I'm glad to see that the first attempt was the only attempt needed!
Whenever I run into an intermittent connectivity or timeout issue, I
first check for a physical cause (loose cable, power supply, etc),
then the next things I check are the parameters for the network
connection, with the MTU size being foremost among those when dealing
with an ISP.
The PPP-over-Ethernet 'standard' typically calls for a maximum MTU of
1492 (versus 1500 for a standard Ethernet connection... some overhead
there). However, since the MTU size you need to use is established by
the maximum packet size that all of the routing equipment between you
and your destination can handle, the optimal MTU size can be anything
but standard. For example, check out this page in the
As you'll see from there, the ISP can choose to use a smaller MTU size
in their configuration of their network devices. The trade-off that
they are trying to balance is between efficient communications for
each individual end-user, and the amount of lag for all users as the
routing equipment can only handle so many requests at a time.
So, why were your Macs fine at first? The answer will be extremely
elusive, and can reside anywhere along the networking connection
between you and the websites you are viewing. Has any equipment in
your office changed? Has your ISP (China Telecom) performed any
maintenance or upgrades on their subscriber-facing equipment in their
local central office? Perhaps they have upgraded their routers, and
lowered the max MTU level in the process. Or, it might even be that
the gateway routers controlled by the Chinese government that allow
access to international networks, have increased overhead requirements
due to increased censorship. It's really hard to say.
The most likely place to look, is the part of the connection that lies
within your own office, followed by the equipment at the other end of
your physical connection (which I am thinking is DSL?). Do you have a
router in your office? If so, that router has a max MTU setting which
may not be properly communicated back to your Macs (but is being
communicated back to the network adapters in your Windows machines).
Or, China Telecom may have upgraded their equipment in the local
central office that manages the PPPoE session(s), and may have lowered
the MTU setting there. If the 'bottleneck' were further on in the
network, then I would expect to see problems on the Windows machines
Since the root cause of the problem might never be found, its probably
best to keep the RMAC utility close at hand on any Mac you may use for
Internet access; I'd even consider keeping it on portable media in
case any of the other Mac users you know ever run into this problem
too. Again, the link to the information page that links to the tool
I'm glad that I was able to help you with this problem, and hope that
your Internet surfin' remains easy and trouble-free! (Or at least as
trouble-free as it can be given your location!)
Google Answers Researcher
Clarification of Answer by
17 Sep 2004 07:50 PDT
One additional thought that I should mention. If there is indeed a
router that is in use in your office, that router will negotiate with
your computer to determine the maximum MTU size during the initial
'handshaking', or discovery stage, that goes on when your computer is
initially connected to the network. There may be an issue here with
the configuration of that router and how it negotiates the max MTU.
So, if there is indeed a router present, you (or the person who
manages the router) may want to look into its configuration to see if
something is misconfigured.
Google Answers Researcher