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

Network File System Version 4                                   C. Lever
Internet-Draft                                                    Oracle
Intended status: Standards Track                             May 2, 2016
Expires: November 3, 2016


    Bi-directional Remote Procedure Call On RPC-over-RDMA Transports
                draft-ietf-nfsv4-rpcrdma-bidirection-03

Abstract

   Recent minor versions of NFSv4 work best when ONC RPC transports can
   send Remote Procedure Call transactions in both directions on the
   same connection.  This document describes how RPC-over-RDMA transport
   endpoints convey RPCs in both directions on a single connection.

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 3, 2016.

Copyright Notice

   Copyright (c) 2016 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
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Understanding RPC Direction . . . . . . . . . . . . . . . . .   3
     2.1.  Forward Direction . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Backward Direction  . . . . . . . . . . . . . . . . . . .   4
     2.3.  Bi-directional Operation  . . . . . . . . . . . . . . . .   4
     2.4.  XID Values  . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Rationale For Bi-Directional RPC-over-RDMA  . . . . . . . . .   5
     3.1.  NFSv4.0 Callback Operation  . . . . . . . . . . . . . . .   5
     3.2.  NFSv4.1 Callback Operation  . . . . . . . . . . . . . . .   6
   4.  Flow Control  . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Backward Credits  . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Managing Receive Buffers  . . . . . . . . . . . . . . . .   7
   5.  Protocol For Backward Operation . . . . . . . . . . . . . . .   8
     5.1.  Sending A Backward Direction Call . . . . . . . . . . . .   8
     5.2.  Sending A Backward Direction Reply  . . . . . . . . . . .   9
     5.3.  Backward Direction Chunks . . . . . . . . . . . . . . . .   9
     5.4.  Backward Direction Retransmission . . . . . . . . . . . .  10
   6.  In the Absence of Backward Direction Support  . . . . . . . .  10
   7.  Backward Direction Upper Layer Binding  . . . . . . . . . . .  11
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   The purpose of this document is to enable bi-directional RPC
   operation on RPC-over-RDMA protocol versions that do not have
   specific protocol facilities for backward direction operation.
   Backward direction RPC transactions enable the operation of NFSv4.1,
   and in particular pNFS.

   For example, using the protocol described in this document, RPC
   transactions can be conveyed in both directions on the same RPC-over-
   RDMA Version One connection without changes to the Version One header
   XDR description.  Therefore this document does not update
   [I-D.ietf-nfsv4-rfc5666bis].

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







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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].

2.  Understanding RPC Direction

   The ONC RPC protocol as described in [RFC5531] is fundamentally a
   message-passing protocol between one server and one or more 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 "Requester."

   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 returned.  A host that
   emits a Reply is referred to as a "Responder."

   Typically, a Call generates a corresponding Reply.  A Reply is never
   sent without a corresponding Call.

   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.

   RPC direction on connectionless RPC transports is not considered in
   this document.

2.1.  Forward Direction

   A traditional ONC RPC client is always a Requester.  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.








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2.2.  Backward Direction

   The ONC RPC specification [RFC5531] does not forbid passing messages
   in the other direction.  An ONC RPC service endpoint can act as a
   Requester, 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 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.

2.3.  Bi-directional Operation

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

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

2.4.  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.

   o  The xid of a Call is arbitrary but is unique among outstanding
      Calls from that Requester.

   o  The xid of a Reply always matches that of the initiating Call.

   When receiving a Reply, a Requester matches the xid value in the
   Reply with a Call it previously sent.








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2.4.1.  XID Generation

   During bi-directional operation, forward and backward direction XIDs
   are typically generated on distinct hosts by possibly different
   algorithms.  There is no co-ordination between forward and backward
   direction XID generation.

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

3.  Rationale For Bi-Directional RPC-over-RDMA

3.1.  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" connection.

   An NFSv4 "delegation" is simply a promise made by a server that it
   will notify a client before another agent is allowed access to a
   file.  With this guarantee, that client can operate as sole accessor
   of the file.  In particular, it can manage the file's data and
   metadata caches aggressively.

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

   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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3.2.  NFSv4.1 Callback Operation

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

   To facilitate operation through NAT routers, all NFSv4.1 transport
   connections are initiated by NFSv4.1 clients.  Therefore NFSv4.1
   servers send callbacks to clients in the backward direction on
   connections established by NFSv4.1 clients.

   NFSv4.1 clients and servers indicate to their peers that a
   backchannel capability is available on a given transport in the
   arguments and results of NFS CREATE_SESSION or BIND_CONN_TO_SESSION
   operations.

   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.

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

   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.

4.  Flow Control

   For an RDMA Send operation to work, the receiving peer must have
   posted an RDMA Receive Work Request (WR) to provide a receive buffer
   in which to land the incoming message.  If a receiver hasn't posted
   enough Receive WRs to land incoming Send operations, the RDMA
   provider is allowed to drop the RDMA connection.

   RPC-over-RDMA transport protocols provide 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 4.3 of [I-D.ietf-nfsv4-rfc5666bis].



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4.1.  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 Requester (ie, an RPC-over-RDMA service
   endpoint) requests credits from the Responder (ie, an RPC-over-RDMA
   client endpoint).  The Responder reports how many credits it has
   granted.  This is the number of backward direction Calls the
   Responder is prepared to handle at once.

   When message direction is not fully determined by context or by an
   accompanying RPC message with a call direction field, it is not
   possible to tell whether the header credit value is a request or
   grant, or whether the value applies to the forward direction or
   backward direction.  In such cases, the receiver MUST NOT use the
   header's credit value.

