RFC 8842

Session Description Protocol (SDP) Offer/Answer Considerations for Datagram Transport Layer Security (DTLS) and Transport Layer Security (TLS)

📅 published January 2021, from ietf

Session Description Protocol (SDP) Offer/Answer Considerations for Datagram Transport Layer Security (DTLS) and Transport Layer Security (TLS) 💖

Okay bestie, here's the title block tea 🫶. This is RFC 8842, a Standards Track doc, dated January 2021, with ISSN 2070-1721. The authors are C. Holmberg (Ericsson) and R. Shpount (TurboBridge). It updates RFC 5763 and RFC 7345, and the title block doesn't list anything it obsoletes 🎀

Abstract

Okay so this doc defines the SDP offer/answer procedures for negotiating and establishing a Datagram Transport Layer Security (DTLS) association 🔐 It also lays out the criteria for when a brand new DTLS association has to be established, which is giving clarity ✨ The document updates RFCs 5763 and 7345 by swapping their common SDP offer/answer procedures for a reference to this spec, so no more copy-paste chaos 💅

This document defines a new SDP media-level attribute, "tls-id". 🧚

This document also defines how the "tls-id" attribute can be used for negotiating and establishing a Transport Layer Security (TLS) connection, in conjunction with the procedures in RFCs 4145 and 8122. Iykyk, it's the TLS side of the same vibe 🦋

Status of This Memo

This is an Internet Standards Track document. 👑

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). If you want more on Internet Standards, check Section 2 of RFC 7841 🌈

Info about the current status of this document, any errata, and how to give feedback on it lives at https://www.rfc-editor.org/info/rfc8842 💜

Copyright (c) 2021 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 (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, because 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 they are provided without warranty as described in the Simplified BSD License. 📜

Table of Contents 🍓

1.  Introduction
2.  Conventions
3.  Establishing a New DTLS Association
  3.1.  General
  3.2.  Change of Local Transport Parameters
  3.3.  Change of ICE ufrag Value
4.  SDP "tls-id" Attribute
5.  SDP Offer/Answer Procedures
  5.1.  General
  5.2.  Generating the Initial SDP Offer
  5.3.  Generating the Answer
  5.4.  Offerer Processing of the SDP Answer
  5.5.  Modifying the Session
6.  ICE Considerations
7.  TLS Considerations
8.  SIP Considerations
9.  RFC Updates
  9.1.  General
  9.2.  Update to RFC 5763
    9.2.1.  Update to Section 1
    9.2.2.  Update to Section 5
    9.2.3.  Update to Section 6.6
    9.2.4.  Update to Section 6.7.1
  9.3.  Update to RFC 7345
    9.3.1.  Update to Section 4
    9.3.2.  Update to Section 5.2.1
    9.3.3.  Update to Section 9.1
10. Security Considerations
11. IANA Considerations
12. References
  12.1.  Normative References
  12.2.  Informative References
Acknowledgements
Authors' Addresses

1. Introduction 🌸

Okay, so [RFC5763] defines the SDP offer/answer procedures for Secure Real-time Transport Protocol using Datagram Transport Layer Security (DTLS-SRTP). Then [RFC7345] defines SDP offer/answer procedures for UDP Transport Layer over Datagram Transport Layer Security (UDPTL-DTLS) ✨ This specification defines general offer/answer procedures for DTLS, based on the ones in [RFC5763]. Other specs that define specific DTLS usages can then reference this one, so the DTLS parts stay the same across all usages. Having common procedures is essential when multiple usages share the same DTLS association [RFC8843]. This document updates [RFC5763] and [RFC7345] by replacing common SDP offer/answer procedures with a reference to this specification 💅

NOTE: Since the publication of [RFC5763], [RFC4474] has been obsoleted by [RFC8224]. Updating the references (and the associated procedures) within [RFC5763] is outside the scope of this document. However, implementers of [RFC5763] applications are encouraged to implement [RFC8224] instead of [RFC4474].

