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Versions: (draft-cel-nfsv4-rpcrdma-bidirection) 00 01 02 03 04 05 06 07 08 RFC 8167

NFSv4                                                           C. Lever
Internet-Draft                                                    Oracle
Intended status: Experimental                               May 29, 2015
Expires: November 30, 2015


   Size-Limited Bi-directional Remote Procedure Call On Remote Direct
                        Memory Access Transports
                draft-ietf-nfsv4-rpcrdma-bidirection-00

Abstract

   Recent minor versions of NFSv4 work best when ONC RPC transports can
   send ONC RPC transactions in both directions.  This document
   describes conventions that enable RPC-over-RDMA version 1 transport
   endpoints to interoperate when operation in both directions is
   necessary.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 30, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Scope Of This Document  . . . . . . . . . . . . . . . . .   3
     1.3.  Understanding RPC Direction . . . . . . . . . . . . . . .   3
       1.3.1.  Forward Direction . . . . . . . . . . . . . . . . . .   4
       1.3.2.  Backward Direction  . . . . . . . . . . . . . . . . .   4
       1.3.3.  Bi-direction  . . . . . . . . . . . . . . . . . . . .   4
       1.3.4.  XID Values  . . . . . . . . . . . . . . . . . . . . .   4
     1.4.  Rationale For RPC-over-RDMA Bi-Direction  . . . . . . . .   5
       1.4.1.  NFSv4.0 Callback Operation  . . . . . . . . . . . . .   5
       1.4.2.  NFSv4.1 Callback Operation  . . . . . . . . . . . . .   6
     1.5.  Design Considerations . . . . . . . . . . . . . . . . . .   6
       1.5.1.  Backward Compatibility  . . . . . . . . . . . . . . .   7
       1.5.2.  Performance Impact  . . . . . . . . . . . . . . . . .   7
       1.5.3.  Server Memory Security  . . . . . . . . . . . . . . .   7
       1.5.4.  Payload Size  . . . . . . . . . . . . . . . . . . . .   7
   2.  Conventions For Backward Operation  . . . . . . . . . . . . .   8
     2.1.  Flow Control  . . . . . . . . . . . . . . . . . . . . . .   8
       2.1.1.  Forward Credits . . . . . . . . . . . . . . . . . . .   9
       2.1.2.  Backward Credits  . . . . . . . . . . . . . . . . . .   9
     2.2.  Managing Receive Buffers  . . . . . . . . . . . . . . . .   9
       2.2.1.  Client Receive Buffers  . . . . . . . . . . . . . . .  10
       2.2.2.  Server Receive Buffers  . . . . . . . . . . . . . . .  10
       2.2.3.  In the Absense of Backward Direction Support  . . . .  10
     2.3.  Backward Direction Retransmission . . . . . . . . . . . .  11
     2.4.  Backward Direction Message Size . . . . . . . . . . . . .  12
     2.5.  Sending A Backward Direction Call . . . . . . . . . . . .  12
     2.6.  Sending A Backward Direction Reply  . . . . . . . . . . .  13
   3.  Limits To This Approach . . . . . . . . . . . . . . . . . . .  13
     3.1.  Payload Size  . . . . . . . . . . . . . . . . . . . . . .  13
     3.2.  Preparedness To Handle Backward Requests  . . . . . . . .  13
     3.3.  Long Term . . . . . . . . . . . . . . . . . . . . . . . .  14
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .  15
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction







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1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

1.2.  Scope Of This Document

   This document describes a set of experimental conventions that apply
   to RPC-over-RDMA version 1, specified in [RFC5666].  When observed,
   these conventions enable RPC-over-RDMA version 1 endpoints to
   concurrently handle RPC transactions that flow from client to server
   and from server to client.

   These conventions can be observed when using the existing the RPC-
   over-RDMA version 1 protocol definition.  Therefore this document
   does not update [RFC5666].

   The purpose of this document is to permit interoperable prototype
   implementations of bi-directional RPC-over-RDMA, enabling the use of
   NFSv4.1 (including pNFS and later NFSv4 minor versions) on RDMA
   transports.

   Providing an Upper Layer Binding for NFSv4.x callback operations is
   outside the scope of this document.

1.3.  Understanding RPC Direction

   The ONC RPC protocol as described in [RFC5531] is fundamentally a
   message-passing protocol involving one server and perhaps multiple
   clients.  ONC RPC transactions are made up of two types of messages.

