Re: [tram] AD Evaluation of draft-ietf-tram-turnbis-23

["Konda, Tirumaleswar Reddy" <[e-mail]>]

Hi Magnus,

Thanks for the detailed review. Please see inline 

> -----Original Message-----
> From: tram <[e-mail]> On Behalf Of Magnus Westerlund
> Sent: Thursday, April 4, 2019 7:30 PM
> To: [e-mail]; [e-mail]
> Subject: [tram] AD Evaluation of draft-ietf-tram-turnbis-23
> 
> 
> 
> Hi,
> 
> I have now finished the AD review. There are a lot of details that I think needs
> fixing or at least feedback on. I also think there are a couple of more serious
> issues here. Especially regarding the handling of MTU and fragmentation, as
> well as IP header fields in address family translation and with non UDP
> transport. We need to conclude on these before we go to IETF last call.
> 
> My Review comments are:
> 
> A. Section 1: "The client can arrange
>    for the server to relay packets to and from certain other hosts
>    (called peers) and can control aspects of how the relaying is done."
> 
> I don't think this is an accurate description of what Turn does. One may easily
> get the impression that the whole packet TURN receives are relayed, rather
> than the transport protocol data part and some of the header information are
> relayed between the client <-> peer, with a knowledge of the peer transport
> address for the client. I think that should be clarified in the introduction.

Sure, updated the text as follows:

When a peer sends a packet to the relayed transport address, the server relays the transport protocol data from the packet to the client. The client knows the peer from which the transport protocol data was relayed by the server. When the server receives an ICMP error packet, the server also relays certain layer 3/4 header fields from the ICMP header to the client. When the client sends a packet to the server, the server relays the transport protocol data from the packet to the appropriate peer using the relayed transport address as the source.

> 
> B. Section 2.7:
> 
> "Applications using TCP can more or less
>    ignore this issue because fragmentation avoidance is now a standard
>    part of TCP, but applications using UDP (and thus any application
>    using this version of TURN) must handle fragmentation avoidance
>    themselves."
> 
> Lower case "must", and as the document uses RFC 2119, this must have the
> meaning of "MUST". A reformulation here is likely in order.

Replaced "must" with "MUST". 

> 
> C. Section 2.7:
> 
>    In this approach, the application exploits the
>    fact that the IP specification [RFC0791] specifies that IP packets up
>    to 576 bytes should never need to be fragmented.
> 
>    As a guideline, sending a maximum of 500 bytes of application data in
>    a single TURN message (by the client on the client-to-server leg) or
>    a UDP datagram (by the peer on the peer-to-server leg) will generally
>    avoid IP fragmentation.  To further reduce the chance of
>    fragmentation, it is recommended that the client use ChannelData
>    messages when transferring significant volumes of data, since the
>    overhead of the ChannelData message is less than Send and Data
>    indications.
> 
> In that first paragraph I think it is worth discussing the IPv6 requirement of
> supporting a 1280 minimal size.

Yes. 

> 
> Secondly, I think recommending only a 500 MTU is way to conservative.
> Those studies I have seen appears to indicate that at least 1220 bytes UDP
> payloads should be be working as well as 500 bytes. See for example figure 12
> in this:
> https://static.googleusercontent.com/media/research.google.com/sv//pubs/arc
> hive/46403.pdf

I have updated the text as follows to address your comments:

   In this approach, the application MUST assume a
   PMTU of 1280 bytes, as IPv6 requires that every link in the Internet
   have an MTU of 1280 octets or greater as specified in [RFC8200].  If
   IPv4 support on legacy or otherwise unusual networks is a
   consideration, the application MAY assume on a PMTU of 576 bytes for
   IPv4 datagrams, as every IPv4 host must be capable of receiving a
   packet whose length is equal to 576 bytes as discussed in [RFC0791].

