The question I have is fairly straight-forward, but the answer may not be. If I place a layer 3 device on Gn between an SGSN and a GGSN, is it possible to adjust MSS in TCP SYN packets? The packets are encapsulated by GTP so it will need to l ook down to the TCP header below the GTP header. If it is, what devices can perform this operation?
Guiseppe, thank you for the reply. The problem is that the packet doesn't lose it's GTP encapsulation until it reaches the GGSN Gi interface. But between the SGSN and GGSN there is transport network with MPLS of which a small part the MPLS encapsulated packets cannot exceed 1500 bytes. There's a lot of overhead: MPLS+labels+GTP header. This makes it necessary to make sure MTU cannot be the normal 1500, but a rather smaller figure. In general TCP packets are the larger ones, and it's not possible to control mobile equipment, nor equipment on the net. So adjusting MSS seems the only way. But if the SGSN is not directly connected but uses an MPLS transit network, part of which the MPLS packet is restricted to 1500 bytes. There may be an option with a GTP-aware firewall to do this, but I'd like to know whether there is any other way.
Thank you again. Sadly, I cannot perform this operaion on the SGSN, since some that are affected belong to different providers. So I was hoping it was possible to alter the tcp mss while the packet was encapsulated by GTP.
It seems to be very difficult if at all possible to do this.
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
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