Re: Fwd: New Version Notification for draft-hinden-6man-hbh-processing-01.txt

Brian E Carpenter <> Thu, 10 June 2021 22:20 UTC

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Subject: Re: Fwd: New Version Notification for draft-hinden-6man-hbh-processing-01.txt
To: Tom Herbert <>, Fernando Gont <>
Cc: "" <>, "" <>, "" <>
References: <> <> <> <>
From: Brian E Carpenter <>
Message-ID: <>
Date: Fri, 11 Jun 2021 10:20:25 +1200
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On 11-Jun-21 03:07, Tom Herbert wrote:
> On Thu, Jun 10, 2021 at 12:41 AM Fernando Gont
> <> wrote:
<big snip>
> If you are pursuing that path, you should also limite the overall IPv6
> header chain length. That;s probably even more important than limiting
> the number of EHs or number of HbH options.
>> Fernando,
>> That limit and other pertinent ones are defined in RFC8504 section 5.3
>> and RFC8883. The limits in RFC8504 are specified for hosts, but it
>> would be straightforward to apply them to routers  where the behavior
>> when a limit is exceeded would be different-- end hosts should drop
>> packets that exceed the limit, intermediate nodes should stop parsing
>> and forward the packet. If a node decides to drop the packet then it
>> can send an RFC8883 ICMP error to inform the sender what the exceeded
>> limit was. Recommended defaults can be provided for limits; e.g. in
>> RFC8504 the limit of HBH or DestOpts is eight meaning that receivers
>> should support up to eight options and senders should be able to send
>> up to eight options with reasonable confidence their packets won't be
>> dropped. A similar default could be established for length of IP
>> header chain (for hosts this isn't immediately necessary since hosts,
>> unlike routers, usually don't have a concept of fixed sized parsing
>> buffer holding headers, although with hardware acceleration that limit
>> might be more applicable to set in the host).

Which all, IMHO, shows that the Internet cannot ever be assumed to be
transparent to extension headers of any kind; either we accept that
extensions are confined to limited domains or we need a probing process
before using them, to determine whether they can survive the trip.

Not much has changed (and it's exactly the same for IPv4 options): unusable
across the Internet.