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write

Write protocol packet data to PCAP or PCAPNG file

    Description

    write(pcapObj,packet,timestamp) writes the protocol packet data to the PCAP file specified in the PCAP file writer object, pcapObj. Input packet specifies the protocol packet and input timestamp specifies the packet arrival time.

    write(pcapngObj,packet,timestamp,interfaceID) writes protocol packet data to a PCAPNG file specified in the PCAPNG file writer object, pcapngObj. Input packet, timestamp, and interfaceID specifies the protocol packet, packet arrival time, and interface identifier, respectively.

    example

    write(___,Name,Value) specifies options using one or more name-value pair arguments in addition to the input argument combinations from any of the previous syntaxes. For example, 'PacketFormat','bits' sets the format of the protocol packets to bits.

    Examples

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    Create a PCAP file writer object, specifying the name of the PCAP file.

    pcapObj = pcapWriter('FileName','write5GNRpacket');
    timestamp = 300;                                           % Timestamp

    5G NR packets do not have a valid link type. As per Tcpdump, if a valid link type is not present, specify the link type of SLL packet.

    linkType = 113;

    Write a global header to the PCAP file.

    writeGlobalHeader(pcapObj,linkType);

    5G NR packets are not directly supported by Wireshark. To enable Wireshark to parse 5G NR packets, add encapsulation and metadata to the 5G NR packet.

    payload = [59; 205];                                       % MAC subPDU (contains truncated buffer status report (BSR) control element)
    radioType = 1;                                             % Frequency division duplexing
    linkDir = 0;                                               % Uplink packet
    rntiType = 3;                                              % Cell-RNTI
    startString = [109; 97; 99; 45; 110; 114];                 % Tag to indicate start of NR MAC signature
    payloadTag = 1;                                            % Payload tag for NR packets
    signature = [startString; radioType; linkDir; rntiType];
    macNRInfoPacket = [signature; payloadTag; payload];

    Construct a user datagram protocol (UDP) header.

    udpPacketLength = 8 + length(macNRInfoPacket);             % Length of header (8 bytes) and payload
    udpHeader = [163; 76;                                      % Source port number
        39; 15;                                                % Destination port number
        fix(udpPacketLength/256); mod(udpPacketLength,256);    % Total length of UDP packet
        0; 0];                                                 % Checksum

    Construct an IPv4 header.

    ipPacketLength = 20 + udpPacketLength;                     % Length of header (20 bytes) and payload
    ipHeader = [69;                                            % Version of IP protocol and priority or traffic class
        0;                                                     % Type of service
        fix(ipPacketLength/256); mod(ipPacketLength,256);      % Total length of the IPv4 packet
        0; 1;                                                  % Identification
        0; 0;                                                  % Flags and fragmentation offset
        64;                                                    % Time to live in seconds
        17;                                                    % UDP protocol number
        0; 0;                                                  % Header checksum
        127; 0; 0; 1;                                          % Source IP address
        127; 0; 0; 1];                                         % Destination IP address

    Construct an SLL header.

    sllHeader = [0; 0;                                         % Packet type
        3; 4;                                                  % Address resolution protocol hardware (ARPHRD) type
        0; 0;                                                  % Link layer address length
        0; 0; 0; 0; 0; 0; 0; 0;                                % Link layer address
        8; 0];                                                 % Protocol type

    Construct an 5G NR packet by adding encapsulation and metadata.

    packet = [sllHeader; ipHeader; udpHeader; macNRInfoPacket];

    Write the 5G NR packet data to the PCAP file.

    write(pcapObj,packet,timestamp);

    Create a PCAPNG file writer object, specifying the name of the PCAPNG file.

    pcapngObj = pcapngWriter('FileName','write5GNRpacket');

    Write the interface block for 5G New Radio (NR). 5G NR packets do not have a valid link type. As per Tcpdump, if a valid link type is not present, specify the link type of SLL packet.

    interface = '5GNR';                                        % Interface name
    linkType = 113;                                            % Link type of SLL packet
    timestamp = 300;                                           % Timestamp
    interfaceID = writeInterfaceDescriptionBlock(pcapngObj,linkType,interface);

    5G NR packets are not directly supported by Wireshark. To enable Wireshark to parse 5G NR packets, add encapsulation and metadata to the 5G NR packet.

    payload = [59; 205];                                       % MAC subPDU (contains short truncated buffer status report (BSR) control element)
    radioType = 1;                                             % Frequency division duplexing
    linkDir = 0;                                               % Uplink packet
    rntiType = 3;                                              % Cell-RNTI
    startString = [109; 97; 99; 45; 110; 114];                 % Tag to indicate the start of NR MAC signature
    payloadTag = 1;                                            % Payload tag for NR packets
    signature = [startString; radioType; linkDir; rntiType];
    macNRInfoPacket = [signature; payloadTag; payload];

