Remote Direct Data Placement C. Bestler Working group Broadcom Internet-Draft L. Coene Expires: March
11,30, 2006 Siemens September 26, 2005 Applicability of Remote Direct Memory Access Protocol (RDMA) and Direct Data Placement (DDP) draft-ietf-rddp-applicability-02.txtdraft-ietf-rddp-applicability-03.txt Status of this Memo This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667.By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomebecomes aware will be disclosed, in accordance with RFC 3668.Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on March 11, 2005.30, 2006. Copyright Notice Copyright (C) The Internet Society (2004).(2005). Abstract This document describes the applicability of Remote Direct Memory Access Protocol (RDMAP) and the Direct Data Placement Protocol (DDP). It comparese and contrasts the different transport options over IP that DDP can use, provides guidance to ULP developers on choosing between available transports and/or how to be indifferent to the specific transport layer used, compares use of DDP with direct use of the supporting transports, and compares DDP over IP transports with non-IP transports that support RDMA functionality. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Direct Placement . . . . . . . . . . . . . . . . . . . . . . . 6 3.13.1. Fewer Required ULP Interactions . . . . . . . . . . . . . 6 3.23.2. Direct Placement using only the LLP . . . . . . . . . . . 6 4. Tagged Messages . . . . . . . . . . . . . . . . . . . . . . . 8 4.14.1. Order Independent Reception . . . . . . . . . . . . . . . 8 4.24.2. Reduced ULP Notifications . . . . . . . . . . . . . . . . 8 4.34.3. Simplified ULP Exchanges . . . . . . . . . . . . . . . . . 9 4.44.4. Order Independent Sending . . . . . . . . . . . . . . . . 10 4.54.5. Tagged Buffers as ULP Credits . . . . . . . . . . . . . . 11 5. RDMA Read . . . . . . . . . . . . . . . . . . . . . . . . . . 13 6. LLP Comparisons . . . . . . . . . . . . . . . . . . . . . . . 14 6.16.1. Multistreaming Implications . . . . . . . . . . . . . . . 14 6.26.2. Out of Order Reception Implications . . . . . . . . . . . 14 6.36.3. Header and Marker Overhead . . . . . . . . . . . . . . . . 14 6.46.4. Middlebox Support . . . . . . . . . . . . . . . . . . . . 14 6.515 6.5. Processing Overhead . . . . . . . . . . . . . . . . . . . 15 6.66.6. Data Integrity Implications . . . . . . . . . . . . . . . 15 220.127.116.11.1. MPA/TCP Specifics . . . . . . . . . . . . . . . . . . 15 18.104.22.168.2. SCTP Specifics . . . . . . . . . . . . . . . . . . . . 16 6.76.7. Non-IP Transports . . . . . . . . . . . . . . . . . . . . 16 22.214.171.124.1. No RDMA Layer Ack . . . . . . . . . . . . . . . . . . 16 6.86.8. Other IP Transports . . . . . . . . . . . . . . . . . . . 17 6.96.9. LLP Independent Session Establishment . . . . . . . . . . 17 126.96.36.199.1. RDMA-only Session Establishment . . . . . . . . . . . 17 6.9.218 6.9.2. RDMA-Conditional Session Establishment . . . . . . . . 18 7. Local Interface Implications . . . . . . . . . . . . . . . . . 20 8. Security considerations . . . . . . . . . . . . . . . . . . . 21 8.18.1. Connection/Association Setup . . . . . . . . . . . . . . . 21 8.28.2. Tagged Buffer Exposure . . . . . . . . . . . . . . . . . . 21 8.38.3. Impact of Encrypted Transports . . . . . . . . . . . . . . 21 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 22. . 23 Intellectual Property and Copyright Statements . . . . . . . . . . 24 1. Introduction Remote Direct Memory Access Protocol (RDMAP) and Direct Data Placement (DDP) work together to provide application independent efficient placemenetplacement of application payload directly into buffers specified by the Upper Layer Protocol (ULP). The DDP protocol is responsible for direct placement of received payload into ULP specified buffers. The RDMAP protocol provides completion notifications to the ULP and support for Data Sink initiated fetch of advertised buffers (RDMA Reads). DDP and RDMAP are both application independent protocols which allow the ULP to perform remote direct data placement. DDP can use multiple standard IP transports including SCTP and TCP. By clarifying the situations where the functionality of these protocols are applicable, this document can guide implementers, application and protocol designers in selecting which protocols to use. The applicability of RDMAP/DDP is driven by their unique capabilities: o The existence of an application independent protocol allows common solutions to be implemented in hardware and/or the kernel. This document will discuss when common data placement procedures are of the greatest benefit to applications as contrasted with application specific solutions built on top of direct use of the underlying transport. o DDP supports both untagged and tagged buffers. Tagged buffers allow the Data Sink ULP to be indifferent to what