Add proposal 174 from Ian Goldberg: Optimistic Data for Tor: Server Side

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+Filename: 174-optimistic-data-server.txt
+Title: Optimistic Data for Tor: Server Side
+Author: Ian Goldberg
+Created: 2-Aug-2010
+Status: Open
+
+Overview:
+
+When a SOCKS client opens a TCP connection through Tor (for an HTTP
+request, for example), the query latency is about 1.5x higher than it
+needs to be.  Simply, the problem is that the sequence of data flows
+is this:
+
+1. The SOCKS client opens a TCP connection to the OP
+2. The SOCKS client sends a SOCKS CONNECT command
+3. The OP sends a BEGIN cell to the Exit
+4. The Exit opens a TCP connection to the Server
+5. The Exit returns a CONNECTED cell to the OP
+6. The OP returns a SOCKS CONNECTED notification to the SOCKS client
+7. The SOCKS client sends some data (the GET request, for example)
+8. The OP sends a DATA cell to the Exit
+9. The Exit sends the GET to the server
+10. The Server returns the HTTP result to the Exit
+11. The Exit sends the DATA cells to the OP
+12. The OP returns the HTTP result to the SOCKS client
+
+Note that the Exit node knows that the connection to the Server was
+successful at the end of step 4, but is unable to send the HTTP query to
+the server until step 9.
+
+This proposal (as well as its upcoming sibling concerning the client
+side) aims to reduce the latency by allowing:
+1. SOCKS clients to optimistically send data before they are notified
+    that the SOCKS connection has completed successfully
+2. OPs to optimistically send DATA cells on streams in the CONNECT_WAIT
+    state
+3. Exit nodes to accept and queue DATA cells while in the
+    EXIT_CONN_STATE_CONNECTING state
+
+This particular proposal deals with #3.
+
+In this way, the flow would be as follows:
+
+1. The SOCKS client opens a TCP connection to the OP
+2. The SOCKS client sends a SOCKS CONNECT command, followed immediately
+    by data (such as the GET request)
+3. The OP sends a BEGIN cell to the Exit, followed immediately by DATA
+    cells
+4. The Exit opens a TCP connection to the Server
+5. The Exit returns a CONNECTED cell to the OP, and sends the queued GET
+    request to the Server
+6. The OP returns a SOCKS CONNECTED notification to the SOCKS client,
+    and the Server returns the HTTP result to the Exit
+7. The Exit sends the DATA cells to the OP
+8. The OP returns the HTTP result to the SOCKS client
+
+Motivation:
+
+This change will save one OP<->Exit round trip (down to one from two).
+There are still two SOCKS Client<->OP round trips (negligible time) and
+two Exit<->Server round trips.  Depending on the ratio of the
+Exit<->Server (Internet) RTT to the OP<->Exit (Tor) RTT, this will
+decrease the latency by 25 to 50 percent.  Experiments validate these
+predictions. [Goldberg, PETS 2010 rump session; see
+https://thunk.cs.uwaterloo.ca/optimistic-data-pets2010-rump.pdf ]
+
+Design:
+
+The current code actually correctly handles queued data at the Exit; if
+there is queued data in a EXIT_CONN_STATE_CONNECTING stream, that data
+will be immediately sent when the connection succeeds.  If the
+connection fails, the data will be correctly ignored and freed.  The
+problem with the current server code is that the server currently
+drops DATA cells on streams in the EXIT_CONN_STATE_CONNECTING state.
+Also, if you try to queue data in the EXIT_CONN_STATE_RESOLVING state,
+bad things happen because streams in that state don't yet have
+conn->write_event set, and so some existing sanity checks (any stream
+with queued data is at least potentially writable) are no longer sound.
+
+The solution is to simply not drop received DATA cells while in the
+EXIT_CONN_STATE_CONNECTING state.  Also do not send SENDME cells in this
+state, so that the OP cannot send more than one window's worth of data
+to be queued at the Exit.  Finally, patch the sanity checks so that
+streams in the EXIT_CONN_STATE_RESOLVING state that have buffered data
+can pass.
+
+If no clients ever send such optimistic data, the new code will never be
+executed, and the behaviour of Tor will not change.  When clients begin
+to send optimistic data, the performance of those clients' streams will
+improve.
+
+After discussion with nickm, it seems best to just have the server
+version number be the indicator of whether a particular Exit supports
+optimistic data.  (If a client sends optimistic data to an Exit which
+does not support it, the data will be dropped, and the client's request
+will fail to complete.)  What do version numbers for hypothetical future
+protocol-compatible implementations look like, though?
+
+Security implications:
+
+Servers (for sure the Exit, and possibly others, by watching the
+pattern of packets) will be able to tell that a particular client
+is using optimistic data.  This will be discussed more in the sibling
+proposal.
+
+On the Exit side, servers will be queueing a little bit extra data, but
+no more than one window.  Clients today can cause Exits to queue that
+much data anyway, simply by establishing a Tor connection to a slow
+machine, and sending one window of data.
+
+Specification:
+
+tor-spec section 6.2 currently says:
+
+    The OP waits for a RELAY_CONNECTED cell before sending any data.
+    Once a connection has been established, the OP and exit node
+    package stream data in RELAY_DATA cells, and upon receiving such
+    cells, echo their contents to the corresponding TCP stream.
+    RELAY_DATA cells sent to unrecognized streams are dropped.
+
+It is not clear exactly what an "unrecognized" stream is, but this last
+sentence would be changed to say that RELAY_DATA cells received on a
+stream that has processed a RELAY_BEGIN cell and has not yet issued a
+RELAY_END or a RELAY_CONNECTED cell are queued; that queue is processed
+immediately after a RELAY_CONNECTED cell is issued for the stream, or
+freed after a RELAY_END cell is issued for the stream.
