Re: [Tsv-art] [nvo3] Tsvart last call review of draft-ietf-nvo3-vmm-04

Linda Dunbar <linda.dunbar@futurewei.com> Tue, 24 March 2020 20:11 UTC

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From: Linda Dunbar <linda.dunbar@futurewei.com>
To: Bob Briscoe <ietf@bobbriscoe.net>, "Bocci, Matthew (Nokia - GB)" <matthew.bocci@nokia.com>, "sarikaya@ieee.org" <sarikaya@ieee.org>
CC: "tsv-art@ietf.org" <tsv-art@ietf.org>, NVO3 <nvo3@ietf.org>, "draft-ietf-nvo3-vmm.all@ietf.org" <draft-ietf-nvo3-vmm.all@ietf.org>
Thread-Topic: [nvo3] [Tsv-art] Tsvart last call review of draft-ietf-nvo3-vmm-04
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Date: Tue, 24 Mar 2020 20:10:30 +0000
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Subject: Re: [Tsv-art] [nvo3] Tsvart last call review of draft-ietf-nvo3-vmm-04
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Bob,


With regard to the purpose of the document, the Abstract stated very clearly:
This document describes virtual machine mobility solutions commonly used in data centers built with overlay-based network. This document is intended for describing the solutions and the impact of moving VMs (or applications) from one Rack to another connected by the Overlay networks.


The Purpose is “for describing the solutions and the impact of moving VMs (or applications) from one Rack to another connected by the Overlay networks.”

Other changes and reply to  your comments are inserted below. Please let us know as soon as possible (hopefully not another 3 months) if they are acceptable?


Linda Dunbar



From: Bob Briscoe <ietf@bobbriscoe.net>
Sent: Monday, March 23, 2020 6:59 AM
To: Bocci, Matthew (Nokia - GB) <matthew.bocci@nokia.com>om>; sarikaya@ieee.org
Cc: tsv-art@ietf.org; NVO3 <nvo3@ietf.org>rg>; draft-ietf-nvo3-vmm.all@ietf.org
Subject: Re: [nvo3] [Tsv-art] Tsvart last call review of draft-ietf-nvo3-vmm-04

Matthew,

It is a long time since I reviewed this (Sep 2018), and I'm sorry for being unresponsive back in Aug 2019 when the -05 revision that aimed to address my -04 review comments was released.

I have to say, IMO this document is still far from suitable for publication as an RFC. The IETF surely cannot publish material that demonstrates such a loose grasp of the subject area, particularly the impact of mobility on layer 4 and above. However, that is not my call. All I can do as a reviewer is identify those of my comments that have not been addressed, and why it is important to address them. Then you, as document shepherd, can decide whether the IESG will need these issues to be addressed.

So, below, I work through my previous top level comments, identifying whether they have been addressed or not. I assume the editorial nits that I identified have been addressed (I haven't checked).

I also have to say that, as a general rule, the only ones of my review comments that have been addressed are the 'easy' ones that could be dealt with using something like find-and-replace techniques. Those that question the subject matter itself, have usually not been addressed at all.

=================================


#. The introduction does not say what the purpose of publishing this draft is.

[NOT ADDRESSED]

[BB] Changing the intended status to Informational has helped to address some of my concerns. However, an informational document still has to have a target readership and a purpose. This vmm document still doesn't seem to have a purpose, or if it does, it doesn't say what's its purpose is.

When it says it...
 ...describes solutions that support VM mobility,

or that:
 ...there is a desire to document comprehensive VM mobility solutions that cover
     both IPv4 and IPv6.

...they are not reasons for the IETF to publish an informational RFC. They are circular reasons - they just say, "we are writing this because it describes something". They do not say why it is relevant for the IETF to publish this description. Is it something that the IETF needs to understand before it moves in to standardize aspects of VM mobility? Does this document provide new insights that have not been understood before? Do multiple VM mobility solutions already exist, but a standard is needed, because they don't interoperate?


#. It does not seem as if the NVO WG has discussed the purpose of using normative text in this draft. See detailed comments.

[NOT ADDRESSED]

[BB] In -07 there are still the same two "MUSTs" as in -04. This is intended to be an informational RFC, so no-one will be required to comply with it. Admittedly, informational RFCs can contain normative keywords in certain special cases (e.g. an informative copy of a specification of a protocol that is outside the IETF's change control). But this draft is not one of those special cases.

By my understanding of what normative keywords are for, these "MUSTs" ought to be replaced with "has to" or "needs to".

[Linda] How about changing to the following?
The Old NVE needs to tunnel these in-flight packets to the New NVE to avoid packets loss.


