Network File System (NFS) Version 4 Minor Version 1 Protocol

1.8.1. RPC and Security

As with previous versions of NFS, the External Data Representation (XDR) and Remote Procedure Call (RPC) mechanisms used for the NFSv4.1 protocol are those defined in [2] and [3]. To meet end-to-end security requirements, the RPCSEC_GSS framework [4] is used to extend the basic RPC security. With the use of RPCSEC_GSS, various mechanisms can be provided to offer authentication, integrity, and privacy to the NFSv4 protocol. Kerberos V5 is used as described in [5] to provide one security framework. With the use of RPCSEC_GSS, other mechanisms may also be specified and used for NFSv4.1 security.¶

To enable in-band security negotiation, the NFSv4.1 protocol has operations that provide the client a method of querying the server about its policies regarding which security mechanisms must be used for access to the server's file system resources. With this, the client can securely match the security mechanism that meets the policies specified at both the client and server.¶

NFSv4.1 introduces parallel access (see Section 1.8.2.2), which is called pNFS. The security framework described in this section is significantly modified by the introduction of pNFS (see Section 12.9), because data access is sometimes not over RPC. The level of significance varies with the storage protocol (see Section 12.2.5) and can be as low as zero impact (see Section 13.12).¶

1.8.2. Protocol Structure

1.8.3. File System Model

The general file system model used for the NFSv4.1 protocol is the same as previous versions. The server file system is hierarchical with the regular files contained within being treated as opaque byte streams. In a slight departure, file and directory names are encoded with UTF-8 to deal with the basics of internationalization.¶

The NFSv4.1 protocol does not require a separate protocol to provide for the initial mapping between path name and filehandle. All file systems exported by a server are presented as a tree so that all file systems are reachable from a special per-server global root filehandle. This allows LOOKUP operations to be used to perform functions previously provided by the MOUNT protocol. The server provides any necessary pseudo file systems to bridge any gaps that arise due to unexported gaps between exported file systems.¶

1.8.4. Locking Facilities

As mentioned previously, NFSv4.1 is a single protocol that includes locking facilities. These locking facilities include support for many types of locks including a number of sorts of recallable locks. Recallable locks such as delegations allow the client to be assured that certain events will not occur so long as that lock is held. When circumstances change, the lock is recalled via a callback request. The assurances provided by delegations allow more extensive caching to be done safely when circumstances allow it.¶

The types of locks are:¶

• Share reservations as established by OPEN operations.¶
• Byte-range locks.¶
• File delegations, which are recallable locks that assure the holder that inconsistent opens and file changes cannot occur so long as the delegation is held.¶
• Directory delegations, which are recallable locks that assure the holder that inconsistent directory modifications cannot occur so long as the delegation is held.¶
• Layouts, which are recallable objects that assure the holder that direct access to the file data may be performed directly by the client and that no change to the data's location that is inconsistent with that access may be made so long as the layout is held.¶

All locks for a given client are tied together under a single client-wide lease. All requests made on sessions associated with the client renew that lease. When the client's lease is not promptly renewed, the client's locks are subject to revocation. In the event of server restart, clients have the opportunity to safely reclaim their locks within a special grace period.¶

2.2.1. RPC-Based Security

Previous NFS versions have been thought of as having a host-based authentication model, where the NFS server authenticates the NFS client, and trusts the client to authenticate all users. Actually, NFS has always depended on RPC for authentication. One of the first forms of RPC authentication, AUTH_SYS, had no strong authentication and required a host-based authentication approach. NFSv4.1 also depends on RPC for basic security services and mandates RPC support for a user-based authentication model. The user-based authentication model has user principals authenticated by a server, and in turn the server authenticated by user principals. RPC provides some basic security services that are used by NFSv4.1.¶

   column descriptions:
   1 == number of pseudo flavor
   2 == name of pseudo flavor
   3 == mechanism's OID
   4 == RPCSEC_GSS service
   5 == NFSv4.1 clients MUST support
   6 == NFSv4.1 servers MUST support

   1      2        3                    4                     5   6
   ------------------------------------------------------------------
   390003 krb5     1.2.840.113554.1.2.2 rpc_gss_svc_none      yes yes
   390004 krb5i    1.2.840.113554.1.2.2 rpc_gss_svc_integrity yes yes
   390005 krb5p    1.2.840.113554.1.2.2 rpc_gss_svc_privacy    no yes

     service@hostname

     nfs

     nfs/hostname

2.4.3. Resolving Client Owner Conflicts

When the server gets an EXCHANGE_ID for a client owner that currently has no state, or that has state but the lease has expired, the server MUST allow the EXCHANGE_ID and confirm the new client ID if followed by the appropriate CREATE_SESSION.¶

When the server gets an EXCHANGE_ID for a new incarnation of a client owner that currently has an old incarnation with state and an unexpired lease, the server is allowed to dispose of the state of the previous incarnation of the client owner if one of the following is true:¶

• The principal that created the client ID for the client owner is the same as the principal that is sending the EXCHANGE_ID operation. Note that if the client ID was created with SP4_MACH_CRED state protection (Section 18.35), the principal MUST be based on RPCSEC_GSS authentication, the RPCSEC_GSS service used MUST be integrity or privacy, and the same GSS mechanism and principal MUST be used as that used when the client ID was created.¶
• The client ID was established with SP4_SSV protection (Section 18.35, Section 2.10.8.3) and the client sends the EXCHANGE_ID with the security flavor set to RPCSEC_GSS using the GSS SSV mechanism (Section 2.10.9).¶
• The client ID was established with SP4_SSV protection, and under the conditions described herein, the EXCHANGE_ID was sent with SP4_MACH_CRED state protection. Because the SSV might not persist across client and server restart, and because the first time a client sends EXCHANGE_ID to a server it does not have an SSV, the client MAY send the subsequent EXCHANGE_ID without an SSV RPCSEC_GSS handle. Instead, as with SP4_MACH_CRED protection, the principal MUST be based on RPCSEC_GSS authentication, the RPCSEC_GSS service used MUST be integrity or privacy, and the same GSS mechanism and principal MUST be used as that used when the client ID was created.¶

If none of the above situations apply, the server MUST return NFS4ERR_CLID_INUSE.¶

If the server accepts the principal and co_ownerid as matching that which created the client ID, and the co_verifier in the EXCHANGE_ID differs from the co_verifier used when the client ID was created, then after the server receives a CREATE_SESSION that confirms the client ID, the server deletes state. If the co_verifier values are the same (e.g., the client either is updating properties of the client ID (Section 18.35) or is attempting trunking (Section 2.10.5), the server MUST NOT delete state.¶

2.6.1. NFSv4.1 Security Tuples

An NFS server can assign one or more "security tuples" to each security policy boundary in its namespace. Each security tuple consists of a security flavor (see Section 2.2.1.1) and, if the flavor is RPCSEC_GSS, a GSS-API mechanism Object Identifier (OID), a GSS-API quality of protection, and an RPCSEC_GSS service.¶

2.6.2. SECINFO and SECINFO_NO_NAME

The SECINFO and SECINFO_NO_NAME operations allow the client to determine, on a per-filehandle basis, what security tuple is to be used for server access. In general, the client will not have to use either operation except during initial communication with the server or when the client crosses security policy boundaries at the server. However, the server's policies may also change at any time and force the client to negotiate a new security tuple.¶

Where the use of different security tuples would affect the type of access that would be allowed if a request was sent over the same connection used for the SECINFO or SECINFO_NO_NAME operation (e.g., read-only vs. read-write) access, security tuples that allow greater access should be presented first. Where the general level of access is the same and different security flavors limit the range of principals whose privileges are recognized (e.g., allowing or disallowing root access), flavors supporting the greatest range of principals should be listed first.¶

2.6.3. Security Error

Based on the assumption that each NFSv4.1 client and server MUST support a minimum set of security (i.e., Kerberos V5 under RPCSEC_GSS), the NFS client will initiate file access to the server with one of the minimal security tuples. During communication with the server, the client may receive an NFS error of NFS4ERR_WRONGSEC. This error allows the server to notify the client that the security tuple currently being used contravenes the server's security policy. The client is then responsible for determining (see Section 2.6.3.1) what security tuples are available at the server and choosing one that is appropriate for the client.¶

2.8.1. Authorization

Authorization to access a file object via an NFSv4.1 operation is ultimately determined by the NFSv4.1 server. A client can predetermine its access to a file object via the OPEN (Section 18.16) and the ACCESS (Section 18.1) operations.¶

Principals with appropriate access rights can modify the authorization on a file object via the SETATTR (Section 18.30) operation. Attributes that affect access rights include mode, owner, owner_group, acl, dacl, and sacl. See Section 5.¶

2.8.2. Auditing

NFSv4.1 provides auditing on a per-file object basis, via the acl and sacl attributes as described in Section 6. It is outside the scope of this specification to specify audit log formats or management policies.¶

2.8.3. Intrusion Detection

NFSv4.1 provides alarm control on a per-file object basis, via the acl and sacl attributes as described in Section 6. Alarms may serve as the basis for intrusion detection. It is outside the scope of this specification to specify heuristics for detecting intrusion via alarms.¶

2.9.1. REQUIRED and RECOMMENDED Properties of Transports

NFSv4.1 works over Remote Direct Memory Access (RDMA) and non-RDMA-based transports with the following attributes:¶

• The transport supports reliable delivery of data, which NFSv4.1 requires but neither NFSv4.1 nor RPC has facilities for ensuring [41].¶
• The transport delivers data in the order it was sent. Ordered delivery simplifies detection of transmit errors, and simplifies the sending of arbitrary sized requests and responses via the record marking protocol [3].¶

Where an NFSv4.1 implementation supports operation over the IP network protocol, any transport used between NFS and IP MUST be among the IETF-approved congestion control transport protocols. At the time this document was written, the only two transports that had the above attributes were TCP and the Stream Control Transmission Protocol (SCTP). To enhance the possibilities for interoperability, an NFSv4.1 implementation MUST support operation over the TCP transport protocol.¶

Even if NFSv4.1 is used over a non-IP network protocol, it is RECOMMENDED that the transport support congestion control.¶

It is permissible for a connectionless transport to be used under NFSv4.1; however, reliable and in-order delivery of data combined with congestion control by the connectionless transport is REQUIRED. As a consequence, UDP by itself MUST NOT be used as an NFSv4.1 transport. NFSv4.1 assumes that a client transport address and server transport address used to send data over a transport together constitute a connection, even if the underlying transport eschews the concept of a connection.¶

2.9.2. Client and Server Transport Behavior

If a connection-oriented transport (e.g., TCP) is used, the client and server SHOULD use long-lived connections for at least three reasons:¶

1. This will prevent the weakening of the transport's congestion control mechanisms via short-lived connections.¶
2. This will improve performance for the WAN environment by eliminating the need for connection setup handshakes.¶
3. The NFSv4.1 callback model differs from NFSv4.0, and requires the client and server to maintain a client-created backchannel (see Section 2.10.3.1) for the server to use.¶

In order to reduce congestion, if a connection-oriented transport is used, and the request is not the NULL procedure:¶

• A requester MUST NOT retry a request unless the connection the request was sent over was lost before the reply was received.¶
• A replier MUST NOT silently drop a request, even if the request is a retry. (The silent drop behavior of RPCSEC_GSS [4] does not apply because this behavior happens at the RPCSEC_GSS layer, a lower layer in the request processing.) Instead, the replier SHOULD return an appropriate error (see Section 2.10.6.1), or it MAY disconnect the connection.¶

