Category: Windows

JSON Enrich Mediator for WSO2 ESB


JSON support for WSO2 ESB [1] was introduced sometime back. But only small number of mediators support manipulating JSON payloads. In this article I am going to introduce a new mediator called JsonEnrichMediator [2], which work quite similar to existing Enrich mediator [3], but aiming JSON payloads. The specialty of this mediator is, since this is working with native JSON payload, JSON payload will not be converted to an XML representation. Hence there won’t be any data loss due to transformations.

Please note that this is a custom mediator I have created and will not ship with WSO2 ESB pack.

Configuring Mediator

  1. Clone the GitHub repository:
  2. Build the repository using maven (mvn clean install)
  3. Copy the built artifact in target folder to ESB_HOME/repository/components/dropins
  4. Download json-path-2.1.0.jar [5], json-smart-2.2.jar [6] and put them to the same folder (dropins).
  5. Start WSO2 ESB (sh bin/

Sample Scenario

For this article I am using a sample scenario which moves a JSON property within the payload. For that you need to add the following API to WSO2 ESB.

<api xmlns="" name="sampleApi" context="/sample">
<resource methods="POST" uri-template="/*">
<log level="full"/>
<source type="custom" clone="false" JSONPath="$"/>
<target type="custom" action="put" JSONPath="$" property="country"/>

The above configuration will take the value pointed by JSONPath “$” and move it to the main body. You can find further details about JSONPath at the location [4].

Once the API is deployed, you need to send following message to ESB.

curl -H "Content-Type: application/json"
-X POST -d '{
"country": "Sri Lanka",
"language" : "Sinhala"

The output of the ESB should look like below

"me": {
"language": "Sinhala"
"country": "Sri Lanka"


I have shown a simple use-case of using JSON Enrich Mediator. You can see the comprehensive documentation at the code repository [2].


[1] WSO2 ESB JSON support :

[2] Code Repository for JSON Enrich Mediator :

[3] WSO2 ESB Enrich Mediator :

[4] JSON Path documentation :

[5] json-path-2.1.0 :

[6] json-smart-2.2.1 :

WSO2 ESB Endpoint Error Handling


WSO2 ESB can be used as an intermediary component to connect different systems. When connecting those systems the availability of those systems is a common issue. Therefore ESB has to handle those undesirable situations carefully and take relevant actions. To cater that requirement outbound-endpoints of the WSO2 ESB can be configured. In this article I discuss two common ways of configuring endpoints.

Two common approaches to configure endpoints are;

  1. Configure with just a timeout (without suspending endpoint)
  2. Configure with a suspend state

Configure with just a timeout

This would suitable if the endpoint failure is not very frequent.

Sample Configuration:

<endpoint name="SimpleTimeoutEP">
    <address uri="http://localhost:9000/StockquoteService">


In this case we only focus on the timeout of the endpoint. The endpoint will stay as Active for ever. If a response does not receive within duration, the responseAction triggers.

duration – in milliseconds

responseAction – when response comes to a time-out message one of the following actions trigger.

  • fault – calls the fault-sequence associated
  • discard – discards the response
  • none – will not take any specific action on response (default action)

The rest of the configuration avoids the endpoint going to suspend state.

If you specify responseAction as “fault”, you can define define customize way of informing the failure to the client in fault-handling sequence or store that message and retry later.

Configure with a suspend state

This approach is useful when connection failures are very often. By suspending endpoint, ESB can save resources without unnecessarily waiting for responses.

In this case endpoint goes through a state transition. The theory behind this behavior is the circuit-breaker pattern. Following are the three states:

  1. Active – Endpoint sends all requests to backend service
  2. Timeout – Endpoint starts counting failures
  3. Suspend – Endpoint limits sending requests to backend service

Sample Configuration:

<endpoint name="Suspending_EP">
    <address uri="http://localhost:9000/StockquoteServicet">
        <errorCodes>101504, 101505</errorCodes>
        <errorCodes>101500, 101501, 101506, 101507, 101508</errorCodes>


In the above configuration:

If endpoint error codes are 101504, 101505; endpoint is moved from active to timeout state.

When the endpoint is in timeout state, it tries 3 attempts with 1 millisecond delays.

If all those retry attempts fail, the endpoint will move to suspend state. If a retry succeed, then endpoint will move to active state.

