FEC Framework A. Begen
Internet-Draft Cisco Systems
Intended status: Standards Track February 08, 2008
Expires: August 11, 2008
SDP Elements for FEC Framework
draft-ietf-fecframe-sdp-elements-00
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Abstract
This document specifies the use of Session Description Protocol (SDP)
to describe the parameters required to signal the Forward Error
Correction (FEC) Framework Configuration Information between the
sender(s) and receiver(s). This document also provides the semantics
for grouping multiple source and repair flows together for the
applications that simultaneously use multiple instances of the FEC
Framework.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 3
3. Forward Error Correction (FEC) and FEC Framework . . . . . . . 3
3.1. Forward Error Correction (FEC) . . . . . . . . . . . . . . 3
3.2. FEC Framework . . . . . . . . . . . . . . . . . . . . . . 4
3.3. FEC Framework Configuration Information . . . . . . . . . 4
4. FEC Framework Descriptors . . . . . . . . . . . . . . . . . . 6
4.1. Transport Protocol Identifiers . . . . . . . . . . . . . . 6
4.2. Media Stream Grouping . . . . . . . . . . . . . . . . . . 7
4.3. Source IP Addresses . . . . . . . . . . . . . . . . . . . 9
4.4. Source Flows . . . . . . . . . . . . . . . . . . . . . . . 9
4.5. Repair Flows . . . . . . . . . . . . . . . . . . . . . . . 9
4.6. Repair Window . . . . . . . . . . . . . . . . . . . . . . 11
4.7. Bandwidth Specification . . . . . . . . . . . . . . . . . 11
5. Scenarios and Examples . . . . . . . . . . . . . . . . . . . . 12
5.1. Session Announcement Considerations . . . . . . . . . . . 12
5.2. Offer/Answer Considerations . . . . . . . . . . . . . . . 12
5.3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.3.1. One Source Flow, One Repair Flow and One FEC Scheme . 13
5.3.2. Two Source Flows, One Repair Flow and One FEC
Scheme . . . . . . . . . . . . . . . . . . . . . . . . 14
5.3.3. Two Source Flows, Two Repair Flows and Two FEC
Schemes . . . . . . . . . . . . . . . . . . . . . . . 15
6. Security Considerations . . . . . . . . . . . . . . . . . . . 16
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
7.1. Transport Protocols . . . . . . . . . . . . . . . . . . . 16
7.2. Attribute Names . . . . . . . . . . . . . . . . . . . . . 17
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
9. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. draft-ietf-fecframe-sdp-elements-00 . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . . 18
10.2. Informative References . . . . . . . . . . . . . . . . . . 19
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . . . 20
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1. Introduction
The Forward Error Correction (FEC) Framework, described in
[I-D.ietf-fecframe-framework], outlines a general framework for using
FEC-based error recovery in packet flows carrying media content.
While a continuous signaling between the sender(s) and receiver(s) is
not required for a Content Delivery Protocol (CDP) that uses the FEC
Framework, a set of parameters pertaining to the FEC Framework MUST
be initially communicated between the sender(s) and receiver(s).
One way to communicate this information is to use the Session
Description Protocol (SDP) [RFC4566]. SDP provides a simple text-
based format for announcements and invitations to describe multimedia
sessions. These SDP announcements and invitations include sufficient
information for the sender(s) and receiver(s) to participate in the
multimedia sessions. SDP also provides a framework for capability
negotiation, which MAY be used to negotiate all or a subset of the
parameters pertaining to the individual sessions.
The purpose of this document is to introduce the SDP elements that
MUST be used by the CDPs using the FEC Framework that choose SDP
[RFC4566] as their session description protocol.
2. Requirements Notation
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].
3. Forward Error Correction (FEC) and FEC Framework
This section gives a brief overview of FEC and the FEC Framework.
3.1. Forward Error Correction (FEC)
Any application that needs a reliable transmission over an unreliable
packet network has to cope with the packet losses. FEC is an
effective approach that provides reliable transmission particularly
in multicast and broadcast applications where the feedback from the
receiver(s) may be potentially limited. In a nutshell, FEC groups
source packets into blocks and applies protection to generate a
desired number of repair packets.
Repair packets MAY be sent on demand or independently of any receiver
feedback. The choice depends on the FEC code used by the
application, the error characteristics of the underlying network, the
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transport scheme (e.g., unicast, multicast, and broadcast), and the
application. At the receiver side, lost packets can be recovered by
erasure decoding provided that a sufficient number of source and
repair packets are received. See [I-D.ietf-fecframe-framework] for
further details.
