NFS version 4 S. Shepler
Internet-Draft Sun Microsystems, Inc.
Expires: April 20, 2006 October 17, 2005
NFS version 4 Minor Version 1
draft-ietf-nfsv4-minorversion1-00
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document is the first I-D that pulls together the major
proposals that have been made for inclusion in NFS version 4 minor
version 1.
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Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Security Negotiation . . . . . . . . . . . . . . . . . . . . . 6
4. Clarification of Security Negotiation in NFSv4.1 . . . . . . . 7
4.1. PUTFH + LOOKUP . . . . . . . . . . . . . . . . . . . . . . 7
4.2. PUTFH + LOOKUPP . . . . . . . . . . . . . . . . . . . . . 7
4.3. PUTFH + SECINFO . . . . . . . . . . . . . . . . . . . . . 7
4.4. PUTFH + Anything Else . . . . . . . . . . . . . . . . . . 8
5. NFSv4.1 Sessions . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Sessions Background . . . . . . . . . . . . . . . . . . . 9
5.1.1. Introduction to Sessions . . . . . . . . . . . . . . . 9
5.1.2. Motivation . . . . . . . . . . . . . . . . . . . . . . 10
5.1.3. Problem Statement . . . . . . . . . . . . . . . . . . 11
5.1.4. NFSv4 Session Extension Characteristics . . . . . . . 12
5.2. Transport Issues . . . . . . . . . . . . . . . . . . . . . 13
5.2.1. Session Model . . . . . . . . . . . . . . . . . . . . 13
5.2.2. Connection State . . . . . . . . . . . . . . . . . . . 14
5.2.3. NFSv4 Channels, Sessions and Connections . . . . . . . 15
5.2.4. Reconnection, Trunking and Failover . . . . . . . . . 17
5.2.5. Server Duplicate Request Cache . . . . . . . . . . . . 18
5.3. Session Initialization and Transfer Models . . . . . . . . 19
5.3.1. Session Negotiation . . . . . . . . . . . . . . . . . 19
5.3.2. RDMA Requirements . . . . . . . . . . . . . . . . . . 20
5.3.3. RDMA Connection Resources . . . . . . . . . . . . . . 21
5.3.4. TCP and RDMA Inline Transfer Model . . . . . . . . . . 22
5.3.5. RDMA Direct Transfer Model . . . . . . . . . . . . . . 24
5.4. Connection Models . . . . . . . . . . . . . . . . . . . . 27
5.4.1. TCP Connection Model . . . . . . . . . . . . . . . . . 28
5.4.2. Negotiated RDMA Connection Model . . . . . . . . . . . 29
5.4.3. Automatic RDMA Connection Model . . . . . . . . . . . 30
5.5. Buffer Management, Transfer, Flow Control . . . . . . . . 30
5.6. Retry and Replay . . . . . . . . . . . . . . . . . . . . . 33
5.7. The Back Channel . . . . . . . . . . . . . . . . . . . . . 34
5.8. COMPOUND Sizing Issues . . . . . . . . . . . . . . . . . . 35
5.9. Data Alignment . . . . . . . . . . . . . . . . . . . . . . 35
5.10. NFSv4 Integration . . . . . . . . . . . . . . . . . . . . 37
5.10.1. Minor Versioning . . . . . . . . . . . . . . . . . . . 37
5.10.2. Slot Identifiers and Server Duplicate Request Cache . 37
5.10.3. COMPOUND and CB_COMPOUND . . . . . . . . . . . . . . . 41
5.10.4. eXternal Data Representation Efficiency . . . . . . . 42
5.10.5. Effect of Sessions on Existing Operations . . . . . . 42
5.10.6. Authentication Efficiencies . . . . . . . . . . . . . 43
5.11. Sessions Security Considerations . . . . . . . . . . . . . 44
5.11.1. Authentication . . . . . . . . . . . . . . . . . . . . 45
6. Directory Delegations . . . . . . . . . . . . . . . . . . . . 47
6.1. Introduction to Directory Delegations . . . . . . . . . . 47
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6.2. Directory Delegation Design (in brief) . . . . . . . . . . 48
6.3. Recommended Attributes in support of Directory
Delegations . . . . . . . . . . . . . . . . . . . . . . . 49
6.4. Delegation Recall . . . . . . . . . . . . . . . . . . . . 50
6.5. Delegation Recovery . . . . . . . . . . . . . . . . . . . 50
7. NFSv4.1 Operations . . . . . . . . . . . . . . . . . . . . . . 51
7.1. LOOKUPP - Lookup Parent Directory . . . . . . . . . . . . 51
7.2. SECINFO -- 33 Obtain Available Security . . . . . . . . . 52
7.3. SECINFO_NO_NAME - Get Security on Unnamed Object . . . . . 55
7.4. CREATECLIENTID - Instantiate Clientid . . . . . . . . . . 57
7.5. CREATESESSION - Create New Session and Confirm Clientid . 63
7.6. BIND_BACKCHANNEL - Create a callback channel binding . . . 68
7.7. DESTROYSESSION - Destroy existing session . . . . . . . . 71
7.8. SEQUENCE - Supply per-procedure sequencing and control . . 72
7.9. CB_RECALLCREDIT - change flow control limits . . . . . . . 73
7.10. CB_SEQUENCE - Supply callback channel sequencing and
control . . . . . . . . . . . . . . . . . . . . . . . . . 74
7.11. GET_DIR_DELEGATION - Get a directory delegation . . . . . 76
7.12. CB_NOTIFY - Notify directory changes . . . . . . . . . . . 79
7.13. CB_RECALL_ANY - Keep any N delegations . . . . . . . . . . 83
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 85
9. Security Considerations . . . . . . . . . . . . . . . . . . . 86
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 86
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 87
Intellectual Property and Copyright Statements . . . . . . . . . . 88
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1. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. Introduction
NFS version 4 Minor Version 1 is defined in this document. Minor
version 1 includes minor extensions for SECINFO usage, sessions, and
directory delegations.
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3. Security Negotiation
The NFSv4.0 specification contains three oversights and ambiguities
with respect to the SECINFO operation.
First, it is impossible for the client to use the SECINFO operation
to determine the correct security triple for accessing a parent
directory. This is because SECINFO takes as arguments the current
file handle and a component name. However, NFSv4.0 uses the LOOKUPP
operation to get the parent directory of the current file handle. If
the client uses the wrong security when issuing the LOOKUPP, and gets
back an NFS4ERR_WRONGSEC error, SECINFO is useless to the client.
The client is left with guessing which security the server will
accept. This defeats the purpose of SECINFO, which was to provide an
efficient method of negotiating security.
Second, there is ambiguity as to what the server should do when it is
passed a LOOKUP operation such that the server restricts access to
the current file handle with one security triple, and access to the
component with a different triple, and remote procedure call uses one
of the two security triples. Should the server allow the LOOKUP?
Third, there is a problem as to what the client must do (or can do),
whenever the server returns NFS4ERR_WRONGSEC in response to a PUTFH
operation. The NFSv4.0 specification says that client should issue a
SECINFO using the parent filehandle and the component name of the
filehandle that PUTFH was issued with. This may not be convenient
for the client.
This document resolves the above three issues in the context of
NFSv4.1.
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4. Clarification of Security Negotiation in NFSv4.1
This section attempts to clarify NFSv4.1 security negotiation issues.
Unless noted otherwise, for any mention of PUTFH in this section, the
reader should interpret it as applying to PUTROOTFH and PUTPUBFH in
addition to PUTFH.
4.1. PUTFH + LOOKUP
The server implementation may decide whether to impose any
restrictions on export security administration. There are at least
three approaches (Sc is the flavor set of the child export, Sp that
of the parent),
a) Sc <= Sp (<= for subset)
b) Sc ^ Sp != {} (^ for intersection, {} for the empty set)
c) free form
To support b (when client chooses a flavor that is not a member of
Sp) and c, PUTFH must NOT return NFS4ERR_WRONGSEC in case of security
mismatch. Instead, it should be returned from the LOOKUP that
follows.
Since the above guideline does not contradict a, it should be
followed in general.
4.2. PUTFH + LOOKUPP
Since SECINFO only works its way down, there is no way LOOKUPP can
return NFS4ERR_WRONGSEC without the server implementing
SECINFO_NO_NAME. SECINFO_NO_NAME solves this issue because via style
"parent", it works in the opposite direction as SECINFO (component
name is implicit in this case).
4.3. PUTFH + SECINFO
This case should be treated specially.
A security sensitive client should be allowed to choose a strong
flavor when querying a server to determine a file object's permitted
security flavors. The security flavor chosen by the client does not
have to be included in the flavor list of the export. Of course the
server has to be configured for whatever flavor the client selects,
otherwise the request will fail at RPC authentication.
In theory, there is no connection between the security flavor used by
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SECINFO and those supported by the export. But in practice, the
client may start looking for strong flavors from those supported by
the export, followed by those in the mandatory set.
4.4. PUTFH + Anything Else
PUTFH must return NFS4ERR_WRONGSEC in case of security mismatch.
This is the most straightforward approach without having to add
NFS4ERR_WRONGSEC to every other operations.
PUTFH + SECINFO_NO_NAME (style "current_fh") is needed for the client
to recover from NFS4ERR_WRONGSEC.
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5. NFSv4.1 Sessions
5.1. Sessions Background
5.1.1. Introduction to Sessions
This draft proposes extensions to NFS version 4 [RFC3530] enabling it
to support sessions and endpoint management, and to support operation
atop RDMA-capable RPC over transports such as iWARP. [RDMAP, DDP]
These extensions enable support for exactly-once semantics by NFSv4
servers, multipathing and trunking of transport connections, and
enhanced security. The ability to operate over RDMA enables greatly
enhanced performance. Operation over existing TCP is enhanced as
well.
While discussed here with respect to IETF-chartered transports, the
proposed protocol is intended to function over other standards, such
as Infiniband. [IB]
The following are the major aspects of this proposal:
Changes are proposed within the framework of NFSv4 minor
versioning. RPC, XDR, and the NFSv4 procedures and operations are
preserved. The proposed extension functions equally well over
existing transports and RDMA, and interoperates transparently with
existing implementations, both at the local programmatic interface
and over the wire.
An explicit session is introduced to NFSv4, and new operations are
added to support it. The session allows for enhanced trunking,
failover and recovery, and authentication efficiency, along with
necessary support for RDMA. The session is implemented as
operations within NFSv4 COMPOUND and does not impact layering or
interoperability with existing NFSv4 implementations. The NFSv4
callback channel is dynamically associated and is connected by the
client and not the server, enhancing security and operation
through firewalls. In fact, the callback channel will be enabled
to share the same connection as the operations channel.
An enhanced RPC layer enables NFSv4 operation atop RDMA. The
session assists RDMA-mode connection, and additional facilities
are provided for managing RDMA resources at both NFSv4 server and
client. Existing NFSv4 operations continue to function as before,
though certain size limits are negotiated. A companion draft to
this document, "RDMA Transport for ONC RPC" [RPCRDMA] is to be
referenced for details of RPC RDMA support.
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Support for exactly-once semantics ("EOS") is enabled by the new
session facilities, by providing to the server a way to bound the
size of the duplicate request cache for a single client, and to
manage its persistent storage.
Block Diagram
+-----------------+-------------------------------------+
| NFSv4 | NFSv4 + session extensions |
+-----------------+------+----------------+-------------+
| Operations | Session | |
+------------------------+----------------+ |
| RPC/XDR | |
+-------------------------------+---------+ |
| Stream Transport | RDMA Transport |
+-------------------------------+-----------------------+
5.1.2. Motivation
NFS version 4 [RFC3530] has been granted "Proposed Standard" status.
The NFSv4 protocol was developed along several design points,
important among them: effective operation over wide-area networks,
including the Internet itself; strong security integrated into the
protocol; extensive cross-platform interoperability including
integrated locking semantics compatible with multiple operating
systems; and protocol extensibility.
The NFS version 4 protocol, however, does not provide support for
certain important transport aspects. For example, the protocol does
not address response caching, which is required to provide
correctness for retried client requests across a network partition,
nor does it provide an interoperable way to support trunking and
multipathing of connections. This leads to inefficiencies,
especially where trunking and multipathing are concerned, and
presents additional difficulties in supporting RDMA fabrics, in which
endpoints may require dedicated or specialized resources. Sessions
can be employed to unify NFS-level constructs such as the clientid,
with transport-level constructs such as transport endpoints. Each
transport endpoint draws on resources via its membership in a
session. Resource management can be more strictly maintained,
leading to greater server efficiency in implementing the protocol.
The enhanced operation over a session affords an opportunity to the
server to implement a highly reliable duplicate request cache, and
thereby export exactly-once semantics.
NFSv4 advances the state of high-performance local sharing, by virtue
of its integrated security, locking, and delegation, and its
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excellent coverage of the sharing semantics of multiple operating
systems. It is precisely this environment where exactly-once
semantics become a fundamental requirement.
Additionally, efforts to standardize a set of protocols for Remote
Direct Memory Access, RDMA, over the Internet Protocol Suite have
made significant progress. RDMA is a general solution to the problem
of CPU overhead incurred due to data copies, primarily at the
receiver. Substantial research has addressed this and has borne out
the efficacy of the approach. An overview of this is the RDDP
Problem Statement document, [RDDPPS].
Numerous upper layer protocols achieve extremely high bandwidth and
low overhead through the use of RDMA. Products from a wide variety
of vendors employ RDMA to advantage, and prototypes have demonstrated
the effectiveness of many more. Here, we are concerned specifically
with NFS and NFS-style upper layer protocols; examples from Network
Appliance [DAFS, DCK+03], Fujitsu Prime Software Technologies [FJNFS,
FJDAFS] and Harvard University [KM02] are all relevant.
By layering a session binding for NFS version 4 directly atop a
standard RDMA transport, a greatly enhanced level of performance and
transparency can be supported on a wide variety of operating system
platforms. These combined capabilities alter the landscape between
local filesystems and network attached storage, enable a new level of
performance, and lead new classes of application to take advantage of
NFS.
5.1.3. Problem Statement
Two issues drive the current proposal: correctness, and performance.
Both are instances of "raising the bar" for NFS, whereby the desire
to use NFS in new classes applications can be accommodated by
providing the basic features to make such use feasible. Such
applications include tightly coupled sharing environments such as
cluster computing, high performance computing (HPC) and information
processing such as databases. These trends are explored in depth in
[NFSPS].
The first issue, correctness, exemplified among the attributes of
local filesystems, is support for exactly-once semantics. Such
semantics have not been reliably available with NFS. Server-based
duplicate request caches [CJ89] help, but do not reliably provide
strict correctness. For the type of application which is expected to
make extensive use of the high-performance RDMA-enabled environment,
the reliable provision of such semantics is a fundamental
requirement.
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Introduction of a session to NFSv4 will address these issues. With
higher performance and enhanced semantics comes the problem of
enabling advanced endpoint management, for example high-speed
trunking, multipathing and failover. These characteristics enable
availability and performance. RFC3530 presents some issues in
permitting a single clientid to access a server over multiple
connections.
A second issue encountered in common by NFS implementations is the
CPU overhead required to implement the protocol. Primary among the
sources of this overhead is the movement of data from NFS protocol
messages to its eventual destination in user buffers or aligned
kernel buffers. The data copies consume system bus bandwidth and CPU
time, reducing the available system capacity for applications.
[RDDPPS] Achieving zero-copy with NFS has to date required
sophisticated, "header cracking" hardware and/or extensive platform-
specific virtual memory mapping tricks.
Combined in this way, NFSv4, RDMA and the emerging high-speed network
fabrics will enable delivery of performance which matches that of the
fastest local filesystems, preserving the key existing local
filesystem semantics, while enhancing them by providing network
filesystem sharing semantics.
