Internet Draft H. Kaplan (ed.) Expires: August 12, 2012 Acme Packet K. Hedayat EXFO N. Venna Saperix P. Jones Cisco Systems, Inc. A. Roychowdhury Hughes Systique Corp. C. SivaChelvan Cisco Systems, Inc. N. Stratton BlinkMind, Inc. March 10, 2012 An Extension to the Session Description Protocol (SDP) for Media Loopback draft-ietf-mmusic-media-loopback-17 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 12, 2012. Kaplan, et al. Expires August 2012 [Page 1] Internet-Draft SDP Media Loopback March 2012 Copyright Notice Copyright (c) 2012 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. Abstract The wide deployment of Voice over IP (VoIP), Text and Video over IP services has introduced new challenges in managing and maintaining real-time voice/real-time Text/video quality, reliability, and overall performance. In particular, media delivery is an area that needs attention. One method of meeting these challenges is monitoring the media delivery performance by looping media back to the transmitter. This is typically referred to as "active monitoring" of services. Media loopback is especially popular in ensuring the quality of transport to the edge of a given VoIP, Real-time Text or Video over IP service. Today in networks that deliver real-time media, short of running 'ping' and 'traceroute' to the edge, service providers are left without the necessary tools to actively monitor, manage, and diagnose quality issues with their service. The extension defined herein adds new SDP media attributes which enables establishment of media sessions where the media is looped back to the transmitter. Such media sessions will serve as monitoring and troubleshooting tools by providing the means for measurement of more advanced VoIP, Real-time Text and Video over IP performance metrics. Table of Contents 1. Introduction..................................................3 1.1 Use Cases Supported.......................................4 2. Terminology...................................................5 3. Offering Entity Behavior......................................6 4. Answering Entity Behavior.....................................6 5. SDP Constructs Syntax..............Error! Bookmark not defined. 5.1 Loopback Type Attribute...................................7 5.2 Loopback Mode Attribute...................................7 Kaplan, et al. Expires August 2012 [Page 2] Internet-Draft SDP Media Loopback March 2012 5.3 Generating the Offer for Loopback Session.................8 5.4 Generating the Answer for Loopback Session................9 5.5 Offerer Processing of the Answer.........................11 5.6 Modifying the Session....................................11 5.7 Establishing Sessions Between Entities Behind NAT........12 6. RTP Requirements.............................................12 7. Payload formats for Packet loopback..........................12 7.1 Encapsulated Payload format..............................13 7.2 Direct loopback RTP payload format.......................15 8. RTCP Requirements............................................16 9. Congestion Control...........................................17 10. Examples....................................................17 10.1 Offer for specific media loopback type..................17 10.2 Offer for choice of media loopback type.................18 10.3 Response to INVITE request rejecting loopback media.....19 11. Security Considerations.....................................19 12. Implementation Considerations...............................20 13. IANA Considerations.........................................20 13.1 SDP Attributes..........................................20 13.2 MIME Types..............................................21 14. Normative References........................................30 1. Introduction The overall quality, reliability, and performance of VoIP, Real-time Text and Video over IP services rely on the performance and quality of the media path. In order to assure the quality of the delivered media there is a need to monitor the performance of the media transport. One method of monitoring and managing the overall quality of real-time VoIP, Text and Video over IP Services is through monitoring the quality of the media in an active session. This type of "active monitoring" of services is a method of proactively managing the performance and quality of VoIP based services. The goal of active monitoring is to measure the media quality of a VoIP, Text or Video over IP session. A way to achieve this goal is to request an endpoint to loop media back to the other endpoint and to provide media statistics (e.g., RTCP and RTCP XR information). Another method involves deployment of special endpoints that always loop incoming media back for sessions. Although the latter method has been used and is functional, it does not scale to support large networks and introduces new network management challenges. Further, it does not offer the granularity of testing a specific endpoint that may be exhibiting problems. Kaplan, et al. Expires August 2012 [Page 3] Internet-Draft SDP Media Loopback March 2012 The extension defined in this memo introduces new SDP media attributes that enable establishment of media sessions where the media is looped back to the transmitter. The offer/answer model [RFC3264] is used to establish a loopback connection. Furthermore, this extension provides guidelines on handling RTP [RFC3550], as well as usage of RTCP [RFC3550] and RTCP XR [RFC3611] for reporting media related measurements. 1.1 Use Cases Supported As a matter of terminology in this document, packets flow from one peer acting as a "loopback source", to the other peer acting as a "loopback mirror", which in turn returns packets to the loopback source. In advance of the session, the peers negotiate to determine which one acts in which role. The negotiation also includes details such as the type of loopback to be used. This specification supports three use cases: "encapsulated packet loopback", "direct loopback", and "media loopback". These are distinguished by the treatment of incoming RTP packets at the loopback mirror. 