





Network Working Group                                      D. Meyer, Ed.
Request for Comments: 4984                                 L. Zhang, Ed.
Category: Informational                                     K. Fall, Ed.
                                                          September 2007


         Report from the IAB Workshop on Routing and Addressing

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Abstract

   This document reports the outcome of the Routing and Addressing
   Workshop that was held by the Internet Architecture Board (IAB) on
   October 18-19, 2006, in Amsterdam, Netherlands.  The primary goal of
   the workshop was to develop a shared understanding of the problems
   that the large backbone operators are facing regarding the
   scalability of today's Internet routing system.  The key workshop
   findings include an analysis of the major factors that are driving
   routing table growth, constraints in router technology, and the
   limitations of today's Internet addressing architecture.  It is hoped
   that these findings will serve as input to the IETF community and
   help identify next steps towards effective solutions.

   Note that this document is a report on the proceedings of the
   workshop.  The views and positions documented in this report are
   those of the workshop participants and not of the IAB.  Furthermore,
   note that work on issues related to this workshop report is
   continuing, and this document does not intend to reflect the
   increased understanding of issues nor to discuss the range of
   potential solutions that may be the outcome of this ongoing work.
















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Key Findings from the Workshop . . . . . . . . . . . . . . . .  4
     2.1.  Problem #1: The Scalability of the Routing System  . . . .  4
       2.1.1.  Implications of DFZ RIB Growth . . . . . . . . . . . .  5
       2.1.2.  Implications of DFZ FIB Growth . . . . . . . . . . . .  6
     2.2.  Problem #2: The Overloading of IP Address Semantics  . . .  6
     2.3.  Other Concerns . . . . . . . . . . . . . . . . . . . . . .  7
     2.4.  How Urgent Are These Problems? . . . . . . . . . . . . . .  8
   3.  Current Stresses on the Routing and Addressing System  . . . .  8
     3.1.  Major Factors Driving Routing Table Growth . . . . . . . .  8
       3.1.1.  Avoiding Renumbering  . . . . . . . . . . . . . . . . . 9
       3.1.2.  Multihoming  . . . . . . . . . . . . . . . . . . . . . 10
       3.1.3.  Traffic Engineering  . . . . . . . . . . . . . . . . . 10
     3.2.  IPv6 and Its Potential Impact on Routing Table Size  . . . 11
   4.  Implications of Moore's Law on the Scaling Problem . . . . . . 11
     4.1.  Moore's Law  . . . . . . . . . . . . . . . . . . . . . . . 12
       4.1.1.  DRAM . . . . . . . . . . . . . . . . . . . . . . . . . 13
       4.1.2.  Off-chip SRAM  . . . . . . . . . . . . . . . . . . . . 13
     4.2.  Forwarding Engines . . . . . . . . . . . . . . . . . . . . 13
     4.3.  Chip Costs . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.4.  Heat and Power . . . . . . . . . . . . . . . . . . . . . . 14
     4.5.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . 15
   5.  What Is on the Horizon . . . . . . . . . . . . . . . . . . . . 15
     5.1.  Continual Growth . . . . . . . . . . . . . . . . . . . . . 15
     5.2.  Large Numbers of Mobile Networks . . . . . . . . . . . . . 16
     5.3.  Orders of Magnitude Increase in Mobile Edge Devices  . . . 16
   6.  What Approaches Have Been Investigated . . . . . . . . . . . . 17
     6.1.  Lessons from MULTI6  . . . . . . . . . . . . . . . . . . . 17
     6.2.  SHIM6: Pros and Cons . . . . . . . . . . . . . . . . . . . 18
     6.3.  GSE/Indirection Solutions: Costs and Benefits  . . . . . . 19
     6.4.  Future for Indirection . . . . . . . . . . . . . . . . . . 20
   7.  Problem Statements . . . . . . . . . . . . . . . . . . . . . . 21
     7.1.  Problem #1: Routing Scalability  . . . . . . . . . . . . . 21
     7.2.  Problem #2: The Overloading of IP Address Semantics  . . . 22
       7.2.1.  Definition of Locator and Identifier . . . . . . . . . 22
       7.2.2.  Consequence of Locator and Identifier Overloading  . . 23
       7.2.3.  Traffic Engineering and IP Address Semantics
               Overload . . . . . . . . . . . . . . . . . . . . . . . 24
     7.3.  Additional Issues  . . . . . . . . . . . . . . . . . . . . 24
       7.3.1.  Routing Convergence  . . . . . . . . . . . . . . . . . 24
       7.3.2.  Misaligned Costs and Benefits  . . . . . . . . . . . . 25
       7.3.3.  Other Concerns . . . . . . . . . . . . . . . . . . . . 25
     7.4.  Problem Recognition  . . . . . . . . . . . . . . . . . . . 26
   8.  Criteria for Solution Development  . . . . . . . . . . . . . . 26
     8.1.  Criteria on Scalability  . . . . . . . . . . . . . . . . . 26
     8.2.  Criteria on Incentives and Economics . . . . . . . . . . . 27



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     8.3.  Criteria on Timing . . . . . . . . . . . . . . . . . . . . 28
     8.4.  Consideration on Existing Systems  . . . . . . . . . . . . 28
     8.5.  Consideration on Security  . . . . . . . . . . . . . . . . 29
     8.6.  Other Criteria . . . . . . . . . . . . . . . . . . . . . . 29
     8.7.  Understanding the Tradeoff . . . . . . . . . . . . . . . . 29
   9.  Workshop Recommendations . . . . . . . . . . . . . . . . . . . 30
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 31
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 31
   12. Informative References . . . . . . . . . . . . . . . . . . . . 31
   Appendix A.  Suggestions for Specific Steps  . . . . . . . . . . . 35
   Appendix B.  Workshop Participants . . . . . . . . . . . . . . . . 35
   Appendix C.  Workshop Agenda . . . . . . . . . . . . . . . . . . . 36
   Appendix D.  Presentations . . . . . . . . . . . . . . . . . . . . 37

1.  Introduction

   It is commonly recognized that today's Internet routing and
   addressing system is facing serious scaling problems.  The ever-
   increasing user population, as well as multiple other factors
   including multi-homing, traffic engineering, and policy routing, have
   been driving the growth of the Default Free Zone (DFZ) routing table
   size at an increasing and potentially alarming rate [DFZ][BGT04].
   While it has been long recognized that the existing routing
   architecture may have serious scalability problems, effective
   solutions have yet to be identified, developed, and deployed.

   As a first step towards tackling these long-standing concerns, the
   IAB held a "Routing and Addressing Workshop" in Amsterdam,
   Netherlands on October 18-19, 2006.  The main objectives of the
   workshop were to identify existing and potential factors that have
   major impacts on routing scalability, and to develop a concise
   problem statement that may serve as input to a set of follow-on
   activities.  This document reports on the outcome from that workshop.

   The remainder of the document is organized as follows: Section 2
   provides an executive summary of the workshop findings.  Section 3
   describes the sources of stress in the current global routing and
   addressing system.  Section 4 discusses the relationship between
   Moore's law and our ability to build large routers.  Section 5
   describes a few foreseeable factors that may exacerbate the current
   problems outlined in Section 2.  Section 6 describes previous work in
   this area.  Section 7 describes the problem statements in more
   detail, and Section 8 discusses the criteria that constrain the
   solution space.  Finally, Section 9 summarizes the recommendations
   made by the workshop participants.






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   The workshop participant list is attached in Appendix B.  The agenda
   can be found in Appendix C, and Appendix D provides pointers to the
   presentations from the workshop.

   Finally, note that this document is a report on the outcome of the
   workshop, not an official document of the IAB.  Any opinions
   expressed are those of the workshop participants and not of the IAB.

2.  Key Findings from the Workshop

   This section provides a concise summary of the key findings from the
   workshop.  While many other aspects of a routing and addressing
   system were discussed, the first two problems described in this
   section were deemed the most important ones by the workshop
   participants.

   The clear, highest-priority takeaway from the workshop is the need to
   devise a scalable routing and addressing system, one that is scalable
   in the face of multihoming, and that facilitates a wide spectrum of
   traffic engineering (TE) requirements.  Several scalability problems
   of the current routing and addressing systems were discussed, most
   related to the size of the DFZ routing table (frequently referred to
   as the Routing Information Base, or RIB) and its implications.  Those
   implications included (but were not limited to) the sizes of the DFZ
   RIB and FIB (the Forwarding Information Base), the cost of
   recomputing the FIB, concerns about the BGP convergence times in the
   presence of growing RIB and FIB sizes, and the costs and power (and
   hence heat dissipation) properties of the hardware needed to route
   traffic in the core of the Internet.

2.1.  Problem #1: The Scalability of the Routing System

   The shape of the growth curve of the DFZ RIB has been the topic of
   much research and discussion since the early days of the Internet
   [H03].  There have been various hypotheses regarding the sources of
   this growth.  The workshop identified the following factors as the
   main driving forces behind the rapid growth of the DFZ RIB:

   o  Multihoming,

   o  Traffic engineering,

   o  Non-aggregatable address allocations (a big portion of which is
      inherited from historical allocations), and

   o  Business events, such as mergers and acquisitions.