RDMA implementations generally have other interesting properties,
such as hardware assisted protocol access, and support for user space
access to I/O. RDMA is compelling here for another reason; hardware
offloaded networking support in itself does not avoid data copies,
without resorting to implementing part of the NFS protocol in the
NIC. Support of RDMA by NFS enables the highest performance at the
architecture level rather than by implementation; this enables
ubiquitous and interoperable solutions.
By providing file access performance equivalent to that of local file
systems, NFSv4 over RDMA will enable applications running on a set of
client machines to interact through an NFSv4 file system, just as
applications running on a single machine might interact through a
local file system.
This raises the issue of whether additional protocol enhancements to
enable such interaction would be desirable and what such enhancements
would be. This is a complicated issue which the working group needs
to address and will not be further discussed in this document.
5.1.4. NFSv4 Session Extension Characteristics
This draft will present a solution based upon minor versioning of
NFSv4. It will introduce a session to collect transport endpoints
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and resources such as reply caching, which in turn enables
enhancements such as trunking, failover and recovery. It will
describe use of RDMA by employing support within an underlying RPC
layer [RPCRDMA]. Most importantly, it will focus on making the best
possible use of an RDMA transport.
These extensions are proposed as elements of a new minor revision of
NFS version 4. In this draft, NFS version 4 will be referred to
generically as "NFSv4", when describing properties common to all
minor versions. When referring specifically to properties of the
original, minor version 0 protocol, "NFSv4.0" will be used, and
changes proposed here for minor version 1 will be referred to as
"NFSv4.1".
This draft proposes only changes which are strictly upward-
compatible with existing RPC and NFS Application Programming
Interfaces (APIs).
5.2. Transport Issues
The Transport Issues section of the document explores the details of
utilizing the various supported transports.
5.2.1. Session Model
The first and most evident issue in supporting diverse transports is
how to provide for their differences. This draft proposes
introducing an explicit session.
A session introduces minimal protocol requirements, and provides for
a highly useful and convenient way to manage numerous endpoint-
related issues. The session is a local construct; it represents a
named, higher-layer object to which connections can refer, and
encapsulates properties important to each associated client.
A session is a dynamically created, long-lived server object created
by a client, used over time from one or more transport connections.
Its function is to maintain the server's state relative to the
connection(s) belonging to a client instance. This state is entirely
independent of the connection itself. The session in effect becomes
the object representing an active client on a connection or set of
connections.
Clients may create multiple sessions for a single clientid, and may
wish to do so for optimization of transport resources, buffers, or
server behavior. A session could be created by the client to
represent a single mount point, for separate read and write
"channels", or for any number of other client-selected parameters.
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The session enables several things immediately. Clients may
disconnect and reconnect (voluntarily or not) without loss of context
at the server. (Of course, locks, delegations and related
associations require special handling, and generally expire in the
extended absence of an open connection.) Clients may connect
multiple transport endpoints to this common state. The endpoints may
have all the same attributes, for instance when trunked on multiple
physical network links for bandwidth aggregation or path failover.
Or, the endpoints can have specific, special purpose attributes such
as callback channels.
The NFSv4 specification does not provide for any form of flow
control; instead it relies on the windowing provided by TCP to
throttle requests. This unfortunately does not work with RDMA, which
in general provides no operation flow control and will terminate a
connection in error when limits are exceeded. Limits are therefore
exchanged when a session is created; These limits then provide maxima
within which each session's connections must operate, they are
managed within these limits as described in [RPCRDMA]. The limits
may also be modified dynamically at the server's choosing by
manipulating certain parameters present in each NFSv4.1 request.
The presence of a maximum request limit on the session bounds the
requirements of the duplicate request cache. This can be used to
advantage by a server, which can accurately determine any storage
needs and enable it to maintain duplicate request cache persistence
and to provide reliable exactly-once semantics.
Finally, given adequate connection-oriented transport security
semantics, authentication and authorization may be cached on a per-
session basis, enabling greater efficiency in the issuing and
processing of requests on both client and server. A proposal for
transparent, server-driven implementation of this in NFSv4 has been
made. [CCM] The existence of the session greatly facilitates the
implementation of this approach. This is discussed in detail in the
Authentication Efficiencies section later in this draft.
5.2.2. Connection State
In RFC3530, the combination of a connected transport endpoint and a
clientid forms the basis of connection state. While has been made to
be workable with certain limitations, there are difficulties in
correct and robust implementation. The NFSv4.0 protocol must provide
a server-initiated connection for the callback channel, and must
carefully specify the persistence of client state at the server in
the face of transport interruptions. The server has only the
client's transport address binding (the IP 4-tuple) to identify the
client RPC transaction stream and to use as a lookup tag on the
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duplicate request cache. (A useful overview of this is in [RW96].)
If the server listens on multiple adddresses, and the client connects
to more than one, it must employ different clientid's on each,
negating its ability to aggregate bandwidth and redundancy. In
effect, each transport connection is used as the server's
representation of client state. But, transport connections are
potentially fragile and transitory.
In this proposal, a session identifier is assigned by the server upon
initial session negotiation on each connection. This identifier is
used to associate additional connections, to renegotiate after a
reconnect, to provide an abstraction for the various session
properties, and to address the duplicate request cache. No
transport-specific information is used in the duplicate request cache
implementation of an NFSv4.1 server, nor in fact the RPC XID itself.
The session identifier is unique within the server's scope and may be
subject to certain server policies such as being bounded in time.
It is envisioned that the primary transport model will be connection
oriented. Connection orientation brings with it certain potential
optimizations, such as caching of per-connection properties, which
are easily leveraged through the generality of the session. However,
it is possible that in future, other transport models could be
accommodated below the session abstraction.
5.2.3. NFSv4 Channels, Sessions and Connections
There are at least two types of NFSv4 channels: the "operations"
channel used for ordinary requests from client to server, and the
"back" channel, used for callback requests from server to client.
As mentioned above, different NFSv4 operations on these channels can
lead to different resource needs. For example, server callback
operations (CB_RECALL) are specific, small messages which flow from
server to client at arbitrary times, while data transfers such as
read and write have very different sizes and asymmetric behaviors.
It is sometimes impractical for the RDMA peers (NFSv4 client and
NFSv4 server) to post buffers for these various operations on a
single connection. Commingling of requests with responses at the
client receive queue is particularly troublesome, due both to the
need to manage both solicited and unsolicited completions, and to
provision buffers for both purposes. Due to the lack of any ordering
of callback requests versus response arrivals, without any other
mechanisms, the client would be forced to allocate all buffers sized
to the worst case.
The callback requests are likely to be handled by a different task
context from that handling the responses. Significant demultiplexing
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and thread management may be required if both are received on the
same queue. However, if callbacks are relatively rare (perhaps due
to client access patterns), many of these difficulties can be
minimized.
Also, the client may wish to perform trunking of operations channel
requests for performance reasons, or multipathing for availability.
This proposal permits both, as well as many other session and
connection possibilities, by permitting each operation to carry
session membership information and to share session (and clientid)
state in order to draw upon the appropriate resources. For example,
reads and writes may be assigned to specific, optimized connections,
or sorted and separated by any or all of size, idempotency, etc.
To address the problems described above, this proposal allows
multiple sessions to share a clientid, as well as for multiple
connections to share a session.
Single Connection model:
NFSv4.1 Session
/ \
Operations_Channel [Back_Channel]
\ /
Connection
|
Multi-connection trunked model (2 operations channels shown):
NFSv4.1 Session
/ \
Operations_Channels [Back_Channel]
| | |
Connection Connection [Connection]
| | |
Multi-connection split-use model (2 mounts shown):
NFSv4.1 Session
/ \
(/home) (/usr/local - readonly)
/ \ |
Operations_Channel [Back_Channel] |
| | Operations_Channel
Connection [Connection] |
| | Connection
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|
In this way, implementation as well as resource management may be
optimized. Each session will have its own response caching and
buffering, and each connection or channel will have its own transport
resources, as appropriate. Clients which do not require certain
behaviors may optimize such resources away completely, by using
specific sessions and not even creating the additional channels and
connections.
5.2.4. Reconnection, Trunking and Failover
Reconnection after failure references stored state on the server
associated with lease recovery during the grace period. The session
provides a convenient handle for storing and managing information
regarding the client's previous state on a per- connection basis,
e.g. to be used upon reconnection. Reconnection to a previously
existing session, and its stored resources, are covered in the
"Connection Models" section below.
One important aspect of reconnection is that of RPC library support.
Traditionally, an Upper Layer RPC-based Protocol such as NFS leaves
all transport knowledge to the RPC layer implementation below it.
This allows NFS to operate over a wide variety of transports and has
proven to be a highly successful approach. The session, however,
introduces an abstraction which is, in a way, "between" RPC and
NFSv4.1. It is important that the session abstraction not have
ramifications within the RPC layer.
One such issue arises within the reconnection logic of RPC.
Previously, an explicit session binding operation, which established
session context for each new connection, was explored. This however
required that the session binding also be performed during reconnect,
which in turn required an RPC request. This additional request
requires new RPC semantics, both in implementation and the fact that
a new request is inserted into the RPC stream. Also, the binding of
a connection to a session required the upper layer to become "aware"
of connections, something the RPC layer abstraction architecturally
abstracts away. Therefore the session binding is not handled in
connection scope but instead explicitly carried in each request.
For Reliability Availability and Serviceability (RAS) issues such as
bandwidth aggregation and multipathing, clients frequently seek to
make multiple connections through multiple logical or physical
channels. The session is a convenient point to aggregate and manage
these resources.
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5.2.5. Server Duplicate Request Cache
Server duplicate request caches, while not a part of an NFS protocol,
have become a standard, even required, part of any NFS
implementation. First described in [CJ89], the duplicate request
cache was initially found to reduce work at the server by avoiding
duplicate processing for retransmitted requests. A second, and in
the long run more important benefit, was improved correctness, as the
cache avoided certain destructive non-idempotent requests from being
reinvoked.
However, such caches do not provide correctness guarantees; they
cannot be managed in a reliable, persistent fashion. The reason is
understandable - their storage requirement is unbounded due to the
lack of any such bound in the NFS protocol, and they are dependent on
transport addresses for request matching.
As proposed in this draft, the presence of maximum request count
limits and negotiated maximum sizes allows the size and duration of
the cache to be bounded, and coupled with a long-lived session
identifier, enables its persistent storage on a per-session basis.
This provides a single unified mechanism which provides the following
guarantees required in the NFSv4 specification, while extending them
to all requests, rather than limiting them only to a subset of state-
related requests:
"It is critical the server maintain the last response sent to the
client to provide a more reliable cache of duplicate non- idempotent
requests than that of the traditional cache described in [CJ89]..."
[RFC3530]
The maximum request count limit is the count of active operations,
which bounds the number of entries in the cache. Constraining the
size of operations additionally serves to limit the required storage
to the product of the current maximum request count and the maximum
response size. This storage requirement enables server- side
efficiencies.
Session negotiation allows the server to maintain other state. An
NFSv4.1 client invoking the session destroy operation will cause the
server to denegotiate (close) the session, allowing the server to
deallocate cache entries. Clients can potentially specify that such
caches not be kept for appropriate types of sessions (for example,
read-only sessions). This can enable more efficient server operation
resulting in improved response times, and more efficient sizing of
buffers and response caches.
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Similarly, it is important for the client to explicitly learn whether
the server is able to implement reliable semantics. Knowledge of
whether these semantics are in force is critical for a highly
reliable client, one which must provide transactional integrity
guarantees. When clients request that the semantics be enabled for a
given session, the session reply must inform the client if the mode
is in fact enabled. In this way the client can confidently proceed
with operations without having to implement consistency facilities of
its own.
5.3. Session Initialization and Transfer Models
Session initialization issues, and data transfer models relevant to
both TCP and RDMA are discussed in this section.
5.3.1. Session Negotiation
The following parameters are exchanged between client and server at
session creation time. Their values allow the server to properly
size resources allocated in order to service the client's requests,
and to provide the server with a way to communicate limits to the
client for proper and optimal operation. They are exchanged prior to
all session-related activity, over any transport type. Discussion of
their use is found in their descriptions as well as throughout this
section.
Maximum Requests
The client's desired maximum number of concurrent requests is
passed, in order to allow the server to size its reply cache
storage. The server may modify the client's requested limit
downward (or upward) to match its local policy and/or resources.
Over RDMA-capable RPC transports, the per-request management of
low-level transport message credits is handled within the RPC
layer. [RPCRDMA]
Maximum Request/Response Sizes
The maximum request and response sizes are exchanged in order to
permit allocation of appropriately sized buffers and request cache
entries. The size must allow for certain protocol minima,
allowing the receipt of maximally sized operations (e.g. RENAME
requests which contains two name strings). Note the maximum
request/response sizes cover the entire request/response message
and not simply the data payload as traditional NFS maximum read or
write size. Also note the server implementation may not, in fact
probably does not, require the reply cache entries to be sized as
large as the maximum response. The server may reduce the client's
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requested sizes.
Inline Padding/Alignment
The server can inform the client of any padding which can be used
to deliver NFSv4 inline WRITE payloads into aligned buffers. Such
alignment can be used to avoid data copy operations at the server
for both TCP and inline RDMA transfers. For RDMA, the client
informs the server in each operation when padding has been
applied. [RPCRDMA]
Transport Attributes
A placeholder for transport-specific attributes is provided, with
a format to be determined. Possible examples of information to be
passed in this parameter include transport security attributes to
be used on the connection, RDMA- specific attributes, legacy
"private data" as used on existing RDMA fabrics, transport Quality
of Service attributes, etc. This information is to be passed to
the peer's transport layer by local means which is currently
outside the scope of this draft, however one attribute is provided
in the RDMA case:
RDMA Read Resources
RDMA implementations must explicitly provision resources to
support RDMA Read requests from connected peers. These values
must be explicitly specified, to provide adequate resources for
matching the peer's expected needs and the connection's delay-
bandwidth parameters. The client provides its chosen value to the
server in the initial session creation, the value must be provided
in each client RDMA endpoint. The values are asymmetric and
should be set to zero at the server in order to conserve RDMA
resources, since clients do not issue RDMA Read operations in this
proposal. The result is communicated in the session response, to
permit matching of values across the connection. The value may
not be changed in the duration of the session, although a new
value may be requested as part of a new session.
5.3.2. RDMA Requirements
A complete discussion of the operation of RPC-based protocols atop
RDMA transports is in [RPCRDMA]. Where RDMA is considered, this
proposal assumes the use of such a layering; it addresses only the
upper layer issues relevant to making best use of RPC/RDMA.
A connection oriented (reliable sequenced) RDMA transport will be
required. There are several reasons for this. First, this model
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most closely reflects the general NFSv4 requirement of long-lived and
congestion-controlled transports. Second, to operate correctly over
either an unreliable or unsequenced RDMA transport, or both, would
require significant complexity in the implementation and protocol not
appropriate for a strict minor version. For example, retransmission
on connected endpoints is explicitly disallowed in the current NFSv4
draft; it would again be required with these alternate transport
characteristics. Third, the proposal assumes a specific RDMA
ordering semantic, which presents the same set of ordering and
reliability issues to the RDMA layer over such transports.
The RDMA implementation provides for making connections to other
RDMA-capable peers. In the case of the current proposals before the
RDDP working group, these RDMA connections are preceded by a
"streaming" phase, where ordinary TCP (or NFS) traffic might flow.
However, this is not assumed here and sizes and other parameters are
explicitly exchanged upon a session entering RDMA mode.
5.3.3. RDMA Connection Resources
On transport endpoints which support automatic RDMA mode, that is,
endpoints which are created in the RDMA-enabled state, a single,
preposted buffer must initially be provided by both peers, and the
client session negotiation must be the first exchange.