If active endpoint receives error codes 101500, 101501, 101506, 101507, 101508; endpoint will directly move to suspend.

After endpoint somehow moves to suspend state, it waits initialDuration before attempting any furthermore. Thereafter it will determine the time period between requests according to following equation.

Min(current suspension duration * progressionFactor, maximumDuration)

In the equation, “current suspension duration” get updated for each reattempt.

Once endpoint succeed in getting a response to a request, endpoint will go back to active state.

If endpoint will get any other error codes (eg: 101503), it will not do any state transition, and remain in active state.


In this article I have shown two basic configurations that would be useful to configure endpoints of WSO2 ESB. You can refer WSO2 ESB documentation for implementing more complex patterns with endpoints.


WSO2 ESB Documentation:

Timeout and Circuit Breaker Pattern in WSO2 Way:

Endpoint Error Codes:

Endpoint Error Handling:

Java Virtual Machine


From this post I thought of discussing how underlying components work together for a successful execution of a Java program. Content in this post consists of a collection of knowledge I gathered after going though several articles on this topic.

Most of us never bothered about internal stuffs related to Java, because IDEs have made our life a lot easier. But it’s worth if you have some idea about internal stuffs as if you happened to work with an enterprise level application, and faced with memory issues. In this article, I though of going from basic concepts to an intermediate level.


In brief “JVM (Java Virtual Machine) is an abstract machine. It is a specification that provides runtime environment in which java bytecode can be executed.” Which implies JVM is merely a concept. I’ll come back to JVM later to discuss more.

JRE (Java Runtime Environment) provides a runtime environment (implementation of JVM). JRE contains the concrete implementation of JVM (eg: HotSpot JVM, JRockit, IBM J9), set of libraries and other files need for JVM. Different vendors release their own JRE, based on a reference.

JDK is for developers to create Java program. JDK consists of JRE + development tools such as ‘javac’.

One important thing to remember is all JVM, JRE and JDK are platform-dependent.

JVM, JRE, JDK (source:


JVM in Detail

It is said that JVM is;

  1. A specification where working of Java Virtual Machine is specified. But implementation provider is independent to choose the algorithm. Its implementation has been provided by Sun and other companies.
  2. An implementation Its implementation is known as JRE (Java Runtime Environment).
  3. Runtime Instance Whenever you run a java program, an instance of JVM is created.

JVM is responsible for following operations:

  • Loads code
  • Verifies code
  • Executes code
  • Provides runtime environment

Internal Architecture of JVM

  1. Classloader : Classloader is used to load class files. The embedded classloader in JVM is also called “primordial class loader”. Depending on the class name, classloader can search for the .class file in directory structure. User can also define classloaders (called “non-primordial class loader”) if required.
  2. Method (Class) Area : Also called non-heap area, it has 2 subsections. “Permanent Generation – This area stores class related data from class definitions, structures, methods, field, method (data and code) and constants. Can be regulated using -XX:PermSize and -XX:MaxPermSize. It can cause java.lang.OutOfMemoryError: PermGen space if it runs out if space”. OutOfMemoryError happens when class definitions are accumulated. The other section, Code Cache is used by JIT to store compiled code (hardware specific native code).
  3. Heap : Area allocated for runtime data. This area is shared by all threads. We can use  -Xms and -Xmx JVM options to tune the heap size. In most of the time “java.lang.OutOfMemoryError” error occurs because heap is getting exhausted. Heap consists of 3 sub-areas:
    1. Eden (Young) “New object or the ones with short life expectancy exist in this area and it is regulated using the -XX:NewSize and -XX:MaxNewSize parameters. GC (garbage collector) minor sweeps this space”
    2. Survivor – “The objects which are still being referenced manage to survive garbage collection in the Eden space end up in this area. This is regulated via the -XX:SurvivorRatio JVM option”
    3. Old (Tenured) – “This is for objects which survive long garbage collections in both the Eden and Survivor space (due to long-time references of course). A special garbage collector takes care of this space. Object de-alloaction in the tenured space is taken care of by GC major”

      Analysis on “java.lang.OutOfMemoryError” can be done by taking a heap-dump when the incident occours. You can refer a case-study on analyzing for a real-world case at here:

