Network Working Group J. Rosenberg
Request for Comments: 2733 dynamicsoft
Category: Standards Track H. Schulzrinne
Columbia University
December 1999
An RTP Payload Format for Generic Forward Error Correction
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 (1999). All Rights Reserved.
Abstract
This document specifies a payload format for generic forward error
correction of media encapsulated in RTP. It is engineered for FEC
algorithms based on the exclusive-or (parity) operation. The payload
format allows end systems to transmit using arbitrary block lengths
and parity schemes. It also allows for the recovery of both the
payload and critical RTP header fields. Since FEC is sent as a
separate stream, it is backwards compatible with non-FEC capable
hosts, so that receivers which do not wish to implement FEC can just
ignore the extensions.
Table of Contents
1 IntrodUCtion ........................................... 2
2 Terminology ............................................ 2
3 Basic Operation ........................................ 3
4 Parity Codes ........................................... 5
5 RTP Media Packet Structure ............................. 6
6 FEC Packet Structure ................................... 7
6.1 RTP Header of FEC Packets .............................. 7
6.2 FEC Header ............................................. 7
7 Protection Operation ................................... 9
8 Recovery Procedures .................................... 10
8.1 Reconstruction ......................................... 10
8.2 Determination of When to Recover ....................... 12
9 Example ................................................ 16
10 Use with Redundant Encodings ........................... 17
11 Indicating FEC Usage in SDP ............................ 20
11.1 FEC as a Separate Stream ............................... 20
11.2 Use with Redundant Encodings ........................... 21
11.3 Usage with RTSP ........................................ 22
12 Security Considerations ................................ 23
13 Acknowledgments ........................................ 24
14 Authors' Addresses ..................................... 24
15 Bibliography ........................................... 25
16 Full Copyright Statement ............................... 26
1 Introduction
The quality of packet voice on the Internet has been mediocre due, in
part, to high packet loss rates. This is especially true on wide-area
connections. Unfortunately, the strict delay requirements of real-
time multimedia usually eliminate the possibility of retransmissions.
It is for this reason that forward error correction (FEC) has been
proposed to compensate for packet loss in the Internet [1] [2]. In
particular, the use of traditional error correcting codes, such as
parity, Reed-Solomon, and Hamming codes, has attracted attention. To
support these mechanisms, protocol support is required.
This document defines a payload format for RTP [3] which allows for
generic forward error correction of real time media. In this context,
generic means that the FEC protocol is (1) independent of the nature
of the media being protected, be it audio, video, or otherwise, (2)
flexible enough to support a wide variety of FEC mechanisms, (3)
designed for adaptivity so that the FEC technique can be modified
easily without out of band signaling, and (4) supportive of a number
of different mechanisms for transporting the FEC packets.
2 Terminology
The following terms are used throughout this document:
Media Payload: is a piece of raw, un-protected user data which
is to be transmitted from the sender. The media payload is
placed inside of an RTP packet.
Media Header: is the RTP header for the packet containing the
media payload.
Media Packet: The combination of a media payload and media
header is called a media packet.
FEC Packet: The forward error correction algorithms at the
transmitter take the media packets as an input. They output
both the media packets that they are passed, and new
packets called FEC packets. The FEC packets are formatted
according to the rules specified in this document.
FEC Header: The FEC header is the header information contained
in an FEC packet.
FEC Payload: The FEC payload is the payload in an FEC packet.
Associated: An FEC packet is said to be "associated" with one or
more media packets when those media packets are used to
generate the FEC packet (by use of the exclusive or
operation).
The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [4].
3 Basic Operation
The payload format described here is used whenever a participant in
an RTP session would like to protect a media stream it is sending
with forward error correction (FEC). The FEC supported by the format
are those codes based on simple exclusive or (xor) parities. The
sender takes some set of packets from the media stream, and applies
an xor operation across the payloads. The sender also applies the xor
operation over components of the RTP headers. Based on the procedures
defined here, the result is an RTP packet containing FEC information.
This packet can be used at the receiver to recover any one of the
packets used to generate the FEC packet. This document does not
mandate the particular set of media packets combined to generate an
FEC packet (such a set [is] referred to as a code). Use of differing
sets results in a tradeoff between overhead, delay, and
recoverability. Section 4 outlines some possible combinations.
