| IRTF Network Coding Research Group (NWCRG) | V. Roca |
| Internet-Draft | INRIA |
| Intended status: Informational | April 12, 2015 |
| Expires: October 14, 2015 |
FECFRAMEv2: Adding Sliding Encoding Window Capabilities to the FEC Framework: Problem Position
draft-roca-nwcrg-fecframev2-problem-position-00
The Forward Error Correction (FEC) Framework (or FECFRAME) (RFC 6363) has been defined by the FECFRAME IETF WG to enable the use of FEC Encoding with real-time flows in a flexible manner. The original FECFRAME specification only considers the use of block FEC codes, wherein the input flow(s) is(are) segmented into a sequence of blocks and FEC encoding performed independently on a per-block basis. This document discusses an extension of FECFRAME in order to enable a sliding (potentially elastic) window encoding of the input flow(s), using convolutional FEC codes for the erasure channel, as an alternative to block FEC codes.
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The Forward Error Correction (FEC) Framework (or FECFRAME) [RFC6363], produced by the FECFRAME IETF WG [fecframe-charter], describes a framework for using Forward Error Correction (FEC) codes with applications in public and private IP networks to provide protection against packet loss. The framework supports applying FEC to arbitrary packet flows over unreliable transport and is primarily intended for real-time, or streaming, media. This framework can be used to define Content Delivery Protocols that provide FEC for streaming media delivery or other packet flows. Content Delivery Protocols defined using this framework can support any FEC scheme (and associated FEC codes) that is compliant with various requirements defined in [RFC6363]. Thus, Content Delivery Protocols can be defined that are not specific to a particular FEC scheme, and FEC schemes can be defined that are not specific to a particular Content Delivery Protocol.
However, it is REQUIRED in [RFC6363] that the FEC scheme operate in a block manner, i.e., the input flow(s) MUST be segmented into a sequence of blocks, and FEC encoding (at a sender/coding node) and decoding (at a receiver/decoding node) MUST be performed independently on a per-block basis. This approach has a major impact on coding and decoding delays. Encoding requires that all the source symbols be known at the encoder. In case of continuous input flow(s), even if source symbols can be sent immediately, repair symbols are necessarily delayed by the block creation time, that directly depends on the block size (i.e., the number of source symbols in this block, k). This block creation time is also the minimum decoding latency any receiver will experience in case of erasures, since no repair symbol for the current block can be received before. A good value for the block size is necessarily a good balance between the minimum decoding latency at the receivers (which must be in line with the most stringent real-time requirement of the flow(s)) and the desired robustness in front of long erasure bursts (which depends on the block size).
On the opposite, a convolutional code associated to a sliding encoding window (of fixed size) or a sliding elastic encoding window (of variable size) removes this minimum decoding delay, since repair symbols can be generated and sent on-the-fly, at any time, from the source symbols present in the current encoding window. Using a sliding encoding window mode is therefore highly beneficial to real-time flows, one of the primary targets of FECFRAME.
The present document introduces the FECFRAME framework specificities, its limits, and options to extend it to sliding (optionally elastic) encoding windows and convolutional codes.
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].
This document uses the following definitions, that are mostly inspired from [RFC5052], [RFC6363] and [nc-taxonomy-id].
Packet Erasure Channel:
Systematic Code:
Code Rate:
Input Symbol:
Output Symbol:
Source Symbol:
Repair Symbol:
Application Data Unit (ADU):
(Source) ADU Flow:
ADU Information (ADUI):
FEC Source Packet:
FEC Repair Packet:
FEC Framework Configuration Information (FFCI):
To Be Completed...
This document uses the following abbreviations.
NC:
PT:
Let us discuss some key aspects of FECFRAME.
FECFRAME is not a full featured Protocol Instantiation (unlike ALC [RFC5775] and NORM [RFC5740] for instance). It is more a shim layer, or more precisely a framework for using FEC inside existing transport protocols. For instance when FECFRAME is used end-to-end inside a single RTP/UDP stream (the simplest use-case), RTP [RFC3550] and UDP are the transport protocols and FECFRAME is more a functional component that performs FEC encoding/decoding and generate RTP compliant repair packets. Even if specific headers are defined for the associated FEC Scheme, FECFRAME is still not a full featured transport protocol.
