Patchwork D2061: sshpeer: initial definition and implementation of new SSH protocol

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Submitter phabricator
Date Feb. 6, 2018, 7:32 p.m.
Message ID <differential-rev-PHID-DREV-x2tniuqjc5o7yfk4m6ro-req@phab.mercurial-scm.org>
Download mbox | patch
Permalink /patch/27384/
State Superseded
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phabricator - Feb. 6, 2018, 7:32 p.m.
indygreg created this revision.
Herald added a subscriber: mercurial-devel.
Herald added a reviewer: hg-reviewers.

REVISION SUMMARY
  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.

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D2061

AFFECTED FILES
  mercurial/configitems.py
  mercurial/help/internals/wireprotocol.txt
  mercurial/sshpeer.py
  mercurial/wireprotoserver.py
  tests/sshprotoext.py
  tests/test-ssh-proto.t

CHANGE DETAILS




To: indygreg, #hg-reviewers
Cc: mercurial-devel
phabricator - Feb. 7, 2018, 2:52 a.m.
indygreg added a comment.


  I want to emphasize that I'm not committed to any implementation detail at this point in time. I've very opened minded about alternatives and making backwards incompatible changes throughout the 4.6 release cycle.
  
  That being said, I am trying to make forward progress on a ton of wire protocol changes. These are blocking my planned work for shallow clone this release cycle. (I don't want to deploy shallow clone on the existing wire protocol for various reasons.) So, I would prefer we //fall forward// and take commits even if there are open bikesheds. I'm more than happy to rework the protocol later. I just don't want my local work to be dozens of changesets ahead of what's reviewed and have to spend hours reworking my code because of a bikeshed. I'd rather commit the flawed work, fix it at the head of my local queue, and move forward. If nothing else, this approach will lead to a more feature complete protocol landing sooner. And only once it is feature complete will we all have the full perspective to bikeshed the protocol.

INLINE COMMENTS

> wireprotocol.txt:241-243
> +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:

I chose the //query string// format here because I don't like reinventing wheels. However, if we wanted to make it a list of space delimited atoms (like the existing capabilities string), I'd be OK with that.

We can always change this later, since we're not locked into any BC guarantees at this juncture.

> wireprotocol.txt:245-247
> +proto
> +   A comma-delimited list of transport protocol versions the client
> +   supports. e.g. ``ssh-v2``.

In the future, I want to advertise:

- compression engine support
- compression engine settings (e.g. max window size for zstandard so a server won't choose a compression level that will result in excessive memory usage for client)
- max concurrent responses limit (in the future, the protocol will gain the ability to stream multiple responses for a single request concurrently)

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D2061

To: indygreg, #hg-reviewers
Cc: mercurial-devel
phabricator - Feb. 7, 2018, 2:52 p.m.
joerg.sonnenberger added inline comments.

INLINE COMMENTS

> test-ssh-proto.t:396
> +  > upgrade 2e82ab3f-9ce3-4b4e-8f8c-6fd1c0e9e23a proto=irrelevant1%2Cirrelevant2
> +  > hello
> +  > between

I'm a bit concerned about the order here. I would prefer to stay with the spirit of the original SSH protocol and go with the following order instead:

- client sends hello to the server
- server sends its capabilities, including the supported "modern" protocol versions
- client upgrades to the "best" version it supports and sends any additional wire capabilities it has

I don't think this changes the number of round-trips, but it slightly affects the number of commands that are "unversioned".

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D2061

To: indygreg, #hg-reviewers
Cc: joerg.sonnenberger, mercurial-devel
phabricator - Feb. 7, 2018, 10:04 p.m.
durin42 added inline comments.

INLINE COMMENTS

> joerg.sonnenberger wrote in test-ssh-proto.t:396
> I'm a bit concerned about the order here. I would prefer to stay with the spirit of the original SSH protocol and go with the following order instead:
> 
> - client sends hello to the server
> - server sends its capabilities, including the supported "modern" protocol versions
> - client upgrades to the "best" version it supports and sends any additional wire capabilities it has
> 
> I don't think this changes the number of round-trips, but it slightly affects the number of commands that are "unversioned".