When sending a reply, the replier MUST send the reply to the same full network address (e.g., if using an IP-based transport, the source port of the requester is part of the full network address) from which the requester sent the request. If using a connection-oriented transport, replies MUST be sent on the same connection from which the request was received.¶

If a connection is dropped after the replier receives the request but before the replier sends the reply, the replier might have a pending reply. If a connection is established with the same source and destination full network address as the dropped connection, then the replier MUST NOT send the reply until the requester retries the request. The reason for this prohibition is that the requester MAY retry a request over a different connection (provided that connection is associated with the original request's session).¶

When using RDMA transports, there are other reasons for not tolerating retries over the same connection:¶

• RDMA transports use "credits" to enforce flow control, where a credit is a right to a peer to transmit a message. If one peer were to retransmit a request (or reply), it would consume an additional credit. If the replier retransmitted a reply, it would certainly result in an RDMA connection loss, since the requester would typically only post a single receive buffer for each request. If the requester 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 reply cache in NFSv4.1. The reply 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 RDMA connection, and stale values must not be replayed from the cache. This implies that the reply cache contents must not be blindly used when replies are sent from it, and credit information appropriate to the channel must be refreshed by the RPC layer.¶

In addition, as described in Section 2.10.6.2, while a session is active, the NFSv4.1 requester MUST NOT stop waiting for a reply.¶

2.9.3. Ports

Historically, NFSv3 servers have listened over TCP port 2049. The registered port 2049 [42] for the NFS protocol should be the default configuration. NFSv4.1 clients SHOULD NOT use the RPC binding protocols as described in [43].¶

2.10.1. Motivation and Overview

Previous versions and minor versions of NFS have suffered from the following:¶

• Lack of support for Exactly Once Semantics (EOS). This includes lack of support for EOS through server failure and recovery.¶
• Limited callback support, including no support for sending callbacks through firewalls, and races between replies to normal requests and callbacks.¶
• Limited trunking over multiple network paths.¶
• Requiring machine credentials for fully secure operation.¶

Through the introduction of a session, NFSv4.1 addresses the above shortfalls with practical solutions:¶

• EOS is enabled by a reply cache with a bounded size, making it feasible to keep the cache in persistent storage and enable EOS through server failure and recovery. One reason that previous revisions of NFS did not support EOS was because some EOS approaches often limited parallelism. As will be explained in Section 2.10.6, NFSv4.1 supports both EOS and unlimited parallelism.¶
• The NFSv4.1 client (defined in Section 1.7) creates transport connections and provides them to the server to use for sending callback requests, thus solving the firewall issue (Section 18.34). Races between responses from client requests and callbacks caused by the requests are detected via the session's sequencing properties that are a consequence of EOS (Section 2.10.6.3).¶
• The NFSv4.1 client can associate an arbitrary number of connections with the session, and thus provide trunking (Section 2.10.5).¶
• The NFSv4.1 client and server produce a session key independent of client and server machine credentials which can be used to compute a digest for protecting critical session management operations (Section 2.10.8.3).¶
• The NFSv4.1 client can also create secure RPCSEC_GSS contexts for use by the session's backchannel that do not require the server to authenticate to a client machine principal (Section 2.10.8.2).¶

A session is a dynamically created, long-lived server object created by a client and 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, and indeed the state exists whether or not the connection exists. A client may have one or more sessions associated with it so that client-associated state may be accessed using any of the sessions associated with that client's client ID, when connections are associated with those sessions. When no connections are associated with any of a client ID's sessions for an extended time, such objects as locks, opens, delegations, layouts, etc. are subject to expiration. The session serves as an object representing a means of access by a client to the associated client state on the server, independent of the physical means of access to that state.¶

A single client may create multiple sessions. A single session MUST NOT serve multiple clients.¶

2.10.2. NFSv4 Integration

Sessions are part of NFSv4.1 and not NFSv4.0. Normally, a major infrastructure change such as sessions would require a new major version number to an Open Network Computing (ONC) RPC program like NFS. However, because NFSv4 encapsulates its functionality in a single procedure, COMPOUND, and because COMPOUND can support an arbitrary number of operations, sessions have been added to NFSv4.1 with little difficulty. COMPOUND includes a minor version number field, and for NFSv4.1 this minor version is set to 1. When the NFSv4 server processes a COMPOUND with the minor version set to 1, it expects a different set of operations than it does for NFSv4.0. NFSv4.1 defines the SEQUENCE operation, which is required for every COMPOUND that operates over an established session, with the exception of some session administration operations, such as DESTROY_SESSION (Section 18.37).¶

    +-----+--------------+-----------+------------+-----------+----
    | tag | minorversion | numops    |SEQUENCE op | op + args | ...
    |     |   (== 1)     | (limited) |  + args    |           |
    +-----+--------------+-----------+------------+-----------+----

    +------------+-----+--------+-------------------------------+--//
    |last status | tag | numres |status + SEQUENCE op + results |  //
    +------------+-----+--------+-------------------------------+--//
            //-----------------------+----
            // status + op + results | ...
            //-----------------------+----

2.10.3. Channels

A channel is not a connection. A channel represents the direction ONC RPC requests are sent.¶

Each session has one or two channels: the fore channel and the backchannel. Because there are at most two channels per session, and because each channel has a distinct purpose, channels are not assigned identifiers.¶

The fore channel is used for ordinary requests from the client to the server, and carries COMPOUND requests and responses. A session always has a fore channel.¶

The backchannel is used for callback requests from server to client, and carries CB_COMPOUND requests and responses. Whether or not there is a backchannel is decided by the client; however, many features of NFSv4.1 require a backchannel. NFSv4.1 servers MUST support backchannels.¶

Each session has resources for each channel, including separate reply caches (see Section 2.10.6.1). Note that even the backchannel requires a reply cache (or, at least, a slot table in order to detect retries) because some callback operations are non-idempotent.¶

2.10.4. Server Scope

Servers each specify a server scope value in the form of an opaque string eir_server_scope returned as part of the results of an EXCHANGE_ID operation. The purpose of the server scope is to allow a group of servers to indicate to clients that a set of servers sharing the same server scope value has arranged to use distinct values of opaque identifiers so that the two servers never assign the same value to two distinct objects. Thus, the identifiers generated by two servers within that set can be assumed compatible so that, in certain important cases, identifiers generated by one server in that set may be presented to another server of the same scope.¶

The use of such compatible values does not imply that a value generated by one server will always be accepted by another. In most cases, it will not. However, a server will not inadvertently accept a value generated by another server. When it does accept it, it will be because it is recognized as valid and carrying the same meaning as on another server of the same scope.¶

When servers are of the same server scope, this compatibility of values applies to the following identifiers:¶

• Filehandle values. A filehandle value accepted by two servers of the same server scope denotes the same object. A WRITE operation sent to one server is reflected immediately in a READ sent to the other.¶
• Server owner values. When the server scope values are the same, server owner value may be validly compared. In cases where the server scope values are different, server owner values are treated as different even if they contain identical strings of bytes.¶

The coordination among servers required to provide such compatibility can be quite minimal, and limited to a simple partition of the ID space. The recognition of common values requires additional implementation, but this can be tailored to the specific situations in which that recognition is desired.¶

Clients will have occasion to compare the server scope values of multiple servers under a number of circumstances, each of which will be discussed under the appropriate functional section:¶

• When server owner values received in response to EXCHANGE_ID operations sent to multiple network addresses are compared for the purpose of determining the validity of various forms of trunking, as described in Section 11.5.2.¶
• When network or server reconfiguration causes the same network address to possibly be directed to different servers, with the necessity for the client to determine when lock reclaim should be attempted, as described in Section 8.4.2.1.¶

When two replies from EXCHANGE_ID, each from two different server network addresses, have the same server scope, there are a number of ways a client can validate that the common server scope is due to two servers cooperating in a group.¶

• If both EXCHANGE_ID requests were sent with RPCSEC_GSS ([4], [9], [27]) authentication and the server principal is the same for both targets, the equality of server scope is validated. It is RECOMMENDED that two servers intending to share the same server scope and server_owner major_id also share the same principal name. In some cases, this simplifies the client's task of validating server scope.¶
• The client may accept the appearance of the second server in the fs_locations or fs_locations_info attribute for a relevant file system. For example, if there is a migration event for a particular file system or there are locks to be reclaimed on a particular file system, the attributes for that particular file system may be used. The client sends the GETATTR request to the first server for the fs_locations or fs_locations_info attribute with RPCSEC_GSS authentication. It may need to do this in advance of the need to verify the common server scope. If the client successfully authenticates the reply to GETATTR, and the GETATTR request and reply containing the fs_locations or fs_locations_info attribute refers to the second server, then the equality of server scope is supported. A client may choose to limit the use of this form of support to information relevant to the specific file system involved (e.g. a file system being migrated).¶

2.10.5. Trunking

Trunking is the use of multiple connections between a client and server in order to increase the speed of data transfer. NFSv4.1 supports two types of trunking: session trunking and client ID trunking.¶

In the context of a single server network address, it can be assumed that all connections are accessing the same server, and NFSv4.1 servers MUST support both forms of trunking. When multiple connections use a set of network addresses to access the same server, the server MUST support both forms of trunking. NFSv4.1 servers in a clustered configuration MAY allow network addresses for different servers to use client ID trunking.¶

Clients may use either form of trunking as long as they do not, when trunking between different server network addresses, violate the servers' mandates as to the kinds of trunking to be allowed (see below). With regard to callback channels, the client MUST allow the server to choose among all callback channels valid for a given client ID and MUST support trunking when the connections supporting the backchannel allow session or client ID trunking to be used for callbacks.¶

Session trunking is essentially the association of multiple connections, each with potentially different target and/or source network addresses, to the same session. When the target network addresses (server addresses) of the two connections are the same, the server MUST support such session trunking. When the target network addresses are different, the server MAY indicate such support using the data returned by the EXCHANGE_ID operation (see below).¶

Client ID trunking is the association of multiple sessions to the same client ID. Servers MUST support client ID trunking for two target network addresses whenever they allow session trunking for those same two network addresses. In addition, a server MAY, by presenting the same major server owner ID (Section 2.5) and server scope (Section 2.10.4), allow an additional case of client ID trunking. When two servers return the same major server owner and server scope, it means that the two servers are cooperating on locking state management, which is a prerequisite for client ID trunking.¶

Distinguishing when the client is allowed to use session and client ID trunking requires understanding how the results of the EXCHANGE_ID (Section 18.35) operation identify a server. Suppose a client sends EXCHANGE_IDs over two different connections, each with a possibly different target network address, but each EXCHANGE_ID operation has the same value in the eia_clientowner field. If the same NFSv4.1 server is listening over each connection, then each EXCHANGE_ID result MUST return the same values of eir_clientid, eir_server_owner.so_major_id, and eir_server_scope. The client can then treat each connection as referring to the same server (subject to verification; see Section 2.10.5.1 below), and it can use each connection to trunk requests and replies. The client's choice is whether session trunking or client ID trunking applies.¶

Session Trunking.

If the eia_clientowner argument is the same in two different EXCHANGE_ID requests, and the eir_clientid, eir_server_owner.so_major_id, eir_server_owner.so_minor_id, and eir_server_scope results match in both EXCHANGE_ID results, then the client is permitted to perform session trunking. If the client has no session mapping to the tuple of eir_clientid, eir_server_owner.so_major_id, eir_server_scope, and eir_server_owner.so_minor_id, then it creates the session via a CREATE_SESSION operation over one of the connections, which associates the connection to the session. If there is a session for the tuple, the client can send BIND_CONN_TO_SESSION to associate the connection to the session.¶

Of course, if the client does not desire to use session trunking, it is not required to do so. It can invoke CREATE_SESSION on the connection. This will result in client ID trunking as described below. It can also decide to drop the connection if it does not choose to use trunking.¶

Client ID Trunking.