#. The draft silently slips back and forth between VM mobility and VM redundancy, without recognizing the differences. See detailed comments.
[Linda] There is no single mention of VM Redundancy in the 07 version. What do you mean?  “Warm Mobility” depends on having relevant information (or status) about the VM at the target location.  That is different from running redundant instance at the target location.

[NOT ADDRESSED]

[BB] In -07 warm standby is still described as it was in -04: state update messages at regular intervals. That is redundancy, not mobility. In particular, Section 7 has VM Mobility in the title, but starts slipping into talking about standby after the second para.

[Linda] In this document, the definition of “WARM Mobility” is referring to having the VM’s relevant information on the target location.

Warm standby is useful for resilience against failures, but it is not mobility. In standby, processing does not /move/, which is what /mobility/ means. Standby never releases the 'Old' address and, because there is no first move there is never a second or subsequent move. So routing and addressing does not become increasingly fragmented.

[Linda] this document doesn’t address the concept of “Warm Standby”.

As the Intro says, the purpose of VM mobility is...

     It is highly desirable [RFC7364<https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Ftools.ietf.org%2Fhtml%2Frfc7364&data=02%7C01%7Clinda.dunbar%40futurewei.com%7Cee683131011247d8707908d7cf21990f%7C0fee8ff2a3b240189c753a1d5591fedc%7C1%7C0%7C637205615584920242&sdata=6YDZF8NsebUKJ9shjxB5QstLjh9M7usEpU%2F69rQqVaM%3D&reserved=0>] to allow

     VMs to be moved dynamically (live, hot, or cold move) from one

     server to another for dynamic load balancing or optimized work

     distribution.

In contrast, warm standby does not ever release the 'Old' processing resources, so it cannot be used for dynamically balancing load and optimizing work distribution.

The following gives the impression that warm 'mobility' is on a spectrum between hot and cold VM mobility:

     The larger the

     duration, the less warm (and hence cold) the Warm VM mobility

     option becomes.


[Linda] are you satisfied with the following change?
The Warm VM mobility refers the backup entities receive backup information at more frequent intervals.  The duration of the interval determines the effectiveness (or benefit) of Warm VM mobility.  The larger the duration, the less effective the Warm VM mobility option becomes.

Warm standby is on a different spectrum to hot and cold mobility. But both hot and cold mobility involve one volley of messages. The only difference is whether processing is stopped or not. Repeating messages is not part way between one message and one message. Regularly repeating messages is not part way between leaving a process running and stopping it.

[Linda] Agree with you. But this document doesn’t cover “Warm Standby”.


This is still muddle-headed thinking - about redundancy, not mobility. I'm not saying redundancy is not important - it's extremely important. I'm just saying it's out of scope for a VM mobility draft.



#. Please adopt different terminology than "source NVE" and "destination NVE", which are really poor choices of terms for an intermediate node. See detailed comments. Why not use "old NVE" and "new NVE", which is what you mean?

[Linda] There is no reference of “source NVE” nor “destination NVE” in the 07 version.


[ADDRESSED] Thank you


#. Applicability is fairly clearly outlined, but it is not clear whether hosts corresponding with the mobile VMs are part of the same controlled environment or on the uncontrolled public Internet. See detailed comments.

[NOT PROPERLY ADDRESSED]

[BB] The introduction now contains the useful scoping sentence:

     There are

     communication among tasks belonging to one tenant and

     communication among tasks belonging to different tenants or with

     external entities.

However, AFAICT, the draft still does not address the case where an internal entity moves but it needs to continue to communicate with an external entity that is not controlled by (or even aware of) the NVA.

[Linda] the behavior on how the Old NVE needs to tunnel packets to the New NVE applies to both external traffic and internal traffic.


A system of tasks within a controlled environment all talking with each other would be like Schroedinger's cat in a box - a quaint thought experiment but not useful. The outside world has to be able to continually give input (requests, data, events, code, etc) and get output (responses, transformed data, notifications, etc). So a VM mobility solution cannot just separate off the outside world as if that's a different problem that is not in scope.

It's not difficult to do this, but it surely has to be done.





#. Section 4.2.1 on L3 VM mobility reads like some potential half-thought-through ideas on how to solve L3 mobility, rather than current practice, let alone best current practice. Either current practice should be described instead, or the scope of the draft should be narrowed solely to L2 VM mobility. See detailed comments.

[NOT ADDRESSED]

[BB] None of the inadequate text in this section has been altered (except terminology find-and-replace that happened to hit words in this section).

The draft still doesn't deal with the reality of where layer-4 connection state is held (in the end applications and the transport stack under them). The authors seem to believe that network elements can close or pause Layer-4 connections. For this, applications have to be built with logic to handle IP address migration. But the scope of this draft is multi-tenant DCs, where the DC infrastructure has no knowledge of which applications each tenant might be using.

