Network Working Group B. Zhang
Internet-Draft Univ. of Arizona
Intended status: Informational L. Zhang
Expires: September 5, 2009 UCLA
March 4, 2009
Evolution Towards Global Routing Scalability
draft-zhang-evolution-00.txt
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Abstract
Internet routing scalability has long been considered a serious
problem. Over the years many efforts have been devoted to address
this problem, however the IETF community as a whole is yet to achieve
a shared understanding on what is the best way forward. We step up a
level to re-examine the problem and the ongoing efforts, and
conclude, to effectively solve the routing scalability problem, we
first need a clear understanding on how to introduce solutions to the
Internet, which is a global scale deployed system. In this draft we
sketch out our reasoning on the need for an evolutionary path towards
scaling the global routing system, instead of attempting a new
design.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Difficulties in Deploying New Solutions . . . . . . . . . . . 4
3. An Evolutionary Path towards Scalable Routing . . . . . . . . 6
3.1. Stage One: Reducing FIB Size . . . . . . . . . . . . . . . 7
3.2. Stage Two: Reducing Multi-AS Virtual Aggregation
Overhead . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3. Stage Three: Reducing RIB Size . . . . . . . . . . . . . . 9
3.4. Stage Four: Insulating the Core from Edge Churns . . . . . 10
3.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 11
4. Evolution versus Incremental Deployability . . . . . . . . . . 12
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. Informative References . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
Internet routing scalability has been a long outstanding problem.
Over the years many efforts have been devoted to solve this problem;
for the last five years we have also been working on a new design to
solve this problem [SIRA]. Since the 2006 IAB Workshop on Internet
Routing and Addressing [RFC4984], new IRTF/IETF efforts have been
devoted to developing a new routing architecture that can provide
effective control over the routing system growth. A number of
proposals have been put on the table; some proposal even developed
running code [LISP]. Yet no clear consensus has emerged in the
community as which proposal(s) may have a good chance to be deployed,
or what is the best way forward.
Assuming the routing scalability problem is real and we can find a
new design that is technically sound, why is it so difficult to agree
on deploying a new design that can solve the problem? We put in the
effort to understand fundamental roadblocks in rolling out our own
design to scalable Inter-domain routing [APT], and came to a new
understanding of the problem at hand: when facing a problem, the
natural approach one tends to take is to develop a new design and
roll it out to replace the old problematic system. That can be an
effective way to solve problems in small scale systems, but it does
not work for the Internet. Instead, the Internet-scale system needs
to resolve problems through an evolutionary path, not a revolutionary
new design.
In this draft we first explain the major difficulties in rolling out
a new design to solve the global routing scalability problem. These
difficulties taught us why the Internet infrastructure needs an
evolutionary path to move forward. We then sketch out a solution
scenario towards resolving routing scalability problem. We also draw
a distinction between an evolution towards final solution direction
versus "a deployable new design."
2. Difficulties in Deploying New Solutions
Two of the few fundamental properties of the Internet are its
distributed governance and its diversity along multiple dimensions.
The Internet is an interconnect of tens of thousands independently
administrated networks, each with its own budget, planning, business
models and operational practices. As a result, not everyone shares
the same view as far as routing scalability is concerned. For
example many customer networks and small regional providers do not
carry the full BGP routing table internally; instead they only
propagate internal routes inside their networks and use default
routing to reach the rest of the Internet through one or a few exit
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points. On the other hand, large networks in general carry the full
routing table internally for efficient delivery to a large number of
destinations. In this case, the former may not feel the pain of
routing table growth but the latter may do.
Even among the networks that do carry the full routing table inside,
some (such as content providers) are able to upgrade their routing
infrastructure every few years to keep up with the demand of ever
growing BGP table; others may not be able to afford doing so. For
example, we have learned from a few large ISPs that, although they
may be able to upgrade their (relatively small number of) core
routers with the latest technology that can handle a million or more
routes, they could not afford upgrading all their edge routers which
may count up to a thousand or more, even though some of them have
been 10 or more years old. As a result, some networks may encounter
the routing scalability problem years earlier than others, some may
experience severe problems while others may not see the problem at
all. Even within the same network, some routers can handle the
increasing routing table size while others cannot. Several incidents
have occurred recently that were caused by edge router RIB overflow.
Although these incidents may be due to other problems (e.g., route
leak-out) that led to inflation of the RIB size, they did show the
fact that a large RIB size can easily push old edge routers to fall
off the cliff.
