The NFS Version 4 Protocol

The NFS Version 4 Protocol

By :-
Kelum Senanayake

[Based on the paper of “The NFS Version 4 Protocol”, Brian Pawlowski,
Spencer Shepler, Carl Beame, Brent Callaghan, Michael Eisler,
David Noveck, David Robinson, Robert Thurlow]

What is NFS 4
 The Network File System (more commonly known as
NFS) is a distributed file system that may be accessed via
a network connection.
 NFS Version 4 is similar to previous versions of NFS in its
straightforward design, simplified error recovery, and
independence of transport protocols and operating
systems for file access in a heterogeneous network.
 NFS, was developed by Sun Microsystems together with
Internet Engineering Task Force (IETF).
 IETF develops and promotes Internet standards,
cooperating closely with the W3C and ISO/IEC standards
bodies.

The IETF process and NFS
1998
Sun/IETF Agreement
BOF, working group forms
Strawman Proposal from Sun
Meetings, writing, e-mail
1999 Prototyping by 5 organizations
Working Group Draft
Additional prototyping
Six working group drafts
Working Group Last Call
2000 IETF Last Call
IESG Review
Assign RFC number
2001 Proposed Standard RFC 3010

2002 Proposed Standard RFC 3530
Two independent implementations
6+ months
Draft Standard
Internet Standard apotheosis

Requirements for NFS Version 4
 Improved access and good performance on the Internet
 Strong security, with security negotiation built into the
protocol
 Enhanced cross-platform interoperability
 Extensibility of the protocol
 Improvements in locking and performance for narrow
data sharing applications

Overview
 The NFS Version 4 protocol is stateful.
 NFS is built on top of the ONC Remote
Procedure Protocol.
 Also uses XDR.
◦ The External Data Representation (XDR) enables
heterogeneous operation by defining a canonical data
encoding over the wire.

Basic NFS Architecture

XDR Protocol RPC Protocol

Structural Changes
 Elimination of ancillary protocols.
◦ Mount protocol, Network Lock Manager protocol.
◦ NFS 4 is a single protocol that uses a well-defined
port with the use of initialized filehandles and fully
integrated Locking in to the protocol.
 The introduction of a COMPOUND RPC procedure
◦ Allows the client to group traditional file operations into a single
request to send to the server.
◦ Reduce network round trip latency for related operations, which
can be costly over a WAN
 NFS 3 was designed to be easy to implement given an
NFS 2 implementation. NFS 4 did not have that
requirement.

Structural Changes… contd
 Introduction of the stateful operations OPEN and
CLOSE
 The regular file CREATE procedure is replaced by the
OPEN operation (with a create bit set) in NFS 4.
◦ The CREATE operation in NFS 4 is used only to create special
file objects such as symbolic links, directories, and special device
nodes.
◦ CREATE and REMOVE in NFS Version 4 subsumes the MKDIR
and RMDIR directory functionality of prior versions of NFS.
 A LOOKUP is very simple.
◦ Only sets the current filehandle to point at the file object
resolved.
◦ Attributes can be obtained with a subsequent GETATTR
operation in the same COMPOUND procedure.

Structural Changes… contd
 Does not assign special semantics to the directory
entries “.” and “..”
◦ Client should explicitly use the LOOKUPP operation.
 The NFS 3 directory scanning operation READDIRPLUS
procedure was dropped.
◦ Now supported by the READDIR operation.

NFS Protocol Stack

NFSv4 (RFC3530)
KerberosV5 (RFC1510)
SPKM-3
LIPKEY (RFC2847)

RPC (RFC1831)
RPCSEC_GSS (RFC2203)
XDR (RFC1832)

TCP*

File system model
 A file system is an implementation of a single file name
space containing files, and provides the basis for
administration and space allocation.
 file system identifier, or fsid, which is a 128-bit per-server
unique identifier.
 A file is a single named object consisting of data and
attributes, residing in a file system.
 A regular file is a simple byte stream – not a directory,
symbolic link or special (device) file.
 A filehandle uniquely identifies a file on a server.