On transport endpoints supporting dynamic negotiation, a more
sophisticated negotiation is possible, but is not discussed in the
current draft.
RDMA imposes several requirements on upper layer consumers.
Registration of memory and the need to post buffers of a specific
size and number for receive operations are a primary consideration.
Registration of memory can be a relatively high-overhead operation,
since it requires pinning of buffers, assignment of attributes (e.g.
readable/writable), and initialization of hardware translation.
Preregistration is desirable to reduce overhead. These registrations
are specific to hardware interfaces and even to RDMA connection
endpoints, therefore negotiation of their limits is desirable to
manage resources effectively.
Following the basic registration, these buffers must be posted by the
RPC layer to handle receives. These buffers remain in use by the
RPC/NFSv4 implementation; the size and number of them must be known
to the remote peer in order to avoid RDMA errors which would cause a
fatal error on the RDMA connection.
The session provides a natural way for the server to manage resource
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allocation to each client rather than to each transport connection
itself. This enables considerable flexibility in the administration
of transport endpoints.
5.3.4. TCP and RDMA Inline Transfer Model
The basic transfer model for both TCP and RDMA is referred to as
"inline". For TCP, this is the only transfer model supported, since
TCP carries both the RPC header and data together in the data stream.
For RDMA, the RDMA Send transfer model is used for all NFS requests
and replies, but data is optionally carried by RDMA Writes or RDMA
Reads. Use of Sends is required to ensure consistency of data and to
deliver completion notifications. The pure-Send method is typically
used where the data payload is small, or where for whatever reason
target memory for RDMA is not available.
Inline message exchange
Client Server
: Request :
Send : ------------------------------> : untagged
: : buffer
: Response :
untagged : <------------------------------ : Send
buffer : :
Client Server
: Read request :
Send : ------------------------------> : untagged
: : buffer
: Read response with data :
untagged : <------------------------------ : Send
buffer : :
Client Server
: Write request with data :
Send : ------------------------------> : untagged
: : buffer
: Write response :
untagged : <------------------------------ : Send
buffer : :
Responses must be sent to the client on the same connection that the
request was sent. It is important that the server does not assume
any specific client implementation, in particular whether connections
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within a session share any state at the client. This is also
important to preserve ordering of RDMA operations, and especially
RMDA consistency. Additionally, it ensures that the RPC RDMA layer
makes no requirement of the RDMA provider to open its memory
registration handles (Steering Tags) beyond the scope of a single
RDMA connection. This is an important security consideration.
Two values must be known to each peer prior to issuing Sends: the
maximum number of sends which may be posted, and their maximum size.
These values are referred to, respectively, as the message credits
and the maximum message size. While the message credits might vary
dynamically over the duration of the session, the maximum message
size does not. The server must commit to preserving this number of
duplicate request cache entires, and preparing a number of receive
buffers equal to or greater than its currently advertised credit
value, each of the advertised size. These ensure that transport
resources are allocated sufficient to receive the full advertised
limits.
Note that the server must post the maximum number of session requests
to each client operations channel. The client is not required to
spread its requests in any particular fashion across connections
within a session. If the client wishes, it may create multiple
sessions, each with a single or small number of operations channels
to provide the server with this resource advantage. Or, over RDMA
the server may employ a "shared receive queue". The server can in
any case protect its resources by restricting the client's request
credits.
While tempting to consider, it is not possible to use the TCP window
as an RDMA operation flow control mechanism. First, to do so would
violate layering, requiring both senders to be aware of the existing
TCP outbound window at all times. Second, since requests are of
variable size, the TCP window can hold a widely variable number of
them, and since it cannot be reduced without actually receiving data,
the receiver cannot limit the sender. Third, any middlebox
interposing on the connection would wreck any possible scheme.
[MIDTAX] In this proposal, maximum request count limits are exchanged
at the session level to allow correct provisioning of receive buffers
by transports.
When operating over TCP or other similar transport, request limits
and sizes are still employed in NFSv4.1, but instead of being
required for correctness, they provide the basis for efficient server
implementation of the duplicate request cache. The limits are chosen
based upon the expected needs and capabilities of the client and
server, and are in fact arbitrary. Sizes may be specified by the
client as zero (requesting the server's preferred or optimal value),
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and request limits may be chosen in proportion to the client's
capabilities. For example, a limit of 1000 allows 1000 requests to
be in progress, which may generally be far more than adequate to keep
local networks and servers fully utilized.
Both client and server have independent sizes and buffering, but over
RDMA fabrics client credits are easily managed by posting a receive
buffer prior to sending each request. Each such buffer may not be
completed with the corresponding reply, since responses from NFSv4
servers arrive in arbitrary order. When an operations channel is
also used for callbacks, the client must account for callback
requests by posting additional buffers. Note that implementation-
specific facilities such as a shared receive queue may also allow
optimization of these allocations.
When a session is created, the client requests a preferred buffer
size, and the server provides its answer. The server posts all
buffers of at least this size. The client must comply by not sending
requests greater than this size. It is recommended that server
implementations do all they can to accommodate a useful range of
possible client requests. There is a provision in [RPCRDMA] to allow
the sending of client requests which exceed the server's receive
buffer size, but it requires the server to "pull" the client's
request as a "read chunk" via RDMA Read. This introduces at least
one additional network roundtrip, plus other overhead such as
registering memory for RDMA Read at the client and additional RDMA
operations at the server, and is to be avoided.
An issue therefore arises when considering the NFSv4 COMPOUND
procedures. Since an arbitrary number (total size) of operations can
be specified in a single COMPOUND procedure, its size is effectively
unbounded. This cannot be supported by RDMA Sends, and therefore
this size negotiation places a restriction on the construction and
maximum size of both COMPOUND requests and responses. If a COMPOUND
results in a reply at the server that is larger than can be sent in
an RDMA Send to the client, then the COMPOUND must terminate and the
operation which causes the overflow will provide a TOOSMALL error
status result.
5.3.5. RDMA Direct Transfer Model
Placement of data by explicitly tagged RDMA operations is referred to
as "direct" transfer. This method is typically used where the data
payload is relatively large, that is, when RDMA setup has been
performed prior to the operation, or when any overhead for setting up
and performing the transfer is regained by avoiding the overhead of
processing an ordinary receive.
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The client advertises RDMA buffers in this proposed model, and not
the server. This means the "XDR Decoding with Read Chunks" described
in [RPCRDMA] is not employed by NFSv4.1 replies, and instead all
results transferred via RDMA to the client employ "XDR Decoding with
Write Chunks". There are several reasons for this.
First, it allows for a correct and secure mode of transfer. The
client may advertise specific memory buffers only during specific
times, and may revoke access when it pleases. The server is not
required to expose copies of local file buffers for individual
clients, or to lock or copy them for each client access.
Second, client credits based on fixed-size request buffers are easily
managed on the server, but for the server additional management of
buffers for client RDMA Reads is not well-bounded. For example, the
client may not perform these RDMA Read operations in a timely
fashion, therefore the server would have to protect itself against
denial-of-service on these resources.
Third, it reduces network traffic, since buffer exposure outside the
scope and duration of a single request/response exchange necessitates
additional memory management exchanges.
There are costs associated with this decision. Primary among them is
the need for the server to employ RDMA Read for operations such as
large WRITE. The RDMA Read operation is a two-way exchange at the
RDMA layer, which incurs additional overhead relative to RDMA Write.
Additionally, RDMA Read requires resources at the data source (the
client in this proposal) to maintain state and to generate replies.
These costs are overcome through use of pipelining with credits, with
sufficient RDMA Read resources negotiated at session initiation, and
appropriate use of RDMA for writes by the client - for example only
for transfers above a certain size.
A description of which NFSv4 operation results are eligible for data
transfer via RDMA Write is in [NFSDDP]. There are only two such
operations: READ and READLINK. When XDR encoding these requests on
an RDMA transport, the NFSv4.1 client must insert the appropriate
xdr_write_list entries to indicate to the server whether the results
should be transferred via RDMA or inline with a Send. As described
in [NFSDDP], a zero-length write chunk is used to indicate an inline
result. In this way, it is unnecessary to create new operations for
RDMA-mode versions of READ and READLINK.
Another tool to avoid creation of new, RDMA-mode operations is the
Reply Chunk [RPCRDMA], which is used by RPC in RDMA mode to return
large replies via RDMA as if they were inline. Reply chunks are used
for operations such as READDIR, which returns large amounts of
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information, but in many small XDR segments. Reply chunks are
offered by the client and the server can use them in preference to
inline. Reply chunks are transparent to upper layers such as NFSv4.
In any very rare cases where another NFSv4.1 operation requires
larger buffers than were negotiated when the session was created (for
example extraordinarily large RENAMEs), the underlying RPC layer may
support the use of "Message as an RDMA Read Chunk" and "RDMA Write of
Long Replies" as described in [RPCRDMA]. No additional support is
required in the NFSv4.1 client for this. The client should be
certain that its requested buffer sizes are not so small as to make
this a frequent occurrence, however.
All operations are initiated by a Send, and are completed with a
Send. This is exactly as in conventional NFSv4, but under RDMA has a
significant purpose: RDMA operations are not complete, that is,
guaranteed consistent, at the data sink until followed by a
successful Send completion (i.e. a receive). These events provide a
natural opportunity for the initiator (client) to enable and later
disable RDMA access to the memory which is the target of each
operation, in order to provide for consistent and secure operation.
The RDMAP Send with Invalidate operation may be worth employing in
this respect, as it relieves the client of certain overhead in this
case.
A "onetime" boolean advisory to each RDMA region might become a hint
to the server that the client will use the three-tuple for only one
NFSv4 operation. For a transport such as iWARP, the server can
assist the client in invalidating the three-tuple by performing a
Send with Solicited Event and Invalidate. The server may ignore this
hint, in which case the client must perform a local invalidate after
receiving the indication from the server that the NFSv4 operation is
complete. This may be considered in a future version of this draft
and [NFSDDP].
In a trusted environment, it may be desirable for the client to
persistently enable RDMA access by the server. Such a model is
desirable for the highest level of efficiency and lowest overhead.
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RDMA message exchanges
Client Server
: Direct Read Request :
Send : ------------------------------> : untagged
: : buffer
: Segment :
tagged : <------------------------------ : RDMA Write
buffer : : :
: [Segment] :
tagged : <------------------------------ : [RDMA Write]
buffer : :
: Direct Read Response :
untagged : <------------------------------ : Send (w/Inv.)
buffer : :
Client Server
: Direct Write Request :
Send : ------------------------------> : untagged
: : buffer
: Segment :
tagged : v------------------------------ : RDMA Read
buffer : +-----------------------------> :
: : :
: [Segment] :
tagged : v------------------------------ : [RDMA Read]
buffer : +-----------------------------> :
: :
: Direct Write Response :
untagged : <------------------------------ : Send (w/Inv.)
buffer : :
5.4. Connection Models
There are three scenarios in which to discuss the connection model.
Each will be discussed individually, after describing the common case
encountered at initial connection establishment.
After a successful connection, the first request proceeds, in the
case of a new client association, to initial session creation, and
then optionally to session callback channel binding, prior to regular
operation.
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Commonly, each new client "mount" will be the action which drives
creation of a new session. However there are any number of other
approaches. Clients may choose to share a single connection and
session among all their mount points. Or, clients may support
trunking, where additional connections are created but all within a
single session. Alternatively, the client may choose to create
multiple sessions, each tuned to the buffering and reliability needs
of the mount point. For example, a readonly mount can sharply reduce
its write buffering and also makes no requirement for the server to
support reliable duplicate request caching.
Similarly, the client can choose among several strategies for
clientid usage. Sessions can share a single clientid, or create new
clientids as the client deems appropriate. For kernel-based clients
which service multiple authenticated users, a single clientid shared
across all mount points is generally the most appropriate and
flexible approach. For example, all the client's file operations may
wish to share locking state and the local client kernel takes the
responsibility for arbitrating access locally. For clients choosing
to support other authentication models, perhaps example userspace
implementations, a new clientid is indicated. Through use of session
create options, both models are supported at the client's choice.
Since the session is explicitly created and destroyed by the client,
and each client is uniquely identified, the server may be
specifically instructed to discard unneeded presistent state. For
this reason, it is possible that a server will retain any previous
state indefinitely, and place its destruction under administrative
control. Or, a server may choose to retain state for some
configurable period, provided that the period meets other NFSv4
requirements such as lease reclamation time, etc. However, since
discarding this state at the server may affect the correctness of the
server as seen by the client across network partitioning, such
discarding of state should be done only in a conservative manner.
Each client request to the server carries a new SEQUENCE operation
within each COMPOUND, which provides the session context. This
session context then governs the request control, duplicate request
caching, and other persistent parameters managed by the server for a
session.
5.4.1. TCP Connection Model
The following is a schematic diagram of the NFSv4.1 protocol
exchanges leading up to normal operation on a TCP stream.
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Client Server
TCPmode : Create Clientid(nfs_client_id4) : TCPmode
: ------------------------------> :
: :
: Clientid reply(clientid, ...) :
: <------------------------------ :
: :
: Create Session(clientid, size S, :
: maxreq N, STREAM, ...) :
: ------------------------------> :
: :
: Session reply(sessionid, size S', :
: maxreq N') :
: <------------------------------ :
: :
: <normal operation> :
: ------------------------------> :
: <------------------------------ :
: : :
No net additional exchange is added to the initial negotiation by
this proposal. In the NFSv4.1 exchange, the CREATECLIENTID replaces
SETCLIENTID (eliding the callback "clientaddr4" addressing) and
CREATESESSION subsumes the function of SETCLIENTID_CONFIRM, as
described elsewhere in this document. Callback channel binding is
optional, as in NFSv4.0. Note that the STREAM transport type is
shown above, but since the transport mode remains unchanged and
transport attributes are not necessarily exchanged, DEFAULT could
also be passed.
5.4.2. Negotiated RDMA Connection Model
One possible design which has been considered is to have a
"negotiated" RDMA connection model, supported via use of a session
bind operation as a required first step. However due to issues
mentioned earlier, this proved problematic. This section remains as
a reminder of that fact, and it is possible such a mode can be
supported.
It is not considered critical that this be supported for two reasons.
One, the session persistence provides a way for the server to
remember important session parameters, such as sizes and maximum
request counts. These values can be used to restore the endpoint
prior to making the first reply. Two, there are currently no
critical RDMA parameters to set in the endpoint at the server side of
the connection. RDMA Read resources, which are in general not
settable after entering RDMA mode, are set only at the client - the
originator of the connection. Therefore as long as the RDMA provider
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supports an automatic RDMA connection mode, no further support is
required from the NFSv4.1 protocol for reconnection.
Note, the client must provide at least as many RDMA Read resources to
its local queue for the benefit of the server when reconnecting, as
it used when negotiating the session. If this value is no longer
appropriate, the client should resynchronize its session state,
destroy the existing session, and start over with the more
appropriate values.
5.4.3. Automatic RDMA Connection Model
The following is a schematic diagram of the NFSv4.1 protocol
exchanges performed on an RDMA connection.
Client Server
RDMAmode : : : RDMAmode
: : :
Prepost : : : Prepost
receive : : : receive
: :
: Create Clientid(nfs_client_id4) :
: ------------------------------> :
: : Prepost
: Clientid reply(clientid, ...) : receive
: <------------------------------ :
Prepost : :
receive : Create Session(clientid, size S, :
: maxreq N, RDMA ...) :
: ------------------------------> :
: : Prepost <=N'
: Session reply(sessionid, size S', : receives of
: maxreq N') : size S'
: <------------------------------ :
: :
: <normal operation> :
: ------------------------------> :
: <------------------------------ :
: : :
5.5. Buffer Management, Transfer, Flow Control
Inline operations in NFSv4.1 behave effectively the same as TCP
sends. Procedure results are passed in a single message, and its
completion at the client signal the receiving process to inspect the
message.
