Network Working Group J. Strombergson

Network Working Group J. Strombergson
Request for Comments: 4194 InformAsic AB
Category: Standards Track L. Walleij
Lunds Tekniska Hogskola
P. Faltstrom
Cisco Systems Inc
October 2005

The S Hexdump Format

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2005).

Abstract

This document specifies the S Hexdump Format (SHF), a new, XML-based
open format for describing binary data in hexadecimal notation. SHF
provides the ability to describe both small and large, simple and
complex hexadecimal data dumps in an open, modern, transport- and
vendor-neutral format.

1. Introduction

In the computing, network, and embedded systems communities, several
different types of data formats for hexadecimal data are being used.
One of the more common formats is known as "S-records" (and several
derivatives), which reportedly originated at the Motorola company.
The S Hexdump Format is named in its honour.

Typical uses of these dump formats include executable object code for
embedded systems (i.e., "firmware"), on-chip flash memories and
filesystems, FPGA configuration bitstreams, graphics and other
application resources, routing tables, etc. Unfortunately, none of
the formats used are truly open, vendor-neutral, and/or well-defined.

Even more problematic is the fact that none of these formats are able
to represent the large data sizes that are getting more and more
common. Data dumps comprised of multiple sub-blocks with different

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RFC 4194 The S Hexdump Format October 2005

Word sizes, and data sizes spanning anywhere from a few Bytes of data
to much larger than 2^32 bits are not handled. Also, the checksums
included in these formats are too simplistic and for larger data
sizes, they provide insufficient ability to accurately detect errors.
Alternatively, the overhead needed for proper error detection is very
large.

Therefore, the S Hexdump format is an effort to provide a modern,
XML-based format that is not too complex for simple tools and
computing environments to implement, generate, parse, and use. Yet
the format is able to handle large data sizes and complex data
structures, and can provide high quality error detection by
leveraging standardized cryptographic hash functions.

One of the simplifications introduced in the format is to disallow
other number systems such as octal or decimal notation, and to allow
for Word sizes of even bytes (8-bit groups) only. This is
intentional and was done to simplify implementations aimed for
practical present-day applications. Formats aimed for esoteric
number systems or odd Word sizes may be implemented elsewhere.

At present, the usage of the SHF format may be mainly for Internet
transport and file storage on development machinery. A parser for
the XML format is presently not easily deployed in hardware devices,
but the parsing and checksumming of the SHF data may be done by a
workstation computer, which in turn converts the SHF tokens to an
ordinary bitstream before the last step (e.g., of a firmware upgrade)
commences.

SHF is a dump format only and shall not be confused with similar
applications, such as binary configuration formats or patches, which
are intended to, for example, alter contents of a core memory. Such
applications require the possibility of modifying individual bits or
groups of bits in the memory of a machine, and is not the intended
usage of the mechanism described in the present document.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].

The key word "Byte" is to be interpreted as a group of 8 bits. The
key word "Octet" is another name for Byte.

The key word "Word" is to be interpreted as a group containing an
integral number of Bytes.

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RFC 4194 The S Hexdump Format October 2005

The key word "Block" is to be interpreted as an ordered sequence of
Words, beginning at a certain address, running from lower to higher
addresses. A Block typically represents a sequence of Words at a
certain address range in the memory of a computer.

The key word "Dump" is to be interpreted as a sequence of Blocks,
which may or may not be in a particular order. A Dump typically
represents some non-continuous, interesting parts of the memory of a
computer, such that the Dump as a whole has a certain meaning, for
example (but not limited to) a complete firmware for an embedded
system.

The expression "2^n" is to be interpreted as the value two (2) raised
to the n:th power. For example, 2^8 equals the value 256.

3. Features and Functionality

The SHF-format has the following features:

o Support for arbitrarily wide data Words

o Support for very large data Blocks

o Support for an arbitrary number of independent data Blocks

o Data integrity detection against errors provided by the RFC3174
specified (see [2]) SHA-1 cryptographic signature

o An XML-based format

In the embedded systems domain, 8- and 16-bit processors are still
used in large numbers and will continue to be used for any
foreseeable future. Simultaneously, more and more systems are using
64-bit and even larger Word sizes.

SHF supports all of these systems by allowing the Word size to be
specified. The Word size MUST be an integer number of Bytes and at
least one (1) Byte.

SHF is able to represent both large and small data Blocks. The data
Block MUST contain at least one (1) Word. Additionally, the data
Block MUST NOT be larger than (2^64)-1 bits.

