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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] F2F Mtg Summary for Framing and Error RecoveryHere is a quick summary of the outcome from the June 27-28 Design Teams Face-to-Face meeting in Palo Alto in the two focal areas: Framing and Error Recovery. The bulk of the meeting was spent discussing framing scenarios, requirements, and alternatives. As soon as the slide decks make it onto Julian's web site, I'll email out that info. Regards, Jim Wendt Networked Storage Architecture / NSSO Hewlett-Packard Company jim_wendt@hp.com 916-785-5198 --------------------------------------------------------------- F2F Meeting Summary for Framing and Error Recovery Design Teams Face-to-Face Meeting / June 27-28 Palo Alto ---------------- Framing: The Bottom Line ---------------------- To cut to the chase, the following rough proposal was generated for handling ULP Framing for iSCSI: A) Proposed changes to "ULP Framing for TCP" I-D are: 1) Modify I-D to include two framing modes: - "Marker mode" for unmodified TCP stacks - "PDU-alignment mode" for modified TCP stacks 2) ULP is responsible for negotiating use of framing protocol and enabling framing behavior on the TCP connection in an unambiguous manner 3) The framing protocol usage, framing mode, and framing operational parameters are negotiated separately in each direction on a TCP connection. Thus there are "Senders" and "Receivers" on a framing TCP connection. An iSCSI Initiator or Target is both a Sender and a Receiver with respect to an framing TCP connection. 4) ULP is responsible for negotiating use of a specific framing mode over the TCP connection by having the receiver request highest framing mode desired from sender (first PDU-alignment, then Marker, then none) and having the sender comply: - if receiver requests, and sender supports, PDU-alignment mode, then sender MUST enable PDU-alignment mode - else if receiver requests, and sender supports, Marker mode, then sender MUST enable Marker mode - else don't use framing protocol 5) ULP is responsible for negotiating framing operational parameters: - Marker period (in Marker mode) - Receiver maximum PDU size (in Marker mode) - Framing keys (in PDU-alignment mode) - ULP packing behavior (in PDU-alignment mode) 6) Change the marker fields to be 16-bits rather than 32-bits (and refer to as "offsets" rather than "pointers") *) An updated version of the "ULP Framing for TCP I-D" reflecting these changes has been posted (7/9/01) to TSVWG for comments (draft-ietf-tsvwg-tcp-ulp-frame-00) B) Proposed changes to iSCSI spec are: 1) Remove Markers appendix from iSCSI spec (Appendix D. Synch and Steering with Fixed Interval Markers) 2) iSCSI spec adds wording to the effect of: * iSCSI initiator and target framing behavior over a TCP connection is defined in draft-ietf-tsvwg-tcp-ulp-frame-00 (or eventual RFC#) * an iSCSI initiator or target is both a sender and receiver with respect to framing behavior * an iSCSI framing sender MUST implement Marker mode, and MAY implement PDU-alignment mode, as defined in <I-D> * an iSCSI framing receiver MAY implement PDU-alignment mode, or Marker mode, or both, or none as defined in <I-D> * an iSCSI receiver on a framing TCP connection dictates use of the highest framing mode desired from sender as follows: - if receiver requests, and sender supports, PDU-alignment mode, then sender MUST enable PDU-alignment mode - else if receiver requests, and sender supports, Marker mode, then sender MUST enable Marker mode - else framing behavior is disabled * Perhaps there is some description of probable framing scenarios capturing the most likely combinations of the following attributes: - initiator or target - software implementation or hardware implementation - unmodified or modified TCP stack - sender AND receiver framing behaviors (no framing, or Marker mode, or PDU-Alignment mode) - values for framing operational parameters 3) > Still need to determine iSCSI mechanism for turning on Framing protocol Marker mode operation 4) > Still need to determine iSCSI mechanism for negotiating framing operational parameters: - Framing mode (if both Marker and PDU Alignment mode are supported) - Marker period (in Marker mode) - Receiver maximum PDU size (in Marker mode) - Framing keys (in PDU-alignment mode, if supported) - ULP packing behavior (in PDU-alignment mode, if supported) ----------- The reasoning for these proposed changes is as follows: 1) Re: Merge Marker mode into "ULP Framing for TCP" I-D a) The TCP-related framing work already has mindshare in TSVWG and this work is embodied in the current framing I-D. Rather than dilute the framing effort with additional I-Ds, all framing related work should be collected into a modified version of the existing framing I-D. b) Other ULPs may also find Marker mode useful in software-only unmodified-TCP client scenarios c) The framing I-D appears to be a reasonable literary vehicle for documenting the collection of framing schemes. The I-D could be extended in the future to include a byte or word stuffing