Subject: Tsvart last call review of draft-ietf-ice-rfc5245bis-16

From: Magnus Westerlund <[email-masked]>
Date: Tue, 30 Jan 2018 05:21:27 -0800

Reviewer: Magnus Westerlund
Review result: Ready with Issues

I've reviewed this document as part of the transport area directorate's ongoing
effort to review key IETF documents. These comments were written primarily for
the transport area directors, but are copied to the document's authors for
their information and to allow them to address any issues raised. When done at
the time of IETF Last Call, the authors should consider this review together
with any other last-call comments they receive. Please always CC tsv-dir@… if
you reply to or forward this review.

Unfortunately there was an error in the datatracker that resulted in that
review requests was not timely sent. Thus, I got this request the day before
the deadline. Therefore I am late with the review.

I know that this is an update to an published RFC. But, I have reviewed it on a
general level without considering what is changed or not.

This draft is on the right track but has open issues, described in the review.

Significant Issues:

A. Section 5.2:

    Lite implementations only utilize host candidates.  A lite
    implementation MUST, for each component of each data stream, allocate
    zero or one IPv4 candidates.  It MAY allocate zero or more IPv6
    candidates, but no more than one per each IPv6 address utilized by
    the host.  Since there can be no more than one IPv4 candidate per
    component of each data stream, if an ICE agent has multiple IPv4
    addresses, it MUST choose one for allocating the candidate.  If a
    host is dual-stack, it is RECOMMENDED that it allocate one IPv4
    candidate and one global IPv6 address.  With the lite implementation,
    ICE cannot be used to dynamically choose amongst candidates.
    Therefore, including more than one candidate from a particular scope
    is NOT RECOMMENDED, since only a connectivity check can truly
    determine whether to use one address or the other.

I find it quite strange that the above text says there can only be single IPv4
based candidate, while for IPv6 a LITE implementation may have one candidate
per IPv6 address. Isn't the LITE implication of having multiple candidates for
the same address family similar? Yes, IPv6 kind of forces the need for dealing
with multiple IPv6 addresses on any host. However, I can see that certain
servers will actually be multi-homed in IPv4 and thus can in a sensible way
actually have multiple IPv4 candidates, and let the clients select which
interface has the best reachability.

Can you please be explicit on what in ICE prevents things to work for IPv4 but
the same case works for IPv6?

B. Section 6.1.1:

    An agent MUST be prepared that the peer might re-determine the roles
    as part of any ICE restart, even if the criteria for doing so are not
    fulfilled.  This can happen if the peer is compliant with an older
    version of this specification.

What does it mean to be prepared for a peer that re-determine the roles? What
is it one MUST do? If the peer changes its role upon an ICE restart, isn't that
going to result in a role mismatch? Thus causing yet another ICE restart, where
also this peer will re-evalute? Isn't that good enough? Or is it something else
it can do?

C. Section 6.1.3:

    The ICE agent has a state determined by the state of the check lists.
    The state is Completed if all check lists are Completed, Failed if
    all check lists are Failed, and Running otherwise.

Does failed really require all the checklists to fail, or simply any to fail if
the others are completed?

D. Section 6.1.4.2:

I don't know if I misunderstand the algorithm here in the bullet list. To me it
appears that it will terminate prior to have initiated all possible tests, as
it appears that it will not unfreeze some of the candidate pairs. If one have
tests running for a foundation, but all other candidate checks have been
started, then the steps are aborted. Is the bullet list rechecked every Ta?

E. Section 7.2.5.2.2.  ICMP Error

    An ICE agent MAY support processing of ICMP errors for connectivity
    checks.  If the agent supports processing of ICMP errors, and if a
    Binging request generates an ICMP error, the agent SHOULD set the
    state of the candidate pair to Failed.

I am a bit worried by this blanket statement on ICMP errors. I think it should
be clarified which ICMP message types that are relevant to consider as errors?
I assume Type 3 (Destination Unreachable) but maybe not all responde codes as
Codes 4, 11,12 may be addressable in other ways, and likely Type 11 (Time
exceeded) with response code 0, response code 1 is not a clear indication of a
non working path.