My comments all still apply, and can still be found quoted way down this email (find text: "#. L3 Mobility").



For useful tutorial on how TCP responds to ICMP destination unreachable messages (defined as soft errors), and the dilemmas surrounding how TCP should respond, see RFC5461 "TCP's Reaction to Soft Errors".
It's probably also worth reading RFC6069 "Making TCP More Robust to Long Connectivity Disruptions (TCP-LCD)".
Other transport protocols (e.g. SCTP, QUIC/UDP, RTP/UDP, etc) face similar dilemmas.

[Linda] Layer 4 connection state handling is out of the scope of NVo3. A separate draft can be written in the Transport Area to deal with Layer Connection state Handling.



# The VM's file system is described as state that moves with the VM (S.6), but VM mobility solutions often move the VM but stitch it back to its (unmoved) storage. Conversely, the storage can also move independent of the VM.

[ADDRESSED]

[BB] Thank you.



#. The draft omits some of the security, transport and management aspects of VM mobility. See detailed comments.

[NOT ADDRESSED]

[BB] The Security Considerations section is unchanged. It still only refers to previously known security issues with overlays in general, and does not discuss security vulnerabilities specific to VM mobility. In particular the need for the transport to recheck for address spoofing after each address change, which was identified in my review, which can still be found quoted below under "#. Gaps".

[Linda] the Security section has the following statement:
In Layer-3 based overlay data center networks, the problem of address spoofing may arise.  An NVE may have untrusted tasks attached.

There is still no mention of the impact of the additional delay / latency, during a mobility event.
[Linda] that is beyond the scope of this document


There also is still no mention of statistics gathering, which would be needed to be able to make decisions on when to migrate VMs. But I guess the decision on when to migrate can be ruled to be at a higher scope than this draft.

[Linda] that is VM manager’s job, out of the scope of NVO3 WG and out of scope of this document.


#. The draft reads as if different sections have been written by different authors and no-one has edited the whole to give it a coherent structure, or to ensure consistency (both technical and editorial) between the parts. See detailed comments.
[Linda] the 07 version should have fixed the problem.

[MOSTLY NOT ADDRESSED]

[BB] I made a number of points to help improve the structure and comprehensibility. Two easy 'find-and-replace' ones have been dealt with, but the three that require more than editorial knowledge have not:

  1.  [ADDRESSED] VM/task was being used in place of L2/L3. This has been taken on board. Thanks.
  2.  [NOT ADDRESSED] Signposting of where sub-cases start and end in S.4.1
  3.  [NOT ADDRESSED] Indecision over whether packets are silently dropped, dropped with an ICMP message, forwarded or tunnelled.
  4.  [NOT ADDRESSED] The order in which the various stages of mobility occur was jumbled. Some stages have been ruled out of scope, but they are still mentioned in jumbled order.
  5.  [ADDRESSED] Replace hypervisor with NVE. Thanks.



#. The quality of the English grammar does not allow a reviewer to concentrate on the technical aspects rather than the English. It would have been useful if one of the English-speaking co-authors had improved the English before submission for review. See detailed comments.

[ADDRESSED] Thanks.

==================
[BB] New problem

Whilst checking over -07, I noticed that the definitions of task, workload and VM are now all interchangeable.

      Tasks:  Task is a program instantiated or running on a virtual

               machine or container.  Tasks in virtual machines or

               containers can be migrated from one server to another.

               We use task, workload and virtual machine

               interchangeably in this document.

Then in 4.2:

     Even though the term VM and Task are used interchangeably in this

     document, the term Task is used in the context of Layer-3

     migration mainly to have slight emphasis on the moving an entity

     (Task) that is instantiated on a VM or a container.

The correct way to deal with confusion between different concepts is not just to say "Well, if you squint up your eyes, all words mean roughly the same thing really."



Bob

On 24/02/2020 13:10, Bocci, Matthew (Nokia - GB) wrote:
Hi Bob, Authors

I am the document shepherd for this draft. It has now been updated to v07 following the shepherd’s review and WG last call.

Please can you let me know where we are with addressing the comments in this review?