  4. Stack : This area is specific to a thread. Each thread has it’s stack which is used to store local variables and regulates method invocation, partial result and return values. This space can be tuned with -Xss JVM option.
  5. Program Counter Register : It contains the address of the Java virtual machine instruction currently being executed.
  6. Native Stack : Used for non-Java code, per thread allocation
  7. Execution Engine : This contains 3 parts;
    1. A virtual processor
    2. Interpreter, which reads the bytecode and execute the instructions
    3. Just-In-Time(JIT) compiler: “It is used to improve the performance. JIT compiles parts of the byte code that have similar functionality at the same time, and hence reduces the amount of time needed for compilation.Here the term ‘compiler’ refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.”


In this post I want to gather a colllection of knowledge I found related to JVM from various articles. All those areticles are mentioned in Reference section below. I would like to thank all those original authors, and hope you will get some knowledge out of this as well.


  1. JVM internals tutorial –
  2. About JVM Memory –
  3. Understanding Java PermGen –
  4. Java HotSpot VM Options –
  5. Java Classloaders –
  6. Heap Dump Analysis with VisualVM –
  7. Real-world Example for Heap Dump Analysis with MAT –

Experience with Azure Big Data Analyzing


This experiment was done in order to have first understanding on how to deal with big data on cloud. So this is my first time experience with lots of new technologies such as Hadoop, Hive etc. Before the experiment I have tried few examples with Hadoop and other related technologies, and yet found that this would be a better way to go.

Use case

This experiment is based on the dataset with annual salaries for different persons. In this case this is about California, USA in 2015. The data set contained names, job titles, basic salary, additional benefits and total salary. So in this, I’ll concentrate on averages of job titles and their averaged total salaries.

Please note that this is an experiment which is solely depends on the dataset. Actual survey reports may contain values different from final results.


Before moving to the rest of the article, you are expected to have better understanding on followings:

  • Your understanding on Microsoft Azure platform is necessary. You can create a free azure account and try to play around with portal. Also you need to understand their concepts such as Resources, Resource Groups etc.
  • You need to have some sort of understanding on big data concepts such as what is big data, Hadoop DFS, Hadoop Eco system, Hive, SQL etc.
  • Better to have done some tutorials in Azure documentation. Specially “Analyze flight delay data”, which laid the foundation for this article.
  • Understanding on HTML, PHP, JS with AJAX is required for understand visualization part.
  • Using tools like Putty, WinSCP if you are in Windows, else commands related to scp and ssh

Planning Deployment

Deployment required for this article is can be shown as follows.

Azure Deployment

Please note that I’ll shutdown HD Insight cluster once it completed its job, else you’ll lose your credits!

The diagram shows different steps that needs in this experiment.

  • First you need a dataset to examine.
  • Then you have to transfer it to Hadoop cluster, and to Hadoop Distributed File System (HDFS) from there.
  • Thereafter you will run required Hive queries to extract the essence of the dataset. After processing we’ll move those data to a SQL database for ease of accessing.
  • Finally you need to develop a PHP application which runs on an App server node to visualize results.

Preparing dataset

You can download San Francisco annual income dataset from following location:

After downloading, you need to open that using excel. I have observed that several job titles contains comma in text. So use find & replace to replace commas in it with hyphen character. Since this is a CSV file, those commas will negatively affect our analysis.

Put the CSV inside a zip folder to minimize data transfer.

Setting up Azure resources

Now you need to create Azure resources required. So let’s start with HD Insight. In this case we need to create a Hadoop-Linux cluster, with 2 worker nodes. Following guide will help to create such cluster quickly.

Also for further information about cluster types and concepts, you may look at following link:

Unfortunately Azure still hasn’t way to deactivate HD Insight cluster when idle. You need to manually delete it, else you’ll be charged for idle hours too.

Thereafter you need to create SQL database. Following tutorial will help on that: (Create a new Azure SQL database section)

Finally you need to create an empty App Service. For further information about App Service you may refer following:

This App Service will contain PHP runtime which will be needed at the last part of the article.

A best practice would be, when creating above resources would be to allocate all the resources to a single resource group, which makes it easy to manage.

Also make sure to give strong passwords and remember those.

Executing process

First you need to transfer zip file created to Hadoop nodes’s file system. You can do it by using scp command or any GUI tool which does scp.