Therefore, although finding a way to put routing table size under
control may be viewed as needed in the long run, different network
can have different degrees of incentive to solve the problem, and
some may not see a need to take any action towards fixing the problem
for the time being.
Yet another important issue is network economics. A new solution
design usually calls for software upgrade or even new hardware, both
require additional investment as well as new expertise in managing
and troubleshooting the new technology. The affordability associated
with deploying a new design varies greatly among different networks.
Even if a network may suffer pain from the growing routing table
size, it still may not be able to deploy a solution if the cost is
considered prohibitively high.
Each network makes its own business decision on whether to deploy a
new design or not, based on its evaluation of the severity of the
problem and the cost of deploying the solution. Given the scale and
diversity of the Internet, it is certain that the buy-in of any new
solution will not be harmonious. Even for those networks that
require a solution to handle routing scalability, the deployment will
likely be a gradual process with several stages. Furthermore, the
day for *all* the networks to deploy a new solution may take forever
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to come.
To summarize: we see that
o Different parties have different perceptions regarding the routing
scalability problem, both due to different operations and
different affordability; some are yet to be convinced that the
routing scalability problem is serious [BGP2008].
o Even for networks where the routing scalability problem shows up,
there are different severity at different routers.
o If any new solution gets rolled out, it is certain to start from
one or a few parties first, and may or may not ever reach the
whole Internet.
The above argues that we should attack the routing scalability
problem with an evolutionary approach. By evolution we mean that the
solution should allow table size reduction to be done only for those
routers whose capacity fall behind the the FIB or RIB growth, and
that the solution should be built on top of the existing system,
rather than bringing in a replacement of it. Building a solution on
top of the existing system makes it much easier to work transparently
with the rest part that does not make the changes or does not make
the change at the same time.
3. An Evolutionary Path towards Scalable Routing
Based on our current understanding of the problem and the solution
space, in this section we sketch out an evolutionary path towards
scalable Inter-domain routing. As the Internet continues to evolve
over time, it is possible that our understanding may also evolve,
thus the path we sketched out in this draft may change. The main
point we want to make is not any particular evolution path, but
rather to show evidence that such an evolutionary path both exists
and is feasible, and that efforts towards solving an existing problem
should aim for an evolutionary path towards architectural change,
rather than attempting a brand new design.
At this time we can see several stages in evolving today's BGP
routing system towards a controllable growth of the core routing
table size. We divide this evolution into stages by identifying
potentially most severe pain at the time that seems warranting a fix,
and we identify a fix that has a reasonable cost, can be carried out
by individual network, and can built on top of the existing
operations, so that it does not break any other parts of the routing
system. Note that such a simple fix necessarily has its limitations.
As the fix gets widely deployed, its limitations are likely to become
more pronounced, and can become the next problem to address, which
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will lead to the next stage of evolving the system forward.
3.1. Stage One: Reducing FIB Size
Over the last month or so we conducted a quick survey on routing
scalability among a small group of people with operational expertise.
The results identified the fast growing FIB size as the highest
priority concern in routing scalability; this is also consistent with
the results from the IAB 2006 workshop on Routing and Addressing
[RFC4984]. Therefore, we consider reducing FIB size is the first
issue to resolve towards scalable BGP routing, and we believe there
is no major disagreement regarding this problem statement.
The proposed solutions for resolving this FIB scalability problem, on
the other hand, differ significantly. Most of the proposals
presented to the IRTF Routing Research Group (including our own, APT)
took on the direction of a basic architectural change. Not only is
an architectural change likely to take long to go through the IETF
standardization process as well as costly to roll out, but also a
more fundamental problem is that it is difficult to make it both show
some immediate benefits to first movers and be compatible with
today's deployed base. We will discuss more about the issues in
introducing a new design in a later section.
A very different solution, Virtual Aggregation (VA), has been
proposed by Francis and Xu [Virtual_Aggregation]. Briefly speaking,
Virtual Aggregation works as follows. An ISP can reduce its routers
FIB size by configuring a router to announce a short prefix, say
1.0.0.0/8 in place of multiple longer prefixes that fall within
1.0.0.0/8, into its own network. This router is called an
Aggregation Point Router (APR) and this prefix is called a virtual
prefix. The APR maintains FIB entries for all the longer prefixes
(e.g., 1.1.0.0/16) covered by the virtual prefix, while other routers
in the network only maintain one FIB entry for the virtual prefix
1.0.0.0/8. When a router receives a packet to be forwarded to
1.1.0.0/16, its FIB will direct the packet to the APR, which checks
its FIB entry for 1.1.0.0./16 and then tunnel the packet to the
egress router for this real prefix. We view Virtual Aggregation as
an evolutionary fix to the FIB scalability problem, because it can be
done by an individual ISP to effectively shrink the FIB of some of
its routers, and the deployment only requires configuration changes
to start with. It has no impact on the routing operation of any
other networks.
