1. Introduction

1. Introduction

This document defines the Extended Report (XR) packet type for the RTP Control Protocol (RTCP) [9], and defines how the use of XR packets can be signaled by an application if it employs the Session Description Protocol (SDP) [4]. XR packets convey information beyond that already contained in the reception report blocks of RTCP's sender report (SR) or Receiver Report (RR) packets. The information is of use across RTP profiles, and so is not appropriately carried in SR or RR profile-specific extensions. Information used for network management falls into this category, for instance.

The definition is broken out over the three sections that follow the Introduction. Section 2 defines the XR packet as consisting of an eight octet header followed by a series of components called report blocks. Section 3 defines the common format, or framework, consisting of a type and a length field, required for all report blocks. Section 4 defines several specific report block types. Other block types can be defined in future documents as the need arises.

The report block types defined in this document fall into three categories. The first category consists of packet-by-packet reports on received or lost RTP packets. Reports in the second category convey reference time information between RTP participants. In the third category, reports convey metrics relating to packet receipts, that are summary in nature but that are more detailed, or of a different type, than that conveyed in existing RTCP packets.

All told, seven report block formats are defined by this document. Of these, three are packet-by-packet block types:

• Loss RLE Report Block (Section 4.1): Run length encoding of reports concerning the losses and receipts of RTP packets.

• Duplicate RLE Report Block (Section 4.2): Run length encoding of reports concerning duplicates of received RTP packets.

• Packet Receipt Times Report Block (Section 4.3): A list of reception timestamps of RTP packets.

There are two reference time related block types:

• Receiver Reference Time Report Block (Section 4.4): Receiver-end wallclock timestamps. Together with the DLRR Report Block mentioned next, these allow non-senders to calculate round-trip times.

• DLRR Report Block (Section 4.5): The delay since the last Receiver Reference Time Report Block was received. An RTP data sender that receives a Receiver Reference Time Report Block can respond with a DLRR Report Block, in much the same way as, in the mechanism already defined for RTCP [9, Section 6.3.1], an RTP data receiver that receives a sender's NTP timestamp can respond by filling in the DLSR field of an RTCP reception report block.

Finally, this document defines two summary metric block types:

• Statistics Summary Report Block (Section 4.6): Statistics on RTP packet sequence numbers, losses, duplicates, jitter, and TTL or Hop Limit values.

• VoIP Metrics Report Block (Section 4.7): Metrics for monitoring Voice over IP (VoIP) calls.

Before proceeding to the XR packet and report block definitions, this document provides an applicability statement (Section 1.1) that describes the contexts in which these report blocks can be used. It also defines (Section 1.2) the normative use of key words, such as MUST and SHOULD, as they are employed in this document.

Following the definitions of the various report blocks, this document describes how applications that employ SDP can signal their use (Section 5). The document concludes with a discussion (Section 6) of numbering considerations for the Internet Assigned Numbers Authority (IANA), of security considerations (Section 7), and with appendices that provide examples of how to implement algorithms discussed in the text.