Experiences with IPv6 and Routing Efficiency

Owen DeLong owen at delong.com
Tue Jan 21 21:58:43 UTC 2014

On Jan 21, 2014, at 02:52 , Frank Habicht <geier at geier.ne.tz> wrote:

> Hi Owen,
> On 1/21/2014 12:13 PM, Owen DeLong wrote:
>> On Jan 18, 2014, at 23:19 , Frank Habicht <geier at geier.ne.tz> wrote:
>>> c) v6 with a few extension headers
>> In this case, it will be at 40+o+n octets into the packet where o is the
>> number of octets contained in headers prior to the TCP header and n is
>> defined as in (b) above.
> my point tried to be that it can be hard for an ASIC to know 'o'
>>> now program a chip to filter based on this port number...
>> I think you might want to be more specific. After all, an ARM 9 is a
>> chip which can easily be programmed to do so (in fact, I can point to
>> iptables/ip6tables as running code which does this on the ARM 9).
> I was thinking about hardware that's forwarding packets "not in software"
> some of those boxes probably want to limit tcp ports 179 and 22.

The difference between hardware and software gets blurrier every day.

For example, many of the "forwarding ASICs" today are actually FPGAs with "software" loaded into them to do certain forwarding tasks.

Yes, I took it to extremes by proposing a more general purpose CPU, but the fact of the matter remains that traversing a header chain in software looking for a known header type that you care about actually isn't complex and can easily be implemented in hardware. The task boils down to:

octet i = &packet header;
while (headers_remaining && header_type != type_of_interest)
  header_type = i[NEXT_HEADER];
  i += (i[HDR_LEN]+1) * 8;
  if (header_type != 0 && header_type != 60 && header_type != 43 && header_type != 44 && header_type != 51 &&
	header_type != 50 && header_type != 60 && header_type != 135) headers_remaining = 0;
if (headers_remaining)
	INSPECT THIS HEADER and act accordingly;
	Punt, header not present;

Not a particularly complex program for a modern ASIC, actually.

Where you run into potential problems (and this can apply to IPv4 as well, though less likely) is if you get an IPv6 packet where the header chain is so unreasonably long that the header you want is not in the first fragment.

With a minimum MTU of 1280 octets, it's almost impossible to create a legitimate packet with a header chain so long that this would be an issue. That is one of the reasons that there are proposals to consider such packets invalid and why I support those proposals. However, those have nothing to do with ASIC difficulty. The fragment problem is hard to solve no matter how much CPU you throw at it because unless you are the end receiving host, you cannot reliably expect to be able to perform reassembly.

>> So... I suppose that whether your complaint has merit depends entirely
>> on whether or not extension headers become more common on IPv6 packets
>> than options have become on IPv4 packets or not and also on how hard it
>> is to build fast-path hardware that bypasses extension headers that it
>> does not care about. Since you only need to parse the first two fields
>                                 ^^^^ ?
>> of each extension header (Next Header Type and Header Length) 
> ... recursively for all extension headers ...

Which in anything but the most contrived of cases is likely n=0 for 95+% of all (unencrypted) packets and likely n<4 for all others.

Further, it's not recursively, it's repetitively. There is a difference. (See above. Note that the loop is _NOT_ recursive, it is repetitive. Recursive would be something like:

sub parse_next_header(header)
	char * i = header;
	register char nh = i[NExT_HEADER];
	if (nh != 0 && nh != 60 && nh != 43 && nh != 44 && nh != 51 && nh != 50 && nh != 60 && nh != 135)
		result = parse_next_header(i+(1+i[HDR_LEN])*8);
	// code to parse this header (modifies result accordingly)
	return (result);

This might be possible to do in an ASIC, but might be more difficult than the iterative solution proposed above.

>> to know
>> everything you need to bypass the current header, it shouldn't be too
>> hard to code that into a chip...
> who's done that so far?

I don't know.

> Up to what number of EHs or octet-length?

Why would the number of HEs or the octet-length (up to the maximum MTU supported by the box) matter in this question?

I understand that if you do it recursively, you get into potential stack resource issues, but with an iterative approach looking for the header of interest, I don't see the issue.


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