Internet-Draft INRIA
Intended status: Standards Track February 7, 2017
Expires: August 11, 2017

Random Linear Codes (RLC) Forward Error Correction (FEC) Scheme for FECFRAME


This document describes a fully-specified FEC scheme for the convolutional Random Linear Codes (RLC) over GF(2^^m), where m equals 1 (binary case), 4 or 8, that can be used to protect arbitrary media streams along the lines defined by FECFRAME extended to convolutional codes. These convolutional FEC codes rely on an encoding window that slides over the source symbols, generating new repair symbols whenever needed. Compared to block FEC codes, these convolutional FEC codes offer key advantages in terms of reduced FEC-related latency while often providing improved erasure recovery capabilities.

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Table of Contents

1. Introduction

Application-Level Forward Erasure Correction (AL-FEC) codes are a key element of telecommunication systems. They are used to recover from packet erasures during content delivery sessions to a large number of receivers (multicast/broadcast transmissions). This is the case with the FLUTE/ALC protocol [RFC6726] in case of reliable file transfers over lossy networks, and the FECFRAME protocol for reliable continuous media transfers over lossy networks.

The present document focusses only on the FECFRAME protocol, used in multicast/broadcast delivery mode, with contents that feature stringent real-time constraints: each source packet has a maximum validity period after which it will not be considered by the destination application.

1.1. Limits of Block Codes with Real-Time Flows

With FECFRAME, there is a single FEC encoding point (either a end-host/server (source) or a middlebox) and a single FEC decoding point (either a end-host (receiver) or middlebox). In this context, currently standardized AL-FEC codes for FECFRAME like Reed-Solomon [RFC6865], LDPC-Staircase [RFC6816], or Raptor/RaptorQ, are all linear block codes: they require the data flow to be segmented into blocks of a predefined maximum size. The block size is a balance between robustness (in particular in front of long erasure bursts for which there is an incentive to increase the block size) and maximum decoding latency (for which there is an incentive to decrease the block size). Therefore, with a multicast/broadcast session, the block code is dimensioned by considering the worst communication channel one wants to support, and this choice impacts all receivers, no matter their individual channel quality.

1.2. Lower Latency and Better Protection with RLC Convolutional Codes

This document introduces a fully-specified FEC scheme that follows a totally different approach: the Random Linear Codes (RLC) over GF(2^^m), where m equals 1, 4 or 8. This FEC scheme is used to protect arbitrary media streams along the lines defined by FECFRAME extended to convolutional codes [fecframe-ext]. This FEC scheme is extremmely efficient in case of media with real-time constraints, sent within a multicast/broadcast session.

The RLC codes belong to the broad class of convolutional AL-FEC codes. The encoding process is based on an encoding window that slides over the set of source packets (in fact source symbols as we will see in Section 3.2), and which is either of fixed or variable size (elastic window). Repair packets (symbols) are generated and sent on-the-fly, after computing a random linear combination of the source symbols present in the current encoding window.

At the receiver, a linear system is managed from the set of received source and repair packets. New variables (representing source symbols) and equations (representing the linear combination of each repair symbol received) are added upon receiving new packets. Variables are removed when they are too old with respect to their validity period (real-time constraints), as well as the associated equations they are involved in (Appendix A introduces an optimisation that extends the time a variable is considered in the system). Erased source symbols are then recovered thanks this linear system whenever its rank permits it.

With RLC (more generally with convolutional codes), the protection of a multicast/broadcast session also needs to be dimensioned by considering the worst communication channel one wants to support. However the receivers experiencing a good to medium channel quality observe a FEC-related latency close to zero [Roca16] since an isolated erased source packet is quickly recovered by the following repair packet. On the opposite, with a block code, recovering an isolated erased source packet always requires waiting the end of the block for the first repair packet to arrive. Additionally, under certain situations (e.g., with a limited FEC-related latency budget and with constant bit rate transmissions after FECFRAME encoding), convolutional codes achieve more easily a target transmission quality (e.g., measured by the residual loss after FEC decoding) by sending fewer repair packets (i.e., higher code rate) than block codes.

