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Versions: 00 01

Transport Area Working Group                   S. Bailey    (Sandburst)
Internet-draft                                 J. Chase          (Duke)
Expires: May 2002                              J. Pinkerton (Microsoft)
                                               A. Romanow       (Cisco)
                                               C. Sapuntzakis   (Cisco)
                                               J. Wendt            (HP)
                                               J. Williams     (Emulex)

                     TCP ULP Framing Protocol (TUF)
                   draft-ietf-tsvwg-tcp-ulp-frame-01


Status of this Memo

     This document is an Internet-Draft and is in full conformance with
     all provisions of Section 10 of RFC2026.

     Internet-Drafts are working documents of the Internet Engineering
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Copyright Notice

     Copyright (C) The Internet Society (2001). All Rights Reserved.


Abstract


     The TCP ULP Framing (TUF) protocol defines a shim layer protocol
     between an Upper Layer Protocol (ULP) and TCP.  TUF also depends on
     a specified TCP segmentation convention between TUF endpoints.
     Together, the shim and segmentation conventions enable a TUF/TCP
     receiver to recognize ULP data units within a TCP segment
     independently of other TCP segments.  This capability simplifies



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     the design of enhanced network interfaces implementing direct data
     placement for ULPs using TCP.  Direct data placement is a key step
     to making IP networking competitive with high-end interconnect
     solutions in data centers and other high-performance application
     domains.


Table Of Contents

     1.     Definitions  . . . . . . . . . . . . . . . . . . . . . .   3
     2.     Overview . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.   Motivation . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.   Approach . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.     Rational For TUF . . . . . . . . . . . . . . . . . . . .   6
     3.1.   Direct Data Placement  . . . . . . . . . . . . . . . . .   7
     3.2.   Direct Data Placement with TCP . . . . . . . . . . . . .   8
     3.2.1. The Simple Case: ULP-unaware Placement . . . . . . . . .   9
     3.2.2. The Complex Case: ULP-aware Placement  . . . . . . . . .   9
     3.2.3. The Problem of ULP-aware Placement with TCP  . . . . . .  10
     3.2.4. Finding ULPDUs In Out-of-order Segments  . . . . . . . .  11
     3.2.5. The TUF Solution . . . . . . . . . . . . . . . . . . . .  12
     3.2.6. TUF's ULP Assumptions  . . . . . . . . . . . . . . . . .  12
     4.     The Protocol . . . . . . . . . . . . . . . . . . . . . .  13
     4.1.   The Framing Protocol Data Unit (FPDU)  . . . . . . . . .  13
     4.1.1. FPDU Format  . . . . . . . . . . . . . . . . . . . . . .  13
     4.1.2. FPDU Size Selection  . . . . . . . . . . . . . . . . . .  14
     4.2.   TUF-conforming TCP Sender Segmentation . . . . . . . . .  15
     4.3.   Negotiating TUF  . . . . . . . . . . . . . . . . . . . .  15
     4.4.   TUF Receiver ULPDU Containment Property Testing  . . . .  16
     5.     Protocol Characteristics . . . . . . . . . . . . . . . .  17
     5.1.   Properties Of TUF-conforming TCP Senders . . . . . . . .  17
     5.2.   Exception Cases  . . . . . . . . . . . . . . . . . . . .  18
     5.2.1. Resegmenting Intermediaries  . . . . . . . . . . . . . .  18
     5.2.2. PMTU Reduction . . . . . . . . . . . . . . . . . . . . .  19
     5.2.3. PMTU Increase  . . . . . . . . . . . . . . . . . . . . .  20
     5.2.4. Receive Window < EMSS  . . . . . . . . . . . . . . . . .  21
     5.2.5. Size of ULPDU + 8 > EMSS . . . . . . . . . . . . . . . .  21
     6.     Security Considerations  . . . . . . . . . . . . . . . .  22
     6.1.   Protocol-specific Security Considerations  . . . . . . .  22
     6.2.   Using IPSec With TUF . . . . . . . . . . . . . . . . . .  22
     6.3.   Using TLS With TUF . . . . . . . . . . . . . . . . . . .  22
     7.     IANA Considerations  . . . . . . . . . . . . . . . . . .  25
            References . . . . . . . . . . . . . . . . . . . . . . .  25
            Authors' Addresses . . . . . . . . . . . . . . . . . . .  26
     A.     Sample Sockets Support For TUF . . . . . . . . . . . . .  27
     A.1    Basic Principles . . . . . . . . . . . . . . . . . . . .  28
     A.2    Enabling TUF . . . . . . . . . . . . . . . . . . . . . .  28
     A.3    Sending Data . . . . . . . . . . . . . . . . . . . . . .  29



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     A.4    Retrieving The Current EMSS or MULPDU  . . . . . . . . .  29
     A.5    Disabling ULPDU Packing  . . . . . . . . . . . . . . . .  29
     A.6    Disabling The Report of Oversized ULPDUs . . . . . . . .  30
            Full Copyright Statement . . . . . . . . . . . . . . . .  30



1.  Definitions

     The following terms and abbreviations are used in this document.

          data delivery - the delivery of received ULP payloads to the
          ULP application, i.e, notifying the application of data
          arrival by completing a receive operation or generating an
          event.

          data placement - the storage of received ULP payloads to host
          memory, pending delivery to the ULP application.

          direct data placement - the storage of received ULP payloads
          directly to application-specified buffers without intermediate
          buffering or copying.

          EMSS - the effective maximum segment size.  EMSS is the TCP
          maximum segment size (MSS) defined in RFC 793 [TCP] and
          exchanged during TCP connection establishment, adjusted by the
          current path maximum transfer unit (MTU) [PathMTU].

          FPDU - framing protocol data unit.  The protocol data unit
          defined by TUF.

          MULPDU - maximum upper layer protocol data unit size.  The
          size of the largest ULPDU that fits in an EMSS-sized FPDU.

          NIC - network interface controller.  The device that provides
          a host's access to a physical network link.

          PDU - protocol data unit.  A self-contained block of control
          and data defined by a particular protocol.

          RDMA - Remote Direct Memory Access protocol.  A data transfer
          protocol which uses memory access-style transfer mode(s) to
          provide generic direct data placement capabilities for
          arbitrary ULPs.

          TUF - TCP ULP Framing protocol.  The protocol defined in this
          document.




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          ULP - upper layer protocol.  The client protocol using the
          services of the transport layer, or TUF.

          ULPDU - upper layer protocol data unit.

          ULPDU containment property - the property that a TCP segment
          contains exactly an integral number of ULPDUs.

2.  Overview

     This section summarizes the motivation for the TCP ULP Framing
     (TUF) protocol and explains its operation in brief.  Section 3
     (`Rational for TUF') develops the rationale for TUF in detail.
     Section 4 (`The Protocol') defines the protocol itself.  Section 5
     (`Protocol Characteristics') examines various properties of the
     protocol's operation.  Implementors may wish to refer directly to
     sections 4 and 5.

