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Fix FIPS_OVERHEAD constant and add CP flag MTU guard

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FIPS_OVERHEAD was 150 but had two bugs: the session AEAD tag (16 bytes)
was listed in the comment but missing from the arithmetic, and the
coordinate budget (~60 bytes) was undersized and didn't belong in a
constant representing fixed data path overhead.

Corrected to 106 bytes (the actual fixed overhead without coordinates).
This increases effective_ipv6_mtu from 1322 to 1366 for standard
Ethernet, well above the IPv6 minimum of 1280.

Added a guard in send_session_data() that computes the total wire size
with coordinates before committing to include them. If adding coords
would exceed the transport MTU, the CP flag is skipped and the warmup
counter is not decremented. This prevents silently producing oversized
packets at tree depth 2+.

Updated design docs (ipv6-adapter, wire-formats, mesh-operation) with
corrected overhead values.
This commit is contained in:
Johnathan Corgan
2026-02-19 14:18:34 +00:00
parent 7df1f21429
commit 999144f59a
7 changed files with 100 additions and 47 deletions
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docs/design/fips-mesh-operation.md
+2 -2
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@@ -515,8 +515,8 @@ routing decisions but retains its own end-to-end encryption and identity.
| LookupResponse | ~400 bytes | Response to discovery | Yes (greedy routed) |
| SessionDatagram + SessionSetup | ~232–402 bytes | Session establishment | Yes (routed) |
| SessionDatagram + SessionAck | ~122 bytes | Session confirmation | Yes (routed) |
| SessionDatagram + Data (minimal) | ~107 bytes + payload | Bulk traffic | Yes (routed) |
| SessionDatagram + Data (with CP) | ~150 bytes + payload | Warmup/recovery | Yes (routed) |
| SessionDatagram + Data (minimal) | 106 bytes + payload | Bulk traffic | Yes (routed) |
| SessionDatagram + Data (with CP) | 106 + coords + payload | Warmup/recovery | Yes (routed) |
| SessionDatagram + CoordsRequired | 70 bytes | Cache miss error | Yes (routed) |
| SessionDatagram + PathBroken | 70+ bytes | Dead-end error | Yes (routed) |
| Disconnect | 2 bytes | Link teardown | No (peer-to-peer) |