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   INTERNET-DRAFT                            P. Culley
   draft-culley-iwarp-mpa-01.txt               Hewlett-Packard Company
                                             U. Elzur
                                               Broadcom Corporation
                                             R. Recio
                                               IBM Corpration
                                             S. Bailey
                                               Sandburst Corporation
                                             et. al.

                                             Expires: April 2003


             Marker PDU Aligned Framing for TCP Specification

1  Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 10 of RFC2026.

   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 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/1id-abstracts.html The list of Internet-Draft
   Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html



2  Abstract

   A framing protocol is defined for TCP that is fully compliant with
   applicable TCP RFCs and fully interoperable with existing TCP
   implementations. The framing mechanism is designed to work as a
   "shim" between TCP and higher-level protocols, preserving the
   reliable, in-order delivery of TCP while adding the preservation of
   higher-level protocol record boundaries.









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   Table of Contents

   1     Status of this Memo.........................................1
   2     Abstract....................................................1
   3     Introduction................................................3
   3.1   Motivation..................................................3
   3.2   Protocol Overview...........................................4
   4     Glossary....................................................6
   5     LLP and ULP requirements....................................7
   5.1   TCP implementation Requirements to support MPA..............7
   5.2   MPA's interactions with the ULP.............................8
   6     FPDU Formats...............................................10
   6.1   Marker Format..............................................11
   7     Data Transfer Semantics....................................12
   7.1   MPA Markers................................................12
   7.2   CRC Calculation............................................13
   7.3   MPA on TCP Sender Segmentation.............................16
   7.3.1 FPDU Size Considerations...................................16
   7.4   MPA Receiver FPDU Identification...........................17
   7.4.1 Re-segmenting Middle boxes and non-conforming senders......18
   8     Connection Semantics.......................................19
   8.1   Connection setup...........................................19
   8.2   Normal Connection Teardown.................................19
   9     Error Semantics............................................20
   10    Security Considerations....................................21
   10.1  Protocol-specific Security Considerations..................21
   10.2  Using IPSec With MPA.......................................21
   11    IANA Considerations........................................22
   12    References.................................................23
   12.1  Normative References.......................................23
   12.2  Informative References.....................................23
   13    Appendix...................................................25
   13.1  Receiver implementation....................................25
   13.1.1  Transport & Network Layer Reassembly Buffers..............25
   14    Author's Addresses.........................................28
   15    Acknowledgments............................................29
   16    Full Copyright Statement...................................32


   Table of Figures

   Figure 1 ULP MPA TCP Layering......................................4
   Figure 2 FPDU Format..............................................10
   Figure 3 Marker Format............................................11
   Figure 4 Example FPDU Format with Marker..........................13
   Figure 5 Annotated Hex Dump of an FPDU............................15
   Figure 6 Annotated Hex Dump of an FPDU with Marker................15






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3  Introduction

   This section discusses the reason for creating MPA on TCP and a
   general overview of the protocol.  Later sections show the MPA
   headers (see section 6 on page 10), and detailed protocol
   requirements and characteristics (see section 7 on page 12), as well
   as Connection Semantics (section 8 on page 19), Error Semantics
   (section 9 on page 20), and Security Considerations (section 10 on
   page 21).

3.1  Motivation

   A generalized framing mechanism for the TCP transport protocol [TCP]
   is desirable to some Upper Layer Protocols (ULP).  One ULP that can
   benefit from the framing mechanism is Direct Data Placement (DDP).
   The ability to locate the Upper Layer Protocol Data Unit (ULPDU)
   boundary is useful to a hardware network adapter that uses DDP to
   directly place the data in the application buffer based on the
   control information carried in the ULPDU header.  This may be done
   without requiring that the packets arrive in order.  One potential
   benefit of this capability is the avoidance of the memory copy
   overhead.  Another potential benefit is the smaller memory
   requirement for handling out of order packets and dropped packets.

   MPA is intended for ULPs that are specifically designed to utilize
   "records" (ULPDUSs) rather than a stream of octets.

   Many approaches have been proposed for the generalized framing
   mechanism.  Some are probabilistic in nature and others are
   deterministic.  A probabilistic approach is characterized by a
   detectable value embedded in the byte stream.  It is probabilistic
   because under some conditions the receiver may incorrectly interpret
   application data as the detectable value.  Under these conditions,
   the protocol may fail with unacceptable frequency.  A deterministic
   approach is characterized by embedded controls at known locations in
   the byte stream.  Because the receiver can guarantee it will only
   examine the data stream at locations that are known to contain the
   embedded control, the protocol can never misinterpret application
   data as being embedded control data.  For unambiguous handling of an
   out of order packet, the deterministic approach is preferred.

   The MPA protocol provides a generalized framing mechanism for TCP
   using the deterministic approach.  It allows the location of the
   ULPDU to be determined in the TCP stream even if the TCP segments
   arrive out of order.








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3.2  Protocol Overview

   MPA is described as a extra layer above TCP and below the ULP.  The
   end-to-end data flow is:

   1.  The ULP negotiates the use of MPA at both ends of a connection.

   2.  The ULP hands data records (ULPDUs) to MPA at the sender.

   3.  MPA creates a Framed Protocol Data Unit (FPDU) by pre-pending a
       header, inserting markers, and appending a CRC after the ULPDU
       and PAD (if any).  MPA delivers the FPDU to TCP.

   4.  The MPA-aware TCP sender puts the FPDUs into the TCP stream.  It
       segments the TCP stream in such a way that each TCP segment
       contains a single FPDU.  TCP then passes each segment to the IP
       layer for transmission.

