[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits] [IPR]

Versions: (draft-lazzaro-avt-mwpp-coding-guidelines) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 4696

INTERNET-DRAFT                                                J. Lazzaro
October 21, 2005                                            J. Wawrzynek
Expires: April 21, 2006                                      UC Berkeley


                  An Implementation Guide for RTP MIDI

              <draft-ietf-avt-rtp-midi-guidelines-13.txt>


Status of this Memo

By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware have
been or will be disclosed, and any of which he or she becomes aware
will be disclosed, in accordance with Section 6 of 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.

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/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 April 21, 2006.


















Lazzaro/Wawrzynek                                               [Page 1]

INTERNET-DRAFT                                           21 October 2005


                                Abstract

     This memo offers non-normative implementation guidance for the
     RTP MIDI payload format.  The memo presents its advice in the
     context of a network musical performance application.  In this
     application two musicians, located in different physical locations,
     interact over a network to perform as they would if located in the
     same room.  Underlying the performances are RTP MIDI sessions over
     unicast UDP.  Algorithms for sending and receiving recovery
     journals (the resiliency structure for the payload format) are
     described in detail.  Although the memo focuses on network musical
     performance, the presented implementation advice is relevant to
     other RTP MIDI applications.






































Lazzaro/Wawrzynek                                               [Page 2]

INTERNET-DRAFT                                           21 October 2005


                           Table of Contents

1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
2. Starting the Session  . . . . . . . . . . . . . . . . . . . . . .   5
3. Session Management: Session Housekeeping  . . . . . . . . . . . .   8
4. Sending Streams: General Considerations . . . . . . . . . . . . .   9
     4.1 Queuing and Coding Incoming MIDI Data . . . . . . . . . . .  13
     4.2 Sending Packets with Empty MIDI Lists . . . . . . . . . . .  14
     4.3 Congestion Control and Bandwidth Management . . . . . . . .  16
5. Sending Streams: The Recovery Journal . . . . . . . . . . . . . .  16
     5.1 Initializing the RJSS . . . . . . . . . . . . . . . . . . .  18
     5.2 Traversing the RJSS . . . . . . . . . . . . . . . . . . . .  21
     5.3 Updating the RJSS . . . . . . . . . . . . . . . . . . . . .  21
     5.4 Trimming the RJSS . . . . . . . . . . . . . . . . . . . . .  22
     5.5 Implementation Notes  . . . . . . . . . . . . . . . . . . .  23
6. Receiving Streams: General Considerations . . . . . . . . . . . .  23
     6.1 The NMP Receiver Design . . . . . . . . . . . . . . . . . .  24
     6.2 High-Jitter Networks, Local Area Networks . . . . . . . . .  26
7. Receiving Streams: The Recovery Journal . . . . . . . . . . . . .  27
     7.1 Chapter W: MIDI Pitch Wheel (0xE) . . . . . . . . . . . . .  32
     7.2 Chapter N: MIDI NoteOn (0x8) and NoteOff (0x9)  . . . . . .  32
     7.3 Chapter C: MIDI Control Change (0xB)  . . . . . . . . . . .  34
     7.4 Chapter P: MIDI Program Change (0xC)  . . . . . . . . . . .  36
8. Security Considerations . . . . . . . . . . . . . . . . . . . . .  37
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . .  37
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  37
B. References  . . . . . . . . . . . . . . . . . . . . . . . . . . .  37
     B.1 Normative References  . . . . . . . . . . . . . . . . . . .  37
     B.2 Informative References  . . . . . . . . . . . . . . . . . .  38
C. Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  38
D. Intellectual Property Rights Statement  . . . . . . . . . . . . .  39
E. Full Copyright Statement  . . . . . . . . . . . . . . . . . . . .  39
F. Change Log  . . . . . . . . . . . . . . . . . . . . . . . . . . .  40


















Lazzaro/Wawrzynek                                               [Page 3]

INTERNET-DRAFT                                           21 October 2005


1.  Introduction

[RTPMIDI] normatively defines a Real Time Protocol (RTP, [RFC3550])
payload format for the MIDI command language [MIDI], for use under any
applicable RTP profile (such as the Audio/Visual Profile (AVP,
[RFC3551])).

However, [RTPMIDI] does not define algorithms for sending and receiving
MIDI streams.  Implementors are free to use any sending or receiving
algorithm that conforms to the normative text in [RTPMIDI] [RFC3550]
[RFC3551] [MIDI].

In this memo, we offer implementation guidance on sending and receiving
MIDI RTP streams.  Unlike [RTPMIDI], this memo is not normative.

RTP is a mature protocol, and excellent RTP reference materials are
available [RTPBOOK].  This memo aims to complement the existing
literature, by focusing on issues that are specific to the MIDI payload
format.

The memo focuses on one application: two-party network musical
performance over wide-area networks, following the interoperability
guidelines in Appendix C.7.2 of [RTPMIDI].  Underlying the performances
are RTP MIDI sessions over unicast UDP transport.  Resiliency is
provided by the recovery journal system [RTPMIDI].  The application also
uses the Real Time Control Protocol (RTCP, [RFC3550]).

As defined in [NMP], a network musical performance occurs when a group
of musicians, located at different physical locations, interact over a
network to perform as they would if located in the same room.

Sections 2-3 of this memo describe session startup and maintenance.
Sections 4-5 cover sending MIDI streams, and Sections 6-7 cover
receiving MIDI streams.

















Lazzaro/Wawrzynek                                               [Page 4]

INTERNET-DRAFT                                           21 October 2005


2.  Starting the Session

In this section, we describe how the application starts a two-player
session.  We assume that the two parties have agreed on a session
configuration, embodied by a pair of Session Description Protocol (SDP,
[SDP]) session descriptions.

One session description (Figure 1) defines how the first party wishes to
receive its stream.  The other session description (Figure 2) defines
how the second party wishes to receive its stream.

The session description in Figure 1 codes that the first party intends
to receive a MIDI stream on IP4 number 192.0.2.94 (coded in the c= line)
at UDP port 16112 (coded in the m= line).  Implicit in the SDP m= line
syntax [SDP] is that the first party also intends to receive an RTCP
stream on 192.0.2.94 at UDP port 16113 (16112 + 1).  The receiver
expects that the PT field of each RTP header in the received stream will
be set to 96 (coded in the m= line).

Likewise, the session description in Figure 2 codes that the second
party intends to receive a MIDI stream on IP4 number 192.0.2.105 at UDP
port 5004, and intends to receive an RTCP stream on 192.0.2.105 at UDP
port 5005 (5004 + 1).  The second party expects that the PT RTP header
field of received stream will be set to 101.


v=0
o=first 2520644554 2838152170 IN IP4 first.example.net
s=Example
t=0 0
c=IN IP4 192.0.2.94
m=audio 16112 RTP/AVP 96
a=rtpmap:96 mpeg4-generic/44100
a=fmtp:96 streamtype=5; mode=rtp-midi; config=""; profile-level-id=12;
cm_unused=ABFGHJKMQTVXYZ; cm_unused=C120-127; ch_never=ADEFMQTVX;
tsmode=buffer; linerate=320000; octpos=last; mperiod=44; rtp_ptime=0;
rtp_maxptime=0; guardtime=44100; render=synthetic; rinit="audio/asc";
url="http://example.net/sa.asc";
cid="xjflsoeiurvpa09itnvlduihgnvet98pa3w9utnuighbuk"

(The a=fmtp line has been wrapped to fit the page to accommodate
 memo formatting restrictions; it comprises a single line in SDP)

         Figure 1 -- Session description for first participant.







Lazzaro/Wawrzynek                                               [Page 5]

INTERNET-DRAFT                                           21 October 2005


v=0
o=second 2520644554 2838152170 IN IP4 second.example.net
s=Example
t=0 0
c=IN IP4 192.0.2.105
m=audio 5004 RTP/AVP 101
a=rtpmap:101 mpeg4-generic/44100
a=fmtp:101 streamtype=5; mode=rtp-midi; config=""; profile-level-id=12;
cm_unused=ABFGHJKMQTVXYZ; cm_unused=C120-127; ch_never=ADEFMQTVX;
tsmode=buffer; linerate=320000;octpos=last;mperiod=44; guardtime=44100;
rtp_ptime=0; rtp_maxptime=0; render=synthetic; rinit="audio/asc";
url="http://example.net/sa.asc";
cid="xjflsoeiurvpa09itnvlduihgnvet98pa3w9utnuighbuk"

(The a=fmtp line has been wrapped to fit the page to accommodate
 memo formatting restrictions; it comprises a single line in SDP)

        Figure 2 -- Session description for second participant.


The session descriptions use the mpeg4-generic media type (coded in the
a=rtpmap line) to specify the use of the MPEG 4 Structured Audio
renderer [MPEGSA].  The session descriptions also use parameters to
customize the stream (Appendix C of [RTPMIDI]).  The parameter values
are identical for both parties, yielding identical rendering
environments for the two client hosts.

























