RE: IPSEC over load-shared T1s (per packet)

["Ben Nagy" <ben () iagu net>]

ObBOFH: One of the T1 RJ connectors must be dirty, which is causing packet
corruption. Give both the telco jacks a good clean (licking them works well)
and see if that fixes the problem. [1]

Seriously, I do have a theory ;)

Does this routing guarantee to preserve sequencing? 

If it's really as you described (packets send one for one via alternate
links) then you have some potential problems brewing, I think.

TCP will "work things out" when packets arrive out of sequence, but with
IPSec it's left up to the implementation. One security concern with most
crypto things is replay protection. IPSec addresses this by using a
mandatory sequence number in the ESP header. The receiveing IPSec doesn't
_have_ to take any notice, but most do. If your receiving IPSec has enabled
replay protection then if one link is going faster half the packets are
going to get dropped (sequence number < current).

This would make your tunneled protocol (say TCP) do the retransission thing,
so it would work itself out eventually, but the speed would indeed suffer
horribly.

See if you can convince your router to preserve "IP flows" and use the two
links in a more sensible manner. That might help.



Best of luck,

ben

PS: Let us know when you work it out? This is an interesting one.

[1] The RJ's are live, for non-network-engineer types. Not enough to kill
you, but it hurts. :)

> -----Original Message-----
> From: firewall-wizards-admin () honor icsalabs com 
> [mailto:firewall-wizards-admin () honor icsalabs com] On Behalf 
> Of TSimons () Delphi-Tech com
> Sent: Thursday, September 18, 2003 3:38 AM
> To: firewall-wizards () honor icsalabs com
>
> Hello All
>
> Recently we doubled our internet bandwith to two T1s from the 
> same provider
> that terminate on in the same router on the NOC side.
>
> We setup IP LOAD-SHARING PER-PACKET on each of the serial 
> links on both
> sides (NOC and Us) in order to get an aggregate 3.0mbit. 
> PER-PACKET routing
> alternates usage of the T1s, one for one...
>
> Since then, VPN performance has taken a dive.  Sniffing out 
> traffic, ESP
> packets are sent 3-4 times before they can be properly decrypted. 
>
> Someone along the way said that using PER-PACKET routing 
> changes the CRC
> value of the packets.  Is this correct, has anyone else seen 
> this issue?  I
> can't see how the CRC is changed, the hop count isn't 
> changing, the lines
> are identical, and they terminate in the same router, so the 
> last hop is the
> F0/0 interface of the router before getting to the firewall.
>
> Thanks,
> ~Todd
>
> __________________________________
> Todd M. Simons
> Senior MIS Engineer
> Dell Tier 1 PA Technician 
> Delphi Technology, Inc.
> New Brunswick, NJ

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