NWCRG J. Detchart Internet-Draft E. Lochin Intended status: Experimental J. Lacan Expires: September 10, 2015 ISAE V. Roca INRIA March 9, 2015 Tetrys, an On-the-Fly Network Coding protocol draft-detchart-nwcrg-tetrys-01 Abstract This document describes Tetrys, an On-The-Fly Network Coding (NC) protocol that can be used to transport delay and loss sensitive data over a lossy network. Tetrys can recover from erasures within a RTT- independent delay, thanks to the transmission of coded packets (redundancy). It can be used for both unicast, multicast and anycast communications. 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 September 10, 2015. Copyright Notice Copyright (c) 2015 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 Detchart, et al. Expires September 10, 2015 [Page 1] Internet-DraftTetrys, an On-the-Fly Network Coding protocol March 2015 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3 2. Definitions, Notations and Abbreviations . . . . . . . . . . 3 3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 4 3.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Packet Format . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1. Common Header Format . . . . . . . . . . . . . . . . . . 6 4.1.1. Header Extensions . . . . . . . . . . . . . . . . . . 7 4.2. Source Packet Format . . . . . . . . . . . . . . . . . . 9 4.3. Coded Packet Format . . . . . . . . . . . . . . . . . . . 9 4.4. Acknowledgement Packet Format . . . . . . . . . . . . . . 10 5. The Coding Coefficient Generator Identifiers . . . . . . . . 12 5.1. Definition . . . . . . . . . . . . . . . . . . . . . . . 12 5.2. Table of Identifiers . . . . . . . . . . . . . . . . . . 12 6. Tetrys Basic Functions . . . . . . . . . . . . . . . . . . . 12 6.1. Encoding . . . . . . . . . . . . . . . . . . . . . . . . 12 6.1.1. Encoding Vector Formats . . . . . . . . . . . . . . . 13 6.1.2. The Elastic Encoding Window . . . . . . . . . . . . . 15 6.2. Decoding . . . . . . . . . . . . . . . . . . . . . . . . 16 7. Security Considerations . . . . . . . . . . . . . . . . . . . 16 8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 16 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 11.1. Normative References . . . . . . . . . . . . . . . . . . 16 11.2. Informative References . . . . . . . . . . . . . . . . . 16 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17 1. Introduction This document describes Tetrys, a novel network coding protocol. Network codes were introduced in the early 2000s [AHL-00] to address the limitations of transmission over the Internet (delay, capacity and packet loss). While the use of network codes is fairly recent in the Internet community, the use of application layer erasure codes in the IETF has already been standardized in the RMT [RMT] and the FECFRAME [FECFRAME] working groups. The protocol presented here can be seen as a network coding extension to standards solutions. The current proposal can be considered as a combination of network erasure coding and feedback mechanisms [Tetrys]. Detchart, et al. Expires September 10, 2015 [Page 2] Internet-DraftTetrys, an On-the-Fly Network Coding protocol March 2015 The main innovation of the Tetrys protocol is in the generation of coded packets from an elastic encoding window, the size of which is periodically updated with the receiver's feedback. This update is done in such a way that any source packets coming from an input flow is included in the encoding window as long as it is not acknowledged or the encoding window did not reach a limit. This mechanism allows for losses on both the forward and return paths and in particular is resilient to acknowledgement losses. With Tetrys, a coded packet is a linear combination of the data source packets belonging to the coding window over a finite field. The choice of the finite field of the coefficients is a trade-off between the best performance (with non-binary coefficients) and the system constraints (binary codes in an energy constrained environment) and is driven by the application. Thanks to the elastic encoding window, the coded packets are built on-the-fly, by using an algorithm or a function to choose the coefficients. The redundancy ratio can be dynamically adjusted, and the coefficients can be generated in different ways along a transmission. This allows to reduce the bandwidth used, compared to FEC block codes, and the decoding delay. 