1.3. Small Transmission Overheads with the RLC FEC Scheme

The RLC FEC scheme is designed so as to reduce the transmission overhead. The main requirement is that each repair packet header must enable a receiver to reconstruct the list of source symbols and the associated random coefficients used during the encoding process. In order to minimize packet overhead, the set of symbols in the encoding window as well as the set of coefficients over GF(2^^m) used in the linear combination are not individually listed in the repair packet header. Instead, each FEC repair packet header contains:

Therefore, no matter the number of source symbols present in the encoding window, each FEC repair packet features a fixed 64-bit long header, also called Repair FEC Payload ID (Figure 7). Similarly, each FEC source packet features a fixed 32-bit lon trailer, also called Explicit Source FEC Payload ID (Figure 5), that contains the ESI of the first source symbol (see the ADUI and source symbol mapping, Section 3.2).

1.4. Document Organization

This fully-specified FEC scheme follows the structure required by [RFC6363], section 5.6. "FEC Scheme Requirements", namely:

3. Procedures:
This section describes procedures specific to this FEC scheme, namely: RLC parameters derivation, ADUI and source symbols mapping, pseudo-random number generator, and coding coefficients generation function;
4. Formats and Codes:
This section defines the Source FEC Payload ID and Repair FEC Payload ID formats, carrying the signaling information associated to each source or repair symbol. It also defines the FEC Framework Configuration Information (FFCI) carrying signaling information for the session;
5. FEC Code Specification:
Finally this section provides the code specification.

2. Definitions and Abbreviations

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 and abbreviations:

Application Data Unit
encoding symbol size (i.e., source or repair symbol), assumed fixed (in bytes)
transmission bandwidth at the output of the FECFRAME sender, assumed fixed (in bits/s)
maximum FEC-related latency within FECFRAME (in seconds)
AL-FEC coding rate
packet loss rate on the erasure channel
encoding window current size at a sender (in symbols)
encoding window maximum size at a sender (in symbols)
decoding window current size at a receiver (in symbols).
decoding window maximum size at a receiver (in symbols)
linear system maximum size (or width) at a receiver (in symbols)
linear system current size (or width) at a receiver (in symbols)
pseudo-random number generator
PRNG defined in Section 3.4 and used in this specification, that returns a new random integer in [0; maxv-1]

3. Procedures

This section introduces the procedures that are used by this FEC scheme.

3.1. RLC parameters derivation

The RLC FEC Scheme relies on several key internal parameters:

Maximum FEC-related latency budget, max_lat (in seconds)
A source ADU flow can have real-time constraints, and therefore any FECFRAME related operation must take place within the validity period of each ADU. When there are multiple flows with different real-time constraints, we consider the most stringent constraints (see [RFC6363], Section 10.2, item 6, for recommendations when several flows are globally protected). This maximum FEC-related latency accounts for all sources of latency added by FEC encoding (sender) and FEC decoding (receiver). Any other source of latency (e.g., added by network communications) is not considered in this latency budget; It can be regarded as the latency budget permitted for all FEC-related operations. This is also an input parameter that enables to derive other internal parameters;
Encoding window current (resp. maximum) size, ew_size (resp. ew_max_size) (in symbols):
these parameters are used by a sender during FEC encoding. More precisely, each repair symbol is a linear combination of the ew_size source symbols present in the encoding window when RLC encoding took place. In all situations, we MUST have ew_size ≤ ew_max_size;
Decoding window current (resp. maximum) size, dw_size (resp. dw_max_size) (in symbols):
these parameters are used by a receiver when managing the linear system used for decoding. dw_size is the current size of the decoding window, i.e., the set of received or erased source symbols that are currently part of the linear system. In all situations, we MUST have dw_size ≤ dw_max_size;

In order to comply with the maximum FEC-related latency budget, assuming a constant transmission bandwidth at the output of the FECFRAME sender (bw_out), encoding symbol size (E), and code rate (cr), we have:

This dw_max_size defines the maximum delay after which an old source symbol may be recovered: after this delay, this old source symbol symbol will be removed from the decoding window.

It is often good practice to choose:

However any value ew_max_size < dw_max_size can be used without impact on the FEC-related latency budget. Finding the optimal value can depend on the erasure channel one wants to support and should be determined after simulations or field trials.

