| Network Working Group | C. Holmberg |
| Internet-Draft | Ericsson |
| Updates: 5763,7345 (if approved) | R. Shpount |
| Intended status: Standards Track | TurboBridge |
| Expires: August 17, 2017 | February 13, 2017 |
Using the SDP Offer/Answer Mechanism for DTLS
draft-ietf-mmusic-dtls-sdp-20.txt
This document defines the SDP offer/answer procedures for negotiating and establishing a DTLS association. The document also defines the criteria for when a new DTLS association must be established. The document updates RFC 5763 and RFC 7345, by replacing common SDP offer/answer procedures with a reference to this specification.
This document defines a new SDP media-level attribute, 'dtls-id'.
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 17, 2017.
Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
[RFC5763] defines SDP offer/answer procedures for SRTP-DTLS. [RFC7345] defines SDP offer/answer procedures for UDPTL-DTLS. This specification defines general offer/answer procedures for DTLS, based on the procedures in [RFC5763]. Other specifications, defining specific DTLS usages, can then reference this specification, in order to ensure that the DTLS aspects are common among all usages. Having common procedures is essential when multiple usages share the same DTLS association [I-D.ietf-mmusic-sdp-bundle-negotiation]. The document updates [RFC5763] and [RFC7345], by replacing common SDP offer/answer procedures with a reference to this specification.
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) [I-D.ietf-ice-rfc5245bis] 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 [I-D.ietf-ice-rfc5245bis] 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 document defines a new SDP attribute, 'dtls-id'. The pair of SDP 'dtls-id' attribute values (the attribute values of the offerer and the answerer) uniquely identifies the DTLS association. Providing a new value of the 'dtls-id' attribute in an SDP offer or answers can be used to indicate whether a new DTLS association is to be established.
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 after a successful SDP offer/answer exchange in the following cases:
NOTE: The first two items above are based on the procedures in [RFC5763]. This specification adds the support for explicit signaling using the SDP 'dtls-id' attribute.
A new DTLS association can only be established as a result of the successful SDP offer/answer exchange. Whenever an entity determines that a new DTLS association is required, the entity MUST initiate an SDP offer/answer exchange, 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 local SDP 'dtls-id' attribute value (see Section 4).
If the underlying transport explicitly prohibits a DTLS association to span multiple transports, and if the transport is changed, the endpoint MUST change its local SDP 'dtls-id' attribute value (see Section 4). An example of such a 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 change its local SDP 'dtls-id' attribute value (see Section 4).
The pair of SDP 'dtls-id' attribute values (the attribute values of the offerer and the answerer) uniquely identifies the DTLS association.
Name: dtls-id
Value: dtls-id-value
Usage Level: media
Charset Dependent: no
Default Value: N/A
Syntax:
dtls-id-value = 6*256(dtls-id-char)
dtls-id-char = ALPHA / DIGIT / "+" / "/" / "-" / "_"
<ALPHA and DIGIT defined in [RFC4566]>
Example:
a=dtls-id:abc3de65cddef001
Every time an endpoint requests to establish a new DTLS association, the endpoint MUST generate a new local 'dtls-id' attribute value. A non-changed local 'dtls-id' attribute value, in combination with non-changed fingerprints, indicates that the endpoint intends to reuse the existing DTLS association.
The 'dtls-id' attribute value MUST be generated using a cryptographic random function and include at least 32 bits of randomness.
No default value is defined for the SDP 'dtls-id' 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 a 'dtls-id' attribute (this could happen if the offerer or answerer represents an existing implementation that has not been updated to support the 'dtls-id' attribute), the offer or answer MUST be treated as if no 'dtls-id' attribute is included. Unless there is another mechanism to explicitly indicate that a new DTLS association is to be established, a modification of one or more of the following characteristics MUST be treated as an indication that an endpoint wants to establish a new DTLS association:
The mux category [I-D.ietf-mmusic-sdp-mux-attributes] for the 'dtls-id' 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-id' attribute applies to the 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-id' attribute. That document is herein updated to make use of these new procedures.
The procedures in this section apply to an SDP media description ("m=" line) associated with DTLS-protected media/data.