frame marker method such as COBS. d) A single framing I-D may help to encourage a single consistent interface with the ULP regardless of which framing mode is employed. e) The iSCSI spec can simply reference the one framing I-D. 2) Re: Make Marker mode mandatory for all iSCSI implementations, and PDU-Alignment mode optional for all iSCSI implementations. a) This allows interoperation of software-only, UNMODIFIED-TCP-stack clients with hardware-accelerated, small-buffer-memory storage arrays. This applies to both 1Gbps-client/1Gbps-array and 1Gbps-client/10Gbps-array scenarios. b) One potentially compelling application for iSCSI involves software-only implementations on mainstream desktops and laptops operating over unmodified TCP stacks to access centralized storage arrays. c) Software implementations are likely to exist far into the future. Individual software-only clients may not operate at 10Gbps, but will be combined together with other clients that aggregate to 10Gbps. d) The only framing mechanisms that can operate completely above a client TCP and not require any modification to the client's standard TCP stack are the interval-based (Marker mode, periodic PDU alignment, fixed length PDU) and byte-stuffing (COBS) framing schemes. All other framing mechanisms (including PDU-Alignment mode) require modification to the client's TCP stack. e) The processing overhead for a client software implementation to insert Markers is small compared to the processing overhead of a byte-stuffing scheme. f) Receivers are allowed to dictate the sender's framing behavior because it is the receiver that is impacted by the presence or absence of framing behavior on the connection. g) Hardware-accelerated receivers can be implemented with minimal buffer memory, meaning that they always rely on framing-based direct data placement processing, only if it is known in advance that every client the receiver could potentially interoperate with is capable of providing the necessary framing-based behavior. These hardware-accelerated receivers will request, and expect that, the sender insert markers (or PDU-Alignment if supported). h) Since a software-implemented receiver may incur extra data movements in processing markers, these receivers can request, and expect that, a sender NOT insert markers, if desired. i) Marker mode doesn't completely eliminate the need for buffer memory on the receiver. The receiver still needs to use "eddy buffers" that temporarily hold incoming data after a dropped segment containing a ULP header up until the next ULP header is located in the packet stream, and which exist for as long as the original ULP header segment is outstanding. But Marker mode does greatly reduce the amount of memory needed as compared to a traditional TCP receiver's reassembly memory requirements (often equal to number-of-connections X round-trip-pipe-size). The Marker mode small memory requirements are dependent upon the period of the marker, and the size of the ULP PDUs being restricted to a reasonably small value. The larger that either one is, the larger the eddy buffer memory requirements. Also, an eddy buffer is required each time a ULP header is dropped, so that multiple ULP header drops in close proximity may cause multiple eddy buffers to be temporarily pending on a connection. j) The PDU-alignment framing mode is preferred. However, it may be several years before all of the different software TCP/IP implementations will be able to support framing behavior. ----------- Open Issues: 1) Acceptability of the PDU-Alignment framing mode's reliance on "key+length" matching across resegmenting middleboxes - In PDU-Alignment mode each TCP segment payload contains one complete framing PDU (consisting of an 8 byte framing header followed by one or more complete ULP PDUs). Thus, every TCP segment has the TCP header followed immediately by the framing header. - In certain cases a single framing PDU must be broken across multiple TCP segments (such as dynamic Path MTU reductions), resulting in TCP segments where a framing header doesn't immediately follow the TCP header. - The framing I-D defines sender behaviors that allow PDU-alignment mode to function deterministically and correctly in all cases where the TCP segmentation flowing from sender to receiver is not altered. - If the TCP segmentation from sender to receiver is altered by an intermediary (resegmenting middlebox), and a framing-header-containing segment drop or reordering has occurred such that the receiver is attempting to locate the next framing header in the segment stream, then the receiver must examine the first 8 bytes of each incoming TCP segment payload for a valid framing header containing valid Key(6B) and Length(2B) fields. - A false-positive occurs if, upon resegmentation by a middlebox, the receiver gets a TCP segment in which the first 8 bytes of the payload indicate a valid framing header (the first 6 bytes match the previously exchanged random key value, and the next 2 bytes contain a valid length), yet the TCP segment payload isn't actually a framing