Note that the decoding beyond maximum latency optimisation (Appendix A) enables an old source symbol to be kept in the linear system beyond the FEC-related latency budget, but not delivered to the receiving application. Here we have: ls_size ≥ dw_max_size

3.2. ADU, ADUI and Source Symbols Mappings

An ADU, coming from the application, cannot be mapped to source symbols directly. Indeed, an erased ADU recovered at a receiver must contain enough information to be assigned to the right application flow (UDP port numbers and IP addresses cannot be used to that purpose as they are not protected by FEC encoding). This requires adding the flow identifier to each ADU before doing FEC encoding.

Additionally, since ADUs are of variable size, padding is needed so that each ADU (with its flow identifier) contribute to an integral number of source symbols. This requires adding the original ADU length to each ADU before doing FEC encoding. Because of these requirements, an intermediate format, the ADUI, or ADU Information, is considered [RFC6363].

For each incoming ADU, an ADUI is created as follows. First of all, 3 bytes are prepended: (Figure 1):

Flow ID (F) (8-bit field):
this unsigned byte contains the integer identifier associated to the source ADU flow to which this ADU belongs. It is assumed that a single byte is sufficient, which implies that no more than 256 flows will be protected by a single FECFRAME instance.
Length (L) (16-bit field):
this unsigned integer contain the length of this ADU, in network byte order (i.e., big endian). This length is for the ADU itself and does not include the F, L, or Pad fields.

Then, zero padding is added to ADU if needed:

Padding (Pad) (variable size field):
this field contains zero padding to align the F, L, ADU and padding up to a size that is multiple of E bytes (i.e., the source and repair symbol length).

Each ADUI contributes to an integral number of source symbols. The data unit resulting from the ADU and the F, L, and Pad fields is called ADU Information (or ADUI). Since ADUs can be of different size, this is also the case for ADUIs.

   symbol length, E              E                     E
< ------------------ >< ------------------ >< ------------------ >
|F| L|                     ADU                     |     Pad     |

Figure 1: ADUI Creation example (here 3 source symbols are created for this ADUI).

Note that neither the initial 3 bytes nor the optional padding are sent over the network. However, they are considered during FEC encoding. It means that a receiver who lost a certain FEC source packet (e.g., the UDP datagram containing this FEC source packet) will be able to recover the ADUI if FEC decoding succeeds. Thanks to the initial 3 bytes, this receiver will get rid of the padding (if any) and identify the corresponding ADU flow.

3.3. Encoding Window Management

Source symbols and the corresponding ADUs are removed from the encoding window:

Source symbols are added to the sliding encoding window each time a new ADU arrives, once the ADU to ADUI and then to source symbols mapping has been performed (Section 3.2). The current size of the encoding window, ew_size, is updated after adding new source symbols. This process may require to remove old source symbols so that: ew_size ≤ ew_max_size.

Note that a FEC codec may feature practical limits in the number of source symbols in the encoding window (e.g., for computational complexity reasons). This factor may further limit the ew_max_lat value, in addition to the maximum FEC-related latency budget (Section 3.1).

3.4. Pseudo-Random Number Generator

The RLC codes rely on the following Pseudo-Random Number Generator (PRNG), identical to the PRNG used with LDPC-Staircase codes ([RFC5170], section 5.7).

The Park-Miler "minimal standard" PRNG [PM88] MUST be used. It defines a simple multiplicative congruential algorithm: Ij+1 = A * Ij (modulo M), with the following choices: A = 7^^5 = 16807 and M = 2^^31 - 1 = 2147483647. A validation criteria of such a PRNG is the following: if seed = 1, then the 10,000th value returned MUST be equal to 1043618065.

Several implementations of this PRNG are known and discussed in the literature. An optimized implementation of this algorithm, using only 32-bit mathematics, and which does not require any division, can be found in [rand31pmc]. It uses the Park and Miller algorithm [PM88] with the optimization suggested by D. Carta in [CA90]. The history behind this algorithm is detailed in [WI08]. Yet, any other implementation of the PRNG algorithm that matches the above validation criteria, like the ones detailed in [PM88], is appropriate.

