Internet Engineering Task Force (IETF) U. Kozat
Request for Comments: 6801 DOCOMO Innovations
Category: Informational A. Begen
ISSN: 2070-1721 Cisco
November 2012
Pseudo Content Delivery Protocol (CDP) for
Protecting Multiple Source Flows in the
Forward Error Correction (FEC) Framework
Abstract
This document provides a pseudo Content Delivery Protocol (CDP) to
protect multiple source flows with one or more repair flows based on
the Forward Error Correction (FEC) Framework and the Session
Description Protocol (SDP) elements defined for the framework. The
purpose of the document is not to provide a full-fledged protocol but
to show how the defined framework and SDP elements can be combined
together to implement a CDP.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see 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/rfc6801.
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Copyright Notice
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Table of Contents
1. Introduction ....................................................3
2. Definitions/Abbreviations .......................................3
3. Construction of a Repair Flow from Multiple Source Flows ........3
3.1. Example: Two Source Flows Protected by a Single
Repair Flow ................................................6
4. Reconstruction of Source Flows from Repair Flow(s) ..............9
4.1. Example: Multiple Source Flows Protected by a
Single Repair Flow .........................................9
5. Security Considerations ........................................10
6. Acknowledgments ................................................10
7. Normative References ...........................................11
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1. Introduction
The Forward Error Correction (FEC) Framework (described in [RFC6363])
and SDP Elements for FEC Framework (described in [RFC6364]) together
define mechanisms sufficient enough to build an actual Content
Delivery Protocol (CDP) with FEC protection. Methods to convey FEC
Framework Configuration Information (described in [RFC6695]), on the
other hand, provide the signaling protocols that may be used as part
of CDP to communicate FEC-Scheme-Specific Information from FEC sender
to a single as well as multiple FEC receivers. This document
provides a guideline on how the mechanisms defined in [RFC6363] and
[RFC6364] can be sufficiently used to design a CDP over a non-trivial
scenario, namely, protection of multiple source flows with one or
more repair flows.
In particular, we provide clarifications and descriptions on how:
o source and repair flows may be uniquely identified,
o source blocks may be generated from one or more source flows,
o repair flows may be paired with the source flows,
o the receiver explicitly and implicitly identifies individual
flows, and
o source blocks are regenerated at the receiver and the missing
source symbols in a source block are recovered.
2. Definitions/Abbreviations
This document uses all the definitions and abbreviations from Section
2 of [RFC6363] minus the RFC 2119 requirements language.
3. Construction of a Repair Flow from Multiple Source Flows
At the sender side, CDP constructs the source blocks (SBs) by
multiplexing transport payloads from multiple flows (see Figures 1
and 2). According to the FEC Framework, each source block is FEC-
protected separately. Each source block is given to the specific FEC
encoder used within the CDP as input and as the outputs Explicit
Source FEC Payload ID, Repair FEC Payload ID, and Repair Payloads
corresponding to that source block are generated. Note that the
Explicit Source FEC Payload ID is optional, and if the CDP has an
implicit means of constructing the source block at the sender/
receiver (e.g., by using any existing sequence numbers in the
payload), the Explicit Source FEC Payload ID might not be output.
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+------------+
s_1 --------> | |
. Source | Source | +--------+ +--------+ +--------+
. Flows | Block |==> ..|SB_(j+1)| | SB_j | |SB_(j-1)| ..
s_n --------> | Generation | +--------+ +--------+ +--------+
+------------+
Figure 1: Source Block Generation for a FEC Scheme
Figure 2 shows the structure of a source block. A CDP must clearly
specify which payload corresponds to which source flow and the length
of each payload.
<------------------ Source Block (SB) ------------------->
+-------...-----+-------...-----+- -+-------...-----+
| Payload_1 | Payload_2 | . . . | Payload_n |
+-------...-----+-------...-----+- -+-------...-----+
\______ _______|______ _______| |______ _______|
\/ \/ \/
FID_1,Len_1 FID_2,Len_2 FID_n,Len_n
Figure 2: Structure of a Source Block
The Flow ID (FID) value provides a unique shorthand identifier for
the source flows. FID is specified and associated with the possibly
wildcarded tuple of {source IP address, source port, destination IP
address, destination port, transport protocol} in the SDP
description. When wildcarded, certain fields in the tuple are not
needed for distinguishing the source flows. The tuple is carried in
the IP and transport headers of the source packets. Since FID is
utilized by the CDP and FEC scheme to distinguish between the source
packets, the tuple must have a one-to-one mapping to a valid FID.
