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for two hosts on Internet when a host send a packet to another host there is a route between them is the route the same on two directions? for example, two hosts A and B

1) the route from A to B
2) the route from B to A

is 1) the same as 2)? any details? thanks

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2 Answers 2

The routes are not necessarily the same. Routes can be changed on the fly, based on a variety of factors. Providers can make decisions on routes based on redundancy, cost, latency, speed, and many other factors. There are businesses that are based solely on changing routing around the internet.

For example: Route A to B might be a direct connection, however the Route from B to A might actually travel from A to D to C to B.

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so how frequently are routes changing? for example, A is in the USA while B is in france. the route between them may change from hour to hour? –  misteryes Apr 25 '13 at 12:27
    
It might change from packet to packet, it might be the same month to month. Typically the connections closest to the servers (next hop, ISP connection, links to ISP core network) will stay very stable, and the middle bit (crossing the ISP core network, trunks betwen providers, routes between countries) will vary more with link congestion, failures, load balancing, etc. –  TessellatingHeckler Apr 27 '13 at 2:33

Routes over many hops are rarely the same.

Take A to B, it could route A - C - E - B

And the B to A could route B - E - F - C - A

Routing is complex, and based off many metrics. Even inside a large private network you could see these discrepancies. Network to network it gets even less predictable or defined.

If you are Point to Point, then yes it would be the same either way as there are no other devices to route through (As in a cross over cable from PC to Laptop network).

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what do you mean of Point to Point? if A is in USA and B is in France, So you mean it is very possible the route from A and B is different from last hour? The dynamics could be serious? –  misteryes Apr 25 '13 at 11:43
    
I said it in the post, but "As in a cross over cable from PC to Laptop network" You said nothing about the environment so I did not want to give a sense of "It never could happen" –  AthomSfere Apr 25 '13 at 11:44
    
I don't follow what you said, what is I did not want to give a sense of "It never could happen" ? –  misteryes Apr 25 '13 at 12:21

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