> 
> 
> D. Section 2.7:
> 
>    [I-D.ietf-tram-stun-pmtud] is an implementation of [RFC4821] that
>    uses STUN to discover the path MTU, and so might be a suitable
>    approach to be used in conjunction with a TURN server that supports
>    the DONT-FRAGMENT attribute.  When the client includes the DONT-
>    FRAGMENT attribute in a Send indication, this tells the server to set
>    the DF bit in the resulting UDP datagram that it sends to the peer.
>    Since some servers may be unable to set the DF bit, the client should
>    also include this attribute in the Allocate request -- any server
>    that does not support the DONT-FRAGMENT attribute will indicate this
>    by rejecting the Allocate request.
> 
> I want to verify that the WG is working on resolving the issues with the STUN
> PMTUD document. I definitely sees the point of the recommendation, but only
> if the WG will conclude on that document in a reasonable time frame.

The authors of the ietf-tram-stun-pmtud draft are working on resolving the DISCUSS from ISEG review.

> 
> E. Section 3:
> 
>    Server-Reflexive Transport Address:  A transport address on the
>       "public side" of a NAT.  This address is allocated by the NAT to
>       correspond to a specific host transport address.
> 
> Shouldn't "public side" be replace with "external side"? That is the terminology
> used by the Behave RFCs.

Okay, replaced "public side" with "external side". 

> 
> F. Section 3:
> 
>    5-tuple:  The combination (client IP address and port, server IP
>       address and port, and transport protocol (currently one of UDP,
>       TCP, or (D)TLS)) used to communicate between the client and the
>       server.
> 
> I see some issues that the 5-tuple aggregates the transport protocol with the
> security protocol. I agree from the TURN protocol perspective the protocol
> combination is the important one. However, writing like you do here you
> obfuscate both that when security is in place you have a combination, as well as
> that there are 4 alternatives defined by this document.

(D)TLS can be considered as pseudo transport protocol. 

> 
> G. Section 4.1:
> 
>    The "turn" and "turns" URI schemes are used to designate a TURN
>    server (also known as a relay) on Internet hosts accessible using the
>    TURN protocol.  The TURN protocol supports sending messages over UDP,
>    TCP, TLS-over-TCP or DTLS-over-UDP.  The "turns" URI scheme MUST be
>    used when TURN is run over TLS-over-TCP or in DTLS-over-UDP, and the
>    "turn" scheme MUST be used otherwise.  The required <host> part of
>    the "turn" URI denotes the TURN server host.  The <port> part, if
>    present, denotes the port on which the TURN server is awaiting
>    connection requests.  If it is absent, the default port is 3478 for
>    both UDP and TCP.  The default port for TURN over TLS and TURN over
>    DTLS is 5349.
> 
> First, I was wondering if this document needed to update RFC 7065? The high
> level obvious is the reference, but otherwise on the surface it doesn't look
> necessary. My main complaint when reading was that it was not obvious what
> not specifying the transport parameter means. Then I saw this in Section 3.1 of
> RFC 7065:
> 
>    The <host>, <port>, and <transport> components are passed without
>    modification to the [RFC5928] algorithm.  <secure> is set to false if
>    <scheme> is equal to "turn", and set to true if <scheme> is equal to
>    "turns" and passed to the [RFC5928] algorithm with the other
>    components.
> 
> After some digging I figured out that RFC5928 is actually updated by RFC
> 7350 that defines the resolution and SRV records for DTLS.
> 
> Can you please update the text in Section 4 to also reference RFC 7350 so that
> one doesn't miss this definitions.

Sure, added the following line to Section 4:
DTLS as a transport protocol for TURN is defined in RFC7350.

> 
> H. Section 5:
> 
>    If the server requires requests to be
>    authenticated, then the server's administrator MUST choose a realm
>    value that will uniquely identify the username and password
>    combination that the client must use, even if the client uses
>    multiple servers under different administrations.
> 
> Just to verify, TURN does not allow a single server address to host multiple
> realms do it? Each Realm needs its own unique server transport address?

No, I don't see any such restriction in TURN.  For example, in an Enterprise network with multiple realms, the TURN server based on the client subnet can choose the realm.

