One document matched: draft-ietf-rddp-mpa-00.txt
INTERNET-DRAFT P. Culley
draft-ietf-rddp-mpa-00.txt Hewlett-Packard Company
U. Elzur
Broadcom Corporation
R. Recio
IBM Corporation
S. Bailey
Sandburst Corporation
J. Carrier
Adaptec
Expires: March 2004
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 an
"adaptation layer" between TCP and the Direct Data Placement [DDP]
protocol, preserving the reliable, in-order delivery of TCP, while
adding the preservation of higher-level protocol record boundaries
that DDP requires.
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Table of Contents
1 Status of this Memo..........................................1
2 Abstract.....................................................1
3 Introduction.................................................5
3.1 Motivation...................................................5
3.2 Protocol Overview............................................5
4 Glossary.....................................................9
5 LLP and DDP requirements....................................11
5.1 TCP implementation Requirements to support MPA..............11
5.1.1 TCP Transmit side...........................................11
5.1.2 TCP Receive side............................................11
5.2 MPA's interactions with DDP.................................12
6 FPDU Formats................................................14
6.1 Marker Format...............................................15
7 Data Transfer Semantics.....................................16
7.1 MPA Markers.................................................16
7.2 CRC Calculation.............................................18
7.3 MPA on TCP Sender Segmentation..............................21
7.3.1 Effects of MPA on TCP Segmentation..........................21
7.3.2 FPDU Size Considerations....................................23
7.4 MPA Receiver FPDU Identification............................24
7.4.1 Re-segmenting Middle boxes and non MPA-aware TCP senders....25
8 Connection Semantics........................................26
8.1 Connection setup............................................26
8.1.1 MPA Request Frame Format....................................29
8.1.2 Example Delayed Startup sequence............................30
8.1.3 Use of "Private Data".......................................33
8.1.4 "Dual Stack" implementations................................36
8.2 Normal Connection Teardown..................................37
9 Error Semantics.............................................38
10 Security Considerations.....................................39
10.1 Protocol-specific Security Considerations...................39
10.2 Using IPsec With MPA........................................39
11 IANA Considerations.........................................40
12 References..................................................41
12.1 Normative References........................................41
12.2 Informative References......................................41
13 Appendix....................................................43
13.1 Analysis of MPA over TCP Operations.........................43
13.1.1 Assumptions...............................................43
13.1.2 The Value of Header Alignment.............................44
13.2 Receiver implementation.....................................52
13.2.1 Network Layer Reassembly Buffers..........................52
13.2.2 TCP Reassembly buffers....................................53
14 Author's Addresses..........................................54
15 Acknowledgments.............................................55
16 Full Copyright Statement....................................58
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Table of Figures
Figure 1 ULP MPA TCP Layering.......................................7
Figure 2 FPDU Format...............................................14
Figure 3 Marker Format.............................................15
Figure 4 Example FPDU Format with Marker...........................17
Figure 5 Annotated Hex Dump of an FPDU.............................20
Figure 6 Annotated Hex Dump of an FPDU with Marker.................20
Figure 7 "MPA Request/Reply Frame".................................29
Figure 8: Example Delayed Startup negotiation......................31
Figure 8: Example Immediate Startup negotiation....................34
Figure 9: Non-aligned FPDU freely placed in TCP octet stream.......46
Figure 10: Aligned FPDU placed immediately after TCP header........48
Revision history
[draft-ietf-rddp-mpa-00] workgroup draft with following changes:
Changed "Start Key" to two separate startup frames to facilitate
identification of incorrect Active/Active startup.
Changed Active/Passive nomenclature to Initiator/Responder to
reduce confusion with TCP startup and verbs doc (which used
opposite sense).
Added "Private Data" to the startup key sequences. This also
required describing the motivation and expected usage models
along with some interface hints. Removed the "Private data"
stuff from appendix.
Added example "Immediate" startup with TCP and explanation.
[draft-culley-iwarp-mpa-03]
Add option to allow receivers to specify Marker use.
Add option that allows both sides to agree not to use CRC.
Added startup declaration "Start Key" with options and larger
MPA mode recognition "key".
Updated MPA/DDP connection startup rules and sequence to deal
with "Start Key".
Added Appendix that provides a more detailed analysis of the
effects of MPA on TCP data streams.
Added appendix that describes a mechanism to deal with "private
data" prior to full MPA/DDP operation.
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[draft-culley-iwarp-mpa-02]
Enhanced descriptions of how MPA is used over an unmodified TCP.
Removed "No Packing" text.
Made MPA an adaptation layer for DDP, instead of a generalized
framing solution.
Added clarifications of the MPA/TCP interaction for optimized
implementations and that any such optimizations are to be used
only when requested by MPA.
Note: a discussion of reasons for these changes can be found in
[ELZER-MPA].
[draft-culley-iwarp-mpa-01] initial draft.
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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 14), and detailed protocol
requirements and characteristics (see section 7 on page 16), as well
as Connection Semantics (section 8 on page 25), Error Semantics
(section 9 on page 38), and Security Considerations (section 10 on
page 39).
3.1 Motivation
The Direct Data Placement protocol [DDP], when used with TCP [RFC793]
requires a mechanism to detect record boundaries. The DDP records
are referred to as Upper Layer Protocol Data Units by this document.
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. Potential
benefits of this capability are the avoidance of the memory copy
overhead and a smaller memory requirement for handling out of order
or dropped packets.
Many approaches have been proposed for a generalized framing
mechanism. Some are probabilistic in nature and others are
deterministic. A probabilistic approach is characterized by a
detectable value embedded in the octet 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 octet 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 framing mechanism for DDP running over
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.
3.2 Protocol Overview
MPA is described as an extra layer above TCP and below DDP. The
operation sequence is:
1. A TCP connection is established by ULP action. This is done
using methods not described by this specification. The ULP may
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exchange some amount of data in streaming mode prior to starting
MPA, but is not required to do so.
2. The Consumer negotiates the use of DDP and MPA at both ends of a
connection. The mechanisms to do this are not described in this
specification. The negotiation may be done in streaming mode, or
by some other mechanism (such as a pre-arranged port number).
3. The ULP activates MPA on each end in the "Startup Phase", either
as an "Initiator" or a "Responder", as determined by the ULP.
This mode verifies the usage of MPA, specifies the use of CRC and
Markers, and allows the ULP to communicate some additional data
via a "private data" exchange. See section 8.1 Connection setup
for more details on the startup process.
4. At the end of the Startup Phase, the ULP puts MPA (and DDP) into
full operation and begins sending DDP data as further described
below. In this document, DDP data chunks are called ULPDUs. For
a description of the DDP data, see [DDP].
Following is a description of data transfer when MPA is in full
operation.
1. DDP determines the Maximum ULPDU (MULPDU) size by querying MPA
for this value. MPA derives this information from TCP, when it
is available, or chooses a reasonable value. This information is
already supported on many TCP implementations, including all
modern flavors of BSD networking, through the TCP_MAXSEG socket
option.
2. DDP creates ULPDUs of MULPDU size or smaller, and hands them to
MPA at the sender.
3. MPA creates a Framed Protocol Data Unit (FPDU) by pre-pending a
header, optionally inserting markers, and appending a CRC field
after the ULPDU and PAD (if any). MPA delivers the FPDU to TCP.
4. The TCP sender puts the FPDUs into the TCP stream. If the TCP
Sender is MPA-aware, it segments the TCP stream in such a way
that a TCP Segment boundary is also the boundary of an 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 (when
present), ULPDU_Length, PAD and the CRC field.
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7. MPA then provides the complete ULPDUs to DDP. MPA may also
separate passing MPA payload to DDP from passing the MPA payload
ordering information.
The layering of PDUs with MPA is shown in Figure 1, below.
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.
+------------------+
| ULP client |
+------------------+ <- Consumer messages
| DDP |
+------------------+ <- ULPDUs
| MPA |
+------------------+ <- FPDUs (containing ULPDUs)
| TCP* |
+------------------+ <- TCP Segments (containing FPDUs)
| IP etc. |
+------------------+
* TCP or MPA-aware TCP.
