S. Bailey (Sandburst)
Internet-draft Expires: July 2002
The Architecture of Direct Data Placement (DDP)
And Remote Direct Memory Access (RDMA)
On Internet Protocols
draft-bailey-roi-ddp-rdma-arch-00
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Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document defines an abstract architecture for Direct Data
Placement (DDP) and Remote Direct Memory Access (RDMA) protocols to
run on Internet Protocol-suite transport protocols. This
architecture does not necessarily reflect the proper way to
implement such protocols, but is, rather, a descriptive tool for
defining and understanding the protocols.
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Table Of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 2
2. Direct Data Placement (DDP) Architecture . . . . . . . . . 2
2.1. Transport Operations . . . . . . . . . . . . . . . . . . . 4
2.2. DDP Operations . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Transport Characterstics In DDP . . . . . . . . . . . . . 8
3. Remote Direct Memory Access (RDMA) Protocol Architecture . 9
3.1. RDMA Operations . . . . . . . . . . . . . . . . . . . . . 10
3.2. Transport Characterstics In RDMA . . . . . . . . . . . . . 12
4. Security Considerations . . . . . . . . . . . . . . . . . 13
5. IANA Considerations . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . 13
Full Copyright Statement . . . . . . . . . . . . . . . . . 14
1. Introduction
This document defines an abstract architecture for Direct Data
Placement (DDP) and Remote Direct Memory Access (RDMA) protocols to
run on Internet Protocol-suite transport protocols. This
architecture does not necessarily reflect the proper way to
implement such protocols, but is, rather, a descriptive tool for
defining and understanding the protocols.
The first section describes the architecture of DDP protocols,
including assumptions of the transports on which DDP is built. The
second section describes the architecture of RDMA protocols layered
on top of DDP.
2. Direct Data Placement (DDP) Architecture
The central idea of general-purpose DDP is that a data sender will
supplement the data it sends with placement information that allows
the receiver's network interface (NI) to place the data directly at
its final destination without any copying. DDP can be used to
steer received data to its final destination for any ULP without
requiring ULP-specific behavior in the NI for each different ULP.
Data sent with DDP information is said to be `DDP-decorated'.
The central component of the DDP architecture is the `buffer',
which is an object with beginning and ending addresses, and a
method (set()) to set the value of an octet at an address. In many
cases, a buffer corresponds directly to a portion of host memory.
However, DDP does not depend on this---a buffer could be a disk
file, or anything else that can be viewed as an addressable
collection of octets. Abstractly, a buffer provides the interface:
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typedef struct {
const address_t start;
const address_t end;
void set(address_t a, uint8_t v);
} buffer_t;
The protocol layering and in-line data flow of DDP is:
Client Protocol
(e.g. ULP or RDMA)
| ^
undecorated messages | | undecorated messages
DDP-decorated messages | | DDP-decorated message reception
v | indications
DDP
^
| transport messages
v
Transport
(e.g. SCTP, DCP)
^
| IP datagrams
v
. . .
In addition to in-line data flow, the client protocol registers
buffers with DDP, and DDP performs buffer update (set()) operations
as a result of receiving DDP-decorated messages.
Undecorated messages correspond directly to messages of the
underlying transport, but must still be distinguished from DDP-
decorated messages in some way.
DDP-decorated messages may be split into multiple, smaller DDP-
decorated messages each in a separate transport message. However,
if the transport is unreliable or unordered, DDP-decorated messages
split across transport messages may or may not provide useful
behavior, in the same way as splitting regular, undecorated
messages across unreliable or unordered transport messages may or
may not provide useful behavior. In other words, the same
considerations apply to building client protocols on different
types of transports with or without the use of DDP.