order (or in what packets) the Data Source sent the data, or what order they are received in. This document will discuss when Data Source flexibility is of benefit to applications. o RDMAP consolidates ULP notifications, thereby minimizing the number of required ULP interactions. o RDMAP defines RDMA Reads, which allow remote access to advertised buffers. This document will review the advantages of using RDMA Reads as contrasted to alternate solutions. Some non-IP transports, such as InfiniBand, directly integrate RDMA features. This document will review the applicability of providing RDMA services over ubiquitous IP transports as opposed to the use of customized transport protocols. Due to the fact that DDP is defined cleanly as a layer over existing IP transports, DDP has simpler ordering rules than some prior RDMA protocols. This may have some implications for application designers. The full capabilities of DDP and RDMAP can only be fully realized by applications that are designed to exploit them. The co-existence of RDMAP/DDP aware local interfaces with traditional socket interfaces will also be explored. Finally, DDP support is defined for at least two IP transports: SCTP and TCP. The rationale for supporting both transports is reviewed, as well as when each would be the appropriate selection. 2. Definitions Advertisement - the act of informing a Remote Peer that a local RDMA Buffer is available to it. A Node makes available an RDMA Buffer for incoming RDMA Read or RDMA Write access by informing its RDMA/ DDP peer of the Tagged Buffer identifiers (STag, base address, and buffer length). This advertisement of Tagged Buffer information is not defined by RDMA/DDP and is left to the ULP. A typical method would be for the Local Peer to embed the Tagged Buffer's Steering Tag, base address, and length in a Send Message destined for the Remote Peer. Data Sink - The peer receiving a data payload. Note that the Data Sink can be required to both send and receive RDMA/DDP Messages to transfer a data payload. Data Source - The peer sending a data payload. Note that the Data Source can be required to both send and receive RDMA/DDP Messages to transfer a data payload. Lower Layer Protocol (LLP) The transport protocol that provides services to DDP. This is an IP transport with any required adaptation layer. Adaptation layers are defined for SCTP and TCP. Steering Tag (STag) An identifier of a Tagged Buffer on a Node, valid as defined within a protocol specification. Tagged Message A DDP message that is directed to a ULP specified buffer based upon imbedded addressing information. In the immediate sense, the destination buffer is specified by the message sender. Untagged Message A DDP message that is directed to a ULP specified buffer based upon a Message Sequence Number being matched with a receiver supplied buffer. The destination buffer is specified by the message receiver. Upper Layer Protocol (ULP) The direct user of RDMAP/DDP services. This may be an application, or a middleware layer such as Sockets Direct Protocol (SDP) or Remote Procedure Calls (RPC). 3. Direct Placement Direct Data Placement optimizes the placement of ULP payload into the correct destination buffers, typically eliminating intermediate copying. Placement is enabled without regard to order of arrival, order of transmission or requiring per-placement interaction with the ULP. RDMAP minimizes the required ULP interactions . This capability is most valuable for applications that require multiple transport layer packets for each required ULP interaction. 3.13.1. Fewer Required ULP Interactions While reducing the number of required ULP interactions is in itself desirable, it is critical for high speed connections. The burst packet rate for a high speed interface could easily exceed the host systems ability to switch ULP contexts. Content access applications are primary examples of applications with both high bandwidth and high content to required ULP interaction ratios. These applications include file access protocols (NAS), storage access (SAN), database access and other application specific forms of content access such as HTTP, XML and email. 