RFC 6128

RTP Control Protocol (RTCP) Port for Source-Specific Multicast (SSM) Sessions

📅 published February 2011, from ietf

RFC 6128 🎀 RTCP Port for SSM Sessions

Okay bestie, here's the rundown on this one 💖

RFC: 6128 Title: RTP Control Protocol (RTCP) Port for Source-Specific Multicast (SSM) Sessions Author: Ali Begen, Cisco (abegen@cisco.com) Category: Standards Track Date: February 2011 ISSN: 2070-1721 Updates: 5760 (RFC 5760) 🦋

Abstract

okay so SDP has this attribute that lets RTP apps say which address and port the RTCP traffic goes to 🧚 In RTP-based source-specific multicast (SSM) sessions, that same attribute gets used to point at the address and RTCP port of the Feedback Target in the SDP description. The catch is that the RTCP port for the SSM session itself can't be set with that same attribute, because it could get ambiguous 🙈 So it has to be worked out from the "m=" line of the media description. That's an unnecessary restriction tbh, and this document gets rid of it by introducing a new SDP attribute ✨

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). Further information on Internet Standards is available in Section 2 of RFC 5741.

Info about the current status of this document, any errata, and how to give feedback on it lives at http://www.rfc-editor.org/info/rfc6128 🌸

Copyright Notice

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, 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 💜

1. Introduction ....................................................2
2. The 'multicast-rtcp' Attribute ..................................3
3. SDP Example .....................................................3
4. Security Considerations .........................................4
5. IANA Considerations .............................................4
   5.1. Registration of SDP Attributes .............................5
6. Acknowledgments .................................................5
7. References ......................................................5
   7.1. Normative References .......................................5
   7.2. Informative References .....................................5

1. Introduction ✨

okay so the Session Description Protocol (SDP) [RFC4566] has an attribute that lets RTP applications [RFC3550] say which address and port go with the RTP Control Protocol (RTCP) traffic [RFC3605]. That attribute is called 'rtcp' 🎀

Now picture this: one or more media senders send RTP packets to a distribution source. The distribution source then multicasts those RTP packets to multicast receivers using a source-specific multicast (SSM) arrangement [RFC5760]. It also multicasts the forward RTCP traffic (sender reports and receiver reports, or their summaries) to the receivers in that same SSM session 🦋

In RTP-based SSM sessions, the 'rtcp' attribute is used to designate the address and the RTCP port of the Feedback Target in the SDP description [RFC5760]. But the RTCP port for the SSM session itself can't be specified by that same attribute, since it could cause ambiguity 🙈 So the multicast RTCP port has to be derived from the "m=" line of the media description (see Section 10.2 of [RFC5760]) by following the +1 rule (see Section 11 of [RFC3550]). Here's the tea though: [RFC3550] lifted the requirement for the +1 rule, because it put an unnecessary restriction on RTCP port selection 💅

So in this specification, we introduce a new SDP attribute to remove that restriction. The new attribute lets the multicast sender use its desired port in the RTCP session. This document updates [RFC5760] 🌈

2. The 'multicast-rtcp' Attribute 💖

In RTP-based SSM sessions, the distribution source can use different multicast RTP and RTCP ports to send the RTP and RTCP packets. Or, the distribution source can use RTP/RTCP port muxing [RFC5761], in which case the RTP and RTCP packets go to the same destination port in the SSM session 🫶

For the cases where the distribution source doesn't want to use the one higher port for RTCP traffic, this document defines a new SDP attribute called 'multicast-rtcp'. With this attribute, the distribution source uses a desired port for the SSM RTCP session. If the 'multicast-rtcp' attribute isn't there, the +1 rule applies, following [RFC5760] ✨

The following ABNF [RFC5234] syntax formally describes the 'multicast-rtcp' attribute:

rtcp-attribute = "a=multicast-rtcp:" port CRLF

Figure 1: ABNF syntax for the 'multicast-rtcp' attribute

Here, the 'port' token is defined as specified in Section 9 of [RFC4566] 🍓

The 'multicast-rtcp' attribute is defined as both a media-level and a session-level attribute. Except where this document says otherwise, the rules of [RFC3550] apply 👑