Exporting file systems
 The export operation makes available only those file
systems, or portions of file systems, desired to be
shared with clients.
 In all versions of NFS, a server contains one or more
file systems that are exported to clients.
 In NFS 4, a server presents a single seamless view of all
the exported file systems to a client.
 The client can notice file system transitions on the
server by observing that the fsid changes.
 The name space describes the set of available files
arranged in a hierarchy.

Pseudo-file systems

Server Local FS Pseudo FS
/
/
A C
A B C
D F
D E F
I
G H I

The COMPOUND procedure
 RPC call defines a single request-response transaction
between the client and server.
 But client may actually be required to transmit a series
of related requests.
 The COMPOUND procedure groups multiple related
operations into a single RPC packet.
 The RPC response to a COMPOUND procedure
contains the replies to all the operations.
 Evaluation of the operations stops on first error.
 It may be unwise to attempt grouping unrelated
operations into a single COMPOUND procedure.

Properties of the COMPOUND procedure
 The set of operations in a COMPOUND procedure is
not atomic.
 Error handling is simple on the server.
 Most operations do not explicitly have a filehandle as an
argument or result.
◦ The server maintains a single filehandle, the current filehandle, as
the argument.

Communication Compound procedures

NFS v3 NFS v4

Important data structures
 Filehandles

 Client ID

 State ID

Filehandles
 A filehandle, is a per server unique identifier for a file
system object.
 Filehandles that are equal refer to the same file system
object.
◦ But no assumptions can be made by the client if the filehandles
differ.
 The current filehandle is used by subsequent operations
in a single COMPOUND procedure.
◦ GETFH operation to fetch the current filehandle.

Client ID
 A client first contacts the server using the
SETCLIENTID operation with a verifier.
 A verifier is a unique, non-repeating 64-bit object.
◦ Generated by the client.
 The server will return a 64-bit clientid.
◦ The clientid is unique.
◦ Will not conflict with those previously granted.
◦ Even across server reboots.
 The clientid is used in client recovery of locking state
following a server reboot.
 After a client reboot, the client will need to get a new
clientid

State ID
 A stateid is a unique 64-bit object that defines the
locking state of a specific file.
 Client requests a lock with clientID and a unique-per-
client lock owner identification.
 Server returns a unique 64-bit object, the stateid.
 Client uses this in subsequent operations as a
shorthand notation to the lock owner information.

Locking
 NFS 4 locking is similar to the Network Lock Manager
(NLM) protocol.
 NLM is used with NFS Versions 2 and 3.
 Locking is tightly coupled to the NFS 4 protocol.
◦ Better support different operating system semantics
and error recovery.

Drawbacks of NLM
 A major failing was the detection and recovery of error
conditions.
 The design assumed that the underlying transport was
reliable and preserved order.
 But an unreliable network easily resulted in orphan
locks on the server.
 If a client crashed and never recovered,
◦ Locks could be permanently abandoned
◦ Prevents any other client from ever acquiring the lock.

Leases
 The key change in NFS 4 locking is the introduction of
leases for lock management.
 A lease is a time-bounded grant of control of the state
of a file, through a lock, from the server to the client.
 A lock granted by the server will remain valid for a fixed
interval.
◦ Time interval can be renewal by the client.
 Client is responsible for contacting the server to
refresh the lease to maintain the lock.
◦ Client failure vs Server failure.
 A client exists in two states: either all the locks held
from a given server are correct or all are lost.
◦ Eliminates another drawback of a lease-based protocol

Mandatory locking & Share reservations

 Mandatory locking - the ability to block I/O operations
by other applications on a file that contains a lock.

 Share reservation - grants a client access to open a file
and the ability to deny other clients open access to the
same file.

