Internet DRAFT - draft-ietf-fecframe-config-signaling
draft-ietf-fecframe-config-signaling
FECFRAME Working Group Rajiv Asati
Internet Draft Cisco Systems
Intended status: Informational
Expires: July 2012
June 8, 2012
Methods to convey FEC Framework Configuration Information
draft-ietf-fecframe-config-signaling-09.txt
Abstract
FEC Framework document [RFC6363] defines the FEC Framework
Configuration Information necessary for the FEC framework operation.
This document describes how to use signaling protocols such as
Session Announcement Protocol (SAP), Session Initiation Protocol
(SIP), Real Time Stream Protocol (RTSP) etc. for determining and
communicating the Configuration information between sender(s) and
receiver(s).
This document doesn't define any new signaling protocol.
Status of this Memo
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Table of Contents
1. Introduction...................................................3
2. Specification Language.........................................4
3. Terminology/Abbreviations......................................4
4. FEC Framework Configuration Information........................5
4.1. Encoding Format...........................................6
5. Signaling Protocol Usage.......................................7
5.1. Signaling Protocol for Multicasting.......................8
5.1.1. Sender Procedure.....................................9
5.1.2. Receiver Procedure..................................12
5.2. Signaling Protocol for Unicasting........................13
5.2.1. SIP.................................................13
5.2.2. RTSP................................................14
6. Security Considerations.......................................15
7. IANA Considerations...........................................15
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8. Acknowledgments...............................................15
9. References....................................................16
9.1. Normative References.....................................16
9.2. Informative References...................................16
Author's Addresses...............................................17
1. Introduction
FEC Framework document [RFC6363] defines the FEC Framework
Configuration Information that governs the overall FEC framework
operation common to any FEC scheme. This information must be
available at both sender and reciever(s).
This document describes how various signaling protocols such as
Session Announcement Protocol (SAP)[RFC2974], Session Initiation
Protocol (SIP)[RFC3261], Real Time Stream Protocol (RTSP)[RFC2326]
etc. could be used by the FEC scheme (and/or Content Delivery
Protocol (CDP))to communicate the Configuration information between
sender and receiver(s). The configuration information may be encoded
in any compatible format such as SDP [RFC4566], XML etc., though
this document references to SDP encoding usage quite extensively.
Note that this document doesn't define any new signaling protocol;
rather it just provides examples of how existing protocols should
be used. Also, the list of signaling protocols for unicast is not
intended to be a complete list.
This document doesn't describe any FEC scheme specific information
(FSSI) (for example, how source blocks are constructed) or any
sender or receiver side operation for a particular FEC scheme (for
example, whether the receiver makes use of one or more repair flows
that are received). Such FEC scheme specifics should be covered in
separate document(s). This document doesn't mandate a particular
encoding format for the configuration information either.
This document is structured such that Section 2 describes the terms
used in this document, section 4 describes the FEC Framework
Configuration Information, section 5 describes how to use signaling
protocol for the multicast and unicast applications, and section 6
describes security consideration.
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2. Specification Language
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. Terminology/Abbreviations
This document makes use of the terms/abbreviations defined in the
FEC Framework document [RFC6363] and defines the following
additional terms:
o Media Sender - Node providing original media flow(s) to the
'FEC Sender'
o Media Receiver - Node performing the Media decoding;
o FEC Sender - Node performing the FEC encoding on the
original media flow(s) to produce the FEC repair flow(s)
o FEC Receiver - Node performing the FEC decoding, as needed,
and providing the original media flow(s) to the Media receiver.
o Sender - Same as FEC Sender
o Receiver - Same as FEC Receiver
o (Media) Flow - A single media instance i.e., an audio stream
or a video stream.
This document deliberately refers to the 'FEC Sender' and 'FEC
Receiver' as the 'Sender' and 'Receiver' respectively.
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4. FEC Framework Configuration Information
The FEC Framework [RFC6363] defines a minimum set of information
that is communicated between the sender and receiver(s) for a proper
operation of an FEC scheme. This information is referred to as "FEC
Framework Configuration Information". This is the information that
the FEC Framework needs in order to apply FEC protection to the
transport flows.
A single instance of the FEC Framework provides FEC protection for
all packets of a specified set of source packet flows, by means of
one or more packet flows consisting of repair packets. As per the
FEC Framework document [RFC6363] section 6.5, the FEC Framework
Configuration Information includes the following for each FEC
Framework instance:
1. Identification of the repair flow(s)
2. Identification of Source Flow(s)
3. Identification of FEC Scheme
4. Length of Explicit Source FEC payload ID
5. FEC Scheme Specific Information (FSSI)
FSSI basically provides an opaque container to encode FEC scheme
specific configuration information such as buffer size, decoding
wait-time etc. Please refer to the FEC Framework document [RFC6363]
for more details.
