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9.2.3.2 Network failure and IP address change
Temporary network failure without change of IP address
Similarly to wireless link failure, temporary network outage, where no changes in IP addresses or any of the EPRs, is handled naturally by the MessageNumber and
acknowledgementretransmission mechanism of WS-ReliableMessaging. In the event of multiple retransmission failures, the DMS shall use WS-ReliableMessaging fault
mechanism to notify the event to some external entities, for example to the dispatcher, which can be configured during Client-DMS configuration exchange, or using
wsa:FaultTo inside the WS-ReliableMessaging CreateSequence message. A maximum number of retransmissions shall be configured at the DMS. WS-ReliableMessaging with
SOAP 1.2 binding provides only a limited set of possible fault events, in which maximum retransmission is not available as an option, and there is no mechanism to add custom
error events. One possible solution might be to consider “unsuccessful transmission after maximum number of retries reached” event as WS-ReliableMessaging
SequenceTerminated fault.
Network failure with IP address change
In IP networking setting, IP address serves both as a network node identifier for the purpose of routing, and as an identifier used by upper layer applications e.g. IP address
is one component of a TCPUDP “socket”. The design of IP networking forces a node to change its IP address whenever it changes its point of connectivity, which consequently
breaks existing application layer sessions that use the IP address. Mobile IP protocol family [IETF-MIPv6] is designed to alleviate this issue, by providing a static address, the
so called Home Address, separate from the dynamic Care-of Address, which depends on the access network, and devising a signaling mechanism to constantly map the Home and
the Care-of addresses at an anchor entity called the Home Agent.
Within the aviation community, the use of IPv6 is being considered as the future underlying protocol of future air traffic management ATM network, with Mobile IPv6
as the basis for providing network layer mobility [ICAO-ATNIPS]. If this solution were in place, changing client’s IP address would not pose any issues to the applications, and
no further specifications would be required for the communications between client and DMS.
When the client’s IP address changes, there are two scenarios that need to be considered depending on the topological location of the DMS. The first case is if the DMS is the next
hop from the client, i.e. it is the wireless access point base station, cf. Figure 6. In such case the followings are most likely the case:
- client’s change of network access point implies change of the serving DMS, - Client’s IP address is allocated by the DMS or by a router that is topologically
close and under the same authoritative control as the DMS.
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Figure 6 - Topologically-close DMS
In both cases, the client’s new IP address will be known to the new DMS after the handover. There are some issues with this scenario:
- Since the serving DMS changes, the DMS EPR information within the client needs to be updated. One possibility may be that the client is configured with a
registry address, from which it could obtain the DMS EPR corresponding to its current location or serving network provider.
- The web services need to be informed of the new DMS. - All client states have to be transferred from the old DMS to the new one;
otherwise some messages may be lost. Another scenario is where the serving DMS is not changed after handover, cf. Figure 7.
In this case the DMS only knows the client’s EPR, and is not aware of the new IP address obtained by the client after the handover. In this scenario, seamless message delivery to
the aircraft client after handover is in general not possible without support from the network layer protocol. The change in the client IP address needs to be propagated in the
network routing infrastructure, so that all intermediate routers within the network will be able to route the message correctly to the client’s new IP address. Similarly, the name-
resolving infrastructure of the network e.g. DNS needs to be updated to map the client’s new IP address to its EPR URL.
Connection lost
New connection established
DMS1
DMS2
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Figure 7 - Topologically-far DMS
In conclusion, WS-ReliableMessaging between a client and a DMS alone is not a sufficient solution to overcome the IP address change problem. Support from the routing
infrastructure, as in Mobile IP, or a network of interconnected register, redirect and name servers that manage client registration, as in the SIP protocol [IETF-SIP], would be
required.
9.2.3.3 Software failure