As the host, you need to mention “”. Along with that, you need to provide ssh credentials.


Then you need to access that node using SSH. In windows you can use Putty tool, others may use terminal.


Unzip the file uploaded


Next, you have to move the csv file to Hadoop File System. Use following commands to create a new directory in HDFS and move the csv file

hdfs dfs -mkdir –p /sf/salary/data

hdfs dfs -put FILENAME.csv /sf/salary/data

Now, you need to create Hive query and execute. You can execute Hive query using a file or in an interactive manner. We’ll do in both manner, but first using the file.

Create the following “salaries.hql” using nano

nano salaries.hql

and add the following queries:

DROP TABLE salaries_raw;
-- Creates an external table over the csv file
 JOB_TITLE string,
 BASE_PAY float,
 OTHER_PAY float,
 BENEFITS float,
 TOTAL_PAY float,
 YEAR string,
 NOTES string,
 AGENCY string,
 STATUS string)
-- The following lines describe the format and location of the file
LOCATION '/sf/salary/data';

-- Drop the salaries table if it exists
DROP TABLE salaries;
-- Create the salaries table and populate it with data
-- pulled in from the CSV file (via the external table defined previously)
 JOB_TITLE AS job_title,
 BASE_PAY AS base_pay,
 TOTAL_PAY AS total_pay,
 TOTAL_PAY_N_BENEFITS AS total_pay_n_benefits
FROM salaries_raw;

You can also locally create “salaries.hql” and upload via SCP.

The queries are self-explanatory, but to make it easy, each query  ends with a semicolon. Table “salaries_raw” is creating to directly extract values in CSV. So first query has one-to-one mapping with csv data. Data to the table is taken from where we stored csv file. Thereafter “salaries” table is created using “salaries_raw” table. The “salaries” table filters values of base_pay, total_pay and total_pay_n_benifits columns only. Those columns are selected, because only those would necessary for the next level.

To execute the HIVE query, use the following command

beeline -u 'jdbc:hive2://localhost:10001/;transportMode=http' -n admin -f salaries.hql

Next part of the Hive query we’ll going to do with interactive manner. You can open interactive shell with command:

beeline -u 'jdbc:hive2://localhost:10001/;transportMode=http' -n admin

and enter following commands:

FROM salaries
WHERE base_pay IS NOT NULL AND total_pay IS NOT NULL AND total_pay_n_benefits IS NOT NULL
GROUP BY job_title;

The above Hive query will output the result to “/sf/salary/output” folder. It’ll group the job title and get the average values of base_pay, total_pay and total_pay_n_benifits columns.

Use “!quit” command to exit from interactive shell.

At this stage, we have successfully completed extracting essence of the dataset. Next, we need to make it ready for presentation.

For presentation, we’re going to copy output data to the SQL database created.

To create a table and do other interactions with SQL database, we need to install FreeTDS to Hadoop node. Use following command to install and verify the connectivity.

sudo apt-get --assume-yes install freetds-dev freetds-bin

TDSVER=8.0 tsql -H <serverName> -U <adminLogin> -P <adminPassword> -p 1433 -D <databaseName>

Once you execute the last command, you’ll be directed to another interactive shell where you can interact with the database you created, when creating SQL node.

Use following commands to create a table to put the output we got

CREATE TABLE [dbo].[salaries](
[job_title] [nvarchar](50) NOT NULL,
[base_pay] float,
[total_pay] float,
[total_pay_n_benefits] float,
([job_title] ASC))

Use “exit” command to exit from SQL interactive session.

To move the data from HDFS to SQL database, we are going to use Sqoop. Following Sqoop command with put output data in HDFS to SQL database.

sqoop export --connect 'jdbc:sqlserver://<serverName>;database=<databaseName>' --username <adminLogin> --password <adminPassword> --table ' salaries' --export-dir 'wasbs:/// sf/salary/output' --fields-terminated-by '\t' -m 1

Once the task is successfully completed, you can again log in to SQL interaction session and execute following to view table results:

SELECT * FROM salaries


Finally you need to use FTP to connect with App Services node, and FTP the following PHP files (including js foolder).