3. SDP Example 🧚

In the session description shown in Figure 2, a source stream is multicast from a distribution source (with a source IP address of 198.51.100.1) to the multicast destination address of 233.252.0.2 and port 41000. The forward RTCP traffic goes to the same multicast group but to port 42000, as specified by the "a=multicast-rtcp:42000" line. A feedback target with an address of 192.0.2.1 and port 43000 is specified by the 'rtcp' attribute 🦋

        v=0
        o=ali 1122334455 1122334466 IN IP4 ssm.example.com
        s='multicast-rtcp' Example
        t=0 0
        a=rtcp-unicast:rsi
        m=video 41000 RTP/AVPF 98
        i=Multicast Stream
        c=IN IP4 233.252.0.2/255
        a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1
        a=rtpmap:98 MP2T/90000
        a=multicast-rtcp:42000
        a=rtcp:43000 IN IP4 192.0.2.1
        a=mid:1

Figure 2: Example SDP showing the use of the 'multicast-rtcp' attribute

4. Security Considerations 🔥

⚠️ Serious security talk ahead, bestie. Read this part carefully!

The 'multicast-rtcp' attribute isn't believed to introduce any significant security risk to multimedia applications. A malevolent third party could use this attribute to redirect the RTCP traffic. But that requires intercepting and rewriting the packets carrying the SDP description. And if an interceptor can do that, many more attacks are possible, including a total change of the addresses and port numbers where the media gets sent 😭

To avoid attacks like this, the SDP description needs to be integrity protected and provided with source authentication. For example, this can be done end-to-end using S/MIME [RFC5652] [RFC5751] when SDP is used in a signaling packet with MIME types (application/sdp). Alternatively, HTTPS [RFC2818] or the authentication method in the Session Announcement Protocol (SAP) [RFC2974] could be used as well 🛡️

5. IANA Considerations 🌸

The following contact information shall be used for all registrations in this document:

Ali Begen
abegen@cisco.com

5.1. Registration of SDP Attributes 💜

This document registers a new attribute name in SDP.

     SDP Attribute ("att-field"):
     Attribute name:     multicast-rtcp
     Long form:          Port in the multicast RTCP session
     Type of name:       att-field
     Type of attribute:  Media or session level
     Subject to charset: No
     Purpose:            Specifies the port for the SSM RTCP session
     Reference:          [RFC6128]
     Values:             See [RFC6128]

6. Acknowledgments 🫶

Thanks to Colin Perkins and Magnus Westerlund for suggesting the name for the 'multicast-rtcp' attribute and for writing text for parts of this specification. Some parts of this specification are based on [RFC3605] and [RFC5760], so thanks also to everyone who contributed to those specifications 🌷

7. References 📚

7.1. Normative References

[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003.

[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006.

[RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control Protocol (RTCP) Extensions for Single-Source Multicast Sessions with Unicast Feedback", RFC 5760, February 2010.

[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008.

7.2. Informative References

[RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP)", RFC 3605, October 2003.

[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and Control Packets on a Single Port", RFC 5761, April 2010.

[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, September 2009.

[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

[RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session Announcement Protocol", RFC 2974, October 2000.

[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Message Specification", RFC 5751, January 2010.

Author's Address 🎀

Ali Begen Cisco 181 Bay Street Toronto, ON M5J 2T3 Canada

EMail: abegen@cisco.com

That's the whole RFC, bestie! 🌈✨ The vibe: the 'multicast-rtcp' attribute lets SSM senders pick their own RTCP port instead of being stuck with the +1 rule 💅👑

📦 where this page comes from

ietf: rfc6128 2011-02
💖 girlified by claude-haiku-5-5 on 2026-10-17: a language model's rewrite of the original, under its terms