   5.  The TCP receiver may be MPA-aware or may not be MPA-aware. If it
       is MPA-aware, it may separate passing the TCP payload to MPA
       from passing the TCP payload ordering information to MPA. In
       either case, RFC compliant TCP wire behavior is observed at both
       the sender and receiver.

   6.  The MPA receiver locates and assembles complete FPDUs within the
       stream, verifies their integrity, and removes MPA markers,
       ULPDU_Length, PAD and CRC.

   7.  MPA then provides the complete ULPDUs to the ULP.  MPA may also
       separate passing MPA payload to the ULP from passing the MPA
       payload ordering information.

   The layering of PDUs with MPA is shown in Figure 1.

               +------------------+
               |     ULP client   |
               +------------------+  <- ULPDUs
               |        MPA       |
               +------------------+  <- FPDUs (containing ULPDUs)
               |        TCP*      |
               +------------------+  <- TCP Segments (containing FPDUs)
               |      IP etc.     |
               +------------------+
                                      * TCP or MPA-aware TCP.

                       Figure 1 ULP MPA TCP Layering







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   MPA-aware TCP is a TCP layer which potentially contains some
   additional semantics as defined in this document.  MPA is
   implemented as a data stream ULP for TCP and is therefore RFC
   compliant.  MPA-aware TCP is RFC compliant.

   MPA with an MPA-aware TCP allows an implementation to recover ULPDUs
   that may be received out of order.  This enables an implementation
   with an appropriate ULP at the receiver to save a significant amount
   of intermediate storage by storing the ULPDUs in the right locations
   in the ULP buffers when they arrive, rather than waiting until full
   ordering can be restored.

   MPA implementations that support recovery of out of order ULPDUs
   should also support a mechanism to indicate the ordering of ULPDUs
   as the sender transmitted them and indicate when missing
   intermediate segments arrive.  These mechanisms allow ULPs to
   reestablish record ordering and report arrival of complete groups of
   records.

   One last area that MPA addresses is data integrity.  Many users of
   TCP have noted that the TCP checksum is not as strong as could be
   desired [CRCTCP].  Studies have shown that the TCP checksum
   indicates segments in error at a much higher rate than the
   underlying link characteristics would indicate.  With these higher
   error rates, the chance that an error will escape detection, when
   using only the TCP checksum for data integrity, becomes a concern.
   A stronger integrity check can reduce the chance of data errors
   being missed.

   MPA includes a CRC check to increase the ULPDU data integrity to the
   level provided by other modern protocols, such as SCTP [SCTP].

   MPA combined with an MPA-aware TCP can only ensure FPDU Alignment
   with the TCP Header if the FPDU is less than or equal to TCP's EMSS.
   Thus if FPDU alignment is desired by the ULP, the ULP must cooperate
   with MPA to ensure FPDUs lengths do not exceed the EMSS under normal
   conditions.
















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4  Glossary

   Delivery - (Delivered, Delivers) - For MPA, Delivery is defined as
       the process of informing the ULP or consumer that a particular
       PDU is ordered for use.  This is specifically different from
       "passing the PDU to the ULP", which may generally occur in any
       order, while the order of "Delivery" is strictly defined.

   EMSS - Effective Maximum Segment Size.  EMSS is the smaller of the
       TCP maximum segment size (MSS) as defined in RFC 793 [TCP], and
       the current path Maximum Transfer Unit (MTU) [PathMTU].

   FPDU - Framing Protocol Data Unit.  The unit of data created by an
       MPA sender.

   FPDU Alignment - the property that a TCP segment begins with an
       FPDU.

   PDU - protocol data unit

   MPA - Marker-based ULP PDU Aligned Framing for TCP protocol.   This
       document defines the MPA protocol.

   MULPDU - Maximum ULPDU. The current maximum size of the record that
       is acceptable for the ULP to pass to MPA for transmission.

   Node - A computing device attached to one or more links of a
       Network. A Node in this context does not refer to a specific
       application or protocol instantiation running on the computer. A
       Node may consist of one or more MPA on TCP devices installed in
       a host computer.

   Remote Peer - The MPA protocol implementation on the opposite end of
       the connection. Used to refer to the remote entity when
       describing protocol exchanges or other interactions between two
       Nodes.

   ULP - Upper Layer Protocol. The protocol layer above the protocol
       layer currently being referenced. The ULP for MPA is expected to
       be DDP [DDP], or an OS, application, adaptation layer, or
       proprietary protocol.  This document does not specify a ULP - it
       provides a set of semantics that allow a ULP to be designed to
       utilize MPA.

   ULPDU - Upper Layer Protocol Data Unit.  The data record defined by
   the layer above MPA.







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5  LLP and ULP requirements

5.1  TCP implementation Requirements to support MPA

   To provide optimum performance, a transmit side TCP implementation
   SHOULD:

   *   With an EMSS large enough to contain the FPDU, segment the
       outgoing TCP stream such that the first octet of every FPDU is
       aligned with the beginning of a TCP segment, and is entirely
       contained in the TCP segment.

   *   Report the current EMSS to the MPA transmit layer.

   When an MPA implementation supports handling out of order ULPDUs,
   the receive side TCP implementation SHOULD:

   *   Pass incoming TCP segments to MPA as soon as they have been
       received and validated, even if not received in order.  The TCP
       layer MUST have committed to keeping each segment before it can
       be passed to the MPA.  This means that the segment must have
       passed the TCP, IP, and lower layer data integrity validation
       (i.e., checksum), must be in the receive window, must not be a
       duplicate, must be part of the same epoch (if timestamps are
       used to verify this) and any other checks required by TCP RFCs.
       The segment MUST NOT be passed to MPA more than once unless
       explicitly requested (see Section 9).

