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Because BSC cannot support the IP packet to be sliced twice,the GPRS customer cannot browse pictures at website

Publication Date:  2012-07-27 Views:  107 Downloads:  0

Issue Description

NE40E version is V6R1,BSC6000 version:V9R8C12SP27, the topology is:BSC6000----(MTU:1500)---PE1(MTU:1500)--- (MTU:1500)---P1--(MTU:1500)-----PE2--(MTU:1500)----SGSN1; The customer complain that the GPRS customer cannot browse pictures at web site properly.

Alarm Information

the GPRS customer cannot browse pictures at website

Handling Process

  1. Because the customer can browse the website without picture ,it means the network is working.

  2. But when use ping –s 1500 BSC6000 loopback interface which encapsulate the customer GPRS data into BSSGP packets at SGSN1, the ping output always are time out.

  3. Then we mirror the SGSN1 downlink stream, we find that the SGSN1 send 2 patches (1packet size is 1500, one is packet size is 76, because the IP packet need be encapsulated by BSSGP protocol) ,but when mirror the PE1 downstream we find it becomes 3 packets(Because the network need add MPLS header) and we cannot find any reply from BSC6000.

  4. But we use normal ping ,we can find the BSC6000 reply .

  5. Then we doubt that it be caused by physical link between the BSC to PE1 cannot support big IP packet, So we reduce the PE1 down link MTU to 1000, but it still do not work.

  6. Because this phenomenon is out of our maintains experiences, then we discuss with R&D about it. We know the BSC cannot support the IP packet to be sliced twice at current version, it need new version to solve it.

  7. We change all the related links which from PE1 to PE2 MTU to 2000 as temporary solution.

Root Cause

Because when we reduce the MTU the problem still cannot be solved. Then we discuss with Wireless R&D, after discussion we know that because BSC6000 version cannot support the IP packet to be sliced twice, when BSC will drop the IP packets which are sliced twice, then it cause GPRS customer cannot browse pictures at website


As maintains experience, we know we need reduce the interface MTU when the physical link is not so good, to let the big packets can be carried by network. But when you reduce MTU the network still cannot carry the big packets properly we should also consider whether all the L3 IP elements in the network can support the IP packets to be sliced twice.