According to RFC 4254 section 5.1, SSH_MSG_CHANNEL_OPEN_CONFIRMATION can contain some type specific data, we have mapping for this extra payload but currently the field is unused.

I propose to add API to allow setting this field when accepting a channel on the server side and to retrieve the value on the client side.

Proposed API for the server side part:

// NewChannelWithPayload is a NewChannel that allows to send an arbitrary
// payload in SSH_MSG_CHANNEL_OPEN_CONFIRMATION.
type NewChannelWithPayload interface {
	NewChannel

	// AcceptWithPayload allows to set an arbitrary payload to send in
	// SSH_MSG_CHANNEL_OPEN_CONFIRMATION.
	AcceptWithPayload(payload []byte) (Channel, <-chan *Request, error)
}

Proposed API for the client side part:

// ChannelWithPayload is a Channel that allows to retrieve the type specific
// data received in SSH_MSG_CHANNEL_OPEN_CONFIRMATION.
type ChannelWithPayload interface {
	Channel

	// Payload returns the channel type specific data received in
	// SSH_MSG_CHANNEL_OPEN_CONFIRMATION.
	Payload() []byte
}

for the client side part we can also evaluate an interface extension for Conn to add something like this:

OpenChannelPayload(name string, data []byte) (Channel, <-chan *Request, []byte, error)

this way the payload is returned directly after opening the channel without the need to call the Payload() method on the channel as proposed above.
I prefer the first option because I'm not a big fan of methods that return multiple values, but that's just a personal preference.

Thank you!

cc @golang/proposal-review