       This is not to imply that the data must be completely ordered
       before use.  An implementation may accept out of order segments,
       SACK them [RFC2018], and pass them to the ULP when the reception
       of the segments needed to fill in the gaps arrive.  Such an
       implementation can "commit" to the data early on, and will not
       overwrite it even if (or when) duplicate data arrives.  MPA
       expects to utilize this "commit" to allow the passing of ULPDUs
       to the ULP when they arrive, independent of ordering.

   *   Provide a mechanism to indicate the ordering of TCP segments as
       the sender transmitted them.  One possible mechanism might be
       attaching the TCP sequence number to each segment.

   *   Provide a mechanism to indicate when a given TCP segment (and
       the prior TCP stream) is complete.  One possible mechanism might
       be to utilize the leading (left) edge of the TCP Receive Window.

   MPA on TCP implementations that do not provide the semantics listed
   above will interoperate with those that do, but may negate many of
   the performance and resource advantages that ULPs designed for MPA
   would expect.




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   The LLP MUST inform MPA when the LLP connection is closed or has
   begun closing the connection (e.g. received a FIN).

5.2  MPA's interactions with the ULP

   ULPs require MPA to maintain ULP record boundaries from the sender
   to the receiver.  When using MPA on TCP to send data, the ULP
   provides records (ULPDUs) to MPA.  MPA will use the reliable
   transmission abilities of TCP to transmit the data, and will insert
   appropriate additional information into the TCP stream to allow the
   MPA receiver to locate the record boundary information.

   As such, MPA accepts complete records (ULPDUs) from the ULP at the
   sender and returns them to the ULP at the receiver.

   MPA provides information to the ULP on the current maximum size of
   the record that is acceptable to send (MULPDU).  The ULP SHOULD be
   able to limit each record size to MULPDU.  The range of MULPDU
   values MUST be between 128 octets and 64768 octets, inclusive.

   The sending ULP MUST NOT post a ULPDU larger than 64768 octets to
   MPA. The ULP MAY post a ULPDU of any size between one and 64768
   octets, however MPA is NOT REQUIRED to support a ULPDU length that
   is greater than the current MULPDU.

   While the maximum theoretical length supported by the MPA header
   ULPDU_Length field is 65535, TCP over IP requires the IP datagram
   maximum length to be 65535 octets. To enable MPA to support FPDU
   Alignment, the maximum size of the ULP payload must fit within an IP
   datagram. Thus the ULPDU limit of 64768 octets was derived by taking
   the maximum IP datagram length, subtracting from it the maximum
   total length of the sum of the IPv4 header, TCP header, IPv4
   options, TCP options, and the worst case MPA overhead, and then
   rounding the result down to a 128 byte boundary.

   On receive, MPA MUST pass each ULPDU with its length to the ULP when
   it has been validated.
















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   If an MPA implementation supports passing out of order ULPDUs to the
   ULP, the MPA implementation SHOULD:

   *   Pass each ULPDU with its length to the ULP as soon as it has
       been fully received and validated.

   *   Provide a mechanism to indicate the ordering of ULPDUs as the
       sender transmitted them.  One possible mechanism might be
       providing the TCP sequence number for each ULPDU.

   *   Provide a mechanism to indicate when a given ULPDU (and prior
       ULPDUs) are complete.  One possible mechanism might be to allow
       the ULP to see the current outgoing TCP Ack sequence number.

   *   Provide an indication to the ULP that the LLP has closed or has
       begun to close the connection (e.g. received a FIN).





































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6  FPDU Formats

   MPA senders create FPDUs out of ULPDUs.  The format of an FPDU shown
   below MUST be used for all MPA FPDUs.  For purposes of clarity,
   markers are not shown in Figure 2.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          ULPDU_Length         |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      |                                                               |
      ~                                                               ~
      ~                            ULPDU                              ~
      |                                                               |
      |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               |          PAD (0-3 octets)     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             CRC                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                           Figure 2 FPDU Format

   ULPDU_Length: 16 bits (unsigned integer).  This is the number of
   octets of the contained ULPDU.  It does not include the length of
   the FPDU header itself, the pad, the CRC, or of any markers that
   fall within the ULPDU. The 16-bit ULPDU Length field is large enough
   to support the largest IP datagrams for IPv4 or IPv6.

   PAD: The PAD field trails the ULPDU and contains between zero and
   three octets of data.  The pad data MUST be set to zero by the
   sender and ignored by the receiver (except for CRC checking).  The
   length of the pad is set so as to make the size of the FPDU an
   integral multiple of four.

   CRC: 32 bits, this CRC is used to verify the entire contents of the
   FPDU, using CRC32c.

   The FPDU adds a minimum of 6 octets to the length of the ULPDU.  In
   addition, the total length of the FPDU will include the length of
   any markers and from 0 to 3 pad bytes added to round-up the ULPDU
   size.












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6.1  Marker Format

   The format of a marker MUST be as specified in Figure 3:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           RESERVED            |            FPDUPTR            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          Figure 3 Marker Format

   RESERVED: The Reserved field MUST be set to zero on transmit and
   ignored on receive (except for CRC calculation).

   FPDUPTR: The FPDU Pointer is a relative pointer, 16-bits long,
   interpreted as an unsigned integer, that indicates the number of
   octets in the TCP stream from the beginning of the FPDU to the first
   octet of the entire marker.



































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7  Data Transfer Semantics

   This section discusses some characteristics and behavior of the MPA
   protocol as well as implications of that protocol.

7.1  MPA Markers

   MPA senders MUST insert a marker into the data stream at a 512 octet
   periodic interval in the TCP Sequence Number Space. The marker
   contains a 16 bit unsigned integer referred to as the FPDUPTR (FPDU
   Pointer).

