Networking and Socket Programming is one of the most important areas of Java programming language, especially for those programmers, who are working in client/server based applications. Knowledge of important protocols e.g. TCP and UDP in detail is very important, especially if you are, for example, in the business of writing high frequency trading applications, which communicate via FIX Protocol or native exchange protocol.
In this article, we will discuss some of the frequently asked questions on networking and socket programming, mostly based around TCP IP protocol. This article is kinda light on NIO though, as it doesn’t include questions regarding multiplexing, selectors, ByteBuffer and FileChannel etc., but it does include classic questions like difference between IO and NIO. Main focus of this post is to make a Java developer familiar with the low level parts e.g. how TCP and UDP protocol works, socket options and writing multi-threaded servers in Java.
Questions discussed here are not really tied up to the Java programming language, and can be used in any programming language, which allows programmers to write client-server applications. By the way, If you are going for an interview in an Investment bank for a core Java developer role, you better prepare well on Java NIO, Socket Programming, TCP, UDP and Networking along with other popular topics e.g. multi-threading, Collections API and Garbage Collection tuning. You can also contribute any question, which is asked to you or related to socket programming and networking and can be useful for Java interviews.
Java Networking and Socket Programming Questions Answers
Here is my list of 15 interview questions related to networking basics, internet protocol and socket programming in Java. Though it doesn’t contain basic questions form API e.g. Server, ServerSocket, it focus on the high level concept of writing scalable server in Java using NIO selectors and how to implement that using threads, the limitations and issues etc. I will probably add few more questions based on some best practices while writing socket based application in Java. If you know a good question on this topic, feel free to suggest.
1. Difference between TCP and UDP protocol?
There are many differences between TCP (Transmission control Protocol) and UDP (User Datagram Protocol), but the main is that TCP is connection oriented, while UDP is connection less. This means TCP provides a guaranteed delivery of messages in the order they were sent, while UDP doesn’t provide any delivery guarantee. Because of this guarantee, TCP is slower than UDP, as it needs to perform more work. TCP is best suited for messages, which you can’t afford to loss, e.g. order and trade messages in electronic trading, wire transfer in banking and finance etc. UDP is more suited for media transmission, where loss of one packet, known as datagrams is affordable and doesn’t affect quality of service.
This answer is enough for most of the interviews, but you need to be more detailed when you are interviewing as Java developer for high frequency trading desk. Some of the points which many candidates forget to mention is about order and data boundary. In TCP, messages are guaranteed to be delivered in the same order as they are sent but data boundary is not preserved, which means multiple messages can be combined and sent together, or the receiver may receive one part of the message in one packet and other part of the message in next packet. Though application will receive full message and in the same order. TCP protocol will do the assembling of the message for you. On the other hand, UDP sends full messages in a datagram packet, if a client receives the packet it is guaranteed that it will get the full message, but there is no guarantee that packets will come in same order they are sent. In short, you must mention the following differences between TCP and UDP protocol while answering during interview :
- TCP is guaranteed delivery, UDP is not guaranteed.
- TCP guarantees order of messages, UDP doesn’t.
- Data boundary is not preserved in TCP, but UDP preserves it.
- TCP is slower compared to UDP.
For a more detailed answer, see my post 9 differences between TCP and UDP protocol.
2. How does TCP handshake works?
Three messages are exchanged as part of a TCP head-shake e.g. Initiator sends SYN, upon receiving this Listener sends SYN-ACK, and finally initiator replied with ACK, at this point TCP connection is moved to ESTABLISHED state. This process is easily understandable by looking at the following diagram.
3. How do you implement reliable transmission in UDP protocol?
This is usually a follow-up of a previous interview question. Though UDP doesn’t provide delivery guarantee at protocol level, you can introduce your own logic to maintain reliable messaging e.g. by introducing sequence numbers and retransmission. If the receiver finds that it has missed a sequence number, it can ask for a replay of that message from the Server. TRDP protocol, which is used in Tibco Rendezvous (a popular high speed messaging middle-ware) uses UDP for faster messaging and provides reliability guarantee by using sequence number and retransmission.