1.1.1 Encapsulated Packet Loopback In the encapsulated packet loopback case, the entire incoming RTP packet is encapsulated as payload within an outer payload type that is specific to this use case and specified below (Section 7.1). The encapsulated packet is returned to the loopback source. The loopback source can generate statistics for one-way path performance up to the RTP level for each direction of travel by examining sequence numbers and timestamps in the outer header and the encapsulated RTP packet payload. The loopback source can also play back the returned media content for evaluation. Because the encapsulating payload extends the packet size, it could encounter difficulties in an environment where the original RTP packet size is close to the path MTU size. The encapsulating payload type therefore offers the possibility of RTP-level fragmentation of the returned packets. The use of this facility could affect statistics derived for the return path. In addition, the increased bit rate required in the return direction may affect these statistics more directly in a restricted-bandwidth situation. Kaplan, et al. Expires August 2012 [Page 4] Internet-Draft SDP Media Loopback March 2012 1.1.2 Direct Loopback In the direct loopback case, the loopback mirror copies the payload of the incoming RTP packet into a new packet, the payload type of which is again specific to this use case and specified below (Section 7.2). The loopback mirror returns the new packet to the packet source. There is no provision in this case for RTP-level fragmentation. This use case has the advantage of keeping the packet size the same in both directions. The packet source can compute only two-way path statistics from the direct loopback packet header, but can play back the returned media content. It has been suggested that the loopback source, knowing that the incoming packet will never be passed to a decoder, can store a timestamp and sequence number inside the payload of the packet it sends to the mirror, then extract that information from the returned direct loopback packet and compute one-way path statistics as in the previous case. Obviously, playout of returned content is no longer possible if this is done. 1.1.3 Media Loopback In the media loopback case, the loopback mirror submits the incoming packet to a decoder appropriate to the incoming payload type. The packet is taken as close as possible to the analog level, then reencoded according to an outgoing format determined by negotiation. The reencoded content is returned to the loopback source as an RTP packet with payload type corresponding to the reencoding format. This usage allows trouble-shooting at the codec level. The capability for path statistics is limited to what is available from RTCP reports. 2. Terminology 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 RFC 2119. Kaplan, et al. Expires August 2012 [Page 5] Internet-Draft SDP Media Loopback March 2012 SDP: Session Description Protocol, as defined in [RFC4566]. This document assumes the SDP offer/answer model is followed, per [RFC3264], but does not assume any specific protocol for carrying the SDP. The following terms are borrowed from [RFC3264] definitions: offer, offerer, answer, answerer, and agent. 3. SDP Offerer Behavior An SDP offerer compliant to this memo and attempting to establish a media session with media loopback MUST include "loopback" media attributes for each individual media description in the offer message. The offerer MUST look for the "loopback" media attributes in the media description(s) of the response from the answer for confirmation that the request is accepted. 4. SDP Answerer Behavior An SDP answerer compliant to this specification and receiving an offer containing media descriptions with the "loopback" media attributes MUST acknowledge the request by including the received "loopback" media attributes for each media description in its asnwer if it agrees to do the loopback. If the answerer does not want to do loopback or wants to reject the "loopback" request for specific media types, it MAY do so as defined in section Error! Reference source not found. of this specification. An answerer MAY reject an offered stream (either with loopback- source or loopback-mirror) if the loopback-type is not specified, the specified loopback-type is not supported, or the endpoint cannot honor the offer for any other reason. The loopback request MUST be rejected by setting the stream's media port number to zero in the answer as defined in RFC 3264 [RFC3264], or by rejecting the entire offer (e.g., by rejecting the session request entirely). Note that an answerer that is not compliant to this specification and which receives an offer with the "loopback" media attributes would ignore the attribute and treat the incoming offer as a normal request. If the offerer does not wish to establish a "normal" RTP session, it would need to terminate the session upon receiving such an answer. Kaplan, et al. Expires August 2012 [Page 6] Internet-Draft SDP Media Loopback March 2012 5. New SDP Attributes Three new SDP media-level attributes are defined: one indicates the type of loopback, and the other two define the mode of the loopback. 5.1 Loopback Type Attribute This specification defines a new 'loopback' attribute, which indicates the type of loopack that the agent is able to do. The loopback type is a property media attribute with the following syntax: a=loopback: Following is the Augmented BNF [RFC5234] for loopback-type: Loopback-attr = "a=loopback:" loopback-type = loopback-choice [1*SP loopback-choice] loopback-choice = loopback-type-pkt / loopback-type-media loopback-type-pkt = "rtp-pkt-loopback" loopback-type-media = "rtp-media-loopback" The loopback type is used to indicate the type of loopback. The loopback-type values are rtp-pkt-loopback, and rtp-media-loopback. rtp-pkt-loopback: In this mode, the RTP packets are looped back to the sender at a point before the encoder/decoder function in the receive direction to a point after the encoder/decoder function in the send direction. This effectively re-encapsulates the RTP payload with the RTP/UDP/IP headers appropriate for sending it in the reverse direction. Any type of encoding related functions, such as packet loss concealment, MUST NOT be part of this type of loopback path. In this mode the RTP packets are looped back with a new payload type and format. Section 7 describes the payload formats that MUST be used for this type of loopback. rtp-media-loopback: This loopback is activated as close as possible to the analog interface and after the decoder so that the RTP packets are subsequently re-encoded prior to transmission back to the sender. 