Keywords: routing and addressing workshop, routing table growth, addressing architecture
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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RFC 4984 IAB Workshop on Routing & Addressing September 2007
[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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RFC 4984 IAB Workshop on Routing & Addressing September 2007
[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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RFC 4984 IAB Workshop on Routing & Addressing September 2007
[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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RFC 4984 IAB Workshop on Routing & Addressing September 2007
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).
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contained in BCP 78, and except as set forth therein, the authors
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