| Network Working Group | C. Holmberg |
| Internet-Draft | Ericsson |
| Updates: 5763,7345 (if approved) | R. Shpount |
| Intended status: Standards Track | TurboBridge |
| Expires: August 24, 2016 | February 21, 2016 |
Using the SDP Offer/Answer Mechanism for DTLS
draft-ietf-mmusic-dtls-sdp-08.txt
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.
This draft defines a new SDP media-level attribute, 'dtls-connection'.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 24, 2016.
Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.
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[RFC5763] defines SDP Offer/Answer procedures for SRTP-DTLS. This draft defines the SDP Offer/Answer [RFC3264] procedures for negotiation DTLS in general, based on the procedures in [RFC5763].
As defined in [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) [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 [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.
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 [RFC2119].
A new DTLS association MUST be established in the following cases:
NOTE: The first two items list above are based on the procedures in [RFC5763]. This draft adds the support for explicit signaling.
Whenever an entity determines, based on the criteria above, that a new DTLS association is required, the entity MUST initiate an associated SDP offer/answer transaction, following the procedures in Section 5.
The sections below describe typical cases where a new DTLS association needs to be established.
If an endpoint modifies its local transport parameters (address and/or port), and if the modification requires a new DTLS association, the endpoint MUST change its DTLS role, change its fingerprint value, and/or use the SDP 'dtls-connection' attribute with a 'new' value Section 4.
If the underlying transport explicitly prohibits a DTLS association to span multiple transports, the SDP 'dtls-connection' attribute MUST be set to 'new' if the transport is changed. An example of such case is when DTLS is carried over SCTP, as described in [RFC6083].
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 Section 4.
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.
The SDP 'connection' attribute [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.
Name: dtls-connection
Value: conn-value
Usage Level: media
Charset Dependent: no
Syntax:
conn-value = "new" / "existing"
Example:
a=dtls-connection:existing
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.
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 (this could happen if the offerer or answerer represents an existing implementation that has not been updated to support the attribute defined in this specification), 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.
The mux category [I-D.ietf-mmusic-sdp-mux-attributes] for the 'dtls-connection' attribute is 'IDENTICAL', which means that the attribute value must be identical across all media descriptions being multiplexed [I-D.ietf-mmusic-sdp-bundle-negotiation].
For RTP-based media, the 'dtls-connection' attribute apply to whole associated media description. The attribute MUST NOT be defined per source (using the SDP 'ssrc' attribute [RFC5576]).
The SDP Offer/Answer [RFC3264] procedures associated with the attribute are defined in Section 5
This section defines the generic SDP offer/answer procedures for negotiating a DTLS association. Additional procedures (e.g. regarding usage of specific SDP attributes etc) for individual DTLS usages (e.g. SRTP-DTLS) are outside the scope of this specification, and need to be specified in a usage specific specification.
NOTE: The procedures in this section are generalizations of procedures first specified in SRTP-DTLS [RFC5763], with the addition of usage of the SDP 'dtls-connection' attribute. That document is herein revised to make use of these new procedures.
The procedures in this section apply to an SDP media description ("m=" line) associated a DTLS-protected media/data stream.
In order to negotiate a DTLS association, the following SDP attributes are used:
This specification does not define the usage of the SDP 'connection' attribute [RFC4145] for negotiating a DTLS connection. However, the attribute MAY be used if the DTLS connection is used together with another protocol, e.g. SCTP or TCP, for which the usage of the attribute has been defined.
Unlike for TCP and TLS connections, endpoints MUST NOT use the SDP 'setup' attribute 'holdconn' value when negotiating a DTLS association.
Endpoints MUST support SHA-256 for generating and verifying the fingerprint value associated with the DTLS association. The use of SHA-256 is preferred.
Endpoints MUST, at a minimum, support TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 and MUST support TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256. UDPTL over DTLS MUST prefer TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 and any other Perfect Forward Secrecy (PFS) cipher suites over non-PFS cipher suites. Implementations SHOULD disable TLS-level compression.