4. What is Network Byte Order? How does two host communicate if they have different byte-ordering?
There are two ways to store two bytes in memory, little endian (least significant byte at the starting address) and big endian (most significant byte at the starting address). They are collectively known as host byte order. For example, an Intel processor stores the 32-bit integer as four consecutive bytes in memory in the order 1-2-3-4, where 1 is the most significant byte. IBM PowerPC processors would store the integer in the byte order 4-3-2-1. Networking protocols such as TCP are based on a specific network byte order, which uses big-endian byte ordering. If two machines are communicating with each other and they have different byte ordering, they are converted to network byte order before sending or after receiving. Therefore, a little endian micro-controller sending to a UDP/IP network must swap the order in which bytes appear within multi byte values before the values are sent onto the network, and must swap the order in which bytes appear in multi byte values received from the network before the values are used. In short, you can also say network byte order is standard of storing byte during transmission, and it uses big endian byte ordering mechanism.
5. What is Nagle’s algorithm?
If an interviewer is testing your knowledge of TCP/IP protocol then it’s very rare for him not to ask this question. Nagle’s algorithm is way of improving performance of TCP/IP protocol and networks by reducing the number of TCP packets that need to be sent over network. It works by buffering small packets until the buffer reaches Maximum Segment Size. This is because small packets, which contain only 1 or 2 bytes of data, have more overhead in terms of a TCP header, which is of 40 bytes. These small packets can also lead to congestion in a slow network. Nagle’s algorithm tries to improve the efficiency of the TCP protocol by buffering them, in order to send a larger packet. Also Nagle’s algorithm has a negative effect on non small writes, so if you are writing large data on packets than it’s better to disable Nagle’s algorithm. In general, Nagle’s algorithm is a defense against a careless application, which sends lots of small packets to network, but it will not benefit or have a negative effect on a well written application, which properly takes care of buffering.
6. What is TCP_NODELAY?
TCP_NODELAY is an option to disable Nagle’s algorithm, provided by various TCP implementations. Since Nagle’s algorithm performs badly with TCP delayed acknowledgement algorithm, it’s better to disable Nagle’s when you are doing write-write-read operation. Where a read after two successive write on socket may get delayed up-to 500 millisecond, until the second write has reached the destination. If latency is more of concern compared to bandwidth usage, e.g. in a network based multi-player game where users want to see action from other players immediately, it’s better to bypass Nagle’s delay by using TCP_NODELAY flag.
7. What is multicasting or multicast transmission? Which Protocol is generally used for multicast? TCP or UDP?
Multi-casting or multicast transmission is one to many distribution, where a message is delivered to a group of subscribers simultaneously in a single transmission from publisher. Copies of messages are automatically created in other network elements e.g. Routers, but only when the topology of network requires it. Tibco Rendezvous supports multicast transmission. Multi-casting can only be implemented using UDP, because it sends full data as datagram package, which can be replicated and delivered to other subscribers. Since TCP is a point-to-point protocol, it can not deliver messages to multiple subscriber, until it has link between each of them. Though, UDP is not reliable, and messages may be lost or delivered out of order. Reliable multicast protocols such as Pragmatic General Multicast (PGM) have been developed to add loss detection and retransmission on top of IP multicast. IP multicast is widely deployed in enterprises, commercial stock exchanges, and multimedia content delivery networks. A common enterprise use of IP multicast is for IPTV applications.
8. What is the difference between Topic and Queue in JMS?
The main difference between Topic and Queue in Java Messaging Service comes when we have multiple consumers to consumer messages. If we set-up multiple listener thread to consume messages from Queue, each messages will be dispatched to only one thread and not all thread. On the other hand in case of Topic each subscriber gets it’s own copy of message.
9. What is the difference between IO and NIO?
The main difference between NIO and IO is that NIO provides asynchronous, non blocking IO, which is critical to write faster and scalable networking systems. On the other hand, most of the utilities from the IO classes are blocking and slow. NIO takes advantage of asynchronous system calls in UNIX systems such as the select() system call for network sockets. Using select(), an application can monitor several resources at the same time and can also poll for network activity without blocking. The select() system call identifies if data is pending or not, then read() or write() may be used knowing that they will complete immediately.