This point will be clearer in the specific example given later in
this section. The length of FID must be a priori fixed and known to
both the receiver and sender. Alternatively, it might be specified
in the FEC-Scheme-Specific Information field in the SDP element
[RFC6364].
The payload length (Len) information is needed to figure out how many
bits, bytes, or symbols (depending on the FEC scheme) from a
particular source flow are included in the source block. If the
payload is not an integer multiple of the specified symbol length,
the remaining portion is padded with zeros (see Figures 3 and 4).
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+------+
+--------+ +--------+ +--------+ | | -------> r_1
.. |SB_(j+1)| | SB_j | |SB_(j-1)| .. ==> | FEC | Repair .
+--------+ +--------+ +--------+ |Scheme| Flows .
| | -------> r_k
+------+
Figure 3: Repair Flow Generation by a FEC Scheme
<------------------ Source Block (SB) ------------------->
| | | | | |
+-------...-----+-------...-----+- -+-------...-----+ |
| Payload_1 | Payload_2 | . . . | Payload_n |0|
+-------...-----+-------...-----+- -+-------...-----+ |
| | | | | |
| Symbol_1 | Symbol_2 | Symbol_3 | . . . | Symbol_m |
|<-------->|<-------->|<-------->| |<-------->|
+------+
Symbol_1,..,Symbol_m => | FEC | => Symbol_u,..,Symbol_1
| Enc. |
+------+
Figure 4: Repair Flow Payload Generation
FEC schemes typically expect a source block of certain size, say, m
symbols. Therefore, the FEC encoder divides each source block into m
symbols (with some padding if the source block is shorter than the
expected m symbols) and generates u repair symbols, which are
functions of the m symbols in the original source block. The repair
symbols are grouped by the FEC scheme into repair payloads with each
repair payload assigned a Repair FEC Payload ID in order to associate
each repair payload with a particular source block at the receiver.
If the payloads in a given source block have sequence numbers that
can uniquely specify their location in the source block, an Explicit
Source FEC Payload ID may not be generated for these payloads.
Otherwise, Explicit Source FEC Payload IDs are generated for each
payload and indicate the order the payloads appear in the source
block.
Note that FID and length information are not actually transmitted
with the source payloads since both information can be gathered by
other means as it will be clear in the next sections.
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3.1. Example: Two Source Flows Protected by a Single Repair Flow
In this section, we present an example of source flow and repair flow
generation by the CDP. We have two source flows with flow IDs of 0
and 1 to be protected by a single repair flow (see Figure 5). The
first source flow is multicast to 233.252.0.1, and the second source
flow is multicast to 233.252.0.2. Both flows use the port number
30000.
SOURCE FLOWS
S1: Source Flow | | INSTANCE #1
|---------| R3: Repair Flow
S2: Source Flow |
Figure 5: Example: Two Source Flows and One Repair Flow
The SDP description below states that the source flow defined by the
tuple {*,*,233.252.0.1,30000} is identified with FID=0 and the source
flow defined by the tuple {*,*,233.252.0.2,30000} is identified with
FID=1 (via the 'id' parameter of the "fec-source-flow" attribute).
The SDP description also states that the repair flow is to be
received at the multicast address of 233.252.0.3 and at port 30000.
v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=FEC Framework Examples
t=0 0
a=group:FEC-FR S1 S2 R3
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=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:S2
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:150ms
a=mid:R3
Figure 6 shows the first and the second source blocks (SB_1 and SB_2)
generated from these two source flows. In this example, SB_1 is of
length 10000 bytes. Suppose that the FEC scheme uses a symbol length
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of 512 bytes. Then, SB_1 can be divided into 20 symbols after
padding the source block for 240 bytes. Assume that the FEC scheme
is rate-2/3 erasure code; hence, it generates 10 repair symbols from
20 original symbols for SB_1. On the other hand, SB_2 is 7000 bytes
long and can be divided into 14 symbols after padding 168 bytes.