Figure 1 ULP MPA TCP Layering
An MPA-aware TCP sender is able to segment the data stream such that
TCP segments begin with FPDUs (FPDU Alignment). This has significant
advantages for receivers. When segments arrive with aligned FPDUs
the receiver usually need not buffer any portion of the segment,
allowing DDP to place it in its destination memory immediately, thus
avoiding copies from intermediate buffers (DDP's reason for
existence).
MPA with an MPA-aware TCP receiver allows a DDP on MPA implementation
to recover ULPDUs that may be received out of order. This enables a
DDP on MPA implementation to save a significant amount of
intermediate storage by placing the ULPDUs in the right locations in
the application buffers when they arrive, rather than waiting until
full ordering can be restored.
The ability of a receiver to recover out of order ULPDUs is optional
and declared to the transmitter during startup. When the receiver
declares that it does not support out of order recovery, the
transmitter does not add the control information to the data stream
needed for out of order recovery.
MPA implementations that support recovery of out of order ULPDUs MUST
support a mechanism to indicate the ordering of ULPDUs as the sender
transmitted them and indicate when missing intermediate segments
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arrive. These mechanisms allow DDP to reestablish record ordering
and report Delivery of complete messages (groups of records).
MPA also addresses enhanced 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 [RFC2960].
This check may be disabled with agreement by providers and
administrators at both ends of a connection. This disabling of CRCs
should only be done when it is clear that the connection through the
network has data integrity at least as good as a CRC (for example
when IPSEC is implemented end to end). DDP's ULP expects this level
of data integrity and therefore the ULP SHOULD NOT have to provide
its own duplicate data integrity and error recovery for lost data
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4 Glossary
Consumer - the ULPs or applications that lie above MPA and DDP. The
Consumer is responsible for making TCP connections, starting MPA
and DDP connections, and generally controlling operations.
Delivery - (Delivered, Delivers) - For MPA, Delivery is defined as
the process of informing DDP that a particular PDU is ordered for
use. This is specifically different from "passing the PDU to
DDP", 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 [RFC793],
and the current path Maximum Transfer Unit (MTU) [RFC1191].
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.
Header Alignment - the property that a TCP segment begins with an
FPDU and the TCP segment includes an integer number of FPDUs.
PDU - protocol data unit
MPA-aware TCP - a TCP implementation that is aware of the receiver
efficiencies of MPA Header Alignment and is capable of sending
TCP segments that begin with an FPDU.
MPA-enabled - MPA is enabled if the MPA protocol is visible on the
wire. When the sender is MPA-enabled, it is inserting framing
and markers. When the receiver is MPA-enabled, it is
interpreting framing and markers.
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 DDP 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.
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ULP - Upper Layer Protocol. The protocol layer above the protocol
layer currently being referenced. The ULP for MPA is DDP [DDP].
ULPDU - Upper Layer Protocol Data Unit. The data record defined by
the layer above MPA (DDP). ULPDU corresponds to DDP's "DDP
Segment".
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5 LLP and DDP requirements
5.1 TCP implementation Requirements to support MPA
The TCP implementation MUST inform MPA when the TCP connection is
closed or has begun closing the connection (e.g. received a FIN).
5.1.1 TCP Transmit side
To provide optimum performance, an MPA-aware transmit side TCP
implementation SHOULD be enabled to:
* With an EMSS large enough to contain the FPDU(s), segment the
outgoing TCP stream such that the first octet of every TCP
Segment begins with an FPDU. Multiple FPDUs MAY be packed into a
single TCP segment as long as they are entirely contained in the
TCP segment.
* Report the current EMSS to the MPA transmit layer.
An MPA-aware TCP transmit side implementation MUST continue to use
the method of segmentation expected by non-MPA applications (and
described in TCP RFCs) when MPA is not enabled on the connection.
When MPA is enabled above an MPA-aware TCP, it SHOULD specifically
enable the segmentation rules described above for the DDP segments
(FPDUs) posted for transmission.
If the transmit side TCP implementation is not able to segment the
TCP stream as indicated above, MPA SHOULD make a best effort to
achieve that result. For example, using the TCP_NODELAY socket
option to disable the Nagle algorithm will usually result in many of
the segments starting with an FPDU.
If the transmit side TCP implementation is not able to report the
EMSS, MPA may assume that TCP will use 1460 octet segments in
creating FPDUs. If the implementation has reason to believe that the
TCP segment size is actually smaller than 1460, it may instead use a
536 octet FPDU.
5.1.2 TCP Receive side
When an MPA receive implementation and the MPA-aware receive side TCP
implementation support handling out of order ULPDUs, the TCP receive
implementation SHOULD be enabled to:
* 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
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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 DDP 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 DDP 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.
DDP on MPA MUST utilize these two mechanisms to establish the
Delivery semantics that DDP's consumers agree to. These
semantics are described fully in [DDP]. These include
requirements on DDP's consumer to respect ownership of buffers
prior to the time that DDP delivers them to the consumer.
An MPA-aware TCP receive side implementation MUST continue to buffer
TCP segments until completely ordered and then deliver them as
expected by non-MPA applications (and described in TCP RFCs) when MPA
is not enabled on the connection. When MPA is enabled above an MPA-
aware TCP, TCP SHOULD enable the in and out of order passing of data,
and the separate ordering information as described above.
When an MPA receive implementation is coupled with a TCP receive
implementation that does not support the preceding mechanisms, TCP
passes and Delivers incoming stream data to MPA in order.
5.2 MPA's interactions with DDP
DDP requires MPA to maintain DDP record boundaries from the sender to
the receiver. When using MPA on TCP to send data, DDP 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 DDP at the sender
and returns them to DDP at the receiver.
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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.
Since FPDU alignment is generally desired by the receiver, DDP must
cooperate with MPA to ensure FPDUs' lengths do not exceed the EMSS
under normal conditions. This is done with the MULPDU mechanism.
MPA provides information to DDP on the current maximum size of the
record that is acceptable to send (MULPDU). DDP SHOULD limit each
record size to MULPDU. The range of MULPDU values MUST be between
128 octets and 64768 octets, inclusive.
The sending DDP MUST NOT post a ULPDU larger than 64768 octets to
MPA. DDP 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 FPDU 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 octet boundary.
On receive, MPA MUST pass each ULPDU with its length to DDP when it
has been validated.
If an MPA implementation supports passing out of order ULPDUs to DDP,
the MPA implementation SHOULD:
* Pass each ULPDU with its length to DDP 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
DDP to see the current outgoing TCP Ack sequence number.
* Provide an indication to DDP that the TCP 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, When CRCs are enabled, this field contains a CRC32C
check value, which is used to verify the entire contents of the FPDU,
using CRC32C. See section 7.2 CRC Calculation on page 18. When CRCs
are not enabled, this field is still present, may contain any value,
and MUST NOT be checked.
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 octets 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 markers are used to identify the start of FPDUs when packets are
received out of order. This is done by locating the markers at fixed
intervals in the data stream (which is correlated to the TCP sequence
number) and using the marker value to locate the preceding FPDU
start.
The MPA receiver's ability to locate out of order FPDUs and pass the
ULPDUs to DDP is implementation dependent. MPA/DDP allows those
receivers that are able to deal with out of order FPDUs in this way
to require the insertion of markers in the data stream. When the
receiver cannot deal with out of order FPDUs in this way, it may
disable the insertion of markers at the sender. All MPA senders MUST
be able to generate markers when their use is declared by the
opposing receiver (see section 8.1 Connection setup on page 26).
When Markers are enabled, 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 octet of the previous FPDU header can be
determined by subtracting the value of the given marker from the
current octet-stream sequence number (i.e. TCP sequence number) of
the first octet 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.
When Markers are enabled (see section 8.1 Connection setup on page
26), the MPA markers MUST be inserted immediately following MPA
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connection establishment, and at every 512th octet of the TCP octet
stream thereafter. As a result, the first marker has an FPDUPTR
value of 0x0000. If the first marker begins at octet sequence number
SeqStart, then markers are inserted such that the first octet of the
marker is at octet 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.
