RTP Payload Format for Flexible Forward Error Correction (FEC)

This document defines new RTP payload formats for the Forward Error Correction (FEC) that is generated by the non-interleaved and interleaved parity codes from a source media encapsulated in RTP . These payload formats may also be used for other types of FEC codes. The type of the source media protected by these parity codes can be audio, video, text or application. The FEC data are generated according to the media type parameters, which are communicated out-of-band (e.g., in SDP). Furthermore, the associations or relationships between the source and repair flows may be communicated in-band or out-of-band. Situations where adaptivitiy of FEC parameters is desired, the endpoint can use the in-band mechanism, whereas when the FEC parameters are fixed, the endpoint may prefer to negotiate them out-of-band. The repair packets proposed in this document protect the source stream packets that belong to the same RTP session.

Both the non-interleaved and interleaved parity codes use the eXclusive OR (XOR) operation to generate the repair symbols. In a nutshell, the following steps take place: The sender determines a set of source packets to be protected by FEC based on the media type parameters. The sender applies the XOR operation on the source symbols to generate the required number of repair symbols. The sender packetizes the repair symbols and sends the repair packet(s) along with the source packets to the receiver(s) (in different flows). The repair packets may be sent proactively or on-demand. Note that the source and repair packets belong to different source and repair flows, and the sender must provide a way for the receivers to demultiplex them, even in the case they are sent in the same 5-tuple (i.e., same source/destination address/port with UDP). This is required to offer backward compatibility for endpoints that do not understand the FEC packets (See ). At the receiver side, if all of the source packets are successfully received, there is no need for FEC recovery and the repair packets are discarded. However, if there are missing source packets, the repair packets can be used to recover the missing information. and describe example block diagrams for the systematic parity FEC encoder and decoder, respectively.

| Systematic | --> +--+ +--+ +--+ +--+ +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+ | Decoder | +==+ +==+ --> | (Receiver) | +==+ +==+ +------------+ Source Packet: +--+ Repair Packet: +==+ Lost Packet: X +--+ +==+]]> In , it is clear that the FEC packets have to be received by the endpoint within a certain amount of time for the FEC recovery process to be useful. In this document, we refer to the time that spans a FEC block, which consists of the source packets and the corresponding repair packets, as the repair window. At the receiver side, the FEC decoder should wait at least for the duration of the repair window after getting the first packet in a FEC block, to allow all the repair packets to arrive. (The waiting time can be adjusted if there are missing packets at the beginning of the FEC block.) The FEC decoder can start decoding the already received packets sooner; however, it should not register a FEC decoding failure until it waits at least for the duration of the repair window. Suppose that we have a group of D x L source packets that have sequence numbers starting from 1 running to D x L, and a repair packet is generated by applying the XOR operation to every L consecutive packets as sketched in . This process is referred to as 1-D non-interleaved FEC protection. As a result of this process, D repair packets are generated, which we refer to as non-interleaved (or row) FEC packets.

If we apply the XOR operation to the group of the source packets whose sequence numbers are L apart from each other, as sketched in . In this case the endpoint generates L repair packets. This process is referred to as 1-D interleaved FEC protection, and the resulting L repair packets are referred to as interleaved (or column) FEC packets.

We generate one non-interleaved repair packet out of L consecutive source packets or one interleaved repair packet out of D non-consecutive source packets. Regardless of whether the repair packet is a non-interleaved or an interleaved one, it can provide a full recovery of the missing information if there is only one packet missing among the corresponding source packets. This implies that 1-D non-interleaved FEC protection performs better when the source packets are randomly lost. However, if the packet losses occur in bursts, 1-D interleaved FEC protection performs better provided that L is chosen large enough, i.e., L-packet duration is not shorter than the observed burst duration. If the sender generates non-interleaved FEC packets and a burst loss hits the source packets, the repair operation fails. This is illustrated in .

The sender may generate interleaved FEC packets to combat with the bursty packet losses. However, two or more random packet losses may hit the source and repair packets in the same column. In that case, the repair operation fails as well. This is illustrated in . Note that it is possible that two burst losses may occur back-to-back, in which case interleaved FEC packets may still fail to recover the lost data.

