Brian Dickson wrote:
It operates in exactly the same way, as if there were two equal cost
routes to two or more routers, each
advertising the existence of one of these servers, on the other side
of a PPLB router - except that it has
the ability to handle the state issue for TCP.
Anyone who operates a network with PPLB towards *external* routes, via
BGP multipath, would
have to be an idiot or a fool, and would certainly have trouble
retaining customers with clue.
I should clarify this even further: the above is *completely
independent* of the "anycast" issue itself.
Consider the following set-up:
A single prefix is announced by a single ASN, for each of which there is
only one instance. (I.e. non-anycast.)
The prefix is used solely for offering services that are front-ended by
a stateful load-balancer pair.
There are two LB's for redundancy reasons. The LB's participate in the
IGP of the ASN, also for redundancy.
The ASN (call it X) has two upstream ASNs, call them A and B.
Each ISP is connected to a separate router, for redundancy.
Inbound traffic from upstream ASN A hits router X-A, and inbound traffic
from upstream B hits router X-B.
Router X-A prefers LB-A, and router X-B prefers LB-B.
LB-A and LB-B are state-independent. The LB support for stateful traffic
(e.g. TCP) works only if all incoming packets
for a particular TCP session hit the same LB.
This is *not* anycast to the world. Some *may* consider it to be
IGP-anycast.
It is an extremely common set-up, perhaps *the* most common
configuration for load balancers.
Anyone using PPLB between the two AS paths, the one containing A and the
one contain B, *will*
absolutely have problems using TCP, as in, it won't work except in
unusual circumstances (one of
the two paths is withdrawn, for instance.)
The real issue is, when any load-balancer is used to affect access to an
advertised service:
- is the LB breaking that access?
- is the LB operated by the operator of the service?
- does the service have customers or potential customers on the other
side of the LB?
- does the operator of the LB offer a competing service?
If the answers to all but the second question are "yes", then the LB
operator may in fact be interfering with someone
else's business. And if they are aware of that interference, it may in
fact be considered tortuous interference.
Especially if that LB set-up is not considered standard in the industry,
and even more so if it contracts any
widely-used "best common practice" document for the industry.
Services that use set-ups like "X" above, would include DNS hosting
companies, web hosting companies, email
hosting companies, content distribution networks, any number of "B2B"
services, etc.
A lot of those companies are big players, with big legal departments.
I know *I* wouldn't want to interfere with their business...
And if I was doing PPLB, I certainly wouldn't be telling everyone
operating services that I could potentially
be interfering with about it, on the mailing list that they all
participate on.
Even if I had been doing PPLB, and *had* told everyone, once I became
aware of this interference issue,
I would very quickly turn off PPLB, and tell everyone I had done so, and
that I was wrong, so as not to get
sued into bankruptcy.
IMHO.
Brian
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