The problem is that the original source A, that we are trying to reach
with a reply, has an address that appears to the responder X to be
routable. But the destination that is reached by that address is either
a black hole or some other entity using the same address.
The reason for the duplication is that, as described in the draft, the
source address for A is a private address. That same address may well
be reachable according to the routing table at X. But it won't get to A.
If the problem is something other than private addressing preventing
reachability, it is likely there is still a mistaken routability
problem, but I can not illustrate the failure without some other case
being described.
Yours,
Joel
On 10/21/14, 10:06 PM, Lizhong Jin wrote:
Inline, thanks.
-----Original Message-----
From: Joel M. Halpern [mailto:jmh(_at_)joelhalpern(_dot_)com]
Sent: 2014年10月22日 0:06
To: lizho(_dot_)jin(_at_)gmail(_dot_)com
Cc: gen-art(_at_)ietf(_dot_)org; mpls(_at_)ietf(_dot_)org;
ietf(_at_)ietf(_dot_)org;
draft-ietf-mpls-lsp-ping-
relay-reply.all
Subject: Re: [mpls] [Gen-art] review:
draft-ietf-mpls-lsp-ping-relay-reply-04
In line.
On 10/21/14, 10:36 AM, lizho(_dot_)jin(_at_)gmail(_dot_)com wrote:
Hi Joel, see inline below, thanks.
Lizhong
2014.10.21，PM9:30，Joel M. Halpern <jmh(_at_)joelhalpern(_dot_)com> wrote ：
If the process for this draft is to use the top address that can be
reached in the routing table, then there is a significant probability
that the original source address, which is always at the top of the
list, will be used. As such, the intended problem will not be
solved.
[Lizhong] let me give an example to explain: the source address A is
firstly added to the stack, then a second routable address B for
replying AS is also added. The reply node will not use address A since
it's not routable, then it will use address B. So it will work and I
don't see the problem.
The whole point of this relay mechanism, as I understand it, is to cope
with
the case when the responder X can not actually reach the source A.
Now suppose that the packet arrives at X with the Address stack A, B, ...
X
examines the stack. The domain of A was numbered using net 10.
The domain of X is numbered using net 10. A's address is probably
routable
in X's routing table. The problem is, that routing will not get to A. X
examines
the stack, determines that A is "routable", and sends the packet. This
fails to
meet the goal.
[Lizhong] The source A you are referring is the initiator, right? The goal
of relay mechanism is to reach the initiator. If X is routable to the
initiator (address A), then it is great, other relay node in the stack will
be skipped.
If the source A you are referring is the interface address of one
intermediate node, then I do not understand "routing will not get to A. X
examines the stack, determines that A is "routable", and sends the packet".
Why routing will not get to A, but A is routable?
Regards
Lizhong
Yours,
Joel