[DCCP]: Add socket option to query the current MPS

This enables applications to query the current value of the Maximum
Packet Size via a socket option, suggested as a SHOULD in (RFC 4340,
p. 102).

This socket option is useful to avoid the annoying bail-out via
`-EMSGSIZE'.  In particular, as fragmentation is not currently
supported (and its use is partly discouraged in RFC 4340).

With this option, it is possible to size buffers accordingly, e.g.

	int buflen = dccp_get_cur_mps(sockfd);

	/* or */
	if (msgsize > dccp_get_cur_mps(sockfd))
		die("message is too large for this path");

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/dccp.h b/include/linux/dccp.h
index 4ed82e2c9f65..0e44a3e5208d 100644
--- a/include/linux/dccp.h
+++ b/include/linux/dccp.h
@@ -202,6 +202,7 @@ struct dccp_so_feat {
 #define DCCP_SOCKOPT_SERVICE            2
 #define DCCP_SOCKOPT_CHANGE_L           3
 #define DCCP_SOCKOPT_CHANGE_R           4
+#define DCCP_SOCKOPT_GET_CUR_MPS        5
 #define DCCP_SOCKOPT_SEND_CSCOV         10
 #define DCCP_SOCKOPT_RECV_CSCOV         11
 #define DCCP_SOCKOPT_CCID_RX_INFO       128