Monday, September 26, 2016

Implementing a Ping client in Python - Part I


In this series of posts,  I will demonstrate how to implement a Ping client using Python.

Ping is one of the most useful tools that a computer programmer might use. It is used to figure out if a target machine is reachable or not.

Ping is implemented using the ICMP Protocol

ICMP Protocol

The Internet Control Message Protocol (ICMP) is defined by the Internet Engineering Task Force. The specifications of the protocol are in RFC 792

The Ping utility uses the ECHO request/response facility of the ICMP protocol. The client sends an ICMP Echo request to the server, and the server responds with an ICMP Echo response. It is a very simple request/response protocol and makes it ideal to use in order to learn low level network programming.

ICMP Packet Description

The following table describes the ICMP Echo Request/Reply Packet

Type (8 bits)Code (8 bits)Checksum (16 bits)
Identifier (16 bits)Sequence Number (16 bits)

The Type is 8 for Echo message, and 0 for reply message.

Code has to be zero.

Identifier can be used to match request/reply packets.

Sequence Number can be used to distinguish multiple request/responses from each other.

The client sends an ICMP Echo packet to the server. An echo packet has the type code 8.

The server responds with an ICMP Response packet. An ICMP Echo response packet has a type code of 0.

Python Implementation Details

I have been programming in Python for quite some time, but have never attempted low level programming at the socket level until now. Normally, in C, C# or Java, one would use byte arrays to send/receive data. However, the only byte array abstraction in Python is the String datatype ( and the StringIO buffer ). It was challenging for me to wrap my head around using a string as a byte buffer. I will mention some of the pitfalls to avoid as I go along.

Ping using C#

At my previous company, I had written a Ping utility using C#/.Net on Windows. That was described here: