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. Work is in progress to support RDS over iWARP, and using DCE to
  guarantee no dropped packets on Ethernet, it may be possible to use RDS over
  UDP in the future.
  
  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
          These constants haven't been assigned yet, because RDS isn't in
          mainline yet. Currently, the kernel module assigns some constant
          and publishes it to user space through two sysctl files
                  /proc/sys/net/rds/pf_rds
                  /proc/sys/net/rds/sol_rds
  
    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.
  
    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.
  
  
  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:
            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