The possible services that Ann and Bill can provide are clearly constrained by
the possible services that the postal service provides. For example, if the postal serv-
ice doesn’t provide a maximum bound on how long it can take to deliver mail
between the two houses (for example, three days), then there is no way that Ann and
Bill can guarantee a maximum delay for mail delivery between any of the cousin
pairs. In a similar manner, the services that a transport protocol can provide are often
constrained by the service model of the underlying network-layer protocol. If the
network-layer protocol cannot provide delay or bandwidth guarantees for transport-
layer segments sent between hosts, then the transport-layer protocol cannot provide
delay or bandwidth guarantees for application messages sent between processes.