   A CALL message, or "call", requests work.  A call is designated by
   the value CALL in the message's msg_type field.  An arbitrary unique
   value is placed in the message's xid field.  A host that originates a
   call is referred to in this document as a "caller."

   A REPLY message, or "reply", reports the results of work requested by
   a call.  A reply is designated by the value REPLY in the message's
   msg_type field.  The value contained in the message's xid field is
   copied from the call whose results are being reported.  A host that
   emits a reply is referred to as a "responder."

   RPC-over-RDMA is a connection-oriented RPC transport.  When a
   connection-oriented transport is used, ONC RPC client endpoints are
   responsible for initiating transport connections, while ONC RPC
   service endpoints wait passively for incoming connection requests.




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   We do not consider RPC direction on connectionless RPC transports in
   this document.

1.3.1.  Forward Direction

   A traditional ONC RPC client is always a caller.  A traditional ONC
   RPC service is always a responder.  This traditional form of ONC RPC
   message passing is referred to as operation in the "forward
   direction."

   During forward direction operation, the ONC RPC client is responsible
   for establishing transport connections.

1.3.2.  Backward Direction

   The ONC RPC standard does not forbid passing messages in the other
   direction.  An ONC RPC service endpoint can act as a caller, in which
   case an ONC RPC client endpoint acts as a responder.  This form of
   message passing is referred to as operation in the "backward
   direction."

   During backward direction operation, the ONC RPC client is
   responsible for establishing transport connections, even though ONC
   RPC calls may come from the ONC RPC server.

   ONC RPC clients and services are optimized to perform and scale well
   while handling traffic in the forward direction, and may not be
   prepared to handle operation in the backward direction.  Not until
   recently has there been a need to handle backward direction
   operation.

1.3.3.  Bi-direction

   A pair of endpoints may choose to use only forward or only backward
   direction operations on a particular transport.  Or, the endpoints
   may send operations in both directions concurrently on the same
   transport.

   Bi-directional operation occurs when both transport endpoints act as
   a caller and a responder at the same time.  As above, the ONC RPC
   client is responsible for establishing transport connections.

1.3.4.  XID Values

   Section 9 of [RFC5531] introduces the ONC RPC transaction identifier,
   or "xid" for short.  The value of an xid is interpreted in the
   context of the message's msg_type field.




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   o  The xid of a call is arbitrary but is unique among outstanding
      calls from that caller.

   o  The xid of a reply always matches that of the initiating call.

   A caller matches the xid value in each reply with a call it
   previously sent.

1.3.4.1.  XIDs with Bi-direction

   During bi-directional operation, the forward and backward directions
   use independent xid spaces.

   In other words, a forward direction caller MAY use the same xid value
   at the same time as a backward direction caller on the same transport
   connection.  Though such concurrent requests use the same xid value,
   they represent distinct ONC RPC transactions.

1.4.  Rationale For RPC-over-RDMA Bi-Direction

1.4.1.  NFSv4.0 Callback Operation

   An NFSv4.0 client employs a traditional ONC RPC client to send NFS
   requests to an NFSv4.0 server's traditional ONC RPC service
   [RFC7530].  NFSv4.0 requests flow in the forward direction on a
   connection established by the client.  This connection is referred to
   as a "forechannel."

   NFSv4.0 introduces the use of callback operations, or "callbacks", in
   Section 10.2 of [RFC7530], for managing file delegation.  An NFSv4.0
   server sets up a traditional ONC RPC client, and an NFSv4.0 client
   sets up a traditional ONC RPC service to handle callbacks.  Callbacks
   flow in the forward direction on a connection established by an
   NFSv4.0 server.  This connection is distinct from connections being
   used as forechannels.  This connection is referred to as a
   "backchannel."

   When an RDMA transport is used as a forechannel, an NFSv4.0 client
   typically provides a TCP callback service.  The client's SETCLIENTID
   operation advertises the callback service endpoint with a "tcp" or
   "tcp6" netid.  The server then connects to this service using a TCP
   socket.

   NFSv4.0 implementations are fully functional without a backchannel in
   place.  In this case, the server does not grant file delegations.
   This might result in a negative performance effect, but functional
   correctness is unaffected.




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1.4.2.  NFSv4.1 Callback Operation

   NFSv4.1 supports file delegation in a similar fashion to NFSv4.0, and
   extends the repertoire of callbacks to manage pNFS layouts, as
   discussed in Chapter 12 of [RFC5661].

