Internet DRAFT - draft-tuexen-opswg-pcapng
draft-tuexen-opswg-pcapng
Network Working Group M. Tuexen, Ed.
Internet-Draft Muenster Univ. of Appl. Sciences
Intended status: Informational F. Risso
Expires: December 28, 2014 Politecnico di Torino
J. Bongertz
Airbus DS CyberSecurity
G. Harris
June 26, 2014
PCAP Next Generation (PCAPNG) Dump File Format
draft-tuexen-opswg-pcapng-00.txt
Abstract
This document describes a format to dump captured packets to a file.
This format is extensible; Wireshark can currently read and write it,
and libpcap can currently read some pcap-ng files.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 28, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. General File Structure . . . . . . . . . . . . . . . . . . . 3
3.1. General Block Structure . . . . . . . . . . . . . . . . . 3
3.2. Block Types . . . . . . . . . . . . . . . . . . . . . . . 4
3.3. Logical Block Hierarchy . . . . . . . . . . . . . . . . . 6
3.4. Physical File Layout . . . . . . . . . . . . . . . . . . 6
3.5. Options . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.6. Data format . . . . . . . . . . . . . . . . . . . . . . . 10
4. Block Definition . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Section Header Block (mandatory) . . . . . . . . . . . . 11
4.2. Interface Description Block (mandatory) . . . . . . . . . 14
4.3. Enhanced Packet Block (optional) . . . . . . . . . . . . 23
4.4. Simple Packet Block (optional) . . . . . . . . . . . . . 27
4.5. Packet Block (obsolete!) . . . . . . . . . . . . . . . . 29
4.6. Name Resolution Block (optional) . . . . . . . . . . . . 31
4.7. Interface Statistics Block (optional) . . . . . . . . . . 34
5. Experimental Blocks (deserved to a further investigation) . . 37
5.1. Alternative Packet Blocks (experimental) . . . . . . . . 37
5.2. Compression Block (experimental) . . . . . . . . . . . . 37
5.3. Encryption Block (experimental) . . . . . . . . . . . . . 38
5.4. Fixed Length Block (experimental) . . . . . . . . . . . . 39
5.5. Directory Block (experimental) . . . . . . . . . . . . . 40
5.6. Traffic Statistics and Monitoring Blocks (experimental) . 41
5.7. Event/Security Block (experimental) . . . . . . . . . . . 41
6. Recommended File Name Extension: .pcapng . . . . . . . . . . 41
7. How to add Vendor / Domain specific extensions . . . . . . . 41
8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 42
9. Security Considerations . . . . . . . . . . . . . . . . . . . 42
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 42
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 42
12.1. Normative References . . . . . . . . . . . . . . . . . . 42
12.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Appendix A. Packet Block Flags Word . . . . . . . . . . . . . . 43
Appendix B. Standardized Block Type Codes . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
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1. Introduction
The problem of exchanging packet traces becomes more and more
critical every day; unfortunately, no standard solutions exist for
this task right now. One of the most accepted packet interchange
formats is the one defined by libpcap, which is rather old and is
lacking in functionality for more modern applications particularly
from the extensibility point of view.
This document proposes a new format for dumping packet traces. The
following goals are being pursued:
Extensibility: It should be possible to add new standard
capabilities to the file format over time, and third parties
should be able to enrich the information embedded in the file with
proprietary extensions, with tools unaware of newer extensions
being able to ignore them.
Portability: A capture trace must contain all the information needed
to read data independently from network, hardware and operating
system of the machine that made the capture.
Merge/Append data: It should be possible to add data at the end of a
given file, and the resulting file must still be readable.
2. Conventions
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 [RFC2119].
3. General File Structure
3.1. General Block Structure
A capture file is organized in blocks, that are appended one to
another to form the file. All the blocks share a common format,
which is shown in Figure 1.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Block Body /
/ variable length, aligned to 32 bits /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Basic block structure.
The fields have the following meaning:
o Block Type (32 bits): unique value that identifies the block.
Values whose Most Significant Bit (MSB) is equal to 1 are reserved
for local use. They can be used to make extensions to the file
format to save private data to the file. The list of currently
defined types can be found in Appendix B
o Block Total Length: total size of this block, in bytes. For
instance, the length of a block that does not have body is 12
bytes.
o Block Body: content of the block.
o Block Total Length: total size of this block, in bytes. This
field is duplicated for permitting backward file navigation.
This structure, shared among all blocks, makes it easy to process a
file and to skip unneeded or unknown blocks. Some blocks can contain
other blocks inside (nested blocks). Some of the blocks are
mandatory, i.e. a dump file is not valid if they are not present,
other are optional.
The General Block Structure allows defining other blocks if needed.
A parser that does non understand them can simply ignore their
content.
3.2. Block Types
The currently standardized Block Type codes are specified in
Appendix B, they have been grouped in the following four categories:
MANDATORY blocks MUST appear at least once in each file:
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o Section Header Block (Section 4.1): it defines the most important
characteristics of the capture file.
o Interface Description Block (Section 4.2): it defines the most
important characteristics of the interface(s) used for capturing
traffic.
OPTIONAL blocks MAY appear in a file:
o Enhanced Packet Block (Section 4.3): it contains a single captured
packet, or a portion of it. It represents an evolution of the
original Packet Block (Section 4.5).
o Simple Packet Block (Section 4.4): it contains a single captured
packet, or a portion of it, with only a minimal set of information
about it.
o Name Resolution Block (Section 4.6): it defines the mapping from
numeric addresses present in the packet dump and the canonical
name counterpart.
o Interface Statistics Block (Section 4.7): it defines how to store
some statistical data (e.g. packet dropped, etc) which can be
useful to understand the conditions in which the capture has been
made.
OBSOLETE blocks SHOULD NOT appear in newly written files (but left
here for reference):
o Packet Block (Section 4.5): it contains a single captured packet,
or a portion of it. It should be considered OBSOLETE, and
superseded by the Enhanced Packet Block (Section 4.3).
EXPERIMENTAL blocks are considered interesting but the authors
believe that they deserve more in-depth discussion before being
defined:
o Alternative Packet Blocks
o Compression Block
o Encryption Block
o Fixed Length Block
o Directory Block
o Traffic Statistics and Monitoring Blocks
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o Event/Security Blocks
3.3. Logical Block Hierarchy
The blocks build a logical hierarchy as they refer to each other.
Figure 2 shows the logical hierarchy of the currently defined blocks
in the form of a "tree view":
Section Header
|
+- Interface Description
| +- Simple Packet
| +- Enhanced Packet
| +- Interface Statistics
|
+- Name Resolution
Figure 2: Logical block Hierarchy of a pcapng file.
For example: each captured packet refers to a specific capture
interface, the interface itself refers to a specific section.
3.4. Physical File Layout
The file MUST begin with a Section Header Block. However, more than
one Section Header Block can be present on the dump, each one
covering the data following it till the next one (or the end of
file). A Section includes the data delimited by two Section Header
Blocks (or by a Section Header Block and the end of the file),
including the first Section Header Block.
In case an application cannot read a Section because of different
version number, it MUST skip everything until the next Section Header
Block. Note that, in order to properly skip the blocks until the
next section, all blocks MUST have the fields Type and Length at the
beginning. This is a mandatory requirement that MUST be maintained
in future versions of the block format.