A DDP-decorated message split across transport messages looks like:
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DDP-decorated message: Transport messages:
stag=s, offset=o, message 1:
notify=y, id=i |type=ddp |
message= |stag=s |
|aabbccddee|-------. |offset=o |
~ ... ~----. \ |notify=n |
|vvwwxxyyzz|-. \ \ |id=? |
| \ `--->|aabbccddee|
| \ ~ ... ~
| +----->|iijjkkllmm|
| |
+ | message 2:
\ | |type=ddp |
\ | |stag=s |
\ + |offset=o+n|
\ \ |notify=y |
\ \ |id=i |
\ `-->|nnooppqqrr|
\ ~ ... ~
`---->|vvwwxxyyzz|
Although this picture suggests that DDP decoration information is
carried in-line with the message payload, components of the DDP
decoration may also be in transport-specific fields, or derived
from transport-specific control information if the transport
permits.
2.1. Transport Operations
For the purposes of this architecture, the transport provides:
void xpt_send(socket_t s, message_t m);
message_t xpt_recv(socket_t s);
msize_t xpt_max_msize(socket_t s);
socket_t
a transport address, including IP addresses, ports and other
transport-specific identifiers.
message_t
a string of octets.
msize_t (unsigned integer)
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a message size.
xpt_send(socket_t s, message_t m)
send a transport message.
xpt_recv(socket_t s)
receive a transport message.
xpt_max_msize(socket_t s)
get the current maximum transport message size. Corresponds,
roughly, to the current path Maximum Transfer Unit (PMTU),
adjusted by underlying protocol overheads.
Real implementations of xpt_send() and xpt_recv() typically return
error indications, but that is not relevant to this architecture.
2.2. DDP Operations
The DDP layer provides:
void ddp_send(socket_t s, message_t m);
void ddp_send_ddp(socket_t s, message_t m, ddp_addr_t d,
ddp_notify_t n);
ddp_recv_t ddp_recv(socket_t s);
bdesc_t ddp_register(socket_t s, buffer_t b);
void ddp_deregister(bhand_t bh);
msizes_t ddp_max_msizes(socket_t s);
ddp_addr_t
the buffer address portion of a DDP-decoration:
typedef struct {
stag_t stag;
address_t offset;
} ddp_addr_t;
stag_t (unsigned integer)
a steering tag. A stag_t identifies the destination buffer
for DDP-decorated messages. stag_ts are generated when the
buffer is registered, communicated to the sender by some
client protocol convention and inserted in DDP-decorated
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messages. stag_t values in this DDP architecture are assumed
to be completely opaque to the client protocol, and
implementation-dependent. However, particular
implementations, such as DDP on a multicast transport (see
below), may provide the buffer holder some control in
selecting stag_ts.
ddp_notify_t
the notification portion of a DDP-decoration:
typedef struct {
bool notify;
ddp_msg_id_t i;
} ddp_notify_t;
ddp_msg_id_t (unsigned integer)
a DDP-decorated message identifier. msg_id_ts are chosen by
the DDP-decorated message receiver (buffer holder),
communicated to the sender by some client protocol convention
and inserted in DDP-decorated messages. Whether a message
reception indication is requested for a DDP-decorated message
is a matter of client protocol convention. Unlike stag_ts,
the structure of msg_id_ts is opaque to DDP, and therefore,
completely in the hands of the client protocol.
bdesc_t
a description of a registered buffer:
typedef struct {
bhand_t bh;
ddp_addr_t a;
} bdesc_t;
`a.offset' is the starting offset of the registered buffer,
which may have no relationship to the `start' or `end'
addresses of that buffer. However, particular implemenations,
such as DDP on a multicast transport (see below), may allow
some client protocol control over the starting offset.
bhand_t
an opaque buffer handle used to unregister a buffer.
ddp_recv_t
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an undecorated message, a DDP-decorated message reception
indication, or a DDP-decorated message reception error:
typedef union {
message_t m;
ddp_msg_id_t i;
ddp_err_t e;
} ddp_recv_t;
ddp_err_t
indicates an error while receiving a DDP-decorated message,
typically `offset' out of bounds, or `stag' is not registered
to the socket.
msizes_t
The maximum undecorated and DDP-decorated messages that fit in
a single transport message:
typedef struct {
msize_t max_undec;
msize_t max_dec;
} msizes_t;
ddp_send(socket_t s, message_t m)
send an undecorated message.
ddp_send_ddp(socket_t s, message_t m, ddp_addr_t d, ddp_notify_t n)
send a DDP-decorated message.
ddp_recv(socket_t s)
get the next received undecorated message, DDP-decorated
message reception indication, or DDP-decorated message error.
ddp_register(socket_t s, buffer_t b)
register a buffer for DDP on a socket. The same buffer may be
registered multiple times on the same or different sockets.
