This paper investigates congestion control in opportunistic networks that use delay-tolerant networking (DTN) as a basis for communication. We propose a mechanism that advertises buffer occupancy information to adjacent nodes and avoids forwarding through nodes with high buffer occupancy. The nodes then achieve global congestion avoidance simply based on locally available information. The proposed mechanism works independent of the routing protocol and is thus applicable to wide array of scenarios. Extensive simulations, with different node mobility models and radio modeling, indicate that the proposed mechanism improves message delivery ratio and decreases end-to-end delay.