Figure 3 shows a typical file configuration, with a single
Section Header that covers the whole file.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SHB v1.0 | Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: File structure example: Typical configuration with a single
Section Header Block.
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Figure 4 shows a file that contains three headers, and is normally
the result of file concatenation. An application that understands
only version 1.0 of the file format skips the intermediate section
and restart processing the packets after the third Section Header.
|-- 1st Section --|-- 2nd Section --|-- 3rd Section --|
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SHB v1.0 | Data | SHB V1.1 | Data | SHB V1.0 | Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: File structure example: three Section Header Blocks in a
single file.
Figure 5 shows a file comparable to a "classic libpcap" file - the
minimum for a useful capture file. It contains a single
Section Header Block (SHB), a single Interface Description Block
(IDB) and a few Enhanced Packet Blocks (EPB).
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SHB | IDB | EPB | EPB | ... | EPB |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: File structure example: a pcapng file similar to a
classical libpcap file.
Figure 6 shows a complex example file. In addition to the minimum
file above, it contains packets captured from three interfaces,
capturing on the third of which begins after packets have arrived on
other interfaces, and also includes some Name Resolution Blocks (NRB)
and an Interface Statistics Block (ISB).
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SHB | IDB | IDB | EPB | NRB |...| IDB | EPB | ISB | NRB | EPB | EPB |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: File structure example: more complex pcapng file.
The last example should make it obvious that the block structure
makes the file format very flexible compared to the classical libpcap
format.
3.5. Options
All the block bodies have the possibility to embed optional fields.
Optional fields can be used to insert some information that may be
useful when reading data, but that is not really needed for packet
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processing. Therefore, each tool can either read the content of the
optional fields (if any), or skip some of them or even all at once.
Skipping all the optional fields at once is straightforward because
most of the blocks are made of a first part with fixed format, and a
second optional part. Therefore, the Block Length field (present in
the General Block Structure, see Section 3.1) can be used to skip
everything till the next block.
Options are a list of Type - Length - Value fields, each one
containing a single value:
o Option Type (2 bytes): it contains the code that specifies the
type of the current TLV record. Option types whose Most
Significant Bit is equal to one are reserved for local use;
therefore, there is no guarantee that the code used is unique
among all capture files (generated by other applications). In
case of vendor-specific extensions that have to be identified
uniquely, vendors MUST request an Option Code whose MSB is equal
to zero.
o Option Length (2 bytes): it contains the actual length of the
following 'Option Value' field without the padding bytes.
o Option Value (variable length): it contains the value of the given
option, aligned to a 32-bit boundary. The actual length of this
field (i.e. without the padding bytes) is specified by the Option
Length field.
If an option's value is a string, the value is not necessarily zero-
terminated. Software that reads these files MUST NOT assume that
strings are zero-terminated, and MUST treat a zero byte as a string
terminator.
Options may be repeated several times (e.g. an interface that has
several IP addresses associated to it) TODO: mention for each option,
if it can/shouldn't appear more than one time. The option list is
terminated by a Option which uses the special 'End of Option' code
(opt_endofopt).
The format of the optional fields is shown in Figure 7.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Code | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Option Value /
/ variable length, aligned to 32 bits /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
/ . . . other options . . . /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Code == opt_endofopt | Option Length == 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Options format.
The following codes can always be present in any optional field:
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+--------------+------+----------+-------------+--------------------+
| Name | Code | Length | Description | Example(s) |
+--------------+------+----------+-------------+--------------------+
| opt_endofopt | 0 | 0 | It delimits | |
| | | | the end of | |
| | | | the | |
| | | | optional | |
| | | | fields. | |
| | | | This block | |
| | | | cannot be | |
| | | | repeated | |
| | | | within a | |
| | | | given list | |
| | | | of options. | |
| opt_comment | 1 | variable | A UTF-8 | "This packet is |
| | | | string | the beginning of |
| | | | containing | all of our |
| | | | a comment | problems" / |
| | | | that is | "Packets 17-23 |
| | | | associated | showing a bogus |
| | | | to the | TCP |
| | | | current | retransmission, as |
| | | | block. | reported in |
| | | | | bugzilla entry |
| | | | | 1486!" / "Captured |
| | | | | at the southern |
| | | | | plant" / "I've |
| | | | | checked again, now |
| | | | | it's working ok" / |
| | | | | ... |
+--------------+------+----------+-------------+--------------------+
3.6. Data format
Endianess
Data contained in each section will always be saved according to the
characteristics (little endian / big endian) of the dumping machine.
This refers to all the fields that are saved as numbers and that span
over two or more bytes.
The approach of having each section saved in the native format of the
generating host is more efficient because it avoids translation of
data when reading / writing on the host itself, which is the most
common case when generating/processing capture dumps.
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Please note: The endianess is indicated by the Section Header Block
(Section 4.1). As this block can appear several times in a pcapng
file, a single file can contain both endianess variants!
Alignment
All fields of this specification uses proper alignment for 16- and
32-bit values. This makes it easier and faster to read/write file
contents if using techniques like memory mapped files.
The alignment bytes (marked in this document e.g. with "aligned to 32
bits") MUST be filled with zero bytes.
Please note: 64-bit values are not aligned to 64-bit boundaries.
This is because the file is naturally aligned to 32-bit boundaries
only. Special care MUST be taken when reading and writing such
values. TODO: the spec is not too consistent wrt how 64-bit values
are saved. in the Packet blocks we clearly specify where the low and
high 32-bits of a 64-bit timestamp should be saved. In the SHB we do
use the endianess of the machine when we save the section length.
4. Block Definition
This section details the format of the body of the blocks currently
defined.
4.1. Section Header Block (mandatory)
The Section Header Block is mandatory. It identifies the beginning
of a section of the capture dump file. The Section Header Block does
not contain data but it rather identifies a list of blocks
(interfaces, packets) that are logically correlated. Its format is
shown in Figure 8.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
0 | Block Type = 0x0A0D0D0A |
+---------------------------------------------------------------+
4 | Block Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8 | Byte-Order Magic |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
12 | Major Version | Minor Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
16 | |
| Section Length |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
24 / /
/ Options (variable) /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Total Length |
+---------------------------------------------------------------+
Figure 8: Section Header Block format.
The meaning of the fields is:
o Block Type: The block type of the Section Header Block is the
integer corresponding to the 4-char string "\r\n\n\r"
(0x0A0D0D0A). This particular value is used for 2 reasons:
1. This number is used to detect if a file has been transferred
via FTP or HTTP from a machine to another with an
inappropriate ASCII conversion. In this case, the value of
this field will differ from the standard one ("\r\n\n\r") and
the reader can detect a possibly corrupted file.
2. This value is palindromic, so that the reader is able to
recognize the Section Header Block regardless of the endianess
of the section. The endianess is recognized by reading the
Byte Order Magic, that is located 8 bytes after the Block
Type.
o Block Total Length: total size of this block, as described in
Section 3.1.
o Byte-Order Magic: magic number, whose value is the hexadecimal
number 0x1A2B3C4D. This number can be used to distinguish
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sections that have been saved on little-endian machines from the
ones saved on big-endian machines.
o Major Version: number of the current mayor version of the format.