The payload format contains information that allows the sender to
tell the receiver exactly which media packets have been used to
generate the FEC. Specifically, each FEC packet contains a bitmask,
called the offset mask, containing 24 bits. If bit i in the mask is
set to 1, the media packet with sequence number N + i was used to
generate this FEC packet. N is called the sequence number base, and
is sent in the FEC packet as well. The offset mask and payload type
are sufficient to signal arbitrary parity based forward error
correction schemes with little overhead.
This document also describes procedures that allow the receiver to
make use of the FEC without having to know the details of specific
codes. This allows the sender much flexibility; it can adapt the code
in use based on network conditions, and be certain the receivers can
still make use of the FEC for recovery.
As the sender generates FEC packets, they are sent to the receivers.
The sender still usually sends the original media stream, as if there
were no FEC. This allows the media stream to still be used by
receivers who are not FEC capable. However, some FEC codes do not
require the original media to be sent; the FEC stream is sufficient
for recovery. These codes have the drawback that all receivers must
be FEC capable. However, they are supported by this format.
The FEC packets are not sent in the same RTP stream as the media
packets. They can be sent as a separate stream, or as a secondary
codec in the redundant codec payload format [5]. When sent as a
separate stream, the FEC packets have their own sequence number
space. Although the timestamps for the FEC packets are derived from
the media packets, they increment monotonically. FEC packet streams
thus work well with any header compression mechanism which requires
fixed deltas between fields in the packet header.
This document does not prescribe the definition of "separate
streams", but leaves this to applications and higher level protocols
to define. For multicast, the separate stream may be implemented by
separate multicast groups, different ports in the same group, or by a
different SSRC within the same group/port. For unicast, different
ports or different SSRC may be used. Each of these approaches has
drawbacks and benefits which depend on the application.
At the receiver, the FEC and original media are received. If no media
packets are lost, the FEC can be ignored. In the event of loss, the
FEC packets can be combined with other media and FEC packets that
have been received, resulting in recovery of missing media packets.
The recovery is exact; the payload is perfectly reconstructed, along
with most components of the header.
RTP packets which contain data formatted according to this
specification (i.e., FEC packets) are signaled using dynamic RTP
payload types.
4 Parity Codes
For brevity, we define the function f(x,y,..) to be the XOR (parity)
operator applied to the packets x,y,... The output of this function
is another packet, called the parity packet. For simplicity, we
assume here that the parity packet is computed as the bitwise XOR of
the input packets. The exact procedure is specified in section 6.
Recovery of data packets using parity codes is accomplished by
generating one or more parity packets over a group of data packets.
To be effective, the parity packets must be generated by linearly
independent combinations of data packets. The particular combination
is called a parity code. One class of codes takes a group of k data
packets, and generates n-k parity packets. There are a large number
of possible parity codes for a given n,k. The payload format does not
mandate a particular code.
For example, consider a parity code which generates a single parity
packet over two data packets. If the original media packets are
a,b,c,d, the packets generated by the sender are:
a b c d <-- media stream
f(a,b) f(c,d) <-- FEC stream
where time increases to the right. In this example, the error
correction scheme (we use the terms scheme and code interchangeably)
introduces a 50% overhead. But if b is lost, a and f(a,b) can be used
to recover b.
Some additional codes are listed below. In each, the original media
stream consists of packets a,b,c,d and so on.
Scheme 1
--------
This scheme is the similar to the one in the example above. However,
instead of sending b, followed by f(a,b), f(a,b) is sent before b.
Doing this clearly requires additional delay at the sender. However,
if allows some bursts of two consecutive packet losses to be
recovered. The packets generated by the sender look like:
a b c d e <-- media stream
f(a,b) f(b,c) f(c,d) f(d,e) <-- FEC stream
Scheme 2
--------
It is not strictly necessary for the original media stream to be
transmitted. In this scheme, only FEC packets are transmitted. This
scheme allows for recovery of all single packet losses and some
consecutive packet losses, but with slightly less overhead than
scheme 1. The packets generated by the sender look like:
f(a,b) f(a,c) f(a,b,c) f(c,d) f(c,e) f(c,d,e) <-- FEC stream
Scheme 3
--------
This scheme requires the receiver to wait an additional four packet
intervals to recover the original media packets. However, it can
recover from one, two or three consecutive packet losses. The packets
generated by the sender look like:
a b c d <-- media stream
f(a,b,c) f(a,c,d) f(a,b,d) <-- FEC stream
5 RTP Media Packet Structure
The formatting of the media packets is unaffected by FEC. If the FEC
is sent as a separate stream, the media packets are sent as if there
was no FEC. If the FEC is being sent as a redundant codec, the media
packets are sent as the main codec as defined in RFC2198 [5].
