Re: [Idr] Benjamin Kaduk's Discuss on draft-ietf-idr-flow-spec-v6-19: (with DISCUSS and COMMENT)

Erik Kline <ek.ietf@gmail.com> Mon, 09 November 2020 04:35 UTC

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From: Erik Kline <ek.ietf@gmail.com>
Date: Sun, 8 Nov 2020 20:35:19 -0800
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To: Robert Raszuk <robert@raszuk.net>
Cc: Benjamin Kaduk <kaduk@mit.edu>, Christoph Loibl <c@tix.at>, "idr@ietf. org" <idr@ietf.org>, The IESG <iesg@ietf.org>, draft-ietf-idr-flow-spec-v6@ietf.org, idr-chairs@ietf.org
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Subject: Re: [Idr] Benjamin Kaduk's Discuss on draft-ietf-idr-flow-spec-v6-19: (with DISCUSS and COMMENT)
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Whether randomized or not, I would imagine the use of flow label matching
would be more the exception than the rule.

On Sat, Nov 7, 2020 at 3:45 AM Robert Raszuk <robert@raszuk.net> wrote:

> Ok 3 octets to keep byte boundary sounds reasonable indeed.
>
> > to keep the flow label matching functionality
>
> On that point I have a mixed feelings.
>
> On one hand original thought for adding this into flow-spec was based on
> manual setting of flow-labels.
>
> Now if that setting is recommended to be automated and random I am really
> afraid that any attempt to retrieve flow label values and to signal them
> with BGP FS may result in massive burden to BGP - at any given time there
> can be millions of flows. Leave alone dynamic nature of those short lived
> flows.
>
> Then even if BGP can signal it, RRs can propagate it then the attempt to
> install such number of ACLs into FS receiver's data plane will either fail
> or the box will crash.
>
> Cheers,
> R.
>
> On Sat, Nov 7, 2020 at 1:02 AM Erik Kline <ek.ietf@gmail.com> wrote:
>
>> FWIW, I had assumed that the 1/2/4 byte formats were for encoding the
>> non-zero parts of a flow label, i.e. flowlabels 1-255 could fit in 1 byte,
>> 256-65535 in 2 bytes, and everything else up to 2^20-1 in 4 bytes.
>>
>> If that's true, it seemed reasonable to me.  Also reasonable, I would
>> think, is to just specify, say, a fixed 3 byte unsigned int.
>>
>> I think it would be valuable to keep the flow label matching
>> functionality, not least because it can be used to group into a flow the
>> fragments where the upper layer header info is not present.
>>
>> On Fri, Nov 6, 2020 at 3:01 PM Robert Raszuk <robert@raszuk.net> wrote:
>>
>>> Hi,
>>>
>>> For the record of part of this discussion regarding size of flowspec v6
>>> flow label field I am perfectly ok to have it fixed size of 20 bit field.
>>>
>>> As a matter of fact if we assume that flow label allocation nature is to
>>> be dynamic by IETF recommendation I am also perfectly fine to remove flow
>>> label match from flow spec v6 all together.
>>>
>>> Kind regards,
>>> Robert.
>>>
>>> On Wed, Nov 4, 2020 at 5:36 PM Benjamin Kaduk <kaduk@mit.edu> wrote:
>>>
>>>> Hi Christoph,
>>>>
>>>> On Wed, Nov 04, 2020 at 12:41:13PM +0100, Christoph Loibl wrote:
>>>> > Hi Benjamin,
>>>> >
>>>> > Thanks for your review of the document. I have edited the document
>>>> and I think cleared your discuss with the changes (please see my comments
>>>> inline).
>>>>
>>>> Also inline, though I will not specifically comment on the (many) bits
>>>> that
>>>> look good as-is.
>>>>
>>>> > Since I have a few more changes to apply the new version is not yet
>>>> online (however I attached the modified version to this message). Since the
>>>> draft submission is closed now I will ask Alvaro to publish the changed
>>>> document today.
