New Omni Layer transaction: Send-to-Many

send-to-many.md

Send-to-Many transactions

A new transaction structure allows to include multiple transfers in one transaction.

The payload of this new transaction type includes one token identifier, which defines the tokens to send. It also includes a list of receiver -> amount mappings to specify, which receiver receives how many tokens. Receivers are specified by actual Bitcoin transaction outputs, which are referenced in the payload. One or more receivers can be defined.

The new transaction type has 7 as identifier.

The payload structure may look like this:

[transaction version] [transaction type] [token identifier to send] [number of outputs following] [receiver output #] [amount to send] ( [output # of receiver] [amount to send] ... )

Example: only one receiver

In the following example, only one receiver is defined, basically mirroring the behavior or regular simple sends.

1.0 Omni are sent to the address specified in output 1.

A single transfer command has the following structure:

Field	Type	Value
Transaction version	Transaction version	0
Transaction type	Transaction type	7 (= Send-to-Many)
Token identifier to send	Token identifier	1 (= Omni)
Number of outputs	Integer-one byte	1
Receiver output #	Integer-one byte	1 (= vout 1)
Amount to send	Number of tokens	1 0000 0000 (= 1.0)

Actual payload:

0000 0007 00000001 01 01 0000000005f5e100

Example: three receivers

Let's imagine a transaction with one input and six outputs. One output for the payload and three outputs for token receivers. Bitcoin values are omitted in this example, but we simply assume the amount of incoming coins is enough to cover the whole transaction. There is another output, which is not relevant for us, and one for change.

The first input has tokens associated with it:

Input Index	Token identifier	Token amount
0	31 (USDT)	100 0000 0000 (= 100.0)

The outputs of the transaction are as follows:

Output Index	Script type
0	Payload with commands
1	Pay-to-pubkey-hash (recipient 1)
2	Pay-to-pubkey-hash (recipient 2)
3	Pay-to-pubkey-hash (not relevant)
4	Pay-to-script-hash (recipient 3)
5	Pay-to-pubkey-hash (our change)

The first output of the transaction contains the payload with the following data:

Field	Type	Value
Transaction version	Transaction version	0
Transaction type	Transaction type	7 (= Send-to-Many)
Token identifier to send	Token identifier	31 (= USDT )
Number of outputs	Integer-one byte	3
Receiver output #	Integer-one byte	1 (= vout 1)
Amount to send	Number of tokens	20 0000 0000 (= 20.0)
Receiver output #	Integer-one byte	2 (= vout 2)
Amount to send	Number of tokens	15 0000 0000 (= 15.0)
Receiver output #	Integer-one byte	4 (= vout 4)
Amount to send	Number of tokens	30 0000 0000 (= 30.0)

Actual payload:

0000 0007 0000001f 03 01 0000000077359400 02 0000000059682f00 04 00000000b2d05e00

20.0 USDT are transferred to the transaction output with index 1, 15.0 USDT to the output with index 2 and 30.0 USDT are transferred to the output with index 4. Given that the sender had a balance of 100.0 USDT, there is a leftover of 35.0 USDT, which were not moved and still belong to the sender.

Notes and remarks

• The transaction fails, if there are not enough tokens for all transfers.
• The transaction fails, if any output references an invalid destination or a destination, which isn't considered as valid destination in terms of the Omni Layer protocol.
• It does not chance anything about Omni Layer's balanced based approach.
• The order of output mappings is not strictly in order. You may first define a send to output #3, then to output #0.
• When constructing the transaction, be careful about the placement of change addresses. If it is inserted randomly, it may affect the mapping.
• Other Omni Layer rules apply, in particular: only the first transaction input specified how many tokens are there to send.
• This is not "send-from-many".
• It is possible to construct a valid transaction with no receiver.

@dexX7 thx for clarifying. A few follow-ups:

1. Is it invalid to specify a payload vout as a recipient?
2. The “change” address idea is to cover the situation when a sender wants to send all of its available balance of a token, but that balance could increase before the Send-to-Many is processed. Some other way would be fine.
3. is it invalid if the specified number of recipients is different (either greater or fewer) than the number of recipients in the payload?
4. The number of recipients can’t be 0, and it’s an unsigned single byte.
5. Can the “almost” aspect cause OC to misinterpret the sender’s intent? > When using Class B (multisig encoding), we "almost" have no limitation.
6. In the extreme case where the vout number overflows one byte, the wrong vout address will be the recipient of 2 amounts and the intended vout will get nothing. (See #8 for mitigation ideas.)
7. We can use origin 0 for the number of outputs, so 0 means 1 output. This eliminates having to treat 0 as an invalid number of outputs.
8. If we use leb128 as the variable length format for the number of outputs and each output number, the typical case (up to 128 outputs) will still use only 1 byte, and it will use more bytes when necessary.

Hey @marvgmail, good questions!

1. That's an interesting one. Only "supported" outputs should be allowed. Currently pay-to-pubkey-hash and pay-to-script-hash are supported. Otherwise the transaction should be invalid.
2. We could add "sendall-to-many" later.
3. If the specified number of recipients is greater than actual recipients, then it's invalid. Otherwise valid, because we generally allow "junk data" after valid payloads. With Class B, we also have fixed length data spaces, which usually result in junk.
4. I think allowing 0 recipients is fine. Unsigned, yes.
5. Omni Core fails during the transaction generation, if a transaction is too large.
6. We need a limitation to 255 recipients.
7. Why not simply allow 0 recipients?
8. I'm really a fan of varlength encoding, but I wouldn't want to add two very new things together (new transaction approach, varlength).

@dexX7, Re …
4. & 7. Why should 0 recipients be valid? That would be Send-to-None, a no-op at the cost of a miner fee.
5. What makes a tx “too large”?
6. Is there a reason for the 255 limitation, other than this tx currently has just 1 byte for the number of recipients?

1. Why shouldn't it be valid? We also allow send-to-selfs for example, which are also no-ops. One less rule is one bit less of complexity. But I'm fine either way. :)
2. We inherit Bitcoin's limits. So far, we have a block size limit, which puts up a cap. There is also a soft rules (i.e. the transaction won't be mined), if it's too big. Last time I recall, it was 100 KB per transaction.
3. This is actually a very good question. When only using Class C/OP_RETURN encoding, we will run into the limit very fast. However, when using Class B/multisig encoding, we have "a lot" space. Assuming 100 KB is indeed the limit and as a very crude estimation, let's assume each recipient with payload data takes 50 byte, then we have space for 2000 recipients. Might be worth to use two bytes then..?

In the long term it would be really good, if we actually implement var length encoding to get rid of quite a few pain points.