Describe the correct default for UDP/RAW path MTU discovery.

diff --git a/man7/raw.7 b/man7/raw.7
index 222266288202aa4c2714212ad1051e3efcc96a1a..60ffc70783661bec3cb7e13f4aa7be0a20c74bfd 100644 (file)
--- a/man7/raw.7
+++ b/man7/raw.7
@@ -127,14 +127,23 @@ In addition all
 socket options valid for datagram sockets are supported.
 
 .SH NOTES
-Raw sockets fragment a packet when its total length exceeds the 
-interface MTU
-(but see BUGS).
-A more network friendly and faster alternative is to implement path MTU 
-discovery as described in the
-.B IP_MTU_DISCOVER 
-section of
-.BR ip (7).
+By default raw sockets do path MTU (Maximum Transmission Unit) discovery. 
+This means the kernel
+will keep track of the MTU to a specific target IP address and return
+.I EMSGSIZE
+when a raw packet write exceeds it. 
+When this happens the application should decrease the packet size. 
+Path MTU discovery can be also turned off using the
+.B IP_MTU_DISCOVER
+socket option or the 
+.B ip_no_pmtu_disc
+sysctl, see 
+.BR ip(7) 
+for details. 
+When turned off raw sockets will fragment outgoing packets
+that exceed the interface MTU. 
+However disabling it is not recommended
+for performance and reliability reasons.
 
 A raw socket can be bound to a specific local address using the 
 .BR bind (2)

diff --git a/man7/udp.7 b/man7/udp.7
index b55b5ba7feeaeed2038720426172081674346308..9268d86e6628152239f6414f40bc32775c5a8346 100644 (file)
--- a/man7/udp.7
+++ b/man7/udp.7
@@ -69,13 +69,23 @@ When the
 flag is set on sending the destination address must refer to an local 
 interface address and the packet is only sent to that interface.  
 
-UDP fragments a packet when its total length exceeds the interface MTU
-(Maximum Transmission Unit).
-A more network friendly alternative is to use path MTU discovery
-as described in the
-.B IP_MTU_DISCOVER 
-section of
-.BR ip (7).
+By default Linux UDP does path MTU (Maximum Transmission Unit) discovery. 
+This means the kernel
+will keep track of the MTU to a specific target IP address and return
+.I EMSGSIZE
+when a UDP packet write exceeds it. 
+When this happens the application should decrease the packet size. 
+Path MTU discovery can be also turned off using the
+.B IP_MTU_DISCOVER
+socket option or the 
+.B ip_no_pmtu_disc
+sysctl, see 
+.BR ip(7) 
+for details. 
+When turned off UDP will fragment outgoing UDP packets
+that exceed the interface MTU. 
+However disabling it is not recommended
+for performance and reliability reasons.
 
 .SH "ADDRESS FORMAT"
 UDP uses the IPv4