Different buffers may also refer to portions of the same
underlying addressable object (buffer aliasing).
ddp_deregister(bhand_t bh)
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unregister a buffer from a socket.
ddp_max_msizes(socket_t s)
get the current maximum undecorated and DDP-decorated message
sizes that will fit in a single transport message.
2.3. Transport Characterstics In DDP
Certain characteristics of the transport on which DDP is mapped
determine the nature of the service provided to client protocols.
Specifically, transports are:
o reliable or unreliable,
o ordered or unordered,
o single source or multisource,
o single destination or multidestination (multicast or anycast).
Some transports support several combinations of these
characteristics. For example, SCTP is reliable, single source,
single destination (point-to-point) and supports both ordered and
unordered modes.
In general, these transport characteristics equally affect
transport and DDP-decorated message delivery. However, there are
several issues specific to DDP-decorated messages.
A key component of DDP, is how operations on the receiving side:
o set()s,
o undecorated messages, and
o DDP-decorated message reception indications
are ordered among themselves, and how they relate to corresponding
operations on the sending side. These relationships depend upon
the characteristics of the underlying transport in a way which is
defined by the DDP protocol. For example, if the transport is
unreliable and unordered, the DDP protocol might specify that the
client protocol is subject to the consequences of transport
messages being lost or duplicated, rather requiring different
characteristics be presented to the client protocol.
Multidestination data delivery is the other transport
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characteristic which may require specific consideration in a DDP
protocol. As mentioned above, the basic DDP model assumes that
buffer address values returned by ddp_register() are opaque to the
client protocol, and can be implementation dependent. The most
natural way to map DDP to a multidestination transport is to
require all receivers produce the same buffer address when
registering a multidestination destination buffer. Restriction of
the DDP model to accomodate multiple destinations involves
engineering tradeoffs comparable to those of providing non-DDP
multidestination transport capability.
3. Remote Direct Memory Access (RDMA) Protocol Architecture
Remote Direct Memory Access (RDMA) extends the capabilities of DDP
with the ability to read from buffers registered to a socket (RDMA
Read). This allows a client protocol to perform arbitrary,
bidirectional data movement without involving the remote client
protocol. When RDMA is implemented in the NI, arbitrary data
movement can be performed without involving the remote host CPU at
all.
In addition, RDMA protocols usually specify a transport-independent
undecorated message service (Send) with characteristics which are
both very efficient to implement in an NI, and convenient for
client protocols.
The RDMA architecture is patterned after the traditional model for
device programming, where the client requests an operation using
Send-like actions (programmed I/O), the server performs the
necessary data transfers for the operation (DMA reads and writes),
and notifies the client of completion. The programmed I/O+DMA
model efficiently supports a high degree of concurrency and
flexibility for both the client and server, even when operations
have a wide range of intrinsic latencies.
RDMA is implemented as a client protocol on top of DDP:
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Client Protocol
| ^
Sends | | Sends
RDMA Read Requests | | RDMA Read Completion indications
RDMA Writes v | RDMA Write Completion indications
RDMA
| ^
undecorated messages | | undecorated messages
DDP-decorated messages | | DDP-decorated message reception
v | indications
DDP
^
| transport messages
v
. . .
In addition to in-line data flow, read (get()) and update (set())
operations are performed on buffers registered with RDMA as a
result of RDMA Read Requests and RDMA Writes, respectively.