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) [RFC8445] is used 😭 One possible way to determine a transport change is based on ufrag [RFC8445] change, but the ufrag value changes both when ICE is negotiated and when ICE restart [RFC8445] occurs. These events don't always require a new DTLS association, but there was previously no way to explicitly indicate in an SDP offer or answer whether a new DTLS association was required. To solve that problem, this document defines a new SDP attribute, "tls-id". The pair of SDP "tls-id" attribute values (the attribute values of the offerer and the answerer) uniquely identifies the DTLS association. Providing a new value of the "tls-id" attribute in an SDP offer or answer can be used to indicate whether a new DTLS association is to be established 🦋

The SDP "tls-id" attribute can be specified when negotiating a Transport Layer Security (TLS) connection, using the procedures in this document together with the procedures in [RFC5763] and [RFC8122]. The unique combination of SDP "tls-id" attribute values can be used to identify the negotiated TLS connection. That unique value can be used, for example, within TLS protocol extensions to tell apart multiple TLS connections and link those connections to specific offer/answer exchanges. The TLS-specific considerations are described in Section 7 🎀

2. Conventions 💜

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. Only the all-caps versions carry that special meaning, so lowercase words are just regular English 🧚

3. Establishing a New DTLS Association 🔥

3.1. General

A new DTLS association must be established between two endpoints after a successful SDP offer/answer exchange in the following cases:

  • The negotiated DTLS setup roles change; or
  • One or more fingerprint values are modified, added, or removed in either an SDP offer or answer; or
  • The intent to establish a new DTLS association is explicitly signaled using SDP, by changing the value of the SDP "tls-id" attribute defined in this document;
NOTE: The first two items above are based on the procedures in [RFC5763]. This specification adds support for explicit signaling using the SDP "tls-id" attribute.

A new DTLS association can only be established as a result of a 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.

In this document, a "new DTLS association" between two endpoints refers to either an initial DTLS association (when no DTLS association is currently established between the endpoints) or a DTLS association replacing a previously established one 💖

3.2. Change of Local Transport Parameters

If an endpoint modifies its local transport parameters (address and/or port), and the modification requires a new DTLS association, the endpoint MUST change its local SDP "tls-id" attribute value (see Section 4) 🌈

If the underlying transport protocol prohibits a DTLS association from spanning multiple 5-tuples (transport/source address/source port/destination address/destination port), and the 5-tuple changes, the endpoint MUST change its local SDP "tls-id" attribute value (see Section 4). An example of this is when DTLS is carried over the Stream Control Transmission Protocol (SCTP), as described in [RFC6083] 🍓

3.3. Change of ICE ufrag Value

If an endpoint uses ICE and modifies a local ufrag value, and the modification requires a new DTLS association, the endpoint MUST change its local SDP "tls-id" attribute value (see Section 4) 🫶

4. SDP "tls-id" Attribute 👑

The pair of SDP "tls-id" attribute values (the attribute values of the offerer and the answerer) uniquely identifies the DTLS association or TLS connection 🦋

Here's the attribute definition, copied exactly as the spec writes it:

Name:  tls-id

Value:  tls-id-value

Usage Level:  media

Charset Dependent:  no

Default Value:  N/A

Syntax:
   tls-id-value = 20*255(tls-id-char)
   tls-id-char = ALPHA / DIGIT / "+" / "/" / "-" / "_"

   <ALPHA and DIGIT defined in RFC 4566>

Example:
   a=tls-id:abc3de65cddef001be82

Every time an endpoint requests to establish a new DTLS association, the endpoint MUST generate a new local "tls-id" attribute value. An unchanged local "tls-id" attribute value, combined with unchanged fingerprints, indicates that the endpoint intends to reuse the existing DTLS association 💅

The "tls-id" attribute value MUST be generated using a strong random function and include at least 120 bits of randomness 🎀

