One document matched: draft-ietf-mmusic-dtls-sdp-01.xml
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<rfc ipr="trust200902" category="std" docName="draft-ietf-mmusic-dtls-sdp-01.txt" submissionType="IETF" xml:lang="en">
<front>
<title>
Using the SDP Offer/Answer Mechanism for DTLS
</title>
<author fullname="Christer Holmberg" initials="C.H." surname="Holmberg">
<organization abbrev="Ericsson">Ericsson</organization>
<address>
<postal>
<street>Hirsalantie 11</street>
<city>Jorvas</city>
<region></region>
<code>02420</code>
<country>Finland</country>
</postal>
<phone></phone>
<email>christer.holmberg@ericsson.com</email>
</address>
</author>
<author fullname="Roman Shpount" initials="R.S." surname="Shpount">
<organization abbrev="TurboBridge">TurboBridge</organization>
<address>
<postal>
<street>4905 Del Ray Avenue, Suite 300</street>
<city>Bethesda</city>
<region>MD</region>
<code>20814</code>
<country>USA</country>
</postal>
<phone>+1 (240) 292-6632</phone>
<email>rshpount@turbobridge.com</email>
</address>
</author>
<date year="2015" />
<area>RAI</area>
<abstract>
<t>
This draft defines the SDP offer/answer procedures for negotiating and establishing
a DTLS association. The draft also defines the criteria for when a new DTLS association
must be established.
</t>
<t>
This draft defines a new SDP media-level attribute, 'dtls-connection'.
</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>
<xref format="default" pageno="false" target="RFC5763"/> defines SDP Offer/Answer procedures
for SRTP-DTLS. This draft defines the SDP Offer/Answer <xref format="default" pageno="false"
target="RFC3264"/> procedures for negotiation DTLS in general, based on the procedures in
<xref format="default" pageno="false" target="RFC5763"/>.
</t>
<t>
This draft also defines a new SDP attribute, 'dtls-connection'. The attribute
is used in SDP offers and answers to explicitly indicate whether a new DTLS
association is to be established.
</t>
<t>
As defined in <xref format="default" pageno="false" target="RFC5763"/>, a new DTLS association
MUST be established when transport parameters are changed. Transport parameter change is not
well defined when Interactive Connectivity Establishment (ICE) <xref format="default"
pageno="false" target="RFC5245"/> is used. One possible way to determine a transport change is
based on ufrag change, but the ufrag value is changed both when ICE is negotiated
and when ICE restart <xref format="default" pageno="false" target="RFC5245"/> occurs. These events
do not always require a new DTLS association to be established, but currently there is no way
to explicitly indicate in an SDP offer or answer whether a new DTLS association is required.
To solve that problem, this draft defines a new SDP attribute, 'dtls-connection'. The attribute
is used in SDP offers and answers to explicitly indicate whether a new DTLS association
is to be established/re-established. The attribute can be used both with and without ICE.
</t>
</section>
<section title="Abbreviations" toc="default">
<t>
TBD
</t>
</section>
<section title="Conventions">
<t>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref target="RFC2119"></xref>.
</t>
</section>
<section title="Establishing a new DTLS Association">
<section title="General" anchor="sec-dtls-gen">
<t>
A new DTLS association MUST be established in the following cases:
<list style="symbols">
<t>
The DTLS roles change;
</t>
<t>
The fingerprint (certificate) value changes; or
</t>
<t>
The establishment of a new DTLS association is explicitly signaled;
</t>
</list>
</t>
<t>
NOTE: The first two items list above are based on the procedures
in <xref format="default" pageno="false" target="RFC5763"/>.
This draft adds the support for explicit signaling.
</t>
<t>
The sections below describe typical cases where a new DTLS association needs to be established.
</t>
</section>
<section title="Change of Local Transport Parameters" anchor="sec-dtls-transport">
<t>
If an endpoint modifies its local transport parameters (IP address and/or port), and if the modification
requires a new DTLS association, the endpoint MUST either change its DTLS role, its fingerprint value and/or
use the SDP 'dtls-connection' attribute with a 'new' value <xref target="sec-dcon-attr"/>.
