Documentation/networking/rds.txt

   1
   2 Overview
   3 ========
   4
   5 This readme tries to provide some background on the hows and whys of RDS,
   6 and will hopefully help you find your way around the code.
   7
   8 In addition, please see this email about RDS origins:
   9 http://oss.oracle.com/pipermail/rds-devel/2007-November/000228.html
  10
  11 RDS Architecture
  12 ================
  13
  14 RDS provides reliable, ordered datagram delivery by using a single
  15 reliable connection between any two nodes in the cluster. This allows
  16 applications to use a single socket to talk to any other process in the
  17 cluster - so in a cluster with N processes you need N sockets, in contrast
  18 to N*N if you use a connection-oriented socket transport like TCP.
  19
  20 RDS is not Infiniband-specific; it was designed to support different
  21 transports.  The current implementation used to support RDS over TCP as well
  22 as IB. Work is in progress to support RDS over iWARP, and using DCE to
  23 guarantee no dropped packets on Ethernet, it may be possible to use RDS over
  24 UDP in the future.
  25
  26 The high-level semantics of RDS from the application's point of view are
  27
  28  *      Addressing
  29         RDS uses IPv4 addresses and 16bit port numbers to identify
  30         the end point of a connection. All socket operations that involve
  31         passing addresses between kernel and user space generally
  32         use a struct sockaddr_in.
  33
  34         The fact that IPv4 addresses are used does not mean the underlying
  35         transport has to be IP-based. In fact, RDS over IB uses a
  36         reliable IB connection; the IP address is used exclusively to
  37         locate the remote node's GID (by ARPing for the given IP).
  38
  39         The port space is entirely independent of UDP, TCP or any other
  40         protocol.
  41
  42  *      Socket interface
  43         RDS sockets work *mostly* as you would expect from a BSD
  44         socket. The next section will cover the details. At any rate,
  45         all I/O is performed through the standard BSD socket API.
  46         Some additions like zerocopy support are implemented through
  47         control messages, while other extensions use the getsockopt/
  48         setsockopt calls.
  49
  50         Sockets must be bound before you can send or receive data.
  51         This is needed because binding also selects a transport and
  52         attaches it to the socket. Once bound, the transport assignment
  53         does not change. RDS will tolerate IPs moving around (eg in
  54         a active-active HA scenario), but only as long as the address
  55         doesn't move to a different transport.
  56
  57  *      sysctls
  58         RDS supports a number of sysctls in /proc/sys/net/rds
  59
  60
  61 Socket Interface
  62 ================
  63
  64   AF_RDS, PF_RDS, SOL_RDS
  65         These constants haven't been assigned yet, because RDS isn't in
  66         mainline yet. Currently, the kernel module assigns some constant
  67         and publishes it to user space through two sysctl files
  68                 /proc/sys/net/rds/pf_rds
  69                 /proc/sys/net/rds/sol_rds
  70
  71   fd = socket(PF_RDS, SOCK_SEQPACKET, 0);
  72         This creates a new, unbound RDS socket.
  73
  74   setsockopt(SOL_SOCKET): send and receive buffer size
  75         RDS honors the send and receive buffer size socket options.
  76         You are not allowed to queue more than SO_SNDSIZE bytes to
  77         a socket. A message is queued when sendmsg is called, and
  78         it leaves the queue when the remote system acknowledges
  79         its arrival.
  80
  81         The SO_RCVSIZE option controls the maximum receive queue length.
  82         This is a soft limit rather than a hard limit - RDS will
  83         continue to accept and queue incoming messages, even if that
  84         takes the queue length over the limit. However, it will also
  85         mark the port as "congested" and send a congestion update to
  86         the source node. The source node is supposed to throttle any
  87         processes sending to this congested port.
  88
  89   bind(fd, &sockaddr_in, ...)
  90         This binds the socket to a local IP address and port, and a
  91         transport.
  92
  93   sendmsg(fd, ...)
  94         Sends a message to the indicated recipient. The kernel will
  95         transparently establish the underlying reliable connection
  96         if it isn't up yet.
  97
  98         An attempt to send a message that exceeds SO_SNDSIZE will
  99         return with -EMSGSIZE
 100
 101         An attempt to send a message that would take the total number
 102         of queued bytes over the SO_SNDSIZE threshold will return
 103         EAGAIN.
 104
 105         An attempt to send a message to a destination that is marked
 106         as "congested" will return ENOBUFS.
 107
 108   recvmsg(fd, ...)
