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Network Working Group                                           B. Zhang
Internet-Draft                                          Univ. of Arizona
Intended status: Informational                                  L. Zhang
Expires: April 29, 2010                                             UCLA
                                                                 L. Wang
                                                        Univ. of Memphis
                                                        October 26, 2009


              Evolution Towards Global Routing Scalability
                   draft-zhang-zhang-evolution-02.txt

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   10, 2008.  The person(s) controlling the copyright in some of this
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Abstract

   Internet routing scalability has long been considered a serious
   problem.  Although many efforts have been devoted to address this
   problem over the years, the IETF community as a whole is yet to
   achieve a shared understanding on what is the best way forward.  In
   this draft, we step up a level to re-examine the problem and the
   ongoing efforts. we conclude that, 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 to introduce a brand new design.




























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Difficulties in Deploying New Solutions  . . . . . . . . . . .  5
   3.  An Evolutionary Path towards Scalable Routing  . . . . . . . .  7
     3.1.  Step One: Local FIB Size Reduction . . . . . . . . . . . .  7
     3.2.  Step Two: Network-Coordinated FIB Size Reduction . . . . .  8
     3.3.  Step Three: Reducing Adjacent AS Virtual Aggregation
           Overhead . . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.4.  Step Four: Reducing RIB Size . . . . . . . . . . . . . . . 11
     3.5.  Step Five: Insulating the Core from Edge Churns  . . . . . 12
     3.6.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . 13
   4.  Evolution versus Incremental Deployability . . . . . . . . . . 14
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   7.  Informative References . . . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17


































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1.  Introduction

   Internet routing scalability has long been an outstanding problem.
   Over the years many efforts, including our own, have been devoted to
   solve this problem.  Since the 2006 IAB Workshop on Internet Routing
   and Addressing [RFC4984], new IRTF/IETF efforts have been devoted to
   developing a scalable routing architecture, and a number of proposals
   have been put on the table [RRG].  We contributed a new design dubbed
   APT [APT]; another new design LISP already has running code [LISP].
   Yet no clear consensus has emerged in the community as to 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 APT[APT],
   and came to a new understanding of the problem at hand: when facing a
   problem, as engineers we naturally tend to design a new system to
   solve the problem, hoping that the new design would be rolled out to
   replace the old problematic one.  This kind of "solution by new
   design" approach can be effective in solving problems in small scale
   (e.g. one could easily replace an old computer with a new one), but
   it does not work for the deployed Internet.  Instead, the Internet-
   scale systems need to resolve problems through an evolutionary path,
   not a revolutionary new design.

   In this draft we first discuss the major difficulties in rolling out
   a new design to solve the global routing scalability problem.  These
   difficulties suggest that the Internet routing infrastructure needs
   an evolutionary path to move forward.  We then show evidences that
   such an evolutionary path indeed exists.  To address the concern
   about getting stuck at "local optimal" with incremental changes, we
   sketch out a solution scenario which demonstrates the feasibility of
   moving the routing system towards a scalable architecture through
   incremental steps (see Section 3).  This evolutionary path is driven
   by the most severe aspect of the routing scalability problem that
   each individual ISP faces at each stage, yet an interesting outcome
   is that the overall routing architecture evolves towards the
   separation between customer networks and provider networks, on which
   our APT design was based.  The major difference between the
   evolutionary design and the initial APT design is that the separation
   is not a starting point of the design, but rather a natural result of
   the evolutionary design.

   We also draw a distinction between an evolutionary process towards a
   final solution direction versus the "incremental deployability" that
   many previously proposed new designs claim to have (see Section 4).
   One must not mistake co-existence between a new design and the



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   existing system as incremental deployability, because the latter
   requires offering immediate gains for first movers, otherwise no one
   has incentive to be first movers.


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 the routing scalability issue is concerned.
   For example, many customer networks and small regional providers do
   not carry the full BGP routing table internally; instead they
   propagate only internal routes inside their networks and use default
   routing to reach the rest of the Internet through one or a few exit
   points.  On the other hand, large networks in general carry the full
   routing table internally for efficient data delivery to a large
   number of destinations.  As a result, 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
   internally, 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 learned from a few large ISPs that,
   although they were able to upgrade the relatively small number of
   core routers with the latest technology that can handle a million or
   more routes, they could not afford to upgrade all their edge routers
   which may count up to a thousand or more, even though some of them
   are more than 10 years old.  Consequently, some networks may
   encounter the FIB or RIB size limitations earlier than others, some
   may experience severe problems while others may not feel 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 or FIB
   overflow.  Although these incidents may be triggered by other
   problems (e.g., route leak-out) that led to the 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 control routing table size is
   necessary in the long run, especially in lieu of increasing IPv6
   deployment, different networks 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.