</t>
</section>
<section title="Change of ICE ufrag value" anchor="sec-dtls-ufrag">
<t>
If an endpoint uses ICE, and modifies a local ufrag value, and if the modification
requires a new DTLS association, the endpoint MUST either change its DTLS role, its fingerprint value and/or
use the SDP 'dtls-connection' attribute with a 'new' value <xref target="sec-dcon-attr"/>.
</t>
</section>
<section title="Multiple SDP fingerprint attributes" anchor="sec-dtls-fingerprint">
<t>
It is possible to associate multiple SDP fingerprint attribute values to
an 'm-' line. If any of the attribute values associated with an 'm-' line are
removed, or if any new attribute values are added, it is considered a fingerprint value change.
</t>
</section>
</section>
<section title="SDP DTLS-Connection Attribute" anchor="sec-dcon-attr">
<section title="General">
<t>
The SDP 'connection' attribute <xref format="default" pageno="false" target="RFC4145"/>
was originally defined for connection-oriented protocols, e.g. TCP and TLS. This section
defines a similar attribute, 'dtls-connection', to be used with DTLS.
</t>
<t>
A 'dtls-connection' attribute value of 'new' indicates that a new DTLS association MUST be
established. A 'dtls-connection' attribute value of 'existing' indicates that a new DTLS
association MUST NOT be established.
</t>
<t>
Unlike the SDP 'connection' attribute for TLS, there is no default value defined
for the 'dtls-connection' attribute. Implementations that wish to use the attribute
MUST explicitly include it in SDP offers and answers. If an offer or answer does
not contain an attribute, other means needs to be used in order for endpoints to
determine whether an offer or answer is associated with an event that requires the
DTLS association to be re-established.
</t>
<t>
The SDP Offer/Answer <xref format="default" pageno="false" target="RFC3264"/>
procedures associated with the attribute are defined in <xref target="sec-oa"/>
</t>
</section>
<section title="ABNF">
<t>
The ABNF <xref format="default" pageno="false" target="RFC5234"/>
grammar for the SDP 'dtls-connection' attributes is:
</t>
<figure>
<artwork align="left"><![CDATA[
dtls-connection-attr = "a=dtls-connection:" conn-value
conn-value = "new" / "existing"
]]></artwork>
</figure>
</section>
</section>
<section title="SDP Offer/Answer Procedures" anchor="sec-oa">
<section title="General">
<t>
This section defines the SDP offer/answer procedures for using the SDP 'dtls-connection'
attribute for DTLS. The section also describes how the usage of the SDP 'setup'
attribute and the SDP 'fingerprint' attribute <xref format="default" pageno="false"
target="RFC4572"/> is affected.
</t>
<t>
The procedures in this section are based on the procedures for SRTP-DTLS
<xref format="default" pageno="false" target="RFC5763"/>, with the addition
of usage of the SDP 'dtls-connection' attribute.
</t>
</section>
<section title="Generating the Initial SDP Offer">
<t>
When the offerer sends the initial offer, and the offerer wants to establish a
DTLS association, it MUST insert an SDP 'dtls-connection' attribute with a 'new' value
in the offer. In addition, the offerer MUST insert an SDP 'setup' attribute according
to the procedures in <xref format="default" pageno="false" target="RFC4145"/>, and
an SDP 'fingerprint' attribute according to the procedures in <xref format="default"
pageno="false" target="RFC4572"/>, in the offer.
</t>
<t>
Unlike for TCP and TLS connections, in case of DTLS associations the SDP 'setup' attribute
'holdconn' value MUST NOT be used.
