From: barmar@alum.mit.edu   
      
   In article ,   
    Jorgen Grahn  wrote:   
      
   > On Thu, 2015-09-24, bit-naughty@hotmail.com wrote:   
   > > On Monday, September 21, 2015 at 10:29:09 PM UTC+5:30, Barry Margolin   
   wrote:   
   > >> BGP is how ISPs learn the address blocks that belong to each other. ISP1   
   > >> tells ISP2 "I have a route for 50.60.128.0/15". ISP2 merges that with   
   > >> all the routing advertisements it has received from other ISPs, and uses   
   > >> that when deciding where to send traffic. (This is an extreme   
   > >> simplification, read the Wikipedia article on BGP for more details.)   
   > >   
   > >   
   > > OK, thanks. For the moment, could you just clear up 2 things for me...? :   
   >   
   > > 1) Suppose I'm watching a Youtube video - it's true that different   
   > > bytes of the same video could travel via different routes to get to   
   > > me, right? Say, I'm in the UK, one byte could come straight from the   
   > > US, and another via South America, say....? I mean, that's what TCP/IP   
   > > does, right?   
   >   
   > IP is designed so it /could/ happen, yes.  Not on the byte level but   
   > on the IP packet level.  And an application or IP stack which cannot   
   > cope with it happening is broken by definition, IMO.   
   >   
   > I think it doesn't happen very often.  Especially spreading one data   
   > flow over several routes has drawbacks, like packets arriving in the   
   > wrong order, sudden MTU changes and so on.   
      
   Also, for efficiency there's lots of caching of routing rules in   
   routers. The cache is usually keyed on the port numbers, so all packets   
   of a particular connection will take the same path.   
      
   >   
   > Perhaps it's more common that packets A->B take a different path than   
   > B->A does?  I'd be interested to hear about it from someone who knows   
   > more about the topology as it looks today.   
      
   Yes, this is not uncommon.   
      
   Networks usually try to use the shortest path to get traffic out of   
   their network to the next ISP in the chain. Suppose ISP1 and ISP2   
   interconnect on both the east and west coasts. A is in ISP1 on the west   
   coast, while B is in ISP2 on the east coast. The A->B traffic will go to   
   the western connection point, then cross the country via ISP2's network.   
   B->A traffic will go to the eastern connection point, then cross the   
   country via ISP1's network.   
      
   --   
   Barry Margolin   
   Arlington, MA   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)