This document is obsolete. Please
refer to RFC 5052 and RFC 5445.
Network Working Group M. Luby Request for Comments: 3452 Digital Fountain Category: Experimental L. Vicisano Cisco J. Gemmell Microsoft L. Rizzo Univ. Pisa M. Handley ICIR J. Crowcroft Cambridge Univ. December 2002
Forward Error Correction (FEC) Building Block
Status of this Memo
This memo defines an Experimental Protocol for the Internet community. It does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document generally describes how to use Forward Error Correction (FEC) codes to efficiently provide and/or augment reliability for data transport. The primary focus of this document is the application of FEC codes to one-to-many reliable data transport using IP multicast. This document describes what information is needed to identify a specific FEC code, what information needs to be communicated out-of-band to use the FEC code, and what information is needed in data packets to identify the encoding symbols they carry. The procedures for specifying FEC codes and registering them with the Internet Assigned Numbers Authority (IANA) are also described. This document should be read in conjunction with and uses the terminology of the companion document titled, "The Use of Forward Error Correction (FEC) in Reliable Multicast".
This document describes how to use Forward Error Correction (FEC) codes to provide support for reliable delivery of content using IP multicast. This document should be read in conjunction with and uses the terminology of the companion document [4], which describes the use of FEC codes within the context of reliable IP multicast transport and provides an introduction to some commonly used FEC codes.
This document describes a building block as defined in RFC 3048 [9]. This document is a product of the IETF RMT WG and follows the general guidelines provided in RFC 3269 [3].
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 [2].
Statement of Intent
This memo contains part of the definitions necessary to fully specify a Reliable Multicast Transport protocol in accordance with RFC 2357. As per RFC 2357, the use of any reliable multicast protocol in the Internet requires an adequate congestion control scheme.
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While waiting for such a scheme to be available, or for an existing scheme to be proven adequate, the Reliable Multicast Transport working group (RMT) publishes this Request for Comments in the "Experimental" category.
It is the intent of RMT to re-submit this specification as an IETF Proposed Standard as soon as the above condition is met.
FEC codes are a valuable basic component of any transport protocol that is to provide reliable delivery of content. Using FEC codes is valuable in the context of IP multicast and reliable delivery because FEC encoding symbols can be useful to all receivers for reconstructing content even when the receivers have received different encoding symbols. Furthermore, FEC codes can ameliorate or even eliminate the need for feedback from receivers to senders to request retransmission of lost packets.
The goal of the FEC building block is to describe functionality directly related to FEC codes that is common to all reliable content delivery IP multicast protocols, and to leave out any additional functionality that is specific to particular protocols. The primary functionality described in this document that is common to all such protocols that use FEC codes are FEC encoding symbols for an object that is included in packets that flow from a sender to receivers. This document for example does not describe how receivers may request transmission of particular encoding symbols for an object. This is because although there are protocols where requests for transmission are of use, there are also protocols that do not require such requests.
The companion document [4] should be consulted for a full explanation of the benefits of using FEC codes for reliable content delivery using IP multicast. FEC codes are also useful in the context of unicast, and thus the scope and applicability of this document is not limited to IP multicast.
This section describes FEC information that is either to be sent out-of-band or in packets. The FEC information is associated with transmission of data about a particular object. There are three classes of packets that may contain FEC information: data packets, session-control packets and feedback packets. They generally contain different kinds of FEC information. Note that some protocols may not use session-control or feedback packets.
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Data packets may sometimes serve as session-control packets as well; both data and session-control packets generally travel downstream from the sender towards receivers and are sent to a multicast channel or to a specific receiver using unicast.
As a general rule, feedback packets travel upstream from receivers to the sender. Sometimes, however, they might be sent to a multicast channel or to another receiver or to some intermediate node or neighboring router that provides recovery services.