Thanks

Matthew

From: Bob Briscoe <ietf@bobbriscoe.net><mailto:ietf@bobbriscoe.net>
Date: Friday, 21 September 2018 at 10:34
To: "sarikaya@ieee.org"<mailto:sarikaya@ieee.org> <sarikaya@ieee.org><mailto:sarikaya@ieee.org>
Cc: "tsv-art@ietf.org"<mailto:tsv-art@ietf.org> <tsv-art@ietf.org><mailto:tsv-art@ietf.org>, NVO3 <nvo3@ietf.org><mailto:nvo3@ietf.org>, IETF <ietf@ietf.org><mailto:ietf@ietf.org>, "draft-ietf-nvo3-vmm.all@ietf.org"<mailto:draft-ietf-nvo3-vmm.all@ietf.org> <draft-ietf-nvo3-vmm.all@ietf.org><mailto:draft-ietf-nvo3-vmm.all@ietf.org>
Subject: Re: [nvo3] [Tsv-art] Tsvart last call review of draft-ietf-nvo3-vmm-04
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Resent to: <sarikaya@ieee.org><mailto:sarikaya@ieee.org>, Linda Dunbar <Linda.dunbar@huawei.com><mailto:Linda.dunbar@huawei.com>, <vumip1@gmail.com><mailto:vumip1@gmail.com>, <tom@herbertland.com><mailto:tom@herbertland.com>, <sadikshi@cisco.com><mailto:sadikshi@cisco.com>, <matthew.bocci@nokia.com><mailto:matthew.bocci@nokia.com>, Sam Aldrin <aldrin.ietf@gmail.com><mailto:aldrin.ietf@gmail.com>, Yizhou Li <liyizhou@huawei.com><mailto:liyizhou@huawei.com>, MARTIN VIGOUREUX <martin.vigoureux@nokia.com><mailto:martin.vigoureux@nokia.com>, Deborah Brungard <db3546@att.com><mailto:db3546@att.com>, <aretana.ietf@gmail.com><mailto:aretana.ietf@gmail.com>, Matthew Bocci <matthew.bocci@nokia.com><mailto:matthew.bocci@nokia.com>
Resent date: Friday, 21 September 2018 at 10:34

Behcet,

Linda made load of responses to my review, some of which I disagree with so I would like to respond to them. I need responses to those two questions first though, 'cos everything else depends on those.


Bob
On 20/09/18 15:30, Behcet Sarikaya wrote:
Dear Bob,
On Wed, Sep 19, 2018 at 9:53 AM Bob Briscoe <ietf@bobbriscoe.net<mailto:ietf@bobbriscoe.net>> wrote:
Behcet,

I would like to make significant responses to many of Linda's responses, but until we get answers to the two pre-requisite questions I've given, I can't be sure how to respond.

So rather than promising a new version with no prior discussion, I believe it would be much more fruitful to engage in this conversation. I'm trying to help.

You already made a detailed review.
Your two points are clarifications from your detailed review.
When I said we will revise I meant we  will revise based on your detailed review.
After we post our revision you can do what ever you wish.

Sincerely,
Behcet
Cheers


Bob
On 19/09/18 15:46, Behcet Sarikaya wrote:
Hi Bob,

Thank you for your comments.
The authors are currently discussing your points and we will come up with a revision soon after the discussions are over.

Regards,
Behcet
On Tue, Sep 18, 2018 at 6:03 PM Bob Briscoe <ietf@bobbriscoe.net<mailto:ietf@bobbriscoe.net>> wrote:
Linda,

Until we can all understand the answers to the following two questions, I don't think we can discuss what track this draft ought to be on, let alone move on to your responses to all my other points.

1/ Applicability

You say this draft solely applies to connections with both ends within the controlled DC environment. But the draft says it's about multi-tenant DCs. Are there any multi-tenant DCs that restrict all VMs to only communicate with other VMs within the same controlled DC environment?

2/ Purpose of publishing as an RFC

When I said:


#. The introduction does not say what the purpose of publishing this draft is.
you responded:


[Linda] The first paragraph on Page 3 has the description why VM Mobility is needed.

Whether VM Mobility is needed was not my question. My question was what is the purpose of the IETF publishing an RFC about VM Mobility? And particularly, what is /this/ RFC intended to achieve?

Are the authors trying to argue for a particular approach vs. others? Are you trying to write a tutorial? Are you trying to give the pros and cons of different approaches? Are you trying to give advice on good practice (with the implication that alternative practices are less good)? Are you trying to clarify ideas by writing them down? Are you trying to outline the implications of VM Mobility for other protocols being developed within the NVO WG?




Bob
On 10/09/18 19:16, Linda Dunbar wrote:
Bob,

Thank you very much for reviewing the draft and provided in-depth comments. I am very sorry for the delayed response due to traveling.

