Internet DRAFT - draft-ietf-fecframe-pseudo-cdp
draft-ietf-fecframe-pseudo-cdp
FEC Framework U. Kozat
Internet-Draft DoCoMo USA Labs
Intended status: Informational A. Begen
Expires: April 20, 2013 Cisco
October 17, 2012
Pseudo Content Delivery Protocol (CDP) for Protecting Multiple Source
Flows in FEC Framework
draft-ietf-fecframe-pseudo-cdp-05
Abstract
This document provides a pseudo Content Delivery Protocol (CDP) to
protect multiple source flows with one or more repair flows based on
the 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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 20, 2013.
Copyright Notice
Copyright (c) 2012 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
Kozat & Begen Expires April 20, 2013 [Page 1]
Internet-Draft Pseudo CDP for Multiple Source Flows October 2012
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.
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.
Kozat & Begen Expires April 20, 2013 [Page 2]
Internet-Draft Pseudo CDP for Multiple Source Flows October 2012
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Definitions/Abbreviations . . . . . . . . . . . . . . . . . . 4
3. Construction of a Repair Flow from Multiple Source Flows . . . 4
3.1. Example: Two Source Flows Protected by a Single Repair
Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Reconstruction of Source Flows from Repair Flow(s) . . . . . . 11
4.1. Example: Multiple Source Flows Protected by a Single
Repair Flow . . . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
8. Normative References . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Kozat & Begen Expires April 20, 2013 [Page 3]
Internet-Draft Pseudo CDP for Multiple Source Flows October 2012
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 provides 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,
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 (SB) by
multiplexing transport payloads from multiple flows (See Figure 1 and
Figure 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 Explicit
Source FEC payload ID is optional and if CDP has 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
Kozat & Begen Expires April 20, 2013 [Page 4]
Internet-Draft Pseudo CDP for Multiple Source Flows October 2012
FEC payload ID might not be output.
+------------+
s_1 --------> | |
. Source | Source | +--------+ +--------+ +--------+
. Flows | Block |==> ..|SB_(j+1)| | SB_j | |SB_(j-1)| ..
s_n --------> | Generation | +--------+ +--------+ +--------+
+------------+
Figure 1: Source Block generation for an 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
Flow ID (FID) value provides a unique short-hand 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 Figure 3 and
Kozat & Begen Expires April 20, 2013 [Page 5]
Internet-Draft Pseudo CDP for Multiple Source Flows October 2012
Figure 4).
+------+
+--------+ +--------+ +--------+ | | -------> r_1
.. |SB_(j+1)| | SB_j | |SB_(j-1)| .. ==> | FEC | Repair .
+--------+ +--------+ +--------+ |Scheme| Flows .
| | -------> r_k
+------+
Figure 3: Repair flow generation by an 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.
Kozat & Begen Expires April 20, 2013 [Page 6]
Internet-Draft Pseudo CDP for Multiple Source Flows October 2012
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.
Kozat & Begen Expires April 20, 2013 [Page 7]
Internet-Draft Pseudo CDP for Multiple Source Flows October 2012
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
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-byte long
and can be divided into 14 symbols after padding 168 bytes. Using
the same encoder, suppose that 7 repair symbols are generated for
SB_2.
Kozat & Begen Expires April 20, 2013 [Page 8]
Internet-Draft Pseudo CDP for Multiple Source Flows October 2012
<-------- 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-
byte symbols, denoted by @1 to @10. Similarly @11 to @17 constitute
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
Kozat & Begen Expires April 20, 2013 [Page 9]
Internet-Draft Pseudo CDP for Multiple Source Flows October 2012
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 and in our
example indicating source block sequence number, length of each
source payload, and the order that the first parity block in a repair
packet is generated are 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 4th symbol and the source block consists
of two source flows carrying 4 and 6 packets from each.
Kozat & Begen Expires April 20, 2013 [Page 10]
Internet-Draft Pseudo CDP for Multiple Source Flows October 2012
+------------------------------------+
| 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 4
packets from the first one and 6 packets from the second one. Flow
IDs state that the packets from source flow 0 precedes 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 an source block
with exact locations of the missing packets. Figure 9 depicts the
case for SB_1. Since the original source block with missing 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.
Kozat & Begen Expires April 20, 2013 [Page 11]
Internet-Draft Pseudo CDP for Multiple Source Flows October 2012
<-------- 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. IANA Considerations
There are no IANA related issues considered in this document.
7. Acknowledgments
The authors would like to thank the FEC Framework Design Team for
their inputs, suggestions and contributions.
8. 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.
Kozat & Begen Expires April 20, 2013 [Page 12]
Internet-Draft Pseudo CDP for Multiple Source Flows October 2012
[RFC6695] Asati, R., "Methods to Convey Forward Error Correction
(FEC) Framework Configuration Information", RFC 6695,
August 2012.
Authors' Addresses
Ulas C. Kozat
DoCoMo USA Labs
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
Kozat & Begen Expires April 20, 2013 [Page 13]