You need to change SQL server host and credentials in “dbconnect.php” file. I’ll leave the rest of code in PHP file as a self-explanatory for you. If you successfully created the app, you should see something similar to following:




In this article I have shown you how I did my first experiment with Azure big data analysis. Along the path I had to cover several other related technologies such as Spark, Azure Stream Analysis. So there are pros and cons with using those technologies. In such cases like analyzing annual income, it’s generally accepted to use Hadoop along with Hive. But if you want to do more frequent activities, you may look in to alternatives.


  1. Project sourcecode –
  2. Get started with Hadoop in Windows –
  3. Analyze Flight Delays with HD Insight –
  4. Use Hive and HiveQL with Hadoop in HDInsight –

Reliable Messaging with WSO2 ESB


Web-Service Reliable Messaging (WS-ReliableMessaging) is a standard which describes a protocol on how to deliver messages reliably between distributed applications. Message failures due to software component, system, network failures can be overcome though this protocol. This protocol describes a transport-independent protocol, such that messages can be exchanged between systems. For further information, please go through the WS-RM specification [1] which completely describes about this topic. For WSO2 ESB, WS-RM is not a novel concept, as it has been there in previous releases. But with new release, WSO2 ESB 4.9.0, WSO2 has separated QoS from fresh pack. Instead, you can installed WS-RM as a feature from p2-repo. Another major changes are that, now WS-RM operates on top of CXF WS-RM [2] , and acting as an inbound endpoint [3].

In this post, I’m not going into comprehensively describe on WS-RM, but going to show how that can be configured in ESB. If you need to read more on WS-RM protocol, I recommend to access WS-RM specification [1], which is a good source for that. Now, let’s move on step-by-step with a sample use-case.

Setting up

First you need to understand that, WS-RM inbound is designed to reliably exchange message between client and WSO2 ESB. So, the message flow diagram can be shown as follows:

Sample Setup Diagram

For this example, I’m using SimpleStockQuote service which comes with WSO2 ESB. You can read more on configuring and starting the service on default port from the documentation. If you have properly configured it, you should be able to access its wsdl via “http://localhost:9000/services/SimpleStockQuoteService?wsdl“.

Next, you need to install “CXF WS Reliable Messaging” feature from p2-repo. About installing features please go through the documentation Installing Features. With this step, you have completed setting up the infrastructure for use-case. Also please note that, this feature requires cxf-bundle, and jetty-bundle. Make-sure, you have no conflicts regarding installation of those bundles.

In order to configure CXF server, we need to give a configuration file. Sample configuration can be also found at CXF Inbound Endpoint Documentation. In that configuration file, you may need to configure the paths to key stores. A sample can be found at [5]. For this sample, configure it’s key store paths and place it in “<ESB_HOME>/repository/conf/cxf” folder.

Now, I’m going to create a new WS-RM inbound endpoint. For that select “Inbound Endpoints” from left panel. Then click “Add Inbound Endpoint“. Then you’ll get a page to initiate an inbound endpoint. At this stage you need to give a name to WS-RM inbound endpoint, and select type as “Custom“. You have to do that because, I have initially mentioned that WS-RM does not come along with fresh ESB pack. Moving to next step, you will get the chance of doing rest of configurations. Following image depicts the configuration of a sample WS-RM inbound endpoint.


At this point, you may already have some idea about the inbound endpoint. I have configured it to listen port 20940, in localhost. The Class of custom inbound should be “org.wso2.carbon.inbound.endpoint.ext.wsrm.InboundRMHttpListener” (without quotes). The configuration “inbound.cxf.rm.config-file” describes where you have placed the CXF server configuration file.

Messages coming into that specified port will go to “sequence” specified, in this case RMIn sequence and faulty messages will go to “fault” sequence. Other configurations related details are described at the official documentation [4].

You can do the above step straight forward by adding the inbound configuration directly from synapse-configuration.