> 
> I. Section 5:
> 
>    For each Allocate request, the server SHOULD generate a
>    new random nonce when the allocation is first attempted following the
>    randomness recommendations in [RFC4086] and SHOULD expire the nonce
>    at least once every hour during the lifetime of the allocation.
> 
> So, I think this text due to its use of RFC 2119 language is a bit confusing in its
> relation to STUNbis. To me it is unclear if the above overrides completely what
> is written in Section 5 of STUNbis:
> 
>    To indicate that it supports this specification, a server MUST
>    prepend the NONCE attribute value with the character string composed
>    of "obMatJos2" concatenated with the (4 character) Base64 [RFC4648]
>    encoding of the 24 bit STUN Security Features as defined in
>    Section 18.1.  The 24 bit Security Feature set is encoded as 3 bytes,
>    with bit 0 as the most significant bit of the first byte and bit 23
>    as the least significant bit of the third byte.  If no security
>    features are used, then a byte array with all 24 bits set to zero
>    MUST be encoded instead.  For the remainder of this document the term
>    "nonce cookie" will refer to the complete 13 character string
>    prepended to the NONCE attribute value.
> 
> Or is the randomness recommendations in TURNbis only for the NONCE
> attribute value part?

Yes, the randomness recommendations in TURNbis is only for the NONCE attribute value part. 

> 
> Secondly, I interpret the text to say that compared to STUN, where a Nonce can
> be long lived after the initial Long Term Credential establishment, in TURN for
> each allocation one will have to to the Allocate, expired nonce->here is your
> new nonce, resend allocate using new nonce, success dance.
> 
> I think part of the problem with the above text in TURNbis is that the sentences
> prior to the quoted one appear to be rehashing of the STUNbis requirements.
> Then the quoted part that actually changes behavior without being clear about
> it. I think it would be good to make it clear what is a change compared to
> STUNbis.

No changes other than the randomness for the NONCE attribute value part.  Added the following line for clarity:

To indicate that the server supports [I-D.ietf-tram-stunbis], the server
 prepends the NONCE attribute value with the "nonce cookie"
(Section 9.2 of [I-D.ietf-tram-stunbis]).

Please see https://tools.ietf.org/html/draft-ietf-tram-turnbis-23#section-18, discusses using the nonce cookie with the STUN security feature for the NONCE attribute 
sent in the Allocate response.

> 
> J. Section 6:
> 
>    Note that, rather than storing the password explicitly, for
>    security reasons, it may be desirable for the server to store the key
>    value, which is a secure hash over the username, realm, and password
>    (see [I-D.ietf-tram-stunbis]).
> 
> Why isn't this recommendation stronger. Are there any reasons why a server
> would actually store the plain text password at all?

Good point, no reason to store the plain text password. Updated text as follows:

For security reasons, the server MUST NOT store the
password explicitly and MUST store the key value, which is a secure
hash over the username, realm, and password (see Section 16.1.3 in
[I-D.ietf-tram-stunbis]).

> 
> K. Section 7.1:
> 
>    The client first picks a host transport address.  It is RECOMMENDED
>    that the client pick a currently unused transport address, typically
>    by allowing the underlying OS to pick a currently unused port for a
>    new socket.
> 
> I think talk about socket in this context should be skipped as it otherwise have
> been eliminated.

The above text is from https://tools.ietf.org/html/rfc5766#section-6.1. I presume you are proposing the following update:
 
    The client first picks a host transport address.  It is RECOMMENDED
    that the client pick a currently unused transport address, typically
    by allowing the underlying OS to pick a currently unused port.

> 
> L. Section 7.1:
> 
>   The client then picks a transport protocol to use between the client
>    and the server.  The transport protocol MUST be one of UDP, TCP, TLS-
>    over-TCP or DTLS-over-UDP.
> 
> I find it strange that this indicates that one should pick the transport protocol to
> use client to server without considering the server capabilities. Especially as the
> client configuration or server discovery procedures will indicate transport
> protocol support.

The server may support more than one transport protocol. 