If the eia_clientowner argument is the same in two different EXCHANGE_ID requests, and the eir_clientid, eir_server_owner.so_major_id, and eir_server_scope results match in both EXCHANGE_ID results, then the client is permitted to perform client ID trunking (regardless of whether the eir_server_owner.so_minor_id results match). The client can associate each connection with different sessions, where each session is associated with the same server.¶

The client completes the act of client ID trunking by invoking CREATE_SESSION on each connection, using the same client ID that was returned in eir_clientid. These invocations create two sessions and also associate each connection with its respective session. The client is free to decline to use client ID trunking by simply dropping the connection at this point.¶

When doing client ID trunking, locking state is shared across sessions associated with that same client ID. This requires the server to coordinate state across sessions and the client to be able to associate the same locking state with multiple sessions.¶

It is always possible that, as a result of various sorts of reconfiguration events, eir_server_scope and eir_server_owner values may be different on subsequent EXCHANGE_ID requests made to the same network address.¶

In most cases, such reconfiguration events will be disruptive and indicate that an IP address formerly connected to one server is now connected to an entirely different one.¶

Some guidelines on client handling of such situations follow:¶

• When eir_server_scope changes, the client has no assurance that any IDs that it obtained previously (e.g., filehandles) can be validly used on the new server, and, even if the new server accepts them, there is no assurance that this is not due to accident. Thus, it is best to treat all such state as lost or stale, although a client may assume that the probability of inadvertent acceptance is low and treat this situation as within the next case.¶
• When eir_server_scope remains the same and eir_server_owner.so_major_id changes, the client can use the filehandles it has, consider its locking state lost, and attempt to reclaim or otherwise re-obtain its locks. It might find that its filehandle is now stale. However, if NFS4ERR_STALE is not returned, it can proceed to reclaim or otherwise re-obtain its open locking state.¶
• When eir_server_scope and eir_server_owner.so_major_id remain the same, the client has to use the now-current values of eir_server_owner.so_minor_id in deciding on appropriate forms of trunking. This may result in connections being dropped or new sessions being created.¶

2.10.6. Exactly Once Semantics

Via the session, NFSv4.1 offers exactly once semantics (EOS) for requests sent over a channel. EOS is supported on both the fore channel and backchannel.¶

Each COMPOUND or CB_COMPOUND request that is sent with a leading SEQUENCE or CB_SEQUENCE operation MUST be executed by the receiver exactly once. This requirement holds regardless of whether the request is sent with reply caching specified (see Section 2.10.6.1.3). The requirement holds even if the requester is sending the request over a session created between a pNFS data client and pNFS data server. To understand the rationale for this requirement, divide the requests into three classifications:¶

• Non-idempotent requests.¶
• Idempotent modifying requests.¶
• Idempotent non-modifying requests.¶

An example of a non-idempotent request is RENAME. Obviously, if a replier executes the same RENAME request twice, and the first execution succeeds, the re-execution will fail. If the replier returns the result from the re-execution, this result is incorrect. Therefore, EOS is required for non-idempotent requests.¶

An example of an idempotent modifying request is a COMPOUND request containing a WRITE operation. Repeated execution of the same WRITE has the same effect as execution of that WRITE a single time. Nevertheless, enforcing EOS for WRITEs and other idempotent modifying requests is necessary to avoid data corruption.¶

Suppose a client sends WRITE A to a noncompliant server that does not enforce EOS, and receives no response, perhaps due to a network partition. The client reconnects to the server and re-sends WRITE A. Now, the server has outstanding two instances of A. The server can be in a situation in which it executes and replies to the retry of A, while the first A is still waiting in the server's internal I/O system for some resource. Upon receiving the reply to the second attempt of WRITE A, the client believes its WRITE is done so it is free to send WRITE B, which overlaps the byte-range of A. When the original A is dispatched from the server's I/O system and executed (thus the second time A will have been written), then what has been written by B can be overwritten and thus corrupted.¶

An example of an idempotent non-modifying request is a COMPOUND containing SEQUENCE, PUTFH, READLINK, and nothing else. The re-execution of such a request will not cause data corruption or produce an incorrect result. Nonetheless, to keep the implementation simple, the replier MUST enforce EOS for all requests, whether or not idempotent and non-modifying.¶

Note that true and complete EOS is not possible unless the server persists the reply cache in stable storage, and unless the server is somehow implemented to never require a restart (indeed, if such a server exists, the distinction between a reply cache kept in stable storage versus one that is not is one without meaning). See Section 2.10.6.5 for a discussion of persistence in the reply cache. Regardless, even if the server does not persist the reply cache, EOS improves robustness and correctness over previous versions of NFS because the legacy duplicate request/reply caches were based on the ONC RPC transaction identifier (XID). Section 2.10.6.1 explains the shortcomings of the XID as a basis for a reply cache and describes how NFSv4.1 sessions improve upon the XID.¶

2.10.7. RDMA Considerations

A complete discussion of the operation of RPC-based protocols over RDMA transports is in [32]. A discussion of the operation of NFSv4, including NFSv4.1, over RDMA is in [33]. Where RDMA is considered, this specification assumes the use of such a layering; it addresses only the upper-layer issues relevant to making best use of RPC/RDMA.¶

                 Sender gather:
     |RPC Request|Pad  bytes|Length| -> |User data...|
     \------+----------------------/      \
             \                             \
              \    Receiver scatter:        \-----------+- ...
         /-----+----------------\            \           \
         |RPC Request|Pad|Length|   ->  |FS buffer|->|FS buffer|->...

2.10.8. Session Security

2.10.9. The Secret State Verifier (SSV) GSS Mechanism

The SSV provides the secret key for a GSS mechanism internal to NFSv4.1 that NFSv4.1 uses for state protection. Contexts for this mechanism are not established via the RPCSEC_GSS protocol. Instead, the contexts are automatically created when EXCHANGE_ID specifies SP4_SSV protection. The only tokens defined are the PerMsgToken (emitted by GSS_GetMIC) and the SealedMessage token (emitted by GSS_Wrap).¶

The mechanism OID for the SSV mechanism is iso.org.dod.internet.private.enterprise.Michael Eisler.nfs.ssv_mech (1.3.6.1.4.1.28882.1.1). While the SSV mechanism does not define any initial context tokens, the OID can be used to let servers indicate that the SSV mechanism is acceptable whenever the client sends a SECINFO or SECINFO_NO_NAME operation (see Section 2.6).¶

The SSV mechanism defines four subkeys derived from the SSV value. Each time SET_SSV is invoked, the subkeys are recalculated by the client and server. The calculation of each of the four subkeys depends on each of the four respective ssv_subkey4 enumerated values. The calculation uses the HMAC [52] algorithm, using the current SSV as the key, the one-way hash algorithm as negotiated by EXCHANGE_ID, and the input text as represented by the XDR encoded enumeration value for that subkey of data type ssv_subkey4. If the length of the output of the HMAC algorithm exceeds the length of key of the encryption algorithm (which is also negotiated by EXCHANGE_ID), then the subkey MUST be truncated from the HMAC output, i.e., if the subkey is of N bytes long, then the first N bytes of the HMAC output MUST be used for the subkey. The specification of EXCHANGE_ID states that the length of the output of the HMAC algorithm MUST NOT be less than the length of subkey needed for the encryption algorithm (see Section 18.35).¶

/* Input for computing subkeys */
enum ssv_subkey4 {
        SSV4_SUBKEY_MIC_I2T     = 1,
        SSV4_SUBKEY_MIC_T2I     = 2,
        SSV4_SUBKEY_SEAL_I2T    = 3,
        SSV4_SUBKEY_SEAL_T2I    = 4
};

The subkey derived from SSV4_SUBKEY_MIC_I2T is used for calculating message integrity codes (MICs) that originate from the NFSv4.1 client, whether as part of a request over the fore channel or a response over the backchannel. The subkey derived from SSV4_SUBKEY_MIC_T2I is used for MICs originating from the NFSv4.1 server. The subkey derived from SSV4_SUBKEY_SEAL_I2T is used for encryption text originating from the NFSv4.1 client, and the subkey derived from SSV4_SUBKEY_SEAL_T2I is used for encryption text originating from the NFSv4.1 server.¶

The PerMsgToken description is based on an XDR definition:¶

/* Input for computing smt_hmac */
struct ssv_mic_plain_tkn4 {
  uint32_t        smpt_ssv_seq;
  opaque          smpt_orig_plain<>;
};

/* SSV GSS PerMsgToken token */
struct ssv_mic_tkn4 {
  uint32_t        smt_ssv_seq;
  opaque          smt_hmac<>;
};

The field smt_hmac is an HMAC calculated by using the subkey derived from SSV4_SUBKEY_MIC_I2T or SSV4_SUBKEY_MIC_T2I as the key, the one-way hash algorithm as negotiated by EXCHANGE_ID, and the input text as represented by data of type ssv_mic_plain_tkn4. The field smpt_ssv_seq is the same as smt_ssv_seq. The field smpt_orig_plain is the "message" input passed to GSS_GetMIC() (see Section 2.3.1 of [7]). The caller of GSS_GetMIC() provides a pointer to a buffer containing the plain text. The SSV mechanism's entry point for GSS_GetMIC() encodes this into an opaque array, and the encoding will include an initial four-byte length, plus any necessary padding. Prepended to this will be the XDR encoded value of smpt_ssv_seq, thus making up an XDR encoding of a value of data type ssv_mic_plain_tkn4, which in turn is the input into the HMAC.¶

The token emitted by GSS_GetMIC() is XDR encoded and of XDR data type ssv_mic_tkn4. The field smt_ssv_seq comes from the SSV sequence number, which is equal to one after SET_SSV (Section 18.47) is called the first time on a client ID. Thereafter, the SSV sequence number is incremented on each SET_SSV. Thus, smt_ssv_seq represents the version of the SSV at the time GSS_GetMIC() was called. As noted in Section 18.35, the client and server can maintain multiple concurrent versions of the SSV. This allows the SSV to be changed without serializing all RPC calls that use the SSV mechanism with SET_SSV operations. Once the HMAC is calculated, it is XDR encoded into smt_hmac, which will include an initial four-byte length, and any necessary padding. Prepended to this will be the XDR encoded value of smt_ssv_seq.¶

The SealedMessage description is based on an XDR definition:¶

/* Input for computing ssct_encr_data and ssct_hmac */
struct ssv_seal_plain_tkn4 {
  opaque          sspt_confounder<>;
  uint32_t        sspt_ssv_seq;
  opaque          sspt_orig_plain<>;
  opaque          sspt_pad<>;
};

/* SSV GSS SealedMessage token */
struct ssv_seal_cipher_tkn4 {
  uint32_t      ssct_ssv_seq;
  opaque        ssct_iv<>;
  opaque        ssct_encr_data<>;
  opaque        ssct_hmac<>;
};

The token emitted by GSS_Wrap() is XDR encoded and of XDR data type ssv_seal_cipher_tkn4.¶

The ssct_ssv_seq field has the same meaning as smt_ssv_seq.¶

The ssct_encr_data field is the result of encrypting a value of the XDR encoded data type ssv_seal_plain_tkn4. The encryption key is the subkey derived from SSV4_SUBKEY_SEAL_I2T or SSV4_SUBKEY_SEAL_T2I, and the encryption algorithm is that negotiated by EXCHANGE_ID.¶

