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Emails with Attachments Cannot Be Sent Because the Competitor's Device Cannot Restructure Disordered Packets

Publication Date:  2012-07-27 Views:  139 Downloads:  0
Issue Description
This NE20E network provided a bearer channel for access to the Internet by mobile phones, which sent access packets through a BSC to SGSN of company N through the NE20E network. There were two channels through the NE20E network: NE20E-A----NE20E-B----NE20E-C and NE20E-A----NE20E-D----NE20E-C. The first channel was a primary one. The customer found that mobile phone users failed to send emails with attachments through the Internet but could access Web pages normally. Large packets could also be successfully pinged. If the services passed the second channel, emails with attachments could be sent. 
Alarm Information
Handling Process
The field engineer disabled data encryption between SGSN of company N and the BSC and confirmed that SGSN directly discarded the disordered packet, which could not be restructured. The substantial solution to the problem lies in the device of company N.
In addition, the NE20E provided the function of preserving order on the MP interface to alleviate the disorder caused by MP fragmentation. However, problems occurred when there was serious disorder due to a limited buffer of 512 for this function.
An example of the configuration is as follows:
[Quidway-Mp-group0/0/0]ppp mp reorder 
Root Cause
The difference between the two channels was that the MP links of NE20E-A----NE20E-D----NE20E-C both contained two channels bound together and, for NE20E-A----NE20E-B----NE20E-C, A----B contained two channels bound together while B----C contained three channels bound together.
Analysis of packets captured found that no packet sent by the BSC exceeded 800 bytes in total length, which could basically indicate that the packet was broken into fragments after being processed by the BSC and the 1514 B packet was broken into two larger fragments and one small fragment. The three fragments reached NE20E-B by way of NE20E-A. Because the link between NE20E-B and NE20E-C was one containing three channels bound together, the three fragments were forwarded through the three channels one by one. The small fragment was first forwarded due to its short transmission time, during which the first two larger fragments were likely not to finish being transmitted. In this case, the last fragment of the fragmented packet reached SGSN first and the first two fragments were absent, and therefore SGSN could not restructure the fragmented packet, which was discarded.