Sequence IDs
 The most problematic part of network locking is dealing
with lock requests that arrive out of order or are
replayed.
 NFS 4 adds to every lock and unlock operation a
monotonically increasing sequence number.
 Server maintains for each lock owner the last sequence
number and the response sent.
◦ Earlier sequence number.
◦ Last sequence number.
◦ Sequence number beyond the next sequence number.

Security Model
 NFS relies on the underlying security model of RPC for
its security services.
 In the area of security, NFS Version 4 improves over
NFS Versions 2 and 3 by
◦ Mandating the use of strong RPC security flavors that depend on
cryptography.
◦ Negotiating the security used via a system that is both secure
and in-band.
◦ Using character strings instead of integers to represent user and
group identifiers.
◦ Supporting access control that is compatible with UNIX and
Windows.
◦ Removing the Mount protocol.

GSS-API framework
 Generic Security Services API
◦ Several security flavors
 NFS is based on RPC and a security flavor based GSS-
API called RPCSEC_GSS.
 RPCSEC_GSS differs from other traditional flavors in
two ways
◦ RPCSEC_GSS does more than authentication such as
performing integrity checksums and encryption of the entire
body of the RPC request and response.
◦ RPCSEC_GSS simply encapsulates the GSS-API messaging
tokens – Adding new security mechanisms (as long as they
conform to GSS-API) does not require significant re-writing.

Mandated strong security
 All versions of NFS are capable of using RPCSEC_GSS.
 NFS 4 implementations must implement security based
on Kerberos Version 5 and LIPKEY

Kerberos versus LIPKEY
 Kerberos has been used on other distributed file
systems.
◦ Andrew File System
◦ Open Software Foundation's Distributed File System
◦ Microsoft's CIFS
 Kerberos is an excellent choice for enterprises and
work groups operating within an Intranet.
◦ Provides centralized control.
◦ Single sign on to the network.
 Kerberos does not work well on the Internet.

Kerberos versus LIPKEY… contd
 NFS Version 4 is also designed to work outside of
intranets on the global Internet.
 LIPKEY - Low Infrastructure Public Key
 LIPKEY uses a symmetric key cipher and server-side
public key certificates.
 The LIPKEY system provides an SSL-like model and
equivalent security for use on the Internet.
 The LIPKEY user experience is similar to that of HTTP
over the Secure Sockets Layer (SSL).

Why not SSL?
 NFS Version 4 does not use SSL.
 SSL does not work over connectionless protocols like
UDP.
 RPC has its own security architecture
◦ It is unclear how to cleanly merge SSL and RPC security.

Migration and replication
 NFS Version 4 has added features to support file system
migration and replication.
 A file system can migrate to a new server.
◦ Clients are notified of the change by means of a special error
code.
 A client is informed of the new location by means of
the fs_locations file attribute.

Modifications for use on the Internet
 Requiring TCP as a transport.
 Defining COMPOUND operation to reduce roundtrip
latency
 Defining a global user identifier name space
 Mandating strong security based on a public key scheme
 Enabling operation through firewalls

Firewall friendly

Port 111
PORTMAP

}
Dynamic
MOUNT
Port 2049 Port 2049
NFSv2/v3
Dynamic NFSv4
LOCK/NLM TCP
Dynamic
STATUS
Dynamic
ACL*

Future Works
 Enhanced “Sessions” based NFS (correctness)
 CCM - session-based security for IPsec
 Migration/replication completion
◦ Core protocol defines client/server failover behaviour
◦ Definition of the server-server file system movement
◦ Transparent reconfiguration from client viewpoint
 Proxy NFS file services (NFS caches)
 Uniform global name space
◦ Features exist in the core protocol to support this
 Support for multi-realm security

Reference
 [1] B. Pawlowski, S. Shepler, C. Beame, B. Callaghan, M. Eisler, D. Noveck, D.
Robinson, and R. Thurlow. “The NFS Version 4 Protocol”. In Proceedings of
the 2nd International System Administration and Networking Conference
(SANE2000), page94, Maastricht, The Netherlands, May 2000.

The NFS Version 4 Protocol

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