>>>> >
>>>> > Cheers  Christoph
>>>> >
>>>> >
>>>> > > ## DISCUSS:
>>>> > > A fairly minor point, but I think that allowing Type 13 (flow label)
>>>> > > component values to be encoded as 2-byte quantities encourages the
>>>> > > selection of non-random flow label values, and thus violates the
>>>> > > guidance from RFC 6437 that these values "should be chosen such that
>>>> > > their bits exhibit a high degree of variability" and that "third
>>>> parties
>>>> > > should be unlikely to be able to guess the next value that a source
>>>> of
>>>> > > flow labels will choose." While having the short 1-byte encoding
>>>> for a
>>>> > > flow label of 0 might be reasonable, a 2-byte label can represent at
>>>> > > most 16 bits of the 20-bit identifier space, discouraging the use
>>>> of the
>>>> > > high 4 bits, when such bits of unpredictability are scarce already.
>>>> > > Let's discuss how big an issue this is and what might be done to
>>>> > > mitigate it.
>>>> >
>>>> > The Type-13 component is not the flow label itself. It is only the
>>>> filter that allows to match on that particular field in the IPv6 header of
>>>> the packets. If the flow-label encoded in the packet (unlikely, but
>>>> possible) can be encoded in 8,16-bits one could encode the match filter in
>>>> a shorter form. However, this may be very unlikely. This specification does
>>>> not choose those flow labels, but allows to match on flow labels in packets
>>>> (usually generated by other systems).
>>>>
>>>> I understand that this specification is describing patterns to match
>>>> flow
>>>> labels and not the flow labels themselves, yes.  However, this document
>>>> is
>>>> saying that, *if* the flow labels themselves have a certain structure,
>>>> then
>>>> the flow specification (from this document) to match the flow labels
>>>> has a
>>>> more compact encoding.  This, in turn, sets up an incentive for the
>>>> party
>>>> assigning flow labels to conform to the specific structure of flow label
>>>> values, in order to take advantage of the more compact encoding.  I
>>>> believe
>>>> that using any kind of substructure when assigning flow label values is
>>>> contrary to the guidance of RFC 6437 (and, relately, harms the overall
>>>> security of the internet), and thus am supplying resistance to a
>>>> mechanism
>>>> that incentivizes using such internal structure in flow label values.
>>>> The discussion that I would like to have here relates to the magnitude
>>>> of
>>>> incentive in question (to use specific substructure when generating flow
>>>> label values), which in turn is related to how much benefit there is
>>>> from
>>>> shaving two bytes off the flow specification encoding.  I do not believe
>>>> that your comment above is trying to make such an assessment on the
>>>> magnitude of the incentive in question or the value of having the
>>>> slightly
>>>> more compact encoding available.
>>>>
>>>> > > Please also confirm that we are providing all the information
>>>> required of
>>>> > > us by RFC 5701 and 5575bis (see comments on Section 6.1); I am not
>>>> sure
>>>> > > whether I am reading the references correctly in these regards.
>>>> >
>>>> > We added the definition of the Local Administrator Field in the
>>>> Extended community to be aligned with the IPv4 rt community:
>>>> >
>>>> > ADDED:
>>>> >        The Local Administrator sub-field contains a number from a
>>>> numbering
>>>> >           space that is administered by the organization to which the
>>>> IP
>>>> >           address carried in the Global Administrator sub-field has
>>>> been
>>>> >           assigned by an appropriate authority.
>>>> >
>>>> > >
>>>> > > There seems to be an error in the sample code (flow_rule_cmp_v6()):
>>>> the
>>>> > > snippet
>>>> > > if comp_a.offset < comp_b.offset:
>>>> > > return A_HAS_PRECEDENCE
>>>> > > if comp_a.offset < comp_b.offset:
>>>> > > return B_HAS_PRECEDENCE
>>>> > > duplicates the condition, whereas the condition should be swapped
>>>> for
>>>> > > correct operation.
>>>> >
>>>> > This in fact is a problem in the code that has been corrected, and a
>>>> test has been added.
>>>>
>>>> I am happy to hear that a test has been added in addition to fixing the
>>>> bug.
>>>>
>>>> > > ## COMMENT:
>>>> > > Thanks for this straightforward and easy-to-read document! Just a
>>>> few
>>>> > > minor comments.
