| RFC : | rfc6364 |
| Title: | |
| Date: | October 2011 |
| Status: | PROPOSED STANDARD |
Internet Engineering Task Force (IETF) A. Begen
Request for Comments: 6364 Cisco
Category: Standards Track October 2011
ISSN: 2070-1721
Session Description Protocol Elements for the
Forward Error Correction (FEC) Framework
Abstract
This document specifies the use of the 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 examples that
show the semantics for grouping multiple source and repair flows
together for the applications that simultaneously use multiple
instances of the FEC Framework.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6364.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Begen Standards Track [Page 1]
RFC 6364 SDP Elements for FEC Framework October 2011
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
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. SDP Elements ....................................................5
4.1. Transport Protocol Identifiers .............................6
4.2. Media Stream Grouping ......................................6
4.3. Source IP Addresses ........................................6
4.4. Source Flows ...............................................6
4.5. Repair Flows ...............................................7
4.6. Repair Window ..............................................8
4.7. Bandwidth Specification ....................................9
5. Scenarios and Examples .........................................10
5.1. Declarative Considerations ................................10
5.2. Offer/Answer Model Considerations .........................10
6. SDP Examples ...................................................11
6.1. One Source Flow, One Repair Flow, and One FEC Scheme ......11
6.2. Two Source Flows, One Repair Flow, and One FEC Scheme .....12
6.3. Two Source Flows, Two Repair Flows, and Two FEC Schemes ...13
6.4. One Source Flow, Two Repair Flows, and Two FEC Schemes ....14
7. Security Considerations ........................................15
8. IANA Considerations ............................................15
8.1. Registration of Transport Protocols .......................15
8.2. Registration of SDP Attributes ............................16
9. Acknowledgments ................................................16
10. References ....................................................17
10.1. Normative References .....................................17
10.2. Informative References ...................................17
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RFC 6364 SDP Elements for FEC Framework October 2011
1. Introduction
The Forward Error Correction (FEC) Framework, described in [RFC6363],
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 has to be initially
communicated between the sender(s) and receiver(s). A signaling
protocol (such as the one described in [FECFRAME-CFG-SIGNAL]) is
required to enable such communication, and the parameters need to be
appropriately encoded so that they can be carried by the signaling
protocol.
One format to encode the parameters is 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 can 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
are used by the CDPs using the FEC Framework that choose SDP
[RFC4566] for their multimedia sessions.
2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "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 reliable transmission over an unreliable
packet network has to cope with packet losses. FEC is an effective
approach that provides reliable transmission, particularly in
multicast and broadcast applications where the feedback from the
receiver(s) is either not available or quite limited.
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RFC 6364 SDP Elements for FEC Framework October 2011
In a nutshell, FEC groups source packets into blocks and applies
protection to generate a desired number of repair packets. These
repair packets can be sent on demand or independently of any receiver
feedback. The choice depends on the FEC scheme or the Content
Delivery Protocol used by the application, the packet loss
characteristics of the underlying network, the transport scheme
(e.g., unicast, multicast, and broadcast), and the application
itself. At the receiver side, lost packets can be recovered by
erasure decoding provided that a sufficient number of source and
repair packets have been received.
3.2. FEC Framework
The FEC Framework [RFC6363] 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 (CDPs). 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 a 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.
3.3. FEC Framework Configuration Information
The FEC Framework [RFC6363] defines a minimum set of information that
has to be communicated between the sender and receiver(s) for proper
operation of a FEC scheme. This information is called the "FEC
Framework Configuration Information". This information includes
unique identifiers for the source and repair flows that carry the
source and repair packets, respectively. It also specifies how the
sender applies protection to the source flow(s) and how the repair
flow(s) can be used to recover lost data.
Multiple instances of the FEC Framework can simultaneously exist at
the sender and the receiver(s) for different source flows, for the
same source flow, or for various combinations of the source flows.
Each instance of the FEC Framework provides the following FEC
Framework Configuration Information:
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RFC 6364 SDP Elements for FEC Framework October 2011
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 among all source flows that
are protected by the same FEC repair flow. Integer
identifiers can be allocated starting from zero and
increasing by one for each flow. However, any random (but
still unique) allocation is also possible. 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.
