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@xxxxxxxxxxxxxxx] >> Sent: 2014年10月22日 0:06 >> To: lizho.jin@xxxxxxxxx >> Cc: gen-art@xxxxxxxx; mpls@xxxxxxxx; ietf@xxxxxxxx; > 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.jin@xxxxxxxxx wrote: >>> Hi Joel, see inline below, thanks. >>> >>> Lizhong >>> >>> >>>> 2014.10.21，PM9:30，Joel M. Halpern <jmh@xxxxxxxxxxxxxxx> 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 > > >