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   All of the above factors can lead to prefix de-aggregation and/or the
   injection of unaggregatable prefixes into the DFZ RIB.  Prefix de-
   aggregation leads to an uncontrolled DFZ RIB growth because, absent
   some non-topologically based routing technology (for example, Routing
   On Flat Labels [ROFL] or any name-independent compact routing
   algorithm, e.g., [CNIR]), topological aggregation is the only known
   practical approach to control the growth of the DFZ RIB.  The
   following section reviews the workshop discussion of the implications
   of the growth of the DFZ RIB.

2.1.1.  Implications of DFZ RIB Growth

   Presentations made at the workshop showed that the DFZ RIB has been
   growing at greater than linear rates for several years [DFZ].  While
   this has the obvious effects on the requirements for RIB and FIB
   memory sizes, the growth driven by prefix de-aggregation also exposes
   the core of the network to the dynamic nature of the edges, i.e., the
   de-aggregation leads to an increased number of BGP UPDATE messages
   injected into the DFZ (frequently referred to as "UPDATE churn").
   Consequently, additional processing is required to maintain state for
   the longer prefixes and to update the FIB.  Note that, although the
   size of the RIB is bounded by the given address space size and the
   number of reachable hosts (i.e., O(m*2^32) for IPv4, where <m> is the
   average number of peers each BGP router may have), the amount of
   protocol activity required to distribute dynamic topological changes
   is not.  That is, the amount of BGP UPDATE churn that the network can
   experience is essentially unbounded.  It was also noted that the
   UPDATE churn, as currently measured, is heavy-tailed [ATNAC2006].
   That is, a relatively small number of Autonomous Systems (ASs) or
   prefixes are responsible for a disproportionately large fraction of
   the UPDATE churn that we observe today.  Furthermore, much of the
   churn may turn out to be unnecessary information, possibly due to
   instability of edge ASs being injected into the global routing system
   [DynPrefix], or arbitrage of some bandwidth pricing model (see [GIH],
   for example, or the discussion of the behavior of AS 9121 in
   [BGP2005]).

   Finally, it was noted by the workshop participants that the UPDATE
   churn situation may be exacerbated by the current Regional Internet
   Registry (RIR) policy in which end sites are allocated Provider-
   Independent (PI) addresses.  These addresses are not topologically
   aggregatable, and as such, bring the churn problem described above
   into the core routing system.  Of course, as noted by several
   participants, the RIRs have no real choice in this matter, as many
   enterprises demand PI addresses that allow them to multihome without
   the "provider lock" that Provider-Allocated (PA) [PIPA] address space
   creates.  Some enterprises also find the renumbering cost associated
   with PA address assignments unacceptable.



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2.1.2.  Implications of DFZ FIB Growth

   One surprising outcome of the workshop was the observation made by
   Tony Li about the relationship between "Moore's Law" [ML] and our
   ability to build cost-effective, high-performance routers (see
   Appendix D).  "Moore's Law" is the empirical observation that the
   transistor density of integrated circuits, with respect to minimum
   component cost, doubles roughly every 24 months.  A commonly held
   wisdom is that Moore's law would save the day by ensuring that
   technology will continue to scale at historical rates that surpass
   the growth rate of routing information handled by core router
   hardware.  However, Li pointed out that Moore's Law does not apply to
   building high-end routers as far as the cost is concerned.

   Moore's Law applies specifically to the high-volume portion of the
   semiconductor industry, while the low-volume, customized silicon used
   in core routing is well off Moore's Law's cost curve.  In particular,
   off-chip SRAM is commonly used for storing FIB data, and the driver
   for low-latency, high-capacity SRAM used to be PC cache memory.
   However, recently cache memory has been migrating directly onto the
   processor die, and cell phones are now the primary driver for off-
   chip SRAM.  Given cell phones require low-power, small-capacity parts
   that are not applicable to high-end routers, the SRAMs that are
   favored for router design are not volume parts and do not track with
   Moore's law.

2.2.  Problem #2: The Overloading of IP Address Semantics

   One of the fundamental assumptions underlying the scalability of
   routing systems was eloquently stated by Yakov Rekhter (and is
   sometimes referred to as "Rekhter's Law"), namely:

        "Addressing can follow topology or topology can follow
         addressing. Choose one."

   The same idea was expressed by Mike O'Dell's design of an alternate
   address architecture for ipv6 [GSE], where the address structure was
   designed specifically to enable "aggressive topological aggregation"
   to scale the routing system.  Noel Chiappa has also written
   extensively on this topic (see, e.g., [EID]).

   There is, however, a difficulty in creating (and maintaining) the
   kind of congruence envisioned by Rekhter's Law in today's Internet.
   The difficulty arises from the overloading of addressing with the
   semantics of both "who" (endpoint identifier, as used by transport
   layer) and "where" (locators for the routing system); some might also
   add that IP addresses are also overloaded with "how" [GIH].  In any




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   event, this kind of overloading is felt to have had deep implications
   for the scalability of the global routing system.

   A refinement to Rekhter's Law, then, is that for the Internet routing
   system to scale, an IP address must be assigned in such a way that it
   is congruent with the Internet's topology.  However, identifiers are
   typically assigned based upon organizational (not topological)
   structure and have stability as a desirable property, a "natural
   incongruence" arises.  As a result, it is difficult (if not
   impossible) to make a single number space serve both purposes
   efficiently.

   Following the logic of the previous paragraphs, workshop participants
   concluded that the so-called "locator/identifier overload" of the IP
   address semantics is one of the causes of the routing scalability
   problem as we see today.  Thus, a "split" seems necessary to scale
   the routing system, although how to actually architect and implement
   such a split was not explored in detail.

2.3.  Other Concerns

   In addition to the issues described in Section 2.1 and Section 2.2,
   the workshop participants also identified the following three
   pressing, but "second tier", issues.

   The first one is a general concern with IPv6 deployment.  It is
   commonly believed that the IPv4 address space has put an effective
   constraint on the IPv4 RIB growth.  Once this constraint is lifted by
   the deployment of IPv6, and in the absence of a scalable routing
   strategy, the rapid DFZ RIB size growth problem today can potentially
   be exacerbated by IPv6's much larger address space.  The only routing
   paradigm available today for IPv6 is a combination of Classless
   Inter-Domain Routing (CIDR) [RFC4632] and Provider-Independent (PI)
   address allocation strategies [PIPA] (and possibly SHIM6 [SHIM6] when
   that technology is developed and deployed).  Thus, the opportunity
   exists to create a "swamp" (unaggregatable address space) that can be
   many orders of magnitude larger than what we faced with IPv4.  In
   short, the advent of IPv6 and its larger address space further
   underscores both the concerns raised in Section 2.1, and the
   importance of resolving the architectural issue raised in
   Section 2.2.

   The second issue is slow routing convergence.  In particular, the
   concern was that growth in the number of routes that service
   providers must carry will cause routing convergence to become a
   significant problem.





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   The third issue is the misalignment of costs and benefits in today's
   routing system.  While the IETF does not typically consider the
   "business model" impacts of various technology choices, many
   participants felt that perhaps the time has come to review that
   philosophy.

2.4.  How Urgent Are These Problems?

   There was a fairly universal agreement among the workshop
   participants that the problems outlined in Section 2.1 and
   Section 2.2 need immediate attention.  This need was not because the
   participants perceived a looming, well-defined "hit the wall" date,
   but rather because these are difficult problems that to date have
   resisted solution, are likely to get more unwieldy as IPv6 deployment
   proceeds, and the development and deployment of an effective solution
   will necessarily take at least a few years.

3.  Current Stresses on the Routing and Addressing System

   The primary concern voiced by the workshop participants regarding the
   state of the current Internet routing system was the rapid growth of
   the DFZ RIB.  The number of entries in 2005 ranged from about 150,000
   entries to 175,000 entries [BGP2005]; this number has reached 200,000
   as of October 2006 [CIDRRPT], and is projected to increase to 370,000
   or more within 5 years [Fuller].  Some workshop participants
   projected that the DFZ could reach 2 million entries within 15 years,
   and there might be as many as 10 million multihomed sites by 2050.

   Another related concern was the number of prefixes changed, added,
   and withdrawn as a function of time (i.e., BGP UPDATE churn).  This
   has a detrimental impact on routing convergence, since UPDATEs
   frequently necessitate a re-computation and download of the FIB.  For
   example, a BGP router may observe up to 500,000 BGP updates in a
   single day [DynPrefix], with the peak arrival rates over 1000 updates
   per second.  Such UPDATE churn problems are not limited to DFZ
   routes; indeed, the number of internal routes carried by large ISPs
   also threatens convergence times, given that such internal routes
   include more specifics, Virtual Private Network (VPN) routes, and
   other routes that do not appear in the DFZ [ATNAC2006].

3.1.  Major Factors Driving Routing Table Growth

   The growth of the DFZ RIB results from the addition of more prefixes
   to the table.  Although some of this growth is organic (i.e., results
   simply from growth of the Internet), a large portion of the growth
   results from de-aggregation of address prefixes (i.e., more specific





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   prefixes).  In this section, we discuss in more detail why this trend
   is accelerating and may be cause for concern.

