TCN-URI shall be mapped to IP addresses in accordance to the IP addressing scheme defined in IEC 61375-2-5.
The resolution of URIs to IP addresses is in general managed by the TCN-DNS service as specified in 5.4.2.
5.4.5.2 Mapping to IP MC group addresses
IEC 61375-2-5 assigns the IP address range 239.192.0.0/14 for train level IP MC group addressing. To use this address range in conjunction with the defined TCN-URI scheme, especially for supporting groups, which are limited to one consist, it is necessary to define a decomposition of the assigned IP MC group address range.
The general decomposition is defined as:
11101111.110000rr.zzzzzzzz.zzzzzzzz with fields described in Table 18.
Table 18 – General decomposition of IP MC groups addresses
Subnet number part:
[r] Defines the range.
‘00’B = all-train groups
‘01’B = ETB-related groups
‘10’B = consist-limited groups
‘11’B = open for other use [z] For further decomposition
All-train groups are groups with group members related to different ETBs1 in the train.
The decomposition for all-train groups is defined as:
11101111.11000000.gggggggg.gggggggg with fields described in Table 19.
Table 19 – Decomposition of all-train groups
Host number part:
[g] All train group number (value range: 0 .. 65535) 0 .. 255 = as specified in IEC 61375-2-5
256 .. 65534 = usable for user defined all-train groups 65535 = reserved for future use
EXAMPLE An IP MC group containing EDs from the operational network and the multimedia network is an all-train group.
An all-train group address shall be assigned to a function if FUNCTION_INFO parameter ‘etbId’
is set to 255 (see 5.3.3.2.4).
NOTE 1 Usage of TRDP for communication to all-train groups is problematic as all-train groups may contain EDs related to different ETBs, and etbTopoCnt values are different for the ETBs. The etbTopoCnt is contained in the TRDP header and used by destination EDs to verify the correctness of the received message, which will fail if an etbTopoCnt value related to another ETB is contained. Using a gateway in between ETB altering the etbTopoCnt value might be a solution, but is complex.
ETB-related groups are groups with group members related to one ETB.
The decomposition for ETB-related groups is defined as:
11101111.11000000.bbgggggg.gggggggg with fields described in Table 20.
_____________
1 For the definition of ‘… related to ETB’ see 4.2.2.6.2.
Table 20 – Decomposition of ETB-related groups
Subnet number part:
[b] ETB Identifier (etbId) value = 0..3
Host number part:
[g] Group number (value range: 0 .. 16383) 0 = all ED
1 = all ECSP (ETB0 only)
2..16382 = usable for user defined ETB-related groups 16383 = reserved for future use
EXAMPLE An IP MC group containing EDs from the operational network only is a ETB-related group. An IP MC group containing EDs from the multimedia network is another ETB-related group.
Consist-limited groups are ETB-related groups where all group members belong to one consist.
The decomposition for consist-limited groups is defined as:
11101111.11000001.bbcccccc.gggggggg with fields described in Table 21.
Table 21 – Decomposition of consist-limited groups
Subnet number part:
[b] ETB Identifier (etbId) value = 0..3 [c] trnCstNo (value range: 1 .. 63)
0 = local consist
Host number part:
[g] Group number (value range: 0 .. 254)
0 = all ED in all CN of the consist connected to the ETB 1 = ECSP within consist (ETB0 only)
2 .. 254 = usable for user defined consist-limited groups 255 = reserved for future use
Group identifiers need to be unique inside a consist. The same 8-bit group identifier value range is used for both vehicle-limited groups and consist-limited groups, so it must be shared.
For example, the value of grpHMI is different for this group defined on consist level (“grpHMI.aVeh.lVeh”) and for this group defined on vehicle level (e.g. “grpHMI.veh02.lCst”).
Notice that “all ED” is coded by setting all grp-id bits to 0, and not to 1. This convention is used in order to avoid confusion, because grp-id bits set to 1 could also mean “all groups”.