Current value is 1. This value should change if the format
changes in such a way that tools that can read the new format
could not read the old format (i.e., the code would have to check
the version number to be able to read both formats).
o Minor Version: number of the current minor version of the format.
Current value is 0. This value should change if the format
changes in such a way that tools that can read the new format can
still automatically read the new format but code that can only
read the old format cannot read the new format.
o Section Length: 64-bit value specifying the length in bytes of the
following section, excluding the Section Header Block itself.
This field can be used to skip the section, for faster navigation
inside large files. Section Length equal -1 (0xFFFFFFFFFFFFFFFF)
means that the size of the section is not specified, and the only
way to skip the section is to parse the blocks that it contains.
Please note that if this field is valid (i.e. not -1), its value
is always aligned to 32 bits, as all the blocks are aligned to
32-bit boundaries. Also, special care should be taken in
accessing this field: since the alignment of all the blocks in the
file is 32-bit, this field is not guaranteed to be aligned to a
64-bit boundary. This could be a problem on 64-bit workstations.
o Options: optionally, a list of options (formatted according to the
rules defined in Section 3.5) can be present.
Adding new block types or options would not necessarily require that
either Major or Minor numbers be changed, as code that does not know
about the block type or option could just skip it; only if skipping a
block or option does not work should the minor version number be
changed.
Aside from the options defined in Section 3.5, the following options
are valid within this block:
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+--------------+------+----------+-------------------+--------------+
| Name | Code | Length | Description | Example(s) |
+--------------+------+----------+-------------------+--------------+
| shb_hardware | 2 | variable | An UTF-8 string | "x86 |
| | | | containing the | Personal |
| | | | description of | Computer" / |
| | | | the hardware used | "Sun Sparc |
| | | | to create this | Workstation" |
| | | | section. | / ... |
| shb_os | 3 | variable | An UTF-8 string | "Windows XP |
| | | | containing the | SP2" / |
| | | | name of the | "openSUSE |
| | | | operating system | 10.2" / ... |
| | | | used to create | |
| | | | this section. | |
| shb_userappl | 4 | variable | An UTF-8 string | "dumpcap |
| | | | containing the | V0.99.7" / |
| | | | name of the | ... |
| | | | application used | |
| | | | to create this | |
| | | | section. | |
+--------------+------+----------+-------------------+--------------+
4.2. Interface Description Block (mandatory)
The Interface Description Block is mandatory. This block is needed
to specify the characteristics of the network interface on which the
capture has been made. In order to properly associate the captured
data to the corresponding interface, the Interface Description Block
MUST be defined before any other block that uses it; therefore, this
block is usually placed immediately after the Section Header Block.
However, if capturing on an interface starts in the middle of a
capture, an Interface Description Block can appear after other
blocks, including blocks for packets on interfaces for which
Interface Description Blocks have already appeared.
An Interface Description Block is valid only inside the section which
it belongs to. The structure of a Interface Description Block is
shown in Figure 9.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
0 | Block Type = 0x00000001 |
+---------------------------------------------------------------+
4 | Block Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8 | LinkType | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
12 | SnapLen |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
16 / /
/ Options (variable) /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Total Length |
+---------------------------------------------------------------+
Figure 9: Interface Description Block format.
The meaning of the fields is:
o Block Type: The block type of the Interface Description Block is
1.
o Block Total Length: total size of this block, as described in
Section 3.1.
o LinkType: a value that defines the link layer type of this
interface. The list of Standardized Link Layer Type codes is
available in the tcpdump.org link-layer header types registry [1].
o SnapLen: maximum number of bytes dumped from each packet. The
portion of each packet that exceeds this value will not be stored
in the file. A value of zero indicates no limit.
o Options: optionally, a list of options (formatted according to the
rules defined in Section 3.5) can be present.
Interface ID: Tools that write / read the capture file associate a
progressive 32-bit number (starting from '0') to each Interface
Definition Block. This number is unique within each Section and
uniquely identifies the interface (inside the current section);
therefore, two Sections can have interfaces identified by the same
identifiers. This unique identifier is referenced by other blocks
(e.g. Packet Block) to point out the interface the block refers to
(e.g. the interface that was used to capture the packet).
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In addition to the options defined in Section 3.5, the following
options are valid within this block:
+----------+----+-------+-----------+-------------------------------+
| Name | Co | Lengt | Descripti | Example(s) |
| | de | h | on | |
+----------+----+-------+-----------+-------------------------------+
| if_name | 2 | Varia | A UTF-8 | "eth0" / "\Device\NPF_{AD1CE6 |
| | | ble | string co | 75-96D0-47C5-ADD0-2504B9126B6 |
| | | | ntaining | 8}" / ... |
| | | | the name | |
| | | | of the | |
| | | | device | |
| | | | used to | |
| | | | capture | |
| | | | data. | |
| if_descr | 3 | Varia | A UTF-8 | "Broadcom NetXtreme" / "First |
| iption | | ble | string co | Ethernet Interface" / ... |
| | | | ntaining | |
| | | | the descr | |
| | | | iption of | |
| | | | the | |
| | | | device | |
| | | | used to | |
| | | | capture | |
| | | | data. | |
| if_IPv4a | 4 | 8 | Interface | 192 168 1 1 255 255 255 0 |
| ddr | | | network | |
| | | | address | |
| | | | and | |
| | | | netmask. | |
| | | | This | |
| | | | option | |
| | | | can be | |
| | | | repeated | |
| | | | multiple | |
| | | | times | |
| | | | within | |
| | | | the same | |
| | | | Interface | |
| | | | Descripti | |
| | | | on Block | |
| | | | when | |
| | | | multiple | |
| | | | IPv4 | |
| | | | addresses | |
| | | | are | |
| | | | assigned | |
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| | | | to the in | |
| | | | terface. | |
| if_IPv6a | 5 | 17 | Interface | 2001:0db8:85a3:08d3:1319:8a2e |
| ddr | | | network | :0370:7344/64 is written (in |
| | | | address | hex) as "20 01 0d b8 85 a3 08 |
| | | | and | d3 13 19 8a 2e 03 70 73 44 |
| | | | prefix | 40" |
| | | | length | |
| | | | (stored | |
| | | | in the | |
| | | | last | |
| | | | byte). | |
| | | | This | |
| | | | option | |
| | | | can be | |
| | | | repeated | |
| | | | multiple | |
| | | | times | |
| | | | within | |
| | | | the same | |
| | | | Interface | |
| | | | Descripti | |
| | | | on Block | |
| | | | when | |
| | | | multiple | |
| | | | IPv6 | |
| | | | addresses | |
| | | | are | |
| | | | assigned | |
| | | | to the in | |
| | | | terface. | |
