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 ===
 RDS
 ===
 
 Overview
 ========
 
 This readme tries to provide some background on the hows and whys of RDS,
 and will hopefully help you find your way around the code.
 
 In addition, please see this email about RDS origins:
 http://oss.oracle.com/pipermail/rds-devel/2007-November/000228.html
 
 RDS Architecture
 ================
 
 RDS provides reliable, ordered datagram delivery by using a single
 reliable connection between any two nodes in the cluster. This allows
 applications to use a single socket to talk to any other process in the
 cluster - so in a cluster with N processes you need N sockets, in contrast
 to N*N if you use a connection-oriented socket transport like TCP.
 
 RDS is not Infiniband-specific; it was designed to support different
 transports.  The current implementation used to support RDS over TCP as well
 as IB.
 
 The high-level semantics of RDS from the application's point of view are
 
  *      Addressing
 
         RDS uses IPv4 addresses and 16bit port numbers to identify
         the end point of a connection. All socket operations that involve
         passing addresses between kernel and user space generally
         use a struct sockaddr_in.
 
         The fact that IPv4 addresses are used does not mean the underlying
         transport has to be IP-based. In fact, RDS over IB uses a
         reliable IB connection; the IP address is used exclusively to
         locate the remote node's GID (by ARPing for the given IP).
 
         The port space is entirely independent of UDP, TCP or any other
         protocol.
 
  *      Socket interface
 
         RDS sockets work *mostly* as you would expect from a BSD
         socket. The next section will cover the details. At any rate,
         all I/O is performed through the standard BSD socket API.
         Some additions like zerocopy support are implemented through
         control messages, while other extensions use the getsockopt/
         setsockopt calls.
 
         Sockets must be bound before you can send or receive data.
         This is needed because binding also selects a transport and
         attaches it to the socket. Once bound, the transport assignment
         does not change. RDS will tolerate IPs moving around (eg in
         a active-active HA scenario), but only as long as the address
         doesn't move to a different transport.
 
  *      sysctls
 
         RDS supports a number of sysctls in /proc/sys/net/rds
 
 
 Socket Interface
 ================
 
   AF_RDS, PF_RDS, SOL_RDS
         AF_RDS and PF_RDS are the domain type to be used with socket(2)
         to create RDS sockets. SOL_RDS is the socket-level to be used
         with setsockopt(2) and getsockopt(2) for RDS specific socket
         options.
 
   fd = socket(PF_RDS, SOCK_SEQPACKET, 0);
         This creates a new, unbound RDS socket.
 
   setsockopt(SOL_SOCKET): send and receive buffer size
         RDS honors the send and receive buffer size socket options.
         You are not allowed to queue more than SO_SNDSIZE bytes to
         a socket. A message is queued when sendmsg is called, and
         it leaves the queue when the remote system acknowledges
         its arrival.
 
         The SO_RCVSIZE option controls the maximum receive queue length.
         This is a soft limit rather than a hard limit - RDS will
         continue to accept and queue incoming messages, even if that
         takes the queue length over the limit. However, it will also
         mark the port as "congested" and send a congestion update to
         the source node. The source node is supposed to throttle any
         processes sending to this congested port.
 
   bind(fd, &sockaddr_in, ...)
         This binds the socket to a local IP address and port, and a
         transport, if one has not already been selected via the
         SO_RDS_TRANSPORT socket option
 
   sendmsg(fd, ...)
         Sends a message to the indicated recipient. The kernel will
         transparently establish the underlying reliable connection
         if it isn't up yet.
 
         An attempt to send a message that exceeds SO_SNDSIZE will
         return with -EMSGSIZE
 
         An attempt to send a message that would take the total number
         of queued bytes over the SO_SNDSIZE threshold will return
         EAGAIN.
 
         An attempt to send a message to a destination that is marked
         as "congested" will return ENOBUFS.
 
   recvmsg(fd, ...)
         Receives a message that was queued to this socket. The sockets
         recv queue accounting is adjusted, and if the queue length
         drops below SO_SNDSIZE, the port is marked uncongested, and
         a congestion update is sent to all peers.
 
         Applications can ask the RDS kernel module to receive
         notifications via control messages (for instance, there is a
         notification when a congestion update arrived, or when a RDMA
         operation completes). These notifications are received through
         the msg.msg_control buffer of struct msghdr. The format of the
         messages is described in manpages.
 
   poll(fd)
         RDS supports the poll interface to allow the application
         to implement async I/O.
 