No default value is defined for the SDP "tls-id" attribute. Implementations that want to use the attribute MUST explicitly include it in SDP offers and answers. If an offer or answer does not contain a "tls-id" attribute (this could happen if the offerer or answerer represents an existing implementation that hasn't been updated to support the "tls-id" attribute), a change to one or more of the following characteristics MUST be treated as an indication that an endpoint wants to establish a new DTLS association, unless there is another mechanism to explicitly indicate that a new DTLS association is to be established 😭

  • DTLS setup role; or
  • fingerprint set; or
  • local transport parameters
NOTE: A modification of the ufrag value is not treated as an indication that an endpoint wants to establish a new DTLS association. To indicate that a new DTLS association is to be established, one or more of the characteristics listed above have to be modified.

The mux category [RFC8859] for the "tls-id" attribute is "IDENTICAL", which means the attribute value applies to all media descriptions being multiplexed [RFC8843]. However, as described in [RFC8843], to avoid duplication, the attribute is only associated with the "m=" line representing the offerer/answerer BUNDLE tag 🧚

For RTP-based media, the "tls-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 procedures [RFC3264] associated with the attribute are defined in Section 5 🌈

5. SDP Offer/Answer Procedures 🦋

5.1. General ✨

okay bestie, this section is the generic SDP offer/answer vibe check for negotiating a DTLS association 💅 It covers the stuff that works for everyone. Extra rules for specific DTLS usages (like DTLS-SRTP) are out of scope here and need their own usage-specific doc 🎀

NOTE: The procedures in this section are generalizations of procedures first specified in the DTLS-SRTP document [RFC5763], with the addition of usage of the SDP tls-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 🦋

okay so when an offerer or answerer wants a brand new DTLS association, it has to make sure the packets from the new association and the old one can still be told apart 🧚 With an ordered transport (like SCTP) that's easy: you just send the new association's packets after all the old ones have gone out. With an unordered transport like UDP, though, old-association packets can show up after the answer SDP and the first new-association packets have already arrived 😭 The only way to sort them out is the 5-tuple. So if an unordered transport is used for the DTLS association, a new 3-tuple (transport/source address/source port) MUST be allocated by at least one of the endpoints so the DTLS packets can be demultiplexed 🔥

When an offerer needs a new DTLS association and an unordered transport (like UDP) is in use, the offerer MUST allocate a new 3-tuple for the offer. That way it can disambiguate packets from the new association from packets from any other DTLS association 💜 This typically means using a local address and/or port, or a set of ICE candidates (see Section 6), that weren't recently used for any other DTLS association.

When an answerer needs a new DTLS association, an unordered transport is used, and the offerer did not allocate a new 3-tuple, the answerer MUST allocate a new 3-tuple for the answer. Same energy: it needs to be able to tell the new association's packets apart from every other one 👑 This typically means a local address and/or port, or a set of ICE candidates (see Section 6), that weren't recently used for any other DTLS association.

To negotiate a DTLS association, these SDP attributes are used:

  • The SDP setup attribute, defined in [RFC4145], is used to negotiate the DTLS roles 🎭
  • The SDP fingerprint attribute, defined in [RFC8122], is used to provide one or more fingerprint values; and
  • The SDP tls-id attribute, defined in this specification, is used to identify the DTLS association 🪪

This specification does not define how the SDP connection attribute [RFC4145] is used 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 its usage has been defined.

Unlike TCP and TLS connections, endpoints MUST NOT use the SDP setup attribute holdconn value when negotiating a DTLS association. Not it, not ever 🚫

Endpoints MUST support the hash functions as defined in [RFC8122] 🔐

The certificate received during the DTLS handshake [RFC6347] MUST match a certificate fingerprint received in SDP fingerprint attributes, following the procedures defined in [RFC8122]. If the fingerprints don't match the hashed certificate, then an endpoint MUST tear down the media session immediately (see [RFC8122]). no notes, just bye 💀

SDP offerers and answerers might reuse certificates across multiple DTLS associations and provide identical fingerprint values for each one 🌸 The combination of the SDP tls-id attribute values from the offerer and the answerer is what identifies each individual DTLS association.