Lazzaro/Wawrzynek                                               [Page 6]

INTERNET-DRAFT                                           21 October 2005


We now show example code that implements the actions the parties take
during the session.  The code is written in C, and uses the sockets API
and other POSIX systems calls.  We show code for the first party (the
second party takes a symmetric set of actions).

Figure 3 shows how the first party initializes a pair of socket
descriptors (rtp_fd and rtcp_fd) to send and receive UDP packets.  After
the code in Figure 3 runs, the first party may check for new RTP or RTCP
packets by calling recv() on rtp_fd or rtcp_fd.

Applications may use recv() to receive UDP packets on a socket using one
of two general methods: "blocking" or "non-blocking".

A call to recv() on a blocking UDP socket puts the calling thread to
sleep until a new packet arrives.

A call to recv() on a non-blocking socket acts to poll the device: the
recv() call returns immediately, with a return value that indicates the
polling result.  In this case, a positive return value signals the size
of a new received packet, and a negative return value (coupled with an
errno value of EAGAIN) indicates no new packet was available.

The choice of blocking or non-blocking sockets is a critical application
choice.  Blocking sockets offer the lowest potential latency (as the OS
wakes the caller as soon as a packet has arrived).  However, audio
applications that use blocking sockets must adopt a multi-threaded
program architecture, so that audio samples may be generated on a
"rendering thread" while the "network thread" sleeps while awaiting the
next packet.  The architecture must also support a thread communication
mechanism, so that the network thread has a mechanism to send MIDI
commands the rendering thread.

In contrast, audio applications that use non-blocking sockets may be
coded using a single thread, that alternatives between audio sample
generation and network polling.  This architecture trades off increased
network latency (as a packet may arrive between polls) for a simpler
program architecture.  For simplicity, our example uses non-blocking
sockets and presumes a single run loop.  Figure 4 shows how the example
configures its sockets to be non-blocking.

Figure 5 shows how to use recv() to check a non-blocking socket for new
packets.

The first party also uses rtp_fd and rtcp_fd to send RTP and RTCP
packets to the second party.  In Figure 6, we show how to initialize
socket structures that address the second party.  In Figure 7, we show
how to use one of these structures in a sendto() call to send an RTP
packet to the second party.



Lazzaro/Wawrzynek                                               [Page 7]

INTERNET-DRAFT                                           21 October 2005


Note that the code shown in Figures 3-7 assumes a clear network path
between the participants.  The code may not work if firewalls or Network
Address Translation (NAT) devices are present in the network path.


3.  Session Management: Session Housekeeping

After the two-party interactive session is set up, the parties begin to
send and receive RTP packets.  In Sections 4-7, we discuss RTP MIDI
sending and receiving algorithms.  In this section, we describe session
"housekeeping" tasks that the participants also perform.

One housekeeping task is the maintenance of the 32-bit SSRC value that
uniquely identifies each party.  Section 8 of [RFC3550] describes SSRC
issues in detail, as does Section 2.1 in [RTPMIDI].  Another
housekeeping task is the sending and receiving of RTCP.  Section 6 of
[RFC3550] describes RTCP in detail.

Another housekeeping task concerns security.  As detailed in Appendix G
of [RTPMIDI], per-packet authentication is strongly recommended for use
with MIDI streams, because the acceptance of rogue packets may lead to
the execution of arbitrary MIDI commands.

A final housekeeping task concerns the termination of the session.  In
our two-party example, the session terminates upon the exit of one of
the participants.  A clean termination may require active effort by a
receiver, as a MIDI stream stopped at an arbitrary point may cause stuck
notes and other indefinite artifacts in the MIDI renderer.

The exit of a party may be signalled in several ways.  Session
management tools may offer a reliable signal for termination (such as
the SIP BYE method [RFC3261]).  The (unreliable) RTCP BYE packet
[RFC3550] may also signal the exit of a party.  Receivers may also sense
the lack of RTCP activity and timeout a party, or may use transport
methods to detect an exit.
















Lazzaro/Wawrzynek                                               [Page 8]

INTERNET-DRAFT                                           21 October 2005


4.  Sending Streams: General Considerations

In this section, we discuss sender implementation issues.

The sender is a real-time data-driven entity.  On an on-going basis, the
sender checks to see if the local player has generated new MIDI data.
At any time, the sender may transmit a new RTP packet to the remote
player, for the reasons described below:

  1. New MIDI data has been generated by the local player, and the
     sender decides it is time to issue a packet coding the data.

  2. The local player has not generated new MIDI data, but the
     sender decides too much time has elapsed since the last
     RTP packet transmission.  The sender transmits a packet in
     order to relay updated header and recovery journal data.

In both cases, the sender generates a packet that consists of an RTP
header, a MIDI command section, and a recovery journal.  In the first
case, the MIDI list of the MIDI command section codes the new MIDI data.
In the second case, the MIDI list is empty.






























Lazzaro/Wawrzynek                                               [Page 9]

INTERNET-DRAFT                                           21 October 2005


#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>

  int rtp_fd, rtcp_fd;       /* socket descriptors */
  struct sockaddr_in addr;   /* for bind address   */

  /*********************************/
  /* create the socket descriptors */
  /*********************************/

  if ((rtp_fd = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
    ERROR_RETURN("Couldn't create Internet RTP socket");

  if ((rtcp_fd = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
    ERROR_RETURN("Couldn't create Internet RTCP socket");


  /**********************************/
  /* bind the RTP socket descriptor */
  /**********************************/

  memset(&(addr.sin_zero), 0, 8);
  addr.sin_family = AF_INET;
  addr.sin_addr.s_addr = htonl(INADDR_ANY);
  addr.sin_port = htons(16112); /* port 16112, from SDP */

  if (bind(rtp_fd, (struct sockaddr *)&addr,
        sizeof(struct sockaddr)) < 0)
     ERROR_RETURN("Couldn't bind Internet RTP socket");


  /***********************************/
  /* bind the RTCP socket descriptor */
  /***********************************/

  memset(&(addr.sin_zero), 0, 8);
  addr.sin_family = AF_INET;
  addr.sin_addr.s_addr = htonl(INADDR_ANY);
  addr.sin_port = htons(16113); /* port 16113, from SDP */

  if (bind(rtcp_fd, (struct sockaddr *)&addr,
        sizeof(struct sockaddr)) < 0)
      ERROR_RETURN("Couldn't bind Internet RTCP socket");


        Figure 3 -- Setup code for listening for RTP/RTCP packets.




Lazzaro/Wawrzynek                                              [Page 10]

INTERNET-DRAFT                                           21 October 2005


#include <unistd.h>
#include <fcntl.h>

  /***************************/
  /* set non-blocking status */
  /***************************/

  if (fcntl(rtp_fd, F_SETFL, O_NONBLOCK))
    ERROR_RETURN("Couldn't unblock Internet RTP socket");

  if (fcntl(rtcp_fd, F_SETFL, O_NONBLOCK))
    ERROR_RETURN("Couldn't unblock Internet RTCP socket");

    Figure 4 -- Code to set socket descriptors to be non-blocking.


#include <errno.h>
#define UDPMAXSIZE 1472     /* based on Ethernet MTU of 1500 */

unsigned char packet[UDPMAXSIZE+1];
int len, normal;

 while ((len = recv(rtp_fd, packet, UDPMAXSIZE + 1, 0)) > 0)
  {
    /*  process packet[].  If (len == UDPMAXSIZE + 1), recv()
     *  may be returning a truncated packet -- process with care
     */
  }

  /* line below sets "normal" to 1 if the recv() return */
  /*   status indicates no packets are left to process  */

 normal = (len < 0) && (errno == EAGAIN);

 if (!normal)
  {
    /*
     *  recv() return status indicates an empty UDP payload
     *  (len == 0) or an error condition (coded by (len < 0)
     *  and (errno != EAGAIN)).  Examine len and errno, and
     *  take appropriate recovery action.
     */
  }


        Figure 5 -- Code to check rtp_fd for new RTP packets.





Lazzaro/Wawrzynek                                              [Page 11]

INTERNET-DRAFT                                           21 October 2005


#include <arpa/inet.h>
#include <netinet/in.h>

struct sockaddr_in * rtp_addr;      /* RTP destination IP/port  */
struct sockaddr_in * rtcp_addr;     /* RTCP destination IP/port */


  /* set RTP address, as coded in Figure 2's SDP */

  rtp_addr = calloc(1, sizeof(struct sockaddr_in));
  rtp_addr->sin_family = AF_INET;
  rtp_addr->sin_port = htons(5004);
  rtp_addr->sin_addr.s_addr = inet_addr("192.0.2.105");

  /* set RTCP address, as coded in Figure 2's SDP */

  rtcp_addr = calloc(1, sizeof(struct sockaddr_in));
  rtcp_addr->sin_family = AF_INET;
  rtcp_addr->sin_port = htons(5005);   /* 5004 + 1 */
  rtcp_addr->sin_addr.s_addr = rtp_addr->sin_addr.s_addr;


    Figure 6 -- Initializing destination addresses for RTP and RTCP.


unsigned char packet[UDPMAXSIZE];  /* RTP packet to send   */
int size;                          /* length of RTP packet */


  /* first fill packet[] and set size ... then: */

  if (sendto(rtp_fd, packet, size, 0, rtp_addr,
          sizeof(struct sockaddr))  == -1)
    {
      /*
       * try again later if errno == EAGAIN or EINTR
       *
       * other errno values --> an operational error
       */
    }


           Figure 7 -- Using sendto() to send an RTP packet.