2.1.  Motivation

     The IP protocols are not usually used for high-performance high
     speed data transfers due to overhead in TCP processing. Instead, a
     number of special purpose protocols have been used. The domain of
     application for such high speed buffer transfer includes storage,
     video delivery and processing, and various applications of cluster
     computing, such as scalable database or application service.  For
     reasons discussed below, today, there is great industry interest in
     developing an IP standard for low overhead high bandwidth data
     transfer, which would decrease the costs of high speed
     interconnects and supplant special purpose protocols.

     The approach typically used for low overhead transfers is called
     direct data placement, in which the network interface places data
     directly in application buffers, avoiding the latency and memory
     bandwidth costs associated with copying.  Direct data placement can
     in principal be done with either of IP's reliable transports--SCTP
     or TCP.  This document considers what is needed to do direct data
     placement with TCP.

     In order to place data directly in application buffers, the network
     interface needs to use information in the Upper Layer Protocol Data
     Units (ULPDUs) contained in the TCP stream.  This can be
     accomplished routinely except when TCP segments arrive out of
     order.  If TCP segments arrive out of order, the location of the
     ULPDUs in the TCP segment cannot be found.  The TUF protocol
     addresses this problem of finding ULPDU headers in the TCP stream,
     even when TCP segments arrive out of order.




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2.2.  Approach

     TUF is implemented as a shim layer between an ULP and TCP.  The
     end-to-end data flow is:

     0.   Use of TUF is negotiated end-to-end by the ULP.

     1.   The ULP delivers a data stream with ULPDUs delimited to TUF.

     2.   TUF inserts a header and delivers the shimmed ULPDUs to TCP.

     3.   The TUF-aware TCP sender preserves boundaries of shimmed
          ULPDUs (TUF FPDUs) as much as possible when delivering
          segments to the IP layer.

     4.   The receiving TCP delivers shimmed ULPDUs to the receiving TUF
          layer.

     5.   TUF removes the shim and delivers the ULPDUs to the ULP.

     In other words, the layering of TUF is:

                          ULP client
                               ^
                               |
                               | ULPDUs (in octet stream)
                               |
                               v
                              TUF
                               ^
                               |
                               | FPDUs (containing ULPDUs)
                               |
                               v
                      TUF-conforming TCP
                               ^
                               |
                               | TCP Segments (each containing an FPDU)
                               |
                               v
                             . . .


     Note that while the semantics of this protocol layering must be
     maintained, the receiving network interface may use the information
     in the framed ULPDUs to place the data in memory on the host.
     Whatever the case, the data is only delivered to the ULP when all
     preceding TCP data has arrived.



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3.  Rational For TUF

     This document defines the TUF protocol as a shim layer between an
     Upper Layer Protocol (ULP) and TCP.  TUF also depends on a TCP
     segmentation convention between TUF/TCP endpoints specified in this
     document.  Taken together they provide the capability for a TUF/TCP
     receiver to recognize ULPDUs by processing each TCP segment
     independently, without requiring state from previous segments.

     The purpose of TUF is to enable practical designs for enhanced
     network interfaces (NICs) implementing direct data placement for
     TCP-based ULPs.  The purpose of direct data placement is to
     eliminate the need for a host to copy received data after it
     arrives in host memory.  This copying incurs CPU, memory and bus
     costs that are substantial and are not masked by advancing hardware
     technology.

     A general and practical solution to the receive copy problem has
     eluded the IP networking community for almost two decades.  There
     is a long history of research and experimental schemes to reduce or
     eliminate receiver copying overhead for IP networking in general,
     and for TCP/IP communication in particular.  While these systems
     have convincingly demonstrated the potential performance benefits
     of reducing copy costs, all such schemes suffer from one or more of
     the following limitations: they require a significant restructuring
     of operating system buffering and/or APIs; they are limited to
     specific modes of communication (e.g., bulk data transfer) or
     specific application ULPs; they do not scale on multiprocessor
     hosts; their benefits depend on specific properties of the network
     (e.g., large MTUs) or host buffer size and alignment.  Moreover,
     all such schemes require some degree of support from NICs to
     separate payloads from headers and/or ensure that their placement
     in host memory meets specific requirements (e.g., for page
     placement and alignment).

     Inherent copying costs for IP communication are one motivation to
     use alternative non-IP technologies for high-speed networking.  A
     number of specialized technologies have been developed for high
     speed data transfers in which network interfaces transfer data from
     application buffer to application buffer without software touching
     the data.  Some examples include the VAXCluster Interconnect in
     1983, Fibre Channel (FC) in 1994, and today InfiniBand (IB) and
     Virtual Interface Architecture (VIA).  These alternatives have
     eroded the popularity of IP technologies in application domains
     including network storage, video processing and delivery, and
     cluster computing for scientific applications and scalable
     database-related services.




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     Until recently, several factors have limited interest in promoting
     IP networking as a solution in these application domains.  First,
     the competing network technologies offered significantly higher
     link speeds than the network hardware available for use with IP.
     Second, these application domains were a relatively small segment
     of the network market.  Recently, however, Ethernet networks have
     closed the bandwidth gap and even exceeded the bandwidth of
     alternatives such as FibreChannel, at much lower cost.  At the same
     time, an increasing number of applications are server-hosted in
     data centers to enable sharing and access from a growing number of
     IP-connected client devices and locations.  With the growth in
     importance and number of data centers, high-speed interconnection
     within the data center is now central to the everyday operation of
     Internet services.

     Thus, technology changes have created an opportunity and demand to
     extend the benefits of IP technologies to high-performance
     application domains, while simultaneously increasing the importance
     of those domains.  The ubiquity of IP offers economies of scale
     heavily favoring IP in these domains.  For example, reliance on
     specialized non-IP technologies for high-performance domains
     creates a need to support multiple protocols and redundant network
     infrastructure in data centers, and it compromises portability and
     interoperability of data center solutions.  Moreover, comprehensive
     support for network management and security is developing rapidly
     in the IP space.  Use of IP technologies would allow data centers
     to benefit from these enhancements.

3.1.  Direct Data Placement

     Direct data placement is a key step toward making IP networking
     competitive in data centers and other high-performance domains.
     Direct data placement refers to the ability of a NIC to place data
     directly from the network into designated application buffers,
     without intermediate copying.  Direct data placement is attractive
     relative to other solutions to the receive copy problem.  It is the
     only solution that can be implemented in a way that is compatible
     with existing operating systems, since the receiving NIC takes over
     most of the responsibility to avoid receive copying.  Also, direct
     data placement generalizes easily to a range of ULPs.  In
     particular, the establishment of an IETF standard for an IP
     transport-based direct data placement protocol, which would allow
     NICs to directly place data independent of the application ULP
     using it.