RDMA operations are performed solely by the server in this proposal,
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as described in the previous "RDMA Direct Model" section. Since
server RDMA operations do not result in a completion at the client,
and due to ordering rules in RDMA transports, after all required RDMA
operations are complete, a Send (Send with Solicited Event for iWARP)
containing the procedure results is performed from server to client.
This Send operation will result in a completion which will signal the
client to inspect the message.
In the case of client read-type NFSv4 operations, the server will
have issued RDMA Writes to transfer the resulting data into client-
advertised buffers. The subsequent Send operation performs two
necessary functions: finalizing any active or pending DMA at the
client, and signaling the client to inspect the message.
In the case of client write-type NFSv4 operations, the server will
have issued RDMA Reads to fetch the data from the client-advertised
buffers. No data consistency issues arise at the client, but the
completion of the transfer must be acknowledged, again by a Send from
server to client.
In either case, the client advertises buffers for direct (RDMA style)
operations. The client may desire certain advertisement limits, and
may wish the server to perform remote invalidation on its behalf when
the server has completed its RDMA. This may be considered in a
future version of this draft.
In the absence of remote invalidation, the client may perform its
own, local invalidation after the operation completes. This
invalidation should occur prior to any RPCSEC GSS integrity checking,
since a validly remotely accessible buffer can possibly be modified
by the peer. However, after invalidation and the contents integrity
checked, the contents are locally secure.
Credit updates over RDMA transports are supported at the RPC layer as
described in [RPCRDMA]. In each request, the client requests a
desired number of credits to be made available to the connection on
which it sends the request. The client must not send more requests
than the number which the server has previously advertised, or in the
case of the first request, only one. If the client exceeds its
credit limit, the connection may close with a fatal RDMA error.
The server then executes the request, and replies with an updated
credit count accompanying its results. Since replies are sequenced
by their RDMA Send order, the most recent results always reflect the
server's limit. In this way the client will always know the maximum
number of requests it may safely post.
Because the client requests an arbitrary credit count in each
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request, it is relatively easy for the client to request more, or
fewer, credits to match its expected need. A client that discovered
itself frequently queuing outgoing requests due to lack of server
credits might increase its requested credits proportionately in
response. Or, a client might have a simple, configurable number.
The protocol also provides a per-operation "maxslot" exchange to
assist in dynamic adjustment at the session level, described in a
later section.
Occasionally, a server may wish to reduce the total number of credits
it offers a certain client on a connection. This could be
encountered if a client were found to be consuming its credits
slowly, or not at all. A client might notice this itself, and reduce
its requested credits in advance, for instance requesting only the
count of operations it currently has queued, plus a few as a base for
starting up again. Such mechanisms can, however, be potentially
complicated and are implementation-defined. The protocol does not
require them.
Because of the way in which RDMA fabrics function, it is not possible
for the server (or client back channel) to cancel outstanding receive
operations. Therefore, effectively only one credit can be withdrawn
per receive completion. The server (or client back channel) would
simply not replenish a receive operation when replying. The server
can still reduce the available credit advertisement in its replies to
the target value it desires, as a hint to the client that its credit
target is lower and it should expect it to be reduced accordingly.
Of course, even if the server could cancel outstanding receives, it
cannot do so, since the client may have already sent requests in
expectation of the previous limit.
This brings out an interesting scenario similar to the client
reconnect discussed earlier in "Connection Models". How does the
server reduce the credits of an inactive client?
One approach is for the server to simply close such a connection and
require the client to reconnect at a new credit limit. This is
acceptable, if inefficient, when the connection setup time is short
and where the server supports persistent session semantics.
A better approach is to provide a back channel request to return the
operations channel credits. The server may request the client to
return some number of credits, the client must comply by performing
operations on the operations channel, provided of course that the
request does not drop the client's credit count to zero (in which
case the connection would deadlock). If the client finds that it has
no requests with which to consume the credits it was previously
granted, it must send zero-length Send RDMA operations, or NULL NFSv4
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operations in order to return the resources to the server. If the
client fails to comply in a timely fashion, the server can recover
the resources by breaking the connection.
While in principle, the back channel credits could be subject to a
similar resource adjustment, in practice this is not an issue, since
the back channel is used purely for control and is expected to be
statically provisioned.
It is important to note that in addition to maximum request counts,
the sizes of buffers are negotiated per-session. This permits the
most efficient allocation of resources on both peers. There is an
important requirement on reconnection: the sizes posted by the server
at reconnect must be at least as large as previously used, to allow
recovery. Any replies that are replayed from the server's duplicate
request cache must be able to be received into client buffers. In
the case where a client has received replies to all its retried
requests (and therefore received all its expected responses), then
the client may disconnect and reconnect with different buffers at
will, since no cache replay will be required.
5.6. Retry and Replay
NFSv4.0 forbids retransmission on active connections over reliable
transports; this includes connected-mode RDMA. This restriction must
be maintained in NFSv4.1.
If one peer were to retransmit a request (or reply), it would consume
an additional credit on the other. If the server retransmitted a
reply, it would certainly result in an RDMA connection loss, since
the client would typically only post a single receive buffer for each
request. If the client retransmitted a request, the additional
credit consumed on the server might lead to RDMA connection failure
unless the client accounted for it and decreased its available
credit, leading to wasted resources.
RDMA credits present a new issue to the duplicate request cache in
NFSv4.1. The request cache may be used when a connection within a
session is lost, such as after the client reconnects. Credit
information is a dynamic property of the connection, and stale values
must not be replayed from the cache. This implies that the request
cache contents must not be blindly used when replies are issued from
it, and credit information appropriate to the channel must be
refreshed by the RPC layer.
Finally, RDMA fabrics do not guarantee that the memory handles
(Steering Tags) within each rdma three-tuple are valid on a scope
outside that of a single connection. Therefore, handles used by the
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direct operations become invalid after connection loss. The server
must ensure that any RDMA operations which must be replayed from the
request cache use the newly provided handle(s) from the most recent
request.
5.7. The Back Channel
The NFSv4 callback operations present a significant resource problem
for the RDMA enabled client. Clearly, callbacks must be negotiated
in the way credits are for the ordinary operations channel for
requests flowing from client to server. But, for callbacks to arrive
on the same RDMA endpoint as operation replies would require
dedicating additional resources, and specialized demultiplexing and
event handling. Or, callbacks may not require RDMA sevice at all
(they do not normally carry substantial data payloads). It is highly
desirable to streamline this critical path via a second
communications channel.
The session callback channel binding facility is designed for exactly
such a situation, by dynamically associating a new connected endpoint
with the session, and separately negotiating sizes and counts for
active callback channel operations. The binding operation is
firewall-friendly since it does not require the server to initiate
the connection.
This same method serves as well for ordinary TCP connection mode. It
is expected that all NFSv4.1 clients may make use of the session
facility to streamline their design.
The back channel functions exactly the same as the operations channel
except that no RDMA operations are required to perform transfers,
instead the sizes are required to be sufficiently large to carry all
data inline, and of course the client and server reverse their roles
with respect to which is in control of credit management. The same
rules apply for all transfers, with the server being required to flow
control its callback requests.
The back channel is optional. If not bound on a given session, the
server must not issue callback operations to the client. This in
turn implies that such a client must never put itself in the
situation where the server will need to do so, lest the client lose
its connection by force, or its operation be incorrect. For the same
reason, if a back channel is bound, the client is subject to
revocation of its delegations if the back channel is lost. Any
connection loss should be corrected by the client as soon as
possible.
This can be convenient for the NFSv4.1 client; if the client expects
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to make no use of back channel facilities such as delegations, then
there is no need to create it. This may save significant resources
and complexity at the client.
For these reasons, if the client wishes to use the back channel, that
channel must be bound first, before using the operations channel. In
this way, the server will not find itself in a position where it will
send callbacks on the operations channel when the client is not
prepared for them.
There is one special case, that where the back channel is bound in
fact to the operations channel's connection. This configuration
would be used normally over a TCP stream connection to exactly
implement the NFSv4.0 behavior, but over RDMA would require complex
resource and event management at both sides of the connection. The
server is not required to accept such a bind request on an RDMA
connection for this reason, though it is recommended.
5.8. COMPOUND Sizing Issues
Very large responses may pose duplicate request cache issues. Since
servers will want to bound the storage required for such a cache, the
unlimited size of response data in COMPOUND may be troublesome. If
COMPOUND is used in all its generality, then the inclusion of certain
non-idempotent operations within a single COMPOUND request may render
the entire request non-idempotent. (For example, a single COMPOUND
request which read a file or symbolic link, then removed it, would be
obliged to cache the data in order to allow identical replay).
Therefore, many requests might include operations that return any
amount of data.
It is not satisfactory for the server to reject COMPOUNDs at will
with NFS4ERR_RESOURCE when they pose such difficulties for the
server, as this results in serious interoperability problems.
Instead, any such limits must be explicitly exposed as attributes of
the session, ensuring that the server can explicitly support any
duplicate request cache needs at all times.
5.9. Data Alignment
A negotiated data alignment enables certain scatter/gather
optimizations. A facility for this is supported by [RPCRDMA]. Where
NFS file data is the payload, specific optimizations become highly
attractive.
Header padding is requested by each peer at session initiation, and
may be zero (no padding). Padding leverages the useful property that
RDMA receives preserve alignment of data, even when they are placed
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into anonymous (untagged) buffers. If requested, client inline
writes will insert appropriate pad bytes within the request header to
align the data payload on the specified boundary. The client is
encouraged to be optimistic and simply pad all WRITEs within the RPC
layer to the negotiated size, in the expectation that the server can
use them efficiently.
It is highly recommended that clients offer to pad headers to an
appropriate size. Most servers can make good use of such padding,
which allows them to chain receive buffers in such a way that any
data carried by client requests will be placed into appropriate
buffers at the server, ready for filesystem processing. The
receiver's RPC layer encounters no overhead from skipping over pad
bytes, and the RDMA layer's high performance makes the insertion and
transmission of padding on the sender a significant optimization. In
this way, the need for servers to perform RDMA Read to satisfy all
but the largest client writes is obviated. An added benefit is the
reduction of message roundtrips on the network - a potentially good
trade, where latency is present.
The value to choose for padding is subject to a number of criteria.
A primary source of variable-length data in the RPC header is the
authentication information, the form of which is client-determined,
possibly in response to server specification. The contents of
COMPOUNDs, sizes of strings such as those passed to RENAME, etc. all
go into the determination of a maximal NFSv4 request size and
therefore minimal buffer size. The client must select its offered
value carefully, so as not to overburden the server, and vice- versa.
The payoff of an appropriate padding value is higher performance.
Sender gather:
|RPC Request|Pad bytes|Length| -> |User data...|
\------+---------------------/ \
\ \
\ Receiver scatter: \-----------+- ...
/-----+----------------\ \ \
|RPC Request|Pad|Length| -> |FS buffer|->|FS buffer|->...
In the above case, the server may recycle unused buffers to the next
posted receive if unused by the actual received request, or may pass
the now-complete buffers by reference for normal write processing.
For a server which can make use of it, this removes any need for data
copies of incoming data, without resorting to complicated end-to-end
buffer advertisement and management. This includes most kernel-based
and integrated server designs, among many others. The client may
perform similar optimizations, if desired.
Padding is negotiated by the session creation operation, and
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subsequently used by the RPC RDMA layer, as described in [RPCRDMA].
5.10. NFSv4 Integration
The following section discusses the integration of the proposed RDMA
extensions with NFSv4.0.
5.10.1. Minor Versioning
Minor versioning is the existing facility to extend the NFSv4
protocol, and this proposal takes that approach.
Minor versioning of NFSv4 is relatively restrictive, and allows for
tightly limited changes only. In particular, it does not permit
adding new "procedures" (it permits adding only new "operations").
Interoperability concerns make it impossible to consider additional
layering to be a minor revision. This somewhat limits the changes
that can be proposed when considering extensions.
To support the duplicate request cache integrated with sessions and
request control, it is desirable to tag each request with an
identifier to be called a Slotid. This identifier must be passed by
NFSv4 when running atop any transport, including traditional TCP.
Therefore it is not desirable to add the Slotid to a new RPC
transport, even though such a transport is indicated for support of
RDMA. This draft and [RPCRDMA] do not propose such an approach.
Instead, this proposal conforms to the requirements of NFSv4 minor
versioning, through the use of a new operation within NFSv4 COMPOUND
procedures as detailed below.
If sessions are in use for a given clientid, this same clientid
cannot be used for non-session NFSv4 operation, including NFSv4.0.
Because the server will have allocated session-specific state to the
active clientid, it would be an unnecessary burden on the server
implementor to support and account for additional, non- session
traffic, in addition to being of no benefit. Therefore this proposal
prohibits a single clientid from doing this. Nevertheless, employing
a new clientid for such traffic is supported.
5.10.2. Slot Identifiers and Server Duplicate Request Cache
The presence of deterministic maximum request limits on a session
enables in-progress requests to be assigned unique values with useful
properties.
The RPC layer provides a transaction ID (xid), which, while required
to be unique, is not especially convenient for tracking requests.
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The transaction ID is only meaningful to the issuer (client), it
cannot be interpreted at the server except to test for equality with
previously issued requests. Because RPC operations may be completed
by the server in any order, many transaction IDs may be outstanding
at any time. The client may therefore perform a computationally
expensive lookup operation in the process of demultiplexing each
reply.
In the proposal, there is a limit to the number of active requests.
This immediately enables a convenient, computationally efficient
index for each request which is designated as a Slot Identifier, or
slotid.
When the client issues a new request, it selects a slotid in the
range 0..N-1, where N is the server's current "totalrequests" limit
granted the client on the session over which the request is to be
issued. The slotid must be unused by any of the requests which the
client has already active on the session. "Unused" here means the
client has no outstanding request for that slotid. Because the slot
id is always an integer in the range 0..N-1, client implementations
can use the slotid from a server response to efficiently match
responses with outstanding requests, such as, for example, by using
the slotid to index into a outstanding request array. This can be
used to avoid expensive hashing and lookup functions in the
performace-critical receive path.
The sequenceid, which accompanies the slotid in each request, is
important for a second, important check at the server: it must be
able to be determined efficiently whether a request using a certain
slotid is a retransmit or a new, never-before-seen request. It is
not feasible for the client to assert that it is retransmitting to
implement this, because for any given request the client cannot know
the server has seen it unless the server actually replies. Of
course, if the client has seen the server's reply, the client would
not retransmit!
The sequenceid must increase monotonically for each new transmit of a
given slotid, and must remain unchanged for any retransmission. The
server must in turn compare each newly received request's sequenceid
with the last one previously received for that slotid, to see if the
new request is:
A new request, in which the sequenceid is greater than that
previously seen in the slot (accounting for sequence wraparound).
The server proceeds to execute the new request.
A retransmitted request, in which the sequenceid is equal to that
last seen in the slot. Note that this request may be either
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complete, or in progress. The server performs replay processing
in these cases.
A misordered duplicate, in which the sequenceid is less than that
previously seen in the slot. The server must drop the incoming
request, which may imply dropping the connection if the transport
is reliable, as dictated by section 3.1.1 of [RFC3530].
This last condition is possible on any connection, not just
unreliable, unordered transports. Delayed behavior on abandoned TCP
connections which are not yet closed at the server, or pathological
client implementations can cause it, among other causes. Therefore,
the server may wish to harden itself against certain repeated
occurrences of this, as it would for retransmissions in [RFC3530].
It is recommended, though not necessary for protocol correctness,
that the client simply increment the sequenceid by one for each new
request on each slotid. This reduces the wraparound window to a
minimum, and is useful for tracing and avoidance of possible
implementation errors.
