ZA.1Segmenting/Reassembly of connectionless messages
Themechanisms for the connectionless segmenting reassembly are not compatible withthe CCITT Blue Book (1988).
Twomessages (XUDT and XUDTS) had to be introduced in the ITU-T White Book (1993)because the UDT and
UDTSmessages did not foresee the possibility of adding optional parameters.
Theintroduction of this feature in the network should therefore be executed incarefully planned stages:
1)before the introduction of any implementations using segmenting/reassembly, allrelay nodes that will be passed
shallbe equipped with the ability to accept and route XUDT(S) messages;
2)new implementations using segmenting/reassembly are introduced. In this phase,all messages from applications
haveto be sent as UDT/UDTS messages, as long as they fit within one UDT/UDTSmessage. For applications
accordingto CCITT Blue Book (1988), there shall not be any change in behaviour of SCCP.This maximizes the
chancesfor successful interworking;
3) assoon as the network is completely retrofitted according to the second or latereditions of the present document,
it isallowed to segment messages that would otherwise fit into an UDT/UDTS message.This may be done e.g.
torestrict the mean length of the messages sent over the network. In this caseall messages larger than a certain
limit(the value "X" from the ITU-T White Book (1993)) are subjected tosegmenting;
4)the final goal is the complete replacement of UDT/UDTS messages by XUDT/XUDTSmessages. For a certain
periodof time, it is nevertheless required to be able to receive and relay UDTmessages;
5) ifinterworking with Blue Book nodes cannot be avoided, there is a limitedcompatibility mechanism allowing
theoriginating node to fall back if communication is not successful using the XUDTmessages. This would
requirethe last intermediate node before the Blue Book node to have the SCCPcompatibility capabilities of
ITU-TRecommendations Q.713 and Q.714 (07/1996 edition) and the use of the return onerror procedure. Even
so,there would be some cases where this will fail.
It isout of the scope of the present document to define whether and when each ofthese stages is to be achieved.
Twoother messages (LUDT and LUDTS) had to be introduced in the ITU-T 1996 edition,to allow optimal use of
MTP-3bcapabilities. Interworking with earlier ITU-T editions requires an intermediatenode to have the appropriate
SCCPcompatibility capabilities. Interworking with ITU-T Blue Book (1988) edition ishowever not guaranteed.
我找了一下规范，里面有解释 WHITE BOOK，BLUE BOOK 在SCCP协议集里的区别。两个参数集里的参数都是一样的。只是赋值的不同。大多数项目都采用的是比较新的WHITE参数集。但BLUE BOOK也是兼容的。
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