NOTE: There are cases where the SDP tls-id attribute value generated by the offerer ends up being used for multiple DTLS associations. For that reason, the combination of the attribute values from the offerer and the answerer is needed to identify a DTLS association. An example is when the offerer sends an updated offer (Section 5.5) without changing its attribute value, but the answerer decides a new DTLS association should be created. The answerer will generate a new local attribute value for the new DTLS association (Section 5.3), while the offerer keeps using the same attribute value it used for the current association. Another example is when the Session Initiation Protocol (SIP) [RFC3261] is used for signaling and an offer is forked to multiple answerers. The attribute value generated by the offerer will be used for the DTLS associations established by each answerer 🍓

5.2. Generating the Initial SDP Offer 🌈

When an offerer sends the initial offer, it MUST insert an SDP setup attribute [RFC4145] with an actpass attribute value, plus one or more SDP fingerprint attributes following the procedures in [RFC8122]. On top of that, the offerer MUST insert an SDP tls-id attribute with a unique attribute value in the offer ✨

Here's the plot twist: because the offerer put in actpass, the offerer MUST be prepared to receive a DTLS ClientHello message [RFC6347] from the answerer (if the answerer is establishing a new DTLS association) before the offerer gets the SDP answer 😭

If the offerer receives a DTLS ClientHello message, and a DTLS association gets established before the offerer receives the SDP answer carrying the fingerprint for that association, then any data received on that DTLS association before the fingerprint MUST be considered to be coming from an unverified source 🚨 What the offerer does with that data, and with sending data back to the unverified source, is outside the scope of this document 🙅‍♀️

5.3. Generating the Answer 💖

When an answerer sends an answer, it MUST insert an SDP setup attribute following the procedures in [RFC4145], plus one or more SDP fingerprint attributes following the procedures in [RFC8122] 🎀 If the answerer decides, based on the criteria in Section 3.1, that a new DTLS association is to be established, the answerer MUST insert an SDP tls-id attribute with a new unique attribute value in the associated answer. Note that the offerer and the answerer each generate their own local tls-id values, and the combination of both identifies the DTLS association 🪪

If the answerer receives an offer that requires a new DTLS association, and the answerer doesn't accept establishing one, the answerer MUST reject the m= lines associated with the suggested DTLS association [RFC3264] 🚫

If the answerer receives an offer that does not require a new DTLS association, and the answerer decides a new DTLS association is not to be established, the answerer MUST insert an SDP tls-id attribute with the previously assigned attribute value in the associated answer. The answerer MUST also insert an SDP setup attribute with an attribute value that doesn't change the previously negotiated DTLS roles, plus one or more SDP fingerprint attributes whose values don't change the previously sent fingerprint set, in the associated answer 🌸

If the answerer receives an offer that does not contain an SDP tls-id attribute, the answerer MUST NOT insert a tls-id attribute in the answer. Not even a little 🙈

If a new DTLS association is to be established, and the answerer inserts an SDP setup attribute with an active attribute value in the answer, the answerer MUST initiate a DTLS handshake [RFC6347] by sending a DTLS ClientHello message towards the offerer 🤝

Even though an offerer is required to insert an SDP setup attribute with an actpass attribute value in initial offers (Section 5.2) and subsequent offers (Section 5.5), the answerer MUST be able to receive initial and subsequent offers with other attribute values. That's for backward compatibility with older implementations that might insert other attribute values in initial and subsequent offers 🧚

5.4. Offerer Processing of the SDP Answer 🦋

When an offerer receives an answer that establishes a new DTLS association based on the criteria in Section 3.1, the offerer's role decides what happens next. If the offerer becomes the DTLS client (based on the value of the SDP setup attribute [RFC4145]), the offerer MUST establish a DTLS association. If the offerer becomes the DTLS server, it MUST wait for the answerer to establish the DTLS association 🫶