         POLLIN handling is pretty straightforward. When there's an
         incoming message queued to the socket, or a pending notification,
         we signal POLLIN.
 
         POLLOUT is a little harder. Since you can essentially send
         to any destination, RDS will always signal POLLOUT as long as
         there's room on the send queue (ie the number of bytes queued
         is less than the sendbuf size).
 
         However, the kernel will refuse to accept messages to
         a destination marked congested - in this case you will loop
         forever if you rely on poll to tell you what to do.
         This isn't a trivial problem, but applications can deal with
         this - by using congestion notifications, and by checking for
         ENOBUFS errors returned by sendmsg.
 
   setsockopt(SOL_RDS, RDS_CANCEL_SENT_TO, &sockaddr_in)
         This allows the application to discard all messages queued to a
         specific destination on this particular socket.
 
         This allows the application to cancel outstanding messages if
         it detects a timeout. For instance, if it tried to send a message,
         and the remote host is unreachable, RDS will keep trying forever.
         The application may decide it's not worth it, and cancel the
         operation. In this case, it would use RDS_CANCEL_SENT_TO to
         nuke any pending messages.
 
   ``setsockopt(fd, SOL_RDS, SO_RDS_TRANSPORT, (int *)&transport ..), getsockopt(fd, SOL_RDS, SO_RDS_TRANSPORT, (int *)&transport ..)``
         Set or read an integer defining  the underlying
         encapsulating transport to be used for RDS packets on the
         socket. When setting the option, integer argument may be
         one of RDS_TRANS_TCP or RDS_TRANS_IB. When retrieving the
         value, RDS_TRANS_NONE will be returned on an unbound socket.
         This socket option may only be set exactly once on the socket,
         prior to binding it via the bind(2) system call. Attempts to
         set SO_RDS_TRANSPORT on a socket for which the transport has
         been previously attached explicitly (by SO_RDS_TRANSPORT) or
         implicitly (via bind(2)) will return an error of EOPNOTSUPP.
         An attempt to set SO_RDS_TRANSPORT to RDS_TRANS_NONE will
         always return EINVAL.
 
 RDMA for RDS
 ============
 
   see rds-rdma(7) manpage (available in rds-tools)
 
 
 Congestion Notifications
 ========================
 
   see rds(7) manpage
 
 
 RDS Protocol
 ============
 
   Message header
 
     The message header is a 'struct rds_header' (see rds.h):
 
     Fields:
 
       h_sequence:
           per-packet sequence number
       h_ack:
           piggybacked acknowledgment of last packet received
       h_len:
           length of data, not including header
       h_sport:
           source port
       h_dport:
           destination port
       h_flags:
           Can be:
 
           =============  ==================================
           CONG_BITMAP    this is a congestion update bitmap
           ACK_REQUIRED   receiver must ack this packet
           RETRANSMITTED  packet has previously been sent
           =============  ==================================
 
       h_credit:
           indicate to other end of connection that
           it has more credits available (i.e. there is
           more send room)
       h_padding[4]:
           unused, for future use
       h_csum:
           header checksum
       h_exthdr:
           optional data can be passed here. This is currently used for
           passing RDMA-related information.
 
   ACK and retransmit handling
 
       One might think that with reliable IB connections you wouldn't need
       to ack messages that have been received.  The problem is that IB
       hardware generates an ack message before it has DMAed the message
       into memory.  This creates a potential message loss if the HCA is
       disabled for any reason between when it sends the ack and before
       the message is DMAed and processed.  This is only a potential issue
       if another HCA is available for fail-over.
 
       Sending an ack immediately would allow the sender to free the sent
       message from their send queue quickly, but could cause excessive
       traffic to be used for acks. RDS piggybacks acks on sent data
       packets.  Ack-only packets are reduced by only allowing one to be
       in flight at a time, and by the sender only asking for acks when
       its send buffers start to fill up. All retransmissions are also
       acked.
 
   Flow Control
 
       RDS's IB transport uses a credit-based mechanism to verify that
       there is space in the peer's receive buffers for more data. This
       eliminates the need for hardware retries on the connection.
 