Lazzaro/Wawrzynek                                              [Page 12]

INTERNET-DRAFT                                           21 October 2005


Figure 8 shows the 5 steps a sender takes to issue a packet.  This
algorithm corresponds to the code fragment for sending RTP packets shown
in Figure 7 of Section 2.  Steps 1, 2, and 3 occur before the sendto()
call in the code fragment.  Step 4 corresponds to the sendto() call
itself.  Step 5 may occur once Step 3 completes.


 Algorithm for Sending a Packet:

  1. Generate the RTP header for the new packet.  See Section 2.1
     of [RTPMIDI] for details.

  2. Generate the MIDI command section for the new packet.  See
     Section 3 of [RTPMIDI] for details.

  3. Generate the recovery journal for the new packet.  We discuss
     this process in Section 5.2.  The generation algorithm examines
     the Recovery Journal Sending Structure (RJSS), a stateful
     coding of a history of the stream.

  4. Send the new packet to the receiver.

  5. Update the RJSS to include the data coded in the MIDI command
     section of the packet sent in step 4.  We discuss the update
     procedure in Section 5.3.


         Figure 8 -- A 5 step algorithm for sending a packet.


In the sections that follow, we discuss specific sender implementation
issues in detail.

4.1 Queuing and Coding Incoming MIDI Data

Simple senders transmit a new packet as soon as the local player
generates a complete MIDI command.  The system described in [NMP] uses
this algorithm.  This algorithm minimizes the sender queuing latency, as
the sender never delays the transmission of a new MIDI command.

In a relative sense, this algorithm uses bandwidth inefficiently, as it
does not amortize the overhead of a packet over several commands.  This
inefficiency may be acceptable for sparse MIDI streams (see Appendix A.4
of [NMP]).  More sophisticated sending algorithms [GRAME] improve
efficiency by coding small groups of commands into a single packet, at
the expense of increasing the sender queuing latency.





Lazzaro/Wawrzynek                                              [Page 13]

INTERNET-DRAFT                                           21 October 2005


Senders assign a timestamp value to each command issued by the local
player (Appendix C.3 of [RTPMIDI]).  Senders may code the timestamp
value of the first MIDI list command in two ways.  The most efficient
method is to set the RTP timestamp of the packet to the timestamp value
of the first command.  In this method, the Z bit of the MIDI command
section header (Figure 2 of [RTPMIDI]) is set to 0, and the RTP
timestamps increment at a non-uniform rate.

However, in some applications, senders may wish to generate a stream
whose RTP timestamps increment at a uniform rate.  To do so, senders may
use the Delta Time MIDI list field to code a timestamp for the first
command in the list.  In this case, the Z bit of the MIDI command
section header is set to 1.

Senders should strive to maintain a constant relationship between the
RTP packet timestamp and the packet sending time: if two packets have
RTP timestamps that differ by 1 second, the second packet should be sent
1 second after the first packet.  To the receiver, variance in this
relationship is indistinguishable from network jitter.  Latency issues
are discussed in detail in Section 6.

Senders may alter the running status coding of the first command in the
MIDI list, in order to comply with the coding rules defined in Section
3.2 of [RTPMIDI].  The P header bit (Figure 2 of [RTPMIDI]) codes this
alteration of the source command stream.

4.2 Sending Packets with Empty MIDI Lists

During a session, musicians might refrain from generating MIDI data for
extended periods of time (seconds or even minutes).  If an RTP stream
followed the dynamics of a silent MIDI source, and stopped sending RTP
packets, system behavior might be degraded in the following ways:

  o  The receiver's model of network performance may fall out
     of date.

  o  Network middleboxes (such as Network Address Translators)
     may "time-out" the silent stream and drop the port and IP
     association state.

  o  If the session does not use RTCP, receivers may misinterpret
     the silent stream as a dropped network connection.

Senders avoid these problems by sending "keep-alive" RTP packets during
periods of network inactivity.  Keep-alive packets have empty MIDI
lists.





Lazzaro/Wawrzynek                                              [Page 14]

INTERNET-DRAFT                                           21 October 2005


Session participants may specify the frequency of keep-alive packets
during session configuration with the MIME parameter "guardtime"
(Appendix C.4.2 of [RTPMIDI]).  The session descriptions shown in
Figures 1-2 use guardtime to specify a keep-alive sending interval of 1
second.

Senders may also send empty packets to improve the performance of the
recovery journal system.  As we describe in Section 6, the recovery
process begins when a receiver detects a break in the RTP sequence
number pattern of the stream.  The receiver uses the recovery journal of
the break packet to guide corrective rendering actions, such as ending
stuck notes and updating out-of-date controller values.

Consider the situation where the local player produces a MIDI NoteOff
command (which the sender promptly transmits in a packet), but then 5
seconds pass before the player produces another MIDI command (which the
sender transmits in a second packet).  If the packet coding the NoteOff
is lost, the receiver is not be aware of the packet loss incident for 5
seconds, and the rendered MIDI performance contains a note that sounds
for 5 seconds too long.

To handle this situation, senders may transmit empty packets to "guard"
the stream during silent sections.  The guard packet algorithm defined
in Section 7.3 of [NMP], as applied to the situation described above,
sends a guard packet after 100 ms of player inactivity, and sends a
second guard packet 100 ms later.  Subsequent guard packets are sent
with an exponential backoff, with a limiting period of 1 second (set by
the "guardtime" parameter in Figures 1-2).  The algorithm terminates
once MIDI activity resumes, or once RTCP receiver reports indicate that
the receiver is up to date.

The perceptual quality of guard packet sending algorithms is a quality
of implementation issue for RTP MIDI applications.  Sophisticated
implementations may tailor the guard packet sending rate to the nature
of the MIDI commands recently sent in the stream, to minimize the
perceptual impact of moderate packet loss.

As an example of this sort of specialization, the guard packet algorithm
described in [NMP] protects against the transient artifacts that occur
when NoteOn commands are lost.  The algorithm sends a guard packet 1 ms
after every packet whose MIDI list contains a NoteOn command.  The Y bit
in Chapter N note logs (Appendix A.6 of [RTPMIDI]) supports this use of
guard packets.

Congestion control and bandwidth management are key issues in guard
packet algorithms.  We discuss these issues in the next section.





Lazzaro/Wawrzynek                                              [Page 15]

INTERNET-DRAFT                                           21 October 2005


4.3 Congestion Control and Bandwidth Management

The congestion control section of [RTPMIDI] discusses the importance of
congestion control for RTP MIDI streams, and references the normative
text in [RFC3550] and [RFC3551] that concerns congestion control.  To
comply the requirements described in those normative documents, RTP MIDI
senders may use several methods to control the sending rate:

  o As described in Section 4.1, senders may pack several MIDI
    commands into a single packet, thereby reducing stream bandwidth
    (at the expense of increasing sender queuing latency).

  o Guard packet algorithms (Section 4.2) may be designed in
    a parametric way, so that the tradeoff between artifact
    reduction and stream bandwidth may be tuned dynamically.

  o The recovery journal size may be reduced, by adapting the
    techniques described in Section 5 of this memo.  Note that
    in all cases, the recovery journal sender must conform to
    the normative text in Section 4 of [RTPMIDI].

  o The incoming MIDI stream may be modified, to reduce the
    number of MIDI commands without significantly altering the
    performance.  Lossy "MIDI filtering" algorithms are well
    developed in the MIDI community, and may be directly applied
    to RTP MIDI rate management.

RTP MIDI senders incorporate these rate control methods into feedback
systems to implement congestion control and bandwidth management.
Sections 10 and 6.4.4 of [RFC3550] and Section 2 in [RFC3551] describe
feedback systems for congestion control in RTP, and Section 6 of [SDP]
describes bandwidth management in media sessions.


5.  Sending Streams: The Recovery Journal

In this section, we describe how senders implement the recovery journal
system.  The implementation we describe uses the default "closed-loop"
recovery journal semantics (Appendix C.2.2.2 of [RTPMIDI]).

We begin by describing the Recovery Journal Sending Structure (RJSS).
Senders use the RJSS to generate the recovery journal section for RTP
MIDI packets.

