I would like to know if I am connected to the network as a validator, how I my node will be protected of DDoS attacks.
That is something the owner of the node has to do and it is not part of the Ethereum protocol. It is also worth mentioning that it does not specifically relate to Proof of Stake. Today miners and pools gets DDoSed already.
On the contrary, it is worth mentioning that the incentive to DDoS others is much higher in POW compared to POS (or specifically Casper). The reasons is that POW is a zero sum game where other miners gain the reward if some miners can not submit a block. Casper in contrast is designed as a coordination game where every participant gets most if all others can include their blocks.
I don't believe your node will be protected. Any server connectable from the internet is at risk for a DDoS attack. The question should be how likely will it be. An attacker has not much to gain from just attacking you, if his goal is somehow influencing the network-wide consensus he needs to influence all validator nodes.
I do believe that the risk and rewards are the same regardless if the network is using PoW or PoS.
Your Node is 'protected' by your ISP, or Service Provider, and any additional steps you take to mitigate such attacks.
It's important to understand DOS (Denial of Service): https://en.wikipedia.org/wiki/Denial-of-service_attack so we are on the same page.
Also note that there is DDoSCoin where attackers earn Credit for launching DoS Attacks and providing Proof-of-Handshake (like the response of a TLS Server).
What protects me from SPAM (a form of Denial of Service, since my time is wasted, taking my time from other Tasks that I wish to do) is my Email Provider. I check in at XYZ to determine if I have email - they own the email address, not me.
If I decide that the email is SPAM because it contains a specific word or is from a specific person then I can block it. I will NEVER see the address or that word again until I unblock. That means that the DoS Attack ends (on me) but not necessarily on my Service Provider.
If enough people make the same complaint or the person increases the frequency of their rejected SPAMs past a certain point my ISP's Software sends a polite message to that person's ISP explaining the problem and asking them to Cease and Desist. There are Laws in effect in most Countries and Router Owners can be contacted to request blocking.
That's where DDoS (distributed DoS) comes in, the blame is distributed and in the case of a Virus causing your unwitting attacker to become a Bot (from the word Robot) the person who's Computer is a small part of the 'DDos Problem' is unaware and blocking them doesn't fix your problem it might only cause a small or medium sized problem for them - a situation that the Virus Creator could care less about.
It's like saying what prevents someone from crank calling you or sending you (snail mail) Junk Mail. Nothing really "prevents" it, but you can get someone else to handle the problem for you; that decreases the Attacker's effectiveness and satisfaction, and increases their costs and risk.
The Attacker need only separate themselves from cost and risk, being satisfied with not actually obtaining their Goals and then the Attacker is "The Winner" - if you call that winning.
It's like people who climb tall Buildings for a YouTube Video, risking imprisonment and possibly (in a Bonnie and Clyde, Jesse James, or D. B. Cooper World) gaining notoriety.
If someone asks your Computer to check a Coin for authenticity, check your Work or Service, even to bounce any and all emails sent to you (even if you don't reply) that takes CPU Cycles and it costs you something - it's 'mitigated' somewhat by chance and value of someone attacking you.
Even the world's largest Army can't prevent the tiniest attack due to cost and some larger and more costly attacks are also not prevented by large Armies.
You are going to "mitigate" your time being wasted (your CPU Cycles being burned and the need to make Encrypted Backups) by doing the least amount of work possible while obtaining a result sufficient that you won't have to do a different vector of work. Someone can still try to breakin and toss a bucket of water on you and your Computer if you don't have that DoS Attack vector covered.
Where there's a way and a will there's a will and a way.