   An increasing fraction of the more-specific prefixes found in the DFZ
   are due to deliberate action on the part of operators [ATNAC2006].
   Motivations to advertise these more-specifics include:

   o  Traffic Engineering, where load is balanced across multiple links
      through selective advertisement of more-specific routes on
      different links to adjust the amount of traffic received on each;
      and

   o  Attempts to prevent prefix-hijacking by other operators who might
      advertise more-specifics to steer traffic toward them; there are
      several known instances of this behavior today [BHB06].

3.1.1.  Avoiding Renumbering

   The workshop participants noted that customers generally prefer to
   have PI address space.  Doing so gives them additional agility in
   selecting ISPs and helps them avoid the need to renumber.  Many end-
   systems use DHCP to assign addresses, so a cursory analysis might
   suggest renumbering might involve modification of a modest number of
   routers and servers (perhaps rather than end hosts) at a site that
   was forced to renumber.

   In reality, however, renumbering can be more cumbersome because IP
   addresses are often used for other purposes such as access control
   lists.  They are also sometimes hard-coded into applications used in
   environments where failure of the DNS would be catastrophic (e.g.,
   some remote monitoring applications).  Although renumbering may be a
   mild inconvenience for some sites and guidelines have been developed
   for renumbering a network without a flag day [RFC4192], for others,
   the necessary changes are sufficiently difficult so as to make
   renumbering effectively impossible.

   For these reasons, PI address space is sought by a growing number of
   customers.  Current RIR policy reflects this trend, and their policy
   is to allocate PI prefixes to all customers who claim a need.
   Routing PI prefixes requires additional entries in the DFZ routing
   and forwarding tables.  At present, ISPs do not typically charge to
   route PI prefixes.  Therefore, the "costs" of the additional
   prefixes, in terms of routing table entries and processing overhead,
   is born by the global routing system as a whole, rather than directly
   by the users of PI space.  The workshop participants observed that no
   strong disincentive exists to discourage the increasing use of PI
   address space.




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3.1.2.  Multihoming

   Multihoming refers generically to the case in which a site is served
   by more than one ISP [RFC4116].  There are several reasons for the
   observed increase in multihoming, including the increased reliance on
   the Internet for mission- and business-critical applications and the
   general decrease in cost to obtain Internet connectivity.
   Multihoming provides backup routing -- Internet connection
   redundancy; in some circumstances, multihoming is mandatory due to
   contract or law.  Multihoming can be accomplished using either PI or
   PA address space, and multihomed sites generally have their own AS
   numbers (although some do not; this generally occurs when such
   customers are statically routed).

   A multihomed site using PI address space has its prefixes present in
   the forwarding and routing tables of each of its providers.  For PA
   space, each prefix allocated from one provider's address allocation
   will be aggregatable for that provider but not the others.  If the
   addresses are allocated from a 'primary' ISP (i.e., one that the site
   uses for routing unless a failure occurs), then the additional
   routing table entries only appear during path failures to that
   primary ISP.  A problem with multihoming arises when a customer's PA
   IP prefixes are advertised by AS(es) other than their 'primary'
   ISP's.  Because of the longest-matching prefix forwarding rule, in
   this case, the customer's traffic will be directed through the non-
   primary AS(s).  In response, the primary ISP is forced to de-
   aggregate the customer's prefix in order to keep the customer's
   traffic flowing through it instead of the non-primary AS(s).

3.1.3.  Traffic Engineering

   Traffic engineering (TE) is the act of arranging for certain Internet
   traffic to use or avoid certain network paths (that is, TE puts
   traffic where capacity exists, or where some set of parameters of the
   path is more favorable to the traffic being placed there).  TE is
   performed by both ISPs and customer networks, for three primary
   reasons:

   o  First, as mentioned above, to match traffic with network capacity,
      or to spread the traffic load across multiple links (frequently
      referred to as "load balancing").

   o  Second, to reduce costs by shifting traffic to lower cost paths or
      by balancing the incoming and outgoing traffic volume to maintain
      appropriate peering relations.






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   o  Finally, TE is sometimes deployed to enforce certain forms of
      policy (e.g., Canadian government traffic may not be permitted to
      transit through the United States).

   Few tools exist for inter-domain traffic engineering today.  Network
   operators usually achieve traffic engineering by "tweaking" the
   processing of routing protocols to achieve desired results.  At the
   BGP level, if the address range requiring TE is a portion of a larger
   PA address aggregate, network operators implementing TE are forced to
   de-aggregate otherwise aggregatable prefixes in order to steer the
   traffic of the particular address range to specific paths.

   In today's highly competitive environment, providers require TE to
   maintain good performance and low cost in their networks.  However,
   the current practice of TE deployment results in an increase of the
   DFZ RIB; although individual operators may have a certain gain from
   doing TE, it leads to an overall increased cost for the Internet
   routing infrastructure as a whole.

3.2.  IPv6 and Its Potential Impact on Routing Table Size

   Due to the increased IPv6 address size over IPv4, a full immediate
   transition to IPv6 is estimated to lead to the RIB and FIB sizes
   increasing by a factor of about four.  The size of the routing table
   based on a more realistic assumption, that of parallel IPv4 and IPv6
   routing for many years, is less clear.  An increasing amount of
   allocated IPv6 address prefixes is in PI space.  ARIN [ARIN] has
   relaxed its policy for allocation of such space and has been
   allocating /48 prefixes when customers request PI prefixes.  Thus,
   the same pressures affecting IPv4 address allocations also affect
   IPv6 allocations.

4.  Implications of Moore's Law on the Scaling Problem

   [Editor's note: The information in this section is gathered from
   presentations given at the workshop.  The presentation slides can be
   retrieved from the pointer provided in Appendix D.  It is worth
   noting that this information has generated quite a bit of discussion
   since the workshop, and as such requires further community input.]

   The workshop heard from Tony Li about the relationship between
   Moore's law and the ability to build cost-effective, high-performance
   routers.  The scalability of the current routing subsystem manifests
   itself in the forwarding table (FIB) and routing table (RIB) of the
   routers in the core of the Internet.  The implementation choices for
   FIB storage are on-chip SRAM, off-chip SRAM, or DRAM.  DRAM is
   commonly used in lower end devices.  RIB storage is done via DRAM.




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   [Editor's note: The exact implementation of a high-performance
   router's RIB and FIB memories is the subject of much debate; it is
   also possible that alternative designs may appear in the future.]

   The scalability question then becomes whether these memory
   technologies can scale faster than the size of the full routing
   table.  Intrinsic in this statement is the assumption that core
   routers will be continually and indefinitely upgraded on a periodic
   basis to keep up with the technology curve and that the costs of
   those upgrades will be passed along to the general Internet
   community.

4.1.  Moore's Law

   In 1965, Gordon Moore projected that the density of transistors in
   integrated circuits could double every two years, with respect to
   minimum component cost.  The period was subsequently adjusted to be
   between 18-24 months and this conjecture became known as Moore's Law
   [ML].  The semiconductor industry has been following this density
   trend for the last 40 or so years.

   The commonly held wisdom is that Moore's law will save the day by
   ensuring that technology will continue to scale at the historical
   rate that will surpass the growth rate of routing information.
   However, it is vital to understand that Moore's law comes out of the
   high-volume portion of the semiconductor industry, where the costs of
   silicon are dominated by the actual fabrication costs.  The
   customized silicon used in core routers is produced in far lower
   volume, typically in the 1,000-10,000 parts per year, whereas
   microprocessors are running in the tens of millions per year.  This
   places the router silicon well off the cost curve, where the
   economies of scale are not directly inherited, and yield improvements
   are not directly inherited from the best current practices.  Thus,
   router silicon benefits from the technological advances made in
   semiconductors, but does not follow Moore's law from a cost
   perspective.

   To date, this cost difference has not shown clearly.  However, the
   growth in bandwidth of the Internet and the steady climb of the speed
   of individual links has forced router manufacturers to apply more
   sophisticated silicon technology continuously.  There has been a new
   generation of router hardware that has grown at about 4x the
   bandwidth every three years, and increases in routing table size have
   been absorbed by the new generations of hardware.  Now that router
   hardware is nearing the practical limits of per-lambda bandwidth, it
   is possible that upgrades solely for meeting the forwarding table
   scaling will become more visible.




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4.1.1.  DRAM

   In routers, DRAM is used for storing the RIB and, in lower-end
   routers, is also used for storing the FIB.  Historically, DRAM
   capacity grows at about 4x every 3.3 years.  This translates to 2.4x
   every 2 years, so DRAM capacity actually grows faster than Moore's
   law would suggest.  DRAM speed, however, only grows about 10% per
   year, or 1.2x every 2 years [DRAM] [Molinero].  This is an issue
   because BGP convergence time is limited by DRAM access speeds.  In
   processing a BGP update, a BGP speaker receives a path and must
   compare it to all of the other paths it has stored for the prefix.
   It then iterates over all of the prefixes in the update stream.  This
   results in a memory access pattern that has proven to limit the
   effectiveness of processor caching.  As a result, BGP convergence
   time degrades at the routing table growth rate, divided by the speed
   improvement rate of DRAM.  In the long run, this is likely to become
   a significant issue.