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
DDP. MPA Receivers MUST pass the ULPDU data and the ULPDU Length to
DDP and not the markers, headers, and CRC.
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7.2 CRC Calculation
An MPA implementation MUST implement CRC support and MUST either:
(1) always use CRCs
or
(2) only negotiate the non-use of CRC on the explicit request of the
system administrator, via an interface not defined in this spec.
The default configuration for a connection MUST be to use CRCs.
(3) The MPA provider at either peer MAY ignore its administrator's
request that CRCs not be used.
The decision for one host to request CRC suppression MAY be made on
an administrative basis for any path that provides equivalent
protection from undetected errors as an end-to-end CRC32c.
The process MUST be invisible to the ULP.
After receipt of an MPA startup declaration indicating that its peer
requires CRCs, an MPA instance MUST continue generating and checking
CRCs until the connection terminates. If an MPA instance has
declared that it does not require CRCs, it MUST turn off CRC checking
immediately after receipt of an MPA mode declaration indicating that
its peer also does not require CRCs. It MAY continue generating
CRCs. See section 8.1 Connection setup on page 26 for details on the
MPA startup.
When sending an FPDU, the sender MUST include a CRC field. When CRCs
are enabled, the CRC field in the MPA FPDU MUST be computed using the
CRC32C polynomial 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 the ULPDU 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 the ULPDU
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, and CRC checking is enabled, 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 38)
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, when enabled for MPA, on TCP
implementations that support this, and with an EMSS large enough to
contain at least one FPDU, segment the outbound TCP stream such that
each TCP segment begins with an FPDU, and fully contains all included
FPDUs.
Implementation note: To achieve the previous segmentation rule,
TCP's Nagle [RFC0896] 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 sender SHOULD try to preserve original TCP segmentation
boundaries on a retransmission.
7.3.1 Effects of MPA on TCP Segmentation
Applications expected to see strong advantages from Direct Data
Placement include transaction-based applications and throughput
applications. Request/response protocols typically send one FPDU per
TCP segment and then wait for a response. Therefore, the application
is expected to set TCP parameters such that it can trade off latency
and wire efficiency. This is accomplished by setting the TCP_NODELAY
socket option.
When latency is not critical, and the application provides data in
chunks larger than EMSS at one time, the TCP implementation may
"pack" any available stream data into TCP segments so that the
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segments are filled to the EMSS. If the amount of data available is
not enough to fill the TCP segment when it is prepared for
transmission, TCP can send the segment partly filled, or use the
Nagle algorithm to wait for the ULP to post more data (discussed
below).
DDP/MPA senders will fill TCP segments to the EMSS with a single FPDU
when a DDP message is large enough. Since the DDP message may not
exactly fit into TCP segments, a "message tail" often occurs that
results in an FPDU that is smaller than a single TCP segment. If a
"message tail", small DDP messages, or the start of a larger DDP
message are available, MPA MAY "pack" the resulting FPDUs into TCP
segments. When this is done, the TCP segments can be more fully
utilized, but, due to the size constraints of FPDUs, segments may not
be filled to the EMSS.
Note that MPA receivers must do more processing of a TCP segment
that contains multiple FPDUs, this may affect the performance of
some receiver implementations.
TCP implementations often utilize the "Nagle" [RFC0896] algorithm to
ensure that segments are filled to the EMSS whenever the round trip
latency is large enough that the source stream can fully fill
segments before Acks arrive. The algorithm does this by delaying the
transmission of TCP segments until a ULP can fill a segment, or until
an ACK arrives from the far side. The algorithm thus allows for
smaller segments when latencies are shorter to keep the ULP's end to
end latency to reasonable levels.
The Nagle algorithm is not mandatory to use [RFC1122].
It is up to the ULP to decide if Nagle is useful with DDP/MPA. Note
that many of the applications expected to take advantage of MPA/DDP
prefer to avoid the extra delays caused by Nagle. In such scenarios
it is anticipated there will be minimal opportunity for packing at
the transmitter and receivers may choose to optimize their
performance for this anticipated behavior.
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7.3.2 FPDU Size Considerations
MPA defines the Maximum Upper Layer Protocol Data Unit (MULPDU) as
the size of the largest ULPDU fitting in an FPDU. For an empty TCP
Segment, MULPDU is EMSS minus the FPDU overhead (6 octets) minus
space for markers and pad octets.
The maximum ULPDU Length for a single ULPDU when markers are
present 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 maximum ULPDU Length for a single ULPDU when markers are NOT
present MUST be computed as:
MULPDU = EMSS - (6 + EMSS mod 4)
As a further optimization of the wire efficiency an MPA
implementation MAY dynamically adjust the MULPDU (see section 7.3.1.
for latency and wire efficiency trade-offs). When one or more FPDUs
are already packed into a TCP Segment, MULPDU MAY be reduced
accordingly.
DDP 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 octets and is not required to follow the segmentation rules
in Section 7.3 MPA on TCP Sender Segmentation on page 21.
If one or more FPDUs are already packed into a TCP segment, such that
the remaining room is less than 128 octets, MPA MUST NOT provide a
MULPDU smaller than 128. In this case, MPA would typically provide a
MULPDU for the next full sized segment, but may still pack the next
FPDU into the small remaining room, provide that the next FPDU is
small enough to fit.
The value 128 is chosen as to allow DDP designers room for the DDP
Header and some user data.
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7.4 MPA Receiver FPDU Identification
An MPA receiver MUST first verify the FPDU before passing the ULPDU
to DDP. To do this, the receiver MUST:
* locate the start of the FPDU unambiguously,
* verify its CRC (if CRC checking is enabled).
If the above conditions are true, the MPA receiver passes the ULPDU
to DDP.
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: For receivers that support out of order reception of FPDUs (see
section 7.1 MPA Markers on page 16) 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.
The ULPDU Length field (see section 6) MUST be used to determine if
the entire FPDU is present before forwarding the ULPDU to DDP.
CRC calculation is discussed in section 7.2 on page 18 above.
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7.4.1 Re-segmenting Middle boxes and non MPA-aware TCP senders
Since MPA on MPA-aware TCP senders start FPDUs on TCP segment
boundaries, a receiving DDP 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 MPA-aware, 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 (when enabled) 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.
Some of the cases that a receiver may have to contend with are listed
below as a reminder to the implementer:
* A single Aligned and complete FPDU, either in order, or out of
order: This can be passed to DDP as soon as validated, and
Delivered when ordering is established.
* Multiple FPDUs in a TCP segment, aligned and fully contained,
either in order, or out of order: These can be passed to DDP as
soon as validated, and Delivered when ordering is established.
* Incomplete FPDU: The receiver should buffer until the remainder
of the FPDU arrives. If the remainder of the FPDU is already
available, this can be passed to DDP as soon as validated, and
Delivered when ordering is established.
* Unaligned FPDU start: The partial FPDU must be combined with its
preceding portion(s). If the preceding parts are already
available, and the whole FPDU is present, this can be passed to
DDP as soon as validated, and Delivered when ordering is
established. If the whole FPDU is not available, the receiver
should buffer until the remainder of the FPDU arrives.
* Combinations of Unaligned or incomplete FPDUs (and potentially
other complete FPDUs) in the same TCP segment: If any FPDU is
present in its entirety, or can be completed with portions
already available, it can be passed to DDP as soon as validated,
and Delivered when ordering is established.
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8 Connection Semantics
8.1 Connection setup
MPA requires that the consumer MUST activate MPA, and any TCP
enhancements for MPA, 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 (if enabled) and to correctly locate the first FPDU.
MPA, and any TCP enhancements for MPA are enabled by the ULP in both
directions at once at an endpoint.
This can be accomplished several ways, and is left up to DDP's ULP:
* DDP's ULP MAY require DDP on MPA startup immediately after TCP
connection setup. This has the advantage that no streaming mode
negotiation is needed. An example of such a protocol is shown in
Figure 9: Example Immediate Startup negotiation on page 34.