5.2 Loopback Mode Attribute The loopback mode defines two value media attributes that are used to indicate the mode of the loopback. These attributes are Kaplan, et al. Expires August 2012 [Page 7] Internet-Draft SDP Media Loopback March 2012 additional mode attributes like sendonly, recvonly, etc. The syntax of the loopback mode media attributes are based on the following: a=:... The loopback-mode values are 'loopback-source' and 'loopback- mirror'. loopback-source: This attribute specifies that the entity that generated the SDP is the media source and expects the receiver of the SDP message to act as a loopback-mirror. loopback-mirror: This attribute specifies that the entity that generated the SDP will mirror (echo) all received media back to the sender of the RTP stream. No media is generated locally by the looping back entity for transmission in the mirrored stream. is a media format description. The format description has the semantics as defined in section 5.14 of RFC 4566[RFC4566]. When loopback-mode is specified as loopback-source, the media format corresponds to the RTP payload types the entity that generated the SDP is willing to send. When loopback-mode is specified as loopback-mirror, the media format corresponds to the RTP payload types the mirror is willing to receive. The "m=" line in the SDP MUST include all the payload types that will be used during the loopback session including those specified in the loopback-mode attribute line. The complete payload space for the call is specified in the "m=" line and the rtpmap attribute is used to map from the payload type number to an encoding name denoting the payload format to be used. 5.3 Generating the Offer for Loopback Session If an offerer wishes to make a loopback request, it MUST include both the loopback-type and loopback-mode attributes in a valid SDP offer: Example: m=audio 41352 RTP/AVP 0 8 100 a=loopback:rtp-media-loopback a=loopback-source:0 8 100 a=rtpmap:0 pcmu/8000 a=rtpmap:8 pcma/8000 a=rtpmap:100 G7221/16000/1 A loopback offer in a given media description MUST NOT contain the standard mode attributes sendonly, recvonly, sendrecv, or inactive. Kaplan, et al. Expires August 2012 [Page 8] Internet-Draft SDP Media Loopback March 2012 The loopback-mode attributes (loopback-source and loopback-mirror) replace the standard attributes. The offerer may offer more than one loopback-type in the SDP offer. The port number and the address in the offer (m/c= lines) indicate where the offerer would like to send and receive the media stream. The payload type numbers indicate the value of the payload the offerer expects to send and receive. If the offerer is the loopback-source, the subset of payload types indicated in the a=loopback-source line are the payload types for the codecs the offerer is willing to send. However, the answer might indicate a different payload type number for the same codec in the loopback- mirror line. In that case, the offerer MUST send the payload type received in the answer. If the offerer is the loopback-mirror, the subset of payload types indicated in the a=loopback-mirror line are the payload types for the codecs the offerer is willing to receive. If loopback-type is rtp-pkt-loopback, the loopback-mirror MUST send and the loopback-source MUST receive the looped back packets encoded in one of the two payload formats (encapsulated RTP or direct loopback) as defined in section 7. Example: m=audio 41352 RTP/AVP 0 8 112 a=loopback:rtp-pkt-loopback a=loopback-source:0 8 a=rtpmap:112 encaprtp/8000 Example: m=audio 41352 RTP/AVP 0 8 112 a=loopback:rtp-pkt-loopback a=loopback-source:0 8 a=rtpmap:112 rtploopback/8000 5.4 Generating the Answer for Loopback Session As with the offer, an SDP answer for loopback MUST NOT contain the standard mode attributes sendonly, recvonly, sendrecv, or inactive. The port number and the address in the answer (m/c= lines) indicate where the answerer would like to receive the media stream. The payload type numbers indicate the value of the payload types the answerer expects to send and receive. The loopback-mode attributes (a=loopback-source or a=loopback-miror) MUST contain at least one codec the answerer is willing to send or receive depending on whether it is the loopback-source or the loopback-mirror. In addition, the "m=" line MUST contain at least one codec that the answerer is willing to send or receive depending on whether it is the loopback-mirror or the loopback-source. Kaplan, et al. Expires August 2012 [Page 9] Internet-Draft SDP Media Loopback March 2012 If the offerer is the loopback-source, the answerer MUST be a loopback-mirror and the subset of payload types indicated in the a=loopback-mirror line are the payload types for the codecs the answerer is willing to receive. Similarly, if the offerer is the loopback-mirror, the answerer MUST be aloopback-source and the subset of payload types indicated in the a=loopback-source line are the payload types for the codecs the answerer is willing to send. If an answerer wishes to accept the loopback request it MUST include both the loopback mode and loopback type attributes in the answer. When a stream is offered with the loopback-source attribute, the corresponding stream in the response MUST be loopback-mirror and vice versa, provided that answerer is capable of supporting the requested loopback-type. For example, if the offer contains the loopback-source attribute: m=audio 41352 RTP/AVP 0 8 a=loopback:rtp-media-loopback a=loopback-source:0 8 The answer that is capable of supporting the offer MUST contain the loopback-mirror attribute: m=audio 41352 RTP/AVP 0 8 a=loopback:rtp-media-loopback a=loopback-mirror:0 8 If a stream is offered with multiple loopback type attributes, the answer MUST include only one of the loopback types that are accepted by the answerer. The answerer SHOULD give preference to the first loopback-type in the SDP offer. For example, if the offer