3. The FEC Encoding ID, identifying the FEC scheme.
4. The length of the Explicit Source FEC Payload ID (in octets).
5. Zero or more FEC-Scheme-Specific Information (FSSI) elements,
each consisting of a name and a value where the valid element
names and value ranges are defined by the FEC scheme.
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 (which is carried in a Sender-
Side FEC-Scheme-Specific Information (SS-FSSI) container) from the
rest of the FSSI is to provide the receiver or the third-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 FSSI container.
The variable-length SS-FSSI and FSSI containers transmit the
information in textual representation and contain zero or more
distinct elements, whose descriptions are provided by the fully
specified FEC schemes.
4. SDP Elements
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] for their multimedia sessions.
Example SDP descriptions can be found in Section 6.
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RFC 6364 SDP Elements for FEC Framework October 2011
4.1. Transport Protocol Identifiers
This specification defines a new transport protocol identifier for
the FEC schemes that take a UDP-formatted input stream and append an
Explicit Source FEC Payload ID, as described in Section 5.3 of
[RFC6363], to generate a source flow. This new protocol identifier
is called 'FEC/UDP'. To use input streams that are formatted
according to another <proto> (as listed in the table for the 'proto'
field in the "Session Description Protocol (SDP) Parameters"
registry), the corresponding 'FEC/<proto>' transport protocol
identifier MUST be registered with IANA by following the instructions
specified in [RFC4566].
Note that if a FEC scheme does not use the Explicit Source FEC
Payload ID as described in Section 4.1 of [RFC6363], 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 5.4 of [RFC6363]. For detailed registration
information, refer to Section 8.1.
4.2. Media Stream Grouping
In the FEC Framework, the 'group' attribute and the FEC grouping
semantics defined in [RFC5888] and [RFC5956], respectively, are used
to associate source and repair flows.
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.
4.4. Source Flows
The FEC Framework allows that multiple source flows MAY be grouped
and protected together by 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'.
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RFC 6364 SDP Elements for FEC Framework October 2011
The syntax for the new attribute in ABNF [RFC5234] is as follows:
fec-source-flow-line = "a=fec-source-flow:" SP source-id
[";" SP tag-length] CRLF
source-id = "id=" src-id
src-id = 1*DIGIT ; Represented as 32-bit non-negative
; integers, and leading zeros are ignored
tag-length = "tag-len=" tlen
tlen = %x31-39 *DIGIT
The REQUIRED parameter 'id' is used to identify the source flow.
Parameter 'id' MUST be an integer.
The 'tag-len' parameter is used to specify the length of the Explicit
Source FEC Payload ID field (in octets). In the case that an
Explicit Source FEC Payload ID is used, the 'tag-len' parameter MUST
exist and indicate its length. Otherwise, the 'tag-len' parameter
MUST NOT exist.
4.5. Repair Flows
A repair flow MUST contain only repair packets formatted as described
in [RFC6363] for a single FEC Framework instance; i.e., packets
belonging to source flows or other repair flows from a different FEC
Framework instance cannot 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 (CHAR and CTL
are defined in [RFC5234]):
fec-repair-flow-line = "a=fec-repair-flow:" SP fec-encoding-id
[";" SP flow-preference]
[";" SP sender-side-scheme-specific]
[";" SP scheme-specific] CRLF
fec-encoding-id = "encoding-id=" enc-id
enc-id = 1*DIGIT ; FEC Encoding ID
flow-preference = "preference-lvl=" preference-level-of-the-flow
preference-level-of-the-flow = 1*DIGIT
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RFC 6364 SDP Elements for FEC Framework October 2011
sender-side-scheme-specific = "ss-fssi=" sender-info
sender-info = element *( "," element )
element = name ":" value
name = token
token = 1*<any CHAR except CTLs or separators>
value = *<any CHAR except CTLs or separators>
separator = "(" / ")" / "<" / ">" / "@"
/ "," / ";" / ":" / "\" / DQUOTE
/ "/" / "[" / "]" / "?" / "="
/ "{" / "}" / SP / HTAB
scheme-specific = "fssi=" scheme-info
scheme-info = element *( "," element )
The REQUIRED parameter 'encoding-id' is used to identify the FEC
scheme used to generate this repair flow. These identifiers (in the
range of [0 - 255]) are registered by the FEC schemes that use the
FEC Framework and are maintained by IANA.
