Hi,
I have some comments to the draft. Please find them below under the
section header they belong to.
1. Introduction
“Limited VLAN space”
The 12-bit overlay network ID limitation of the Ethernet header is not
there any more for several years. There are different types of VLANs.
The I-SID can be considered as a 24-bit VLAN ID, which is sometimes
called I-VLAN. Furthermore, there exist B-VLANs and S-VLANs aside the
C-VLANs that were defined at the first place. The combination of these
VLANs provides flexibility for network virtualization; altogether the
Ethernet header today provides 60 bits for network virtualization.
It would be better to remove this because the limitation is not there today.
“FIB explosion due to handling of large number of MACs/IP addresses”
FIB explosion might be there in a L2 network if the already available L2
network virtualization tools are not used. FIB explosion is not likely
in a PBB network.
It would be better to update the bullet accordingly or remove it.
“Spanning Tree limitations”
L2 networks are not limited to a single spanning tree for several years.
Shortest Path Bridging (SPB) provides shortest path forwarding and uses
physical links that might have been blocked by a single spanning tree
instance. Moreover, MSTP also provides multiple spanning tree instances,
which allow the use of the physical links.
It would be better to remove the bullet.
“Broadcast storms”
Broadcast storms are not necessarily there in today’s L2 networks, e.g.
there are no broadcast storms at all in an SPBM network. Furthermore,
applying the existing L2 network virtualization techniques, the
broadcast due to unknown destination is kept within the virtual network,
thus it is not a broadcast storm.
It would be better to update the bullet such that it exactly specifies
the problem or remove it.
“Inefficient Broadcast/Multicast handling”
It is not clear what is meant here, it would be better to be more
specific. (If it is about L2, then the statement is not valid. Handling
of multicast is very efficient in L2, both shortest path trees and
shared threes can be used today.)
“Limited mobility/portability support”
It is not clear what is meant here. Is it about VM migration? If so,
then the statement is not exact for L2, VDP supports VM migration.
(Furthermore, L2 networks typically automatically learn the new location
of a station after its movement.)
“Lack of service auto-discovery"
If it is about L2, then that is not the case. SPB has in-built service
auto-discovery.
It would be better to be more specific in the bullet or remove it.
The issue list is introduced as “Existing virtual network models used
for data center networks have known limitations, specifically in the
context of multiple tenants. These issues can be summarized as”
As the comments to the bullets show the list is not accurate given the
existing virtual network models available for data centers today.
Therefore, it would be better to bring this issue list up to date to
today’s networking technologies.
(I understand that the WG aims to provide a solution over L3.
Nevertheless, it seems to me that it would be better to give other
motivation for it than L2 has issues, given that no issue has been
pointed out that is still valid for Ethernet today.)
4.1. Pros & Cons
“Unicast tunneling state management is handled at the edge of the
network. Intermediate transport nodes are unaware of such state. Note
that this is not the case when multicast is enabled in the core network.”
This statement may depend on the solution applied or the terms used here
might not be entirely clear. Even if the core only provides point to
point tunnels, then those tunnels have to be established in the core,
hence maintenance of some states is required in the core nodes. If the
core provides multipoint tunnels as well, then of course more state is
required to maintain a multipoint tunnel in the core than a point to
point tunnel. The type of connectivity that needs to be provided by the
core depends on the actual location of VMs belonging to the same
group/tenant. If VMs of a group are only behind two NVEs, then it is
enough to provide a point to point connectivity/tunnel by the core
independently of whether the traffic among the VMs is unicast or
multicast. If VMs of a group are behind more than two NVEs, then the
core has to provide a multipoint connectivity between those NVEs; the
way it is provided is solution dependent.
“Load balancing may not be optimal as the hash algorithm may not work
well due to the limited number of combinations of tunnel source and
destination addresses”
There are other possibilities of load balancing besides hash.
It would be better to update the sentence to “Hash-based load balancing
may not be optimal as the hash algorithm may not work well due to the
limited number of combinations of tunnel source and destination addresses”
4.2.1. Data plane vs Control plane driven
“Multicasting in the core network for dynamic learning can lead to
significant scalability limitations.”
Multicast should be kept within the virtual network even while it is
transmitted in the core, which can be aided by service auto-discovery.
Having these features, the scalability limitations should not be
significant.
“Specific forwarding rules must be enforced to prevent loops from
happening. This can be achieved using a spanning tree protocol or a
shortest path tree, or using a split-horizon mesh.”
“a spanning tree protocol” is not the same category as “a shortest path
tree” or “split horizon mesh” here. The first one is a protocol, while
the latter two are forwarding rules as said in the beginning of the
sentence.
It would be better to remove “protocol” from the sentence or resolve it
another way.
“the amount of state to be distributed is a function of the number of
virtual machines.”
This ‘function’ is not that easy to draw, it depends on a lot of
factors, furthermore it is most likely that the state to be maintained
does not depend on the way the state is installed, i.e. it is the same
both for the control and the data plane driven cases.
For example, there is no need to maintain any state if the connectivity
is point to point through the core, i.e. a group of VMs are only behind
two NVEs.
We can take a look on mapping tables e.g. by having an example of three
NVEs: NVE-A, NVE-B and NVE-C providing the connectivity through the core
for a group of VMs. If VM1 behind NVE-A communicates with VN2 behind
NVE-B, then the mapping table in NVE-A is VN2->NVE-B, i.e. NVE-A has to
address the packets to NVE-B for the transmission through the core if
any VM behind NVE-A sends them to VN-2. Similarly the mapping table is
VN1->NVE-A in NVE-B. It does not matter how these mapping tables are
maintained, the same states should be there. That is, states are the
same both for data and control plane driven mapping tables.
If state here means a mapping state, then it would be better to update
the cited sentence. Moreover, maybe this section is not the best
location for it. One way out is to remove the paragraph. Alternatively
it can be updated, e.g. “the amount of state to be distributed depends
on the network scenario, the grouping and the number of virtual machines.”
If the state means something else, then it would be useful to clarify
what exactly meant here.
4.2.6. Interaction between network overlays and underlays
Is it really the case that the DC provider wants to give visibility of
its infrastructure to its Tenants? Isn’t it that the Tenant gets is
overlay and the underlay should be completely hidden? For example, if
the Tenant buys L2aaS, does/should it bother with the fact that L2
overlay happens to be provided by an L3 underlay? Maybe the direction of
SLAs would be more appropriate here than providing visibility of the
underlay.
Best regards,
János
On 6/18/2012 11:51 PM, Benson Schliesser wrote:
Dear NVO3 Participants -
This message begins a two week Call for Adoption of
http://tools.ietf.org/html/draft-lasserre-nvo3-framework-02 by the NVO3 working
group, ending on 02-July-2012.
Please respond to the NVO3 mailing list with any statements of approval or
disapproval, along with any additional comments that might explain your
position. Also, if any NVO3 participant is aware of IPR associated with this
draft, please inform the mailing list and/or the NVO3 chairs.
Thanks,
-Benson& Matthew
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