From 65faf8dff03b7ed638341b6105b26addae28d641 Mon Sep 17 00:00:00 2001
Message-Id: <65faf8dff03b7ed638341b6105b26addae28d641.1714771621.git.mdw@distorted.org.uk>
From: Mark Wooding <mdw@chiark.greenend.org.uk>
Date: Wed, 24 Dec 2008 23:29:15 +0000
Subject: [PATCH 1/1] tripe-keys: Add a subcommand to print the correct tunnel
 MTU.
Organization: Straylight/Edgeware

From: Mark Wooding <mdw@distorted.org.uk>

This probably isn't the right place for it, but it was easy.  The right
thing to do is to add path-MTU discovery to the server, but that will
get really messy.
---
 keys/tripe-keys.8.in | 14 ++++++++++++++
 keys/tripe-keys.in   | 25 +++++++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/keys/tripe-keys.8.in b/keys/tripe-keys.8.in
index eb2604b8..ce3c1ec1 100644
--- a/keys/tripe-keys.8.in
+++ b/keys/tripe-keys.8.in
@@ -55,6 +55,8 @@ tripe-keys \- simple centralized key management for tripe
 .B "rebuild"
 .br
 .B "clean"
+.br
+.BR "mtu " [ \fIpath-mtu ]
 .
 .\"--------------------------------------------------------------------------
 .SH "DESCRIPTION"
@@ -215,6 +217,18 @@ might have written to a directory.  In particular, it deletes
 and their associated
 .B .old
 files.
+.TP
+.BR "mtu " [ \fIpath-mtu ]
+Write, as a decimal number on standard output, the recommended MTU for a
+TrIPE tunnel interface, given that the
+.I path-mtu
+between two peers is as specified.  The default is 1500, which is very
+commonly correct, but you should check using a tool such as
+.BR tracepath (8).
+Getting the MTU too big will lead to unnecessary fragmentation of
+TrIPE's UDP datagrams; getting it too small will fail to utilize the
+underlying network effectively.  If in doubt, it's therefore better to
+underestimate.
 .
 .\"--------------------------------------------------------------------------
 .SH "SEE ALSO"
diff --git a/keys/tripe-keys.in b/keys/tripe-keys.in
index b195342c..b0dbf945 100644
--- a/keys/tripe-keys.in
+++ b/keys/tripe-keys.in
@@ -473,6 +473,30 @@ def cmd_clean(args):
         r == 'keyring' or r == 'keyring.pub' or r.startswith('peer-')):
       zap(i)
 
+###--------------------------------------------------------------------------
+### Commands: mtu
+
+def cmd_mtu(args):
+  mtu, = (lambda mtu = '1500': (mtu,))(*args)
+  mtu = int(mtu)
+
+  blksz = C.gcciphers[conf['cipher']].blksz
+
+  index = conf['mac'].find('/')
+  if index == -1:
+    tagsz = C.gcmacs[conf['mac']].tagsz
+  else:
+    tagsz = int(conf['mac'][index + 1:])/8
+
+  mtu -= 20                             # Minimum IP header
+  mtu -= 8                              # UDP header
+  mtu -= 1                              # TrIPE packet type octet
+  mtu -= tagsz                          # MAC tag
+  mtu -= 4                              # Sequence number
+  mtu -= blksz                          # Initialization vector
+
+  print mtu
+
 ###--------------------------------------------------------------------------
 ### Main driver.
 
@@ -485,6 +509,7 @@ commands = {'help': (cmd_help, 0, 1, ''),
             'upload': (cmd_upload, 0, 0, ''),
             'update': (cmd_update, 0, 0, ''),
             'clean': (cmd_clean, 0, 0, ''),
+            'mtu': (cmd_mtu, 0, 1, '[PATH-MTU]'),
             'generate': (cmd_generate, 1, 1, 'TAG'),
             'rebuild': (cmd_rebuild, 0, 0, '')}
 
-- 
[mdw]