Measurement of Round-Trip Time and Fractional Loss Using Session Traversal Utilities for NAT (STUN)
draft-ietf-tram-stun-path-data-05

[Ballot discuss]
The RespTransCnt mechanism seems to be a bit fragile and error prone possibly leading to wrong conclusions on the client (please see my example below). If you agree with my assessment, it is probably useful to evaluate whether the added complexity of this RespTransCnt mechanism is worth it for the potentially unreliable results it produces.

Consider the following two cases (copy paste with a monospace font for better readability)

Case 1: Upstream loss of first "re"transmission

|  Upstream loss  |
|  Client  Server |
+-+-+-+-+-+-+-+-+-+
|  1        x    |
|                |
|  2        2,1  |
|    2,1          |

Case 2: Downstream loss of response to first "re"transmission with re-ordering

| Downstream loss |
|  Client  Server |
+-+-+-+-+-+-+-+-+-+
|  1        1,2  |
|    x            |
|  2        2,1  |
|    2,1          |

How does the client differentiate between these two cases?

[Ballot comment]
Section 5: IANA Considerations

Shouldn't the range for this option be in the 0x8000-0xBFFF range instead of the 0x8000-0xFFFF range as currently stated by the draft?

Section 6:

I think this requirement is backwards and needs to be reworded.

"Unauthenticated STUN message MUST NOT include the PATH-CHARACTERISTIC attribute in order to prevent on-path attacker from influencing decision-making."

Suggest rewording to.

"The PATH-CHARACTERISTIC attribute MUST NOT be included in unauthenticated STUN messages in order to prevent an on-path attacker from influencing decision-making."

I also agree with Alissa about the vagueness of the attribute name.

[Ballot comment]
I don't think this is appropriate to include a URL as affiliation (see callstats.io on the first page).

Below is Lionel Morand's OPS DIR review:

I think the draft is ready for publication as it is, even if some clarifications would help the reader for a correct use of the proposed solution. The comments listed below should not block the publication process but it would be nice if authors could address them if a new version of the draft is required.

Main comments:

- The whole solution is introduced as an improvement of the ICE prioritization formula. With the PATH-CHARACTERISTIC attribute, it is understood that there are additional criteria for selecting the most suitable candidate. But finally, it is not clearly said how the loss and RTT info modify/impact/improve the formula recommended in the RFC 5245. If it is left outside the document, please indicate it in the text.

- Moreover, the case with stateful agent handling the RespTransCnt field is pretty clear. But I think that more information should be given to the reader about the difference for the client to be able to rely or not on the RespTransCnt field.

Other comments:

- In section 1. Introduction:

  The ICE [RFC5245] mechanism uses a prioritization formula to order
  the candidate pairs and perform connectivity checks, in which the
  most preferred address pairs are tested first and when a sufficiently
  good pair is discovered, that pair is used for communications and
  further connectivity tests are stopped.

It is maybe too obvious and not essential for people involved in this work but could help the reader to know that ICE is a technique for NAT traversal and not a general purpose solution.

- In section 3.  Path characteristics determination mechanism

  This document defines a new comprehension-optional STUN attribute
  PATH-CHARACTERISTIC.  PATH-CHARACTERISTIC will have a STUN Type TBD-
  CA.  This type is in the comprehension-optional range, which means
  that STUN agents can safely ignore the attribute if they do not
  understand it.

It is said in another section but could be good to clarify that "safely ignore" means that when the new attribute is not supported by the server, the client naturally fallbacks to existing STUN/ICE procedures.

Instead of having a format for the request (section 3.1) and the response (section 3.2) that are the same, it would be clearer to have a section describing the format of the attribute (section 3.1)  and addition clarifying the use in the request (section 3.2) and the response (section 3.3).

- In section 3.1. The PATH-CHARACTERISTIC attribute in request

  The PATH-CHARACTERISTIC attribute in a STUN request takes a 4-byte
  Value.

I don't know what is the current IETF policy but I would prefer "32-bit value" instead of "4-byte value".

        0                  1                  2                  3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Reserved, should be 0  |  ReTransCnt  |  RespTransCnt |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Figure 1: PATH-CHARACTERISTIC attribute in request

Why are the two first octects marked as "reserved"? I'm assuming that these bits are present for alignment with a fixed size of STUN attributes but it should be clarified.
And if they are "should" should be replaced by "must". I think it would be easier to left "reserved" in the figure and describe its contents in the description given below the figure.