 109         Receives a message that was queued to this socket. The sockets
 110         recv queue accounting is adjusted, and if the queue length
 111         drops below SO_SNDSIZE, the port is marked uncongested, and
 112         a congestion update is sent to all peers.
 113
 114         Applications can ask the RDS kernel module to receive
 115         notifications via control messages (for instance, there is a
 116         notification when a congestion update arrived, or when a RDMA
 117         operation completes). These notifications are received through
 118         the msg.msg_control buffer of struct msghdr. The format of the
 119         messages is described in manpages.
 120
 121   poll(fd)
 122         RDS supports the poll interface to allow the application
 123         to implement async I/O.
 124
 125         POLLIN handling is pretty straightforward. When there's an
 126         incoming message queued to the socket, or a pending notification,
 127         we signal POLLIN.
 128
 129         POLLOUT is a little harder. Since you can essentially send
 130         to any destination, RDS will always signal POLLOUT as long as
 131         there's room on the send queue (ie the number of bytes queued
 132         is less than the sendbuf size).
 133
 134         However, the kernel will refuse to accept messages to
 135         a destination marked congested - in this case you will loop
 136         forever if you rely on poll to tell you what to do.
 137         This isn't a trivial problem, but applications can deal with
 138         this - by using congestion notifications, and by checking for
 139         ENOBUFS errors returned by sendmsg.
 140
 141   setsockopt(SOL_RDS, RDS_CANCEL_SENT_TO, &sockaddr_in)
 142         This allows the application to discard all messages queued to a
 143         specific destination on this particular socket.
 144
 145         This allows the application to cancel outstanding messages if
 146         it detects a timeout. For instance, if it tried to send a message,
 147         and the remote host is unreachable, RDS will keep trying forever.
 148         The application may decide it's not worth it, and cancel the
 149         operation. In this case, it would use RDS_CANCEL_SENT_TO to
 150         nuke any pending messages.
 151
 152
 153 RDMA for RDS
 154 ============
 155
 156   see rds-rdma(7) manpage (available in rds-tools)
 157
 158
 159 Congestion Notifications
 160 ========================
 161
 162   see rds(7) manpage
 163
 164
 165 RDS Protocol
 166 ============
 167
 168   Message header
 169
 170     The message header is a 'struct rds_header' (see rds.h):
 171     Fields:
 172       h_sequence:
 173           per-packet sequence number
 174       h_ack:
 175           piggybacked acknowledgment of last packet received
 176       h_len:
 177           length of data, not including header
 178       h_sport:
 179           source port
 180       h_dport:
 181           destination port
 182       h_flags:
 183           CONG_BITMAP - this is a congestion update bitmap
 184           ACK_REQUIRED - receiver must ack this packet
 185           RETRANSMITTED - packet has previously been sent
 186       h_credit:
 187           indicate to other end of connection that
 188           it has more credits available (i.e. there is
 189           more send room)
 190       h_padding[4]:
 191           unused, for future use
 192       h_csum:
 193           header checksum
 194       h_exthdr:
 195           optional data can be passed here. This is currently used for
 196           passing RDMA-related information.
 197
 198   ACK and retransmit handling
 199
 200       One might think that with reliable IB connections you wouldn't need
 201       to ack messages that have been received.  The problem is that IB
 202       hardware generates an ack message before it has DMAed the message
 203       into memory.  This creates a potential message loss if the HCA is
 204       disabled for any reason between when it sends the ack and before
 205       the message is DMAed and processed.  This is only a potential issue
 206       if another HCA is available for fail-over.
 207
 208       Sending an ack immediately would allow the sender to free the sent
 209       message from their send queue quickly, but could cause excessive
 210       traffic to be used for acks. RDS piggybacks acks on sent data
 211       packets.  Ack-only packets are reduced by only allowing one to be
 212       in flight at a time, and by the sender only asking for acks when
 213       its send buffers start to fill up. All retransmissions are also
 214       acked.
 215
 216   Flow Control
 217
 218       RDS's IB transport uses a credit-based mechanism to verify that
 219       there is space in the peer's receive buffers for more data. This
 220       eliminates the need for hardware retries on the connection.
 221
 222   Congestion
 223
 224       Messages waiting in the receive queue on the receiving socket
 225       are accounted against the sockets SO_RCVBUF option value.  Only
 226       the payload bytes in the message are accounted for.  If the
 227       number of bytes queued equals or exceeds rcvbuf then the socket
 228       is congested.  All sends attempted to this socket's address
 229       should return block or return -EWOULDBLOCK.