F. Section 7.2.5.2.3.  Timeout

    If the Binding request times out, the ICE agent MUST set the
    candidate pair state to Failed.

Isn't this erroneous? Timeout for the connectivity check is happening when all
the (re-)transmissions have timed out, isn't it? or as simple as missing the
word "transaction"?

G. Section 14.3:

     Num-Of-Pairs: the number of pairs of candidates
     with STUN or TURN servers.

I don't understand this definition. What does "with STUN or TURN servers" mean?
Candidate pairs where the local side is server-reflexive or relay?

H. Section 14.3:

     Num-Waiting: the number of checks in the check list in the
     Waiting state.

     Num-In-Progress: the number of checks in the In-Progress state.

Is "the number of checks" only per single checklist or across all the check
lists?

I. Section 17.2.3:

When VAD is being
   used, keepalives will be sent during silence periods.

I would claim that this is only true for when VAD without any comfort noise is
used. A lot of codecs with VAD operations still generates comfort noise on a
frequency of a couple packets a second, way more often then the minimal for ICE
keep-alives.

Minor/Editorial Issues:

1.  Section 5.1.2:

This section doesn't make it clear that higher priority values are more
prioritized over lower values. That really should be defined here. Now that
information only becomes evident implicitly in section 5.1.2.1.

2. Section 2.1.

    In order to execute ICE, an ICE agent has to identify all of its
    address candidates.

I think this sentence is raising a too high requirement. An ICE agent has
attempt to identify as many of the address candidates as possible. The better
coverage of the potential candidates the more likely it is to function. I would
also argue that there are multiple cases where you will not figure out that
there are candidates that you don't know about. An obvious example is in cases
of two NATs between the local address realm and the STUN server, the agent
can't figure out that there was an address given to the flow in the middle
address realm between the two NATs. That can be only learn if the agent has a
STUN server in that address realm. Secondly, there are cases where policy may
be applied to exclude certain interfaces and their related candidates.

I also noted that this first paragraph and the second has a strange relation.
The first part of first paragraph is general, then there is the part of the
host candidate. Then the second part starts with STUN and TURN derived
candidates. Maybe the first paragraph should be split between the general and
the host part, or some other bridging is needed.

3. Section 2.3:

If the transactions above succeed, the agents will set
    the nominated flag for the pairs, and will cancel any future checks
    for that component of the data stream.

Although what is stated is normal, it is not guaranteed to happen, I know this
is intended as a simplified overview, but ignoring that there can occur some
shuffling back and forth if high priority checks complete after low priority
ones should at least be hinted or at least allowed by the use of words.

4. Section 3.

As RFC 7825 do describe a significant enough different usage of ICE from SIP, I
think it would be good to actually included an informational reference to this
usage.

5. Section 5.1.1.4:

An ICE agent SHOULD
    monitor the interfaces it uses, invalidate candidates whose base has
    gone away, and acquire new candidates as appropriate when new
    interfaces appear.

I am missing discussion of new addresses here. If the base disappears, it might
be that there is a new IP address that one should use. That doesn't necessary
imply a new interface.

6. Section 7.2.5.1:

    If the Binding request generates a 487 (Role Conflict) error
    response, and if the ICE agent included an ICE-CONTROLLED attribute
    in the request, the agent MUST switch to the controlling role. If
    the agent included an ICE-CONTROLLING attribute in the request, the
    agent MUST switch to the controlled role.

I think the first sentence should have a forward reference to Section 7.3.1.1
where the rest of the solution is described.

7. Section 7.3:

If the agent is using Diffserv Codepoint markings [RFC2475] in its
    data packets, it SHOULD apply the same markings to Binding responses.

I find this sentence a bit unclear. Is it intended to say:

If the agent receiving the binding request, intended to use DSCP markings !=0
for the data, it SHOULD set, the same marking to binding responses.

or

If the agent receives a binding request with DSCP markings, then it should
apply to corresponding code point when forming the binding response?