The ssct_iv field is the initialization vector (IV) for the encryption algorithm (if applicable) and is sent in clear text. The content and size of the IV MUST comply with the specification of the encryption algorithm. For example, the id-aes256-CBC algorithm MUST use a 16-byte initialization vector (IV), which MUST be unpredictable for each instance of a value of data type ssv_seal_plain_tkn4 that is encrypted with a particular SSV key.¶

The ssct_hmac field is the result of computing an HMAC using the value of the XDR encoded data type ssv_seal_plain_tkn4 as the input text. The key is the subkey derived from SSV4_SUBKEY_MIC_I2T or SSV4_SUBKEY_MIC_T2I, and the one-way hash algorithm is that negotiated by EXCHANGE_ID.¶

The sspt_confounder field is a random value.¶

The sspt_ssv_seq field is the same as ssvt_ssv_seq.¶

The field sspt_orig_plain field is the original plaintext and is the "input_message" input passed to GSS_Wrap() (see Section 2.3.3 of [7]). As with the handling of the plaintext by the SSV mechanism's GSS_GetMIC() entry point, the entry point for GSS_Wrap() expects a pointer to the plaintext, and will XDR encode an opaque array into sspt_orig_plain representing the plain text, along with the other fields of an instance of data type ssv_seal_plain_tkn4.¶

The sspt_pad field is present to support encryption algorithms that require inputs to be in fixed-sized blocks. The content of sspt_pad is zero filled except for the length. Beware that the XDR encoding of ssv_seal_plain_tkn4 contains three variable-length arrays, and so each array consumes four bytes for an array length, and each array that follows the length is always padded to a multiple of four bytes per the XDR standard.¶

For example, suppose the encryption algorithm uses 16-byte blocks, and the sspt_confounder is three bytes long, and the sspt_orig_plain field is 15 bytes long. The XDR encoding of sspt_confounder uses eight bytes (4 + 3 + 1-byte pad), the XDR encoding of sspt_ssv_seq uses four bytes, the XDR encoding of sspt_orig_plain uses 20 bytes (4 + 15 + 1-byte pad), and the smallest XDR encoding of the sspt_pad field is four bytes. This totals 36 bytes. The next multiple of 16 is 48; thus, the length field of sspt_pad needs to be set to 12 bytes, or a total encoding of 16 bytes. The total number of XDR encoded bytes is thus 8 + 4 + 20 + 16 = 48.¶

GSS_Wrap() emits a token that is an XDR encoding of a value of data type ssv_seal_cipher_tkn4. Note that regardless of whether or not the caller of GSS_Wrap() requests confidentiality, the token always has confidentiality. This is because the SSV mechanism is for RPCSEC_GSS, and RPCSEC_GSS never produces GSS_wrap() tokens without confidentiality.¶

There is one SSV per client ID. There is a single GSS context for a client ID / SSV pair. All SSV mechanism RPCSEC_GSS handles of a client ID / SSV pair share the same GSS context. SSV GSS contexts do not expire except when the SSV is destroyed (causes would include the client ID being destroyed or a server restart). Since one purpose of context expiration is to replace keys that have been in use for "too long", hence vulnerable to compromise by brute force or accident, the client can replace the SSV key by sending periodic SET_SSV operations, which is done by cycling through different users' RPCSEC_GSS credentials. This way, the SSV is replaced without destroying the SSV's GSS contexts.¶

SSV RPCSEC_GSS handles can be expired or deleted by the server at any time, and the EXCHANGE_ID operation can be used to create more SSV RPCSEC_GSS handles. Expiration of SSV RPCSEC_GSS handles does not imply that the SSV or its GSS context has expired.¶

The client MUST establish an SSV via SET_SSV before the SSV GSS context can be used to emit tokens from GSS_Wrap() and GSS_GetMIC(). If SET_SSV has not been successfully called, attempts to emit tokens MUST fail.¶

The SSV mechanism does not support replay detection and sequencing in its tokens because RPCSEC_GSS does not use those features (see "Context Creation Requests", Section 5.2.2 of [4]). However, Section 2.10.10 discusses special considerations for the SSV mechanism when used with RPCSEC_GSS.¶

2.10.10. Security Considerations for RPCSEC_GSS When Using the SSV Mechanism

When a client ID is created with SP4_SSV state protection (see Section 18.35), the client is permitted to associate multiple RPCSEC_GSS handles with the single SSV GSS context (see Section 2.10.9). Because of the way RPCSEC_GSS (both version 1 and version 2, see [4] and [9]) calculate the verifier of the reply, special care must be taken by the implementation of the NFSv4.1 client to prevent attacks by a man-in-the-middle. The verifier of an RPCSEC_GSS reply is the output of GSS_GetMIC() applied to the input value of the seq_num field of the RPCSEC_GSS credential (data type rpc_gss_cred_ver_1_t) (see Section 5.3.3.2 of [4]). If multiple RPCSEC_GSS handles share the same GSS context, then if one handle is used to send a request with the same seq_num value as another handle, an attacker could block the reply, and replace it with the verifier used for the other handle.¶

There are multiple ways to prevent the attack on the SSV RPCSEC_GSS verifier in the reply. The simplest is believed to be as follows.¶

• Each time one or more new SSV RPCSEC_GSS handles are created via EXCHANGE_ID, the client SHOULD send a SET_SSV operation to modify the SSV. By changing the SSV, the new handles will not result in the re-use of an SSV RPCSEC_GSS verifier in a reply.¶
• When a requester decides to use N SSV RPCSEC_GSS handles, it SHOULD assign a unique and non-overlapping range of seq_nums to each SSV RPCSEC_GSS handle. The size of each range SHOULD be equal to MAXSEQ / N (see Section 5 of [4] for the definition of MAXSEQ). When an SSV RPCSEC_GSS handle reaches its maximum, it SHOULD force the replier to destroy the handle by sending a NULL RPC request with seq_num set to MAXSEQ + 1 (see Section 5.3.3.3 of [4]).¶
• When the requester wants to increase or decrease N, it SHOULD force the replier to destroy all N handles by sending a NULL RPC request on each handle with seq_num set to MAXSEQ + 1. If the requester is the client, it SHOULD send a SET_SSV operation before using new handles. If the requester is the server, then the client SHOULD send a SET_SSV operation when it detects that the server has forced it to destroy a backchannel's SSV RPCSEC_GSS handle. By sending a SET_SSV operation, the SSV will change, and so the attacker will be unavailable to successfully replay a previous verifier in a reply to the requester.¶

Note that if the replier carefully creates the SSV RPCSEC_GSS handles, the related risk of a man-in-the-middle splicing a forged SSV RPCSEC_GSS credential with a verifier for another handle does not exist. This is because the verifier in an RPCSEC_GSS request is computed from input that includes both the RPCSEC_GSS handle and seq_num (see Section 5.3.1 of [4]). Provided the replier takes care to avoid re-using the value of an RPCSEC_GSS handle that it creates, such as by including a generation number in the handle, the man-in-the-middle will not be able to successfully replay a previous verifier in the request to a replier.¶

2.10.11. Session Mechanics - Steady State

2.10.12. Session Inactivity Timer

The server MAY maintain a session inactivity timer for each session. If the session inactivity timer expires, then the server MAY destroy the session. To avoid losing a session due to inactivity, the client MUST renew the session inactivity timer. The length of session inactivity timer MUST NOT be less than the lease_time attribute (Section 5.8.1.11). As with lease renewal (Section 8.3), when the server receives a SEQUENCE operation, it resets the session inactivity timer, and MUST NOT allow the timer to expire while the rest of the operations in the COMPOUND procedure's request are still executing. Once the last operation has finished, the server MUST set the session inactivity timer to expire no sooner than the sum of the current time and the value of the lease_time attribute.¶

2.10.13. Session Mechanics - Recovery

2.10.14. Parallel NFS and Sessions

A client and server can potentially be a non-pNFS implementation, a metadata server implementation, a data server implementation, or two or three types of implementations. The EXCHGID4_FLAG_USE_NON_PNFS, EXCHGID4_FLAG_USE_PNFS_MDS, and EXCHGID4_FLAG_USE_PNFS_DS flags (not mutually exclusive) are passed in the EXCHANGE_ID arguments and results to allow the client to indicate how it wants to use sessions created under the client ID, and to allow the server to indicate how it will allow the sessions to be used. See Section 13.1 for pNFS sessions considerations.¶

5.8.1. Definitions of REQUIRED Attributes

5.8.2. Definitions of Uncategorized RECOMMENDED Attributes

The definitions of most of the RECOMMENDED attributes follow. Collections that share a common category are defined in other sections.¶

5.11.1. Attribute 56: dir_notif_delay

The dir_notif_delay attribute is the minimum number of seconds the server will delay before notifying the client of a change to the directory's attributes.¶

5.11.2. Attribute 57: dirent_notif_delay

The dirent_notif_delay attribute is the minimum number of seconds the server will delay before notifying the client of a change to a file object that has an entry in the directory.¶

5.12.1. Attribute 62: fs_layout_type

The fs_layout_type attribute (see Section 3.3.13) applies to a file system and indicates what layout types are supported by the file system. When the client encounters a new fsid, the client SHOULD obtain the value for the fs_layout_type attribute associated with the new file system. This attribute is used by the client to determine if the layout types supported by the server match any of the client's supported layout types.¶

5.12.2. Attribute 66: layout_alignment

When a client holds layouts on files of a file system, the layout_alignment attribute indicates the preferred alignment for I/O to files on that file system. Where possible, the client should send READ and WRITE operations with offsets that are whole multiples of the layout_alignment attribute.¶

5.12.3. Attribute 65: layout_blksize

When a client holds layouts on files of a file system, the layout_blksize attribute indicates the preferred block size for I/O to files on that file system. Where possible, the client should send READ operations with a count argument that is a whole multiple of layout_blksize, and WRITE operations with a data argument of size that is a whole multiple of layout_blksize.¶

5.12.4. Attribute 63: layout_hint

The layout_hint attribute (see Section 3.3.19) may be set on newly created files to influence the metadata server's choice for the file's layout. If possible, this attribute is one of those set in the initial attributes within the OPEN operation. The metadata server may choose to ignore this attribute. The layout_hint attribute is a subset of the layout structure returned by LAYOUTGET. For example, instead of specifying particular devices, this would be used to suggest the stripe width of a file. The server implementation determines which fields within the layout will be used.¶

5.12.5. Attribute 64: layout_type

This attribute lists the layout type(s) available for a file. The value returned by the server is for informational purposes only. The client will use the LAYOUTGET operation to obtain the information needed in order to perform I/O, for example, the specific device information for the file and its layout.¶

5.12.6. Attribute 68: mdsthreshold

This attribute is a server-provided hint used to communicate to the client when it is more efficient to send READ and WRITE operations to the metadata server or the data server. The two types of thresholds described are file size thresholds and I/O size thresholds. If a file's size is smaller than the file size threshold, data accesses SHOULD be sent to the metadata server. If an I/O request has a length that is below the I/O size threshold, the I/O SHOULD be sent to the metadata server. Each threshold type is specified separately for read and write.¶

The server MAY provide both types of thresholds for a file. If both file size and I/O size are provided, the client SHOULD reach or exceed both thresholds before sending its read or write requests to the data server. Alternatively, if only one of the specified thresholds is reached or exceeded, the I/O requests are sent to the metadata server.¶

For each threshold type, a value of zero indicates no READ or WRITE should be sent to the metadata server, while a value of all ones indicates that all READs or WRITEs should be sent to the metadata server.¶

