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       RDS - Reliable Datagram Sockets


       #include <sys/socket.h>
       #include <netinet/in.h>


       This  is an implementation of the RDS socket API. It provides reliable,
       in-order  datagram  delivery  between  sockets  over   a   variety   of

       Currently,  RDS  can be transported over Infiniband, and loopback.  RDS
       over TCP is disabled, but will be re-enabled in the near future.

       RDS uses standard AF_INET addresses as described in ip(7)  to  identify
       end points.

   Socket Creation
       RDS  is  still  in  development  and  as  such does not have a reserved
       protocol  family  constant.   Applications   must   read   the   string
       representation  of  the  protocol  family  value from the pf_rds sysctl
       parameter file described below.

       rds_socket = socket(pf_rds, SOCK_SEQPACKET, 0);

   Socket Options
       RDS  sockets  support  a  number  of   socket   options   through   the
       setsockopt(2)  and  getsockopt(2)  calls. The following generic options
       (with socket level SOL_SOCKET) are of specific importance:

              Specifies the  size  of  the  receive  buffer.  See  section  on
              "Congestion Control" below.

              Specifies   the   size   of   the   send  buffer.  See  "Message
              Transmission" below.

              Specifies the send timeout when trying to enqueue a message on a
              socket with a full queue in blocking mode.

       In  addition  to  these,  RDS  supports  a  number of protocol specific
       options (with socket level SOL_RDS).  Just as  with  the  RDS  protocol
       family, an official value has not been assigned yet, so the kernel will
       assign a value dynamically.  The assigned value can be  retrieved  from
       the sol_rds sysctl parameter file.

       RDS  specific  socket  options  will be described in a separate section

       A new RDS socket has no local address when it is  first  returned  from
       socket(2).   It  must  be  bound  to a local address by calling bind(2)
       before any messages can be sent or received. This will also attach  the
       socket  to  a  specific  transport,  based on the type of interface the
       local address is attached to.  From that point on, the socket can  only
       reach destinations which are available through this transport.

       For  instance,  when  binding to the address of an Infiniband interface
       such as ib0, the socket will use the Infiniband transport.  If  RDS  is
       not  able  to  associate  a  transport  with the given address, it will
       return EADDRNOTAVAIL.

       An RDS socket can only be bound to one address and only one socket  can
       be  bound  to a given address/port pair. If no port is specified in the
       binding address then an unbound port is selected at random.

       RDS does not allow the application to bind a previously bound socket to
       another  address.  Binding  to  the  wildcard address INADDR_ANY is not
       permitted either.

       The default mode of operation for RDS is to use unconnected socket, and
       specify  a destination address as an argument to sendmsg.  However, RDS
       allows sockets to be connected to a remote end point using  connect(2).
       If  a  socket  is  connected,  calling  sendmsg  without  specifying  a
       destination address will use the previously given remote address.

   Congestion Control
       RDS does not have explicit congestion  control  like  common  streaming
       protocols   such  as  TCP.  However,  sockets  have  two  queue  limits
       associated with them; the send queue size and the receive  queue  size.
       Messages are accounted based on the number of bytes of payload.

       The send queue size limits how much data local processes can queue on a
       local socket (see the following section). If that  limit  is  exceeded,
       the  kernel will not accept further messages until the queue is drained
       and messages have been delivered to  and  acknowledged  by  the  remote

       The receive queue size limits how much data RDS will put on the receive
       queue of a socket before marking  the  socket  as  congested.   When  a
       socket  becomes congested, RDS will send a congestion map update to the
       other participating hosts, who are then expected to stop  sending  more
       messages to this port.

       There  is a timing window during which a remote host can still continue
       to send messages to a congested port;  RDS  solves  this  by  accepting
       these  messages  even if the socket’s receive queue is already over the

       As the application pulls incoming messages off the receive queue  using
       recvmsg(2),  the  number  of bytes on the receive queue will eventually
       drop below the receive queue size, at which  point  the  port  is  then
       marked  uncongested,  and  another  congestion  update  is  sent to all
       participating hosts. This tells them  to  allow  applications  to  send
       additional messages to this port.

       The  default values for the send and receive buffer size are controlled
       by the A given  RDS  socket  has  limited  transmit  buffer  space.  It
       defaults  to  the  system  wide  socket  send  buffer  size  set in the
       wmem_default and rmem_default sysctls, respectively. They can be  tuned
       by  the application through the SO_SNDBUF and SO_RCVBUF socket options.