The RJSS is a hierarchical representation of the checkpoint history of
the stream.  The checkpoint history holds the MIDI commands that are at
risk to packet loss (Appendix A.1 of [RTPMIDI] precisely defines the
checkpoint history).  The layout of the RJSS mirrors the hierarchical



Lazzaro/Wawrzynek                                              [Page 16]

INTERNET-DRAFT                                           21 October 2005


structure of the recovery journal bitfields.

Figure 9 shows a RJSS implementation for a simple sender.  The leaf
level of the RJSS hierarchy (the jsend_chapter structures) corresponds
to channel chapters (Appendices A.2-9 in [RTPMIDI]).  The second level
of the hierarchy (jsend_channel) corresponds to the channel journal
header (Figure 9 in [RTPMIDI]).  The top level of the hierarchy
(jsend_journal) corresponds to the recovery journal header (Figure 8 in
[RTPMIDI]).

Each RJSS data structure may code several items:

  1. The current contents of the recovery journal bitfield
     associated with the RJSS structure (jheader[], cheader[],
     or a chapter bitfield).

  2. A seqnum variable.  Seqnum codes the extended RTP sequence
     number of the most recent packet that added information to the
     RJSS structure.  If the seqnum of a structure is updated, the
     seqnums of all structures above it in the recovery journal
     hierarchy are also updated.  Thus, a packet that caused an
     update to a specific jsend_chapter structure would update the
     seqnum values of this structure and of the jsend_channel and
     jsend_journal structures that contain it.

  3. Ancillary variables used by the sending algorithm.

A seqnum variable is set to zero if the checkpoint history contains no
information at the level or at any level below it.  This coding scheme
assumes that the first sequence number of a stream is normalized to 1,
and limits the total number of stream packets to 2^32 - 1.

The cm_unused and ch_never parameters in Figures 1-2 define the subset
of MIDI commands supported by the sender (see Appendix C.2.3 of
[RTPMIDI] for details).  The sender transmits most voice commands, but
does not transmit system commands.  The sender assumes the MIDI source
uses note commands in the typical way, and does not use the Chapter E
note resiliency tools (Appendix A.7 of [RTPMIDI]).  The sender does not
support Control Change commands for controller numbers with All Notes
Off (123-127), All Sound Off (120), and Reset All Controllers (121)
semantics, and does not support enhanced Chapter C encoding (Appendix
A.3.3 of [RTPMIDI]).

We chose this subset of MIDI commands to simplify the example.  In
particular, the command restrictions ensure that all commands are
active, all note commands are N-active, and all Control Change commands
are C-active (see Appendix A.1 of [RTPMIDI] for definitions of active,
N-active, and C-active).



Lazzaro/Wawrzynek                                              [Page 17]

INTERNET-DRAFT                                           21 October 2005


In the sections that follow, we describe the tasks a sender performs to
manage the recovery journal system.

5.1 Initializing the RJSS

At the start of a stream, the sender initializes the RJSS.   All seqnum
variables are set to zero, including all elements of note_seqnum[] and
control_seqnum[].

The sender initializes jheader[] to form a recovery journal header that
codes an empty journal.  The S bit of the header is set to 1, and the A,
Y, R, and TOTCHAN header fields are set to zero.  The checkpoint packet
sequence number field is set to the sequence number of the upcoming
first RTP packet (per Appendix A.1 of [RTPMIDI]).





































Lazzaro/Wawrzynek                                              [Page 18]

INTERNET-DRAFT                                           21 October 2005


  typedef unsigned char  uint8;      /* must be 1 octet  */
  typedef unsigned short uint16;     /* must be 2 octet  */
  typedef unsigned long  uint32;     /* must be 4 octets */

  /**************************************************************/
  /* leaf level hierarchy: Chapter W, Appendix A.5 of [RTPMIDI] */
  /**************************************************************/

  typedef struct jsend_chapterw {  /* Pitch Wheel (0xE) */
   uint8  chapterw[RFC3550]; /* bitfield Figure A.5.1 [RTPMIDI] */
   uint32 seqnum;      /* extended sequence number, or 0 */
  } jsend_chapterw;

  /**************************************************************/
  /* leaf level hierarchy: Chapter N, Appendix A.6 of [RTPMIDI] */
  /**************************************************************/

  typedef struct jsend_chaptern { /* Note commands (0x8, 0x9) */

   /* chapter N maximum size is 274 octets: a 2 octet header, */
   /* and a maximum of 128 2-octet logs and 16 OFFBIT octets  */

   uint8  chaptern[274];     /* bitfield Figure A.6.1 [RTPMIDI] */
   uint16 size;              /* actual size of chaptern[]     */
   uint32 seqnum;            /* extended seq number, or 0     */
   uint32 note_seqnum[128];  /* most recent note seqnum, or 0 */
   uint32 note_tstamp[128];  /* NoteOn execution timestamp    */
   uint32 bitfield_ptr[128]; /* points to a chapter log, or 0 */
  } jsend_chaptern;

  /**************************************************************/
  /* leaf level hierarchy: Chapter C, Appendix A.3 of [RTPMIDI] */
  /**************************************************************/

  typedef struct jsend_chapterc {     /* Control Change (0xB) */

   /* chapter C maximum size is 257 octets: a 1 octet header */
   /* and a maximum of 128 2-octet logs                      */

   uint8  chapterc[257];    /* bitfield Figure A.3.1 [RTPMIDI] */
   uint16 size;             /* actual size of chapterc[]      */
   uint32 seqnum;           /* extended sequence number, or 0 */
   uint32 control_seqnum[128]; /* most recent seqnum, or 0    */
   uint32 bitfield_ptr[128]; /* points to a chapter log, or 0 */
  } jsend_chapterc;

      Figure 9 -- Recovery Journal Sending Structure (part 1)




Lazzaro/Wawrzynek                                              [Page 19]

INTERNET-DRAFT                                           21 October 2005


  /**************************************************************/
  /* leaf level hierarchy: Chapter P, Appendix A.2 of [RTPMIDI] */
  /**************************************************************/

  typedef struct jsend_chapterp { /* MIDI Program Change (0xC) */

   uint8  chapterp[RFC3551]; /* bitfield Figure A.2.1 [RTPMIDI] */
   uint32 seqnum;      /* extended sequence number, or 0 */

  } jsend_chapterp;

  /***************************************************/
  /* second-level of hierarchy, for channel journals */
  /***************************************************/

  typedef struct jsend_channel {

   uint8  cheader[RFC3551]; /* header Figure 9 [RTPMIDI]) */
   uint32 seqnum;     /* extended sequence number, or 0  */

   jsend_chapterp chapterp;           /* chapter P info  */
   jsend_chapterc chapterc;           /* chapter C info  */
   jsend_chapterw chapterw;           /* chapter W info  */
   jsend_chaptern chaptern;           /* chapter N info  */

  } jsend_channel;

  /*******************************************************/
  /* top level of hierarchy, for recovery journal header */
  /*******************************************************/

   typedef struct jsend_journal {

   uint8 jheader[RFC3551]; /* header Figure 8, [RTPMIDI] */
                     /* Note: Empty journal has a header */

   uint32 seqnum;    /* extended sequence number, or 0   */
                     /* seqnum = 0 codes empty journal   */

   jsend_channel channels[16];  /* channel journal state */
                                /* index is MIDI channel */

   } jsend_journal;



    Figure 9 (continued) -- Recovery Journal Sending Structure




Lazzaro/Wawrzynek                                              [Page 20]

INTERNET-DRAFT                                           21 October 2005


In jsend_chaptern, elements of note_tstamp[] are set to zero.  In
jsend_chaptern and jsend_chapterc, elements of bitfield_ptr[] are set to
the null pointer index value (bitfield_ptr[] is an array whose elements
point to the first octet of the note or control log associated with the
array index).

5.2 Traversing the RJSS

Whenever an RTP packet is created (Step 3 in the algorithm defined in
Figure 8), the sender traverses the RJSS to create the recovery journal
for the packet.  The traversal begins at the top level of the RJSS.  The
sender copies jheader[] into the packet, and then sets the S bit of
jheader[] to 1.

The traversal continues depth-first, visiting every jsend_channel whose
seqnum variable is non-zero.  The sender copies the cheader[] array into
the packet, and then sets the S bit of cheader[] to 1.  After each
cheader[] copy, the sender visits each leaf-level chapter, in order of
its appearance in the chapter journal Table of Contents (first P, then
C, then W, then N, as shown in Figure 9 of [RTPMIDI]).

If a chapter has a non-zero seqnum, the sender copies the chapter
bitfield array into the packet, and then sets the S bit of the RJSS
array to 1.  For chaptern[], the B bit is also set to 1.  For the
variable-length chapters (chaptern[] and chapterc[]), the sender checks
the size variable to determine the bitfield length.

Before copying chaptern[], the sender updates the Y bit of each note log
to code the onset of the associated NoteOn command (Figure A.6.3 in
[RTPMIDI]).  To determine the Y bit value, the sender checks the
note_tstamp[] array for note timing information.