5.4.5.3 Joining IP multicast groups
For joining IP MC groups some aspects should be considered.
a) All-train and ETB-related groups have defined IP MC addresses which will not change when train composition changes, so it is sufficient for EDs to join the group when the system is booting.
b) Consist-limited groups are dynamic IP Multicast groups possessing IP multicast group addresses which may change after each train inauguration. Due to their dynamic nature, all ED joining such a group need to leave the group with a new train inauguration, and join again with the new IP MC address. Another, recommended, solution is to translate IP MC
addresses while routing IP telegrams from ETB to ECN (ETBN). During this network address translation (NAT, see RFC2663, RFC2766, RFC3022) the [c] bits representing the train consist number within the IP MC destination address can be substituted with 0 (local consist). In that case it is sufficient for EDs to join the group when the system is booting.
c) EDs, which should be capable to receive TRDP telegrams addressed to both consist-limited as well as ETB-realted or all-train groups, should join both IP MC groups.
EXAMPLE The function ‘fctDoor’ may be both train wide (ETB0-related) addressable (e.g. TCN-URI
‘fctDoor.anyVeh.aCst.aCltrn.lTrn’) as well as consist-limited (e.g. TCN-URI ‘fctDoor.anyVeh.cst01.anyClTrn.lTrn’). If we assume that NAT is used and that the fctId=50 for the fctDoor, the following two IP MC addresses need to be joined: 239.193.0.50 and 239.194.1.50.
5.4.5.4 Well known TCN-URI
Some TCN-URI mappings to IP addresses are predefined and do not require an explicit resolving of the URI via TCN-DNS. Those TCN-URI are listed in Table 22.
Table 22 – Well-known TCN-URI
TCN-URI (host part) Scope IP address Description
grpAll.aVeh.aCst.aClTrn.lTrn D 239.193.0.0 Broadcast to all end devices related to ETB0.
grpAll.aVeh.lCst.lClTrn.lTrn D 239.194.0.0 Broadcast to all end devices related to ETB0 within the local consist
grpAll.aVeh.lCst.lClTrn.lTrn D 239.255.0.0 Alternative broadcast to all end devices within the local consist if all ED are connected to the same CN (only one CN connected to the ETB)
NOTE this address is defined in IEC 61375-3-4
lDev.lVeh.lCst.lClTrn.lTrn S,D 127.0.0.1 Own device (local loop-back) grpECSP.anyVeh.aCst.aClTrn.lTrn D 239.193.0.1 Broadcast to all ECSP
EXAMPLE
The following example gives some hints for the implementation of grpAll.aVeh.lCst.lClTrn.lTrn in a consist with multiple CN.
Every ED in a consist subscribes to 239.194.0.0.
ETBNs use ETB inauguration result to add a rule to their MC routing table for address 239.194.X.0 where X represents their own consist (X = trnCstNo + (etbId × 64)). All other addresses from range 239.194.X.0 are not routed to ECN.
ETBNs translate MC address 239.194.X.0 to 239.194.0.0 when routing it from ETB to ECN.
In case of multiple CNs in one consist: packet with destination address 239.194.0.0 has to be handled by ETBNs in this consist as well in order to deliver it to all CNs. One solution is to route it from source CN to ETB and translate the destination address to 239.194.X.0. Another solution is that the ETBN in the source CN tunnels the packet to other ETBNs in the consist using IP unicast, and the destination ETBNs then retransmit the packet as multicast into their CN. The first approach is easier to implement but the packet is sent to all ETBNs in the train. Tunneling approach requires more support from ETBNs, on the other hand the packet never leaves consist boundary.
5.4.5.5 IP routing in networks with multiple ETBs
Basic rules for dynamic IP routing management are defined in IEC 61375-2-5.
The following additional rule shall be observed:
For transmitting IP packets to a destination ED, which is related to another ETB than the ETB the source ED is related to, the IP packet shall be transferred over the ETB the destination ED is related to.
NOTE 1 For a definition of ‘ED related to ETB’ see 4.2.2.6.2.
NOTE 2 The reason for this rule is that a sending ED cannot expect that an IP packet can be routed between the ETBs within the destination consist. It therefore needs to be routed within the sender’s consist to the ETB the destination ED is related to.
EXAMPLE This example uses the train architecture shown in Figure 21. If the source ED in consist ‘B’ wants to send to the ED with fctDevNo = 10020 in consist ‘E’, it has to route the packet first to its local multimedia ECN and then via the ETB1 to consist ‘E’.