Protobuf definition for a tree

message Tree {
    enum HashFunction {
        md5 = 1;  
        sha = 2;
        sha224 = 3;
        sha256 = 4;
        sha384 = 5;
        sha512 = 6;
    }
    required bytes treeId = 1;
    required HashFunction hashFunction = 2;
    required bytes root = 3;
    required int64 size = 4;
    map<string,string> backend = 5;
    optional string treeName = 6;
}