployed), and hence result in a change
in the scaling characteristics of these devices. In this case, there
is further motivation to avoid large number of devices in the network
where possible. Further to this, the smaller PoP profile may result
in physical constraints around the deployment of additional network
elements, particularly due to the availability of power and physical
space to deploy such elements.
1.3. Developing Requirements for Scaled L3VPN Environments
Whilst the collected scaling considerations outlined in this document
are based on the author's collective experience within various
Service Provider networks, and discussions with operators of similar
networks, it should be noted that the problems outlined in this
document are not static. With the growth in the use of IP as the
underlying transport of many services, the demand for L3VPN
environments has grown. As such, this has meant that various
technologies are being considered to allow growth of these networks
at a lower cost point to a wider footprint than was previously
required. A network operator must therefore consider the extent to
which the service layer must be built - both to meet economic and
technical requirements. With newer aggregation methods, the service
layer edge (and hence the L3VPN PE) acquires responsibility for
inter-working between newer dynamic aggregation technologies, and the
existing IP network. As such, these edge functionalities result in
further requirements for loading onto these network elements.
*** Author's note: Do we want to put anything about NNI for footprint
extension here? Datacenter edge - perhaps Ning's problem around the
L3VPN edge in his datacentres? ***
1.4. 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].
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2. 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
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.
2.1. Best Common Practice
Ultimately, the decision as to which PE-CE routing protocols to
support is a business decision much more often than it is a technical
one, because there are few use cases where something other than BGP
and static routing as PE-CE routing protocols is a technical
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requirement. If a provider chooses to support additional protocols,
especially IGPs, they should consider the effects that these have on
the overall scaling profile of the PE routers and the network as a
whole when determining if and to what extent they will support other
protocols.
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
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.
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2.2. Common Problems at Scale Limits
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
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
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driven by the local churn.
***specific items related to different PE-CE protocols?***
3. Multicast
Multicast support within a VPN [RFC6513] 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. Additional scaling considerations around Multicast in a
VPN are related to the size and number of multicast streams. While
this is a consideration whenever Multicast is used even outside of a
VPN because of the bandwidth utilization it may generate in the core,
the additional overhead of implementing multicast within a VPN makes
this a more siginificant consideration in this case. Related to the
previous consideration is the stream fanout - the amount of P and PE
router paths in the network that could potentially carry a given
multicast stream based on the number of PEs that are configured with
a given Multicast-enabled VRF, and the number that actually do carry
the stream based on actual receivers joining the stream behind that
PE.
*** This section is quite weak. We're looking for contributors who
can assist in fleshing this out ***
3.1. Best Common Practices
Multicast BCPs???
3.2. Common Problems at Scale Limits
Multicast tree interruptions
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PIM neighbor adjacency drops
4. 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.
4.1. Best Common Practices
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
(e.g. in provisioning logic/rules) 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
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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
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.
4.2. Common Problems at Scale Limits
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
implementations 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 likelihood 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
observed isn't caused by an event on the same router. Failures
become increasingly non-deterministic and difficult for operators to
manage and address.
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5. 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
hierarchical 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
distribution 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 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.
It is fairly common for the contract or Service Level Agreement
between SP and customer to include a maximum limit as to how many
routes can be carried in a VRF. At its most basic, this maximum can
be used as a method to estimate the number of VRFs that can be
present on a PE given its scaling limitations. However, there is a
wide gulf between a contractual limitation of no more than N routes
per VRF with a corresponding configured limit and the fact that many
customers will not carry nearly that many routes. This leads to the
potential for significant stranded capacity. Therefore the provider
needs a way to have different tiers of "maximum routes allowed" so
that the capacity management can be done in such a way as to enable
better loading of PE routers to take this relationship into account
(e.g. populating a PE with a combination of high-scale and low-scale
VRFs). The alternative to this method would be to assume a standard
maximum routes per VRF, and then similarly to the way that carriers
use statistical multiplexing and oversubscription to assume that not
all customers will have their pipes full of bandwidth at the same
time, make some assumptions about control plane capacity. This may
come in the form of an average that is calculated based on the actual
size of the routes in each VRF. This has many challenges. Among
them- Should it be calculated per-PE? Network-wide? What happens
when there are too many VRFs that exceed the average on a given PE?