This PRNG produces, natively, a 31-bit value between 1 and 0x7FFFFFFE (2^^31-2) inclusive. Since it is desired to scale the pseudo-random number between 0 and maxv-1 inclusive, one must keep the most significant bits of the value returned by the PRNG (the least significant bits are known to be less random, and modulo-based solutions should be avoided [PTVF92]). The following algorithm MUST be used:




In this document, pmms_rand(maxv) denotes the PRNG function that implements the Park-Miller "minimal standard" algorithm, defined above, and that scales the raw value between 0 and maxv-1 inclusive, using the above scaling algorithm.

Additionally, the pmms_srand(seed) function must be provided to enable the initialization of the PRNG with a seed before calling pmms_rand(maxv) the first time. The seed is a 31-bit integer between 1 and 0x7FFFFFFE inclusive. In this specification, the seed is restricted to a value between 1 and 0xFFFF inclusive, as this is the Repair_Key 16-bit field value of the Repair FEC Payload ID (Section 4.1.3).

3.5. Coding Coefficients Generation Function

The coding coefficients, used during the encoding process, are generated at the RLC encoder by the following function each time a new repair symbol needs to be produced:

 * Fills in the table of coding coefficients (of the right size)
 * provided with the appropriate number of coding coefficients to
 * use for the repair symbol key provided.
 * (in) repair_key    key associated to this repair symbol
 * (in) cc_tab[]      pointer to a table of the right size to store
 *                    coding coefficients. All coefficients are
 *                    stored as bytes, regardless of the m parameter,
 *                    upon return of this function.
 * (in) cc_nb[]       number of entries in the table. This value is
 *                    equal to the current encoding window size.
 * (in) m             Finite Field GF(2^^m) parameter.
 * (out)              returns an error code
int generate_coding_coefficients (UINT16    repair_key,
                                  UINT8     cc_tab[],
                                  UINT16    cc_nb,
                                  UINT8     m)
    UINT32    i;

    if (repair_key == 0) {
        return SOMETHING_WENT_WRONG;
    if (m == 1) {
        /* 0 is a valid coefficient value in binary GF */
        for (i = 0 ; i < cc_nb ; i ++) {
            cc_tab[i] = (UINT8) pmms_rand(2);
    } else {
        /* coefficient 0 is avoided in non-binary GF to consider each
         * source symbol */
        UINT32    maxv;
        maxv = get_gf_size(); /* i.e., 16 if m=4 or 256 if m=8 */
        for (i = 0 ; i < cc_nb ; i ++) {
            do {
                cc_tab[i] = (UINT8) pmms_rand(maxv);
            } while (cc_tab[i] == 0)

Figure 2: Coding Coefficients Generation Function pseudo-code

4. RLC FEC Scheme for Arbitrary ADU Flows

4.1. Formats and Codes

4.1.1. FEC Framework Configuration Information

The FEC Framework Configuration Information (or FFCI) includes information that MUST be communicated between the sender and receiver(s). More specifically, it enables the synchronization of the FECFRAME sender and receiver instances. It includes both mandatory elements and scheme-specific elements, as detailed below. Mandatory Information

  • FEC Encoding ID: the value assigned to this fully specified FEC scheme MUST be XXXX, as assigned by IANA (Section 9).

When SDP is used to communicate the FFCI, this FEC Encoding ID is carried in the 'encoding-id' parameter. FEC Scheme-Specific Information

The FEC Scheme-Specific Information (FSSI) includes elements that are specific to the present FEC scheme. More precisely:

Encoding symbol length (E):
a non-negative integer that indicates the length of each encoding symbol in bytes;

This element is required both by the sender (RLC encoder) and the receiver(s) (RLC decoder).

When SDP is used to communicate the FFCI, this FEC scheme-specific information is carried in the 'fssi' parameter in textual representation as specified in [RFC6364]. For instance:


If another mechanism requires the FSSI to be carried as an opaque octet string (for instance, after a Base64 encoding), the encoding format consists of the following 2 octets:

Encoding symbol length (E) field (16-bits):
Length, in number of bytes, of the source and repair symbols.

