Subject: Re: common blackholes, Re: I-D ACTION:draft-ietf-tcpimpl-pmtud-00.txt

From: Joe Touch <[email-masked]>
Date: Thu, 14 Jan 1999 11:21:06 -0800 (PST)

> From: John Hanley <[email-masked]>
> 
> Thanks for writing up these MTU issues.
> 
> FYI, there is a common form of blackhole which may be located very close
> to a TCP server (such as a web server).  It might be worth mentioning.
> 
> Various vendors have layer-4 "load balancers" which accept port 80 requests
> sent to a "virtual IP" and divvy up the work among multiple backend web
> servers.  Such a device will use a table or a socket identifier hash
> to map particular request packets to a particular server.  When such a
> device recieves an ICMP packet saying "fragmentation required", it is
> essential that is use that *same* mapping to send the ICMP to the
> proper backend server.  (Flooding to all servers Would Be Bad :-(

There are a whole slew of proiblems that are the result of this kind of
transparent caching, which break the MUSTs in the host- (1122,
standard) and gateway-requirements (1812, proposed standard) RFCs, and
the recommendations in the NAT (1631, informational) and load-sharing
(2391, informational) RFCs.

I'm collecting a list of such problems for an I-D for the (pending)
WREC (web replication and caching) WG.

The problems I know of are related to IPSEC, spoofing detectors,
traceroute and ping, and TCP control block sharing. This generalizes
the ICMP problems I know of for traceroute. 

If there are other known problems, please post or send me direct
e-mail, and I'll post the results here in a few weeks when they're
colleted.

Thanks,

Joe