This may be accomplished by using a well-known port, or a service
locator protocol to locate an appropriate port on which DDP on
MPA is expected to operate.
* DDP's ULP MAY negotiate the start of DDP on MPA sometime after a
normal TCP startup, using TCP streaming data exchanges on the
same connection. The exchange establishes that DDP on MPA (as
well as other ULPs) will be used, and exactly locates the point
in the octet stream where MPA is to begin operation. Note that
such a negotiation protocol is outside the scope of this
specification. A simplified example of such a protocol is shown
in Figure 8: Example Delayed Startup negotiation on page 31.
An MPA endpoint operates in two distinct phases.
The "Startup Phase" is used to verify correct MPA setup, exchange CRC
and Marker configuration, and optionally pass "private data" between
endpoints prior to completing a DDP connection. During this phase,
specifically formatted frames are exchanged as TCP byte streams
without using CRCs or Markers. During this phase a DDP endpoint need
not be "bound" to the MPA connection. In fact, the choice of DDP
endpoint and its operating parameters may not be known until the
consumer supplied "private data" (if any) has been examined by the
consumer.
The second distinct phase is "Full operation" during which FPDUs are
sent using all the rules that pertain (CRCs, Markers, MULPDU
restrictions etc.). A DDP endpoint MUST be "bound" to the MPA
connection at entry to this phase.
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When "private data" is passed between ULPs in the "Startup Phase",
the ULP is responsible for interpreting that data, and then placing
MPA into "Full operation".
Note: The following text differentiates the two endpoints by calling
them "Initiator" and "Responder". This is quite arbitrary and is
NOT related to the TCP startup (SYN, SYN/ACK sequence). The
Initiator is the side that sends first in the MPA startup
sequence (the MPA Request Frame).
The ULP is responsible for determining which side is "Initiator" or
"Responder". For "Client/Server" type ULPs this is easy. For peer-
peer ULPs (which might utilize a TCP style "active/active" startup),
some mechanism (not defined by this specification) must be
established, or some streaming mode data exchanged prior to MPA
startup to determine the side which starts in "Initiator" and which
starts in "Responder" MPA mode.
The following rules apply to MPA connection startup phase:
1. When MPA is started in the "Initiator" mode, the MPA
implementation MUST send a valid "MPA Request Frame".
2. When MPA is started in the "Responder" mode, the MPA
implementation MUST wait until a "MPA Request Frame" is received
and validated before sending any MPA data and before starting to
interpret any data received as FPDUs and passing any received
ULPDUs to DDP. After the received "MPA Request Frame" is
validated, the MPA implementation MUST either send a valid "MPA
Reply Frame", or close the connection.
Note: Since the receiver's ability to deal with markers is
unknown until the Request and Reply frames have been
received, sending FPDUs before this occurs is not possible.
Note: The requirement to wait on a Request Frame before sending a
Reply frame is a design choice, it makes for well ordered
sequence of events at each end, and avoids having to specify
how to deal with situations where both ends start at the same
time.
3. MPA "Initiator" mode implementations MUST receive and validate a
"MPA Reply Frame" before sending any FPDUs, and before starting
to interpret any data received as FPDUs and passing any received
ULPDUs to DDP.
4. MPA "Responder" mode implementations MUST receive and validate at
least one FPDU before sending any FPDUs or markers.
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Note: this requirement is present to allow the Initiator time to
get its receiver into full operation before an FPDU arrives,
avoiding potentially difficult requirements on the receiver.
5. If a received "Key" does not match the expected value, (See 8.1.1
MPA Request Frame Format below) the TCP/DDP connection MUST be
closed, and an error returned to the ULP.
6. The received "Private Data" fields may be used by consumers at
either end to further validate the connection, and set up DDP or
other ULP parameters. The ULP MAY close the TCP/MPA/DDP
connection as a result of validating the "Private Data" fields.
7. When the first FPDU is to be sent, then if markers are enabled,
the first octets sent are the special marker 0x00000000, followed
by the start of the FPDU (the FPDU's "ULPDU Length" field). If
markers are not enabled, the first octets sent are the start of
the FPDU (the FPDU's "ULPDU Length" field).
8. MPA implementations MUST use the difference between the MPA
Request Frame and the MPA Reply Frame to check for incorrect
"Initiator/Initiator" startups. Implementations SHOULD put a
timeout on waiting for the MPA Reply Frame when started in
"Responder" mode, to detect incorrect "Responder/Responder"
startups.
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8.1.1 MPA Request Frame Format
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 | |
+ Key (16 bytes containing "MPA ID Req Frame") +
4 | (4D 50 41 20 49 44 20 52 65 71 20 46 72 61 6D 65) |
+ Or (16 bytes containing "MPA ID Rep Frame") +
8 | (4D 50 41 20 49 44 20 52 65 70 20 46 72 61 6D 65) |
+ +
12 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
16 |M|C| Res | Rev | PD_Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ~
~ Private Data ~
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7 "MPA Request/Reply Frame"
Key: This field contains the "key" used to authenticate that the
sender is an MPA sender. Initiator mode senders must set this
field to the fixed value "MPA ID Req frame" or (in byte order) 4D
50 41 20 49 44 20 52 65 71 20 46 72 61 6D 65 (in hexadecimal).
Responder mode receivers MUST check this field for the same
value, and close the connection and report an error locally if
any other value is detected. Responder mode senders must set this
field to the fixed value "MPA ID Rep frame" or (in byte order) 4D
50 41 20 49 44 20 52 65 70 20 46 72 61 6D 65 (in hexadecimal).
Initiator mode receivers MUST check this field for the same
value, and close the connection and report an error locally if
any other value is detected.
M: This bit, when sent in an "MPA Request Frame" or an "MPA Reply
Frame", declares a receiver's requirement for Markers. When in a
received "MPA Request Frame" or "MPA Reply Frame" and the value
is '0', markers MUST NOT be added to the data stream by the
sender. When '1' markers MUST be added as described in section
7.1 MPA Markers on page 16.
C: This bit declares an endpoint's preferred CRC usage. When this
field is '0' in the "MPA Request Frame" and the "MPA Reply
Frame", CRCs MUST not be checked and need not be generated by
either endpoint. When this bit is '1' in either the "MPA Request
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Frame" or "MPA Reply Frame", CRCs MUST be generated and checked
by both endpoints.
Res: This field is reserved for future use. It must be set to zero
when sending, and not checked on reception.
Rev: This field contains the Revision of MPA. For this version of
the specification senders MUST set this field to zero. MPA
receivers compliant with this version of the specification MUST
check this field for zero, and close the connection and report an
error locally if any other value is detected.
PD_Length: This field MUST contain the length in Octets of the
Private Data field. A value of zero indicates that there is no
private data field present at all. The private data field may be
as long as 65535 Octets.
Private Data: This field may contain any value defined by ULPs or may
not be present. ULPs define how to set and validate this field.
8.1.2 Example Delayed Startup sequence
A variety of startup sequences are possible when using MPA on TCP.
Following is an example of an MPA/DDP startup that occurs after TCP
has been running for a while and has exchanged some amount of
streaming data. This example does not use any private data (an
example that does is shown later in 8.1.3.2 Example Immediate Startup
using Private Data on page 34), although it is perfectly legal to
include the private data. Note that since the example does not use
any Private Data, there are no ULP interactions shown between
receiving "Startup frames" and putting MPA into "Full operation".