This lends to a very efficient encoding. When little (or no) FEC is
used, there are mostly media packets being sent. This means that the
overhead (present in FEC packets only) tracks the amount of FEC in
use.
6 FEC Packet Structure
An FEC packet is constructed by placing an FEC header and FEC payload
in the RTP payload, as shown in Figure 1:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RTP Header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FEC Header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FEC Payload
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: FEC Packet Structure
6.1 RTP Header of FEC Packets
The version field is set to 2. The padding bit is computed via the
protection operation, defined below. The extension bit is also
computed via the protection operation. The SSRC value will generally
be the same as the SSRC value of the media stream it protects. It MAY
be different if the FEC stream is being demultiplexed via the SSRC
value. The CC value is computed via the protection operation. The
CSRC list is never present, independent of the value of the CC field.
The extension is never present, independent of the value of the X
bit. The marker bit is computed via the protection operation.
The sequence number has the standard definition: it MUST be one
higher than the sequence number in the previously transmitted FEC
packet. The timestamp MUST be set to the value of the media RTP clock
at the instant the FEC packet is transmitted. This results in the TS
value in FEC packets to be monotonically increasing, independent of
the FEC scheme.
The payload type for the FEC packet is determined through dynamic,
out of band means. According to RFC1889 [3], RTP participants which
cannot recognize a payload type must discard it. This provides
backwards compatibility. The FEC mechanisms can then be used in a
multicast group with mixed FEC-capable and FEC-incapable receivers.
6.2 FEC Header
This header is 12 bytes. The format of the header is shown in Figure
2, and consists of an SN base field, length recovery field, E field,
PT recovery field, mask field and TS recovery field.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SN base length recovery
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
E PT recovery mask
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TS recovery
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Parity Header Format
The length recovery field is used to determine the length of any
recovered packets. It is computed via the protection operation
applied to the unsigned network-ordered 16 bit representation of the
sums of the lengths (in bytes) of the media payload, CSRC list,
extension and padding of media packets associated with this FEC
packet (in other words, the CSRC list, extension, and padding, if
present, are "counted" as part of the payload). This allows the FEC
procedure to be applied even when the lengths of the media packets
are not identical. For example, assume an FEC packet is being
generated by xor'ing two media packets together. The length of the
two media packets are 3 (0b011) and 5 (0b101) bytes, respectively.
The length recovery field is then encoded as 0b011 xor 0b101 = 0b110.
The E bit indicates a header extension. Implementations conforming to
this version of the specification MUST set this bit to zero.
The PT recovery field is oBTained via the protection operation
applied to the payload type values of the media packets associated
with the FEC packet.
The mask field is 24 bits. If bit i in the mask is set to 1, then the
media packet with sequence number N + i is associated with this FEC
packet, where N is the SN Base field in the FEC packet header. The
least significant bit corresponds to i=0, and the most significant to
i=23.
The SN base field MUST be set to the minimum sequence number of those
media packets protected by FEC. This allows for the FEC operation to
extend over any string of at most 24 packets.
The TS recovery field is computed via the protection operation
applied to the timestamps of the media packets associated with this
FEC packet. This allows the timestamp to be completely recovered.
The payload of the FEC packet is the protection operation applied to
the concatenation of the CSRC list, RTP extension, media payload, and
padding of the media packets associated with the FEC packet.
Note that it's possible for the FEC packet to be slightly larger than
the media packets it protects (due to the presence of the FEC
header). This could cause difficulties if this results in the FEC
packet exceeding the Maximum Transmission Unit size for the path
along which it is sent.
7 Protection Operation
The protection operation involves concatenating specific fields from
the RTP header of the media packet, appending the payload, padding
with zeroes, and then computing the xor across the resulting bit
strings. The resulting bit string is used to generate the FEC packet.
The following procedure MAY be followed for the protection operation.