3.23.2. Direct Placement using only the LLP Direct data placement can be achieved without RDMA. Pre-posting of receive buffers could allow a non-RDMA network stack to place data directly to user buffers. The degree to which DDP optimizes depends on which transport is being compared with, and on the nature of the local interface. Without RDMAP/DDP pre-posting buffers requires the receiving side to accurately predict the required buffers and their sizes. This is not feasible for all ULPs. By contrast, DDP only requires the ULP to predict the sequence and size of incoming untagged messages. An application that could predict incoming messages and required nothing more than direct placement into buffers might be able to do so with a properly designed local interface to SCTP or TCP. Doing so for TCP requires making predictions at a byte level rather than a message level. The main benefit of DDP for such an application would be that pre-postingpre- posting of receive buffers is a mandated local interface capability, and that predictions can be made on a per-message basis (not per byte). The LLP can also be used directly if ULP specific knowledge is built into the protocol stack to allow "parse and place" handling of received packets. Such a solution either requires interaction with the ULP, or that the protocol stack have knowledge of ULP specific syntax rules. DDP achieves the benefits of directly placing incoming payload without requiring tight coupling between the ULP and the protocol stack. However, "parse and place" capabilities can certainly provide equivalent services to a limited number of ULPs. 4. Tagged Messages This section covers the major benefits from the use of Tagged Messages. A more critical advantage of DDP is the ability of the Data Source to use tagged buffers. Tagging messages allows the Data Source to choose the ordering and packetization of its payload deliveries. With direct data placement based solely upon pre-posted receives, the packetization and delivery of payload must be agreed by the ULP peers in advance. Even if there is an encoding of what is being transferred, as is common with middleware solutions, this information is not understood at the application independent layers. The directions on where to place the incoming data cannot be accessed without switching to the ULP first. DDP provides a standardized 'packing list' which can be interpreted without requiring ULP interaction. Indeed, it is designed to be implementable in hardware. 4.14.1. Order Independent Reception Tagged messages are directed to a buffer based on an included Steering Tag. Additionally, no notice is provided to the ULP for each individual Tagged Message's arrival. Together these allow tagged messages received out-of-order to be processed without intermediate buffering or additional notifications to the ULP. 4.24.2. Reduced ULP Notifications RDMAP further reduces required ULP interactions consolidating completion notifications of tagged messages with the completion notification of a trailing untagged message. For most ULPs this radically reduces the number of ULP required interactions even further. While RDMAP consolidation of notices is beneficial to most applications. Itapplications, it may be detrimental to some applications that benefit from streamed delivery to enable ULP processing of received data as promptly as possible. A ULP that uses RDMAP cannot begin processing any portion of an exchange until it receives notification that the entire exchange has been placed. An "exchange" here is a set of zero or more tagged messages and a single terminating untagged message. An application that would prefer to begin work on the received payload, no matter what order it arrived in, as soon as possible might prefer to work directly with the LLP. RDMAP is optimized for applications that are more concerned when the entire exchange is complete. An application that benefits from being able to begin processing of each received packet as quickly as possible may find RDMAP interferes with that goal. Such an application might be able to retain most of the benefits of RDMAP by using the DDP layer directly. However, in addition to taking on the responsibilities of the RDMAP layer, the application would likely have more difficulty finding support for a DDP-only API. Many hardware implementations may choose to tightly couple RDMAP and DDP, and might not provide an API directly to DDP services. These features minimize the required interactions with the ULP. This can be extremely beneficial for applications that use multiple transport layer packets to accomplish what is a single ULP interaction. 4.34.3. Simplified ULP Exchanges The notification rules for Tagged Messages allows ULPs to create multi-message "exchanges" consisting of zero or more tagged messages that represent a single step in the ULP interaction. The receiving ULP is notified that the untagged message has arrived, and implicitly of any associated tagged messages. A ULP where all exchanges would naturally be only the untagged message would derive virtually no benefit from the use of RDMAP/DDP as opposed to SCTP. But while tagged buffers are the justification for RDMAP/DDP, untagged buffers are still necessary. Without untagged buffers the only method to exchange buffer advertisements would involve out-of-band communications and/or sharing of compile time constants. Most RDMA-aware ULPs use untagged buffers for requests and responses. Buffer advertisements are typically done within these untagged messages. Limiting use of untagged