This document specifies the FEC information that must be carried in data packets and the other FEC information that must be communicated either out-of-band or in data packets. This document does not specify out-of-band methods nor does it specify the way out-of-band FEC information is associated with FEC information carried in data packets. These methods must be specified in a complete protocol instantiation that uses the FEC building block. FEC information is classified as follows:
1) FEC Encoding ID
Identifies the FEC encoder being used and allows receivers to select the appropriate FEC decoder. The value of the FEC Encoding ID MUST be the same for all transmission of data related to a particular object, but MAY vary across different transmissions of data about different objects, even if transmitted to the same set of multicast channels and/or using a single upper-layer session. The FEC Encoding ID is subject to IANA registration.
2) FEC Instance ID
Provides a more specific identification of the FEC encoder being used for an Under-Specified FEC scheme. This value is not used for Fully-Specified FEC schemes. (See Section 3.1 for the definition of Under-Specified and Fully-Specified FEC schemes.) The FEC Instance ID is scoped by the FEC Encoding ID, and is subject to IANA registration.
3) FEC Payload ID
Identifies the encoding symbol(s) in the payload of the packet. The types and lengths of the fields in the FEC Payload ID, i.e., the format of the FEC Payload ID, are determined by the FEC Encoding ID. The full specification of each field MUST be uniquely determined by the FEC Encoding ID for Fully-Specified FEC schemes, and MUST be uniquely determined by the combination of the FEC Encoding ID and the FEC Instance ID for Under-Specified FEC schemes. As an example, for the Under-Specified FEC scheme with
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FEC Encoding ID 129 defined in Section 5.1, the fields in the FEC Payload ID are a 32-bit Source Block Number followed by a 32-bit Encoding Symbol ID, where the full specification of both of these fields depends on the FEC Instance ID.
4) FEC Object Transmission Information
This is information regarding the encoding of a specific object needed by the FEC decoder. As an example, for the Under-Specified FEC scheme with FEC Encoding ID 129 defined in Section 5.1, this information might include the lengths of the different source blocks that make up the object and the overall object length. This might also include specific parameters of the FEC encoder.
The FEC Encoding ID, FEC Instance ID (for Under-Specified FEC schemes) and the FEC Object Transmission Information can be sent to a receiver within the data packet headers, within session control packets, or by some other means. In any case, the means for communicating this to a receiver is outside the scope of this document. The FEC Payload ID MUST be included in the data packet header fields, as it provides a description of the encoding symbols contained in the packet.
The FEC Encoding ID is a numeric index that identifies a specific FEC scheme OR a class of encoding schemes that share the same FEC Payload ID format.
An FEC scheme is a Fully-Specified FEC scheme if the encoding scheme is formally and fully specified, in a way that independent implementors can implement both encoder and decoder from a specification that is an IETF RFC. The FEC Encoding ID uniquely identifies a Fully-Specified FEC scheme. Companion documents of this specification may specify Fully-Specified FEC schemes and associate them with FEC Encoding ID values.
These documents MUST also specify a format for the FEC Payload ID and specify the information in the FEC Object Transmission Information.
It is possible that a FEC scheme may not be a Fully-Specified FEC scheme, because either a specification is simply not available or a party exists that owns the encoding scheme and is not willing to disclose the algorithm or specification. We refer to such an FEC encoding schemes as an Under-Specified FEC scheme. The following holds for an Under-Specified FEC scheme:
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o The fields and their formats of the FEC Payload ID and the specific information in the FEC Object Transmission Information MUST be defined for the Under-Specified FEC scheme.
o A value for the FEC Encoding ID MUST be reserved and associated with the fields and their formats of the FEC Payload ID and the specific information in the FEC Object Transmission Information. An already reserved FEC Encoding ID value MUST be reused if the associated FEC Payload ID has the same fields and formats and the FEC Object Transmission Information has same information as the ones needed for the new Under-Specified FEC scheme.
o A value for the FEC Instance ID MUST be reserved.
