[Ilugc] [OT?] Many sites not opening - BSNL BB
- From: rajasuperman@xxxxxxxxx (Raja Subramanian)
- Date: Sun, 1 Apr 2012 23:19:29 +0530
On Sun, Apr 1, 2012 at 8:21 PM, Natarajan V <rajanvn at gmail.com> wrote:
The following is what *think* might have happened.
MTU is largest size of each packet. The default value is 1500. But
seems like this size is too big for some networks.
Actually the issue is more like this:
The link between your DSL modem and DSLAM (other end ISP equipment
your DSL modem connects to) uses encapsulated PPP - it's either PPP over
Ethernet or ATM.
Since 1500 is the MTU on Ethernet networks, when you add PPPoA/E
the packet size increases due to encapsulation overhead.
PPPoE add 8 bytes overhead. So the MTU should be 1492 bytes to
ensure that even after adding encapsulation overhead you are still
at the 1500 byte MTU limit.
* I don't use BSNL to verify, so please let us know if MTU 1492 works for you.
Note that the same issue is present when you use VPN tunnels - you
need to use a lower MTU on the tunnel to accommodate VPN overhead.
There are a couple of ways around manual MTU setting:
Routers can fragment and reassemble packets so that they can pass through
packets with smaller MTU sizes, but this has a lot of overhead. In fact it has
so much overhead that IPv6 does not support fragmentation/reassembly.
It simply drops the packet, sends a "packet to big" ICMP error condition.
This forces sender/receiver to perform path MTU discovery.
When I observed
more closely, some sites just didn't load at all.
The othersites (such
as thehindu & slashdot), start to load, and they keep waiting for
ever. The site has been waiting indefinitely for some content to be
loaded from other domains whose routes accept smaller than 1500.
That's why some sites stopped loading midway, while others never
loaded at all.
Sites which load are successfully able to perform path MTU discovery
so they automatically discover the lower MTU on your DSL modem.
To be certain use "tracepath" which is similar to traceroute but reports
the path MTU at each hop. It will be great to see the tracepath output
for your various websites.
- Raja
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