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   Yet another important issue in solution evaluation 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 new solution if the cost is considered prohibitively high.
   Instead, people tend to look for simple twists of the existing
   systems that can provide effective relief from the RIB/FIB growth
   pressure.  One such simple patch was presented at October 2008 NANOG
   meeting [NANOG44].

   Each network makes its own business decision on whether to deploy a
   new design, based on its evaluation of the severity of the problem
   and its affordability 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 consisting of several stages.
   Furthermore, the day for the global Internet as a whole to deploy a
   new solution may take forever to come.

   To summarize: we see that

   o  Different parties have different perceptions regarding the routing
      scalability problem due to their differences in economical
      conditions and operational practices; some are yet to be convinced
      that the routing scalability problem is serious [BGP2008].
   o  For networks that face the routing scalability problem, there can
      still be different severity at different routers.
   o  Networks that experience routing scalability problems are also
      likely to have affordability concerns for new solutions.
   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
      entire Internet.

   The above argues that we should attack the routing scalability
   problem with an evolutionary approach.  By evolution we mean that (1)
   the solution should be deployable by individual AS who deems an
   action necessary, without needing coordination with neighbor ASes;
   (2) the solution can bring immediate gain to a single first-mover;
   (3) even within the AS, the solution should enable the routing table
   size reduction at only those routers whose capacity fall behind the
   the FIB or RIB growth curve; and (4) the solution should be an
   incremental step on top of the existing system to minimize the cost
   while being effective.  Building a solution on top of the existing
   system makes it much cheaper and easier to roll out, and makes it



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   likely to work transparently with the rest of the system 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
   scaling Inter-domain routing.  As the Internet continues to evolve
   over time, it is likely that our understanding will also evolve, thus
   the specific path we sketched out in this draft may, or is likely to,
   change.  The main point we argue in this draft is not any specific
   evolutionary path that the Internet may take, but rather we aim to
   show strong evidences that such an evolutionary path both exists and
   is feasible; that we should aim for an evolutionary path to address
   routing scalability problem, rather than attempting a brand new
   design; and, most of all, that such incremental steps should not
   result in "local maximal" situation, instead they can indeed move the
   overall system towards the routing architecture that our new design,
   APT, aimed for.

   At this time we can see several steps in evolving today's BGP routing
   system towards a controllable growth of the routing table size.  We
   identify potentially most severe pain at each step that warrants a
   fix.  We then identify a fix that has a reasonable cost, can be
   carried out by individual networks, and can be built on top of the
   existing operations, so that it does not break any other parts of the
   global routing system.  Note that any such 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.  At the same time, other aspects of the routing
   scalability problems that were not addressed by these fixes may
   become more severe.  These issues will lead to the next step of
   evolving the system forward.

3.1.  Step One: Local FIB Size Reduction

   During early 2009 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 the first issue
   to addres, 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



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   presented to the IRTF Routing Research Group (including our own
   earlier work, 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 it suffers from a more fundamental problem: the
   difficulty to bring immediate benefits to first movers and to be
   compatible with today's deployed base.  We will discuss more about
   the difficulties in deploying a new design with architectural changes
   in Section 4.

   A different direction to reduce FIB has also be proposed.  Proposals
   in this direction do not propose immediate architectual changes,
   instead they take pragmatic approaches to reducing FIB size.  A
   simple idea to compress FIB size has been suggested by multiple
   people independently for some time.  It works in the following way:
   if all longer prefixes, say those under 1.0.0.0/8, share the same
   next hop with their covering prefix 1.0.0.0/8, then only 1.0.0.0/8
   needs to be installed in the FIB.  A recent study [FIBAggregate]
   refined the basic idea to an effective FIB Aggregation scheme, and
   proposed an efficient FIB update mechanism when the next hop of
   either the covering or some covered prefix(es) changes and when
   prefixes need to be added or removed from the FIB.  Preliminary
   evaluation shows that different FIB Aggregation techniques can reduce
   the FIB size by 50% to 70% or more with no impact on the correctness
   of packet forwarding.

   FIB aggregation requires no protocol changes.  However the
   effectiveness of FIB aggregation depends on the aggregatability of
   covered and covering prefixes, hence has a lower bound on how much it
   can reduce FIB size (i.e. it cannot reduce FIB to an arbitrarily
   small size).  Another proposal for FIB size reduction is Virtual
   Aggregation (VA) by Francis and Xu [Virtual_Aggregation].  VA has the
   potential to reduce the FIB size by a factor of 10 or more.

