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Based on kernel version 4.1. Page generated on 2015-06-28 12:13 EST.

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