- In section 3.2. .  The PATH-CHARACTERISTIC attribute in response

  When a server receives a STUN request that includes a PATH-
  CHARACTERISTIC attribute, it processes the request as per the STUN
  protocol [RFC5389] plus the specific rules mentioned here.

It is maybe obvious or already described in RFC 5389, but I assume that the server must include the new attribute in the response if not received in the request.

  o  If the server is stateless or does not want to remember the
      transaction ID then it would populate value 0 for the RespTransCnt
      field in PATH-CHARACTERISTIC attribute sent in the response.  If
      the server is stateful then it populates RespTransCnt with the
      number of responses it has sent for the STUN request.

Is there any recommendation between "stateless vs stateful" for an optimal support of this new attribute? If it is, we could find something like "SHOULD be stateful but MAYBE stateless"...

- In section 3.3.  Example Operation

It should be clarified that the server is stateful in this example. Otherwise, the RespTransCnt field in PATH-CHARACTERISTIC attribute in the response is of no use. Could be "stateful server" instead of "server" in the description of the example.

Moreover, as commented earlier, The case with stateful agent handling the RespTransCnt field is pretty clear. But I think that more information should be given to the reader about the difference for the client to be able to rely or not on the RespTransCnt field.

- In section 4. Use cases

  o  When an endpoint has multiple interfaces (for example 3G, 4G,
      WiFi, VPN, etc.), an ICE agent can choose the interfaces for
      application data according to the path characteristics.  After
      STUN responses to STUN checks are received, the ICE agent using
      regular nomination can sort the ICE candidate pairs according to
      the path characteristics (loss and RTT) discovered using STUN.
      The controlling agent can then assign the highest priority to
      candidate pair which best fulfills the desired path
      characteristics.  However, it should be noted that the path
      capacity or throughput is not determined by these STUN checks.  If
      an endpoint needs to pick paths based on capacity, it would have
      to send application data on those paths.

This text illustrates the main comment given above. It is said:

      the ICE agent using
      regular nomination can sort the ICE candidate pairs according to
      the path characteristics (loss and RTT) discovered using STUN.

But there is no clue on how the agent will do.

[Ballot comment]
I agree with Alissa's comments. In addition:

- Is there any expected interaction between this and the ongoing 5245bis work?

- 3, last paragraph: "distinguish STUN responses from the re-transmitted
  requests."
I think you mean to distinguish the response to one request from a response to a retransmitted request. But it sounds like you mean to distinguish the response from the request itself.

-4 : resends of the same request:
you’ve used the term retransmission so far. I assume this means the same—any reason not to be consistent?

[Ballot comment]
= General =

I find the name the name PATH-CHARACTERISTIC a bit general for this, since it specifies a single specific characteristic while one could imagine other characteristics one might want to specify in the future. Would suggest naming this something more specific.

= Section 1 =

I think it will be important for future ICE specs to document expected usage of this feature, and those won't necessarily be limited to continuous nomination. I don't think there is anything to be done in this draft, but just noting here that close coordination between TRAM and ICE will be necessary to make sure this happens going forward.

= Section 3.3 =

Section 3.1 defines ReTransCnt as "Number of times request is re-transmitted with the same transaction ID to the server," but then the examples in 3.3 all show the client populating this field with 1 in its first request. If the number is supposed to be about *re-transmits* (not transmits, but re-transmits), I would have expected it to be 0 in the first request, then 1 upon an actual re-transmit. Why do these start at 1? It may be that simply re-wording the use of "re-transmits" in 3.1 would fix this.

Also, in the first two examples, why do the ReTransCnt values keep incrementing after a successful response? For example, in the normal case, why does the client re-transmit its request with ReTransCnt equal to 2 when it received a successful response?

[Ballot comment]
The draft looks good, I would just add in the security section that the information could be observed passively if not encrypted (especially since that is an option) and used for reconnaissance and later attacks.

[Ballot comment]
A few questions that could potentially be further clarified in the document:

1) How often should the request be retransmitted to get a qualified loss measure?
2) What's the frequency of the retransmissions/pacing or wating between retransmissions?
3) How big is the overhead when retransmitting several times?

Thanks!