   If the FPDUPTR's value is non-zero, the FPDU Pointer is a 16 bit
   relative back-pointer. FPDUPTR MUST contain the number of octets in
   the TCP stream from the beginning of the current FPDU to the first
   octet of the marker, unless the marker falls between FPDUs. Thus the
   location of the first byte of the previous FPDU header can be
   determined by subtracting the value of the given marker from the
   current byte-stream sequence number (e.g. TCP sequence number) of
   the first byte of the marker. Note that this computation must take
   into account that the TCP sequence number could have wrapped between
   the marker and the header.

   An FPDUPTR value of 0x0000 is a special case - it is used when the
   marker falls exactly between FPDUs.  In this case, the marker MUST
   be placed in the following FPDU and viewed as being part of that
   FPDU (e.g. for CRC calculation). Thus an FPDUPTR value of 0x0000
   means that immediately following the marker is an FPDU header.

   Since all FPDUs are integral multiples of 4 octets, the bottom two
   bits of the FPDUPTR as calculated by the sender are zero.  MPA
   reserves these bits so they MUST be treated as zero for computation
   at the receiver.

   The MPA markers MUST be inserted immediately following MPA
   connection establishment, and at every 512th octet of the TCP byte
   stream thereafter.  As a result, the first marker has an FPDUPTR
   value of 0x0000.  If the first marker begins at byte sequence number
   SeqStart, then markers are inserted such that the first byte of the
   marker is at byte sequence number SeqNum if the remainder of (SeqNum
   - SeqStart) mod 512 is zero.  Note that SeqNum can wrap.

   For example, if the TCP sequence number were used to calculate the
   insertion point of the marker, the starting TCP sequence number is
   unlikely to be zero, and 512 octet multiples are unlikely to fall on
   a modulo 512 of zero. If the MPA connection is started at TCP
   sequence number 11, then the 1st marker will begin at 11, and
   subsequent markers will begin at 523, 1035, etc.





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   If an FPDU is large enough to contain multiple markers, they MUST
   all point to the same point in the TCP stream: the first octet of
   the FPDU.

   If a marker interval contains multiple FPDUs (the FPDUs are small),
   the marker MUST point to the start of the FPDU containing the marker
   unless the marker falls between FPDUs, in which case the marker MUST
   be zero.

   The following example shows an FPDU containing a marker.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       ULPDU Length (0x0010)   |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      |                                                               |
      +                                                               +
      |                         ULPDU (octets 0-9)                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            (0x0000)           |        FPDU ptr (0x000C)      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        ULPDU (octets 10-15)                   |
      |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               |          PAD (2 octets:0,0)   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              CRC                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Figure 4 Example FPDU Format with Marker

   MPA Receivers MUST preserve ULPDU boundaries when passing data to
   the ULP. MPA Receivers MUST pass the ULPDU data and the ULPDU Length
   to the ULP and not the markers, headers, and CRC.

7.2  CRC Calculation

   When sending an FPDU, the sender MUST include a valid CRC field.
   The CRC field in the MPA FPDU, MUST be computed in the manner
   described in the iSCSI Protocol [iSCSI] document for Header and Data
   Digests.













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   The fields which MUST be included in the CRC calculation when
   sending an FPDU are as follows:

   1)  If the first octet of the FPDU is the "ULPDU Length" field, the
       CRC-32c is calculated from the first octet of the "ULPDU Length"
       header, through all ULP payload and markers (if present), to the
       last octet of the PAD (if present), inclusive. If there is a
       marker immediately following the PAD, the marker is included in
       the CRC calculation for this FPDU.

   2)  If the first octet of the FPDU is a marker, (i.e. the marker
       fell between FPDUs, and thus is required to be included in the
       second FPDU), the CRC-32c is calculated from the first octet of
       the marker, through the "ULPDU Length" header, through all ULP
       payload and markers (if present), to the last octet of the PAD
       (if present), inclusive.

   3)  After calculating the CRC-32c, the resultant value is placed
       into the CRC field at the end of the FPDU.

   When an FPDU is received, the receiver MUST first perform the
   following:

   1)  Calculate the CRC of the incoming FPDU in the same fashion as
       defined above.

   2)  Verify that the calculated CRC-32c value is the same as the
       received CRC-32c value found in the FPDU CRC field.  If not, the
       receiver MUST treat the FPDU as an invalid FPDU.

   The procedure for handling invalid FPDUs is covered in the Error
   Section (see section 9 on page 20)

   The following is an annotated hex dump of an example FPDU sent as
   the first FPDU on the stream.  As such, it starts with a marker. The
   FPDU contains 24 octets of the contained ULPDU, which are all zeros.
   The CRC32c has been correctly calculated and can be used as a
   reference.  See the [DDP] and [RDMA] specification for definitions
   of the DDP Control field, Queue, MSN, MO, and Send Data.














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       Octet Contents  Annotation
       Count

       0000    00 00   Marker: Reserved
       0002    00 00           FPDUPTR
       0004    00 2a   Length
       0006    40 03   DDP Control Field, Send with Last flag set
       0008    00 00   Reserved (STag position with no STag)
       000a    00 00
       000c    00 00   Queue = 0
       000e    00 00
       0010    00 00   MSN = 1
       0012    00 01
       0014    00 00   MO = 0
       0016    00 00
       0018    00 00
                       Send Data (24 octets of zeros)
       002e    00 00
       0030    4C 86   CRC32c
       0032    B3 84
                  Figure 5 Annotated Hex Dump of an FPDU

   The following is an example sent as the second FPDU of the stream
   where the first FPDU (which is not shown here) had a length of 492
   octets and was also a Send to Queue 0 with Last Flag set.  This
   example contains a marker.