The certificate received during the DTLS handshake MUST match the fingerprint received in the SDP "fingerprint" attribute. If the fingerprint does not match the hashed certificate, then the endpoint MUST tear down the media session immediately. Note that it is permissible to wait until the other side's fingerprint has been received before establishing the connection; however, this may have undesirable latency effects.
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 [RFC4145], and an SDP 'fingerprint' attribute according to the procedures in [RFC4572], in the offer.
If the offerer inserts the SDP 'setup' attribute with an 'actpass' or 'passive' value, the offerer MUST be prepared to receive a DTLS ClientHello message (if a new DTLS association is established by the answerer) from the answerer before it receives the SDP answer.
If an answerer receives an offer that contains an SDP 'dtls-connection' attribute with a 'new' value, or if the answerer receives an offer that contains an 'dtls-connection' attribute with an 'existing' value and the answerer determines (based on the criteria for establishing a new DTLS association) that a new DTLS association is to be established, the answerer MUST insert a 'new' value in the associated answer. In addition, the answerer MUST insert an SDP 'setup' attribute according to the procedures in [RFC4145], and an SDP 'fingerprint' attribute according to the procedures in [RFC4572], in the answer.
If an answerer receives an offer that contains an SDP 'dtls-connection' attribute with a 'new' value, and if the answerer does not accept the establishment of a new DTLS association, the answerer MUST reject the "m=" lines associated with the suggested DTLS association [RFC3264].
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 is not to be established, the answerer MUST insert a 'dtls-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 previously sent fingerprint, in the answer.
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.
If a new DTLS association is to be established, and if the answerer inserts an SDP 'setup' attribute with an 'active' value in the answer, the answerer MUST initiate a DTLS handshake by sending a DTLS ClientHello message towards the offerer.
When an offerer receives an answer that contains an SDP 'dtls-connection' attribute with a 'new' value, and if the offerer becomes DTLS client (based on the value of the SDP 'setup' attribute value [RFC4145]), the offerer MUST establish a DTLS association. If the offerer becomes DTLS server, it MUST wait for the answerer to establish the DTLS association.
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.
An offerer needs to be able to handle error conditions that can occur during an offer/answer transaction, e.g. if an answer contains an SDP 'dtls-connection' attribute with an 'existing' value even if no DTLS connection exists, or if the answer contains a new fingerprint value for an existing DTLS connection. If such error case occurs, the offerer SHOULD terminate the associated DTLS connection (if it exists) and send a new offer in order to terminate each media stream using the DTLS association, by setting the associated port value to zero [RFC4145].
When the offerer sends a subsequent offer, and if 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 [RFC4145], and an SDP 'fingerprint' attribute according to the procedures in [RFC4572], in the offer.
when the offerer sends a subsequent offer, and the offerer does not want to establish a new DTLS association, and 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 previously sent fingerprint, in the offer.
NOTE: When a new DTLS association is established, each endpoint needs to be prepared to receive data on both the new and old DTLS associations as long as both are alive.
When ICE is used, the ICE connectivity checks are performed before the DTLS handshake begins. Note that if aggressive nomination mode is used, multiple candidate pairs may be marked valid before ICE finally converges on a single candidate pair.
An ICE restart [RFC5245] does not by default require a new DTLS association to be established.
As defined in [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.
If DTLS is transported on top of a connection-oriented transport protocol (e.g. TCP or SCTP), where all IP packets are acknowledged, all DTLS packets associated with a previous DTLS association MUST be acknowledged (or timed out) before a new DTLS association can be established on the same transport.
When the Session Initiation Protocol (SIP) [RFC3261] is used as the signal protocol for establishing a multimedia session, dialogs [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.
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 'dtls-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.
This section updates specifications that use DTLS-protected media, in order to reflect the procedures defined in this specification.
Update to section 5:
--------------------
OLD TEXT:
5. Establishing a Secure Channel
The two endpoints in the exchange present their identities as part of
the DTLS handshake procedure using certificates. This document uses
certificates in the same style as described in "Connection-Oriented
Media Transport over the Transport Layer Security (TLS) Protocol in
the Session Description Protocol (SDP)" [RFC4572].