contains: m=audio 41352 RTP/AVP 0 8 112 a=loopback:rtp-media-loopback rtp-pkt-loopback a=loopback-source:0 8 a=rtpmap:112 encaprtp/8000 The answer that is capable of supporting the offer and chooses to loopback the media using the rtp-media-loopback type MUST contain: m=audio 41352 RTP/AVP 0 8 a=loopback:rtp-media-loopback a=loopback-mirror:0 8 Kaplan, et al. Expires August 2012 [Page 10] Internet-Draft SDP Media Loopback March 2012 As specified in section 7, if the loopback-type is rtp-pkt-loopback, either the encapsulated RTP payload format or direct loopback RTP payload format MUST be used for looped back packets. For example, if the offer contains: m=audio 41352 RTP/AVP 0 8 112 113 a=loopback:rtp-pkt-loopback a=loopback-source:0 8 a=rtpmap:112 encaprtp/8000 a=rtpmap:113 rtploopback/8000 The answer that is capable of supporting the offer must contain one of the following: m=audio 41352 RTP/AVP 0 8 112 a=loopback:rtp-pkt-loopback a=loopback-mirror:0 8 a=rtpmap:112 encaprtp/8000 m=audio 41352 RTP/AVP 0 8 113 a=loopback:rtp-pkt-loopback a=loopback-mirror:0 8 a=rtpmap:113 rtploopback/8000 The previous examples used the 'encaprtp' and 'rtploopback' encoding names, which will be defined in sections 7.1.3 and 7.2.3. 5.5 Offerer Processing of the Answer If the received answer does not contain a=loopback-mirror or a=loopback-source, it is assumed that the loopback extensions are not supported by the remote agent. This is not a protocol failure, and instead merely completes the SDP offer/answer exchange with whatever normal rules apply; the offerer MAY decide to end the established RTP session (if any) through normal means of the upper- layer signaling protocol (e.g., by sending a SIP BYE). 5.6 Modifying the Session At any point during the loopback session, either participant MAY issue a new offer to modify the characteristics of the previous session, as defined in section 8 of RFC 3264 [RFC3264]. This also includes transitioning from a normal media processing mode to loopback mode, and vice a versa. Kaplan, et al. Expires August 2012 [Page 11] Internet-Draft SDP Media Loopback March 2012 5.7 Establishing Sessions Between Entities Behind NAT ICE/STUN/TURN provide a general solution to establishing media sessions between entities that are behind NATs. Loopback sessions that involve one or more end points behind NATs SHOULD use these general solutions wherever possible. 6. RTP Requirements A loopback-mirror that is compliant to this specification and accepting a media with rtp-pkt-loopback loopback-type MUST loopback the incoming RTP packets using either the encapsulated RTP payload format or the direct loopback RTP payload format as defined in section 7 of this specification. An answering entity that is compliant to this specification and accepting a media with the loopback type rtp-media-loopback MUST transmit all received media back to the sender, unless congestion feedback or other lower-layer constraints prevent it from doing so. The incoming media MUST be treated as if it were to be played (e.g. the media stream MAY receive treatment from PLC algorithms). The answering entity MUST re-generate all the RTP header fields as it would when transmitting media. The answering entity MAY choose to encode the loopback media according to any of the media descriptions supported by the offering entity. Furthermore, in cases where the same media type is looped back, the answering entity MAY choose to preserve number of frames/packet and bitrate of the encoded media according to the received media. 7. Payload formats for Packet loopback The payload formats described in this section MUST be used by a loopback-mirror when rtp-pkt-loopback is the specified loopback-type. Two different formats are specified here - an encapsulated RTP payload format and a direct loopback RTP payload format. The encapsulated RTP payload format should be used when the incoming RTP header information needs to be preserved during the loopback operation. This is useful in cases where loopback source needs to measure performance metrics in both directions. However, this comes at the expense of increased packet size as described in section 7.1. The direct loopback RTP payload format should be used when bandwidth requirement prevents the use of encapsulated RTP payload format. Kaplan, et al. Expires August 2012 [Page 12] Internet-Draft SDP Media Loopback March 2012 To keep the implementation of loopback-mirrors simple it is mandated that no payload format other than encapsulated or direct loopback formats can be used in the packets generated by a loopback-mirror. As described in RFC 3550 [RFC3550], sequence numbers and timestamps in the RTP header are generated with initial random values for security reasons. If this were not mandated and the source payload is sequence number aware, the loopback-mirror will be required to understand that payload format to generate looped back packets that do not violate RFC 3550 [RFC3550]. Requiring looped back packets to be in one of the two formats means loopback-mirror does not have to look into the actual payload received before generating the loopback packets. 7.1 Encapsulated Payload format A received RTP packet is encapsulated in the payload section of the RTP packet generated by a loopback-mirror. Each received packet MUST be encapsulated in a different packet, the encapsulated packet MUST be fragmented only if required (for example: due to MTU limitations). 7.1.1 Usage of RTP Header fields Payload Type (PT): The assignment of an RTP payload type for this packet format is outside the scope of this document; it is either specified by the RTP profile under which this payload format is used or more likely signaled dynamically out-of-band (e.g., using SDP; section 7.1.3 defines the name binding). Marker (M) bit: If the received RTP packet is looped back in multiple RTP packets, the M bit is set to 1 in the last packet, otherwise it is set to 0. Extension (X) bit: Defined by the RTP Profile used. Sequence Number: The RTP sequence number SHOULD be generated by the loopback-mirror in the usual manner with a constant random offset as described in RFC 3550 [RFC3550]. Timestamp: The RTP timestamp denotes the sampling instant for when the loopback-mirror is transmitting this packet to the loopback- source. The RTP timestamp MUST use the same clock rate used by the loopback-source. The initial value of the timestamp SHOULD be random for security reasons (see Section 5.1 of RFC 3550 [RFC3550]). SSRC: set as described in RFC 3550 [RFC3550]. Kaplan, et al. Expires August 2012 [Page 13] Internet-Draft SDP Media Loopback March 2012 CC and CSRC fields are used as described in RFC 3550 [RFC3550]. 