In networks where the source packets are lost both randomly and in bursts, the sender ought to generate both non-interleaved and interleaved FEC packets. This type of FEC protection is known as 2-D parity FEC protection. At the expense of generating more FEC packets, thus increasing the FEC overhead, 2-D FEC provides superior protection against mixed loss patterns. However, it is still possible for 2-D parity FEC protection to fail to recover all of the lost source packets if a particular loss pattern occurs. An example scenario is illustrated in .

2-D parity FEC protection also fails when at least two rows are missing a source and the FEC packet and the missing source packets (in at least two rows) are aligned in the same column. An example loss pattern is sketched in . Similarly, 2-D parity FEC protection cannot repair all missing source packets when at least two columns are missing a source and the FEC packet and the missing source packets (in at least two columns) are aligned in the same row.

The overhead is defined as the ratio of the number of bytes belonging to the repair packets to the number of bytes belonging to the protected source packets. Generally, repair packets are larger in size compared to the source packets. Also, not all the source packets are necessarily equal in size. However, if we assume that each repair packet carries an equal number of bytes carried by a source packet, we can compute the overhead for different FEC protection methods as follows: 1-D Non-interleaved FEC Protection: Overhead = 1/L 1-D Interleaved FEC Protection: Overhead = 1/D 2-D Parity FEC Protection: Overhead = 1/L + 1/D where L and D are the number of columns and rows in the source block, respectively.

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 .

This document uses a number of definitions from .

L: Number of columns of the source block. D: Number of rows of the source block. bitmask: Run-length encoding of packets protected by a FEC packet. If the bit i in the mask is set to 1, the source packet number N + i is protected by this FEC packet. Here, N is the sequence number base, which is indicated in the FEC packet as well.

This section defines the formats of the source and repair packets.

The source packets MUST contain the information that identifies the source block and the position within the source block occupied by the packet. Since the source packets that are carried within an RTP stream already contain unique sequence numbers in their RTP headers , we can identify the source packets in a straightforward manner and there is no need to append additional field(s). The primary advantage of not modifying the source packets in any way is that it provides backward compatibility for the receivers that do not support FEC at all. In multicast scenarios, this backward compatibility becomes quite useful as it allows the non-FEC-capable and FEC-capable receivers to receive and interpret the same source packets sent in the same multicast session.

The repair packets MUST contain information that identifies the source block they pertain to and the relationship between the contained repair symbols and the original source block. For this purpose, we use the RTP header of the repair packets as well as another header within the RTP payload, which we refer to as the FEC header, as shown in . Note that all the source stream packets that are protected by a particular FEC packet need to be in the same RTP session.

RTP Payload | Repair Symbols | / +------------------------------+ __|]]> The RTP header is formatted according to with some further clarifications listed below: Marker (M) Bit: This bit is not used for this payload type, and SHALL be set to 0. Payload Type: The (dynamic) payload type for the repair packets is determined through out-of-band means. Note that this document registers new payload formats for the repair packets (Refer to for details). According to , an RTP receiver that cannot recognize a payload type must discard it. This provides backward compatibility. If a non-FEC-capable receiver receives a repair packet, it will not recognize the payload type, and hence, will discard the repair packet. Sequence Number (SN): The sequence number has the standard definition. It MUST be one higher than the sequence number in the previously transmitted repair packet. The initial value of the sequence number SHOULD be random (unpredictable, based on ). Timestamp (TS): The timestamp SHALL be set to a time corresponding to the repair packet's transmission time. Note that the timestamp value has no use in the actual FEC protection process and is usually useful for jitter calculations. Synchronization Source (SSRC): The SSRC value SHALL be randomly assigned as suggested by . This allows the sender to multiplex the source and repair flows on the same port, or multiplex multiple repair flows on a single port. The repair flows SHOULD use the RTCP CNAME field to associate themselves with the source flow. In some networks, the RTP Source, which produces the source packets and the FEC Source, which generates the repair packets from the source packets may not be the same host. In such scenarios, using the same CNAME for the source and repair flows means that the RTP Source and the FEC Source MUST share the same CNAME (for this specific source-repair flow association). A common CNAME may be produced based on an algorithm that is known both to the RTP and FEC Source . This usage is compliant with . Note that due to the randomness of the SSRC assignments, there is a possibility of SSRC collision. In such cases, the collisions MUST be resolved as described in . The format of the FEC header is shown in .