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Initiator Responder
+---------------------------+
|ULP streaming mode |
| <Hello> request to |
| transition to DDP/MPA | +--------------------------+
| mode (optional) | --------> |ULP gets request; |
+---------------------------+ |enables MPA Responder mode|
|with last (optional) |
|streaming mode <Hello Ack>|
|for MPA to send. |
+---------------------------+ |MPA waits for incoming |
|ULP receives streaming | <-------- | <MPA Request frame> |
| <Hello Ack>; | +--------------------------+
|Enters MPA Initiator mode; |
|MPA sends |
| <MPA Request Frame>; |
|MPA waits for incoming | +--------------------------+
| <MPA Reply Frame | - - - - > |MPA receives |
+---------------------------+ | <MPA Request Frame> |
|Consumer binds DDP to MPA,|
|MPA sends the |
| <MPA Reply Frame>. |
|DDP/MPA enables FPDU |
+---------------------------+ |decoding, but does not |
|MPA receives the | < - - - - |send any FPDUs. |
| <MPA Reply Frame> | +--------------------------+
|Consumer binds DDP to MPA, |
|DDP/MPA begins full |
|operation. |
|MPA sends first FPDU (as | +--------------------------+
|DDP ULPDUs become | ========> |MPA Receives first FPDU. |
|available). | |MPA sends first FPDU (as |
+---------------------------+ |DDP ULPDUs become |
<====== |available. |
+--------------------------+
Figure 8: Example Delayed Startup negotiation
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An example Delayed Startup sequence is described below:
* Active and passive sides start up a TCP connection in the
ususal fashion, probably using sockets APIs. They exchange
some amount of streaming mode data. At some point one side
(the MPA Initiator) sends streaming mode data that
effectively says "Hello, Lets go into MPA/DDP mode."
* When the remote side (the MPA Responder) gets this streaming mode
message, the consumer would send a last streaming mode message
that effectively says "I Acknowledge your Hello, and am now in
MPA Responder Mode". The exchange of these messages establishes
the exact point in the TCP stream where MPA is enabled. The
Responding Consumer enables MPA in the Responder mode and waits
for the initial MPA startup message.
* The Initiating Consumer would enable MPA startup in the
Initiator mode which then sends the MPA Request Frame. It is
assumed that no "Private Data" messages are needed for this
example, although it is possible to do so. The Initiating
MPA (and Consumer) would also wait for the MPA connection to
be accepted.
* The Responding MPA would receive the initial "MPA Request Frame"
and would inform the consumer that this message arrived. The
Consumer can then accept the MPA/DDP connection or close the TCP
connection.
* To accept the connection request, the Responding Consumer would
use an appropriate API to bind the TCP/MPA connections to a DDP
endpoint, thus enabling MPA/DDP into full operation. In the
process of going to full operation, MPA sends the "MPA Reply
Frame". MPA/DDP waits for the first incoming FPDU before sending
any FPDUs.
* If the initial TCP data was not a properly formatted MPA Request
Frame the Consumer can close or reset the TCP connection
immediately.
* The Initiating MPA would receive the MPA Reply Frame and
would report this message to the Consumer. The Consumer can
then accept the MPA/DDP connection, or close or reset the TCP
connection to abort the process.
* On determining that the Connection is acceptable, the
Initiating Consumer would use an appropriate API to bind the
TCP/MPA connections to a DDP endpoint thus enabling MPA/DDP
into full operation. MPA/DDP would begin sending DDP
messages as MPA FPDUs.
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8.1.3 Use of "Private Data"
This section is advisory in nature, in that it suggests a method that
a ULP can deal with pre-DDP connection information exchange.
8.1.3.1 Motivation
Prior RDMA protocols have been developed that provide "private data"
via out of band mechanisms. As a result, many applications now
expect some form of "private data" to be available for application
use prior to setting up the DDP/RDMA connection. For example,
An RDMA Endpoint (referred to as a Queue Pair, or QP, in InfiniBand
and the [Verbs]) must be associated with a Protection Domain. No
receive operations may be posted to the endpoint before it is
associated with a Protection Domain. Indeed under both the
InfiniBand and proposed iWARP verbs [Verbs] an endpoint/QP is created
within a Protection Domain.
There are some applications where the choice of Protection Domain is
dependent upon the identity of the remote ULP client. For example, if
a user session requires multiple connections, it is highly desirable
for all of those connections to use a single Protection Domain.
InfiniBand, the DAT APIs and the IT-API all provide for the active
side ULP to provide "Private Data" when requesting a connection. This
data is passed to the ULP to allow it to determine whether to accept
the connection, and if so with which endpoint (and implicitly which
Protection Domain).
The Private Data can also be used to ensure that both ends of the
connection have configured their RDMA endpoints compatibly on such
matters as the RDMA Read capacity. Further ULP-specific uses are also
presumed, such as establishing the identity of the client.
Private Data is also allowed for when accepting the connection, to
allow completion of any negotiation on RDMA resources and for other
ULP reasons.
There are several potential ways to exchange this "Private Data".
For Example, the InfiniBand specification includes a connection
management protocol that allows a small amount of "private data" to
be exchanged using datagrams before actually starting the RDMA
connection.
This draft allows for small amounts of "Private Data" to be exchanged
as part of the MPA startup sequence. The actual Private Data fields
are carried in the MPA Request Frame, and the MPA Reply Frame.
If larger amounts of private data or more negotiation is necessary,
TCP streaming mode messages may be exchanged prior to enabling MPA.
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8.1.3.2 Example Immediate Startup using Private Data
Initiator Responder
+---------------------------+
|TCP SYN sent | +--------------------------+
+---------------------------+ --------> |TCP gets SYN packet; |
+---------------------------+ | Sends SYN-Ack |
|TCP gets SYN-Ack | <-------- +--------------------------+
| Sends Ack |
+---------------------------+ --------> +--------------------------+
+---------------------------+ |Consumer enables MPA |
|Enters MPA Initiator mode; | |Responder Mode, waits for |
|MPA sends | | <MPA Request frame> |
| <MPA Request Frame>; | +--------------------------+
|MPA waits for incoming | +--------------------------+
| <MPA Reply Frame | - - - - > |MPA receives |
+---------------------------+ | <MPA Request Frame> |
|Consumer examines "Private|
|Data", provides MPA with |
|return "Private Data", |
|binds DDP to MPA, and |
|enables MPA to send an |
| <MPA Reply Frame>. |
|DDP/MPA enables FPDU |
+---------------------------+ |decoding, but does not |
|MPA receives the | < - - - - |send any FPDUs. |
| <MPA Reply Frame> | +--------------------------+
|Consumer examines "Private |
|Data", binds DDP to MPA, |
|and enables DDP/MPA to |
|begin full operation. |
|MPA sends first FPDU (as | +--------------------------+
|DDP ULPDUs become | ========> |MPA Receives first FPDU. |
|available). | |MPA sends first FPDU (as |
+---------------------------+ |DDP ULPDUs become |
<====== |available. |
+--------------------------+
Figure 9: Example Immediate Startup negotiation
Note: the exact order of when MPA is started in the TCP connection
sequence is implementation dependent; the above diagram shows one
possible sequence. Also, the Initiator "Ack" to the Responder's
"SYN-Ack" may be combined into the same TCP segment containing
the MPA Request Frame (as is allowed by TCP RFCs).
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The example immediate startup sequence is described below:
* The passive side (Responding Consumer) would listen on the TCP
destination port, to indicate its readiness to accept a
connection.
* The active side (Initiating Consumer) would request a
connection from a TCP endpoint (that expected to upgrade to
MPA/DDP/RDMA and expected the private data) to a destination
address and port.
* The Initiating Consumer would initiate a TCP connection to
the destination port. Acceptance/rejection of the connection
would proceed as per normal TCP connection establishment.
* The passive side (Responding Consumer) would receive the TCP
connection request as usual allowing normal TCP gatekeepers, such
as INETD and TCPserver, to exercise their normal
safeguard/logging functions. On acceptance of the TCP
connection, the Responding consumer would enable MPA in the
Responder mode and wait for the initial MPA startup message.
* The Initiating Consumer would enable MPA startup in the
Initiator mode to send an initial "MPA Request Frame" with
its included "Private Data" message to send. The Initiating
MPA (and Consumer) would also wait for the MPA connection to
be accepted, and any returned private data.
* The Responding MPA would receive the initial "MPA Request Frame"
with the "Private Data" message and would pass the Private Data
through to the consumer. The Consumer can then accept the
MPA/DDP connection, close the TCP connection, or reject the MPA
connection with a return message.