The usage of signaling protocols described in this document requires
that the application layer responsible for the FEC Framework
instance provide the value for each of the configuration information
parameter (listed above) encoded as per the chosen encoding format.
In case of failure to receive the complete information, the
signaling protocol module must return an error for the Operation,
Administration and Maintenance (OAM) purposes and optionally convey
this error to the application layer. Please refer to the figure 1 of
the FEC Framework document [RFC6363] for further illustration.
This document does not make any assumption that the 'FEC sender' and
'Media Sender' functionalities are implemented on the same device,
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though that may be the case. Similarly, this document does not make
any assumption that 'FEC receiver' and 'Media Receiver'
functionalities are implemented on the same device, though that may
be the case. There may also be more than one Media Sender.
4.1. Encoding Format
The FEC Framework Configuration Information (listed above in section
4) may be encoded in any format such as SDP, XML etc. as chosen or
prefered by a particular FEC Framework instance. The selection of
such encoding format or syntax is independent of the signaling
protocol and beyond the scope of this document.
Whatever encoding format is selected for a particular FEC framework
instance, it must be known to the signaling protocol. This is to
provide a means (e.g. a field such as Payload Type) in the signaling
protocol message(s) to convey the chosen encoding format for the
configuration information so that the Payload i.e., configuration
information can be correctly parsed as per the semantics of the
chosen encoding format at the receiver. Please note that the
encoding format is not a negotiated parameter, but rather a property
of a particular FEC Framework instance and/or its implementation.
Additionally, the encoding format for each FEC Framework
configuration parameter must be defined in terms of a sequence of
octets that can be embedded within the payload of the signaling
protocol message(s). The length of the encoding format must either
be fixed, or derived by examining the encoded octets themselves.
For example, the initial octets may include some kind of length
indication.
Independent of the encoding formats supported by an FEC scheme, each
instance of the FEC Framework must use a single encoding format to
describe all of the configuration information associated with that
instance. The signaling protocol specified in this document should
not validate the encoded information, though it may validate the
syntax or length of the encoded information.
The reader may refer to the SDP elements document [RFC6364], which
describes the usage of 'SDP' encoding format as an example encoding
format for FEC Framework Configuration Information.
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5. Signaling Protocol Usage
FEC Framework [RFC6363] requires certain FEC Framework Configuration
Information to be available to both sender and receiver(s). This
configuration information is almost always formulated at the sender
(or on behalf of a sender), and somehow made available at the
receiver(s). While one may envision a static method to populate the
configuration information at both sender and receiver(s), it would
not be optimal since it would (a) require the knowledge of every
receiver in advance, (b) require the time and means to configure
each receiver and sender, and (c) increase the misconfiguration
possibility. Hence, there is a benefit in using a dynamic method
i.e., signaling protocol to convey the configuration information
between sender and one or more receivers.
Since the configuration information may be needed at a particular
receiver versus many receivers (depending on the multimedia stream
being unicast e.g. Video on Demand, or multicast e.g. Broadcast or
IPTV), we need two types of signaling protocols - one to deliver the
configuration information to many receivers via multicasting
(described in section 5.1), and the other to deliver the
configuration information to one and only one receiver via
unicasting (described in section 5.2).
Figure 1 below illustrates a sample topology showing the FEC sender
and FEC receiver (that may or may not be the Media Sender and Media
Receiver respectively) such that FEC_Sender1 is serving
FEC_Receiver11,12,13 via the multicast signaling protocol, whereas
the FEC_Sender2 is serving only FEC_Receiver2 via the unicast
signaling protocol.
FEC_Sender2---------| |--------FEC_Receiver2
| |
FEC_Sender1-------IP/MPLS network
|-----------FEC_Receiver11
|-----------FEC_Receiver12
|-----------FEC_Receiver13
Figure 1 Topology using Sender and Receiver
The rest of the document continues to use the terms 'Sender' and
'Receiver' to refer to the 'FEC Sender' and 'FEC Receiver'
respectively.
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5.1. Signaling Protocol for Multicasting
This specification describes using Session Announcement Protocol
(SAP) version 2 [RFC2974] as the signaling protocol to multicast the
configuration information from one sender to many receivers. The
apparent advantage is that the server doesn't need to maintain any
state for any receiver using SAP.
SAP messages are carried over UDP over IP with destination UDP
port being 9875 and source UDP port being any available number,
as described in RFC2974. The SAP message(s) MUST contain an
authentication header using PGP authentication.