The FEC header consists of the following fields: The R bit MUST be set to 1 to indicate a retransmission packet, and MUST be set to 0 for repair packets. The F field (1 bit) indicates the type of the mask. Namely:

The P, X, CC, M and PT recovery fields are used to determine the corresponding fields of the recovered packets. The Length recovery (16 bits) field is used to determine the length of the recovered packets. The TS recovery (32 bits) field is used to determine the timestamp of the recovered packets. The SSRC count (8 bits) field describes the number of SSRCs protected by the FEC packet. 0 is not a valid value, and the packet MUST be ignored. The Reserved (24 bits) field are reserved for future use. It MUST be set to zero by senders and ignored by receivers (see , Section 4.2). The SSRC_i (32 bits) field describes the SSRC of the packets protected by this particular FEC packet. If a FEC packet contains protects multiple SSRCs (indicated by the SSRC Count > 1), there will be multiple blocks of data containing the SSRC, SN base and Mask fields. The SN base_i (16 bits) field indicates the lowest sequence number, taking wrap around into account, of the source packets for a particular SSSRC (indicated in SSRC_i) protected by this repair packet. If the F-bit is set to 0, it represents that the source packets of all the SSRCs protected by this particular repair packet are indicated by using a flexible bitmask. Mask is a run-length encoding of packets for a particular SSRC_i protected by the FEC packet. Where a bit j set to 1 indicates that the source packet with sequence number (SN base_i + j + 1) is protected by this FEC packet. The k-bit in the bitmasks indicates if it is 15-, 46-, or a 109-bitmask. k=0 denotes that there is one more k-bit set, and k=1 denotes that it is the last block of bit mask. While parsing the header, the current count of the number of k-bit gives the size of the bit mask v = count(k). Size of next bitmask = 2^(v+3)-1. Editor's note: If we limit the number of k-bits to 3, we could essentially remove the last k-bit.

If the F-bit is set to 1, it represents that the source packets of all the SSRCs protected by this particular repair packet are indicated by using fixed offsets.

Consequently, the following conditions occur for M and N values:

0, N=0, is Row FEC, and no column FEC will follow Hence, FEC = SN, SN+1, SN+2, ... , SN+(M-1), SN+M. If M>0, N=1, is Row FEC, and column FEC will follow. Hence, FEC = SN, SN+1, SN+2, ... , SN+(M-1), SN+M. and more to come If M>0, N>1, indicates column FEC of every M packet in a group of N packets starting at SN base. Hence, FEC = SN+(Mx0), SN+(Mx1), ... , SN+(MxN). ]]> By setting R to 1, F to 1, this FEC protects only one packet, i.e., the FEC payload carries just the packet indicated by SN Base_i, which is effectively retransmitting the packet. Note that the parsing of this packet is different. The sequence number (SN base_i) replaces the length recovery in the FEC packet. The SSRC_count which would be 1, M and N would be set to 0, and the reserved bits from the FEC header are removed. By doing this, we save 64 bits.

The details on setting the fields in the FEC header are provided in . It should be noted that a mask-based approach (similar to the ones specified in and ) may not be very efficient to indicate which source packets in the current source block are associated with a given repair packet. In particular, for the applications that would like to use large source block sizes, the size of the mask that is required to describe the source-repair packet associations may be prohibitively large. The 8-bit fields proposed in indicate a systematized approach. Instead the approach in this document uses the 8-bit fields to indicate packet offsets protected by the FEC packet. The approach in is inherently more efficient for regular patterns, it does not provide flexibility to represent other protection patterns (e.g., staircase).

This section provides the media subtype registration for the non-interleaved and interleaved parity FEC. The parameters that are required to configure the FEC encoding and decoding operations are also defined in this section. If no specific FEC code is specified in the subtype, then the FEC code defaults to the parity code defined in this specification.

This registration is done using the template defined in and following the guidance provided in . Note to the RFC Editor: In the following sections, please replace "XXXX" with the number of this document prior to publication as an RFC.

