  We consider the problem of aggregating data in a dynamic graph, that is,
aggregating the data that originates from all nodes in the graph to a specific
node, the sink. We are interested in giving lower bounds for this problem,
under different kinds of adversaries. In our model, nodes are endowed with
unlimited memory and unlimited computational power. Yet, we assume that
communications between nodes are carried out with pairwise interactions, where
nodes can exchange control information before deciding whether they transmit
their data or not, given that each node is allowed to transmit its data at most
once. When a node receives a data from a neighbor, the node may aggregate it
with its own data. We consider three possible adversaries: the online adaptive
adversary, the oblivious adversary , and the randomized adversary that chooses
the pairwise interactions uniformly at random. For the online adaptive and the
oblivious adversary, we give impossibility results when nodes have no knowledge
about the graph and are not aware of the future. Also, we give several tight
bounds depending on the knowledge (be it topology related or time related) of
the nodes. For the randomized adversary, we show that the Gathering algorithm,
which always commands a node to transmit, is optimal if nodes have no knowledge
at all. Also, we propose an algorithm called Waiting Greedy, where a node
either waits or transmits depending on some parameter, that is optimal when
each node knows its future pairwise interactions with the sink.