If self-signed certificates are used, the content of the
subjectAltName attribute inside the certificate MAY use the uniform
resource identifier (URI) of the user. This is useful for debugging
purposes only and is not required to bind the certificate to one of
the communication endpoints. The integrity of the certificate is
ensured through the fingerprint attribute in the SDP. The
subjectAltName is not an important component of the certificate
verification.
The generation of public/private key pairs is relatively expensive.
Endpoints are not required to generate certificates for each session.
The offer/answer model, defined in [RFC3264], is used by protocols
like the Session Initiation Protocol (SIP) [RFC3261] to set up
multimedia sessions. In addition to the usual contents of an SDP
[RFC4566] message, each media description ("m=" line and associated
parameters) will also contain several attributes as specified in
[RFC5764], [RFC4145], and [RFC4572].
When an endpoint wishes to set up a secure media session with another
endpoint, it sends an offer in a SIP message to the other endpoint.
This offer includes, as part of the SDP payload, the fingerprint of
the certificate that the endpoint wants to use. The endpoint SHOULD
send the SIP message containing the offer to the offerer's SIP proxy
over an integrity protected channel. The proxy SHOULD add an
Identity header field according to the procedures outlined in
[RFC4474]. The SIP message containing the offer SHOULD be sent to
the offerer's SIP proxy over an integrity protected channel. When
the far endpoint receives the SIP message, it can verify the identity
of the sender using the Identity header field. Since the Identity
header field is a digital signature across several SIP header fields,
in addition to the body of the SIP message, the receiver can also be
certain that the message has not been tampered with after the digital
signature was applied and added to the SIP message.
The far endpoint (answerer) may now establish a DTLS association with
the offerer. Alternately, it can indicate in its answer that the
offerer is to initiate the TLS association. In either case, mutual
DTLS certificate-based authentication will be used. After completing
the DTLS handshake, information about the authenticated identities,
including the certificates, are made available to the endpoint
application. The answerer is then able to verify that the offerer's
certificate used for authentication in the DTLS handshake can be
associated to the certificate fingerprint contained in the offer in
the SDP. At this point, the answerer may indicate to the end user
that the media is secured. The offerer may only tentatively accept
the answerer's certificate since it may not yet have the answerer's
certificate fingerprint.
When the answerer accepts the offer, it provides an answer back to
the offerer containing the answerer's certificate fingerprint. At
this point, the offerer can accept or reject the peer's certificate
and the offerer can indicate to the end user that the media is
secured.
Note that the entire authentication and key exchange for securing the
media traffic is handled in the media path through DTLS. The
signaling path is only used to verify the peers' certificate
fingerprints.
The offer and answer MUST conform to the following requirements.
o The endpoint MUST use the setup attribute defined in [RFC4145].
The endpoint that is the offerer MUST use the setup attribute
value of setup:actpass and be prepared to receive a client_hello
before it receives the answer. The answerer MUST use either a
setup attribute value of setup:active or setup:passive. Note that
if the answerer uses setup:passive, then the DTLS handshake will
not begin until the answerer is received, which adds additional
latency. setup:active allows the answer and the DTLS handshake to
occur in parallel. Thus, setup:active is RECOMMENDED. Whichever
party is active MUST initiate a DTLS handshake by sending a
ClientHello over each flow (host/port quartet).
o The endpoint MUST NOT use the connection attribute defined in
[RFC4145].
o The endpoint MUST use the certificate fingerprint attribute as
specified in [RFC4572].
o The certificate presented during the DTLS handshake MUST match the
fingerprint exchanged via the signaling path in the SDP. The
security properties of this mechanism are described in Section 8.
o If the fingerprint does not match the hashed certificate, then the
endpoint MUST tear down the media session immediately. Note that
it is permissible to wait until the other side's fingerprint has
been received before establishing the connection; however, this
may have undesirable latency effects.
NEW TEXT:
5. Establishing a Secure Channel
The two endpoints in the exchange present their identities as part of
the DTLS handshake procedure using certificates. This document uses
certificates in the same style as described in "Connection-Oriented
Media Transport over the Transport Layer Security (TLS) Protocol in
the Session Description Protocol (SDP)" [RFC4572].