1.1. Requirements Notation 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. Definitions, Notations and Abbreviations The terminology used in this document is presented below. It is aligned with the FECFRAME terminology as well as with recent activities in the Network Coding Research Group. Source symbol: a symbol that has to be transmitted between the ingress and egress of the network. Coded symbol: a linear combination over a finite field of a set of source symbols. Source symbol ID: a sequence number to identify the source symbols. Coded symbol ID: a sequence number to identify the coded symbols. Encoding vector: a set of the encoding coefficients and input symbol IDs. One or both sets can be null. Detchart, et al. Expires September 10, 2015 [Page 3] Internet-DraftTetrys, an On-the-Fly Network Coding protocol March 2015 Source packet: a source packet contains a source symbol with its associated IDs. Coded packet: a coded packet contains a coded symbol, the coded symbol's ID and encoding vector. Input symbol: a symbol at the input of the Tetrys Encoding Building Block. Output symbol: a symbol generated by the Tetrys Encoding Building Block. For a non systematic mode, all output symbols are coded symbols. For a systematic mode, output symbols can be the input symbols and a number of coded symbols that are linear combinations of the input symbols + the encoding vectors. Feedback packet: a feedback packet is a packet containing information about the decoded or received source symbols. It can also bring additional information about the Packet Error Rate or the number of various packets in the receiver decoding window Elastic Encoding Window: an encoder-side buffer that stores all the non-acknowledged source packets of the input flow that are part of the coding process. Coding Coefficient Generator Identifier: a unique identifier to define a function or an algorithm allowing to generate the coefficients used to compute the coded packets. Code rate: Define the rate of generating and sending the redundancy. 3. Architecture -- Editor's note: The architecture used in this document should be aligned with the future NC Architecture document [NWCRG-ARCH]. -- 3.1. Use Cases Tetrys is well suited, but not limited to the use case where there is a single flow originated by a single source, with intra stream coding that takes place at a single encoding node. Transmission can be over a single path or multiple paths. In addition, the flow can be sent in unicast, multicast, or anycast mode. This is the simplest use- case, that is very much inline with currently proposed scenarios for end-to-end streaming. Detchart, et al. Expires September 10, 2015 [Page 4] Internet-DraftTetrys, an On-the-Fly Network Coding protocol March 2015 3.2. Overview +-----------+ +-----------+ | Tetrys | output packets | Tetrys | source | Sender |----------------->| Receiver | source ---------->| | feedback packets | |----------> symbols | |<-----------------| | symbols +-----------+ +-----------+ Figure 1: Tetrys Architecture. The Tetrys protocol features several key functionalities: o On-the-fly encoding; o Decoding; o Signaling, to carry in particular the symbol identifiers in the encoding window and the associated coding coefficients when meaningful, in a manner that was previously used in FEC; o Feedback management; o Elastic window management; o Channel estimation; o Dynamic adjustment of the code rate and flow control; o Congestion control management (if appropriate); -- Editor's note: must be discussed -- o Tetrys packet header creation and processing; o -- Editor's note: something else? -- These functionalities are provided by several building blocks: o The Tetrys Building Block: this BB is used during encoding and decoding processes. It must be noted that Tetrys does not mandate a specific building block. Instead any building block compatible with the elastic encoding window feature of Tetrys can be used. o The Window Management Building Block: this building block is in charge of managing the encoding encoding window at a Tetrys sender. Detchart, et al. Expires September 10, 2015 [Page 5] Internet-DraftTetrys, an On-the-Fly Network Coding protocol March 2015 -- Editor's note: Is it worth moving it in a dedicated BB? To be discussed -- o Other ? In order to enable future components and services to be added dynamically, Tetrys adds a header extension mechanism, compatible with that of LCT, NORM, FECFRAME [REFS]. 4. Packet Format 4.1. Common Header Format All types of Tetrys packets share the same common header format (see Figure 2). 