Using the same encoder, suppose that seven repair symbols are
generated for SB_2.
<-------- Source Block 1 -------->
+------------+-------------------+
| $1 $2 $3 $4| #1 #2 #3 #4 #5 #6 | 0..00
+------------+-------------------+
\__________________ __________________/
\/
@1 @2 @3 @4 @5 @6 @7 @8 @9 @10
<---- Source Block 2 ---->
+----------------+-------+
| $5 $6 $7 $8 $9 | #7 #8 |0..00
+----------------+-------+
\______________ _____________/
\/
@11 @12 @13 @14 @15 @16 @17
$: 1000-byte payload from source flow 1
#: 1000-byte payload from source flow 2
@: Repair symbol
Figure 6: Source Block with Two Source Flows
The information on the unit of payload length, FEC scheme, symbol
size, and coding rates can be specified in the FEC-Scheme-Specific
Information (FSSI) field of the SDP element. If the values of the
payload lengths from each source flow and the order of appearance of
source flows in every source block are fixed during the session,
these values may be also provided in the FSSI field. To carry FSSI
information to the FEC receivers, one may use the signaling methods
described in [RFC6695]. In our example, we will consider the case
where the ordering is fixed and known both at the sender and the
receiver, but the payload lengths will be variable from one source
block to another. We assume that the payload of a source flow with
an FID smaller than another flow's FID precedes other payloads in a
source block.
The FEC scheme gets the source blocks as input and generates the
parity blocks for each source block to protect the whole source
block. In the example, the repair payloads for SB_1 consist of 512-
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byte symbols, denoted by @1 to @10. Similarly, @11 to @17
constitutes the repair payloads for SB_2. The FEC scheme outputs the
repair payloads along with the Repair FEC Payload IDs. In our
example, Repair FEC Payload ID provides information on the source
block sequence number and the order the repair symbols are generated.
For instance, @3 is the third FEC repair symbol for SB_1, and the
three tuple {@3,SB_1,3} can uniquely deliver this information. In
our example, the FEC scheme also provides Explicit Source FEC Payload
IDs that carry information to indicate which source symbols
correspond to which source block sequence number and the relative
position in the source block. For instance, the two tuple {SB_2,2}
can be attached to $6 as the Explicit Source FEC Payload ID to
indicate that $6 is protected together with packets belonging to
SB_2, and $6 is the second payload in SB_2.
The source packets are generated from the source symbols by
concatenating consecutive symbols in one packet. There should not be
any fragmentation of a source symbol; e.g., symbols #7 and #8 can be
concatenated in one transport payload of 2000 bytes (the
implementation should make sure that the size of the resulting source
packet -- payload plus the overhead -- is not larger than the path
MTU), but one portion of symbol #7 should not be put in one source
packet and the remaining portion in another source packet. The
simplest implementation is to place each source symbol in a different
source packet as shown in Figure 7.
+------------------------------------+
| IP header {233.252.0.1} |
+------------------------------------+
| Transport header {30000} |
+------------------------------------+
| Original Transport Payload {$6} |
+------------------------------------+
| Source FEC Payload ID {SB_2,2} |
+------------------------------------+
Figure 7: Example of a Source Packet for IPv4
The repair packets are generated from the repair symbols belonging to
the same source block by grouping consecutive symbols in one packet.
There should not be any fragmentation of a repair symbol; e.g.,
symbols @4, @5, and @6 can be concatenated in one transport payload
of 1536 bytes, but @6 should not be divided into smaller sub-symbols
and spread over multiple repair packets. The Repair FEC Payload ID
must carry sufficient information for the decoding process. In our
example, for instance, indicating source block sequence number,
length of each source payload, and the order that the first parity
symbol in the repair packet among all the parity symbols generated
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for the same source block is sufficient. The exact header format of
Repair FEC Payload ID may be specified in the FSSI field of the SDP
element. In Figure 8, for instance, the repair symbols @4, @5, and
@6 are concatenated together. The Payload ID {SB_1,4,4,6} states
that the repair symbols protect SB_1, the first repair symbol in the
payload is generated as the fourth symbol and the source block
consists of two source flows carrying four and six packets from each.