| if_MACad | 6 | 6 | Interface | 00 01 02 03 04 05 |
| dr | | | Hardware | |
| | | | MAC | |
| | | | address | |
| | | | (48 | |
| | | | bits). | |
| if_EUIad | 7 | 8 | Interface | 02 34 56 FF FE 78 9A BC |
| dr | | | Hardware | |
| | | | EUI | |
| | | | address | |
| | | | (64 | |
| | | | bits), if | |
| | | | available | |
| | | | . | |
| if_speed | 8 | 8 | Interface | 100000000 for 100Mbps |
| | | | speed (in | |
| | | | bps). | |
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| if_tsres | 9 | 1 | Resolutio | 6 |
| ol | | | n of time | |
| | | | stamps. | |
| | | | If the | |
| | | | Most Sign | |
| | | | ificant | |
| | | | Bit is | |
| | | | equal to | |
| | | | zero, the | |
| | | | remaining | |
| | | | bits | |
| | | | indicates | |
| | | | the resol | |
| | | | ution of | |
| | | | the | |
| | | | timestamp | |
| | | | as as a | |
| | | | negative | |
| | | | power of | |
| | | | 10 (e.g. | |
| | | | 6 means m | |
| | | | icrosecon | |
| | | | d resolut | |
| | | | ion, time | |
| | | | stamps | |
| | | | are the | |
| | | | number of | |
| | | | microseco | |
| | | | nds since | |
| | | | 1/1/1970) | |
| | | | . If the | |
| | | | Most Sign | |
| | | | ificant | |
| | | | Bit is | |
| | | | equal to | |
| | | | one, the | |
| | | | remaining | |
| | | | bits | |
| | | | indicates | |
| | | | the resol | |
| | | | ution as | |
| | | | as | |
| | | | negative | |
| | | | power of | |
| | | | 2 (e.g. | |
| | | | 10 means | |
| | | | 1/1024 of | |
| | | | second). | |
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| | | | If this | |
| | | | option is | |
| | | | not | |
| | | | present, | |
| | | | a resolut | |
| | | | ion of | |
| | | | 10^-6 is | |
| | | | assumed | |
| | | | (i.e. tim | |
| | | | estamps | |
| | | | have the | |
| | | | same reso | |
| | | | lution of | |
| | | | the | |
| | | | standard | |
| | | | 'libpcap' | |
| | | | timestamp | |
| | | | s). | |
| if_tzone | 10 | 4 | Time zone | TODO: give a good example |
| | | | for GMT | |
| | | | support | |
| | | | (TODO: | |
| | | | specify | |
| | | | better). | |
| if_filte | 11 | varia | The | 00 "tcp port 23 and host |
| r | | ble | filter | 192.0.2.5" |
| | | | (e.g. | |
| | | | "capture | |
| | | | only TCP | |
| | | | traffic") | |
| | | | used to | |
| | | | capture | |
| | | | traffic. | |
| | | | The first | |
| | | | byte of | |
| | | | the | |
| | | | Option | |
| | | | Data | |
| | | | keeps a | |
| | | | code of | |
| | | | the | |
| | | | filter | |
| | | | used | |
| | | | (e.g. if | |
| | | | this is a | |
| | | | libpcap | |
| | | | string, | |
| | | | or BPF | |
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| | | | bytecode, | |
| | | | and | |
| | | | more). | |
| | | | More | |
| | | | details | |
| | | | about | |
| | | | this | |
| | | | format | |
| | | | will be | |
| | | | presented | |
| | | | in | |
| | | | Appendix | |
| | | | XXX | |
| | | | (TODO). | |
| | | | (TODO: | |
| | | | better | |
| | | | use | |
| | | | different | |
| | | | options | |
| | | | for | |
| | | | different | |
| | | | fields? | |
| | | | e.g. if_f | |
| | | | ilter_pca | |
| | | | p, if_fil | |
| | | | ter_bpf, | |
| | | | ...) | |
| if_os | 12 | varia | A UTF-8 | "Windows XP SP2" / "openSUSE |
| | | ble | string co | 10.2" / ... |
| | | | ntaining | |
| | | | the name | |
| | | | of the | |
| | | | operating | |
| | | | system of | |
| | | | the | |
| | | | machine | |
| | | | in which | |
| | | | this | |
| | | | interface | |
| | | | is instal | |
| | | | led. This | |
| | | | can be | |
| | | | different | |
| | | | from the | |
| | | | same info | |
| | | | rmation | |
| | | | that can | |
| | | | be | |
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| | | | contained | |
| | | | by the | |
| | | | Section | |
| | | | Header | |
| | | | Block | |
| | | | (Section | |
| | | | 4.1) | |
| | | | because | |
| | | | the | |
| | | | capture | |
| | | | can have | |
| | | | been done | |
| | | | on a | |
| | | | remote | |
| | | | machine. | |
| if_fcsle | 13 | 1 | An | 4 |
| n | | | integer | |
| | | | value | |
| | | | that | |
| | | | specified | |
| | | | the | |
| | | | length of | |
| | | | the Frame | |
| | | | Check | |
| | | | Sequence | |
| | | | (in bits) | |
| | | | for this | |
| | | | interface | |
| | | | . For | |
| | | | link | |
| | | | layers | |
| | | | whose FCS | |
| | | | length | |
| | | | can | |
| | | | change | |
| | | | during | |
| | | | time, the | |
| | | | Packet | |
| | | | Block | |
| | | | Flags | |
| | | | Word can | |
| | | | be used | |
| | | | (see | |
| | | | Appendix | |
| | | | A). | |
| if_tsoff | 14 | 8 | A 64-bit | 1234 |
| set | | | integer | |
| | | | value | |
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| | | | that | |
| | | | specifies | |
| | | | an offset | |
| | | | (in | |
| | | | seconds) | |
| | | | that must | |
| | | | be added | |
| | | | to the | |
| | | | timestamp | |
| | | | of each | |
| | | | packet to | |
| | | | obtain | |
| | | | the | |
| | | | absolute | |
| | | | timestamp | |
| | | | of a | |
| | | | packet. | |
| | | | If the | |
| | | | option is | |
| | | | missing, | |
| | | | the times | |
| | | | tamps | |
| | | | stored in | |
| | | | the | |
| | | | packet | |
| | | | MUST be c | |
| | | | onsidered | |
| | | | absolute | |
| | | | timestamp | |
| | | | s. The | |
| | | | time zone | |
| | | | of the | |
| | | | offset | |
| | | | can be | |
| | | | specified | |
| | | | with the | |
| | | | option | |
| | | | if_tzone. | |
| | | | TODO: | |
| | | | won't a i | |
| | | | f_tsoffse | |
| | | | t_low for | |
| | | | fractiona | |
| | | | l second | |
| | | | offsets | |
| | | | be useful | |
| | | | for | |
| | | | highly sy | |
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| | | | ncronized | |
| | | | capture | |
| | | | systems? | |
+----------+----+-------+-----------+-------------------------------+
4.3. Enhanced Packet Block (optional)
An Enhanced Packet Block is the standard container for storing the
packets coming from the network. The Enhanced Packet Block is
optional because packets can be stored either by means of this block
or the Simple Packet Block, which can be used to speed up dump
generation. The format of an Enhanced Packet Block is shown in
Figure 10.
The Enhanced Packet Block is an improvement over the original Packet
Block (Section 4.5):
o it stores the Interface Identifier as a 32-bit integer value.
This is a requirement when a capture stores packets coming from a
large number of interfaces
o differently from the Packet Block (Section 4.5), the number of
packets dropped by the capture system between this packet and the
previous one is not stored in the header, but rather in an option
of the block itself.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
0 | Block Type = 0x00000006 |
+---------------------------------------------------------------+
4 | Block Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8 | Interface ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
12 | Timestamp (High) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
16 | Timestamp (Low) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
20 | Captured Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
24 | Packet Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
28 / /
/ Packet Data /
/ variable length, aligned to 32 bits /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
/ Options (variable) /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Total Length |
+---------------------------------------------------------------+
Figure 10: Enhanced Packet Block format.
