I have been looking for an answer to that question (the one in the title) and the best thing I've found was:
In DNS Protocol design, UDP transport Block size (payload size) has been limited to 512-Bytes to optimize performance whilst generating minimal network traffic.
my question is: how exactly does this enhance performance and are there any other reasons for this limitation when using UDP ?
The 512 byte payload guarantees that DNS packets can be reassembled if fragmented in transit. Also, generally speaking there's less chance of smaller packets being randomly dropped.
The IPv4 standard specifies that every host must be able to reassemble packets of 576 bytes or less. With an IPv4 header (20 bytes, though it can be as high as 60 bytes w/ options) and an 8 byte UDP header, a DNS packet with a 512 byte payload will be smaller than 576 bytes.
As @RyanRies says: DNS can use TCP for larger payloads and for zone transfers and DNSSEC. There's a lot more latency when TCP comes into play because, unlike UDP, there's a handshake between the client and server before any data begins to flow.
Modern DNS is not actually limited to 512 bytes payload for UDP anymore.
With EDNS0 in use a larger payload size can be specified, which is also commonly the case for DNSSEC-aware clients.
The support for larger payloads over UDP has been a double-edged sword, however, it is in part the reason why using nameservers for amplification attacks has become more popular as you can achieve better amplification if the attacker uses a query that gets a large response.
See rfc2671 for the nitty-gritty details of EDNS0
TCP offers several capabilities over UDP like retransmission to guarantee data is received. A large UDP datagram can be fragmented into multiple IP packets. This is done on layer 3 when it's detected that, given the datagram size, the packet will be larger than the link MTU. Because UDP can't handle retransmission, if any of the datagram fragments are lost the entire UDP datagram is lost. This is quite inefficient. For large datagrams, DNS prefers to break them down to multiple ordered TCP segments. Each of these segments is packaged into its own packet and can be retransmitted if not received by the destination. Duplicate segments can also be detected. Basically, TCP handles failure much better hence more efficient for large datagrams.
UDP based protocols like DNS cap the UDP datagram size to around 512 bytes because this size guarantees the datagram won't be fragmented and hence losing one fragment leads to losing the entire datagram. It's around 512 because, in the IPv4 specifications, hosts should be able to handle packets that are at least 576 bytes large (which is usually well below most link MTUs). Since the IPv4 header can be up to 60 bytes large (20-byte header plus up to 40 bytes of options), 516 bytes (576 - 60) is the maximum size of the IP payload to guarantee not fragmenting.
DNS operations for example, queries, and zone maintenance operations by default use port 53. For performance reasons, queries use the UDP protocol with a block-size limit of 512 bytes. TCP can be optionally negotiated on a transaction-by-transaction basis for query operations, but due to the performance overhead incurred with TCP, this is essentially a theoretical capability. Historically, exceeding the 512-byte response size limit was typically avoided at all costs, and indeed the limit of 13 IPv4 root-servers was the maximum that could be returned in a single 512-byte UDP transaction.
Ron Aitchison - Pro DNS and BIND 10 - 2011
Its a QOS thing.
Because UDP is stateless, error-handling of packets isn't possible.
Thus, by keeping packets to a max size, there is a greater change they will reach their destination, reducing the impact of the absence of error handling.
