[tor-commits] [torspec/master] Document current circid selection behavior

nickm at torproject.org nickm at torproject.org
Thu May 13 17:12:01 UTC 2021

commit e24e9cb0c4f9bfbb7907549b064a7efc50568581
Author: Nick Mathewson <nickm at torproject.org>
Date:   Thu May 13 13:11:56 2021 -0400

    Document current circid selection behavior
 tor-spec.txt | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/tor-spec.txt b/tor-spec.txt
index 62b7d5d..d044542 100644
--- a/tor-spec.txt
+++ b/tor-spec.txt
@@ -1000,6 +1000,12 @@ see tor-design.pdf.
    belong to any circuit: CircID 0 must not be used for circuits.  No
    other CircID value, including 0x8000 or 0x80000000, is reserved.
+   Existing Tor implementations choose their CircID values at random from
+   among the available unused values.  To avoid distinguishability, new
+   implementations should do the same. Implementations MAY give up and stop
+   attempting to build new circuits on a channel, if a certain number of
+   randomly chosen CircID values are all in use (today's Tor stops after 64).
 5.1.2. EXTEND and EXTENDED cells
    To extend an existing circuit, the client sends an EXTEND or EXTEND2

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