When an offerer or answerer indicates that it wants to establish a new DTLS association, it needs to make sure that media packets in the existing DTLS association and new DTLS association can be de-multiplexed. In case of an ordered transport (e.g., SCTP) this can be done simply by sending packets for the new DTLS association after all packets for the existing DTLS association have been sent. In case of an unordered transport, such as UDP, packets for the old DTLS association can arrive after the answer SDP was received and after the first packets for the new DTLS association were received. The only way to de-multiplex packets belonging to the old and new DTLS association is on the basis of transport 5-tuple. Because of this, if an unordered transport is used for the DTLS association, a new transport (3-tuple) MUST be allocated by at least one of the end points so that DTLS packets can be de-multiplexed.
When an offerer needs to establish a new DTLS association, and if an unordered transport (e.g., UDP) is used, the offerer MUST allocate a new transport (3-tuple) for the offer in such a way that the offerer can disambiguate any packets associated with the new DTLS association from any packets associated with any other DTLS association. This typically means using a local address and/or port, or a set of ICE candidates (see Section 6), which were not recently used for any other DTLS association.
When an answerer needs to establish a new DTLS association, if an unordered transport is used, and if the offerer did not allocate a new transport, the answerer MUST allocate a new transport for the answer in such a way that it can disambiguate any packets associated with the new DTLS association from any packets associated with any other DTLS association. This typically means using a local address and/or port, or a set of ICE candidates (see Section 6), which were not recently used for any other DTLS association.
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 association. However, the attribute MAY be used if the DTLS association 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 the cipher suites as defined in [I-D.ietf-mmusic-4572-update].
The certificate received during the DTLS handshake MUST match a certificate fingerprints received in SDP 'fingerprint' attributes according to the procedures defined in [I-D.ietf-mmusic-4572-update]. If fingerprints do not match the hashed certificate, then an endpoint MUST tear down the media session immediately (see [I-D.ietf-mmusic-4572-update]). Note that it is permissible to wait until the other side's fingerprint(s) has been received before establishing the connection; however, this may have undesirable latency effects.
SDP offerers and answerers might reuse certificates across multiple DTLS associations, and provide identical fingerprint values for each DTLS association. The combination of the SDP 'dtls-id' attribute values of the SDP offerer and answerer identifies each individual DTLS association.
When an offerer sends the initial offer, the offerer MUST insert an SDP 'setup' attribute according to the procedures in [RFC4145], and one or more SDP 'fingerprint' attributes according to the procedures in [I-D.ietf-mmusic-4572-update]. In addition, the offerer MUST insert in the offer an SDP 'dtls-id' attribute with a unique value.
If the offerer inserts the SDP 'setup' attribute with an 'actpass' or 'passive' attribute 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 the offerer receives the SDP answer.
When an answerer sends an answer, the answerer MUST insert in the answer an SDP 'setup' attribute according to the procedures in [RFC4145], and one or more SDP 'fingerprint' attributes according to the procedures in [I-D.ietf-mmusic-4572-update]. If the answerer determines, based on the criteria specified in Section 3.1, that a new DTLS association is to be established, the answerer MUST insert in the associated answer an SDP 'dtls-id' attribute with a new unique value. Note that the offerer and answerer generate their own local 'dtls-id' attribute values, and the combination of both values identify the DTLS association.
If the answerer receives an offer that requires establishment of a new DTLS association, 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 does not require the establishment of a new DTLS association, and if the answerer determines that a new DTLS association is not to be established, the answerer MUST insert an SDP 'dtls-id' attribute with the previously assigned 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 one or more SDP 'fingerprint' attributes values that do not change the previously sent fingerprint set, in the associated answer.
If the answerer receives an offer that does not contain an SDP 'dtls-id' attribute, the answerer MUST NOT insert a 'dtls-id' 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 establishes a new DTLS association based on criteria defined in Section 3.1, 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 does not establish a new DTLS association, the offerer will continue using the previously established DTLS association.
NOTE: A new DTLS association can be established based on changes in either an SDP offer or answer. When communicating with legacy endpoints, an offerer can receive an answer that includes the same fingerprint set and setup role. A new DTLS association MUST still be established if such an answer was received as a response to an offer which requested the establishment of a new DTLS association.
When the offerer sends a subsequent offer, and if the offerer wants to establish a new DTLS association, the offerer MUST insert an SDP 'setup' attribute according to the procedures in [RFC4145], and one or more SDP 'fingerprint' attributes according to the procedures in [I-D.ietf-mmusic-4572-update]. In addition, the offerer MUST insert in the offer an SDP 'dtls-id' attribute with a new unique value.