The OPTIONAL parameter 'preference-lvl' is used to indicate the
preferred order for using the repair flows. The exact usage of the
parameter 'preference-lvl' and the pertaining rules MAY be defined by
the FEC scheme or the CDP. If the parameter 'preference-lvl' does
not exist, it means that the receiver(s) MAY receive and use the
repair flows in any order. However, if a preference level is
assigned to the repair flow(s), the receivers are encouraged to
follow the specified order in receiving and using the repair flow(s).
The OPTIONAL parameters 'ss-fssi' and 'fssi' are 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 (the Sender-Side FSSI) MUST be
communicated in the container specified by the parameter 'ss-fssi'.
Any other FSSI MUST be communicated in the container specified by the
parameter 'fssi'. In both containers, FSSI is transmitted in the
form of textual representation and MAY contain multiple distinct
elements. If the FEC scheme does not require any specific
information, the 'ss-fssi' and 'fssi' parameters MUST NOT exist.
4.6. Repair Window
The repair window is the time that spans a FEC block, which consists
of the source block and the corresponding repair packets.
At the sender side, the FEC encoder processes a block of source
packets and generates a number of repair packets. Then, both the
source and repair packets are transmitted within a certain duration
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RFC 6364 SDP Elements for FEC Framework October 2011
not larger than the value of the repair window. The value of the
repair window impacts the maximum number of source packets that can
be included in a FEC block.
At the receiver side, the FEC decoder should wait at least for the
duration of the repair window after getting the first packet in a FEC
block, to allow all the repair packets to arrive. (The waiting time
can be adjusted if there are missing packets at the beginning of the
FEC block.) The FEC decoder can start decoding the already received
packets sooner; however, it SHOULD NOT register a FEC decoding
failure until it waits at least for the duration of the repair
window.
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 unit CRLF
window-size = %x31-39 *DIGIT ; Represented as
; 32-bit non-negative integers
unit = "ms" / "us"
<unit> is the unit of time specified for the repair window size. Two
units are defined here: 'ms', which stands for milliseconds; and
'us', which stands for microseconds.
The 'a=repair-window' attribute is a media-level attribute, since
each repair flow MAY have a different repair window size.
Specifying the repair window size in an absolute time value does not
necessarily correspond to an integer number of packets or exactly
match with the clock rate used in RTP (in the case of RTP transport),
causing mismatches among subsequent repair windows. However, in
practice, this mismatch does not break anything in the FEC decoding
process.
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.
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RFC 6364 SDP Elements for FEC Framework October 2011
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 a FEC Framework instance or the
media-level bandwidth for the individual repair flows MAY be
specified. In this case, it is RECOMMENDED that the Transport
Independent Application Specific (TIAS) bandwidth modifier [RFC3890]
and the 'a=maxprate' attribute be used, unless the Application-
Specific (AS) bandwidth modifier [RFC4566] is used. The use of the
AS bandwidth modifier 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.
5.1. Declarative 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 can choose protection provided by one or more FEC Framework
instances and subscribe to the respective multicast session(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, since this can easily overwhelm the sender,
particularly in large-scale multicast applications.
5.2. Offer/Answer Model 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 the options available
to this particular receiver, and the receiver answers back to the
sender with its choice(s).
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RFC 6364 SDP Elements for FEC Framework October 2011
Receivers supporting the SDP Capability Negotiation Framework
[RFC5939] MAY also use this framework to negotiate all, or a subset,
of the FEC Framework parameters.
The backward compatibility in the Offer/Answer Model is handled as
specified in [RFC5956].
6. SDP Examples
This section provides SDP examples that can be used by the FEC
Framework.
[RFC5888] defines the media stream identification attribute ('mid')
as a token in ABNF. In contrast, the identifiers for the source
flows are integers and can 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.
In the examples below, random FEC Encoding IDs will be used for
illustrative purposes. Artificial content for the SS-FSSI and FSSI
will also be provided.