The attribute is available on a per-filehandle basis. If the current filehandle refers to a non-pNFS file or directory, the metadata server should return an attribute that is representative of the filehandle's file system. It is suggested that this attribute is queried as part of the OPEN operation. Due to dynamic system changes, the client should not assume that the attribute will remain constant for any specific time period; thus, it should be periodically refreshed.¶

5.13.1. Attribute 69: retention_get

If retention is enabled for the associated file, this attribute's value represents the retention begin time of the file object. This attribute's value is only readable with the GETATTR operation and MUST NOT be modified by the SETATTR operation (Section 5.5). The value of the attribute consists of:¶

const RET4_DURATION_INFINITE    = 0xffffffffffffffff;
struct retention_get4 {
        uint64_t        rg_duration;
        nfstime4        rg_begin_time<1>;
};

The field rg_duration is the duration in seconds indicating how long the file will be retained once retention is enabled. The field rg_begin_time is an array of up to one absolute time value. If the array is zero length, no beginning retention time has been established, and retention is not enabled. If rg_duration is equal to RET4_DURATION_INFINITE, the file, once retention is enabled, will be retained for an infinite duration.¶

If (as soon as) rg_duration is zero, then rg_begin_time will be of zero length, and again, retention is not (no longer) enabled.¶

5.13.2. Attribute 70: retention_set

This attribute is used to set the retention duration and optionally enable retention for the associated file object. This attribute is only modifiable via the SETATTR operation and MUST NOT be retrieved by the GETATTR operation (Section 5.5). This attribute corresponds to retention_get. The value of the attribute consists of:¶

struct retention_set4 {
        bool            rs_enable;
        uint64_t        rs_duration<1>;
};

If the client sets rs_enable to TRUE, then it is enabling retention on the file object with the begin time of retention starting from the server's current time and date. The duration of the retention can also be provided if the rs_duration array is of length one. The duration is the time in seconds from the begin time of retention, and if set to RET4_DURATION_INFINITE, the file is to be retained forever. If retention is enabled, with no duration specified in either this SETATTR or a previous SETATTR, the duration defaults to zero seconds. The server MAY restrict the enabling of retention or the duration of retention on the basis of the ACE4_WRITE_RETENTION ACL permission. The enabling of retention MUST NOT prevent the enabling of event-based retention or the modification of the retention_hold attribute.¶

The following rules apply to both the retention_set and retentevt_set attributes.¶

• As long as retention is not enabled, the client is permitted to decrease the duration.¶
• The duration can always be set to an equal or higher value, even if retention is enabled. Note that once retention is enabled, the actual duration (as returned by the retention_get or retentevt_get attributes; see Section 5.13.1 or Section 5.13.3) is constantly counting down to zero (one unit per second), unless the duration was set to RET4_DURATION_INFINITE. Thus, it will not be possible for the client to precisely extend the duration on a file that has retention enabled.¶
• While retention is enabled, attempts to disable retention or decrease the retention's duration MUST fail with the error NFS4ERR_INVAL.¶
• If the principal attempting to change retention_set or retentevt_set does not have ACE4_WRITE_RETENTION permissions, the attempt MUST fail with NFS4ERR_ACCESS.¶

5.13.3. Attribute 71: retentevt_get

Gets the event-based retention duration, and if enabled, the event-based retention begin time of the file object. This attribute is like retention_get, but refers to event-based retention. The event that triggers event-based retention is not defined by the NFSv4.1 specification.¶

5.13.4. Attribute 72: retentevt_set

Sets the event-based retention duration, and optionally enables event-based retention on the file object. This attribute corresponds to retentevt_get and is like retention_set, but refers to event-based retention. When event-based retention is set, the file MUST be retained even if non-event-based retention has been set, and the duration of non-event-based retention has been reached. Conversely, when non-event-based retention has been set, the file MUST be retained even if event-based retention has been set, and the duration of event-based retention has been reached. The server MAY restrict the enabling of event-based retention or the duration of event-based retention on the basis of the ACE4_WRITE_RETENTION ACL permission. The enabling of event-based retention MUST NOT prevent the enabling of non-event-based retention or the modification of the retention_hold attribute.¶

5.13.5. Attribute 73: retention_hold

Gets or sets administrative retention holds, one hold per bit position.¶

This attribute allows one to 64 administrative holds, one hold per bit on the attribute. If retention_hold is not zero, then the file MUST NOT be deleted, renamed, or modified, even if the duration on enabled event or non-event-based retention has been reached. The server MAY restrict the modification of retention_hold on the basis of the ACE4_WRITE_RETENTION_HOLD ACL permission. The enabling of administration retention holds does not prevent the enabling of event-based or non-event-based retention.¶

If the principal attempting to change retention_hold does not have ACE4_WRITE_RETENTION_HOLD permissions, the attempt MUST fail with NFS4ERR_ACCESS.¶

6.4.1. Setting the Mode and/or ACL Attributes

In the case where a server supports the sacl or dacl attribute, in addition to the acl attribute, the server MUST fail a request to set the acl attribute simultaneously with a dacl or sacl attribute. The error to be given is NFS4ERR_ATTRNOTSUPP.¶

6.4.3. Creating New Objects

If a server supports any ACL attributes, it may use the ACL attributes on the parent directory to compute an initial ACL attribute for a newly created object. This will be referred to as the inherited ACL within this section. The act of adding one or more ACEs to the inherited ACL that are based upon ACEs in the parent directory's ACL will be referred to as inheriting an ACE within this section.¶

Implementors should standardize what the behavior of CREATE and OPEN must be depending on the presence or absence of the mode and ACL attributes.¶

1. If just the mode is given in the call:¶

   In this case, inheritance SHOULD take place, but the mode MUST be applied to the inherited ACL as described in Section 6.4.1.1, thereby modifying the ACL.¶

2. If just the ACL is given in the call:¶

   In this case, inheritance SHOULD NOT take place, and the ACL as defined in the CREATE or OPEN will be set without modification, and the mode modified as in Section 6.4.1.2.¶

3. If both mode and ACL are given in the call:¶

   In this case, inheritance SHOULD NOT take place, and both attributes will be set as described in Section 6.4.1.3.¶

4. If neither mode nor ACL is given in the call:¶

   In the case where an object is being created without any initial attributes at all, e.g., an OPEN operation with an opentype4 of OPEN4_CREATE and a createmode4 of EXCLUSIVE4, inheritance SHOULD NOT take place (note that EXCLUSIVE4_1 is a better choice of createmode4, since it does permit initial attributes). Instead, the server SHOULD set permissions to deny all access to the newly created object. It is expected that the appropriate client will set the desired attributes in a subsequent SETATTR operation, and the server SHOULD allow that operation to succeed, regardless of what permissions the object is created with. For example, an empty ACL denies all permissions, but the server should allow the owner's SETATTR to succeed even though WRITE_ACL is implicitly denied.¶

   In other cases, inheritance SHOULD take place, and no modifications to the ACL will happen. The mode attribute, if supported, MUST be as computed in Section 6.3.2, with the MODE4_SUID, MODE4_SGID, and MODE4_SVTX bits clear. If no inheritable ACEs exist on the parent directory, the rules for creating acl, dacl, or sacl attributes are implementation defined. If either the dacl or sacl attribute is supported, then the ACL4_DEFAULTED flag SHOULD be set on the newly created attributes.¶

struct nfsacl41 {
        aclflag4        na41_flag;
        nfsace4         na41_aces<>;
};

const ACL4_AUTO_INHERIT         = 0x00000001;
const ACL4_PROTECTED            = 0x00000002;
const ACL4_DEFAULTED            = 0x00000004;

11.1.1. Terminology Related to Trunking

It is particularly important to clarify the distinction between trunking detection and trunking discovery. The definitions we present are applicable to all minor versions of NFSv4, but we will focus on how these terms apply to NFS version 4.1.¶

• Trunking detection refers to ways of deciding whether two specific network addresses are connected to the same NFSv4 server. The means available to make this determination depends on the protocol version, and, in some cases, on the client implementation.¶

  In the case of NFS version 4.1 and later minor versions, the means of trunking detection are as described in this document and are available to every client. Two network addresses connected to the same server can always be used together to access a particular server but cannot necessarily be used together to access a single session. See below for definitions of the terms "server-trunkable" and "session-trunkable".¶

• Trunking discovery is a process by which a client using one network address can obtain other addresses that are connected to the same server. Typically, it builds on a trunking detection facility by providing one or more methods by which candidate addresses are made available to the client, who can then use trunking detection to appropriately filter them.¶

  Despite the support for trunking detection, there was no description of trunking discovery provided in RFC 5661 [66], making it necessary to provide those means in this document.¶

The combination of a server network address and a particular connection type to be used by a connection is referred to as a "server endpoint". Although using different connection types may result in different ports being used, the use of different ports by multiple connections to the same network address in such cases is not the essence of the distinction between the two endpoints used. This is in contrast to the case of port-specific endpoints, in which the explicit specification of port numbers within network addresses is used to allow a single server node to support multiple NFS servers.¶

Two network addresses connected to the same server are said to be server-trunkable. Two such addresses support the use of client ID trunking, as described in Section 2.10.5.¶

Two network addresses connected to the same server such that those addresses can be used to support a single common session are referred to as session-trunkable. Note that two addresses may be server-trunkable without being session-trunkable, and that, when two connections of different connection types are made to the same network address and are based on a single file system location entry, they are always session-trunkable, independent of the connection type, as specified by Section 2.10.5, since their derivation from the same file system location entry, together with the identity of their network addresses, assures that both connections are to the same server and will return server-owner information, allowing session trunking to be used.¶

11.1.2. Terminology Related to File System Location

Regarding the terminology that relates to the construction of multi-server namespaces out of a set of local per-server namespaces:¶

• Each server has a set of exported file systems that may be accessed by NFSv4 clients. Typically, this is done by assigning each file system a name within the pseudo-fs associated with the server, although the pseudo-fs may be dispensed with if there is only a single exported file system. Each such file system is part of the server's local namespace, and can be considered as a file system instance within a larger multi-server namespace.¶
• The set of all exported file systems for a given server constitutes that server's local namespace.¶
• In some cases, a server will have a namespace more extensive than its local namespace by using features associated with attributes that provide file system location information. These features, which allow construction of a multi-server namespace, are all described in individual sections below and include referrals (Section 11.5.6), migration (Section 11.5.5), and replication (Section 11.5.4).¶
• A file system present in a server's pseudo-fs may have multiple file system instances on different servers associated with it. All such instances are considered replicas of one another. Whether such replicas can be used simultaneously is discussed in Section 11.11.1, while the level of coordination between them (important when switching between them) is discussed in Sections 11.11.2 through 11.11.8 below.¶
• When a file system is present in a server's pseudo-fs, but there is no corresponding local file system, it is said to be "absent". In such cases, all associated instances will be accessed on other servers.¶

Regarding the terminology that relates to attributes used in trunking discovery and other multi-server namespace features:¶

• File system location attributes include the fs_locations and fs_locations_info attributes.¶

• File system location entries provide the individual file system locations within the file system location attributes. Each such entry specifies a server, in the form of a hostname or an address, and an fs name, which designates the location of the file system within the server's local namespace. A file system location entry designates a set of server endpoints to which the client may establish connections. There may be multiple endpoints because a hostname may map to multiple network addresses and because multiple connection types may be used to communicate with a single network address. However, except where explicit port numbers are used to designate a set of servers within a single server node, all such endpoints MUST designate a way of connecting to a single server. The exact form of the location entry varies with the particular file system location attribute used, as described in Section 11.2.¶