I don't feel super-strongly. I think my bias is to let the upgrade happen prior to the hello so that we can have custom servers in the future not have to support the bad old framing. I'll land the patch as is, but we can continue discussing it a bit here or on the list if I haven't convinced you. :)

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D2061

To: indygreg, #hg-reviewers
Cc: durin42, joerg.sonnenberger, mercurial-devel

Patch

diff --git a/tests/test-ssh-proto.t b/tests/test-ssh-proto.t
--- a/tests/test-ssh-proto.t
+++ b/tests/test-ssh-proto.t
@@ -388,3 +388,107 @@ 
   0
   0
   0
+
+Send an upgrade request to a server that doesn't support that command
+
+  $ hg -R server serve --stdio << EOF
+  > upgrade 2e82ab3f-9ce3-4b4e-8f8c-6fd1c0e9e23a proto=irrelevant1%2Cirrelevant2
+  > hello
+  > between
+  > pairs 81
+  > 0000000000000000000000000000000000000000-0000000000000000000000000000000000000000
+  > EOF
+  0
+  384
+  capabilities: lookup changegroupsubset branchmap pushkey known getbundle unbundlehash batch streamreqs=generaldelta,revlogv1 $USUAL_BUNDLE2_CAPS_SERVER$ unbundle=HG10GZ,HG10BZ,HG10UN
+  1
+  
+
+  $ hg --config experimental.sshpeer.advertise-v2=true --debug debugpeer ssh://user@dummy/server
+  running * "*/tests/dummyssh" 'user@dummy' 'hg -R server serve --stdio' (glob)
+  sending upgrade request: * proto=exp-ssh-v2-0001 (glob)
+  devel-peer-request: hello
+  sending hello command
+  devel-peer-request: between
+  devel-peer-request:   pairs: 81 bytes
+  sending between command
+  remote: 0
+  remote: 384
+  remote: capabilities: lookup changegroupsubset branchmap pushkey known getbundle unbundlehash batch streamreqs=generaldelta,revlogv1 $USUAL_BUNDLE2_CAPS_SERVER$ unbundle=HG10GZ,HG10BZ,HG10UN
+  remote: 1
+  url: ssh://user@dummy/server
+  local: no
+  pushable: yes
+
+Send an upgrade request to a server that supports upgrade
+
+  $ SSHSERVERMODE=upgradev2 hg -R server serve --stdio << EOF
+  > upgrade this-is-some-token proto=exp-ssh-v2-0001
+  > hello
+  > between
+  > pairs 81
+  > 0000000000000000000000000000000000000000-0000000000000000000000000000000000000000
+  > EOF
+  upgraded this-is-some-token exp-ssh-v2-0001
+  383
+  capabilities: lookup changegroupsubset branchmap pushkey known getbundle unbundlehash batch streamreqs=generaldelta,revlogv1 $USUAL_BUNDLE2_CAPS_SERVER$ unbundle=HG10GZ,HG10BZ,HG10UN
+
+  $ SSHSERVERMODE=upgradev2 hg --config experimental.sshpeer.advertise-v2=true --debug debugpeer ssh://user@dummy/server
+  running * "*/tests/dummyssh" 'user@dummy' 'hg -R server serve --stdio' (glob)
+  sending upgrade request: * proto=exp-ssh-v2-0001 (glob)
+  devel-peer-request: hello
+  sending hello command
+  devel-peer-request: between
+  devel-peer-request:   pairs: 81 bytes
+  sending between command
+  protocol upgraded to exp-ssh-v2-0001
+  url: ssh://user@dummy/server
+  local: no
+  pushable: yes
+
+Verify the peer has capabilities
+
+  $ SSHSERVERMODE=upgradev2 hg --config experimental.sshpeer.advertise-v2=true --debug debugcapabilities ssh://user@dummy/server
+  running * "*/tests/dummyssh" 'user@dummy' 'hg -R server serve --stdio' (glob)
+  sending upgrade request: * proto=exp-ssh-v2-0001 (glob)
+  devel-peer-request: hello
+  sending hello command
+  devel-peer-request: between
+  devel-peer-request:   pairs: 81 bytes
+  sending between command
+  protocol upgraded to exp-ssh-v2-0001
+  Main capabilities:
+    batch
+    branchmap
+    $USUAL_BUNDLE2_CAPS_SERVER$
+    changegroupsubset
+    getbundle
+    known
+    lookup
+    pushkey
+    streamreqs=generaldelta,revlogv1
+    unbundle=HG10GZ,HG10BZ,HG10UN
+    unbundlehash
+  Bundle2 capabilities:
+    HG20
+    bookmarks
+    changegroup
+      01
+      02
+    digests
+      md5
+      sha1
+      sha512
+    error
+      abort
+      unsupportedcontent
+      pushraced
+      pushkey
+    hgtagsfnodes
+    listkeys
+    phases
+      heads
+    pushkey
+    remote-changegroup
+      http
+      https
diff --git a/tests/sshprotoext.py b/tests/sshprotoext.py
--- a/tests/sshprotoext.py
+++ b/tests/sshprotoext.py
@@ -53,6 +53,35 @@ 
 
         super(prehelloserver, self).serve_forever()
 