In the FECFRAME architecture, most technical details are in the FEC Scheme. In particular a FEC Scheme defines:
FECFRAME is pretty flexible in the way it can be used. In particular FECFRAME:
FECFRAME works in conjunction with SDP (or a similar protocol) to specify high level per FECFRAME Instance (i.e., per-session) signaling. This information, called FEC Framework Configuration Information [RFC6363], describes:
In practice, the FEC Scheme is valid for the whole FECFRAME Instance duration, since no update mechanism has been defined to carry a new SDP session description reliably and in real-time to all the potential receivers. This is different from ALC or NORM where the FEC Scheme selection is made on a per-object manner (rather than per-session).
The FECFRAME framework has a certain number of features and restrictions. We discuss each of them below, at the light of the use-cases identified for Network Coding.
FECFRAME, as described in [RFC6363], MUST be associated to block FEC codes. For instance ([RFC6363], section 5.1) says:
Therefore the input flow(s) are segmented into a sequence of blocks, FEC encoding being performed independently on a per-block basis.
However this is not a fundamental limitation, in the sense that the same FECFRAME architecture can be used with sliding (potentially elastic) encoding windows, associated with convolutional codes. To that purpose it is sufficient:
The FECFRAME architecture, as specified in [RFC6363], MUST feature a single encoding node and a single decoding node. These nodes may be the source and destination nodes, or may be middle-boxes, or any combination.
The question of having multiple in-network re-coding operations is not considered in [RFC6363]. The question whether this is feasible and appropriate, given the typical FECFRAME use-cases, is an open question that remains to be discussed.
FECFRAME, as specified in [RFC6363], can globally protect several flows that can originate either from a single source or from multiple sources. This also means that FECFRAME can perform both intra-flow coding or inter-flow coding. The only requirement is that those flows be identified and signaled to the FECFRAME encoder and decoder via the FEC Framework Configuration Information (e.g., it can be detailed in the SDP description).
From this point of view, FECFRAME is already in line with advanced network-coding use-cases.
FECFRAME, as specified in [RFC6363], does not specify nor restrict how the source flow(s) and repair flow(s) are to be transmitted: whether they go through the same path (e.g., when they are sent to the same UDP connection) or through multiple paths is irrelevant to FECFRAME as it is an operational and management aspect. However, it is anticipated that when several repair flows are generated, offering different protections levels (e.g., through different code-rates), these repair flows will often use different paths, to better accommodate receiver heterogeneity.
From this point of view, FECFRAME is already in line with advanced network-coding use-cases.
FECFRAME, as specified in [RFC6363], has been designed only with the transport layer use-case in mind. It MUST be located between the application and transport layers.
The question of having FECFRAME used in lower protocol layers is not considered in [RFC6363]. Whether this is feasible and appropriate, given the typical FECFRAME use-cases, is an open question that remains to be discussed.
A few key architectural aspects follow, proving the feasibility of adding sliding encoding window support to FECFRAMEv2. We assume hereafter that FECFRAMEv2 follows the initial spirit of FECFRAME, i.e., is applied in end-to-end (see Section 4.2). Extending FECFRAMEv2 to other situations, for instance with in-network re-coding, is not considered here and remains to be discussed.
+----------------------+
| Application |
+----------------------+
|
| (1) Application Data Units (ADUs)
v
+----------------------+ +----------------+
| FEC Framework v2 | | |
| |-------------------------->| FEC Scheme |
|(2) Update of sliding | (3) Source Symbols of | |
| encoding window | the sliding window | |
| |<--------------------------| |
| | (4) Explicit Source | |
|(7) Construct FEC | FEC Payload ID(s) |(5) FEC Encoding|
| source and repair |<--------------------------| (optional) |
| packet(s) | (6) Repair FEC Payload ID | |
+----------------------+ + Repair symbol(s) +----------------+
|
| (8) FEC source packets and FEC repair packets
v
+----------------------+
| Transport Layer |
| (e.g., UDP) |
+----------------------+
Figure 1: Architecture of FECFRAMEv2 in sliding encoding window mode, at a sender/coding node.