At the same time, since all packets destined to a prefix that has
been aggregated will go through the APR, this step of evolutionary
fix introduces both additional delivery delay (i.e., path stretch)
and encapsulation cost. Furthermore, the APR can also become a
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concentration point of traffic. Several operational steps can be
applied to mitigate these problems.
o Do not aggregate prefixes that carry heavy volumes of traffic to
prevent the traffic from path stretch or contributing to the APR
load.
o One can adjust APR load by adjusting the number of virtual
prefixes, using more APRs to share the load.
o One may be able to configure an APR at the POP where adjacent
prefixes are announced into one's network; properly positioning
APRs can minimize the path stretch.
o Finally, if an APR receives heaving volume of traffic from certain
ingress routers, the APR can send to those ingress routers the FIB
entries that their traffic are destined to, and if the ingress
routers cache these entries, they can encapsulate the packets
towards the egress router themselves.
This last technique makes an APR perform more or less in the same way
as a Default Mapper (DM) in our APT design [APT], however with one
fundamental difference. Deploying an APR does not require any new
protocol or a new functional box (the DM node) that the APT
deployment would require. Instead, an operator can simply configure
a router to be an APR, without needing any changes to other routers
that benefit from reduced FIB size. Only when the APR rollout
becomes successful and the APR load becomes an issue, then the
operator may consider additional changes to make the ingress routers
handle caching.
We believe that the deployment of Virtual Aggregation (VA) can
effectively reduce the FIB size at some routers. How many ISPs would
deploy VA? How much time can VA buy us in curtailing the FIB size
growth? It seems only time can tell. But if we look ahead one step,
as the Internet continues to grow, and as IPv6 deployment starts
rolling out, more networks may face the FIB size problem and adopt
Virtual Aggregation as a solution. When two or more adjacent ASes
all deploy virtual aggregation, packets that traverse these ASes will
experience the cumulated path stretch and encapsulation cost of all
the ASes along their paths. The need to resolve this new problem (of
cumulated path stretch and cost) can naturally lead to the next step
of evolution towards better routing scalability.
3.2. Stage Two: Reducing Multi-AS Virtual Aggregation Overhead
Assuming the AS path a packet takes is W-X-Y-Z, and both X and Y have
deployed Virtual Aggregation. In this case we would like to see that
X's APR encapsulates the packet directly to the egress router of Y
instead of X's own. This will minimize the path stretch and the
packet will only need to be encapsulated/decapsulated once.
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To enable such inter-AS Virtual Aggregation, X's APR needs to know
Y's egress router for the destination prefix. This mapping
information (i.e., mapping from a destination prefix to an egress
router) needs to be propagated somehow from Y to X. The least
resistant approach is to piggyback such mapping information on
existing BGP announcements. Francis and Xu have proposed such an
extension to BGP, which carries the mapping information in a new BGP
attribute [InterDomainVA]; the APT team was also looking into more or
less the same design when the above mentioned draft was published.
We argue that this second step is feasible by the following
reasoning. First, this second step towards better routing
scalability will take effect only after at least two networks (X and
Y) have deployed VA and benefited from it. Therefore we reason they
would not want to move away from VA but would like to minimize its
cost in path stretch and encapsulation, to improve the traffic
performance for their customers. Second, the required BGP
implementation changes are backward compatible, meaning that networks
that have deployed this solution and networks that have not deployed
this solution can still communicate without problems.
As a side note we would also like to point out that this virtual
aggregation mapping exchange *closely* resembles the early design of
mapping information exchange between Default Mappers in APT that the
APT team proposed earlier [APT-00]. Again a fundamental difference
between what we discussed in this section and that early design back
in 2007 is that this draft sketches out an evolutionary path forward,
which does not require a protocol change as a starting point, nor any
information exchange across multiple ASes. Rather, the need for
mapping exchange arises only after the FIB size reduction has been
achieved, and the mapping exchange can start with two adjacent ASes
after each of them has deployed virtual aggregation.