5.3 Updating the RJSS

After an RTP packet is sent, the sender updates the RJSS to refresh the
checkpoint history (Step 5 in the sending algorithm defined in Figure
8).  For each command in the MIDI list of the sent packet, the sender
performs the update procedure we now describe.

The update procedure begins at the leaf level.  The sender generates a
new bitfield array for the chapter associated with the MIDI command,
using the chapter-specific semantics defined in Appendix A of [RTPMIDI].

For Chapter N and Chapter C, the sender uses the bitfield_ptr[] array to
locate and update an existing log for a note or controller.  If a log
does not exist, the sender adds a log to the end of the chaptern[] or
chapterc[] bitfield, and changes the bitfield_ptr[] value to point to
the log.  For Chapter N, the sender also updates note_tstamp[].



Lazzaro/Wawrzynek                                              [Page 21]

INTERNET-DRAFT                                           21 October 2005


The sender also clears the S bit of the chapterp[], chapterw[], or
chapterc[] bitfield.  For chaptern[], the sender clears the S bit or the
B bit of the bitfield, as described in Appendix A.6 of [RTPMIDI].

Next, the sender refreshes the upper levels of the RJSS hierarchy.  At
the second-level, the sender updates the cheader[] bitfield of the
channel associated with the command.  The sender sets the S bit of
cheader[] to 0.  If the new command forced the addition of a new chapter
or channel journal, the sender may also update other cheader[] fields.
At the top-level, the sender updates the top-level jheader[] bitfield in
a similar manner.

Finally, the sender updates the seqnum variables associated with the
changed bitfield arrays.  The sender sets the seqnum variables to the
extended sequence number of the packet.

5.4 Trimming the RJSS

At regular intervals, receivers send RTCP receiver reports to the sender
(as described in Section 6.4.2 of [RFC3550]).  These reports include the
extended highest sequence number received (EHSNR) field.  This field
codes the highest sequence number that the receiver has observed from
the sender, extended to disambiguate sequence number rollover.

When the sender receives an RTCP receiver report, it runs the RJSS
trimming algorithm.  The trimming algorithm uses the EHSNR to trim away
parts of the RJSS, and thus reduce the size of recovery journals sent in
subsequent RTP packets.  The algorithm conforms to the closed-loop
sending policy defined in Appendix C.2.2.2 of [RTPMIDI].

The trimming algorithm relies on the following observation: if the EHSNR
indicates that a packet with sequence number K has been received, MIDI
commands sent in packets with sequence numbers J <= K may be removed
from the RJSS without violating the closed-loop policy.

To begin the trimming algorithm, the sender extracts the EHSNR field
from the receiver report, and adjusts the EHSNR to reflect the sequence
number extension prefix of the sender.  Then, the sender compares the
adjusted EHSNR value with seqnum fields at each level of the RJSS,
starting at the top level.

Levels whose seqnum is less than or equal to the adjusted EHSNR are
trimmed, by setting the seqnum to zero.  If necessary, the jheader[] and
cheader[] arrays above the trimmed level are adjusted to match the new
journal layout.  The checkpoint packet sequence number field of
jheader[] is updated to match the EHSNR.

At the leaf level, the sender trims the size of the variable-length



Lazzaro/Wawrzynek                                              [Page 22]

INTERNET-DRAFT                                           21 October 2005


chaptern[] and chapterc[] bitfields.  The sender loops through the
note_seqnum[] or control_seqnum[] array, and removes chaptern[] or
chapterc[] logs whose seqnum value is less than or equal to the adjusted
EHSNR.  The sender sets the associated bitfield_ptr[] to null, and
updates the LENGTH field of the associated cheader[] bitfield.

Note that the trimming algorithm does not add information to the
checkpoint history.  As a consequence, the trimming algorithm does not
clear the S bit (and for chaptern[], the B bit) of any recovery journal
bitfield.  As a second consequence, the trimming algorithm does not set
RJSS seqnum variables to the EHSNR value.

5.5 Implementation Notes

For pedagogical purposes, the recovery journal sender we describe has
been simplified in several ways.  In practice, an implementation would
use enhanced versions of the traversing, updating, and trimming
algorithms presented in Sections 5.2-4.


6.  Receiving Streams: General Considerations

In this section, we discuss receiver implementation issues.

To begin, we imagine that an ideal network carries the RTP stream.
Packets are never lost or reordered, and the end-to-end latency is
constant.  In addition, we assume that all commands coded in the MIDI
list of a packet share the same timestamp (an assumption coded by the
"rtp_ptime" and "rtp_maxptime" values in Figures 1-2; see Appendix C.4.1
of [RTPMIDI] for details).

Under these conditions, a simple algorithm may be used to render a high-
quality performance.  Upon the receipt of an RTP packet, the receiver
immediately executes the commands coded in the MIDI command section of
the payload.  Commands are executed in order of their appearance in the
MIDI list.  The command timestamps are ignored.















Lazzaro/Wawrzynek                                              [Page 23]

INTERNET-DRAFT                                           21 October 2005


Unfortunately, this simple algorithm breaks down once we relax our
assumptions about the network and the MIDI list:

  1. If we permit lost and reordered packets to occur in the
     network, the algorithm may produce unrecoverable rendering
     artifacts, violating the mandate defined in Section 4 of [RTPMIDI].

  2. If we permit the network to exhibit variable latency, the
     algorithm modulates the network jitter onto rendered MIDI
     command stream.

  3. If we permit a MIDI list to code commands with different
     timestamps, the algorithm adds temporal jitter to the
     rendered performance, as it ignores MIDI list timestamps.

In this section, we discuss interactive receiver design techniques under
these relaxed assumptions.  Section 6.1 describes a receiver design for
high-performance Wide Area Networks (WANs), and Section 6.2 discusses
design issues for other types of networks.

6.1 The NMP Receiver Design

The Network Musical Performance (NMP) system [NMP] is an interactive
performance application that uses an early version of the RTP MIDI
payload format.  NMP is designed for use between universities within the
State of California, using the high-performance CalREN2 network.

In the NMP system, network artifacts may affect how a musician hears the
performances of remote players.  However, the network does not affect
how a musician hears his own performance.

Several aspects of CalREN2 network behavior (as measured in 2001
timeframe, as documented in [NMP]) guided the NMP system design:

  o  The median symmetric latency (1/2 the round-trip time)
     of packets sent between network sites is comparable to the
     acoustic latency between two musicians located in the same
     room.  For example, the latency between Berkeley and Stanford
     is 2.1 ms, corresponding to an acoustic distance of 2.4 feet
     (0.72 meters).  These campuses are 40 miles (64 km) apart.
     Preserving the benefits of the underlying network latency
     at the application level was a key NMP design goal.

  o  For most times of day, the nominal temporal jitter is
     quite short.  For Berkeley-Stanford, the standard deviation
     of the round-trip time was under 200 microseconds.

  o  For most times of day, a few percent (0-4%) of the packets



Lazzaro/Wawrzynek                                              [Page 24]

INTERNET-DRAFT                                           21 October 2005


     sent arrive significantly late (> 40 ms), probably due
     to a queuing transient somewhere in the network path.
     More rarely (< 0.1%), a packet is lost during the transient.

  o  At predictable times during the day (before lunchtime,
     at the end of the workday, etc), network performance
     deteriorates (10-20% late packets) in a manner that makes
     the network unsuitable for low-latency interactive use.

  o  CalREN2 has deeply over-provisioned bandwidth, relative to
     MIDI bandwidth usage.

The NMP sender freely uses network bandwidth to improve the performance
experience.  As soon as a musician generates a MIDI command, an RTP
packet coding the command is sent to the other players.  This sending
algorithm reduces latency at the cost of bandwidth.  In addition, guard
packets (described in Section 4.2) are sent at frequent intervals, to
minimize the impact of packet loss.

The NMP receiver maintains a model of the stream, and uses this model as
the basis of its resiliency system.  Upon the receipt of a packet, the
receiver predicts the RTP sequence number and the RTP timestamp (with
error bars) of the packet.  Under normal network conditions, about 95%
of received packets fit the predictions [NMP].  In this common case, the
receiver immediately executes the MIDI command coded in the packet.

Note that the NMP receiver does not use a playout buffer; the design is
optimized for lowest latency at the expense of command jitter.  Thus,
the NMP receiver design does not completely satisfy the interoperability
text in Appendix C.7.2 of [RTPMIDI], which requires that receivers in
network musical performance applications be capable of using a playout
buffer.

Occasionally, an incoming packet fits the sequence number prediction,
but falls outside the timestamp prediction error bars (see Appendix B of
[NMP] for timestamp model details).  In most cases, the receiver still
executes the command coded in the packet.  However, the receiver
discards NoteOn commands with non-zero velocity.  By discarding late
commands that sound notes, the receiver prevents "straggler notes" from
disturbing a performance.  By executing all other late commands, the
receiver quiets "soft stuck notes" immediately, and updates the state of
the MIDI system.

