diff --git a/README.md b/README.md
index 5644dbe..140410e 100644
--- a/README.md
+++ b/README.md
@@ -30,7 +30,7 @@ sessions across the mesh.
 
 - **Self-organizing mesh routing** — spanning tree coordinates and bloom
   filter candidate selection, no global routing tables
-- **Multi-transport** — UDP/IP overlay today; designed for Ethernet,
+- **Multi-transport** — UDP, TCP, and Ethernet today; designed for
   Bluetooth, serial, radio, and Tor
 - **Noise encryption** — hop-by-hop link encryption plus independent
   end-to-end session encryption
@@ -100,7 +100,10 @@ peers:                                 # peers:
 ```
 
 The `nsec` field accepts bech32 (`nsec1...`) or hex-encoded secret keys.
-Omit it entirely for an ephemeral identity that changes each restart.
+Omit `nsec` for an ephemeral identity that changes each restart, or set
+`node.identity.persistent: true` to auto-generate and reuse a stable
+identity via key file (see
+[fips-configuration.md](docs/design/fips-configuration.md#identity-nodeidentity)).
 
 See [docs/design/fips-configuration.md](docs/design/fips-configuration.md) for
 the full configuration reference.
@@ -180,7 +183,7 @@ testing/      Docker-based integration test harnesses
 ## Status & Roadmap
 
 FIPS is at **v0.1.0 (alpha)**. The core protocol works end-to-end over
-UDP/IP overlays but has not been tested beyond small meshes.
+UDP, TCP, and Ethernet but has not been tested beyond small meshes.
 
 ### What works today
 
diff --git a/docs/design/fips-configuration.md b/docs/design/fips-configuration.md
index 9aa7b1c..5b1e6fa 100644
--- a/docs/design/fips-configuration.md
+++ b/docs/design/fips-configuration.md
@@ -70,7 +70,19 @@ handle infrastructure concerns only.
 
 | Parameter | Type | Default | Description |
 |-----------|------|---------|-------------|
-| `node.identity.nsec` | string | *(generate random)* | Hex-encoded secret key. If omitted, an ephemeral identity is generated on each start. |
+| `node.identity.nsec` | string | *(none)* | Secret key in nsec (bech32) or hex format. If omitted, behavior depends on `persistent`. |
+| `node.identity.persistent` | bool | `false` | Persist identity across restarts via key file. |
+
+Identity resolution follows a three-tier priority:
+
+1. **Explicit `nsec`** in config — always used when present, regardless of `persistent`
+2. **Persistent key file** — when `persistent: true` and no `nsec`, loads from `fips.key`
+   adjacent to the config file; if no key file exists, generates a new keypair and saves it
+3. **Ephemeral** — when `persistent: false` (default) and no `nsec`, generates a fresh
+   keypair on each start
+
+Key files (`fips.key` with mode 0600, `fips.pub` with mode 0644) are written adjacent
+to the highest-priority config file for operator visibility, even in ephemeral mode.
 
 ### General
 
@@ -442,7 +454,8 @@ The full YAML structure with all defaults:
 ```yaml
 node:
   identity:
-    nsec: null                       # hex secret key (null = generate ephemeral)
+    nsec: null                       # secret key in nsec or hex (null = depends on persistent)
+    persistent: false                # true = load/save fips.key; false = ephemeral each start
   leaf_only: false
   tick_interval_secs: 1
   base_rtt_ms: 100
diff --git a/docs/design/fips-intro.md b/docs/design/fips-intro.md
index e4ff9a4..b325ff1 100644
--- a/docs/design/fips-intro.md
+++ b/docs/design/fips-intro.md
@@ -739,9 +739,12 @@ for wireless mesh routing and is used in protocols including
 and [Babel](https://www.irif.fr/~jch/software/babel/). FIPS computes ETX
 per-link from MMP loss measurements for future use in candidate ranking.
 
-The CE (Congestion Experienced) echo flag reserves space for
+The CE (Congestion Experienced) echo flag provides hop-by-hop
 [ECN](https://en.wikipedia.org/wiki/Explicit_Congestion_Notification)
-signaling, following the TCP/IP ECN echo pattern (RFC 3168).
+signaling, following the TCP/IP ECN echo pattern (RFC 3168). Transit nodes
+detect congestion via MMP loss/ETX metrics or kernel buffer drops and set
+the CE flag on forwarded frames; destination nodes mark ECN-capable IPv6
+packets accordingly.
 
 ### Cryptographic Primitives