7.1.2 RTP Payload Structure The RTP header in the encapsulated packet MUST be followed by the payload header defined in this section. If the received RTP packet has to be looped back in multiple packets due to fragmentation, the RTP header in each packet MUST be followed by the payload header defined in this section. The header is devised so that the loopback-source can decode looped back packets in the presence of moderate packet loss [RFC3550]. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | receive timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | F | R | CC |M| PT | sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | transmit timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | synchronization source (SSRC) identifier | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | contributing source (CSRC) identifiers | | .... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The 12 octets after the receive timestamp are identical to the RTP header in the received packet except for the first 4 bits of the first octet. Receive Timestamp: 32 bits The Receive timestamp denotes the sampling instant for when the last octet of the received media packet that is being encapsulated by the loopback-mirror is received from the loopback-source. The Receive timestamp MUST be based on the same clock used by the loopback-source. The initial value of the timestamp SHOULD be random for security reasons (see Section 5.1 of RFC 3550 [RFC3550]). Fragmentation (F): 2 bits First Fragment (00) /Last Fragment (01) /No Fragmentation(10)/ Intermediate Fragment (11). This field identifies how much of the received packet is encapsulated in this packet by the loopback- mirror. If the received packet is not fragmented, this field is Kaplan, et al. Expires August 2012 [Page 14] Internet-Draft SDP Media Loopback March 2012 set to 10; otherwise the packet that contains the first fragments sets this field to 00, the packet that contains the last fragment sets this field to 01, all other packets set this field to 11. Reserved: 2 bits This field is reserved for future definition. In the absence of such a definition, the bits in this field MUST be set to zero and MUST be ignored by the receiver. Any padding octets in the original packet MUST NOT be included in the loopback packet generated by a loopback-mirror. The loopback-mirror MAY add padding octets if required. 7.1.3 Usage of SDP The payload type number for the encapsulated stream can be negotiated using a mechanism like SDP. There is no static payload type assignment for the encapsulated stream, so dynamic payload type numbers MUST be used. The binding to the name is indicated by an rtpmap attribute. The name used in this binding is "encaprtp". The following is an example SDP fragment for encapsulated RTP. m=audio 41352 RTP/AVP 112 a=rtpmap:112 encaprtp/8000 7.2 Direct loopback RTP payload format The direct loopback RTP payload format can be used in scenarios where the 16 byte overhead of the encapsulated payload format is significant. This payload format MUST NOT be used in cases where the MTU on the loopback path will cause fragmentation of looped back RTP packets. When using this payload format, the receiver MUST loop back each received packet in a separate RTP packet. 7.2.1 Usage of RTP Header fields Payload Type (PT): The assignment of an RTP payload type for this packet format is outside the scope of this document; it is either specified by the RTP profile under which this payload format is used or more likely signaled dynamically out-of-band (e.g., using SDP; section 7.2.3 defines the name binding). Marker (M) bit: Set to the value in the received packet. Kaplan, et al. Expires August 2012 [Page 15] Internet-Draft SDP Media Loopback March 2012 Extension (X) bit: Defined by the RTP Profile used. Sequence Number: The RTP sequence number SHOULD be generated by the loopback-mirror in the usual manner with a constant random offset. Timestamp: The RTP timestamp denotes the sampling instant for when the loopback-mirror is transmitting this packet to the loopback-source. The RTP timestamp MUST be based on the same clock used by the loopback-source. The initial value of the timestamp SHOULD be random for security reasons (see Section 5.1 of RFC 3550 [RFC3550]). SSRC: set as described in RFC 3550 [RFC3550]. CC and CSRC fields are used as described in RFC 3550 [RFC3550]. 7.2.2 RTP Payload Structure This payload format does not define any payload specific headers. The loopback-mirror simply copies the payload data from the payload portion of the packet received from the loopback-source. 7.2.3 Usage of SDP The payload type number for the payload loopback stream can be negotiated using a mechanism like SDP. There is no static payload type assignment for the stream, so dynamic payload type numbers MUST be used. The binding to the name is indicated by an rtpmap attribute. The name used in this binding is "rtploopback". The following is an example SDP fragment for direct loopback RTP format. m=audio 41352 RTP/AVP 112 a=rtpmap:112 rtploopback/8000 8. RTCP Requirements The use of the loopback attribute is intended for monitoring of media quality of the session. Consequently the media performance information should be exchanged between the offering and the answering entities. An offering or answering entity that is compliant to this specification SHOULD support RTCP per [RFC3550] and RTCP-XR per RFC 3611 [RFC3611]. Furthermore, if the client or Kaplan, et al. Expires August 2012 [Page 16] Internet-Draft SDP Media Loopback March 2012 the server choose to support RTCP-XR, they SHOULD support RTCP-XR Loss RLE report block, Duplicate RLE report block, Statistics Summary report block, and VoIP Metric Reports Block per sections 4.1, 4.2, 4.6, and 4.7 of RFC 3611 [RFC3611]. The client and the server MAY support other RTCP-XR reporting blocks as defined by RFC 3611 [RFC3611]. 