Figure 1 (adapted from [RFC6865]) illustrates the general architecture of FECFRAMEv2 when working in sliding encoding window mode. The difference with respect to the [RFC6363] architecture lies in steps 2 to 6. Instead of creating a source block, composed of a certain number of ADUs plus their associated flow/length/padding information (see for instance [RFC6865]), FECFRAMEv2 in sliding encoding window mode updates this window (step 2) and communicates the set of symbols to the FEC Scheme (step 3). This latter returns the Explicit Source FEC Payload ID(s) (step 4) when applicable so that the new symbol(s) can be sent immediately. If needed, one or several repair symbols are created by the FEC Scheme (step 5), and communicated to FECFRAME along with their Repair FEC Payload ID (step 6). The packets are then sent (steps 7 and 8).
When FECFRAMEv2 work in the traditional block mode, Figure 2 and Figure 3 of [RFC6363] remain valid.
Let us now detail the ADU / source symbol mapping. As in FECFRAME, each ADU is first prepended with its {flow ID, length} information, and potentially zero padded to align with the target symbol length (see below). This augmented ADU is called ADUI (see Figure 2).
Then source symbols are mapped to ADUIs. There is not necessarily a 1-1 bijection between ADUIs and symbols, because incoming packets can have largely varying sizes. In that case it MAY make sense to have small symbols of size significantly lower than the PMTU, as in [RFC6681]. On the opposite, [RFC6816] and [RFC6865] more conservatively associate one symbol per ADUI. (NB: this is different from ALC or NORM where source and repair packets are all of the same size, except maybe the packet containing the last source symbol of an object).
The source and repair symbol length can therefore be determined in different ways. It can be of fixed length, indicated in the FEC Framework Configuration Information (FFCI). Otherwise, for situations where there is a single source symbol per ADUI, the symbol length:
Figure 2 illustrates the creation of the ADUI, from incoming ADUs, and the correspondence to source symbols in the case there is a single source symbol per ADUI. It assumes the symbol length is aligned with the maximum ADUI length currently in the sliding encoding window (here corresponding to the ADU of index i+2).
Symbol Length (i.e., max ADU size in window + 3) <-------------------------------------------------> +----+------+----------------------+--------------+ |F[i]| L[i] | ADU[i] | Pad[i] | => src symbol i +----+------+----------+-----------+--------------+ |Fi+1|L[i+1]| ADU[i+1] | Pad[i+1] | => src symbol i+1 +----+------+----------+--------------------------+ |Fi+2|L[i+2]| ADU[i+2] (currently the largest ADU)| => src symbol i+2 +----+------+--------+----------------------------+ |Fi+3|L[i+3]|ADU[i+3]| Pad[i+3] | => src symbol i+3 +----+------+--------+----------------------------+
Figure 2: ADUI and source symbols, simple case with a single source symbol per ADUI, of length equal to the maximum ADUI size in the sliding encoding window.
When FECFRAMEv2 works in a sliding encoding window mode and the symbol length is chosen on a per-window basis, coding operations can require to adjust the L and Pad fields of each ADUI present in the current sliding encoding window. Indeed, as the maximum length of ADUs in the window varies, the various paddings and therefore L fields need to be updated accordingly. A FECFRAMEv2 implementation must accommodate with this constraint.
Let us now detail the sliding window update process at a sender. Two kinds of limitations exist that impact the sliding window management:
The most stringent limitation defines the maximum window size in terms of either number source symbols or number of ADUs (depending on the relationship between them, see Section 5.2, they can be equal or not).
Source symbols are added to the sliding encoding window as ADUs arrive.
Source symbols (and the corresponding ADUs) are removed from the sliding encoding window:
Any **source** symbol of a flow MUST be uniquely identified during the full duration where this symbol is useful.