The Enhanced Packet Block has the following fields:
o Block Type: The block type of the Enhanced Packet Block is 6.
o Block Total Length: total size of this block, as described in
Section 3.1.
o Interface ID: it specifies the interface this packet comes from;
the correct interface will be the one whose Interface Description
Block (within the current Section of the file) is identified by
the same number (see Section 4.2) of this field. The interface ID
MUST be valid, which means that an matching interface description
block MUST exist.
o Timestamp (High) and Timestamp (Low): high and low 32-bits of a
64-bit quantity representing the timestamp. The timestamp is a
single 64-bit unsigned integer representing the number of units
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since 1/1/1970 00:00:00 UTC. The way to interpret this field is
specified by the 'if_tsresol' option (see Figure 9) of the
Interface Description block referenced by this packet. Please
note that differently from the libpcap file format, timestamps are
not saved as two 32-bit values accounting for the seconds and
microseconds since 1/1/1970. They are saved as a single 64-bit
quantity saved as two 32-bit words.
o Captured Len: number of bytes captured from the packet (i.e. the
length of the Packet Data field). It will be the minimum value
among the actual Packet Length and the snapshot length (defined in
Figure 9). The value of this field does not include the padding
bytes added at the end of the Packet Data field to align the
Packet Data Field to a 32-bit boundary.
o Packet Len: actual length of the packet when it was transmitted on
the network. It can be different from Captured Len if the user
wants only a snapshot of the packet.
o Packet Data: the data coming from the network, including link-
layer headers. The actual length of this field is Captured Len.
The format of the link-layer headers depends on the LinkType field
specified in the Interface Description Block (see Section 4.2) and
it is specified in the entry for that format in the the
tcpdump.org link-layer header types registry [2].
o Options: optionally, a list of options (formatted according to the
rules defined in Section 3.5) can be present.
In addition to the options defined in Section 3.5 and in the Packet
Block (Table 1), the following options are valid within this block:
+---------------+------+----------+--------------------+------------+
| Name | Code | Length | Description | Example(s) |
+---------------+------+----------+--------------------+------------+
| epb_flags | 2 | 4 | A flags word | 0 |
| | | | containing link- | |
| | | | layer information. | |
| | | | A complete | |
| | | | specification of | |
| | | | the allowed flags | |
| | | | can be found in | |
| | | | Appendix A. | |
| epb_hash | 3 | variable | This option | 2 EC 1D 87 |
| | | | contains a hash of | 97 / 3 45 |
| | | | the packet. The | 6e c2 17 |
| | | | first byte | 7c 10 1e |
| | | | specifies the | 3c 2e 99 |
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| | | | hashing algorithm, | 6e c2 9a |
| | | | while the | 3d 50 8e |
| | | | following bytes | |
| | | | contain the actual | |
| | | | hash, whose size | |
| | | | depends on the | |
| | | | hashing algorithm, | |
| | | | and hence from the | |
| | | | value in the first | |
| | | | byte. The hashing | |
| | | | algorithm can be: | |
| | | | 2s complement | |
| | | | (algorithm byte = | |
| | | | 0, size=XXX), XOR | |
| | | | (algorithm byte = | |
| | | | 1, size=XXX), | |
| | | | CRC32 (algorithm | |
| | | | byte = 2, size = | |
| | | | 4), MD-5 | |
| | | | (algorithm byte = | |
| | | | 3, size=XXX), | |
| | | | SHA-1 (algorithm | |
| | | | byte = 4, | |
| | | | size=XXX). The | |
| | | | hash covers only | |
| | | | the packet, not | |
| | | | the header added | |
| | | | by the capture | |
| | | | driver: this gives | |
| | | | the possibility to | |
| | | | calculate it | |
| | | | inside the network | |
| | | | card. The hash | |
| | | | allows easier | |
| | | | comparison/merging | |
| | | | of different | |
| | | | capture files, and | |
| | | | reliable data | |
| | | | transfer between | |
| | | | the data | |
| | | | acquisition system | |
| | | | and the capture | |
| | | | library. (TODO: | |
| | | | the text above | |
| | | | uses "first bit", | |
| | | | but shouldn't this | |
| | | | be "first | |
| | | | byte"?!?) | |
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| epb_dropcount | 4 | 8 | A 64-bit integer | 0 |
| | | | value specifying | |
| | | | the number of | |
| | | | packets lost (by | |
| | | | the interface and | |
| | | | the operating | |
| | | | system) between | |
| | | | this packet and | |
| | | | the preceding one. | |
+---------------+------+----------+--------------------+------------+
4.4. Simple Packet Block (optional)
The Simple Packet Block is a lightweight container for storing the
packets coming from the network. Its presence is optional.
A Simple Packet Block is similar to a Packet Block (see Section 4.5),
but it is smaller, simpler to process and contains only a minimal set
of information. This block is preferred to the standard Packet Block
when performance or space occupation are critical factors, such as in
sustained traffic dump applications. A capture file can contain both
Packet Blocks and Simple Packet Blocks: for example, a capture tool
could switch from Packet Blocks to Simple Packet Blocks when the
hardware resources become critical.
The Simple Packet Block does not contain the Interface ID field.
Therefore, it MUST be assumed that all the Simple Packet Blocks have
been captured on the interface previously specified in the first
Interface Description Block.
Figure 11 shows the format of the Simple Packet Block.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
0 | Block Type = 0x00000003 |
+---------------------------------------------------------------+
4 | Block Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8 | Packet Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
12 / /
/ Packet Data /
/ variable length, aligned to 32 bits /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Total Length |
+---------------------------------------------------------------+
Figure 11: Simple Packet Block format.
The Simple Packet Block has the following fields:
o Block Type: The block type of the Simple Packet Block is 3.
o Block Total Length: total size of this block, as described in
Section 3.1.
o Packet Len: actual length of the packet when it was transmitted on
the network. Can be different from captured len if the packet has
been truncated by the capture process.
o Packet Data: the data coming from the network, including link-
layer headers. The length of this field can be derived from the
field Block Total Length, present in the Block Header, and it is
the minimum value among the SnapLen (present in the Interface
Description Block) and the Packet Len (present in this header).
The format of the link-layer headers depends on the LinkType field
specified in the Interface Description Block (see Section 4.2) and
it is specified in the entry for that format in the the
tcpdump.org link-layer header types registry [3].
The Simple Packet Block does not contain the timestamp because this
is often one of the most costly operations on PCs. Additionally,
there are applications that do not require it; e.g. an Intrusion
Detection System is interested in packets, not in their timestamp.
A Simple Packet Block cannot be present in a Section that has more
than one interface because of the impossibility to refer to the
correct one (it does not contain any Interface ID field).
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The Simple Packet Block is very efficient in term of disk space: a
snapshot whose length is 100 bytes requires only 16 bytes of
overhead, which corresponds to an efficiency of more than 86%.
4.5. Packet Block (obsolete!)
The Packet Block is marked obsolete, better use the Enhanced Packet
Block instead!
