| Internet Engineering Task Force | S. Yang |
| Internet-Draft | X. Huang |
| Intended status: Informational | CUHK(SZ) |
| Expires: October 26, 2019 | R.W. Yeung |
| CUHK | |
| J.K. Zao | |
| NCTU | |
| April 24, 2019 |
BATS Coding Scheme for Multi-hop Data Transport
draft-yang-nwcrg-bats-01
This document describes a BATS coding scheme for communication through multi-hop networks. BATS code is a class of efficient linear network coding scheme with a matrix generalization of fountain codes as the outer code, and batch-based linear network coding as the inner code.
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This document specifies a BATS code scheme for data delivery in multi-hop networks. The BATS code described here includes an outer code and an inner code. The outer code is a matrix generalization of fountain codes (see also the RapterQ code described in RFC 6330), which inherits the advantages of reliability and efficiency and possesses the extra desirable property of being network coding compatible. The inner code is formed by linear network coding for combating packet loss, improving the multicast efficiency, etc. A detailed design and analysis of BATS codes are provided in the BATS monograph.
The BATS coding scheme can be applied in multi-hop networks formed by wireless communication links, which are inherently unreliable due to interference. Existing network protocols like TCP/IP use end-to-end retransmission and store-and-forward at the relays, so that packet loss would accumulate along the way.
The BATS coding scheme can be used for various data delivery applications like file transmission, video streaming, etc.
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 RFC 2119.
A BATS coding scheme includes an outer code encoder (also called encoder), an inner code encoder (also called recoder) and a decoder. The BATS coding scheme described in this document can be used by a Data Delivery Protocol (DDP) with the following procedures.
Suppose that the DDP has F octets of data for transmission. The construction of source packets from the data depends on two parameters K and T:
If F is smaller than T*K, the data MUST be padded to have T*K octets, so that the data can be partitioned into K source packets, each of which has T octets.
A source node has the data for transmission. The DDP will first pad and partition the data into K source packets, each containing T octets. The DDP provides the BATS encoder with the following information:
Using this information, the (outer code) encoder generates a batch for each batch ID. For the batch ID ID[j], the encoder returns the DDP that contains
The DDP will use the batches to form DDP packets to be transmitted to other network nodes towards the destination nodes. The DDP MUST deliver with each coded packet its
The DDP MUST deliver the following information to each recoder:
The DDP MUST deliver the following information to each decoder:
The packet length information MUST be known by all the nodes.
An intermediate node does not need the data, but only helps to deliver the data to the destination nodes. At an intermediate node, the DDP only receives the DDP packets from the other network nodes, and should be able to extract coded packets and the corresponding batch information from these packets.
The DDP provides the recoder with the following information:
The recoder uses the information provided by the DDP to generate M' recoded packets, each containing T' octets. The DDP uses the M' recoded packets to form the DDP packets for transmitting.
A destination node needs the data transmitted by the source node. At the destination node, the DDP receives DDP packets from the other network nodes, and should be able to extract coded packets and the corresponding batch information from these packets.
The DDP provides the decoder with the following information:
The decoder uses this information to decode the K source packets. If successful, the decoder returns the recovered K source packets to the DDP, which will use the source packets to form the source data.
The recommendation for the parameters M, K, T, and T' is shown as follows:
It is RECOMMENDED that K is at least 128. However, the encoder/decoder SHALL support an arbitrary positive integer value of K.
A DDP can form a DDP packet includes a header and a payload.
The BATS packet header has 40 bits (5 octets) and includes F, M, K, q, ID, and d
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F | K | M |q| ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| d |
+-+-+-+-+-+-+-+-+
Figure: BATS packet header format.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| coefficient vector | coded data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure: BATS packet payload format.
The playload has T' octets, where the first O octets are called the coeffcient vector and the remaining T octets are the coded data.
The T octets of a source packets are treated as a column vector of T elements in GF(256). Linear algebra and matrix operations over finite fields are assumed in this section.
Suppose that a pseudo-random number generator Rand() is given.
Let b[0], b[1], ...,b[K-1] be the K source packets. A batch with batch ID ID[j] is generated using the following steps.
First, a degree d[j] = DD() is sampled using the given degree distribution DD() and pseudo-random generator Rand() (initialized with any seed).
