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.
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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
Bailey and others Expires May 2002 [Page 26]
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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.
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Bailey and others Expires May 2002 [Page 30]
