[plug] OT: TCP/IP 'routing theory?' question from a student

[Matt Kemner]

On Thu, 1 Nov 2001, Daniel wrote:

> What I think happens at an ISP -

> I believe they can route traffic for optimum performance, each with a
> fall back position where if one goes down traffic goes through
> another, and if it comes back up bandwidth is immediately utilized.

ISPs use a system called BGP to announce their IP addresses to the world
at large - and specify which link they prefer their traffic to go down..
That way if that link disappears, the announcements are still going out
the other link which will then take precedence.

> At home on my humble linux box firewall -
> I have one line out to one ISP.  I also have a modem I can dial out while
> my account to another ISP is still current.
> Can I use both transparently to machines networked to the box. ie if I am
> downloading one file it can use full bandwidth from both?

In a word, no.  There are a few dodgy tricks you can employ to use
bandwidth from both (such as running a proxy server that talks to proxies
at each ISP with round-robin) but you will rarely get full bandwidth from
both.

> (I am aware of some multiplexing that can combine bandwidth from two lines
> if the software is on client machine and at ISP - these ISPs aren't likely
> to work together on this)

If they are 2 different ISPs it will be almost impossible for them to work
together on this, even if they wanted to... :)

 - Matt