Commit 7c559a9e authored by Gerrit Renker's avatar Gerrit Renker Committed by David S. Miller
Browse files

[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: default avatarGerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: default avatarIan McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: default avatarArnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent bc849872

Documentation/networking/dccp.txt

+3 −0
Original line number Diff line number Diff line
@@ -41,6 +41,9 @@ the socket will fall back to 0 (which means that no meaningful service code 
is present). Connecting sockets set at most one service option; for

listening sockets, multiple service codes can be specified.



DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the current maximum packet

size (application payload size) in bytes, see RFC 4340, section 14.



DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are used for setting the

partial checksum coverage (RFC 4340, sec. 9.2). The default is that checksums

always cover the entire packet and that only fully covered application data is

include/linux/dccp.h

+1 −0
Original line number Diff line number Diff line
@@ -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

net/dccp/proto.c

+4 −0
Original line number Diff line number Diff line
@@ -587,6 +587,10 @@ static int do_dccp_getsockopt(struct sock *sk, int level, int optname, 
	case DCCP_SOCKOPT_SERVICE:

		return dccp_getsockopt_service(sk, len,

					       (__be32 __user *)optval, optlen);

	case DCCP_SOCKOPT_GET_CUR_MPS:

		val = dp->dccps_mss_cache;

		len = sizeof(val);

		break;

	case DCCP_SOCKOPT_SEND_CSCOV:

		val = dp->dccps_pcslen;

		len = sizeof(val);

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