       Octet Contents  Annotation
       Count

       01ec    00 2a   Length
       01ee    40 03   DDP Control Field: Send with Last Flag set
       01f0    00 00   Reserved (STag position with no STag)
       01f2    00 00
       01f4    00 00   Queue = 0
       01f6    00 00
       01f8    00 00   MSN = 2
       01fa    00 02
       01fc    00 00   MO = 0
       01fe    00 00
       0200    00 00   Marker: Reserved
       0202    00 14           FPDUPTR
       0204    00 00
                       Send Data (24 octets of zeros)
       021a    00 00
       021c    A1 9C   CRC32c
       021e    D1 03
            Figure 6 Annotated Hex Dump of an FPDU with Marker





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7.3  MPA on TCP Sender Segmentation

   The various TCP RFCs allow considerable choice in segmenting a TCP
   stream.  In order to optimize FPDU recovery at the MPA receiver, MPA
   specifies additional segmentation rules.

   MPA MUST encapsulate the ULPDU such that there is exactly one ULPDU
   contained in one FPDU.

   An MPA-aware TCP sender SHOULD segment the outbound TCP stream such
   that there is exactly one FPDU per TCP segment.

   An MPA-aware TCP sender SHOULD, with an EMSS large enough to contain
   the FPDU, segment the outgoing TCP stream such that the first octet
   of every FPDU is aligned with the beginning of a TCP segment, and is
   entirely contained in the TCP segment.

        Implementation note: To achieve the previous segmentation rule,
        TCP's Nagle [NagleDAck] algorithm SHOULD be disabled.

   There are exceptions to the above rule.  Once an ULPDU is provided
   to MPA, the MPA on TCP sender MUST transmit it or fail the
   connection; it cannot be repudiated.  As a result, during changes in
   MTU and EMSS, or when TCP's Receive Window size (RWIN) becomes too
   small, it may be necessary to send FPDUs that do not conform to the
   segmentation rule above.

   A possible, but less desirable, alternative is to use IP
   fragmentation on accepted FPDUs to deal with MTU reductions or
   extremely small EMSS.

   The sender MUST still format the FPDU according to FPDU format as
   shown in Figure 2.

   On a retransmission, TCP does not necessarily preserve original TCP
   segmentation boundaries. This can lead to the loss of FPDU alignment
   and containment within a TCP segment during TCP retransmissions. An
   MPA-Aware TCP SHOULD try to preserve original TCP segmentation
   boundaries on a retransmission.

7.3.1  FPDU Size Considerations

   MPA defines the Maximum Upper Layer Protocol Data Unit (MULPDU) as
   the size of the largest ULPDU fitting in an EMSS-sized FPDU.  MULPDU
   is EMSS minus the FPDU overhead (6 octets) minus space for markers
   and pad octets.







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     The maximum ULPDU Length for a single ULPDU MUST be computed as:

        MULPDU = EMSS - (6 + 4 * Ceiling(EMSS / 512) + EMSS mod 4)

   The formula above accounts for the worst-case number of markers.

   The ULP SHOULD provide ULPDUs that are as large as possible, but
   less than or equal to MULPDU.

   If the TCP implementation needs to adjust EMSS to support MTU
   changes, the MULPDU value is changed accordingly.

   In certain rare situations, the EMSS may shrink to very small sizes.
   If this occurs, the MPA on TCP sender MUST not shrink the MULPDU
   below 128 bytes and is not required to follow the segmentation rules
   in Section 7.3 MPA on TCP Sender Segmentation on page 16.  The value
   128 is chosen as to allow ULP designers a reasonable amount of room
   to implement their protocol.  Typical WAN scenarios will not reduce
   the EMSS below 512 octets.

7.4  MPA Receiver FPDU Identification

   An MPA receiver MUST first verify the FPDU before passing the ULPDU
   to the ULP.  To do this, the receiver MUST:

   *   locate the start of the FPDU unambiguously,

   *   verify its CRC.

   If the above conditions are true, the MPA receiver passes the ULPDU
   to the ULP.

   To detect the start of the FPDU unambiguously one of the following
   MUST be used:

   1:  In an ordered TCP stream, the ULPDU Length field in the current
       FPDU when FPDU has a valid CRC, can be used to identify the
       beginning of the next FPDU.

   2:  A Marker can always be used to locate the beginning of an FPDU
       (in FPDUs with valid CRCs).  Since the location of the marker is
       known in the octet stream (sequence number space), the marker
       can always be found.

   3:  Having found an FPDU by means of a Marker, following contiguous
       FPDUs can be found by using the ULPDU Lengths (from FPDUs with
       valid CRCs) to establish the next FPDU boundary.






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   The ULPDU Length field MUST be used to determine if the entire FPDU
   is present before forwarding the ULPDU to the ULP.

   CRC calculation is discussed in section 7.2 on page 13 above.

7.4.1  Re-segmenting Middle boxes and non-conforming senders

   Since fully conforming MPA on TCP senders start FPDUs on TCP segment
   boundaries, a receiving ULP on MPA on TCP implementation may be able
   to optimize the reception of data in various ways.

   However, MPA receivers MUST NOT depend on FPDU Alignment on TCP
   segment boundaries.

   Some MPA senders may be unable to conform to the sender requirements
   because their implementation of TCP is not designed with MPA in
   mind.  Even if the sender is fully conformant, the network may
   contain "middle boxes" which modify the TCP stream by changing the
   segmentation.  This is generally interoperable with TCP and its
   users and MPA must be no exception.

