2007/12/17, Iljitsch van Beijnum <iljitsch@muada.com>:
It seems to me that the basic architecture behind proposals like LISP
is that we separate two things:
1 the dynamic inter-AS connectivity (I'll call this the ground floor)
2 the mapping from prefixes to ASes (I'll call this the second floor)
Where 2 itself can be split into:
2a the fairly static mapping of prefixes to a set of ASes
2b the dynamic reachability status of each individual prefix->AS
relationship
LISP is of course somewhat messy because it wants to be highly
backward compatible. In a more radical approach, there isn't even any
reason for (locator) addresses on the ground floor to be globally
unique: see HRA.
In TIDR I proposed to use a specific set of addresses
for locators (240.0.0.0) that would be assigned to
transit AS-es. Transit AS-es would then originate
"locator prefixes" that would be use for the dynamic
inter-AS connectivity of the "ground floor". One of
the main benefits of this is the protection of the
inter-domain routing infrastructe because a packet
coming from a non-transit AS with destination within
the 240.0.0.0 block will be stopped as it enters
the inter-domain routing infrastructure. In other
words the first non-transit AS will discard that
packet. And a transit AS will never accept IP
packets with an IP destination in this range UNLESS
it is a tunneled packet.
Doing routing calculations per prefix rather than per AS could be
considered a design flaw in BGP. For some time now, the number of
In TIDR we have the possibility of announcing one
or more "locator prefixes" per AS.
prefixes per AS has been stable at close to 8.5. Interestingly, new
ASes tend to have far fewer than 8 prefixes, so what's really
happening is that the influx of new ASes and the addition of new
prefixes coincidentally happen at the same rate. In the two floor
system, we immediately gain an order of magnitude in processing
reduction, but this is a one time thing that may not even translate to
IPv6, where the AS-to-prefix ratio is about 1.4. the real savings will
have to come from the ability to prune leaf ASes from the ground
floor:
Pruning of leaf AS-es is the base of the reasoning
that I followed in the TIDR draft. Leaf AS-es don´t
participate in the inter-domain packet forwarding,
while at the same time contribute to the size of
the global BGP table. They also consume AS numbers.
They don't play in the football team but they have
a number reserved for their shirt. :-)
And finally, they are 5/6 of the total number
of AS-es, roughly speaking.
rather than map a prefix to a single leaf AS, we need to map a
prefix to multiple transit ASes, or we're stuck with the one-time gain
if we assume the number of prefixes per AS isn't going to go up. (Some
people argue that this is exactly what will happen as the IPv4 space
fragments when it runs out, in my opinion, this isn't all that likely.)
In TIDR, "identifier prefixes" of a leaf AS can
be mapped by all the upstream ISPs. Only transit
AS-es are allowed to map "identifier prefixes" to
"locator prefixes".
Another thing that slightly worries me is that LISP is only focussed
on IP-in-IP tunneling, ignoring all the work that has been done on
MPLS,
I think this point is important. In TIDR one can
specify several tunneling techniques, IP-in-IP,
GRE, or even an MPLS label. Two peering AS-es
could announce between them the mapping of
"identifier prefixes" to specific MPLS labels.
Regards,
Juanjo