More rarely, an incoming packet does not fit the sequence number
prediction.   The receiver keeps track of the highest sequence number
received in the stream, and predicts that an incoming packet will have a
sequence number one greater than this value.  If the sequence number of
an incoming packet is greater than the prediction, a packet loss has



Lazzaro/Wawrzynek                                              [Page 25]

INTERNET-DRAFT                                           21 October 2005


occurred.  If the sequence number of the received packet is less than
the prediction, the packet has been received out of order.  All sequence
number calculations are modulo 2^16, and use standard methods (described
in [RFC3550]) to avoid tracking errors during rollover.

If a packet loss has occurred, the receiver examines the journal section
of the received packet, and uses it to gracefully recover from the loss
episode.  We describe this recovery procedure in Section 7 of this memo.
The recovery process may result in the execution of one or more MIDI
commands.  After executing the recovery commands, the receiver processes
the MIDI command encoded in the packet, using the timestamp model test
described above.

If a packet is received out of order, the receiver ignores the packet.
The receiver takes this action because a packet received out of order is
always preceded by a packet that signalled a loss event.  This loss
event triggered the recovery process, which may have executed recovery
commands.  The MIDI command coded in the out-of-order packet might, if
executed, duplicate these recovery commands, and this duplication might
endanger the integrity of the stream.  Thus, ignoring the out-of-order
packet is the safe approach.

6.2 High-Jitter Networks, Local Area Networks

The NMP receiver targets a network with a particular set of
characteristics: low nominal jitter, low packet loss, and occasional
outlier packets that arrive very late.  In this section, we consider how
networks with different characteristics impact receiver design.

Networks with significant nominal jitter cannot use the buffer-free
receiver design described in Section 6.1.  For example, the NMP system
performs poorly for musicians that use dial-up modem connections,
because the buffer-free receiver design modulates modem jitter onto the
performances.  Receivers designed for high-jitter networks should use a
substantial playout buffer.  References [GRAME] and [CCRMA] describe how
to use playout buffers in latency-critical applications.

Receivers intended for use on Local Area Networks (LANs) face a
different set of issues.  A dedicated LAN fabric built with modern
hardware is in many ways a predictable environment.  The network
problems addressed by the NMP receiver design (packet loss and outlier
late packets) might only occur under extreme network overload
conditions.

Systems designed for this environment may choose to configure streams
without the recovery journal system (Appendix C.2.1 of [RTPMIDI]).
Receivers may also wish to forego, or simplify, the detection of outlier
late packets.  Receivers should monitor the RTP sequence numbers of



Lazzaro/Wawrzynek                                              [Page 26]

INTERNET-DRAFT                                           21 October 2005


incoming packets, to detect network unreliability.

However, in some respects, LAN applications may be more demanding than
WAN applications.  In LAN applications, musicians may be receiving
performance feedback from audio that is rendered from the stream.  The
tolerance a musician has for latency and jitter in this context may be
quite low.

To reduce the perceived jitter, receivers may use a small playout buffer
(in the range of 100us to 2ms).  The buffer does add a small amount of
latency to the system, that may be annoying to some players.  Receiver
designs should include buffer tuning parameters, to let musicians adjust
the tradeoff between latency and jitter.


7.  Receiving Streams: The Recovery Journal

In this section, we describe the recovery algorithm used by the NMP
receiver [NMP].  In most ways, the recovery techniques we describe are
generally applicable to interactive receiver design.  However, a few
aspects of the design are specialized for the NMP system:

  o The recovery algorithm covers a subset of the MIDI command
    set.  MIDI Systems (0xF), Poly Aftertouch (0xA), and Channel
    Aftertouch (0xD) commands are not protected, and Control
    Change (0xB) commands protection is simplified.  Note commands
    for a particular note number are assumed to follow the typical
    NoteOn->NoteOff->NoteOn->NoteOff pattern.  The cm_unused and
    ch_never parameters in Figures 1-2 specify this coverage.

  o The NMP system does not use a playout buffer.  Therefore, the
    recovery algorithm does not address interactions with a
    playout buffer.

At a high level, the receiver algorithm works as follows.  Upon the
detection of a packet loss, the receiver examines the recovery journal
of the packet that ends the loss event.  If necessary, the receiver
executes one or more MIDI commands to recover from the loss.

To prepare for recovery, a receiver maintains a data structure, the
Recovery Journal Receiver Structure (RJRS).  The RJRS codes information
about the MIDI commands the receiver executes (both incoming stream
commands and self-generated recovery commands).  At the start of the
stream, the RJRS is initialized to code that no commands have been
executed.   Immediately after executing a MIDI command, the receiver
updates the RJRS with information about the command.





Lazzaro/Wawrzynek                                              [Page 27]

INTERNET-DRAFT                                           21 October 2005


We now describe the recovery algorithm in detail.  We begin with two
definitions that classify loss events.  These definitions assume that
the packet that ends the loss event has RTP sequence number I.

  o Single-packet loss.  A single-packet loss occurs if the last
    packet received before the loss event (excluding out-of-order
    packets) has the sequence number I-2 (modulo 2^16).

  o Multi-packet loss.  A multi-packet loss occurs if the last
    packet received before the loss event (excluding out-of-order
    packets) has a sequence number less than I-2 (modulo 2^16).

Upon the detection of a packet loss, the recovery algorithm examines the
recovery journal header (Figure 8 of [RTPMIDI]) to check for special
cases:

  o If the header field A is 0, the recovery journal has no channel
    journals, and so no action is taken.

  o If a single-packet loss has occurred, and the header S bit is
    1, the lost packet has a MIDI command section with an empty
    MIDI list.  No action is taken.

If these checks fail, the algorithm parses the recovery journal body.
For each channel journal (Figure 9 in [RTPMIDI]) in the recovery
journal, the receiver compares the data in each chapter journal
(Appendix A of [RTPMIDI]) to the RJRS data for the chapter.  If the data
are inconsistent, the algorithm infers that MIDI command(s) related to
the chapter journal have been lost.  The recovery algorithm executes
MIDI commands to repair this loss, and updates the RJRS to reflect the
repair.

For single-packet losses, the receiver skips channel and chapter
journals whose S bits are set to 1.  For multi-packet losses, the
receiver parses each channel and chapter journal and checks for
inconsistency.

In the sections that follow, we describe the recovery steps that are
specific to each chapter journal.  We cover 4 chapter journal types: P
(Program Change, 0xC), C (Control Change, 0xB), W (Pitch Wheel, 0xE),
and N (Note, 0x8 and 0x9).  Chapters are parsed in the order of their
appearance in the channel journal (P, then W, then N, then C).

The sections below reference the C implementation of the RJRS shown in
Figure 10.  This structure is hierarchical, reflecting the recovery
journal architecture.  At the leaf level, specialized data structures
(jrec_chapterw, jrec_chaptern, jrec_chapterc, and jrec_chapterp) code
state variables for a single chapter journal type.  A mid-level



Lazzaro/Wawrzynek                                              [Page 28]

INTERNET-DRAFT                                           21 October 2005


structure (jrec_channel) represents a single MIDI channel, and a top-
level structure (jrec_stream) represents the entire MIDI stream.

















































Lazzaro/Wawrzynek                                              [Page 29]

INTERNET-DRAFT                                           21 October 2005


  typedef unsigned char  uint8;       /* must be 1 octet  */
  typedef unsigned short uint16;      /* must be 2 octets */
  typedef unsigned long  uint32;      /* must be 4 octets */


  /*****************************************************************/
  /* leaf level of hierarchy: Chapter W, Appendix A.5 of [RTPMIDI] */
  /*****************************************************************/

  typedef struct jrec_chapterw {   /* MIDI Pitch Wheel (0xE) */

   uint16 val;           /* most recent 14-bit wheel value   */

  } jrec_chapterw;


  /*****************************************************************/
  /* leaf level of hierarchy: Chapter N, Appendix A.6 of [RTPMIDI] */
  /*****************************************************************/

  typedef struct jrec_chaptern { /* Note commands (0x8, 0x9) */

   /* arrays of length 128 --> one for each MIDI Note number */

   uint32 time[128];    /* exec time of most recent NoteOn */
   uint32 extseq[128];  /* extended seqnum for that NoteOn */
   uint8  vel[128];     /* NoteOn velocity (0 for NoteOff) */

  } jrec_chaptern;


  /*****************************************************************/
  /* leaf level of hierarchy: Chapter C, Appendix A.3 of [RTPMIDI] */
  /*****************************************************************/

  typedef struct jrec_chapterc {     /* Control Change (0xB) */

   /* array of length 128 --> one for each controller number */

   uint8 value[128];   /* Chapter C value tool state */
   uint8 count[128];   /* Chapter C count tool state */
   uint8 toggle[128];  /* Chapter C toggle tool state */

  } jrec_chapterc;



     Figure 10 -- Recovery Journal Receiving Structure (part 1)



Lazzaro/Wawrzynek                                              [Page 30]

