Liu,
the arguments you use against existing IGPs would be valid 20 years ago,
but not today. First, links have high bandwidth, CPUs are fast and any
serious IGP implementation has addressed the bottlenecks you are talking
about. These days IGPs can support thousands of nodes in an area without
any problem, and converge sub-second, with precomputed backups, even
withing few tens of miliseconds. There are real deployments that clearly
prove it, it's not an academic statement. Even the periodic flooding can
be avoided completely using RFC 4136.
regards,
Peter
On 5/15/14 07:08 , [email protected] wrote:
Greeting all:
I read comments on our draft, thank you for your comments.
And some questions had already been replied in our latest FAR
presentation material(not been presented at the meeting because of
hard-deadline):
--------"Draft is highly subjective. Data Centers are using existing
protocols without problems."
Why OSPF and other conventional routing methods do not work well in a
large-scale network with several thousands of routers?
As everyone knows, the OSPF protocol uses multiple databases, more
topological exchange information (as seen in the following example) and
complicated algorithm. It requires routers to consume more memory and
CPU processing capability. But the processing rate of CPU on the
protocol message per second is very limited. When the network expands,
CPU will quickly approach its processing limits, and at this time OSPF
can not continue to expand the scale of the management. The SPF
algorithm itself does not thoroughly solve these problems.
On the contrary, FAR does not have the convergence time delay and the
additional CPU overheads, which SPF requires. Because in the initial
stage, FAR already knows the regular information of the whole network
topology and does not need to periodically do SPF operation.
One of the examples of "more topological exchange information":
In the OSPF protocol, LSA floods every 1800 seconds. Especially in the
larger network, the occupation of CPU and band bandwidth will soon reach
the router’s performance bottleneck.
In order to reduce these adverse effects, OSPF introduced the concept of
Area, which still has not solved the problem thoroughly). By dividing
the OSPF Area into several areas, the routers in the same area do not
need to know the topological details outside their area. (In comparison
with FAR, after OSPF introducing the concept of Area, the equivalent
paths cannot be selected in the whole network scope)
OSPF can achieve the following results by Area :
1) Routers only need to maintain the same link state databases as other
routers within the same Area, without the necessity of maintaining the
same link state database as all routers in the whole OSPF domain.
2) The reduction of the link state databases means dealing with
relatively fewer LSA, which reduces the CPU consumption of routers;
3) The large number of LSAs flood only within the same Area.
But, its negative effect is that the smaller number of routers which can
be managed in each OSPF area.
On the contrary, because FAR does not have the above disadvantages, FAR
can also manage large-scale network even without dividing Areas.
The aging time of OSPF is set in order to adapt to routing
transformation and protocol message exchange happened frequently in the
irregular topology. Its negative effect is:
when the network does not change, the LSA needs to be refreshed every
1800 seconds to reset the aging time. In the regular topology, as the
routings are fixed, it does not need the complex protocol message
exchange and aging rules to reflect the routing changes, as long as LFA
mechanism in the FAR is enough.
Therefore, in FAR, we can omit many unnecessary processing and the
packet exchange. The benefits are fast convergence speed and much larger
network scale than other dynamic routing protocol.
Now there are some successful implementations of simplified routings in
the regular topology in the HPC environment.
Conclusion: As FAR needs few routing entries and the topology is
regular, the database does not need to be updated regularly. Without the
need for aging, there is no need for CPU and bandwidth overhead brought
by LSA flood every 30 minutes, so the expansion of the network has no
obvious effect on the performance of FAR, which is contrary to OSPF.
--------"Network convergence doesn't follow link state
dynamics - Fast reroute exists. "
Comparison of convergence time:
The settings of OSPF spf_delay and spf_hold_time can affect the change
of convergence time. The convergence time of the network with 2480 nodes
is about 15-20 seconds(as seen in the following pages); while the FAR
does not need to calculate the SFP, so there is no such convergence time.
These issues *still exist*in rapid convergence technology of OSPF and
ISIS (such as I-SPF). The convergence speed and network scale constraint
each other. FAR does not have the above problems, and the convergence
time is almost negligible.
And test data is been include in another pptx material named OSPF in
DCN(2).pptx, which can be download from IETF.
Looking forward to further discussion.
Best.
Richard Bin Liu
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