buffers to requests and responses by moving all bulk data using tagged transfers can greatly simplify the amount of prediction that the Data Sink must perform in pre-posting receive buffers. For example, a typical RDMA enabled interaction would consist of the following: Client sends transaction request to server's as an untagged message. This message includes buffer advertisements for the buffers where the results are to be placed. The Server sends multiple tagged messages to the advertised buffers. The Server sends transaction reply as an untagged message to the client. Client receives single notification, indicating completion of the interaction. With this type of exchange the pacing and required size of untagged buffers is highly predictable. The variability of response sizes is absorbed by tagged transfers. 4.44.4. Order Independent Sending Use of tagged messages is especially applicable when the Data Sink does not know the actual size, structure or location of the content it is requesting (or updating). For example, suppose the Data Sink ULP needs to fetch four related pieces of data into a four separate buffers. With SCTP the Data Sink ULP could receive four messages into four separate buffers, only having to predict the maximum size of each. However it would have to dictate the order in which the Data Source supplied the separate pieces. If the Data Source found it advantageous to fetch them in a different order it would have to use intermediate buffering to re-orderre- order the pieces into the expected order even though the application only required that all four be delivered and did not truly have an ordering requirement. Techniques such as RAID striping and mirroring represent this same problem, but one step further. What appears to be a single resource to the Data Sink is actually stored in separate locations by the Data Source. Non RDMA protocols would either require the Data Source to fetch the material in the desired order or force the Data Source to use its own holding buffers to assemble an image of the destination buffer. While sometimes referred to as a "buffer-to-buffer" solution, RDMA more fundamentally enables remote buffer access. The ULP is free to work with larger remote buffers than it has locally. This reduces buffering requirements and the number of times the data must be copied in an end-to-end transfer. There are numerous reasons why the Data Sink would not know the true order or location of the requested data. It could be different for each client, different records selected and/or different sort orders, RAID striping, file fragmentation, volume fragmentation, volume mirroring and server-side dynamic compositing of content (such as server side includes for HTTP). In all of these cases the Data Source is free to assemble the desired data in the Data Sinks buffer in whatever order the component data becomes available to it. It is not constrained on ordering. It does not have to assemble an image in its own memory before creating it in the Data Sink's buffers. Note that while DDP enables use of tagged messages for bulk transfer, there are some application scenarios where untagged messages would still be used for bulk transfer. For example, under the Direct Access File Server (DAFS) protocol the file server does not expose its own memory to its clients. A client wishing to write may advertise a buffer which the server will issue RDMA Reads upon. However, when performing a small write it may be preferable to include the data in the untagged message rather than incurring an additional round trip with the RDMA Read and its response. 4.54.5. Tagged Buffers as ULP Credits The handling of end-to-end buffer credits differs considerably with DDP than when the ULP directly uses either TCP or SCTP. With both TCP and SCTP buffer credits are based upon the receiver granting transmit permission based on the total number of bytes. These credits reflect system buffering resources and/or simple flow control. They do not represent ULP resources. DDP defines no standard flow control, but presumes the existince of a ULP mechanism. The presumed mechanism is that the Data Sink ULP has issued credits to the Data Source allowing the Data Source to send a specific number of untagged messages. The ULP peers must ensure that the sender is aware of the maximum size that can be sent to any specific target buffer. One method of doing so is to use a standard size for all untagged buffers within a given connection. For example, DAFS specifies an initial size requirement for session establishment, during which the untagged buffer size for the remainder of the session is negotiated. Tagged buffers are ULP resources advertised directly from ULP to ULP. A DDP put to a known tagged buffer is constrained only by transport level flow control, not by available system buffering. Either tagged or untagged