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Network Working Group                                              A. LiRequest for Comments: 3558                                          UCLACategory: Standards Track                                      July 2003RTP Payload Format for Enhanced Variable Rate Codecs (EVRC)and Selectable Mode Vocoders (SMV)Status of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2003).  All Rights Reserved.Abstract   This document describes the RTP payload format for Enhanced Variable   Rate Codec (EVRC) Speech and Selectable Mode Vocoder (SMV) Speech.   Two sub-formats are specified for different application scenarios.  A   bundled/interleaved format is included to reduce the effect of packet   loss on speech quality and amortize the overhead of the RTP header   over more than one speech frame.  A non-bundled format is also   supported for conversational applications.Table of Contents1. Introduction ...................................................22. Background .....................................................23. The Codecs Supported ...........................................33.1. EVRC ......................................................33.2. SMV .......................................................33.3. Other Frame-Based Vocoders ................................44. RTP/Vocoder Packet Format ......................................44.1. Interleaved/Bundled Packet Format .........................54.2. Header-Free Packet Format .................................64.3. Determining the Format of Packets .........................75. Packet Table of Contents Entries and Codec Data Frame Format ...75.1. Packet Table of Contents entries ..........................75.2. Codec Data Frames .........................................86. Interleaving Codec Data Frames .................................97. Bundling Codec Data Frames ....................................128. Handling Missing Codec Data Frames ............................12Li                          Standards Track                     [Page 1]

RFC 3558          RTP Payload Format for EVRC and SMV          July 20039. Implementation Issues .........................................129.1. Interleaving Length .......................................129.2. Validation of Received Packets ............................139.3. Processing the Late Packets ...............................1310. Mode Request .................................................1311. Storage Format ...............................................1412. IANA Considerations ..........................................1512.1. Registration of Media Type EVRC ..........................1512.2. Registration of Media Type EVRC0 .........................1612.3. Registration of Media Type SMV ...........................1712.4. Registration of Media Type SMV0 ..........................1813. Mapping to SDP Parameters ....................................1914. Security Considerations ......................................2015. Adding Support of Other Frame-Based Vocoders .................2016. Acknowledgements .............................................2117. References ...................................................2117.1 Normative ................................................2117.2 Informative ..............................................2218. Author's Address .............................................2219. Full Copyright Statement .....................................231. Introduction   This document describes how speech compressed with EVRC [1] or SMV   [2] may be formatted for use as an RTP payload type.  The format is   also extensible to other codecs that generate a similar set of frame   types.  Two methods are provided to packetize the codec data frames   into RTP packets: an interleaved/bundled format and a zero-header   format.  The sender may choose the best format for each application   scenario, based on network conditions, bandwidth availability, delay   requirements, and packet-loss tolerance.   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 inRFC 2119 [3].2. Background   The 3rd Generation Partnership Project 2 (3GPP2) has published two   standards which define speech compression algorithms for CDMA   applications: EVRC [1] and SMV [2].  EVRC is currently deployed in   millions of first and second generation CDMA handsets.  SMV is the   preferred speech codec standard for CDMA2000, and will be deployed in   third generation handsets in addition to EVRC.  Improvements and new   codecs will keep emerging as technology improves, and future handsets   will likely support multiple codecs.Li                          Standards Track                     [Page 2]

RFC 3558          RTP Payload Format for EVRC and SMV          July 2003   The formats of the EVRC and SMV codec frames are very similar.  Many   other vocoders also share common characteristics, and have many   similar application scenarios.  This parallelism enables an RTP   payload format to be designed for EVRC and SMV that may also support   other, similar vocoders with minimal additional specification work.   This can simplify the protocol for transporting vocoder data frames   through RTP and reduce the complexity of implementations.3. The Codecs Supported3.1. EVRC   The Enhanced Variable Rate Codec (EVRC) [1] compresses each 20   milliseconds of 8000 Hz, 16-bit sampled speech input into output   frames in one of the three different sizes: Rate 1 (171 bits), Rate   1/2 (80 bits), or Rate 1/8 (16 bits).  In addition, there are two   zero bit codec frame types: null frames and erasure frames.  Null   frames are produced as a result of the vocoder running at rate 0.   Null frames are zero bits long and are normally not transmitted.   Erasure frames are the frames substituted by the receiver to the   codec for the lost or damaged frames.  Erasure frames are also zero   bits long and are normally not transmitted.   The codec chooses the output frame rate based on analysis of the   input speech and the current operating mode (either normal or one of   several reduced rate modes).  For typical speech patterns, this   results in an average output of 4.2 kilobits/second for normal mode   and a lower average output for reduced rate modes.3.2. SMV   The Selectable Mode Vocoder (SMV) [2] compresses each 20 milliseconds   of 8000 Hz, 16-bit sampled speech input into output frames of one of   the four different sizes: Rate 1 (171 bits), Rate 1/2 (80 bits), Rate   1/4 (40 bits), or Rate 1/8 (16 bits).  In addition, there are two   zero bit codec frame types: null frames and erasure frames.  Null   frames are produced as a result of the vocoder running at rate 0.   Null frames are zero bits long and are normally not transmitted.   Erasure frames are the frames substituted by the receiver to the   codec for the lost or damaged frames.  Erasure frames are also zero   bits long and are normally not transmitted.   The SMV codec can operate in six modes.  Each mode may produce frames   of any of the rates (full rate to 1/8 rate) for varying percentages   of time, based on the characteristics of the speech samples and the   selected mode.  The SMV mode can change on a   frame-by-frame basis.  The SMV codec does not need additional   information other than the codec data frames to correctly decode theLi                          Standards Track                     [Page 3]