The client may however, for implementation-specific reasons, choose a
different algorithm. For example it might maintain a single sequence
space for all slots in the session - e.g. employing the RPC XID
itself. The sequenceid, in any case, is never interpreted by the
server for anything but to test by comparison with previously seen
values.
The server may thereby use the slotid, in conjunction with the
sessionid and sequenceid, within the SEQUENCE portion of the request
to maintain its duplicate request cache (DRC) for the session, as
opposed to the traditional approach of ONC RPC applications that use
the XID along with certain transport information [RW96].
Unlike the XID, the slotid is always within a specific range; this
has two implications. The first implication is that for a given
session, the server need only cache the results of a limited number
of COMPOUND requests. The second implication derives from the first,
which is unlike XID-indexed DRCs, the slotid DRC by its nature cannot
be overflowed. Through use of the sequenceid to identify
retransmitted requests, it is notable that the server does not need
to actually cache the request itself, reducing the storage
requirements of the DRC further. These new facilities makes it
practical to maintain all the required entries for an effective DRC.
The slotid and sequenceid therefore take over the traditional role of
the port number in the server DRC implementation, and the session
replaces the IP address. This approach is considerably more portable
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and completely robust - it is not subject to the frequent
reassignment of ports as clients reconnect over IP networks. In
addition, the RPC XID is not used in the reply cache, enhancing
robustness of the cache in the face of any rapid reuse of XIDs by the
client.
It is required to encode the slotid information into each request in
a way that does not violate the minor versioning rules of the NFSv4.0
specification. This is accomplished here by encoding it in a control
operation within each NFSv4.1 COMPOUND and CB_COMPOUND procedure.
The operation easily piggybacks within existing messages. The
implementation section of this document describes the specific
proposal.
In general, the receipt of a new sequenced request arriving on any
valid slot is an indication that the previous DRC contents of that
slot may be discarded. In order to further assist the server in slot
management, the client is required to use the lowest available slot
when issuing a new request. In this way, the server may be able to
retire additional entries.
However, in the case where the server is actively adjusting its
granted maximum request count to the client, it may not be able to
use receipt of the slotid to retire cache entries. The slotid used
in an incoming request may not reflect the server's current idea of
the client's session limit, because the request may have been sent
from the client before the update was received. Therefore, in the
downward adjustment case, the server may have to retain a number of
duplicate request cache entries at least as large as the old value,
until operation sequencing rules allow it to infer that the client
has seen its reply.
The SEQUENCE (and CB_SEQUENCE) operation also carries a "maxslot"
value which carries additional client slot usage information. The
client must always provide its highest-numbered outstanding slot
value in the maxslot argument, and the server may reply with a new
recognized value. The client should in all cases provide the most
conservative value possible, although it can be increased somewhat
above the actual instantaneous usage to maintain some minimum or
optimal level. This provides a way for the client to yield unused
request slots back to the server, which in turn can use the
information to reallocate resources. Obviously, maxslot can never be
zero, or the session would deadlock.
The server also provides a target maxslot value to the client, which
is an indication to the client of the maxslot the server wishes the
client to be using. This permits the server to withdraw (or add)
resources from a client that has been found to not be using them, in
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order to more fairly share resources among a varying level of demand
from other clients. The client must always comply with the server's
value updates, since they indicate newly established hard limits on
the client's access to session resources. However, because of
request pipelining, the client may have active requests in flight
reflecting prior values, therefore the server must not immediately
require the client to comply.
It is worthwhile to note that Sprite RPC [BW87] defined a "channel"
which in some ways is similar to the slotid proposed here. Sprite
RPC used channels to implement parallel request processing and
request/response cache retirement.
5.10.3. COMPOUND and CB_COMPOUND
Support for per-operation control can be piggybacked onto NFSv4
COMPOUNDs with full transparency, by placing such facilities into
their own, new operation, and placing this operation first in each
COMPOUND under the new NFSv4 minor protocol revision. The contents
of the operation would then apply to the entire COMPOUND.
Recall that the NFSv4 minor revision is contained within the COMPOUND
header, encoded prior to the COMPOUNDed operations. By simply
requiring that the new operation always be contained in NFSv4 minor
COMPOUNDs, the control protocol can piggyback perfectly with each
request and response.
In this way, the NFSv4 RDMA Extensions may stay in compliance with
the minor versioning requirements specified in section 10 of
[RFC3530].
Referring to section 13.1 of the same document, the proposed session-
enabled COMPOUND and CB_COMPOUND have the form:
+-----+--------------+-----------+------------+-----------+----
| tag | minorversion | numops | control op | op + args | ...
| | (== 1) | (limited) | + args | |
+-----+--------------+-----------+------------+-----------+----
and the reply's structure is:
+------------+-----+--------+-------------------------------+--//
|last status | tag | numres | status + control op + results | //
+------------+-----+--------+-------------------------------+--//
//-----------------------+----
// status + op + results | ...
//-----------------------+----
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The single control operation within each NFSv4.1 COMPOUND defines the
context and operational session parameters which govern that COMPOUND
request and reply. Placing it first in the COMPOUND encoding is
required in order to allow its processing before other operations in
the COMPOUND.
5.10.4. eXternal Data Representation Efficiency
RDMA is a copy avoidance technology, and it is important to maintain
this efficiency when decoding received messages. Traditional XDR
implementations frequently use generated unmarshaling code to convert
objects to local form, incurring a data copy in the process (in
addition to subjecting the caller to recursive calls, etc). Often,
such conversions are carried out even when no size or byte order
conversion is necessary.
It is recommended that implementations pay close attention to the
details of memory referencing in such code. It is far more efficient
to inspect data in place, using native facilities to deal with word
size and byte order conversion into registers or local variables,
rather than formally (and blindly) performing the operation via
fetch, reallocate and store.
Of particular concern is the result of the READDIR operation, in
which such encoding abounds.
5.10.5. Effect of Sessions on Existing Operations
The use of a session replaces the use of the SETCLIENTID and
SETCLIENTID_CONFIRM operations, and allows certain simplification of
the RENEW and callback addressing mechanisms in the base protocol.
The cb_program and cb_location which are obtained by the server in
SETCLIENTID_CONFIRM must not be used by the server, because the
NFSv4.1 client performs callback channel designation with
BIND_BACKCHANNEL. Therefore the SETCLIENTID and SETCLIENTID_CONFIRM
operations becomes obsolete when sessions are in use, and a server
should return an error to NFSv4.1 clients which might issue either
operation.
Another favorable result of the session is that the server is able to
avoid requiring the client to perform OPEN_CONFIRM operations. The
existence of a reliable and effective DRC means that the server will
be able to determine whether an OPEN request carrying a previously
known open_owner from a client is or is not a retransmission.
Because of this, the server no longer requires OPEN_CONFIRM to verify
whether the client is retransmitting an open request. This in turn
eliminates the server's reason for requesting OPEN_CONFIRM - the
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server can simply replace any previous information on this
open_owner. Client OPEN operations are therefore streamlined,
reducing overhead and latency through avoiding the additional
OPEN_CONFIRM exchange.
Since the session carries the client liveness indication with it
implicitly, any request on a session associated with a given client
will renew that client's leases. Therefore the RENEW operation is
made unnecessary when a session is present, as any request (including
a SEQUENCE operation with or without additional NFSv4 operations)
performs its function. It is possible (though this proposal does not
make any recommendation) that the RENEW operation could be made
obsolete.
An interesting issue arises however if an error occurs on such a
SEQUENCE operation. If the SEQUENCE operation fails, perhaps due to
an invalid slotid or other non-renewal-based issue, the server may or
may not have performed the RENEW. In this case, the state of any
renewal is undefined, and the client should make no assumption that
it has been performed. In practice, this should not occur but even
if it did, it is expected the client would perform some sort of
recovery which would result in a new, successful, SEQUENCE operation
being run and the client assured that the renewal took place.
5.10.6. Authentication Efficiencies
NFSv4 requires the use of the RPCSEC_GSS ONC RPC security flavor
[RFC2203] to provide authentication, integrity, and privacy via
cryptography. The server dictates to the client the use of
RPCSEC_GSS, the service (authentication, integrity, or privacy), and
the specific GSS-API security mechanism that each remote procedure
call and result will use.
If the connection's integrity is protected by an additional means
than RPCSEC_GSS, such as via IPsec, then the use of RPCSEC_GSS's
integrity service is nearly redundant (See the Security
Considerations section for more explanation of why it is "nearly" and
not completely redundant). Likewise, if the connection's privacy is
protected by additional means, then the use of both RPCSEC_GSS's
integrity and privacy services is nearly redundant.
Connection protection schemes, such as IPsec, are more likely to be
implemented in hardware than upper layer protocols like RPCSEC_GSS.
Hardware-based cryptography at the IPsec layer will be more efficient
than software-based cryptography at the RPCSEC_GSS layer.
When transport integrity can be obtained, it is possible for server
and client to downgrade their per-operation authentication, after an
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appropriate exchange. This downgrade can in fact be as complete as
to establish security mechanisms that have zero cryptographic
overhead, effectively using the underlying integrity and privacy
services provided by transport.
Based on the above observations, a new GSS-API mechanism, called the
Channel Conjunction Mechanism [CCM], is being defined. The CCM works
by creating a GSS-API security context using as input a cookie that
the initiator and target have previously agreed to be a handle for
GSS-API context created previously over another GSS-API mechanism.
NFSv4.1 clients and servers should support CCM and they must use as
the cookie the handle from a successful RPCSEC_GSS context creation
over a non-CCM mechanism (such as Kerberos V5). The value of the
cookie will be equal to the handle field of the rpc_gss_init_res
structure from the RPCSEC_GSS specification.
The [CCM] Draft provides further discussion and examples.
5.11. Sessions Security Considerations
The NFSv4 minor version 1 retains all of existing NFSv4 security; all
security considerations present in NFSv4.0 apply to it equally.
Security considerations of any underlying RDMA transport are
additionally important, all the more so due to the emerging nature of
such transports. Examining these issues is outside the scope of this
draft.
When protecting a connection with RPCSEC_GSS, all data in each
request and response (whether transferred inline or via RDMA)
continues to receive this protection over RDMA fabrics [RPCRDMA].
However when performing data transfers via RDMA, RPCSEC_GSS
protection of the data transfer portion works against the efficiency
which RDMA is typically employed to achieve. This is because such
data is normally managed solely by the RDMA fabric, and intentionally
is not touched by software. Therefore when employing RPCSEC_GSS
under CCM, and where integrity protection has been "downgraded", the
cooperation of the RDMA transport provider is critical to maintain
any integrity and privacy otherwise in place for the session. The
means by which the local RPCSEC_GSS implementation is integrated with
the RDMA data protection facilities are outside the scope of this
draft.
It is logical to use the same GSS context on a session's callback
channel as that used on its operations channel(s), particularly when
the connection is shared by both. The client must indicate to the
server:
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- what security flavor(s) to use in the call back. A special
callback flavor might be defined for this.
- if the flavor is RPCSEC_GSS, then the client must have previously
created an RPCSEC_GSS session with the server. The client offers to
the server the the opaque handle<> value from the rpc_gss_init_res
structure, the window size of RPCSEC_GSS sequence numbers, and an
opaque gss_cb_handle.
This exchange can be performed as part of session and clientid
creation, and the issue warrants careful analysis before being
specified.
If the NFS client wishes to maintain full control over RPCSEC_GSS
protection, it may still perform its transfer operations using either
the inline or RDMA transfer model, or of course employ traditional
TCP stream operation. In the RDMA inline case, header padding is
recommended to optimize behavior at the server. At the client, close
attention should be paid to the implementation of RPCSEC_GSS
processing to minimize memory referencing and especially copying.
These are well-advised in any case!
The proposed session callback channel binding improves security over
that provided by NFSv4 for the callback channel. The connection is
client-initiated, and subject to the same firewall and routing checks
as the operations channel. The connection cannot be hijacked by an
attacker who connects to the client port prior to the intended
server. The connection is set up by the client with its desired
attributes, such as optionally securing with IPsec or similar. The
binding is fully authenticated before being activated.
5.11.1. Authentication
Proper authentication of the principal which issues any session and
clientid in the proposed NFSv4.1 operations exactly follows the
similar requirement on client identifiers in NFSv4.0. It must not be
possible for a client to impersonate another by guessing its session
identifiers for NFSv4.1 operations, nor to bind a callback channel to
an existing session. To protect against this, NFSv4.0 requires
appropriate authentication and matching of the principal used. This
is discussed in Section 16, Security Considerations of [RFC3530].
The same requirement when using a session identifier applies to
NFSv4.1 here.
Going beyond NFSv4.0, the presence of a session associated with any
clientid may also be used to enhance NFSv4.1 security with respect to
client impersonation. In NFSv4.0, there are many operations which
carry no clientid, including in particular those which employ a
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stateid argument. A rogue client which wished to carry out a denial
of service attack on another client could perform CLOSE, DELEGRETURN,
etc operations with that client's current filehandle, sequenceid and
stateid, after having obtained them from eavesdropping or other
approach. Locking and open downgrade operations could be similarly
attacked.
When an NFSv4.1 session is in place for any clientid, countermeasures
are easily applied through use of authentication by the server.
Because the clientid and sessionid must be present in each request
within a session, the server may verify that the clientid is in fact
originating from a principal with the appropriate authenticated
credentials, that the sessionid belongs to the clientid, and that the
stateid is valid in these contexts. This is in general not possible
with the affected operations in NFSv4.0 due to the fact that the
clientid is not present in the requests.
In the event that authentication information is not available in the
incoming request, for example after a reconnection when the security
was previously downgraded using CCM, the server must require the
client re-establish the authentication in order that the server may
validate the other client-provided context, prior to executing any
operation. The sessionid, present in the newly retransmitted
request, combined with the retransmission detection enabled by the
NFSv4.1 duplicate request cache, are a convenient and reliable
context for the server to use for this contingency.
The server should take care to protect itself against denial of
service attacks in the creation of sessions and clientids. Clients
who connect and create sessions, only to disconnect and never use
them may leave significant state behind. (The same issue applies to
NFSv4.0 with clients who may perform SETCLIENTID, then never perform
SETCLIENTID_CONFIRM.) Careful authentication coupled with resource
checks is highly recommended.
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6. Directory Delegations
6.1. Introduction to Directory Delegations
The major addition to NFS version 4 in the area of caching is the
ability of the server to delegate certain responsibilities to the
client. When the server grants a delegation for a file to a client,
the client receives certain semantics with respect to the sharing of
that file with other clients. At OPEN, the server may provide the
client either a read or write delegation for the file. If the client
is granted a read delegation, it is assured that no other client has
the ability to write to the file for the duration of the delegation.
If the client is granted a write delegation, the client is assured
that no other client has read or write access to the file. This
reduces network traffic and server load by allowing the client to
perform certain operations on local file data and can also provide
stronger consistency for the local data.
Directory caching for the NFS version 4 protocol is similar to
previous versions. Clients typically cache directory information for
a duration determined by the client. At the end of a predefined
timeout, the client will query the server to see if the directory has
been updated. By caching attributes, clients reduce the number of
GETATTR calls made to the server to validate attributes.
Furthermore, frequently accessed files and directories, such as the
current working directory, have their attributes cached on the client
so that some NFS operations can be performed without having to make
an RPC call. By caching name and inode information about most
recently looked up entries in DNLC (Directory Name Lookup Cache),
clients do not need to send LOOKUP calls to the server every time
these files are accessed.
This caching approach works reasonably well at reducing network
traffic in many environments. However, it does not address
environments where there are numerous queries for files that do not
exist. In these cases of "misses", the client must make RPC calls to
the server in order to provide reasonable application semantics and
promptly detect the creation of new directory entries. Examples of
high miss activity are compilation in software development
environments. The current behavior of NFS limits its potential
scalability and wide-area sharing effectiveness in these types of
environments. Other distributed stateful filesystem architectures
such as AFS and DFS have proven that adding state around directory
contents can greatly reduce network traffic in high miss
environments.