4.1.2.  Off-chip SRAM

   Storing the FIB in off-chip SRAM is a popular design decision.  For
   high-speed interfaces, this requires low-latency, high-capacity
   parts.  The driver for this type of SRAM was formerly PC cache
   memory.  However, this cache memory has recently been migrating
   directly onto the processor die, so that the volumes of cache memory
   have fallen off.  Today, the primary driver for off-chip SRAM is cell
   phones, which require low-power, small-capacity parts that are not
   applicable to high-end router design.  As a result, the SRAMs that
   are favored for router design are not volume parts.  They have fallen
   off the cost curve and do not track with Moore's law.

4.2.  Forwarding Engines

   For many years, router companies have been building special-purpose
   silicon to provide high-speed packet-forwarding capabilities.  This
   has been necessary because the architectural limitations of general
   purpose CPUs make them incapable of providing the high-bandwidth, low
   latency, low-jitter I/O interface for making high speed forwarding
   decisions.

   As a result, the forwarding engines being built for high-end routers
   are some of the most sophisticated Application-specific Integrated
   Circuits (ASICs) being built, and are currently only one
   technological step behind general-purpose CPUs.  This has been
   largely driven by the growth in bandwidth and has already pushed the
   technology well beyond the knee in the price/performance curve.
   Given that this level of technology is already a requirement to meet
   the performance goals, using on-chip SRAM is an interesting design



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   alternative.  If this choice is selected, then growth in the
   available FIB is tightly coupled to process technology improvements,
   which are driven by the general-purpose CPU market.  While this
   growth rate should suffice, in general, the forwarding engine market
   is decidedly off the high-volume price curve, resulting in spiraling
   costs to support basic forwarding.

   Moreover, if there is any change in Moore's law or decrease in the
   rate of processor technology evolution, the forwarding engine could
   quickly become the technological leader of silicon technology.  This
   would rapidly result in forwarding technology becoming prohibitively
   expensive.

4.3.  Chip Costs

   Each process technology step in chip development has come at
   increasing cost.  The milestone of sending a completed chip design to
   a fabricator for manufacturing is known as 'tapeout', and is the
   point where the designer pays for the fixed overhead of putting the
   chip into production.  The costs of taping out a chip have been
   rising about 1.5x every 2 years, driven by new process technology.
   The actual design and development costs have been rising similarly,
   because each new generation of technology increases the device count
   by roughly a factor of 2.  This allows new features and chip
   architectures, which inevitably lead to an increase in complexity and
   labor costs.  If new chip development was driven solely by the need
   to scale up memory, and if memory structures scaled, then we would
   expect labor costs to remain fixed.  Unfortunately, memory structures
   typically do not seem to scale linearly.  Individual memory
   controllers have a non-negligible cost, leading to the design for an
   internal on-chip interconnect of memories.  The net result is that we
   can expect that chip development costs to continue to escalate
   roughly in line with the increases in tapeout costs, leading to an
   ongoing cost curve of about 1.5x every 2 years.  Since each
   technology step roughly doubles memory, that implies that if demand
   grows faster than about (2x/1.5x) = 1.3x per year, then technology
   refresh will not be able to remain on a constant cost curve.

4.4.  Heat and Power

   Transistors consume power both when idle ("leakage current") and when
   switching.  The smaller and hotter the transistors, the larger the
   leakage current.  The overall power consumption is not linear with
   the density increase.  Thus, as the need for more powerful routers
   increases, cooling technology grows more taxed.  At present, the
   existing air cooling system is starting to be a limiting factor for
   scaling high-performance routers.




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   A key metric for system evaluation is now the unit of forwarding
   bandwidth per Watt-- [(Mb/s)/W].  About 60% of the power goes to the
   forwarding engine circuits, with the rest divided between the
   memories, route processors, and interconnect.  Using parallelization
   to achieve higher bandwidths can aggravate the situation, due to
   increased power and cooling demands.

   [Editor's note: Many in the community have commented that heat, power
   consumption, and the attendant heat dissipation, along with size
   limitations of fabrication processes for high speed parallel I/O
   interfaces, are the current limiting factors.]

4.5.  Summary

   Given the uncontrolled nature of its growth rate, there is some
   concern about the long-term prospects for the health and cost of the
   routing subsystem of the Internet.  The ongoing growth will force
   periodic technology refreshes.  However, the growth rate can possibly
   exceed the rate that can be supported at constant cost based on the
   development costs seen in the router industry.  Since high-end
   routing is based on low-volume technology, the cost advantages that
   the bulk of the broader computing industry see, based on Moore's law,
   are not directly inherited.  This leads to a sustainable growth rate
   of 1.3x/2yrs for the forwarding table and 1.2x/2yrs for the routing
   table.  Given that the current baseline growth is at 1.3x/2yrs
   [CIDRRPT], with bursts that even exceed Moore's law, the trend is for
   the costs of technology refresh to continue to grow, indefinitely,
   even in constant dollars.

5.  What Is on the Horizon

   Routing and addressing are two fundamental pieces of the Internet
   architecture, thus any changes to them will likely impact almost all
   of the "IP stack", from applications to packet forwarding.  In
   resolving the routing scalability problems, as agreed upon by the
   workshop attendees, we should aim at a long-term solution.  This
   requires a clear understanding of various trends in the foreseeable
   future: the growth in Internet user population, the applications, and
   the technology.

5.1.  Continual Growth

   The backbone operators expect that the current Internet user
   population base will continue to expand, as measured by the traffic
   volume, the number of hosts connected to the Internet, the number of
   customer networks, and the number of regional providers.





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5.2.  Large Numbers of Mobile Networks

   Boeing's Connexion service pioneered the deployment of commercial
   mobile networks that may change their attachment points to the
   Internet on a global scale.  It is believed that such in-flight
   Internet connectivity would likely become commonplace in the not-too-
   distant future.  When that happens, there can be multiple thousands
   of airplane networks in the air at any given time.

   Given that today's DFZ RIB already handles over 200,000 prefixes
   [CIDRRPT], several thousands of mobile networks, each represented by
   a single prefix announcement, may not necessarily raise serious
   routing scalability or stability concerns.  However, there is an open
   question regarding whether this number can become substantially
   larger if other types of mobile networks, such as networks on trains
   or ships, come into play.  If such mobile networks become
   commonplace, then their impact on the global routing system needs to
   be assessed.

5.3.  Orders of Magnitude Increase in Mobile Edge Devices

   Today's technology trend indicates that billions of hand-held gadgets
   may come online in the next several years.  There were different
   opinions regarding whether this would, or would not, have a
   significant impact on global routing scalability.  The current
   solutions for mobile hosts, namely Mobile IP (e.g., [RFC3775]),
   handle the mobility by one level of indirection through home agents;
   mobile hosts do not appear any different, from a routing perspective,
   than stationary hosts.  If we follow the same approach, new mobile
   devices should not present challenges beyond the increase in the size
   of the host population.

   The workshop participants recognized that the increase in the number
   of mobile devices can be significant, and that if a scalable routing
   system supporting generic identity-locator separation were developed
   and introduced, billions of mobile gadgets could be supported without
   bringing undue impact on global routing scalability and stability.

   Further investigation is needed to gain a complete understanding of
   the implications on the global routing system of connecting many new
   mobile hand-held devices (including mobile sensor networks) to the
   Internet.









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6.  What Approaches Have Been Investigated

   Over the years, there have been many efforts designed to investigate
   scalable inter-domain routing for the Internet [IDR-REQS].  To
   benefit from the insights obtained from these past results, the
   workshop reviewed several major previous and ongoing IETF efforts:

   1.  The MULTI6 working group's exploration of the solution space and
       the lessons learned,

   2.  The solution to multihoming being developed by the SHIM6 Working
       Group, and its pros and cons,

   3.  The GSE proposal made by O'Dell in 1997, and its pros and cons,
       and

   4.  Map-and-Encap [RFC1955], a general indirection-based solution to
       scalable multihoming support.

6.1.  Lessons from MULTI6

   The MULTI6 working group was chartered to explore the solution space
   for scalable support of IPv6 multihoming.  The numerous proposals
   collected by MULTI6 working group generally fell into one of two
   major categories: resolving the above-mentioned conflict by using
   provider-independent address assignments, or by assigning multiple
   address prefixes to multihomed sites, one for each of its providers,
   so that all the addresses can be topologically aggregatable.

   The first category includes proposals of (1) simply allocating
   provider-independent address space, which is effectively the current
   practice, and (2) assigning IP addresses based on customers'
   geographical locations.  The first approach does not scale; the
   second approach represents a fundamental change to the Internet
   routing system and its economic model, and imposes undue constraints
   on ISPs.  These proposals were found to be incomplete, as they
   offered no solutions to the new problems they introduced.

   The majority of the proposals fell into the second category--
   assigning multiple address blocks per site.  Because IP addresses
   have been used as identifiers by higher-level protocols and
   applications, these proposals face a fundamental design decision
   regarding which layer should be responsible for mapping the multiple
   locators (i.e., the multiple addresses received from ISPs) to an
   identifier.  A related question involves which nodes are responsible
   for handling multiple addresses.  One can implement a multi-address
   scheme at either each individual host or at edge routers of a site,
   or even both.  Handling multiple addresses by edge routers provides



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   the ability to control the traffic flow of the entire site.
   Conversely, handling multiple addresses by individual hosts offers
   each host the flexibility to choose different policies for selecting
   a provider; it also implies changes to all the hosts of a multihomed
   site.

