comparison mercurial/help/internals/wireprotocol.txt @ 35976:48a3a9283f09

sshpeer: initial definition and implementation of new SSH protocol

The existing SSH protocol has several design flaws. Future commits
will elaborate on these flaws as new features are introduced
to combat these flaws. For now, hopefully you can take me for my
word that a ground up rewrite of the SSH protocol is needed.

This commit lays the foundation for a new SSH protocol by defining
a mechanism to upgrade the SSH transport channel away from the
default (version 1) protocol to something modern (which we'll call
"version 2" for now).

This upgrade process is detailed in the internals documentation
for the wire protocol. The gist of it is the client sends a
request line preceding the "hello" command/line which basically
says "I'm requesting an upgrade: here's what I support." If the
server recognizes that line, it processes the upgrade request and
the transport channel is switched to use the new version of the
protocol. If not, it sends an empty response, which is how all
Mercurial SSH servers from the beginning of time reacted to unknown
commands. The upgrade request is effectively ignored and the client
continues to use the existing version of the protocol as if nothing
happened.

The new version of the SSH protocol is completely identical to
version 1 aside from the upgrade dance and the bytes that follow.
The immediate bytes that follow the protocol switch are defined to
be a length framed "capabilities: " line containing the remote's
advertised capabilities. In reality, this looks very similar to
what the "hello" response would look like. But it will evolve
quickly.

The methodology by which the protocol will evolve is important.

I'm not going to introduce the new protocol all at once. That would
likely lead to endless bike shedding and forward progress would
stall. Instead, I intend to tricle out new features and diversions
from the existing protocol in small, incremental changes.

To support the gradual evolution of the protocol, the on-the-wire
advertised protocol name contains an "exp" to denote "experimental"
and a 4 digit field to capture the sub-version of the protocol.
Whenever we make a BC change to the wire protocol, we can increment
this version and lock out all older clients because it will appear
as a completely different protocol version. This means we can incur
as many breaking changes as we want. We don't have to commit to
supporting any one feature or idea for a long period of time. We
can even evolve the handshake mechanism, because that is defined
as being an implementation detail of the negotiated protocol version!
Hopefully this lowers the barrier to accepting changes to the
protocol and for experimenting with "radical" ideas during its
development.

In core, sshpeer received most of the attention. We haven't even
implemented the server bits for the new protocol in core yet.
Instead, we add very primitive support to our test server, mainly
just to exercise the added code paths in sshpeer.

Differential Revision: https://phab.mercurial-scm.org/D2061

# no-check-commit because of required foo_bar naming

35975:40d94ea51402

Note that output from the ``hello`` command is terminated by a ``\n``. This is
part of the response payload and not part of the wire protocol adding a newline
after responses. In other words, the length of the response contains the
trailing ``\n``.

SSH Version 1 Transport
-----------------------

The SSH transport (version 1) is a custom text-based protocol suitable for
use over any bi-directional stream transport. It is most commonly used with

If the server receives an unknown command, it will send an empty ``string``
response.

The server terminates if it receives an empty command (a ``\n`` character).

Capabilities
============

Servers advertise supported wire protocol features. This allows clients to

35976:48a3a9283f09

Note that output from the ``hello`` command is terminated by a ``\n``. This is
part of the response payload and not part of the wire protocol adding a newline
after responses. In other words, the length of the response contains the
trailing ``\n``.

Clients supporting version 2 of the SSH transport send a line beginning
with ``upgrade`` before the ``hello`` and ``between`` commands. The line
(which isn't a well-formed command line because it doesn't consist of a
single command name) serves to both communicate the client's intent to
switch to transport version 2 (transports are version 1 by default) as
well as to advertise the client's transport-level capabilities so the
server may satisfy that request immediately.

The upgrade line has the form:

    upgrade <token> <transport capabilities>

That is the literal string ``upgrade`` followed by a space, followed by
a randomly generated string, followed by a space, followed by a string
denoting the client's transport capabilities.