> 
> Secondly, what is the motivation for the MUST in the second sentence?
> This is appears to have strange interactions with any future extensions and
> have no impact. A client must obvious chose a client to server transport
> protocol it supports, and one that the server supports or the client hope it will
> support.

https://tools.ietf.org/html/rfc5766#section-6.1 has the same text, I have updated the text as follows:
The client then picks a transport protocol to use between the client and the server based on the transport protocols supported by the server.  
The TURN server MUST enable one of the transport protocols, UDP, TCP, TLS- over-TCP or DTLS-over-UDP.

> 
> M. Section 7.1:
> 
>    The client also picks a server transport address, which SHOULD be
>    done as follows.  The client uses one or more procedures described in
>    [RFC8155] to discover a TURN server and uses the TURN server
>    resolution mechanism defined in [RFC5928] to get a list of server
>    transport addresses that can be tried to create a TURN allocation.
> 
> 1) So RFC 8155 states that it has no recommendation on server selection, nor
> on how to related local configuration to any auto-discovery servers.
> To me it appears that the protocol utilizing TURN should consider what makes
> sense in their deployments. Thus, does the need for such consideration needs to
> be highlighted somewhere in this specification.

https://tools.ietf.org/html/rfc8155#section-3 discusses using auto-discovery in the absence of local configuration or if the local configuration does not work. https://tools.ietf.org/html/rfc8155#section-9 explains the mechanisms to authenticate the discovered TURN server.  

I am not sure about the additional considerations you are looking in this specification.

> 
> 2) The RFC5928 reference  should be complemented with an RFC 7350
> reference.

Done.

> 
> N. Section 7.1:
> 
>   Clients MUST NOT include a REQUESTED-ADDRESS-
>    FAMILY attribute in an Allocate request that contains a RESERVATION-
>    TOKEN attribute, for the reasons outlined in [RFC6156].
> 
> Considering that this document obsoletes RFC 6156, I think it is bad form to
> reference that document, rather than to move the necessary motivation into
> this document.

Done, updated text as follows:
Clients MUST NOT include a REQUESTED-ADDRESS-FAMILY attribute in an Allocate request that contains a RESERVATION-TOKEN attribute, for the reason that server uses the previously reserved transport address corresponding to the included token and the client cannot obtain a relayed transport address of a specific address type.

> 
> O. Section 7.1:
> 
>    If the client wishes to obtain a relayed transport address of a
>    specific address type then it includes a REQUESTED-ADDRESS-FAMILY
>    attribute in the request.
> 
> I don't find anything describing what the default behavior is if one doesn't
> include the attribute, but the TURN server supports multiple address families.
> 
> I see that bullet 7 in Section 7.2 indicates that IPv4 is default.
> Should the text in 7.1 be explicit about this default?

The text in Section 7.1 is added for TURN clients not supporting TURNbis specification to provide backward compatibility. 

> 
> P. Section 7.2:
> 
>          Otherwise, if the attribute is included but
>         specifies a protocol other that UDP, the server rejects the
>         request with a 442 (Unsupported Transport Protocol) error.
> 
> Assuming that you don't support an extension for that transport protocol.
> Should that be made explicit?

I did not get the above comment. The attribute REQUESTED-TRANSPORT is originally defined in TURN (see https://tools.ietf.org/html/rfc5766#section-6.2), and TURNbis does not update the use of the attribute.


> 
> Q. Section 8.3:
> 
>    If the client receives a success response to its Refresh request with
>    a non-zero lifetime, it updates its copy of the allocation data
>    structure with the time-to-expiry value contained in the response.
> 
>    If the client receives a 437 (Allocation Mismatch) error response to
>    a request to delete the allocation, then the allocation no longer
>    exists and it should consider its request as having effectively
>    succeeded.
> 
> So there are no considerations for the client if it receives an error response to a
> refresh request when Lifetime was non-zero?
> 
> I am bit surprised by that. I assume that there are certain cases when the
> request could be resent, and others where the meaning is that the allocation is
> gone.

TURNbis does not update the refresh behavior already defined in https://tools.ietf.org/html/rfc5766#section-7.3.  I will add the following line:
If the client receives an error response to its request to refresh the allocation, it should consider the allocation no longer exists.