An Under-Specified FEC scheme is fully identified by the tuple (FEC Encoding ID, FEC Instance ID). The tuple MUST identify a single scheme that has at least one implementation. The party that owns this tuple MUST be able to provide information on how to obtain the Under-Specified FEC scheme identified by the tuple, e.g., a pointer to a publicly available reference-implementation or the name and contacts of a company that sells it, either separately or embedded in another product.
Different Under-Specified FEC schemes that share the same FEC Encoding ID -- but have different FEC Instance IDs -- also share the same fields and corresponding formats of the FEC Payload ID and specify the same information in the FEC Object Transmission Information.
This specification reserves the range 0-127 for the values of FEC Encoding IDs for Fully-Specified FEC schemes and the range 128-255 for the values of Under-Specified FEC schemes.
3.2. FEC Payload ID and FEC Object Transmission Information
A document that specifies an FEC scheme and reserves a value of FEC Encoding ID MUST define the fields and their packet formats for the FEC Payload ID and specify the information in the FEC Object Transmission Information according to the needs of the encoding scheme. This applies to documents that reserve values of FEC Encoding IDs for both Fully-Specified and Under-Specified FEC schemes.
The specification of the fields and their packet formats for the FEC Payload ID MUST specify the meaning of the fields and their format down to the level of specific bits. The total length of all the
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fields in the FEC Payload ID MUST have a length that is a multiple of a 4-byte word. This requirement facilitates the alignment of packet fields in protocol instantiations.
The FEC building block applies to creating and sending encoding symbols for objects that are to be reliably transported using IP multicast or unicast. The FEC building block does not provide higher level session support. Thus, for example, many objects may be transmitted within the same session, in which case a higher level building block may carry a unique Transport Object ID (TOI) for each object in the session to allow the receiver to demultiplex packets within the session based on the TOI within each packet. As another example, a receiver may subscribe to more than one session at a time.
In this case a higher level building block may carry a unique Transport Session ID (TSI) for each session to allow the receiver to demultiplex packets based on the TSI within each packet.
Other building blocks may supply direct support for carrying out-of- band information directly relevant to the FEC building block to receivers. For example, the length of the object is part of the FEC Object Transmission Information that may in some cases be communicated out-of-band to receivers, and one mechanism for providing this to receivers is within the context of another building block that provides this information.
Some protocols may use FEC codes as a mechanism for repairing the loss of packets. Within the context of FEC repair schemes, feedback packets are (optionally) used to request FEC retransmission. The FEC-related information present in feedback packets usually contains an FEC Block ID that defines the block that is being repaired, and the number of Repair Symbols requested. Although this is the most common case, variants are possible in which the receivers provide more specific information about the Repair Symbols requested (e.g., an index range or a list of symbols accepted). It is also possible to include multiple requests in a single feedback packet. This document does not provide any detail about feedback schemes used in combination with FEC nor the format of FEC information in feedback packets. If feedback packets are used in a complete protocol instantiation, these details must be provided in the protocol instantiation specification.
The FEC building block does not provide any support for congestion control. Any complete protocol MUST provide congestion control that conforms to RFC 2357 [5], and thus this MUST be provided by another building block when the FEC building block is used in a protocol.
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A more complete description of the applicability of FEC codes can be found in the companion document [4].
This section specifies the FEC Encoding ID, the associated FEC Payload ID format, and the specific information in the FEC Object Transmission Information for a number of known Under-Specified FEC schemes. Under-Specified FEC schemes that use the same FEC Payload ID fields, formats, and specific information in the FEC Object Transmission Information (as for one of the FEC Encoding IDs specified in this section) MUST use the corresponding FEC Encoding ID. Other FEC Encoding IDs may be specified for other Under- Specified FEC schemes in companion documents.
5.1. Small Block, Large Block and Expandable FEC Codes
This subsection reserves the FEC Encoding ID value 128 for the Under-Specified FEC schemes described in [4] that are called Small Block FEC codes, Large Block FEC codes and Expandable FEC codes.