   Congestion
 
       Messages waiting in the receive queue on the receiving socket
       are accounted against the sockets SO_RCVBUF option value.  Only
       the payload bytes in the message are accounted for.  If the
       number of bytes queued equals or exceeds rcvbuf then the socket
       is congested.  All sends attempted to this socket's address
       should return block or return -EWOULDBLOCK.
 
       Applications are expected to be reasonably tuned such that this
       situation very rarely occurs.  An application encountering this
       "back-pressure" is considered a bug.
 
       This is implemented by having each node maintain bitmaps which
       indicate which ports on bound addresses are congested.  As the
       bitmap changes it is sent through all the connections which
       terminate in the local address of the bitmap which changed.
 
       The bitmaps are allocated as connections are brought up.  This
       avoids allocation in the interrupt handling path which queues
       sages on sockets.  The dense bitmaps let transports send the
       entire bitmap on any bitmap change reasonably efficiently.  This
       is much easier to implement than some finer-grained
       communication of per-port congestion.  The sender does a very
       inexpensive bit test to test if the port it's about to send to
       is congested or not.
 
 
 RDS Transport Layer
 ===================
 
   As mentioned above, RDS is not IB-specific. Its code is divided
   into a general RDS layer and a transport layer.
 
   The general layer handles the socket API, congestion handling,
   loopback, stats, usermem pinning, and the connection state machine.
 
   The transport layer handles the details of the transport. The IB
   transport, for example, handles all the queue pairs, work requests,
   CM event handlers, and other Infiniband details.
 
 
 RDS Kernel Structures
 =====================
 
   struct rds_message
     aka possibly "rds_outgoing", the generic RDS layer copies data to
     be sent and sets header fields as needed, based on the socket API.
     This is then queued for the individual connection and sent by the
     connection's transport.
 
   struct rds_incoming
     a generic struct referring to incoming data that can be handed from
     the transport to the general code and queued by the general code
     while the socket is awoken. It is then passed back to the transport
     code to handle the actual copy-to-user.
 
   struct rds_socket
     per-socket information
 
   struct rds_connection
     per-connection information
 
   struct rds_transport
     pointers to transport-specific functions
 
   struct rds_statistics
     non-transport-specific statistics
 
   struct rds_cong_map
     wraps the raw congestion bitmap, contains rbnode, waitq, etc.
 
 Connection management
 =====================
 
   Connections may be in UP, DOWN, CONNECTING, DISCONNECTING, and
   ERROR states.
 
   The first time an attempt is made by an RDS socket to send data to
   a node, a connection is allocated and connected. That connection is
   then maintained forever -- if there are transport errors, the
   connection will be dropped and re-established.
 
   Dropping a connection while packets are queued will cause queued or
   partially-sent datagrams to be retransmitted when the connection is
   re-established.
 
 
 The send path
 =============
 
   rds_sendmsg()
     - struct rds_message built from incoming data
     - CMSGs parsed (e.g. RDMA ops)
     - transport connection alloced and connected if not already
     - rds_message placed on send queue
     - send worker awoken
 
   rds_send_worker()
     - calls rds_send_xmit() until queue is empty
 
   rds_send_xmit()
     - transmits congestion map if one is pending
     - may set ACK_REQUIRED
     - calls transport to send either non-RDMA or RDMA message
       (RDMA ops never retransmitted)
 
   rds_ib_xmit()
     - allocs work requests from send ring
     - adds any new send credits available to peer (h_credits)
     - maps the rds_message's sg list
     - piggybacks ack
     - populates work requests
     - post send to connection's queue pair
 
 The recv path
 =============
 
   rds_ib_recv_cq_comp_handler()
     - looks at write completions
     - unmaps recv buffer from device
     - no errors, call rds_ib_process_recv()
     - refill recv ring
 
   rds_ib_process_recv()
     - validate header checksum
     - copy header to rds_ib_incoming struct if start of a new datagram
     - add to ibinc's fraglist
     - if competed datagram:
          - update cong map if datagram was cong update
          - call rds_recv_incoming() otherwise
          - note if ack is required
 
   rds_recv_incoming()
     - drop duplicate packets
     - respond to pings
     - find the sock associated with this datagram
     - add to sock queue
     - wake up sock
     - do some congestion calculations
   rds_recvmsg
     - copy data into user iovec
     - 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
   TCP 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.
 

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