consumable resources (e.g. available bandwidth)
this change will happen more often and can
introduce routing oscillations as traffic shifts
back and forth between alternate paths. Further-
more, frequently changing routes can increase
the delay variation (jitter) experienced by the
end users.
A third problem with today’s routing is that if
the existing path cannot admit a new flow, the
associated traffic cannot be forwarded even if
an adequate path exists.
2.6 Need for better Routing
Protocols
The Internet is growing fast – the number of
connected hosts is doubled almost every year,
while the volume of traffic is doubling every six
to ten months. This growth has been sustained
for several years, and all measures indicate that
it may well continue at the same rate for several
years, [HUIT00].
Internet providers must invest continuously to
build up network capacity, but they also have
to cope with another problem – as the Internet
grows, the number of routers that have to be
propagated by routing protocols also grows,
resulting in more routing traffic.
Let us look at one example: a problem with BGP
is that a small fraction of the routes contributes
an inordinate amount of updates. This phe-
nomenon, informally known as “route flap”, can
be caused by software or hardware bugs, by the
interaction between BGP and network conges-
tion described in the previous section, or by
local decisions. Whatever the cause, it is neces-
sary to mitigate its effects. If a misbehaving
router sends too many updates at too short inter-
vals, its neighbours that try to process all the
updates will exhaust their computing resource,
and may fall into a congested state that triggers
further instabilities.
One solution, proposed in [RFC2439], is to limit
the rate at which updates are accepted for any
given path.
The problems mentioned above are only ex-
amples of what can happen in a large Internet.
There are many other challenges in the area of
routing. Some of these are:
- Problems related to interconnection between
ASs;- How to manage large ASs – since IGP
requires that all the routers know each other
within the AS, a good idea is to divide the AS
into several small “sub domains”; - How the routing tables can be aggregated to
cope with the growth of the Internet routing
table; - Is IPv6 the solution?
- How to manage large ASs – since IGP
In the Internet today, routing is typically done in
a distributed fashion. Routes are optimized for a
single arbitrary metric, administrative weight, or
hop count. For any source-destination pair, all
the packets follow the current “shortest path”
(i.e. lowest cost path). Alternatively, fully
acceptable routes are not used if they represent
higher “cost”2).The current IP routing protocols were designed
for “elastic traffic”, such as TCP based applica-
tions like FTP, HTTP, etc., which are insensitive
to delay and delay-variations.In order to support the traffic growth and new
types of services that are planned to be trans-
ported over IP-networks and the corresponding
QoS-requirements, we need better routing proto-
cols.3 Constraint-based Routing
Constraint-based routing refers to a class of rout-
ing systems that compute routes through a net-
work subject to satisfaction of a set of con-
straints and requirements. In the most general
setting, constraint-based routing may also seek
to optimise overall network performance while
minimising costs.The constraints and requirements may be im-
posed by the network itself or by administrative
policies. Constraints may include bandwidth,
hop count, delay, and policy instruments such as
resource class attributes. Constraints may also
include domain specific attributes of certain net-
work technologies and contexts that impose
restrictions on the solution space of the routing
function. Path oriented technologies such as
MPLS have made constraint-based routing
feasible and attractive in public IP networks.Constraint-based routing is in general applicable
to traffic aggregates as well as flows, and may
be subject to a wide variety of constraints that
may include policy restrictions.2)Some routing protocols, such as OSPF, do support alternative routes with equal cost, so a split of
traffic among several equal-cost paths are accepted.