> 
> R. Section 11.5:
> 
>    When the server receives an ICMP packet, the server verifies that the
>    type is either 3, 11 or 12 for an ICMPv4 [RFC0792] packet or either
>    1, 2, or 3 for an ICMPv6 [RFC4443] packet.  It also verifies that the
>    IP packet in the ICMP packet payload contains a UDP header.  If
>    either of these conditions fail, then the ICMP packet is silently
>    dropped.
> 
> Why is not ICMPv6 Type 4 (Parameter Problem) included when Parameter
> Problem are for ICMPv4?

I don't get the above comment, please explain the question with an example. 

> 
> S. Section 12.5:
> 
>    Over TCP and TLS-over-TCP, the ChannelData message MUST be padded to
>    a multiple of four bytes in order to ensure the alignment of
>    subsequent messages.  The padding is not reflected in the length
>    field of the ChannelData message, so the actual size of a ChannelData
>    message (including padding) is (4 + Length) rounded up to the nearest
>    multiple of 4.  Over UDP, the padding is not required but MAY be
>    included.
> 
> Are there anywhere written what one may or may not include of the TURN
> related messages in a single UDP datagram. Because it is technically feasible I
> belive to include two Send Indications in one UDP datagram to the server.

As per https://tools.ietf.org/html/draft-ietf-tram-turnbis-23#section-11.2, one send indication in one UDP datagram to the server.

> 
> T. Section 13.1:
> 
>    Flow Label
> 
>       Preferred behavior: The relay sets the Flow label to 0.  The relay
>       can choose to set the Flow label to a different value if it
>       supports the IPv6 Flow Label field [RFC6437].
> 
>       Alternate behavior: The relay sets the Flow label to the default
>       value for outgoing packets.
> 
> I think this section should be improved in two ways.
> 
> First of all I think it should clarify that each allocation to peer flow is its own
> IPv6 Flow in perspective of the IPv6 Flow label.
> 
> Secondly, I think in the spirit of RFC 6473 the preferred behavior should be to
> set the IPv6 flow label if the implementation is capable, or leave it to the
> underlying network stack to actually set it. Compared to RFC 7915, the TURN
> server actually have an understanding of what are different flows without
> having examining additional protocol layers.

Updated text as follows:

Preferred behavior:
The TURN server can use the relayed transport address, peer transport address and UDP protocol number 
to generate and set the flow label value in the IPv6 packet as discussed in Section 3 of [RFC6437]. If the TURN server is incapable of 
generating the flow label value from the IPv6 packet's 5-tuple, it sets the Flow label to 0. 

(Please note the proposed update deviates from the behavior defined https://tools.ietf.org/html/rfc6156#section-8.1)   


> 
> Thirdly, I don't understand what the alternative behavior actually is.
> This as it is either setting it to 0, or classifying things as flow and then provide it
> with a random flow label. I think the right alternative behavior is to set it 0.

Updated text to say:

       Alternate behavior: The relay sets the Flow label to the default
       value of zero for outgoing packets.

> 
> U. Section 13. 1,2,3:
> 
>       For both preferred and alternate behavior, the DONT-FRAGMENT
>       attribute MUST be ignored by the server.
> 
> I don't understand why just because one performs v4<->v6 or v6 to v4
> translation or v6 to v6 relaying now can ignore the DONT-FRAGEMENT
> attribute. If the client has request to not fragment and the server is unable to
> do that when forwarding to the peer it needs to generate an ICMP attribute
> response saying the packet is to big. Otherwise the relay breaks the Path MTU
> discovery mechanisms.

TURNbis is using the translation algorithms specified in RFC7915. 