There are unclarity of which agent is referenced and whom "it" is in the
sentence.

8. Section 8.3.1:

   The procedures in Section 8 require that an ICE agent continue to
   listen for STUN requests and continue to generate triggered checks
   for a data stream, even once processing for that stream completes.

That reference to Section 8, should that in fact be to Section 8.1
specifically? It looks strange with a self reference, which in some aspect a
reference to section 8 means.

9. Section 15:

4.57566E+18 (note that
   an implementation would represent this as a 64-bit integer so as not
   to lose precision).

Why the floating point representation? Priorities are integer numbers and thus
should be presented as such in this example.

10. Section 17.2.1:

   First and foremost, ICE makes use of TURN and STUN servers, which
   would typically be located in the network operator's data centers.

Is really network operator's data centers the right entity here? I would claim
it is the service operators data centers, they may contract STUN services from
network operators, but if the service operator isn't providing any STUN service
for its own service, then ICE is unlikely to work.

11. Section 18.2:

Local IPv6 addresses can be preferred.

I think this sentence needs to clarify that it means local to host, rather than
any form of relayed or translated address, rather than a local scope only IPv6
address.

12. Section 18.5:

   A number of NAT boxes are now being deployed into the market that try
   to provide "generic" ALG functionality.  These generic ALGs hunt for
   IP addresses, either in text or binary form within a packet, and
   rewrite them if they match a binding.

Are actually these generic ALG functionality relevant today? They proved to be
a very bad idea very quickly. A note that this was a consideration at the time
RFC 5245 was published.

Cheers

Magnus Westerlund

---

From: Christer Holmberg <[email-masked]>
Date: Tue, 30 Jan 2018 14:34:46 +0000

Hi,

Please see inline. As requested, I include my replies to the comments you
already provided earlier.

>the same case works for IPv6?

This is text from RFC 5245. I agree it is confusing, and unfortunately I
don’t have a good answer.

I guess my approach would be to suggest that we simply remove the
restriction. In addition, there is generic text about dual-stack etc
elsewhere, 
and I don’t see anything ICE lite specific.

OLD:

"Lite implementations only utilize host candidates.  A lite
   determine whether to use one address or the other."

NEW:

"Lite implementations only utilize host candidates.
With the lite implementation, ICE cannot be used to dynamically choose
amongst candidates. Therefore, including more than one candidate from a
particular IP address family is NOT RECOMMENDED, since only a connectivity
check can Truly determine whether to use one address or the other."

---

>also this peer will re-evalute? Isn't that good enough? Or is it

The roles are re-negotiated during the ICE restart: it may, or may not,
result in a role mismatch.

"Prepared" means that the peer might change its role even though it does
not fulfil the 5245bis criteria for being allowed to do so.

---

>the others are completed?

If there are one or more completed, the session can still continue. As
described in section 8.1.2:

"If at least one of the check lists for other data streams is
    Completed, the controlling agent SHOULD remove the failed data
    stream from the session while sending updated candidate list to
    its peer."

---

>started, then the steps are aborted. Is the bullet list rechecked every

No. Whenever Ta fires, one check list in Running state is checked. When
all check lists have been checked, it will start over from the top of the
list.

Or, did I misunderstand your issue?

---

>exceeded) with response code 0, response code 1 is not a clear indication

This is from RFC 5245.

I don’t think the ICE WG should go through all different codes and
combinations, and determine what should be considered an error, and what
not.

If you can provide something (table, guidance etc), we are happy to
include it. Otherwise I’d like to keep it as it is, and let
implementations deal with it, as at least I am not aware that this would
Have caused issues in ICE deployments.

---

>the (re-)transmissions have timed out, isn't it? or as simple as missing

Correct. I will change to "Binding request transaction"

---

>Candidate pairs where the local side is server-reflexive or relay?

The text comes from RFC 5245, but I think it’s wrong. There are no pairs
during the gathering phase.