A Packet Block is the standard container for storing the packets
coming from the network. The Packet Block is optional because
packets can be stored either by means of this block or the Simple
Packet Block, which can be used to speed up dump generation. The
format of a packet block is shown in Figure 12.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
0 | Block Type = 0x00000002 |
+---------------------------------------------------------------+
4 | Block Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8 | Interface ID | Drops Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
12 | Timestamp (High) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
16 | Timestamp (Low) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
20 | Captured Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
24 | Packet Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
28 / /
/ Packet Data /
/ variable length, aligned to 32 bits /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
/ Options (variable) /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Total Length |
+---------------------------------------------------------------+
Figure 12: Packet Block format.
The Packet Block has the following fields:
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o Block Type: The block type of the Packet Block is 2.
o Block Total Length: total size of this block, as described in
Section 3.1.
o Interface ID: it specifies the interface this packet comes from;
the correct interface will be the one whose Interface Description
Block (within the current Section of the file) is identified by
the same number (see Section 4.2) of this field. The interface ID
MUST be valid, which means that an matching interface description
block MUST exist.
o Drops Count: a local drop counter. It specifies the number of
packets lost (by the interface and the operating system) between
this packet and the preceding one. The value xFFFF (in
hexadecimal) is reserved for those systems in which this
information is not available.
o Timestamp (High) and Timestamp (Low): timestamp of the packet.
The format of the timestamp is the same already defined in the
Enhanced Packet Block (Section 4.3).
o Captured Len: number of bytes captured from the packet (i.e. the
length of the Packet Data field). It will be the minimum value
among the actual Packet Length and the snapshot length (SnapLen
defined in Figure 9). The value of this field does not include
the padding bytes added at the end of the Packet Data field to
align the Packet Data Field to a 32-bit boundary.
o Packet Len: actual length of the packet when it was transmitted on
the network. Can be different from Captured Len if the user wants
only a snapshot of the packet.
o Packet Data: the data coming from the network, including link-
layer headers. The actual length of this field is Captured Len.
The format of the link-layer headers depends on the LinkType field
specified in the Interface Description Block (see Section 4.2) and
it is specified in the entry for that format in the the
tcpdump.org link-layer header types registry [4].
o Options: optionally, a list of options (formatted according to the
rules defined in Section 3.5) can be present.
In addition to the options defined in Section 3.5, the following
options are valid within this block:
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+------------+------+----------+----------------+-------------------+
| Name | Code | Length | Description | Example(s) |
+------------+------+----------+----------------+-------------------+
| pack_flags | 2 | 4 | Same as | 0 |
| | | | epb_flags of | |
| | | | the enhanced | |
| | | | packet block. | |
| pack_hash | 3 | variable | Same as | 2 EC 1D 87 97 / 3 |
| | | | epb_hash of | 45 6e c2 17 7c 10 |
| | | | the enhanced | 1e 3c 2e 99 6e c2 |
| | | | packet block. | 9a 3d 50 8e |
+------------+------+----------+----------------+-------------------+
Table 1
4.6. Name Resolution Block (optional)
The Name Resolution Block is used to support the correlation of
numeric addresses (present in the captured packets) and their
corresponding canonical names and it is optional. Having the literal
names saved in the file, this prevents the need of a name resolution
in a delayed time, when the association between names and addresses
can be different from the one in use at capture time. Moreover, the
Name Resolution Block avoids the need of issuing a lot of DNS
requests every time the trace capture is opened, and allows to have
name resolution also when reading the capture with a machine not
connected to the network.
A Name Resolution Block is normally placed at the beginning of the
file, but no assumptions can be taken about its position. Name
Resolution Blocks can be added in a second time by tools that process
the file, like network analyzers.
The format of the Name Resolution Block is shown in Figure 13.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
0 | Block Type = 0x00000004 |
+---------------------------------------------------------------+
4 | Block Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8 | Record Type | Record Value Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
12 / Record Value /
/ variable length, aligned to 32 bits /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. . . . other records . . . .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Record Type == end_of_recs | Record Value Length == 00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
/ Options (variable) /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Total Length |
+---------------------------------------------------------------+
Figure 13: Name Resolution Block format.
The Name Resolution Block has the following fields:
o Block Type: The block type of the Name Resolution Block is 4.
o Block Total Length: total size of this block, as described in
Section 3.1.
This is followed by a zero-terminated list of records (in the TLV
format), each of which contains an association between a network
address and a name. There are three possible types of records:
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+-----------------+------+----------+-----------------+-------------+
| Name | Code | Length | Description | Example(s) |
+-----------------+------+----------+-----------------+-------------+
| nres_endofrecor | 0 | 0 | It delimits the | |
| d | | | end of name | |
| | | | resolution | |
| | | | records. This | |
| | | | record is | |
| | | | needed to | |
| | | | determine when | |
| | | | the list of | |
| | | | name resolution | |
| | | | records has | |
| | | | ended and some | |
| | | | options (if | |
| | | | any) begin. | |
| nres_ip4record | 1 | Variable | Specifies an | 127 0 0 1 |
| | | | IPv4 address | "localhost" |
| | | | (contained in | |
| | | | the first 4 | |
| | | | bytes), | |
| | | | followed by one | |
| | | | or more zero- | |
| | | | terminated | |
| | | | strings | |
| | | | containing the | |
| | | | DNS entries for | |
| | | | that address. | |
| nres_ip6record | 2 | Variable | Specifies an | 20 01 0d b8 |
| | | | IPv6 address | 00 00 00 00 |
| | | | (contained in | 00 00 00 00 |
| | | | the first 16 | 12 34 56 78 |
| | | | bytes), | "somehost" |
| | | | followed by one | |
| | | | or more zero- | |
| | | | terminated | |
| | | | strings | |
| | | | containing the | |
| | | | DNS entries for | |
| | | | that address. | |
+-----------------+------+----------+-----------------+-------------+
Table 2
Each Record Value is aligned to a 32-bit boundary. The corresponding
Record Value Length reflects the actual length of the Record Value;
it does not include the lengths of the Record Type or the Record
Value Length.
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After the list of Name Resolution Records, optionally, a list of
options (formatted according to the rules defined in Section 3.5) can
be present.
In addiction to the options defined in Section 3.5, the following
options are valid within this block:
+---------------+------+----------+--------------+------------------+
| Name | Code | Length | Description | Example(s) |
+---------------+------+----------+--------------+------------------+
| ns_dnsname | 2 | Variable | A UTF-8 | "our_nameserver" |
| | | | string | |
| | | | containing | |
| | | | the name of | |
| | | | the machine | |
| | | | (DNS server) | |
| | | | used to | |
| | | | perform the | |
| | | | name | |
| | | | resolution. | |
| ns_dnsIP4addr | 3 | 4 | The IPv4 | 192 168 0 1 |
| | | | address of | |
| | | | the DNS | |
| | | | server. | |
| ns_dnsIP6addr | 4 | 16 | The IPv6 | 20 01 0d b8 00 |
| | | | address of | 00 00 00 00 00 |
| | | | the DNS | 00 00 12 34 56 |
| | | | server. | 78 |
+---------------+------+----------+--------------+------------------+
4.7. Interface Statistics Block (optional)
The Interface Statistics Block contains the capture statistics for a
given interface and it is optional. The statistics are referred to
the interface defined in the current Section identified by the
Interface ID field. An Interface Statistics Block is normally placed
at the end of the file, but no assumptions can be taken about its
position - it can even appear multiple times for the same interface.