Second, initialize Rand() with ID[j] as the seed. Using Rand() sample d packets among all the source packets. Suppose the indices of the packets sampled are i_1, i_2, ..., i_d. Let B[j] = (b[i_1], b[i_2], ..., b[i_d]).
Third, sample a dxM generator matrix G[j] with all the entries uniformly distribution among {0,1,...,255}.
Fourth, form the batch X[j] = B[j]*G[j].
Last, append coefficient vectors to the packets of the batches. Let x[j,i], i=0,1,...,M-1, be the (i+1)th column of X[j]. The coefficient vector of x[j,i] is the (i+1)th column of the MxM identity matrix with entries in GF(q), which can be represented by ceil(M*log2(q)/8) octets. The coded packet x'[j,i] is formed by appending the coefficient vector before x[j,i].
The inner code comprises (random) linear network coding applied on the coded packets belonging to the same batch. At a particular network node, recoded packets are generated by (random) linear combinations of the received coded packets of a batch. The recoded packets have the same batch ID, sparse degree and coded packet length.
Suppose that coded packets x'[j,i], i = 0, 1, ..., r-1, which belonging to the same batch j, have been received at an intermediate node. Using the recommended packet format, it can be verified that the corresponding packet header of these coded packets are the same. Then a recoded packet can be generated by one of the following two approaches:
A recoder can combine these two approaches to generate recoded packets. For example, when M' > r, the recoder can output x'[j,i], i = 0, 1, ..., r-1 as r systematic recoded packets and generate M'-r recoded packets using linear combinations of randomly chosen coeffcients.
A belief propagation (BP) decoder is described.
The decoder receives a sequence of batches Y'[j], j = 0, 1, ..., n-1, each of which is a T'-row matrix over GF(256). Let Y[j] be the submatrix of the last T rows of Y'[j]. When q = 256, let H[j] be the first M rows of Y'[j]; when q = 2, let H[j] be the matrix over GF(256) formed by embedding each bit in the first M/8 rows of Y'[j] into GF(256).
The decoder is initialized with the following steps similar to those of encoding. For each batch j,
A batch j is said to be decodable if the system of linear equations Y[j] = B[j]G[j]H[j] with B[j] as the variable matrix has a unique solution, i.e., rank(G[j]H[j]) = d[j]. The BP decoding algorithm has mutiple iterations. Each iteration is formed by the following steps:
The BP decoder repeats the above steps until no batches are decodable during the decoding step.
This memo includes no request to IANA.
Subsuming both Random Linear Network Codes (RLNC) and fountain codes, BATS codes naturally inherit both their desirable capability of offering confidentiality protection as well as their vulnerability towards pollution attacks.
Since the transported messages are linearly combined with random coefficients at each recoding node, it is statistically impossible to recover individual messages by capturing the coded messages at any one or small number of nodes. As long as the coding matrices of the transported messages cannot be fully recovered, any attempt of decoding is equivalent to randomly guessing the transported messages. Thus, with the use of BATS codes, information confidentiality throughout the data transport process is assured.
The only thread towards confidentiality exists in the form of eavesdropping onto the initial encoding process, which takes place at the encoding nodes. In these nodes, the transported data are presented in plain text and can be read along their transfer paths. Hence, information isolation between the encoding process and all other user processes running on the node must be assured.
In addition, the authenticity and trustworthiness of the encoding, recoding and decoding program running on all the nodes must be attested by a trusted authority. Such a measure is also necessary in countering pollution attacks.
Like all network codes, BATS codes are vulnerable under pollution attacks. In these attacks, one or more compromised coding node(s) can pollute the coded messages or inject forged messages into the coding network. These attacks can prevent the receivers from recovering the transported data correctly. Although error detection mechanisms can be put in place to prevent the receivers from getting the wrong messages, detection and discard of the polluted messages still constitute a form of denial-of-service (DoS) attack.
The research community has long been investigating the use of various signature schemes (including homomorphic signatures) to identify the forged messages and stall the attacks (see Zhao07, Yu08, Agrawal09). Nevertheless, these countermeasures are regarded as being computationally too expensive to be employed in broadband communications. A practical approach to protect against pollution attacks consists of the following system-level countermeasures:
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
This becomes an Appendix.