[a] For IPv4 to IPv6: The translator uses the DF bit in the IPv4 header to not fragment the IPv6 packet (see https://tools.ietf.org/html/rfc7915#section-4), DON'T-FRAGMENT attribute can be ignored. 
[b] For IPv6 to IPv4: The translation algorithm in https://tools.ietf.org/html/rfc7915#section-5 is used.  If the IPv6 endpoint wants to discover the path MTU, it will send IPv6 packets without Fragment header and DON'T-FRAGMENT attribute can be ignored.
[c] For IPv6 to IPv6: If the IPv6 endpoint wants to discover the path MTU, it will send IPv6 packets without Fragment header and DON'T-FRAGMENT attribute can be ignored.

Please note ignoring the DONT-FRAGMENT attribute is picked from https://tools.ietf.org/html/rfc6156#section-8 and discussed in the BEHAVE WG https://mailarchive.ietf.org/arch/msg/behave/oabu0VtUMBE-X3-vhgjdSFCboxc.  

> 
> V. Section 13.2:
> 
> Fragmentation:
> 
>       If the incoming packet included a Fragment header and the outgoing
>       packet size exceeds the outgoing link's MTU, the relay MUST
>       fragment the outgoing packet into fragments of no more than 1280
>       bytes.  The relay sets the fields of the Fragment header as
>       appropriate for a packet originating from the server.
> 
> Is really the intention here to refragment the packet without providing any
> indication that the practical MTU is smaller than what the peer or client thinks
> it is?

Yes, TURNbis is following the existing behavior already defined in https://tools.ietf.org/html/rfc6156#section-8.1. 

> 
> W. Section 14:
> 
>       Preferred Behavior: Set the outgoing value to the incoming value,
>       UNLESS the server is doing Active Queue Management, the incoming
>       ECN field is ECT(1) (=0b01) or ECT(0) (=0b10), and the server
>       wishes to indicate that congestion has been experienced, in which
>       case set the outgoing value to CE (=0b11).
> 
> First of all do you have any special meaning with all capital UNLESS?
> 
> Secondly, I don't know how wise it is to have partial definitions of what the ECN
> behavior is. Maybe simpler to say that that the relay is performing AQM it can
> support the ECN marking behavior as defined in RFC 3168.

The above text is same as https://tools.ietf.org/html/rfc5766#section-12, replaced "UNLESS" with "unless".

> 
> X. Section 14:
> 
> How are these IP level fields handled when the transport is TCP or TLS/TCP?
> There there are not necessary a one to one mapping between datagrams and
> TCP packets. And if there are no support then there are need for clarification of
> behaviors when using TCP based transport.

Section 14 already clarifies that the descriptions in this section do not apply to TURN messages sent over TCP or TLS transport from the server to the client.

> 
> Y. Section 16.4.  DATA
> 
>    The DATA attribute is present in all Send and Data indications.  The
>    value portion of this attribute is variable length and consists of
>    the application data (that is, the data that would immediately follow
>    the UDP header if the data was been sent directly between the client
>    and the peer).  If the length of this attribute is not a multiple of
>    4, then padding must be added after this attribute.
> 
> Can you please clarify if one would get all the remaining bytes of the IP packet
> payload or only the number of bytes the UDP length field indicates ?
> 
> The reason I ask is that there is possibility that there are a discrepancy between
> these fields, something the proposal for an experimental specification for UDP
> options attempt to utilize.
> https://datatracker.ietf.org/doc/draft-ietf-tsvwg-udp-options/
> 
> Note, I only want you to clarify what is actually included. I would assume using
> the UDP length field makes sense in this case.

The application data does not include the options discussed in draft-ietf-tsvwg-udp-options.  
Please clarify the specific update you are looking in this section. 


> 
> Z. The use of "relay" in the meaning of "Turn Server". There are many
> occurrences of relay in this meaning. Either make it explicit to be a synonym or
> change it to server.

Sure, updated draft.

> 
> AA. Section 16.7
> 
> rffu is lacking a definition. I would suggest making it upper case and copy the
> definition from 16.8.

Updated.

> 
> AB. Section 16.12:
> 
> Rsvd in Format figure does not match definition below which uses "Reserved".

Fixed.

