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[ScreenOS] How to enable Jumbo frames to reassemble fragmented ESP packets



Article ID: KB21816 KB Last Updated: 13 Jan 2014Version: 2.0

Fragmented ESP packets (Protocol 50) are reassembled and forwarded as per the jumbo size MTU, when the jumbo frame is enabled (set envar max-frame-size=9830). This article explains how to reassemble them.

  • NS-5400 is acting as the passthrough for ESP packets.

  • Only the NS-5400 has Jumbo Frame support enabled.

  • ISG and SSG firewalls are configured with MTU size of 1500 bytes.

With Jumbo Frames disabled on the NS-5400, fragmented ESP packets are forwarded and their destination is reached.  

With Jumbo Frames enabled on the NS-5400, the firewall reassembles the ESP packets and then forwards the packet with the size greater than the adjacent device's MTU (1500); so the packet is dropped.

With set envar max-frame-size=9830

SSG receives and drops the reassembled ESP packet, since the packet size is greater than the MTU of 1500 bytes:

00397.0: ethernet5/0(i) len=1518:0010db8f2bcb->001122334455/0800
              vhl=45, tos=00, id=32965, frag=0000, ttl=62 tlen=1504
              esp:spi 3698861298

****** 00397.0: <Trust/ethernet5/0> packet received [1504]******
  ipid = 32965(80c5), @2d558910
  packet dropped, err ip len
Without max-frame-size=9830:

SSG receives the fragmented ESP packets and continues to forward them to their destination.

00679.0: ethernet5/0(i) len=786:0010db8f2bcb->001122334455/0800
              vhl=45, tos=00, id=32965, frag=2000, ttl=62 tlen=772
              frag offset=0 more fragment=1
              esp:spi 3698861298

00679.0: ethernet5/0(i) len=766:0010db8f2bcb->001122334455/0800
              vhl=45, tos=00, id=32965, frag=005e, ttl=62 tlen=752
              frag offset=752 more fragment=0


The following workarounds are necessary:

  1. Remove the Jumbo Frame size setting (unset env max-frame-size), so that the ISG/NS-5000 firewall will use the MTU of 1500 bytes.

  2. Enable the Jumbo Frame support end-to-end, so that the adjacent device is also able to handle Jumbo Frames.

  3. Prevent the ESP packets from fragmenting in the first place:

Setting the TCP MSS for VPN traffic. 

                                         set flow tcp-mss <mss value>

Setting the Path MTU for VPN traffic.

                                          set flow path-mtu

Note: The destination device receiving this packets might not be able to handle this packets as its interface MTU might be set to process normal packets.

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