Final answer:
The principal duty of the porters involves physical labor to build and maintain societal infrastructure, possibly at great personal cost, and to conduct themselves ethically and obediently.
Step-by-step explanation:
The principal duty of the porters, based on the provided texts, is to develop and maintain the infrastructure required for the functioning of society. This involves physically demanding tasks, like the toil of a serf or sweeper, creation and maintenance of ports and roads, and even sacrificing their lives, as suggested by the phrase "mark them with your dead."
Additionally, there is an ethical aspect to their duty to refrain from violence and to follow directions in their line of work. This sense of duty is also reciprocal, as indicated in posters that suggest obligation is a two-way street, highlighting that while porters have responsibilities, they should also be supported and encouraged in their roles.