   The presence of markers in MPA allows an MPA receiver to recover the
   FPDUs despite these obstacles, although it may be necessary to
   utilize additional buffering at the receiver to do so.





























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8  Connection Semantics

8.1  Connection setup

   MPA requires that the ULP MUST activate the framing mode on a TCP
   half connection at the same location in the octet stream at both the
   sender and the receiver. This is required in order for the marker
   scheme to correctly locate the markers.

   MPA MAY be utilized separately in each direction, or enabled in both
   directions at once; it is up to the ULP.

   This can be accomplished several ways, and is left up to the ULP:

   *   The ULP MAY require MPA framing immediately after TCP connection
       setup.  This has the advantage that no additional negotiation is
       needed (at least for MPA).  In this case the marker MUST be the
       first four octets sent (this marker has the special value
       0x0000, meaning it belongs to the FPDU that follows).

   *   The ULP MAY negotiate the start of MPA.  The exchange
       establishes that MPA (as well as other ULPs) will be used, and
       exactly locates the point in the octet stream where MPA is to
       begin operation.  Again, the marker is the first four octets
       sent (this marker has the special value 0x0000, meaning it
       belongs to the FPDU that follows).  Note that such a negotiation
       protocol is outside the scope of this specification.

8.2  Normal Connection Teardown

   Each half connection of MPA terminates when the ULP closes the
   corresponding TCP half connection.

   A mechanism SHOULD be provided by MPA to the ULP for the ULP to be
   made aware that a graceful close of the LLP connection has been
   received by the LLP (e.g. FIN is received).

















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9  Error Semantics

   The following errors MUST be detected by MPA and the codes SHOULD be
   provided to the ULP:

       Code Error

       1   TCP connection closed, terminated or lost.  This includes
           lost by timeout, too many retries, RST received or FIN
           received.

       2   Received MPA CRC does not match the calculated value for the
           FPDU.

       3   In the event that the CRC is valid, received MPA marker and
           'ULPDU Length' fields do not agree on the start of a FPDU.
           If the FPDU start determined from previous ULPDU Length
           fields does not match with the MPA marker position, MPA
           SHOULD deliver an error to the ULP.  It may not be possible
           to make this check as a segment arrives, but the check
           SHOULD be made when a gap creating an out of order sequence
           is closed and any time a marker points to an already
           identified FPDU.  It is OPTIONAL for a receiver to check
           each marker, if multiple markers are present in an FPDU, or
           if the segment is received in order.

   When conditions 2 or 3 above are detected, an MPA-aware TCP
   implementation MAY choose to silently drop the TCP segment rather
   than reporting the error to the ULP.  In this case, the sending TCP
   will retry the segment, usually correcting the error, unless the
   problem was at the source.  In that case, the source will usually
   exceed the number of retries and terminate the connection.

   Once MPA delivers an error of any type, it MUST not deliver any
   additional FPDUs on that half connection.

   MPA MUST NOT close the TCP connection following a reported error.
   Closing the connection is the responsibility of the ULP.















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10 Security Considerations

   This section discusses the security considerations for MPA.

10.1 Protocol-specific Security Considerations

   The vulnerabilities of MPA to third-party attacks are no greater
   than any other protocol running over TCP.  A third party, by sending
   packets into the network that are delivered to an MPA receiver,
   could launch a variety of attacks that take advantage of how MPA
   operates.  For example, a third party could send random packets that
   are valid for TCP, but contain no FPDU headers.  An MPA receiver
   reports an error to the ULP when any packet arrives that cannot be
   validated as an FPDU when properly located on an FPDU boundary.
   This would have a severe impact on performance.  Communication
   security mechanisms such as IPsec [IPSEC] or TLS [TLS] may be used
   to prevent such attacks.  Independent of how MPA operates, a third
   party could use ICMP packets to reduce the path MTU to such a small
   size that performance would likewise be severely impacted.  Range
   checking on path MTU sizes in ICMP packets may be used to prevent
   such attacks.

10.2 Using IPSec With MPA

   IPsec can be used to protect against the packet injection attacks
   outlined above.  Because IPsec is designed to secure individual IP
   packets, MPA can run above IPsec without change.  IPsec packets are
   processed (e.g., integrity checked and decrypted) in the order they
   are received, and an MPA receiver will process the decrypted FPDUs
   contained in these packets in the same manner as FPDUs contained in
   unsecured IP packets.






















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11 IANA Considerations

   If a well-known port is chosen as the mechanism to identify a ULP on
   MPA on TCP, the well-known port must be registered with IANA.
   Because the use of the port is ULP specific, registration of the
   port with IANA is left to the ULP.















































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12 References

12.1 Normative References

   [iSCSI] Satran, Julian, draft-ietf-iscsi-15.txt, July 30, 2002 (work
       in progress)

   [PathMTU] Mogul, J., and Deering, S., "Path MTU Discovery", RFC
       1191, November 1990.

   [RFC2018] Mathis, M., ahdavi, J., Floyd, S., Romanow, A., "TCP
       Selective Acknowledgment Options", RFC 2018, October 1996.

   [RFC2026] Bradner, S., "The Internet Standards Process -- Revision
       3", BCP 9, RFC 2026, October 1996.

   [TCP] Postel, J., "Transmission Control Protocol - DARPA Internet
       Program Protocol Specification", RFC 793, September 1981.

12.2 Informative References

   [CRCTCP] Stone J., Partridge, C., "When the CRC and TCP checksum
       disagree", ACM Sigcomm, Sept. 2000.