At the high level, a sender, acting as the SAP announcer, signals
the FEC Framework Configuration Information for each FEC Framework
instance available at the sender, using the SAP message(s). The
configuration information, encoded in a suitable format as per the
section 4.1, is carried in the Payload of the SAP message(s). A
receiver, acting as the SAP listener, listens on a well-known UDP
port and at least one well known multicast group IP address (as
explained in the section 5.1.1). This enables the receiver to
receive the SAP message(s) and obtains the FEC Framework
Configuration Information for each FEC Framework Instance.
Using the configuration information, the receiver becomes aware of
available FEC protection options, corresponding multicast trees (S,G
or *,G addresses) etc. The receiver may subsequently subscribe to
one or more multicast trees to receive the FEC streams using out-of-
band multicasting techniques such as PIM [RFC4601]. This, however,
is outside the scope of this document.
Figure 2 below illustrates the SAP packet format (it is reprinted
from the RFC2974) -
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| V=1 |A|R|T|E|C| auth len | msg id hash |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: originating source (32 or 128 bits) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| optional authentication data |
: .... :
*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
| optional payload type |
+ +-+- - - - - - - - - -+
| |0| |
+ - - - - - - - - - - - - - - - - - - - - +-+ |
| |
: payload :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 SAP Message format
While the RFC2974 includes explanation for each field, it is worth
discussing the 'Payload' and 'Payload Type' fields. The 'Payload'
field is used to carry the FEC Framework Configuration Information.
Subsequently, the optional 'Payload Type' field, which is a MIME
content type specifier, is used to describe the encoding format used
to encode the Payload.
For example, the 'Payload Type' field may be application/sdp if
the FEC Framework Configuration Information is encoded in SDP
format and carried in the SAP payload. Similarly, it would be
application/xml if the FEC Framework Configuration Information was
encoded in XML format.
Section 5.1.1 describes the sender procedure, whereas the section
5.1.2 describes the receiver procedure in the context of config
signaling using RFC2974.
5.1.1. Sender Procedure
The sender signals the FEC framework configuration for each FEC
framework instance in a periodic SAP announcement message [RFC2974].
The SAP announcement message is sent to a well known multicast IP
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address and UDP port, as specified in [RFC2974]. The announcement is
multicast with the same scope as the session being announced.
The SAP module at the sender obtains the FEC Framework Configuration
Information per Instance from the 'FEC Framework' module and places
that in the SAP payload accordingly. A single SAP (announcement)
message must carry the FEC Framework Configuration Information for a
single FEC Framework Instance. The SAP message is then sent over UDP
over IP.
While it is possible to aggregate multiple SAP (announcement)
messages in a single UDP datagram as long as the resulting UDP
datagram length is less than the IP MTU of the outgoing interface,
this specification does not recommend it since there is no length
field in the SAP header to identify SAP message boundary. Hence,
this specification recommends single SAP announcement message to
be sent in a UDP datagram.
The IP packet carrying the SAP message must be sent to destination
IP address of one of the following depending on the selected scope:
- 224.2.127.254 (if IPv4 global scope 224.0.1.0-238.255.255.255
is selected for the FEC stream), or
- FF0X:0:0:0:0:0:2:7FFE (if IPv6 multicasting is selected for the
FEC stream, where X is the 4-bit scope value), or
- the highest multicast address (239.255.255.255, for example) in
the relevant administrative scope zone (if IPv4 administrative
scope 239.0.0.0-239.255.255.255 is selected for the FEC stream)
As defined in RFC2974, the IP packet carrying SAP message must use
destination UDP port being 9875 and source UDP port bein any
available number. The default IP TTL value (or Hop Limit value)
should be 255 at the sender, though the sender implementation may
allow it to be any other value to implicitly create the multicast
boundary for SAP announcements. The IP DSCP field may be set to any
value that indicates a desired QoS treatment in the IP network.
The IP packet carrying the SAP message must be sent with source IP
address that is reachable by the receiver. The sender may assign the
same IP address in the "originating source" field of the SAP
message, as the one used in the source IP address of the IP packet.
Furthermore, the FEC Framework Configuration Information must not
include any of the reserved multicast group IP addresses for the FEC
streams (i.e., source or repair flows), though it may use the same
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IP address as the 'originating source' address to identify the FEC
streams (i.e., source or repair flows). Please refer to IANA
assignments for multicast addresses.
The sender must periodically send the 'SAP announcement' message to
ensure that the receiver doesn't purge the cached entry(s) from the
database and doesn't trigger the deletion of FEC Framework
Configuration Information.