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-id' attribute with the previously assigned value in the offer. In addition, the offerer MUST insert an SDP 'setup' attribute, and one or more SDP 'fingerprint' attributes with values that do not change the previously sent fingerprint set, in the offer. The value of the 'setup' attribute SHOULD be set to 'actpass', in order to allow the answerer to establish a new DTLS association with a different role, but MAY be set to the current negotiated role ('active' or 'passive'). It MUST NOT be set to a value that changes the current negotiated role.
NOTE: When a new DTLS association is being 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 the Interactive Connectivity Establishment (ICE) mechanism [I-D.ietf-ice-rfc5245bis] 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.
NOTE: Aggressive nomination has been deprecated from ICE, but must still be supported for backwards compatibility reasons.
When a new DTLS association is established over an unordered transport, in order to disambiguate any packets associated with the newly established DTLS association, at least one of the endpoints MUST allocate a completely new set of ICE candidates which were not recently used for any other DTLS association. This means the answerer cannot initiate a new DTLS association unless the offerer initiated ICE restart [I-D.ietf-ice-rfc5245bis]. If the answerer wants to initiate a new DTLS association, it needs to initiate an ICE restart and a new offer/answer exchange on its own. However, an ICE restart does not by default require a new DTLS association to be established.
NOTE: 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.
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 instance of that transport (5-tuple).
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 an INVITE request is often referred to as an 'empty INVITE', or an 'offer-less INVITE'. The receiving endpoint will include the SDP offer in a response to the request. When the endpoint generates such SDP offer, if a previously established DTLS association exists, the offerer SHOULD insert an SDP 'dtls-id' attribute, and one or more SDP 'fingerprint' attributes, with previously assigned attribute values. If a previously established DTLS association did not exist, the offer SHOULD be generated based on the same rules as a new offer (see Section 5.2). Regardless of the previous existence of a DTLS association, the SDP 'setup' attribute MUST be included according to the rules defined in [RFC4145] and if ICE is used, ICE restart MUST be initiated.
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 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, a fingerprint of
a 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 DTLS 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)
[I-D.ietf-ice-rfc5245bis] considerations for DTLS-protected media
are described in [RFCXXXX].
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)
[I-D.ietf-ice-rfc5245bis] 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-id' 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-id' attribute to the table for SDP media level attributes.
Attribute name: dtls-id
Type of attribute: media-level
Subject to charset: no
Purpose: Indicates whether a new DTLS association is to be
established/re-established.
Appropriate Values: see Section 4
Contact name: Christer Holmberg
Mux Category: IDENTICAL
Thanks to Justin Uberti, Martin Thomson, Paul Kyzivat, Jens Guballa, Charles Eckel and Gonzalo Salgueiro for providing comments and suggestions on the document.
[RFC EDITOR NOTE: Please remove this section when publishing]
Changes from draft-ietf-mmusic-sdp-dtls-19
Changes from draft-ietf-mmusic-sdp-dtls-18
Changes from draft-ietf-mmusic-sdp-dtls-17
Changes from draft-ietf-mmusic-sdp-dtls-16
Changes from draft-ietf-mmusic-sdp-dtls-15
Changes from draft-ietf-mmusic-sdp-dtls-14
Changes from draft-ietf-mmusic-sdp-dtls-13
Changes from draft-ietf-mmusic-sdp-dtls-12
Changes from draft-ietf-mmusic-sdp-dtls-11
Changes from draft-ietf-mmusic-sdp-dtls-10
Changes from draft-ietf-mmusic-sdp-dtls-08
Changes from draft-ietf-mmusic-sdp-dtls-07
Changes from draft-ietf-mmusic-sdp-dtls-06
Changes from draft-ietf-mmusic-sdp-dtls-05
Changes from draft-ietf-mmusic-sdp-dtls-04
Changes from draft-ietf-mmusic-sdp-dtls-03
Changes from draft-ietf-mmusic-sdp-dtls-02
Changes from draft-ietf-mmusic-sdp-dtls-01
Changes from draft-ietf-mmusic-sdp-dtls-00
Changes from draft-holmberg-mmusic-sdp-dtls-01
Changes from draft-holmberg-mmusic-sdp-dtls-00