Thanks for your detailed answer.
-----Original Message----- From: Erik Nordmark [mailto:[EMAIL PROTECTED]] Sent: Tuesday, 5 February 2002 2:35 PM To: [EMAIL PROTECTED] Cc: [EMAIL PROTECTED] Subject: Re: ND - Neighbor Adv - 'O' Flag - clarification > As per ND specification the Neighbor Advertisement message contains "O" > Flag (OverRide flag). > What is the purpose of this flag. Or when the nodes is supposed set this > Flag. Or when the nodes are not supposed to set this flag. The mechanical aspects of when it is set are covered in RFC 2460. > I suppose whenever the receiver finds a difference between the sent link > address and the address what it has in its ND cache it can modify the value. Correct. That is what RFC 2460 says. In addition there is some effect on the reachability state as specified in the rfc. > Why is this extra bit...? That question I don't think is clearly answered in the RFC. The motivation is that there are two possible approaches for dealing with information such as the link-layer address received in a ND packet: 1. The last received information takes precedence 2. The first received information takes precedence Normally #1 makes sense - the link layer address might change e.g. due to a replacement of the network adaptor or some form of failover between multiple adapters. But in the case of a neighbor solicitation #2 makes more sense - it is desirable by default to pick the first entity to respond since there is some correlation between the speed to respond and the load and/or speed of the node. Also, in the case of proxy neighbor advertisements it might make sense to make the precedence be determined by the sender of the NA. In the case of Mobile IP there is a home agent that proxies for a mobile node that is away from home. But in the case when the mobile moves home and start sending NA messages you'd like those message to take precedence over any proxy advertisements sent by the HA. (Of course the HA will be notified by the MN to stop sending the proxy advertisement but before it does the above rule makes sense.) The single override bit provides the flexibility to do all of the above. Erik -------------------------------------------------------------------- IETF IPng Working Group Mailing List IPng Home Page: http://playground.sun.com/ipng FTP archive: ftp://playground.sun.com/pub/ipng Direct all administrative requests to [EMAIL PROTECTED] --------------------------------------------------------------------