The format of the Interface Statistics Block is shown in Figure 14.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
0 | Block Type = 0x00000005 |
+---------------------------------------------------------------+
4 | Block Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8 | Interface ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
12 | Timestamp (High) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
16 | Timestamp (Low) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
20 / /
/ Options (variable) /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Total Length |
+---------------------------------------------------------------+
Figure 14: Interface Statistics Block format.
The fields have the following meaning:
o Block Type: The block type of the Interface Statistics Block is 5.
o Block Total Length: total size of this block, as described in
Section 3.1.
o Interface ID: it specifies the interface these statistics refers
to; the correct interface will be the one whose Interface
Description Block (within the current Section of the file) is
identified by same number (see Section 4.2) of this field. Please
note: in former versions of this document, this field was 16 bits
only. As this differs from its usage in other places of this doc
and as this block was not used "in the wild" before (as to the
knowledge of the authors), it seems reasonable to change it to 32
bits!
o Timestamp: time this statistics refers to. The format of the
timestamp is the same already defined in the Enhanced Packet Block
(Section 4.3).
o Options: optionally, a list of options (formatted according to the
rules defined in Section 3.5) can be present.
All the statistic fields are defined as options in order to deal with
systems that do not have a complete set of statistics. Therefore, In
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addiction to the options defined in Section 3.5, the following
options are valid within this block:
+------------------+------+--------+-------------------+------------+
| Name | Code | Length | Description | |
+------------------+------+--------+-------------------+------------+
| isb_starttime | 2 | 8 | Time in which the | 97 c3 04 |
| | | | capture started; | 00 aa 47 |
| | | | time will be | ca 64 |
| | | | stored in two | (Little |
| | | | blocks of four | Endian, |
| | | | bytes each. The | decodes to |
| | | | format of the | 06/29/2012 |
| | | | timestamp is the | 06:16:50 |
| | | | same already | UTC) |
| | | | defined in the | |
| | | | Enhanced Packet | |
| | | | Block (Section | |
| | | | 4.3). | |
| isb_endtime | 3 | 8 | Time in which the | 96 c3 04 |
| | | | capture ended; ; | 00 73 89 |
| | | | time will be | 6a 65 |
| | | | stored in two | (Little |
| | | | blocks of four | Endian, |
| | | | bytes each. The | decodes to |
| | | | format of the | 06/29/2012 |
| | | | timestamp is the | 06:17:00 |
| | | | same already | UTC) |
| | | | defined in the | |
| | | | Enhanced Packet | |
| | | | Block (Section | |
| | | | 4.3). | |
| isb_ifrecv | 4 | 8 | Number of packets | 100 |
| | | | received from the | |
| | | | physical | |
| | | | interface | |
| | | | starting from the | |
| | | | beginning of the | |
| | | | capture. | |
| isb_ifdrop | 5 | 8 | Number of packets | 0 |
| | | | dropped by the | |
| | | | interface due to | |
| | | | lack of resources | |
| | | | starting from the | |
| | | | beginning of the | |
| | | | capture. | |
| isb_filteraccept | 6 | 8 | Number of packets | 100 |
| | | | accepted by | |
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| | | | filter starting | |
| | | | from the | |
| | | | beginning of the | |
| | | | capture. | |
| isb_osdrop | 7 | 8 | Number of packets | 0 |
| | | | dropped by the | |
| | | | operating system | |
| | | | starting from the | |
| | | | beginning of the | |
| | | | capture. | |
| isb_usrdeliv | 8 | 8 | Number of packets | 0 |
| | | | delivered to the | |
| | | | user starting | |
| | | | from the | |
| | | | beginning of the | |
| | | | capture. The | |
| | | | value contained | |
| | | | in this field can | |
| | | | be different from | |
| | | | the value | |
| | | | 'isb_filteraccept | |
| | | | - isb_osdrop' | |
| | | | because some | |
| | | | packets could | |
| | | | still lay in the | |
| | | | OS buffers when | |
| | | | the capture | |
| | | | ended. | |
+------------------+------+--------+-------------------+------------+
All the fields that refer to packet counters are 64-bit values,
represented with the byte order of the current section. Special care
must be taken in accessing these fields: since all the blocks are
aligned to a 32-bit boundary, such fields are not guaranteed to be
aligned on a 64-bit boundary.
5. Experimental Blocks (deserved to a further investigation)
5.1. Alternative Packet Blocks (experimental)
Can some other packet blocks (besides the ones described in the
previous paragraphs) be useful?
5.2. Compression Block (experimental)
The Compression Block is optional. A file can contain an arbitrary
number of these blocks. A Compression Block, as the name says, is
used to store compressed data. Its format is shown in Figure 15.
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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
+---------------------------------------------------------------+
| Block Type = ? |
+---------------------------------------------------------------+
| Block Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Compr. Type | |
+-+-+-+-+-+-+-+-+ |
| |
| Compressed Data |
| |
| variable length, byte-aligned |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Total Length |
+---------------------------------------------------------------+
Figure 15: Compression Block format.
The fields have the following meaning:
o Block Type: The block type of the Compression Block is not yet
assigned.
o Block Total Length: total size of this block, as described in
Section 3.1.
o Compression Type: specifies the compression algorithm. Possible
values for this field are 0 (uncompressed), 1 (Lempel Ziv), 2
(Gzip), other?? Probably some kind of dumb and fast compression
algorithm could be effective with some types of traffic (for
example web), but which?
o Compressed Data: data of this block. Once decompressed, it is
made of other blocks.
5.3. Encryption Block (experimental)
The Encryption Block is optional. A file can contain an arbitrary
number of these blocks. An Encryption Block is used to store
encrypted data. Its format is shown in Figure 16.
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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
+---------------------------------------------------------------+
| Block Type = ? |
+---------------------------------------------------------------+
| Block Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encr. Type | |
+-+-+-+-+-+-+-+-+ |
| |
| Encrypted Data |
| |
| variable length, byte-aligned |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Total Length |
+---------------------------------------------------------------+
Figure 16: Encryption Block format.
The fields have the following meaning:
o Block Type: The block type of the Encryption Block is not yet
assigned.
o Block Total Length: total size of this block, as described in
Section 3.1.
o Encryption Type: specifies the encryption algorithm. Possible
values for this field are ??? (TODO) NOTE: this block should
probably contain other fields, depending on the encryption
algorithm. To be defined precisely.
o Encrypted Data: data of this block. Once decrypted, it originates
other blocks.
5.4. Fixed Length Block (experimental)
The Fixed Length Block is optional. A file can contain an arbitrary
number of these blocks. A Fixed Length Block can be used to optimize
the access to the file. Its format is shown in Figure 17. A Fixed
Length Block stores records with constant size. It contains a set of
Blocks (normally Packet Blocks or Simple Packet Blocks), of which it
specifies the size. Knowing this size a priori helps to scan the
file and to load some portions of it without truncating a block, and
is particularly useful with cell-based networks like ATM.
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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
+---------------------------------------------------------------+
| Block Type = ? |
+---------------------------------------------------------------+
| Block Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cell Size | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| Fixed Size Data |
| |
| variable length, byte-aligned |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Total Length |
+---------------------------------------------------------------+
Figure 17: Fixed Length Block format.
