Question

I’m currently experimenting with the Icmp protocol and I would like to implement a new class IcmpPacket.
I read about the protocol on Wikipedia and I also found an article about pinging with Icmp on MSDN.
I’m a bit confused because of some differences between both websites.

Wikipedia defines the structure of a Icmp packet as the following:
1 Byte Type
1 Byte Code
2 Byte Checksum
4 Byte Rest of header (whatever this is)

On MSDN there is an example given for such an IcmpPacket class (extract of the class)

public class IcmpPacket { 
    public byte Type;
    public byte SubCode;
    public UInt16 CheckSum;
    public UInt16 Identifier;
    public UInt16 SequenceNumber;
    public byte[] Data;
}

the first three attributes are clear but what about Identifier and SequenceNumber (=> Rest of header?)

Another question I got is if I would like to use an IcmpPacket to ping a server, do I really need Identifier, SequenceNumber and Data? (My hoster said that the smallest IcmpPacket possible is about 28Bytes large, so if taking Identifier and SequenceNumber, do I have to fill the remaining 20 Bytes with Data?)

P.S.: Is there maybe a website out which well describes the structure if Icmp?

Answer 1

what about Identifier and SequenceNumber?

Identifier and SequenceNumber are values meaningful to your implementation to match echo requests with the corresponding echo replies. RFC 792 offers a suggestion for using them…

  For example, the identifier might be used like a port in TCP or UDP to
  identify a session, and the sequence number might be incremented
  on each echo request sent.  The echoer returns these same values
  in the echo reply.

if I would like to use an IcmpPacket to ping a server, do I really need Identifier, SequenceNumber and Data?

Identifier, SequenceNumber and Data fields must exist in the frame…

My hoster said that the smallest IcmpPacket possible is about 28Bytes large, so if taking Identifier and SequenceNumber, do I have to fill the remaining 20 Bytes with Data?

If they are talking about a 28 byte IPv4 packet, then the smallest packet must include the minimum 20-byte IPv4 header + 8 bytes of ICMP header.

Is there maybe a website out which well describes the structure if Icmp?

You should read RFC 792 online…