While the time interval between repetitions of an announcement can
be calculated as per the very sophisticated but complex method
explained in [RFC2974], this document recommends a simpler method in
which the user specifies the time interval in the range of 1-200
seconds with suggested default value being 60 seconds. In this
method, the 'time interval' may be signaled in the SAP message
payload e.g. within the FEC Framework Configuration Information.
Note that SAP doesn't allow the time-interval to be signaled in
the SAP header. Hence, the usage of simpler method requires the
time-interval to be included in the FEC Framework Configuration
Information, if the default time interval (=60 seconds) for SAP
message repeations is not used. For example, the usage of "r="
(repeat time) field in SDP may convey the time-interval value, if
SDP encoding format is used.
The time interval must be chosen to ensure that SAP announcement
messages are sent out before the corresponding multicast routing
entry e.g. (S,G) or (*,G) (corresponding to the SAP multicast
tree(s)) on the router(s) times out. (It is worth noting that the
default time-out period for the multicast routing entry is 210
seconds, per the PIM specification [RFC4601], though the time-out
period may be set to another value as allowed by the router
implementation.)
A SAP implementation may also support the complex method for
determining the SAP announcement time interval, and provide the
option to select it.
The sender may choose to delete the announced FEC Framework
Configuration Information, as defined in section 4 of RFC2974. The
explicit deletion is useful if the sender no longer desires to send
anymore FEC streams.
If the sender needs to modify the announced FEC Framework
Configuration Information for one or more FEC instances, then the
sender must send a new announcement message with a different
'Message Identifier Hash' value as per the rules described in
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section 5 of RFC2974 [RFC2974]. Such announcement message should be
sent immediately (without having to wait for the time-interval) to
ensure that the modifications are received by the receiver as soon
as possible. The sender must also send the SAP deletion message to
delete the previous SAP announcement message (i.e., with the
previous 'Message Identifier Hash' value).
5.1.2. Receiver Procedure
The receiver must listen on UDP port 9875 for packets arriving with
IP destination address of either 224.2.127.254 (if IPv4 global scope
session is used for the FEC stream), or FF0X:0:0:0:0:0:2:7FFE (if
IPv6 is selected, where X is the 4-bit scope value), or the highest
IP address (239.255.255.255, for example) in the relevant
administrative scope zone (if IPv4 administrative scope 239.0.0.0-
239.255.255.255 is selected for the FEC stream). These IP addresses
are mandated for SAP usage by RFC2974 [RFC2974].
The receiver, upon receiving a SAP announcement message, creates an
entry, if it doesn't already exist, in a local database and passes
the FEC Framework Configuration Information from the SAP Payload
field to the 'FEC Framework' module. Each entry also maintains a
time-out value, which is (re)set to five times the time-interval
value, which is either the default = 60 seconds, or the value
signaled by the sender.
Note that SAP doesn't allow the time-interval to be signaled in
the SAP header. Hence, the time-interval should be included in the
FEC Framework Configuration Information. For example, the usage of
"r=" (repeat time) field in SDP to convey the time-interval value,
if SDP encoding format is used.
The time-out value associated with each entry is reset when the
corresponding announcement (please see section 5 of [RFC2974]) is
received. If the time-out value for any entry reaches zero, then
that entry must be deleted from the database, as described in
section 4 of [RFC2974]. The receiver, upon receiving a SAP delete
message, must delete the matching SAP entry in its database, as
described in section 4 of [RFC2974].
The deletion of SAP entry must result in the receiver no longer
using the relevant FEC Framework Configuration Information for the
corresponding instance, and must no longer subscribe to any related
FEC streams.
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5.2. Signaling Protocol for Unicasting
This document describes leveraging any signaling protocol that is
already used by the unicast application, for exchanging the FEC
Framework Configuration Information between two nodes.
For example, a multimedia (VoD) client may send a request via
unicasting for a particular content to the multimedia (VoD) server,
which may offer various options such as encodings, bitrates,
transport etc. for the content. The client selects the suitable
options and answers to the server, paving the way for the content to
be unicast on the chosen transport from server to the client. This
offer/answer signaling, described in [RFC3264], is commonly utilized
by many application protocols such as SIP, RTSP etc.
The fact that two nodes desiring unicast communication almost always
rely on an application to first exchange the application related
parameters via the signaling protocol makes it logical to enhance
such signaling protocol(s) to (a) convey the desire for the FEC
protection and (b) subsequently also exchange FEC parameters i.e.,
FEC Framework Configuration Information. This enables the node
acting as the offerer to offer 'FEC Framework Configuration
Information' for each of available FEC instances, and the node
acting as the answerer conveying the chosen FEC Framework
instance(s) to the offerer. The usage of FEC framework instance is
explained the FEC Framework document [RFC6363].