> 
> AC. Section 16.12:
> 
> Reason Phrase (variable). This field is under defined. First of all the format of
> the text string is not defined, which it is in Stunbis Error-Code attribute is
> defined as.
> 
>    The reason phrase MUST be a UTF-8 [RFC3629] encoded
>    sequence of less than 128 characters (which can be as long as 509
>    bytes when encoding them or 763 bytes when decoding them).

Thanks, updated section to use UTF-8 encoding.  

> 
> Secondly, I think it might be worth pointing out the padding here. It is not clear
> if there is any form of string termination and how the padding which will be
> interpreted as NULL characters 0-3 depending on the alignment. I note this
> later issue appears to be existing in STUNBis also.

Padding is discussed in STUN for variable length attributes, please see https://tools.ietf.org/html/draft-ietf-tram-stunbis-18#section-14.  

> 
> AD. Section 16.2:
> 
> Why isn't the text here say that this attribute may also be used to indicate the
> desired lifetime in allocation requests?

https://tools.ietf.org/html/rfc5766#section-14.2 did not discuss desired lifetime. Anyways, added the following line:
The TURN client can include the LIFETIME attribute with the desired lifetime in Allocate and Refresh requests.

> 
> AE. Section 16.9: DONT-FRAGMENT
> 
> I think it would be good to indicate the use of the attribute in Allocate Requests
> also.

The above point is discussed in Section 7.1:

<snip>
   If the client wishes to later use the DONT-FRAGMENT attribute in one
   or more Send indications on this allocation, then the client SHOULD
   include the DONT-FRAGMENT attribute in the Allocate request.  This
   allows the client to test whether this attribute is supported by the
   server.
</snip>

> 
> AF. Section  24.2
> 
> [Protocol-Numbers]
>               "IANA Protocol Numbers Registry", 2005,
>               <http://www.iana.org/assignments/protocol-numbers>.
> 
> This is a normative reference based on how it is used in Section 16.8.

Okay, moved as Normative reference.

> 
> AG. Seciton 18.
> 
>    Sometime before the 20 minute lifetime is up, the client refreshes
>    the allocation.  This is done using a Refresh request.  As before,
>    the client includes the latest username, realm, and nonce values in
>    the request.  The client also includes the SOFTWARE attribute,
>    following the recommended practice of always including this attribute
>    in Allocate and Refresh messages.  When the server receives the
>    Refresh request, it notices that the nonce value has expired, and so
>    replies with 438 (Stale Nonce) error given a new nonce value.  The
>    client then reattempts the request, this time with the new nonce
>    value.  This second attempt is accepted, and the server replies with
>    a success response.  Note that the client did not include a LIFETIME
>    attribute in the request, so the server refreshes the allocation for
>    the default lifetime of 10 minutes (as can be seen by the LIFETIME
>    attribute in the success response).
> 
> I notice that this part nor the text before appears to discuss the permission
> lifetime that will expire after 5 minutes. Thus, it needs to be refreshed prior to
> the Allocation.

I have added text to discuss refreshing the channel binding, https://tools.ietf.org/html/rfc5766#section-16 did not discuss refreshing the permissions and channel bindings.  

> 
> AH. Section 18.
> 
> I am missing any example having to do with Address Family interactions.

There was no ask from the WG members to add an example and even https://tools.ietf.org/html/rfc6156 did not have an translation example.

> 
> AI. DTLS Cipher Suits
> 
> Am I understanding it correct that the definition of what DTLS cipher suit that a
> TURN Server currently is to support is the ones in RFC 7525 ?

No, the cipher suites and (D)TLS profile is discussed in https://tools.ietf.org/html/draft-ietf-tram-stunbis-21#section-6.2.3 and it follows the recommendations in RFC7525. 

> 
> In that case, I think the wording is a bit weak as the only reference I find are in
> Section 2.1:
> 
> If (D)TLS transport is
>    used between the TURN client and the TURN server, the guidance given
>    in [RFC7525] MUST be followed to avoid attacks on (D)TLS.

RFC7525 provides various recommendations to avoid attacks on (D)TLS.

Best Regards,
-Tiru

> 
> 
> Cheers
> 
> Magnus Westerlund
> 
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