An RDMA `buffer' extends a DDP buffer with a get() operation that
retrieves the value of the octet at address `a':
typedef struct {
const address_t start;
const address_t end;
void set(address_t a, uint8_t v);
uint8_t get(address_t a);
} buffer_t;
3.1. RDMA Operations
The RDMA layer provides:
void rdma_send(socket_t s, message_t m);
void rdma_write(socket_t s, message_t m, ddp_addr_t d,
rdma_notify_t n);
void rdma_read(socket_t s, ddp_addr_t s, ddp_addr_t d);
rdma_recv_t rdma_recv(socket_t s);
bdesc_t rdma_register(socket_t s, buffer_t b, bmode_t mode);
void rdma_deregister(bhand_t bh);
msizes_t rdma_max_msizes(socket_t s);
Although, for clarity, these data transfer interfaces are
synchronous, rdma_read() and possibly rdma_send() (in the presence
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of Send flow control), can require an arbitrary amount of time to
complete. To express the full concurrency and interleaving of RDMA
data transfer, these interfaces are also defined to be
multithreaded. For example, a client protocol may perform an
rdma_send(), while an rdma_read() operation is in progress.
rdma_notify_t
RDMA Write notification information:
typedef struct {
bool notify;
rdma_write_id_t i;
} rdma_notify_t;
identical in function to ddp_notify_t, except that the type
rdma_write_id_t may not be equivalent to ddp_msg_id_t.
rdma_write_id_t (unsigned integer)
an RDMA Write identifier.
rdma_recv_t
a Send message, an RDMA Write completion identifier, or an
RDMA error:
typedef union {
message_t m;
rdma_write_id_t i;
rdma_err_t e;
} rdma_recv_t;
rdma_err_t
an RDMA protocol error indication. RDMA errors include buffer
addressing errors corresponding to ddp_err_ts, and buffer
protection violations (e.g. RDMA Writing a buffer only
registered for reading).
bmode_t
buffer registration mode (permissions). Any combination of
permitting RDMA Read (BMODE_READ) and RDMA Write (BMODE_WRITE)
operations.
rdma_send(socket_t s, message_t m)
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Send a message.
rdma_write(socket_t s, message_t m, ddp_addr_t d, rdma_notify_t n)
RDMA Write to remote buffer address d.
rdma_read(socket_t s, ddp_addr_t s, ddp_addr_t d)
RDMA Read from remote buffer address s to local buffer address
d.
rdma_recv(socket_t s);
get the next received Send message, RDMA Write completion
identifier, or RDMA error.
rdma_register(socket_t s, buffer_t b, bmode_t mode)
register a buffer for RDMA on a socket (for read access, write
access or both). As with DDP, the same buffer may be
registered multiple times on the same or different sockets,
and different buffers may refer to portions of the same
underlying addressable object.
rdma_deregister(bhand_t bh)
unregister a buffer from a socket.
rdma_max_msizes(socket_t s)
get the current maximum Send (max_undec) and RDMA Read or
Write (max_dec) operations that will fit in a single transport
message. The values returned by rdma_max_msizes() are closely
related to the values returned by ddp_max_msizes(), but may
not be equal.
3.2. Transport Characterstics In RDMA
As with DDP, RDMA can be used on transports with a variety of
different characteristics that manifest themselves directly in the
service provided by RDMA.
Like DDP, an RDMA protocol must specify how:
o set()s,
o get()s,
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o Send messages, and
o RDMA Read completions
are ordered among themselves and how they relate to corresponding
operations on the remote peer(s). These relationships are likely
to be a function of the underlying transport characteristics.
There are some additional characteristics of RDMA which may
translate poorly to unreliable or multipoint transports due to
attendent complexities in managing endpoint state:
o Send flow control
o RDMA Read
These difficulties can be overcome by placing restrictions on the
service provided by RDMA. However, many RDMA clients, especially
those that separate data transfer and application logic concerns,
are likely to depend upon capabilities only provided by RDMA on a
point-to-point, reliable transport.
4. Security Considerations
Security considerations are not addressed in this document. Any
security considerations resulting from the use of DDP or RDMA must
be addressed in the relevant standards.
5. IANA Considerations
IANA considerations are not addressed in by this document. Any
IANA considerations resulting from the use of DDP or DMA must be
addressed in the relevant standards.
Author's Address
Stephen Bailey
Sandburst Corporation
600 Federal Street
Andover, MA 01810
USA
Email: steph@sandburst.com
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