RFC 3558          RTP Payload Format for EVRC and SMV          July 2003   data of various modes; therefore, the mode of the encoder does not   need to be transmitted with the encoded frames.   The SMV codec chooses the output frame rate based on analysis of the   input speech and the current operating mode.  For typical speech   patterns, this results in an average output of 4.2 kilobits/second   for Mode 0 in two way conversation (approximately 50% active speech   time and 50% in eighth rate while listening) and lower for other   reduced rate modes.  SMV is more bandwidth efficient than EVRC.  EVRC   is equivalent in performance to SMV mode 1.3.3. Other Frame-Based Vocoders   Other frame-based vocoders can be carried in the packet format   defined in this document, as long as they possess the following   properties:      o The codec is frame-based;      o blank and erasure frames are supported;      o the total number of rates is less than 17;      o the maximum full rate frame can be transported in a single RTP        packet using this specific format.   Vocoders with the characteristics listed above can be transported   using the packet format specified in this document with some   additional specification work; the pieces that must be defined are   listed inSection 15.4. RTP/Vocoder Packet Format   The vocoder speech data may be transmitted in either of the two RTP   packet formats specified in the following two subsections, as   appropriate for the application scenario.  In the packet format   diagrams shown in this document, bit 0 is the most significant bit.Li                          Standards Track                     [Page 4]

RFC 3558          RTP Payload Format for EVRC and SMV          July 20034.1. Interleaved/Bundled Packet Format   This format is used to send one or more vocoder frames per packet.   Interleaving or bundling MAY be used.  The RTP packet for this format   is as follows:    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                      RTP Header [4]                           |   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+   |R|R| LLL | NNN | MMM |  Count  |  TOC  |  ...  |  TOC  |padding|   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |        one or more codec data frames, one per TOC entry       |   |                             ....                              |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   The RTP header has the expected values as described in the RTP   specification [4].  The RTP timestamp is in 1/8000 of a second units   for EVRC and SMV.  For any other vocoders that use this packet   format, the timestamp unit needs to be defined explicitly.  The M bit   should be set as specified in the applicable RTP profile, for   example,RFC 3551 [5].  Note thatRFC 3551 [5] specifies that if the   sender does not suppress silence, the M bit will always be zero.   When multiple codec data frames are present in a single RTP packet,   the timestamp is that of the oldest data represented in the RTP   packet.  The assignment of an RTP payload type for this packet format   is outside the scope of this document; it is specified by the RTP   profile under which this payload format is used.   The first octet of a Interleaved/Bundled format packet is the   Interleave Octet.  The second octet contains the Mode Request and   Frame Count fields.  The Table of Contents (ToC) field then follows.   The fields are specified as follows:   Reserved (RR): 2 bits      Reserved bits.  MUST be set to zero by sender, SHOULD be ignored      by receiver.   Interleave Length (LLL): 3 bits      Indicates the length of interleave; a value of 0 indicates      bundling, a special case of interleaving.  SeeSection 6 andSection 7 for more detailed discussion.Li                          Standards Track                     [Page 5]

