There are a couple of different issues, that are all too easy to
confuse. (No, I don't think Brian, Scott, Dave, etc. have individually
confused them. But I think that the discussion may have switched from
one to another unintentionally.
There are prefixes on RLocs. Such prefixes appear in routing. Clearly,
the presence of such a prefix in routing does not tell you enough to
judge the reachability or liveness of an individual xTR within that
prefix. (The absence of a prefix, that's useful. But that's the easy
part of the problem.)
As far as I know, in terms of liveness testing of ETRs, there is no
granularity finer than the ETR to test. So if we are using an ID / RLoc
split, the liveness testing we are interested in is the reachability /
efficacy of a given ETR. [There is the additional complication of the
ETR being live, but not having reachability to the endpoint. That is a
different problem than the liveness problem described in the draft.
There are many additional problems...)
Conversely, although the liveness testing must be on the basis of
individual ETRs, it does seem likely that many hosts in a site will be
trying to reach endpoints behind the same set of ETRs. As such, having
a border guy (or someone else, if you really want to complicate life)
testing / monitoring that liveness / reachability on behalf of the
various sources within the site would seem likely to
1) reduce the probing traffic
2) increase the odds of having accurate data when packets need to be sent.
Yours,
Joel
Brian E Carpenter wrote:
Scott,
As an endpoint, you are interested in reachability of specific
endpoints. You don't care if a whole prefix is reachable, if the
particular endpoint isn't reachable within that prefix. So if you
delegate liveness monitoring to an agent, you are asking it to monitor
specific locators.
The e2e principle seems to make it clear that, ultimately, liveness is
the endpoint's responsibility, but that is usually interpreted as a
statement about what the transport layer or above should do. And they
already do it, 100% independently of what we do in layer 3 and below.
I think that limiting layer 3 liveness detection to the prefix level
(except presumably in the case of Mobile IP) is a perfectly good
engineering compromise, unless we're planning to repudiate [Saltzer].
Brian
[Saltzer] End-To-End Arguments in System Design, J.H. Saltzer,
D.P.Reed, D.D.Clark, ACM TOCS, Vol 2, Number 4, November 1984, pp
277-288.
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