Depending on the FEC Scheme being used, a **repair** symbol of a flow MAY or MAY not need to be uniquely identified during the full duration where this symbol is useful. For instance, being able to identify a repair symbol is OPTIONAL with Random Linear Codes (RLC), since the coding window content and associated coding vector are transmitted within the Repair FEC Payload ID. But being able to identify a repair symbol is REQUIRED with FEC Schemes that use this symbol identifier during the encoding and decoding processes (this is the case for instance with any block FEC code and some of the convolutional FEC codes).
In block mode, the so called encoding symbols are uniquely identified both by their Source Block Number (SBN) and Encoding Symbol ID (ESI), the first k ESI values denoting source symbols, the remaining n-k ESI values the repair symbols [RFC5052]. In sliding encoding window mode, there is no SBN concept (as there is no block) and there is no way to split the ESI space into source and repair identifiers. Therefore FECFRAMEv2 adds the the Repair (R) bit to distinguish two identifier spaces, one for source symbols (when the "R" bit is 0) and one for repair symbols (when the "R" bit is 1).
0 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |R| IDV (7 bits)| +-+-+-+-+-+-+-+-+
Figure 3: Symbol Identifier, 8-bit version.
0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |R| ID Value (IDV) (15 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Symbol Identifier, 16-bit version.
Figure 3 and Figure 4 illustrate Symbol ID (SID) format for situations where 8 bits or 16 bits are sufficient to uniquely identify any symbol. Other formats (e.g., 24-bit or 32-bit SID formats) are possible when meaningful. The SID is composed of:
Repair (R) flag (1-bit field):
ID Value (IDV) 7-bit (resp. 15-bit) field:
Therefore each source (and optionally repair) symbol of a flow is uniquely identified by its Symbol ID: SID = {R; IDV}. Different flows have different flow identifiers within the current FECFRAME instance, so the {flowID, SID} tuple enables to identify any symbol.
IDV being a b-bit long field, wrap-arounds will occur after the maximum value is achieved. This has two consequences:
A convolutional FEC Scheme for the erasure channel is required for FECFRAMEv2 working in sliding encoding window mode.
Several convolutional codes are possible, Random Linear Codes (RLC) being one option among others. Depending on the code, the Source and Repair FEC Payload IDs will be adapted. Some codes will also require the transmission of the coding vectors in addition to the current sliding window composition. These considerations, among others, will be specified in the FEC Scheme.
For FECFRAMEv2, the SDP session-level description needs to indicate whether:
Other details (list to be defined) also need to be specified in the session-level description.
The target use-cases for FECFRAME and Tetrys [tetrys-id] are rather similar: essentially end-to-end, with unicast or multicast flows, with a single or multiple sources and flows.
In terms of coding, FECFRAMEv2 and Tetrys follow the same sliding window approach, potentially elastic. The main difference comes from the manner the sliding window progresses.
Feedback flows are missing from FECFRAMEv2, but probably exist in the upper RTP/RTCP protocol (if used), in a simplified manner as they provide global statistics rather than per source packet reception/decoding status. Adding a detailed symbol-level acknowledgement mechanism to FECFRAMEv2, although feasible, is not in line with the original FECFRAME philosophy. Tetrys also performs code rate adjustment, based on feedback information. The same can be done with FECFRAMEv2 in case an RTCP feedback flow is available, with global reception/loss statistics.
If Tetrys is a full featured protocol, with a specific header, FECFRAME is more a shim layer. Whether it is appropriate or not to turn FECFRAMEv2 into something more elaborate, thereby breaking backward compatibility and the initial philosophy of FECFRAME, remains an open question.
Some of the key concepts of DNC [dnc-id] can be added to FECFRAMEv2 fairly easily: coding decisions and flow management decisions may depend on timing aspects thanks to a timestamp carried in the various packets. Very often, real-time flows will already contain a timestamp as part of their RTP header which facilitates these decisions.
TBD
TBD
N/A
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC3550] | Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. |
| [RFC5052] | Watson, M., Luby, M. and L. Vicisano, "Forward Error Correction (FEC) Building Block", RFC 5052, August 2007. |
| [RFC6363] | Watson, M., Begen, A. and V. Roca, "Forward Error Correction (FEC) Framework", RFC 6363, October 2011. |