On 6 August 2012 21:31, Mark Trickett <marktrickett@bigpond.com> wrote:
Hello Craig,
On Sun, 2012-08-05 at 23:28 +1000, Craig Sanders wrote:
cc-ed back to the list. please don't reply privately when the discussion belongs on the list.
I try to keep things on the list, but also try to not unduly clutter. I am still sorting out the balance point.
On Sat, Aug 04, 2012 at 08:14:26PM +1000, Mark Trickett wrote:
On Sat, 2012-08-04 at 08:59 +1000, Craig Sanders wrote:
On Fri, Aug 03, 2012 at 09:45:57PM +1000, Allan Duncan wrote:
It won't help you with your download problem, but it helps explain strange numbers when MPLS gets invovled.
well spotted.
But since I am not an Internode customer, I cannot get to that page. I
I meant well-spotted RE the fact of MPLS, not the particular web page.
For those watching from the side lines, here's my summary of what the doco on the Internode site says: They firstly explain in a brief paragraph about how packet Time To Live (TTL) values work. Each time a packet is passed by a router the TTL value is decreased until it reaches zero. At that point the router making it zero discard the packet and send a message back to the originator saying it never reached its destination. It then explains how traceroute takes advantage of TTL by making use of deliberately low values that force intermediary routing devices to drop packets and send a response back to traceroute. This return response is used to calculate the response time. A simple example of 'normal' traceroute is given of tracerouting from Perth to Sydney via Adelaide. A packet with TTL of 1 is sent, gets to Adelaide and is dropped. Response sent from Adelaide back to Perth and round trip calculated at 30ms out + 30ms back = 60ms. A second packet is sent with TTL 2, passes Adelaide and gets to Sydney. Dropped at Sydney, response sent back, round trip time 100ms (30 + 20 + 20 + 30). Under MPLS that pathway is pre-calculated *before* a packet is ever sent. Once calculated a packet is more or less committed to following that path. Further, intermediate routers don't bother trying to calculate paths for packets that don't involve them "trying to stay as stupid as possible, avoiding any particular knowledge about how the various network endpoints are talking to each other." When one of these MPLS routers generates the TTL zero message that message is sent along the very same pathway as the originating packet. So in the Perth-Adelaide-Sydney example, when Adelaide gets the TTL=1 packet, the response message is sent via Sydney before returning to Perth. Hence the increased response times. HTH (or perhaps muddies the water further :^o ) -- Colin Fee tfeccles@gmail.com