The FEC Payload ID is composed of a Source Block Number and an Encoding Symbol ID structured as follows:
The Source Block Number identifies from which source block of the object the encoding symbol(s) in the payload are generated. These blocks are numbered consecutively from 0 to N-1, where N is the number of source blocks in the object.
The Encoding Symbol ID identifies which specific encoding symbol(s) generated from the source block are carried in the packet payload. The exact details of the correspondence between Encoding Symbol IDs and the encoding symbol(s) in the packet payload are dependent on the particular encoding algorithm used as identified by the FEC Encoding ID and by the FEC Instance ID, and these details may be proprietary.
The FEC Object Transmission Information has the following specific information:
o The FEC Encoding ID 128.
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o The FEC Instance ID associated with the FEC Encoding ID 128 to be used.
o The total length of the object in bytes.
o The number of source blocks that the object is partitioned into, and the length of each source block in bytes.
To understand how this out-of-band information is communicated, one must look outside the scope of this document. One example may be that the source block lengths may be derived by a fixed algorithm from the object length. Another example may be that all source blocks are the same length and this is what is passed out-of-band to the receiver. A third example could be that the full sized source block length is provided and this is the length used for all but the last source block, which is calculated based on the full source block length and the object length.
This subsection reserves the FEC Encoding ID value 129 for the Under-Specified FEC schemes described in [4] that are called Small Block Systematic FEC codes. For Small Block Systematic FEC codes, each source block is of length at most 65536 source symbols.
Although these codes can generally be accommodated by the FEC Encoding ID described in Section 5.1, a specific FEC Encoding ID is defined for Small Block Systematic FEC codes to allow more flexibility and to retain header compactness. The small source block length and small expansion factor that often characterize systematic codes may require the data source to frequently change the source block length. To allow the dynamic variation of the source block length and to communicate it to the receivers with low overhead, the block length is included in the FEC Payload ID.
The FEC Payload ID is composed of the Source Block Number, Source Block Length and the Encoding Symbol ID:
The Source Block Number identifies from which source block of the object the encoding symbol(s) in the payload are generated. These blocks are numbered consecutively from 0 to N-1, where N is the number of source blocks in the object.
The Source Block Length is the length in units of source symbols of the source block identified by the Source Block Number.
The Encoding Symbol ID identifies which specific encoding symbol(s) generated from the source block are carried in the packet payload. Each encoding symbol is either an original source symbol or a redundant symbol generated by the encoder. The exact details of the correspondence between Encoding Symbol IDs and the encoding symbol(s) in the packet payload are dependent on the particular encoding algorithm used as identified by the FEC Encoding ID and by the FEC Instance ID, and these details may be proprietary.
The FEC Object Transmission Information has the following specific information:
o The FEC Encoding ID 129.
o The FEC Instance ID associated with the FEC Encoding ID 129 to be used.
o The total length of the object in bytes.
o The maximum number of encoding symbols that can be generated for any source block. This field is provided for example to allow receivers to preallocate buffer space that is suitable for decoding to recover any source block.
o For each source block, the length in bytes of encoding symbols for the source block.
How this out-of-band information is communicated is outside the scope of this document. As an example the length in bytes of encoding symbols for each source block may be the same for all source blocks. As another example, the encoding symbol length may be the same for all source blocks of a given object and this length is communicated for each object. As a third example, it may be that there is a threshold value I, and for all source blocks consisting of less than I source symbols, the encoding symbol length is one fixed number of bytes, but for all source blocks consisting of I or more source symbols, the encoding symbol length is a different fixed number of bytes.
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Note that each encoding symbol, i.e., each source symbol and redundant symbol, must be the same length for a given source block, and this implies that each source block length is a multiple of its encoding symbol length. If the original source block length is not a multiple of the encoding symbol length, it is up to the sending application to appropriately pad the original source block to form the source block to be encoded, and to communicate this padding to the receiving application. The form of this padding, if used, and how it is communicated to the receiving application, is outside the scope of this document, and must be handled at the application level.