     The TUF protocol is necessary to permit easily deployable enhanced
     NICs supporting direct data placement.  Such NICs already exist and
     their usage is growing rapidly, but their development is impeded by



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     the lack of standards.  Direct data placement is unnecessarily
     difficult and expensive to design and implement for existing TCP-
     based ULPs; the key objective of TUF is to define transport
     conventions to simplify the design of these NICs.  A related
     impediment is that in the absence of a general direct data
     placement protocol these products are limited to specific ULPs such
     as iSCSI.  TUF, and possibly additional, higher layer protocol
     definitions outside the scope of this document, would encourage the
     market by ensuring interoperability of product offerings from
     different vendors.

     This document defines a framing protocol (TUF) and TCP segmentation
     conventions that enable simple support of direct data placement for
     a class of TCP-based ULPs.  It does not propose a generic direct
     placement ULP, such as an RDMA protocol, or any facility for direct
     data placement, but only the foundations for building such a
     facility on TCP.  A key objective of TUF is to do this in a way
     that is compatible with existing standards and with the spirit of
     TCP's stream communication model.  TUF can simplify support for
     direct data placement for ULPs such as iSCSI, and it can serve as a
     basis for a future RDMA proposal.

     The key limitation of TUF as a solution to the receive copy problem
     is that it works only if the ULP standard and the sending and
     receiving implementations all support it.  Impact on the sender and
     ULPs is minimal, but ULPs must be adapted to allow use of TUF at
     the ULP/transport boundary.  The necessary modifications may be
     quite small.  Use of TUF is a negotiated option between the sender
     and receiver for each ULP session, preserving interoperability
     among senders and receivers that do not support TUF.

3.2.  Direct Data Placement with TCP

     Direct data placement is widely used to accomplish high-performance
     data transfer in non-IP technologies such as block storage channels
     (SCSI, Fibre Channel, etc.), and other specialized high performance
     networks like InfiniBand.  This section considers how direct
     placement can be done with TCP.

     The Internet Protocol suite provides two transports that are prime
     candidates for use with direct data placement -- SCTP and TCP.  The
     framing features of the SCTP Stream Control Transmission Protocol
     [SCTP] make it more directly adaptable for direct data placement
     for future ULPs using SCTP.  However, the maturity and ubiquity of
     TCP make it desirable to define a flexible method for direct data
     placement for TCP-based ULPs as well.

     There has been a great deal of `moral confusion' concerning the



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     interaction of direct data placement with TCP's ordering
     guarantees.  These ordering guarantees do not prohibit direct data
     placement, even if data is placed as it arrives out of order.

     TCP guarantees data delivery to the application ULP as an ordered,
     sequential stream [RFC793].  Data is delivered only when TCP has
     notified the application of its arrival and transferred ownership
     of the receive data buffer.  TCP does not specify how received data
     is stored prior to its delivery, and it does not preclude placement
     of data in application buffers out of order, as long as no data is
     delivered until all preceding data has also been delivered.  Out-
     of-order placement greatly simplifies direct data placement NICs
     because it streamlines data paths and eliminates the need for a TCP
     reassembly buffer on the NIC.

     An implementation performing direct data placement must still
     respect all TCP delivery semantics.  For example, if a checksum
     integrity check fails, the data must not be placed in ULP-supplied
     buffers, because, for example, the TCP ports and the TCP sequence
     number are not trustworthy.

3.2.1.  The Simple Case: ULP-unaware Placement

     Direct data placement into a ULP client-supplied buffer designated
     to hold the next data delivered to the ULP, regardless of the
     contents of the received data, is one of the simplest possible
     forms of direct data placement.  This form of direct data placement
     is already fully supported by existing TCP mechanisms.  New NIC
     products currently, or soon to be available, which claim to offer
     `full zero copy operation' typical provide only this ULP-unaware
     form of direct data placement.

     While ULP-unaware direct data placement works well for ULPs like
     FTP where the entire contents of a TCP connection are known to be
     nothing but a single stream of bulk client data, most widely used
     ULPs, e.g. HTTP [HTTP], BEEP [BEEP] and storage protocols,
     multiplex control and data, and possibly even interleave data from
     different requests on the same TCP connection.  The simple ULP-
     unaware direct data placement is inadequate to avoid data copies
     for these ULPs.

3.2.2.  The Complex Case: ULP-aware Placement

     An explicit goal of this proposal is to support out-of-order direct
     data placement for ULPs that provide additional transport-like
     features such as control and data multiplexing, layered above TCP
     (e.g., iSCSI or a generic direct data placement protocol such as
     RDMA).  In many ULPs, such as storage protocols, control



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     information contained in the ULP uniquely identifies the
     destination application buffer of each particular piece of data.

     For example, suppose a client requests a read operation using a
     network storage ULP, specifying the destination buffer for the
     requested data.  The requesting ULP includes control information in
     the request (e.g., in the ULPDU header) uniquely identifying that
     buffer, and the responder includes that information in the read
     response.  For some protocols, the identifier is a unique request
     ID, allowing the client ULP to identify the buffer indirectly
     through a table of pending requests.  If the storage protocol uses
     RDMA, the response may specify the buffer directly by means of a
     region identifier.

     A network interface that understands the relevant ULP control
     information can use it to place the incoming data (e.g., read
     response payload) directly in the correct buffer.  In this case,
     data placement is guided by ULPDU headers embedded in the TCP data
     stream.  The NIC accesses these headers as hints for placement of
     the ULP payloads--a form of integrated layer processing for each
     TCP segment as it arrives.  This is compatible with TCP's ordering
     properties if completion of ULP header processing and delivery of
     the payload data to the application are strictly in order.

3.2.3.  The Problem of ULP-aware Placement with TCP

     The problem with performing direct data placement as a function of
     ULP control information in TCP is that it may be difficult to
     locate the ULP control information (ULPDU headers) within a TCP
     segment.

     If all TCP segments are received in sequence order, ULP control
     information can be unambiguously located by the rules that permit
     any ULP implementation to do so.  For example, each ULPDU may
     contain a length field that implicitly specifies the location of
     the beginning of the subsequent ULPDU.

     If TCP segments are not received in sequence order, without taking
     additional measures, it may not be possible to unambiguously locate
     ULP control information needed for direct data placement.  For
     example, if ULPDU length information is in a TCP segment that is
     delayed or lost in transmission, assuming the ULPDU length is the
     only means of locating the beginning of the subsequent ULPDU, it is
     impossible to locate ULP control information for ULPDUs in
     subsequent TCP segments until the lost or delayed TCP segment is
     received.  ULP control information, and the data whose placement
     depends on it may even be in different TCP segments.  If the ULP
     control information is in a TCP segment that is delayed or lost, it



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     is impossible to directly place the data until the ULP control
     information is received.

