On Sun, Apr 1, 2012 at 8:21 PM, Natarajan V <[email protected]> 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 _______________________________________________ ILUGC Mailing List: http://www.ae.iitm.ac.in/mailman/listinfo/ilugc