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | V | Reserved | C |S| HDR_LEN | Packet Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Congestion Control Information (CCI, length = 32*C bits) | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Transport Session Identifier (TSI, length = 32*S bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Header Extensions (if applicable) | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Common Header Format -- Editor's note: this format inherits from the LCT header format (RFC 5651) with slight modifications. -- o Tetrys version number (V): 4 bits. Indicates the Tetrys version number. The Tetrys version number for this specification is 1. o Reserved (Resv): 9 bits. These bits are reserved. In this version of the specification, they MUST be set to zero by senders and MUST be ignored by receivers. o Congestion control flag (C): 2 bits. C=0 indicates the Congestion Control Information (CCI) field is 0 bits in length. C=1 indicates the CCI field is 32 bits in length. C=2 indicates the CCI field is 64 bits in length. C=3 indicates the CCI field is 96 bits in length. Detchart, et al. Expires September 10, 2015 [Page 6] Internet-DraftTetrys, an On-the-Fly Network Coding protocol March 2015 -- Editor's note: version number and congestion control to be discussed -- o Transport Session Identifier flag (S): 1 bit. This is the number of full 32-bit words in the TSI field. The TSI field is 32*S bits in length, i.e., the length is either 0 bits or 32 bits. o Header length (HDR_LEN): 8 bits. Total length of the Tetrys header in units of 32-bit words. The length of the Tetrys header MUST be a multiple of 32 bits. This field can be used to directly access the portion of the packet beyond the Tetrys header, i.e., to the first other header if it exists, or to the packet payload if it exists and there is no other header, or to the end of the packet if there are no other headers or packet payload. o Packet Type: 8 bits. Type of packet. o Congestion Control Information (CCI): 0, 32, 64, or 96 128 bits Used to carry congestion control information. For example, the congestion control information could include layer numbers, logical channel numbers, and sequence numbers. This field is opaque for the purpose of this specification. This field MUST be 0 bits (absent) if C=0. This field MUST be 32 bits if C=1. This field MUST be 64 bits if C=2. This field MUST be 96 bits if C=3. o Transport Session Identifier (TSI): 0, 16, 32, or 48 bits The TSI uniquely identifies a session among all sessions from a particular sender. The TSI is scoped by the IP address of the sender, and thus the IP address of the sender and the TSI together uniquely identify the session. Although a TSI in conjunction with the IP address of the sender always uniquely identifies a session, whether or not the TSI is included in the Tetrys header depends on what is used as the TSI value. If the underlying transport is UDP, then the 16-bit UDP source port number MAY serve as the TSI for the session. If the TSI value appears multiple times in a packet, then all occurrences MUST be the same value. If there is no underlying TSI provided by the network, transport or any other layer, then the TSI MUST be included in the Tetrys header. 4.1.1. Header Extensions Header Extensions are used in Tetrys to accommodate optional header fields that are not always used or have variable size. The presence of Header Extensions can be inferred by the Tetrys header length (HDR_LEN). If HDR_LEN is larger than the length of the standard header, then the remaining header space is taken by Header Extension fields. Detchart, et al. Expires September 10, 2015 [Page 7] Internet-DraftTetrys, an On-the-Fly Network Coding protocol March 2015 If present, Header Extensions MUST be processed to ensure that they are recognized before performing any congestion control procedure or otherwise accepting a packet. The default action for unrecognized Header Extensions is to ignore them. This allows the future introduction of backward-compatible enhancements to Tetrys without changing the Tetrys version number. Non-backward-compatible Header Extensions CANNOT be introduced without changing the Tetrys version number. There are two formats for Header Extension fields, as depicted in Figure Figure 3. The first format is used for variable-length extensions, with Header Extension Type (HET) values between 0 and 127. The second format is used for fixed-length (one 32-bit word) extensions, using HET values from 127 to 255. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HET (<=127) | HEL | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + . . . Header Extension Content (HEC) . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HET (>=128) | Header Extension Content (HEC) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Header Extension Format o Header Extension Type (HET): 8 bits The type of the Header Extension. This document defines a number of possible types. Additional types may be defined in future versions of this specification. HET values from 0 to 127 are used for variable- length Header Extensions. HET values from 128 to 255 are used for fixed-length 32-bit Header Extensions. o Header Extension Length (HEL): 8 bits The length of the whole Header Extension field, expressed in multiples of 32-bit words. This field MUST be present for variable-length extensions (HETs between 0 and 127) and MUST NOT be present for fixed-length extensions (HETs between 128 and 255). o Header Extension Content (HEC): variable length The content of the Header Extension. The format of this sub-field depends on the Header Extension Type. For fixed-length Header Extensions, the Detchart, et al. Expires September 10, 2015 [Page 8] Internet-DraftTetrys, an On-the-Fly Network Coding protocol March 2015 HEC is 24 bits. For variable-length Header Extensions, the HEC field has variable size, as specified by the HEL field. Note that the length of each Header Extension field MUST be a multiple of 32 bits. Also note that the total size of the Tetrys header, including all Header Extensions and all optional header fields, cannot exceed 255 32-bit words. 