Docs: CG 2.4.6 Add warning about MTU in domestic router

diff --git a/Documentation/contributor/source-code.itexi b/Documentation/contributor/source-code.itexi
index b27228d2f34abb6f1409ca3335741fdf90ba9476..27e30b165a60b326cf7bc87af252181f42484802 100644 (file)
--- a/Documentation/contributor/source-code.itexi
+++ b/Documentation/contributor/source-code.itexi
@@ -1452,6 +1452,31 @@ should add these lines to @file{.git/config}:
 @end example
 @end itemize
 
+@knownissues
+Encryption protocols, including ssh, generally do not permit packet
+fragmentation to avoid introducing a point of insecurity.  This
+means that the maximum packet size must not exceed the smallest
+MTU (Maximum Transmission Unit) set in the routers along the path.
+This smallest MTU is determined by a procedure during call set-up
+which relies on the transmission over the path of ICMP packets.
+If any of the routers in the path block ICMP packets this mechanism
+fails, resulting in the possibility of packets being transmitted
+which exceed the MTU of one of the routers.  If this happens the
+packet is discarded, causing the ssh session to hang, timeout or
+terminate with the error message
+
+@example
+ssh: connect to host <host ip addr> port 22: Bad file number
+fatal: The remote end hung up unexpectedly
+@end example
+
+depending on precisely when in the proceedings the first large
+packet is transmitted.  Most routers on the internet have MTU
+set to 1500, but routers installed in homes to connect via
+broadband may use a slightly smaller MTU for efficient transmission
+over ATM.  If this problem is encountered a possible work-around is
+to set the MTU in the local router to 1500.
+
 @node Git on Windows
 @section Git on Windows