9. Congestion Control All the participants in a loopback session SHOULD implement congestion control mechanisms as defined by the RTP profile under which the loopback mechanism is implemented. For audio video profiles, implementations SHOULD conform to the mechanism defined in Section 2 of RFC 3551. 10. Examples This section provides examples for media descriptions using SDP for different scenarios. The examples are given for SIP-based transactions and are abbreviated and do not show the complete signaling for convenience. 10.1 Offer for specific media loopback type An agent sends an SDP offer which looks like: v=0 o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com s=Example i=An example session e=alice@example.com c=IN IP4 host.atlanta.example.com t=0 0 m=audio 49170 RTP/AVP 0 a=loopback:rtp-media-loopback a=loopback-source:0 a=rtpmap:0 pcmu/8000 The agent is offering to source the media and expects the answering agent to mirror the RTP stream per rtp-media-loopback loopback type. An answering agent sends an SDP answer which looks like: v=0 o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com Kaplan, et al. Expires August 2012 [Page 17] Internet-Draft SDP Media Loopback March 2012 s=Example i=An example session e=bob@example.com c=IN IP4 host.biloxi.example.com t=0 0 m=audio 49270 RTP/AVP 0 a=loopback:rtp-media-loopback a=loopback-mirror:0 a=rtpmap:0 pcmu/8000 The answerer is accepting to mirror the media from the offerer at the media level. 10.2 Offer for choice of media loopback type An agent sends an SDP offer which looks like: v=0 o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com s=Example i=An example session e=alice@example.com c=IN IP4 host.atlanta.example.com t=0 0 m=audio 49170 RTP/AVP 0 112 113 a=loopback:rtp-media-loopback rtp-pkt-loopback a=loopback-source:0 a=rtpmap:0 pcmu/8000 a=rtpmap:112 encaprtp/8000 a=rtpmap:113 rtploopback/8000 The offerer is offering to source the media and expects the answerer to mirror the RTP stream at either the media or rtp level. An answering agent sends an SDP answer which looks like: v=0 o=box 1234567890 1122334455 IN IP4 host.biloxi.example.com s=Example i=An example session e=bob@example.com c=IN IP4 host.biloxi.example.com t=0 0 m=audio 49270 RTP/AVP 0 112 a=loopback:rtp-pkt-loopback a=loopback-mirror:0 a=rtpmap:0 pcmu/8000 Kaplan, et al. Expires August 2012 [Page 18] Internet-Draft SDP Media Loopback March 2012 a=rtpmap:112 encaprtp/8000 The answerer is accepting to mirror the media from the offerer at the packet level using the encapsulated RTP payload format. 10.3 Answerer rejecting loopback media An agent sends an SDP offer which looks like: v=0 o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com s=Example i=An example session e=user@example.com c=IN IP4 host.atlanta.example.com t=0 0 m=audio 49170 RTP/AVP 0 a=loopback:rtp-media-loopback a=loopback-source:0 a=rtpmap:0 pcmu/8000 The offerer is offering to source the media and expects the answerer to mirror the RTP stream at the media level. An answering agent sends an SDP answer which looks like: v=0 o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com s=Example i=An example session e=user@example.com c=IN IP4 host.biloxi.example.com t=0 0 m=audio 0 RTP/AVP 0 a=loopback:rtp-media-loopback a=loopback-mirror:0 a=rtpmap:0 pcmu/8000 11. Security Considerations The security considerations of [RFC3264] apply. Furthermore, given that media loopback may be automated without the end user's knowledge, the server of the media loopback should be aware of denial of service attacks. It is recommended that sessions with media loopback are authenticated and the frequency of such sessions is limited by the server. Kaplan, et al. Expires August 2012 [Page 19] Internet-Draft SDP Media Loopback March 2012 12. Implementation Considerations The media loopback approach described in this document is a complete solution that would work under all scenarios. However, it is believed that the solution may not be light-weight enough for the common case. In light of this concern, this section clarifies which features of the loopback proposal MUST be implemented for all implementations and which features MAY be deferred if the complete solution is not desired. All implementations MUST support the rtp-pkt-loopback option for loopback-type attribute. In addition, for the loopback-mode attribute, all implementations of an offerer MUST at a minimum be able to act as a loopback-source. All implementation MUST also at a minimum support the direct media loopback payload type. The rtp- media-loopback attribute MAY be implemented in complete implementations of this draft. 13. IANA Considerations 13.1 SDP Attributes This document defines three new media-level SDP attributes. IANA has registered the following attributes: Contact name: Kaynam Hedayat . Attribute name: "loopback". Type of attribute: Media level. Subject to charset: No. Purpose of attribute: The 'loopback' attribute is used to indicate the type of media loopback. Allowed attribute values: The parameters to 'loopback' may be one or more of "rtp-pkt-loopback" and "rtp-media-loopback". See section 5 of this document for syntax. Contact name: Kaynam Hedayat . Attribute name: "loopback-source". Type of attribute: Media level. Subject to charset: No. Purpose of attribute: The 'loopback-source' attribute specifies that the sender is the media Kaplan, et al. Expires August 2012 [Page 20] Internet-Draft SDP Media Loopback March 2012 source and expects the receiver to act as a loopback-mirror. Allowed attribute values: The parameter to 'loopback-source' is a media format ("") description as defined in RFC 4566 Section 5.14. Contact name: Kaynam Hedayat . Attribute name: "loopback-mirror". Type of attribute: Media level. Subject to charset: No. Purpose of attribute: The 'loopback-mirror' attribute specifies that the receiver will mirror (echo) all received media back to the sender of the RTP stream. Allowed attribute values: The parameter to 'loopback-mirror' is a media format ("") description as defined in RFC 4566 Section 5.14. 