Currently, India-China border has dispute regions in: western sector: now around Aksai Chin- started around 1950s when China started constructing road connecting Xinjiang to Tibet through Aksai Chin. Dongra kingdom and China decided two areas as boundary: 1. Karakoram Range 2. Pangong Lake but did not demarcate the area between them.
Under the princely state, the process of demarcation was under taken by Johnson and called Johnson's Line, later modified called as Modified Johnson Boundary. But this was not communicated to China because it was very weak at that time and so was not controlling Xinjiang.
Central sector: Uttarakhand- now both the countries have very good understanding
Eastern sector: 1. Regarding Arunachal Pradesh: China considers Leh, Nepal, Sikkim, Bhutan& Arunachal Pradesh also as part of Tibet. Most of the recent disputes even 1962 war are centered around this region
2. Around India-China-Bhutan tri junction- current dispute is related to this point
Mc Mohan line for eastern boundary is presented as natural boundary between India- Tibet during 1913 conference by Britain. China withdrew from this conference since Tawang valley was allotted to India in the conference, which is not accepted by China. So even now China doesn't give legitimacy to Mc Mohan line.
During 1950's China was willing to accept Mc Mohan Line on liberal terms if India agrees to accede Aksai Chin to China. But due to political pressure, then government headed by Jawaharlal Nehru was not ready to accept. Though ready to concede now, Chinese government is not willing as it has consolidated its power over Aksai Chin over the years.
Current conflict revolves around India-China-Bhutan tri junction. Though the border dispute aggravated many times between China & India, it is different now because, the disputed area i.eDoklam belongs to Bhutan and Indian troops are defending it because of our relations with Bhutan. The region is very close to Siliguri pass or Chicken neck which connects North East - India with mainland India. This region also hosts thousands of Tibetan refugees. It is also important as Doklam facilitates easier setting up of military bases in tri-junction. The dispute around this region between China & Bhutan is not new. But recently China started constructing road in this region aggravating the conflict.
Current stand of is not only because of border dispute but also because of other factors:
Irritants to Chinese:
Indian refusal to participate in Chinese Belt & Road Initiative, India participating in various exercises with Chinese rivals like Malabar Exercise (which China views as Asian NATO), where India, US, Japan participate and Australia is also keen to join, India's stand on South China sea, Recent visit of Dalai lama to Tawang valley.
Irritants to Indian side:
China's reluctance to recognize MasoodAzhar as international terrorist, China's denial to allow India into NSG (works on consensus), Continuing CPEC (China-Pak Economic Corridor) through PoK
Between two countries:
War is very least possible because, both the countries are nuclear powers and well aware of devastating effects of war irrespective of whether they are winning or losing. Despite hundreds of minor conflicts not even a single bullet is been fired on border since 1962 Indo-China war. This shoes the restraint on both the sides. Both countries have also deep understanding of concentrating on economic ties for mutual benefit rather than border dispute.
Both the countries know that their capabilities have tremendously improved since 1962 war. Based on geopolitical conditions, no third country might join in the aid of either India or China. So both sides know that there will not be clear victory in case of war.
Despite their differences there are many points on which both the countries need co-operation on international platforms.
Apart from above, China wants India to recognize that China is stronger than India militarily, economically. It wants India to accept that China is like a world power now. India's indifferent behavior towards China might want it to show its superiority and try to insult India as it has done in 1962 war. So possibility of war though almost nil cannot be completely eliminated. It is high time for both the countries to start diplomatic dialogues and resolve the issue before it aggravates to dangerous level.