RFC 3558          RTP Payload Format for EVRC and SMV          July 2003   Interleave Index (NNN): 3 bits      Indicates the index within an interleave group.  MUST have a value      less than or equal to the value of LLL.  Values of NNN greater      than the value of LLL are invalid.  Packet with invalid NNN values      SHOULD be ignored by the receiver.   Mode Request (MMM): 3 bits      The Mode Request field is used to signal Mode Request information.      SeeSection 10 for details.   Frame Count (Count): 5 bits      The number of ToC fields (and vocoder frames) present in the      packet is the value of the frame count field plus one.  A value of      zero indicates that the packet contains one ToC field, while a      value of 31 indicates that the packet contains 32 ToC fields.   Padding (padding): 0 or 4 bits      This padding ensures that codec data frames start on an octet      boundary.  When the frame count is odd, the sender MUST add 4 bits      of padding following the last TOC.  When the frame count is even,      the sender MUST NOT add padding bits.  If padding is present, the      padding bits MUST be set to zero by sender, and SHOULD be ignored      by receiver.   The Table of Contents field (ToC) provides information on the codec   data frame(s) in the packet.  There is one ToC entry for each codec   data frame.  The detailed formats of the ToC field and codec data   frames are specified inSection 5.   Multiple data frames may be included within a Interleaved/Bundled   packet using interleaving or bundling as described inSection 6 andSection 7.4.2. Header-Free Packet Format   The Header-Free Packet Format is designed for maximum bandwidth   efficiency and low latency.  Only one codec data frame can be sent in   each Header-Free format packet.  None of the payload header fields   (LLL, NNN, MMM, Count) nor ToC entries are present.  The codec rate   for the data frame can be determined from the length of the codec   data frame, since there is only one codec data frame in each   Header-Free packet.   Use of the RTP header fields for Header-Free RTP/Vocoder Packet   Format is the same as described inSection 4.1 for   Interleaved/Bundled RTP/Vocoder Packet Format.  The detailed format   of the codec data frame is specified inSection 5.Li                          Standards Track                     [Page 6]

RFC 3558          RTP Payload Format for EVRC and SMV          July 2003    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                      RTP Header [4]                           |   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+   |                                                               |   +          ONLY one codec data frame            +-+-+-+-+-+-+-+-+   |                                               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+4.3. Determining the Format of Packets   All receivers SHOULD be able to process both packet formats.  The   sender MAY choose to use one or both packet formats.   A receiver MUST have prior knowledge of the packet format to   correctly decode the RTP packets.  When packets of both formats are   used within the same session, different RTP payload type values MUST   be used for each format to distinguish the packet formats.  The   association of payload type number with the packet format is done   out-of-band, for example by SDP during the setup of a session.5. Packet Table of Contents Entries and Codec Data Frame Format5.1. Packet Table of Contents entries   Each codec data frame in a Interleaved/Bundled packet has a   corresponding Table of Contents (ToC) entry.  The ToC entry indicates   the rate of the codec frame.  (Header-Free packets MUST NOT have a   ToC field.)   Each ToC entry is occupies four bits.  The format of the bits is   indicated below:       0 1 2 3      +-+-+-+-+      |fr type|      +-+-+-+-+   Frame Type: 4 bits      The frame type indicates the type of the corresponding codec data      frame in the RTP packet.Li                          Standards Track                     [Page 7]

RFC 3558          RTP Payload Format for EVRC and SMV          July 2003   For EVRC and SMV codecs, the frame type values and size of the   associated codec data frame are described in the table below:   Value   Rate      Total codec data frame size (in octets)   ---------------------------------------------------------     0     Blank      0    (0 bit)     1     1/8        2    (16 bits)     2     1/4        5    (40 bits; not valid for EVRC)     3     1/2       10    (80 bits)     4     1         22    (171 bits; 5 padded at end with zeros)     5     Erasure    0    (SHOULD NOT be transmitted by sender)   All values not listed in the above table MUST be considered reserved.   A ToC entry with a reserved Frame Type value SHOULD be considered   invalid.  Note that the EVRC codec does not have 1/4 rate frames,   thus frame type value 2 MUST be considered a reserved value when the   EVRC codec is in use.   Other vocoders that use this packet format need to specify their own   table of frame types and corresponding codec data frames.5.2. Codec Data Frames   The output of the vocoder MUST be converted into codec data frames   for inclusion in the RTP payload.  The conversions for EVRC and SMV   codecs are specified below.  (Note: Because the EVRC codec does not   have Rate 1/4 frames, the specifications of 1/4 frames does not apply   to EVRC codec data frames).  Other vocoders that use this packet   format need to specify how to convert vocoder output data into   frames.   The codec output data bits as numbered in EVRC and SMV are packed   into octets.  The lowest numbered bit (bit 1 for Rate 1, Rate 1/2,   Rate 1/4 and Rate 1/8) is placed in the most significant bit   (internet bit 0) of octet 1 of the codec data frame, the second   lowest bit is placed in the second most significant bit of the first   octet, the third lowest in the third most significant bit of the   first octet, and so on.  This continues until all of the bits have   been placed in the codec data frame.   The remaining unused bits of the last octet of the codec data frame   MUST be set to zero.  Note that in EVRC and SMV this is only   applicable to Rate 1 frames (171 bits) as the Rate 1/2 (80 bits),   Rate 1/4 (40 bits, SMV only) and Rate 1/8 frames (16 bits) fit   exactly into a whole number of octets.   Following is a detailed listing showing a Rate 1 EVRC/SMV codec   output frame converted into a codec data frame:Li                          Standards Track                     [Page 8]