header. - While it is felt that the probability of a false-positive in these resegmenting-middlebox scenarios will be sufficiently low, further analysis work may be may be required in this area. - Note that this mechanism is NOT a scanning technique for locating start-of-frame across an arbitrary byte stream. It only provides an indication of PDU alignment or not. The first 8 bytes of the TCP segment payload are examined to determine if the segment contains the start of a ULP PDU. 2) None of the current framing schemes take TCP data integrity into account. It either needs to be decided: a) how to detect when a data integrity problem occurs within a framing header, and what to do about it (even if it just kills the TCP connection), b) or that a sufficient level of data integrity needs to be provided for all protocols running over TCP via a more holistic approach. 3) Do Markers work at 10Gbps - The feasibility of markers at 10Gbps has been questioned. It would be beneficial to hear specifics regarding why Markers won't work at 10Gbps. Markers don't allow for a no-memory direct data placement NIC since eddy-buffers are required. So, support for clients with unmodified TCP stacks comes at a cost, which is the cost of supporting eddy buffers on the NIC. - One question is whether the eddy buffers can be contained entirely in the ASIC or need to be in off-chip memory. ---------------- Error Recovery: The Bottom Line ---------------------- 1) Information was presented regarding estimated iSCSI header and data digest error rates, and possible approaches to iSCSI error recovery. The error rates info is summarized as follows: a) "Good Internet" - 1500 byte MTU / 8192 byte iSCSI PDU - TCP checksum mismatch 1 in 90,000 - Checksum escape 1 in 135M to 1 in 10B - For bandwidth of 30Mbps @ 100msec RTT > 8 to 600 digest errors per year > 1 header digest every 2 months to 10 years - For bandwidth of 300Mbps @ 10msec RTT > 80 to 6,000 digest errors per year > 1 to 70 header digest errors per year b) "Bad Internet" - 1500 byte MTU / 8192 byte iSCSI PDU - TCP checksum mismatch 1 in 11,000 - Checksum escape 1 in 16M to 1 in 1B - For bandwidth of 10Mbps @ 100msec RTT > .5 to 33 digest errors per week > 26 to 1,650 digest errors per year > 0.3 to 20 header digest errors per year - For bandwidth of 100Mbps @ 10msec RTT > 5 to 335 digest errors per week > 260 to 16,500 digest errors per year > 3 to 200 header digest errors per year c) 1Gbps iSCSI connection - assuming current TCP yields 70% bandwidth utilization - at 100msec > packet loss less than 1 in 50M > .5 to 40 digest errors per year > 1 header digest error every 2..100 years - at 10msec > packet loss less than 1 in 500,000 > 60 to 4,000 digest errors per year > 1 to 40 header digest errors per year d) 10Gbps iSCSI connection - assuming current TCP yields 70% bandwidth utilization - at 100msec > packet loss less than 1 in 5 billion > 1 digest error every 3 mo to 10 years > 1 header digest error every 20 to 1000 years - at 10msec > packet loss less than 1 in 50M > 6 to 400 digest errors per year > 1 header digest error every 3 mo to 12 years e) The frequency of framing header corruption escaping the TCP checksum mechanism is on the order of the frequency of the iSCSI header escaping, but depends on the mechanism used as well as the MSS and iSCSI PDU sizes: - Markers (at 2k intervals) - 1/3 as likely as iSCSI headers. - Framing (w/o chunking) - 1.5 times as likely - Framing (w/ chunks) - 1/2 as likely as iSCSI headers. These assumed an 8k iSCSI PDU size, except for Framing (w/o chunking), which assumed a 1k iSCSI PDU to fit in a single segment. All schemes had a framing header size of 8 bytes, and assumed an MSS of 1460. 2) No definitive conclusions were reached during the F2F in regards to Error Recovery mechanisms. - Further work needs to be done in this area. - Mallikarjun Chadalapaka, Mark Bakke, and others can help move the work forward in this area. 2) It would be valuable to collect information regarding TCP checksum mismatch rates on production systems. If anyone has access to fairly busy systems and can collect the following information, you can forward it to Mark Bakke (mbakke@cisco.com). You'll want to collect three data items: a) sysUpTime b) tcpInSegs - total number of inbound TCP segments c) tcpInErrs - total number of inbound TCP segments with errors (most likely checksum mismatches, but some implementations may count other error discards here as well) ---------------- Slide Decks ------------------------------------ Sorry, but these materials aren't on the web yet. Hopefully they will be in the next week or two. I'll email when they are available on a server somewhere. * "iSCSI Framing Presentation" - slides/spreadsheet - Matt Wakeley * "TCP Framing Discussion" - slides - Jim Wendt * "Recovering From iSCSI Digest Errors" - slides - Mark Bakke * "Expected iSCSI digest error rates on Internet connections" - spreadsheet - Mark Bakke * CRC and checksum performance - slides - Jonathan Stone ---------------- Framing Discussion Summary ---------------------- /iSCSI