The two-day talks in New Delhi over the world's highest battlefield follow local media reports in the past few months that the two sides were inching towards a blueprint for a troop pullout.
Both sides on Tuesday fielded large teams of bureaucrats and military officers, headed by their respective defence secretaries.
Tariq Waseem Ghazi, Pakistan's defence secretary, and his Indian counterpart, Shekhar Dutt, were also to hold separate one-on-one discussions later in the day.
Ghazi said after arriving in New Delhi on Monday that he was optimistic.
"We have come here with an open, positive frame of mind. We are ready to discuss all possibilities ... we are open to all proposals," he said.
The Siachen dialogue - part of a wide-ranging peace process - comes a day ahead of a peace conference involving Manmohan Singh, the Indian prime minister, and some Kashmiri separatist groups in Srinagar.
Thousands of soldiers have died in Siachen, high in the Himalayas, with more fatalities due to freezing temperatures, high altitude sickness and avalanches than to enemy fire.
"We have come here with an open, positive frame of mind," he said. "We are ready to discuss all possibilities ... we are open to all proposals"
Tariq Waseem Ghazi,
Pakistani defence secretary
Military experts estimate that 7,000 Indian soldiers and 4,000 Pakistani soldiers are stationed on the glacier.
Though diplomatic, commercial, sporting and transport links between India and Pakistan have improved since their peace process started in January 2004, they have made little headway over Kashmir, the cause of two of their three wars.
A sticking point is India's demand that troop positions be marked on the ground and on a map as evidence in case the area is occupied by Pakistan after a pullout deal is reached.
The region has witnessed no fighting since late 2003, when a ceasefire came into place on the militarised Kashmir frontier.
Pakistani analysts say Islamabad is uneasy about marking positions, fearing it will legitimise India's hold in Siachen.
Analysts do not expect a breakthrough in the third round of Siachen talks since January 2004 but say dialogue is a good way of building trust.