The token can be anything. However, a random UUID is recommended. (Use
of version 4 UUIDs is recommended because version 1 UUIDs can leak the
client's MAC address.)

The transport capabilities string is a URL/percent encoded string
containing key-value pairs defining the client's transport-level
capabilities. The following capabilities are defined:

proto
   A comma-delimited list of transport protocol versions the client
   supports. e.g. ``ssh-v2``.

If the server does not recognize the ``upgrade`` line, it should issue
an empty response and continue processing the ``hello`` and ``between``
commands. Here is an example handshake between a version 2 aware client
and a non version 2 aware server:

   c: upgrade 2e82ab3f-9ce3-4b4e-8f8c-6fd1c0e9e23a proto=ssh-v2
   c: hello\n
   c: between\n
   c: pairs 81\n
   c: 0000000000000000000000000000000000000000-0000000000000000000000000000000000000000
   s: 0\n
   s: 324\n
   s: capabilities: lookup changegroupsubset branchmap pushkey known getbundle ...\n
   s: 1\n
   s: \n

(The initial ``0\n`` line from the server indicates an empty response to
the unknown ``upgrade ..`` command/line.)

If the server recognizes the ``upgrade`` line and is willing to satisfy that
upgrade request, it replies to with a payload of the following form:

   upgraded <token> <transport name>\n

This line is the literal string ``upgraded``, a space, the token that was
specified by the client in its ``upgrade ...`` request line, a space, and the
name of the transport protocol that was chosen by the server. The transport
name MUST match one of the names the client specified in the ``proto`` field
of its ``upgrade ...`` request line.

If a server issues an ``upgraded`` response, it MUST also read and ignore
the lines associated with the ``hello`` and ``between`` command requests
that were issued by the server. It is assumed that the negotiated transport
will respond with equivalent requested information following the transport
handshake.

All data following the ``\n`` terminating the ``upgraded`` line is the
domain of the negotiated transport. It is common for the data immediately
following to contain additional metadata about the state of the transport and
the server. However, this isn't strictly speaking part of the transport
handshake and isn't covered by this section.

Here is an example handshake between a version 2 aware client and a version
2 aware server:

   c:  upgrade 2e82ab3f-9ce3-4b4e-8f8c-6fd1c0e9e23a proto=ssh-v2
   c:  hello\n
   c:  between\n
   c:  pairs 81\n
   c:  0000000000000000000000000000000000000000-0000000000000000000000000000000000000000
   s: upgraded 2e82ab3f-9ce3-4b4e-8f8c-6fd1c0e9e23a ssh-v2\n
   s: <additional transport specific data>

The client-issued token that is echoed in the response provides a more
resilient mechanism for differentiating *banner* output from Mercurial
output. In version 1, properly formatted banner output could get confused
for Mercurial server output. By submitting a randomly generated token
that is then present in the response, the client can look for that token
in response lines and have reasonable certainty that the line did not
originate from a *banner* message.

SSH Version 1 Transport
-----------------------

The SSH transport (version 1) is a custom text-based protocol suitable for
use over any bi-directional stream transport. It is most commonly used with

If the server receives an unknown command, it will send an empty ``string``
response.

The server terminates if it receives an empty command (a ``\n`` character).

SSH Version 2 Transport
-----------------------

**Experimental**

Version 2 of the SSH transport behaves identically to version 1 of the SSH
transport with the exception of handshake semantics. See above for how
version 2 of the SSH transport is negotiated.

Immediately following the ``upgraded`` line signaling a switch to version
2 of the SSH protocol, the server automatically sends additional details
about the capabilities of the remote server. This has the form:

   <integer length of value>\n
   capabilities: ...\n

e.g.

   s: upgraded 2e82ab3f-9ce3-4b4e-8f8c-6fd1c0e9e23a ssh-v2\n
   s: 240\n
   s: capabilities: known getbundle batch ...\n

Following capabilities advertisement, the peers communicate using version
1 of the SSH transport.

Capabilities
============

Servers advertise supported wire protocol features. This allows clients to