   [DDP] H. Shah et al., "Direct Data Placement over Reliable
       Transports", RDMA Consortium Draft Specification
       draft-shah-iwarp-ddp-01.txt, October 2002
   [IPSEC]  Atkinson, R., Kent, S., "Security Architecture for the
       Internet Protocol", RFC 2401, November 1998.

   [NagleDAck] Minshall G., Mogul, J., Saito, Y., Verghese, B.,
       "Application performance pitfalls and TCP's Nagle algorithm",
       Workshop on Internet Server Performance, May 1999.

   [RDMA] R. Recio et al., "RDMA Protocol Specification", RDMA
       Consortium Draft Specification
       draft-recio-iwarp-rdmap-01.txt, October 2002


   [SCTP] R. Stewart et al., "Stream Control Transmission Protocol",
       RFC 2960, October 2000.

   [STONE] Stone, J., "Checksums in the Internet", Doctoral
       dissertation - August 2001

   [TLS] Dierks, T. and others, "The TLS Protocol, Version 1.0", RFC
       2246, January 1999.






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   [Williams93] Williams, R., "A PAINLESS GUIDE TO CRC ERROR DETECTION
       ALGORITHMS" - Internet publication, August 1993,
       http://www.geocities.com/SiliconValley/Pines/8659/crc.htm.

   [RFC792] Internet Control Message Protocol. J. Postel. Sep-01-1981

   [RFC1122] Requirements for Internet hosts - communication layers.
       R.T.     Braden. Oct-01-1989.













































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13 Appendix

   This appendix is for information only and is NOT part of the
   standard.

13.1 Receiver implementation

13.1.1 Transport & Network Layer Reassembly Buffers

   The use of reassembly buffers (either TCP reassembly buffers or IP
   fragmentation reassembly buffers) is implementation dependent. When
   MPA is enabled, reassembly buffers are needed if FPDU Alignment is
   lost or if IP fragmentation occurs. This is because the incoming out
   of order segment may not contain enough information for MPA to
   process all of the FPDU. In the usual case this should be a
   transient condition due to a reduction in the path MTU, so a
   solution does not need to be high performance. For cases where a re-
   segmenting middle box is present, the presence of markers
   significantly reduces the amount of buffering needed.

   Recovery from IP Fragmentation must be transparent to the MPA
   Consumers.































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13.1.1.1 Network Layer Reassembly Buffers

   Most IP implementations set the IP Don't Fragment bit. Thus upon a
   path MTU change, intermediate devices drop the IP datagram if it is
   too large and reply with an ICMP message which tells the source TCP
   that the path MTU has changed. This causes TCP to emit segments
   conformant with the new path MTU size. Thus IP fragments under most
   conditions should never occur at the receiver. But it is possible.

   There are several options for implementation of network layer
   reassembly buffers:

   1.  drop any IP fragments, and reply with an ICMP message according
       to [RFC792] (fragmentation needed and DF set) to tell the Remote
       Peer to resize its TCP segment

   2.  support an IP reassembly buffer, but have it of limited size
       (possibly the same size as the local link's MTU). The end Node
       would normally never advertise a path MTU larger than the local
       link MTU. It is recommended that a dropped IP fragment cause an
       ICMP message to be generated according to RFC792.

   3.  multiple IP reassembly buffers, of effectively unlimited size.

   4.  support an IP reassembly buffer for the largest IP datagram (64
       KB).

   5.  support for a large IP reassembly buffer which could span
       multiple IP datagrams.

   An implementation should support at least 2 or 3 above, to avoid
   dropping packets that have traversed the entire fabric.

   There is no end-to-end ACK for IP reassembly buffers, so there is no
   flow control on the buffer. The only end-to-end ACK is a TCP ACK,
   which can only occur when a complete IP datagram is delivered to
   TCP. Because of this, under worst case, pathological scenarios, the
   largest IP reassembly buffer is the TCP receive window (to buffer
   multiple IP datagrams that have all been fragmented).

   Note that if the Remote Peer does not implement re-segmentation of
   the data stream upon receiving the ICMP reply updating the path MTU,
   it is possible to halt forward progress because the opposite peer
   would continue to retransmit using a transport segment size that is
   too large. This deadlock scenario is no different than if the fabric
   MTU (not last hop MTU) was reduced after connection setup, and the
   remote Node's behavior is not compliant with [RFC1122].






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13.1.1.2 TCP Reassembly buffers

   A TCP reassembly buffer is also needed. TCP reassembly buffers are
   needed if FPDU Alignment is lost when using TCP with MPA or when the
   MPA FPDU spans multiple TCP segments (which is an exceptional case).
   This is a transient condition that only occurs when a path MTU has
   been reduced, unless there is a middle-box in the fabric that is re-
   segmenting the TCP stream.

   Since lost FPDU Alignment often means that FPDUs are incomplete, an
   MPA on TCP implementation must have a reassembly buffer large enough
   to recover an FPDU that is less than or equal to the MTU of the
   locally attached link (this should be the largest possible
   advertised TCP path MTU). If the MTU is smaller than 140 octets, the
   buffer MUST be at least 140 octets long to support the minimum FPDU
   size.  The 140 octets allows for the minimum MULPDU of 128, 2 octets
   of pad, 2 of ULPDU_Length, 4 of CRC, and space for a possible
   marker. As usual, additional buffering may provide better
   performance.

   Note that if the TCP segment were not stored, it is possible to
   deadlock the MPA algorithm. If the path MTU is reduced, FPDU
   Alignment requires the source TCP to re-segment the data stream to
   the new path MTU. The source MPA will detect this condition and
   reduce the MPA segment size, but any FPDUs already posted to the
   source TCP will be re-segmented and lose FPDU Alignment. If the
   destination does not support a TCP reassembly buffer, these segments
   can never be successfully transmitted and the protocol deadlocks.