RFC 3558          RTP Payload Format for EVRC and SMV          July 2003   The codec data frame for a EVRC/SMV codec Rate 1 frame is 22 octets   long.  Bits 1 through 171 from the EVRC/SMV codec Rate 1 frame are   placed as indicated, with bits marked with "Z" set to zero.  EVRC/SMV   codec Rate 1/8, Rate 1/4 and Rate 1/2 frames are converted similarly,   but do not require zero padding because they align on octet   boundaries.                        Rate 1 codec data frame    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|   |0|0|0|0|0|0|0|0|0|1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|3|3|   |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|2|   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   :                                                               :   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1| | | | | |   |4|4|4|4|4|5|5|5|5|5|5|5|5|5|5|6|6|6|6|6|6|6|6|6|6|7|7|Z|Z|Z|Z|Z|   |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| | | | | |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+6. Interleaving Codec Data Frames   As indicated inSection 4.1, more than one codec data frame MAY be   included in a single Interleaved/Bundled packet by a sender.  This is   accomplished by interleaving or bundling.   Bundling is used to spread the transmission overhead of the RTP and   payload header over multiple vocoder frames.  Interleaving   additionally reduces the listener's perception of data loss by   spreading such loss over non-consecutive vocoder frames.  EVRC, SMV,   and similar vocoders are able to compensate for an occasional lost   frame, but speech quality degrades exponentially with consecutive   frame loss.   Bundling is signaled by setting the LLL field to zero and the Count   field to greater than zero.  Interleaving is indicated by setting the   LLL field to a value greater than zero.   The discussions on general interleaving apply to the bundling (which   can be viewed as a reduced case of interleaving) with reduced   complexity.  The bundling case is discussed in detail inSection 7.   Senders MAY support interleaving and/or bundling.  All receivers that   support Interleave/Bundling packet format MUST support both   interleaving and bundling.Li                          Standards Track                     [Page 9]

RFC 3558          RTP Payload Format for EVRC and SMV          July 2003   Given a time-ordered sequence of output frames from the codec   numbered 0..n, a bundling value B (the value in the Count field plus   one), and an interleave length L where n = B * (L+1) - 1, the output   frames are placed into RTP packets as follows (the values of the   fields LLL and NNN are indicated for each RTP packet):   First RTP Packet in Interleave group:      LLL=L, NNN=0      Frame 0, Frame L+1, Frame 2(L+1), Frame 3(L+1), ... for a total of      B frames   Second RTP Packet in Interleave group:      LLL=L, NNN=1      Frame 1, Frame 1+L+1, Frame 1+2(L+1), Frame 1+3(L+1), ... for a      total of B frames   This continues to the last RTP packet in the interleave group:   L+1 RTP Packet in Interleave group:      LLL=L, NNN=L      Frame L, Frame L+L+1, Frame L+2(L+1), Frame L+3(L+1), ... for a      total of B frames   Within each interleave group, the RTP packets making up the   interleave group MUST be transmitted in value-increasing order of the   NNN field.  While this does not guarantee reduced end-to-end delay on   the receiving end, when packets are delivered in order by the   underlying transport, delay will be reduced to the minimum possible.   Receivers MAY signal the maximum number of codec data frames (i.e.,   the maximum acceptable bundling value B) they can handle in a single   RTP packet using the OPTIONAL maxptime RTP mode parameter identified   inSection 12.   Receivers MAY signal the maximum interleave length (i.e., the maximum   acceptable LLL value in the Interleaving Octet) they will accept   using the OPTIONAL maxinterleave RTP mode parameter identified inSection 12.   The parameters maxptime and maxinterleave are exchanged at the   initial setup of the session.  In one-to-one sessions, the sender   MUST respect these values set be the receiver, and MUST NOT   interleave/bundle more packets than what the receiver signals that it   can handle.  This ensures that the receiver can allocate a known   amount of buffer space that will be sufficient for all   interleaving/bundling used in that session.  During the session, the   sender may decrease the bundling value or interleaving length (so   that less buffer space is required at the receiver), but never exceedLi                          Standards Track                    [Page 10]