Type name: audio Subtype name: flexfec Required parameters: rate: The RTP timestamp (clock) rate. The rate SHALL be larger than 1000 Hz to provide sufficient resolution to RTCP operations. However, it is RECOMMENDED to select the rate that matches the rate of the protected source RTP stream. repair-window: The time that spans the source packets and the corresponding repair packets. The size of the repair window is specified in microseconds. Optional parameters: L: indicates the number of columns of the source block that are protected by this FEC block and it applies to all the source SSRCs. L is a positive integer. D: indicates the number of rows of the source block that are protected by this FEC block and it applies to all the source SSRCs. D is a positive integer. ToP: indicates the type of protection applied by the sender: 0 for 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC protection, and 2 for 2-D parity FEC protection. The ToP value of 3 is reserved for future uses. Encoding considerations: This media type is framed (See Section 4.8 in the template document ) and contains binary data. Security considerations: See of [RFCXXXX]. Interoperability considerations: None. Published specification: [RFCXXXX]. Applications that use this media type: Multimedia applications that want to improve resiliency against packet loss by sending redundant data in addition to the source media. Fragment identifier considerations: None. Additional information: None. Person & email address to contact for further information: Varun Singh <varun@callstats.io> and IETF Audio/Video Transport Payloads Working Group. Intended usage: COMMON. Restriction on usage: This media type depends on RTP framing, and hence, is only defined for transport via RTP . Author: Varun Singh <varun@callstats.io>. Change controller: IETF Audio/Video Transport Working Group delegated from the IESG. Provisional registration? (standards tree only): Yes.

Type name: video Subtype name: flexfec Required parameters: rate: The RTP timestamp (clock) rate. The rate SHALL be larger than 1000 Hz to provide sufficient resolution to RTCP operations. However, it is RECOMMENDED to select the rate that matches the rate of the protected source RTP stream. repair-window: The time that spans the source packets and the corresponding repair packets. The size of the repair window is specified in microseconds. Optional parameters: L: indicates the number of columns of the source block that are protected by this FEC block and it applies to all the source SSRCs. L is a positive integer. D: indicates the number of rows of the source block that are protected by this FEC block and it applies to all the source SSRCs. D is a positive integer. ToP: indicates the type of protection applied by the sender: 0 for 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC protection, and 2 for 2-D parity FEC protection. The ToP value of 3 is reserved for future uses. Encoding considerations: This media type is framed (See Section 4.8 in the template document ) and contains binary data. Security considerations: See of [RFCXXXX]. Interoperability considerations: None. Published specification: [RFCXXXX]. Applications that use this media type: Multimedia applications that want to improve resiliency against packet loss by sending redundant data in addition to the source media. Fragment identifier considerations: None. Additional information: None. Person & email address to contact for further information: Varun Singh <varun@callstats.io> and IETF Audio/Video Transport Payloads Working Group. Intended usage: COMMON. Restriction on usage: This media type depends on RTP framing, and hence, is only defined for transport via RTP . Author: Varun Singh <varun@callstats.io>. Change controller: IETF Audio/Video Transport Working Group delegated from the IESG. Provisional registration? (standards tree only): Yes.

Type name: text Subtype name: flexfec Required parameters: rate: The RTP timestamp (clock) rate. The rate SHALL be larger than 1000 Hz to provide sufficient resolution to RTCP operations. However, it is RECOMMENDED to select the rate that matches the rate of the protected source RTP stream. repair-window: The time that spans the source packets and the corresponding repair packets. The size of the repair window is specified in microseconds. Optional parameters: L: indicates the number of columns of the source block that are protected by this FEC block and it applies to all the source SSRCs. L is a positive integer. D: indicates the number of rows of the source block that are protected by this FEC block and it applies to all the source SSRCs. D is a positive integer. ToP: indicates the type of protection applied by the sender: 0 for 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC protection, and 2 for 2-D parity FEC protection. The ToP value of 3 is reserved for future uses. Encoding considerations: This media type is framed (See Section 4.8 in the template document ) and contains binary data. Security considerations: See of [RFCXXXX]. Interoperability considerations: None. Published specification: [RFCXXXX]. Applications that use this media type: Multimedia applications that want to improve resiliency against packet loss by sending redundant data in addition to the source media. Fragment identifier considerations: None. Additional information: None. Person & email address to contact for further information: Varun Singh <vvarun@callstats.io> and IETF Audio/Video Transport Payloads Working Group. Intended usage: COMMON. Restriction on usage: This media type depends on RTP framing, and hence, is only defined for transport via RTP . Author: Varun Singh <varun@callstats.io>. Change controller: IETF Audio/Video Transport Working Group delegated from the IESG. Provisional registration? (standards tree only): Yes.