3.2. FEC Framework
The FEC Framework [I-D.ietf-fecframe-framework] outlines a general
framework for using FEC codes in multimedia applications that stream
audio, video or other types of multimedia content. It defines the
common components and aspects of Content Delivery Protocols (CDP).
The FEC Framework also defines the requirements for the FEC schemes
that need to be used within a CDP. However, the details of the FEC
schemes are not specified within the FEC Framework. For example, the
FEC Framework defines what configuration information has to be known
at the sender and receiver(s) at minimum, but the FEC Framework
neither specifies how the FEC repair packets are generated and used
to recover missing source packets, nor dictates how the configuration
information is communicated between the sender and receiver(s).
These are rather specified by the individual FEC schemes or CDPs.
For a proper operation, the information required by the FEC Framework
and the details of an FEC scheme have to be communicated between the
sender and receiver(s). One way to provide this information is to
use the Session Description Protocol (SDP) [RFC4566]. SDP provides a
commonly used text-based format for announcements and invitations
that describe multimedia sessions. These SDP announcements and
invitations include sufficient information for clients to participate
in multimedia sessions. By using the SDP capability negotiation
framework, all or a subset of the parameters pertaining to the FEC
Framework MAY also be negotiated between the sender and receiver(s).
The purpose of this document is to introduce the SDP elements that
MUST be used by the CDPs using the FEC Framework that choose SDP
[RFC4566] as their session description protocol.
Note that there are many similarities between the FEC Framework
[I-D.ietf-fecframe-framework] and the FEC Building Block [RFC5052],
which describes a framework that uses FEC codes to provide
reliability to bulk data transfer applications running over IP
multicast or broadcast. See [I-D.ietf-fecframe-framework] for
further details.
3.3. FEC Framework Configuration Information
The FEC Framework defines a minimum set of information that MUST be
communicated between the sender and receiver(s) for a proper
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operation of an FEC scheme. This information is called the FEC
Framework Configuration Information. This information specifies how
the sender applies protection to the source flow(s) and how the
repair flow(s) can be used to recover lost data. In other words,
this information specifies the relationship(s) between the source and
repair flows.
The FEC Framework Configuration Information includes identifiers for
unique identification of the source and repair flows that carry the
source and repair packets, respectively. For example, a packet flow
that is transmitted over UDP is uniquely identified by the tuple of
{Source IP Address, Destination IP Address, Source UDP port,
Destination UDP port}. However, an integer identifier MAY be used
internally within the FEC scheme as a shorthand to identify this
flow.
Multiple instances of the FEC Framework MAY simultaneously exist at
the sender and the receiver(s) for different source flows, for the
same source flow, or for various combinations of source flows. Each
instance of the FEC Framework MUST provide the following FEC
Framework Configuration Information:
1. Identification of the repair flows.
2. For each source flow protected by the repair flow(s):
a. Definition of the source flow.
b. An integer identifier for this flow definition (i.e., tuple).
This identifier MUST be unique amongst all source flows that are
protected by the same FEC repair flow. The identifiers SHOULD be
allocated starting from zero and increasing by one for each flow.
A source flow identifier need not be carried in source packets
since source packets are directly associated with a flow by virtue
of their packet headers. Note that an application MAY wildcard
some of the fields if only a subset of the fields of the tuple
(e.g., {Destination IP Address, Destination UDP port} ) is
sufficient.
3. The FEC Encoding ID that identifies the FEC scheme.
4. The length of the Explicit Source FEC Payload ID (in bytes).
This value MAY be zero indicating that no Explicit Source FEC
Payload ID is used by the FEC scheme. If it is nonzero, however,
it means that the Explicit Source FEC Payload ID is used. In this
case, only one FEC scheme MUST be used for this source flow,
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unless the generic tag (defined in [I-D.ietf-fecframe-framework])
is used by all of the FEC schemes protecting this source flow.
5. An opaque container for the FEC-Scheme-Specific Information
required only by the sender. This information is referred to as
the Sender-Side FEC-Scheme-Specific Information (SS-FSSI).
6. An opaque container for the FEC-Scheme-Specific Information
required by the receiver. This information is referred to as the
Receiver-Side FEC-Scheme-Specific Information (RS-FSSI).