RFC 3558          RTP Payload Format for EVRC and SMV          July 2003   the maximum value set by the receiver.  This prevents the situation   where a receiver needs to allocate more buffer space in the middle of   a session but is unable to do so.   Additionally, senders have the following restrictions:   o  MUST NOT bundle more codec data frames in a single RTP packet than      indicated by maxptime (seeSection 12) if it is signaled.   o  SHOULD NOT bundle more codec data frames in a single RTP packet      than will fit in the MTU of the underlying network.   o  Once beginning a session with a given maximum interleaving value      set by maxinterleave inSection 12, MUST NOT increase the      interleaving value (LLL) to exceed the maximum interleaving value      that is signaled.   o  MAY change the interleaving value, but MUST do so only between      interleave groups.   o  Silence suppression MUST only be used between interleave groups.      A ToC with Frame Type 0 (Blank Frame,Section 5.1) MUST be used      within interleaving groups if the codec outputs a blank frame.      The M bit in the RTP header is not set for these blank frames, as      the stream is continuous in time.  Because there is only one time      stamp for each RTP packet, silence suppression used within an      interleave group would cause ambiguities when reconstructing the      speech at the receiver side, and thus is prohibited.   Given an RTP packet with sequence number S, interleave length (field   LLL) L, interleave index value (field NNN) N, and bundling value B,   the interleave group consists of this RTP packet and other RTP   packets with sequence numbers from S-N mod 65536 to S-N+L mod 65536   inclusive.  In other words, the interleave group always consists of   L+1 RTP packets with sequential sequence numbers.  The bundling value   for all RTP packets in an interleave group MUST be the same.   The receiver determines the expected bundling value for all RTP   packets in an interleave group by the number of codec data frames   bundled in the first RTP packet of the interleave group received.   Note that this may not be the first RTP packet of the interleave   group if packets are delivered out of order by the underlying   transport.Li                          Standards Track                    [Page 11]

RFC 3558          RTP Payload Format for EVRC and SMV          July 20037. Bundling Codec Data Frames   As discussed inSection 6, the bundling of codec data frames is a   special reduced case of interleaving with LLL value in the Interleave   Octet set to 0.   Bundling codec data frames indicates that multiple data frames are   included consecutively in a packet, because the interleaving length   (LLL) is 0.  The interleaving group is thus reduced to a single RTP   packet, and the reconstruction of the codec data frames from RTP   packets becomes a much simpler process.   Furthermore, the additional restrictions on senders are reduced to:   o  MUST NOT bundle more codec data frames in a single RTP packet than      indicated by maxptime (seeSection 12) if it is signaled.   o  SHOULD NOT bundle more codec data frames in a single RTP packet      than will fit in the MTU of the underlying network.8. Handling Missing Codec Data Frames   The vocoders covered by this payload format support erasure frames as   an indication when frames are not available.  The erasure frames are   normally used internally by a receiver to advance the state of the   voice decoder by exactly one frame time for each missing frame.   Using the information from packet sequence number, time stamp, and   the M bit, the receiver can detect missing codec data frames from RTP   packet loss and/or silence suppression, and generate corresponding   erasure frames.  Erasure frames MUST also be used in storage format   to record missing frames.9. Implementation Issues9.1. Interleaving Length   The vocoder interpolates the missing speech content when given an   erasure frame.  However, the best quality is perceived by the   listener when erasure frames are not consecutive.  This makes   interleaving desirable as it increases speech quality when packet   loss occurs.   On the other hand, interleaving can greatly increase the end-to-end   delay.  Where an interactive session is desired, either   Interleaved/Bundled packet format with interleaving length (field   LLL) 0 or Header-Free packet format is RECOMMENDED.Li                          Standards Track                    [Page 12]

RFC 3558          RTP Payload Format for EVRC and SMV          July 2003   When end-to-end delay is not a primary concern, an interleaving   length (field LLL) of 4 or 5 is RECOMMENDED as it offers a reasonable   compromise between robustness and latency.9.2. Validation of Received Packets   When receiving an RTP packet, the receiver SHOULD check the validity   of the ToC fields and match the length of the packet with what is   indicated by the ToC fields.  If any invalidity or mismatch is   detected, it is RECOMMENDED to discard the received packet to avoid   potential severe degradation of the speech quality.  The discarded   packet is treated following the same procedure as a lost packet, and   the discarded data will be replaced with erasure frames.   On receipt of an RTP packet with an invalid value of the LLL or NNN   fields, the RTP packet SHOULD be treated as lost by the receiver for   the purpose of generating erasure frames as described inSection 8.   On receipt of an RTP packet in an interleave group with other than   the expected frame count value, the receiver MAY discard codec data   frames off the end of the RTP packet or add erasure codec data frames   to the end of the packet in order to manufacture a substitute packet   with the expected bundling value.  The receiver MAY instead choose to   discard the whole interleave group.9.3. Processing the Late Packets   Assume that the receiver has begun playing frames from an interleave   group.  The time has come to play frame x from packet n of the   interleave group.  Further assume that packet n of the interleave   group has not been received.  As described inSection 8, an erasure   frame will be sent to the receiving vocoder.   Now, assume that packet n of the interleave group arrives before   frame x+1 of that packet is needed.  Receivers should use frame x+1   of the newly received packet n rather than substituting an erasure   frame.  In other words, just because packet n was not available the   first time it was needed to reconstruct the interleaved speech, the   receiver should not assume it is not available when it is   subsequently needed for interleaved speech reconstruction.10.  Mode Request   The Mode Request signal requests a particular encoding mode for the   speech encoding in the reverse direction.  All implementations are   RECOMMENDED to honor the Mode Request signal.  The Mode Request   signal SHOULD only be used in one-to-one sessions.  In multi-party   sessions, any received Mode Request signals SHOULD be ignored.Li                          Standards Track                    [Page 13]