Other procedures MAY be followed, but the end result MUST be
identical to the one described here. For each media packet to be
protected, a bit string is generated by concatenating the following
fields together in the order specifed:
o Padding Bit (1 bit)
o Extension Bit (1 bit)
o CC bits (4 bits)
o Marker bit (1 bit)
o Payload Type (7 bits)
o Timestamp (32 bits)
o Unsigned network-ordered 16 bit representation of the sum of
the lengths (in bytes) of the CSRC List, length of the padding,
length of the extension, and length of the media payload (16
bits)
o if CC is nonzero, the CSRC List (variable length)
o if X is 1, the Header Extension (variable length)
o the payload (variable length)
o Padding, if present (variable length)
Note that the Padding Bit (first entry above) forms the most
significant bit of the bit string.
If the lengths of the bit strings are not equal, each bit string that
is shorter than the length of the longest, MUST be padded to the
length of the longest. Any value for the pad may be used. The pad
MUST be added at the end of the bit string.
The parity operation is then applied across the bit strings. The
result is the bit string used to build the FEC packet. Call this the
FEC bit string.
The first (most significant) bit in the FEC bit string is written
into the Padding Bit of the FEC packet. The second bit in the FEC bit
string is written into the Extension bit of the FEC packet. The next
four bits of the FEC bit string are written into the CC field of the
FEC packet. The next bit of the FEC bit string is written into the
marker bit of the FEC packet. The next 7 bits of the FEC bit string
are written into the PT recovery field in the FEC packet header. The
next 32 bits of the FEC bit string are written into the TS recovery
field in the packet header. The next 16 bits are written into the
length recovery field in the FEC packet header. The remaining bits
are set to be the payload of the FEC packet.
8 Recovery Procedures
The FEC packets allow end systems to recover from the loss of media
packets. All of the header fields of the missing packets, including
CSRC lists, extensions, padding bits, marker and payload type, are
recoverable. This section describes the procedure for performing
this recovery.
Recovery requires two distinct operations. The first determines which
packets (media and FEC) must be combined in order to recover a
missing packet. Once this is done, the second step is to actually
reconstruct the data. The second step MUST be performed as described
below. The first step MAY be based on any algorithm chosen by the
implementer. Different algorithms result in a tradeoff between
complexity and the ability to recover missing packets if at all
possible.
8.1 Reconstruction
Let T be the list of packets (FEC and media) which can be combined to
recover some media packet xi. The procedure is as follows:
1. For the media packets in T, compute the bit string as
described in the protection operation of the previous
section.
2. For the FEC packet in T, compute the bit string in the same
fashion, except use the PT Recovery instead of Payload Type,
TS Recovery instead of Timestamp, and always set the CSRC
list, extension, and padding to null.
3. If any of the bit strings generated from the media packets
are shorter than the bit string generated from the FEC
packet, pad them to be the same length as the bit string
generated from the FEC. The padding MUST be added at the
end of the bit string, and MAY be of any value.
4. Perform the exclusive or (parity) operation across the bit
strings, resulting in a recovery bit string.
5. Create a new packet with the standard 12 byte RTP header
and no payload.
6. Set the version of the new packet to 2.
7. Set the Padding bit in the new packet to the first bit in
the recovery bit string.
8. Set the Extension bit in the new packet to the second bit
in the recovery bit string.
9. Set the CC field to the next four bits in the recovery bit
string.
10. Set the marker bit in the new packet to the next bit in the
recovery bit string.
11. Set the payload type in the new packet to the next 7 bits
in the recovery bit string.
12. Set the SN field in the new packet to xi.
13. Set the TS field in the new packet to the next 32 bits in
the recovery bit string.
14. Take the next 16 bits of the recovery bit string. Whatever
unsigned integer this represents (assuming network-order),
take that many bytes from the recovery bit string and
append them to the new packet. This represents the CSRC
list, extension, payload, and padding.
15. Set the SSRC of the new packet to the SSRC of the media
stream it's protecting.
This procedure will completely recover both the header and payload of
an RTP packet.
8.2 Determination of When to Recover
The previous section discussed how to recover a media packet with
sequence number xi when all of the packets needed to recover it were
available. The decision about whether to attempt recovery of some
media packet xi, and how to determine if sufficient data is available
to recover it, is left to the implementer. However, this section
provides a simple algorithm which MAY be used for this purpose.