FSSI includes the information that is specific to the FEC scheme used
by the CDP. FSSI is used to communicate the information that cannot
be adequately represented otherwise and is essential for proper FEC
encoding and decoding operations. The motivation behind separating
the FSSI required only by the sender from the rest of the FSSI is to
provide the receiver or the 3rd party entities a means of controlling
the FEC operations at the sender. Any FSSI other than the one solely
required by the sender MUST be communicated via the RS-FSSI
container.
The variable-length opaque SS-FSSI and RS-FSSI containers transmit
the information in the form of an octet string. The FEC schemes
define the structure of this octet string, which MAY contain multiple
distinct elements. If the FEC scheme does not require any specific
information, the FSSI MAY be null. For the fully-specified FEC
schemes, a full description of the encoded information in both
containers MUST be provided. See [I-D.ietf-fecframe-framework] for
details.
4. FEC Framework Descriptors
This section defines the SDP elements that MUST be used to describe
the FEC Framework Configuration Information in multimedia sessions by
the CDPs that choose SDP [RFC4566] as their session description
protocol. Example SDP configurations can be found in Section 5.
4.1. Transport Protocol Identifiers
This specification defines a class of new transport protocol
identifiers for SDP media descriptions. For all existing identifiers
<proto>, this specification defines the identifier 'FEC/<proto>'.
This identifier MAY be used as the transport protocol identifier in
the media descriptions for the source data to indicate that the FEC
Source Packet format defined in Section 6.3 of
[I-D.ietf-fecframe-framework] is used, where the original transport
payload field is formatted according to <proto>. However, if the FEC
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scheme does not use the Explicit Source FEC Payload ID as described
in Section 6.3 of [I-D.ietf-fecframe-framework], then the original
transport protocol identifier MUST be used to support backward
compatibility with the receivers that do not support FEC at all.
This specification also defines another transport protocol
identifier, 'UDP/FEC', to indicate the FEC Repair Packet format
defined in Section 6.4 of [I-D.ietf-fecframe-framework].
4.2. Media Stream Grouping
The FEC Framework [I-D.ietf-fecframe-framework] states that multiple
instances of the FEC Framework MAY exist at the sender and the
receiver(s), and a source flow MAY be protected by multiple FEC
Framework instances. Furthermore, within a single FEC Framework
instance, multiple source flows MAY be protected by multiple repair
flows. However, each repair flow MUST provide protection for a
single FEC Framework instance. An example scenario is shown in
Figure 1. Here, source flows 0 and 1 are grouped together and
protected by repair flow 3; source flow 0 is also protected by repair
flow 4; source flows 1 and 2 are grouped together and protected by
repair flows 5, 6 and 7.
The motivation behind grouping source flows before applying FEC
protection is that a better coding performance can be achieved by
doing so and many receivers may benefit from this grouping. For
example, consider a layered video source that consists of one base
layer (e.g., source flow 0) and one enhancement layer (e.g., source
flow 1), where each layer is carried in a separate flow. Repair flow
3 protects the combination of the base and enhancement layers for the
receivers who receive both layers, and repair flow 4 protects the
base layer only, for the receivers who want the base layer only, or
who receive both layers but prefer FEC protection for the base layer
only due to their bandwidth and/or processing limitations.
Using multiple FEC Framework instances for a single source flow
provides flexibility to the receivers. Some instances may use larger
or smaller source block sizes, which accommodate the receivers that
have looser and tighter latency requirements, respectively.
Different instances may also provide FEC protection at different
redundancy levels. This enables the receivers experiencing different
packet loss rates to choose the repair flows that are tailored to
their needs.
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____| FEC FRAMEWORK
/ | INSTANCE
/ | 3: Repair Flow
/
SOURCE FLOWS / __| FEC FRAMEWORK
0: Source Flow |___/ |---' | INSTANCE
1: Source Flow | |____ | 4: Repair Flow
2: Source Flow | \
\ | FEC FRAMEWORK
\_| INSTANCE
| 5: Repair Flow
| 6: Repair Flow
| 7: Repair Flow
Figure 1: Example scenario with multiple FEC Framework instances
The 'group' attribute and the FEC grouping semantics defined in
[RFC4756] are used to associate source and repair flows together with
the following additional requirement:
In the case that the Explicit Source FEC Payload ID is used, then
only one FEC scheme MUST be used for this source flow, unless the
generic tag is used by all of the FEC schemes for the Source FEC
Payload ID, as defined in [I-D.ietf-fecframe-framework].