Replies to your comments are inserted below marked by [Linda]:


-----Original Message-----
From: Bob Briscoe [mailto:ietf@bobbriscoe.net]
Sent: Monday, September 03, 2018 9:45 PM
To: tsv-art@ietf.org<mailto:tsv-art@ietf.org>
Cc: nvo3@ietf.org<mailto:nvo3@ietf.org>; ietf@ietf.org<mailto:ietf@ietf..org>; draft-ietf-nvo3-vmm.all@ietf.org<mailto:draft-ietf-nvo3-vmm.all@ietf.org>
Subject: Tsvart last call review of draft-ietf-nvo3-vmm-04

Reviewer: Bob Briscoe
Review result: Not Ready

I have been selected as the Transport Directorate reviewer for this draft. The Transport Directorate seeks to review all transport or transport-related drafts as they pass through IETF last call and IESG review, and sometimes on special request. The purpose of the review is to provide assistance to the Transport ADs. For more information about the Transport Directorate Reviews and the Transport Area Review Team, please see https://trac..ietf.org/trac/tsv/wiki/TSV-Directorate-Reviews<https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Ftrac.ietf.org%2Ftrac%2Ftsv%2Fwiki%2FTSV-Directorate-Reviews&data=02%7C01%7Clinda.dunbar%40futurewei.com%7Cee683131011247d8707908d7cf21990f%7C0fee8ff2a3b240189c753a1d5591fedc%7C1%7C0%7C637205615584930231&sdata=JBO6IkFeB6ratjlYoEIE%2BnTCiah41a5VKBzt4K2eBPg%3D&reserved=0>

In this case, very very few of the review comments relate to transport issues, although the greatest issue concerns a desire that the network could pause or stop connections during L3 VM Mobility, which is certainly a transport issue.

[Linda] There is “Hot Migration” with transport service continuing, and there is a “Cold Migration”, which is a common practice in many data centers, which stop the task running on the old place and move to the new place before restart as described in the Task Migration.
Is it helpful to add this description to the draft?


==Summary==

The technical aspects of the draft concerning L2 VM mobility (within a subnet) seem sound. However, this is only part of the draft, which has the following
issues:

#. The introduction does not say what the purpose of publishing this draft is.
It seems that, rather than describing a specific protocol or protocols, it intends to describe the overall system procedure that would typically be used in DCs for VM mobility. It is tagged as a BCP, but it does not say who needs this BCP, why it is useful for the IETF to publish this BCP, how wide the authors' knowledge is of current practice (given DCs are private), or why this is a BCP rather than a protocol spec.

[Linda] The first paragraph on Page 3 has the description why VM Mobility is needed. Is it helpful to move this paragraph to the beginning of the Introduction Section?
“Virtualization which is being used in almost all of today’s data
centers enables many virtual machines to run on a single physical
computer or compute server. Virtual machines (VM) need hypervisor
running on the physical compute server to provide them shared
processor/memory/storage. Network connectivity is provided by the
network virtualization edge (NVE) [RFC8014]. Being able to move VMs
dynamically, or live migration, from one server to another allows for
dynamic load balancing or work distribution and thus it is a highly
desirable feature [RFC7364].”


The draft starts out (S.3) as if it intends to say what a good VM Mobility protocol should or shouldn't do, but the rest of the document doesn't give any reasoning for these recommendations, it just asserts what appears to be one view of how a whole VM Mobility system works, sometimes referring to one example protocol RFC for a component part, but more often with no references or details.

[Linda] Is it helpful to move the paragraph above to the beginning of the Introduction Section? So that audience is aware of why VM Mobility is needed. And then follow up with what a good VM Mobility protocol should or shouldn't do?

#. It does not seem as if the NVO WG has discussed the purpose of using normative text in this draft. See detailed comments.

[Linda] The “Intended status” of the draft is “Best Current Practice”. So all the text are not “normative”. Is it Okay?

#. The draft silently slips back and forth between VM mobility and VM redundancy, without recognizing the differences. See detailed comments.

[Linda] There is only one usage of “redundancy” in the entire document, used under the context of “Hot standby option”, indicating  the “redundancy” of “the VMs in both primary and secondary domains have identical information and can provide services simultaneously as in load-share mode of operation” being expensive.

#. Please adopt different terminology than "source NVE" and "destination NVE", which are really poor choices of terms for an intermediate node. See detailed comments. Why not use "old NVE" and "new NVE", which is what you mean?
[Linda] Thanks for the suggestion. We will change to “Old NVE”, and “new NVE”.

#. Applicability is fairly clearly outlined, but it is not clear whether hosts corresponding with the mobile VMs are part of the same controlled environment or on the uncontrolled public Internet. See detailed comments.
[Linda] “Hosts” are the App running on the VM. It is the under the same controlled environment. Not on uncontrolled public internet.


#. Section 4.2.1 on L3 VM mobility reads like some potential half-thought-through ideas on how to solve L3 mobility, rather than current practice, let alone best current practice. Either current practice should be described instead, or the scope of the draft should be narrowed solely to L2 VM mobility. See detailed comments.
[Linda] This is refereeing to “Cold Migration”, which is a common practice in many data centers.