6.1. One Source Flow, One Repair Flow, and One FEC Scheme
SOURCE FLOWS | INSTANCE #1
S1: Source Flow |--------| R1: Repair Flow
|
Figure 1: Scenario #1
In this example, we have one source video flow (mid:S1) and one FEC
repair flow (mid:R1). We form one FEC group with the
"a=group:FEC-FR S1 R1" line. The source and repair flows are sent to
the same port on different multicast groups. The repair window is
set to 150 ms.
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RFC 6364 SDP Elements for FEC Framework October 2011
v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=FEC Framework Examples
t=0 0
a=group:FEC-FR S1 R1
m=video 30000 RTP/AVP 100
c=IN IP4 233.252.0.1/127
a=rtpmap:100 MP2T/90000
a=fec-source-flow: id=0
a=mid:S1
m=application 30000 UDP/FEC
c=IN IP4 233.252.0.2/127
a=fec-repair-flow: encoding-id=0; ss-fssi=n:7,k:5
a=repair-window:150ms
a=mid:R1
6.2. Two Source Flows, One Repair Flow, and One FEC Scheme
SOURCE FLOWS
S2: Source Flow | | INSTANCE #1
|---------| R2: Repair Flow
S3: Source Flow |
Figure 2: Scenario #2
In this example, we have two source video flows (mid:S2 and mid:S3)
and one FEC repair flow (mid:R2) protecting both source flows. We
form one FEC group with the "a=group:FEC-FR S2 S3 R2" line. The
source and repair flows are sent to the same port on different
multicast groups. The repair window is set to 150500 us.
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RFC 6364 SDP Elements for FEC Framework October 2011
v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=FEC Framework Examples
t=0 0
a=group:FEC-FR S2 S3 R2
m=video 30000 RTP/AVP 100
c=IN IP4 233.252.0.1/127
a=rtpmap:100 MP2T/90000
a=fec-source-flow: id=0
a=mid:S2
m=video 30000 RTP/AVP 101
c=IN IP4 233.252.0.2/127
a=rtpmap:101 MP2T/90000
a=fec-source-flow: id=1
a=mid:S3
m=application 30000 UDP/FEC
c=IN IP4 233.252.0.3/127
a=fec-repair-flow: encoding-id=0; ss-fssi=n:7,k:5
a=repair-window:150500us
a=mid:R2
6.3. Two Source Flows, Two Repair Flows, and Two FEC Schemes
SOURCE FLOWS | INSTANCE #1
S4: Source Flow |--------| R3: Repair Flow
S5: Source Flow |--------| INSTANCE #2
| R4: Repair Flow
Figure 3: Scenario #3
In this example, we have two source video flows (mid:S4 and mid:S5)
and two FEC repair flows (mid:R3 and mid:R4). The source flows
mid:S4 and mid:S5 are protected by the repair flows mid:R3 and
mid:R4, respectively. We form two FEC groups with the
"a=group:FEC-FR S4 R3" and "a=group:FEC-FR S5 R4" lines. The source
and repair flows 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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RFC 6364 SDP Elements for FEC Framework October 2011
v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=FEC Framework Examples
t=0 0
a=group:FEC-FR S4 R3
a=group:FEC-FR S5 R4
m=video 30000 RTP/AVP 100
c=IN IP4 233.252.0.1/127
a=rtpmap:100 MP2T/90000
a=fec-source-flow: id=0
a=mid:S4
m=video 30000 RTP/AVP 101
c=IN IP4 233.252.0.2/127
a=rtpmap:101 MP2T/90000
a=fec-source-flow: id=1
a=mid:S5
m=application 30000 UDP/FEC
c=IN IP4 233.252.0.3/127
a=fec-repair-flow: encoding-id=0; ss-fssi=n:7,k:5
a=repair-window:200ms
a=mid:R3
m=application 30000 UDP/FEC
c=IN IP4 233.252.0.4/127
a=fec-repair-flow: encoding-id=0; ss-fssi=n:14,k:10
a=repair-window:400ms
a=mid:R4
6.4. One Source Flow, Two Repair Flows, and Two FEC Schemes
SOURCE FLOWS | INSTANCE #1
S6: Source Flow |--------| R5: Repair Flow
|
|--------| INSTANCE #2
| R6: Repair Flow
Figure 4: Scenario #4
In this example, we have one source video flow (mid:S6) and two FEC
repair flows (mid:R5 and mid:R6) with different preference levels.
