Subject: [rbridge] A question about the Rbridge ID

From: Xu Xiaohu <[email-masked]>
Date: Sat, 28 Aug 2010 09:39:42 +0800

Hi Radia,

As stated in RBRIDGE specification:
   "To save room in the TRILL header and simplify forwarding lookups, a
   dynamic nickname acquisition protocol is run among the RBridges to
   select 2-octet nicknames for RBridges, unique within the campus,
   which are an abbreviation for the 6-octet IS-IS system ID of the
   RBridge.  The 2-octet nicknames are used to specify the ingress and
   egress RBridges in the TRILL header."

My question is why not consider to use IP addresses directly as Rbridge
identifiers. To realize free configuration, these IP addresses could also be
automatically generated in the similar way as the TRILL nickname does.

Some conceivable benefits of using IP address as Rbridge ID include:

1) The core Rbridge could directly use the proven IP forwarding chips.

2) Matured OAM tools for IP, e.g., PING and Traceroute, can be used
directly. Or do you have any plan to develop TRILL nickname based PING or
Traceroute tools?

3) Features in the IP header, e.g., QoS, Fragmentation can be inherited
naturally. For example, there is no need to use complex technology to deal
with large packet as stated in Rbridge specification, "... If traffic
engineering tools know which links support larger than minimally acceptable
data packet sizes, paths can be computed that can support large data
packets."

Do you believe saving room in the packet header is a strong reason for
developing a totally new TRILL header, rather than reusing the proven IP
header for the TRILL Rbridge? Or would you please give some other reasons
for this option?

Best wishes,
Xiaohu  

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From: Donald Eastlake <[email-masked]>
Date: Sat, 28 Aug 2010 18:00:16 -0400

Hi Xiaohu,

I'll try to give an answer to your questions below:

> automatically generated in the similar way as the TRILL nickname does.

An obvious disadvantage is that IP addresses are twice as big as TRILL
nicknames for IPv4 (worse for IPv6 but I can't see why you would use
IPv6). But the main thing is that TRILL has already made its decision.

> 1) The core Rbridge could directly use the proven IP forwarding chips.

There was consideration of using various existing headers (IP,
802.1ah, MPLS) early on in the development of the TRILL standard but
these were all rejected for one reason or another. It is my
understanding that leading switching chip companies now all have TRILL
chips so chip availability is not a problem.

> directly. Or do you have any plan to develop TRILL nickname based PING or

RBridges are designed so that an RBridge level Traceroute could be
added fairly easily. There are two drafts that I know of which are
being developed for TRILL OAM tools of one sort or another. OAM is
listed on the proposed new Charter for the TRILL WG.

> data packet sizes, paths can be computed that can support large data

Constrained routing does not seem like a complex technology. But, in
any case, people will be expecting Layer 2 QoS, that is frame
priority, Data Center Bridging ( see
http://tools.ietf.org/html/draft-eastlake-trill-rbridge-dcb-00 ), and
things like that. It was also a goal to be able to incrementally
replace 802.1Q customer bridges with RBridges and it seems like a
complex design problem to try to do that with IP facilities. Finally,
people engineering Layer 2 networks, particularly in things like Data
Centers, generally have large MTUs, sometimes 9K jumbo frames, so
exceeding the MTU is rarely a problem and if it is many people think
the fragmentation is an inefficient solution.

> header for the TRILL Rbridge? Or would you please give some other reasons

I would say it doesn't matter any more. The TRILL header has already
been developed, the TRILL base protocol specification is approved as
an IETF standard, hardware has already been developed or adapted for
TRILL, TRILL implementations are far enough along that a publicly
announced TRILL interoperability event has been held at UNH IOL and it
looks like a second one will be scheduled early next year. I don't see
any reason for TRILL to change the basic design at this point.

> Xiaohu

Thanks,
Donald

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From: Xu Xiaohu <[email-masked]>
Date: Mon, 30 Aug 2010 09:11:36 +0800

Hi Donald,

> -----邮件原件-----
> 发件人: [email-masked] [mailto:[email-masked]] 代表
> Donald Eastlake
> 发送时间: 2010年8月29日 6:00
> 收件人: Xu Xiaohu
> 抄送: [email-masked]; [email-masked]
> 主题: Re: [rbridge] A question about the Rbridge ID
> 
> Hi Xiaohu,
> 

Yes, TRILL chips are available. However, the economy of scale for IP chips is not available for TRILL chips.

Thanks for your reply anymore.

Best wishes,
Xiaohu