</t>
</section>
<section title="Generating the Answer">
<t>
If an answerer receives an offer that contains an SDP 'dtls-connection' attribute with a 'new'
value, the answerer MUST insert a 'new' value in the associated answer. The same applies
if the answerer receives an offer that contains an SDP 'dtls-connection' attribute with a 'new'
value, but the answerer determines (based on the criteria for establishing a new DTLS
association) that a new DTLS association is to be established. In addition, the answerer MUST
insert an SDP 'setup' attribute according to the procedures in <xref format="default"
pageno="false" target="RFC4145"/>, and an SDP 'fingerprint' attribute according to the
procedures in <xref format="default" pageno="false" target="RFC4572"/>, in the answer.
</t>
<t>
If the answerer does not accept the establishment of the DTLS association, it MUST
reject the "m=" lines associated with the suggested DTLS association <xref format="default"
pageno="false" target="RFC3264"/>.
</t>
<t>
If an answerer receives an offer that contains a 'dtls-connection' attribute with an 'existing' value,
and if the answerer determines that a new DTLS association does not need to be established,
it MUST insert a connection attribute with an 'existing' value in the associated answer.
In addition, the answerer MUST insert an SDP 'setup' attribute with a value that does not
change the previously negotiated DTLS roles, and an SDP 'fingerprint' attribute with a value
that does not change the fingerprint, in the answer.
</t>
<t>
If the answerer receives an offer that does not contain an SDP 'dtls-connection' attribute,
the answerer MUST NOT insert a 'dtls-connection' attribute in the answer.
</t>
<t>
If a new DTLS association is to be established, and if the answerer
becomes DTLS client, the answerer MUST initiate the procedures for establishing
the DTLS association. If the answerer becomes DTLS server, it MUST wait for the offerer to
establish the DTLS association.
</t>
</section>
<section title="Offerer Processing of the SDP Answer">
<t>
When an offerer receives an answer that contains an SDP 'dtls-connection' attribute with
a 'new' value, and if the offerer becomes DTLS client, the offerer MUST establish
a DTLS association. If the offerer becomes DTLS server, it MUST wait for the answerer to
establish the DTLS association.
</t>
<t>
If the answer contains an SDP 'dtls-connection' attribute with an 'existing' value, the offerer
will continue using the previously established DTLS association. It is considered an error
case if the answer contains a 'dtls-connection' attribute with an 'existing' value, and a DTLS
association does not exist.
</t>
</section>
<section title="Modifying the Session">
<t>
When the offerer sends a subsequent offer, and the offerer wants to establish a new
DTLS association, the offerer MUST insert an SDP 'dtls-connection' attribute with a 'new'
value in the offer. In addition, the offerer MUST insert an SDP 'setup' attribute
according to the procedures in <xref format="default" pageno="false" target="RFC4145"/>,
and an SDP 'fingerprint' attribute according to the procedures in <xref format="default"
pageno="false" target="RFC4572"/>, in the offer.
</t>
<t>
when the offerer sends a subsequent offer, and the offerer does not want to establish
a new DTLS association, if a previously established DTLS association exists, the
offerer MUST insert an SDP 'dtls-connection' attribute with an 'existing' value in the offer.
In addition, the offerer MUST insert an SDP 'setup' attribute with a value that does
not change the previously negotiated DTLS roles, and an SDP 'fingerprint' attribute with
a value that does not change the fingerprint, in the offer.
</t>
</section>
</section>
<section title="ICE Considerations" anchor="sec-dtls-reest-ice">
<t>
An ICE restart <xref format="default" pageno="false" target="RFC5245"/>
does not by default require a new DTLS association to be established.
</t>
<t>
As defined in <xref format="default" pageno="false" target="RFC5763"/>,
each ICE candidate associated with a component is treated as being part of the
same DTLS association. Therefore, from a DTLS perspective it is not considered
a change of local transport parameters when an endpoint switches between those
ICE candidates.
</t>
</section>
<section title="SIP Considerations">
<t>
When the Session Initiation Protocol (SIP) <xref format="default" pageno="false"
target="RFC3261"/> is used as the signal protocol for establishing a multimedia
session, dialogs <xref format="default" pageno="false" target="RFC3261"/> might be
established between the caller and multiple callees. This is referred to as forking.