buffers allows bypassing of system buffer resources. Use of tagged buffers additionally allows the Data Source to choose what order to exercise the credits in. To the extent allowed by the ULP, tagged buffers are also divisible resources. The Data Sink can advertise a single 100 KB buffer, and then receive notifications from its peer that it had written 50 KB, 20 KB and 30 KB to that buffer in three successive transactions. ULP-management of tagged buffer resources, independent of transport and DDP layer credits, is an additional benefit of RDMA protocols. Large bulk transfers cannot be blocked by limited general purpose buffering capacity. Applications can flow control based upon higher level abstractions, such as number of outstanding requests, independent of the amount of data that must be transferred. However, use of system buffering, as offered by direct use of the underlying transports, can be preferable under certain circumstances. One example would be when the number of target ULP buffers is sufficiently large, and the rate at which any writes arrive is sufficiently low, that pinning all the target ULP buffers in memory would be undesirable. The maximum transfer rate, and hence the maximum amount of system buffering required, may be more stable and predictable than the total ULP buffer exposure. Another would be the Data Sink wishes to receive a stream of data at a predictable rate, but does not know in advance what the size of each data packet will be. This is common from streaming media that has been encoded with a variable bit rate. With DDP the Data Sink would either have to use untagged buffers large enough for the largest packet, or advertise a circular buffer. If for security or other reasons the Data Sink did not want the size of its buffer to be publicly known, using the underlying SCTP transport directly may be preferable because of their byte-oriented credits. 5. RDMA Read RDMA Reads are a further service provided by RDMAP. RDMA Reads allow the Data Sink to fetch exactly the portion of the peer ULP buffer required on a "just in time" basis. This can be done without requiring per-fetch support from the Data Source ULP. Storage servers may wish to limit the maximum write buffer allocated to any single session. The storage server may be a very minimal layer between the client and the disk storage media, or the server may merely wish to limit the total resources that would be required if all clients could push the entire payload they wished written at their own convenience. In either case, there is little benefit in transferring data from the Data Source far in advance of when it will be written to the persistent storage media. RDMA Reads allow the Storage Server to fetch the payload on a "just in time" basis. In this fashion a relatively small number of block sized buffers can be used to execute a single transaction that specified writing a large file, or a Storage Server with numerous clients can fetch buffers from the individual clients in the order that is most convenient to the server. This same capability can be used when the desired portion of the advertised buffer is not known in advance. For example the advertised buffer could contain performance statistics. The data sink could request the portions of the data it required, without requiring an interaction with the Data Source ULP. This is applicable for many applications that publish semi-volatile data that does not require transactional validity checking (i.e., authorized users have read access to the entire set of data). It is less applicable when there are ULP consistency checks that must be performed upon the data. Such applications would be better served by having the client send a request, and having the server use RDMA Writes to publish the requested data. Neither RDMAP or DDP provide mechanisms for bundling multiple disjoint updates into an atomic operation. Therefore use of an advertised buffer as a data resource is subject to the same caveats as any randomly updated data resource, such as flat files, that do not enforce their own cosnsistency. 6. LLP Comparisons Normally the choice of underlying IP transport is irrelevant to the ULP. RDMAP and DDP provides the same services over either. There may be performance impacts of the choice, however. It is the responsibility of the ULP to determine which IP transport is best suited to its needs. SCTP provides for preservation of message boundaries. Each DDP segment will be delivered within a single SCTP packet. The equivalent services are only available with TCP through the use of the MPA adaptation layer. 