path: root/Documentation/networking/rds.txt

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