3.2.4.  Finding ULPDUs In Out-of-order Segments

     Early attempts at ULP-aware direct data placement in TCP took the
     approach of only directly placing data for TCP segments received
     in-order.  Otherwise, data was copied through a reassembly buffer
     as in a traditional implementation.  Unfortunately packet loss, and
     attendant out-of-order reception is a frequent, continuous
     characteristic of both wide-area, and switched local area networks
     of almost any size, as TCP adjusts to varying congestion
     conditions.  Under these conditions, a large portion of the data
     transferred ends up being copied, rather than being directly
     placed.

     Another solution to this problem is to build a reassembly buffer
     into the network interface.  Data received out-of-order can be held
     in the network interface reassembly buffer until all preceding data
     is received, and then direct placement can be performed on the
     reassembled data.  Within certain implementation assumptions, this
     is reasonable approach, but, unfortunately there are a number of
     issues including very large memory requirements, limited
     scalability, and increased latency, that make the reassembly
     approach undesirable.

     The size of reassembly buffer needed in the network interface is a
     direct function of the bandwidth * delay product of all active TCP
     connections.  Reasonable assumptions on the active bandwidth *
     delay product can imply a large amount of reassembly memory.
     Furthermore, this large reassembly memory must run at high
     speed---more than two times the link speed, to maintain full link
     bandwidth.

     Finally, performing reassembly in the network interface requires
     that the bandwidth from the network interface to host memory be not
     just equal, but substantially greater than the maximum bandwidth of
     the network link, to ensure that the reassembly buffer is drained
     when reassembly is complete.  System bus and interconnect bandwidth
     are particularly scarce and expensive resources in most systems.

     What is needed to permit ULP-aware direct data placement without
     reassembly buffering is a way to ensure that the ULP control
     information and the data associated with it is highly likely to be
     contained completely within a single TCP segment, and a way for a
     receiver to validate this containment property on TCP segments it
     receives.  If the receiver can determine that a ULPDU starts at the
     beginning of a TCP segment, the receiver can perform ULP-aware



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     direct placement for that ULPDU, and subsequent ULPDUs contained in
     that TCP segment.  The property that a ULPDU is completely
     contained within a TCP segment is called the `ULPDU containment
     property'.

3.2.5.  The TUF Solution

     The TUF protocol defines a shim layer above TCP and below the ULP
     that allows the receiver to validate the ULPDU containment property
     for each TCP segment received, independently of any other TCP
     segment.  The TUF protocol also defines a segmentation behavior for
     the TCP sender that ensures the ULPDU containment property holds as
     often as possible while still respecting the protocol requirements
     for TCP senders.

     The TUF-specified TCP segmentation behavior ensures that the ULPDU
     containment property is maintained as long as the receiver window
     size is at least equal to the effective MSS (EMSS), the path MTU
     (PMTU) does not change, and the TCP stream is not resegmented by an
     intermediary.  In conditions where the TCP receiver window size is
     smaller than EMSS, or the PMTU changes, the segmentation behavior
     further ensures that once the relevant condition is restored, the
     ULPDU containment property will be satisfied again.

     For the high-performance applications that this protocol targets,
     small receiver window sizes, and PMTU changes are rare transients.
     Thus, the specified protocol ensures that ULP control information
     and its associated data are virtually always together in a single
     TCP segment.

3.2.6.  TUF's ULP Assumptions

     A key assumption of TUF is that ULPs running on TUF can adjust
     ULPDU sizes to fit completely within an EMSS-sized TCP segment.
     Clearly, if a ULPDU does not fit within an EMSS-sized TCP segment,
     the ULPDU containment property can not be satisfied.  Most storage
     protocols (e.g. iSCSI), and other performance-targeted protocols
     (e.g. RDMA protocols) support this capability.  ULPs that can not
     adjust ULPDU sizes to fit within an EMSS-sized TCP segment, but
     still want the performance advantages of direct data placement, can
     be mapped on top of an intermediate protocol (e.g. an RDMA
     protocol) that does support this data `chunking'.

     TUF does not change the stream delivery semantics of TCP to the
     ULP, through the TUF implementation.  It merely inserts a shim
     header that can be used by direct placement network interfaces to
     verify the ULPDU containment property.  The shim header is inserted
     by the sending TUF implementation and removed by the receiving TUF



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     implementation, leaving a stream to be delivered to the ULP.

4.  The Protocol

     This section defines the TUF protocol itself.  The first two
     sections are the core of the protocol defining:

     o    the shim layer PDUs, called FPDUs,

     o    a TCP-conforming segmentation behavior which ensures the ULPDU
          containment property holds under most conditions.

     The remaining sections cover other aspects of the protocol which
     are primarily implications of the core protocol:

     o    what ULP-specified negotiations to enable TUF must accomplish,

     o    how receivers can process received TCP segments to establish
          whether the ULPDU containment property holds.

4.1.  The Framing Protocol Data Unit (FPDU)

     TUF sends groups of one or more complete ULPDUs in a framing
     protocol data unit (FPDU).

4.1.1.  FPDU Format

     The format of an FPDU is:

     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Length               |             Key               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              Key                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                                                               |
     ~                                                               ~
     ~                            ULPDUs                             ~
     |                                                               |
     |                                                               |
     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            ULPDUs             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


     Length: 16 bits (unsigned integer)



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          This is the length in octets of the set of framed ULPDUs.  It
          does not include the length of the FPDU header itself.

     Key: 48 bits (unsigned integer)

          This is used by the receiver to validate the ULPDU containment
          property.  It is selected at random by the sender, and
          initially signaled to the receiver in a ULP-specified way,
          before the receiver attempts to test the ULPDU containment
          property.  All FPDUs sent on the same connection in the same
          direction must use the same key value.  A good quality random
          number generator MUST be used to generate the initial key.
          RFC 1750 discusses relevant characteristics and provides
          references for good quality random number generation
          [RFC1750].

     The length of an FPDU is 8 + L octets, where L is the length of the
     set of framed ULPDUs.  The 16-bit length field is sufficient to
     permit a TCP segment with an FPDU to completely fill a maximum-size
     IPv4 or IPv6 datagram.

4.1.2.  FPDU Size Selection

     Each FPDU SHOULD contain as many contiguous, complete ULPDUs as
     will fit within the current EMSS, unless ULPDU packing is disabled.
     If ULPDU packing is disabled each FPDU SHALL contain a single
     ULPDU.  ULPDU packing mode may be negotiated, or specified a priori
     by a ULP.  Disabling ULPDU packing is analogous to disabling the
     Nagle algorithm in TCP.