   During the process of evaluating all the proposals, two major lessons
   were learned:

   o  Changing anything in the current practice is hard: for example,
      inserting an additional header into the protocol would impact IP
      fragmentation processing, and the current congestion control
      assumes that each TCP connection follows a single routing path.
      In addition, operators ask for the ability to perform traffic
      engineering on a per-site basis, and specification of site policy
      is often interdependent with the IP address structure.

   o  The IP address has been used as an identifier and has been
      codified into many Internet applications that manipulate IP
      addresses directly or include IP addresses within the application
      layer data stream.  IP addresses have also been used as
      identifiers in configuring network policies.  Changing the
      semantics of an IP address, for example, using only the last 64-
      bit as identifiers as proposed by GSE, would require changes to
      all such applications and network devices.

6.2.  SHIM6: Pros and Cons

   The SHIM6 working group took the second approach from the MULTI6
   working group's investigation, i.e., supporting multihoming through
   the use of multiple addresses.  SHIM6 adopted a host-based approach,
   where the host IP stack includes a "shim" that presents a stable
   "upper layer identifier" (ULID) to the upper layer protocols, but may
   rewrite the IP packets sent and received so that a currently working
   IP address is used in the transmitted packets.  When needed, a SHIM6
   header is also included in the packet itself, to signal to the remote
   stack.

   With SHIM6, protocols above the IP layer use the ULID to identify
   endpoints (e.g., for TCP connections).  The current design suggests
   choosing one of the locators as the ULID (borrowing a locator to be
   used as an identifier).  This approach makes the implementation
   compatible with existing IPv6 upper layer protocol implementations
   and applications.  Many of these applications have inherited the long
   time practice of using IP addresses as identifiers.

   SHIM6 is able to isolate upper layer protocols from multiple IP layer
   addresses.  This enables a multihomed site to use provider-allocated



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   prefixes, one from each of its multiple providers, to facilitate
   provider-based prefix aggregation.  However, this gain comes with
   several significant costs.  First, SHIM6 requires modifications to
   all host stack implementations to support the shim processing.
   Second, the shim layer must maintain the mapping between the
   identifier and the multiple locators returned from IPv6 AAAA name
   resolution, and must take the responsibility to try multiple locators
   if failures ever occur during the end-to-end communication.  At this
   time, the host has little information to determine the order of
   locators it should use in reaching a multihomed destination, however,
   there is ongoing effort in addressing this issue.

   Furthermore, as a host-based approach, SHIM6 provides little control
   to the service provider for effective traffic engineering.  At the
   same time, it also imposes additional state information on the host
   regarding the multiple locators of the remote communication end.
   Such state information may not be a significant issue for individual
   user hosts, but can lead to larger resource demands on large
   application servers that handle hundreds of thousands of simultaneous
   TCP connections.

   Yet another major issue with the SHIM6 solution is the need for
   renumbering when a site changes providers.  Although a multihomed
   site is assigned multiple address blocks, none of them can be treated
   as a persistent identifier for the site.  When the site changes one
   of its providers, it must purge the address block of that provider
   from the entire site.  The current practice of using the IP address
   as both an identifier and a locator has been strengthened by the use
   of IP addresses in access control lists present in various types of
   policy-enforcement devices (e.g., firewalls).  If SHIM6's ULIDs are
   to be used for policy enforcement, a change of providers may
   necessitate the re-configuration of many such devices.

6.3.  GSE/Indirection Solutions: Costs and Benefits

   The use of indirection for scalable multihoming was discussed at the
   workshop, including the GSE [GSE] and indirection approaches, such as
   Map-and-Encap [RFC1955], in general.  The GSE proposal changes the
   IPv6 address structure to bear the semantics of both an identifier
   and a locator.  The first n bytes of the 16-byte IPv6 address are
   called the Routing Goop (RG), and are used by the routing system
   exclusively as a locator.  The last 8 bytes of the IPv6 address
   specify an interface on an end-system.  The middle (16 - n - 8) bytes
   are used to identify site local topology.  The border routers of a
   site re-write the source RG of each outgoing packet to make the
   source address part of the source provider's address aggregation;
   they also re-write the destination RG of each incoming packet to hide
   the site's RG from all the internal routers and hosts.  Although GSE



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   designates the lower 8 bytes of the IPv6 address as identifiers, the
   extent to which GSE could be made compatible with increasingly-
   popular cryptographically-generated addresses (CGA) remains to be
   determined [dGSE].

   All identifier/locator split proposals require a mapping service that
   can return a set of locators corresponding to a given identifier.  In
   addition, these proposals must also address the problem of detecting
   locator failures and redirecting data flows to remaining locators for
   a multihomed site.  The Map-and-Encap proposal did not address these
   issues.  GSE proposed to use DNS for providing the mapping service,
   but it did not offer an effective means for locator failure recovery.
   GSE also requires host stack modifications, as the upper layers and
   applications are only allowed to use the lower 8-bytes, rather than
   the entire, IPv6 address.

6.4.  Future for Indirection

   As the saying goes, "There is no problem in computer science that
   cannot be solved by an extra level of indirection".  The GSE proposal
   can be considered a specific instantiation of a class of indirection-
   based solutions to scalable multihoming.  Map-and-Encap [RFC1955]
   represents a more general form of this indirection solution, which
   uses tunneling, instead of locator rewriting, to cross the DFZ and
   support provider-based prefix aggregation.  This class of solutions
   avoids the provider and customer conflicts regarding PA and PI
   prefixes by putting each in a separate name space, so that ISPs can
   use topologically aggregatable addresses while customers can have
   their globally unique and provider-independent identifiers.  Thus, it
   supports scalable multihoming, and requires no changes to the end
   systems when the encapsulation is performed by the border routers of
   a site.  It also requires no changes to the current practice of both
   applications as well as backbone operations.

   However, all gains of an effective solution are accompanied with
   certain associated costs.  As stated earlier in this section, a
   mapping service must be provided.  This mapping service not only
   brings with it the associated complexity and cost, but it also adds
   another point of failure and could also be a potential target for
   malicious attacks.  Any solution to routing scalability is
   necessarily a cost/benefit tradeoff.  Given the high potential of its
   gains, this indirection approach deserves special attention in our
   search for scalable routing solutions.








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7.  Problem Statements

   The fundamental goal of this workshop was to develop a prioritized
   problem statement regarding routing and addressing problems facing us
   today, and the workshop spent a considerable amount of time on
   reaching that goal.  This section provides a description of the
   prioritized problem statement, together with elaborations on both the
   rationale and open issues.

   The workshop participants noted that there exist different classes of
   stakeholders in the Internet community who view today's global
   routing system from different angles, and assign different priorities
   to different aspects of the problem set.  The prioritized problem
   statement in this section is the consensus of the participants in
   this workshop, representing primarily large network operators and a
   few router vendors.  It is likely that a different group of
   participants would produce a different list, or with different
   priorities.  For example, freedom to change providers without
   renumbering might make the top of the priority list assembled by a
   workshop of end users and enterprise network operators.

7.1.  Problem #1: Routing Scalability

   The workshop participants believe that routing scalability is the
   most important problem facing the Internet today and must be solved,
   although the time frame in which these problems need solutions was
   not directly specified.  The routing scalability problem includes the
   size of the DFZ RIB and FIB, the implications of the growth of the
   RIB and FIB on routing convergence times, and the cost, power (and
   hence, heat dissipation) and ASIC real estate requirements of core
   router hardware.

   It is commonly believed that the IPv4 RIB growth has been constrained
   by the limited IPv4 address space.  However, even under this
   constraint, the DFZ IPv4 RIB has been growing at what appears to be
   an accelerating rate [DFZ].  Given that the IPv6 routing architecture
   is the same as the IPv4 architecture (with substantially larger
   address space), if/when IPv6 becomes widely deployed, it is natural
   to predict that routing table growth for IPv6 will only exacerbate
   the situation.

   The increasing deployment of Virtual Private Network/Virtual Routing
   and Forwarding (VPN/VRF) is considered another major factor driving
   the routing system growth.  However, there are different views
   regarding whether this factor has, or does not have, a direct impact
   to the DFZ RIB.  A common practice is to delegate specific routers to
   handle VPN connections, thus backbone routers do not necessarily hold




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   state for individual VPNs.  Nevertheless, VPNs do represent
   scalability challenges in network operations.

7.2.  Problem #2: The Overloading of IP Address Semantics

   As we have reported in Section 3, multihoming, along with traffic
   engineering, appear to be the major factors driving the growth of the
   DFZ RIB.  Below, we elaborate their impact on the DFZ RIB.

7.2.1.  Definition of Locator and Identifier

   Roughly speaking, the Internet comprises a large number of transit
   networks and a much larger number of customer networks containing
   hosts that are attached to the backbone.  Viewing the Internet as a
   graph, transit networks have branches and customer networks with
   hosts hang at the edges as leaves.

   As its name suggests, locators identify locations in the topology,
   and a network's or host's locator should be topologically constrained
   by its present position.  Identifiers, in principle, should be
   network-topology independent.  That is, even though a network or host
   may need to change its locator when it is moved to a different set of
   attachment points in the Internet, its identifier should remain
   constant.