+
+The earlier part of this section will be addressed in the sibling
+proposal.
+
+Compatibility:
+
+There are compatibility issues, as mentioned above.  OPs MUST NOT send
+optimistic data to Exit nodes whose version numbers predate (something).
+OPs MAY send optimistic data to Exit nodes whose version numbers match
+or follow that value.  (But see the question about independent server
+reimplementations, above.)
+
+Implementation:
+
+Here is a simple patch.  It seems to work with both regular streams and
+hidden services, but there may be other corner cases I'm not aware of.
+(Do streams used for directory fetches, hidden services, etc. take a
+different code path?)
+
+diff --git a/src/or/connection.c b/src/or/connection.c
+index 7b1493b..f80cd6e 100644
+--- a/src/or/connection.c
++++ b/src/or/connection.c
+@@ -2845,7 +2845,13 @@ _connection_write_to_buf_impl(const char *string, size_t len,
+     return;
+   }
+ 
+-  connection_start_writing(conn);
++  /* If we receive optimistic data in the EXIT_CONN_STATE_RESOLVING
++   * state, we don't want to try to write it right away, since
++   * conn->write_event won't be set yet.  Otherwise, write data from
++   * this conn as the socket is available. */
++  if (conn->state != EXIT_CONN_STATE_RESOLVING) {
++      connection_start_writing(conn);
++  }
+   if (zlib) {
+     conn->outbuf_flushlen += buf_datalen(conn->outbuf) - old_datalen;
+   } else {
+@@ -3382,7 +3388,11 @@ assert_connection_ok(connection_t *conn, time_t now)
+     tor_assert(conn->s < 0);
+ 
+   if (conn->outbuf_flushlen > 0) {
+-    tor_assert(connection_is_writing(conn) || conn->write_blocked_on_bw ||
++    /* With optimistic data, we may have queued data in
++     * EXIT_CONN_STATE_RESOLVING while the conn is not yet marked to writing.
++     * */
++    tor_assert(conn->state == EXIT_CONN_STATE_RESOLVING ||
++	    connection_is_writing(conn) || conn->write_blocked_on_bw ||
+             (CONN_IS_EDGE(conn) && TO_EDGE_CONN(conn)->edge_blocked_on_circ));
+   }
+ 
+diff --git a/src/or/relay.c b/src/or/relay.c
+index fab2d88..e45ff70 100644
+--- a/src/or/relay.c
++++ b/src/or/relay.c
+@@ -1019,6 +1019,9 @@ connection_edge_process_relay_cell(cell_t *cell, circuit_t *circ,
+   relay_header_t rh;
+   unsigned domain = layer_hint?LD_APP:LD_EXIT;
+   int reason;
++  int optimistic_data = 0;  /* Set to 1 if we receive data on a stream
++			       that's in the EXIT_CONN_STATE_RESOLVING
++			       or EXIT_CONN_STATE_CONNECTING states.*/
+ 
+   tor_assert(cell);
+   tor_assert(circ);
+@@ -1038,9 +1041,20 @@ connection_edge_process_relay_cell(cell_t *cell, circuit_t *circ,
+   /* either conn is NULL, in which case we've got a control cell, or else
+    * conn points to the recognized stream. */
+ 
+-  if (conn && !connection_state_is_open(TO_CONN(conn)))
+-    return connection_edge_process_relay_cell_not_open(
+-             &rh, cell, circ, conn, layer_hint);
++  if (conn && !connection_state_is_open(TO_CONN(conn))) {
++    if ((conn->_base.state == EXIT_CONN_STATE_CONNECTING ||
++	    conn->_base.state == EXIT_CONN_STATE_RESOLVING) &&
++	rh.command == RELAY_COMMAND_DATA) {
++	/* We're going to allow DATA cells to be delivered to an exit
++	 * node in state EXIT_CONN_STATE_CONNECTING or
++	 * EXIT_CONN_STATE_RESOLVING.  This speeds up HTTP, for example. */
++	log_warn(domain, "Optimistic data received.");
++	optimistic_data = 1;
++    } else {
++	return connection_edge_process_relay_cell_not_open(
++		 &rh, cell, circ, conn, layer_hint);
++    }
++  }
+ 
+   switch (rh.command) {
+     case RELAY_COMMAND_DROP:
+@@ -1090,7 +1104,9 @@ connection_edge_process_relay_cell(cell_t *cell, circuit_t *circ,
+       log_debug(domain,"circ deliver_window now %d.", layer_hint ?
+                 layer_hint->deliver_window : circ->deliver_window);
+ 
+-      circuit_consider_sending_sendme(circ, layer_hint);
++      if (!optimistic_data) {
++	  circuit_consider_sending_sendme(circ, layer_hint);
++      }
+ 
+       if (!conn) {
+         log_info(domain,"data cell dropped, unknown stream (streamid %d).",
+@@ -1107,7 +1123,9 @@ connection_edge_process_relay_cell(cell_t *cell, circuit_t *circ,
+       stats_n_data_bytes_received += rh.length;
+       connection_write_to_buf(cell->payload + RELAY_HEADER_SIZE,
+                               rh.length, TO_CONN(conn));
+-      connection_edge_consider_sending_sendme(conn);
++      if (!optimistic_data) {
++	  connection_edge_consider_sending_sendme(conn);
++      }
+       return 0;
+     case RELAY_COMMAND_END:
+       reason = rh.length > 0 ?
+
+Performance and scalability notes:
+
+There may be more RAM used at Exit nodes, as mentioned above, but it is
+transient.