var cancellationToken = default(CancellationToken);
var pipe = new Pipe();
var pipeReader = pipe.Reader;
ProxyProtocolHeader? value = null;
var parser = new ProxyProtocolParser();
ReadResult readResult;
bool done = false;
do
{
    cancellationToken.ThrowIfCancellationRequested();
    readResult = await pipeReader.ReadAsync(cancellationToken)
        .ConfigureAwait(false);
    if (readResult.IsCanceled)
    {
        continue;
    }
    done = parser.TryParse(readResult.Buffer, out var advanceTo, out value);
    if (!done && readResult.IsCompleted)
    {
        throw new InvalidOperationException("Incomplete PROXY protocol header");
    }
    pipeReader.AdvanceTo(advanceTo.Consumed, advanceTo.Examined);
}
while (!done);
// use the value
_ = value;