RFC 3558          RTP Payload Format for EVRC and SMV          July 2003   In addition, the Mode Request signal MAY also be sent through non-RTP   means, which is out of the scope of this specification.   The three-bit Mode Request field is used to signal the receiver to   set a particular encoding mode to its audio encoder.  If the Mode   Request field is set to a valid value in RTP packets from node A to   node B, it is a request for node B to change to the requested   encoding mode for its audio encoder and therefore the bit rate of the   RTP stream from node B to node A.  Once a node sets this field to a   value, it SHOULD continue to set the field to the same value in   subsequent packets until the requested mode is different.  This   design helps to eliminate the scenario of getting the codec stuck in   an unintended state if one of the packets that carries the Mode   Request is lost.  An otherwise silent node MAY send an RTP packet   containing a blank frame in order to send a Mode Request.   Each codec type using this format SHOULD define its own   interpretation of the Mode Request field.  Codecs SHOULD follow the   convention that higher values of the three-bit field correspond to an   equal or lower average output bit rate.   For the EVRC codec, the Mode Request field MUST be interpreted   according to Tables 2.2.1.2-1 and 2.2.1.2-2 of the EVRC codec   specifications [1].   For SMV codec, the Mode Request field MUST be interpreted according   to Table 2.2-2 of the SMV codec specifications [2].11.  Storage Format   The storage format is used for storing speech frames, e.g., as a file   or e-mail attachment.   The file begins with a magic number to identify the vocoder that is   used.  The magic number for EVRC corresponds to the ASCII character   string "#!EVRC\n", i.e., "0x23 0x21 0x45 0x56 0x52 0x43 0x0A".  The   magic number for SMV corresponds to the ASCII character string   "#!SMV\n", i.e., "0x23 0x21 0x53 0x4d 0x56 0x0a".   The codec data frames are stored in consecutive order, with a single   TOC entry field, extended to one octet, prefixing each codec data   frame.  The ToC field is extended to one octet by setting the four   most significant bits of the octet to zero.  For example, a ToC value   of 4 (a full-rate frame) is stored as 0x04.   Speech frames lost in transmission and non-received frames MUST be   stored as erasure frames (frame type 5, see definition inSection5.1) to maintain synchronization with the original media.Li                          Standards Track                    [Page 14]

RFC 3558          RTP Payload Format for EVRC and SMV          July 200312.  IANA Considerations   Four new MIME sub-types as described in this section have been   registered by the IANA.   The MIME-names for the EVRC and SMV codec are allocated from the IETF   tree since all the vocoders covered are expected to be widely used   for Voice-over-IP applications.12.1.  Registration of Media Type EVRC   Media Type Name:           audio   Media Subtype Name:        EVRC   Required Parameter:        none   Optional parameters:      The following parameters apply to RTP transfer only.      ptime:    Defined as usual for RTP audio (seeRFC 2327).      maxptime: The maximum amount of media which can be encapsulated in         each packet, expressed as time in milliseconds.  The time SHALL         be calculated as the sum of the time the media present in the         packet represents.  The time SHOULD be a multiple of the         duration of a single codec data frame (20 msec).  If not         signaled, the default maxptime value SHALL be 200 milliseconds.      maxinterleave: Maximum number for interleaving length (field LLL         in the Interleaving Octet).  The interleaving lengths used in         the entire session MUST NOT exceed this maximum value.  If not         signaled, the maxinterleave length SHALL be 5.   Encoding considerations:      This type is defined for transfer of EVRC-encoded data via RTP      using the Interleaved/Bundled packet format specified in Sections      4.1, 6, and 7 ofRFC 3558.  It is also defined for other transfer      methods using the storage format specified in Section 11 ofRFC3558.   Security considerations:      SeeSection 14 "Security Considerations" ofRFC 3558.   Public specification:      The EVRC vocoder is specified in 3GPP2 C.S0014.  Transfer methods      are specified inRFC 3558.Li                          Standards Track                    [Page 15]

RFC 3558          RTP Payload Format for EVRC and SMV          July 2003   Additional information:      The following information applies for storage format only.      Magic number: #!EVRC\n (seeSection 11 of RFC 3558)      File extensions: evc, EVC      Macintosh file type code: none      Object identifier or OID: none   Intended usage:      COMMON.  It is expected that many VoIP applications (as well as      mobile applications) will use this type.   Person & email address to contact for further information:      Adam Li      adamli@icsl.ucla.edu   Author/Change controller:      Adam Li      adamli@icsl.ucla.edu      IETF Audio/Video Transport Working Group12.2. Registration of Media Type EVRC0   Media Type Name:           audio   Media Subtype Name:        EVRC0   Required Parameters:       none   Optional parameters:       none   Encoding considerations:   none      This type is only defined for transfer of EVRC-encoded data via      RTP using the Header-Free packet format specified inSection 4.2      of RFC 3558.   Security considerations:      SeeSection 14 "Security Considerations" ofRFC 3558.   Public specification:      The EVRC vocoder is specified in 3GPP2 C.S0014.  Transfer methods      are specified inRFC 3558.   Additional information:    none   Intended usage:      COMMON.  It is expected that many VoIP applications (as well as      mobile applications) will use this type.Li                          Standards Track                    [Page 16]

