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+=========================
+Tranport Network ID Check
+=========================
+.. meta::
+    :author: zzz
+    :created: 2019-02-28
+    :thread: http://zzz.i2p/topics/1577
+    :lastupdated: 2019-02-28
+    :status: Open
+
+.. contents::
+
+
+Overview
+========
+
+NTCP2 (proposal 111) does not reject connections from different network IDs
+at the Session Request phase.
+The connection must currently be rejected at the Session Confirmed phase,
+when Bob checks Alice's RI.
+
+Similarly, SSU does not reject connections from different network IDs
+at the Session Request phase.
+The connection must currently be rejected after the Session Confirmed phase,
+when Bob checks Alice's RI.
+
+This proposal changes the Session Request phase of both transports to incorporate the
+network ID, in a backwards-compatible way.
+
+
+Motivation
+==========
+
+Connections from the wrong network should be rejected, and the
+peer should be blacklisted, as soon as possible.
+
+
+Design
+======
+
+Ideally we would XOR in the network ID somewhere in the Session Request.
+Since this must be backwards-compatible, we will XOR in (id - 2)
+so it will be a no-op for the current network ID value of 2.
+
+
+Specification
+=============
+
+For NTCP2, XOR (id - 2) into the obfuscated X value in Session Request.
+
+For SSU, replace the XOR of the protocol version (currently 0) with
+an XOR of (id - 2) in the HMAC-MD5 calculation.
+
+
+Notes
+=====
+
+
+Issues
+======
+
+- Should we make a similar change to NTCP 1 as well?
+- Should we make changes to reseeds to prevent reseeding for the wrong network?
+
+
+Migration
+=========
+
+This is backwards-compatible for the current network ID value of 2.
+If any people are running networks (test or otherwise) with a different network ID value,
+this change is backwards-incompatible.
+However, we are not aware of anybody doing this.
+If it's a test network only, it's not an issue, just update all of the routers at once.