   Blocking Behavior
       The  sendmsg(2)  and  recvmsg(2)  calls  can  block  in  a  variety  of
       situations.   Whether a call blocks or returns with an error depends on
       the non-blocking setting of the file descriptor  and  the  MSG_DONTWAIT
       message  flag. If the file descriptor is set to blocking mode (which is
       the default), and the MSG_DONTWAIT flag is not  given,  the  call  will

       In addition, the SO_SNDTIMEO and SO_RCVTIMEO socket options can be used
       to specify a timeout (in seconds)  after  which  the  call  will  abort
       waiting, and return an error. The default timeout is 0, which tells RDS
       to block indefinitely.

   Message Transmission
       Messages may be sent using sendmsg(2) once the  RDS  socket  is  bound.
       Message  length  cannot exceed 4 gigabytes as the wire protocol uses an
       unsigned 32 bit integer to express the message length.

       RDS does not support out of band data. Applications are allowed to send
       to unicast addresses only; broadcast or multicast are not supported.

       A  successful sendmsg(2) call puts the message in the socket’s transmit
       queue where it will remain until either  the  destination  acknowledges
       that the message is no longer in the network or the application removes
       the message from the send queue.

       Messages can be removed from the send queue with the RDS_CANCEL_SENT_TO
       socket option described below.

       While  a  message  is  in  the  transmit  queue  its  payload bytes are
       accounted for.  If an attempt is made to send a message while there  is
       not  sufficient  room on the transmit queue, the call will either block
       or return EAGAIN.

       Trying to send to a destination that is marked congested  (see  above),
       the call will either block or return ENOBUFS.

       A  message sent with no payload bytes will not consume any space in the
       destination’s send buffer but will result in a message receipt  on  the
       destination.  The  receiver  will  not get any payload data but will be
       able to see the sender’s address.

       Messages sent to a port to which no socket is bound  will  be  silently
       discarded  by  the  destination host. No error messages are reported to
       the sender.

   Message Receipt
       Messages may be received with recvmsg(2) on an RDS socket  once  it  is
       bound  to  a  source  address.  RDS will return messages in-order, i.e.
       messages from the same sender will arrive in the same  order  in  which
       they were be sent.

       The address of the sender will be returned in the sockaddr_in structure
       pointed to by the msg_name field, if set.

       If the MSG_PEEK flag is given, the first  message  on  the  receive  is
       returned without removing it from the queue.

       The memory consumed by messages waiting for delivery does not limit the
       number of messages that can be queued for receive. RDS does attempt  to
       perform congestion control as described in the section above.

       If the length of the message exceeds the size of the buffer provided to
       recvmsg(2), then  the  remainder  of  the  bytes  in  the  message  are
       discarded and the MSG_TRUNC flag is set in the msg_flags field. In this
       truncating case recvmsg(2)  will  still  return  the  number  of  bytes
       copied,  not  the  length of entire messge.  If MSG_TRUNC is set in the
       flags argument to recvmsg(2), then it will return the number  of  bytes
       in  the  entire  message.  Thus  one  can  examine the size of the next
       message in the receive queue without incurring a  copying  overhead  by
       providing  a  zero  length buffer and setting MSG_PEEK and MSG_TRUNC in
       the flags argument.

       The sending address of a zero-length message will still be provided  in
       the msg_name field.

   Control Messages
       RDS   uses   control  messages  (a.k.a.  ancillary  data)  through  the
       msg_control and msg_controllen fields  in  sendmsg(2)  and  recvmsg(2).
       Control  messages  generated by RDS have a cmsg_level value of sol_rds.
       Most control messages are related to the zerocopy  interface  added  in
       RDS version 3, and are described in rds-rdma(7).

       The  only  exception  is  the  RDS_CMSG_CONG_UPDATE  message,  which is
       described in the following section.

       RDS supports the poll(2) interface in a  limited  fashion.   POLLIN  is
       returned  when  there  is  a message (either a proper RDS message, or a
       control message) waiting in the socket’s  receive  queue.   POLLOUT  is
       always returned while there is room on the socket’s send queue.

       Sending   to   congested  ports  requires  special  handling.  When  an
       application tries to send to a congested destination, the  system  call
       will  return ENOBUFS.  However, it cannot poll for POLLOUT, as there is
       probably still room on the transmit queue, so the call to poll(2) would
       return immediately, even though the destination is still congested.

       There  are  two  ways  of  dealing with this situation. The first is to
       simply poll for POLLIN.  By default, a process sleeping in  poll(2)  is
       always  woken  up  when  the  congestion  map  is updated, and thus the
       application can retry any previously congested sends.

       The second option is explicit congestion monitoring,  which  gives  the
       application more fine-grained control.