RFC 3558          RTP Payload Format for EVRC and SMV          July 2003   Person & email address to contact for further information:      Adam Li      adamli@icsl.ucla.edu   Author/Change controller:      Adam Li      adamli@icsl.ucla.edu      IETF Audio/Video Transport Working Group12.3. Registration of Media Type SMV   Media Type Name:           audio   Media Subtype Name:        SMV   Required Parameter:        none   Optional parameters:   The following parameters apply to RTP transfer only.      ptime:    Defined as usual for RTP audio (seeRFC 2327).      maxptime: The maximum amount of media which can be encapsulated         in each packet, expressed as time in milliseconds.  The time         SHALL be calculated as the sum of the time the media present         in the packet represents.  The time SHOULD be a multiple of the         duration of a single codec data frame (20 msec).  If not         signaled, the default maxptime value SHALL be 200         milliseconds.      maxinterleave: Maximum number for interleaving length (field LLL         in the Interleaving Octet).  The interleaving lengths used in         the entire session MUST NOT exceed this maximum value.  If not         signaled, the maxinterleave length SHALL be 5.   Encoding considerations:      This type is defined for transfer of SMV-encoded data via RTP      using the Interleaved/Bundled packet format specified inSection4.1, 6, and 7 ofRFC 3558.  It is also defined for other transfer      methods using the storage format specified in Section 11 ofRFC3558.   Security considerations:      SeeSection 14 "Security Considerations" ofRFC 3558.   Public specification:      The SMV vocoder is specified in 3GPP2 C.S0030-0 v2.0.      Transfer methods are specified inRFC 3558.Li                          Standards Track                    [Page 17]

RFC 3558          RTP Payload Format for EVRC and SMV          July 2003   Additional information:      The following information applies to storage format only.      Magic number: #!SMV\n (seeSection 11 of RFC 3558)      File extensions: smv, SMV      Macintosh file type code: none      Object identifier or OID: none   Intended usage:      COMMON.  It is expected that many VoIP applications (as well as      mobile applications) will use this type.   Person & email address to contact for further information:      Adam Li      adamli@icsl.ucla.edu   Author/Change controller:      Adam Li      adamli@icsl.ucla.edu      IETF Audio/Video Transport Working Group12.4. Registration of Media Type SMV0   Media Type Name:           audio   Media Subtype Name:        SMV0   Required Parameter:        none   Optional parameters:       none   Encoding considerations:   none      This type is only defined for transfer of SMV-encoded data via RTP      using the Header-Free packet format specified inSection 4.2 of      RFC 3558.   Security considerations:      SeeSection 14 "Security Considerations" ofRFC 3558.   Public specification:      The SMV vocoder is specified in 3GPP2 C.S0030-0 v2.0.  Transfer      methods are specified inRFC 3558.   Additional information:    none   Intended usage:      COMMON.  It is expected that many VoIP applications (as well as      mobile applications) will use this type.Li                          Standards Track                    [Page 18]

RFC 3558          RTP Payload Format for EVRC and SMV          July 2003   Person & email address to contact for further information:      Adam Li      adamli@icsl.ucla.edu   Author/Change controller:      Adam Li      adamli@icsl.ucla.edu      IETF Audio/Video Transport Working Group13.  Mapping to SDP Parameters   Please note that this section applies to the RTP transfer only.   The information carried in the MIME media type specification has a   specific mapping to fields in the Session Description Protocol (SDP)   [6], which is commonly used to describe RTP sessions.  When SDP is   used to specify sessions employing the EVRC or EMV codec, the mapping   is as follows:      o  The MIME type ("audio") goes in SDP "m=" as the media name.      o  The MIME subtype (payload format name) goes in SDP "a=rtpmap"         as the encoding name.      o  The parameters "ptime" and "maxptime" go in the SDP "a=ptime"         and "a=maxptime" attributes, respectively.      o  The parameter "maxinterleave" goes in the SDP "a=fmtp"         attribute by copying it directly from the MIME media type         string as "maxinterleave=value".   Some examples of SDP session descriptions for EVRC and SMV encodings   follow below.   Example of usage of EVRC:      m=audio 49120 RTP/AVP 97      a=rtpmap:97 EVRC/8000      a=fmtp:97 maxinterleave=2      a=maxptime:80   Example of usage of SMV      m=audio 49122 RTP/AVP 99      a=rtpmap:99 SMV0/8000      a=fmtp:99Li                          Standards Track                    [Page 19]