10. How do you write a multi-threaded server in Java?
A multi-threaded server is the one which can serve multiple clients without blocking. Java provides excellent support to develop such a server. Prior to Java 1.4, you could write multi-threaded server using traditional socket IO and threads. This had a severe limitation on scalability, because it creates a new thread for each connection and you can only create a fixed number of threads, depending upon machine’s and platform’s capability. Though this design can be improved by using thread pools and worker threads, it still a resource intensive design. After JDK 1.4 and NIO’s introduction, writing scalable and multi-threaded server become bit easier. You can easily create it in single thread by using Selector, which takes advantage of asynchronous and non-blocking IO model of Java NIO.
11. What is an ephemeral port?
In TCP/IP, a connection usually contains four things, Server IP, Server port, Client IP and Client Port. Out of these four, 3 are well known in most of the time, what is not known is client port, this is where ephemeral ports come into picture. Ephemeral ports are dynamic ports assigned by your machine’s IP stack, from a specified range, known as ephemeral port range, when a client connection explicitly doesn’t specify a port number. These are short lived, temporary ports, which can be reused once connection is closed. However, most of IP software, doesn’t reuse an ephemeral port, until the whole range is exhausted. Similar to TCP, UDP protocol also uses an ephemeral port, while sending datagrams . In Linux, the ephemeral port range is from 32768 to 61000, while in windows the default ephemeral port range is 1025 to 5000. Similarly, different operating systems have different ephemeral port ranges.
11. What is sliding window protocol?
Sliding window protocol is a technique for controlling transmitted data packets between two network computers where reliable and sequential delivery of data packets is required, such as provided by Transmission Control Protocol (TCP). In the sliding window technique, each packet includes a unique consecutive sequence number, which is used by the receiving computer to place data in the correct order. The objective of the sliding window technique is to use the sequence numbers to avoid duplicate data and to request missing data.
12. When do you get “too many files open” error?
Just like a File connection, a Socket Connection also needs file descriptors. Since every machine has limited number of file descriptors, it’s possible that they may run out of file descriptors. When this happens, you will see the “too many files open“ error. You can check how many file descriptors per process are allowed on a UNIX based system by executing the ulimit -n command or simply count entries on /proc/ /fd/.
14) What is TIME_WAIT state in TCP protocol? When does a socket connection goes to TIME_WAIT state?
When one end of a TCP Connection closes it by making system call, it goes into TIME_WAIT state. Since TCP packets can arrive in the wrong order, the port must not be closed immediately in order to allow late packets to arrive. That’s why that end of the TCP connection goes into TIME_WAIT state. For example, if a client closes a socket connection, then it will go to TIME_WAIT state, similarly if server closes connection then you will see TIME_WAIT there. You can check the status of your TCP and UDP sockets by using these networking commands in UNIX.
15. What will happen if you have too many socket connections in TIME_WAIT state on Server?
When a socket connection or port goes into TIME_WAIT state, it doesn’t release the file descriptor associated with it. The file descriptor is only released when TIME_WAIT state is gone, i.e. after some specified configured time. If too many connections are in TIME_WAIT state than your Server may ran out of file descriptors and start throwing “too many files open” error, and stop accepting new connections.
That’s all about in this list of networking and socket programming interview questions and answers. Though I have originally intended this list for Java programmers it is equally useful for any programmer. In fact, this is bare minimum knowledge of sockets and protocols every programmer should have. I have found that C and C++ programmers are better answering these questions than an average Java programmer. One reason for this may be because Java programmers have got so many useful libraries, e.g. Apache MINA, which handle all the low level work for them. Anyway, knowledge of fundamentals is very important and everything else is just an excuse, but at the same time I also recommend using tried and tested libraries like Apache MINA for production code.
| Reference: | 15 Java Socket Programming, Networking Interview Questions and Answers from our JCG partner Javin Paul at the Javarevisited blog. |
