| A tactical network is inherently mobile. Mobility causes frequent change in link quality as nodes may continuously change distance with respect to each other and go behind objects. This change in link quality results in packet losses that are sometimes due to higher BER (Bit Error Rate) and other times due to route changes. These packet losses are quite different than packet losses due to congestion, and frequently require alternative handling and recovery mechanisms to achieve efficient reliable transport. Consequently, an effective congestion control mechanism needs to distinguish between them. Congestion control mechanisms that do not take this into consideration tend to perform poorly in tactical environments. TCP is an example of a protocol that performs poorly in a MANET (Mobile Ad-hoc NETwork) environment for this specific reason. TCP assumes that packet losses due to any other reason than congestion are extremely rare. Thus TCP invokes its congestion control mechanism at every occurrence of loss. This assumption of TCP works well in wireline networks where packet loss due to BER or link loss is extremely small and thus can be safely interpreted solely as a sign of congestion. MANET environments, however, require differentiation between the types and causes of packet loss, in order to efficiently handle and recover from the loss.
In this paper, we provide a methodical comparison between different reliable unicast transport layer options in the tactical environment. We first develop a list of criteria that serves as a baseline for comparing reliable unicast transport layer mechanisms in the tactical environment. Based on the criteria, we then rank standard TCP and all its MANET friendly variants. We show that protocols which distinguish between packet losses due to congestion and other reasons are generally ranked higher.
Moreover, we discuss explicit congestion notification (ECN) which, unlike wireline networks, proves to be an essential building block for realizing any effective congestion control mechanism in the tactical environment.
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