  The network addresses used in file system location entries typically appear without port number indications and are used to designate a server at one of the standard ports for NFS access, e.g., 2049 for TCP or 20049 for use with RPC-over-RDMA. Port numbers may be used in file system location entries to designate servers (typically user-level ones) accessed using other port numbers. In the case where network addresses indicate trunking relationships, the use of an explicit port number is inappropriate since trunking is a relationship between network addresses. See Section 11.5.2 for details.¶

• File system location elements are derived from location entries, and each describes a particular network access path consisting of a network address and a location within the server's local namespace. Such location elements need not appear within a file system location attribute, but the existence of each location element derives from a corresponding location entry. When a location entry specifies an IP address, there is only a single corresponding location element. File system location entries that contain a hostname are resolved using DNS, and may result in one or more location elements. All location elements consist of a location address that includes the IP address of an interface to a server and an fs name, which is the location of the file system within the server's local namespace. The fs name can be empty if the server has no pseudo-fs and only a single exported file system at the root filehandle.¶
• Two file system location elements are said to be server-trunkable if they specify the same fs name and the location addresses are such that the location addresses are server-trunkable. When the corresponding network paths are used, the client will always be able to use client ID trunking, but will only be able to use session trunking if the paths are also session-trunkable.¶
• Two file system location elements are said to be session-trunkable if they specify the same fs name and the location addresses are such that the location addresses are session-trunkable. When the corresponding network paths are used, the client will be able to able to use either client ID trunking or session trunking.¶

Discussion of the term "replica" is complicated by the fact that the term was used in RFC 5661 [66] with a meaning different from that used in this document. In short, in [66] each replica is identified by a single network access path, while in the current document, a set of network access paths that have server-trunkable network addresses and the same root-relative file system pathname is considered to be a single replica with multiple network access paths.¶

Each set of server-trunkable location elements defines a set of available network access paths to a particular file system. When there are multiple such file systems, each of which containing the same data, these file systems are considered replicas of one another. Logically, such replication is symmetric, since the fs currently in use and an alternate fs are replicas of each other. Often, in other documents, the term "replica" is not applied to the fs currently in use, despite the fact that the replication relation is inherently symmetric.¶

11.4.1. GETATTR within an Absent File System

As mentioned above, an exception is made for GETATTR in that attributes may be obtained for a filehandle within an absent file system. This exception only applies if the attribute mask contains at least one attribute bit that indicates the client is interested in a result regarding an absent file system: fs_locations, fs_locations_info, or fs_status. If none of these attributes is requested, GETATTR will result in an NFS4ERR_MOVED error.¶

When a GETATTR is done on an absent file system, the set of supported attributes is very limited. Many attributes, including those that are normally REQUIRED, will not be available on an absent file system. In addition to the attributes mentioned above (fs_locations, fs_locations_info, fs_status), the following attributes SHOULD be available on absent file systems. In the case of RECOMMENDED attributes, they should be available at least to the same degree that they are available on present file systems.¶

change_policy:

This attribute is useful for absent file systems and can be helpful in summarizing to the client when any of the location-related attributes change.¶

fsid:

This attribute should be provided so that the client can determine file system boundaries, including, in particular, the boundary between present and absent file systems. This value must be different from any other fsid on the current server and need have no particular relationship to fsids on any particular destination to which the client might be directed.¶

mounted_on_fileid:

For objects at the top of an absent file system, this attribute needs to be available. Since the fileid is within the present parent file system, there should be no need to reference the absent file system to provide this information.¶

Other attributes SHOULD NOT be made available for absent file systems, even when it is possible to provide them. The server should not assume that more information is always better and should avoid gratuitously providing additional information.¶

When a GETATTR operation includes a bit mask for one of the attributes fs_locations, fs_locations_info, or fs_status, but where the bit mask includes attributes that are not supported, GETATTR will not return an error, but will return the mask of the actual attributes supported with the results.¶

Handling of VERIFY/NVERIFY is similar to GETATTR in that if the attribute mask does not include fs_locations, fs_locations_info, or fs_status, the error NFS4ERR_MOVED will result. It differs in that any appearance in the attribute mask of an attribute not supported for an absent file system (and note that this will include some normally REQUIRED attributes) will also cause an NFS4ERR_MOVED result.¶

11.4.2. READDIR and Absent File Systems

A READDIR performed when the current filehandle is within an absent file system will result in an NFS4ERR_MOVED error, since, unlike the case of GETATTR, no such exception is made for READDIR.¶

Attributes for an absent file system may be fetched via a READDIR for a directory in a present file system, when that directory contains the root directories of one or more absent file systems. In this case, the handling is as follows:¶

• If the attribute set requested includes one of the attributes fs_locations, fs_locations_info, or fs_status, then fetching of attributes proceeds normally and no NFS4ERR_MOVED indication is returned, even when the rdattr_error attribute is requested.¶
• If the attribute set requested does not include one of the attributes fs_locations, fs_locations_info, or fs_status, then if the rdattr_error attribute is requested, each directory entry for the root of an absent file system will report NFS4ERR_MOVED as the value of the rdattr_error attribute.¶
• If the attribute set requested does not include any of the attributes fs_locations, fs_locations_info, fs_status, or rdattr_error, then the occurrence of the root of an absent file system within the directory will result in the READDIR failing with an NFS4ERR_MOVED error.¶
• The unavailability of an attribute because of a file system's absence, even one that is ordinarily REQUIRED, does not result in any error indication. The set of attributes returned for the root directory of the absent file system in that case is simply restricted to those actually available.¶

11.5.1. Combining Multiple Uses in a Single Attribute

A file system location attribute will sometimes contain information relating to the location of multiple replicas, which may be used in different ways:¶

• File system location entries that relate to the file system instance currently in use provide trunking information, allowing the client to find additional network addresses by which the instance may be accessed.¶
• File system location entries that provide information about replicas to which access is to be transferred.¶
• Other file system location entries that relate to replicas that are available to use in the event that access to the current replica becomes unsatisfactory.¶

In order to simplify client handling and to allow the best choice of replicas to access, the server should adhere to the following guidelines:¶

• All file system location entries that relate to a single file system instance should be adjacent.¶
• File system location entries that relate to the instance currently in use should appear first.¶
• File system location entries that relate to replica(s) to which migration is occurring should appear before replicas that are available for later use if the current replica should become inaccessible.¶

11.5.2. File System Location Attributes and Trunking

Trunking is the use of multiple connections between a client and server in order to increase the speed of data transfer. A client may determine the set of network addresses to use to access a given file system in a number of ways:¶

• When the name of the server is known to the client, it may use DNS to obtain a set of network addresses to use in accessing the server.¶
• The client may fetch the file system location attribute for the file system. This will provide either the name of the server (which can be turned into a set of network addresses using DNS) or a set of server-trunkable location entries. Using the latter alternative, the server can provide addresses it regards as desirable to use to access the file system in question. Although these entries can contain port numbers, these port numbers are not used in determining trunking relationships. Once the candidate addresses have been determined and EXCHANGE_ID done to the proper server, only the value of the so_major_id field returned by the servers in question determines whether a trunking relationship actually exists.¶

When the client fetches a location attribute for a file system, it should be noted that the client may encounter multiple entries for a number of reasons, such that when it determines trunking information, it may need to bypass addresses not trunkable with one already known.¶

The server can provide location entries that include either names or network addresses. It might use the latter form because of DNS-related security concerns or because the set of addresses to be used might require active management by the server.¶

Location entries used to discover candidate addresses for use in trunking are subject to change, as discussed in Section 11.5.7 below. The client may respond to such changes by using additional addresses once they are verified or by ceasing to use existing ones. The server can force the client to cease using an address by returning NFS4ERR_MOVED when that address is used to access a file system. This allows a transfer of client access that is similar to migration, although the same file system instance is accessed throughout.¶

11.5.3. File System Location Attributes and Connection Type Selection

Because of the need to support multiple types of connections, clients face the issue of determining the proper connection type to use when establishing a connection to a given server network address. In some cases, this issue can be addressed through the use of the connection "step-up" facility described in Section 18.36. However, because there are cases in which that facility is not available, the client may have to choose a connection type with no possibility of changing it within the scope of a single connection.¶

The two file system location attributes differ as to the information made available in this regard. The fs_locations attribute provides no information to support connection type selection. As a result, clients supporting multiple connection types would need to attempt to establish connections using multiple connection types until the one preferred by the client is successfully established.¶

The fs_locations_info attribute includes the FSLI4TF_RDMA flag, which is convenient for a client wishing to use RDMA. When this flag is set, it indicates that RPC-over-RDMA support is available using the specified location entry. A client can establish a TCP connection and then convert that connection to use RDMA by using the step-up facility.¶

Irrespective of the particular attribute used, when there is no indication that a step-up operation can be performed, a client supporting RDMA operation can establish a new RDMA connection, and it can be bound to the session already established by the TCP connection, allowing the TCP connection to be dropped and the session converted to further use in RDMA mode, if the server supports that.¶

11.5.4. File System Replication

The fs_locations and fs_locations_info attributes provide alternative file system locations, to be used to access data in place of or in addition to the current file system instance. On first access to a file system, the client should obtain the set of alternate locations by interrogating the fs_locations or fs_locations_info attribute, with the latter being preferred.¶

In the event that the occurrence of server failures, communications problems, or other difficulties make continued access to the current file system impossible or otherwise impractical, the client can use the alternate locations as a way to get continued access to its data.¶

The alternate locations may be physical replicas of the (typically read-only) file system data supplemented by possible asynchronous propagation of updates. Alternatively, they may provide for the use of various forms of server clustering in which multiple servers provide alternate ways of accessing the same physical file system. How the difference between replicas affects file system transitions can be represented within the fs_locations and fs_locations_info attributes, and how the client deals with file system transition issues will be discussed in detail in later sections.¶

Although the location attributes provide some information about the nature of the inter-replica transition, many aspects of the semantics of possible asynchronous updates are not currently described by the protocol, which makes it necessary for clients using replication to switch among replicas undergoing change to familiarize themselves with the semantics of the update approach used. Due to this lack of specificity, many applications may find the use of migration more appropriate because a server can propagate all updates made before an established point in time to the new replica as part of the migration event.¶

11.5.5. File System Migration

When a file system is present and becomes inaccessible using the current access path, the NFSv4.1 protocol provides a means by which clients can be given the opportunity to have continued access to their data. This may involve using a different access path to the existing replica or providing a path to a different replica. The new access path or the location of the new replica is specified by a file system location attribute. The ensuing migration of access includes the ability to retain locks across the transition. Depending on circumstances, this can involve:¶

• The continued use of the existing clientid when accessing the current replica using a new access path.¶
• Use of lock reclaim, taking advantage of a per-fs grace period.¶
• Use of Transparent State Migration.¶

Typically, a client will be accessing the file system in question, get an NFS4ERR_MOVED error, and then use a file system location attribute to determine the new access path for the data. When fs_locations_info is used, additional information will be available that will define the nature of the client's handling of the transition to a new server.¶

In most instances, servers will choose to migrate all clients using a particular file system to a successor replica at the same time to avoid cases in which different clients are updating different replicas. However, migration of an individual client can be helpful in providing load balancing, as long as the replicas in question are such that they represent the same data as described in Section 11.11.8.¶

• In the case in which there is no transition between replicas (i.e., only a change in access path), there are no special difficulties in using of this mechanism to effect load balancing.¶
• In the case in which the two replicas are sufficiently coordinated as to allow a single client coherent, simultaneous access to both, there is, in general, no obstacle to the use of migration of particular clients to effect load balancing. Generally, such simultaneous use involves cooperation between servers to ensure that locks granted on two coordinated replicas cannot conflict and can remain effective when transferred to a common replica.¶
• In the case in which a large set of clients is accessing a file system in a read-only fashion, it can be helpful to migrate all clients with writable access simultaneously, while using load balancing on the set of read-only copies, as long as the rules in Section 11.11.8, which are designed to prevent data reversion, are followed.¶