INTERNET-DRAFT                                           21 October 2005


  /*****************************************************************/
  /* leaf level of hierarchy: Chapter P, Appendix A.2 of [RTPMIDI] */
  /*****************************************************************/

  typedef struct jrec_chapterp { /* MIDI Program Change (0xC) */

   uint8 prognum;       /* most recent 7-bit program value  */
   uint8 prognum_qual;  /* 1 once first 0xC command arrives */

   uint8 bank_msb;     /* most recent Bank Select MSB value */
   uint8 bank_msb_qual;   /* 1 once first 0xBn 0x00 arrives */

   uint8 bank_lsb;     /* most recent Bank Select LSB value */
   uint8 bank_lsb_qual;   /* 1 once first 0xBn 0x20 arrives */

  } jrec_chapterp;



  /***************************************************/
  /* second-level of hierarchy, for MIDI channels    */
  /***************************************************/

  typedef struct jrec_channel {

   jrec_chapterp chapterp;  /* Program Change (0xC) info  */
   jrec_chapterc chapterc;  /* Control Change (0xB) info  */
   jrec_chapterw chapterw;  /* Pitch Wheel (0xE) info  */
   jrec_chaptern chaptern;  /* Note (0x8, 0x9) info  */

  } jrec_channel;



  /***********************************************/
  /* top level of hierarchy, for the MIDI stream */
  /***********************************************/

   typedef struct jrec_stream {

   jrec_channel channels[16];  /* index is MIDI channel */

   } jrec_stream;




    Figure 10 (continued) -- Recovery Journal Receiving Structure



Lazzaro/Wawrzynek                                              [Page 31]

INTERNET-DRAFT                                           21 October 2005


7.1 Chapter W: MIDI Pitch Wheel (0xE)

Chapter W of the recovery journal protects against the loss of MIDI
Pitch Wheel (0xE) commands.  A common use of the Pitch Wheel command is
to transmit the current position of a rotary "pitch wheel" controller
placed on the side of MIDI piano controllers.  Players use the pitch
wheel to dynamically alter the pitch of all depressed keys.

The NMP receiver maintains the jrec_chapterw structure (Figure 10) for
each voice channel in jrec_stream, to code pitch wheel state
information.  In jrec_chapterw, val holds the 14-bit data value of the
most recent Pitch Wheel command that has arrived on a channel.  At the
start of the stream, val is initialized to the default pitch wheel value
(0x2000).

At the end of a loss event, a receiver may find a Chapter W (Appendix
A.5 in [RTPMIDI]) bitfield in a channel journal.  This chapter codes the
14-bit data value of the most recent MIDI Pitch Wheel command in the
checkpoint history.  If the Chapter W and jrec_chapterw pitch wheel
values do not match, one or more commands have been lost.

To recover from this loss, the NMP receiver immediately executes a MIDI
Pitch Wheel command on the channel, using the data value coded in the
recovery journal.  The receiver then updates the jrec_chapterw variables
to reflect the executed command.

7.2 Chapter N: MIDI NoteOn (0x8) and NoteOff (0x9)

Chapter N of the recovery journal protects against the loss of MIDI
NoteOn (0x9) and NoteOff (0x8) commands.  If a NoteOn command is lost, a
note is skipped.  If a NoteOff command is lost, a note may sound
indefinitely.  Recall that NoteOn commands with a velocity value of 0
have the semantics of NoteOff commands.

The recovery algorithms in this section only work for MIDI sources that
produce NoteOn->NoteOff->NoteOn->NoteOff patterns for a note number.
Piano keyboard and drum pad controllers produce these patterns.  MIDI
sources that use NoteOn->NoteOn->NoteOff->NoteOff patterns for legato
repeated notes, such as guitar and wind controllers, require more
sophisticated recovery strategies.  Chapter E (not used in this example)
supports recovery algorithms for atypical note command patterns (see
Appendix A.7 of [RTPMIDI] for details).

The NMP receiver maintains a jrec_chaptern structure (Figure 10) for
each voice channel in jrec_stream, to code note-related state
information.  State is kept for each of the 128 note numbers on a
channel, using three arrays of length 128 (vel[], seq[], and time[]).
The arrays are initialized to zero at the start of a stream.



Lazzaro/Wawrzynek                                              [Page 32]

INTERNET-DRAFT                                           21 October 2005


The vel[n] array element holds information about the most recent note
command for note number n.  If this command is a NoteOn command, vel[n]
holds the velocity data for the command.  If this command is a NoteOff
command, vel[n] is set to 0.

The time[n] and extseq[n] array elements code information about the most
recently executed NoteOn command.  The time[n] element holds the
execution time of the command, referenced to the local timebase of the
receiver.  The extseq[n] element holds the RTP extended sequence number
of the packet associated with the command.  For incoming stream
commands, extseq[n] codes the packet of the associated MIDI list.  For
commands executed to perform loss recovery, extseq[n] codes the packet
of the associated recovery journal.

The Chapter N recovery journal bitfield (Figure A.6.1 in [RTPMIDI])
consists of two data structures: a bit array coding recently-sent
NoteOff commands that are vulnerable to packet loss, and a note log list
coding recently-sent NoteOn commands that are vulnerable to packet loss.

At the end of a loss event, Chapter N recovery processing begins with
the NoteOff bit array.  For each set bit in the array, the receiver
checks the corresponding vel[n] element in jrec_chaptern.  If vel[n] is
non-zero, a NoteOff command, or a NoteOff->NoteOn->NoteOff command
sequence, has been lost.  To recover from this loss, the receiver
immediately executes a NoteOff command for the note number on the
channel, and sets vel[n] to 0.

The receiver then parses the note log list, using the S bit to skip over
"safe" logs in the single-packet loss case.  For each at-risk note log,
the receiver checks the corresponding vel[n] element.

If vel[n] is zero, a NoteOn command, or a NoteOn->NoteOff->NoteOn
command sequence, has been lost.  The receiver may execute the most
recent lost NoteOn (to play the note) or may take no action (to skip the
note), based on criteria we describe at the end of this section.
Whether the note is played or skipped, the receiver updates the vel[n],
time[n], and extseq[n] elements as if the NoteOn executed.

If vel[n] is non-zero, the receiver performs several checks to test if a
NoteOff->NoteOn sequence has been lost.

  o If vel[n] does not match the note log velocity, the note log
    must code a different NoteOn command, and thus a NoteOff->NoteOn
    sequence has been lost.

  o If extseq[n] is less than the (extended) checkpoint packet
    sequence numbed coded in the recovery journal header (Figure 8
    of [RTPMIDI]), the vel[n] NoteOn command is not in the checkpoint



Lazzaro/Wawrzynek                                              [Page 33]

INTERNET-DRAFT                                           21 October 2005


    history, and thus a NoteOff->NoteOn sequence has been lost.

  o If the Y bit is set to 1, the NoteOn is musically "simultaneous"
    with the RTP timestamp of the packet.  If time[n] codes a time
    value that is clearly not recent, a NoteOff->NoteOn sequence has
    been lost.

If these tests indicate a lost NoteOff->NoteOn sequence, the receiver
immediately executes a NoteOff command.   The receiver decides if the
most graceful action is to play or to skip the lost NoteOn, using the
criteria we describe at the end of this section.  Whether or not the
receiver issues a NoteOn command, the vel[n], time[n], and extseq[n]
arrays are updated as if it did.

Note that the tests above do not catch all lost NoteOff->NoteOn
commands.  If a fast NoteOn->NoteOff->NoteOn sequence occurs on a note
number, with identical velocity values for both NoteOn commands, a lost
NoteOff->NoteOn does not result in the recovery algorithm generating a
NoteOff command.  Instead, the first NoteOn continues to sound, to be
terminated by the future NoteOff command.   In practice, this (rare)
outcome is not musically objectionable.

The number of tests in this resiliency algorithm may seem excessive.
However, in some common cases, a subset of the tests are not useful.
For example, MIDI streams that assigns the same velocity value to all
note events are often produced by inexpensive keyboards.  The vel[n]
tests are not useful for these streams.

Finally, we discuss how the receiver decides whether to play or to skip
a lost NoteOn command.  The note log Y bit is set if the NoteOn is
"simultaneous" with the RTP timestamp of the packet holding the note
log.  If Y is 0, the receiver does not execute a NoteOn command.  If Y
is 1, and if the packet has not arrived late, the receiver immediately
executes a NoteOn command for the note number, using the velocity coded
in the note log.

7.3 Chapter C: MIDI Control Change (0xB)

Chapter C (Appendix A.3 in [RTPMIDI]) protects against the loss of MIDI
Control Change commands.   A Control Change command alters the 7-bit
value of one of the 128 MIDI controllers.

Chapter C offers three tools for protecting a Control Change command:
the value tool (for graded controllers such as sliders) the toggle tool
(for on/off switches) and the count tool (for momentary-contact
switches).  Senders choose a tool to encode recovery information for a
controller, and encode the tool type along with the data in the journal
(Figures A.3.2 and A.3.3 in [RTPMIDI]).