   For various reasons, NFSv4.1 requires that all transport connections
   be initiated by NFSv4.1 clients.  Therefore, NFSv4.1 servers send
   callbacks to clients in the backward direction on connections
   established by NFSv4.1 clients.

   An NFSv4.1 client or server indicates to its peer that a backchannel
   capability is available on a given transport when sending a
   CREATE_SESSION or BIND_CONN_TO_SESSION operation.

   NFSv4.1 clients may establish distinct transport connections for
   forechannel and backchannel operation, or they may combine
   forechannel and backchannel operation on one transport connection
   using bi-directional operation.

   When an RDMA transport is used as a forechannel, an NFSv4.1 client
   must additionally connect using a transport with backward direction
   capability to use as a backchannel.  Without a backward direction
   RPC-over-RDMA capability, TCP is the only choice at present for an
   NFSv4.1 backchannel connection.

   Some implementations find it more convenient to use a single combined
   transport (ie. a transport that is capable of bi-directional
   operation).  This simplifies connection establishment and recovery
   during network partitions or when one endpoint restarts.

   As with NFSv4.0, if a backchannel is not in use, an NFSv4.1 server
   does not grant delegations.  But because of its reliance on callbacks
   to manage pNFS layout state, pNFS operation is not possible without a
   backchannel.

1.5.  Design Considerations

   As of this writing, the only use case for backward direction ONC RPC
   messages is the NFSv4.1 backchannel.  The conventions described in
   this document take advantage of certain characteristics of NFSv4.1
   callbacks, namely:

   o  NFSv4.1 callbacks typically bear small argument and result
      payloads

   o  NFSv4.1 callback payloads are insensitive to alignment relative to
      system pages



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   o  NFSv4.1 callbacks are infrequent relative to forechannel
      operations

1.5.1.  Backward Compatibility

   Existing clients that implement RPC-over-RDMA version 1 should
   interoperate correctly with servers that implement RPC-over-RDMA with
   backward direction support, and vice versa.

   The approach taken here avoids altering the RPC-over-RDMA version 1
   XDR specification.  Keeping the XDR the same enables existing RPC-
   over-RDMA version 1 implementations to interoperate with
   implementations that support operation in the backward direction.

1.5.2.  Performance Impact

   Support for operation in the backward direction should never impact
   the performance or scalability of forward direction operation, where
   the bulk of ONC RPC transport activity typically occurs.

1.5.3.  Server Memory Security

   RDMA transfers involve one endpoint exposing a section of its memory
   to the other endpoint, which then drives RDMA READ and WRITE
   operations to access or modify the exposed memory.  RPC-over-RDMA
   client endpoints expose their memory, and RPC-over-RDMA server
   endpoints initiate RDMA data transfer operations.

   If RDMA transfers are not used for backward direction operations,
   there is no need for servers to expose their memory to clients.
   Further, this avoids the client complexity required to drive RDMA
   transfers.

1.5.4.  Payload Size

   Small RPC-over-RDMA messages are conveyed using only RDMA SEND
   operations.  SEND is used to transmit both ONC RPC calls and replies.

   To send a large payload, an RPC-over-RDMA client endpoint registers a
   region of memory known as a chunk, and transmits its coordinates to a
   server endpoint, who uses an RDMA transfer to move data to or from
   the client.  See Sections 3.1, 3.2, and 3.4 of [RFC5666].

   To transmit RPC-over-RDMA messages larger than the receive buffer
   size (typically 1024 bytes), a chunk must be used.  For example, in
   an RDMA_NOMSG type message, the entire RPC header and Upper Layer
   payload are contained in chunks.  See Section 5.1 of [RFC5666] for
   details.



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   If chunks are not allowed to be used for conveying backward direction
   messages, an RDMA_NOMSG type message cannot be used to convey a
   backward direction message using the conventions described in this
   document.  Therefore, backward direction messages sent using the
   conventions in this document can be no larger than a single receive
   buffer.

   Stipulating such a limit on backward direction message size assumes
   that either Upper Layer Protocol consumers of backward direction
   messages can advertise this limit to peers, or that ULP consumers can
   agree by convention on a maximum size of their backchannel payloads.

   In addition, using only inline forms of RPC-over-RDMA messages and
   never populating the RPC-over-RDMA chunk lists means that the RPC
   header's msg_type field is always at a fixed location in messages
   flowing in the backward direction, allowing efficient detection of
   the direction of an RPC-over-RDMA message.