Type name: application Subtype name: flexfec Required parameters: rate: The RTP timestamp (clock) rate. The rate SHALL be larger than 1000 Hz to provide sufficient resolution to RTCP operations. However, it is RECOMMENDED to select the rate that matches the rate of the protected source RTP stream. repair-window: The time that spans the source packets and the corresponding repair packets. The size of the repair window is specified in microseconds. Optional parameters: L: indicates the number of columns of the source block that are protected by this FEC block and it applies to all the source SSRCs. L is a positive integer. D: indicates the number of rows of the source block that are protected by this FEC block and it applies to all the source SSRCs. D is a positive integer. ToP: indicates the type of protection applied by the sender: 0 for 1-D interleaved FEC protection, 1 for 1-D non-interleaved FEC protection, and 2 for 2-D parity FEC protection. The ToP value of 3 is reserved for future uses. Encoding considerations: This media type is framed (See Section 4.8 in the template document ) and contains binary data. Security considerations: See of [RFCXXXX]. Interoperability considerations: None. Published specification: [RFCXXXX]. Applications that use this media type: Multimedia applications that want to improve resiliency against packet loss by sending redundant data in addition to the source media. Fragment identifier considerations: None. Additional information: None. Person & email address to contact for further information: Varun Singh <varun@callstats.io> and IETF Audio/Video Transport Payloads Working Group. Intended usage: COMMON. Restriction on usage: This media type depends on RTP framing, and hence, is only defined for transport via RTP . Author: Varun Singh <varun@callstats.io>. Change controller: IETF Audio/Video Transport Working Group delegated from the IESG. Provisional registration? (standards tree only): Yes.

Applications that are using RTP transport commonly use Session Description Protocol (SDP) to describe their RTP sessions. The information that is used to specify the media types in an RTP session has specific mappings to the fields in an SDP description. In this section, we provide these mappings for the media subtypes registered by this document. Note that if an application does not use SDP to describe the RTP sessions, an appropriate mapping must be defined and used to specify the media types and their parameters for the control/description protocol employed by the application. The mapping of the media type specification for "non-interleaved-parityfec" and "interleaved-parityfec" and their parameters in SDP is as follows: The media type (e.g., "application") goes into the "m=" line as the media name. The media subtype goes into the "a=rtpmap" line as the encoding name. The RTP clock rate parameter ("rate") also goes into the "a=rtpmap" line as the clock rate. The remaining required payload-format-specific parameters go into the "a=fmtp" line by copying them directly from the media type string as a semicolon-separated list of parameter=value pairs. SDP examples are provided in .

When offering 1-D interleaved parity FEC over RTP using SDP in an Offer/Answer model , the following considerations apply: Each combination of the L and D parameters produces a different FEC data and is not compatible with any other combination. A sender application may desire to offer multiple offers with different sets of L and D values as long as the parameter values are valid. The receiver SHOULD normally choose the offer that has a sufficient amount of interleaving. If multiple such offers exist, the receiver may choose the offer that has the lowest overhead or the one that requires the smallest amount of buffering. The selection depends on the application requirements. The value for the repair-window parameter depends on the L and D values and cannot be chosen arbitrarily. More specifically, L and D values determine the lower limit for the repair-window size. The upper limit of the repair-window size does not depend on the L and D values. Although combinations with the same L and D values but with different repair-window sizes produce the same FEC data, such combinations are still considered different offers. The size of the repair-window is related to the maximum delay between the transmission of a source packet and the associated repair packet. This directly impacts the buffering requirement on the receiver side and the receiver must consider this when choosing an offer. There are no optional format parameters defined for this payload. Any unknown option in the offer MUST be ignored and deleted from the answer. If FEC is not desired by the receiver, it can be deleted from the answer.

In declarative usage, like SDP in the Real-time Streaming Protocol (RTSP) or the Session Announcement Protocol (SAP) , the following considerations apply: The payload format configuration parameters are all declarative and a participant MUST use the configuration that is provided for the session. More than one configuration may be provided (if desired) by declaring multiple RTP payload types. In that case, the receivers should choose the repair flow that is best for them.

This section provides a complete specification of the 1-D and 2-D parity codes and their RTP payload formats.

The following sections specify the steps involved in generating the repair packets and reconstructing the missing source packets from the repair packets.

