wdiff draft-bailey-roi-ddp-rdma-arch-00.txt draft-bailey-roi-ddp-rdma-arch-01.txt

S.
Internet-Draft Stephen Bailey (Sandburst)
Internet-draft
Expires: July 2002 May 2003 Tom Talpey (NetApp)

The Architecture of Direct Data Placement (DDP)
And Remote Direct Memory Access (RDMA)
On Internet Protocols
draft-bailey-roi-ddp-rdma-arch-00
draft-bailey-roi-ddp-rdma-arch-01

Status of this Memo

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all provisions of Section 10 of RFC2026.

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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. transports. 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.

Table Of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 2
2. Direct Data Placement (DDP) Architecture . . . . . . . . . 2 . . . . . . . . . . . . . 3
2.1. Direct Data Placement (DDP) Protocol Architecture . . . 3
2.1.1. Transport Operations . . . . . . . . . . . . . . . . . . . 4
2.2. 5
2.1.2. DDP Operations . . . . . . . . . . . . . . . . . . . . . . 5
2.3. 6
2.1.3. Transport Characterstics In Characteristics in DDP . . . . . . . . . . . . . 8
3. 9
2.2. Remote Direct Memory Access (RDMA) Protocol Architecture . 9
3.1. . . 10
2.2.1. RDMA Operations . . . . . . . . . . . . . . . . . . . . . 10
3.2. 11
2.2.2. Transport Characterstics In Characteristics in RDMA . . . . . . . . . . . 14
3. Security Considerations . . . . . . . . . . . . . . . 12 . 14
4. Security IANA Considerations . . . . . . . . . . . . . . . . . 13 . 15
5. IANA Considerations Acknowledgements . . . . . . . . . . . . . . . . . . . . 15
References . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . 13 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . 16
Full Copyright Statement . . . . . . . . . . . . . . . . . 14 17

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. transports [RDDP, ROM]. 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 part of the document describes the architecture of DDP
protocols, including what assumptions of are made about the transports
on which DDP is built. The second section part describes the architecture
of RDMA protocols layered on top of DDP.

Before introducing the protocols, three definitions will be useful
to guide discussion:

o Placement - writing to a data buffer.

o Delivery - informing the Upper Layer Protocol (ULP) (e.g.
RDMA) that a particular message is available for use.
Delivery therefore may be viewed as the "control" signal
associated with a unit of data. Note that the order of
delivery is defined more strictly than it is for placement.

o Completion - informing the ULP or application that a
particular RDMA operation has finished. A completion, for
instance, may require the delivery of several messages, or it
may also reflect that some local processing has finished.

The goal of the DDP protocol is to allow the efficient placement of
data into buffers designated by Upper Layer Protocols (e.g. RDMA).
This is described in detail in [ROM]. Efficiency may be
characterized by the minimization of the number of transfers of the
data over the receiver's system buses.

The goal of the RDMA protocol is to provide the semantics to enable
Remote Direct Memory Access between peers in a way consistent with
application requirements. The RDMA protocol provides facilities
immediately useful to existing and future networking, storage, and
other application protocols. [DAFS, FIBRE, IB, MYR, SDP, SRVNET,
VI]

The DDP and RDMA protocols work together to achieve their
respective goals. RDMA provides facilities to a ULP for
identifying buffers, controlling the transfer of data between ULP
peers, and providing completion notifications to the ULP. RDMA
uses the features of DDP to steer payloads to specific buffers at
the Data Sink. ULPs that do not require the features of RDMA may
be layered directly on top of DDP.

The DDP and RDMA protocols are transport independent. The
following figure shows the relationship between RDMA, DDP, Upper
Layer Protocols and Transport.

+---------------------------------------------------+
| ULP |
+---------+------------+----------------------------+
| | | RDMA |
| | +----------------------------+
| | DDP |
| +-----------------------------------------+
| Transport |
+---------------------------------------------------+

2. Architecture

The Architecture section is presented in two parts: Direct Data
Placement Protocol architecture and Remote Direct Memory Access
Protocol architecture.

