diff --git a/tools/ping/README.md b/tools/ping/README.md
index d5b536d..8c0c513 100644
--- a/tools/ping/README.md
+++ b/tools/ping/README.md
@@ -35,4 +35,53 @@
 
     ndnpingserver -p 4 ndn:/edu/arizona
 
-A list of the available options can be found with `man ndnpingserver`.
\ No newline at end of file
+A list of the available options can be found with `man ndnpingserver`.
+
+## ndnping Protocol
+
+This section briefly describes ndnping's protocol, in order to allow alternate implementations
+to be compatible with this implementation.
+The current protocol version is **ndnping protocol version 1**.
+This version number will be incremented in case there's an incompatible change.
+
+### Probe Interests
+
+The client expresses probe Interests that are intended to be forwarded to the server
+without being aggregated.
+The Interests MUST have one of the following name structures:
+
+    ndn:/<prefix>/ping/<seq>
+    ndn:/<prefix>/ping/<client-identifier>/<seq>
+
+where:
+
+* `<prefix>` is the server prefix that contains zero or more NameComponents.
+* `<seq>` is one NameComponent whose value is an unsigned 64-bit integer, represented as
+  a decimal number in ASCII encoding.
+* `<client-identifier>` is one NameComponent with arbitrary value.
+
+If a client expresses multiple probe Interests, it is RECOMMENDED for those Interests to
+have consecutive increasing integers in `<seq>` field.
+
+If a client uses the name structure with `<client-identifier>` field, it is RECOMMENDED to use
+one or more printable characters only, to make it easier for a human operator to read the logs.
+A client implementation MAY restrict this field to be non-empty and have printable characters only.
+
+The probe Interest SHOULD carry MustBeFresh selector by default.
+A client implementation MAY allow the operator to turn off MustBeFresh selector.
+
+### Reply Data
+
+The server SHOULD register the prefix `ndn:/<prefix>/ping`, and MUST reply to probe Interests
+under this prefix with Data.
+The Data MUST have the same name as the probe Interest.
+
+The Data MUST have BLOB as its ContentType.
+The Data MAY have arbitrary payload in its Content field.
+A server implementation MAY allow the operator to configure the length and contents of the payload.
+
+The Data SHOULD have a valid signature.
+The server SHOULD use DigestSha256 signature by default.
+A server implementation MAY allow the operator to configure a different signature algorithm.
+The client SHOULD NOT verify the signature by default.
+A client implementation MAY allow the operator to enable signature verification.