My suggestion is to change Num-Of-Pairs to Num-Of-Cands, and say:

"Num-Of-Cands: the number of server-reflexive and relay candidates"

---

>Is "the number of checks" only per single checklist or across all the

Per single check list.

---

>frequency of a couple packets a second, way more often then the minimal

What about:

"In deployments that are not utilizing Voice Activity Detection (VAD),
without any comfort noise,
the keepalives are never..."

=======

>information only becomes evident implicitly in section 5.1.2.1.

I suggest the following:

"This priority will be used by ICE to determine the order of the
   connectivity checks and the relative preference for candidates.
Higher priority values give more priority over lower values."

---

>be applied to exclude certain interfaces and their related candidates.

I suggest to replace with first sentence and simply say:

"In order to execute ICE, an ICE agent identifies and gathers one or more
address candidates."

>the host part, or some other bridging is needed.

I suggest to split the first paragraph into two paragraphs, where the
second paragraph begins with the "At least one viable candidate뒰�
sentence. 

---

>ones should at least be hinted or at least allowed by the use of words.

One of the tasks have been to simplify section 2, because it was too
detailed for people who just wanted to get an overview of ICE. For
example, we removed the text about role conflicts.

So, I would prefer to not cover your case in section 2. I don¹t think it
is essential for people who want to get an overview. Implementers
obviously will need to read the whole spec.

---

>think it would be good to actually included an informational reference to

I can do that. However, note that RFC 7825 references RFC 5245. It even
references specific sections, which may not be the same in 5245bis.

In addition, RFC 7825 uses ³aggressive nomination² terminology, which has
been removed from 5245bis.

What about:

"RFC 7825 defines an ICE usage for the Real-Time Streaming Protocol
(RTSP). Note, however, that the ICE usage is based on RFC 5245."

---

>be that there is a new IP address that one should use. That doesn't

What about:

"Host candidates do not time out, but the candidate addresses may
change or disappear for a number of reasons. An ICE agent SHOULD
monitor the interfaces it uses, invalidate candidates whose base has
gone away, and acquire new candidates as appropriate when new
<new>IP addresses (on new or currently used interfaces)</new> appear."

---

>where the rest of the solution is described.

I will add a reference.

---

>apply to corresponding code point when forming the binding response?

It means that it will use the same markings in Binding responses that it
uses in data packets (audio, video, etc).

>reference to section 8 means.

It is the STUN server.

Would the following be more clear?

"If the agent is using Diffserv Codepoint markings [RFC2475] in data
packets that it sends, the agent SHOULD apply the same markings to Binding
responses."

---

>should be presented as such in this example.

This is from RFC 5245, and unfortunately I don’t know.

---

>for its own service, then ICE is unlikely to work.

Could we simply say “located in data centers”?

---

>any form of relayed or translated address, rather than a local scope only

I could say:

"Local IPv6 host addresses"

---

>a very bad idea very quickly. A note that this was a consideration at the

I don’t know whether the functionality is relevant. But, I guess it
doesn’t hurt to have the text.

Regards,

Christer

---

From: Ben Campbell <[email-masked]>
Date: Tue, 30 Jan 2018 10:49:40 -0600

Commenting on the first issue:

> On Jan 30, 2018, at 8:34 AM, Christer Holmberg <[email-masked]> wrote:
> 
>> Significant Issues:
>> 
>> A. Section 5.2:
>> 
> don¡¯t have a good answer.
> 
> and I don¡¯t see anything ICE lite specific.
> 
> OLD:
> 
> check can Truly determine whether to use one address or the other.”
> 

We should avoid making non-critical changes to text that was unchanged from 5245 at this point in the process. If people think this is important to change, it needs working group discussion. This is a hazard of bis drafts—it’s really hard to tell when you are done. :-)

I am guessing that the original motivation was that, since ICE-lite cannot select among candidates, you want the minimum number of candidates necessary per address family. Multiple IPv6 candidates are allowed because of the nature of IPv6. But I think if you are truly multi-homed, you probably shouldn’t be using ICE-lite.

Ben.