# The VM's file system is described as state that moves with the VM (S.6), but VM mobility solutions often move the VM but stitch it back to its (unmoved) storage. Conversely, the storage can also move independent of the VM.
[Linda] It depends. When a VM move to a different zone, the storage/file can becomes inaccessible.

#. The draft omits some of the security, transport and management aspects of VM mobility. See detailed comments.
[Linda] Can you provide some text?

#. The draft reads as if different sections have been written by different authors and no-one has edited the whole to give it a coherent structure, or to ensure consistency (both technical and editorial) between the parts. See detailed comments.

[Linda] we can improve.


#. The quality of the English grammar does not allow a reviewer to concentrate on the technical aspects rather than the English. It would have been useful if one of the English-speaking co-authors had improved the English before submission for review. See detailed comments.
[Linda] can you help?  Becoming a co-author to improve?

==Detailed Comments==

===#. Normative statements===

In the body of the document, there is just one occurrence of normative text (actually two "MUST"s, but both state a common requirement - just written separately for IPv4 and IPv6). This merely serves to imply that everything else the document says is less important or optional, which was probably not the intention.
[Linda] The goal is to indicate any solution in moving the VM “MUST” follow this rule. They make sense, aren’t they?

At the start there is a requirements section, which states what a VM Mobility protocol "SHOULD" or "SHOULD NOT" do. I think this is intended as a set of goals for the rest of the document. If so, these "SHOULDs" are not intended to apply to implementations, so they ought not to be capitalized.

[Linda] okay, will change.


The first requirement, "Data center network SHOULD support virtual machine mobility in IPv6", is written as a requirement on all DC networks, not on implementations. I assume this was intended to read as "Data center network virtual machine mobility protocols SHOULD support IPv6". Even then, it doesn't really add anything to say VM mobility should support v6 and it should support v4. A L2 solution won't. While undoubtedly, a L3 solution will at least support one of them.
[Linda]Agree. Will change it to “Data center that support IPv6 address should …”

I'm not sure that 'protocol' is the right word anyway; I think 'VM Mobility procedure' would be a better phrase, because it includes steps such as suspending the VM, which is more than a protocol.
[Linda] yes. Will change to “Procedure”.

The requirement "Virtual machine mobility protocol MAY support host routes to accomplish virtualization", is not followed up at all in the rest of the draft.
Even if this requirement stays, the last 3 words should be deleted.

[Linda] will change to “Host Route can be used to support the Virtual Machine Mobility Procedure.”

By the end of the draft, the solution falls far short of the most relevant "Requirements" anyway, so one assumes the title of the section ought to have been "Goals". Specifically, even in the simpler case of L2 VM mobility, S.4.1 says that triangular routing and tunnelling persist "until a neighbour cache entry times out". A cache timeout is about 10 orders of magnitude longer than the requirement to only persist "while handling packets in flight", which would be a few milliseconds at most (the time for packets to clear the network that were already launched into flight when the old VM stopped).

Whatever, it would be preferable for the draft to give rationale for these requirements, rather than just assert them. This would help to shed light on the merits of the different trade offs that solutions choose.

[Linda] Agree, will add.

===#. Mobility vs. Redundancy===

Redundancy and mobility have a lot of similarities, but they have different goals. With mobility, it is necessary to know the exact instant when one set of state is identical to the other so it can hand over. With redundancy, the aim is to keep two (or more) sets of state evolving through the same sequence of changes, but there is no need to know the point at which one is the same as the other was at a certain point.
[Linda] Agree with what you said. There is only one usage of “redundancy” in the entire document, used under the context of “Hot standby option”, indicating  the “redundancy” of  “the VMs in both primary and secondary domains have identical information and can provide services simultaneously as in load-share mode of operation” being expensive.

The draft slips from mobility to resilience in the following places:
* S.2. Terminology: Warm VM Mobility is defined without any ending, as if it is permanent replication. * S.7. "Handling of Hot, Warm and Cold Virtual Machine Mobility" is actually all about redundancy, and doesn't address mobility explicitly.

[Linda] Will add the definition “Hot Migration”, “cold migration”, and “warm migration”.

===#. Terminology===

Packets run from the source at A to the destination at B via NVE1, then via NVE2. Please don't call NVE1 and NVE2 the source NVE and the destination NVE.
In future, no-one will thank you for the apparent contradictions when they continually stumble over phrases like this one in S.4.1: "...send their packets to the source NVE"..

The term "packets in flight" is used incorrectly to refer to all the packets sent to the old NVE after the VM has moved, even if they were launched into flight long after the old VM stopped receiving packets.

[Linda] thank for the comments. Will change.