RFC 3558          RTP Payload Format for EVRC and SMV          July 2003   Note that the payload format (encoding) names are commonly shown in   upper case.  MIME subtypes are commonly shown in lower case.  These   names are case-insensitive in both places.  Similarly, parameter   names are case-insensitive both in MIME types and in the default   mapping to the SDP a=fmtp attribute.14.  Security Considerations   RTP packets using the payload format defined in this specification   are subject to the security considerations discussed in the RTP   specification [4], and any appropriate profile (for example [5]).   This implies that confidentiality of the media streams is achieved by   encryption.  Because the data compression used with this payload   format is applied end-to-end, encryption may be performed after   compression so there is no conflict between the two operations.   A potential denial-of-service threat exists for data encoding using   compression techniques that have non-uniform receiver-end   computational load.  The attacker can inject pathological datagrams   into the stream which are complex to decode and cause the receiver to   become overloaded.  However, the encodings covered in this document   do not exhibit any significant non-uniformity.   As with any IP-based protocol, in some circumstances, a receiver may   be overloaded simply by the receipt of too many packets, either   desired or undesired.  Network-layer authentication may be used to   discard packets from undesired sources, but the processing cost of   the authentication itself may be too high.  In a multicast   environment, pruning of specific sources may be implemented in future   versions of IGMP [7] and in multicast routing protocols to allow a   receiver to select which sources are allowed to reach it.   Interleaving may affect encryption.  Depending on the used encryption   scheme there may be restrictions on, for example, the time when keys   can be changed.  Specifically, the key change may need to occur at   the boundary between interleave groups.15.  Adding Support of Other Frame-Based Vocoders   As described above, the RTP packet format defined in this document is   very flexible and designed to be usable by other frame-based   vocoders.   Additional vocoders using this format MUST have properties as   described inSection 3.3.Li                          Standards Track                    [Page 20]

RFC 3558          RTP Payload Format for EVRC and SMV          July 2003   For an eligible vocoder to use the payload format mechanisms defined   in this document, a new RTP payload format document needs to be   published as a standards track RFC.  That document can simply refer   to this document and then specify the following parameters:      o Define the unit used for RTP time stamp;      o Define the meaning of the Mode Request bits;      o Define corresponding codec data frame type values for ToC;      o Define the conversion procedure for vocoders output data frame;      o Define a magic number for storage format, and complete the        corresponding MIME registration.16.  Acknowledgements   The following authors have made significant contributions to this   document: Adam H. Li, John D. Villasenor, Dong-Seek Park, Jeong-Hoon   Park, Keith Miller, S. Craig Greer, David Leon, Nikolai Leung,   Marcello Lioy, Kyle J. McKay, Magdalena L. Espelien, Randall Gellens,   Tom Hiller, Peter J. McCann, Stinson S. Mathai, Michael D. Turner,   Ajay Rajkumar, Dan Gal, Magnus Westerlund, Lars-Erik Jonsson, Greg   Sherwood, and Thomas Zeng.17.  References17.1 Normative   [1]  3GPP2 C.S0014, "Enhanced Variable Rate Codec, Speech Service        Option 3 for Wideband Spread Spectrum Digital Systems", January        1997.   [2]  3GPP2 C.S0030-0 v2.0, "Selectable Mode Vocoder, Service Option        for Wideband Spread Spectrum Communication Systems", May 2002.   [3]  Bradner, S., "Key words for use in RFCs to Indicate Requirement        Levels",BCP 14,RFC 2119, March 1997.   [4]  Schulzrinne, H., Casner, S., Jacobson, V. and R. Frederick,        "RTP: A Transport Protocol for Real-Time Applications",RFC3550, July 2003.   [5]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video        Conferences with Minimal Control",RFC 3551, July 2003.   [6]  Handley, M. and V. Jacobson, "SDP: Session Description        Protocol",RFC 2327, April 1998.Li                          Standards Track                    [Page 21]

RFC 3558          RTP Payload Format for EVRC and SMV          July 200317.2 Informative   [7]  Deering, S., "Host Extensions for IP Multicasting", STD 5,RFC1112, August 1989.18.  Author's Address   Adam H. Li   Image Communication Lab   Electrical Engineering Department   University of California   Los Angeles, CA 90095   USA   Phone: +1 310 825 5178   EMail: adamli@icsl.ucla.eduLi                          Standards Track                    [Page 22]

RFC 3558          RTP Payload Format for EVRC and SMV          July 200319.  Full Copyright Statement   Copyright (C) The Internet Society (2003).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Li                          Standards Track                    [Page 23]