If self-signed certificates are used, the content of the
subjectAltName attribute inside the certificate MAY use the uniform
resource identifier (URI) of the user. This is useful for debugging
purposes only and is not required to bind the certificate to one of
the communication endpoints. The integrity of the certificate is
ensured through the fingerprint attribute in the SDP. The
subjectAltName is not an important component of the certificate
verification.
The generation of public/private key pairs is relatively expensive.
Endpoints are not required to generate certificates for each session.
The offer/answer model, defined in [RFC3264], is used by protocols
like the Session Initiation Protocol (SIP) [RFC3261] to set up
multimedia sessions.
When an endpoint wishes to set up a secure media session with another
endpoint, it sends an offer in a SIP message to the other endpoint.
This offer includes, as part of the SDP payload, the fingerprint of
the certificate that the endpoint wants to use. The endpoint SHOULD
send the SIP message containing the offer to the offerer's SIP proxy
over an integrity protected channel. The proxy SHOULD add an
Identity header field according to the procedures outlined in
[RFC4474]. The SIP message containing the offer SHOULD be sent to
the offerer's SIP proxy over an integrity protected channel. When
the far endpoint receives the SIP message, it can verify the identity
of the sender using the Identity header field. Since the Identity
header field is a digital signature across several SIP header fields,
in addition to the body of the SIP message, the receiver can also be
certain that the message has not been tampered with after the digital
signature was applied and added to the SIP message.
The far endpoint (answerer) may now establish a DTLS association with
the offerer. Alternately, it can indicate in its answer that the
offerer is to initiate the TLS association. In either case, mutual
DTLS certificate-based authentication will be used. After completing
the DTLS handshake, information about the authenticated identities,
including the certificates, are made available to the endpoint
application. The answerer is then able to verify that the offerer's
certificate used for authentication in the DTLS handshake can be
associated to the certificate fingerprint contained in the offer in
the SDP. At this point, the answerer may indicate to the end user
that the media is secured. The offerer may only tentatively accept
the answerer's certificate since it may not yet have the answerer's
certificate fingerprint.
When the answerer accepts the offer, it provides an answer back to
the offerer containing the answerer's certificate fingerprint. At
this point, the offerer can accept or reject the peer's certificate
and the offerer can indicate to the end user that the media is
secured.
Note that the entire authentication and key exchange for securing the
media traffic is handled in the media path through DTLS. The
signaling path is only used to verify the peers' certificate
fingerprints.
The offerer and answerer MUST follow the SDP offer/answer procedures
defined in [RFCXXXX].
Update to section 6.6:
----------------------
OLD TEXT:
6.6. Session Modification
Once an answer is provided to the offerer, either endpoint MAY
request a session modification that MAY include an updated offer.
This session modification can be carried in either an INVITE or
UPDATE request. The peers can reuse the existing associations if
they are compatible (i.e., they have the same key fingerprints and
transport parameters), or establish a new one following the same
rules are for initial exchanges, tearing down the existing
association as soon as the offer/answer exchange is completed. Note
that if the active/passive status of the endpoints changes, a new
connection MUST be established.
NEW TEXT:
6.6. Session Modification
Once an answer is provided to the offerer, either endpoint MAY
request a session modification that MAY include an updated offer.
This session modification can be carried in either an INVITE or
UPDATE request. The peers can reuse an existing DTLS association,
or establish a new one, following the procedures in [RFCXXXX].
Update to section 6.7.1:
------------------------
OLD TEXT:
6.7.1. ICE Interaction
Interactive Connectivity Establishment (ICE), as specified in
[RFC5245], provides a methodology of allowing participants in
multimedia sessions to verify mutual connectivity. When ICE is being
used, the ICE connectivity checks are performed before the DTLS
handshake begins. Note that if aggressive nomination mode is used,
multiple candidate pairs may be marked valid before ICE finally
converges on a single candidate pair. Implementations MUST treat all
ICE candidate pairs associated with a single component as part of the
same DTLS association. Thus, there will be only one DTLS handshake
even if there are multiple valid candidate pairs. Note that this may
mean adjusting the endpoint IP addresses if the selected candidate
pair shifts, just as if the DTLS packets were an ordinary media
stream.
