new proposal 155

i2p2www/spec/proposals/155-streaming-mtu-ecies.rst

@@ -0,0 +1,141 @@
+========================================
+Streaming MTU for ECIES Destinations
+========================================
+.. meta::
+    :author: zzz
+    :created: 2020-05-06
+    :thread: http://zzz.i2p/topics/2886
+    :lastupdated: 2020-05-06
+    :status: Open
+    :target: 0.9.47
+
+.. contents::
+
+
+
+Overview
+========
+
+
+Summary
+-------
+
+ECIES reduces exisiting session (ES) message overhead by about 90 bytes.
+Therefore we can increase the MTU by about 90 bytes for ECIES connections.
+See [ECIES]_, [STREAMING-SPEC]_, and [STREAMING-OPTIONS]_.
+
+Without increasing the MTU, in many cases the overhead savings aren't really 'saved',
+as the messages will be padded out to use two full tunnel messages anyway.
+
+This proposal does not require any change to the specifications.
+It is posted as a proposal solely to facilitate discussion and consensus-building
+of the recommended value and of the implementation details.
+
+
+Goals
+-----
+
+- Increase negotiated MTU
+- Maximize usage of 1 KB tunnel messages
+- Do not change streaming protocol
+
+
+Design
+======
+
+Use the existing MAX_PACKET_SIZE_INCLUDED option and MTU negotiation.
+Streaming continues to use the minimum of the sent and received MTU.
+The default remains 1730 for all connections, no matter what keys are used.
+
+Implementations are encouraged to include the option in all SYN packets, in both directions,
+although this is not a requirement.
+
+If a destination is ECIES-only, send the higher value (either as Alice or Bob).
+If a destination is dual-key, behavior may vary:
+
+If dual-key client is outside the router (in an external application),
+it may not "know" the key being used at the far-end, and Alice must
+send the standard value of 1730.
+
+If dual-key client is inside the router, the information of what key
+is being used may or may not be known to the client.
+The leaseset may not have been fetched yet, or the internal API interfaces
+may not easily make that information available to the client.
+If the information is available, Alice may send the higher value;
+otherwise, Alice must send the standard value of 1730.
+
+A dual-key client as Bob may send the higher value in response,
+even if no value or a value of 1730 was received from Alice;
+however, there is no provision for negotiating upwards in streaming,
+so the MTU should remain at 1730.
+
+
+
+Specification
+=============
+
+ECIES and dual-key ECIES destinations may send an MTU of up to 1820.
+The default remains 1730 for all connections, no matter what keys are used.
+Clients must use the minimum of the sent and received MTU, as usual.
+
+This will be added as a note to [STREAMING-OPTIONS]_. 
+No change to [STREAMING-SPEC]_.
+
+
+
+Justification
+=============
+
+See [CALCULATION]_ for why the current value is 1730.
+See [ECIES]_ for why the ECIES overhead is 90 bytes less than ElGamal.
+
+
+
+Notes
+=====
+
+As noted in [STREAMING-OPTIONS],
+the data in the SYN packets sent from Alice to Bob may exceed Bob's MTU.
+This is a weakness in the streaming protocol.
+
+It may be advisable, in dual-key clients, to limit the data in the sent SYN packets
+to 1730 bytes, while sending an MTU option of 1820.
+Once an 1820 MTU is received from Bob, Alice may increase the actual maximum
+payload sent.
+
+
+
+Issues
+======
+
+The 1820 value is preliminary. To be confirmed and possibly adjusted.
+
+
+
+
+Migration
+=========
+
+No backward compatibility issues.
+This is an existing option and MTU negotiation is already part of the specification.
+
+Older ECIES destinations will support 1730.
+Any client receiving a higher value will respond with 1730, and the far-end
+will negotiate downward, as usual.
+
+
+
+References
+==========
+
+.. [ECIES]
+   {{ proposal_url('144') }}#overhead
+
+.. [CALCULATION]
+   https://github.com/i2p/i2p.i2p/blob/master/apps/streaming/java/src/net/i2p/client/streaming/impl/ConnectionOptions.java#L220
+
+.. [STREAMING-OPTIONS]
+    {{ site_url('docs/api/streaming', True) }}
+
+.. [STREAMING-SPEC]
+    {{ spec_url('streaming') }}#flags-and-option-data-fields