In other cases, the client might not have sufficient guarantees of data similarity or coherence to function properly (e.g., the data in the two replicas is similar but not identical), and the possibility that different clients are updating different replicas can exacerbate the difficulties, making the use of load balancing in such situations a perilous enterprise.¶

The protocol does not specify how the file system will be moved between servers or how updates to multiple replicas will be coordinated. It is anticipated that a number of different server-to-server coordination mechanisms might be used, with the choice left to the server implementer. The NFSv4.1 protocol specifies the method used to communicate the migration event between client and server.¶

In the case of various forms of server clustering, the new location may be another server providing access to the same physical file system. The client's responsibilities in dealing with this transition will depend on whether a switch between replicas has occurred and the means the server has chosen to provide continuity of locking state. These issues will be discussed in detail below.¶

Although a single successor location is typical, multiple locations may be provided. When multiple locations are provided, the client will typically use the first one provided. If that is inaccessible for some reason, later ones can be used. In such cases, the client might consider the transition to the new replica to be a migration event, even though some of the servers involved might not be aware of the use of the server that was inaccessible. In such a case, a client might lose access to locking state as a result of the access transfer.¶

When an alternate location is designated as the target for migration, it must designate the same data (with metadata being the same to the degree indicated by the fs_locations_info attribute). Where file systems are writable, a change made on the original file system must be visible on all migration targets. Where a file system is not writable but represents a read-only copy (possibly periodically updated) of a writable file system, similar requirements apply to the propagation of updates. Any change visible in the original file system must already be effected on all migration targets, to avoid any possibility that a client, in effecting a transition to the migration target, will see any reversion in file system state.¶

11.5.6. Referrals

Referrals allow the server to associate a file system namespace entry located on one server with a file system located on another server. When this includes the use of pure referrals, servers are provided a way of placing a file system in a location within the namespace essentially without respect to its physical location on a particular server. This allows a single server or a set of servers to present a multi-server namespace that encompasses file systems located on a wider range of servers. Some likely uses of this facility include establishment of site-wide or organization-wide namespaces, with the eventual possibility of combining such together into a truly global namespace, such as the one provided by AFS (the Andrew File System) [65].¶

Referrals occur when a client determines, upon first referencing a position in the current namespace, that it is part of a new file system and that the file system is absent. When this occurs, typically upon receiving the error NFS4ERR_MOVED, the actual location or locations of the file system can be determined by fetching a locations attribute.¶

The file system location attribute may designate a single file system location or multiple file system locations, to be selected based on the needs of the client. The server, in the fs_locations_info attribute, may specify priorities to be associated with various file system location choices. The server may assign different priorities to different locations as reported to individual clients, in order to adapt to client physical location or to effect load balancing. When both read-only and read-write file systems are present, some of the read-only locations might not be absolutely up-to-date (as they would have to be in the case of replication and migration). Servers may also specify file system locations that include client-substituted variables so that different clients are referred to different file systems (with different data contents) based on client attributes such as CPU architecture.¶

If the fs_locations_info attribute lists multiple possible targets, the relationships among them may be important to the client in selecting which one to use. The same rules specified in Section 11.5.5 below regarding multiple migration targets apply to these multiple replicas as well. For example, the client might prefer a writable target on a server that has additional writable replicas to which it subsequently might switch. Note that, as distinguished from the case of replication, there is no need to deal with the case of propagation of updates made by the current client, since the current client has not accessed the file system in question.¶

Use of multi-server namespaces is enabled by NFSv4.1 but is not required. The use of multi-server namespaces and their scope will depend on the applications used and system administration preferences.¶

Multi-server namespaces can be established by a single server providing a large set of pure referrals to all of the included file systems. Alternatively, a single multi-server namespace may be administratively segmented with separate referral file systems (on separate servers) for each separately administered portion of the namespace. The top-level referral file system or any segment may use replicated referral file systems for higher availability.¶

Generally, multi-server namespaces are for the most part uniform, in that the same data made available to one client at a given location in the namespace is made available to all clients at that namespace location. However, there are facilities provided that allow different clients to be directed to different sets of data, for reasons such as enabling adaptation to such client characteristics as CPU architecture. These facilities are described in Section 11.17.3.¶

Note that it is possible, when providing a uniform namespace, to provide different location entries to different clients in order to provide each client with a copy of the data physically closest to it or otherwise optimize access (e.g., provide load balancing).¶

11.5.7. Changes in a File System Location Attribute

Although clients will typically fetch a file system location attribute when first accessing a file system and when NFS4ERR_MOVED is returned, a client can choose to fetch the attribute periodically, in which case, the value fetched may change over time.¶

For clients not prepared to access multiple replicas simultaneously (see Section 11.11.1), the handling of the various cases of location change are as follows:¶

• Changes in the list of replicas or in the network addresses associated with replicas do not require immediate action. The client will typically update its list of replicas to reflect the new information.¶
• Additions to the list of network addresses for the current file system instance need not be acted on promptly. However, to prepare for a subsequent migration event, the client can choose to take note of the new address and then use it whenever it needs to switch access to a new replica.¶
• Deletions from the list of network addresses for the current file system instance do not require the client to immediately cease use of existing access paths, although new connections are not to be established on addresses that have been deleted. However, clients can choose to act on such deletions by preparing for an eventual shift in access, which becomes unavoidable as soon as the server returns NFS4ERR_MOVED to indicate that a particular network access path is not usable to access the current file system.¶

For clients that are prepared to access several replicas simultaneously, the following additional cases need to be addressed. As in the cases discussed above, changes in the set of replicas need not be acted upon promptly, although the client has the option of adjusting its access even in the absence of difficulties that would lead to the selection of a new replica.¶

• When a new replica is added, which may be accessed simultaneously with one currently in use, the client is free to use the new replica immediately.¶
• When a replica currently in use is deleted from the list, the client need not cease using it immediately. However, since the server may subsequently force such use to cease (by returning NFS4ERR_MOVED), clients might decide to limit the need for later state transfer. For example, new opens might be done on other replicas, rather than on one not present in the list.¶

11.11.1. File System Transitions and Simultaneous Access

The fs_locations_info attribute (described in Section 11.17) may indicate that two replicas may be used simultaneously, although some situations in which such simultaneous access is permitted are more appropriately described as instances of trunking (see Section 11.5.4.1). Although situations in which multiple replicas may be accessed simultaneously are somewhat similar to those in which a single replica is accessed by multiple network addresses, there are important differences since locking state is not shared among multiple replicas.¶

Because of this difference in state handling, many clients will not have the ability to take advantage of the fact that such replicas represent the same data. Such clients will not be prepared to use multiple replicas simultaneously but will access each file system using only a single replica, although the replica selected might make multiple server-trunkable addresses available.¶

Clients who are prepared to use multiple replicas simultaneously can divide opens among replicas however they choose. Once that choice is made, any subsequent transitions will treat the set of locking state associated with each replica as a single entity.¶

For example, if one of the replicas become unavailable, access will be transferred to a different replica, which is also capable of simultaneous access with the one still in use.¶

When there is no such replica, the transition may be to the replica already in use. At this point, the client has a choice between merging the locking state for the two replicas under the aegis of the sole replica in use or treating these separately until another replica capable of simultaneous access presents itself.¶

11.11.2. Filehandles and File System Transitions

There are a number of ways in which filehandles can be handled across a file system transition. These can be divided into two broad classes depending upon whether the two file systems across which the transition happens share sufficient state to effect some sort of continuity of file system handling.¶

When there is no such cooperation in filehandle assignment, the two file systems are reported as being in different handle classes. In this case, all filehandles are assumed to expire as part of the file system transition. Note that this behavior does not depend on the fh_expire_type attribute and supersedes the specification of the FH4_VOL_MIGRATION bit, which only affects behavior when fs_locations_info is not available.¶

When there is cooperation in filehandle assignment, the two file systems are reported as being in the same handle classes. In this case, persistent filehandles remain valid after the file system transition, while volatile filehandles (excluding those that are only volatile due to the FH4_VOL_MIGRATION bit) are subject to expiration on the target server.¶

11.11.3. Fileids and File System Transitions

In NFSv4.0, the issue of continuity of fileids in the event of a file system transition was not addressed. The general expectation had been that in situations in which the two file system instances are created by a single vendor using some sort of file system image copy, fileids would be consistent across the transition, while in the analogous multi-vendor transitions they would not. This poses difficulties, especially for the client without special knowledge of the transition mechanisms adopted by the server. Note that although fileid is not a REQUIRED attribute, many servers support fileids and many clients provide APIs that depend on fileids.¶

It is important to note that while clients themselves may have no trouble with a fileid changing as a result of a file system transition event, applications do typically have access to the fileid (e.g., via stat). The result is that an application may work perfectly well if there is no file system instance transition or if any such transition is among instances created by a single vendor, yet be unable to deal with the situation in which a multi-vendor transition occurs at the wrong time.¶

Providing the same fileids in a multi-vendor (multiple server vendors) environment has generally been held to be quite difficult. While there is work to be done, it needs to be pointed out that this difficulty is partly self-imposed. Servers have typically identified fileid with inode number, i.e. with a quantity used to find the file in question. This identification poses special difficulties for migration of a file system between vendors where assigning the same index to a given file may not be possible. Note here that a fileid is not required to be useful to find the file in question, only that it is unique within the given file system. Servers prepared to accept a fileid as a single piece of metadata and store it apart from the value used to index the file information can relatively easily maintain a fileid value across a migration event, allowing a truly transparent migration event.¶

In any case, where servers can provide continuity of fileids, they should, and the client should be able to find out that such continuity is available and take appropriate action. Information about the continuity (or lack thereof) of fileids across a file system transition is represented by specifying whether the file systems in question are of the same fileid class.¶

Note that when consistent fileids do not exist across a transition (either because there is no continuity of fileids or because fileid is not a supported attribute on one of instances involved), and there are no reliable filehandles across a transition event (either because there is no filehandle continuity or because the filehandles are volatile), the client is in a position where it cannot verify that files it was accessing before the transition are the same objects. It is forced to assume that no object has been renamed, and, unless there are guarantees that provide this (e.g., the file system is read-only), problems for applications may occur. Therefore, use of such configurations should be limited to situations where the problems that this may cause can be tolerated.¶

11.11.4. Fsids and File System Transitions

Since fsids are generally only unique on a per-server basis, it is likely that they will change during a file system transition. Clients should not make the fsids received from the server visible to applications since they may not be globally unique, and because they may change during a file system transition event. Applications are best served if they are isolated from such transitions to the extent possible.¶

Although normally a single source file system will transition to a single target file system, there is a provision for splitting a single source file system into multiple target file systems, by specifying the FSLI4F_MULTI_FS flag.¶

11.11.5. The Change Attribute and File System Transitions

Since the change attribute is defined as a server-specific one, change attributes fetched from one server are normally presumed to be invalid on another server. Such a presumption is troublesome since it would invalidate all cached change attributes, requiring refetching. Even more disruptive, the absence of any assured continuity for the change attribute means that even if the same value is retrieved on refetch, no conclusions can be drawn as to whether the object in question has changed. The identical change attribute could be merely an artifact of a modified file with a different change attribute construction algorithm, with that new algorithm just happening to result in an identical change value.¶

When the two file systems have consistent change attribute formats, and this fact is communicated to the client by reporting in the same change class, the client may assume a continuity of change attribute construction and handle this situation just as it would be handled without any file system transition.¶

11.11.6. Write Verifiers and File System Transitions

In a file system transition, the two file systems might be cooperating in the handling of unstably written data. Clients can determine if this is the case by seeing if the two file systems belong to the same write-verifier class. When this is the case, write verifiers returned from one system may be compared to those returned by the other and superfluous writes can be avoided.¶

When two file systems belong to different write-verifier classes, any verifier generated by one must not be compared to one provided by the other. Instead, the two verifiers should be treated as not equal even when the values are identical.¶

11.11.7. READDIR Cookies and Verifiers and File System Transitions

In a file system transition, the two file systems might be consistent in their handling of READDIR cookies and verifiers. Clients can determine if this is the case by seeing if the two file systems belong to the same readdir class. When this is the case, readdir class, READDIR cookies, and verifiers from one system will be recognized by the other, and READDIR operations started on one server can be validly continued on the other simply by presenting the cookie and verifier returned by a READDIR operation done on the first file system to the second.¶

When two file systems belong to different readdir classes, any READDIR cookie and verifier generated by one is not valid on the second and must not be presented to that server by the client. The client should act as if the verifier were rejected.¶

11.11.8. File System Data and File System Transitions

When multiple replicas exist and are used simultaneously or in succession by a client, applications using them will normally expect that they contain either the same data or data that is consistent with the normal sorts of changes that are made by other clients updating the data of the file system (with metadata being the same to the degree indicated by the fs_locations_info attribute). However, when multiple file systems are presented as replicas of one another, the precise relationship between the data of one and the data of another is not, as a general matter, specified by the NFSv4.1 protocol. It is quite possible to present as replicas file systems where the data of those file systems is sufficiently different that some applications have problems dealing with the transition between replicas. The namespace will typically be constructed so that applications can choose an appropriate level of support, so that in one position in the namespace, a varied set of replicas might be listed, while in another, only those that are up-to-date would be considered replicas. The protocol does define three special cases of the relationship among replicas to be specified by the server and relied upon by clients:¶

• When multiple replicas exist and are used simultaneously by a client (see the FSLIB4_CLSIMUL definition within fs_locations_info), they must designate the same data. Where file systems are writable, a change made on one instance must be visible on all instances at the same time, regardless of whether the interrogated instance is the one on which the modification was done. This allows a client to use these replicas simultaneously without any special adaptation to the fact that there are multiple replicas, beyond adapting to the fact that locks obtained on one replica are maintained separately (i.e., under a different client ID). In this case, locks (whether share reservations or byte-range locks) and delegations obtained on one replica are immediately reflected on all replicas, in the sense that access from all other servers is prevented regardless of the replica used. However, because the servers are not required to treat two associated client IDs as representing the same client, it is best to access each file using only a single client ID.¶
• When one replica is designated as the successor instance to another existing instance after the return of NFS4ERR_MOVED (i.e., the case of migration), the client may depend on the fact that all changes written to stable storage on the original instance are written to stable storage of the successor (uncommitted writes are dealt with in Section 11.11.6 above).¶
• Where a file system is not writable but represents a read-only copy (possibly periodically updated) of a writable file system, clients have similar requirements with regard to the propagation of updates. They may need a guarantee that any change visible on the original file system instance must be immediately visible on any replica before the client transitions access to that replica, in order to avoid any possibility that a client, in effecting a transition to a replica, will see any reversion in file system state. The specific means of this guarantee varies based on the value of the fss_type field that is reported as part of the fs_status attribute (see Section 11.18). Since these file systems are presumed to be unsuitable for simultaneous use, there is no specification of how locking is handled; in general, locks obtained on one file system will be separate from those on others. Since these are expected to be read-only file systems, this is not likely to pose an issue for clients or applications.¶

When none of these special situations applies, there is no basis within the protocol for the client to make assumptions about the contents of a replica file system or its relationship to previous file system instances. Thus, switching between nominally identical read-write file systems would not be possible because either the client does not use the fs_locations_info attribute, or the server does not support it.¶

11.11.9. Lock State and File System Transitions

While accessing a file system, clients obtain locks enforced by the server, which may prevent actions by other clients that are inconsistent with those locks.¶

When access is transferred between replicas, clients need to be assured that the actions disallowed by holding these locks cannot have occurred during the transition. This can be ensured by the methods below. Unless at least one of these is implemented, clients will not be assured of continuity of lock possession across a migration event:¶

• Providing the client an opportunity to re-obtain his locks via a per-fs grace period on the destination server, denying all clients using the destination file system the opportunity to obtain new locks that conflict with those held by the transferred client as long as that client has not completed its per-fs grace period. Because the lock reclaim mechanism was originally defined to support server reboot, it implicitly assumes that filehandles will, upon reclaim, be the same as those at open. In the case of migration, this requires that source and destination servers use the same filehandles, as evidenced by using the same server scope (see Section 2.10.4) or by showing this agreement using fs_locations_info (see Section 11.11.2 above).¶

  Note that such a grace period can be implemented without interfering with the ability of non-transferred clients to obtain new locks while it is going on. As long as the destination server is aware of the transferred locks, it can distinguish requests to obtain new locks that contrast with existing locks from those that do not, allowing it to treat such client requests without reference to the ongoing grace period.¶

• Locking state can be transferred as part of the transition by providing Transparent State Migration as described in Section 11.12.¶

Of these, Transparent State Migration provides the smoother experience for clients in that there is no need to go through a reclaim process before new locks can be obtained; however, it requires a greater degree of inter-server coordination. In general, the servers taking part in migration are free to provide either facility. However, when the filehandles can differ across the migration event, Transparent State Migration is the only available means of providing the needed functionality.¶

It should be noted that these two methods are not mutually exclusive and that a server might well provide both. In particular, if there is some circumstance preventing a specific lock from being transferred transparently, the destination server can allow it to be reclaimed by implementing a per-fs grace period for the migrated file system.¶

11.12.1. Transparent State Migration and pNFS

When pNFS is involved, the protocol is capable of supporting:¶

• Migration of the Metadata Server (MDS), leaving the Data Servers (DSs) in place.¶
• Migration of the file system as a whole, including the MDS and associated DSs.¶
• Replacement of one DS by another.¶
• Migration of a pNFS file system to one in which pNFS is not used.¶
• Migration of a file system not using pNFS to one in which layouts are available.¶

Note that migration, per se, is only involved in the transfer of the MDS function. Although the servicing of a layout may be transferred from one data server to another, this not done using the file system location attributes. The MDS can effect such transfers by recalling or revoking existing layouts and granting new ones on a different data server.¶

Migration of the MDS function is directly supported by Transparent State Migration. Layout state will normally be transparently transferred, just as other state is. As a result, Transparent State Migration provides a framework in which, given appropriate inter-MDS data transfer, one MDS can be substituted for another.¶

Migration of the file system function as a whole can be accomplished by recalling all layouts as part of the initial phase of the migration process. As a result, I/O will be done through the MDS during the migration process, and new layouts can be granted once the client is interacting with the new MDS. An MDS can also effect this sort of transition by revoking all layouts as part of Transparent State Migration, as long as the client is notified about the loss of locking state.¶

In order to allow migration to a file system on which pNFS is not supported, clients need to be prepared for a situation in which layouts are not available or supported on the destination file system and so direct I/O requests to the destination server, rather than depending on layouts being available.¶

Replacement of one DS by another is not addressed by migration as such but can be effected by an MDS recalling layouts for the DS to be replaced and issuing new ones to be served by the successor DS.¶

Migration may transfer a file system from a server that does not support pNFS to one that does. In order to properly adapt to this situation, clients that support pNFS, but function adequately in its absence, should check for pNFS support when a file system is migrated and be prepared to use pNFS when support is available on the destination.¶

11.13.1. Client Transition Notifications

When there is a change in the network access path that a client is to use to access a file system, there are a number of related status indications with which clients need to deal:¶

• If an attempt is made to use or return a filehandle within a file system that is no longer accessible at the address previously used to access it, the error NFS4ERR_MOVED is returned.¶

  Exceptions are made to allow such filehandles to be used when interrogating a file system location attribute. This enables a client to determine a new replica's location or a new network access path.¶

  This condition continues on subsequent attempts to access the file system in question. The only way the client can avoid the error is to cease accessing the file system in question at its old server location and access it instead using a different address at which it is now available.¶

• Whenever a client sends a SEQUENCE operation to a server that generated state held on that client and associated with a file system no longer accessible on that server, the response will contain the status bit SEQ4_STATUS_LEASE_MOVED, indicating that there has been a lease migration.¶

  This condition continues until the client acknowledges the notification by fetching a file system location attribute for the file system whose network access path is being changed. When there are multiple such file systems, a location attribute for each such file system needs to be fetched. The location attribute for all migrated file systems needs to be fetched in order to clear the condition. Even after the condition is cleared, the client needs to respond by using the location information to access the file system at its new location to ensure that leases are not needlessly expired.¶

Unlike NFSv4.0, in which the corresponding conditions are both errors and thus mutually exclusive, in NFSv4.1 the client can, and often will, receive both indications on the same request. As a result, implementations need to address the question of how to coordinate the necessary recovery actions when both indications arrive in the response to the same request. It should be noted that when processing an NFSv4 COMPOUND, the server will normally decide whether SEQ4_STATUS_LEASE_MOVED is to be set before it determines which file system will be referenced or whether NFS4ERR_MOVED is to be returned.¶

Since these indications are not mutually exclusive in NFSv4.1, the following combinations are possible results when a COMPOUND is issued:¶

• The COMPOUND status is NFS4ERR_MOVED, and SEQ4_STATUS_LEASE_MOVED is asserted.¶

  In this case, transition recovery is required. While it is possible that migration discovery is needed in addition, it is likely that only the accessed file system has transitioned. In any case, because addressing NFS4ERR_MOVED is necessary to allow the rejected requests to be processed on the target, dealing with it will typically have priority over migration discovery.¶

• The COMPOUND status is NFS4ERR_MOVED, and SEQ4_STATUS_LEASE_MOVED is clear.¶

  In this case, transition recovery is also required. It is clear that migration discovery is not needed to find file systems that have been migrated other than the one returning NFS4ERR_MOVED. Cases in which this result can arise include a referral or a migration for which there is no associated locking state. This can also arise in cases in which an access path transition other than migration occurs within the same server. In such a case, there is no need to set SEQ4_STATUS_LEASE_MOVED, since the lease remains associated with the current server even though the access path has changed.¶

• The COMPOUND status is not NFS4ERR_MOVED, and SEQ4_STATUS_LEASE_MOVED is asserted.¶

  In this case, no transition recovery activity is required on the file system(s) accessed by the request. However, to prevent avoidable lease expiration, migration discovery needs to be done.¶

• The COMPOUND status is not NFS4ERR_MOVED, and SEQ4_STATUS_LEASE_MOVED is clear.¶

  In this case, neither transition-related activity nor migration discovery is required.¶

Note that the specified actions only need to be taken if they are not already going on. For example, when NFS4ERR_MOVED is received while accessing a file system for which transition recovery is already occurring, the client merely waits for that recovery to be completed, while the receipt of the SEQ4_STATUS_LEASE_MOVED indication only needs to initiate migration discovery for a server if such discovery is not already underway for that server.¶

The fact that a lease-migrated condition does not result in an error in NFSv4.1 has a number of important consequences. In addition to the fact that the two indications are not mutually exclusive, as discussed above, there are number of issues that are important in considering implementation of migration discovery, as discussed in Section 11.13.2.¶