How Finely Do We Need to Control Internet Traffic?
==================================================
The Internet has grown tremendously both in the capacity of traffic
that it can carry and in the actual traffic that it does carry. At the
present, the cost of this capacity, measured either in the rates that
ISPs charge or in the cost of leasing dark fiber, is at the lowest
that it has ever been. With this increase in capacity and drop in
cost, one would expect to see a corresponding lack of interest in
controlling small aggregates of traffic. Yet, a desire for higher
performance, increased reliability and new services is driving a
curious trend toward controlling finer and finer amounts of traffic on
the Internet.
One such example is the almost religious debate between MPLS and
traditional IP routing. MPLS offers fine grained control over traffic,
with the ability to dictate the specific path through a network for
traffic from an ingress interface on one end to an egress interface on
another end of the network. IP routing proponents often cite the
common practice of over-provisioning networks given current market
conditions, which seems counter to the need for fine-grained traffic
control. A second such example is the common practice of multihoming,
which has led to de-aggregation. As more and more stub networks
purchase connectivity from more than one ISP, they find they have a
choice of multiple paths for sending and receiving traffic. Many
companies such as NetVMG, Opnix, Proficient Networks, Routescience,
and Sockeye provide devices that control the paths of egress traffic
to individual IP addresses. It is commonly believed that stub networks
are purposely de-aggregating their network block announcements to
split ingress traffic between inter-AS paths. This has brought out the
worries of associated routing table growth and protocol overhead. A
more direct example of this phenomenon is the current topic in
networking research of overlay networks. Many overlay networks rely on
application layer forwarding and providing better customized paths for
individual traffic flows than the underlying Internet can
provide. However, given all the feverish research and industrial
activity in all these areas of networking, their advantages in terms
of performance, reliability and enabling of new applications are still
debated.
Future thrusts into finer control of Internet traffic will undoubtedly
be influenced by the structure of the Internet. Certainly a possible
future is one where nothing is different - since the introduction of
SS7, the PSTN has not changed for over 20 years. Alternatively,
overlay networking may become common, and/or the current ISP model of
the Internet may change.
Two core research issues need to be pursued in this area. The first
will determine how overlay networks should evolve to the future
Internet. Many overlay networks cannot scale to all the hosts on the
current Internet. However, what happens when all the hosts on the
Internet are part of multiple disjoint overlay networks? Peering
agreements between nodes at the overlay level may become
commonplace. Routing decisions by different overlay networks may
interfere with each other by changing traffic patterns in the
underlying network. This can lead to an unstable system or one that is
not much better than the underlying Internet. Can overlay networks
co-exist or will measurements and routing decisions have to be
coordinated to still promise improvements over the current Internet?
Instead, should we abandon overlay networking but use the techniques
developed for it to improve routing in the underlying IP network? A
global, distributed measurement infrastructure can be built to detect
the capacities and utilization of various Internet paths. A control
network that reconfigures IP routes dynamically based on these
measurements can then be put in place. Such an approach can improve on
what IP routing offers today but less than what overlay networks
promise. However, this approach can be more scalable than overlay
network forwarding.
The other long term research issue considers Internet routing if the
current hierarchical nature of the AS topology no longer holds in the
future. Given the turmoil that many ISPs are facing, one can imagine a
future Internet without a core consisting of a few large ISPs. In
order to send a packet from California to New York, a path traversing
several small networks may be employed, instead of a path through a
single continental ISP's network. This Internet may be composed of a
large number of small ASes that peer with each other for transit, with
no clear hierarchy. Peering may be dramatically different where ASes
no longer determine peering tactics based on size or AS hierarchy
position. No longer will most of the traffic traverse a few global
sized, well engineered ISPs. If instead the majority of traffic
traverses the same few paths, and if smaller, less-provisioned
networks comprise these paths, will congestion occur more rapidly? We
may need to rethink the fundamental decisions of the current wide-area
routing architecture. Will fast routing convergence or agility in
re-routing around congestion become even more critical? Will
multi-path routing become a necessity, and hot potato routing a more
common occurrence? Perhaps the current two level IGP/EGP hierarchy
will not be sufficient. We may have to consider a third level, or
current overlay networks may fill that need. A more traditional
peering hierarchy may then appear at the overlay network level.
Clearly other issues will also be involved in determining how finely
we need to control traffic in the future. New services may dictate
stringent performance or security requirements that may require fine
control. Without any current, compelling services with these
requirements, we should be careful to not dismiss technologies for
fine traffic control : we may have a chicken and egg problem.
Acknowledgements : I want to thank Supratik Bhattacharyya, Chen-Nee
Chuah, Adam Costello and Gianluca Iannaccone for their feedback.