3.2.  Step Two: Network-Coordinated FIB Size Reduction

   Briefly, Virtual Aggregation works as follows.  An ISP can reduce its
   routers FIB size by configuring a router, dubbed Aggregation Point
   Router (APR), to announce a short prefix, say 1.0.0.0/8, into its own
   network in place of multiple longer prefixes that fall within
   1.0.0.0/8.  This short 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 it announces, while other routers in
   the network only maintain the virtual prefix 1.0.0.0/8.  When a
   router R receives a packet to be forwarded to 1.1.0.0/16, R's FIB
   will direct the packet to the APR, and the APR then encapsulates the
   packet to the egress router for the actual prefix 1.1.0.0./16.




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   Both FIB Aggregation and Virtual Aggregation represent evolutionary
   steps towards scaling the FIB size.  Each of them can be done by an
   individual ISP to effectively shrink the FIB size for its routers,
   and makes no impact on the routing operations of any other networks.
   Virtual Aggregation can be more effective in FIB size reduction,
   however because all packets destined to the prefixes that have been
   aggregated will go through the APR, Virtual Aggregation introduces
   additional delivery delay (i.e., path stretch), encapsulation
   overhead, as well as the potential of the APR becoming a traffic load
   concentration point.  Several operational steps can be applied to
   mitigate these problems.

   o  Do not aggregate prefixes that carry heavy volumes of traffic
      (popular prefixes).  Based on the assumption that most traffic
      falls into a small percentage of prefixes, avoiding aggregation of
      popular prefixes can prevent majority of data traffic from path
      stretch and prevent the APR from overload.
   o  One can also control APR load by using more APRs to share the
      load.
   o  Proper positioning of APRs can minimize the path stretch
      [VA_performance].
   o  Finally, if an APR receives heaving volume of traffic from certain
      ingress router, the APR can send to this ingress router the FIB
      entries that its traffic are destined to, so that the ingress
      router can cache the FIB entries and encapsulate the packets
      towards the egress routers directly.  This will both reduce the
      APR load and eliminate the path stretch.

   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 necessarily
   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.  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 FIB Aggregation (FA) and Virtual Aggregation (VA) can
   be deployed sequentially or in parallel to effectively reduce the FIB
   size at most routers.  Virtual Aggregation can be viewed as a poor
   man's map-encap within one AS.  The APR holds the mapping table from
   the virtual prefix to all the egress routers through which the
   specific prefixes can be reached.  This mapping information is
   directly derived from BGP routing updates without a new mapping
   distribution system.  The APR then encapsulates packets to those
   egress routers.




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   How much time can FA and 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 FA and VA
   as solutions.  When two or more adjacent ASes all deploy Virtual
   Aggregation, packets that traverse these ASes will experience the
   cumulated path stretches and encapsulation/decapsulation cost of all
   the ASes along their paths.  The need to resolve this new problem (of
   cumulated path stretch and overhead) can naturally lead to the next
   step of evolution towards better routing scalability.

3.3.  Step Three: Reducing Adjacent 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.  Then instead of X's own egress router,
   we would like to see that X's APR encapsulates the packet directly to
   the egress router of Y that connects Y to Z. This will reduce the
   path stretch and the packet will only need to be encapsulated/
   decapsulated once instead of two times.

   To enable such inter-AS Virtual Aggregation, X's APR needs to know
   Y's egress router for a given destination prefix P. This mapping
   information (i.e., mapping from a destination prefix P to an egress
   router) needs to be propagated from Y to X. The least resistant
   approach is to piggyback such mapping information on the existing BGP
   announcement for prefix P. 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 show the feasibility of this second step by the following
   reasoning.  First, this second step towards better routing
   scalability will take place only after at least two adjacent networks
   (X and Y in our example) have deployed VA and benefited from it.
   Therefore we reason that they would not want to move away from VA but
   would like to minimize VA's cost in path stretch and encapsulation,
   to improve the performance for their customers.  Second, the required
   BGP implementation changes are backward compatible, meaning that
   networks that have deployed this solution can easily interwork with
   networks that have not deployed this solution.  Furthermore, adjacent
   VA-enabled ASes may not need to exchange the mapping for all of their
   prefixes.  Again if we take the common assumption that majority of
   traffic falls within a relatively small number of prefixes, then AS Y
   may only need to send AS X the mapping for a small number of prefixes
   to improve the performance for a bulk part of the traffic.