+class upgradev2server(wireprotoserver.sshserver):
+    """Tests behavior for clients that issue upgrade to version 2."""
+    def serve_forever(self):
+        name = wireprotoserver.SSHV2
+        l = self._fin.readline()
+        assert l.startswith(b'upgrade ')
+        token, caps = l[:-1].split(b' ')[1:]
+        assert caps == b'proto=%s' % name
+
+        # Filter hello and between requests.
+        l = self._fin.readline()
+        assert l == b'hello\n'
+        l = self._fin.readline()
+        assert l == b'between\n'
+        l = self._fin.readline()
+        assert l == 'pairs 81\n'
+        self._fin.read(81)
+
+        # Send the upgrade response.
+        self._fout.write(b'upgraded %s %s\n' % (token, name))
+        servercaps = wireproto.capabilities(self._repo, self)
+        rsp = b'capabilities: %s' % servercaps
+        self._fout.write(b'%d\n' % len(rsp))
+        self._fout.write(rsp)
+        self._fout.write(b'\n')
+        self._fout.flush()
+
+        super(upgradev2server, self).serve_forever()
+
 def performhandshake(orig, ui, stdin, stdout, stderr):
     """Wrapped version of sshpeer._performhandshake to send extra commands."""
     mode = ui.config(b'sshpeer', b'handshake-mode')
@@ -85,6 +114,8 @@ 
         wireprotoserver.sshserver = bannerserver
     elif servermode == b'no-hello':
         wireprotoserver.sshserver = prehelloserver
+    elif servermode == b'upgradev2':
+        wireprotoserver.sshserver = upgradev2server
     elif servermode:
         raise error.ProgrammingError(b'unknown server mode: %s' % servermode)
 
diff --git a/mercurial/wireprotoserver.py b/mercurial/wireprotoserver.py
--- a/mercurial/wireprotoserver.py
+++ b/mercurial/wireprotoserver.py
@@ -32,6 +32,12 @@ 
 HGTYPE2 = 'application/mercurial-0.2'
 HGERRTYPE = 'application/hg-error'
 
+# Names of the SSH protocol implementations.
+SSHV1 = 'ssh-v1'
+# This is advertised over the wire. Incremental the counter at the end
+# to reflect BC breakages.
+SSHV2 = 'exp-ssh-v2-0001'
+
 class abstractserverproto(object):
     """abstract class that summarizes the protocol API
 
diff --git a/mercurial/sshpeer.py b/mercurial/sshpeer.py
--- a/mercurial/sshpeer.py
+++ b/mercurial/sshpeer.py
@@ -8,13 +8,15 @@ 
 from __future__ import absolute_import
 
 import re
+import uuid
 
 from .i18n import _
 from . import (
     error,
     pycompat,
     util,
     wireproto,
+    wireprotoserver,
 )
 
 def _serverquote(s):
@@ -162,15 +164,24 @@ 
         hint = ui.config('ui', 'ssherrorhint')
         raise error.RepoError(msg, hint=hint)
 
-    # The handshake consists of sending 2 wire protocol commands:
-    # ``hello`` and ``between``.
+    # The handshake consists of sending wire protocol commands in reverse
+    # order of protocol implementation and then sniffing for a response
+    # to one of them.
+    #
+    # Those commands (from oldest to newest) are:
     #
-    # The ``hello`` command (which was introduced in Mercurial 0.9.1)
-    # instructs the server to advertise its capabilities.
+    # ``between``
+    #   Asks for the set of revisions between a pair of revisions. Command
+    #   present in all Mercurial server implementations.
     #
-    # The ``between`` command (which has existed in all Mercurial servers
-    # for as long as SSH support has existed), asks for the set of revisions
-    # between a pair of revisions.
+    # ``hello``
+    #   Instructs the server to advertise its capabilities. Introduced in
+    #   Mercurial 0.9.1.
+    #
+    # ``upgrade``
+    #   Requests upgrade from default transport protocol version 1 to
+    #   a newer version. Introduced in Mercurial 4.6 as an experimental
+    #   feature.
     #
     # The ``between`` command is issued with a request for the null
     # range. If the remote is a Mercurial server, this request will
@@ -186,6 +197,18 @@ 
     # RFC 822 like lines. Of these, the ``capabilities:`` line contains
     # the capabilities of the server.
     #
+    # The ``upgrade`` command isn't really a command in the traditional
+    # sense of version 1 of the transport because it isn't using the
+    # proper mechanism for formatting insteads: instead, it just encodes
+    # arguments on the line, delimited by spaces.
+    #
+    # The ``upgrade`` line looks like ``upgrade <token> <capabilities>``.
+    # If the server doesn't support protocol upgrades, it will reply to
+    # this line with ``0\n``. Otherwise, it emits an
+    # ``upgraded <token> <protocol>`` line to both stdout and stderr.
+    # Content immediately following this line describes additional
+    # protocol and server state.
+    #
     # In addition to the responses to our command requests, the server
     # may emit "banner" output on stdout. SSH servers are allowed to
     # print messages to stdout on login. Issuing commands on connection
@@ -195,6 +218,14 @@ 
 
     requestlog = ui.configbool('devel', 'debug.peer-request')
 
