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Network Working Group B. Callaghan Request for Comments: 1761 R. Gilligan Category: Informational Sun Microsystems, Inc.

                                                         February 1995
             Snoop Version 2 Packet Capture File Format

Status of this Memo

 This memo provides information for the Internet community.  This memo
 does not specify an Internet standard of any kind.  Distribution of
 this memo is unlimited.


 This paper describes the file format used by "snoop", a packet
 monitoring and capture program developed by Sun.  This paper is
 provided so that people can write compatible programs to generate and
 interpret snoop packet capture files.

1. Introduction

 The availability of tools to capture, display and interpret packets
 traversing a network has proven extremely useful in debugging
 networking problems.  The ability to capture packets and store them
 for later analysis allows one to de-couple the tasks of collecting
 information about a network problem and analysing that information.
 The "snoop" program, developed by Sun, has the ability to capture
 packets and store them in a file, and can interpret the packets
 stored in capture files.  This RFC describes the file format that the
 snoop program uses to store captured packets.  This paper was written
 so that others may write programs to interpret the capture files
 generated by snoop, or create capture files that can be interpreted
 by snoop.

Callaghan & Gilligan [Page 1] RFC 1761 Snoop Packet Capture File Format February 1995

2. File Format

 The snoop packet capture file is an array of octets structured as
      |                        |
      |      File Header       |
      |                        |
      |                        |
      |     Packet Record      |
      ~        Number 1        ~
      |                        |
      .                        .
      .                        .
      .                        .
      |                        |
      |     Packet Record      |
      ~        Number N        ~
      |                        |
 The File Header is a fixed-length field containing general
 information about the packet file and the format of the packet
 records it contains.  One or more variable-length Packet Record
 fields follow the File Header field.  Each Packet Record field holds
 the data of one captured packet.

3. File Header

 The structure of the File Header is as follows:
  |                                                               |
  +                     Identification Pattern                    +
  |                                                               |
  |                       Version Number = 2                      |
  |                         Datalink Type                         |

Callaghan & Gilligan [Page 2] RFC 1761 Snoop Packet Capture File Format February 1995

      Identification Pattern:
              A 64-bit (8 octet) pattern used to identify the file as
              a snoop packet capture file.  The Identification Pattern
              consists of the 8 hexadecimal octets:
                      73 6E 6F 6F 70 00 00 00
              This is the ASCII string "snoop" followed by three null
      Version Number:
              A 32-bit (4 octet) unsigned integer value representing
              the version of the packet capture file being used.  This
              document describes version number 2.  (Version number 1
              was used in early implementations and is now obsolete.)
      Datalink Type:
              A 32-bit (4 octet) field identifying the type of
              datalink header used in the packet records that follow.
              The datalink type codes are listed in the table below:
              Datalink Type           Code
              -------------           ----
              IEEE 802.3              0
              IEEE 802.4 Token Bus    1
              IEEE 802.5 Token Ring   2
              IEEE 802.6 Metro Net    3
              Ethernet                4
              HDLC                    5
              Character Synchronous   6
              IBM Channel-to-Channel  7
              FDDI                    8
              Other                   9
              Unassigned              10 - 4294967295

4. Packet Record Format

 Each packet record holds a partial or complete copy of one packet as
 well as some descriptive information about that packet.  The packet
 may be truncated in order to limit the amount of data to be stored in
 the packet file.  In addition, the packet record may be padded in
 order for it to align on a convenient machine-dependent boundary.
 Each packet record holds 24 octets of descriptive information about
 the packet, followed by the packet data, which is variable-length,
 and an optional pad field.  The descriptive information is structured

Callaghan & Gilligan [Page 3] RFC 1761 Snoop Packet Capture File Format February 1995

 as six 32-bit (4-octet) integer values.
 The structure of the packet record is as follows:
  |                        Original Length                        |
  |                        Included Length                        |
  |                      Packet Record Length                     |
  |                        Cumulative Drops                       |
  |                       Timestamp Seconds                       |
  |                     Timestamp Microseconds                    |
  |                                                               |
  .                                                               .
  .                          Packet Data                          .
  .                                                               .
  +                                               +- - - - - - - -+
  |                                               |     Pad       |
      Original Length
              32-bit unsigned integer representing the length in
              octets of the captured packet as received via a network.
      Included Length
              32-bit unsigned integer representing the length of the
              Packet Data field.  This is the number of octets of the
              captured packet that are included in this packet record.
              If the received packet was truncated, the Included
              Length field will be less than the Original Length
      Packet Record Length
              32-bit unsigned integer representing the total length of
              this packet record in octets.  This includes the 24
              octets of descriptive information, the length of the
              Packet Data field, and the length of the Pad field.

Callaghan & Gilligan [Page 4] RFC 1761 Snoop Packet Capture File Format February 1995

      Cumulative Drops
              32-bit unsigned integer representing the number of
              packets that were lost by the system that created the
              packet file between the first packet record in the
              file and this one.  Packets may be lost because of
              insufficient resources in the capturing system, or for
              other reasons.  Note: some implementations lack the
              ability to count dropped packets.  Those
              implementations may set the cumulative drops value to
      Timestamp Seconds
              32-bit unsigned integer representing the time, in
              seconds since January 1, 1970, when the packet arrived.
      Timestamp Microseconds
              32-bit unsigned integer representing microsecond
              resolution of packet arrival time.
      Packet Data
              Variable-length field holding the packet that was
              captured, beginning with its datalink header.  The
              Datalink Type field of the file header can be used to
              determine how to decode the datalink header.  The length
              of the Packet Data field is given in the Included Length
              Variable-length field holding zero or more octets that
              pads the packet record out to a convenient boundary.

5. Data Format

 All integer values are stored in "big-endian" order, with the high-
 order bits first.

6. Security Considerations

 Security issues are not discussed in this memo.

Callaghan & Gilligan [Page 5] RFC 1761 Snoop Packet Capture File Format February 1995

Authors' Addresses

 Brent Callaghan
 Sun Microsystems, Inc.
 2550 Garcia Avenue
 Mailstop UMTV05-44
 Mountain View, CA 94043-1100
 Phone: 1-415-336-1051
 Robert E. Gilligan
 Sun Microsystems, Inc.
 2550 Garcia Avenue
 Mailstop UMTV05-44
 Mountain View, CA 94043-1100
 Phone: 1-415-336-1012

Callaghan & Gilligan [Page 6]

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