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  xfr Path MTU Probe
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  Address family: IPv4 (28 bytes IP+UDP overhead)

  Payload      client → server    server → client
     1472      pass (2/3)         BLOCKED (0/3)
     8972      local MTU limit    untested

  Largest payload client → server: 1472 bytes  (path MTU ≈ 1500)
  Largest payload server → client: none observed
  (server → client is a lower bound, capped by the client → server path)
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