   From an ISP's viewpoint, identifiers identify customer networks and
   customer hosts.  Note that the word "identifier" used here is defined
   in the context of the Internet routing system; the definition may
   well be different when the word "identifier" is used in other
   contexts.  As an example, a non-routable, provider-independent IP
   prefix for an enterprise network could serve as an identifier for
   that enterprise.  This block of IP addresses can be used to route
   packets inside the enterprise network.  However, they are independent
   from the DFZ topology, which is why they are not globally routable on
   the Internet.

   Note that in cases such as the last example, the definition of
   locators and identifiers can be context-dependent.  Following the
   example further, a PI address may be routable in an enterprise but
   not the global network.  If allowed to be visible in the global
   network, such addresses might act as identifiers from a backbone
   operator's point of view but locators from an enterprise operator's
   point of view.








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7.2.2.  Consequence of Locator and Identifier Overloading

   In today's Internet architecture, IP addresses have been used as both
   locators and identifiers.  Combined with the use of CIDR to perform
   route aggregation, a problem arises for either providers or customers
   (or both).

   Consider, for example, a campus network C that received prefix
   x.y.z/24 from provider P1.  When C multihomes with a second provider
   P2, both P1 and P2 must announce x.y.z/24 so that C can be reached
   through both providers.  In this example, the prefix x.y.z/24 serves
   both as an identifier for C, as well as a (non-aggregatable) locator
   for C's two attachment points to the transit system.

   As far as the DFZ RIB is concerned, the above example shows that
   customer multihoming blurs the distinction between PA and PI
   prefixes.  Although C received a PA prefix x.y.z/24 from P1, C's
   multihoming forced this prefix to be announced globally (equivalent
   to a PI prefix), and forced the prefix's original owner, provider P1,
   to de-aggregate.  As a result, today's multihoming practice leads to
   a growth of the routing table size in proportion to the number of
   multihomed customers.  The only practical way to scale a routing
   system today is topological aggregation, which gets destroyed by
   customer multihoming.

   Although multihoming may blur the PA/PI distinction, there exists a
   big difference between PA and PI prefixes when a customer changes its
   provider(s).  If the customer has used a PA prefix from a former
   provider P1, the prefix is supposed to be returned to P1 upon
   completion of the change.  The customer is supposed to get a new
   prefix from its new provider, i.e., renumbering its network.  It is
   necessary for providers to reclaim their PA prefixes from former
   customers in order to keep the topological aggregatiblity of their
   prefixes.  On the other hand, renumbering is considered very painful,
   if not impossible, by many Internet users, especially large
   enterprise customers.  It is not uncommon for IP addresses in such
   enterprises to penetrate deeply into various parts of the networking
   infrastructure, ranging from applications to network management
   (e.g., policy databases, firewall configurations, etc.).  This shows
   how fragile the system becomes due to the overloading of IP addresses
   as both locators and identifiers; significant enterprise operations
   could be disrupted due to the otherwise simple operation of switching
   IP address prefix assignment.








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7.2.3.  Traffic Engineering and IP Address Semantics Overload

   In today's practice, traffic engineering (TE) is achieved by de-
   aggregating IP prefixes.  One can effectively adjust the traffic
   volume along specific routing paths by adjusting the prefix lengths
   and the number of prefixes announced through those paths.  Thus, the
   very means of TE practice directly conflicts with constraining the
   routing table growth.

   On the surface, traffic engineering induced prefix de-aggregation
   seems orthogonal to the locator-identifier overloading problem.
   However, this may not necessarily be true.  Had all the IP prefixes
   been topologically aggregatable to start with, it would make re-
   aggregation possible or easier, when the finer granularity prefix
   announcements propagate further away from their origins.

7.3.  Additional Issues

7.3.1.  Routing Convergence

   There are two kinds of routing convergence issues, eBGP (global
   routing) convergence and IGP (enterprise or provider) routing
   convergence.  Upon isolated topological events, eBGP convergence does
   not suffer from extensive path explorations in most cases [PathExp],
   and convergence delay is largely determined by the minimum route
   advertisement interval (MRAI) timer [RFC4098], except those cases
   when a route is withdrawn.  Route withdrawals tend to suffer from
   path explorations and hence slow convergence; one participant's
   experience suggests that the withdrawal delays often last up to a
   couple of minutes.  One may argue that, if the destination becomes
   unreachable, a long convergence delay would not bring further damage
   to applications.  However, there are often cases where a more
   specific route (a longer prefix) has failed, yet the destination can
   still be reached through an aggregated route (a shorter prefix).  In
   these cases, the long convergence delay does impact application
   performance.

   While IGPs are designed to and do converge more quickly than BGP
   might, the workshop participants were concerned that, in addition to
   the various special purpose routes that IGPs must carry, the rapid
   growth of the DFZ RIB size can effectively slow down IGP convergence.
   The IGP convergence delay can be due to multiple factors, including

   1.  Delays in detecting physical failures,

   2.  The delay in loading updated information into the FIB, and





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   3.  The large size of the internal RIB, often twice as big as the DFZ
       RIB, which can lead to both longer route computation time and
       longer FIB loading time.

   The workshop participants hold different views regarding (1) the
   severity of the routing convergence problem; and (2) whether it is an
   architectural problem, or an implementation issue.  However, people
   generally agree that if we solve the routing scalability problem,
   that will certainly help reduce the convergence delay or make the
   problem a much easier one to handle because of the reduced number of
   routes to process.

7.3.2.  Misaligned Costs and Benefits

   Today's rapid growth of the DFZ RIB is driven by a few major factors,
   including multihoming and traffic engineering, in addition to the
   organic growth of the Internet's user base.  There is a powerful
   incentive to deploy each of the above features, as they bring direct
   benefits to the parties who make use of them.  However, the
   beneficiaries may not bear the direct costs of the resulting routing
   table size increase, and there is no measurable or enforceable
   constraint to limit such increase.

   For example, suppose that a service provider has two bandwidth-
   constrained transoceanic links and wants to split its prefix
   announcements in order to fully load each link.  The origin AS
   benefits from performing the de-aggregation.  However, if the de-
   aggregated announcements propagate globally, the cost is born by all
   other ASs.  That is, the costs of this type of TE practice are not
   contained to the beneficiaries.  Multihoming provides a similar
   example (in this case, the multihomed site achieves a benefit, but
   the global Internet incurs the cost of carrying the additional
   prefix(es)).

   The misalignment of cost and benefit in the current routing system
   has been a driver for acceleration of the routing system size growth.

7.3.3.  Other Concerns

   Mobility was among the most frequently mentioned issues at the
   workshop.  It is expected that billions of mobile gadgets may be
   connected to the Internet in the near future.  There was also a
   discussion on network mobility as deployed in the Connexion service
   provided by Boeing over the last few years.  However, at this time it
   seems unclear (1) whether the Boeing-like network mobility support
   would cause a scaling issue in the routing system, and (2) exactly
   what would be the impact of billions of mobile hosts on the global




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   routing system.  These discussions were covered in Section 5 of this
   report.

   Routing security is another issue that was brought up a number of
   times during the workshop.  The consensus from the workshop
   participants was that, however important routing security may be, it
   was out of scope for this workshop, whose main goal was to produce a
   problem statement about addressing and routing scalability.  It was
   duly considered that security must be one of the top design goals
   when we get to a solution development stage.  It was also noted that,
   if we continue to allow the routing table to grow indefinitely, then
   it may be impossible to add security enhancements in the future.

7.4.  Problem Recognition

   The first step in solving a problem is recognizing its existence as
   well as its importance.  However, recognizing the severity of the
   routing scaling issue can be a challenge by itself, because there
   does not exist a specific hard limit on routing system scalability
   that can be easily demonstrated, nor is there any specific answer to
   the question of how much time we may have in developing a solution.
   Nevertheless, a general consensus among the workshop participants is
   that we seem to be running out of time.  The current RIB scaling
   leads to both accelerated hardware cost increases, as explained in
   Section 4, as well as pressure for shorter depreciation cycles, which
   in turn also translates to cost increases.

8.  Criteria for Solution Development

   Any common problem statement may admit multiple different solutions.
   This section provides a set of considerations, as identified from the
   workshop discussion, over the solution space.  Given the
   heterogeneity among customers and providers of the global Internet,
   and the elasticity of the problem, none of these considerations
   should inherently preclude any specific solution.  Consequently,
   although the following considerations were initially deemed as
   constraints on solutions, we have instead opted to adopt the term
   'criteria' to be used in guiding solution evaluations.

8.1.  Criteria on Scalability

   Clearly, any proposed solution must solve the problem at hand, and
   our number one problem concerns the scalability of the Internet's
   routing and addressing system(s) as outlined in previous sections.
   Under the assumption of continued growth of the Internet user
   population, continued increases of multihoming and RFC 2547 VPN
   [RFC2547] deployment, the solution must enable the routing system to
   scale gracefully, as measured by the number of



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   o  DFZ Internet routes, and

   o  Internal routes.

   In addition, scalable support for traffic engineering (TE) must be
   considered as a business necessity, not an option.  Capacity planning
   involves placing circuits based on traffic demand over a relatively
   long time scale, while TE must work more immediately to match the
   traffic load to the existing capacity and to match the routing policy
   requirements.