+------------------------------------+
| IP header {233.252.0.3} |
+------------------------------------+
| Transport header {30000} |
+------------------------------------+
| Repair FEC Payload ID {SB_1,4,4,6} |
+------------------------------------+
| Repair Symbols {@4,@5,@6} |
+------------------------------------+
Figure 8: Example of a Repair Packet for IPv4
4. Reconstruction of Source Flows from Repair Flow(s)
Here we provide an example for reconstructing multiple source flows
from a single repair flow.
4.1. Example: Multiple Source Flows Protected by a Single Repair Flow
At the receiver, source flows 1 and 2 are received at
{233.252.0.1,30000} and {233.252.0.2,30000}, while the repair flow is
received at {233.252.0.3,30000}. The CDP can map these tuples to the
flow IDs using the SDP elements. Accordingly, the payloads received
at {233.252.0.1,30000} and {233.252.0.2,30000} are mapped to flow IDs
0 and 1, respectively.
The CDP passes the flow IDs and received payloads along with the
Explicit Source FEC Payload ID to the FEC scheme defined in the SDP
description. The CDP also passes the received repair packet payloads
and Repair FEC Payload ID to the FEC scheme. The FEC scheme can
construct the original source block with missing packets by using the
information given in the FEC Payload IDs. The FEC Repair Payload ID
provides the information that SB_1 has packets from two flows with
four packets from the first one and six packets from the second one.
Flow IDs state that the packets from source flow 0 precede the
packets from source flow 1. Explicit Source FEC Payload IDs, on the
other hand, provide the information about which source payload
appears in what order. Therefore, the FEC scheme can depict a source
block with exact locations of the missing packets. Figure 9 depicts
the case for SB_1. Since the original source block with missing
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packets can be constructed at the decoder and the FEC scheme knows
the coding rate (e.g., it might be carried in the FSSI field in the
SDP description), a proper decoding operation can start as soon as
the repair symbols are provided to the FEC scheme.
<-------- Source Block 1 -------->
+------------+-------------------+
| $1 $2 X X | #1 X #3 #4 #5 #6 |
+------------+-------------------+
O: Symbols received from the source flow 1 for SB_1
#: Symbols received from the source flow 2 for SB_1
X: Lost source symbols
Figure 9: Source Block Regeneration
When the FEC scheme can recover any missing symbol while more repair
symbols are arriving, it provides the recovered blocks along with the
source flow IDs of the recovered blocks as outputs to the CDP. The
receiver knows how long to wait to repair the remaining missing
packets (e.g., specified by the 'repair-window' attribute in the SDP
description). After the associated timer expires, the CDP hands over
whatever could be recovered from the source flow to the application
layer and continues with processing the next source block.
5. Security Considerations
For the general security considerations related to the FEC Framework,
refer to [RFC6363]. For the security considerations related to the
SDP elements in the FEC Framework, refer to [RFC6364]. There are no
additional security considerations that apply to this document.
6. Acknowledgments
The authors would like to thank the FEC Framework design team for
their inputs, suggestions, and contributions.
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7. Normative References
[RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error
Correction (FEC) Framework", RFC 6363, October 2011.
[RFC6364] Begen, A., "Session Description Protocol Elements for the
Forward Error Correction (FEC) Framework", RFC 6364,
October 2011.
[RFC6695] Asati, R., "Methods to Convey Forward Error Correction
(FEC) Framework Configuration Information", RFC 6695,
August 2012.
Authors' Addresses
Ulas C. Kozat
DOCOMO Innovations
3240 Hillview Avenue
Palo Alto, CA 94304-1201
USA
Phone: +1 650 496 4739
EMail: kozat@docomolabs-usa.com
Ali Begen
Cisco
181 Bay Street
Toronto, ON M5J 2T3
Canada
EMail: abegen@cisco.com
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