The 'group' attribute MAY be used to group multiple repair flows with
one or more source flows. Note that [RFC3388] prohibits an "m" line
identified by its 'mid' attribute from appearing in more than one
"a=group:FEC" line. Thus, [RFC3388] mandates us to write
a=group:FEC 0 1 2 3 4 5 6 7
for the scenario sketched in Figure 1. This limitation prevents us
from indicating particular associations between the source and repair
flows by using an "a=group:FEC" line per FEC Framework instance
[RFC4756].
Editor's note: The FEC grouping and flow association issues are
currently under discussion in FECFRAME and MMUSIC WGs. This section
will be updated once a decision is made.
The FEC Framework also supports additivity among the repair flows,
meaning that multiple repair flows MAY be decoded jointly to improve
the recovery chances of the missing packets. In addition, the sender
MAY assign different levels of priority to each repair flow. See
Section 4.5 for details.
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4.3. Source IP Addresses
The 'source-filter' attribute of SDP ("a=source-filter") as defined
in [RFC4570] is used to express the source addresses or fully
qualified domain names in the FEC Framework.
Editor's note: Additional requirements or exceptions regarding
source filters are TBD.
4.4. Source Flows
The FEC Framework allows that multiple source flows MAY be grouped
and protected together by a single or multiple FEC Framework
instances. For this reason, as described in Section 3.3, individual
source flows MUST be identified with unique identifiers. For this
purpose, we introduce the attribute 'fec-source-flow'.
The syntax for the new attribute in ABNF [RFC5234] is as follows:
fec-source-flow-line = "a=fec-source-flow:" source-id
[";" SP tag-length] CRLF
source-id = "id=" src-id
src-id = 1*DIGIT
tag-length = "tag-len=" tlen
tlen = *DIGIT
The MANDATORY parameter 'id' is used to identify the source flow.
Note that the parameter 'id' MUST be an integer.
The OPTIONAL 'tag-len' parameter is used to specify the length of the
Explicit Source FEC Payload ID field (in bytes) and MUST be used
according to the requirements listed in Section 4.2. If no value is
specified for the 'tag-len' parameter, it indicates a value of zero.
4.5. Repair Flows
A repair flow MUST contain only repair packets formatted as described
in [I-D.ietf-fecframe-framework] for a single FEC Framework instance.
In other words, packets belonging to source flows or other repair
flows from a different FEC Framework instance MUST NOT be sent within
this flow. We introduce the attribute 'fec-repair-flow' to describe
the repair flows.
The syntax for the new attribute in ABNF is as follows:
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fec-repair-flow-line = "a=fec-repair-flow:" fec-encoding-id
[";" SP flow-priority] [";" SP sender-side-scheme-specific]
[";" SP receiver-side-scheme-specific] CRLF
fec-encoding-id = "encoding-id=" enc-id
enc-id = 1*DIGIT ; FEC Encoding ID
flow-priority = "priority=" priority-of-the-flow
priority-of-the-flow = *DIGIT
sender-side-scheme-specific = "ss-fssi=" sender-info
sender-info = *CHAR
receiver-side-scheme-specific = "rs-fssi=" receiver-info
receiver-info = *CHAR
The MANDATORY parameter 'encoding-id' is used to identify the FEC
scheme used to generate this repair flow. These identifiers MUST be
registered with IANA by the FEC schemes that use the FEC Framework.
The OPTIONAL parameter 'priority' is used to indicate the priorities
of the repair flows when multiple repair flows are grouped together
to be used in an additive manner within a single FEC Framework
instance. The exact usage of the parameter 'priority' and the
pertaining rules SHOULD be defined by the FEC scheme or the CDP. If
no value is specified for the parameter 'priority', it means that the
receiver(s) MAY receive and use the repair flows in any order.
However, if a priority is assigned to the repair flow(s), the
receivers MUST follow the specified order in receiving and using the
repair flow(s).
The OPTIONAL parameters 'ss-fssi' and 'rs-fssi' are opaque containers
to convey the FEC-Scheme-Specific Information (FSSI) that includes
the information that is specific to the FEC scheme used by the CDP
and is necessary for proper FEC encoding and decoding operations.
The FSSI required only by the sender (called Sender-Side FSSI) MUST
be communicated in the container specified by the parameter 'ss-
fssi'. Any other FSSI (called Receiver-Side FSSI) MUST be
communicated in the container specified by the parameter 'rs-fssi'.