     TUF SHALL present the size of the largest ULPDU size fitting in an
     EMSS-sized FPDU (MULPDU) to the ULP.  MULPDU is EMSS - the FPDU
     header size (8 octets).  ULPs SHOULD submit as large ULPDUs as
     possible to TUF, up to MULPDU, subject to limits imposed by
     specific ULP properties.  The ULP MAY also chose to pack several
     ULPDUs into an EMSS-sized unit before submitting them as one ULPDU
     to TUF.  Depending upon the ULP, ULP packing may improve data
     transfer efficiency, and is unlikely to have any detrimental
     effect.

     A TUF implementation probing for PMTU increase SHOULD present an
     increased MULPDU value to the ULP until a large enough FPDU to
     perform the probe results.

     Under exceptional circumstances, the EMSS can become too small to
     accommodate even a single ULPDU.  For example, a ULP may define
     fixed-sized PDUs that are incompressible, or variable size PDUs
     with some absolute minimum size, such as the size of a data PDU



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     containing a minimum amount of data.  It is possible for the EMSS
     to shrink to as small as 8 octets [PathMTU].  If the EMSS is too
     small to accommodate an incompressible ULPDU, the FPDU MUST contain
     only that ULPDU.  ULPs using TUF SHOULD NOT define ULPDUs with a
     minimum size greater than 128 octets.

4.2.  TUF-conforming TCP Sender Segmentation

     TCP senders are allowed substantial freedom in the choice of how to
     segment an outgoing TCP stream.  Within the confines of the
     receiver-advertised receive window, and the sender computed
     congestion window, any segmentation is permitted.  Virtually all
     TCP implementations do attempt to segment outgoing TCP streams into
     EMSS-sized segments where possible because it improves performance.

     TUF-conforming TCP sender behavior ensures that the ULPDU
     containment property holds most of the time.  To do this, a TUF-
     conforming TCP sender MUST respect a single additional rule in
     performing segmentation:

          A TUF-conforming TCP sender MUST segment the outgoing TCP
          stream such that the first octet of every FPDU is sent at the
          beginning of a TCP segment

4.3.  Negotiating TUF

     Negotiating the use of TUF is the responsibility of the ULP.  The
     use of TUF MAY be negotiated separately for each direction on a
     connection.  The negotiation procedure MUST ensure that when TUF is
     enabled or disabled, the remote peer will not transmit its first
     TCP segment in the new mode until it is certain that the local peer
     has actually enabled or disabled TUF.

     TUF operation is characteristically requested by the receiver and
     offered by the sender.  Before enabling TUF, the relevant
     parameters:

     1.   the sender's 48-bit key

     2.   ULPDU packing mode

     MUST be established at each peer.

     A natural way to enable the use of TUF is a ULP-defined negotiation
     exchange of the TUF parameters culminating in enabling TUF, if
     requested, for each transfer direction.  A three-way handshake
     protocol can be used to ensure that the point at which TUF is
     enabled is unambiguous and each end has time to perform local state



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     changes.  A connection on which TUF is enabled is likely to be the
     same connection on which the negotiation occurs, but this is not
     required.  A new connection could also use TUF from its initial
     establishment, if the TUF parameters and modes are known through
     some out-of-band mechanism.

     Use of TUF could be disabled during a connection using a similar
     ULP-defined three-way handshake.

     Other alternatives to parameter exchange include stipulating some
     parameters a priori.  For example, a ULP could specify that TUF
     with ULPDU packing enabled is always used in both directions.  In
     this case, only the 48-bit keys need to be exchanged before TUF is
     enabled.  Or, a ULP could determine TUF characteristics on the
     basis of the TCP port number.

4.4.  TUF Receiver ULPDU Containment Property Testing

     A TUF receiver that wishes to use ULP control information to
     perform direct data placement must first verify the ULPDU
     containment property.  To do this, the receiver MUST establish that
     the TCP segment contains exactly one FPDU.  Abstractly, this can be
     done by assuming the TCP segment payload begins with an FPDU, and
     verifying the following properties of that putative FPDU:

     o    The received TCP segment payload length equals the FPDU length
          plus the length of the FPDU header (8 octets).

     o    The 48-bit key equals the value signaled to the receiver when
          TUF was enabled for the connection.

     If these conditions are true, the TUF receiver MAY assume that the
     ULPDU containment property holds, and use ULP control information
     to directly place data in the contained ULPDUs.

     TUF DOES NOT provide any information that a TUF receiver can use to
     locate ULP control information beyond the ULPDU containment
     property.  In particular, a TUF receiver MUST NOT scan TCP segments
     in an attempt to locate FPDUs that do not begin at the beginning of
     a TCP segment.  However, even if the ULPDU containment property
     does not hold, a TUF receiver may still be able to reliably locate
     and use ULP control information.  For example, if a received TCP
     segment contains the next unreceived data in the TCP stream, the
     location of ULPDUs in that segment are unambiguous.  The behavior
     of a TUF receiver acting on ULP control information located with
     properties other than the ULPDU containment property is not
     specified here.




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5.  Protocol Characteristics

     This section discusses some characteristics and behavior which are
     implications of the TUF protocol.

5.1.  Properties Of TUF-conforming TCP Senders

     The general practice of TCP senders to send as much data as
     possible within a TCP segment (up to EMSS) implies that an FPDU
     whose size is less than or equal to EMSS, and whose first octet
     begins a TCP segment will be sent entirely within a single TCP
     segment.  This ensures the ULPDU containment property for that TCP
     segment.

     A TUF-conforming TCP sender still obeys all requirements of TCP.
     While the segmentation of a TUF-conforming TCP sender will have
     distinctive characteristics when viewed from the network wire, the
     same segmentation behavior could also result from a stock TCP
     sender.

     The one property of a TUF-conforming TCP sender which arguably
     departs from traditional expectations is that a TUF-conforming TCP
     sender may not produce TCP segments which are as close in size to
     EMSS as a stock TCP sender.  The need to ensure the ULPDU
     containment property may result in TCP segments which are not as
     full as if the property did not need to hold.  While this is
     abstractly true, in practice, several characteristics combine to
     minimize this effect.  Specifically:

     o    Packing ULPDUs into FPDUs gives behavior similar to that of
          stock TCP segmentation, albeit with coarser granularity.

     o    ULPs which benefit from data-dependent direct data placement
          (candidates for TUF) usually transfer large amounts of data in
          bulk.  This means that most ULPDUs are data-carrying, and will
          be EMSS-sized.  Even when control is interleaved with data,
          the combination of a small number of control ULPDUs with a
          data ULPDU can be packed to fill an EMSS-sized segment.

     Therefore, a TUF-conforming TCP sender seems likely to behave
     similarly to a stock TCP sender under most circumstances.  However,
     applications that both send and receive data over the same TCP
     connection, where there might be dependencies between incoming and
     outgoing data, are often subject to excessive delays attributable
     to TCP's Nagle algorithm and/or delayed-ACK algorithm [NagleDAck].
     These algorithms generally perform best when TCP always sends full-
     EMSS segments.  Because TUF can generate sub-EMSS segments as a by-
     product of aligning FPDU boundaries with TCP segment boundaries,



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     TUF might be especially vulnerable to the known problems with the
     Nagle and/or delayed-ACK algorithms.

