#
#
# patch "constants.hh"
#  from [adc7ef399cc939ee0b1ec47b97510f7ebdcaf2fd]
#    to [66134f17373632d6b83ae6cd293b319ed2219c8c]
# 
# patch "netcmd.cc"
#  from [303dbc50dc5e0ab9e0eb381d1faad7fbab1c592c]
#    to [db8fdc942bab5c77a2335b0663baf8df116f62e0]
# 
# patch "netsync.cc"
#  from [11a86d0793f450923d9f6e3eaedf696e4d6bfe20]
#    to [2eeffe750a843e30a2c13e4621f6f62f0ebb0096]
#
============================================================
--- constants.hh	adc7ef399cc939ee0b1ec47b97510f7ebdcaf2fd
+++ constants.hh	66134f17373632d6b83ae6cd293b319ed2219c8c
@@ -130,6 +130,9 @@ namespace constants
   // the current netcmd/netsync protocol version
   u8 const netcmd_current_protocol_version = 7;
 
+  // the oldest netcmd/netsync protocol version that we're compatible with
+  u8 const netcmd_minimum_protocol_version = 6;
+
   // minimum size of any netcmd on the wire
   std::size_t const netcmd_minsz = (1     // version
                                     + 1   // cmd code
============================================================
--- netcmd.cc	303dbc50dc5e0ab9e0eb381d1faad7fbab1c592c
+++ netcmd.cc	db8fdc942bab5c77a2335b0663baf8df116f62e0
@@ -46,7 +46,7 @@ read_netcmd_item_type(string const & in,
     }
 }
 
-netcmd::netcmd() : version(constants::netcmd_current_protocol_version),
+netcmd::netcmd() : version(0),
                    cmd_code(error_cmd)
 {}
 
@@ -112,12 +112,15 @@ netcmd::read(string_queue & inbuf, chain
     default:
       // if the versions don't match, we will throw the more descriptive
       // error immediately after this switch.
-      if (extracted_ver == version)
+      if (extracted_ver <= constants::netcmd_current_protocol_version &&
+          extracted_ver >= constants::netcmd_minimum_protocol_version)
         throw bad_decode(F("unknown netcmd code 0x%x")
                           % widen<u32,u8>(cmd_byte));
     }
   // Ignore the version on usher_cmd packets.
-  if (extracted_ver != version && cmd_code != usher_cmd)
+  if (extracted_ver <= constants::netcmd_current_protocol_version &&
+      extracted_ver >= constants::netcmd_minimum_protocol_version &&
+      cmd_code != usher_cmd)
     throw bad_decode(F("protocol version mismatch: wanted '%d' got '%d'\n"
                        "%s")
                      % widen<u32,u8>(version)
============================================================
--- netsync.cc	11a86d0793f450923d9f6e3eaedf696e4d6bfe20
+++ netsync.cc	2eeffe750a843e30a2c13e4621f6f62f0ebb0096
@@ -156,7 +156,31 @@
 // drop the TCP stream at any point, if too much data is received or too
 // much idle time passes; no commitments or transactions are made.
 //
+// Version Negotiation
+// -------------------
 //
+// Before the exchange begin, the client may receive one or more
+// "usher <message>" packets, any number sent by "usher" proxies and
+// one sent by more recent servers. It ignores the protocol version
+// field on these packets, but replys with a "usher_reply <host> <include>"
+// packet containing its own maximum supported protocol version.
+//
+// Older server begin by sending a "hello" packet (see below) that contains
+// their only supported protocol version. New servers first send a "usher"
+// packet and use the response to determine the client's maximum protocol
+// version, and then use the lesser of that or their own maximum version
+// in the "hello" packet and all later packets.
+//
+// When the client receive the "hello" packet it uses that version as the
+// protocol version for all remaining packets.
+//
+// If the "usher_reply" packet indicates a version that's older than the
+// minimum version supported by the server, the server sends an error packet
+// and closes the connection.
+//
+// If the "hello" packet indicates a version not supported by the client,
+// it sends an error packet and closes the connection.
+//
 // Authentication and setup
 // ------------------------
 //