6.16.1. Multistreaming Implications SCTP also provides multi-streaming. When the same pair of hosts have need for multiple DDP streams this can be a major advantage. A single SCTP association carries multiple DDP streams, consolidating connection setup, congestion control and acknowledgements. Completions are controlled by the DDP Source Sequence Number (DDP-SSN)(DDP- SSN) on a per stream basis. Therefore combining multiple DDP Streams into a single SCTP association cannot result in a dropped packet carrying data for one stream delaying completions on others. 6.26.2. Out of Order Reception Implications The use of unordered Data Chunks with SCTP guarantees that the DDP layer will be able to perform placements when IP datagrams are received out of order. Placement of out-of-order DDP Segments carried over MPA/TCP is not guaranteed, but certainly allowed. The ability of the MPA receiver to process out-of-order DDP Segments may be impaired when TCPalignment of TCP segments and MPA FPDUs is lost. Using SCTP, each DDP Segment is encoded in a single Data Chunk and never spread over multiple IP datagrams. 6.36.3. Header and Marker Overhead MPA and TCP headers together are smaller than the headers used by SCTP and its adaptation layer. However, this advantage can be considerably reduced by the insertion of MPA markers. In any event the different in ULP payload per IP Datagram is not likely to be a signifigant factor. 6.46.4. Middlebox Support Even with the MPA adaptation layer, DDP traffic carried over MPA/TCP will appear to all network middleboxes as a normal TCP connection. In many environmenetsenvironments there may be a requirement to use only TCP connections to satisfy existing network elements and/or to facilitate monitoring and control of connections. While SCTP is certainly just as monitorable and controllable as TCP, there is no guarantee that the network management infrastructure has the required support for both. 6.56.5. Processing Overhead A DDP stream delivered via MPA/TCP will requirerequired more processing effort thanthat one delivered over SCTP. However this extra work may be justified for many deployments where full SCTP support is unavailable in the intermediate network. 6.6endpoints of the network, or where middleboxes impair the usability of SCTP. 6.6. Data Integrity Implications Both the SCTP and MPA/TCP adaptation provide end-to-end CRC32c protection against data corruption, or its equivalent. A ULP that requires a greater degree of protection may add it own. However, DDP and RDMAP headers will only be guaranteed to have the equivalent of end-to-end CRC32c protection. A ULP that requires data integrity checking more thorough than an end-to-end CRC32c should first invalidate all STags that reference a buffer before applying their own integrity check. 188.8.131.52.1. MPA/TCP Specifics It is mandatory for MPA/TCP implementations to implement CRC32c, but it is NOT mandatory to use the CRC32c during an RDMA connection. The activating or deactivating of the CRC in MPA/TCP is an administrative configuration operation at the local and remote end. The administration of the CRC(ON/OFF) is invisible to the ULP. Applications SHOULD trust that this administrative option will only be used when the end-to-end protection is at least as effective as a transport layer CRC32c. Applications SHOULD NOT apply additional protection as a guard against this administrative option being turned on inadvertently. Administrators MUST NOT enable CRC32c suppression unless the end-to-endend-to- end protection is truly equivalent. If the CRC is active/used for one direction/end , then the use of the CRC is mandatory in both directions/ends. If both ends have been configured NOT to use the CRC, then this is allowed as long as an equivalent protection(comparable or better than/to CRC) from undetected errors on the connection is provided. 184.108.40.206.2. SCTP Specifics SCTP provides CRC32c protection automatically. The adaptation to SCTP provides for no option to suppress SCTP CRC32c protection. 6.76.7. Non-IP Transports DDP is defined to operate over ubiquitous IP transports such as SCTP and TCP. This enabled a new DDP-enabled node to be added anywhere to an IP network. No DDP-specific support from middle-boxes is required. There are non-IP transport fabric offering RDMA capabilities. Because these capabilities are integrated with the transport protocol they have some technical advantages when compared to RDMA over IP. For example fencing of RDMA operations can be based upon transport level acks. Because DDP is cleanly layered over an IP transport, any explicit RDMA layer ack must be separate from the transport layer ack. There may be deployments where the benefits of RDMA/transport integration outweigh the benefits of being on an IP network. 