* To accept the connection request, the Responding Consumer would
use an appropriate API to bind the TCP/MPA connections to a DDP
endpoint, thus enabling MPA/DDP into full operation. In the
process of going to full operation, MPA sends the "MPA Reply
Frame" which includes the Consumer supplied "Private Data"
containing any appropriate consumer response. MPA/DDP waits for
the first incoming FPDU before sending any FPDUs.
* If the initial TCP data was not a properly formatted MPA Request
Frame, or if the Consumer Private Data was not acceptable, the
Consumer can close or reset the TCP connection immediately.
* To reject the MPA connection request, the Responding Consumer
would send an MPA Reply Frame with any ULP supplied "Private Data
Response" (with reason for rejection) and close the TCP
connection.
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* The Initiating MPA would receive the MPA Reply Frame with the
"Private Data Response" message and would report this message
to the Consumer, including the supplied Private Data. The
Consumer can then accept and finalize the MPA/DDP connection,
or close or reset the TCP connection to abort the process.
* On determining from the "Private Data Response" that the
Connection is acceptable, the Initiating Consumer would use
an appropriate API to bind the TCP/MPA connections to a DDP
endpoint thus enabling MPA/DDP into full operation. MPA/DDP
would begin sending DDP messages as MPA FPDUs.
8.1.4 "Dual Stack" implementations
MPA/DDP implementations are commonly expected to be implemented as
part of a "Dual stack" architecture. One "stack" is the traditional
TCP stack, usually with a sockets interface API. The second stack is
the MPA/DDP "stack" with its own API, and potentially separate code
or hardware to deal with the MPA/DDP data. Of course,
implementations may vary, so the following comments are of an
advisory nature only.
The use of the two "stacks" offers advantages:
TCP connection setup is usually done with the TCP stack. This
allows use of the usual naming and addressing mechanisms. It
also means that any mechanisms used to "harden" the connection
setup against security threats are also used when starting
MPA/DDP.
Some applications may have been originally designed for TCP, but
are "enhanced" to utilize MPA/DDP after a negotiation reveals
the capability to do so. The negotiation process takes place in
TCP's streaming mode, using the usual TCP APIs.
Some new applications, designed for RDMA or DDP, still need to
exchange some data prior to starting MPA/DDP. This exchange can
be of arbitrary length or complexity, but often consists of only
a small amount of "private data", perhaps only a single message.
Using the TCP streaming mode for this exchange allows this to be
done using well understood methods.
The main disadvantage of using two stacks is the conversion of an
active TCP connection between them. This process must be done with
care to prevent loss of data.
To avoid some of the problems when using a "dual stack" architecture
the following additional restrictions may be required by the
implementation:
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1. Enabling the DDP/MPA stack SHOULD be done only when no incoming
stream data is expected. This is typically managed by the ULP
protocol. When following the recommended startup sequence, the
"Responder" side enters DDP/MPA mode, sends the last streaming
mode data, and then waits for the "MPA Request frame". No
additional streaming mode data is expected. The "Initiator" side
ULP receives the last streaming mode data, and then enters
DDP/MPA mode. Again, no additional streaming mode data is
expected.
2. The DDP/MPA MAY provide the ability to send a "Last streaming
message" as part of its "Responder" DDP/MPA enable function.
This allows the DDP/MPA stack to more easily manage the
conversion to DDP/MPA mode (and avoid problems with a very fast
return of the "MPA Request Frame" from the Initiator side).
Note: Regardless of the "stack" architecture used, TCP's rules must
be followed. For example, if network data is lost, re-segmented
or re-ordered, TCP must recover appropriately even when this
occurs while switching stacks.
8.2 Normal Connection Teardown
Each half connection of MPA terminates when DDP closes the
corresponding TCP half connection.
A mechanism SHOULD be provided by MPA to DDP for DDP 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 DDP or other consumer:
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 (if
enabled) 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 DDP. 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.
4 Invalid MPA Request Frame or MPA Response Frame received.
In this case, the TCP connection MUST be immediately closed.
DDP and other ULPs should treat this similar to code 1,
above.
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 DDP. 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 pass or deliver
any additional FPDUs on that half connection.
For Error codes 2 and 3, MPA MUST NOT close the TCP connection
following a reported error. Closing the connection is the
responsibility of DDP's ULP.
Note that since MPA will not deliver any FPDUs on a half
connection following an error detected on the receive side of
that connection, DDP's ULP is expected to tear down the
connection. This may not occur until after one or more last
messages are transmitted on the opposite half connection. This
allows a diagnostic error message to be sent.
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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 DDP 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 [RFC2401] may be used to prevent such attacks. Independent
of how MPA operates, a third party could use ICMP messages 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 DDP on
MPA on TCP, the well-known port must be registered with IANA.
Because the use of the port is DDP specific, registration of the port
with IANA is left to DDP.
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12 References
12.1 Normative References
[iSCSI] Satran, J., "iSCSI", draft-ietf-ips-iscsi-20.txt (work in
progress), January 2003.
[RFC1191] Mogul, J., and Deering, S., "Path MTU Discovery", RFC 1191,
November 1990.
[RFC2018] Mathis, M., Mahdavi, 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.
[RFC793] 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", draft-ietf-rddp-ddp-00.txt (Work in progress),
February 2003
[RFC2401] Atkinson, R., Kent, S., "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC0896] J. Nagle, "Congestion Control in IP/TCP Internetworks", RFC
896, January 1984.
[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",
draft-ietf-rddp-rdmap-00.txt, February 2003
[RFC2960] R. Stewart et al., "Stream Control Transmission Protocol",
RFC 2960, October 2000.
[RFC792] Postel, J., "Internet Control Message Protocol". September
1981
[RFC1122] Braden, R.T., "Requirements for Internet hosts -
communication layers". October 1989.
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[ELZUR-MPA] Elzur, U., "Analysis of MPA over TCP Operations" draft-
elzur-iwarp-mpa-tcp-analysis-00.txt, February 2003.
[Verbs] J. Hilland et al., "RDMA Protocol Verbs Specification" draft-
hilland-rddp-verbs-00.txt, April 2003.
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13 Appendix
This appendix is for information only and is NOT part of the
standard.
13.1 Analysis of MPA over TCP Operations
This appendix analyzes the impact of MPA (Marker PDU Aligned Framing
for TCP [MPA]) on the TCP sender, receiver, and wire protocol.
One of MPA's high level goals is to provide enough information, when
combined with the Direct Data Placement Protocol [DDP], to enable
out-of-order placement of DDP payload into the final Upper Layer
Protocol (ULP) buffer. Note that DDP separates the act of placing
data into a ULP buffer from that of notifying the ULP that the ULP
buffer is available for use. In DDP terminology, the former is
defined as "Placement", and the later is defined as "Delivery". MPA
supports in-order delivery of the data to the ULP, including support
for Direct Data Placement in the final ULP buffer location when TCP
segments arrive out-of-order. Effectively, the goal is to use the
pre-posted ULP buffers as the TCP receive buffer, where the
reassembly of the ULP Protocol Data Unit (PDU) by TCP (with MPA and
DDP) is done in place, in the ULP buffer, with no data copies.
This Appendix walks through the advantages and disadvantages of the
TCP sender modifications proposed by MPA:
1) that MPA require the TCP sender to do "Header Alignment", where a
TCP segment is required to begin with an MPA Framing Protocol Data
Unit (FPDU) (if there is payload present).
2) that there be an integral number of FPDUs in a TCP segment (under
conditions where the Path MTU is not changing).
This Appendix concludes that the scaling advantages of Header
Alignment are strong, based primarily on fairly drastic TCP receive
buffer reduction requirements and simplified receive handling. The
analysis also shows that there is little effect to TCP wire behavior.
13.1.1 Assumptions
13.1.1.1 MPA is layered beneath DDP [DDP]
MPA is an adaptation layer between DDP and TCP. DDP requires
preservation of DDP segment boundaries and a CRC32C digest covering
the DDP header and data. MPA adds these features to the TCP stream
so that DDP over TCP has the same basic properties as DDP over SCTP.