   As a side note we would also like to point out that this virtual
   aggregation mapping exchange *closely* resembles the early design of



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   APT mapping information exchange between Default Mappers [APT-00].
   The content of the mapping exchanges is somewhat different, but a
   more fundamental difference between what we discussed in this section
   and that early APT design back in 2007 is the following: here we
   sketch out an evolutionary path forward, which does not require, as a
   starting point, any protocol change or information exchange across
   multiple ASes that the early APT design does.  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.

3.4.  Step Four: Reducing RIB Size

   Piggybacking the virtual aggregation mapping information on BGP can
   work well when the mapping table is small.  When more networks have
   adopted Virtual Aggregation, the mapping table is likely to grow
   large, 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 today, would be the RIB size growth: A BGP router
   will receive the same mapping information from multiple neighboring
   BGP routers, and store all of it in its Adj-RIBs-IN.  Thus BGP
   routers may end up with storing multiple copies of the same mapping
   information.  For example assuming ISP ASes W, X, Y, and Z have a
   full-mesh connectivity among themselves, and AS-W propagates a
   mapping entry [eggress router R, customer prefix P], then X will
   receive 3 copies of this mappy entry from Y, Z, and W, respectively.

   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.  Since the
   mapping information is global in scope (i.e. the pair of [eggress
   router R, customer prefix P] is independent from which path one uses
   to reach egress router R), this separate session can apply certain
   special rules to remove duplicate entries.

   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 of its 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.  Careful engineering analysis and



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   configuration can eliminate the need for many routers to keep full
   RIB; among those that 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
   would run a different session (e.g., on 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.

   When the RIB size for most routers (other than the APRs) is reduced,
   what are the prefixes that get dropped out of the RIB?  Since APRs
   (or ingress routers, if they are upgraded to handle caching) must
   encapsulate packets towards egress routers that connect to the more
   specific prefixes that have been aggregated out, the ASes must
   exchange the reachability information about their own topologies, so
   that routers in different ASes know how to reach each other.  The
   prefixes that got aggregated out of the core routing system would be
   those that belong to the edge ASes.  As such, Virtual Aggregation
   plus mapping exchange effectively drives the overall routing system
   towards the separation of edge site prefixes from the transit network
   routing, a scalable routing architecture that the APT design has
   depicted[eFIT_IPv6].

   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 APT's Default Mappers that use BGP to
   exchange mapping information.  But once again we must not forget an
   essential difference: we reach this perceived stage towards scalable
   routing through an evolutionary path, instead of requiring
   installation of a new design from day one.

3.5.  Step Five: 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
   would be reflected as mapping updates, which are disseminated through



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   the interconnections of APRs.  We perceive this as a benefit, as
   other non-APR routers can be sheltered from updates due to edge
   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 mapping dissemination system from the edge
   dynamics.  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 to reach the affected customer site.
   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, in the APT design, the failure handling actions are data-
   driven, i.e., a link failure to an edge network is not reported
   unless and until there are data packets that are heading towards the
   failed link.  We are actively working on an evolutionary solution
   that can provide equivalent data-driven handling of edge failures as
   APT does.

3.6.  Summary

   If we can imagine a picture where all the networks in the Internet
   had deployed all the steps of routing scalability improvement we
   sketched above, then the Internet routing system would have converged
   to a new map-encap routing architecture that resembles APT.  Then
   what is the fundamental difference between the evolutionary path
   described in this draft and the deployment of APT?

   First, we emphasize that the fundamental goal is to reduce the
   routing system size, and that the separation of edges and core (or
   EIDs from Locators as in LISP's terminology) itself should *not* be a
   required starting point.  Second, we show that the evolutionary path,
   which goes through several steps with clearly identified benefits and
   minimal cost at each step, can naturally converge towards the
   separation as a result!  We make two points from this last statement:
   (1) This could also be used as an evidence that the evolution can
   indeed lead to architectural changes, as it moves the system towards
   the same point that a new design points to. (2) We used the phrase
   "converge towards separation", rather than "achieving separation",



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   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 of the evolution, some may not have
   even 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 economical and
   operational practice dimensions.


4.  Evolution versus Incremental Deployability

   So far our discussion has focused on a possible evolutionary path of
   the routing system towards a scalable design.  In this section we
   would like to broaden the discussion to a more general question: Many
   new designs make the claim that they are incrementally deployable.
   So what is the difference between an "incrementally deployable" new
   design and an evolutionary path?