     Further work, including implementation experience with TUF, as well
     as existing and future proposals for improvements to the Nagle
     and/or delayed-ACK algorithms, might be necessary to optimize TUF
     performance while fully preserving the congestion-avoidance
     features of TCP.  This work is currently outside the scope of this
     document.

5.2.  Exception Cases

     The complete operational specification of TUF is contained in the
     rules for forming FPDUs, and sending those FPDUs in TCP segments.
     However, the operation of TUF will be subject to a variety of
     transient or exceptional conditions.  The behavior of TUF under
     those conditions is discussed below to illustrate specifically how
     TUF addresses them.

5.2.1.  Resegmenting Intermediaries

     Resegmenting TCP-layer intermediaries (middleboxes) are one of the
     most formidable obstacles to maintaining the ULPDU containment
     property.  In the presence of such an intermediary, the
     segmentation chosen by the sender may not be the segmentation at
     the receiver.  While such intermediaries may or may not be common
     in particular networks, in many cases the presence or absence of
     such resegmenting behavior is beyond the control or even knowledge
     of the end points using TUF.  Therefore, TUF must detect such
     resegmentation by design.

     A primary reason for the presence of a random key in the FPDU
     header is to detect such resegmentation.  An alternative to the
     random key which has been proposed, is to use ULP-specific
     validation criteria to determine the ULPDU containment property.
     For example, some ULP PDUs include relatively strong data integrity
     checks such as CRCs, and other ULP control information can often be
     validated against various ULP-specific criteria.

     While such ULP-specific validation criteria may involve checking
     many more bits than the combination of the FPDU's 16-bit length and
     48-bit key, ULP-specific validation criteria may not actually offer
     a strong guarantee of the ULPDU containment property.  For certain
     data streams, the probability of a false-positive indication of the
     ULPDU containment property can be extremely high.

     Assume that the intermediary resegments to a granularity of no
     finer than G octets (e.g. 4).  Also assume that the TCP data stream



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     contains predominantly application data.  If the ULP is a storage
     protocol, simply transferring a file containing a continuous,
     repeated stream of well-formed ULPDUs which are some multiple of G
     in size increases the probability of a false-positive indication of
     the ULPDU containment property to approximately:

             1 / (sizeof(repeated ULPDU)/G)

     If the well-formed ULPDUs are relatively small (e.g. 32 octets
     where G=4 octets), the probability of a false-positive indication
     of the ULPDU containment property is approximately 1/8, for EACH
     TCP segment which does not actually begin with a ULPDU.  Clearly,
     in this case, it would take only a very small number of TCP
     segments which do not begin with an actual ULPDU before the `fake'
     ULPDU in the application data is interpreted as an actual ULPDU.
     The consequences of such a false-positive interpretation could be
     dire, for example executing a destructive operation request.

     The 48-bit random key in the FPDU results in a low probability of a
     false-positive indication of the ULPDU containment property because
     it is effectively secret with respect to the application data
     stream.

     Note that although this analysis may appear to be security-minded,
     prompting the image of a sighted third-party adversary that can
     `sniff' the 48-bit key, it is actually considering a safety, rather
     than a security property.  The security properties of TUF are
     discussed in Section 6 (`Security Considerations') below.

     Even though TUF can detect the presence of a resegmenting
     intermediary, such an intermediary will almost certainly
     substantially reduce the chance of the ULPDU containment property
     being satisfied.  A TUF implementation which detects a very low
     incidence of the ULPDU containment property for a sustained
     interval (>> RTT) may assume that a resegmenting intermediary is in
     operation and SHOULD discontinue the use of ULP control information
     found using the ULPDU containment property.  In such cases, the ULP
     MAY elect to disable the use of TUF altogether, or simply just stop
     exploiting the ULPDU containment property.

5.2.2.  PMTU Reduction

     When a PMTU reduction is detected by a TUF-compliant TCP, the TUF-
     compliant TCP sender may send FPDUs already committed to the TCP
     layer in one of two ways:

     o    send unsegmented FPDUs in TCP segments of the old EMSS size,
          and rely on IP fragmentation to deliver the segments,



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     o    segment FPDUs to fit in TCP segments which respect the new
          EMSS size.

     Stock TCPs face a similar choice on PMTU change, and both
     alternatives are used in practice.

     In the case that a TUF-compliant TCP chooses to segment FPDUs, it
     SHOULD segment them in such a way that, in the absence of
     resegmentation by an intermediary, the segments are guaranteed not
     to give a false-positive indication of the ULPDU containment
     property.  There are various ways to ensure this.  For example, no
     matter how the FPDU is segmented, the first segment is guaranteed
     not to give a false-positive indication of the ULPDU containment
     property---the 48-bit key will match, but the length will not.  In
     the worst possible case, each subsequent TCP segment could be sent
     with fewer than 8 octets of data, also guaranteed not to give a
     false-positive indication of the ULPDU containment property.  More
     efficient approaches are possible, but PMTU reduction is a rare
     event, and reacting to it is only a transient condition.
     Eventually a new MULPDU will be presented to the ULP, and FPDUs
     that fit in the new EMSS will result.  During the transient
     condition, performance will suffer temporarily no matter how FPDUs
     are segmented.

     No matter what segmentation is chosen by a TUF-compliant TCP sender
     when segmenting an FPDU, if the segments pass through a
     resegmenting intermediary, the correctness of the ULPDU containment
     property remains strictly a matter of probability.

5.2.3.  PMTU Increase

     As described in `FPDU Size Selection' above, a TUF-compliant TCP
     probing for PMTU increase will present an increased MULPDU value to
     the ULP.  This should eventually lead to an FPDU large enough to
     actually perform the PMTU increase probe.  The MULPDU value should
     not be further adjusted until the probe is actually performed.
     This behavior is similar to when a stock TCP would like to perform
     a PMTU increase, but less data is available than would fill the
     desired segment.

     Also, note that depending on the ULP, the actual distribution of
     FPDU sizes may have a granularity coarser than a single octet.  An
     FPDU with an particular, desired TCP segment size may never be
     generated.  Therefore when probing for PMTU increase, a TUF-
     compliant TCP must be satisfied with an FPDU that produces a TCP
     segment size that is `close' to the desired size.

     Finally, note that in cases where PMTU grows and shrinks relatively



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     frequently, better performance may result from not probing for PMTU
     increase at all, or probing very rarely.  This is because the
     performance disruption resulting from PMTU decrease can be
     substantial, and in many cases, implementations of TUF will be in
     hardware, so performance may less sensitive to differences in PMTU.

