Subject: [rbridge] Should RBridges decrement the IP TTL?

From: Radia.Perlman at Sun.COM (Radia Perlman)
Date: Tue May 4 21:36:02 2004

One of the variations on the RBridge theme has the RBridge use the TTL 
in the IP
header when forwarding IP packets. This avoids extra encapsulation overhead.
I think the tradeoffs are:

in favor of using TTL: no extra bytes of overhead in the case of an 
on-campus IP packet,
but there will still need to be encapsulation (to preserve the layer 2 
address) in the case of
off-campus IP packet.

against: the TTL will be decrementing, making the fact that there are 
internal hops visible.

Opinions?

Radia

---

From: aidan at nicta.com.au (Aidan Williams)
Date: Wed May 5 02:04:33 2004

Radia Perlman wrote:
> One of the variations on the RBridge theme has the RBridge use the 
> TTL in the IP header when forwarding IP packets. This avoids extra 
> encapsulation overhead. I think the tradeoffs are:
> 
> on-campus IP packet, but there will still need to be encapsulation 
> (to preserve the layer 2 address) in the case of off-campus IP 
> packet.
> 

An un-encapsulated variant of rbridge would avoid the issue of
fragmentation resulting from the encapsulation overhead as well.

A bare IP packet approach could be done if rbridges used the destination 
IP address rather than the MAC address to make forwarding decisions. 
The next hop for IP destinations outside the campus would be resolved 
using the same information that hosts have (e.g. a default route).  The 
default route could be propagated to all rbridges using the link state 
routing protocol already in use.

> against: the TTL will be decrementing, making the fact that there are

Various link-local protocols use the TTL to detect and drop packets that 
have been forwarded.  A sender sets the TTL to the maximum allowable 
value and the receiver drops packets with TTL != max.  Messing with the 
TTL will affect these protocols.

This is not an insuperable problem if link-local is defined to be 
"really local to this link" rather than "heard by all the hosts in my 
subnet" as has been suggested in Dave Thaler's multilink draft. 
Unfortunately, the "really local to this link" definition makes sense 
for some protocols in an rbridge-style network (e.g. IPv6 neighbour 
discovery, IGMP) but not others (e.g. service discovery, multicast DNS).

Not decrementing the TTL re-introduces the problem of loops
.. but you knew that already. ;-)

- aidan