4.2. Source Packet Format A source packet is the encapsulation of a source symbol, a source symbol ID and a Common Packet Header. The source symbols can have variable sizes. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | / Common Packet Header / | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Symbol ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | / Payload / | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: Source Packet Format Common Packet Header: a common packet header where Packet Type=0. Source Symbol ID: the sequence number to identify a source symbol. Payload: the payload (source symbol) 4.3. Coded Packet Format A coded packet is the encapsulation of a coded symbol, a coded symbol ID, the associated encoding vector and the Common Packet Header. As the source symbols can have variable sizes, each source symbol size need to be encoded, and the result must be stored in the coded packet. The Encoded Payload Size is 16 bits. Detchart, et al. Expires September 10, 2015 [Page 9] Internet-DraftTetrys, an On-the-Fly Network Coding protocol March 2015 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | / Common Packet Header / | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Coded Symbol ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | / Encoding Vector / | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Encoded Payload Size | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | / Payload / | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: Coded Packet Format Common Packet Header: a common packet header where Packet Type=1. Coded Symbol ID: the sequence number to identify a coded symbol. Encoding Vector: an encoding vector to define the linear combination used (coefficients, and source symbols). Encoded Payload Size: the coded payload size used if the source symbols are of variable size. Payload: the coded symbol. 4.4. Acknowledgement Packet Format A Tetrys Decoding Building Block MAY send back to a Tetrys Encoding Building Block some Acknowledgement packets. They contain information about what it is received and/or decoded, and other information such as a packet loss rate or the size of the decoding buffers. The acknowledgement packets are OPTIONAL hence they could be omitted or lost in transmission without impacting the basic protocol performance. Detchart, et al. Expires September 10, 2015 [Page 10] Internet-DraftTetrys, an On-the-Fly Network Coding protocol March 2015 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | / Common Packet Header / | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Nb of missing source symbols | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Nb of not already used coded symbols | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Transport Session Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | First Source Symbol ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SACK size | | +-+-+-+-+-+-+-+-+ + | | / SACK Vector / | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: Acknowledgement Packet Format Common Packet Header: a common packet header where Packet Type=2. Nb missing source symbols: the number of missing source symbols in the receiver. Nb of not already used coded symnbols: the number of not already used coded symbols in the receiver that have not already been used for decoding. Meaning the number of linear combinations containing at least 2 unknown source symbols. Transport Session Identifier (TSI): the unique identifier for the session (CAN be 0bit, depending of the Common Packet Header's field S) First Source Symbol ID: ID of the first source symbol to acknowledge. SACK size: the size of the SACK vector in 32-bit words. For instance, with value 2, the SACK vector is 64 bits long. SACK vector: bit vector indicating the acknowledged symbols following the first source symbol ID. The "First Source Symbol" is not included in this bit vector. A bit equal to 1 at position i means that the source symbol of ID equal to "First Source Symbol ID" + i + 1 is acknowledged by this acknowledgment packet. Detchart, et al. Expires September 10, 2015 [Page 11] Internet-DraftTetrys, an On-the-Fly Network Coding protocol March 2015 5. The Coding Coefficient Generator Identifiers 5.1. Definition The Coding Coefficient Generator Identifiers define a function or an algorithm to build the coding coefficients used to generate the coded symbols. They MUST be known by all the Building Blocks. 