The RTP header of a repair packet is formed based on the guidelines given in . The FEC header includes 12 octets (or upto 28 octets when the longer optional masks are used). It is constructed by applying the XOR operation on the bit strings that are generated from the individual source packets protected by this particular repair packet. The set of the source packets that are associated with a given repair packet can be computed by the formula given in . The bit string is formed for each source packet by concatenating the following fields together in the order specified: The first 64 bits of the RTP header (64 bits). Unsigned network-ordered 16-bit representation of the source packet length in bytes minus 12 (for the fixed RTP header), i.e., the sum of the lengths of all the following if present: the CSRC list, extension header, RTP payload and RTP padding (16 bits). By applying the parity operation on the bit strings produced from the source packets, we generate the FEC bit string. The FEC header is generated from the FEC bit string as follows: The first (most significant) 2 bits in the FEC bit string are skipped. The MSK bits in the FEC header are set to the appropriate value, i.e., it depends on the chosen bitmask length. The next bit in the FEC bit string is written into the P recovery bit in the FEC header. The next bit in the FEC bit string is written into the X recovery bit in the FEC header. The next 4 bits of the FEC bit string are written into the CC recovery field in the FEC header. The next bit is written into the M recovery bit in the FEC header. The next 7 bits of the FEC bit string are written into the PT recovery field in the FEC header. The next 16 bits are skipped. The next 32 bits of the FEC bit string are written into the TS recovery field in the FEC header. The next 16 bits are written into the length recovery field in the FEC header. Depending on the chosen MSK value, the bit mask of appropriate length will be set to the appropriate values. As described in , the SN base field of the FEC header MUST be set to the lowest sequence number of the source packets protected by this repair packet. When MSK represents a bitmask (MSK=00,01,10), the SN base field corresponds to the lowest sequence number indicated in the bitmask. When MSK=11, the following considerations apply: 1) for the interleaved FEC packets, this corresponds to the lowest sequence number of the source packets that forms the column, 2) for the non-interleaved FEC packets, the SN base field MUST be set to the lowest sequence number of the source packets that forms the row. The repair packet payload consists of the bits that are generated by applying the XOR operation on the payloads of the source RTP packets. If the payload lengths of the source packets are not equal, each shorter packet MUST be padded to the length of the longest packet by adding octet 0's at the end. Due to this possible padding and mandatory FEC header, a repair packet has a larger size than the source packets it protects. This may cause problems if the resulting repair packet size exceeds the Maximum Transmission Unit (MTU) size of the path over which the repair flow is sent.

This section describes the recovery procedures that are required to reconstruct the missing source packets. The recovery process has two steps. In the first step, the FEC decoder determines which source and repair packets should be used in order to recover a missing packet. In the second step, the decoder recovers the missing packet, which consists of an RTP header and RTP payload. In the following, we describe the RECOMMENDED algorithms for the first and second steps. Based on the implementation, different algorithms MAY be adopted. However, the end result MUST be identical to the one produced by the algorithms described below. Note that the same algorithms are used by the 1-D parity codes, regardless of whether the FEC protection is applied over a column or a row. The 2-D parity codes, on the other hand, usually require multiple iterations of the procedures described here. This iterative decoding algorithm is further explained in .

We denote the set of the source packets associated with repair packet p* by set T(p*). Note that in a source block whose size is L columns by D rows, set T includes D source packets plus one repair packet for the FEC protection applied over a column, and L source packets plus one repair packet for the FEC protection applied over a row. Recall that 1-D interleaved and non-interleaved FEC protection can fully recover the missing information if there is only one source packet missing in set T. If there are more than one source packets missing in set T, 1-D FEC protection will not work.

The first step is associating the source and repair packets. If the endpoint relies entirely on out-of-band signaling (MSK=11, and M=N=0), then this information may be inferred from the media type parameters specified in the SDP description. Furthermore, the payload type field in the RTP header, assists the receiver distinguish an interleaved or non-interleaved FEC packet. Mathematically, for any received repair packet, p*, we can determine the sequence numbers of the source packets that are protected by this repair packet as follows:

where p*_snb denotes the value in the SN base field of p*'s FEC header, X_1 is set to L and 1 for the interleaved and non-interleaved FEC packets, respectively, and

where X_2 is set to D and L for the interleaved and non-interleaved FEC packets, respectively.

When using fixed size bitmasks (16-, 48-, 112-bits), the SN base field in the FEC header indicates the lowest sequence number of the source packets that forms the FEC packet. Finally, the bits maked by "1" in the bitmask are offsets from the SN base and make up the rest of the packets protected by the FEC packet. The bitmasks are able to represent arbitrary protection patterns, for example, 1-D interleaved, 1-D non-interleaved, 2-D, staircase.

