prop289: Add documentation for the circuit FIFO list

Part of #26288

Signed-off-by: David Goulet <dgoulet@torproject.org>

src/core/or/circuit_st.h

@@ -106,9 +106,23 @@ struct circuit_t {
   int deliver_window;
   /** FIFO containing the digest of the cells that are just before a SENDME is
    * sent by the client. It is done at the last cell before our package_window
-   * goes down to 0 which is when we expect a SENDME. The protocol doesn't
-   * allow more than 10 outstanding SENDMEs worth of data meaning this list
-   * should only contain at most 10 digests of 4 bytes each. */
+   * goes down to 0 which is when we expect a SENDME.
+   *
+   * Our current circuit package window is capped to 1000
+   * (CIRCWINDOW_START_MAX) which is also the start value. The increment is
+   * set to 100 (CIRCWINDOW_INCREMENT) which means we don't allow more than
+   * 1000/100 = 10 outstanding SENDME cells worth of data. Meaning that this
+   * list can not contain more than 10 digests of DIGEST_LEN bytes (20).
+   *
+   * At position i in the list, the digest corresponds to the
+   * ((CIRCWINDOW_INCREMENT * i) - 1)-nth cell received since we expect the
+   * (CIRCWINDOW_INCREMENT * i)-nth cell to be the SENDME and thus containing
+   * the previous cell digest.
+   *
+   * For example, position 2 (starting at 0) means that we've received 299
+   * cells and the 299th cell digest is kept at index 2.
+   *
+   * At maximum, this list contains 200 bytes plus the smartlist overhead. */
   smartlist_t *sendme_last_digests;
 
   /** Temporary field used during circuits_handle_oom. */