   It was recognized that different parties in the Internet may have
   different specific TE requirements.  For example,

   o  End site TE: based on locally determined performance or cost
      policies, end sites may wish to control the traffic volume exiting
      to, or entering from specific providers.

   o  Small ISP to transit ISP TE: operators may face tight resource
      constraints and wish to influence the volume of entering traffic
      from both customers and providers along specific routing paths to
      best utilize the limited resources.

   o  Large ISP TE: given the densely connected nature of the Internet
      topology, a given destination normally can be reached through
      different routing paths.  An operator may wish to be able to
      adjust the traffic volume sent to each of its peers based on
      business relations with its neighbor ASs.

   At this time, it remains an open issue whether a scalable TE solution
   would be necessarily inside the routing protocol, or can be
   accomplished through means that are external to the routing system.

8.2.  Criteria on Incentives and Economics

   The workshop attendees concluded that one important reason for
   uncontrolled routing growth was the misalignment of incentives.  New
   entries are added to the routing system to provide benefit to
   specific parties, while the cost is born by everyone in the global
   routing system.  The consensus of the workshop was that any proposed
   solutions should strive to provide incentives to reward practices
   that reduce the overall system cost, and punish the "bad" behavior
   that imposes undue burden on the global system.

   Given the global scale and distributed nature of the Internet, there
   can no longer (ever) be a flag day on the Internet.  To bootstrap the
   deployment of new solutions, the solutions should provide incentives
   to first movers.  That is, even when a single party starts to deploy



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   the new solution, there should be measurable benefits to balance the
   costs.

   Independent of what kind of solutions the IETF develops, if any, it
   is unlikely that the resulting routing system would stay constant in
   size.  Instead, the workshop participants believed the routing system
   will continue to grow, and that ISPs will continue to go through
   system and hardware upgrade cycles.  Many attendees expressed a
   desire that the scaling properties of the system can allow the
   hardware to keep up with the Internet growth at a rate that is
   comparable to the current costs, for example, allowing one to keep a
   5-year hardware depreciation cycle, as opposed to a situation where
   scaling leads to accelerated cost increases.

8.3.  Criteria on Timing

   Although there does not exist a specific hard deadline, the unanimous
   consensus among the workshop participants is that the solution
   development must start now.  If one assumes that the solution
   specification can get ready within a 1 - 2 year time frame, that will
   be followed by another 2-year certification cycle.  As a result, even
   in the best case scenario, we are facing a 3 - 5 year time frame in
   getting the solutions deployed.

8.4.  Consideration on Existing Systems

   The routing scalability problem is a shared one between IPv4 and
   IPv6, as IPv6 simply inherited IPv4's CIDR-style "Provider-based
   Addressing".  The proposed solutions should, and are also expected
   to, solve the problem for both IPv4 and IPv6.

   Backwards compatibility with the existing IPv4 and IPv6 protocol
   stack is a necessity.  Although a wide deployment of IPv6 is yet to
   happen, there has been substantial investment into IPv6
   implementation and deployment by various parties.  IPv6 is considered
   a legacy with shipped code.  Thus, a highly desired feature of any
   proposed solution is to avoid imposing backwards-incompatible changes
   on end hosts (either IPv4 or IPv6).

   In the routing system itself, the solutions must allow incremental
   changes from the current operational Internet.  The solutions should
   be backward compatible with the routing protocols in use today,
   including BGP, OSPF, IS-IS, and others, possibly with incremental
   enhancements.

   The above backward-compatibility considerations should not constrain
   the exploration of the solution space.  We need to first find right
   solutions, and look into their backward-compatibility issues after



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   that.  This way enables us to gain a full understanding of the
   tradeoffs, and what potential gains, if any, that we may achieve by
   relaxing the backward-compatibility concerns.

   As a rule of thumb for successful deployment, for any new design, its
   chance of success is higher if it makes fewer changes to the existing
   system.

8.5.  Consideration on Security

   Security should be considered from day one of solution development.
   If nothing else, the solutions must not make securing the routing
   system any worse than the situation today.  It is highly desirable to
   have a solution that makes it more difficult to inject false routing
   information, and makes it easier to filter out DoS traffic.

   However, securing the routing system is not considered a requirement
   for the solution development.  Security is important; having a
   working system in the first place is even more important.

8.6.  Other Criteria

   A number of other criteria were also raised that fall into various
   different categories.  They are summarized below.

   o  Site renumbering forced by the routing system should be avoided.

   o  Site reconfiguration driven by the routing system should be
      minimized.

   o  The solutions should not force ISPs to reveal internal topology.

   o  Routing convergence delay must be under control.

   o  End-to-end data delivery paths should be stable enough for good
      Voice over IP (VoIP) performance.

8.7.  Understanding the Tradeoff

   As the old saying goes, every coin has two sides.  If we let the
   routing table continue to grow at its present rate, rapid hardware
   and software upgrade and replacement cycles for deployed core routing
   equipment may become cost prohibitive.  In the worst case, the
   routing table growth may exceed our ability to engineer the global
   routing system in a cost-effective way.  On the other hand, solutions
   for stopping or substantially slowing down the growth in the Internet
   routing table will necessarily bring their own costs, perhaps showing
   up elsewhere and in different forms.  Examples of such tradeoffs are



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   presented in Section 6, where we examined the gains and costs of a
   few different approaches to scalable multihoming support (SHIM6, GSE,
   and a general tunneling approach).  A major task in the solution
   development is to understand who may have to give up what, and
   whether that makes a worthy tradeoff.

   Before ending this discussion on the solution criteria, it is worth
   mentioning the shortest presentation at the workshop, which was made
   by Tony Li (the presentation slides can be found from Appendix D).
   He asked a fundamental question: what is at stake?  It is the
   Internet itself.  If the routing system does not scale with the
   continued growth of the Internet, eventually the costs might spiral
   out of control, the digital divide widen, and the Internet growth
   slow down, stop, or retreat.  Compared to this problem, he considered
   that none of the criteria mentioned so far (except solving the
   problem) was important enough to block the development and deployment
   of an effective solution.

9.  Workshop Recommendations

   The workshop attendees would like to make the following
   recommendations:

   First of all, the workshop participants would like to reiterate the
   importance of solving the routing scalability problem.  They noted
   that the concern over the scalability and flexibility of the routing
   and addressing system has been with us for a very long time, and the
   current growth rate of the DFZ RIB is exceeding our ability to
   engineer the routing infrastructure in an economically feasible way.
   We need to start developing a long-term solution that can last for
   the foreseeable future.

   Second, because the participants of this workshop consisted of mostly
   large service providers and major router vendors, the workshop
   participants recommend that IAB/IESG organize additional workshops or
   use other venues of communication to reach out to other stakeholders,
   such as content providers, retail providers, and enterprise
   operators, both to communicate to them the outcome of this workshop,
   and to solicit the routing/addressing problems they are facing today,
   and their requirements on the solution development.

   Third, the workshop participants recommend conducting the solution
   development in an open, transparent way, with broad-ranging
   participation from the larger networking community.  A majority of
   the participants indicated their willingness to commit resources
   toward developing a solution.  We must also invite the participation
   from the research community in this process.  The locator-identifier
   split represents a fundamental architectural issue, and the IAB



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   should lead the investigation into understanding of both how to make
   this architectural change and the overall impact of the change.

   Fourth, given the goal of developing a long-term solution, and the
   fact that development and deployment cycles will necessarily take
   some time, it may be helpful (or even necessary) to buy some time
   through engineering feasible short- or intermediate-term solutions
   (e.g., FIB compression).

   Fifth, the workshop participants believe the next step is to develop
   a roadmap from here to the solution deployment.  The IAB and IESG are
   expected to take on the leadership role in this roadmap development,
   and to leverage on the momentum from this successful workshop to move
   forward quickly.  The roadmap should provide clearly defined short-,
   medium-, and long-term objectives to guide the solution development
   process, so that the community as a whole can proceed in an
   orchestrated way, seeing exactly where we are going when engineering
   necessary short-term fixes.

   Finally, the workshop participants also made a number of suggestions
   that the IETF might consider when examining the solution space.
   These suggestions are captured in Appendix A.

10.  Security Considerations

   While the security of the routing system is of great concern, this
   document introduces no new protocol or protocol usage and as such
   presents no new security issues.

11.  Acknowledgments

   Jari Arkko, Vince Fuller, Darrel Lewis, Tony Li, Eric Rescorla, and
   Ted Seely made many insightful comments on earlier versions of this
   document.  Finally, many thanks to Wouter Wijngaards for the fine
   notes he took during the workshop.

12.  Informative References

   [RFC1955]    Hinden, R., "New Scheme for Internet Routing and
                Addressing (ENCAPS) for IPNG", RFC 1955, June 1996.

   [RFC2547]    Rosen, E. and Y. Rekhter, "BGP/MPLS VPNs", RFC 2547,
                March 1999.

   [RFC3775]    Johnson, D., Perkins, C., and J. Arkko, "Mobility
                Support in IPv6", RFC 3775, June 2004.





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   [RFC4098]    Berkowitz, H., Davies, E., Hares, S., Krishnaswamy, P.,
                and M. Lepp, "Terminology for Benchmarking BGP Device
                Convergence in the Control Plane", RFC 4098, June 2005.

   [RFC4116]    Abley, J., Lindqvist, K., Davies, E., Black, B., and V.
                Gill, "IPv4 Multihoming Practices and Limitations",
                RFC 4116, July 2005.

