  This paper investigates low-latency streaming codes for a three-node relay
network. The source transmits a sequence of messages (streaming messages) to
the destination through the relay between them, where the first-hop channel
from the source to the relay and the second-hop channel from the relay to the
destination are subject to packet erasures. Every source message must be
recovered perfectly at the destination subject to a fixed decoding delay of $T$
time slots. In any sliding window of $T+1$ time slots, we assume no more than
$N_1$ and $N_2$ erasures are introduced by the first-hop channel and second-hop
channel respectively. Under this channel loss assumption, we fully characterize
the maximum achievable rate in terms of $T$, $N_1$ and $N_2$. The achievability
is proved by using a symbol-wise decode-forward strategy where the source
symbols within the same message are decoded by the relay with different delays.
The converse is proved by analyzing the maximum achievable rate for each
channel when the erasures in the other channel are consecutive (bursty). In
addition, we show that traditional message-wise decode-forward strategies,
which require the source symbols within the same message to be decoded by the
relay with the same delay, are sub-optimal in general.
