Subject: [armd] review of draft-ietf-armd-problem-statement-02

From: Lucy yong <[email-masked]>
Date: Thu, 10 May 2012 19:29:24 +0000

Hi,

I read this draft and think it describes clearly. I support it.

Here are some editing suggestion and comments.
Notation: > for original text, < suggested text.

>the issue is complicated by routers having many interfaces on which
   address resolution must be performed or with IEEE 802.1Q domains,
   where individual VLANs form their own broadcast domains.

< the issue is complicated by routers having many interfaces on which
   address resolution must be performed or within IEEE 802.1Q domains
   where individual VLANs form their own broadcast domains.

>This document is a product of the ARMD WG and identifies potential
   issues associated with address resolution in datacenters with massive
   number of hosts.
<This document identifies potential
   issues associated with address resolution in datacenters with massive
   number of hosts.

>   Broadcast Domain:  The set of all links, repeaters, and switches that
      are traversed in order to reach all nodes that are members of a
      given L2 domain.  For example, when sending a broadcast packet on
      a VLAN, the domain would include all the links and switches that
      the packet traverses when broadcast traffic is sent.

Comment:  in DC, will a link or switch be physical link or virtual link, physical switch or virtual switch? It is better to state explicitly.

>  As the size of an L2 network increases, the level of
   broadcast traffic from protocols like ARP increases.

<  As the size of an L2 broadcast domain increases, the level of
   broadcast traffic from protocols like ARP increases.

> That is, split large L2 networks into multiple smaller L2 networks,
   each operating as its own L3/IP subnet.  Numerous data center
   networks have been designed with this principle, e.g., with each rack
   placed within its own L3 IP subnet.  By doing so, the broadcast
   domain (and address resolution) is confined to one Top of Rack
   switch, which works well from a scaling perspective.  Unfortunately,
   this conflicts in some ways with the current trend towards dynamic
   work load shifting in data centers and increased virtualization as
   discussed below.

Comment: In DC, split large L2 network into multiple smaller L2 network is for security trust design. Multiple L2 networks are on the same L3 subnetwork so they all can support the same application, but they are isolated by L2 network for security reason, which also reduces ARP issue.

>  First, it uses broadcast, and any network with a large number of
   attached hosts will see a correspondingly large amount of broadcast ARP traffic.
Comment: it is not necessary true. A lot of trust designs prevent from host-to-host communications.

>  Additionally, If no response
   is received, the router has to send the ARP/ND query multiple times.
< Additionally, if no response
   is received, the router has to send the ARP/ND query multiple times.

>  Although address-resolution traffic remains local to one L2 network,
   some data center designs terminate L2 subnets at individual
   aggregation switches/routers (e.g., see Section 4.4.2).

< Although address-resolution traffic remains local to one L2 network,
   some data center designs terminate L2 domain at individual
   aggregation switches/routers (e.g., see Section 4.4.2).

Regards,
Lucy

---

From: Thomas Narten <[email-masked]>
Date: Fri, 25 May 2012 16:15:52 -0400

Hi Lucy.

> Comment: in DC, will a link or switch be physical link or virtual
>  link, physical switch or virtual switch? It is better to state
>  explicitly.

I'm not sure we need say whether a link is physical or virtual. I'm
inclined to leave the definition as is. How would it make a
difference?

> Comment: In DC, split large L2 network into multiple smaller L2 network is 
> for security trust design. Multiple L2 networks are on the same L3
> subnetwork so they all can support the same application, but they
> are isolated by L2 network for security reason, which also reduces
> ARP issue.

Agreed. But do we need to say that in the document? (by saying
"comment", I assume you are not asking for text changes.) There are
lots of factors for any particular design choice, and we can't really
describe them all. So I'm inclined to only list those most related to
address resolution.

> Comment: it is not necessary true. A lot of trust designs prevent from host

I think the same comment applies here as above.

One thing I note is that using terms like "any network" or "L2
network" is not very precise. To be precise, we often need to be
talking about an L2 domain, i.e, a specific VLAN.

Thanks!

Thomas