mindmeld, what effect will this have on Cisco business?
Faster 'Net growth rate raises fears about routers
By CAROLYN DUFFY MARSAN
Network World, 04/02/01
Alarm bells are ringing in the Internet engineering
community over an obscure statistic that indicates the
'Net is growing - in size and complexity - at a faster
rate than today's routers can handle.
This recent finding has prompted the Internet
Engineering Task Force (IETF) to embark on an
overhaul of the communications protocol that handles
routing across the Internet's backbone.
At stake for large companies may be the need to buy
more powerful network gear earlier than originally
intended.
"The sky is not falling, but the sky is hanging a little
low," says Fred Baker, a routing pioneer with Cisco
and a member of the IETF's Internet Architecture
Board. "The issue needs to be addressed soon."
Forum: Internet routing and multihoming
What do you think? Discuss this article.
The statistic in question is the number of entries in the
backbone's routing table, the master list of network
destinations that is stored in backbone routers and
used to determine the best path from one network to
another. The size and traffic in the routing table are
indicators of the overall health of the Internet, in
particular how well the individual networks that
comprise the Internet are communicating with each
other.
After years of predictable growth, the size of the
routing table and traffic in it exploded during the past
six months, topping 104,000 entries in March,
compared with 75,000 a year ago. Even more
troubling is evidence that frequent updates to the
routing table entries by network managers are causing
instability in the Internet's backbone routing
infrastructure.
Nobody knows how big or how active the routing
table can get before the Internet's core routers start
crashing. But current projections show that if the
growth goes unchecked, the Internet could face a
router processing-power crunch in as soon as 18
months.
"It's not the size of the table, but the number of
updates per second that kills a router stone dead,"
explains Geoff Huston, a Telstra official who tracks
this statistic for the IETF. "By the time the table
gets to around 200,000 entries, we may be pushing a
default-free router well beyond its processing
capability."
In addition, the churn in the Internet's routing table
means that it is taking longer to propagate accurate
routing information globally across the Internet.
The time it takes the Internet to process a route
withdrawal or a route announcement is getting longer,
Huston says. "This is again a processor overload
issue."
What's driving the increase in routing table entries is
the rising popularity of multihoming on corporate
networks. Multihoming is the term used to describe a
network configuration in which one Internet server is
connected to two different ISPs for improved
reliability and redundancy. A multihomed network
requires a separate entry in the routing table for each
ISP.
Most large companies and dot-coms multihome their
networks. Huston estimates that 70% of the
announcements in today's routing table are related to
multihoming.
Multihoming is popular because the cost of
transmission circuits is plummeting, making it less
expensive to buy Internet access services from two or
more ISPs. At the same time, companies are more
concerned about the reliability of their networks and
less willing to trust one service provider.
"Half of the companies that are multihomed should
have gotten better service from their providers," says
Patrik Faltstrom, a Cisco engineer and co-chair of the
IETF's Applications Area. "ISPs haven't done a good
enough job explaining to their customers that they
don't need to multihome."
For network managers with multihomed networks, the
growing size and complexity of the Internet's routing
table means they may need to buy bigger, more
expensive routers and upgrade them more frequently,
experts say. That's because routers must store a view
of the routing table for each ISP they use. The router
processing problem is worse for backbone providers,
which store hundreds of views of the routing table in
their routers.
"When you talk about the size of the routing tables, it's
a symptom that you're talking about, like a cough,''
Baker says. "The issue isn't that the routing tables are
too big or multihoming is bad. It's that these trends are
driving equipment costs and putting more burdens on
the routing protocols."
In response, the IETF plans to revamp the 6-year-old
standard used in multihoming and backbone routing:
the Border Gateway Protocol (BGP) 4. Network
managers run BGP4 on routers to load balance and
back up their Internet traffic across multiple ISPs.
The IETF is concerned about whether BGP4 "can
scale up to carrying millions or even tens of millions of
distinct routing entries," Huston says. "I believe we
can scale up BGP to about two to three times of
today without too much drama. . . . We could tweak
BGP to scale up to 10 to 20 times the size of today if
we had better use of route attributes that allow
selective aggregation."
Selective aggregation would reduce the number and
frequency of changes to the routing tables. For
example, when the Internet's backbone loses a link
today, hundreds of messages are sent to the routing
tables saying that individual routes are down. In the
future, BGP might be able to announce this message
once. Similarly, routers can communicate only with a
neighbor or the whole Internet using BGP4. In the
future, routers might also be able to communicate with
the other routers along one path of the Internet.
To tackle these questions of how best to revamp
BGP4, the IETF has launched a new effort called
Prefix Taxonomy Ongoing Measurement and Inter
Network Experiment (Ptomaine). The BGP4 redesign
should be done in about a year.
The need to revamp BGP "is a problem we've been
playing ostrich with for years,'' says Randy Bush,
co-chair of the IETF's operations and management
area and vice president of IP Networking at Verio.
"An upgrade to BGP will give network managers
improved tools that allow them to meet their
redundancy and reliability needs with multihoming and
balance their traffic without being bad 'Net citizens''
by overloading the routing tables.
Ultimately, the IETF may need to develop a new
framework for routing, in which load balancing and
other traffic engineering messages used in multihomed
networks are carried by protocols other than BGP
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