3.3. Stage Three: Reducing RIB Size
Piggybacking the virtual aggregation mapping information on BGP can
work well when the mapping table is small, i.e., the number of
networks that have adopted virtual aggregation is small. When more
networks have adopted virtual aggregation, the mapping table will
grow big, which may make it no longer feasible to piggyback all the
mapping information on the existing BGP sessions. The main problem,
as we can perceive now, would be the RIB size growth: A BGP router
will receive the same mapping information from every neighboring BGP
router, and store all of it in its Adj-RIBs-IN and Local-RIB. Thus
every BGP router will have to store multiple copies of the mapping
table. This issue was pointed out back in 2007 when the early APT
design was discussed, and one suggestion to get around the problem is
to use separate BGP sessions for mapping information exchange.
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Another factor is that, after a network X has deployed virtual
aggregation for a while and has gained sufficient operational
experience, it may become clear that many routers no longer need to
keep the full RIB table. If an internal router has small FIB and
relies on APRs to route packets towards all other destinations, it
does not need a full RIB to build its FIB. Theoretically speaking,
all border routers of X that connect to legacy networks (i.e., those
that have not deployed VA) would still need to keep the full RIB in
order to make BGP announcements into the legacy neighbors. However
in practice, only the customer-facing border routers need a full RIB.
The other border routers, those that face either peer or provider
legacy neighbors, only need to announce X's own customer prefixes to
them. Again, careful engineering analysis and configuration can
eliminate the need for many routers to keep full RIB, and those which
keep the full RIB will be the ones serving as APRs.
As we perceive what may happen further into the future, the picture
becomes more blurry, hence what we try to forecast here may or may
not bear great accuracy for what may happen in the future. Having
said that, we perceive that the combination of the aforementioned two
factors (relieving regular routers from storing mapping table and
full RIB table) would lead to moving the mapping dissemination from
the regular BGP instance (which is used for inter-domain routing) to
a separate BGP instance only between APRs via multi-hop BGP sessions.
Though the protocol is still BGP for the ease of deployment, APRs run
a different session (e.g., a different TCP port) for mapping
dissemination purpose only. Other regular routers run regular BGP
instance for inter-domain routing purpose, but are relieved from
bearing the overhead of storing and propagating mapping information
or the full RIB table.
Again we cannot help but to point out the close resemblance between
the system we depicted above and the original APT design. On the
surface, it seems the only noticeable difference is just the names:
here we have APRs instead of DMs that use BGP to exchange mapping
information. But once again we must not forget an essential
difference: we reach this perceived stage three towards scalable
routing through an evolutionary path, instead of requiring
installation of a new design from day one.
3.4. Stage Four: Insulating the Core from Edge Churns
In the current Internet, flaps of customer prefixes are propagated to
the rest of the Internet in the form of BGP updates, i.e., routing
churns. With virtual aggregation and mapping exchange, these churns
can be reflected as mapping updates, which are disseminated through
the interconnects of APRs. We perceive this as a benefits, as other
non-APR routers can be sheltered from updates due to edge
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instabilities.
Our earlier measurement and analysis study [TopologyGrowth] has shown
that most Internet topology growth comes from the addition of
customer edge ASes. It is conceivable that as the number of customer
sites continues to increase, the amount of churns may become too much
to handle in a cost-effective way. A solution to this edge churn
problem is to insulate the edge dynamics from the mapping
dissemination system. Based on the current BGP data, our estimation
shows that, if we could remove BGP updates induced by customer prefix
instabilities, we would have reduced the total amount of routing
churns by an order of magnitude [eFIT_IPv6]. Ideally, when the link
connecting a customer site to a provider fails, the mapping system
should propagate this failure information only when the failure has a
long duration, so that every network will be aware of this failure
and choose an alternative path. But long lasting failures probably
do not happen frequently. Short failures, which are frequent, should
not be propagated through the mapping system. Instead, they should
be handled by other means. For example, APT has a failure handling
mechanism in which the failure handling actions are data-driven,
i.e., a link failure to an edge network is not responded to unless
and until there are data packets that are heading towards the failed
link customer site. We are actively working on an evolutionary
solution that can provide equivalent data-driven handling of edge
failures as APT does.
3.5. Summary
If we imagine a picture where all the networks in the Internet had
deployed all the stages of routing scalability improvement we
sketched above, then the Internet routing would have deployed a new
map-encap routing architecture like APT. The prefixes that got
aggregated out of the core routing system would be those that belong
to the edge ASes, as the ISPs still must exchange routing
reachability among themselves to be able to tunnel packets toward
their egress routes. As such, the separation of transit networks
from the edge sites, as the APT design had proposed, would have been
achieved.