The SHF Dump MUST contain at least one (1) data Block. The maximum
number of Blocks supported is 2^64. Each data Block in the Dump MAY
have different Word sizes and start at different addresses.

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RFC 4194 The S Hexdump Format October 2005

The checksum (or message digest) used to verify the correctness or
data integrity of each Block is 20 Bytes (160 bits) long. The digest
MUST be calculated on the data actually represented by the SHF data
Block, NOT the representation, i.e., NOT the ASCII-code. SHA-1 is
only able to calculate a digest for a data Block no larger than
(2^64)-1 bits and this limits the size of each data Block in SHF to
(2^64)-1 bits.

4. SHF XML Specification

The SHF format consists of an XML data structure representing a Dump.
The Dump consists of a Dump header section and one (1) or more Block
sections containing data. Each Block of data is independent of any
other Block.

A short, symbolic example of an SHF Dump is illustrated by the
following structure:

<dump name="(Human readable string)" blocks="(64-bit value)">
<block name="(Human readable string)" start_address="(64-bit
value)" word_size="(64-bit value)" length="(64-bit value)"
checksum="(20-Byte digest)">
(Data)
</block>
</dump>

4.1. Header Section

The header section comprises the Dump tag, which includes the
following attributes:

o name: A compulsory string of arbitrary length used by any
interested party to identify the specific SHF Dump.

o blocks: An optional 64-bit hexadecimal value representing the
number of Blocks in the specific SHF Dump. Whenever available,
this value should be supplied. However, there are potential
scenarios where the number of Blocks cannot be given beforehand.
If the value is present, it should be verified by implementers; if
the value is untrue, the behaviour is implementation-defined.

After the opening Dump tag, one or more subsections of Blocks must
follow. Finally, the complete SHF Dump ends with a closing Dump tag.

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RFC 4194 The S Hexdump Format October 2005

4.2. Block Subsection

The Block subsection contains a Block tag and a number of data words.
The Block tag includes the following attributes:

o name: A compulsory string of arbitrary length used by any
interested party to identify the specific Block.

o start_address: A compulsory, 64-bit hexadecimal value representing
the start address in Bytes for the data in the Block.

o word_size: A compulsory 64-bit hexadecimal value representing the
number of Bytes (the width) of one Word of the data.

o length: A compulsory hexadecimal representation of an unsigned
64-bit integer indicating the number of Words following inside the
Block element. If this value turns out to be untrue, the Block
MUST be discarded.

o checksum: A compulsory hexadecimal representation of the 20 Byte
SHA-1 digest of the data in the Block.

The total size of the data in the Block (in bits) is given by the
expression (8 * word_size * length). The expression MUST NOT be
larger than (2^64)-1.

After the opening Block tag, a hexadecimal representation of the
actual data in the Block follows. Finally, the Block section ends
with a closing Block tag.

5. SHF Rules and Limits

There are several rules and limits in SHF:

o All attribute values representing an actual value and the data
MUST be in hexadecimal notation. The only attribute excluded from
this rule is the name attribute in the Dump and Block tags. This
restriction has been imposed for ease of reading the dump: a
reader shall not be uncertain about whether a figure is in hex
notation or not, and can always assume it is hexadecimal.

o All attribute values, with the exception of the checksum, MAY omit
leading zeros. Conversely, the checksum MUST NOT omit leading
zeros.

o The data represented in a Block MUST NOT be larger than (2^64)-1
bits.

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RFC 4194 The S Hexdump Format October 2005

o The size of a Word MUST NOT be larger than (2^64)-1 bits. This
implies that a Block with a Word defined to the maximum width
cannot contain more than one Word. An SHF consumer shall assure
that it can handle a certain Word length before beginning to parse
blocks of an SHF Dump. Failure to do so may cause buffer
overflows and endanger the stability and security of the system
running the consuming application.

o The attribute values representing an actual value MUST be in
big-endian format. This means that the most significant
hexadecimal digits are to be put to the left in a hexadecimal
Word, address, or similar field. For example, the address value
1234 represents the address 1234 and not 3412. While some
computing architectures may be using little-endian Words as their
native format, it is the responsibility of any SHF producer
running on such an architecture to swap the attribute values to a
big-endian format. The reverse holds for a consumer receiving the
big-endian SHF attributes: if the consumer is little-endian, the
values have to be swapped around.

o Likewise, the words inside a Dump MUST be stored in a big-endian
format if the word size is larger than one Byte. Here, the same
need for swapping Bytes around may arise, as mentioned in the
previous paragraph.