Inbound Endpoint:

<inboundEndpoint xmlns="" name="RM_INBOUND_NEW_EXT" sequence="RMIn" onError="fault" class="org.wso2.carbon.inbound.endpoint.ext.wsrm.InboundRMHttpListener" suspend="false">
      <parameter name="inbound.cxf.rm.port">20940</parameter>
      <parameter name="inbound.cxf.rm.config-file">repository/conf/cxf/server.xml</parameter>
      <parameter name="coordination">true</parameter>
      <parameter name=""></parameter>
      <parameter name="inbound.behavior">listening</parameter>
      <parameter name="sequential">true</parameter>

RMIn sequence:

<sequence xmlns="" name="RMIn" onError="fault">
      <property name="PRESERVE_WS_ADDRESSING" value="true"/>

<header xmlns:wsrm="" name="wsrm:Sequence" action="remove"/>

<header xmlns:wsa="" name="wsa:To" action="remove"/>

<header xmlns:wsa="" name="wsa:FaultTo" action="remove"/>
      <log level="full"/>

<address uri="http://localhost:9000/services/SimpleStockQuoteService"/>

Now you have completed the sample setting up. One more step to go. Let’s test this sample.

Running the sample

For that, ESB provides the client which can send reliable messages. Go to <ESB_HOME>/samples/axis2Client folder from terminal and apply following command:

ant stockquote -Dsymbol=IBM -Dmode=quote -Daddurl=http://localhost:20940 -Dwsrm=true

The command will send a getQuote request to ESB using WS-RM and projects the expected result.

Message flow

As specified in the WS-RM spec [1], between client and ESB, several messages exchange in this scenario. If you use a packet capturing tool like Wireshark, you’ll see those messages. I have already attached the message flow [6],  I observed to make it more clear. In brief, following messages  are exchanged, you can follow the messages at text file with these points:

  1. “CreateSequence” message to initiate reliable messaging
  2. “CreateSequenceResponse” from ESB to client
  3. Actual message with data from client to ESB. This is the 1st and the last message in this case. ESB will send this message to backend server and get the response.
  4. “SequenceAcknowledgement” message along with the response from backend server send from ESB to client
  5. “TerminateSequence” message from client to ESB


Through this post, I wanted to introduce you to the new approach of implementing WS-ReliableMessaging. This implementation will come along with WSO2 ESB 4.9.0 release,and prior have different approach than this. Therefor this post will help anyone who is interested in doing WS-RM with newer ESB versions.


[1] WS-ReliableMessaging spec. –

[2] CXF WS-RM –

[3] WSO2 ESB, Inbound Endpoint –

[4] CXF WS-RM Inbound Endpoint –

[5] Sample CXF configuration –

[6] Message flow – link-to-file

Concepts of IPv6


IPv6 was there for around 20 years but still it is considered as a new thing, Regardless of how much organizations are forced to adopt to this still it’s just 10% in world. So in this post I thought of writing on the initial concepts of IPv6. Because as a newcomer to this topic, you will come-across so many technical terms even hard to remember, hex-numbers which are not human-friendly compared to IPv4 address and concepts around multicast which may hard to understand at once. Even for me it was not easy at the first time, so I thought of digging a bit in it and here’s my experience on it. Please note that through this I’m not going to cover every single point of IPv6 and assumed that you are already familiarized with IPv4 basics. So let’s start.

Why IPv6

You may already aware that IPv4 address-space has almost exceeded and there should be a new addressing schema. So IPv6 came with IP address with 128 bits long, so according to current situation this is far more enough since it consists 2^128 addresses!

IPv6 address can be divided into  chunks of 16-bits. so there are 8-parts in a single address. Each part represent by 4 hexadecimal numbers. And there are short-hand methods of writing a IPv6 address using :: (double-colon) and omitting leading zeros inside a chunk. So that’s it about a IPv6 address for more please refer [2].

Multicast in IPv6

Multicast is sending a message to a particular group of nodes which listens to an IPv6 address. This technique has replaced the Broadcast and ARP concepts which are using in IPv4. In one way we can see removing broadcast has reduced the unnecessary traffic-jam. But as a consequence of that, a bit complex mechanism has used to identify nodes and interfaces. So in IPv6 interfaces tend more to talk with each other compared to IPv4 and get the things done.

Working in IPv6

Now lets see how IPv6 works in a more practical aspect. For clear explanation, let’s take a case where a host connected to router via a network.

After Host-A boots up

When the host boots-up, it is automatically configured for several IPv6 address.

1. Link-local address

An address that will use within a link. Packets with link-local address as src. or dest. never get forwarded by routers. The creation of this link-local is happen with the help of EUI-64 (which uses MAC address of the interface or may be random 64 bits generated). For that FE80:: /64 bits become the prefix and link-local IPv6 address get created. After the process of Duplicate Address Detection (DAD) process, this link-local may become the global unique IPv6 address of that node (will discuss later about this).

