NTP Interleaved Modes

RFC 5905 describes the operations of NTPv4 in basic client/server, symmetric, and broadcast mode. The transmit timestamp is one of the four timestamps included in every NTP packet used for time synchronization. A packet that strictly follows RFC 5905, i.e. it contains a transmit timestamp corresponding to the packet itself, is said to be in basic mode. There are, at least, four options where a transmit timestamp can be captured i.e. by NTP daemon, by network drivers, or at the MAC or physical layer of the OSI model. A typical transmit timestamp in a software NTP implementation in the basic mode is the one captured by the NTP daemon using the system clock, before the computation of message digest and before the packet is passed to the operating system, and does not include any processing and queuing delays in the system, network drivers, and hardware. These delays may add a significant error to the offset and network delay measured by clients and peers of the server. For best accuracy, the transmit timestamp should be captured as close to the wire as possible, but that is difficult to implement in the current packet since this timestamp is available only after the packet transmission. The protocol described in RFC 5905 does not specify any mechanism for the server to provide its clients and peers with this more accurate timestamp. Different mechanisms could be used to exchange this more accurate timestamp. This document describes interleaved modes, in which an NTP packet contains a transmit timestamp corresponding to the previous packet that was sent to the client or peer. This transmit timestamp could be captured at one of the any four places mentioned above. More specifically, this document: Introduces and specifies a new interleaved client/server mode. Specifies the interleaved symmetric mode based on the NTP reference implementation with some modifications. Specifies the interleaved broadcast mode based purely on the NTP reference implementation. The protocol does not change the NTP packet header format. Only the semantics of some timestamp fields is different. NTPv4 that supports client/server and broadcast interleaved modes is compatible with NTPv4 without this capability as well as with all previous NTP versions. The protocol requires both servers and clients/peers to keep some state specific to the interleaved mode. It prevents traffic amplification that would be possible if the timestamp was sent in a separate message in order to keep the servers stateless. This document assumes familiarity with RFC 5905.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

The interleaved client/server mode is similar to the basic client/ server mode. The only difference between the two modes is in the meaning of the transmit and origin timestamp fields. A client request in the basic mode has an origin timestamp equal to the transmit timestamp from the previous server response, or is zero. A server response in the basic mode has an origin timestamp equal to the transmit timestamp from the client's request. The transmit timestamps correspond to the packets in which they are included. A client request in the interleaved mode has an origin timestamp equal to the receive timestamp from the previous server response. A server response in the interleaved mode has an origin timestamp equal to the receive timestamp from the client's request. The transmit timestamps correspond to the previous packets that were sent to the server or client. A server which supports the interleaved mode needs to save pairs of local receive and transmit timestamps. The server SHOULD discard old timestamps to limit the amount of memory needed to support clients using the interleaved mode. The server MAY separate the timestamps by IP addresses, but it SHOULD NOT separate them by port numbers, i.e. clients are allowed to change their source port between requests. When the server receives a request, it SHOULD compare the origin timestamp with all receive timestamps it has saved (for the IP address). If a match is found, the server SHOULD respond with a packet in the interleaved mode, which contains the transmit timestamp corresponding to the packet which had the matching receive timestamp. If no match is found, the server MUST NOT respond in the interleaved mode. The server MAY always respond in the basic mode. In both cases, the server SHOULD save the new receive and transmit timestamps. Both servers and clients that support the interleaved mode MUST NOT send a packet that has a transmit timestamp equal to the receive timestamp in order to reliably detect whether received packets conform to the interleaved mode. The first request from a client is always in the basic mode and so is the server response. It has a zero origin timestamp and zero receive timestamp. Only when the client receives a valid response from the server, it will be able to send a request in the interleaved mode. The client SHOULD limit the number of requests in the interleaved mode per server response to prevent processing of very old timestamps in case a large number of packets is lost. An example of packets in a client/server exchange using the interleaved mode is shown in Figure . The packets in the basic and interleaved mode are indicated with B and I respectively. The timestamps t1', t3' and t11' point to the same transmissions as t1, t3 and t11, but they may be less accurate. The first exchange is in the basic mode followed by a second exchange in the interleaved mode. For the third exchange, the client request is in the interleaved mode, but the server response is in the basic mode, because the server did not have the pair of timestamps t6 and t7 (e.g. they were dropped to save timestamps for other clients using the interleaved mode).