If forking occurs, separate DTLS associations MUST be established between the caller
and each callee.
</t>
<t>
It is possible to send an INVITE request which does not contain an SDP offer. Such
INVITE request is often referred to as an 'empty INVITE', or an 'offerless INVITE'.
The receiving endpoint will include the SDP offer in a response associated with the
response. When the endpoint generates such SDP offer, it MUST assign an SDP connection
attribute, with a 'new' value, to each 'm-' line that describes DTLS protected media.
If ICE is used, the endpoint MUST allocate a new set of ICE candidates, in order to
ensure that two DTLS association would not be running over the same transport.
</t>
</section>
<section title="RFC Updates">
<t>
Here we will add the RFC updates that are needed.
</t>
</section>
<section title="Security Considerations">
<t>
This draft does not modify the security considerations associated with DTLS, or
the SDP offer/answer mechanism. The draft simply clarifies the procedures for
negotiating and establishing a DTLS association.
</t>
</section>
<section anchor="section.iana" title="IANA Considerations">
<section anchor="section.iana.attr" title="Registration of New SDP Attribute">
<t>
This document updates the "Session Description Protocol Parameters" registry
as specified in Section 8.2.2 of <xref target="RFC4566" pageno="false" format="default"/>.
Specifically, it adds the SDP attributes in <xref target="section.iana.attr"
pageno="false" format="default"/> to the table for SDP media level attributes.
</t>
<figure>
<artwork align="left"><![CDATA[
Attribute name: dtls-connection
Type of attribute: media-level
Subject to charset: no
Purpose: TBD
Appropriate Values: see Section X
Contact name: Christer Holmberg
]]></artwork>
</figure>
</section>
</section>
<section title="Acknowledgements">
<t>
Thanks to Justin Uberti, Martin Thomson, Paul Kyzivat and Jens Guballa
for providing comments and suggestions on the draft.
</t>
</section>
<section title="Change Log">
<t>[RFC EDITOR NOTE: Please remove this section when publishing]</t>
<t>Changes from draft-ietf-mmusic-sdp-dtls-00
<list style="symbols">
<t>- SDP 'connection' attribute replaced with new 'dtls-connection' attribute.</t>
<t>- IANA Considerations added.</t>
<t>- E-mail regarding 'dtls-connection-id' attribute added as Annex.</t>
</list>
</t>
<t>Changes from draft-holmberg-mmusic-sdp-dtls-01
<list style="symbols">
<t>- draft-ietf-mmusic version of draft submitted.</t>
<t>- Draft file name change (sdp-dtls -> dtls-sdp) due to collision with another expired draft.</t>
<t>- Clarify that if ufrag in offer is unchanged, it must be unchanged in associated answer.</t>
<t>- SIP Considerations section added.</t>
<t>- Section about multiple SDP fingerprint attributes added.</t>
</list>
</t>
<t>Changes from draft-holmberg-mmusic-sdp-dtls-00
<list style="symbols">
<t>- Editorial changes and clarifications.</t>
</list>
</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119"?>
<?rfc include="reference.RFC.3261"?>
<?rfc include="reference.RFC.3264"?>
<?rfc include="reference.RFC.4145"?>
<?rfc include="reference.RFC.4566"?>
<?rfc include="reference.RFC.4572"?>
<?rfc include="reference.RFC.5234"?>
<?rfc include="reference.RFC.5245"?>
<?rfc include="reference.RFC.5763"?>
</references>
<section title="Design Considerations" toc="default">
<section title="dtls-connection versus dtls-connection-id" toc="default">
<t>
The text below is from an e-mail sent by Roman to the
MMUSIC mailing list, 1st October 2015. It is intended
to serve as background reading when discussing the
way forward regarding the SDP attribute.