>>>> > >
>>>> > > Section 3.3
>>>> > >
>>>> > > This component uses the Numeric Operator (numeric_op) described in
>>>> > > [I-D.ietf-idr-rfc5575bis] Section 4.2.1.1. Type 3 component values
>>>> > > SHOULD be encoded as single octet (numeric_op len=00).
>>>> > >
>>>> > > Why only SHOULD? (Likewise for Section 3.4, 3.5, etc.)
>>>> > >
>>>> >
>>>> > This is because we want these types to be aligned with the IPv4 types
>>>> as much as possible in order to allow for reusable code.
>>>>
>>>> Ah, of course.  I should have checked that before commenting; sorry.
>>>>
>>>> > > Section 3.7
>>>> > >
>>>> > > Contains a list of {numeric_op, value} pairs that are used to match
>>>> > > the 20-bit Flow Label IPv6 header field ([RFC8200] Section 3).
>>>> > >
>>>> > > [It seems that RFC 8200 §3 just points to RFC 8200 §6, which itself
>>>> > > mostly points to RFC 6437. I don't know if it is useful to
>>>> > > short-circuit some of those references.]
>>>> >
>>>> > Flow Spec does not care about what and how this Flow Label is
>>>> actually encoded. It matches against this particular 20-bit field in the
>>>> header. I think it is better to keep this reference.
>>>> >
>>>> > > Section 3.8.1
>>>> > >
>>>> > > The following example demonstrates the prefix encoding for: "packets
>>>> > > from :🔢5678:9A00:0/64-104 to 2001:DB8::/32 and upper-layer-
>>>> > >
>>>> > > Where is the "/64-104" notation introduced? I did not see it in RFC
>>>> > > 4291. (It also appears in §3.8.2, though the latter uses "/65-104"
>>>> > > to demonstrate the operation of padding.)
>>>> >
>>>> > Appended this to the Type-1 compontent section:
>>>> >
>>>> >    Note: This Flow Specification component can be represented by the
>>>> >    notation ipv6address/length if offset is 0, or ipv6address/offset-
>>>> >    length.  The ipv6address in this notation is the textual IPv6
>>>> >    representation of the pattern shifted to the right by the number of
>>>> >    offset bits.  See also Section 3.8.
>>>>
>>>> Thanks!
>>>>
>>>> > >
>>>> > > Section 4
>>>> > >
>>>> > > The definition for the order of traffic filtering rules from
>>>> > > [I-D.ietf-idr-rfc5575bis] Section 5.1 is reused with new
>>>> > > consideration for the IPv6 prefix offset. As long as the offsets are
>>>> > > equal, the comparison is the same, retaining longest-prefix-match
>>>> > > semantics. If the offsets are not equal, the lowest offset has
>>>> > > precedence, as this flow matches the most significant bit.
>>>> > >
>>>> > > I will note just to confirm my understanding that this procedure
>>>> seems
>>>> > > to give higher precedence to, e.g., 1234::/1-4 than to
>>>> > > :🔢5678:9a00/81-128 even though the latter is inspecting more bits
>>>> > > of the address/prefix. (To be clear, some choice has to be made and
>>>> I
>>>> > > have no reason to prefer a different one over this one, I am just
>>>> > > exploring the consequences of this choice.)
>>>> >
>>>> > Yes this is the case. In general the lower offset generates a lower
>>>> number of consecutive IPv6 address blocks that would match (offset=0
>>>> matches IP addresses only from a single prefix, offset=1 matches 2 blocks,
>>>> offset=2 4, ...), this can be extended to offset=127 which matches every
>>>> second IPv6 address.
>>>> >
>>>> > > It is again (as per 5575bis) surprising that we allow for a
>>>> "not-equal"
>>>> > > comparison of differently encoded (e.g., flow label) values that
>>>> have
>>>> > > the same semantic meaning, but I expect the same response (and no
>>>> > > document change) as when I made the comment the first time.
>>>> >
>>>> > While we add IPv6 functionality into Flow-Spec (rfc5575bis) changing
>>>> any of the fundamental behaviour is out of the scope of this document.
>>>> >
>>>> > > Section 5
>>>> > >
>>>> > > The validation procedure is the same as specified in
>>>> > > [I-D.ietf-idr-rfc5575bis] Section 6 with the exception that item a)
>>>> > > of the validation procedure should now read as follows:
>>>> > >
>>>> > > Are there also any AFI/SAFI differences from 5575bis to consider in
>>>> > > terms of "routes received over"/etc.?