   With few exceptions, NFSv4.1 servers can break down callback requests
   so they fit within this limit.  There are potentially large NFSv4.1
   callback operations, such as a CB_GETATTR operation where a large ACL
   must be conveyed.  Although we are not aware of any NFSv4.1
   implementation that uses CB_GETATTR, this state of affairs is not
   guaranteed in perpetuity.

2.  Conventions For Backward Operation

   Performing backward direction ONC RPC operations over an RPC-over-
   RDMA transport can be accomplished within limits by observing the
   conventions described in the following subsections.  For reference,
   the XDR description of RPC-over-RDMA version 1 is contained in
   Section 4.3 of [RFC5666].

2.1.  Flow Control

   For an RDMA SEND operation to work, the receiving consumer must have
   posted an RDMA RECV Work Request to provide a receive buffer in which
   to capture the incoming message.  If a receiver hasn't posted enough
   RECV WRs to catch incoming SEND operations, the RDMA provider is
   allowed to drop the RDMA connection.

   The RPC-over-RDMA version 1 protocol provides built-in send flow
   control to prevent overrunning the number of pre-posted receive
   buffers on a connection's receive endpoint.  This is fully discussed
   in Section 3.3 of [RFC5666].






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2.1.1.  Forward Credits

   An RPC-over-RDMA credit is roughly the capability to handle one RPC-
   over-RDMA transaction.  Each forward direction RPC-over-RDMA call
   requests a number of credits from the responder.  Each forward
   direction reply informs the caller how many credits the responder is
   prepared to handle in total.  The value of the request and grant are
   carried in each RPC-over-RDMA message's rdma_credit field.

   Practically speaking, the critical value is the value of the
   rdma_credit field in RPC-over-RDMA replies.  When a caller is
   operating correctly, it sends no more outstanding requests at a time
   than the responder's advertised forward direction credit value.

   The credit value is a guaranteed minimum.  However, a receiver can
   post more receive buffers than its credit value.  There is no
   requirement in the RPC-over-RDMA protocol for a receiver to indicate
   a credit overrun.  Operation continues as long as there are enough
   receive buffers to handle incoming messages.

2.1.2.  Backward Credits

   Credits work the same way in the backward direction as they do in the
   forward direction.  However, forward direction credits and backward
   direction credits are accounted separately.

   In other words, the forward direction credit value is the same
   whether or not there are backward direction resources associated with
   an RPC-over-RDMA transport connection.  The backward direction credit
   value MAY be different than the forward direction credit value.  The
   rdma_credit field in a backward direction RPC-over-RDMA message MUST
   NOT contain the value zero.

   A backward direction caller (an RPC-over-RDMA service endpoint)
   requests credits from the responder (an RPC-over-RDMA client
   endpoint).  The responder reports how many credits it can grant.
   This is the number of backward direction calls the responder is
   prepared to handle at once.

   When an RPC-over-RDMA server endpoint is operating correctly, it
   sends no more outstanding requests at a time than the client
   endpoint's advertised backward direction credit value.

2.2.  Managing Receive Buffers

   An RPC-over-RDMA transport endpoint must pre-post receive buffers
   before it can receive and process incoming RPC-over-RDMA messages.
   If a sender transmits a message for a receiver which has no prepared



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   receive buffer, the RDMA provider is allowed to drop the RDMA
   connection.

2.2.1.  Client Receive Buffers

   Typically an RPC-over-RDMA caller posts only as many receive buffers
   as there are outstanding RPC calls.  A client endpoint without
   backward direction support might therefore at times have no pre-
   posted receive buffers.

   To receive incoming backward direction calls, an RPC-over-RDMA client
   endpoint must pre-post enough additional receive buffers to match its
   advertised backward direction credit value.  Each outstanding forward
   direction RPC requires an additional receive buffer above this
   minimum.

   When an RDMA transport connection is lost, all active receive buffers
   are flushed and are no longer available to receive incoming messages.
   When a fresh transport connection is established, a client endpoint
   must re-post a receive buffer to handle the reply for each
   retransmitted forward direction call, and a full set of receive
   buffers to handle backward direction calls.

2.2.2.  Server Receive Buffers

   A forward direction RPC-over-RDMA service endpoint posts as many
   receive buffers as it expects incoming forward direction calls.  That
   is, it posts no fewer buffers than the number of RPC-over-RDMA
   credits it advertises in the rdma_credit field of forward direction
   RPC replies.

   To receive incoming backward direction replies, an RPC-over-RDMA
   server endpoint must pre-post a receive buffer for each backward
   direction call it sends.