When the client receives a response, it performs all tests described in RFC 5905, except now the sanity check for bogus packet needs to compare the origin timestamp with both transmit and receive timestamps from the request in order to be able to detect if the response is in the basic or interleaved mode. The client SHOULD NOT update its NTP state when an invalid response is received to not lose the timestamps which will be needed to complete a measurement when the following response in the interleaved mode is received. If the packet passed the tests and conforms to the interleaved mode, the client can compute the offset and delay using the formulas from RFC 5905 and one of two different sets of timestamps. The first set is RECOMMENDED for clients that filter measurements based on the delay. The corresponding timestamps from Figure are written in parentheses. T1 - local transmit timestamp of the previous request (t1) T2 - remote receive timestamp from the previous response (t2) T3 - remote transmit timestamp from the latest response (t3) T4 - local receive timestamp of the previous response (t4) The second set gives a more accurate measurement of the current offset, but the delay is much more sensitive to a frequency error between the server and client due to a much longer interval between T1 and T4. T1 - local transmit timestamp of the latest request (t5) T2 - remote receive timestamp from the latest response (t6) T3 - remote transmit timestamp from the latest response (t3) T4 - local receive timestamp of the previous response (t4) Clients MAY filter measurements based on the mode. The maximum number of dropped measurements in the basic mode SHOULD be limited in case the server does not support or is not able to respond in the interleaved mode. Clients that filter measurements based on the delay will implicitly prefer measurements in the interleaved mode over the basic mode, because they have a shorter delay due to a more accurate transmit timestamp (T3). The server MAY limit saving of the receive and transmit timestamps to requests which have an origin timestamp specific to the interleaved mode in order to not waste resources on clients using the basic mode. Such an optimization will delay the first interleaved response of the server to a client by one exchange. A check for a non-zero origin timestamp works with clients that implement NTP data minimization. To detect requests in the basic mode from clients that do not implement the data minimization, the server can encode in low-order bits of the receive and transmit timestamps below precision of the clock a bit indicating whether the timestamp is a receive timestamp. If the server receives a request with a non-zero origin timestamp which does not indicate it is receive timestamp of the server, the request is in the basic mode and it is not necessary to save the new receive and transmit timestamp.

The interleaved symmetric mode uses the same principles as the interleaved client/server mode. A packet in the interleaved symmetric mode has a transmit timestamp which corresponds to the previous packet sent to the peer and an origin timestamp equal to the receive timestamp from the last packet received from the peer. In order to prevent the peer from matching the transmit timestamp with an incorrect packet when the peers' transmissions do not alternate (e.g. they use different polling intervals) and a previous packet was lost, the use of the interleaved mode in symmetric associations requires additional restrictions. Peers which have an association need to count valid packets received between their transmissions to determine in which mode a packet should be formed. A valid packet in this context is a packet which passed all NTP tests for duplicate, replayed, bogus, and unauthenticated packets. Other received packets may update the NTP state to allow the (re)initialization of the association, but they do not change the selection of the mode. A peer A SHOULD send a peer B a packet in the interleaved mode only when the following conditions are met: The peer A has an active association with the peer B which was specified with an option enabling the interleaved mode, OR the peer A received at least one valid packet in the interleaved mode from the peer B. The peer A did not send a packet to the peer B since it received the last valid packet from the peer B. The previous packet that the peer A sent to the peer B was the only response to a packet received from the peer B. An example of packets exchanged in a symmetric association is shown in Figure . The minimum polling interval of the peer A is twice as long as the maximum polling interval of the peer B. The first packets sent by the peers are in the basic mode. The second and third packet sent by the peer A is in the interleaved mode. The second packet sent by the peer B is in the interleaved mode, but the following packets sent by the peer are in the basic mode, because multiple responses are sent per request.

If the peer A has no association with the peer B and it responds with symmetric passive packets, it does not need to count the packets in order to meet the restrictions, because each request has at most one response. The peer SHOULD process the requests in the same way as a server which supports the interleaved client/server mode. It MUST NOT respond in the interleaved mode if the request was not in the interleaved mode. The peers SHOULD compute the offset and delay using one the two sets of timestamps specified in the client/server section. They MAY switch between them to minimize the interval between T1 and T4 in order to reduce the error in the measured delay.

A packet in the interleaved broadcast mode contains two transmit timestamps. One corresponds to the packet itself and is saved in the transmit timestamp field. The other corresponds to the previous packet and is saved in the origin timestamp field. The packet is compatible with the basic mode, which uses a zero origin timestamp. A client which does not support the interleaved mode ignores the origin timestamp and processes all packets as if they were in the basic mode. A client which supports the interleaved mode SHOULD check if the origin timestamp is not zero to detect packets in the interleaved mode. The client SHOULD also compare the origin timestamp with the transmit timestamp from the previous packet to detect lost packets. If the difference is larger than a specified maximum (e.g. 1 second), the packet SHOULD NOT be used for synchronization. The client SHOULD compute the offset using the origin timestamp from the received packet and the local receive timestamp of the previous packet. If the client needs to measure the network delay, it SHOULD use the interleaved client/server mode.

The interleaved modes described in this document are based on the reference NTP implementation written by David Mills. The authors would like to thank Kristof Teichel for his useful comments.

This memo includes no request to IANA.

Security issues that apply to the basic modes apply also to the interleaved modes. They are described in The Security of NTP's Datagram Protocol. Clients and peers SHOULD NOT leak the receive timestamp in packets sent to other peers or clients (e.g. as a reference timestamp) to prevent off-path attackers from easily getting the origin timestamp needed to make a valid response in the interleaved mode. Clients SHOULD randomize all bits of both receive and transmit timestamps, as recommended for the transmit timestamp in the NTP client data minimization, to make it more difficult for off-path attackers to guess the origin timestamp. Protecting symmetric associations in the interleaved mode against replay attacks is even more difficult than in the basic mode, because the NTP state needs to be protected not only between the reception and transmission in order to send the peer a packet with a valid origin timestamp, but all the time to not lose the timestamps which will be needed to complete a measurement when the following packet in the interleaved mode is received.