BTW, I think s/before/after/ in: "that have old ARP or neighbor cache entry before VM or task migration".

I think: s/IP-based VM mobility/L3 VM mobility/ throughout, because "based"
sounds (to me) like the mobility control protocol is over (i.e. based on) IP.

===#. Applicability===

In section 4.2 it says that the protocol mostly used as the IP based task migration protocol is ILA. This implies that all hosts corresponding with the mobile VMs are either part of the same controlled environment, or they are proxied via nodes that are part of the same controlled environment (I only have passing knowledge of ILA, but I understand that it depends on ILA routers on the path). If I am correct, this aspect of scope needs to be made clear from the start.

Also under the heading of applicabiliy, the sentence "Since migrations should be relatively rare events" appears very late in the document (S.4.2.1). The assumed level of churn ought to be stated nearer the start.

[Linda] yes, under the same controlled environment.

===#. L3 Mobility===
L2 VM mobility is independent of the application, because resolution of L2 mappings is delegated to the stack. In contrast, L3 VM mobility is only feasible under certain conditions, because an application needs an IP address to open a socket (resolution of DNS names is not delegated to the stack, and apps can use IP addresses directly anyway).

Examples of the 'certain conditions':
a) /All/ applications used in the whole DC load balancing scheme contain IP address migration logic for /all/ their connections;
b) VMs running solely applications that support IP address migration register this fact with the NVA, and it only select such VMs for mobility.
c) An abstraction is layered over /all/ the IP addresses exposed to applications (at both ends) so that the IP addresses that applications use are solely identifiers  (e.g. ILA, LISP, HIP), not also locators.

The introduction says the draft is about VM mobility in a multi-tenant DC, so the DC admin will not know the range of applications being used. This excludes condition (a) above. When the draft says "...if all applications running are known to handle this gracefully...", it doesn't quantify just how restrictive this condition is, and it gives no explanation of how this knowledge might be 'known' or which function within the system 'knows' it.

S.4.2.1 contains what seems like plenty of arm-waving.
* "TCP connections could be automatically closed in the network stack during a migration event."
        o There is no TCP connection state in the network stack.
        o Even if the network starts to drop every packet, the TCP connection
        state persists in the end-points for a duration of the order of 30-90
        minutes (OS-dependent) before TCP deems the connection is broken.
       o Other transport protocols have similar designs (including the app-layer
        of protocols over UDP).
* "More involved approach to connection migration":
        o pausing the connection [does this refer to an actual feature of any
        L4 protocol?]
        o packaging connection state and sending to target [does
        this assume logic written into the application, or is this assuming the
        stack handles this and the app is restricted to using some form of
        separate identifier/locator addresses?]
        o instantiating connection state in the peer stack [ditto?].

There's some arm-waving in S.7 too:
  "Cold Virtual Machine mobility is facilitated by the VM initially
   sending an ARP or Neighbor Discovery message at the destination NVE
   but the source NVE not receiving any packets inflight."
   [How is it arranged for the source NVE not to receive any packets in flight?]

And in S.7:
  "In hot
   standby option, regarding TCP connections, one option is to start
   with and maintain TCP connections to two different VMs at the same
   time."
   [This sounds like resilience logic has been written into the application,
   which would be a special case but not something VM mobility infrastructure
   could depend on.]

[Linda] will add.

===#. Gaps===
#. Security Considerations: repeats issues in other drafts that are not specific to mobility, but it does not mention any security issues specifically due to VM mobility. It says that address spoofing may arise in a DC (sort-of implying it is worse than in non-DC environments, but not saying why). The handshake at the start of a connection (e.g. TCP, SCTP, QUIC) checks for source address spoofing. So L3 VM mobility would be more vulnerable to source address spoofing in cases where the mobile VM was the connection initiator and there was not a new handshake after the move. However, this draft does not contain any detailed mobility protocols, so it is not possible to identify any specific security flaws.

#. Transport Issues: Effect of delay on the transport: Cold mobility introduces significant delay, and other forms less, but still some delay. It should be pointed out that some applications (e.g. real-time) will therefore not be useful if subjected to VM mobility. Similarly, even a short period of delay will drive most congestion controls to severely reduce throughput. These points might be self-evident, but perhaps they should be stated explicitly.

BTW, in the L3 VM mobility case, the draft often refers to TCP connections, but the address bindings of any transport protocols would have to be migrated due to VM mobility (e.g. SCTP; sequences of datagrams over UDP; streams over UDP such as with RTP, QUIC).

#. Management Issues: perhaps the draft ought to recommend statistics gathering (e.g. time taken, amount of duplicate data) to aid a DC's future decisions on the cost-benefit of moving a VM. The OPSDIR review says a BCP does not /have/ to describe management issues, but this document seems to describe a whole system procedure, not just a protocol, which then surely includes the management plane.