Note that Simple Traversal of the UDP Protocol through NAT (STUN)
packets are sent directly over UDP, not over DTLS. [RFC5764]
describes how to demultiplex STUN packets from DTLS packets and SRTP
packets.
NEW TEXT:
6.7.1. ICE Interaction
The Interactive Connectivity Establishment (ICE) [RFC5245]
considerations for DTLS-protected media are described in
[RFCXXXX].
Note that Simple Traversal of the UDP Protocol through NAT (STUN)
packets are sent directly over UDP, not over DTLS. [RFC5764]
describes how to demultiplex STUN packets from DTLS packets and SRTP
packets.
Update to section 4:
--------------------
OLD TEXT:
4. SDP Offerer/Answerer Procedures
4.1. General
An endpoint (i.e., both the offerer and the answerer) MUST create an
SDP media description ("m=" line) for each UDPTL-over-DTLS media
stream and MUST assign a UDP/TLS/UDPTL value (see Table 1) to the
"proto" field of the "m=" line.
The procedures in this section apply to an "m=" line associated with
a UDPTL-over-DTLS media stream.
In order to negotiate a UDPTL-over-DTLS media stream, the following
SDP attributes are used:
o The SDP attributes defined for UDPTL over UDP, as described in
[ITU.T38.2010]; and
o The SDP attributes, defined in [RFC4145] and [RFC4572], as
described in this section.
The endpoint MUST NOT use the SDP "connection" attribute [RFC4145].
In order to negotiate the TLS roles for the UDPTL-over-DTLS transport
connection, the endpoint MUST use the SDP "setup" attribute
[RFC4145].
If the endpoint supports, and is willing to use, a cipher suite with
an associated certificate, the endpoint MUST include an SDP
"fingerprint" attribute [RFC4572]. The endpoint MUST support SHA-256
for generating and verifying the SDP "fingerprint" attribute value.
The use of SHA-256 is preferred. UDPTL over DTLS, at a minimum, MUST
support TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 and MUST support
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256. UDPTL over DTLS MUST prefer
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 and any other Perfect Forward
Secrecy (PFS) cipher suites over non-PFS cipher suites.
Implementations SHOULD disable TLS-level compression.
If a cipher suite with an associated certificate is selected during
the DTLS handshake, the certificate received during the DTLS
handshake MUST match the fingerprint received in the SDP
"fingerprint" attribute. If the fingerprint does not match the
hashed certificate, then the endpoint MUST tear down the media
session immediately. Note that it is permissible to wait until the
other side's fingerprint has been received before establishing the
connection; however, this may have undesirable latency effects.
4.2. Generating the Initial Offer
The offerer SHOULD assign the SDP "setup" attribute with a value of
"actpass", unless the offerer insists on being either the sender or
receiver of the DTLS ClientHello message, in which case the offerer
can use either a value of "active" (the offerer will be the sender of
ClientHello) or "passive" (the offerer will be the receiver of
ClientHello). The offerer MUST NOT assign an SDP "setup" attribute
with a "holdconn" value.
If the offerer assigns the SDP "setup" attribute with a value of
"actpass" or "passive", the offerer MUST be prepared to receive a
DTLS ClientHello message before it receives the SDP answer.
4.3. Generating the Answer
If the answerer accepts the offered UDPTL-over-DTLS transport
connection, in the associated SDP answer, the answerer MUST assign an
SDP "setup" attribute with a value of either "active" or "passive",
according to the procedures in [RFC4145]. The answerer MUST NOT
assign an SDP "setup" attribute with a value of "holdconn".
If the answerer assigns an SDP "setup" attribute with a value of
"active" value, the answerer MUST initiate a DTLS handshake by
sending a DTLS ClientHello message on the negotiated media stream,
towards the IP address and port of the offerer.
4.4. Offerer Processing of the Answer
When the offerer receives an SDP answer, if the offerer ends up being
active it MUST initiate a DTLS handshake by sending a DTLS
ClientHello message on the negotiated media stream, towards the IP
address and port of the answerer.