5.2. Table of Identifiers 0000: GF256 Vandermonde based coefficients. Each coefficient is build as alpha^( (source_id*repair_id) % 255). 0001: GF16 Vandermonde based coefficients. Each coefficient is build as alpha^( (source_id*repair_id) % 15). 0010: SRLC. Others: To be discussed. 6. Tetrys Basic Functions 6.1. Encoding At the beginning of a transmission, a Tetrys Encoding Building Block MUST choose an initial code rate (added redundancy) as it doesn't know the packet loss rate of the channel. In steady state, the Tetrys Encoding Building Block generates coded symbols when it receives some information from the decoding blocks. When a Tetrys Encoding Building Block needs to generate a coded symbol, it considers the set of source symbols stored in the Elastic Encoding Window. These source symbols are the set of source symbols which are not yet acknowledged by the receiver. A Tetrys Encoding Building Block SHOULD set a limit of the Elastic Encoding Window size. This allows to reduce the complexity by considering less source symbols. It also provides a coping mechanism if all the acknowledgment packets are lost. At the generation of a coded symbol, the Tetrys Encoding Building Block generates an encoding vector containing the IDs of the source symbols stored in the Elastic Encoding Window. For each source symbol, a finite field coefficient is determined using a Coding Coefficient Generator. This generator CAN take as input the source symbol ID and the coded symbol ID and CAN determine a coefficient in a deterministic way. A classical example of such deterministic function is a generator matrix where the rows are indexed by the Detchart, et al. Expires September 10, 2015 [Page 12] Internet-DraftTetrys, an On-the-Fly Network Coding protocol March 2015 source symbol IDs and the columns by the coded symbol IDs. For example, the entries of this matrix can be built from a Vandermonde structure, like Reed-Solomon codes, or from a sparse binary matrix, like Low-Density Generator Matrix codes. Finally, the coded symbol is the sum of the source symbols multiplied by their corresponding coefficients. 6.1.1. Encoding Vector Formats The encoding vectors are sent in each coded symbols. They contain the source symbol IDs and/or the coefficients. To avoid the overhead of transmitting all the source symbol IDs, the following algorithm is used to compress them. 6.1.1.1. Transmitting the source symbol IDs The source symbol IDs are organized as a sorted list of 32-bit integers. Instead of sending the full list, a differential transform to reduce the number of bits needed to represent an ID is used. 6.1.1.1.1. Compressing the Source symbol IDs Assume the symbol IDs used in the combination are: [1..3],[5..6],[8..10]. 1. Keep the first element in the packet as the first_source_id: 1. 2. Apply a differential transform to the others elements ([3,5,6,8,10]) which removes the element i-1 to the element i, starting with the first_source_id as i0, and get the list L => [2,2,1,2,2] 3. Compute b, the number of bits needed to store all the elements, which is ceil(log2(max(L))): here, 2 bits. 4. Write b in the corresponding field, and write all the b * [(2 * NB blocks) - 1] elements in a bit vector, here: 10 10 01 10 10. 6.1.1.1.2. Decompressing the Source symbol IDs When a Tetrys Decoding Building Block wants to reverse the operations, this algorithm is used: 1. Rebuild the list of the transmitted elements by reading the bit vector and b: [10 10 01 10 10] => [2,2,1,2,2] Detchart, et al. Expires September 10, 2015 [Page 13] Internet-DraftTetrys, an On-the-Fly Network Coding protocol March 2015 2. Apply the reverse transform by adding successively the elements, starting with first_source_id: [1,1+2,(1+2)+2,(1+2+2)+1,...] => [1,3,5,6,8,10] 3. Rebuild the blocks using the list and first_source_id: [1..3],[5..6],[8..10]. 6.1.1.2. Encoding Vector Format The encoding vector CAN be used to store the source symbol IDs included in the associated coded symbol, the coefficients used in the combination, or both. It CAN be used to send only the number of source symbols included in the coded symbol. If the source IDs are stored, the nb of blocks MUST be different from 0. The encoding vector format uses a 4-bit Coding Coefficient Generator Identifier to identity the algorithm to generate the coefficients, and contains a set of blocks for the source symbol IDs used in the combination. In this format, the number of blocks is stored as a 8-bit unsigned integer. To reduce the overhead, a compressed way to store the symbol IDs is used: the IDs are not stored as themselves, but stored as the difference between the previous. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | EV_LEN | CCGI |I|C| | NB_BLOCKS | NB_COEFS | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | FIRST_SOURCE_ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | b_id | | +-+-+-+-+-+-+-+ id_bit_vector +-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | b_coef | | +-+-+-+-+-+-+-+ coef_bit_vector +-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: Encoding Vector Format o Encoding Vector Length (EV_LEN): size in units of 32-bit words. o Coding Coefficient Generator Identifier (CCGI): 8-bit ID to identify the algorithm or the function used to generate the coefficients (see Section 5). As a CCGI is included in each Detchart, et al. Expires September 10, 2015 [Page 14] Internet-DraftTetrys, an On-the-Fly Network Coding protocol March 2015 encoded vector, it can dynamically change between the generation of 2 coded symbols. o Store the IDs flag (I): 1 bit to know if an encoding vector contains the list of the IDs used. MUST be 1 if the Encoding Vector stores the source symbol IDs. o Number of blocks used to store the source symbol IDs (NB_BLOCKS): the number of blocks used to store all the source symbol IDs. o Number of coefficients (NB_COEFS): The number of the coefficients used to generate the associated coded symbol. o The first source Identifier (FIRST_SOURCE_ID): the first source symbol ID used in the combination. o Number of bits for each edge block (b_id): the number of bits needed to store the edge (see Section 6.1.1.1). o The compressed edge blocks (id_bit_vector): equal to b_id * (NB_BLOCKS * 2 - 1). o Number of bits needed to store each coefficient (b_coef): the number of bits used to store the coefficients. o The coefficients (coef_bit_vector): The coefficients stored (as a vector of b_coef * NB_COEFS). o Padding: padding to have an Encoding Vector size multiple of 32-bit (for the id and coefficient part). 6.1.2. The Elastic Encoding Window When an input source symbol is passed to a Tetrys Encoding Building Block, it is added to the Elastic Encoding Window. This window MUST have a limit set by the encoding building Block (depending of the use case: unicast, multicast, file transfer, real-time transfer, ...). If the Elastic Encoding Window reached its limit, the window slides over the symbols: the first (oldest) symbols are removed. Then, a packet containing this symbol can be sent onto the network. As an element of the coding window, this symbol is included in the next linear combinations created to generate the coded symbols. As explained below, the receiver sends periodic feedback indicating the received or decoded source symbols. In the case of a unicast transmission, when the sender receives the information that a source symbol was received and/or decoded by the receiver, it removes this symbol from the coding window. In a multicast transmission, a source Detchart, et al. Expires September 10, 2015 [Page 15] Internet-DraftTetrys, an On-the-Fly Network Coding protocol March 2015 symbol is removed from the coding window only when all the receivers have received or decoded it. 6.2. Decoding A classical matrix inversion is sufficient to recover the source symbols. 7. Security Considerations N/A 8. Privacy Considerations N/A 9. IANA Considerations N/A 10. Acknowledgments N/A 11. References 11.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 11.2. Informative References [AHL-00] Ahlswede, R., Ning Cai, , Li, S., and R. Yeung, "Network information flow", IEEE Transactions on Information Theory vol.46, no.4, pp.1204,1216, July 2000. [FECFRAME] Watson, M., Begen, A., and V. Roca, "Forward Error Correction (FEC) Framework", Request for Comments 6363, October 2011. [NWCRG-ARCH] NWCRG, , "Network Coding Architecture", TBD TBD. [RMT] Vicisano, L., Gemmel, J., Rizzo, L., Handley, M., and J. Crowcroft, "Forward Error Correction (FEC) Building Block", Request for Comments 3452, December 2002. Detchart, et al. Expires September 10, 2015 [Page 16] Internet-DraftTetrys, an On-the-Fly Network Coding protocol March 2015 [Tetrys] Lacan, J. and E. Lochin, "Rethinking reliability for long- delay networks", International Workshop on Satellite and Space Communications 2008 (IWSSC08), October 2008. Authors' Addresses Jonathan Detchart ISAE 10, avenue Edouard-Belin BP 54032 Toulouse CEDEX 4 31055 France Email: jonathan.detchart@isae.fr Emmanuel Lochin ISAE 10, avenue Edouard-Belin BP 54032 Toulouse CEDEX 4 31055 France Email: emmanuel.lochin@isae.fr Jerome Lacan ISAE 10, avenue Edouard-Belin BP 54032 Toulouse CEDEX 4 31055 France Email: jerome.lacan@isae.fr Vincent Roca INRIA 655, av. de l'Europe Inovallee; Montbonnot ST ISMIER cedex 38334 France Email: vincent.roca@inria.fr URI: http://privatics.inrialpes.fr/people/roca/ Detchart, et al. Expires September 10, 2015 [Page 17]