A Ping extension for ActivityPub

-- rationale

Diagnosing communication failures between federated servers often requires
sending test messages. There is no dedicated activity type for this purpose,
however, and thus many operators use normal notes. This creates unnecessary
noise. It would be better to have a side effect free message that can be
triggered and sent on demand.

The proposed Ping and corresponding Pong activities are similar to the ICMP
echo request and echo reply messages. (c.f. the familiar ping tool.)

Other online social contexts often use the term ping to refer to a variety
of activities. The activity here is unrelated to any user visible activity or
action.

-- message format

The ping message has a type of Ping. Here, user pinger on server
h1.example.com is sending a Ping to testrcpt on h2.example.com.

{
	"@context": "https://www.w3.org/ns/activitystreams",
	"type": "Ping",
	"id": "https://h1.example.com/u/pinger/ping/r4nd0m1d",
	"actor": "https://h1.example.com/u/pinger",
	"to": "https://h2.example.com/u/testrcpt"
}

The Pong message is similar, but includes an object field quoting the Ping id
field.

{
	"@context": "https://www.w3.org/ns/activitystreams",
	"type": "Pong",
	"id": "https://h2.example.com/u/testrcpt/pong/0pp0s1t3",
	"actor": "https://h2.example.com/u/testrcpt",
	"to": "https://h1.example.com/u/pinger",
	"object": "https://h1.example.com/u/pinger/ping/r4nd0m1d"
}

Ping and Pong id fields look like URLs, but need not be fetchable. They are
only intended as transient messages.

-- semantics

The Ping message should be sent from one actor to another, delivered to their
inbox. Upon receipt of a Ping message, a server should reply with a Pong
message. The Pong reply should quote the id of the Ping (just the id, not the
whole message) in the object field.

Random ids may used. They should be probabilistically unique.

The usual access and verification checks performed for other messages should
be performed for Ping and Pong as well. (If HTTP signatures are in use,
messages should be signed by senders and verified by receivers.)

Ping and Pong messages should be queued using the normal facilities. (Don't
fast track.) Messages should not be retried. After one failure, drop the
message.

As these messages are intended as administrator aids, they should not be
displayed to end users. They should not cause any lasting change in the state
of either the sending or receiving server.

Rate limiting and abuse controls apply as usual. Servers may choose to impose
length restrictions on maximum id length. A minimum of 256 bytes should be
supported.

Servers which do not understand the Ping activity will hopefully ignore it.

-- usage

It is unspecified how one initiaties a ping, but it is expected to be a manual
operation performed by a system adminstrator. This will generate traffic,
which may then be logged. The admin reads the logs and solves the problem.
Specific problem solving instructions are not provided here.

-- future

It may be helpful to have a variant of Ping that does perform retries to test
recovery after disconnect.