2. Solicited Node Multicast Address – Group address

Each time a global or link-local address get assigned to a host, it’ll automatically get assigned to a group with Solicited Node Multicast Address. So this group address is created by taking the last 24 bits of the link-local or global IPv6 address and prefixing FF02::1:FF to it. So the important of this address is since this is generating using a host’s link-local or global address, other parties who are not aware about this node can send message through this. Will see this at Neighbor Discovery.

3. All node multicast address – Group address

Additionally each node with IPv6 capability also joins FF02::1. This is a bit similar to Broadcasting in IPv4, but seems it’s using occasionally otherwise there’s no-point of the fact reducing the traffic in IPv6.

Neighbor Discovery

Neighbor Discovery is finding neighbors in a network. So let’s take a look at a sample.

Host-A with MAC addresses

There’s a host-A with Link-local (may be Global) IPv6 address configured and now Router-R1 wants to send a message to host-A. so, host-A IPv6 configurations are as follows:

Link-local :- fe80::200:aaff:feaa:aaaa
Solicited Node Multicast Address :- ff02::1:ffaa:aaaa

So if R1 sends a ping request to fe80::200:aaff:feaa:aaaa, since it’s not aware about that neighbor, it sends the Neighbor Solicitation Request to ff02::1:ffaa:aaaa which can be derived from the given IP address. Then the request should go to host-A. Then host A can directly directly address R1 since it knows information about R1 from Neighbor Solicitation Request and send the Neighbor Advertisement.

The interesting thing is what is happening behind the screen. If you think more deeper, you will realize that for any kind of message transferring need to know hardware address (MAC). So host-A listening to  Solicited Node Multicast Address has the meaning of host-A listening to a special MAC address 33:33:ff:aa:aa:aa , which is derived by prefixing 33:33 to the last 32-bits of the  Solicited Node Multicast Address. Since R1 can derive the Solicited Node Multicast Address, it definitely can guess that MAC address too. Since Neighbor Solicitation has R1’s MAC and Neighbor Advertisement has host-A’s MAC, now they can communicate with each other directly, and now they are Neighbors!

If R1 wants to find all its neighbors it can ping to All node multicast address FF02::1 . Very similar to earlier case, this All node multicast address uses 33:33:00:00:00:01 as the MAC address. but the specialty of this case is request contains R1’s IP address but, not containing MAC (echo request). So what receiving node should do? since they should respond, they use R1’s IP address and derive Solicited Node Multicast Address of R1, and send Neighbor Solicitation Request with node’s MAC address. Now once R1 gets Neighbor Solicitation Request from node, it sends Neighbor Advertisement to each node mentioning it’s MAC address. So now all nodes know R1 & R1 knows each node. So they are Happy!

Duplicate Address Detection

This starts with if a node gets a new IPv6 address. Assume a case where out previous router-R1 gets a new IPv6 address 3333::3. Now R1 needs to make-sure no-one else is not using it. So it follows a similar pattern to figure a neighbor. It sends a Neighbor Solicitation Request with IP address :: (since still not sure of new one) and real MAC address to imaginary Solicited Node Multicast Address ; ff02::1:ff00:3 (you know how to generate this since we did this several times). Now R1 waits. if it didn’t get Neighbor Advertisement the derived assumption of the imaginary Solicited Node Multicast Address is correct, no one using that IPv6 address. If it gets Neighbor Advertisement, it has no permission to assign that.

Stateless Address Autoconfiguration

Once a router is enabled for IPv6 unicast-routing, interface assigns a new address called All routers multicast address (FF02::2). Also routers sends a Routing Advertisement to All node multicast address (FF02::1), so any interesting node can use that information to configure a global IPv6 address. This Routing Advertisement launches at regular intervals containing information about the network address etc. But rather waiting for a Routing Advertisement, a node can send a Router Solicitation message and get Routing Advertisement and continue configuration. Configuration take place on EUI-64/random generated with the help of above described Duplicate Address Detection.


So, it’s a pretty much lengthy post, but tried my best to express things more summarized manner. Though you may learn IPv6 in lectures, you may not have the experience of working with IPv6. So in that case the video course [1] did help me a lot. So, I would recommend you to follow that and see. Have a great time with IPv6!