   When the existing transport connection is lost, all active receive
   buffers are flushed and are no longer available to receive incoming
   messages.  When a fresh transport connection is established, a server
   endpoint must re-post a receive buffer to handle the reply for each
   retransmitted backward direction call, and a full set of receive
   buffers for receiving forward direction calls.

2.2.3.  In the Absense of Backward Direction Support

   An RPC-over-RDMA transport endpoint might not support backward
   direction operation.  There might be no mechanism in the
   implementation to do so.  Or the Upper Layer Protocol consumer might




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   not yet have configured the transport to handle backward direction
   traffic.

   A loss of the RDMA connection may result if the receiver is not
   prepared to receive an incoming message.  Thus a denial-of-service
   could result if a sender continues to send backchannel messages after
   every transport reconnect to an endpoint that is not prepared to
   receive them.

   Generally, for RPC-over-RDMA version 1 transports, the Upper Layer
   Protocol consumer is responsible for informing its peer when it has
   no support for the backward direction.  Otherwise even a simple
   backward direction NULL probe from a peer would result in a lost
   connection.

   An NFSv4.1 server should never send backchannel messages to an
   NFSv4.1 client before the NFSv4.1 client has sent a CREATE_SESSION or
   a BIND_CONN_TO_SESSION operation.  As long as an NFSv4.1 client has
   prepared appropriate backchannel resources before sending one of
   these operations, denial-of-service is avoided.  Legacy versions of
   NFS should never send backchannel operations.

   Therefore, an Upper Layer Protocol consumer MUST NOT perform backward
   direction ONC RPC operations unless the peer consumer has indicated
   it is prepared to handle them.  A description of Upper Layer Protocol
   mechanisms used for this indication is outside the scope of this
   document.

2.3.  Backward Direction Retransmission

   In rare cases, an ONC RPC transaction cannot be completed within a
   certain time.  This can be because the transport connection was lost,
   the call or reply message was dropped, or because the Upper Layer
   consumer delayed or dropped the ONC RPC request.  Typically, the
   caller sends the transaction again, reusing the same RPC XID.  This
   is known as an "RPC retransmission".

   In the forward direction, the caller is the ONC RPC client.  The
   client is always responsible for establishing a transport connection
   before sending again.

   In the backward direction, the caller is the ONC RPC server.  Because
   an ONC RPC server does not establish transport connections with
   clients, it cannot send a retransmission if there is no transport
   connection.  It must wait for the ONC RPC client to re-establish the
   transport connection before it can retransmit ONC RPC transactions in
   the backward direction.




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   If an ONC RPC client has no work to do, it may be some time before it
   re-establishes a transport connection.  Backward direction callers
   must be prepared to wait indefinitely before a connection is
   established before a pending backward direction ONC RPC call can be
   retransmitted.

2.4.  Backward Direction Message Size

   RPC-over-RDMA backward direction messages are transmitted and
   received using the same buffers as messages in the forward direction.
   Therefore they are constrained to be no larger than receive buffers
   posted for forward messages.  Typical implementations have chosen to
   use 1024-byte buffers.

   It is expected that the Upper Layer Protocol consumer establishes an
   appropriate payload size limit for backward direction operations,
   either by advertising that size limit to its peers, or by convention.
   If that is done, backward direction messages would not exceed the
   size of receive buffers at either endpoint.

   If a sender transmits a backward direction message that is larger
   than the receiver is prepared for, the RDMA provider drops the
   message and the RDMA connection.

   If a sender transmits an RDMA message that is too small to convey a
   complete and valid RPC-over-RDMA and RPC message in either direction,
   the receiver MUST NOT use any value in the fields that were
   transmitted.  Namely, the rdma_credit field MUST be ignored, and the
   message dropped.

2.5.  Sending A Backward Direction Call

   To form a backward direction RPC-over-RDMA call message on an RPC-
   over-RDMA version 1 transport, an ONC RPC service endpoint constructs
   an RPC-over-RDMA header containing a fresh RPC XID in the rdma_xid
   field (see Section 1.3.4 for full requirements).

   The rdma_vers field MUST contain the value one.  The number of
   requested credits is placed in the rdma_credit field (see
   Section 2.1).

   The rdma_proc field in the RPC-over-RDMA header MUST contain the
   value RDMA_MSG.  All three chunk lists MUST be empty.

   The ONC RPC call header MUST follow immediately, starting with the
   same XID value that is present in the RPC-over-RDMA header.  The call
   header's msg_type field MUST contain the value CALL.




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2.6.  Sending A Backward Direction Reply

   To form a backward direction RPC-over-RDMA reply message on an RPC-
   over-RDMA version 1 transport, an ONC RPC client endpoint constructs
   an RPC-over-RDMA header containing a copy of the matching ONC RPC
   call's RPC XID in the rdma_xid field (see Section 1.3.4 for full
   requirements).

   The rdma_vers field MUST contain the value one.  The number of
   granted credits is placed in the rdma_credit field (see Section 2.1).

   The rdma_proc field in the RPC-over-RDMA header MUST contain the
   value RDMA_MSG.  All three chunk lists MUST be empty.

   The ONC RPC reply header MUST follow immediately, starting with the
   same XID value that is present in the RPC-over-RDMA header.  The
   reply header's msg_type field MUST contain the value REPLY.

3.  Limits To This Approach

3.1.  Payload Size

   The major drawback to the approach described in this document is the
   limit on payload size in backward direction requests.

   o  Some NFSv4.1 callback operations can have potentially large
      arguments or results.  For example, CB_GETATTR on a file with a
      large ACL; or CB_NOTIFY, which can provide a large, complex
      argument.

   o  Any backward direction operation protected by RPCSEC_GSS may have
      additional header information that makes it difficult to send
      backward direction operations with large arguments or results.

   o  Larger payloads could potentially require the use of RDMA data
      transfers, which are complex and make it more difficult to detect
      backward direction requests.  The msg_type field in the ONC RPC
      header would no longer be at a fixed location in backward
      direction requests.

3.2.  Preparedness To Handle Backward Requests

   A second drawback is the exposure of the client transport endpoint to
   backward direction calls before it has posted receive buffers to
   handle them.

   Clients that do not support backward direction operation typically
   drop messages they do not recognize.  However, this does not allow



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   bi-direction-capable servers to quickly identify clients that cannot
   handle backward direction requests.

   The conventions in this document rely on Upper Layer Protocol
   consumers to decide when backward direction transport operation is
   appropriate.

3.3.  Long Term

   To address the limitations described in this section in the long run,
   a new version of the RPC-over-RDMA protocol would be required.  The
   use of the conventions described in this document to enable backward
   direction operation is thus a transitional approach that is
   appropriate only while RPC-over-RDMA version 1 is the predominantly
   deployed version of the RPC-over-RDMA protocol.

4.  Security Considerations

   As a consequence of limiting the size of backward direction RPC-over-
   RDMA messages, the use of RPCSEC_GSS integrity and confidentiality
   services (see [RFC2203]) in the backward direction may be challenging
   due to the size of the additional RPC header information required for
   RPCSEC_GSS.

5.  IANA Considerations

   This document does not require actions by IANA.

6.  Acknowledgements

   Tom Talpey was an indispensable resource, in addition to creating the
   foundation upon which this work is based.  Our warmest regards go to
   him for his help and support.

   Dave Noveck provided excellent review, constructive suggestions, and
   navigational guidance throughout the process of drafting this
   document.

   Dai Ngo was a solid partner and collaborator.  Together we
   constructed and tested independent prototypes of the conventions
   described in this document.

   The author wishes to thank Bill Baker for his unwavering support of
   this work.  In addition, the author gratefully acknowledges the
   expert contributions of Karen Deitke, Chunli Zhang, Mahesh
   Siddheshwar, and Tom Tucker.





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   Special thanks go to the nfsv4 Working Group chair Spencer Shepler
   and the WG Editor Tom Haynes for their support.

7.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2203]  Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
              Specification", RFC 2203, September 1997.

   [RFC5531]  Thurlow, R., "RPC: Remote Procedure Call Protocol
              Specification Version 2", RFC 5531, May 2009.

   [RFC5661]  Shepler, S., Eisler, M., and D. Noveck, "Network File
              System (NFS) Version 4 Minor Version 1 Protocol", RFC
              5661, January 2010.

   [RFC5666]  Talpey, T. and B. Callaghan, "Remote Direct Memory Access
              Transport for Remote Procedure Call", RFC 5666, January
              2010.

   [RFC7530]  Haynes, T. and D. Noveck, "Network File System (NFS)
              Version 4 Protocol", RFC 7530, March 2015.

Author's Address

   Charles Lever
   Oracle Corporation
   1015 Granger Avenue
   Ann Arbor, MI  48104
   US

   Phone: +1 734 274 2396
   Email: chuck.lever@oracle.com
















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