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13.1.1.2 MPA preserves DDP message framing
MPA was designed as a framing layer specifically for DDP and was not
intended as a general-purpose framing layer for any other ULP using
TCP.
A framing layer allows ULPs using it to receive indications from the
transport layer only when complete ULPDUs are present. As a framing
layer, MPA is not aware of the content of the DDP PDU, only that it
has received and, if necessary, reassembled a complete PDU for
delivery to the DDP.
13.1.1.3 The size of the ULPDU passed to MPA is less than EMSS under
normal conditions
To make reception of a complete DDP PDU on every received segment
possible, DDP passes to MPA a PDU that is no larger than the EMSS of
the underlying fabric. Each FPDU that MPA creates contains sufficient
information for the receiver to directly place the ULP payload in the
correct location in the correct receive buffer.
Edge cases when this condition does not occur are dealt with, but do
not need to be on the fast path
13.1.1.4 Out-of-order placement but NO out-of-order delivery
DDP receives complete DDP PDUs from MPA. Each DDP PDU contains the
information necessary to place its ULP payload directly in the
correct location in host memory.
Because each DDP segment is self-describing, it is possible for DDP
segments received out of order to have their ULP payload placed
immediately in the ULP receive buffer.
Data delivery to the ULP is guaranteed to be in the order the data
was sent. DDP only indicates data delivery to the ULP after TCP has
acknowledged the complete byte stream.
13.1.2 The Value of Header Alignment
Significant receiver optimizations can be achieved when Header
Alignment and complete FPDUs are the common case. The optimizations
allow utilizing significantly fewer buffers on the receiver and less
computation per FPDU. The net effect is the ability to build a "Flow-
Through" receiver that enables TCP-based solutions to scale to 10G
and beyond in an economical way. The optimizations are especially
relevant to hardware implementations of receivers that process
multiple protocol layers - Data Link Layer (e.g., Ethernet), Network
and Transport Layer (e.g., TCP/IP), and even some ULP on top of TCP
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(e.g., MPA/DDP). As network speed increases, there is an increasing
desire to use a hardware based receiver in order to achieve an
efficient high performance solution.
A TCP receiver, under worst case conditions, has to allocate buffers
(BufferSizeTCP) whose capacities are a function of the bandwidth-
delay product. Thus:
BufferSizeTCP = K * bandwidth [octets/S] * Delay [S].
Where bandwidth is the end-to-end bandwidth of the connection, delay
is the round trip delay of the connection, and K is an implementation
dependent constant.
Thus BufferSizeTCP scales with the end-to-end bandwidth (10x more
buffers for a 10x increase in end-to-end bandwidth). As this
buffering approach may scale poorly for hardware or software
implementations alike, several approaches allow reduction in the
amount of buffering required for high-speed TCP communication.
The MPA/DDP approach is to enable the ULP's buffer to be used as the
TCP receive buffer. If the application pre-posts a sufficient amount
of buffering, and each TCP segment has sufficient information to
place the payload into the right application buffer, when an out-of-
order TCP segment arrives it could potentially be placed directly in
the ULP buffer. However, placement can only be done when a complete
FPDU with the placement information is available to the receiver, and
the FPDU contents contain enough information to place the data into
the correct ULP buffer (e.g., there is a DDP header available).
For the case when the FPDU is not aligned with the TCP segment, it
may take, on average, 2 TCP segments to assemble one FPDU. Therefore,
the receiver has to allocate BufferSizeNAF (Buffer Size, Non-Aligned
FPDU) octets:
BufferSizeNAF = K1* EMSS * number_of_connections + K2 * EMSS
Where K1 and K2 are implementation dependent constants and EMSS is
the effective maximum segment size.
For example, a 1 Gbps link with 10,000 connections and an EMSS of
1500B would require 15 MB of memory. Often the number of connections
used scales with the network speed, aggravating the situation for
higher speeds.
A Header Aligned FPDU would allow the receiver to allocate
BufferSizeAF (Buffer Size, Aligned FPDU) octets:
BufferSizeAF = K2 * EMSS
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for the same conditions. A Header Aligned receiver may require memory
in the range of ~100s of KB - which is feasible for an on-chip memory
and enables a "Flow-Through" design, in which the data flows through
the NIC and is placed directly in the destination buffer. Assuming
most of the connections support Header Alignment, the receiver
buffers no longer scale with number of connections.
Additional optimizations can be achieved in a balanced I/O sub-system
-- where the system interface of the network controller provides
ample bandwidth as compared with the network bandwidth. For almost
twenty years this has been the case and the trend is expected to
continue - while Ethernet speeds have scaled by 1000 (from 10
megabit/sec to 10 gigabit/sec), I/O bus bandwidth of volume CPU
architectures has scaled from ~2 MB/sec to ~2 GB/sec (PC-XT bus to
PCI-X DDR). Under these conditions, the Header Aligned FPDU approach
allows BufferSizeAF to be indifferent to network speed. It is
primarily a function of the local processing time for a given frame.
Thus when the Header Aligned FPDU approach is used, receive buffering
is expected to scale gracefully (i.e. less than linear scaling) as
network speed is increased.
13.1.2.1 Impact of lack of Header Alignment on the receiver
computational load and complexity
The receiver must perform IP and TCP processing, and then perform
FPDU CRC checks, before it can trust the FPDU header placement
information. For simplicity of the description, the assumption is
that a FPDU is carried in no more than 2 TCP segments. In reality,
with no Header Alignment, an FPDU can be carried by more than 2 TCP
segments (e.g., if the PMTU was reduced).
----++-----------------------------++-----------------------++-----
+---||---------------+ +--------||--------+ +----------||----+
| TCP Seg X-1 | | TCP Seg X | | TCP Seg X+1 |
+---||---------------+ +--------||--------+ +----------||----+
----++-----------------------------++-----------------------++-----
FPDU #N-1 FPDU #N
Figure 10: Non-aligned FPDU freely placed in TCP octet stream
The receiver algorithm for processing TCP segments (e.g., TCP segment
#X in Figure 10: Non-aligned FPDU freely placed in TCP octet stream)
carrying non-aligned FPDUs (in-order or out-of-order) includes:
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Data Link Layer processing (whole frame) - typically including a
CRC calculation.
1. Network Layer processing (assuming not an IP fragment, the
whole Data Link Layer frame contains one IP datagram. IP
fragments should be reassembled in a local buffer. This is not
a performance optimization goal)
2. Transport Layer processing -- TCP protocol processing, header
and checksum checks.
a. Classify incoming TCP segment using the 5 tuple (IP SRC,
IP DST, TCP SRC Port, TCP DST Port, protocol)
3. Find FPDU message boundaries.
a. Get MPA state information for the connection
If the TCP segment is in-order, use the receiver managed
MPA state information to calculate where the previous
FPDU message (#N-1) ends in the current TCP segment X.
(previously, when the MPA receiver processed the first
part of FPDU #N-1, it calculated the number of bytes
remaining to complete FPDU #N-1 by using the MPA
Length field).
Get the stored partial CRC for FPDU #N-1
Complete CRC calculation for FPDU #N-1 data (first
portion of TCP segment #X)
Check CRC calculation for FPDU #N-1
If no FPDU CRC errors, placement is allowed
Locate the local buffer for the first portion of
FPDU#N-1, CopyData(local buffer of first portion
of FPDU #N-1, host buffer address, length)
Compute host buffer address for second portion of FPDU
#N-1
CopyData (local buffer of second portion of FPDU #N-1,
host buffer address for second portion, length)
Calculate the octet offset into the TCP segment for
the next FPDU #N.
Start Calculation of CRC for available data for FPDU
#N
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Store partial CRC results for FPDU #N
Store local buffer address of first portion of FPDU #N
No further action is possible on FPDU #N, before it is
completely received
If TCP out-of-order, receiver must buffer the data until
at least one complete FPDU is received. Typically
buffering for more than one TCP segment per connection
is required. Use the MPA based Markers to calculate
where FPDU boundaries are.