13.2 MIME Types The IANA has registered the following MIME types: 13.2.1 audio/encaprtp To: ietf-types@iana.org Subject: Registration of media type audio/encaprtp Type name: audio Subtype name: encaprtp Required parameters: rate:RTP timestamp clock rate, which is equal to the sampling rate. The typical rate is 8000; other rates may be specified. Optional parameters: none Encoding considerations: This media type is framed binary data. Security considerations: See Section 12 of this document. Kaplan, et al. Expires August 2012 [Page 21] Internet-Draft SDP Media Loopback March 2012 Interoperability considerations: none Published specification: This MIME type is described fully within this document. Applications which use this media type: Applications wishing to monitor and ensure the quality of transport to the edge of a given VoIP, Real-Time Text or Video Over IP Service. Additional information: none Person & email address to contact for further information: Kaynam Hedayat EMail: kaynam.hedayat@exfo.com Intended usage: COMMON Restrictions on usage: This media type depends on RTP framing, and hence is only defined for transfer via RTP. Transfer within other framing protocols is not defined at this time. Author: Kaynam Hedayat. Change controller: IETF Audio/Video Transport working group delegated from the IESG. 13.2.2 video/encaprtp To: ietf-types@iana.org Subject: Registration of media type video/encaprtp Type name: video Subtype name: encaprtp Required parameters: rate:RTP timestamp clock rate, which is equal to the sampling rate. The typical rate is 8000; other rates may be specified. Optional parameters: none Kaplan, et al. Expires August 2012 [Page 22] Internet-Draft SDP Media Loopback March 2012 Encoding considerations: This media type is framed binary data. Security considerations: See Section 12 of this document. Interoperability considerations: none Published specification: This MIME type is described fully within this document. Applications which use this media type: Applications wishing to monitor and ensure the quality of transport to the edge of a given VoIP, Real-Time Text or Video Over IP Service. Additional information: none Person & email address to contact for further information: Kaynam Hedayat EMail: kaynam.hedayat@exfo.com Intended usage: COMMON Restrictions on usage: This media type depends on RTP framing, and hence is only defined for transfer via RTP. Transfer within other framing protocols is not defined at this time. Author: Kaynam Hedayat. Change controller: IETF Audio/Video Transport working group delegated from the IESG. 13.2.3 text/encaprtp To: ietf-types@iana.org Subject: Registration of media type text/encaprtp Type name: text Subtype name: encaprtp Required parameters: Kaplan, et al. Expires August 2012 [Page 23] Internet-Draft SDP Media Loopback March 2012 rate:RTP timestamp clock rate, which is equal to the sampling rate. The typical rate is 8000; other rates may be specified. Optional parameters: none Encoding considerations: This media type is framed binary data. Security considerations: See Section 12 of this document. Interoperability considerations: none Published specification: This MIME type is described fully within this document. Applications which use this media type: Applications wishing to monitor and ensure the quality of transport to the edge of a given VoIP, Real-Time Text or Video Over IP Service. Additional information: none Person & email address to contact for further information: Kaynam Hedayat EMail: kaynam.hedayat@exfo.com Intended usage: COMMON Restrictions on usage: This media type depends on RTP framing, and hence is only defined for transfer via RTP. Transfer within other framing protocols is not defined at this time. Author: Kaynam Hedayat. Change controller: IETF Audio/Video Transport working group delegated from the IESG. 13.2.4 application/encaprtp To: ietf-types@iana.org Subject: Registration of media type application/encaprtp Kaplan, et al. Expires August 2012 [Page 24] Internet-Draft SDP Media Loopback March 2012 Type name: application Subtype name: encaprtp Required parameters: rate:RTP timestamp clock rate, which is equal to the sampling rate. The typical rate is 8000; other rates may be specified. Optional parameters: none Encoding considerations: This media type is framed binary data. Security considerations: See Section 12 of this document. Interoperability considerations: none Published specification: This MIME type is described fully within this document. Applications which use this media type: Applications wishing to monitor and ensure the quality of transport to the edge of a given VoIP, Real-Time Text or Video Over IP Service. Additional information: none Person & email address to contact for further information: Kaynam Hedayat EMail: kaynam.hedayat@exfo.com Intended usage: COMMON Restrictions on usage: This media type depends on RTP framing, and hence is only defined for transfer via RTP. Transfer within other framing protocols is not defined at this time. Author: Kaynam Hedayat. Change controller: IETF Audio/Video Transport working group delegated from the IESG. 13.2.5 audio/rtploopback Kaplan, et al. Expires August 2012 [Page 25] Internet-Draft SDP Media Loopback March 2012 To: ietf-types@iana.org Subject: Registration of media type audio/rtploopback Type name: audio Subtype name: rtploopback Required parameters: rate:RTP timestamp clock rate, which is equal to the sampling rate. The typical rate is 8000; other rates may be specified. Optional parameters: none Encoding considerations: This media type is framed binary data. Security considerations: See Section 12 of this document. Interoperability considerations: none Published specification: This MIME type is described fully within this document. Applications which use this media type: Applications wishing to monitor and ensure the quality of transport to the edge of a given VoIP, Real-Time Text or Video Over IP Service. Additional information: none Person & email address to contact for further information: Kaynam Hedayat EMail: kaynam.hedayat@exfo.com Intended usage: COMMON Restrictions on usage: This media type depends on RTP framing, and hence is only defined for transfer via RTP. Transfer within other framing protocols is not defined at this time. Author: Kaynam Hedayat. Change controller: IETF Audio/Video Transport working Kaplan, et al. Expires August 2012 [Page 