If the offerer indicated a desire to reuse an existing DTLS association, and the answerer does not request establishing a new one, the offerer keeps using the previously established DTLS association 💅

A new DTLS association can be established based on changes in either an SDP offer or answer 🔥 When talking to legacy endpoints, an offerer can receive an answer that includes the same fingerprint set and setup role. A new DTLS association will still be established if such an answer comes back in response to an offer that asked for a new DTLS association, because the transport parameters would have been changed in the offer 🌈

5.5. Modifying the Session ✨

When an offerer sends a subsequent offer and wants to establish a new DTLS association, the offerer MUST insert an SDP setup attribute [RFC4145] with an actpass attribute value, plus one or more SDP fingerprint attributes following the procedures in [RFC8122] 🎀 On top of that, the offerer MUST insert an SDP tls-id attribute with a new unique attribute value in the offer 💖

When an offerer sends a subsequent offer and does not want a new DTLS association, and a previously established DTLS association exists, the offerer MUST insert an SDP setup attribute with an actpass attribute value in the offer. It also needs one or more SDP fingerprint attributes whose values don't change the previously sent fingerprint set. Finally, the offerer MUST insert an SDP tls-id attribute with the previously assigned attribute value in the offer 🪪

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 for as long as both are alive 🫶

6. ICE Considerations 🧊

Okay so when ICE ([RFC8445]) is in use, the connectivity checks happen before the DTLS handshake even starts 🫶. Also, if aggressive nomination mode is used, several candidate pairs might get marked valid before ICE finally settles on one pair 👀.

NOTE: Aggressive nomination has been deprecated from ICE but must still be supported for backwards compatibility reasons [RFC8445].

Now here's the important bit 💅. When a new DTLS association gets established over an unordered transport, at least one of the endpoints MUST allocate a completely new set of ICE candidates that weren't recently used for any other DTLS association. That means the answerer can't start a new DTLS association unless the offerer started an ICE restart [RFC8445]. If the answerer wants a new one, it has to kick off its own ICE restart and a new offer/answer exchange. Still, an ICE restart doesn't require a new DTLS association by default ✨.

NOTE: Simple Traversal of the UDP Protocol through NAT (STUN) packets are sent directly over UDP, not over DTLS. [RFC7983] describes how to demultiplex STUN packets from DTLS packets and SRTP packets.

Each ICE candidate tied to a component counts as part of the same DTLS association 🦋. So from DTLS's point of view, switching between those ICE candidates is NOT a change of local transport parameters. Iconic behaviour, honestly 😭.

7. TLS Considerations 🔐

The procedures in this document can also be used to negotiate and establish a TLS connection, with one restriction (more on that below) 💖.

As specified in [RFC4145], the SDP "connection" attribute is what says whether a new TLS connection should be established. An offerer and answerer MUST make sure the "connection" attribute value and the "tls-id" attribute value don't clash about whether a new TLS connection is happening 🙈.

NOTE: Even though the SDP "connection" attribute can tell you whether a new TLS connection is wanted, the unique combination of SDP "tls-id" attribute values is what identifies a TLS connection 🧚. That unique value can be used, for example, inside TLS protocol extensions to tell apart multiple TLS connections and link them to specific offer/answer exchanges. One such extension is defined in [RFC8844].

If an offerer or answerer puts an SDP "connection" attribute with a "new" value in the offer/answer and also includes an SDP "tls-id" attribute, the "tls-id" value MUST be new and unique 🌸.

If an offerer or answerer puts an SDP "connection" attribute with an "existing" value in the offer/answer, a TLS connection was already established, and the offerer/answerer had previously included an SDP "tls-id" attribute tied to that same TLS connection in an earlier offer/answer, then it MUST also include an SDP "tls-id" attribute with the value that was previously assigned in the offer/answer 🎀.