5.2.4.  Receive Window < EMSS

     A TUF-compliant TCP sender that is presented with a receive window
     smaller than EMSS may be required to segment FPDUs.  The TCP window
     probe is a limiting case of this condition where the advertised
     receive window is 0, and the amount of data typically sent in
     response is a single octet.

     In this case, a TUF-compliant TCP sender will segment in accordance
     to the requirements of TCP, and the rule defined in `TUF-conforming
     TCP Sender Segmentation' above.  In addition, as when resegmenting
     in response to PMTU decrease, a TUF-compliant TCP sender SHOULD
     segment in such a way that, in the absence of a resegmenting
     intermediary, segments are guaranteed not to give a false-positive
     indication of the ULPDU containment property.  In situations where
     the receive window is smaller than EMSS, data transfer performance
     is likely to be limited independently of any segmentation behavior
     by the TCP sender.  Furthermore, ULP implementations that choose to
     use TUF will almost certainly be designed to maintain a receiver
     window larger than EMSS, so a small receiver window should occur
     extremely infrequently.

5.2.5.  Size of ULPDU + 8 > EMSS

     In cases where EMSS shrinks below the minimum size of a ULPDU that
     a ULP wants to send, TUF will create FPDUs that are larger than
     EMSS, and a TUF-compliant TCP sender will face the same
     alternatives as during PMTU reduction:

     o    send unsegmented FPDUs and rely on IP fragmentation to deliver
          the segments

     o    segment FPDUs to fit in TCP segments which respect the EMSS
          size

     A ULP which is presented with an MULPDU value that is too small to
     accommodate PDUs necessary operation SHOULD simply attempt to use
     ULPDUs which are as small as possible

     If the EMSS shrinks to a pathologically small size, then a TUF
     implementation SHOULD discontinue the use of ULP control
     information found using the ULPDU containment property.  In such



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     cases, the ULP MAY elect to disable the use of TUF altogether, or
     simply just stop exploiting the ULPDU containment property.

     A path MTU which results in an EMSS < 128 + 8 octets is an
     extremely unlikely occurrence and when it does occur, poor data
     transfer performance is a likely result, independent of TCP sender
     segmentation behavior.

6.  Security Considerations

     This section discusses both protocol-specific considerations and
     the implications of using TUF with existing security mechanisms.

6.1.  Protocol-specific Security Considerations

     A third-party that can inject spoofed packets into the network
     which can be delivered to a TUF receiver could launch a variety of
     attacks that exploit TUF-specific behavior.  For example a blind
     third-party adversary could inject random packets which appear in
     the valid TCP window and do not begin with valid FPDU headers.  A
     barrage of such packets might cause a TUF receiver to conclude that
     a resegmenting intermediary is present and disable the use of TUF
     and direct data placement.  This would substantially degrade
     performance.  However, it would probably also have more dire
     consequences than performance, such as causing the ULP to interpret
     the bogus data as valid.  Furthermore, such a third-party could
     also degrade performance just as effectively in a TUF-independent
     way by injecting spoofed ICMP packets which result in reduction of
     the path MTU to an inefficiently small size.

     Fundamentally, the vulnerabilities of TUF to active third-party
     interference are no more acute than to TCP without TUF.  In both
     cases, a communication security mechanism such as IPSec is the only
     way to completely prevent such attacks.

6.2.  Using IPSec With TUF

     Since IPSec is designed to secure arbitrary IP packet streams,
     including streams where packets are lost, TUF can run cleanly on
     top of IPSec without any change.  IPSec packets may be decrypted in
     the order they are received, and a TUF receiver may test and
     exploit the ULPDU containment property just as if the IP datagram
     were unsecured.

6.3.  Using TLS With TUF

     Using TLS [TLS] with TUF, particularly trying to exploit the ULPDU
     containment property to locate ULP control information, is not a



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     straightforward process.  TUF can be directly layered on top of
     TLS, but many of the advantages of TUF are lost.  This document
     does not define a way of using TLS with TUF that could offer better
     performance than stock reassembly buffer-based implementations.
     That task is left to a different document, if there is sufficient
     motivation to address the problems.  This section does outlines
     some of the known complications of trying to do better than stock
     reassembly buffer-based implementations using TLS with TUF.

     TLS is a record-oriented protocol.  TLS records are PDUs with a
     similar structure to ULPDUs defined in application ULPs.  As with
     other ULPs, the only way to avoid a complete reassembly buffer is
     to be able to find TLS PDUs in the presence of lost TCP segments.
     The ULPDU containment property could be used to do this, which
     suggests that TLS itself should be layered on top of TUF.  In this
     case, the FPDU header will travel in the clear, but this will
     probably not present serious vulnerabilities other than denial of
     service attacks comparable to what is already possible without TUF.

     Once the TLS records are located and processed it still remains to
     locate the ULPDUs.  The simplest way to do this would be to have
     the TLS implementation be TUF-compliant, and ensure the ULPDU
     containment property within each TLS record.  In this case, the
     protocol layering would look like:



























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                         ULP client
                              ^
                              |
                              | ULPDUs (in octet stream)
                              |
                              v
                      TUF-conforming TLS
                              ^
                              |
                              | TLS records (containing ULPDUs)
                              |
                              v
                             TUF
                              ^
                              |
                              | FPDUs (each containing a TLS record)
                              |
                              v
                     TUF-conforming TCP
                              ^
                              |
                              | TCP Segments (each containing an FPDU)
                              |
                              v
                            . . .


     An obvious complications of using TLS with TUF is that ciphers
     defined for use with TLS do not offer independence across TLS
     records.  The most common cipher used with TLS is RC4, which is a
     stream cipher.  Efficient decryption of an RC4 stream depends upon
     the entire preceding data stream.  In other words, it is simply not
     feasible to decrypt TLS records encrypted with RC4 in any order
     other than the TCP stream order.  This clearly defeats the purpose
     of TUF.

     TLS is also defined to work with block ciphers such as 3DES in
     Cipher Block Chaining (CBC) mode.  In this case, the dependency of
     the decryption operation on data in previous TLS records is less
     severe.  To decrypt the current TLS record only requires ciphertext
     from the previous TLS record.  While this does not allow complete
     independence of processing TLS records, a lost or delayed TCP
     segment containing a TLS record only prevents decrypting the
     immediately subsequent TLS record, not all TLS records after it.

     TLS compression presents another complication to using TLS with
     TUF.  TLS compression algorithms are allowed to increase the
     content length by up to 1024 octets.  If the content length does



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     increase, the TLS record may not fit within an EMSS-sized TCP
     segment, even if the uncompressed ULPDU does.  If the risk of
     exceeding an EMSS-sized TCP segment is small, it may be acceptable
     to occasionally send FPDUs containing TLS records that span several
     TCP segments, or use IP fragmentation.  Some TLS compression
     algorithms may never increase the content length, or only increase
     it by some small, manageable amount.