+    # Generate a random token to help identify responses to version 2
+    # upgrade request.
+    token = bytes(uuid.uuid4())
+    upgradecaps = [
+        ('proto', wireprotoserver.SSHV2),
+    ]
+    upgradecaps = util.urlreq.urlencode(upgradecaps)
+
     try:
         pairsarg = '%s-%s' % ('0' * 40, '0' * 40)
         handshake = [
@@ -204,6 +235,11 @@ 
             pairsarg,
         ]
 
+        # Request upgrade to version 2 if configured.
+        if ui.configbool('experimental', 'sshpeer.advertise-v2'):
+            ui.debug('sending upgrade request: %s %s\n' % (token, upgradecaps))
+            handshake.insert(0, 'upgrade %s %s\n' % (token, upgradecaps))
+
         if requestlog:
             ui.debug('devel-peer-request: hello\n')
         ui.debug('sending hello command\n')
@@ -217,12 +253,31 @@ 
     except IOError:
         badresponse()
 
+    # Assume version 1 of wire protocol by default.
+    protoname = wireprotoserver.SSHV1
+    reupgraded = re.compile(b'^upgraded %s (.*)$' % re.escape(token))
+
     lines = ['', 'dummy']
     max_noise = 500
     while lines[-1] and max_noise:
         try:
             l = stdout.readline()
             _forwardoutput(ui, stderr)
+
+            # Look for reply to protocol upgrade request. It has a token
+            # in it, so there should be no false positives.
+            m = reupgraded.match(l)
+            if m:
+                protoname = m.group(1)
+                ui.debug('protocol upgraded to %s\n' % protoname)
+                # If an upgrade was handled, the ``hello`` and ``between``
+                # requests are ignored. The next output belongs to the
+                # protocol, so stop scanning lines.
+                break
+
+            # Otherwise it could be a banner, ``0\n`` response if server
+            # doesn't support upgrade.
+
             if lines[-1] == '1\n' and l == '\n':
                 break
             if l:
@@ -235,20 +290,39 @@ 
         badresponse()
 
     caps = set()
-    for l in reversed(lines):
-        # Look for response to ``hello`` command. Scan from the back so
-        # we don't misinterpret banner output as the command reply.
-        if l.startswith('capabilities:'):
-            caps.update(l[:-1].split(':')[1].split())
-            break
 
-    # Error if we couldn't find a response to ``hello``. This could
-    # mean:
+    # For version 1, we should see a ``capabilities`` line in response to the
+    # ``hello`` command.
+    if protoname == wireprotoserver.SSHV1:
+        for l in reversed(lines):
+            # Look for response to ``hello`` command. Scan from the back so
+            # we don't misinterpret banner output as the command reply.
+            if l.startswith('capabilities:'):
+                caps.update(l[:-1].split(':')[1].split())
+                break
+    elif protoname == wireprotoserver.SSHV2:
+        # We see a line with number of bytes to follow and then a value
+        # looking like ``capabilities: *``.
+        line = stdout.readline()
+        try:
+            valuelen = int(line)
+        except ValueError:
+            badresponse()
+
+        capsline = stdout.read(valuelen)
+        if not capsline.startswith('capabilities: '):
+            badresponse()
+
+        caps.update(capsline.split(':')[1].split())
+        # Trailing newline.
+        stdout.read(1)
+
+    # Error if we couldn't find capabilities, this means:
     #
     # 1. Remote isn't a Mercurial server
     # 2. Remote is a <0.9.1 Mercurial server
     # 3. Remote is a future Mercurial server that dropped ``hello``
-    #    support.
+    #    and other attempted handshake mechanisms.
     if not caps:
         badresponse()
 
diff --git a/mercurial/help/internals/wireprotocol.txt b/mercurial/help/internals/wireprotocol.txt
--- a/mercurial/help/internals/wireprotocol.txt
+++ b/mercurial/help/internals/wireprotocol.txt
@@ -218,6 +218,95 @@ 
 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
 -----------------------
 
@@ -281,6 +370,31 @@ 
 
 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
 ============
 
diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -574,6 +574,9 @@ 
 coreconfigitem('experimental', 'update.atomic-file',
     default=False,
 )
+coreconfigitem('experimental', 'sshpeer.advertise-v2',
+    default=False,
+)
 coreconfigitem('extensions', '.*',
     default=None,
     generic=True,