The FEC building block does not provide any support for congestion control. Any complete protocol MUST provide congestion control that conforms to RFC 2357 [5], and thus this MUST be provided by another building block when the FEC building block is used in a protocol.
There are no other specific requirements from other building blocks for the use of this FEC building block. However, any protocol that uses the FEC building block will inevitably use other building blocks for example to provide support for sending higher level session information within data packets containing FEC encoding symbols.
Data delivery can be subject to denial-of-service attacks by attackers which send corrupted packets that are accepted as legitimate by receivers. This is particularly a concern for multicast delivery because a corrupted packet may be injected into the session close to the root of the multicast tree, in which case the corrupted packet will arrive to many receivers. This is particularly a concern for the FEC building block because the use of even one corrupted packet containing encoding data may result in the decoding of an object that is completely corrupted and unusable. It is thus RECOMMENDED that the decoded objects be checked for integrity before delivering objects to an application. For example, an MD5 hash [8] of an object may be appended before transmission, and the MD5 hash is computed and checked after the object is decoded but before it is delivered to an application. Moreover, in order to obtain strong cryptographic integrity protection a digital signature verifiable by the receiver SHOULD be computed on top of such a hash value. It is also RECOMMENDED that a packet authentication protocol such as TESLA [7] be used to detect and discard corrupted packets upon arrival. Furthermore, it is RECOMMENDED that Reverse Path Forwarding checks be enabled in all network routers and switches
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along the path from the sender to receivers to limit the possibility of a bad agent successfully injecting a corrupted packet into the multicast tree data path.
Another security concern is that some FEC information may be obtained by receivers out-of-band in a session description, and if the session description is forged or corrupted then the receivers will not use the correct protocol for decoding content from received packets. To avoid these problems, it is RECOMMENDED that measures be taken to prevent receivers from accepting incorrect session descriptions, e.g., by using source authentication to ensure that receivers only accept legitimate session descriptions from authorized senders.
Values of FEC Encoding IDs and FEC Instance IDs are subject to IANA registration. FEC Encoding IDs and FEC Instance IDs are hierarchical: FEC Encoding IDs scope ranges of FEC Instance IDs. Only FEC Encoding IDs that correspond to Under-Specified FEC schemes scope a corresponding set of FEC Instance IDs.
The FEC Encoding ID is a numeric non-negative index. In this document, the range of values for FEC Encoding IDs is 0 to 255. Values from 0 to 127 are reserved for Fully-Specified FEC schemes and Values from 128 to 255 are reserved for Under-Specified FEC schemes, as described in more detail in Section 3.1. This specification already assigns the values 128 and 129, as described in Section 5.
Each FEC Encoding ID assigned to an Under-Specified FEC scheme scopes an independent range of FEC Instance IDs (i.e., the same value of FEC Instance ID can be reused for different FEC Encoding IDs). An FEC Instance ID is a numeric non-negative index.
This document defines a name-space for FEC Encoding IDs named:
ietf:rmt:fec:encoding
IANA has established and manages the new registry for the "ietf:rmt:fec:encoding" name-space. The values that can be assigned within the "ietf:rmt:fec:encoding" name-space are numeric indexes in the range [0, 255], boundaries included. Assignment requests are granted on a "Specification Required" basis as defined in RFC 2434 [6]: An IETF RFC MUST exist and specify the FEC Payload ID fields and formats as well as the FEC Object Transmission Information for the value of "ietf:rmt:fec:encoding" (FEC Encoding ID) being assigned by IANA (see Section 3.1 for more details). Note that the values 128
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and 129 of "ietf:rmt:fec:encoding" are already assigned by this document as described in Section 5.