7.  IANA Considerations

     If framing is enabled a priori for a ULP by connecting to a well-
     known port, this well-known port would be registered for the framed
     ULP with IANA.

8.  References

     [BEEP]
          Rose, M., "The Blocks Extensible Exchange Protocol Core", RFC
          3080, March 2001.

     [HTTP]
          Fielding, R. and others, "Hypertext Transfer Protocol --
          HTTP/1.1.", RFC 2616, June 1999.
          http://www.ietf.org/internet-drafts/draft-ietf-tsvwg-
          initwin-00.txt.

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

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

     [RFC1750]
          Eastlake, D., Crocker, S., Schiller., J., "Randomness
          Recommendations for Security.", RFC 1750, December 1994.

     [RFC2581]
          Allman, M., and others, "TCP Congestion Control," RFC 2581,
          April 1999.

     [SCTP]
          Stewart, R.R. and others, "Stream Control Transmission
          Protocol," RFC2960, October 2000.





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     [Stevens]
          Stevens, W. Richard, "Unix Network Programming Volume 1,"
          Prentice Hall, 1998, ISBN 0-13-490012-X.

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

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

Authors' Addresses


     Stephen Bailey
     Sandburst Corporation
     600 Federal Street
     Andover, MA  01810
     USA

     Phone: +1 978 689 1614
     Email: steph@sandburst.com


     Jeff Chase
     Department of Computer Science
     Duke University
     Durham, NC 27708-0129
     USA

     Phone: +1 919 660 6559
     Email: chase@cs.duke.edu


     Jim Pinkerton
     Microsoft, Inc.
     1 Microsoft Way
     Redmond, WA 98052
     USA

     EMail: jpink@microsoft.com









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     Allyn Romanow
     Cisco Systems
     170 W Tasman Drive
     San Jose, CA 95134
     USA

     Phone: +1 408 525 8836
     Email: allyn@cisco.com


     Constantine Sapuntzakis
     Cisco Systems
     170 W Tasman Drive
     San Jose, CA 95134
     USA

     Phone: +1 408 525 5497
     EMail: csapuntz@cisco.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



Appendix A. Sample Sockets Support For TUF

     The sockets support for TUF described below is only a sketch.  It
     is provided as an aid to understanding TUF.  Implementing this
     interface is not a requirement for a TUF implementation.

     Other software interfaces are possible.  The described interface



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     draws from the sockets interface for UDP.  The described interface
     might be natural for applications already designed to support both
     TCP and UCP, or that do network input and output in complete PDU
     units.  For applications that perform octet-at-a-time style input
     and output, an alternative interface that draws from the tradition
     of the TCP URG pointer interface (e.g. using a MSG_OOB flag to
     send()) is equally possible.  An implementation may even offer
     several different interfaces to TUF.

     That said, the sockets support sketched below might well provide
     the basis for a complete, standard interface to be described
     outside this draft.

A.1 Basic Principles

     The sockets support for TUF takes the form of a set of socket
     options that may be set or requested to enable the appropriate
     behavior.

     A socket may be in one of two TUF-related modes in the send
     direction:

     1.   TUF-compliant TCP sender mode.  No data (FPDU headers) is
          added to the TCP octet stream, but each data buffer presented
          in a sending operation is to be sent according to the rules of
          TCP and TUF-compliant TCP senders.  This mode provides direct
          access to a TUF-compliant TCP sender for purposes such as
          implementing TUF.

     2.   TUF sender mode.  An FPDU header is added to data presented by
          an integral number of sending operations, and the FPDU is
          passed to a TUF-compliant TCP sender for transmission

     A socket may be in one TUF-related mode in the receive direction:

     1.   TUF receiver mode.  FPDUs are expected in each TCP segment.

     If a socket receiving operation is used to retrieve received data
     (as opposed to the data being directly placed), FPDU headers are
     removed before the data is returned.

A.2 Enabling TUF









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          /* Pick a sending mode */
          if (sendMode == TUF_TCP)
            mode = TUF_SEND_TCP
          else
            mode = TUF_SEND;

          mode |= TUF_RECEIVE;

          setsockopt (s, SOL_TCP, TUF_MODE, &mode, sizeof(mode));


A.3 Sending Data

     The standard socket sending operations, including send(), sendto(),
     sendmsg(), writev(), and others are used to send ULPDUs in TUF.
     The EMSGSIZE error should be returned if the buffer passed to the
     sending operation would result in an FPDU that does not fit in an
     EMSS-sized TCP segment, unless oversized ULPDU errors are disabled,
     as described below.

     When the path EMSS increases, the sending operation MAY return
     EMSGSIZE once to inform the client of the change.

A.4 Retrieving The Current EMSS or MULPDU


          getsockopt (s, SOL_TCP, TUF_MULPDU, &emss, sizeof(emss));


     If the socket is in TUF_SEND_TCP mode, this call returns the TCP
     EMSS.  If the socket is in TUF_SEND mode, the call returns the
     maximum ULPDU that can be submitted in a sending operation without
     requiring fragmentation of the associated FPDU.

     The number should not count any octets that go towards TCP options.

A.5 Disabling ULPDU Packing


          flag = 0;
          setsockopt (s, SOL_TCP, TUF_PACK_PDUS, &flag, sizeof(flag));


     This call disables TUF from packing more than one ULPDU into an
     FPDU.  By default, ULP PDU packing is enabled.






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A.6 Disabling The Report of Oversized ULPDUs


          flag = 0;
          setsockopt (s, SOL_TCP, TUF_REPORT_OVERSIZED, &flag,
                      sizeof(flag));

     This call disables sending operations from returning EMSGSIZE in
     response to oversized ULPDUs.  It may be called at any time on a
     socket, whether connected or not.  It is used to continue ULP
     operation when MULPDU is already known to be too small to permit
     some ULPDUs to be sent with out segmentation.  Oversized ULPDU
     reporting can be enabled again if PMTU is discovered to have
     increased.

Full Copyright Statement

     Copyright (C) The Internet Society (2001). All Rights Reserved.

     This document and translations of it may be copied and furnished to
     others, and derivative works that comment on or otherwise explain
     it or assist in its implementation may be prepared, copied,
     published and distributed, in whole or in part, without restriction
     of any kind, provided that the above copyright notice and this
     paragraph are included on all such copies and derivative works.
     However, this document itself may not be modified in any way, such
     as by removing the copyright notice or references to the Internet
     Society or other Internet organizations, except as needed for the
     purpose of developing Internet standards in which case the
     procedures for copyrights defined in the Internet Standards process
     must be followed, or as required to translate it into languages
     other than English.

     The limited permissions granted above are perpetual and will not be
     revoked by the Internet Society or its successors or assigns.

     This document and the information contained herein is provided on
     an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
     ENGINEERING TASK FORCE DISCLAIMS 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.









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