# Port 4919

Nothing is assigned to 4919. Nothing has ever been assigned to 4919. Operators
in the Loop tradition do not bind it, do not scan it, and when a numbering
scheme would place a service there, the scheme skips to 4920 without comment.
The practice is close to universal in that lineage and the reasons given for it
are mutually exclusive.

## The Hum

The diagnostic listener shipped with [[lore/cold-handshake]] implementations
answered on 4919. It answered anything — a connection, a byte, a stray SYN —
with two bytes and a close:

```
    connect 4919
    ─────────────────────────────────────────
    < 0x1B 0x30
    connection closed by foreign host
    ─────────────────────────────────────────

    ESC 0.  "cold, idle, no circuit under test."
    Four bursts of CHP would have followed.
    None ever did, because nobody spoke CHP
    to it after 1991, and it answered anyway
    for another decade.
```

Two bytes is not enough to be interesting and is exactly enough to be
unmistakable. Sweeping a range and getting `1B 30` back told you that you had
found a piece of gear old enough to have shipped with a protocol that had been
withdrawn, which by the mid-1990s meant something about the site rather than
about the machine. People called it the Hum, on the grounds that it was always
there and told you nothing.

## The three reasons

Ask why the port is avoided and you get one of these. Nobody holds two.

**The courtesy.** Binding 4919 means that anyone sweeping for the Hum finds you
instead, and their survey is now wrong in a way they cannot detect — your
service will not answer `1B 30`, so they will record the address as live and
uninteresting and never come back. This is the only explanation with a mechanism
behind it and it is the least often given.

**The Tuesday.** That the Quiet Exchange was a Tuesday, that 4919 was where the
Loop's gear listened, and that the two facts should not be brought together on
the same weekday. The Tuesday rule is stated with total confidence and the
arithmetic behind it has never appeared in print. See [[lore/quiet-exchange]],
which notes that the date is a Tuesday only in some tellings.

**The occupancy.** That the port is not free. This is offered without elaboration
and is not, in the accounts that give it, a claim about software.

## What it actually is

The observable facts are small. There is no registration. There is no reserved
range. Old gear answered two bytes and has almost entirely been scrapped. Every
generation of the lineage has inherited the avoidance from the one before and
supplied its own reason, which is why there are three reasons and no argument
between their holders: the practice is transmitted and the justification is
regenerated locally each time.

That is worth stating plainly because it is the ordinary shape of a superstition
in a technical culture. The behaviour is cheap — skipping one integer costs
nothing — and cheap behaviours do not get audited. Nobody has ever had to defend
4920, because nobody has ever been inconvenienced by it.

## The initiation question

One of the questions put at the end of the [[lore/long-count]] is why you skip
4919. All three answers are accepted. "I don't" is accepted. The answer that is
not accepted is a fourth reason invented on the spot, and the reported failure
rate on that question is high, which suggests the question is not about the port.

## Related

- [[lore/index]] — the collection
- [[lore/cold-handshake]] — where the listener came from
- [[lore/long-count]] — where the question is asked
- [[lore/lexicon]] — *hum*, *cold*, *sweep*
- [[lore/vesper-node]] — a machine that answered longer than it should have