 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
|   Encoding Symbol Length (E)  |

Figure 3: FSSI Encoding Format

4.1.2. Explicit Source FEC Payload ID

A FEC source packet MUST contain an Explicit Source FEC Payload ID that is appended to the end of the packet as illustrated in Figure 4.

|           IP Header            |
|        Transport Header        |
|              ADU               |
| Explicit Source FEC Payload ID |

Figure 4: Structure of an FEC Source Packet with the Explicit Source FEC Payload ID

More precisely, the Explicit Source FEC Payload ID is composed of the following field (Figure 5):

Encoding Symbol ID (ESI) (32-bit field):
this unsigned integer identifies the first source symbol of the ADUI corresponding to this FEC source packet. The ESI is incremented for each new source symbol, and after reaching the maximum value (2^32-1), wrapping to zero occurs.
 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
|                   Encoding Symbol ID (ESI)                    |

Figure 5: Source FEC Payload ID Encoding Format

4.1.3. Repair FEC Payload ID

A FEC repair packet MUST contain a Repair FEC Payload ID that is prepended to the repair symbol as illustrated in Figure 6. There MUST be a single repair symbol per FEC repair packet.

|           IP Header            |
|        Transport Header        |
|     Repair FEC Payload ID      |
|         Repair Symbol          |

Figure 6: Structure of an FEC Repair Packet with the Repair FEC Payload ID

More precisely, the Repair FEC Payload ID is composed of the following fields (Figure 7):

Repair_Key (16-bit field):
this unsigned integer is used as a seed by the coefficient generation function Section 3.5, in order to generate the desired number of coding coefficients. Value 0 MUST NOT be used.
Number of Source Symbols in the Encoding Window, NSS (16-bit field):
this unsigned integer indicates the number of source symbols in the encoding window when this repair symbol was generated.
ESI of first source symbol in encoding window, FSS_ESI (32-bit field):
this unsigned integer indicates the ESI of the first source symbol in the encoding window when this repair symbol was generated.

 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
|       Repair_Key              |  NSS (# source symbols in ew) |
|                            FSS_ESI                            |

Figure 7: Repair FEC Payload ID Encoding Format

4.1.4. Additional Procedures

The following procedure applies:

  • The ESI of source symbols MUST start with value 0 for the first source symbol and MUST be managed sequentially. Wrapping to zero will happen after reaching the maximum 32-bit value.

5. FEC Code Specification

TBD... Describe a typical sender's operation, when using the RLC FEC scheme. Describe a typical receiver operation, when using the RLC FEC scheme.

(summary, to be detailed): The FECFRAME sender generates a linear combination of source symbols, using the coding coefficients generation function and sends it within an FEC repair packet. This linear combination encompasses all the source symbols currently in the encoding window. FEC repair packets are sent immediately after having been created, inter-mixed with FEC source packets.

(summary, to be detailed): A FECFRAME receiver, upon receiving a FEC repair packet, adds an equation to the linear system it maintains (or no equation if this repair packet does not change the linear system rank). Whenever possible, decoding is performed in order to recover erased source symbols if any.

6. Implementation Status

Editor's notes:

  • RFC Editor, please remove this section motivated by RFC 6982 before publishing the RFC. Thanks.

An implementation of the RLC convolutional FEC Scheme for FECFRAME exists:

  • Organisation: Inria
  • Description: This is an implementation of the RLC Convolutional FEC Scheme. It relies on a modified version of our OpenFEC (http://openfec.org) FEC code library. It is integrated in our FECFRAME software (see [fecframe-ext]).
  • Maturity: prototype.
  • Coverage: this software complies with the RLC FEC Scheme (limited to m=8 as of end of January, 2017).
  • Lincensing: proprietary.
  • Contact: vincent.roca@inria.fr

7. Security Considerations


8. Operations and Management Considerations

9. IANA Considerations

This document registers one value in the "FEC Framework (FECFRAME) FEC Encoding IDs" registry [RFC6363] as follows:

  • XXX refers to the convolutional Random Linear Codes (RLC) FEC Scheme for Arbitrary Packet Flows, as defined in Section XXX of this document.