If an offerer or answerer receives an offer/answer with conflicting attribute values, it MUST process that offer/answer as misformed 🚫.

An endpoint MUST NOT make assumptions about whether its peer supports the SDP "tls-id" attribute. So to avoid any ambiguity, both offerers and answerers MUST always use the "connection" attribute together with the "tls-id" attribute 💜.

NOTE: As defined in [RFC4145], if the SDP "connection" attribute isn't explicitly present, the implicit default value is "new".

Here's the SDP example. It's based on the example in Section 3.4 of [RFC8122], with the SDP "tls-id" attribute added in 🍓:

m=image 54111 TCP/TLS t38
c=IN IP4 192.0.2.2
a=tls-id:abc3de65cddef001be82
a=setup:passive
a=connection:new
a=fingerprint:SHA-256 \
 12:DF:3E:5D:49:6B:19:E5:7C:AB:4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF: \
 3E:5D:49:6B:19:E5:7C:AB:4A:AD
a=fingerprint:SHA-1 \
 4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB

8. SIP Considerations 📞

When the Session Initiation Protocol (SIP) [RFC3261] is the signalling protocol used to set up a multimedia session, dialogs [RFC3261] might get established between the caller and several callees. This is called forking 🍴. If forking happens, a separate DTLS association gets established between the caller and each callee.

When forking happens, an SDP offerer can receive DTLS ClientHello messages and SDP answers from multiple remote locations 🌈. Because of that, the offerer might have to wait for several SDP answers (from different remote locations) until it gets a certificate fingerprint that matches the certificate tied to a specific DTLS handshake. The offerer MUST NOT declare a fingerprint mismatch until it's sure it won't receive SDP answers from any additional remote locations 🕵️‍♀️.

It's possible to send an INVITE request that doesn't contain an SDP offer. People often call that an "empty INVITE" or an "offerless INVITE" 🙃. The receiving endpoint will put the SDP offer in a response to the request. When the endpoint generates that SDP offer and a DTLS association was already established before, the offerer MUST insert an SDP "tls-id" attribute and one or more SDP "fingerprint" attributes, using their previously assigned values ♻️. If no DTLS association was established before, the offer MUST be generated using the same rules as a new offer (see Section 5.2). Whether or not a DTLS association existed before, the SDP "setup" attribute MUST be included according to the rules in [RFC4145]. And if ICE is used, ICE restart MUST be initiated, following the third-party call-control considerations described in [RFC8839] 🔄.

9. RFC Updates 📝

9.1. General

This section updates specifications that use DTLS-protected media, so they reflect the procedures defined in this specification 💅.

9.2. Update to RFC 5763

9.2.1. Update to Section 1

The reference to [RFC4572] gets replaced with a reference to [RFC8122] 🔁.

9.2.2. Update to Section 5

The text in [RFC5763], Section 5 ("Establishing a Secure Channel") gets changed. The generic SDP offer/answer procedures for DTLS are swapped out for a reference to this specification 🦋. Here's the NEW TEXT:

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)" [RFC8122]. 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. Generating 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 wants 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]. 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, is 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 with a 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 RFC 8842.

9.2.3. Update to Section 6.6

The text in [RFC5763], Section 6.6 ("Session Modification") gets changed too. The generic SDP offer/answer procedures for DTLS are replaced with a reference to this specification 🌸. Here's the NEW TEXT:

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 RFC 8842.

9.2.4. Update to Section 6.7.1

The text in [RFC5763], Section 6.7.1 ("ICE Interaction") gets changed as well, with the ICE procedures swapped out for a reference to this specification 🧊. Here's the NEW TEXT:

The Interactive Connectivity Establishment (ICE) [RFC8445] considerations for DTLS-protected media are described in RFC 8842.