usage scenarios A wide variance of usage scenarios were strongly represented: * High-speed short-distance storage LANs * High-speed long-distance storage WANs * Multitudes of low-end clients using software iSCSI client implementations and unmodified software TCP stacks * Multiple first-generation 1Gbps clients aggregating to next-generation 10Gbps storage arrays * A variety of IP networks and paths with potential for both TCP-level resegmenting middleboxes and dynamic changes in Path MTU. /Memory-based solutions * It was felt that 1Gbps memory-based solutions are feasible and may be cost-competitive (e.g. there is no usage of direct data placement nor framing mechanisms in this case) * There were different opinions regarding whether 10Gbps memory-based solutions would be cost-competitive or feasible for 10Gbps. * There was discussion regarding the comparative cost of memory-based and no-memory iSCSI HBAs and infrastructure relative to Fibre Channel * There were concerns regarding next-generation 10Gbps storage arrays that want to support first-generation clients. The next-generation 10Gbps storage arrays can only implement no-memory solutions if the first-generation clients were mandated to implement support for framing (thus making direct data placement possible on the storage array). * Hybrid schemes were discussed where a next-generation 10Gbps storage array would contain a moderate amount of memory to handle non-framing first-generation clients while using full framing and direct data placement with no memory buffers for 10Gbps clients * Matt Wakeley has created a spreadsheet for high-speed memory subsystem costs /Direct data placement alternatives * Discussion of various levels at which direct data placement information can ride: - Above TCP (iSCSI task tags, RDMA protocol) - At transport (TCP RDMA option) - Below transport (TAF) /iSCSI layering scenarios and evolution * iSCSI can be layered: - over normal TCP - over Markers over normal TCP - over Framing TCP - over RDMA+chunking over Framing TCP * Layering iSCSI over RDMA+chunking doesn't seem likely for first-generation iSCSI implementations /Framing alternatives * Framing mechanism classes: - Intervalic (Periodic Markers, Periodically aligned headers, Fixed size ULP PDUs) - Framing aware TCP (ala ULP Framing over TCP I-D) - TCP message boundary indications (Reserved bit, TCP option, URG pointer, PSH bit, etc) - Byte stuffing (COBS, 7B/8B, etc) * Framing mechanism characteristics: - sender TCP modifications required? - receiver memory requirements (full TCP receive window, eddy buffers, IP reassembly buffers) - level of TCP changes (none, behavioral, header fields) - support ULP PDU > TCP MSS - software processing overhead - hardware implementation complexity - handle dynamic Path MTU changes - handle resegmenting TCP middlebox - require [dynamic] chunking above TCP - emit short segments more often than typical - added protocol bytes overhead - tied to TCP sequence number processing - increase probability of segment drops - TCP aesthetics /Markers and ULP Framing merge * Proposal to merge Marker mechanism into current "ULP Framing for TCP I-D" and have iSCSI mandate implementation of Marker mode * See "Framing: The Bottom Line" section above ---------------- Error Recovery Discussion Summary --------------- /Mark Bakke slides and spreadsheet * Mark presented slides and a spreadsheet discussing: - expected iSCSI header and data digest error rates given link bandwidth, RTT, probability of segment drop, and probability of TCP checksum escape - recommended iSCSI error handling approaches for Header digest and data digest errors - <slides link> - <spreadsheet link> /Discussion regarding iSCSI error recovery complexity * It was felt that 90% of the recovery complexity already exists for the sake of session recovery (aftet a TCP connection failure) and if only "within-command" recovery was eliminated, it wouldn't substantially simplify the protocol or its specification. However, this assertion needs to be validated. * It was felt that complete command recovery would probably be a dequate for the expected error incidence (not a noticable impact), but it hasn't been shown how adopting this approach would reduce complexity. /Discussion re: IPSec SA's * There was some discussion regarding use of IPSec and concerns that the set of Security Associations would not fit into on-chip memory, forcing the SAs to be cached in off-chip memory. /Jonathan Stone slides * Jonathan presented data analysis from his soon-to-be-completed dissertation regarding the nature of empirically-observed transport-level errors, and the error detection performance of CRC and checksum algorithms on such. ---------------- List of Attendees ----------------------- Mark Bakke, Stephen Bailey, Uri Elzur, Somesh Gupta, Randy Haagens, John Hufferd, Jim Pinkerton, Venkat Rangan, Allyn Romanow, Costa Sapuntzakis, Julian Satran, Jonathan Stone, Matt Wakeley, Jim Wendt, Jim Williams --------------------------------------------------------------
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