>>>> > >
>>>> >
>>>> > Flow-Spec V6 uses a different AFI/SAFI but this is already covered in
>>>> the rfc5575bis text when it comes to validation:
>>>> >
>>>> >    The validation process described below validates Flow
>>>> Specifications
>>>> >    against unicast routes received over the same AFI but the
>>>> associated
>>>> >    unicast routing information SAFI:
>>>>
>>>> Thank you for the extra explanation; I suspected that there was
>>>> something
>>>> like this that I was missing.
>>>>
>>>> >
>>>> > > Section 6.1
>>>> > >
>>>> > > This extended community uses the same encoding as the IPv6 address
>>>> > > specific Route Target extended community [RFC5701] Section 2 with
>>>> the
>>>> > > high-order octet of the Type always set to 0x80 and the Sub-Type
>>>> > > always TBD.
>>>> > >
>>>> > > RFC 5701 suggests that since we are allocating the "TBD" sub-type
>>>> value,
>>>> > > we should also state what the semantics of the "local administrator"
>>>> > > 2-octet field are.
>>>> > >
>>>> >
>>>> > This has been edited into the document. See also the DISCUSS section.
>>>> >
>>>> > > Interferes with: All BGP Flow Specification redirect Traffic
>>>> > > Filtering Actions (with itself and those specified in
>>>> > > [I-D.ietf-idr-rfc5575bis] Section 7.4).
>>>> > >
>>>> > > I think we are supposed to also state what action to take when
>>>> > > encountering interfering actions.
>>>> >
>>>> > This is already in rfc5575bis Section 7.7, and also applies to this
>>>> document.
>>>> >
>>>> > >
>>>> > > Section 7
>>>> > >
>>>> > > It was a little surprising to me that the inability to match (e.g.)
>>>> TCP
>>>> > > flags or Port values on non-initial IP fragments was not reiterated
>>>> in
>>>> > > the security considerations of 5575bis. I don't think it would make
>>>> > > sense to mention that just in this document's security
>>>> considerations,
>>>> > > though -- if we are to change anything it should be in 5575bis. But
>>>> I
>>>> > > guess we had that conversation already, at
>>>> > >
>>>> https://mailarchive.ietf.org/arch/msg/idr/zvpLPjllvKWFuyraZUVz8lB8Wz0/
>>>> > > and thus no change is expected.
>>>> >
>>>> > While IPv4 and IPv6 fragmentation serves the same pupose the encoding
>>>> is different so we needed to redefine thi section. FS is working stateless
>>>> on the packets and can only match on what is in the packet.
>>>>
>>>> Exactly.  I am proposing that this restriction (working stateless and
>>>> cannot match [application-layer headers] things not in the packet) is
>>>> mentioned again in the security considerations section.  I believe that
>>>> this limitation can be described in a way that is agnostic to IPv4 vs
>>>> IPv6,
>>>> and thus would be better placed in 5575bis than in this document.
>>>>
>>>> > I cannot access the url from above.
>>>>
>>>> Huh, interesting.  Perhaps if you log in with your datatracker
>>>> credentials?
>>>> I had specifically tried to find one in the idr archive, since the iesg
>>>> archive is definitely non-public.
>>>> The referred-to message is "Re: [Idr] Benjamin Kaduk's Discuss on
>>>> draft-ietf-idr-rfc5575bis-22: (with DISCUSS and COMMENT)" from  Fri, 24
>>>> April 2020 02:12 UTC, Message-ID: <20200424021209.GR27494@kduck.mit.edu
>>>> >.
>>>>
>>>> Thanks again,
>>>>
>>>> Ben
>>>>
>>>> > >
>>>> > > Section 8.1.2
>>>> > >
>>>> > > (side note) it seems a little weird to have the IPv4 version be the
>>>> one
>>>> > > that gets the unqualified (e.g., "Destination Prefix") name.
>>>> >
>>>> > This is because we are not changing names of rfc5575bis to avoid
>>>> confusion.
>>>> >
>>>> >
>>>>
>>>>