</t>
<figure>
<artwork align="left"><![CDATA[
The "dtls-ufrag" has little to do with ICE and exists
in a completely different layer. We can call this
attribute "dtls-connection-id" if this will makes it
less spooky. The problem that I am trying to resolve
with new attribute is related to when new DTLS association
needs to be established. I would argue that original
intent was, that new DTLS association needs to be
established on change of one of the end points or
DTLS association setup attributes (setup role or
fingerprint).
Originally, end point change was detected based on
transport 5-tuple change. This, of cause, does not
work for ICE, where 5-tuple is not known in advance
and all 5-tuples associated with the same ICE component
should be treated as the same connection. One option was
to detect end point change when ICE is used based on
ICE ufrag change, but this does not work either since
ufrag can change due to ICE restart, but the same
endpoints will continue to communicate.
I would also argue that setting up new DTLS association
on 5-tuple change does not always work for non-ICE case
either, since we can have an end point which can initiate
a re-INVITE when it detects the local IP changes due to
DHCP lease expiration or any other reason. This transport
change does not necessarily require DTLS association
change, and new DTLS handshake is undesirable since it
will delay the media flow re-establishment but several
network round trips.
So, we need to detect when two new end-points are
communicating and new DTLS association needs to be
setup. What we originally proposed is that end point
will simply tell that it is setting up a new session
by using SDP connection attribute or some renamed
version of it.
What I am saying here is that end point cannot always
identify if it needs to setup a new DTLS association.
The problem arises when new offer is generated in
response to an offerless INVITE. In such case, an end
point does not know if it is continuing to communicate
with the same end-point or if this offer is intended
to be sent to a new end point.
There are two solution possible to this:
1. We specify that if an end points generates an offer in
response to an offer-less INVITE it should always assume
it is communicating with a new end point, it MUST add
"connection:new" and MUST make sure that none of the
existing transports can be possibly reused for this new
DTLS association by allocating new IP:port for non ICE
or a complete new set of ICE candidates in case of ICE.
This will work, but it is wasteful when offer-less INVITE
re-establishes connection between two existing end points.
In such cases additional ports will be consumed, TURN
tunnels will be allocated, and time spent on creating a
DTLS session when all of this can be simply reused.
2. Instead of asking the end point which generates the
offer to determine if it is establishing a new DTLS
association, we will ask the end point to identify itself.
So, instead of SDP connection attribute, an end point
will provide some sort of randomly generated end point
identifier in the new attribute (dtls-ufrag or
dtls-connection-id). When the connection ID pair stays
the same, the existing DTLS association continues to run
over the negotiated transport. If one of the connection
IDs changes, this would mean new DTLS association would
need to be established. This nicely uncouples end point
change identification from transport and makes negotiation
follow the original intent.
In case of response to an offer-less INVITE, an offer with
the existing connection ID will be generated. If this offer
is sent to a new end point, both end points will detect
that new DTLS association is required due to connection ID
change of the answering end point. If this offer will be
sent to an end point which is already a part of the existing
DTLS association, no new DTLS association will be necessary,
since both connection IDs will stay the same.
This also gives us path to a more "strategic" solution in the
future. DTLS handshake can be extended to include the
connection ID. Each DTLS handshake can negotiate a association
identifier similar to SSRC which can be used in the all
subsequent DTLS messages for this association. This way
multiple DTLS associations can be multiplexed over the single
transport and each of them can be tied to an m= line in
offer/answer. This, of cause, is not part of the current
draft and is outside of MMUSIC chapter, but does provide a
natural extension path for DTLS in the future.
In general Christer and I are trying to understand if there
is interest in formalizing the dtls-connection-id option
(more complex) or if we should stick with SDP
connection:new/existing attribute and force new DTLS association
always be established in response to offer-less INVITE (simpler
option but can waste resources).
Please let us know if these options need further clarification
or if you have any additional questions or opinions.
]]></artwork>
</figure>
</section>
</section>
</back>
</rfc>
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