From: Michael Kerrisk <mtk.manpages@gmail.com>
Date: Sat, 25 Nov 2006 19:17:22 +0000 (+0000)
Subject: Describe the correct default for UDP/RAW path MTU discovery.
X-Git-Tag: man-pages-2.43~12
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=edd260bc5dd1c6cd4edf5b65b58ffdcb97b29ca1;p=thirdparty%2Fman-pages.git

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

diff --git a/man7/raw.7 b/man7/raw.7
index 2222662882..60ffc70783 100644
--- 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 b55b5ba7fe..9268d86e66 100644
--- 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