This document also defines a name-space for FEC Instance IDs named:
ietf:rmt:fec:encoding:instance
The "ietf:rmt:fec:encoding:instance" name-space is a sub-name-space associated with the "ietf:rmt:fec:encoding" name-space. Each value of "ietf:rmt:fec:encoding" assigned in the range [128, 255] has a separate "ietf:rmt:fec:encoding:instance" sub-name-space that it scopes. Values of "ietf:rmt:fec:encoding" in the range [0, 127] do not scope a "ietf:rmt:fec:encoding:instance" sub-name-space.
The values that can be assigned within each "ietf:rmt:fec:encoding:instance" sub-name-space are non-negative numeric indices. Assignment requests are granted on a "First Come First Served" basis as defined in RFC 2434 [6]. The same value of "ietf:rmt:fec:encoding:instance" can be assigned within multiple distinct sub-name-spaces, i.e., the same value of "ietf:rmt:fec:encoding:instance" can be used for multiple values of "ietf:rmt:fec:encoding".
Requestors of "ietf:rmt:fec:encoding:instance" assignments MUST provide the following information:
o The value of "ietf:rmt:fec:encoding" that scopes the "ietf:rmt:fec:encoding:instance" sub-name-space. This must be in the range [128, 255].
o Point of contact information
o A pointer to publicly accessible documentation describing the Under-Specified FEC scheme, associated with the value of "ietf:rmt:fec:encoding:instance" assigned, and a way to obtain it (e.g., a pointer to a publicly available reference-implementation or the name and contacts of a company that sells it, either separately or embedded in a product).
It is the responsibility of the requestor to keep all the above information up to date.
The IETF has been notified of intellectual property rights claimed in regard to some or all of the specification contained in this document. For more information consult the online list of claimed rights.
Brian Adamson contributed to this document by shaping Section 5.2 and providing general feedback. We also wish to thank Vincent Roca, Justin Chapweske and Roger Kermode for their extensive comments.
[1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[3] Kermode, R. and L. Vicisano, "Author Guidelines for Reliable Multicast Transport (RMT) Building Blocks and Protocol Instantiation documents", RFC 3269, April 2002.
[4] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M. and J. Crowcroft, "The Use of Forward Error Correction (FEC) in Reliable Multicast", RFC 3453, December 2002.
[5] Mankin, A., Romanow, A., Bradner, S. and V. Paxson, "IETF Criteria for Evaluating Reliable Multicast Transport and Application Protocols", RFC 2357, June 1998.
[6] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
[7] Perrig, A., Canetti, R., Song, D. and J. Tygar, "Efficient and Secure Source Authentication for Multicast", Network and Distributed System Security Symposium, NDSS 2001, pp. 35-46, February 2001.
[8] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992.
[9] Whetten, B., Vicisano, L., Kermode, R., Handley, M., Floyd, S. and M. Luby, "Reliable Multicast Transport Building Blocks for One-to-Many Bulk-Data Transfer", RFC 3048, January 2001.
Michael Luby Digital Fountain, Inc. 39141 Civic Center Drive Suite 300 Fremont, CA 94538
EMail: luby@digitalfountain.com
Lorenzo Vicisano Cisco Systems, Inc. 170 West Tasman Dr., San Jose, CA, USA, 95134
EMail: lorenzo@cisco.com
Jim Gemmell Microsoft Research 455 Market St. #1690 San Francisco, CA, 94105
EMail: jgemmell@microsoft.com
Luigi Rizzo Dip. di Ing. dell'Informazione Universita` di Pisa via Diotisalvi 2, 56126 Pisa, Italy
EMail: luigi@iet.unipi.it
Mark Handley ICSI Center for Internet Research 1947 Center St. Berkeley CA, USA, 94704
EMail: mjh@icir.org
Jon Crowcroft Marconi Professor of Communications Systems University of Cambridge Computer Laboratory William Gates Building J J Thomson Avenue Cambridge CB3 0FD
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