Delegation of directory contents is proposed as an extension for
NFSv4. Such an extension would provide similar traffic reduction
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benefits as with file delegations. By allowing clients to cache
directory contents (in a read-only fashion) while being notified of
changes, the client can avoid making frequent requests to interrogate
the contents of slowly-changing directories, reducing network traffic
and improving client performance.
These extensions allow improved namespace cache consistency to be
achieved through delegations and synchronous recalls alone without
asking for notifications. In addition, if time-based consistency is
sufficient, asynchronous notifications can provide performance
benefits for the client, and possibly the server, under some common
operating conditions such as slowly-changing and/or very large
directories.
6.2. Directory Delegation Design (in brief)
A new operation GET_DIR_DELEGATION is used by the client to ask for a
directory delegation. The delegation covers directory attributes and
all entries in the directory. If either of these change the
delegation will be recalled synchronously. The operation causing the
recall will have to wait before the recall is complete. Any changes
to directory entry attributes will not cause the delegation to be
recalled.
In addition to asking for delegations, a client can also ask for
notifications for certain events. These events include changes to
directory attributes and/or its contents. If a client asks for
notification for a certain event, the server will notify the client
when that event occurs. This will not result in the delegation being
recalled for that client. The notifications are asynchronous and
provide a way of avoiding recalls in situations where a directory is
changing enough that the pure recall model may not be effective while
trying to allow the client to get substantial benefit. In the
absence of notifications, once the delegation is recalled the client
has to refresh its directory cache which might not be very efficient
for very large directories.
The delegation is read only and the client may not make changes to
the directory other than by performing NFSv4 operations that modify
the directory or the associated file attributes so that the server
has knowledge of these changes. In order to keep the client
namespace in sync with the server, the server will notify the client
holding the delegation of the changes made as a result. This is to
avoid any subsequent GETATTR or READDIR calls to the server. If a
client holding the delegation makes any changes to the directory, the
delegation will not be recalled.
Delegations can be recalled by the server at any time. Normally, the
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server will recall the delegation when the directory changes in a way
that is not covered by the notification, or when the directory
changes and notifications have not been requested.
Also if the server notices that handing out a delegation for a
directory is causing too many notifications to be sent out, it may
decide not to hand out a delegation for that directory or recall
existing delegations. If another client removes the directory for
which a delegation has been granted, the server will recall the
delegation.
Both the notification and recall operations need a callback path to
exist between the client and server. If the callback path does not
exist, then delegation can not be granted. Note that with the
session extensions [talpey] that should not be an issue. In the
absense of sessions, the server will have to establish a callback
path to the client to send callbacks.
6.3. Recommended Attributes in support of Directory Delegations
supp_dir_attr_notice - notification delays on directory attributes
supp_child_attr_notice - notification delays on child attributes
These attributes allow the client and server to negotiate the
frequency of notifications sent due to changes in attributes. These
attributes are returned as part of a GETATTR call on the directory.
The supp_dir_attr_notice value covers all attribute changes to the
directory and the supp_child_attr_notice covers all attribute changes
to any child in the directory.
These attributes are per directory. The client needs to get these
values by doing a GETATTR on the directory for which it wants
notifications. However these attributes are only required when the
client is interested in getting attribute notifications. For all
other types of notifications and delegation requests without
notifications, these attributes are not required.
When the client calls the GET_DIR_DELEGATION operation and asks for
attribute change notifications, it will request a notification delay
that is within the server's supported range. If the client violates
what supp_attr_file_notice or supp_attr_dir_notice values are, the
server should not commit to sending notifications for that change
event.
A value of zero for these attributes means the server will send the
notification as soon as the change occurs. It is not recommended to
set this value to zero since that can put a lot of burden on the
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server. A value of N means that the server will make a best effort
guarentee that attribute notification are not delayed by more than
that. nfstime4 values that compute to negative values are illegal.
6.4. Delegation Recall
The server will recall the directory delegation by sending a callback
to the client. It will use the same callback procedure as used for
recalling file delegations. The server will recall the delegation
when the directory changes in a way that is not covered by the
notification. However the server will not recall the delegation if
attributes of an entry within the directory change. Also if the
server notices that handing out a delegation for a directory is
causing too many notifications to be sent out, it may decide not to
hand out a delegation for that directory. If another client tries to
remove the directory for which a delegation has been granted, the
server will recall the delegation.
The server will recall the delegation by sending a CB_RECALL callback
to the client. If the recall is done because of a directory changing
event, the request making that change will need to wait while the
client returns the delegation.
6.5. Delegation Recovery
Crash recovery has two main goals, avoiding the necessity of breaking
application guarantees with respect to locked files and delivery of
updates cached at the client. Neither of these applies to
directories protected by read delegations and notifications. Thus,
the client is required to establish a new delegation on a server or
client reboot.
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7. NFSv4.1 Operations
7.1. LOOKUPP - Lookup Parent Directory
If the NFSv4 minor version is 1, then following replaces section
14.2.14 of the NFSv4.0 specification. The LOOKUPP operation's "over
the wire" format is not altered, but the semantics are slightly
modified to account for the addition of SECINFO_NO_NAME.
SYNOPSIS
(cfh) -> (cfh)
ARGUMENT
/* CURRENT_FH: object */
void;
RESULT
struct LOOKUPP4res {
/* CURRENT_FH: directory */
nfsstat4 status;
};
DESCRIPTION
The current filehandle is assumed to refer to a regular directory
or a named attribute directory. LOOKUPP assigns the filehandle
for its parent directory to be the current filehandle. If there
is no parent directory an NFS4ERR_NOENT error must be returned.
Therefore, NFS4ERR_NOENT will be returned by the server when the
current filehandle is at the root or top of the server's file
tree.
As for LOOKUP, LOOKUPP will also cross mountpoints.
If the current filehandle is not a directory or named attribute
directory, the error NFS4ERR_NOTDIR is returned.
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If the requester's security flavor does not match that configured
for the parent directory, then the server SHOULD return
NFS4ERR_WRONGSEC (a future minor revision of NFSv4 may upgrade
this to MUST) in the LOOKUPP response. However, if the server
does so, it MUST support the new SECINFO_NO_NAME operation, so
that the client can gracefully determine the correct security
flavor. See the discussion of the SECINFO_NO_NAME operation for a
description.
ERRORS
NFS4ERR_ACCESS NFS4ERR_BADHANDLE NFS4ERR_FHEXPIRED NFS4ERR_IO
NFS4ERR_MOVED NFS4ERR_NOENT NFS4ERR_NOFILEHANDLE NFS4ERR_NOTDIR
NFS4ERR_RESOURCE NFS4ERR_SERVERFAULT NFS4ERR_STALE
NFS4ERR_WRONGSEC
7.2. SECINFO -- 33 Obtain Available Security
If the NFSv4 minor version is 1, then following replaces section
14.2.31 of the NFSv4.0 specification. The SECINFO operation's "over
the wire" format is not altered, but the semantics are slightly
modified to account for the addition of SECINFO_NO_NAME.
SYNOPSIS
(cfh), name -> { secinfo }
ARGUMENT
struct SECINFO4args {
/* CURRENT_FH: directory */
component4 name;
};
RESULT
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enum rpc_gss_svc_t {/* From RFC 2203 */
RPC_GSS_SVC_NONE = 1,
RPC_GSS_SVC_INTEGRITY = 2,
RPC_GSS_SVC_PRIVACY = 3
};
struct rpcsec_gss_info {
sec_oid4 oid;
qop4 qop;
rpc_gss_svc_t service;
};
union secinfo4 switch (uint32_t flavor) {
case RPCSEC_GSS:
rpcsec_gss_info flavor_info;
default:
void;
};
typedef secinfo4 SECINFO4resok<>;
union SECINFO4res switch (nfsstat4 status) {
case NFS4_OK:
SECINFO4resok resok4;
default:
void;
};
DESCRIPTION
The SECINFO operation is used by the client to obtain a list of
valid RPC authentication flavors for a specific directory
filehandle, file name pair. SECINFO should apply the same access
methodology used for LOOKUP when evaluating the name. Therefore,
if the requester does not have the appropriate access to LOOKUP
the name then SECINFO must behave the same way and return
NFS4ERR_ACCESS.
The result will contain an array which represents the security
mechanisms available, with an order corresponding to the server's
preferences, the most preferred being first in the array. The
client is free to pick whatever security mechanism it both desires
and supports, or to pick in the server's preference order the
first one it supports. The array entries are represented by the
secinfo4 structure. The field 'flavor' will contain a value of
AUTH_NONE, AUTH_SYS (as defined in [RFC1831]), or RPCSEC_GSS (as
defined in [RFC2203]). The field flavor can also any other
security flavor registered with IANA.
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For the flavors AUTH_NONE and AUTH_SYS, no additional security
information is returned. The same is true of many (if not most)
other security flavors, including AUTH_DH. For a return value of
RPCSEC_GSS, a security triple is returned that contains the
mechanism object id (as defined in [RFC2743]), the quality of
protection (as defined in [RFC2743]) and the service type (as
defined in [RFC2203]). It is possible for SECINFO to return
multiple entries with flavor equal to RPCSEC_GSS with different
security triple values.
On success, the current filehandle retains its value.
If the name has a length of 0 (zero), or if name does not obey the
UTF-8 definition, the error NFS4ERR_INVAL will be returned.
IMPLEMENTATION
The SECINFO operation is expected to be used by the NFS client
when the error value of NFS4ERR_WRONGSEC is returned from another
NFS operation. This signifies to the client that the server's
security policy is different from what the client is currently
using. At this point, the client is expected to obtain a list of
possible security flavors and choose what best suits its policies.
As mentioned, the server's security policies will determine when a
client request receives NFS4ERR_WRONGSEC. The operations which
may receive this error are: LINK, LOOKUP, LOOKUPP, OPEN, PUTFH,
PUTPUBFH, PUTROOTFH, RESTOREFH, RENAME, and indirectly READDIR.
LINK and RENAME will only receive this error if the security used
for the operation is inappropriate for saved filehandle. With the
exception of READDIR, these operations represent the point at
which the client can instantiate a filehandle into the "current
filehandle" at the server. The filehandle is either provided by
the client (PUTFH, PUTPUBFH, PUTROOTFH) or generated as a result
of a name to filehandle translation (LOOKUP and OPEN). RESTOREFH
is different because the filehandle is a result of a previous
SAVEFH. Even though the filehandle, for RESTOREFH, might have
previously passed the server's inspection for a security match,
the server will check it again on RESTOREFH to ensure that the
security policy has not changed.
If the client wants to resolve an error return of
NFS4ERR_WRONGSEC, the following will occur:
* For LOOKUP and OPEN, the client will use SECINFO with the same
current filehandle and name as provided in the original LOOKUP
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or OPEN to enumerate the available security triples.
* For LINK, PUTFH, PUTROOTFH, PUTPUBFH, RENAME, and RESTOREFH,
the client will use SECINFO_NO_NAME { style = current_fh }.
The client will prefix the SECINFO_NO_NAME operation with the
appropriate PUTFH, PUTPUBFH, or PUTROOTFH operation that
provides the file handled originally provided by the PUTFH,
PUTPUBFH, PUTROOTFH, or RESTOREFH, or for the failed LINK or
RENAME, the SAVEFH.
* ========================================================= NOTE:
In NFSv4.0, the client was required to use SECINFO, and had to
reconstruct the parent of the original file handle, and the
component name of the original filehandle.
========================================================
* For LOOKUPP, the client will use SECINFO_NO_NAME { style =
parent } and provide the filehandle with equals the filehandle
originally provided to LOOKUPP.
The READDIR operation will not directly return the
NFS4ERR_WRONGSEC error. However, if the READDIR request included
a request for attributes, it is possible that the READDIR
request's security triple did not match that of a directory entry.
If this is the case and the client has requested the rdattr_error
attribute, the server will return the NFS4ERR_WRONGSEC error in
rdattr_error for the entry.
See the section "Security Considerations" for a discussion on the
recommendations for security flavor used by SECINFO and
SECINFO_NO_NAME.
ERRORS
7.3. SECINFO_NO_NAME - Get Security on Unnamed Object
Obtain available security mechanisms with the use of the parent of an
object or the current filehandle.
SYNOPSIS
(cfh), secinfo_style -> { secinfo }
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ARGUMENT
enum secinfo_style_4 {
current_fh = 0,
parent = 1
};
typedef secinfo_style_4 SECINFO_NO_NAME4args;
RESULT
typedef SECINFO4res SECINFO_NO_NAME4res;
DESCRIPTION
Like the SECINFO operation, SECINFO_NO_NAME is used by the client
to obtain a list of valid RPC authentication flavors for a
specific file object. Unlike SECINFO, SECINFO_NO_NAME only works
with objects are accessed by file handle.
There are two styles of SECINFO_NO_NAME, as determined by the
value of the secinfo_style_4 enumeration. If "current_fh" is
passed, then SECINFO_NO_NAME is querying for the required security
for the current filehandle. If "parent" is passed, then
SECINFO_NO_NAME is querying for the required security of the
current filehandles's parent. If the style selected is "parent",
then SECINFO should apply the same access methodology used for
LOOKUPP when evaluating the traversal to the parent directory.
Therefore, if the requester does not have the appropriate access
to LOOKUPP the parent then SECINFO_NO_NAME must behave the same
way and return NFS4ERR_ACCESS.
Note that if PUTFH, PUTPUBFH, or PUTROOTFH return
NFS4ERR_WRONGSEC, this is tantamount to the server asserting that
the client will have to guess what the required security is,
because there is no way to query. Therefore, the client must
iterate through the security triples available at the client and
reattempt the PUTFH, PUTROOTFH or PUTPUBFH operation. In the
unfortunate event none of the MANDATORY security triples are
supported by the client and server, the client SHOULD try using
others that support integrity. Failing that, the client can try
using other forms (e.g. AUTH_SYS and AUTH_NONE), but because such
forms lack integrity checks, this puts the client at risk.
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The server implementor should pay particular attention to the
section "Clarification of Security Negotiation in NFSv4.1" for
implementation suggestions for avoiding NFS4ERR_WRONGSEC error
returns from PUTFH, PUTROOTFH or PUTPUBFH.
Everything else about SECINFO_NO_NAME is the same as SECINFO. See
the previous discussion on SECINFO.
IMPLEMENTATION
See the previous dicussion on SECINFO.
ERRORS
NFS4ERR_ACCESS NFS4ERR_BADCHAR NFS4ERR_BADHANDLE NFS4ERR_BADNAME
NFS4ERR_BADXDR NFS4ERR_FHEXPIRED NFS4ERR_INVAL NFS4ERR_MOVED
NFS4ERR_NAMETOOLONG NFS4ERR_NOENT NFS4ERR_NOFILEHANDLE
NFS4ERR_NOTDIR NFS4ERR_RESOURCE NFS4ERR_SERVERFAULT NFS4ERR_STALE
7.4. CREATECLIENTID - Instantiate Clientid
Create a clientid
SYNOPSIS
client -> clientid
ARGUMENT
struct CREATECLIENTID4args {
nfs_client_id4 clientdesc;
};
RESULT
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struct CREATECLIENTID4resok {
clientid4 clientid;
verifier4 clientid_confirm;
};
union SETCLIENTID4res switch (nfsstat4 status) {
case NFS4_OK:
CREATECLIENTID4resok resok4;
case NFS4ERR_CLID_INUSE:
void;
default:
void;
};
DESCRIPTION
The client uses the CREATECLIENTID operation to register a
particular client identifier with the server. The clientid
returned from this operation will be necessary for requests that
create state on the server and will serve as a parent object to
sessions created by the client. In order to verify the clientid
it must first be used as an argument to CREATESESSION.