2.1. Direct Data Placement (DDP) Protocol 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 destination, without requiring ULP-specific layer-
specific behavior in the NI for each different ULP. layer. Data sent with such
DDP information is said to be `DDP-decorated'. `tagged'.

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 user
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:

typedef struct {
const address_t start;
const address_t end;
void set(address_t a, uint8_t data_t v);
} buffer_t; ddp_buffer_t;

address_t

a reference to local memory

data_t

an octet data value.

The protocol layering and in-line data flow of DDP is:

Client Protocol
(e.g. ULP or RDMA)
| ^
undecorated
untagged messages | | undecorated messages
DDP-decorated untagged message delivery
tagged messages | | DDP-decorated tagged message reception delivery
v | indications
DDP
DDP+---> data placement
^
| transport messages
v
Transport
(e.g. SCTP, DCP) DCP, framed TCP)
^
| 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 tagged 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

DDP messages may be split into multiple, smaller DDP-
decorated messages DDP 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 arbitrary upper layer 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 DDP message split across transport messages looks like:

DDP-decorated

DDP 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 in-
line with the message payload, components of the DDP
decoration information
may also be in transport-specific fields, or derived from
transport-specific control information if the transport permits.

2.1.

2.1.1. 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) (scalar)

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.

2.1.2. 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 ddp_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: tagged message:

typedef struct {
stag_t stag;
address_t offset;
} ddp_addr_t;

stag_t (unsigned integer) (scalar)

a steering tag. Steering Tag. A stag_t identifies the destination buffer
for DDP-decorated tagged messages. stag_ts are generated when the buffer is
registered, communicated to the sender by some client protocol
convention and inserted in DDP-decorated DDP 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: DDP message, used to signal that
the message represents the final fragment of a multi-segmented
DDP message:

typedef struct {
bool
boolean_t notify;
ddp_msg_id_t i;
} ddp_notify_t;

ddp_msg_id_t (unsigned integer) (scalar)

a DDP-decorated DDP message identifier. msg_id_ts are chosen by the DDP-decorated DDP
message receiver (buffer holder), communicated to the sender
by some client protocol convention and inserted in DDP-decorated DDP
messages. Whether a message reception indication is requested
for a DDP-decorated DDP 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,
implementations, 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 deregister a buffer.

ddp_recv_t

an undecorated untagged message, a DDP-decorated tagged message reception indication, or
a DDP-decorated tagged 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 tagged message, typically
`offset' out of bounds, or `stag' is not registered to the
socket.

msizes_t

The maximum undecorated untagged and DDP-decorated tagged messages that fit in a single
transport message:

typedef struct {
msize_t max_undec; max_untagged;
msize_t max_dec; max_tagged;
} msizes_t;

ddp_send(socket_t s, message_t m)
send an undecorated untagged message.

ddp_send_ddp(socket_t s, message_t m, ddp_addr_t d, ddp_notify_t n)

send a DDP-decorated tagged message.

ddp_recv(socket_t s)

get the next received undecorated untagged message, DDP-decorated tagged message
reception indication, or DDP-decorated tagged message error.

ddp_register(socket_t s, buffer_t ddp_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)
unregister

remove a buffer registration from a socket. buffer.

ddp_max_msizes(socket_t s)

get the current maximum undecorated untagged and DDP-decorated tagged message sizes that
will fit in a single transport message.

2.3.

2.1.3. Transport Characterstics Characteristics 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 [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 DDP message delivery. However, there are several
issues specific to DDP-decorated DDP messages.

A key component of DDP, DDP is how the following operations on the
receiving side:

o set()s,

o undecorated messages, and

o DDP-decorated message reception indications side are ordered among themselves, and how they relate to
corresponding operations on the sending side. side:

o set()s,

o untagged message reception indications, and

o tagged message reception indications.

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
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 accommodate multiple destinations involves
engineering tradeoffs comparable to those of providing non-DDP
multidestination transport capability.

2.2. 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. client.
When RDMA is implemented in the NI, hardware, arbitrary data movement can
be performed without involving the remote host CPU at all.