Then what is the new contribution of this draft? First, we emphasize
that our goal is to reduce the FIB/RIB table size; the separation
itself should *not* be a required starting point. Second, we show an
evolutionary path towards resolving routing scalability through
several stages, with clearly identified benefits and minimal cost at
each stage. Furthermore, we show that the evolutionary path, as we
sketched out in this draft, can naturally converge towards the
separation as a result. We make two points from this last
statements: (1) This could also be used as an evidence that, if we
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could afford starting anew, then a separation design would lead to
scalable routing! (2) We used the phrase "converge towards
separation", rather than "achieving separation", because we believe
that, even after a long time and many networks have adopted the
solution, it is most likely that some networks will remain at various
early stages, some may not have made a single change. This is the
nature of the Internet, due to its two properties that we mentioned
at the beginning: its distributed governance, and its diversity along
multiple dimensions
4. Evolution versus Incremental Deployability
Many new designs have plans for incremental deployment. So what is
the difference between incremental deployability and evolution?
A new design requires that the whole system eventually make the
change, and the benefit of the new design will be achieved only after
a significant portion of the system has deployed the design.
Incremental deployment techniques are used to glue the part that has
made the change and the part that has not. It makes the system
function at the intermediate stage, but it does not provide any
incentive for individual networks to make the change.
A typical example is to use MBone tunnels to incrementally deploy IP
multicast. Although tunneling provides a technique to connect
multicast islands together, the whole deployment plan does not have
enough incentive for networks to join the MBone. The main benefit of
deploying IP multicast is to improve the performance and scalability
of large scale group communication applications, but this benefit
exists only after the majority of the Internet has deployed IP
multicast. Early adopters would not see enough deployment benefits
to justify the cost.
The evolutionary approach recognizes that changes to the Internet is
a gradual process with possibly several stages, and at each stage,
networks that make the changes must have the incentive to do so.
More specifically,
1. Each stage focuses on an immediate problem with enough economic
impact that warrants a fix.
2. Each stage offers a solution that solves the problem, does not
break other parts of the Internet, and can be deployed with a
reasonable cost considering the specific problem.
3. As the solution is being deployed by more and more networks, its
downside may become more pronounced and eventually requires a
fix, which leads to the next stage of the evolution.
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An evolutionary design emphasizes the evolution path that the
Internet would take, while a revolutionary new design focuses on the
final outcome once the changes are done by all participants. We
believe that we must take an evolutionary approach towards a scalable
Internet routing architecture.
5. Acknowledgements
The authors are part of the APT team. The APT effort is funded by
NSF.
6. Security Considerations
This draft is a discussion on the Internet's necessity to follow an
evolutionary path towards the future. There is no direct impact on
the Internet security.
7. Informative References
[APT] Jen, D., Meisel, M., Massey, D., Wang, L., Zhang, B., and
L. Zhang, "APT: A Practical Transit Mapping Service",
draft-jen-apt-01, November 2007.
[APT-00] Jen, D., Meisel, M., Massey, D., Wang, L., Zhang, B., and
L. Zhang, "APT: A Practical Transit Mapping Service",
draft-jen-apt-00, July 2007.
[BGP2008] Huston, G., "BGP IN 2008 - what's changed", APRICOT
presentation, 2009, <http://apricot2009.net/
index.php?option=content&task=view&id=51>.
[InterDomainVA]
Xu, X. and P. Francis, "Simple Tunnel Endpoint Signaling
in BGP", draft-xu-tunnel-00, February 2009.
[LISP] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
"Location/ID Separation Protocol (LISP)",
draft-farinacci-lisp-12, March 2009.
[RFC4984] Meyer, D., Zhang, L., and K. Fall, "Report from the IAB
Workshop on Routing and Addressing", RFC 4984,
September 2007.
[SIRA] Zhang, B. and et. al., "A Secure and Scalable Internet
Routing Architecture", ACM SIGCOMM 2006 Poster Session.
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[TopologyGrowth]
Oliveira, R., Zhang, B., and L. Zhang, "Observing the
Evolution of Internet AS Topology", ACM SIGCOMM 2007.
[Virtual_Aggregation]
Francis, P., Xu, X., and H. Billani, "FIB Suppression with
Virtual Aggregation and Default Routes",
draft-francis-idr-intra-va-01, September 2008.
[eFIT_IPv6]
Massey, D. and et. al., "A Scalable Routing System Design
for Future Internet", ACM SIGCOMM 2007 IPv6 Workshop.
Authors' Addresses
Beichuan Zhang
Univ. of Arizona
Email: bzhang@arizona.edu
Lixia Zhang
UCLA
Email: lixia@cs.ucla.edu
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