On Jun 1, 2011, at 7:52 PM, Conrad Shultz wrote: > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > On 6/1/11 5:34 PM, G S wrote: >> But since this test doesn't actually check the routes to these >> hosts, how would it be able to provide meaningful results even in >> these cases? > > Note: I am _not_ an SCNetworkReachability guru, so take this all with a > grain of salt. > > How do you know that the routes aren't tested? Quoting from your first > message: > > "A remote host is considered reachable when a data packet, sent by an > application into the network stack, can leave the local device." > > If the kernel cannot route the packet, this would be interpreted by any > sane implementation as "cannot leave the local device." > > A kernel routing table lookup would cover VPN availability issues, LANs > with no gateway (or otherwise defined default route), etc. > >> I figured the specific-host test pinged the host. > > I can't imagine it would. Ping would lead to many, many false negatives > regarding host availability. A great many sites block ICMP echo > requests (I just tried "ping apple.com" for example - and get no response). > > Since the OS has no way of knowing what particular port/service is of > interest, your only good option is to try to connect to the service of > interest and handle failure appropriately. > > (Of course, even if you seemingly can't connect by this approach, it is > fundamentally unknowable whether the receiving host actually got your > test message: http://en.wikipedia.org/wiki/Two_Generals%27_Problem) > > - -- > Conrad Shultz
Totally right. Reachability may not be the best name for it. Think of it as Request-Response-Receivability (Can I get response from the host when I request it? and does that response not have any error in the header?) _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