   When a complete FPDU is received, processing continues normally.























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14 Author's Addresses

   Stephen Bailey
       Sandburst Corporation
       600 Federal Street
       Andover, MA  01810 USA
       Phone: +1 978 689 1614
       Email: steph@sandburst.com

   Paul R. Culley
       Hewlett-Packard Company
       20555 SH 249
       Houston, Tx. USA 77070-2698
       Phone:  281-514-5543
       Email:  paul.culley@hp.com

   Uri Elzur
       Broadcom
       16215 Alton Parkway
       CA, 92618
       Phone: 949.585.6432
       Email:  uri@broadcom.com

   Renato J Recio
       IBM
       Internal Zip 9043
       11400 Burnett Road
       Austin,  Texas  78759
       Phone:  512-838-3685
       Email:  recio@us.ibm.com

   John Carrier
       Adaptec Inc.
       691 South Milpitas Blvd.
       Milpitas, CA 95035
       Phone:  360-378-8526
       Email:  John_Carrier@adaptec.com
















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15 Acknowledgments

   Dwight Barron
       Hewlett-Packard Company
       20555 SH 249
       Houston, Tx. USA 77070-2698
       Phone: 281-514-2769
       Email: dwight.barron@hp.com

   Jeff Chase
       Department of Computer Science
       Duke University
       Durham, NC 27708-0129 USA
       Phone: +1 919 660 6559
       Email: chase@cs.duke.edu

   Ted Compton
       EMC Corporation
       Research Triangle Park, NC 27709, USA
       Phone: 919-248-6075
       Email: compton_ted@emc.com

   Dave Garcia
       Hewlett-Packard Company
       19333 Vallco Parkway
       Cupertino, Ca. USA 95014
       Phone: 408.285.6116
       Email: dave.garcia@hp.com

   Hari Ghadia
       Adaptec, Inc.
       691 S. Milpitas Blvd.,
       Milpitas, CA 95035  USA
       Phone: +1 (408) 957-5608
       Email: hari_ghadia@adaptec.com

   Howard C. Herbert
       Intel Corporation
       MS CH7-404
       5000 West Chandler Blvd.
       Chandler, Arizona 85226
       Phone: 480-554-3116
       Email: howard.c.herbert@intel.com










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   Jeff Hilland
       Hewlett-Packard Company
       20555 SH 249
       Houston, Tx. USA 77070-2698
       Phone: 281-514-9489
       Email: jeff.hilland@hp.com

   Mike Ko
       IBM
       650 Harry Rd.
       San Jose, CA 95120
       Phone: (408) 927-2085
       Email: mako@us.ibm.com

   Mike Krause
       Hewlett-Packard Corporation, 43LN
       19410 Homestead Road
       Cupertino, CA 95014 USA
       Phone: +1 (408) 447-3191
       Email: krause@cup.hp.com

   Dave Minturn
       Intel Corporation
       MS JF1-210
       5200 North East Elam Young Parkway
       Hillsboro, Oregon  97124
       Phone: 503-712-4106
       Email: dave.b.minturn@intel.com

   Jim Pinkerton
       Microsoft, Inc.
       One Microsoft Way
       Redmond, WA, USA 98052
       Email: jpink@microsoft.com

   Hemal Shah
       Intel Corporation
       MS PTL1
       1501 South Mopac Expressway, #400
       Austin, Texas  78746
       Phone: 512-732-3963
       Email: hemal.shah@intel.com

   Allyn Romanow
       Cisco Systems
       170 W Tasman Drive
       San Jose, CA 95134 USA
       Phone: +1 408 525 8836
       Email: allyn@cisco.com




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   Tom Talpey
       Network Appliance
       375 Totten Pond Road
       Waltham, MA 02451 USA
       Phone: +1 (781) 768-5329
       EMail: thomas.talpey@netapp.com

   Patricia Thaler
       Agilent Technologies, Inc.
       1101 Creekside Ridge Drive, #100
       M/S-RG10
       Roseville, CA 95678
       Phone: +1-916-788-5662
       email: pat_thaler@agilent.com

   Jim Wendt
       Hewlett Packard Corporation
       8000 Foothills Boulevard MS 5668
       Roseville, CA 95747-5668 USA
       Phone: +1 916 785 5198
       Email: jim_wendt@hp.com

   Jim Williams
       Emulex Corporation
       580 Main Street
       Bolton, MA 01740 USA
       Phone: +1 978 779 7224
       Email: jim.williams@emulex.com

























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16 Full Copyright Statement

   This document and the information contained herein is provided on an
   "AS IS" basis and ADAPTEC INC., AGILENT TECHNOLOGIES INC., BROADCOM
   CORPORATION, CISCO SYSTEMS INC., DUKE UNIVERSITY, EMC CORPORATION,
   EMULEX CORPORATION, HEWLETT-PACKARD COMPANY, INTERNATIONAL BUSINESS
   MACHINES CORPORATION, INTEL CORPORATION, MICROSOFT CORPORATION,
   NETWORK APPLIANCE INC., SANDBURST CORPORATION, THE INTERNET SOCIETY,
   AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
   THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
   ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
   PARTICULAR PURPOSE.

   Copyright (c) 2002 ADAPTEC INC., BROADCOM CORPORATION, CISCO SYSTEMS
   INC., EMC CORPORATION, HEWLETT-PACKARD COMPANY, INTERNATIONAL
   BUSINESS MACHINES CORPORATION, INTEL CORPORATION, MICROSOFT
   CORPORATION, NETWORK APPLIANCE INC., All Rights Reserved



































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