The fields have the following meaning:
o Block Type: The block type of the Fixed Length Block is not yet
assigned.
o Block Total Length: total size of this block, as described in
Section 3.1.
o Cell size: the size of the blocks contained in the data field.
o Fixed Size Data: data of this block.
5.5. Directory Block (experimental)
If present, this block contains the following information:
o number of indexed packets (N)
o table with position and length of any indexed packet (N entries)
A directory block MUST be followed by at least N packets, otherwise
it MUST be considered invalid. It can be used to efficiently load
portions of the file to memory and to support operations on memory
mapped files. This block can be added by tools like network
analyzers as a consequence of file processing.
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5.6. Traffic Statistics and Monitoring Blocks (experimental)
One or more blocks could be defined to contain network statistics or
traffic monitoring information. They could be use to store data
collected from RMON or Netflow probes, or from other network
monitoring tools.
5.7. Event/Security Block (experimental)
This block could be used to store events. Events could contain
generic information (for example network load over 50%, server
down...) or security alerts. An event could be:
o skipped, if the application doesn't know how to do with it
o processed independently by the packets. In other words, the
applications skips the packets and processes only the alerts
o processed in relation to packets: for example, a security tool
could load only the packets of the file that are near a security
alert; a monitoring tool could skip the packets captured while the
server was down.
6. Recommended File Name Extension: .pcapng
The recommended file name extension for the "PCAP Next Generation
Dump File Format" specified in this document is ".pcapng".
On Windows and OS X, files are distinguished by an extension to their
filename. Such an extension is technically not actually required, as
applications should be able to automatically detect the pcapng file
format through the "magic bytes" at the beginning of the file, as
some other UN*X desktop environments do. However, using name
extensions makes it easier to work with files (e.g. visually
distinguish file formats) so it is recommended - though not required
- to use .pcapng as the name extension for files following this
specification.
Please note: To avoid confusion (like the current usage of .cap for a
plethora of different capture file formats) other file name
extensions than .pcapng should be avoided!
7. How to add Vendor / Domain specific extensions
TODO - explain the preferred way to add new block types and new
options for existing blocks in more detail.
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8. Conclusions
The file format proposed in this document should be very versatile
and satisfy a wide range of applications. In the simplest case, it
can contain a raw dump of the network data, made of a series of
Simple Packet Blocks. In the most complex case, it can be used as a
repository for heterogeneous information. In every case, the file
remains easy to parse and an application can always skip the data it
is not interested in; at the same time, different applications can
share the file, and each of them can benefit of the information
produced by the others. Two or more files can be concatenated
obtaining another valid file.
9. Security Considerations
TBD.
10. IANA Considerations
TBD.
11. Acknowledgments
Loris Degioanni and Gianluca Varenni were coauthoring this document
before it was submitted to the IETF.
The authors wish to thank Anders Broman, Ulf Lamping, Richard Sharpe
and many others for their invaluable comments.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
12.2. URIs
[1] http://www.tcpdump.org/linktypes.html
[2] http://www.tcpdump.org/linktypes.html
[3] http://www.tcpdump.org/linktypes.html
[4] http://www.tcpdump.org/linktypes.html
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Appendix A. Packet Block Flags Word
The Packet Block Flags Word is a 32-bit value that contains link-
layer information about the packet.
The meaning of the bits is the following:
+--------+----------------------------------------------------------+
| Bit | Description |
| Number | |
+--------+----------------------------------------------------------+
| 0-1 | Inbound / Outbound packet (00 = information not |
| | available, 01 = inbound, 10 = outbound) |
| 2-4 | Reception type (000 = not specified, 001 = unicast, 010 |
| | = multicast, 011 = broadcast, 100 = promiscuous). |
| 5-8 | FCS length, in bytes (0000 if this information is not |
| | available). This value overrides the if_fcslen option of |
| | the Interface Description Block, and is used with those |
| | link layers (e.g. PPP) where the length of the FCS can |
| | change during time. |
| 9-15 | Reserved (MUST be set to zero). |
| 16-31 | link-layer-dependent errors (Bit 31 = symbol error, Bit |
| | 30 = preamble error, Bit 29 = Start Frame Delimiter |
| | error, Bit 28 = unaligned frame error, Bit 27 = wrong |
| | Inter Frame Gap error, Bit 26 = packet too short error, |
| | Bit 25 = packet too long error, Bit 24 = CRC error, |
| | other?? are 16 bit enough?). |
+--------+----------------------------------------------------------+
Appendix B. Standardized Block Type Codes
Every Block is uniquely identified by a 32-bit integer value, stored
in the Block Header.
As pointed out in Section 3.1, Block Types codes whose Most
Significant Bit (bit 31) is set to 1 are reserved for local use by
the application.
All the remaining Block Type codes (0x00000000 to 0x7FFFFFFF) are
standardized by this document. A request should be sent to the
authors of this document to add a new Standard Block Type code to the
specification.
Here is a list of the Standardized Block Type Codes.
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+-----------------------+-------------------------------------------+
| Block Type Code | Description |
+-----------------------+-------------------------------------------+
| 0x00000000 | Reserved ??? |
| 0x00000001 | Interface Description Block (Section 4.2) |
| 0x00000002 | Packet Block (Section 4.5) |
| 0x00000003 | Simple Packet Block (Section 4.4) |
| 0x00000004 | Name Resolution Block (Section 4.6) |
| 0x00000005 | Interface Statistics Block (Section 4.7) |
| 0x00000006 | Enhanced Packet Block (Section 4.3) |
| 0x00000007 | IRIG Timestamp Block (requested by |
| | Gianluca Varenni |
| | <gianluca.varenni@cacetech.com>, CACE |
| | Technologies LLC) |
| 0x00000008 | Arinc 429 in AFDX Encapsulation |
| | Information Block (requested by Gianluca |
| | Varenni <gianluca.varenni@cacetech.com>, |
| | CACE Technologies LLC) |
| 0x0A0D0D0A | Section Header Block (Section 4.1) |
| 0x0A0D0A00-0x0A0D0AFF | Reserved. Used to detect trace files |
| | corrupted because of file transfers using |
| | the HTTP protocol in text mode. |
| 0x000A0D0A-0xFF0A0D0A | Reserved. Used to detect trace files |
| | corrupted because of file transfers using |
| | the HTTP protocol in text mode. |
| 0x000A0D0D-0xFF0A0D0D | Reserved. Used to detect trace files |
| | corrupted because of file transfers using |
| | the HTTP protocol in text mode. |
| 0x0D0D0A00-0x0D0D0AFF | Reserved. Used to detect trace files |
| | corrupted because of file transfers using |
| | the FTP protocol in text mode. |
+-----------------------+-------------------------------------------+
Authors' Addresses
Michael Tuexen (editor)
Muenster University of Applied Sciences
Stegerwaldstrasse 39
Steinfurt 48565
DE
Email: tuexen@fh-muenster.de
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Fulvio Risso
Politecnico di Torino
Corso Duca degli Abruzzi, 24
Torino 10129
IT
Email: fulvio.risso@polito.it
Jasper Bongertz
Airbus Defence and Space CyberSecurity
Kanzlei 63c
Meerbusch 40667
DE
Email: jasper@packet-foo.com
Guy Harris
Email: guy@alum.mit.edu
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