    Construct a UDP header.

    udpPacketLength = 8 + length(macNRInfoPacket);             % Length of header (8 bytes) and payload
    udpHeader = [163; 76;                                      % Source port number
        39; 15;                                                % Destination port number
        fix(udpPacketLength/256); mod(udpPacketLength,256);    % Total length of UDP packet
        0; 0];                                                 % Checksum

    Construct an IPv4 header.

    ipPacketLength = 20 + udpPacketLength;                     % Length of header (20 bytes) and payload
    ipHeader = [69;                                            % Version of IP protocol and Priority or Traffic Class
        0;                                                     % Type of service
        fix(ipPacketLength/256);mod(ipPacketLength,256);       % Total length of the IPv4 packet
        0; 1;                                                  % Identification
        0; 0;                                                  % Flags and fragmentation offset
        64;                                                    % Time to live in seconds
        17;                                                    % UDP protocol number
        0; 0;                                                  % Header checksum
        127; 0; 0; 1;                                          % Source IP address
        127; 0; 0; 1];                                         % Destination IP address

    Construct an SLL header.

    sllHeader = [0;0;                                          % Packet type
        3; 4;                                                  % Address resolution protocol hardware (ARPHRD) type
        0; 0;                                                  % Link layer address length
        0; 0; 0; 0; 0; 0; 0; 0;                                % Link layer address
        8; 0];                                                 % Protocol type

    Construct an 5G NR packet by adding encapsulation and metadata.

    packet = [sllHeader; ipHeader; udpHeader; macNRInfoPacket];

    Write the 5G NR packet data to the PCAPNG file.

    packetComment = 'This is 5G NR MAC packet';                % Packet comment
    write(pcapngObj,packet,timestamp,interfaceID,'PacketComment',packetComment);

    Input Arguments

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    Note

    The pcapWriter and pcapngWriter objects do not overwrite the existing PCAP or PCAPNG files, respectively. Each time when you create these objects, specify a unique PCAP or PCAPNG file name.

    PCAP file writer object, specified as a pcapWriter object.

    Protocol packet, specified as one of these values.

    • Binary-valued vector – This value represents bits.

    • Character vector – This value represents octets in hexadecimal format.

    • String scalar – This value represents octets in hexadecimal format.

    • Numeric vector with each element in the range [0, 255] – This value represents octets in decimal format.

    • n-by-2 character array – In this value, each row represents an octet in hexadecimal format .

    Data Types: char | string | double

    Packet arrival time in POSIX® microseconds elapsed since 1/1/1970, specified as a nonnegative integer.

    Data Types: double

    PCAPNG file writer object, specified as a pcapngWriter object.

    Unique identifier for an interface, specified as a nonnegative scalar.

    Data Types: double

    Name-Value Arguments

    Specify optional comma-separated pairs of Name,Value arguments. Name is the argument name and Value is the corresponding value. Name must appear inside quotes. You can specify several name and value pair arguments in any order as Name1,Value1,...,NameN,ValueN.

    Example: 'PacketFormat','bits' specifies the format of the protocol packet as bits.

    Format of the protocol packet, specified as the comma-separated pair consisting of PacketFormat and 'octets' or 'bits'. If this value is specified as 'octets', packet is specified as one of these values.

    • Binary-valued vector – This value represents bits.

    • Character vector – This value represents octets in hexadecimal format.

    • String scalar – This value represents octets in hexadecimal format.

    • Numeric vector with each element in the range [0, 255] – This value represents octets in decimal format.

    • n-by-2 character array – In this value, each row represents an octet in hexadecimal format .

    Data Types: char | string | double

    Comment for the protocol packet, specified as the comma-separated pair consisting of PacketComment and a character vector or a string scalar.

    Dependencies

    To enable this name-value pair argument, specify the pcapngObj input argument.

    Data Types: char | string

    References

    [1] Tuexen, M. “PCAP Next Generation (Pcapng) Capture File Format.” 2020. https://www.ietf.org/.

    [2] Group, The Tcpdump. “Tcpdump/Libpcap Public Repository.” Accessed May 20, 2020. https://www.tcpdump.org.

    [3] “Development/LibpcapFileFormat - The Wireshark Wiki.” Accessed May 20, 2020. https://www.wireshark.org.

    Extended Capabilities

    C/C++ Code Generation
    Generate C and C++ code using MATLAB® Coder™.

    Introduced in R2020b