When value of M is non-zero, the 8-bit fields indicate the offset of packets protected by an interleaved (N>0) or non-interleaved (N=0) FEC packet. Using a combination of interleaved and non-interleaved FEC packets can form 2-D protection patterns. Mathematically, for any received repair packet, p*, we can determine the sequence numbers of the source packets that are protected by this repair packet are as follows:

0: p*_snb, p*_snb+(Mx1), p*_snb+(Mx2),..., p*_snb+(Mx(N-1)), p*_snb+(MxN)]]>

For a given set T, the procedure for the recovery of the RTP header of the missing packet, whose sequence number is denoted by SEQNUM, is as follows: For each of the source packets that are successfully received in T, compute the 80-bit string by concatenating the first 64 bits of their RTP header and the unsigned network-ordered 16-bit representation of their length in bytes minus 12. For the repair packet in T, compute the FEC bit string from the first 80 bits of the FEC header. Calculate the recovered bit string as the XOR of the bit strings generated from all source packets in T and the FEC bit string generated from the repair packet in T. Create a new packet with the standard 12-byte RTP header and no payload. Set the version of the new packet to 2. Skip the first 2 bits in the recovered bit string. Set the Padding bit in the new packet to the next bit in the recovered bit string. Set the Extension bit in the new packet to the next bit in the recovered bit string. Set the CC field to the next 4 bits in the recovered bit string. Set the Marker bit in the new packet to the next bit in the recovered bit string. Set the Payload type in the new packet to the next 7 bits in the recovered bit string. Set the SN field in the new packet to SEQNUM. Skip the next 16 bits in the recovered bit string. Set the TS field in the new packet to the next 32 bits in the recovered bit string. Take the next 16 bits of the recovered bit string and set the new variable Y to whatever unsigned integer this represents (assuming network order). Convert Y to host order. Y represents the length of the new packet in bytes minus 12 (for the fixed RTP header), i.e., the sum of the lengths of all the following if present: the CSRC list, header extension, RTP payload and RTP padding. Set the SSRC of the new packet to the SSRC of the source RTP stream. This procedure recovers the header of an RTP packet up to (and including) the SSRC field.

Following the recovery of the RTP header, the procedure for the recovery of the RTP payload is as follows: Append Y bytes to the new packet. For each of the source packets that are successfully received in T, compute the bit string from the Y octets of data starting with the 13th octet of the packet. If any of the bit strings generated from the source packets has a length shorter than Y, pad them to that length. The padding of octet 0 MUST be added at the end of the bit string. Note that the information of the first 8 octets are protected by the FEC header. For the repair packet in T, compute the FEC bit string from the repair packet payload, i.e., the Y octets of data following the FEC header. Note that the FEC header may be 12, 16, 32 octets depending on the length of the bitmask. Calculate the recovered bit string as the XOR of the bit strings generated from all source packets in T and the FEC bit string generated from the repair packet in T. Append the recovered bit string (Y octets) to the new packet generated in .

In 2-D parity FEC protection, the sender generates both non-interleaved and interleaved FEC packets to combat with the mixed loss patterns (random and bursty). At the receiver side, these FEC packets are used iteratively to overcome the shortcomings of the 1-D non-interleaved/interleaved FEC protection and improve the chances of full error recovery. The iterative decoding algorithm runs as follows: Set num_recovered_until_this_iteration to zero Set num_recovered_so_far to zero Recover as many source packets as possible by using the non-interleaved FEC packets as outlined in and , and increase the value of num_recovered_so_far by the number of recovered source packets. Recover as many source packets as possible by using the interleaved FEC packets as outlined in and , and increase the value of num_recovered_so_far by the number of recovered source packets. If num_recovered_so_far > num_recovered_until_this_iteration---num_recovered_until_this_iteration = num_recovered_so_far---Go to step 3Else---Terminate The algorithm terminates either when all missing source packets are fully recovered or when there are still remaining missing source packets but the FEC packets are not able to recover any more source packets. For the example scenarios when the 2-D parity FEC protection fails full recovery, refer to . Upon termination, variable num_recovered_so_far has a value equal to the total number of recovered source packets. Example: Suppose that the receiver experienced the loss pattern sketched in .

The receiver executes the iterative decoding algorithm and recovers source packets #1 and #11 in the first iteration. The resulting pattern is sketched in .