The algorithm is described below in C code. The code assumes that
several functions exist. recover_packet() takes the sequence number
of a packet, and an FEC packet. Using the FEC packet and data packets
received previously, the data packet with the given sequence number
is recovered. add_fec_to_pending_list() adds the given FEC packet to
a linked list of FEC packets which have not yet been used for
recovery. wait_for_packet() waits for a packet, FEC or data, from the
network. remove_from_pending_list() removes the FEC packet from the
pending list. The structure packet contains a boolean variable fec
which is true when the packet is FEC, false if it's media. When its
an FEC packet, the mask and snbase field contain those values from
the FEC packet header. When it's a media packet, the sn variable
contains the sequence number of the packet. The global array A
indicates which media packets have been received, and which have not.
It is indexed by the sequence number of the packet.
The function fec_recovery implements the algorithm. It waits for
packets, and when it receives an FEC packet, calls recover_with_fec()
to attempt to use it to recover. If no recovery is possible, the FEC
packet is stored for later attempts. If the received packet was a
media packet, its presence is noted, and any old FEC packets are
checked to see if recovery is now possible. Recovered packets are
treated as if they were received, triggering further attempts at
recovery.
A real implementation will need to use a circular buffer instead of
the simple array (A in the code) in order to avoid running off the
end of the buffer. In addition, the code below does not attempt to
free up FEC packets that are old and were never used. Normally, such
discarding is done based on time constraints introduced by the
playout buffer. If an FEC data protects packets whose play time has
elapsed, the FEC is no longer needed.
typedef struct packet_s {
BOOLEAN fec; /* FEC or media */
int sn; /* SN of the packet, for media only */
BOOLEAN mask[24]; /* Mask, FEC only */
int snbase; /* SN Base, FEC only */
struct packet_s *next;
} packet;
BOOLEAN A[65535];
packet *pending_list;
packet *recover_with_fec(packet *fec_pkt) {
packet *data_pkt;
int pkts_present, /* number of packets from the mask that are
present */
pkts_needed, /* number of packets needed is the number of ones
in the mask minus 1 */
pkt_to_recover, /* sn of the packet we are recovering */
i;
pkts_present = 0;
/* The number of packets needed is the number of ones in the mask
minus 1. The code below increments pkts_needed by the number
of ones in the mask, so we initialize this to -1 so that the
final count is correct */
pkts_needed = -1;
/* Go through all 24 bits in the mask, and check if we have
all but one of the media packets */
for(i = 0; i < 24; i++) {
/* If the packet is here and in the mask, increment counter */
if(A[i+fec_pkt->snbase] && fec_pkt->mask[i]) pkts_present++;
/* Count the number of packets needed as well */
if(fec_pkt->mask[i]) pkts_needed++;
/* The packet to recover is the one with a bit in the
mask that's not here yet */
if(!A[i+fec_pkt->snbase] && fec_pkt->mask[i])
pkt_to_recover = i+fec_pkt->snbase;
}
/* If we can recover, do so. Otherwise, return NULL */
if(pkts_present == pkts_needed) {
data_pkt = recover_packet(pkt_to_recover, fec_pkt);
} else {
data_pkt = NULL;
}
return(data_pkt);
}
void fec_recovery() {
packet *p, /* packet received or regenerated */
*fecp, /* fec packet from pending list */
*pnew; /* new packets recovered */
while(1) {
p = wait_for_packet(); /* get packet from network */
while(p) {
/* if it's an FEC packet, try to recover with it. If we can't,
store it for later potential use. If we can recover, act as
if the recovered packet is received and try to recover some
more. Otherwise, if it's a data packet, mark it as received,
and check if we can now recover a data packet with the list
of pending FEC packets */
if(p->fec == TRUE) {
pnew = recover_with_fec(p);
if(pnew)
A[pnew->sn] = TRUE;
else
add_fec_to_pending_list(p);
/* We assign pnew to p since the while loop will continue
to recover based on p not being NULL */
p = pnew;
} else {
/* Mark this data packet as here */
A[p->sn] = TRUE;
free(p);
p = NULL;
/* Go through pending list. Try and recover a packet using
each FEC. If we are successful, add the data packet to
the list of received packets, remove the FEC packet from
the pending list, since we've used it, and then try to
recover some more */
for(fecp = pending_list; fecp != NULL; fecp = fecp->next) {
pnew = recover_with_fec(fecp);
if(pnew) {
/* The packet is now here, as we've recovered it */
A[pnew->sn] = TRUE;
/* One FEC packet can only be used once to recover,
so remove it from the pending list */
remove_fec_from_pending_list(fecp);
p = pnew;
break;
}
} /*for*/
} /*p->fec was false */
} /* while p*/
} /* while 1 */
}
9 Example
Consider 2 media packets to be sent, x and y, from SSRC 2. Their
sequence numbers are 8 and 9, respectively, with timestamps of 3 and
5, respectively. Packet x uses payload type 11, and packet y uses
payload type 18. Packet x is has 10 bytes of payload, and packet y
11. Packet y has its marker bit set. The RTP headers for packets x
and y are shown in Figures 3 and 4 respectively.