The source flow mid:S6 is protected by both of the repair flows. We
form two FEC groups with the "a=group:FEC-FR S6 R5" and
"a=group:FEC-FR S6 R6" lines. The source and repair flows are sent
to the same port on different multicast groups. The repair window is
set to 200 ms for both FEC groups.
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RFC 6364 SDP Elements for FEC Framework October 2011
v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=FEC Framework Examples
t=0 0
a=group:FEC-FR S6 R5
a=group:FEC-FR S6 R6
m=video 30000 RTP/AVP 100
c=IN IP4 233.252.0.1/127
a=rtpmap:100 MP2T/90000
a=fec-source-flow: id=0
a=mid:S6
m=application 30000 UDP/FEC
c=IN IP4 233.252.0.3/127
a=fec-repair-flow: encoding-id=0; preference-lvl=0; ss-fssi=n:7,k:5
a=repair-window:200ms
a=mid:R5
m=application 30000 UDP/FEC
c=IN IP4 233.252.0.4/127
a=fec-repair-flow: encoding-id=1; preference-lvl=1; ss-fssi=t:3
a=repair-window:200ms
a=mid:R6
7. Security Considerations
There is a weak threat if the SDP is modified in a way that it shows
an incorrect association and/or grouping of the source and repair
flows. Such attacks can result in failure of FEC protection and/or
mishandling of other media streams. It is RECOMMENDED that the
receiver perform an integrity check on SDP to only trust SDP from
trusted sources. The receiver MUST also follow the security
considerations of SDP [RFC4566]. For other general security
considerations related to SDP, refer to [RFC4566]. For the security
considerations related to the use of source address filters in SDP,
refer to [RFC4570].
The security considerations for the FEC Framework also apply. Refer
to [RFC6363] for details.
8. IANA Considerations
8.1. Registration of Transport Protocols
This specification updates the "Session Description Protocol (SDP)
Parameters" registry as defined in Section 8.2.2 of [RFC4566].
Specifically, it adds the following values to the table for the
'proto' field.
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RFC 6364 SDP Elements for FEC Framework October 2011
Type SDP Name Reference
------ ---------- -----------
proto FEC/UDP [RFC6364]
proto UDP/FEC [RFC6364]
8.2. Registration of SDP Attributes
This document registers new attribute names in SDP.
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: Provide parameters for a FEC source flow
Reference: [RFC6364]
Values: See [RFC6364]
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: Provide parameters for a FEC repair flow
Reference: [RFC6364]
Values: See [RFC6364]
SDP Attribute ("att-field"):
Attribute name: repair-window
Long form: Pointer to FEC Repair Window
Type of name: att-field
Type of attribute: Media level
Subject to charset: No
Purpose: Indicate the size of the repair window
Reference: [RFC6364]
Values: See [RFC6364]
9. Acknowledgments
The author would like to thank the FEC Framework Design Team for
their inputs, suggestions, and contributions.
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RFC 6364 SDP Elements for FEC Framework October 2011
10. References
10.1. Normative References
[RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error
Correction (FEC) Framework", RFC 6363, October 2011.
[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.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888, June 2010.
[RFC5956] Begen, A., "Forward Error Correction Grouping Semantics in
the Session Description Protocol", RFC 5956,
September 2010.
[RFC3890] Westerlund, M., "A Transport Independent Bandwidth
Modifier for the Session Description Protocol (SDP)",
RFC 3890, September 2004.
[RFC5234] Crocker, D., Ed., 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.
10.2. Informative References
[FECFRAME-CFG-SIGNAL]
Asati, R., "Methods to convey FEC Framework Configuration
Information", Work in Progress, September 2011.
[RFC5939] Andreasen, F., "Session Description Protocol (SDP)
Capability Negotiation", RFC 5939, September 2010.
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RFC 6364 SDP Elements for FEC Framework October 2011
Author's Address
Ali Begen
Cisco
181 Bay Street
Toronto, ON M5J 2T3
Canada
EMail: abegen@cisco.com
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