       With  explicit  monitoring, the application polls for POLLIN as before,
       and additionally uses the RDS_CONG_MONITOR socket option to  install  a
       64bit  mask  value in the socket, where each bit corresponds to a group
       of ports. When a congestion update arrives, RDS checks the set of ports
       that  became  uncongested against the bit mask installed in the socket.
       If they overlap, a control messages is enqueued on the socket, and  the
       application is woken up. When it calls recvmsg(2), it will be given the
       control message containing the bitmap.  on the socket.

       The  congestion  monitor  bitmask  can  be  set   and   queried   using
       setsockopt(2)  with  RDS_CONG_MONITOR,  and a pointer to the 64bit mask

       Congestion   updates   are   delivered   to   the    application    via
       RDS_CMSG_CONG_UPDATE  control  messages.  These  control  messages  are
       always  delivered  by  themselves  (or  possibly   additional   control
       messages), but never along with a RDS data message. The cmsg_data field
       of the control message is an 8 byte datum  containing  the  64bit  mask

       Applications  can  use the following macros to test for and set bits in
       the bitmask:

       #define RDS_CONG_MONITOR_SIZE   64
       #define RDS_CONG_MONITOR_BIT(port)  (((unsigned int) port) % RDS_CONG_MONITOR_SIZE)
       #define RDS_CONG_MONITOR_MASK(port) (1 << RDS_CONG_MONITOR_BIT(port))

   Canceling Messages
       An application can cancel (flush) messages from the  send  queue  using
       the  RDS_CANCEL_SENT_TO  socket  option  with setsockopt(2).  This call
       takes an optional sockaddr_in address structure as argument. If  given,
       only  messages  to  the  destination  specified  by  this  address  are
       discarded. If no address is given, all pending messages are  discarded.

       Note  that  this affects messages that have not yet been transmitted as
       well  as  messages  that  have  been  transmitted,  but  for  which  no
       acknowledgment from the remote host has been received yet.

       If  sendmsg(2)  succeeds,  RDS  guarantees  that the  message  will  be
       visible  to recvmsg(2) on a socket bound to the destination address  as
       long as that destination socket remains open.

       If  there  is  no  socket  bound on the  destination,  the  message  is
       silently  dropped.   If  the sending RDS can’t be sure that there is no
       socket bound then it will try to send the message indefinitely until it
       can be sure or the sent message is canceled.

       If  a socket is closed then all pending sent messages on the socket are
       canceled and may or may not be seen by the receiver.

       The RDS_CANCEL_SENT_TO socket option can be used to cancel all  pending
       messages to a given destination.

       If  a  receiving socket is closed with pending messages then the sender
       considers  those messages as  having  left  the  network and  will  not
       retransmit them.

       A  message will only be seen by recvmsg(2) once,  unless  MSG_PEEK  was
       specified.  Once  the message has been delivered it is removed from the
       sending socket’s transmit queue.

       All messages sent from the same socket to the same destination will  be
       delivered  in  the  order  they’re  sent.  Messages sent from different
       sockets, or to different destinations, may be delivered in any order.


       These  parameteres  may  only  be  accessed  through  their  files   in
       /proc/sys/net/rds.  Access through sysctl(2) is not supported.

       pf_rds This  file  contains  the  string representation of the protocol
              family constant passed to socket(2) to create a new RDS  socket.

              This file contains the string representation of the socket level
              parameter that is passed to getsockopt(2) and  setsockopt(2)  to
              manipulate RDS socket options.

       max_unacked_bytes and max_unacked_packets
              These   parameters   are   used   to   tune  the  generation  of
              acknowledgements. By default, the system receiving RDS  messages
              does not send back explicit acknowledgements unless it transmits
              a message of its own (in which case the ACK is piggybacked  onto
              the  outgoing  message),  or when the sending system requests an

              However, the sender needs to see an ACK from  time  to  time  so
              that  it can purge old messages from the send queue. The unacked
              bytes and packet counters are used to keep  track  of  how  much
              data  has been sent without requesting an ACK. The default is to
              request an acknowledgement every 16 packets,  or  every  16  MB,
              whichever comes first.

       reconnect_delay_min_ms and reconnect_delay_max_ms
              RDS  uses  host-to-host  connections  to  transport RDS messages
              (both for  the  TCP  and  the  Infiniband  transport).  If  this
              connection  breaks, RDS will try to re-establish the connection.
              Because this reconnect may be triggered by  both  hosts  at  the
              same  time and fail, RDS uses a random backoff before attempting
              a reconnect.  These  two  parameters  specify  the  minimum  and
              maximum  delay  in  milliseconds.  The  default values are 1 and
              1000, respectively.


       rds-rdma(7), socket(2), bind(2), sendmsg(2), recvmsg(2), getsockopt(2),