4.5. Modifying the Session
Once an offer/answer exchange has been completed, either endpoint MAY
send a new offer in order to modify the session. The endpoints can
reuse the existing DTLS association if the key fingerprint values and
transport parameters indicated by each endpoint are unchanged.
Otherwise, following the rules for the initial offer/answer exchange,
the endpoints can negotiate and create a new DTLS association and,
once created, delete the previous DTLS association, following the
same rules for the initial offer/answer exchange. Each endpoint
needs to be prepared to receive data on both the new and old DTLS
associations as long as both are alive.
NEW TEXT:
4. SDP Offerer/Answerer Procedures
An endpoint (i.e., both the offerer and the answerer) MUST create an
SDP media description ("m=" line) for each UDPTL-over-DTLS media
stream and MUST assign a UDP/TLS/UDPTL value (see Table 1) to the
"proto" field of the "m=" line.
The offerer and answerer MUST follow the SDP offer/answer procedures
defined in [RFCXXXX] in order to negotiate the DTLS association
associated with the UDPTL-over-DTLS media stream. In addition,
the offerer and answerer MUST use the SDP attributes defined for
UDPTL over UDP, as defined in [ITU.T38.2010].
Update to section 5.2.1:
------------------------
OLD TEXT:
5.2.1. ICE Usage
When Interactive Connectivity Establishment (ICE) [RFC5245] is being
used, the ICE connectivity checks are performed before the DTLS
handshake begins. Note that if aggressive nomination mode is used,
multiple candidate pairs may be marked valid before ICE finally
converges on a single candidate pair. User Agents (UAs) MUST treat
all ICE candidate pairs associated with a single component as part of
the same DTLS association. Thus, there will be only one DTLS
handshake even if there are multiple valid candidate pairs. Note
that this may mean adjusting the endpoint IP addresses if the
selected candidate pair shifts, just as if the DTLS packets were an
ordinary media stream. In the case of an ICE restart, the DTLS
handshake procedure is repeated, and a new DTLS association is
created. Once the DTLS handshake is completed and the new DTLS
association has been created, the previous DTLS association is
deleted.
NEW TEXT:
5.2.1. ICE Usage
The Interactive Connectivity Establishment (ICE) [RFC5245]
considerations for DTLS-protected media are described in
[RFCXXXX].
This specification does not modify the security considerations associated with DTLS, or the SDP offer/answer mechanism. In addition to the introduction of the SDP 'dtls-connection' attribute, the specification simply clarifies the procedures for negotiating and establishing a DTLS association.
This document updates the "Session Description Protocol Parameters" registry as specified in Section 8.2.2 of [RFC4566]. Specifically, it adds the SDP dtls-connection attribute to the table for SDP media level attributes.
Attribute name: dtls-connection
Type of attribute: media-level
Subject to charset: no
Purpose: Indicate whether a new DTLS association is to be established/re-established.
Appropriate Values: see Section 4
Contact name: Christer Holmberg
Category: IDENTICAL
Thanks to Justin Uberti, Martin Thomson, Paul Kyzivat and Jens Guballa for providing comments and suggestions on the draft.
[RFC EDITOR NOTE: Please remove this section when publishing]
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| [RFC5576] | Lennox, J., Ott, J. and T. Schierl, "Source-Specific Media Attributes in the Session Description Protocol (SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009. |
| [RFC6083] | Tuexen, M., Seggelmann, R. and E. Rescorla, "Datagram Transport Layer Security (DTLS) for Stream Control Transmission Protocol (SCTP)", RFC 6083, DOI 10.17487/RFC6083, January 2011. |
| [I-D.ietf-mmusic-sdp-mux-attributes] | Nandakumar, S., "A Framework for SDP Attributes when Multiplexing", Internet-Draft draft-ietf-mmusic-sdp-mux-attributes-12, January 2016. |
| [I-D.ietf-mmusic-sdp-bundle-negotiation] | Holmberg, C., Alvestrand, H. and C. Jennings, "Negotiating Media Multiplexing Using the Session Description Protocol (SDP)", Internet-Draft draft-ietf-mmusic-sdp-bundle-negotiation-25, January 2016. |