IPv6 Routing Header - How does it work?

[mcmanus at ducksong.com (Patrick R. McManus)]

[Vriz: Fri, Apr 19, 2002 at 10:29:13AM +0800]
> I see. Actually, I'm not trying to state a route for my packet to go.
> It's just that I would like to find out the route my packet has taken from a
> source to a destination.
> If I'm not wrong, I think traceroute in linux does that. However, I would
> like to know how it is done.

ah! This is a much easier question!

yes, traceroute shows you the path your packets take from you to the
destination.

It does this by manipulating the TTL (time to live) field in the IP
header. TTL (in practice) specifies the maximum number of hops a
packet may travel. Each time a router forwards a packet it reduces the
TTL by one (technically max(1,# sec q'd at router), but generally
implmented as just 1) - if a packet has a TTL of 0 it gets dropped and
the original sender is notified via an ICMP_TIME_EXCEEDED ICMP
message. The normal purpose for a TTL is to prevent loops from filling
the network with endlessly forwarded packets.

traceroute starts out by sending its probe with a TTL of just 1. The
first router gets the packet, decrements the TTL and discovers that it
can't forward it anymore - so it sends the ICMP notification to the
original sender and drops the packet. The original sender uses the src
address of that ICMP message to identify the first hop router. It then
repeats the process with a TTL of 2 to find the second hop router,
etc..

There are a whole slew of reasons why this fails to work now and again
in practice (links that filter icmp, and routers using private rfc 1918
address space combined with links that filter private addresses being
the 2 most common.), but its a pretty good technique overall.

when this works, this tells you the path from your src to the
destination. It doesn't tell you anything about the path from the
other end towards you - and asymmetric paths are common. Figuring the
latter out is much trickier.

> And if my packet has travelled from a source to a destination using a
> specific route, does that mean it's going to be more or less unchanged for
> my other packets (same source and destination)?

they call this 'fate sharing' - and you can assume this with a
fair degree of certainty - but its hardly absolute. There is some
routing that takes port numbers into account, as well as switches and
routers that simply load balance their traffic - but the latter
introduces reordering (you send packets 1,2,3 and they arrive 1,3,2)
which has a bad impact on tcp performance and so networks try and
engineer so that doesn't happen too much.

-Patrick