In both containers, FSSI is transmitted in the form of an octet
string. The FEC schemes define the structure of this octet string,
which MAY contain multiple distinct elements. If the FEC scheme does
not require any specific information, the 'ss-fssi' and 'rs-fssi'
parameters MAY be null and ignored.
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4.6. Repair Window
An FEC encoder processes a block of source packets and generates a
number of repair packets, which are then transmitted within a certain
duration. At the receiver side, the FEC decoder tries to decode all
the packets received within the repair window to recover the missing
packets, if there are any. Repair window stands for the time that
spans the source packets and the corresponding repair packets.
Assuming that there is no issue of delay variation, the FEC decoder
SHOULD NOT wait longer than the repair window since additional
waiting would not help the recovery process.
This document specifies a new attribute to describe the size of the
repair window in milliseconds and microseconds.
The syntax for the attribute in ABNF is as follows:
repair-window-line = "a=repair-window:" window-size
[SP unit] CRLF
window-size = 1*DIGIT
unit = ms / us
<unit> is the unit of time the repair window size is specified with.
Currently, two units are defined: "ms", which stands for
milliseconds and "us", which stands for microseconds. The default
unit is "ms". Alternative units MAY be defined in the future by
registering them with IANA.
The 'a=repair-window' attribute is a media-level attribute since each
repair flow MAY have a different repair window value.
4.7. Bandwidth Specification
The bandwidth specification as defined in [RFC4566] denotes the
proposed bandwidth to be used by the session or media. The
specification of bandwidth is OPTIONAL.
In the context of the FEC Framework, the bandwidth specification can
be used to express the bandwidth of the repair flows or the bandwidth
of the session. If included in the SDP, it SHALL adhere to the
following rules:
The session-level bandwidth for an FEC Framework instance MAY be
specified. In this case, it is RECOMMENDED to use the Transport
Independent Application Specific (TIAS) bandwidth modifier [RFC3890]
and the 'a=maxprate' attribute for the session.
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The media-level bandwidth for the individual repair flows MAY also be
specified. In this case, it is RECOMMENDED to use the TIAS bandwidth
modifier [RFC3890].
The Application Specific (AS) bandwidth modifier [RFC4566] MAY be
used instead of TIAS, however, this is NOT RECOMMENDED since TIAS
allows the calculation of the bitrate according to the IP version and
transport protocol, whereas AS does not. Thus, in TIAS-based bitrate
calculations, the packet size SHALL include all headers and payload,
excluding the IP and UDP headers. In AS-based bitrate calculations,
the packet size SHALL include all headers and payload, plus the IP
and UDP headers.
For the ABNF syntax information of the TIAS and AS, refer to
[RFC3890] and [RFC4566], respectively.
5. Scenarios and Examples
This section discusses the considerations for session announcement
and offer/answer models. SDP examples that can be used by the FEC
Framework are also provided.
5.1. Session Announcement Considerations
In multicast-based applications, the FEC Framework Configuration
Information pertaining to all FEC protection options available at the
sender MAY be advertised to the receivers as a part of a session
announcement. This way, the sender can let the receivers know all
available options for FEC protection. Based on their needs, the
receivers MAY choose protection provided by one or more FEC Framework
instances and subscribe to the respective multicast group(s) to
receive the repair flow(s). Unless explicitly required by the CDP,
the receivers SHOULD NOT send an answer back to the sender specifying
their choices.
5.2. Offer/Answer Considerations
In unicast-based applications, a sender and receiver MAY adopt the
offer/answer model [RFC3264] to set the FEC Framework Configuration
Information. In this case, the sender offers all available options
to the receiver and the receiver answers back to the sender with its
choice(s). Note that some FEC protection options MAY be offered to
only a particular set of (e.g., premium) receivers.
Receivers supporting the SDP Capability Negotiation Framework
[I-D.ietf-mmusic-sdp-capability-negotiation] MAY also use this
framework to negotiate all or a subset of the FEC Framework
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parameters.
The backward compatibility in offer/answer model is handled as
specified in [RFC3388]. If a receiver receives an offer containing
FEC grouping and it does not understand the FEC grouping semantics,
it MAY respond with an answer that ignores the grouping attribute or
MAY refuse the request. In the first case, the offerer MUST
establish the connection without FEC. In the second case, if the
offerer still wishes to establish the session, it SHOULD retry the
request with an offer without FEC.