26] Internet-Draft SDP Media Loopback March 2012 group delegated from the IESG. 13.2.6 video/rtploopback To: ietf-types@iana.org Subject: Registration of media type video/rtploopback Type name: video Subtype name: rtploopback Required parameters: rate:RTP timestamp clock rate, which is equal to the sampling rate. The typical rate is 8000; other rates may be specified. Optional parameters: none Encoding considerations: This media type is framed binary data. Security considerations: See Section 12 of this document. Interoperability considerations: none Published specification: This MIME type is described fully within this document. Applications which use this media type: Applications wishing to monitor and ensure the quality of transport to the edge of a given VoIP, Real-Time Text or Video Over IP Service. Additional information: none Person & email address to contact for further information: Kaynam Hedayat EMail: kaynam.hedayat@exfo.com Intended usage: COMMON Restrictions on usage: This media type depends on RTP framing, and hence is only defined for transfer via RTP. Transfer within other framing protocols is not defined at this time. Kaplan, et al. Expires August 2012 [Page 27] Internet-Draft SDP Media Loopback March 2012 Author: Kaynam Hedayat. Change controller: IETF Audio/Video Transport working group delegated from the IESG. 13.2.7 text/rtploopback To: ietf-types@iana.org Subject: Registration of media type text/rtploopback Type name: text Subtype name: rtploopback Required parameters: rate:RTP timestamp clock rate, which is equal to the sampling rate. The typical rate is 8000; other rates may be specified. Optional parameters: none Encoding considerations: This media type is framed binary data. Security considerations: See Section 12 of this document. Interoperability considerations: none Published specification: This MIME type is described fully within this document. Applications which use this media type: Applications wishing to monitor and ensure the quality of transport to the edge of a given VoIP, Real-Time Text or Video Over IP Service. Additional information: none Person & email address to contact for further information: Kaynam Hedayat EMail: kaynam.hedayat@exfo.com Intended usage: COMMON Kaplan, et al. Expires August 2012 [Page 28] Internet-Draft SDP Media Loopback March 2012 Restrictions on usage: This media type depends on RTP framing, and hence is only defined for transfer via RTP. Transfer within other framing protocols is not defined at this time. Author: Kaynam Hedayat. Change controller: IETF Audio/Video Transport working group delegated from the IESG. 13.2.8 application/rtploopback To: ietf-types@iana.org Subject: Registration of media type application/rtploopback Type name: application Subtype name: rtploopback Required parameters: rate:RTP timestamp clock rate, which is equal to the sampling rate. The typical rate is 8000; other rates may be specified. Optional parameters: none Encoding considerations: This media type is framed binary data. Security considerations: See Section 12 of this document. Interoperability considerations: none Published specification: This MIME type is described fully within this document. Applications which use this media type: Applications wishing to monitor and ensure the quality of transport to the edge of a given VoIP, Real-Time Text or Video Over IP Service. Additional information: none Person & email address to contact for further information: Kaplan, et al. Expires August 2012 [Page 29] Internet-Draft SDP Media Loopback March 2012 Kaynam Hedayat EMail: kaynam.hedayat@exfo.com Intended usage: COMMON Restrictions on usage: This media type depends on RTP framing, and hence is only defined for transfer via RTP. Transfer within other framing protocols is not defined at this time. Author: Kaynam Hedayat. Change controller: IETF Audio/Video Transport working group delegated from the IESG. 14. Normative References [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with the Session Description Protocol (SDP)", RFC 3264, June 2002. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC3611] Almeroth, K., Caceres, R., Clark, A., Cole, R., Duffield, N., Friedman, T., Hedayat, K., Sarac, K. and M. Westerlund, "RTP Control Protocol Extended Reports (RTCP XR)", RFC 3611, November 2003. [RFC5234] Crocker, P. Overell, "Augmented ABNF for Syntax Specification: ABNF", RFC 5234, October 2005. [RFC2119] Bradner, S.,"Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2736] Handley, M., Perkins, C., "Guidelines for Writers of RTP Payload Format Specifications", RFC 2736, BCP 0036, December 1999. Kaplan, et al. Expires August 2012 [Page 30] Internet-Draft SDP Media Loopback March 2012 [RFC3551] Schulzrinne, H., Casner, S., "RTP Profile for Audio and Video Conferences with Minimial Control", STD 65, RFC 3551, July 2003. [RFC4566] Handley, M., Jacobson, V., Perkins, C., "SDP: Session Description Protocol", RFC 4566, July 2006. [RFC4855] Casner, S., "Media Type Registration of RTP Payload Formats", RFC 4855, February 2007. Authors' Addresses Hadriel Kaplan Acme Packet 100 Crosby Drive Bedford, MA 01730 USA EMail: hkaplan@acmepacket.com URI: http://www.acmepacket.com Kaynam Hedayat EXFO 285 Mill Road Chelmsford, MA 01824 US Phone: +1 978 367 5611 EMail: kaynam.hedayat@exfo.com URI: http://www.exfo.com/ Nagarjuna Venna Saperix 738 Main Street, #398 Waltham, MA 02451 US Phone: +1 978 367 5703 EMail: vnagarjuna@saperix.com URI: http://www.saperix.com/ Paul E. Jones Kaplan, et al. Expires August 2012 [Page 31] Internet-Draft SDP Media Loopback March 2012 Cisco Systems, Inc. 7025 Kit Creek Rd. Research Triangle Park, NC 27709 US Phone: +1 919 392 6948 EMail: paulej@packetizer.com URI: http://www.cisco.com/ Arjun Roychowdhury Hughes Systique Corp. 15245 Shady Grove Rd, Ste 330 Rockville MD 20850 US Phone: +1 301 527 1629 EMail: arjun@hsc.com URI: http://www. hsc.com/ Chelliah SivaChelvan Cisco Systems, Inc. 2200 East President George Bush Turnpike Richardson, TX 75082 US Phone: +1 972 813 5224 EMail: chelliah@cisco.com URI: http://www.cisco.com/ Nathan Stratton BlinkMind, Inc. 2027 Briarchester Dr. Katy, TX 77450 Phone: +1 832 330 3810 EMail: nathan@robotics.net URI: http://www.robotics.net/ Kaplan, et al. Expires August 2012 [Page 32]