 230
 231       Applications are expected to be reasonably tuned such that this
 232       situation very rarely occurs.  An application encountering this
 233       "back-pressure" is considered a bug.
 234
 235       This is implemented by having each node maintain bitmaps which
 236       indicate which ports on bound addresses are congested.  As the
 237       bitmap changes it is sent through all the connections which
 238       terminate in the local address of the bitmap which changed.
 239
 240       The bitmaps are allocated as connections are brought up.  This
 241       avoids allocation in the interrupt handling path which queues
 242       sages on sockets.  The dense bitmaps let transports send the
 243       entire bitmap on any bitmap change reasonably efficiently.  This
 244       is much easier to implement than some finer-grained
 245       communication of per-port congestion.  The sender does a very
 246       inexpensive bit test to test if the port it's about to send to
 247       is congested or not.
 248
 249
 250 RDS Transport Layer
 251 ==================
 252
 253   As mentioned above, RDS is not IB-specific. Its code is divided
 254   into a general RDS layer and a transport layer.
 255
 256   The general layer handles the socket API, congestion handling,
 257   loopback, stats, usermem pinning, and the connection state machine.
 258
 259   The transport layer handles the details of the transport. The IB
 260   transport, for example, handles all the queue pairs, work requests,
 261   CM event handlers, and other Infiniband details.
 262
 263
 264 RDS Kernel Structures
 265 =====================
 266
 267   struct rds_message
 268     aka possibly "rds_outgoing", the generic RDS layer copies data to
 269     be sent and sets header fields as needed, based on the socket API.
 270     This is then queued for the individual connection and sent by the
 271     connection's transport.
 272   struct rds_incoming
 273     a generic struct referring to incoming data that can be handed from
 274     the transport to the general code and queued by the general code
 275     while the socket is awoken. It is then passed back to the transport
 276     code to handle the actual copy-to-user.
 277   struct rds_socket
 278     per-socket information
 279   struct rds_connection
 280     per-connection information
 281   struct rds_transport
 282     pointers to transport-specific functions
 283   struct rds_statistics
 284     non-transport-specific statistics
 285   struct rds_cong_map
 286     wraps the raw congestion bitmap, contains rbnode, waitq, etc.
 287
 288 Connection management
 289 =====================
 290
 291   Connections may be in UP, DOWN, CONNECTING, DISCONNECTING, and
 292   ERROR states.
 293
 294   The first time an attempt is made by an RDS socket to send data to
 295   a node, a connection is allocated and connected. That connection is
 296   then maintained forever -- if there are transport errors, the
 297   connection will be dropped and re-established.
 298
 299   Dropping a connection while packets are queued will cause queued or
 300   partially-sent datagrams to be retransmitted when the connection is
 301   re-established.
 302
 303
 304 The send path
 305 =============
 306
 307   rds_sendmsg()
 308     struct rds_message built from incoming data
 309     CMSGs parsed (e.g. RDMA ops)
 310     transport connection alloced and connected if not already
 311     rds_message placed on send queue
 312     send worker awoken
 313   rds_send_worker()
 314     calls rds_send_xmit() until queue is empty
 315   rds_send_xmit()
 316     transmits congestion map if one is pending
 317     may set ACK_REQUIRED
 318     calls transport to send either non-RDMA or RDMA message
 319     (RDMA ops never retransmitted)
 320   rds_ib_xmit()
 321     allocs work requests from send ring
 322     adds any new send credits available to peer (h_credits)
 323     maps the rds_message's sg list
 324     piggybacks ack
 325     populates work requests
 326     post send to connection's queue pair
 327
 328 The recv path
 329 =============
 330
 331   rds_ib_recv_cq_comp_handler()
 332     looks at write completions
 333     unmaps recv buffer from device
 334     no errors, call rds_ib_process_recv()
 335     refill recv ring
 336   rds_ib_process_recv()
 337     validate header checksum
 338     copy header to rds_ib_incoming struct if start of a new datagram
 339     add to ibinc's fraglist
 340     if competed datagram:
 341       update cong map if datagram was cong update
 342       call rds_recv_incoming() otherwise
 343       note if ack is required
 344   rds_recv_incoming()
 345     drop duplicate packets
 346     respond to pings
 347     find the sock associated with this datagram
 348     add to sock queue
 349     wake up sock
 350     do some congestion calculations
 351   rds_recvmsg
 352     copy data into user iovec
 353     handle CMSGs
 354     return to application
 355
 356