220.127.116.11.1. No RDMA Layer Ack DDP does not provide for its own acknowledgements. The only form of ack provided at the RDMAP layer is an RDMA Read Response. DDP and RDMAP rely almost entirely upon other layers for flow control and pacing. The LLP is relied upon to guarantee delivery and avoid network congestion, and ULP level acking is relied upon for ULP pacing and to avoid ULP buffer overruns. Previous RDMA protocols, such as InfiniBand, have been able to use their integration with the transport layer to provide stronger ordering guarantees. It is important that application designers that require such guarantees to provide them through ULP interaction. Specifically: There is no ability for a local interface to "fence" outbound messages to guarantee that prior tagged messages have been placed prior to sending a tagged message. The only guarantees available from the other side would be an RDMA Read Response (coming from the RDMAP layer) or a response from the ULP layer. Remember that the normal ordering rules only guarantee when the Data Sink ULP will be notified of untagged messages, it does not control when data is placed into receive buffers. Re-use of tagged buffers must be done with extreme care. The fact that an untagged message indicates that all prior tagged messages have been placed does not guarantee that no later tagged message have. The best strategy is to only change the state of any given advertised buffers with with untagged messages. As covered elsewhere in this document, flow control of untagged messages MUST be provided by the ULP itself. 6.86.8. Other IP Transports Both TCP and SCTP provide DDP with reliable transport with TCP friendly rate control. As currently DDP is defined to work over reliable transports and implicitly relies upon some form of rate control. DDP is fully compatible with a non-reliable protocol. Out-of-order placement is obviously not dependent on whether the other DDP Segments ever actually arrive. However, RDMAP requires the LLP to provide reliable service. An alternate completion handling protocol would be required if DDP were to be deployed over an unreliable IP transport. As noted in the prior section on tagged buffers as ULP credits, neither RDMAP or DDP provide any flow control for tagged messages. If no transport layer flow control is provided, an RDMAP/DDP application would be only limited by the link layer rate, almost inevitably resulting in severe network congestion. RDMAP encourages applications to be ignorant of the underlying transport PMTU. The ULP is only notified when all messages ending in a single untagged message have completed. The ULP is not aware of the granularity or ordering of the underlying message. This approach assumes that the ULP is only interested in the complete set of messages, and has no use for a subset of them. 6.96.9. LLP Independent Session Establishment For an RDMAP/DDP application, the transport services provided by a pair of SCTP Streams and by a TCP connection both provide the same service (reliable delivery of DDP Segments between two connected RDMAP/DDP endpoints). 18.104.22.168.1. RDMA-only Session Establishment It is also possible to allow for transport neutral establishment of RDMAP/DDP sessions between endpoints. Combined, these two features would allow most applications to be unconcerned as to which LLP was actually in use. Specifically, the procedures for DDP Stream Session establishment discussed in section 3 of the SCTP mapping, and section 13.3 of the MPA/TCP mapping, both allow for the exchange of ULP specific data ("Private Data") before enabling the exchange of DDP Segments. This delays can allow for proper selection and/or configuration of the endpoints based upon the exchanged data. For example, each DDP Stream Session associated with a single client session might be assigned to the same DDP Protection Domain. To be transport neutral, the applications should exchange Private Data as part of session establishment messages to determine how the RDMA endpoints are to be configured. One side must be the Initiator, and the other the Responder. With SCTP, a pair of SCTP streams can be used for sequential sessions. With MPA/TCP each connection can be used for at most one session. However, the same source/destination pair of ports can be re-used sequentially subject to normal TCP rules. Both SCTP limits the Stream Session Initiate control message to a single SCTP Data Chunk, or 516 bytes whichever is greater.and MPA only limitslimit the private data in a MPA Request or Response frame to 65,535 bytes. An IP transport neutral application SHOULD limit itselfsize to 512 bytes or lessa maximum of Private Data in the connection establishment phase. An application that wanted to be InfiniBand compatible as well would need to limit the size even further.512 bytes. MPA/TCP requires the end of the TCP connection that initiated the conversion to MPA mode to send the first DDP Segment. SCTP does not have this requirement. ULPs which wish to be transport neutral should require the initiating end to send the first message. A zero-lengthzero- length RDMA Write can be used for this purpose if the ULP logic itself does naturally support this restriction. 