9.3. Update to RFC 7345

9.3.1. Update to Section 4 ✨

Okay bestie, the subsections (4.1 - 4.5) in [RFC7345], Section 4 ("SDP Offerer/Answerer Procedures") are getting removed and replaced with the new text below 💅

NEW TEXT:

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 RFC 8842 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].

9.3.2. Update to Section 5.2.1 🌸

The text in [RFC7345], Section 5.2.1 ("ICE Usage") gets updated. The ICE procedures are swapped out for a reference to this spec, so no more duplicate instructions 🧚

NEW TEXT:

The Interactive Connectivity Establishment (ICE) [RFC8445]
considerations for DTLS-protected media are described in RFC 8842.

9.3.3. Update to Section 9.1 🦋

A reference to [RFC8122] is getting added to [RFC7345], Section 9.1 ("Normative References") so everything is properly linked up 🔗

NEW TEXT:

[RFC8122]   Lennox, J. and C.  Holmberg, "Connection-Oriented
            Media Transport over the Transport Layer Security
            (TLS) Protocol in the Session Description Protocol
            (SDP)", RFC 8122, DOI 10.17487/RFC8122, March 2017,
            <https://www.rfc-editor.org/info/rfc8122>.

10. Security Considerations 🔒

Okay, security time, and I'm being very serious about this one 🚨 This spec does not change the security considerations that come with DTLS or the SDP offer/answer mechanism. The only new thing is the SDP "tls-id" attribute. Otherwise, this document just clarifies how a DTLS association gets negotiated and set up. Nothing spicy beyond that 💅

This spec also does not change the actual TLS connection setup procedures. The SDP tls-is attribute can't be used on its own to link an SDP offer/answer exchange to a TLS connection setup. So this document doesn't bring in any new security worries about that correlation either. Phew 😭

11. IANA Considerations 📋

This document updates the "Session Description Protocol Parameters" registry, as laid out in Section 8.2.2 of [RFC4566]. Specifically, it adds the SDP "tls-id" attribute to the table for SDP media-level attributes, like so ✨

Attribute name:  tls-id

Type of attribute:  Media-level

Subject to charset:  No

Purpose:  Indicates whether a new DTLS association or TLS connection
   is to be established/re-established.

Appropriate Values:  See Section 4

Contact name:  Christer Holmberg

Mux Category:  IDENTICAL

12. References 📚

12.1. Normative References 🧠

[RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119,
           DOI 10.17487/RFC2119, March 1997,
           <https://www.rfc-editor.org/info/rfc2119>.

[RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
           A., Peterson, J., Sparks, R., Handley, M., and E.
           Schooler, "SIP: Session Initiation Protocol", RFC 3261,
           DOI 10.17487/RFC3261, June 2002,
           <https://www.rfc-editor.org/info/rfc3261>.

[RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
           with Session Description Protocol (SDP)", RFC 3264,
           DOI 10.17487/RFC3264, June 2002,
           <https://www.rfc-editor.org/info/rfc3264>.

[RFC4145]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in
           the Session Description Protocol (SDP)", RFC 4145,
           DOI 10.17487/RFC4145, September 2005,
           <https://www.rfc-editor.org/info/rfc4145>.

[RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
           Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
           July 2006, <https://www.rfc-editor.org/info/rfc4566>.

[RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
           for Establishing a Secure Real-time Transport Protocol
           (SRTP) Security Context Using Datagram Transport Layer
           Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
           2010, <https://www.rfc-editor.org/info/rfc5763>.

[RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
           Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
           January 2012, <https://www.rfc-editor.org/info/rfc6347>.

[RFC7345]  Holmberg, C., Sedlacek, I., and G. Salgueiro, "UDP
           Transport Layer (UDPTL) over Datagram Transport Layer
           Security (DTLS)", RFC 7345, DOI 10.17487/RFC7345, August
           2014, <https://www.rfc-editor.org/info/rfc7345>.