How does one add control plane capacity to a "full" router? This may
be a manageable model in a network with a robust and flexible
provisioning system, such that high-scale VRFs can be moved between
PE routers to balance the load, but each of these moves likely
represents an outage for the customer and the potential for other
errors to creep in, and is not likely to be attractive due to the
operational costs of managing the network. In other words, it
doesn't scale, but for a completely different reason.
5.1. Best Common Practices
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
(***reference?***), 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,
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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.
*** 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... ***
5.2. Common problems at scale limits
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
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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.
6. Known issues and gaps
6.1. 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
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.
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
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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.
6.2. Multicast
Issues in multicast VPN scale?
6.3. Network Events
Guidance on interface event dampening values (research and testing),
correlation tools to help determine root cause in a cascade failure,
6.4. 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
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.
6.5. 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
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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. These conservative recommendations avoid
risk to both the vendor and the implementer by catering to the lowest
common denominator, but they have the adverse effect of 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
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.
However, 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.
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6.6. 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
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.
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6.7. High Availability and Network Resiliency
In many cases, L3VPN services are carrying significant amounts of
business-critical data. Customers and carriers design their networks
to be robust enough to absorb single and sometimes even dual faults
with little or no impact to the network as a whole. However, the
expectations as to the frequency and duration of outages due to
either scheduled or unscheduled events continue to go higher and
higher. This is leading more providers to adopt features such as
Non-Stop Forwarding and Non-Stop Routing, as well as things like In-
Service Software Upgrades to improve the chances that outages will be
transparent to the underlying customers, networks, and applications
using the network elements. As these become more common within the
L3VPN space, they must be considered when evaluating PE scale.
Often, the machinery necessary to make these reliability enhancements
work requires duplication and sharing of state between multiple
elements. At its most basic level, this state sharing takes more
resources and more time the more state there is to be shared, so
increases in the different scaling vectors discussed in this document
will cause proportional increases in the complexity and resource
requirements necessary for the combined feature set. In more complex
scenarios and implementations, it may contribute to the complexity
associated with capacity planning, and may make the response even
more non-deterministic as scale increases.
7. To-Do list
RFC EDITOR: Please remove this section before publication.
Still not discussed in the document:
use of centralized/dedicated Route-reflectors vs more distributed RRs
for scale - geographic artifacts/suboptimal routing based on RR
placement
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
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tunneling for PE-PE transport
Future scaling considerations (MPLS-TP at the edge, interworking with
L2 technologies, significant increases in density, etc)
8. 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. Thanks also
to Yakov Rekhter, Luay Jalil and Jeff Loughridge for their reviews
and comments.
9. IANA Considerations
This draft makes no request to IANA..
10. 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.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11.2. Informative References
[EIGRP] Wikipedia.org, "Enhanced Interior Gateway Routing
Protocol", <http://en.wikipedia.org/wiki/
Enhanced_Interior_Gateway_Routing_Protocol>.
[I-D.ietf-l3vpn-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-11 (work in progress),
January 2012.
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[I-D.shishio-grow-isp-rfd-implement-survey]
Pelsser, C., Bush, R., Kawamura, S., and S. Tsuchiya,
"Route Flap Damping Deployment Status Survey",
draft-shishio-grow-isp-rfd-implement-survey-03 (work in
progress), December 2011.
[I-D.ymbk-rfd-usable]
Pelsser, C., Patel, K., Maennel, O., Mohapatra, P., and R.
Bush, "Making Route Flap Damping Usable",
draft-ymbk-rfd-usable-02 (work in progress),
December 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.
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[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.
[RFC6513] Rosen, E. and R. Aggarwal, "Multicast in MPLS/BGP IP
VPNs", RFC 6513, February 2012.
[Y.1731] ITU-T, "OAM functions and mechanisms for Ethernet based
networks", <http://www.itu.int/rec/T-REC-Y.1731/en>.
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
BT
London,
UK
Phone: +
Email: rob.shakir@bt.com
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