22.214.171.124.2. RDMA-Conditional Session Establishment It is sometimes desirable for the active side of a session to connect with the passive side before knowing whether the passive side supports RDMA. This style of session establishment can be supported with either TCP or SCTP, but not as transparently as for RDMA-only sessions. Pre-existingPre- existing non-RDMA servers are also far more likely to be using TCP than SCTP. With TCP. a normal TCP connection is established. It is then used by the ULP to determine whether or not to convert to MPA mode and use RDMA. This will typically be integral with other session establishment negotiations. With SCTP, the establishment of an association tests whether RDMA is supported. If not supported, the application simply requests the association without the RDMA adaptation indication. In key difference is that with SCTP the determination as to whether the peer can support RDMA is made before the transport layer association/connection is established while with TCP the established connection itself is used to determine whether RDMA is supported. 7. Local Interface Implications Full utilization of DDP and RDMAP capabilities requires a local interface that explicitly requests these services. Protocols such as Sockets Direct Protocol (SDP) can allow applications to keep their traditional byte-stream or message-stream interface and still enjoy many of the benefits of the optimized wire level protocols. 8. Security considerations 8.18.1. Connection/Association Setup Both the SCTP and TCP adaptations allow for existing procedures to be followed for the establishment of the SCTP association or TCP connection. Use of DDP does not impair the use of any security measures to filter, validate and/or log the remote end of an association/connection. 8.28.2. Tagged Buffer Exposure DDP only exposes ULP memory to the extent explicitly allowed by ULP actions. These include posting of receive operations and enabling of Steering Tags. Neither RDMAP or DDP place requirements on how ULP's advertise buffers. A ULP may use a single Steering Tag for multiple buffer advertisements. However, the ULP should be aware that enforcement on STag usage is likely limited to the overall range that is enabled. If the remote peer writes into the 'wrong' advertised buffer, neither the DDP or RDMAP layer will be aware of this. Nor is there any report to the ULP on how the remote peer specifically used tagged buffers. Unless the ULP peers have an adequate basis for mutual trust, the receiving ULP might be well advised to use a distinct STag for each interaction, and to invalidate it after each use or to require its peer to use the RDMAP option to invalidate the STag with its responding untagged message. 8.38.3. Impact of Encrypted Transports While DDP is cleanly layered over the LLP, its maximum benefit may be limited when the LLP Stream is secured with a streaming cypher, such as Transport Layer Security (TLS). If the LLP must decrypt in order, it cannot provide out-of-order DDP Segments to the DDP layer for placement purposes. IPsec tunnel mode encrypts entire IP Datagrams. IPsec transport mode encrypts TCP Segments or SCTP packets. In neither case should IPsec preclude providing out-of-order DDP Segments to the DDP layer for placement. Note that end-to-end use of IPsec cryptographic integrity protection may allow suppression of MPA CRC generation and checking under certain circumstances. This is one example where the LLP may be judged to have "or equivalent" protection to an end-to-end CRC32c. 99. References  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999.  Kent, S. and R. Atkinson, "IP Encapsulating Security Payload (ESP)", RFC 2406, November 1998.  Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Zhang, L.L., and V. Paxson, "Stream Control Transmission Protocol", RFC 2960, October 2000.  Coene, L., "Stream Control Transmission Protocol Applicability Statement", RFC 3257, April 2002.  Recio, R., "An RDMA Protocol Specification", draft-ietf-rddp-rdmap-02draft-ietf-rddp-rdmap-05 (work in progress), September 2004.July 2005.  Shah, H., "Direct Data Placement over Reliable Transports", draft-ietf-rddp-ddp-03draft-ietf-rddp-ddp-05 (work in progress), August 2004.July 2005.  Stewart, R., "Stream Control Transmission Protocol (SCTP) Remote Direct Memory Access (RDMA) Direct Data Placement (DDP) Adaptationn", draft-ietf-rddp-sctp-02 (work in progress), August 2004.2005.  Culley, P., "Marker PDU Aligned Framing for TCP Specification", draft-ietf-rddp-mpa-01draft-ietf-rddp-mpa-02 (work in progress), July 2004.February 2005.  "Direct Access File System versino 1.0", September 2001.  Pinkerton, J., "Sockets Direct Protocol (SDP) for iWARP over TCP 1.0", October 2003. Authors' Addresses Caitlin Bestler 1241 W. 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