[1] IPv6 video course by Keith Barker –

[2] Tutorial on IPv6 –


Basics of Virtualization


Today with Cloud Computing, “Virtualization” has become a buzz word. Many hosting services tend to use this technology since it’s more cost effective. So during last few weeks, I too get involved with this term, so thought of having look at what it really means. In this case a book[1] and a video[2] I found was really helpful. But I need to say that technology is rapidly changing. So you may find there are new ways to do “Virtualization”.

What is Virtualization

You have to understand that virtualization is not cloud computing, but a component of Cloud Computing. As the book[1] says: “… the process of converting physical IT resources into virtual IT resources.” So first you need to have physical resources to do virtualization. Resouces in the definition can be;

  1. Servers
  2. Storage
  3. Networks etc.

To explain virtualization, there are few other terms you need to understand.

  • Host – The actual physical hardware use for virtualization
  • Host Operating System – The operating system which runs on-top-of Host hardware
  • Guest Operating System – The operating system which runs on-top-of virtualized layer
  • Hypervisor (Virtual Machine Monitor) – The software piece which enables virtualized software to access underlying hardware

Why Virtualization

There are several reasons why people are so interested about virtualization

  • Hardware Independence – Virtualized software is decoupled with underlying hardware, so that it can be moved from one-to-another just as moving a file
  • Server Consolidation – Instead of having multiple servers to run many servers, now all virtualized servers can run on the same server and share underlying hardware
  • Resource Replication – Virtual servers saves on virtual hard disks, which can apply simple file operations. So for en example, you can replicate the same server by copy the virtual hard disk

How to Virtualize

Types of virtualization depends on where the Hypervisor lies. So from the top-level, there are 2 kinds of virtualizations you can see (as mentioned in [1] & [2]):

  1. Hardware-based Virtualization
  2. Operating System-based Virtualization

So let’s dive more deep into these 2 areas.

Hardware-based Virtualization

This means, take the bare hardware and install hypervisor. So hypervisor will provide basic functionality. Then you can install operating systems on-top-of hypervisor. So in this case you don’t need any host operating system. Therefore you need to have a client machine which need to manage the hypervisor.  Those management software can do magical things such as moving operating systems between several hardware servers etc. This kind of virtualization is optimized for performance. The issue with this approach is compatibility with hardware devices. Because device drivers may not be available for hypervisors when compared to operating systems. So following diagram shows how hardware-based virtualization can be shown (taken from [1]).

Hardware Based Hypervisor
Hardware Based Hypervisor



Operating System-based Virtualization

This kind of virtualization you may have already experienced too (without knowing 🙂 ). In this case hypervisor  is used to abstract the host operating system. Simple use-case is using Virtual Box to run Windows on-top of Ubuntu, where Virtual Box act as the hypervisor. This approach will eliminates the  hardware compatibility issues but incurs more overhead as host operating system needs resources in addition to guests. So following diagram shows how operating system-based virtualization can be shown (taken from [1]).

Operating System-Based Virtualization
Operating System-Based Virtualization


As I mentioned in the introduction, today there are new concepts introduced to this field. One such interesting thing is Docker[3], which has become more popular recently. So before the conclusion of this post I thought of keeping a note on Docker too.


Docker from outside can be view as similar approach to Operating System-based virtualization. But the difference is that Docker has able to eliminate the overhead of the guest operating system. With Docker they have introduced the concept of “Containers” which can run on Linux platforms currently and with additional library on Windows too. In Docker world the hypervisor is Docker Engine which enables you to do all sorts of things to a Docker Container. You can play with Docker through the online try-it console[4].




Through this post, I tried to express my view on basic concepts on Virtualization in computer world. In here I mentioned why virtualization is important and benefits of it. After that I expressed how virtualization can be achieved and brief introduction to Docker too. Other than mentioned things various companies tries to make this virtualization more efficient and various technologies are upcoming. So this is just a glimpse of the virtualization world and if you are interested, you can refer the given resources and expand your knowledge!



[1] Cloud Computing: Concepts, Technology & Architecture by Thomas Erl

[2] Introduction to Virtualization – 

[3] Docker – 

[4] Docker Try It! –