1 files changed, 45 insertions, 7 deletions

diff --git a/Documentation/networking/dccp.txt b/Documentation/networking/dccp.txt
index a62fdf7a6bff..d718bc2ff1cf 100644
--- a/Documentation/networking/dccp.txt
+++ b/Documentation/networking/dccp.txt
@@ -1,18 +1,20 @@
 DCCP protocol
-============
+=============
 
 
 Contents
 ========
-
 - Introduction
 - Missing features
 - Socket options
+- Sysctl variables
+- IOCTLs
+- Other tunables
 - Notes
 
+
 Introduction
 ============
-
 Datagram Congestion Control Protocol (DCCP) is an unreliable, connection
 oriented protocol designed to solve issues present in UDP and TCP, particularly
 for real-time and multimedia (streaming) traffic.
@@ -29,22 +31,41 @@ It has a base protocol and pluggable congestion control IDs (CCIDs).
 DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol
 is at http://www.ietf.org/html.charters/dccp-charter.html
 
+
 Missing features
 ================
-
 The Linux DCCP implementation does not currently support all the features that are
 specified in RFCs 4340...42.
 
 The known bugs are at:
-        http://linux-net.osdl.org/index.php/TODO#DCCP
+        http://www.linuxfoundation.org/collaborate/workgroups/networking/todo#DCCP
 
 For more up-to-date versions of the DCCP implementation, please consider using
 the experimental DCCP test tree; instructions for checking this out are on:
-http://linux-net.osdl.org/index.php/DCCP_Testing#Experimental_DCCP_source_tree
+http://www.linuxfoundation.org/collaborate/workgroups/networking/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,
@@ -112,6 +133,7 @@ DCCP_SOCKOPT_CCID_TX_INFO
 On unidirectional connections it is useful to close the unused half-connection
 via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs.
 
+
 Sysctl variables
 ================
 Several DCCP default parameters can be managed by the following sysctls
@@ -145,6 +167,7 @@ rx_ccid = 2
 seq_window = 100
         The initial sequence window (sec. 7.5.2) of the sender. This influences
         the local ackno validity and the remote seqno validity windows (7.5.1).
+        Values in the range Wmin = 32 (RFC 4340, 7.5.2) up to 2^32-1 can be set.
 
 tx_qlen = 5
         The size of the transmit buffer in packets. A value of 0 corresponds
@@ -155,15 +178,30 @@ sync_ratelimit = 125 ms
         sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit
         of this parameter is milliseconds; a value of 0 disables rate-limiting.
 
+
 IOCTLS
 ======
 FIONREAD
         Works as in udp(7): returns in the `int' argument pointer the size of
         the next pending datagram in bytes, or 0 when no datagram is pending.
 
+
+Other tunables
+==============
+Per-route rto_min support
+        CCID-2 supports the RTAX_RTO_MIN per-route setting for the minimum value
+        of the RTO timer. This setting can be modified via the 'rto_min' option
+        of iproute2; for example:
+                > ip route change 10.0.0.0/24   rto_min 250j dev wlan0
+                > ip route add    10.0.0.254/32 rto_min 800j dev wlan0
+                > ip route show dev wlan0
+        CCID-3 also supports the rto_min setting: it is used to define the lower
+        bound for the expiry of the nofeedback timer. This can be useful on LANs
+        with very low RTTs (e.g., loopback, Gbit ethernet).
+
+
 Notes
 =====
-
 DCCP does not travel through NAT successfully at present on many boxes. This is
 because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT
 support for DCCP has been added.