6. SHF DTD

The contents of the element named "block" and the attributes
"blocks", "address", "word_size" and "checksum" should only contain
the characters that are valid hexbyte sequences. These are:

whitespace ::= (#x20 | #x9 | #xC | #xD | #xA)
hexdigit ::= [0-9A-Fa-f]
hexbytes ::= whitespace* hexdigit (hexdigit|whitespace)*

A parser reading in an SHF file should silently ignore any other
characters that (by mistake) appear in any of these elements or
attributes. These alien characters should be treated as if they did
not exist. Also note that "whitespace" has no semantic meaning; it
is only valid for the reason of improving the human readability of
the Dump. Whitespace may be altogether removed and the hexbyte
sequences concatenated if desired. Notice that the fact that word
size is to be given in a number of bytes implies that the number of
hexadecimal digits inside a block need to be even. Malformed blocks
should be ignored by implementations.

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RFC 4194 The S Hexdump Format October 2005


<?xml version="1.0" encoding="UTF-8"?>

<!ELEMENT dump (block)+>
<!ATTLIST dump
name CDATA #REQUIRED
blocks CDATA #IMPLIED>

<!ELEMENT block (#PCDATA)>
<!ATTLIST block
name CDATA #REQUIRED
address CDATA #REQUIRED
word_size CDATA #REQUIRED
length CDATA #REQUIRED
checksum CDATA #REQUIRED>

7. SHF Examples

This section contains three different SHF examples, illustrating the
usage of SHF and the attributes in SHF.

The first example is a simple SHF Dump with a single Block of data:

<?xml version="1.0" encoding="UTF-8"?>
<dump name="Simple SHF example" blocks="01">
<block name="Important message in hex format" address="0400"
word_size="01" length="1f"
checksum="5601b6acad7da5c7b92036786250b053f05852c3">
41 6c 6c 20 79 6f 75 72 20 62 61 73 65 20 61 72
65 20 62 65 6c 6f 6e 67 20 74 6f 20 75 73 0a
</block>
</dump>

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RFC 4194 The S Hexdump Format October 2005

The second example is a program in 6502 machine code residing at
memory address 0x1000, which calculates the 13 first Fibonacci
numbers and stores them at 0x1101-0x110d:

<?xml version="1.0" encoding="UTF-8"?>
<dump name="6502 Fibonacci" blocks="02">
<block name="Code" address="1000" word_size="01" length="2a"
checksum="5cab5bf8ee299af1ad17e8093d941914eb5930c7">
a9 01 85 20 85 21 20 1e 10 20 1e 10 18 a5 21 aa
65 20 86 20 85 21 20 1e 10 c9 c8 90 ef 60 ae 00
11 a5 21 9d 00 11 ee 00 11 60
</block>
<block name="Mem" address="1100" word_size="01" length="e"
checksum="c8c2001c42b0226a5d9f7c2f24bd47393166487a">
01 00 00 00 00 00 00 00 00 00 00 00 00 00
</block>
</dump>

The final example contains a Block of 40-bit wide data:

<?xml version="1.0" encoding="UTF-8"?>
<dump name="Example of an SHF dump with wide data words" blocks="00001">
<block name="SMIL memory dump" address="000" word_size="5"
length="1A" checksum="ff2033489aff0e4e4f0cd7901afc985f7a213c97">
00100 00200 00000 00090 00000 00036 00300 00400
00852 00250 00230 00858 00500 00600 014DC 00058
002A8 000B8 00700 00800 000B0 00192 00100 00000
00900 00A00 00000 0000A 40000 00000 00B00 00C00
00000 00000 00000 00001 00D00 00E00 00000 00100
0CCCC CCCCD 00F00 01000 00000 00010 80000 00000
00100 00790 00000 00234
</block>
</dump>

8. SHF Security Considerations

The SHF format is a format for representing hexadecimal data that one
wants to transfer, manage, or transform. The format itself does not
guarantee that the represented data is not falsely represented,
malicious, or otherwise dangerous.

The data integrity of the SHF file as a whole is to be provided, if
needed, by means external to the SHF file, such as the generic
signing mechanism described by RFC 3275 [3].

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RFC 4194 The S Hexdump Format October 2005

9. IANA Considerations

This section contains the registration information for the MIME type
to SHF. The media type has been chosen to comply with the guidelines
in [4].

o Registration: application/shf+xml
o MIME media type name: application
o MIME subtype name: shf+xml
o Required parameters: charset

Required parameters: charset

This parameter must exist and must be set to "UTF-8". No other
character sets are allowed for transporting SHF data. The character
set designator MUST be uppercase.