   [RFC4192]    Baker, F., Lear, E., and R. Droms, "Procedures for
                Renumbering an IPv6 Network without a Flag Day",
                RFC 4192, September 2005.

   [RFC4632]    Fuller, V. and T. Li, "Classless Inter-domain Routing
                (CIDR): The Internet Address Assignment and Aggregation
                Plan", BCP 122, RFC 4632, August 2006.

   [IDR-REQS]   Doria, A. and E. Davies, "Analysis of IDR requirements
                and History", Work in Progress, February 2007.

   [ARIN]       "American Registry for Internet Numbers",
                 http://www.arin.net/index.shtml.

   [PIPA]       Karrenberg, D., "IPv4 Address Allocation and Assignment
                Policies for the RIPE NCC Service Region",
                RIPE-387 http://www.ripe.net/docs/ipv4-policies.html,
                2006.

   [SHIM6]      "Site Multihoming by IPv6 Intermediation (shim6)",
                 http://www.ietf.org/html.charters/shim6-charter.html.

   [EID]        Chiappa, J., "Endpoints and Endpoint Names: A Proposed
                Enhancement to the Internet Architecture",
                 http://www.chiappa.net/~jnc/tech/endpoints.txt, 1999.

   [GSE]        O'Dell, M., "GSE - An Alternate Addressing Architecture
                for IPv6", Work in Progress, 1997.

   [dGSE]       Zhang, L., "An Overview of Multihoming and Open Issues
                in GSE", IETF Journal, http://www.isoc.org/tools/blogs/
                ietfjournal/?p=98#more-98, 2006.

   [PathExp]    Oliveira, R. and et. al., "Quantifying Path Exploration
                in the Internet", Internet Measurement Conference (IMC)
                2006, http://www.cs.ucla.edu/~rveloso/papers/
                imc175f-oliveira.pdf.






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   [DynPrefix]  Oliveira, R. and et. al., "Measurement of Highly Active
                Prefixes in BGP", IEEE GLOBECOM 2005
                http://www.cs.ucla.edu/~rveloso/papers/activity.pdf.

   [BHB06]      Boothe, P., Hielbert, J., and R. Bush, "Short-Lived
                Prefix Hijacking on the Internet", NANOG 36
                http://www.nanog.org/mtg-0602/pdf/boothe.pdf, 2006.

   [ROFL]       Caesar, M. and et. al., "ROFL: Routing on Flat Labels",
                SIGCOMM 2006, http://www.sigcomm.org/sigcomm2006/
                discussion/showpaper.php?paper_id=34, 2006.

   [CNIR]       Abraham, I. and et. al., "Compact Name-Independent
                Routing with Minimum Stretch", ACM Symposium on Parallel
                Algorithms and Architectures,
                http://citeseer.ist.psu.edu/710757.html, 2004.

   [BGT04]      Bu, T., Gao, L., and D. Towsley, "On Characterizing BGP
                Routing Table Growth", J. Computer and Telecomm
                Networking V45N1, 2004.

   [Fuller]     Fuller, V., "Scaling issues with ipv6 routing+
                multihoming",  http://www.iab.org/about/workshops/
                routingandaddressing/vaf-iab-raws.pdf, 2006.

   [H03]        Huston, G., "Analyzing the Internet's BGP Routing
                Table",  http://www.potaroo.net/papers/ipj/
                2001-v4-n1-bgp/bgp.pdf, 2003.

   [BGP2005]    Huston, G., "2005 -- A BGP Year in Review",  http://
                www.apnic.net/meetings/21/docs/sigs/routing/
                routing-pres-huston-routing-update.pdf.

   [DFZ]        Huston, G., "Growth of the BGP Table - 1994 to Present",
                 http://bgp.potaroo.net, 2006.

   [GIH]        Huston, G., "Wither Routing?",
                 http://www.potaroo.net/ispcol/2006-11/raw.html, 2006.

   [ATNAC2006]  Huston, G. and G. Armitage, "Projecting Future IPv4
                Router Requirements from Trends in Dynamic BGP
                Behaviour",  http://www.potaroo.net/papers/phd/
                atnac-2006/bgp-atnac2006.pdf, 2006.

   [CIDRRPT]    "The CIDR Report",  http://www.cidr-report.org.






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   [ML]         "Moore's Law",
                Wikipedia http://en.wikipedia.org/wiki/Moore's_law,
                2006.

   [Molinero]   Molinero-Fernandez, P., "Technology trends in routers
                and switches", PhD thesis, Stanford University  http://
                klamath.stanford.edu/~molinero/thesis/html/
                pmf_thesis_node5.html, 2005.

   [DRAM]       Landler, P., "DRAM Productivity and Capacity/Demand
                Model", Global Economic Workshop http://
                www.sematech.org/meetings/archives/GES/19990514/docs/
                07_econ.pdf, 1999.






































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Appendix A.  Suggestions for Specific Steps

   At the end of the workshop there was a lively round-table discussion
   regarding specific steps that IETF may consider undertaking towards a
   quick solution development, as well as potential issues to avoid.
   Those steps included:

   o  Finding a home (mailing list) to continue the discussion started
      from the workshop with wider participation.  [Editor's note: Done
      -- This action has been completed.  The list is ram@iab.org.]

   o  Considering a special process to expedite solution development,
      avoiding the lengthy protocol standardization cycles.  For
      example, IESG may charter special design teams for the solution
      investigation.

   o  If a working group is to be formed, care must be taken to ensure
      that the scope of the charter is narrow and specific enough to
      allow quick progress, and that the WG chair be forceful enough to
      keep the WG activity focused.  There was also a discussion on
      which area this new WG should belong to; both routing area ADs and
      Internet area ADs are willing to host it.

   o  It is desirable that the solutions be developed in an open
      environment and free from any Intellectual Property Right claims.

   Finally, given the perceived severity of the problem at hand, the
   workshop participants trust that IAB/IESG/IETF will take prompt
   actions.  However, if that were not to happen, operators and vendors
   would be most likely to act on their own and get a solution deployed.

Appendix B.  Workshop Participants

   Loa Anderson (IAB)
   Jari Arkko (IESG)
   Ron Bonica
   Ross Callon (IESG)
   Brian Carpenter (IAB)
   David Conrad (IANA)
   Leslie Daigle (IAB Chair)
   Elwyn Davies (IAB)
   Terry Davis
   Weisi Dong
   Aaron Falk (IRTF Chair)
   Kevin Fall (IAB)
   Dino Farinacci
   Vince Fuller
   Vijay Gill



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   Russ Housley (IESG)
   Geoff Huston
   Daniel Karrenberg
   Dorian Kim
   Olaf Kolkman (IAB)
   Darrel Lewis
   Tony Li
   Kurtis Lindqvist (IAB)
   Peter Lothberg
   David Meyer (IAB)
   Christopher Morrow
   Dave Oran (IAB)
   Phil Roberts (IAB Executive Director)
   Jason Schiller
   Peter Schoenmaker
   Ted Seely
   Mark Townsley (IESG)
   Iljitsch van Beijnum
   Ruediger Volk
   Magnus Westerlund (IESG)
   Lixia Zhang (IAB)

Appendix C.  Workshop Agenda

   IAB Routing and Addressing Workshop Agenda
               October 18-19
            Amsterdam, Netherlands

   DAY 1: the proposed goal is to collect, as complete as possible, a
   set of scalability problems in the routing and addressing area facing
   the Internet today.

   0815-0900: Welcome, framing up for the 2 days
              Moderator: Leslie Daigle

   0900-1200: Morning session
              Moderator: Elwyn Davies
              Strawman topics for the morning session:
              - Scalability
              - Multihoming support
              - Traffic Engineering
              - Routing Table Size: Rate of growth, Dynamics
                (this is not limited to DFZ, include iBGP)
              - Causes of the growth
              - Pains from the growth
                (perhaps "Impact on routers" can come here?)
              - How big a problem is BGP slow convergence?




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   1015-1030: Coffee Break

   1200-1300: Lunch

   1330-1730: Afternoon session: What are the top 3 routing problems
              in your network?
              Moderator: Kurt Erik Lindqvist

   1500-1530: Coffee Break

   Dinner at Indrapura (http://www.indrapura.nl), sponsored by Cisco

   ---------
   DAY 2: The proposed goal is to formulate a problem statement

   0800-0830: Welcome

   0830-1000: Morning session: What's on the table
              Moderator: Elwyn Davies
              - shim6
              - GSE

   1000-1030: Coffee Break

   1030-1200: Problem Statement session #1: document the problems
              Moderator: David Meyer

   1200-1300: Lunch

   1300-1500: Problem Statement session # 2, cont;
              Moderator: Dino Farinacci
               - Constraints on solutions

   1500-1530: Coffee Break

   1530-1730: Summary and Wrap-up
              Moderator: Leslie Daigle

Appendix D.  Presentations

   The presentations from the workshop can be found on

      http://www.iab.org/about/workshops/routingandaddressing








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Authors' Addresses

   David Meyer (editor)

   EMail: dmm@1-4-5.net


   Lixia Zhang (editor)

   EMail: lixia@cs.ucla.edu


   Kevin Fall (editor)

   EMail: kfall@intel.com




































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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

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Meyer, et al.                Informational                     [Page 39]

