Subject: [rbridge] RE: rbridge Digest, Vol 1, Issue 2

From: Konrad.Roeder at T-Mobile.com (Roeder, Konrad)
Date: Wed May 5 18:40:40 2004

Radia,
You 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:
>
>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

I think it gets down to a fundamental question is RBridge a Layer 2 or Layer 3 protocol? (Or both?)

If the RBridges decrements the IP header's TTL, then how do we handle the interaction with Traceroute?  Does RBridge reply as if it were a router (correctly sends back an ICMP message when TTL goes from 1 to 0) or a bridge (ignores traceroute)?  Will this break traceroute?  Does an RBridge have an IP address to provide in the ICMP message?

If RBridge is a Layer 2 protocol, then it probably should not meddle with TTL.
If RBridge is a Layer 3 protocol, then it should behave like a router and decrement TTL.

Konrad

Konrad Roeder
Broadband Wireless Network Engineer

T-Mobile USA, Inc.
Office:  425-748-2381
PCS:   425-444-2076
FAX:    425-748-3050
12920 SE 38th Street
Bellevue, WA  98006

e-mail: [email-masked]

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From: Radia.Perlman at Sun.COM (Radia Perlman)
Date: Thu May 6 09:21:48 2004

I'm not sure it's useful to try to answer a philosophical question like 
whether RBridge is
layer 2 or layer 3. I don't think there's any agreed-upon definition, or 
whether it's too useful
to categorize. One might say it is layer 3 because it participates in 
some layer 3 protocols
(like ARP and ND).

Someone privately told me there are certain IP protocols that will not 
work if the TTL
gets decremented, like "local broadcast". Not sure which protocols that, 
but that indeed would
answer the question. It is essential that RBridge not break any existing 
IPv4 or v6 behavior.
RBridges are presenting the campus as a single IP subnet, and any 
protocols the IP wants to
use on a single IP subnet should still work if you replace bridges with 
RBridges.

I have less sympathy with traceroute, since this is just a perception 
thing...there will be more
visible hops than there were before, but a hop is when something is 
being forwarded, so not
sure why people wouldn't want to see the bridge hops as well as the 
layer 3 hops.

But that's irrelevant. If there are IP protocols that would *break* if 
the TTL is decremented (and
it looks like there are), then we shouldn't decrement the TTL.

This means that all packets (IP on-campus, IP off-campus, and non-IP) 
will have an RBridge
encapsulation header that includes a hop count.

There are another couple of interesting issues with this direction, 
which I"ll bring up in another note.

If there are people that would like to see RBridges use the layer 3 TTL, 
please speak up.

Radia

Roeder, Konrad wrote:

>_______________________________________________
>rbridge mailing list
>[email-masked]
>http://www.postel.org/mailman/listinfo/rbridge
>  
>

---

From: touch at ISI.EDU (Joe Touch)
Date: Thu May 6 09:38:59 2004

> to categorize. One might say it is layer 3 because it participates in 

It may be useful to distinguish between what is provided to the outside 
of an rbridge (i.e., what it emulates) and how it operates internally.

An rbridge (at least to me) emulates an L2 bridge - in which case the 
TTL would not be decremented at all.

When such a device emulates an L3 router, it is closer (IMO) to Virtual 
Routers (see the draft; this was developed in the X-Bone project
for recursive Internets) - which are VERY closely related, but each [L2 
and L3] present unique issues. This draft focuses on the L2-providing 
device.

Internally, either may be implemented with L2 encapsulation, L3 
encapsulation, or carrier pidgeon ;-)

ARP and ND are interesting cases because they are L3 protocols that use 
L2 information, i.e., they are 'glue' layers of a sort. They must be 
interfaced to the edge of a campus of rbridges carefully, but I don't 
see any showstoppers.

Other more complex cases would be MPLS and other so-called layer 2.5 
protocols. I confess they never made much sense in that regard to me; 
IMO, MPLS is just a different L2 protocol that is layered over other L2 
protocols.

> gets decremented, like "local broadcast".

RFC1812 talks specifically about how to process all-1's broadcast (local 
broadcast), and how it MUST NOT be forwarded if the TTL is decremented. 
There are other cases, e.g., subnet broadcast, which SHOULD NOT me 
forwarded (although described as permitted in 1812, this is depricated 
in RFC2644).

In many ways, the definition of an IP subnet is "that in which the IP 
TTL is not decremented".

I agree with Radia that this is more important for things like:
	- broadcast
	- multicast

which would affect:
	- ARP/RARP
	- DHCP
	- BOOTP
	- IGMP
	- ICMP
	etc...

(see draft-ietf-pilc-link-design for details - notably the broadcast and 
multicast sections, which I was responsible for).

Joe
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From: alper.yegin at samsung.com (Alper Yegin)
Date: Fri May 14 16:07:55 2004

> interfaced to the edge of a campus of rbridges carefully, but I don't

Mobile IPv4 has its ARP-like side: The foreign agent (the first hop
router to the mobile node) is advised to parse the L2 headers to learn
the L2 address of the node.

> > gets decremented, like "local broadcast".

Mobile IPv4, router discovery are some examples.

Alper