Since the if condition holds true, the receiver runs a new iteration. In the second iteration, source packets #2 and #10 are recovered, resulting in a full recovery as sketched in .

This section provides two SDP examples. The examples use the FEC grouping semantics defined in .

In this example, we have one source video stream and one FEC repair stream. The source and repair streams are multiplexed on different SSRCs. The repair window is set to 200 ms.

In this example, we have one source video stream (ssrc:1234) and one FEC repair streams (ssrc:2345). We form one FEC group with the "a=ssrc-group:FEC-FR 1234 2345" line. The source and repair streams are multiplexed on different SSRCs. The repair window is set to 200 ms.

FEC is an effective approach to provide applications resiliency against packet losses. However, in networks where the congestion is a major contributor to the packet loss, the potential impacts of using FEC SHOULD be considered carefully before injecting the repair flows into the network. In particular, in bandwidth-limited networks, FEC repair flows may consume most or all of the available bandwidth and consequently may congest the network. In such cases, the applications MUST NOT arbitrarily increase the amount of FEC protection since doing so may lead to a congestion collapse. If desired, stronger FEC protection MAY be applied only after the source rate has been reduced . In a network-friendly implementation, an application SHOULD NOT send/receive FEC repair flows if it knows that sending/receiving those FEC repair flows would not help at all in recovering the missing packets. However, it MAY still continue to use FEC if considered for bandwidth estimation instead of speculatively probe for additional capacity . It is RECOMMENDED that the amount of FEC protection is adjusted dynamically based on the packet loss rate observed by the applications. In multicast scenarios, it may be difficult to optimize the FEC protection per receiver. If there is a large variation among the levels of FEC protection needed by different receivers, it is RECOMMENDED that the sender offers multiple repair flows with different levels of FEC protection and the receivers join the corresponding multicast sessions to receive the repair flow(s) that is best for them. Editor's note: Additional congestion control considerations regarding the use of 2-D parity codes should be added here.

RTP packets using the payload format defined in this specification are subject to the security considerations discussed in the RTP specification and in any applicable RTP profile. The main security considerations for the RTP packet carrying the RTP payload format defined within this memo are confidentiality, integrity and source authenticity. Confidentiality is achieved by encrypting the RTP payload. Integrity of the RTP packets is achieved through a suitable cryptographic integrity protection mechanism. Such a cryptographic system may also allow the authentication of the source of the payload. A suitable security mechanism for this RTP payload format should provide confidentiality, integrity protection, and at least source authentication capable of determining if an RTP packet is from a member of the RTP session. Note that the appropriate mechanism to provide security to RTP and payloads following this memo may vary. It is dependent on the application, transport and signaling protocol employed. Therefore, a single mechanism is not sufficient, although if suitable, using the Secure Real-time Transport Protocol (SRTP) is recommended. Other mechanisms that may be used are IPsec and Transport Layer Security (TLS) (RTP over TCP); other alternatives may exist.

New media subtypes are subject to IANA registration. For the registration of the payload formats and their parameters introduced in this document, refer to .

Some parts of this document are borrowed from . Thus, the author would like to thank the editor of and those who contributed to . Thanks to Bernard Aboba , Rasmus Brandt , Roni Even , Stefan Holmer , Jonathan Lennox , and Magnus Westerlund for providing valuable feedback on earlier versions of this draft.

Note to the RFC-Editor: please remove this section prior to publication as an RFC.

FEC packet format changed as per discussions in IETF96, Berlin. Removed section on non-parity codes and flexfec-raptor.

FEC packet format changed as per discussions in IETF94, Tokyo. Added section on non-parity codes. Registration of application/flexfec-raptor.

FEC packet format changed as per discussions in IETF93, Prague. Replaced non-interleaved-parityfec and interleaved-parity-fec with flexfec. SDP simplified for the case when association to RTP is made in the FEC header and not in the SDP.

Initial WG version, based on draft-singh-payload-1d2d-parity-scheme-00.

This is the initial version, which is based on draft-ietf-fecframe-1d2d-parity-scheme-00. The following are the major changes compared to that document: Updated packet format with 16-, 48-, 112- bitmask. Updated the sections on: repair packet construction, source packet construction. Updated the media type registration and aligned to RFC6838.

Some details were added regarding the use of CNAME field. Offer-Answer and Declarative Considerations sections have been completed. Security Considerations section has been completed. The timestamp field definition has changed.