IEEE 1588 PTP Clock Modes and Transports

IEEE 1588 PTP Clock Modes and Transports

IEEE-1588 PTPv2 has various clock-modes that can work with different Transport-layer technologies like Ethernet, IPv4, IPv6 as well as MPLS, DeviceNet, ControlNet, etc. Here we look at the various scenarios and cases on how PTP can serve different network use-cases with its different configuration combinations.

Ethernet Mode

S. No.TransportClock ModeDelay MechanismClock Steps
1EthernetBoundaryEnd-to-End1
2EthernetBoundaryE2E2
3EthernetBoundaryPeer-to-Peer1
4EthernetBoundaryP2P2
5EthernetTransparentE2E1
6EthernetTransparentE2E2
7EthernetTransparentP2P1
8EthernetTransparentP2P2
Table 1: IEEE-1588 Clock Modes with Ethernet Transport

The PTP communication in Ethernet mode using the specific Multicast MAC address(es). These come in two varieties, the general Multicast address (01-1B-19-00-00-00) and the reserved Multicast address (01-80-C2-00-00-0E). While the former is used to deliver General (Announce etc) and event (Sync, Delay_Req etc) messages, the latter is specifically used for peer-delay (Pdelay_Req etc) messages.

1588 Sync and Delay Req

Let us look at the PTP messages and then map them to each of the modes and transports tabulated above.

S. No.MessageTypeDestination MACTimestamps
1SyncEvent01-1B-19-00-00-00T1, T2
2Follow_UpGeneral01-1B-19-00-00-00T2′
3Delay_ReqEvent01-1B-19-00-00-00 *T3
4Delay_RespGeneral01-1B-19-00-00-00 *T4
5PDelay_ReqEvent01-80-C2-00-00-0ET1, T2
6PDelay_RespEvent01-80-C2-00-00-0ET3, T4
7#P_Follow_UpGeneral01-80-C2-00-00-0ET3′
8AnnounceGeneral01-1B-19-00-00-00NA
9ManagementGeneral01-1B-19-00-00-00NA
10SignalingGeneral01-1B-19-00-00-00NA

Table 2: PTP Message Types and Destination

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Lock

The reason that there are two types of DMACs (Destination MAC addresses) is because some PTP messages (like Announce and Sync etc) need to go to all the switches in the L2 (Layer-2) network while others like PDelay_* messages need to be exchanged only between the immediate next-hop switch (thus they use the BPDU (Bridge Packet Data Unit) multicast address of 01-80-C2-00-00-0E). The SMAC (Source MAC) of each outgoing PTP message is the unique MAC of the port from which the packet is egressing and the PTP EtherType used is 0x88F7.

Delay Req and Peer Delay 1588

We need to see messages (Table-2) in light of clock-modes (Table-1), as we see in the following tabulated data. For each row/mode, the messages marked in BOLD are applicable while the ones in GREYed out are not.

Clock ModeApplicable IngressApplicable EgressComments
Ethernet BC_E2E_1sSync
*Follow_Up
Delay_Req
Delay_Resp
PDelay_Req
PDelay_Resp
P_Follow_Up

Announce
Management
Signaling
Sync
Follow_Up
Delay_Req
Delay_Resp
PDelay_Req
PDelay_Resp
P_Follow_Up
Announce
Management
Signaling
*Note: In case an upstream 2-step BC device is connected, a Follow-Up may be received and should be absorbed.
Ethernet BC_E2E_2sSync
#Follow_Up
Delay_Req
Delay_Resp
PDelay_Req
PDelay_Resp
P_Follow_Up
Announce
Management
Signaling
Sync
Follow_Up
Delay_Req
Delay_Resp

PDelay_Req
PDelay_Resp
P_Follow_Up

Announce
Management
Signaling
#Note: A Follow-Up may or may not arrive (based on the upstream device is 2-step or 1-step), however, it is always generated.
Ethernet BC_P2P_1sSync
*Follow_Up
Delay_Req
Delay_Resp
PDelay_Req
PDelay_Resp

*P_Follow_Up
Announce
Management
Signaling
Sync
Follow_Up
Delay_Req
Delay_Resp
PDelay_Req
PDelay_Resp
P_Follow_Up
Announce
Management
Signaling
*Note: In case an upstream 2-step BC device is connected, a Follow-Up or P_Follow_Up may be received and should be absorbed.
Ethernet BC_P2P_2sSync
#Follow_Up
Delay_Req
Delay_Resp
PDelay_Req
PDelay_Resp

#P_Follow_Up
Announce
Management
Signaling
Sync
#Follow_Up
Delay_Req
Delay_Resp
PDelay_Req
PDelay_Resp

#P_Follow_Up
Announce
Management
Signaling
#Note: A Follow_Up/P_Follow_Up may or may not arrive (based on the upstream device is 2-step or 1-step), however, it is always generated.
Ethernet TC_E2E_1sSync
*Follow_Up
Delay_Req
Delay_Resp
PDelay_Req
PDelay_Resp
P_Follow_Up

Announce
Management
Signaling
Sync
*Follow_Up
Delay_Req
Delay_Resp
PDelay_Req
PDelay_Resp
P_Follow_Up
Announce
Management
Signaling
*Note: In case an upstream 2-step is connected, the Follow_Up is passed as-is, while CF (Correction Field) is updated in Sync.
Ethernet TC_E2E_2sSync
#Follow_Up
Delay_Req
Delay_Resp
PDelay_Req
PDelay_Resp
P_Follow_Up

Announce
Management
Signaling
Sync
#Follow_Up
Delay_Req
Delay_Resp
PDelay_Req
PDelay_Resp
P_Follow_Up

Announce
Management
Signaling
#Note: A Follow-Up may or may not arrive (based on the upstream device is 2-step or 1-step), however, it is always generated with the updated Correction Field (CF)
Ethernet TC_P2P_1sSync
*Follow_Up
Delay_Req
Delay_Resp
PDelay_Req
PDelay_Resp
#P_Follow_Up
Announce
Management
Signaling
Sync
*Follow_Up
Delay_Req
Delay_Resp
PDelay_Req
PDelay_Resp

P_Follow_Up
Announce
Management
Signaling
*Note: In case an upstream 2-step is connected, the Follow_Up is passed as-is, while CF (Correction Field) is updated in Sync.

#Note: A P_Follow_Up may arrive if the upstream is 2-step, but it will be consumed and never be generated.
Ethernet TC_P2P_2sSync
#Follow_Up
Delay_Req
Delay_Resp
PDelay_Req
PDelay_Resp
*P_Follow_Up

Announce
Management
Signaling
Sync
#Follow_Up
Delay_Req
Delay_Resp
PDelay_Req
PDelay_Resp
*P_Follow_Up
Announce
Management
Signaling
#Note: A Follow_Up may or may not arrive (based on the upstream device is 2-step or 1-step), however, it is always generated with the updated Correction Field (CF)

*Note: P_Follow_Up may or may not arrive, however, it is always generated with the required timestamps.
Table 3: PTP messages applicable to Clock Modes

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