Lazzaro/Wawrzynek                                              [Page 34]

INTERNET-DRAFT                                           21 October 2005


A few uses of Control Change commands are not solely protected by
Chapter C.  The protection of controllers 0 and 32 (Bank Select MSB and
Bank Select LSB) is shared between Chapter C and Chapter P (Section
7.4).

Chapter M (Appendix A.4 of [RTPMIDI]) also protects the Control Change
command.  However, the NMP system does not use this chapter, because
MPEG 4 Structured Audio [MPEGSA] does not use the controllers protected
by this chapter.

The Chapter C bitfield consists of a list of controller logs.  Each log
codes the controller number, the tool type, and the state value for the
tool.

The NMP receiver maintains the jrec_chapterc structure (Figure 10) for
each voice channel in jrec_stream, to code Control Change state
information.  The value[] array holds the most recent data values for
each controller number.  At the start of the stream, value[] is
initialized to the default controller data values specified in [MPEGSA].

The count[] and toggle[] arrays hold the count tool and toggle tool
state values.  At the start of a stream, these arrays are initialized to
zero.  Whenever a Control Command executes, the receiver updates the
count[] and toggle[] state values, using the algorithms defined in
Appendix A.3 of [RTPMIDI].

At the end of a loss event, the receiver parses the Chapter C controller
log list, using the S bit to skip over "safe" logs in the single-packet
loss case.  For each at-risk controller number n, the receiver
determines the tool type in use (value, toggle, or count), and compares
the data in the log to the associated jrec_chapterc array element
(value[n], toggle[n], or count[n]).  If the data do not match, one or
more Control Change commands have been lost.

The method the receiver uses to recover from this loss depends on the
tool type and the controller number.  For graded controllers protected
by the value tool, the receiver executes a Control Change command using
the new data value.

For the toggle and count tools, the recovery action is more complex.
For example, the Damper Pedal (Sustain) controller (number 64) is
typically used as a sustain pedal for piano-like sounds, and is
typically coded using the toggle tool.  If Damper Pedal (Sustain)
Control Change command(s) are lost, the receiver takes different actions
depending on the starting and ending state of the lost sequence, to
ensure "ringing" piano notes are "damped" to silence.





Lazzaro/Wawrzynek                                              [Page 35]

INTERNET-DRAFT                                           21 October 2005


After recovering from the loss, the receiver updates the value[],
toggle[], and count[] arrays to reflect the Chapter C data and the
executed commands.

7.4 Chapter P: MIDI Program Change (0xC)

Chapter P of the recovery journal protects against the loss of MIDI
Program Change (0xC) commands.

The 7-bit data value of the Program Change command selects one of 128
possible timbres for the channel.  To increase the number of possible
timbres, Control Change (0xB) commands may be issued prior to the
Program Change command, to select a "program bank".  The Bank Select MSB
(number 0) and Bank Select LSB (number 32) controllers specify the
14-bit bank number that subsequent Program Change commands reference.

The NMP receiver maintains the jrec_chapterp structure (Figure 10) for
each voice channel in jrec_stream, to code Program Change state
information.

The prognum variable of jrec_chapterp holds the data value for the most
recent Program Change command that has arrived on the stream.  The
bank_msb and bank_lsb variables of jrec_chapterp code the Bank Select
MSB and Bank Select LSB controller data values that were in effect when
that Program Change command arrived.  The prognum_qual, bank_msb_qual
and bank_lsb_qual variables are initialized to 0, and are set to 1 to
qualify the associated data values.

Chapter P fields code the data value for the most recent Program Change
command, and the MSB and LSB bank values in effect for that command.

At the end of a loss event, the receiver checks Chapter P to see if the
recovery journal fields match the data stored in jrec_chapterp.  If
these checks fail, one or more Program Change commands have been lost.

To recover from this loss, the receiver takes the following steps.  If
the B bit in Chapter P is set (Figure A.2.1 in [RTPMIDI]), Control
Change bank command(s) have preceded the Program Change command.  The
receiver compares the bank data coded by Chapter P with the current bank
data for the channel (coded in jrec_channelc).

If the bank data do not agree, the receiver issues Control Change
command(s) to align the stream with Chapter P.  The receiver then
updates jrec_channelp and jrec_channelc variables to reflect the
executed command(s).  Finally, the receiver issues a Program Change
command that reflects the data in Chapter P, and updates the prognum and
qual_prognum fields in jrec_channelp.




Lazzaro/Wawrzynek                                              [Page 36]

INTERNET-DRAFT                                           21 October 2005


Note that this method relies on Chapter P recovery to precede Chapter C
recovery during channel journal processing.  This ordering ensures that
lost Bank Select Control Change commands that occur after a lost Program
Change command in a stream are handled correctly.


8.  Security Considerations

Security considerations for the RTP MIDI payload format are discussed in
the Security Considerations section of [RTPMIDI].


9.  IANA Considerations

IANA considerations for the RTP MIDI payload format are discussed in the
IANA Considerations section of [RTPMIDI].


A.  Acknowledgments

This memo was written in conjunction with [RTPMIDI], and the
Acknowledgments section of [RTPMIDI] also applies to this memo.


B.  References

B.1 Normative References

[RTPMIDI] Lazzaro, J., and J. Wawrzynek.  "RTP Payload Format for MIDI",
work in progress, draft-ietf-avt-rtp-midi-format-13.txt.

[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson.
"RTP: A transport protocol for real-time applications", RFC 3550, July
2003.

[RFC3551] Schulzrinne, H., and S. Casner.  "RTP Profile for Audio and
Video Conferences with Minimal Control", RFC 3551, July 2003.

[MIDI] MIDI Manufacturers Association.  "The Complete MIDI 1.0
Detailed Specification", 1996.

[SDP] Handley, M., Jacobson, V., and C. Perkins.  "SDP: Session
Description Protocol", draft-ietf-mmusic-sdp-new-25.txt.

[MPEGSA] International Standards Organization.  "ISO/IEC 14496
MPEG-4", Part 3 (Audio), Subpart 5 (Structured Audio), 2001.

B.2 Informative References



Lazzaro/Wawrzynek                                              [Page 37]

INTERNET-DRAFT                                           21 October 2005


[NMP] Lazzaro, J. and J. Wawrzynek.  "A Case for Network Musical
Performance", 11th International Workshop on Network and Operating
Systems Support for Digital Audio and Video (NOSSDAV 2001) June 25-26,
2001, Port Jefferson, New York.

[RFC3261] Rosenberg, J, Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler.  "SIP: Session
Initiation Protocol", RFC 3261, June 2002.

[GRAME] Fober, D., Orlarey, Y. and S. Letz.  "Real Time Musical Events
Streaming over Internet", Proceedings of the International Conference
on WEB Delivering of Music 2001, pages 147-154.

[CCRMA] Chafe C., Wilson S., Leistikow R., Chisholm D., and G. Scavone.
"A simplified approach to high quality music and sound over IP",
COST-G6 Conference on Digital Audio Effects (DAFx-00), Verona, Italy,
December 2000.

[RTPBOOK] Perkins, C.  "RTP: Audio and Video for the Internet",
Addison-Wesley, ISBN 0-672-32249-8, 2003.


C.  Authors' Addresses

John Lazzaro (corresponding author)
UC Berkeley
CS Division
315 Soda Hall
Berkeley CA 94720-1776
Email: lazzaro@cs.berkeley.edu

John Wawrzynek
UC Berkeley
CS Division
631 Soda Hall
Berkeley CA 94720-1776
Email: johnw@cs.berkeley.edu














Lazzaro/Wawrzynek                                              [Page 38]

INTERNET-DRAFT                                           21 October 2005


D.  Intellectual Property Rights Statement

The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in this
document or the extent to which any license under such rights might or
might not be available; nor does it represent that it has made any
independent effort to identify any such rights.  Information on the
procedures with respect to rights in RFC documents can be found in BCP
78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an attempt
made to obtain a general license or permission for the use of such
proprietary rights by implementers or users of this specification can be
obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary rights
that may cover technology that may be required to implement this
standard.  Please address the information to the IETF at ietf-
ipr@ietf.org.


E.  Full Copyright Statement

Copyright (C) The Internet Society (2005).  This document is subject to
the rights, licenses and restrictions contained in BCP 78, and except as
set forth therein, the authors retain all their rights.

This document and the information contained herein are provided
on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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.







Lazzaro/Wawrzynek                                              [Page 39]

INTERNET-DRAFT                                           21 October 2005


F.  Change Log

[Note to RFC Editors: this Appendix, and its Table of Contents listing,
should be removed from the final version of the memo]

References were changed to keep this document in sync with draft-ietf-
avt-rtp-midi-format-13.txt.












































Lazzaro/Wawrzynek                                              [Page 40]


Html markup produced by rfcmarkup 1.107, available from http://tools.ietf.org/tools/rfcmarkup/