5.3. Examples
Editor's note: More examples showing the usage of multiple FEC
Framework instances, additivity of the repair flows and
prioritization of the repair flows will be provided once the issues
related to FEC grouping and flow association are resolved.
Editor's note: As of now, no FEC Encoding ID has been registered
with IANA. In the examples below, an FEC Encoding ID of zero and an
encoding (i.e., payload format) of 'parityfec' will be used for
illustrative purposes. Artificial content for the SS-FSSI and RS-
FSSI will also be provided.
[RFC3388] defines the media stream identification attribute ('mid')
as a token in ABNF. In contrast, the identifiers for the source
flows MUST be integers and SHOULD be allocated starting from zero and
increasing by one for each flow. To avoid any ambiguity, using the
same values for identifying the media streams and source flows is NOT
RECOMMENDED, even when 'mid' values are integers.
5.3.1. One Source Flow, One Repair Flow and One FEC Scheme
SOURCE FLOWS | INSTANCE #1
0: Source Flow |---------| 1: Repair Flow
Figure 6: Scenario #1
In this example, we have one source video stream (mid:S1) and one FEC
repair stream (mid:R1). We form one FEC group with the "a=group:FEC
S1 R1" line. The source and repair streams are sent to the same port
on different multicast groups. The repair window is set to 150 ms.
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v=0
o=ali 1122334455 1122334466 IN IP4 fec.rocks.com
s=FEC Framework Examples
t=0 0
a=group:FEC S1 R1
m=video 30000 RTP/AVP 100
c=IN IP4 224.1.1.1/127
a=rtpmap:100 MP2T/90000
a=fec-source-flow: id=0
a=mid:S1
m=video 30000 RTP/AVP 110
c=IN IP4 224.1.2.1/127
a=rtpmap:110 parityfec/90000
a=fec-repair-flow: encoding-id=0; ss-fssi=1Q2A3Z; rs-fssi=4W5S6X
a=repair-window: 150
a=mid:R1
5.3.2. Two Source Flows, One Repair Flow and One FEC Scheme
SOURCE FLOWS | INSTANCE #1
0: Source Flow |_________| 2: Repair Flow
1: Source Flow |
Figure 8: Scenario #2
In this example, we have two source video streams (mid:S1 and mid:S2)
and one FEC repair stream (mid:R1), protecting both source streams.
We form one FEC group with the "a=group:FEC S1 S2 R1" line. The
source and repair streams are sent to the same port on different
multicast groups. The repair window is set to 150500 us.
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v=0
o=ali 1122334455 1122334466 IN IP4 fec.rocks.com
s=FEC Framework Examples
t=0 0
a=group:FEC S1 S2 R1
m=video 30000 RTP/AVP 100
c=IN IP4 224.1.1.1/127
a=rtpmap:100 MP2T/90000
a=fec-source-flow: id=0
a=mid:S1
m=video 30000 RTP/AVP 101
c=IN IP4 224.1.1.2/127
a=rtpmap:101 MP2T/90000
a=fec-source-flow: id=1
a=mid:S2
m=video 30000 RTP/AVP 110
c=IN IP4 224.1.2.1/127
a=rtpmap:110 parityfec/90000
a=fec-repair-flow: encoding-id=0; ss-fssi=1Q2A3Z; rs-fssi=4W5S6X
a=repair-window: 150500 us
a=mid:R1
5.3.3. Two Source Flows, Two Repair Flows and Two FEC Schemes
SOURCE FLOWS | INSTANCE #1
0: Source Flow |---------| 2: Repair Flow
1: Source Flow |_
\-------| INSTANCE #2
| 3: Repair Flow
Figure 10: Scenario #3
In this example, we have two source video streams (mid:S1 and mid:S2)
and two FEC repair streams (mid:R1 and mid:R2). The source streams
mid:S1 and mid:S2 are protected by the repair streams mid:R1 and
mid:R2, respectively. We form two FEC groups with the "a=group:FEC
S1 R1" and "a=group:FEC S2 R2" lines. The source and repair streams
are sent to the same port on different multicast groups. The repair
window is set to 200 ms and 400 ms for the first and second FEC
group, respectively.