[Linda] can you become a co-author and add those in?

===#. Incoherent Structure===

S.4.1. happens to talk about VMs moving, while S.4.2. happens to talk about tasks moving, but this is not the distinguishing aspect of these two sections (anyway, S.2. says "the draft uses task and VM interchangeably"): * "4.1 VM Migration" is about "L2 VM Mobility" so this ought to be the section heading, *
"4.2 Task Migration" is about "L3 VM Mobility" so this ought to be the section heading. It would also help not to switch from VM to task across these sections
- it's just a distraction.

S.4.1 needs better signposting of where each sub-case ends (Subsections might be useful to solve this): * IPv4 * end-user client * 2 paras starting "All NVEs communicating with this virtual machine..." [Not clear that the end-user case has ended and we have returned to the general IPv4 case?] * IPv6 [Strictly, it still hasn't said whether the end-user client case has ended.] [Also, it doesn't explain why there is no need for an end-user client case under IPv6?] Sections 5 & 6 seem to be about either L2 or L3 mobility, whereas Sections 7 &
8 seem to be restricted to L2.

The draft vacillates over what to do with packets arriving at the old NVE in the L3 case (see also L3 mobility above): * S4.2 first says packets are dropped, possibly with an ICMP error message;
  o then later it says they are silently dropped;
  o then in the very next sentence it says either silently drop them or forward
  them to the new location
* S.5 says they should not be lost, but instead delivered to the destination hypervisor
  o then it describes how they are tunnelled (which is not the same as
  "forwarding").

The order in which all the stages of mobilty are given is jumbled up across sections that also appear in arbitrary order: * S.5 prepares, establishes uses then stops a tunnel, but it doesn't say where the other stages fit between these steps
        o When tunneling packets, it talks about the *migrating* VM not the
        *migrated* VM, which implies tunnelling has started before the new VM
        is running. Does this imply there is a huge buffer? o It says "Stop
        Tunneling Packets - When source NVE stops receiving packets destined
        to..." but it is never clear when a source has stopped sending packets
        to a destination, unless it explicitly closes the connection (e.g. with
        a FIN in the case of TCP). Often there are long gaps between packets,
        because many flows are 'thin' (meaning the application frequently has
        nothing to send). These gaps can last for milliseconds, hours or even
        days without any implication that the connection has ended.
* Then S.6. describes moving state, but doesn't say that this is not after the previous tunnelling steps (or where it fits within those steps). * Then S.7 describes hot, warm and cold mobility, but doesn't lay out the tunnelling or steps to move state in each case. * Then S.8 says it's about VM life-cycle, but just gives the very first 3 steps for allocation of resources to a VM, then abruptly ends, without even starting the VM, let alone getting to move it.

S.5 exhibits another inconsistency by talking about the hypervisor, not the NVE.

==#. Nits==

Nits with the English are too numerous to mention them all. Below are pointers to general problems as well as some individual instances.

S.4
  "Layer 2 and Layer 3 protocols are described next.  In the following
   sections, we examine more advanced features."
        s/following/subsequent/

S.4.1
Expand WSC, MSC and NVA on first use.

s/the VM moves in the same link/the VM moves in the same subnet/

"i.e. end-user clients ask for the same MAC address upon migration. [...] to ensure that the same IPv4 address is assigned to the VM." I think s/IPv4/MAC/ was intended?

"  All NVEs communicating with this virtual machine uses the old ARP
   entry.  If any VM in those NVEs need to talk to the new VM in the
   destination NVE, it uses the old ARP entry."
Repetition: these 2 sentences say the same. (The mistake is also repeated when these 2 sentences are repeated for IPv6).

S.4.2.1
s/Push the new mapping to hosts./Push the new mapping to communicating hosts./

S.5.
The IPv4/IPv6 pairs of paras for "tunnel estabilshment" and "tunneling packets"
only differ in the words "IPv4"/"IPv6". So in each case a single para could be given for IP (irrespective of whether v4 or v6).

Thank you very much.

Linda Dunbar







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Bob Briscoe                               http://bobbriscoe.net/<https://nam11.safelinks.protection.outlook.com/?url=http%3A%2F%2Fbobbriscoe.net%2F&data=02%7C01%7Clinda.dunbar%40futurewei.com%7Cee683131011247d8707908d7cf21990f%7C0fee8ff2a3b240189c753a1d5591fedc%7C1%7C0%7C637205615584960213&sdata=1Ru92LgwFSaV1yfc%2BXXmdM17uAnI8gjozuUVy71fgjo%3D&reserved=0>