IMPLEMENTATION
A server's client record is a 5-tuple:
1. clientdesc.id:
The long form client identifier, sent via the client.id
subfield of the CREATECLIENTID4args structure
2. clientdesc.verifier:
A client-specific value used to indicate reboots, sent via
the clientdesc.verifier subfield of the CREATECLIENTID4args
structure
3. principal:
The RPCSEC_GSS principal sent via the RPC headers
4. clientid:
The shorthand client identifier, generated by the server
and returned via the clientid field in the
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CREATECLIENTID4resok structure
5. confirmed:
A private field on the server indicating whether or not a
client record has been confirmed. A client record is
confirmed if there has been a successful CREATESESSION
operation to confirm it. Otherwise it is unconfirmed. An
unconfirmed record is established by a CREATECLIENTID call.
Any unconfirmed record that is not confirmed within a lease
period may be removed.
The following identifiers represent special values for the fields
in the records.
id_arg:
The value of the clientdesc.id subfield of the
CREATECLIENTID4args structure of the current request.
verifier_arg:
The value of the clientdesc.verifier subfield of the
CREATECLIENTID4args structure of the current request.
old_verifier_arg:
A value of the clientdesc.verifier field of a client record
received in a previous request; this is distinct from
verifier_arg.
principal_arg:
The value of the RPCSEC_GSS principal for the current request.
old_principal_arg:
A value of the RPCSEC_GSS principal received for a previous
request. This is distinct from principal_arg.
clientid_ret:
The value of the clientid field the server will return in the
CREATECLIENTID4resok structure for the current request.
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old_clientid_ret:
The value of the clientid field the server returned in the
CREATECLIENTID4resok structure for a previous request. This is
distinct from clientid_ret.
Since CREATECLIENTID is a non-idempotent operation, we must
consider the possibility that replays may occur as a result of a
client reboot, network partition, malfunctioning router, etc.
Replays are identified by the value of the client field of
CREATECLIENTID4args and the method for dealing with them is
outlined in the scenarios below.
The scenarios are described in terms of what client records whose
clientdesc.id subfield have value equal to id_arg exist in the
server's set of client records. Any cases in which there is more
than one record with identical values for id_arg represent a
server implementation error. Operation in the potential valid
cases is summarized as follows.
1. Common case
If no client records with clientdesc.id matching id_arg
exist, a new shorthand client identifier clientid_ret is
generated, and the following unconfirmed record is added to
the server's state.
{ id_arg, verifier_arg, principal_arg, clientid_ret, FALSE
}
Subsequently, the server returns clientid_ret.
2. Router Replay
If the server has the following confirmed record, then this
request is likely the result of a replayed request due to a
faulty router or lost connection.
{ id_arg, verifier_arg, principal_arg, clientid_ret, TRUE }
Since the record has been confirmed, the client must have
received the server's reply from the initial CREATECLIENTID
request. Since this is simply a spurious request, there is
no modification to the server's state, and the server makes
no reply to the client.
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3. Client Collision
If the server has the following confirmed record, then this
request is likely the result of a chance collision between
the values of the clientdesc.id subfield of
CREATECLIENTID4args for two different clients.
{ id_arg, *, old_principal_arg, clientid_ret, TRUE }
Since the value of the clientdesc.id subfield of each
client record must be unique, there is no modification of
the server's state, and NFS4ERR_CLID_INUSE is returned to
indicate the client should retry with a different value for
the clientdesc.id subfield of CREATECLIENTID4args.
This scenario may also represent a malicious attempt to
destroy a client's state on the server. For security
reasons, the server MUST NOT remove the client's state when
there is a principal mismatch.
4. Replay
If the server has the following unconfirmed record then
this request is likely the result of a client replay due to
a network partition or some other connection failure.
{ id_arg, verifier_arg, principal_arg, clientid_ret, FALSE
}
Since the response to the CREATECLIENTID request that
created this record may have been lost, it is not
acceptable to drop this duplicate request. However, rather
than processing it normally, the existing record is left
unchanged and clientid_ret, which was generated for the
previous request, is returned.
5. Change of Principal
If the server has the following unconfirmed record then
this request is likely the result of a client which has for
whatever reasons changed principals (possibly to change
security flavor) after calling CREATECLIENTID, but before
calling CREATESESSION.
{ id_arg, verifier_arg, old_principal_arg, clientid_ret,
FALSE}
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Since the client has not changed, the principal field of
the unconfirmed record is updated to principal_arg and
clientid_ret is again returned. There is a small
possibility that this is merely a collision on the client
field of CREATECLIENTID4args between unrelated clients, but
since that is unlikely, and an unconfirmed record does not
generally have any filesystem pertinent state, we can
assume it is the same client without risking loss of any
important state.
After processing, the following record will exist on the
server.
{ id_arg, verifier_arg, principal_arg, clientid_ret, FALSE}
6. Client Reboot
If the server has the following confirmed client record,
then this request is likely from a previously confirmed
client which has rebooted.
{ id_arg, old_verifier_arg, principal_arg, clientid_ret,
TRUE }
Since the previous incarnation of the same client will no
longer be making requests, lock and share reservations
should be released immediately rather than forcing the new
incarnation to wait for the lease time on the previous
incarnation to expire. Furthermore, session state should
be removed since if the client had maintained that
information across reboot, this request would not have been
issued. If the server does not support the
CLAIM_DELEGATE_PREV claim type, associated delegations
should be purged as well; otherwise, delegations are
retained and recovery proceeds according to RFC3530. The
client record is updated with the new verifier and its
status is changed to unconfirmed.
After processing, clientid_ret is returned to the client
and the following record will exist on the server.
{ id_arg, verifier_arg, principal_arg, clientid_ret, FALSE
}
7. Reboot before confirmation
If the server has the following unconfirmed record, then
this request is likely from a client which rebooted before
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sending a CREATESESSION request.
{ id_arg, old_verifier_arg, *, clientid_ret, FALSE }
Since this is believed to be a request from a new
incarnation of the original client, the server updates the
value of clientdesc.verifier and returns the original
clientid_ret. After processing, the following state exists
on the server.
{ id_arg, verifier_arg, *, clientid_ret, FALSE }
ERRORS
NFS4ERR_BADXDR NFS4ERR_CLID_INUSE NFS4ERR_INVAL NFS4ERR_RESOURCE
NFS4ERR_SERVERFAULT
7.5. CREATESESSION - Create New Session and Confirm Clientid
Start up session and confirm clientid.
SYNOPSIS
clientid, session_args -> sessionid, session_args
ARGUMENT
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struct CREATESESSION4args {
clientid4 clientid;
bool persist;
count4 maxrequestsize;
count4 maxresponsesize;
count4 maxrequests;
count4 headerpadsize;
switch (bool clientid_confirm) {
case TRUE:
verifier4 setclientid_confirm;
case FALSE:
void;
}
switch (channelmode4 mode) {
case DEFAULT:
void;
case STREAM:
streamchannelattrs4 streamchanattrs;
case RDMA:
rdmachannelattrs4 rdmachanattrs;
};
};
RESULT
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typedef opaque sessionid4[16];
struct CREATESESSION4resok {
sessionid4 sessionid;
bool persist;
count4 maxrequestsize;
count4 maxresponsesize;
count4 maxrequests;
count4 headerpadsize;
switch (channelmode4 mode) {
case DEFAULT:
void;
case STREAM:
streamchannelattrs4 streamchanattrs;
case RDMA:
rdmachannelattrs4 rdmachanattrs;
};
};
union CREATESESSION4res switch (nfsstat4 status) {
case NFS4_OK:
CREATESESSION4resok resok4;
default:
void;
};
DESCRIPTION
This operation is used by the client to create new session objects
on the server. Additionally the first session created with a new
shorthand client identifier serves to confirm the creation of that
client's state on the server. The server returns the parameter
values for the new session.
IMPLEMENTATION
To describe the implementation, the same notation for client
records introduced in the description of CREATECLIENTID is used
with the following addition.
clientid_arg: The value of the clientid field of the
CREATESESSION4args structure of the current request.
Since CREATESESSION is a non-idempotent operation, we must
consider the possibility that replays may occur as a result of a
client reboot, network partition, malfunctioning router, etc.
Replays are identified by the value of the clientid and sessionid
fields of CREATESESSION4args and the method for dealing with them
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is outlined in the scenarios below.
The processing of this operation is divided into two phases:
clientid confirmation and session creation. In case the state for
the provided clientid has not been verified, it is confirmed
before the session is created. Otherwise the clientid
confirmation phase is skipped and only the session creation phase
occurs. Note that since only confirmed clients may create
sessions, the clientid confirmation stage does not depend upon
sessionid_arg.
CLIENTID CONFIRMATION
The operational cases are described in terms of what client
records whose clientid field have value equal to clientid_arg
exist in the server's set of client records. Any cases in which
there is more than one record with identical values for clientid
represent a server implementation error. Operation in the
potential valid cases is summarized as follows.
1. Common Case
If the server has the following unconfirmed record, then
this is the expected confirmation of an unconfirmed record.
{ *, *, principal_arg, clientid_arg, FALSE }
The confirmed field of the record is set to TRUE and
processing of the operation continues normally.
2. Stale Clientid
If the server contains no records with clientid equal to
clientid_arg, then most likely the client's state has been
purged during a period of inactivity, possibly due to a
loss of connectivity. NFS4ERR_STALE_CLIENTID is returned,
and no changes are made to any client records on the
server.
3. Principal Change or Collision
If the server has the following record, then the client has
changed principals after the previous CREATECLIENTID
request, or there has been a chance collision between
shortand client identifiers.
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{ *, *, old_principal_arg, clientid_arg, * }
Neither of these cases are permissible. Processing stops
and NFS4ERR_CLID_INUSE is returned to the client. No
changes are made to any client records on the server.
SESSION CREATION
To determine whether this request is a replay, the server examines
the sessionid argument provided by the client. If the sessionid
matches the identifier of a previously created session, then this
request must be interpreted as a replay. No new state is created
and a reply with the parameters of the existing session is
returned to the client. If a session corresponding to the
sessionid does not already exist, then the request is not a replay
and is processed as follows.
NOTE: It is the responsibility of the client to generate
appropriate values for sessionid. Since the ordering of messages
sent on different transport connections is not guaranteed,
immediately reusing the sessionid of a previously destroyed
session may yield unpredictable results. Client implementations
should avoid recently used sessionids to ensure correct behavior.
The server examines the persist, maxrequestsize, maxresponsesize,
maxrequests and headerpadsize arguments. For each argument, if
the value is acceptable to the server, it is recommended that the
server use the provided value to create the new session. If it is
not acceptable, the server may use a different value, but must
return the value used to the client. These parameters have the
following interpretation.
persist:
True if the client desires server support for "reliable"
semantics. For sessions in which only idempotent operations
will be used (e.g. a read-only session), clients should set
this value to false. If the server does not or cannot provide
"reliable" semantics this value must be set to false on return.
maxrequestsize:
The maximum size of a COMPOUND request that will be sent by the
client including RPC headers.
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maxresponsesize:
The maximum size of a COMPOUND reply that the client will
accept from the server including RPC headers. The server must
not increase the value of this parameter. If a client sends a
COMPOUND request for which the size of the reply would exceed
this value, the server will return NFS4ERR_RESOURCE.
maxrequests:
The maximum number of concurrent COMPOUND requests that the
client will issue on the session. Subsequent COMPOUND requests
will each be assigned a slot identifier by the client on the
range 0 to maxrequests - 1 inclusive. A slot id cannot be
reused until the previous request on that slot has completed.
headerpadsize:
The maximum amount of padding the client is willing to apply to
ensure that write payloads are aligned on some boundary at the
server. The server should reply with its preferred value, or
zero if padding is not in use. The server may decrease this
value but must not increase it.
The server creates the session by recording the parameter values
used and if the persist parameter is true and has been accepted by
the server, allocating space for the duplicate request cache
(DRC).
If the session state is created successfully, the server
associates it with the session identifier provided by the client.
This identifier must be unique among the client's active sessions
but there is no need for it to be globally unique. Finally, the
server returns the negotiated values used to create the session to
the client.
ERRORS
NFS4ERR_BADXDR NFS4ERR_CLID_INUSE NFS4ERR_RESOURCE
NFS4ERR_SERVERFAULT NFS4ERR_STALE_CLIENTID
7.6. BIND_BACKCHANNEL - Create a callback channel binding
Establish a callback channel on the connection.
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SYNOPSIS
ARGUMENT
struct BIND_BACKCHANNEL4args {
clientid4 clientid;
uint32_t callback_program;
uint32_t callback_ident;
count4 maxrequestsize;
count4 maxresponsesize;
count4 maxrequests;
switch (channelmode4 mode) {
case DEFAULT:
void;
case STREAM:
streamchannelattrs4 streamchanattrs;
case RDMA:
rdmachannelattrs4 rdmachanattrs;
};
};
RESULT
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struct BIND_BACKCHANNEL4resok {
count4 maxrequestsize;
count4 maxresponsesize;
count4 maxrequests;
switch (channelmode4 mode) {
case DEFAULT:
void;
case STREAM:
streamchannelattrs4 streamchanattrs;
case RDMA:
rdmachannelattrs4 rdmachanattrs;
};
};
union BIND_BACKCHANNEL4res switch (nfsstat4 status) {
case NFS4_OK:
BIND_BACKCHANNEL4resok resok4;
default:
void;
};
DESCRIPTION
The BIND_BACKCHANNEL operation serves to establish the current
connection as a designated callback channel for the specified
session. Normally, only one callback channel is bound, however if
more than one are established, they are used at the server's
prerogative, no affinity or preference is specified by the client.
The arguments and results of the BIND_BACKCHANNEL call are a
subset of the session parameters, and used identically to those
values on the callback channel only. However, not all session
operation channel parameters are relevant to the callback channel,
for example header padding (since writes of bulk data are not
performed in callbacks).
IMPLEMENTATION
No discussion at this time.
ERRORS
TBD
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7.7. DESTROYSESSION - Destroy existing session
Destroy existing session.
SYNOPSIS
void -> status
ARGUMENT
struct DESTROYSESSION4args {
sessionid4 sessionid;
};
RESULT
struct SESSION_DESTROYres {
nfsstat status;
};
DESCRIPTION
The SESSION_DESTROY operation closes the session and discards any
active state such as locks, leases, and server duplicate request
cache entries. Any remaining connections bound to the session are
immediately unbound and may additionally be closed by the server.
This operation must be the final, or only operation in any
request. Because the operation results in destruction of the
session, any duplicate request caching for this request, as well
as previously completed requests, will be lost. For this reason,
it is advisable to not place this operation in a request with
other state-modifying operations. In addition, a SEQUENCE
operation is not required in the request.
Note that because the operation will never be replayed by the
server, a client that retransmits the request may receive an error
in response, even though the session may have been successfully
destroyed.
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IMPLEMENTATION
No discussion at this time.
ERRORS
TBD
7.8. SEQUENCE - Supply per-procedure sequencing and control
Supply per-procedure sequencing and control
SYNOPSIS
control -> control
ARGUMENT
typedef uint32_t sequenceid4;
typedef uint32_t slotid4;
struct SEQUENCE4args {
clientid4 clientid;
sessionid4 sessionid;
sequenceid4 sequenceid;
slotid4 slotid;
slotid4 maxslot;
};
RESULT
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struct SEQUENCE4resok {
clientid4 clientid;
sessionid4 sessionid;
sequenceid4 sequenceid;
slotid4 slotid;
slotid4 maxslot;
slotid4 target_maxslot;
};
union SEQUENCE4res switch (nfsstat4 status) {
case NFS4_OK:
SEQUENCE4resok resok4;
default:
void;
};
DESCRIPTION
The SEQUENCE operation is used to manage operational accounting
for the session on which the operation is sent. The contents
include the client and session to which this request belongs,
slotid and sequenceid, used by the server to implement session
request control and the duplicate reply cache semantics, and
exchanged slot counts which are used to adjust these values. This
operation must appear once as the first operation in each COMPOUND
sent after the channel is successfully bound, or a protocol error
must result.