Media Packet x
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1 0000 0 0 000 0 0 1 0 1 10 0 0 0 0 0 0 0 0 0 0 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 1 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 1 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version: 2
Padding: 0
Extension: 0
Marker: 0
PTI: 11
SN: 8
TS: 3
SSRC: 2
Figure 3: RTP Header for Media Packet X
An FEC packet is generated from these two. We assume that payload
type 127 is used to indicate an FEC packet. The resulting RTP header
is shown in Figure 5.
The FEC header in the FEC packet is shown in Figure 6.
11 Use with Redundant Encodings
One can consider an FEC packet as a "redundant coding" of the media.
Media Packet y
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1 0000 0 0 010 0 1 0 0 1 00 0 0 0 0 0 0 0 0 0 0 0 1 0 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 1 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 1 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version: 2
Padding: 0
Extension: 0
Marker: 1
PTI: 18
SN: 9
TS: 5
SSRC: 2
Figure 4: RTP Header for Media Packet Y
Because of this, the payload format for encoding of redundant audio
data [5] can be used to carry the FEC data along with the media. The
procedure for this is as follows.
The FEC operation defined above acts on a stream of RTP media
packets. The stream which is operated on is the stream before the
encapsulation defined in RFC2198 [5]. In other words, the media
stream to be protected is encapsulated in standard RTP media packets.
The FEC operation above is performed (with one minor change),
generating a stream of FEC packets. The change to the procedure above
is that if the RTP packets being protected contain an RTP extension,
padding, or a CSRC list, these MUST be removed from the packets, and
the CC field, Padding Bit, and Extension but MUST be set to zero,
before the FEC operation is applied. These modified packets are used
in the procedure above. Note that the sender MUST still send the
original packets (with the CSRC list, padding, and extension in tact)
as the primary encoding in RFC2198. The removal of these fields only
applies to the protection operation.
Once the FEC packets have been generated, the media payload is
extracted from the media packets. This payload is used as the primary
encoding as defined in RFC2198. Then, the FEC header and payload of
the FEC packets is extracted, and treated as a redundant encoding.
Additional redundant encodings, besides FEC, MAY be added to the
packet as well. These encodings will not be protected by FEC,
however.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1 0000 0 0 011 1 1 1 1 1 10 0 0 0 0 0 0 0 0 0 0 0 0 0 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 1 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 1 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version: 2
Padding: 0
Extension: 0
Marker: 1
PTI: 127
SN: 1
TS: 5
SSRC: 2
Figure 5: RTP Header of FEC for Packets X and Y
The redundant encodings header for the primary codec is set as
defined in RFC2198. The redundant encodings header for the FEC data
is set as follows. The block PT is set to the dynamic PT associated
with the FEC format. The block length is set to the sum of the
lengths of the FEC header and payload. The timestamp offset SHOULD be
set to zero. The secondary coder payload includes the FEC header and
FEC payload.
At the receiver, the primary codec and all secondary codecs are
extracted as separate RTP packets. This is done by copying the
sequence number, SSRC, marker bit, CC field, RTP version, and
extension bit from the RTP header of the redundant encodings packet
to the RTP header of each extracted packet. If the secondary codec
contains FEC, the CC field, Extension Bit, and Padding Bit in the RTP
header of the FEC packet MUST be set to zero instead. The payload
type identifier in the extracted packet is copied from the block PT
of the redundant encodings header. The timestamp of the extracted
packet is the difference between the timestamp in the RTP header and
the offset in the block header. The payload of the extracted packet
is the data block. This will result in the FEC stream and media
stream being extracted.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00 0 1 1 0 0 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 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 1 1 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SN base: 8 [min(8,9)]
len. rec.: 1 [8 xor 9]
E: 0
PTI rec.: 25 [11 xor 18]
mask: 3
TS rec.: 6 [3 xor 5]
The payload length is 11 bytes.