(Via drafts-lastcall-comment@iana.org): IESG/Authors/WG Chairs:

IANA has completed its review of draft-ietf-tram-stun-path-data-03.txt. If any part of this review is inaccurate, please let us know.

IANA understands that, upon approval of this document, there is a single action which IANA must complete.

In the STUN Attributes subregistry of the Session Traversal Utilities for NAT (STUN) Parameters registry located at:

http://www.iana.org/assignments/stun-parameters/

a new registration will be made as follows:

Value: [ TBD-at-Registration ]
Name: PATH-CHARACTERISTIC
Reference: [ RFC-to-be ]

The value to be selected will be from the comprehension-optional range (0x8000-0xFFFF).

QUESTION: Should this assignment be made from the IETF Review range (0x8000-0xBFFF), or the Designated Expert range (0xC000-0xFFFF)?

IANA understands that this is the only action required to be completed upon approval of this document.

Note:  The actions requested in this document will not be completed until the document has been approved for publication as an RFC. This message is only to confirm what actions will be performed. 


Thank you,

Sabrina Tanamal
IANA Specialist
ICANN

The following Last Call announcement was sent out:

From: The IESG
To: "IETF-Announce"
CC: tram-chairs@ietf.org, "Simon Perreault" , sperreault@jive.com, spencerdawkins.ietf@gmail.com, draft-ietf-tram-stun-path-data@ietf.org, tram@ietf.org
Reply-To: ietf@ietf.org
Sender:
Subject: Last Call:  (Discovery of path characteristics using STUN) to Proposed Standard


The IESG has received a request from the TURN Revised and Modernized WG
(tram) to consider the following document:
- 'Discovery of path characteristics using STUN'
  as Proposed Standard

The IESG plans to make a decision in the next few weeks, and solicits
final comments on this action. Please send substantive comments to the
ietf@ietf.org mailing lists by 2016-02-11. Exceptionally, comments may be
sent to iesg@ietf.org instead. In either case, please retain the
beginning of the Subject line to allow automated sorting.

Abstract


  A host with multiple interfaces needs to choose the best interface
  for communication.  Oftentimes, this decision is based on a static
  configuration and does not consider the path characteristics, which
  may affect the user experience.

  This document describes a mechanism for an endpoint to discover the
  path characteristics using Session Traversal Utilities for NAT (STUN)
  messages.  The measurement information can then be used to influence
  the endpoint's Interactive Connectivity Establishment (ICE) candidate
  pair selection algorithm.




The file can be obtained via
https://datatracker.ietf.org/doc/draft-ietf-tram-stun-path-data/

IESG discussion can be tracked via
https://datatracker.ietf.org/doc/draft-ietf-tram-stun-path-data/ballot/


No IPR declarations have been submitted directly on this I-D.

> As required by RFC 4858, this is the current template for the Document
> Shepherd Write-Up.
>
> Changes are expected over time. This version is dated 24 February 2012.
>
> (1) What type of RFC is being requested (BCP, Proposed Standard,
> Internet Standard, Informational, Experimental, or Historic)?

Proposed Standard

> Why is this the proper type of RFC?

Because a new STUN attribute and associated processing behaviour are defined.

> Is this type of RFC indicated in the title page header?

Yes.

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>  Are there existing implementations of the protocol? Have a
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>  conclusion that the document had no substantive issues? If
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>  what was its course (briefly)? In the case of a Media Type
>  review, on what date was the request posted?
>
> Personnel
>
>  Who is the Document Shepherd? Who is the Responsible Area
>  Director?

Technical Summary

  This document describes a mechanism for an endpoint to discover the path
  characteristics using Session Traversal Utilities for NAT (STUN) messages.
  The measurement information can then be used to influence the endpoint's
  Interactive Connectivity Establishment (ICE) candidate pair selection
  algorithm.

Working Group Summary

  The proces for this document was straightforward. No controversy, good
  discussion, easy consensus.

Document Quality

  Web browser vendors have participated actively in the discussion with usage by
  their WebRTC implementation in mind.

Personnel

  Document Shepherd: Simon Perreault
  Responsible Area Director: Spencer Dawkins

> (3) Briefly describe the review of this document that was performed by
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The document shepherd has read and reviewed every version of this draft, has
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None.

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No concerns.

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Yes.

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No.

> (9) How solid is the WG consensus behind this document? Does it
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Strong consensus from the whole working group.

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No nits found.

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No.

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No.

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None performed.