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authorSowmini Varadhan <sowmini.varadhan@oracle.com>2016-07-14 13:51:05 +0300
committerDavid S. Miller <davem@davemloft.net>2016-07-15 21:36:58 +0300
commit09204a6cda4b2189775cb6ee78d7ddd179b0647d (patch)
treea04283f527729514c5d804ec71272057a5c48972 /Documentation/networking
parentd67214a29bc8be3216849215aa4fdca057bd8119 (diff)
downloadlinux-09204a6cda4b2189775cb6ee78d7ddd179b0647d.tar.xz
Documentation: RDS: Document Multipath RDS (mprds)
Document the design of mprds, covering a brief description of the motivation, data-structures and modifications to the RDS control plane. Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com> Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'Documentation/networking')
-rw-r--r--Documentation/networking/rds.txt55
1 files changed, 55 insertions, 0 deletions
diff --git a/Documentation/networking/rds.txt b/Documentation/networking/rds.txt
index 1366e1114d55..0235ae69af2a 100644
--- a/Documentation/networking/rds.txt
+++ b/Documentation/networking/rds.txt
@@ -365,4 +365,59 @@ The recv path
handle CMSGs
return to application
+Multipath RDS (mprds)
+=====================
+ Mprds is multipathed-RDS, primarily intended for RDS-over-TCP
+ (though the concept can be extended to other transports). The classical
+ implementation of RDS-over-TCP is implemented by demultiplexing multiple
+ PF_RDS sockets between any 2 endpoints (where endpoint == [IP address,
+ port]) over a single TCP socket between the 2 IP addresses involved. This
+ has the limitation that it ends up funneling multiple RDS flows over a
+ single TCP flow, thus it is
+ (a) upper-bounded to the single-flow bandwidth,
+ (b) suffers from head-of-line blocking for all the RDS sockets.
+
+ Better throughput (for a fixed small packet size, MTU) can be achieved
+ by having multiple TCP/IP flows per rds/tcp connection, i.e., multipathed
+ RDS (mprds). Each such TCP/IP flow constitutes a path for the rds/tcp
+ connection. RDS sockets will be attached to a path based on some hash
+ (e.g., of local address and RDS port number) and packets for that RDS
+ socket will be sent over the attached path using TCP to segment/reassemble
+ RDS datagrams on that path.
+
+ Multipathed RDS is implemented by splitting the struct rds_connection into
+ a common (to all paths) part, and a per-path struct rds_conn_path. All
+ I/O workqs and reconnect threads are driven from the rds_conn_path.
+ Transports such as TCP that are multipath capable may then set up a
+ TPC socket per rds_conn_path, and this is managed by the transport via
+ the transport privatee cp_transport_data pointer.
+
+ Transports announce themselves as multipath capable by setting the
+ t_mp_capable bit during registration with the rds core module. When the
+ transport is multipath-capable, rds_sendmsg() hashes outgoing traffic
+ across multiple paths. The outgoing hash is computed based on the
+ local address and port that the PF_RDS socket is bound to.
+
+ Additionally, even if the transport is MP capable, we may be
+ peering with some node that does not support mprds, or supports
+ a different number of paths. As a result, the peering nodes need
+ to agree on the number of paths to be used for the connection.
+ This is done by sending out a control packet exchange before the
+ first data packet. The control packet exchange must have completed
+ prior to outgoing hash completion in rds_sendmsg() when the transport
+ is mutlipath capable.
+
+ The control packet is an RDS ping packet (i.e., packet to rds dest
+ port 0) with the ping packet having a rds extension header option of
+ type RDS_EXTHDR_NPATHS, length 2 bytes, and the value is the
+ number of paths supported by the sender. The "probe" ping packet will
+ get sent from some reserved port, RDS_FLAG_PROBE_PORT (in <linux/rds.h>)
+ The receiver of a ping from RDS_FLAG_PROBE_PORT will thus immediately
+ be able to compute the min(sender_paths, rcvr_paths). The pong
+ sent in response to a probe-ping should contain the rcvr's npaths
+ when the rcvr is mprds-capable.
+
+ If the rcvr is not mprds-capable, the exthdr in the ping will be
+ ignored. In this case the pong will not have any exthdrs, so the sender
+ of the probe-ping can default to single-path mprds.