Encoding considerations:

This media type may contain binary content; accordingly, when used
over a transport that does not permit binary transfer, an appropriate
encoding must be applied.

Security considerations:

A hex Dump in itself has no other security considerations than what
applies for any other XML file. However, the included binary data
may in decoded form contain any executable code for a target
platform. If additional security is desired, additional transport
security solutions may be applied. For target code contained in a
hex Dump, developers may want to include certificates, checksums, and
the like in hexdump form for the target platform. Such uses are
outside the scope of this document and a matter of implementation.

Interoperability considerations:

n/a

Published specification:

This media type is a proper subset of the XML 1.0 specification [5].
One restriction is made: no entity references other than the five
predefined general entities references ("&", "<", ">",
"'", and """) and numeric entity references may be present.
Neither the "XML" declaration (e.g., <?xml version="1.0" ?>) nor the
"DOCTYPE" declaration (e.g., <!DOCTYPE ...>) need be present. (XML
fragments are allowed.) All other XML 1.0 instructions (e.g., CDATA
blocks, processing instructions, and so on) are allowed.

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RFC 4194 The S Hexdump Format October 2005

Applications that use this media type: any program or individual
wishing to make use of this XML 1.0 subset for hexdump exchange.

Additional information:

o Magic number: There is no single initial Byte sequence that is
always present for SHF files
o File extension: shf
o Macintosh File Type code: none

Intended usage: COMMON.

Author/Change controller: this MIME transport type is controlled by
the IETF.

10. Extensions

The attributes of elements in the SHF XML format may be extended when
need arises. For example, certain applications will want to
represent executable code as an SHF Dump, and may then need an
execution start address to be associated with certain Dump Blocks, so
that the address can be configured as a starting point for the CPU
part of any processor code present in the Block, as opposed to the
raw data, which is already given a start address by way of the
"address" attribute. This can be done by extending the Block tag
with a "start_address" attribute.

Another possible scenario is when a dump is applied to a computer
system with several independent address spaces, such as a system with
two CPUs, each with independent memories. In this case, a user may
want to add an "address_space" attribute.

As long as such new attributes are added, with no attributes being
removed or redefined, the resulting Dump shall be considered a valid
SHF Dump and transported using the application/xml+shf transport
type. Parsers unaware of the modified namespace shall silently
ignore any such extended attributes, or simply duplicate them from
input to output when processing an SHF file as a filter. The
management of such extended attributes is a matter of convention
between different classes of users and not a matter of the IETF.

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RFC 4194 The S Hexdump Format October 2005

11. Additional Information

Contact for further information: c.f., the "Authors' Addresses"
section of this memo.

Acknowledgements: The SMIL memory Dump was kindly provided by Sten
Henriksson at Lund University. Proofreading and good feedback on the
SHF document was generously provided by Peter Lindgren, Tony Hansen,
Larry Masinter, and Clive D.W. Feather. We also want to thank the
Applications area workgroup for their help during development.

12. Normative References

[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[2] Eastlake, 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
(SHA1)", BCP 14, RFC 3174, September 2001.

[3] Eastlake, 3rd, D., Joseph, J., and D. David, "(Extensible Markup
Language) XML-Signature Syntax and Processing", BCP 14,
RFC 3275, March 2002.

[4] Makoto, M., Simon, S., and D. Dan, "(Extensible Markup Language)
XML Media Types", RFC 3023, January 2001.

[5] Bray, Tim, Paoli, Jean, Sperberg-McQueen, C. M. and Maler, Eve,
Yergeau, Francois, "Extensible Markup Language (XML) 1.0 (Third
Edition)", http://www.w3.org/TR/REC-xml.

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RFC 4194 The S Hexdump Format October 2005

Authors' Addresses

Joachim Strombergson
InformAsic AB
Hugo Grauers gata 5a
Gothenburg 411 33
SE

Phone: +46 31 68 54 90
EMail: [email protected]
URI: http://www.InformAsic.com/

Linus Walleij
Lunds Tekniska Hogskola
Master Olofs Vag 24
Lund 224 66
SE

Phone: +46 703 193678
EMail: [email protected]

Patrik Faltstrom
Cisco Systems Inc
Ledasa
273 71 Lovestad
Sweden

EMail: [email protected]
URI: http://www.cisco.com

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RFC 4194 The S Hexdump Format October 2005

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