dccp: Policy-based packet dequeueing infrastructure

This patch adds a generic infrastructure for policy-based dequeueing of 
TX packets and provides two policies:
 * a simple FIFO policy (which is the default) and
 * a priority based policy (set via socket options).
Both policies honour the tx_qlen sysctl for the maximum size of the write
queue (can be overridden via socket options). 

The priority policy uses skb->priority internally to assign an u32 priority
identifier, using the same ranking as SO_PRIORITY. The skb->priority field
is set to 0 when the packet leaves DCCP. The priority is supplied as ancillary
data using cmsg(3), the patch also provides the requisite parsing routines.

Signed-off-by: Tomasz Grobelny <tomasz@grobelny.oswiecenia.net>
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>

1 files changed, 19 insertions, 0 deletions

diff --git a/Documentation/networking/dccp.txt b/Documentation/networking/dccp.txt
index b132e4a3cf0..fcfc1253442 100644
--- a/Documentation/networking/dccp.txt
+++ b/Documentation/networking/dccp.txt
@@ -45,6 +45,25 @@ http://linux-net.osdl.org/index.php/DCCP_Testing#Experimental_DCCP_source_tree
 
 Socket options
 ==============
+DCCP_SOCKOPT_QPOLICY_ID sets the dequeuing policy for outgoing packets. It takes
+a policy ID as argument and can only be set before the connection (i.e. changes
+during an established connection are not supported). Currently, two policies are
+defined: the "simple" policy (DCCPQ_POLICY_SIMPLE), which does nothing special,
+and a priority-based variant (DCCPQ_POLICY_PRIO). The latter allows to pass an
+u32 priority value as ancillary data to sendmsg(), where higher numbers indicate
+a higher packet priority (similar to SO_PRIORITY). This ancillary data needs to
+be formatted using a cmsg(3) message header filled in as follows:
+	cmsg->cmsg_level = SOL_DCCP;
+	cmsg->cmsg_type	 = DCCP_SCM_PRIORITY;
+	cmsg->cmsg_len	 = CMSG_LEN(sizeof(uint32_t));	/* or CMSG_LEN(4) */
+
+DCCP_SOCKOPT_QPOLICY_TXQLEN sets the maximum length of the output queue. A zero
+value is always interpreted as unbounded queue length. If different from zero,
+the interpretation of this parameter depends on the current dequeuing policy
+(see above): the "simple" policy will enforce a fixed queue size by returning
+EAGAIN, whereas the "prio" policy enforces a fixed queue length by dropping the
+lowest-priority packet first. The default value for this parameter is
+initialised from /proc/sys/net/dccp/default/tx_qlen.
 
 DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of
 service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,