10. Acknowledgments

11. References

11.1. Normative References

[fecframe-ext] Roca, V. and A. Begen, "Forward Error Correction (FEC) Framework version 2", Transport Area Working Group (TSVWG) draft-roca-tsvwg-fecframev2 (Work in Progress), October 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC6363] Watson, M., Begen, A. and V. Roca, "Forward Error Correction (FEC) Framework", RFC 6363, DOI 10.17487/RFC6363, October 2011.
[RFC6364] Begen, A., "Session Description Protocol Elements for the Forward Error Correction (FEC) Framework", RFC 6364, DOI 10.17487/RFC6364, October 2011.

11.2. Informative References

[CA90] Carta, D., "Two Fast Implementations of the Minimal Standard Random Number Generator", Communications of the ACM, Vol. 33, No. 1, pp.87-88, January 1990.
[PM88] Park, S. and K. Miller, "Random Number Generators: Good Ones are Hard to Find", Communications of the ACM, Vol. 31, No. 10, pp.1192-1201, 1988.
[PTVF92] Press, W., Teukolsky, S., Vetterling, W. and B. Flannery, "Numerical Recipies in C; Second Edition", Cambridge University Press, ISBN: 0-521-43108-5, 1992.
[rand31pmc] Whittle, R., "31 bit pseudo-random number generator", September 2005.
[RFC5170] Roca, V., Neumann, C. and D. Furodet, "Low Density Parity Check (LDPC) Staircase and Triangle Forward Error Correction (FEC) Schemes", RFC 5170, DOI 10.17487/RFC5170, June 2008.
[RFC6726] Paila, T., Walsh, R., Luby, M., Roca, V. and R. Lehtonen, "FLUTE - File Delivery over Unidirectional Transport", RFC 6726, DOI 10.17487/RFC6726, November 2012.
[RFC6816] Roca, V., Cunche, M. and J. Lacan, "Simple Low-Density Parity Check (LDPC) Staircase Forward Error Correction (FEC) Scheme for FECFRAME", RFC 6816, DOI 10.17487/RFC6816, December 2012.
[RFC6865] Roca, V., Cunche, M., Lacan, J., Bouabdallah, A. and K. Matsuzono, "Simple Reed-Solomon Forward Error Correction (FEC) Scheme for FECFRAME", RFC 6865, DOI 10.17487/RFC6865, February 2013.
[Roca16] Roca, V., Teibi, B., Burdinat, C., Tran, T. and C. Thienot, "Block or Convolutional AL-FEC Codes? A Performance Comparison for Robust Low-Latency Communications", Submitted for publication https://hal.inria.fr/hal-01395937/en/, November 2016.
[WI08] Whittle, R., "Park-Miller-Carta Pseudo-Random Number Generator", http://www.firstpr.com.au/dsp/rand31/, January 2008.

Appendix A. Decoding Beyond Maximum Latency Optimization

This annex introduces non normative considerations. They are provided as suggestions, without any impact on interoperability. For more information see [Roca16].

It is possible to improve the decoding performance of convolutional codes without impacting maximum latency, at the cost of extra CPU overhead. The optimization consists, for a receiver, to extend the linear system beyond the decoding window:

  • ls_max_size > dw_max_size

Usually the following choice is a good trade-off between decoding performance and extra CPU overhead:

  • ls_max_size = 2 * dw_max_size


        late source symbols
 (pot. decoded but not delivered)            dw_max_size
/--------------^-----------------\ /--------------^---------------\
src0 src1 src2 src3 src4 src5 src6 src7 src8 src9 src10 src11 src12

Figure 8: Relationship between parameters to decode beyond maximum latency.

It means that source symbols (and therefore ADUs) may be decoded even if their transport protocol added latency exceeds the maximum value permitted by the application. It follows that these source symbols SHOULD NOT be delivered to the application and SHOULD be dropped once they are no longer needed. However, decoding these late symbols significantly improves the global robustness in bad reception conditions and is therefore recommended for receivers experiencing bad channels[Roca16]. In any case whether or not to use this facility and what exact value to use for the ls_max_size parameter are decisions made by each receiver independantly, without any impact on others, neither the other receivers nor the source.

Author's Address

Vincent Roca INRIA 655, av. de l'Europe Inovallee; Montbonnot ST ISMIER cedex, 38334 France EMail: vincent.roca@inria.fr