When a complete FPDU is available, a similar procedure
to the in-order algorithm above is used. There is
additional complexity, though, because when the
missing segment arrives, this TCP segment must be
run through the CRC engine after the CRC is
calculated for the missing segment.
If we assume Header Alignment, the following diagram and the
algorithm below apply. Note that when using MPA, the receiver is
assumed to actively detect presence or loss of Header Alignment for
every TCP segment received.
+--------------------------+ +--------------------------+
+--|--------------------------+ +--|--------------------------+
| | TCP Seg X | | | TCP Seg X+1 |
+--|--------------------------+ +--|--------------------------+
+--------------------------+ +--------------------------+
FPDU #N FPDU #N+1
Figure 11: Aligned FPDU placed immediately after TCP header
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The receiver algorithm for Header Aligned frames (in-order or out-of-
order) includes:
1) Data Link Layer processing (whole frame) - typically
including a CRC calculation.
2) Network Layer processing (assuming not an IP fragment, the
whole Data Link Layer frame contains one IP datagram. IP
fragments should be reassembled in a local buffer. This is
not a performance optimization goal)
3) Transport Layer processing -- TCP protocol processing, header
and checksum checks.
a. Classify incoming TCP segment using the 5 tuple (IP SRC,
IP DST, TCP SRC Port, TCP DST Port, protocol)
4) Check for Header Alignment. (Described in detail in [MPA]
section 7.4). Assuming Header Alignment for the rest of the
algorithm below.
a. If the header is not aligned, see the algorithm defined
in the prior section.
5) If TCP is in-order or out-of-order the MPA header is at the
beginning of the current TCP payload. Get the FPDU length
from the FPDU header.
6) Calculate CRC over FPDU
7) Check CRC calculation for FPDU #N
8) If no FPDU CRC errors, placement is allowed
9) CopyData(TCP segment #X, host buffer address, length)
10) Loop to #5 until all the FPDUs in the TCP segment are
consumed in order to handle FPDU packing.
Implementation note: In both cases the receiver has to classify the
incoming TCP segment and associate it with one of the flows it
maintains. In the case of no Header Alignment, the receiver is forced
to classify incoming traffic before it can calculate the FPDU CRC. In
the case of Header Alignment the operations order is left to the
implementer.
The Header Aligned receiver algorithm is significantly simpler. There
is no need to locally buffer portions of FPDUs. Accessing state
information is also substantially simplified - the normal case does
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not require retrieving information to find out where a FPDU starts
and ends or retrieval of a partial CRC before the CRC calculation can
commence. This avoids adding internal latencies, having multiple data
passes through the CRC machine, or scheduling multiple commands for
moving the data to the host buffer.
The aligned FPDU approach is useful for in-order and out-of-order
reception. The receiver can use the same mechanisms for data storage
in both cases, and only needs to account for when all the TCP
segments have arrived to enable delivery. . The Header Alignment,
along with the high probability that at least one complete FPDU is
found with every TCP segment, allows the receiver to perform data
placement for out-of-order TCP segments with no need for intermediate
buffering. Essentially the TCP receive buffer has been eliminated and
TCP reassembly is done in place within the ULP buffer.
In case Header Alignment is not found, the receiver should follow the
algorithm for non aligned FPDU reception which may be slower and less
efficient.
13.1.2.2 Header Alignment effects on TCP wire protocol
An MPA-aware TCP exposes its EMSS to MPA. MPA uses the EMSS to
calculate its MULPDU, which it then exposes to DDP, its ULP. DDP
uses the MULPDU to segment its payload so that each FPDU sent by
MPA fits completely into one TCP segment. This has no impact on
wire protocol and exposing this information is already supported
on many TCP implementations, including all modern flavors of BSD
networking, through the TCP_MAXSEG socket option.
In the common case, the ULP (i.e. DDP over MPA) messages provided to
the TCP layer are segmented to MULPDU size. It is assumed that the
ULP message size is bounded by MULPDU, such that a single ULP message
can be encapsulated in a single TCP segment. Therefore, in the common
case, there is no increase in the number of TCP segments emitted. For
smaller ULP messages, the sender can also apply packing, i.e. the
sender packs as many complete FPDUs as possible into one TCP segment.
The requirement to always have a complete FPDU may increase the
number of TCP segments emitted. Typically, a ULP message size varies
from few bytes to multiple EMSS (e.g., 64 Kbytes). In some cases the
ULP may post more than one message at a time for transmission, giving
the sender an opportunity for packing. In the case where more than
one FPDU is available for transmission and the FPDUs are encapsulated
into a TCP segment and there is no room in the TCP segment to include
the next complete FPDU, another TCP segment is sent. In this corner
case some of the TCP segments are not full size. In the worst case
scenario, the ULP may choose a FPDU size that is EMSS/2 +1 and has
multiple messages available for transmission. For this poor choice of
FPDU size, the average TCP segment size is therefore about 1/2 of the
EMSS and the number of TCP segments emitted is approaching 2x of what
is possible without the requirement to encapsulate an integer number
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of complete FPDUs in every TCP segment. This is a dynamic situation
that only lasts for the duration where the sender ULP has multiple
non-optimal messages for transmission and this causes a minor impact
on the wire utilization.
However, it is not expected that requiring Header Alignment will have
a measurable impact on wire behavior of most applications. Throughput
applications with large I/Os are expected to take full advantage of
the EMSS. Another class of applications with many small outstanding
buffers (as compared to EMSS) is expected to use packing when
applicable. Transaction oriented applications are also optimal.
TCP retransmission is another area that can affect sender behavior.
TCP supports retransmission of the exact, originally transmitted
segment (see [RFC0793] section 2.6, [RFC0793] section 3.7 "managing
the window" and [RFC1122] section 4.2.2.15 ). In the unlikely event
that part of the original segment has been received and acknowledged
by the remote peer (e.g., a re-segmenting middle box, as documented
in 7.4.1 Re-segmenting Middle boxes and non MPA-aware TCP senders on
page 25), a better available bandwidth utilization may be possible by
re-transmitting only the missing octets. If an MPA-aware TCP
retransmits complete FPDUs, there may be some marginal bandwidth
loss.
Another area where a change in the TCP segment number may have impact
is that of Slow Start and Congestion Avoidance. Slow-start
exponential increase is measured in segments per second, as the
algorithm focuses on the overhead per segment at the source for
congestion that eventually results in dropped segments. Slow-start
exponential bandwidth growth for MPA-aware TCP is similar to any TCP
implementation. Congestion Avoidance allows for a linear growth in
available bandwidth when recovering after a packet drop. Similar to
the analysis for slow-start, MPA-aware TCP doesn't change the
behavior of the algorithm. Therefore the average size of the segment
versus EMSS is not a major factor in the assessment of the bandwidth
growth for a sender. Both Slow Start and Congestion Avoidance for an
MPA-aware TCP will behave similarly to any TCP sender and allow an
MPA-aware TCP to enjoy the theoretical performance limits of the
algorithms.
In summary, the ULP messages generated at the sender (e.g., the
amount of messages grouped for every transmission request) and
message size distribution has the most significant impact over the
number of TCP segments emitted. The worst case effect for certain
ULPs (with average message size of EMSS/2+1 to EMSS), is bounded by
an increase of up to 2x in the number of TCP segments and
acknowledges. In reality the effect is expected to be marginal.
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13.2 Receiver implementation
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 out of order packets
arrive and Markers are not enabled. Buffers are also 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. For cases where a re-segmenting
middle box is present, or where the TCP sender is not MPA-aware, the
presence of markers significantly reduces the amount of buffering
needed.
Recovery from IP Fragmentation must be transparent to the MPA
Consumers.
13.2.1 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.
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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].
13.2.2 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. Buffers are also needed if
Markers are disabled and out of order packets arrive.
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) 2003 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
Culley et. al. Expires: March 2004 [Page 58]
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