[RFC8122]  Lennox, J. and C. Holmberg, "Connection-Oriented Media
           Transport over the Transport Layer Security (TLS) Protocol
           in the Session Description Protocol (SDP)", RFC 8122,
           DOI 10.17487/RFC8122, March 2017,
           <https://www.rfc-editor.org/info/rfc8122>.

[RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
           2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
           May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8445]  Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
           Connectivity Establishment (ICE): A Protocol for Network
           Address Translator (NAT) Traversal", RFC 8445,
           DOI 10.17487/RFC8445, July 2018,
           <https://www.rfc-editor.org/info/rfc8445>.

[RFC8843]  Holmberg, C., Alvestrand, H., and C. Jennings,
           "Negotiating Media Multiplexing Using the Session
           Description Protocol (SDP)", RFC 8843,
           DOI 10.17487/RFC8843, January 2021,
           <https://www.rfc-editor.org/info/rfc8843>.

[RFC8859]  Nandakumar, S., "A Framework for Session Description
           Protocol (SDP) Attributes When Multiplexing", RFC 8859,
           DOI 10.17487/RFC8859, January 2021,
           <https://www.rfc-editor.org/info/rfc8859>.

12.2. Informative References 🌈

[ITU.T38]  ITU-T, "Procedures for real-time Group 3 facsimile
           communication over IP networks", Recommendation T.38,
           September 2010, <https://www.itu.int/rec/T-REC-T.38/en>.

[RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
           Authenticated Identity Management in the Session
           Initiation Protocol (SIP)", RFC 4474,
           DOI 10.17487/RFC4474, August 2006,
           <https://www.rfc-editor.org/info/rfc4474>.

[RFC4572]  Lennox, J., "Connection-Oriented Media Transport over the
           Transport Layer Security (TLS) Protocol in the Session
           Description Protocol (SDP)", RFC 4572,
           DOI 10.17487/RFC4572, July 2006,
           <https://www.rfc-editor.org/info/rfc4572>.

[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,
           <https://www.rfc-editor.org/info/rfc5576>.

[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,
           <https://www.rfc-editor.org/info/rfc6083>.

[RFC7983]  Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
           Updates for Secure Real-time Transport Protocol (SRTP)
           Extension for Datagram Transport Layer Security (DTLS)",
           RFC 7983, DOI 10.17487/RFC7983, September 2016,
           <https://www.rfc-editor.org/info/rfc7983>.

[RFC8224]  Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
           "Authenticated Identity Management in the Session
           Initiation Protocol (SIP)", RFC 8224,
           DOI 10.17487/RFC8224, February 2018,
           <https://www.rfc-editor.org/info/rfc8224>.

[RFC8839]  Petit-Huguenin, M., Nandakumar, S., Holmberg, C., Keränen,
           A., and R. Shpount, "Session Description Protocol (SDP)
           Offer/Answer Procedures for Interactive Connectivity
           Establishment (ICE)", RFC 8839, DOI 10.17487/RFC8839,
           January 2021, <https://www.rfc-editor.org/info/rfc8839>.

[RFC8844]  Thomson, M. and E. Rescorla, "Unknown Key-Share Attacks on
           Uses of TLS with the Session Description Protocol (SDP)",
           RFC 8844, DOI 10.17487/RFC8844, January 2021,
           <https://www.rfc-editor.org/info/rfc8844>.

Acknowledgements 🫶

Big thanks to Justin Uberti, Martin Thomson, Paul Kyzivat, Jens Guballa, Charles Eckel, Gonzalo Salgueiro, and Paul Jones for giving comments and suggestions on the document 💖 Ben Campbell did the Area Director review, and Paul Kyzivat did the Gen-ART review 👑

Authors' Addresses 🦋

Christer Holmberg
Ericsson
Hirsalantie 11
FI-02420 Jorvas
Finland

Email: christer.holmberg@ericsson.com


Roman Shpount
TurboBridge
4905 Del Ray Avenue, Suite 300
Bethesda, MD 20814
United States of America

Email: rshpount@turbobridge.com
📦 where this page comes from