   We believe that one fundamental difference is that all new designs
   have an implicit assumption that the entire system would eventually
   move to the new design.  No matter how much effort the designer puts
   into the incremental deployment step of a new design, the design
   itself does not start with the assumption that significant portions
   of the system would never adopt it.  Therefore, it is likely the case
   that the assumed benefit of the new design would be achieved only
   after a majority, if not the whole, of the system has deployed the
   design, and that the cost of incremental deployment would be
   minimized only then as well.  The incremental deployment machinery is
   simply to glue together the part that has made the change and the
   rest that has not, to make the system function together at the
   intermediate, and hopefully transient, stage.  However the system as
   whole would be in a sub-optimal state until the new design gets fully
   deployed.  LISP can serve as an example here.

   In contrast, gradual evolutions in a large system depict a picture
   where changes may happen here and there as needed, but there is no
   expectation that the system as a whole must make a change.  Whoever
   adopts a step forward can gain the benefit, without waiting for
   others to take actions.

   The evolutionary approach recognizes that changes to the Internet can
   only be a gradual process with multiple stages.  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.




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   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.

   Like many others, we too hoped that our new design, APT, could be
   eventually deployed everywhere to put the routing scalability under
   control.  We gradually realized that it is infeasible to attempt to
   roll out a new routing framework (i.e. a clean separation of edge
   prefixes from the core routing system) in a vast deployed system.
   The Internet Protocol, IP, was designed to accommodate heterogeneity
   at subnet technology level.  Today, the intrinsic heterogeneity and
   distributed governance in the Internet require the accommodation of
   heterogeneity at the network control plane.  Solutions to routing
   scalability should be control knobs on top of the deployed base to
   those parties who need them, and there should not be an expectation
   that the entire Internet would (eventually) move to a new design.

   An evolutionary process accommodates differences at different parts
   of the system, as new functions are built on top of, hence can
   peacefully co-exist with, the deployed base.  On the other hand, a
   revolutionary new design focuses on the final outcome once the
   replacement of the old by the new is done throughout the Internet.
   The latter would offer a clean picture of the overall system,
   assuming the final stage could be reached.  The former, on the other
   hand, would present a much messier or more complex picture, both
   because we twist old protocols for new functions and because
   different parts may do different things.  As pointed out by Haldane
   over 80 years ago [SizeImpact], in biological systems, "The higher
   animals are not larger than the lower because they are more
   complicated.  They are more complicated because they are larger."  We
   believe the same is true for man-built systems: as the system grows
   larger in size, it is necessarily becoming more complicated.


5.  Acknowledgements

   The authors are part of the APT team.  The APT project is funded by
   US National Science Foundation.  The survey mentioned in Section 3.1
   was conducted by Dan Jen.


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



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   the Internet security.


7.  Informative References

   [APT]      Jen, D., Meisel, M., Massey, D., Wang, L., Zhang, B., and
              L. Zhang, "APT: A Practical Tunneling Architecture for
              Routing Scalability", UCLA Computer Science Departnent
              Technical Report 080004, March 2008.

   [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>.

   [FIBAggregate]
              Zhang, B., Wang, L., Zhao, X., Liu, Y., and L. Zhang, "FIB
              Aggregation", draft-zhang-fibaggregation-01.txt, October
              2009.

   [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.

   [NANOG44]  Roisman, D., "Extending the Life of Layer 3 Switches in a
              256k+ Route World", NANOG44, October 2008, <http://
              www.nanog.org/meetings/nanog44/presentations/Monday/
              Roisman_lightning.pdf>.

   [RFC4984]  Meyer, D., Zhang, L., and K. Fall, "Report from the IAB
              Workshop on Routing and Addressing", RFC 4984,
              September 2007.

   [RRG]      RRG, "IRTF Routing Research Group Home Page", <http://
              tools.ietf.org/group/irtf/trac/wiki/RoutingResearchGroup>.

   [SIRA]     Zhang, B. and et. al., "A Secure and Scalable Internet
              Routing Architecture", ACM SIGCOMM 2006 Poster Session.

   [SizeImpact]
              Haldane, "On Being the Right Size", 1928,



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              <http://irl.cs.ucla.edu/papers/right-size.html>.

   [TopologyGrowth]
              Oliveira, R., Zhang, B., and L. Zhang, "Observing the
              Evolution of Internet AS Topology", ACM SIGCOMM 2007.

   [VA_performance]
              Ballani, B., Francis, P., Jen, D., Xu, X., and L. Zhang,
              "FIB Aggregation", draft-ietf-grow-va-perf-00.txt, July
              2009.

   [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


   Lan Wang
   Univ. of Memphis

   Email: lanwang@cs.memphis.edu












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