Figure 6: FEC Header of Result
To use the FEC and media packets for recovery, the CSRC list,
extension, and padding MUST be removed from the media packets, if
present, and the CC field, Extension Bit, and Padding Bit MUST be set
to zero. These modified media packets, along with the FEC packets,
are then used to recover based on the procedures in section 8. The
recovered media packets will always have no extension, padding, or
CSRC list. An implementation MAY copy these fields into the recovered
packet from another media packet, if available.
Using the redundant encodings payload format also implies that the
marker bit may not be recovered correctly. Applications MUST set the
marker bit to zero in media packets reconstructed using FEC
encapsulated in RFC2198 redundancy.
An advantage of this approach is a reduction in the overhead for
sending FEC packets.
11 Indicating FEC Usage in SDP
FEC packets contain RTP packets with dynamic payload type values. In
addition, the FEC packets can be sent on separate multicast groups or
separate ports from the media. The FEC can even be carried in packets
containing media, using the redundant encodings payload format [5].
These configuration options must be indicated out of band. This
section describes how this can be accomplished using the Session
Description Protocol (SDP), specified in RFC2327 [6].
11.1 FEC as a Separate Stream
In the first case, the FEC packets are sent as a separate stream.
This can mean they are sent on a different port and/or multicast
group from the media. When this is done, several pieces of
information must be conveyed:
o The address and port where the FEC is being sent to
o The payload type number for the FEC
o Which media stream the FEC is protecting
The payload type number for the FEC is conveyed in the m line of the
media it is protecting, listed as if it were another valid encoding
for the stream. There is no static payload type assignment for FEC,
so dynamic payload type numbers MUST be used. The binding to the
number is indicated by an rtpmap attribute. The name used in this
binding is "parityfec".
The presence of the payload type number in the m line of the media it
is protecting does not mean the FEC is sent to the same address and
port as the media. Instead, this information is conveyed through an
fmtp attribute line. The presence of the FEC payload type on the m
line of the media serves only to indicate which stream the FEC is
protecting.
The format for the fmtp line for FEC is:
a=fmtp:<number> <port> <network type> <addresss type> <connection
address>
where 'number' is the payload type number present in the m line. Port
is the port number where the FEC is sent to. The remaining three
items - network type, address type, and connection address - have the
same syntax and semantics as the c line from SDP. This allows the
fmtp line to be partially parsed by the same parser used on the c
lines. Note that since FEC cannot be hierarchically encoded, the
<number of addresses> parameter MUST NOT appear in the connection
address.
The following is an example SDP for FEC:
v=0
o=hamming 2890844526 2890842807 IN IP4 126.16.64.4
s=FEC Seminar
c=IN IP4 224.2.17.12/127
t=0 0
m=audio 49170 RTP/AVP 0 78
a=rtpmap:78 parityfec/8000
a=fmtp:78 49172 IN IP4 224.2.17.12/127
m=video 51372 RTP/AVP 31 79
a=rtpmap:79 parityfec/8000
a=fmtp:79 51372 IN IP4 224.2.17.13/127
The presence of two m lines in this SDP indicates that there are two
media streams - one audio and one video. The media format of 0
indicates that the audio uses PCM, and is protected by FEC with
payload type number 78. The FEC is sent to the same multicast group
and TTL as the audio, but on a port number two higher (49172). The
video is protected by FEC with payload type number 79. The FEC
appears on the same port as the video (51372), but on a different
multicast address.
11.2 Use with Redundant Encodings
When the FEC stream is being sent as a secondary codec in the
redundant encodings format, this must be signaled through SDP. To do
this, the procedures defined in RFC2198 are used to signal the use
of redundant encodings. The FEC payload type is indicated in the same
fashion as any other secondary codec. An rtpmap attribute MUST be
used to indicate a dynamic payload type number for the FEC packets.
The FEC MUST protect only the main codec. In this case, the fmtp
attribute for the FEC MUST NOT be present.
For example:
m=audio 12345 RTP/AVP 121 0 5 100
a=rtpmap:121 red/8000/1
a=rtpmap:100 parityfec/8000
a=fmtp:121 0/5/100
This SDP indicates that there is a single audio stream, which can
consist of PCM (media format 0) , DVI (media format 5), the redundant
encodings (indicated by media format 121, which is bound to red
through the rtpmap attribute), or FEC (media format 100, which is
bound to parityfec through the rtpmap attribute). Although the FEC
format is specified as a possible coding for this stream, the FEC
MUST NOT be sent by itself for this stream. Its presence in the m
line is required only because non-primary codecs must be listed here
according to RFC2198. The fmtp attribute indicates that the
redundant encodings format can be used, with DVI as a secondary
coding and FEC as a tertiary encoding.