IMPLEMENTATION
No discussion at this time.
ERRORS
NFS4ERR_BADSESSION NFS4ERR_BADSLOT
7.9. CB_RECALLCREDIT - change flow control limits
Change flow control limits
SYNOPSIS
targetcount -> status
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ARGUMENT
struct CB_RECALLCREDIT4args {
sessionid4 sessionid;
uint32_t target;
};
RESULT
struct CB_RECALLCREDIT4res {
nfsstat4 status;
};
DESCRIPTION
The CB_RECALLCREDIT operation requests the client to return
session and transport credits to the server, by zero-length RDMA
Sends or NULL NFSv4 operations.
IMPLEMENTATION
No discussion at this time.
ERRORS
NONE
7.10. CB_SEQUENCE - Supply callback channel sequencing and control
Sequence and control
SYNOPSIS
control -> control
ARGUMENT
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typedef uint32_t sequenceid4;
typedef uint32_t slotid4;
struct CB_SEQUENCE4args {
clientid4 clientid;
sessionid4 sessionid;
sequenceid4 sequenceid;
slotid4 slotid;
slotid4 maxslot;
};
RESULT
struct CB_SEQUENCE4resok {
clientid4 clientid;
sessionid4 sessionid;
sequenceid4 sequenceid;
slotid4 slotid;
slotid4 maxslot;
slotid4 target_maxslot;
};
union CB_SEQUENCE4res switch (nfsstat4 status) {
case NFS4_OK:
CB_SEQUENCE4resok resok4;
default:
void;
};
DESCRIPTION
The CB_SEQUENCE operation is used to manage operational accounting
for the callback channel of the session on which the operation is
sent. The contents include the client and session to which this
request belongs, slotid and sequenceid, used by the server to
implement session request control and the duplicate reply cache
semantics, and exchanged slot counts which are used to adjust
these values. This operation must appear once as the first
operation in each CB_COMPOUND sent after the callback channel is
successfully bound, or a protocol error must result.
IMPLEMENTATION
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No discussion at this time.
ERRORS
NFS4ERR_BADSESSION NFS4ERR_BADSLOT
7.11. GET_DIR_DELEGATION - Get a directory delegation
Obtain a directory delegation.
SYNOPSIS
(cfh), requested notification -> (cfh), cookieverf, stateid,
supported notification
ARGUMENT
struct GET_DIR_DELEGATION4args {
dir_notification_type4 notification_type;
attr_notice4 child_attr_delay;
attr_notice4 dir_attr_delay;
};
/*
* Notification types.
*/
const DIR_NOTIFICATION_NONE = 0x00000000;
const DIR_NOTIFICATION_CHANGE_CHILD_ATTRIBUTES = 0x00000001;
const DIR_NOTIFICATION_CHANGE_DIR_ATTRIBUTES = 0x00000002;
const DIR_NOTIFICATION_REMOVE_ENTRY = 0x00000004;
const DIR_NOTIFICATION_ADD_ENTRY = 0x00000008;
const DIR_NOTIFICATION_RENAME_ENTRY = 0x00000010;
const DIR_NOTIFICATION_CHANGE_COOKIE_VERIFIER = 0x00000020;
typedef uint32_t dir_notification_type4;
typedef nfstime4 attr_notice4;
RESULT
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struct GET_DIR_DELEGATION4resok {
verifier4 cookieverf;
/* Stateid for get_dir_delegation */
stateid4 stateid;
/* Which notifications can the server support */
dir_notification_type4 supp_notification;
bitmap4 child_attributes;
bitmap4 dir_attributes;
};
union GET_DIR_DELEGATION4res switch (nfsstat4 status) {
case NFS4_OK:
/* CURRENT_FH: delegated dir */
GET_DIR_DELEGATION4resok resok4;
default:
void;
};
DESCRIPTION
The GET_DIR_DELEGATION operation is used by a client to request a
directory delegation. The directory is represented by the current
filehandle. The client also specifies whether it wants the server
to notify it when the directory changes in certain ways by setting
one or more bits in a bitmap. The server may also choose not to
grant the delegation. In that case the server will return
NFS4ERR_DIRDELEG_UNAVAIL. If the server decides to hand out the
delegation, it will return a cookie verifier for that directory.
If the cookie verifier changes when the client is holding the
delegation, the delegation will be recalled unless the client has
asked for notification for this event. In that case a
notification will be sent to the client.
The server will also return a directory delegation stateid in
addition to the cookie verifier as a result of the
GET_DIR_DELEGATION operation. This stateid will appear in
callback messages related to the delegation, such as notifications
and delegation recalls. The client will use this stateid to
return the delegation voluntarily or upon recall. A delegation is
returned by calling the DELEGRETURN operation.
The server may not be able to support notifications of certain
events. If the client asks for such notifications, the server
must inform the client of its inability to do so as part of the
GET_DIR_DELEGATION reply by not setting the appropriate bits in
the supported notifications bitmask contained in the reply.
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The GET_DIR_DELEGATION operation can be used for both normal and
named attribute directories. It covers all the entries in the
directory except the ".." entry. That means if a directory and
its parent both hold directory delegations, any changes to the
parent will not cause a notification to be sent for the child even
though the child's ".." entry points to the parent.
IMPLEMENTATION
Directory delegation provides the benefit of improving cache
consistency of namespace information. This is done through
synchronous callbacks. A server must support synchronous
callbacks in order to support directory delegations. In addition
to that, asynchronous notifications provide a way to reduce
network traffic as well as improve client performance in certain
conditions. Notifications would not be requested when the goal is
just cache consitency.
Notifications are specified in terms of potential changes to the
directory. A client can ask to be notified whenever an entry is
added to a directory by setting notification_type to
DIR_NOTIFICATION_ADD_ENTRY. It can also ask for notifications on
entry removal, renames, directory attribute changes and cookie
verifier changes by setting notification_type flag appropriately.
In addition to that, the client can also ask for notifications
upon attribute changes to children in the directory to keep its
attribute cache up to date. However any changes made to child
attributes do not cause the delegation to be recalled. If a
client is interested in directory entry caching, or negative name
caching, it can set the notification_type appropriately and the
server will notify it of all changes that would otherwise
invalidate its name cache. The kind of notification a client asks
for may depend on the directory size, its rate of change and the
applications being used to access that directory. However, the
conditions under which a client might ask for a notification, is
out of the scope of this specification.
The client will set one or more bits in a bitmap
(notification_type) to let the server know what kind of
notification(s) it is interested in. For attribute notifications
it will set bits in another bitmap to indicate which attributes it
wants to be notified of. If the server does not support
notifications for changes to a certain attribute, it should not
set that attribute in the supported attribute bitmap
(supp_notification) specified in the reply.
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In addition to that, the client will also let the server know if
it wants to get the notification as soon as the attribute change
occurs or after a certain delay by setting a delay factor,
child_attr_delay for attribute changes to children and
dir_attr_delay for attribute changes to the directory. If this
delay factor is set to zero, that indicates to the server that the
client wants to be notified of any attribute changes as soon as
they occur. If the delay factor is set to N, the server will make
a best effort guarantee that attribute updates are not out of sync
by more than that. One value covers all attribute changes for the
directory and another value covers all attribute changes for all
children in the directory. If the client asks for a delay factor
that the server does not support or that may cause significant
resource consumption on the server by causing the server to send a
lot of notifications, the server should not commit to sending out
notifications for that attribute and therefore must not set the
approprite bit in the child_attributes and dir_attributes bitmaps
in the response.
The server will let the client know about which notifications it
can support by setting appropriate bits in a bitmap. If it agrees
to send attribute notifications, it will also set two attribute
masks indicating which attributes it will send change
notifications for. One of the masks covers changes in directory
attributes and the other covers atttribute changes to any files in
the directory.
The client should use a security flavor that the filesystem is
exported with. If it uses a different flavor, the server should
return NFS4ERR_WRONGSEC.
ERRORS
NFS4ERR_ACCESS NFS4ERR_BADHANDLE NFS4ERR_BADXDR NFS4ERR_FHEXPIRED
NFS4ERR_INVAL NFS4ERR_MOVED NFS4ERR_NOFILEHANDLE NFS4ERR_NOTDIR
NFS4ERR_RESOURCE NFS4ERR_SERVERFAULT NFS4ERR_STALE
NFS4ERR_DIRDELEG_UNAVAIL NFS4ERR_WRONGSEC NFS4ERR_EIO
NFS4ERR_NOTSUPP
7.12. CB_NOTIFY - Notify directory changes
Tell the client of directory changes.
SYNOPSIS
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stateid, notification -> {}
ARGUMENT
struct CB_NOTIFY4args {
stateid4 stateid;
dir_notification4 changes<>;
};
/*
* Notification information sent to the client.
*/
union dir_notification4
switch (dir_notification_type4 notification_type) {
case DIR_NOTIFICATION_CHANGE_CHILD_ATTRIBUTES:
dir_notification_attribute4 change_child_attributes;
case DIR_NOTIFICATION_CHANGE_DIR_ATTRIBUTES:
fattr4 change_dir_attributes;
case DIR_NOTIFICATION_REMOVE_ENTRY:
dir_notification_remove4 remove_notification;
case DIR_NOTIFICATION_ADD_ENTRY:
dir_notification_add4 add_notification;
case DIR_NOTIFICATION_RENAME_ENTRY:
dir_notification_rename4 rename_notification;
case DIR_NOTIFICATION_CHANGE_COOKIE_VERIFIER:
dir_notification_verifier4 verf_notification;
};
/*
* Changed entry information.
*/
struct dir_entry {
component4 file;
fattr4 attrs;
};
struct dir_notification_attribute4 {
dir_entry changed_entry;
};
struct dir_notification_remove4 {
dir_entry old_entry;
nfs_cookie4 old_entry_cookie;
};
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struct dir_notification_rename4 {
dir_entry old_entry;
dir_notification_add4 new_entry;
};
struct dir_notification_verifier4 {
verifier4 old_cookieverf;
verifier4 new_cookieverf;
};
struct dir_notification_add4 {
dir_entry new_entry;
/* what READDIR would have returned for this entry */
nfs_cookie4 new_entry_cookie;
bool last_entry;
prev_entry_info4 prev_info;
};
union prev_entry_info4 switch (bool isprev) {
case TRUE: /* A previous entry exists */
prev_entry4 prev_entry_info;
case FALSE: /* we are adding to an empty
directory */
void;
};
/*
* Previous entry information
*/
struct prev_entry4 {
dir_entry prev_entry;
/* what READDIR returned for this entry */
nfs_cookie4 prev_entry_cookie;
};
RESULT
struct CB_NOTIFY4res {
nfsstat4 status;
};
DESCRIPTION
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The CB_NOTIFY operation is used by the server to send
notifications to clients about changes in a delegated directory.
These notifications are sent over the callback path. The
notification is sent once the original request has been processed
on the server. The server will send an array of notifications for
all changes that might have occurred in the directory. The
dir_notification_type4 can only have one bit set for each
notification in the array. If the client holding the delegation
makes any changes in the directory that cause files or sub
directories to be added or removed, the server will notify that
client of the resulting change(s). If the client holding the
delegation is making attribute or cookie verifier changes only,
the server does not need to send notifications to that client.
The server will send the following information for each operation:
* ADDING A FILE: The server will send information about the new
entry being created along with the cookie for that entry. The
entry information contains the nfs name of the entry and
attributes. If this entry is added to the end of the
directory, the server will set a last_entry flag to true. If
the file is added such that there is atleast one entry before
it, the server will also return the previous entry information
along with its cookie. This is to help clients find the right
location in their DNLC or directory caches where this entry
should be cached.
* REMOVING A FILE: The server will send information about the
directory entry being deleted. The server will also send the
cookie value for the deleted entry so that clients can get to
the cached information for this entry.
* RENAMING A FILE: The server will send information about both
the old entry and the new entry. This includes name and
attributes for each entry. This notification is only sent if
both entries are in the same directory. If the rename is
across directories, the server will send a remove notification
to one directory and an add notification to the other
directory, assuming both have a directory delegation.
* FILE/DIR ATTRIBUTE CHANGE: The client will use the attribute
mask to inform the server of attributes for which it wants to
receive notifications. This change notification can be
requested for both changes to the attributes of the directory
as well as changes to any file attributes in the directory by
using two separate attribute masks. The client can not ask for
change attribute notification per file. One attribute mask
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covers all the files in the directory. Upon any attribute
change, the server will send back the values of changed
attributes. Notifications might not make sense for some
filesystem wide attributes and it is up to the server to decide
which subset it wants to support. The client can negotiate the
frequency of attribute notifications by letting the server know
how often it wants to be notified of an attribute change. The
server will return supported notification frequencies or an
indication that no notification is permitted for directory or
child attributes by setting the supp_dir_attr_notice and
supp_child_attr_notice attributes respectively.
* COOKIE VERIFIER CHANGE: If the cookie verifier changes while a
client is holding a delegation, the server will notify the
client so that it can invalidate its cookies and reissue a
READDIR to get the new set of cookies.
IMPLEMENTATION
ERRORS
NFS4ERR_BAD_STATEID NFS4ERR_INVAL NFS4ERR_BADXDR
NFS4ERR_SERVERFAULT
7.13. CB_RECALL_ANY - Keep any N delegations
Notify client to return delegation and keep N of them.
SYNOPSIS
N -> {}
ARGUMENT
struct CB_RECALLANYY4args {
uint4 dlgs_to_keep;
}
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RESULT
struct CB_RECALLANY4res {
nfsstat4 status;
};
DESCRIPTION
The server may decide that it can not hold all the delegation
state without running out of resources. Since the server has no
knowledge of which delegations are being used more than others, it
can not implement an effective reclaim scheme that avoids
reclaiming frequently used delegations. In that case the server
may issue a CB_RECALL_ANY callback to the client asking it to keep
N delegations and return the rest. The reason why CB_RECALL_ANY
specifies a count of delegations the client may keep as opposed to
a count of delegations the client must yield is as follows. Were
it otherwise, there is a potential for a race between a
CB_RECALL_ANY that had a count of delegations to free with a set
of client originated operations to return delegations. As a
result of the race the client and server would have differing
ideas as to how many delegations to return. Hence the client
could mistakenly free too many delegations. This operation
applies to delegations for a regular file (read or write) as well
as for a directory.
The client can choose to return any type of delegation as a result
of this callback i.e. read, write or directory delegation. The
client can also choose to keep more delegations than what the
server asked for and it is up to the server to handle this
situation. The server must give the client enough time to return
the delegations. This time should not be less than the lease
period.
IMPLEMENTATION
ERRORS
NFS4ERR_RESOURCE
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8. Acknowledgements
Ackknowledgements for SECINFO
Mike Eisler, Tom Talpey, Saadia Khan, Jon Bauman
Acknowledgements for SESSIONS
Tom Talpey, Jon Bauman, Spencer Shepler with input and review by
Charles Antonelli, Brent Callaghan, Mike Eisler, John Howard, Chet
Juszczak, Trond Myklebust, Dave Noveck, John Scott, Mike
Stolarchuk and Mark Wittle.
Acknowledgements for Directory Delegations
Saadia Khan with input and review from David Noveck, Michael
Eisler, Carl Burnett, Ted Anderson and Thomas Talpey.
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9. Security Considerations
To Be Completed.
10. References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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Author's Address
Spencer Shepler
Sun Microsystems, Inc.
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