In addition, RDMA protocols usually specify a transport-independent
undecorated
untagged message service (Send) with characteristics which are both
very efficient to implement in an NI, hardware, 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 layered as a client protocol on top of DDP:

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 data_t v);
uint8_t
data_t get(address_t a);
} buffer_t;

3.1. rdma_buffer_t;

2.2.1. 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 rdma_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
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: information, used to signal that the
message represents the final fragment of a multi-segmented
RDMA message:

typedef struct {
bool
boolean_t 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) (scalar)

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)
Send

send a message. message, delivering it to the next untagged RDMA buffer
at the remote peer.

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 rdma_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

remove a buffer registration from a socket. buffer.

rdma_max_msizes(socket_t s)

get the current maximum Send (max_undec) (max_untagged) and RDMA Read or
Write (max_dec) (max_tagged) 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.

2.2.2. Transport Characterstics Characteristics 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,

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
attendant 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.

3. Security Considerations

Security considerations are not addressed

System integrity must be maintained in this document. Any
security considerations resulting any RDMA solution.
Mechanisms must be specified to prevent RDMA or DDP operations from
impairing system integrity. For example, the use threat caused by
potential buffer overflow needs full examination, and prevention
mechanisms must be spelled out.

Because a Steering Tag exports access to a memory region, one
critical aspect of DDP or RDMA security is the scope of this access. It must
be addressed in possible to individually control specific attributes of the
access provided by a Steering Tag, including remote read access,
remote write access, and others that might be identified. A
specification must provide both implementation requirements
relevant standards.

5. to this issue, and guidelines to assist implementors in
making the appropriate design decisions.

A number of other potential attacks have been envisioned and must
be addressed. Some such examples are outlined in [RDMACON].
Resource issues leading to denial-of-service attacks, overwrites
and other concurrent operations, the ordering of completions as
required by the RDMA protocol, and the granularity of transfer are
all within the required scope of any security analysis of RDMA and
DDP.

4. IANA Considerations

IANA considerations are not addressed in by this document. Any
IANA considerations resulting from the use of DDP or DMA RDMA must be
addressed in the relevant standards.

Author's Address

5. Acknowledgements

The authors wish to acknowledge the valuable contributions of David
Black, Jeff Mogul and Allyn Romanow.

6. References

[DAFS]
Direct Access File System http://www.dafscollaborative.org
http://www.ietf.org/internet-drafts/draft-wittle-dafs-00.txt

[FIBRE]
Fibre Channel Standard
http://www.fibrechannel.com/technology/index.master.html

[IB] InfiniBand Architecture Specification, Volumes 1 and 2,
Release 1.0.a. http://www.infinibandta.org

[MYR]
Myrinet, http://www.myricom.com

[RDDP]
Remote Direct Data Placement Working Group charter,
http://www.ietf.org/html.charters/rddp-charter.html

[RDMACON]
D. Black, M. Speer, J. Wroclawski, "DDP and RDMA Concerns",
http://www.ietf.org/internet-drafts/draft-black-rdma-
concerns-00.txt, Work in Progress, June 2002

[ROM]
A. Romanow, J. Mogul, T. Talpey, S. Bailey, "RDMA over IP
Problem Statement", http://www.ietf.org/internet-drafts/draft-
romanow-rdma-over-ip-problem-statement-01.txt, Work in
Progress, November 2002

[SCTP]
R. Stewart et al., "Stream Transmission Control Protocol",
Standards Track RFC, http://www.ietf.org/rfc/rfc2960

[SDP]
Sockets Direct Protocol v1.0

[SRVNET]
Compaq Servernet,
http://nonstop.compaq.com/view.asp?PAGE=ServerNet

[VI] Virtual Interface Architecture Specification Version 1.0.
http://www.viarch.org/html/collateral/san_10.pdf

Authors' Addresses

Stephen Bailey
Sandburst Corporation
600 Federal Street
Andover, MA 01810 USA
USA

Phone: +1 978 689 1614
Email: steph@sandburst.com

Tom Talpey
Network Appliance
375 Totten Pond Road
Waltham, MA 02451 USA

Phone: +1 781 768 5329
Email: thomas.talpey@netapp.com

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