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Diffstat (limited to 'Documentation/networking')
| -rw-r--r-- | Documentation/networking/rds.txt | 55 |
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. |