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@@ -664,20 +664,32 @@ public:
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/**
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* ECHO request (a.k.a. ping):
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- * <[...] arbitrary payload to be echoed back>
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+ * <[1] 8-bit purpose of echo request>
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+ * <[...] additional arbitrary payload>
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*
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* This generates OK with a copy of the transmitted payload. No ERROR
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* is generated. Response to ECHO requests is optional and ECHO may be
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* ignored if a node detects a possible flood.
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*
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- * There is a de-facto standard for ECHO payload. No payload indicates an
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- * ECHO used for path confirmation. Otherwise the first byte contains
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- * flags, in which currently the only flag is 0x01 for a user-requested
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- * echo. For user-requested echoes the result may be reported back through
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- * the API. Otherwise the payload is for internal use.
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+ * An empty payload is permitted. This is used in some versions for
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+ * path checking and validation. If a payload is present it must
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+ * follow the above format, though the recipient does not have to check
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+ * this. It can simply echo it back.
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+ *
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+ * Echo purpose codes:
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+ * 0x00 - User ECHO request
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+ * 0x01 - Dead path detection
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*
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* Support for fragmented echo packets is optional and their use is not
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* recommended.
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+ *
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+ * Dead path detection is performed by sending ECHOs with the same random
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+ * payload to the best (or every) direct path and then once indirectly
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+ * (such as via a root server). When an OK is received echoing back this
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+ * test payload, all paths that have not yet received this OK are cancelled
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+ * or re-tested. This can be done after a short period of inactivity to
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+ * detect and automatically cancel dead paths without requiring any
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+ * special logic (other than support for ECHO) at the remote end.
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*/
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VERB_ECHO = 8,
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Adam Ierymenko