Documentation/networking/rds.txt - kernel/quantenna - Git at Google


 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.

 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
 	AF_RDS and PF_RDS are the domain type to be used with socket(2)
 	to create RDS sockets. SOL_RDS is the socket-level to be used
 	with setsockopt(2) and getsockopt(2) for RDS specific socket
 	options.

   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

	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.

	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
	AF_RDS and PF_RDS are the domain type to be used with socket(2)
	to create RDS sockets. SOL_RDS is the socket-level to be used
	with setsockopt(2) and getsockopt(2) for RDS specific socket
	options.

	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