gPTP (IEEE Std 802.1AS) 与 PTP (IEEE Std 1588

参考文献为 IEEE 标准 Std 802.1AS。
由于时间关系，没来得及翻译和解释。但总体上看，gPTP相比PTP，是从PTP定义的几种支持模式中选子集，或者是使标准更容易实现、更加通用。比如，gPTP采用peer to peer的方式，因此每个节点都需要支持gPTP协议，由此可见，网络中的路由器、交换机一般都需比较新，或者是直接建设一个新网络，采用新设备，以支持gPTP；而对于PTP，规定同时可支持peer to peer 以及 end to end 的方式，因此，当采用end to end 方式时，即便网络中有老旧的交换设备，也还是可以支持整个网络实现对时的，但是，因为链路中存在不支持PTP的老旧设备，网络中某些节点的时间同步精度会受到影响。相关介绍可见另一篇文章。

Differences between gPTP (IEEE Std 802.1AS) and PTP (IEEE Std 1588-2019)

1. a) gPTP假定PTP实例间的通信仅使用IEEE 802 MAC PDUs 和地址，1588支持多种层2，以及层3层4的通讯方法。gPTP assumes all communication between PTP Instances is done only using IEEE 802 MAC PDUs and addressing, while IEEE Std 1588-2019 supports various layer 2 and layer 3-4 communication methods.
2. b) gPTP specifies a media-independent sublayer that simplifies the integration within a single timing domain of multiple different networking technologies with radically different media access protocols. gPTP specifies a media-dependent sublayer for each medium. The information exchanged between PTP Instances has been generalized to support different packet formats and management schemes appropriate to the particular networking technology. IEEE Std 1588-2019, on the other hand, has introduced a new architecture based on media-independent and media-dependent sublayers (see 6.5.2, Figure 5, and Figure 6 of IEEE Std 1588-2019); however, this architecture is optional. The architecture of IEEE Std 1588-2008 [B10], which is not based on media-independent and media-dependent layers, has been retained for Internet Protocol (IP) version 4, IP version 6, Ethernet LANs, and several industrial automation control protocols. The intent in IEEE Std 1588- 2019 is that the new architecture, based on media-independent and media-dependent layers, will be used for IEEE 802.11 networks, IEEE 802.3 EPON, and CSN using the specifications of gPTP, and that the architecture must be used for transports that define native timing mechanisms if those native timing mechanisms are used.
3. c) In gPTP there are only two types of PTP Instances: PTP End Instances and PTP Relay Instances, while IEEE Std 1588-2019 has Ordinary Clocks, Boundary Clocks, end-to-end Transparent Clocks, and P2P Transparent Clocks. A PTP End Instance corresponds to an IEEE 1588 Ordinary Clock, and a PTP Relay Instance is a type of IEEE 1588 Boundary Clock where its operation is very tightly defined, so much so that a PTP Relay Instance with Ethernet ports can be shown to be mathematically equivalent to a P2P Transparent Clock in terms of how synchronization is performed, as shown in 11.1.3. In addition, a PTP Relay Instance can operate in a mode (i.e., the mode where the variable syncLocked is TRUE; see 10.2.5.15) where the PTP Relay Instance is equivalent to a P2P Transparent Clock in terms of when time-synchronization messages are sent. A time-aware system measures link delay and residence time and communicates these in a correction field. In summary, a PTP Relay Instance conforms to the specifications for a Boundary Clock in IEEE Std 1588-2019, but a PTP Relay Instance does not conform to the complete specifications for a P2P Transparent Clock in IEEE Std 1588-2019 because:
   (1) When syncLocked is FALSE, the PTP Relay Instance sends Sync according to the specifications for a Boundary Clock, and
   (2) The PTP Relay Instance invokes the BMCA and has PTP Port states.
4. d) PTP Instances communicate gPTP information only directly with other PTP Instances. That is, a gPTP domain consists ONLY of PTP Instances. Non-PTP Relay Instances cannot be used to relay gPTP information. In IEEE Std 1588-2019, it is possible to use non-IEEE-1588-aware relays in an IEEE 1588 domain, although this slows timing convergence and introduce extra jitter and wander that must be filtered by any IEEE 1588 clock.
5. e) For full-duplex Ethernet links, gPTP requires the use of the peer-to-peer delay mechanism, while IEEE Std 1588-2019 also allows the use of end-to-end delay measurement.
6. f) For full-duplex Ethernet links, gPTP requires the use of two-step processing (use of Follow_Up and delay_Resp_Follow_Up messages to communicate timestamps), with an optional one-step processing mode that embeds timestamps in the Sync “on the fly” as they are being transmitted (gPTP does not specify one-step processing for peer
   delay messages). IEEE Std 1588-2019 allows either two-step or one-step processing to be required (for both Sync and peer delay messages) depending on a specific profile.
7. g) All PTP Instances in a gPTP domain are logically syntonized; in other words, they all measure time intervals using the same frequency. This is done by the process described in 7.3.3 and is mandatory. Syntonization in IEEE Std 1588-2019 is optional. The syntonization method used by gPTP is supported as an option in IEEE Std 1588-2019, but uses a TLV standardized as part of IEEE Std 1588-2019 (this feature is new for IEEE Std 1588-2019), while gPTP uses the ORGANIZATION_EXTENSION TLV specified in 11.4.4.3.
8. h) Finally, this standard includes formal interface definitions, including primitives, for the time-aware applications (see Clause
   9). IEEE Std 1588-2019 describes external interfaces without describing specific interface primitives.