4.2.  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
   receive buffer, the RDMA provider is allowed to drop the RDMA
   connection.

4.2.1.  Client Receive Buffers

   Typically an RPC-over-RDMA Requester 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.





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   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.

4.2.2.  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 credits granted 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.

5.  Protocol For Backward Operation

   Performing backward direction ONC RPC operations over an RPC-over-
   RDMA transport connection can be accomplished by observing the
   protocol described in the following subsections.  For reference, the
   XDR description of RPC-over-RDMA Version One is contained in
   Section 5.1 of [I-D.ietf-nfsv4-rfc5666bis].

5.1.  Sending A Backward Direction Call

   To form a backward direction RPC-over-RDMA Call message, an ONC RPC
   service endpoint constructs an RPC-over-RDMA header containing a
   fresh RPC XID in the rdma_xid field (see Section 2.4 for full
   requirements).

   The rdma_vers field MUST contain the same value in backward and
   forward direction Call messages on the same connection.

   The number of requested backward direction credits is placed in the
   rdma_credit field (see Section 4).

   Whether presented inline or as a separate chunk, the ONC RPC Call
   header MUST start with the same XID value that is present in the RPC-



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   over-RDMA header, and the header's msg_type field MUST contain the
   value CALL.

5.2.  Sending A Backward Direction Reply

   To form a backward direction RPC-over-RDMA Reply message, 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 2.4 for full requirements).

   The rdma_vers field MUST contain the same value in a backward
   direction Reply message as in the matching Call message.

   The number of granted backward direction credits is placed in the
   rdma_credit field (see Section 4).

   Whether presented inline or as a separate chunk, the ONC RPC Reply
   header MUST start with the same XID value that is present in the RPC-
   over-RDMA header, and the header's msg_type field MUST contain the
   value REPLY.

5.3.  Backward Direction Chunks

   Chunks MAY be used in the backward direction.  They operate the same
   way as in the forward direction (see [I-D.ietf-nfsv4-rfc5666bis] for
   details).

   An implementation might not support any Upper Layer Protocol that has
   DDP-eligible data items.  The Upper Layer Protocol may also use only
   small messages, or it may have a native mechanism for restricting the
   size of backward direction RPC messages, obviating the need to handle
   Long Messages in the backward direction.

   When there is no Upper Layer Protocol requirement for chunks,
   implementers can choose not to provide support for chunks in the
   backward direction.  This avoids the complexity of adding support for
   performing RDMA Reads and Writes in the backward direction.

   When chunks are not implemented, RPC messages in the backward
   direction are always sent using RDMA_MSG, and therefore can be no
   larger than what can be sent inline (that is, without chunks).
   Sending an inline message larger than the receiver's inline threshold
   can result in loss of connection.

   If a backward direction requester provides a non-empty chunk list to
   a responder that does not support chunks, the responder MUST reply
   with an RDMA_ERROR message with rdma_err field set to ERR_CHUNK.




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5.4.  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
   Requester sends the transaction again, reusing the same RPC XID.
   This is known as an "RPC retransmission".

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

   In the backward direction, the Requester 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.

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

6.  In the Absence of Backward Direction Support

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

   If an endpoint is not prepared to receive an incoming backward
   direction message, loss of the RDMA connection might result.  Thus a
   denial-of-service could result if a sender continues to send backward
   direction messages after every transport reconnect to an endpoint
   that is not prepared to receive them.

   When dealing with the possibility that the remote peer has no
   transport level support for backward direction operation, the Upper
   Layer Protocol becomes responsible for informing peers when backward
   direction operation is supported.  Otherwise even a simple backward
   direction NULL probe from a peer could result in a lost connection.

   An NFSv4.1 server does not send backchannel messages to an NFSv4.1
   client before the NFSv4.1 client has sent a CREATE_SESSION or a



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   BIND_CONN_TO_SESSION operation.  As long as an NFSv4.1 client has
   prepared appropriate backchannel resources before sending one of
   these operations announcing support for backchannel operation,
   denial-of-service is avoided.

   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.

7.  Backward Direction Upper Layer Binding

   Since backward direction operation occurs on an already-established
   connection, there is no need to specify RPC bind parameters.

   An Upper Layer Protocol that operates on RPC-over-RDMA transports in
   the backward direction may have DDP-eligible data items.  These are
   specified in an Upper Layer Binding document.

   By default, no data items in a ULP are DDP-eligible.  If there are no
   DDP-eligible data items to document, an explicit Upper Layer Binding
   may not be needed for an Upper Layer Protocol that operates only in
   the backward direction.

   Consult Section 7 of [I-D.ietf-nfsv4-rfc5666bis] for details about
   what else may be contained in a binding.

8.  Security Considerations

   Security considerations for operation on RPC-over-RDMA transports are
   outlined in Section 9 of [I-D.ietf-nfsv4-rfc5666bis].

9.  IANA Considerations

   This document does not require actions by IANA.

10.  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.





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   Dai Ngo was a solid partner and collaborator.  Together we
   constructed and tested independent prototypes of the changes
   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, Steve Wise, and Tom Tucker.

   Special thanks go to the nfsv4 Working Group Chair Spencer Shepler
   and the nfsv4 Working Group Secretary Tom Haynes for their support.

11.  Normative References

   [I-D.ietf-nfsv4-rfc5666bis]
              Lever, C., Simpson, W., and T. Talpey, "Remote Direct
              Memory Access Transport for Remote Procedure Call", draft-
              ietf-nfsv4-rfc5666bis-04 (work in progress), March 2016.

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

   [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
   USA

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









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