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v=0
o=ali 1122334455 1122334466 IN IP4 fec.rocks.com
s=FEC Framework Examples
t=0 0
a=group:FEC S1 R1
a=group:FEC S2 R2
m=video 30000 RTP/AVP 100
c=IN IP4 224.1.1.1/127
a=rtpmap:100 MP2T/90000
a=fec-source-flow: id=0
a=mid:S1
m=video 30000 RTP/AVP 101
c=IN IP4 224.1.1.2/127
a=rtpmap:101 MP2T/90000
a=fec-source-flow: id=1
a=mid:S2
m=video 30000 RTP/AVP 110
c=IN IP4 224.1.2.1/127
a=rtpmap:110 parityfec/90000
a=fec-repair-flow: encoding-id=0; ss-fssi=1Q2A3Z; rs-fssi=4W5S6X
a=repair-window: 200 ms
a=mid:R1
m=video 30000 RTP/AVP 111
c=IN IP4 224.1.2.2/127
a=rtpmap:111 parityfec/90000
a=fec-repair-flow: encoding-id=0; ss-fssi=123QAZ; rs-fssi=456WSX
a=repair-window: 400 ms
a=mid:R2
6. Security Considerations
For the general security considerations related to SDP, refer to
[RFC4566]. For the security considerations related to source/FEC
media stream grouping in SDP and use of source address filters in
SDP, refer to [RFC4756] and [RFC4570], respectively.
7. IANA Considerations
7.1. Transport Protocols
The 'proto' sub-field of the media description line ("m=") describes
the transport protocol used. This document registers the following
values:
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UDP/FEC
Editor's note: Additional transport protocols to be registered are
TBD.
7.2. Attribute Names
As recommended by [RFC4566], the following attribute names should be
registered with IANA.
The contact information for the registrations is:
Ali Begen
abegen@cisco.com
SDP Attribute ("att-field"):
Attribute name: fec-source-flow
Long form: Pointer to FEC Source Flow
Type of name: att-field
Type of attribute: Media level
Subject to charset: No
Purpose: See this document
Reference: This document
Values: See this document
SDP Attribute ("att-field"):
Attribute name: fec-repair-flow
Long form: Pointer to FEC Repair Flow
Type of name: att-field
Type of attribute: Media level
Subject to charset: No
Purpose: See this document
Reference: This document
Values: See this document
SDP Attribute ("att-field"):
Attribute name: repair-window
Long form: Repair Window Size
Type of name: att-field
Type of attribute: Media level
Subject to charset: No
Purpose: See this document
Reference: This document
Values: See this document
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8. Acknowledgments
The author would like to thank the FEC Framework Design Team for
their inputs, suggestions and contributions.
9. Change Log
9.1. draft-ietf-fecframe-sdp-elements-00
This is the initial version, which is based on an earlier individual
submission. The following are the major changes compared to that
document:
o The opaque container in the FEC Framework Configuration
Information (FEC-Scheme-Specific Information) is now divided into
two parts: information needed only by the sender and information
needed by the receiver. The repair flow descriptors are also
updated accordingly.
o "Minimum Buffer Size" is now called "Repair Window." Its size can
also be specified in microseconds in addition to milliseconds.
o Simple examples with complete SDPs are included.
o "Scheme ID" is changed to "Encoding ID" to be consistent with the
framework draft.
o Some other editorial changes.
10. References
10.1. Normative References
[I-D.ietf-fecframe-framework]
Watson, M., "Forward Error Correction (FEC) Framework",
draft-ietf-fecframe-framework-01 (work in progress),
November 2007.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4570] Quinn, B. and R. Finlayson, "Session Description Protocol
(SDP) Source Filters", RFC 4570, July 2006.
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[RFC4756] Li, A., "Forward Error Correction Grouping Semantics in
Session Description Protocol", RFC 4756, November 2006.
[RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H.
Schulzrinne, "Grouping of Media Lines in the Session
Description Protocol (SDP)", RFC 3388, December 2002.
[RFC3890] Westerlund, M., "A Transport Independent Bandwidth
Modifier for the Session Description Protocol (SDP)",
RFC 3890, September 2004.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[I-D.ietf-mmusic-sdp-capability-negotiation]
Andreasen, F., "SDP Capability Negotiation",
draft-ietf-mmusic-sdp-capability-negotiation-08 (work in
progress), December 2007.
10.2. Informative References
[RFC5052] Watson, M., Luby, M., and L. Vicisano, "Forward Error
Correction (FEC) Building Block", RFC 5052, August 2007.
Author's Address
Ali Begen
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
Email: abegen@cisco.com
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