11.3 Usage with RTSP
RTSP [7] can be used to request FEC packets to be sent as a separate
stream. When SDP is used with RTSP, the Session Description does not
include a connection address and port number for each stream.
Instead, RTSP uses the concept of a "Control URL". Control URLs are
used in SDP in two distinct ways.
1. There is a single control URL for all streams. This is
referred to as "aggregate control". In this case, the fmtp
line for the FEC stream is omitted.
2. There is a Control URL assigned to each stream. This is
referred to as "non-aggregate control". In this case, the
fmtp line specifies the Control URL for the stream of FEC
packets. The URL may be used in a SETUP command by an RTSP
client.
The format for the fmtp line for FEC with RTSP and non-aggregate
control is:
a=fmtp:<number> <control URL>
where 'number' is the payload type number present in the m line.
Control URL is the URL used to control the stream of FEC packets.
Note that the Control URL does not need to be an absolute URL. The
rules for converting a relative Control URL to an absolute URL are
given in RFC2326, Section C.1.1.
12 Security Considerations
The use of FEC has implications on the usage and changing of keys for
encryption. As the FEC packets do consist of a separate stream, there
are a number of permutations on the usage of encryption. In
particular:
o The FEC stream may be encrypted, while the media stream is
not.
o The media stream may be encrypted, while the FEC stream is
not.
o The media stream and FEC stream are both encrypted, but using
different keys.
o The media stream and FEC stream are both encrypted, but using
the same key.
The first three of these would require any application level
signaling protocols to be aware of the usage of FEC, and to thus
exchange keys for it and negotiate its usage on the media and FEC
streams separately. In the final case, no such additional mechanisms
are needed. The first two cases present a layering violation, as FEC
packets should really be treated no differently than other RTP
packets. Encrypting just one may also make certain known-plaintext
attacks possible. For these reasons, applications utilizing
encryption SHOULD encrypt both streams.
However, the changing of keys becomes problematic. For example, if
two packets a and b are sent, and FEC packet f(a,b) is sent, and the
keys used for a and b are different, which key should be used to
decode f(a,b)? In general, old keys will likely need to be cached, so
that when the keys change for the media stream, the old key is kept,
and used, until it is determined that the key has changed on the FEC
packets as well.
Another issue with the use of FEC is its impact on network
congestion. Adding FEC in the face of increasing network losses is a
bad idea, as it can lead to increased congestion and eventual
congestion collapse if done on a widespread basis. As a result,
implementers MUST NOT substantially increase the amount of FEC in use
as network losses increase.
13 Acknowledgments
This work is based on an earlier draft on FEC, submitted by Budge and
Mackenzie in 1997. We would also like to thank Steve Casner, Mark
Handley, Orion Hodson and Colin Perkins for their comments. Thanks to
Anders Klemets who wrote the section on usage with RTSP.
14 Authors' Addresses
Jonathan Rosenberg
dynamicsoft
200 Executive Drive
Suite 120
West Orange, NJ 07046
Email: jdrosen@dynamicsoft.com
Henning Schulzrinne
Columbia University
M/S 0401, 1214 Amsterdam Ave.
New York, NY 10027-7003
EMail: schulzrinne@cs.columbia.edu
15 Bibliography
[1] J.C. Bolot and A. V. Garcia, "Control mechanisms for packet audio
in the internet," in Proceedings of the Conference on Computer
Communications (IEEE Infocom) , (San Francisco, California), Mar.
1996.
[2] Perkins, C. and O. Hodson, "Options for Repair of Streaming
media", RFC2354, June 1998.
[3] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:
A Transport Protocol for Real-Time Applications", RFC1889,
January 1996.
[4] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", BCP 14, RFC2119, March 1997.
[5] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., Handley, M.,
Bolot, J.C., Vega-Garcia, A. and S. Fosse-Parisis, "RTP Payload
for Redundant Audio Data", RFC2198, September 1997.
[6] Handley, M. and V. Jacobson, "SDP: Session Description Protocol",
RFC2327, April 1998.
[7] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC2326, April 1998.
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