While enhancing an application's signaling protocol to exchange FEC
parameters is one method (briefly explained above), an alternative
method would be to have a unicast based generic protocol that could
be used by two nodes independent of the application's signaling
protocol. The latter is not covered by this document, of course.
The remainder of this section provides example signaling protocols
and explains how they can be used to exchange FEC Framework
Configuration Information.
5.2.1. SIP
SIP [RFC3261] is an application-level signaling protocol to create,
modify, and terminate multimedia sessions with one or more
participants. SIP also enables the participants to discover one
another and to agree on a characterization of a multimedia session
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they would like to share. SIP runs on either TCP or UDP or SCTP
transport, and uses SDP as the encoding format to describe multmedia
session attributes.
SIP already uses an offer/answer model with SDP, described in
[RFC3264], to exchange the information between two nodes to
establish unicast sessions between them. This document extends the
usage of this model for exchanging the FEC Framework Configuration
Information, explained in section 4. Any SDP specific enhancements
to accommodate the FEC Framework are covered in the SDP Elements
specification [RFC6364].
5.2.2. RTSP
Real-Time Streaming Protocol (RTSP) [RFC2326] is an application-
level signaling protocol for control over the delivery of data with
real-time properties. RTSP provides an extensible framework to
enable controlled, on-demand delivery of real-time data, such as
audio and video. RTSP runs on either TCP or UDP transports.
RTSP already provides an ability to extend the existing method with
new parameters. This specification defines 'FEC Protection Needed'
option-tag (please see section 7 for IANA Considerations) and
prescribes including it in the Require (or Proxy-Require) header of
SETUP (method) request message, so as to request for FEC protection
for the data.
The node receiving such request either responds with "200 OK"
message that includes offers i.e., available FEC options (e.g. FEC
Framework Configuration Information for each Instance) or "551
Option not supported" message. A sample of related message exchange
is shown below -
Node1->Node2: SETUP < ... > RTSP/1.0
CSeq: 1
Transport: <omitted for simplicity>
Require: FEC-protection-needed
Node2->Node1: RTSP/1.0 200 OK
CSeq: 1
Transport: <omitted for simplicity>
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The requesting node (Node1) may then send a new SETUP message to
convey the selected FEC protection to Node2, and proceed with
regular RTSP messaging.
Suffice to say, if the requesting node (Node1) received '551 Option
not supported' response from Node2, then the requesting node (Node1)
may send the SETUP message without using the Require header.
6. Security Considerations
This document recommends SAP message(s) be authenticated to ensure
sender authentication, as described in section 5.1.
There is no additional security consideration other than what's
already covered in [RFC2974] for SAP, [RFC2326] for RTSP, and
[RFC3261] for SIP.
7. IANA Considerations
This document requests IANA to register a new RTSP Option tag
(option-tag) listed below in the RTSP/1.0 Option Tags table of the
"Real Time Streaming Protocol (RTSP)/1.0 Parameters" registry
available from http://www.iana.org/, and provides the following
information in compliance with section 3.8.1 in [RFC2326]:
. Name of option-tag = FEC-protection-needed
. Description = See section 5.2.2
. Change of Control = IETF
8. Acknowledgments
Thanks to Colin Perkins for pointing out the issue with the time-
interval for the SAP messages. Additionally, thanks to Vincent Roca,
Ali Begen, Mark Watson, Ulas Kozat and David Harrington for greatly
improving this document.
This document was prepared using 2-Word-v2.0.template.dot.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6363] Watson, M., "Forward Error Correction (FEC) Framework",
RFC6363, March 2011.
[RFC6364] Begen, A., "Session Description Protocol Elements for
FEC Framework ", RFC6364, October 2011.
[RFC2974] Handley, M., Perkins, C. and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.
9.2. Informative References
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June
2002.
[RFC2326] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[RFC3261] Handley, M., Schulzrinne, H., Schooler, E. and J.
Rosenberg, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC4601] Fenner, etc., "Protocol Independent Multicast - Sparse
Mode (PIM-SM): Protocol Specification", RFC 4601, August
2006.
[RFC3547] Baugher, etc., "The Group Domain of Interpretation", RFC
3547, July 2003.
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Internet-Draft FEC Framework Config Signaling June 8, 2012
Author's Addresses
Rajiv Asati
Cisco Systems,
7025-6 Kit Creek Rd, RTP, NC, 27709-4987
Email: rajiva@cisco.com
Asati Expires July 8, 2012 [Page 17]
