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Author Topic: strange issue with packet encapsulation  (Read 141 times)


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strange issue with packet encapsulation
« on: October 31, 2021, 01:02:53 PM »

I'm experiencing a weird problem for several weeks that I cannot explain nor fix. I'm not even sure where it happens. Any help would be really appreciated.

I have two Internet IPv4 connections (mutual backup) and two corresponding IPv6 tunnels. I have no connectivity issues except this one.

It is occurring on an IPv6 tunnel with endpoint in Prague (Czech rep.). My other tunnel is not affected.

Ping is fine, traceroute is fine, but all downloads except the tiny ones fail.

I spent a lot of time debugging the issue. I have lowered the MTU, clamped the TCP MSS, but it didn't help. Finally I captured the packets for analysis:

1. The TCP connection starts normally (SYN, SYN+ACK, ACK)

2. the first part (about 30-120 kB of data) is transferred without problems, that means an exchange of roughly 30 to 100 data packets incoming and the same amount of ACKs outgoing.

3. but then the size of incoming packets suddenly increases by 20 bytes and something is wrong. The result are dropped packets, retransmissions, resets, etc. and the download fails.

With the wireshark tool I have found out that the additional 20 bytes are another IPv4 header.

A SIT tunnel packet normally looks like this:
[IPv4 header, protocol 41]
[IPv6 packet]

And in the step 3 I'm suddenly receiving this:
[IPv4 header #1, protocol 41]
[IPv4 header #2, protocol 41]
[IPv6 packet]

The IPv4 headers contain correct IPv4 addresses and correct payload lengths (i.e. in the outer header it is 20 bytes larger than in the inner one). It looks like the IPv6-in-IPv4 traffic changes in the middle of download and becomes encapsulated twice.

I captured the packets on a PPPoE interface, they are entering my router (Linux) already malformed.
« Last Edit: October 31, 2021, 01:07:27 PM by xitop6 »


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Re: strange issue with packet encapsulation
« Reply #1 on: November 05, 2021, 03:17:20 PM »

Having IPv4 header #2 when IPv4 header #1 indicates protocol 41 (ipv6) is an obvious error.

This looks like encapsulation error, possibly at HE site. You should send the capture to HE help desk and ask them to have a look at their side.