Re: [Teas] binary wasRe: Yangdoctors early review of draft-ietf-teas-yang-te-types-03 (was -01)

["Tarek Saad (tsaad)" <[e-mail]>]

Hi Jan and Tom,



Thank you much for the review and additional comments. We've uploaded version -04 and -05 of the draft that attempts to address your comments.

Please see inline [TS2] for resolution to raised comments inline below.



    ----- Original Message -----

    From: "Jan Lindblad" <[e-mail]>

    To: "Tarek Saad (tsaad)" <[e-mail]>

    Cc: <[e-mail]>;

    <[e-mail]>; <[e-mail]>;

    <[e-mail]>

    Sent: Monday, January 21, 2019 10:10 AM

    Subject: Re: [Teas] Yangdoctors early review of

    draft-ietf-teas-yang-te-types-03 (was -01)





    Tarek, team,



    > Hi Jan

    >

    > Thanks again for your thorough review and comments. We have uploaded

    version -03 of the draft that addresses your comments. Please see Inline

    [TS] for the resolution action taken.

    > Please let us know if you have any further comments.



    Very good to see progress. There are still a few things I think we

    should discuss. Look for [janl2] inline below.





    > From: Jan Lindblad <[e-mail] <mailto:[e-mail]>>

    > Date: Monday, November 5, 2018 at 4:20 AM

    > To: Tarek Saad <[e-mail] <mailto:[e-mail]>>

    > Cc: "[e-mail] <mailto:[e-mail]>"

    <[e-mail] <mailto:[e-mail]>>,

    "[e-mail]

    <mailto:[e-mail]>"

    <[e-mail]

    <mailto:[e-mail]>>, "[e-mail]

    <mailto:[e-mail]>" <[e-mail] <mailto:[e-mail]>>,

    "[e-mail] <mailto:[e-mail]>" <[e-mail]

    <mailto:[e-mail]>>

    > Subject: Re: Yangdoctors early review of

    draft-ietf-teas-yang-te-types-01

    > Resent-From: <[e-mail] <mailto:[e-mail]>>

    > Resent-To: Tarek Saad <[e-mail] <mailto:[e-mail]>>,

    <[e-mail] <mailto:[e-mail]>>,

    <[e-mail] <mailto:[e-mail]>>,

    <[e-mail] <mailto:[e-mail]>>,

    <[e-mail] <mailto:[e-mail]>>,

    <[e-mail] <mailto:[e-mail]>>, <[e-mail]

    <mailto:[e-mail]>>, <[e-mail]

    <mailto:[e-mail]>>, <[e-mail]

    <mailto:[e-mail]>>, <[e-mail]

    <mailto:[e-mail]>>, Lou Berger <[e-mail]

    <mailto:[e-mail]>>

    > Resent-Date: Monday, November 5, 2018 at 4:19 AM

    >

    > Tarek,

    >

    > See my comments below [janl].

    >

    >>     General questions:

    >>

    >>     #1: There are many locations in the YANG talking about an "ERO

    subobject index"

    >>     (and once RRO index, record route subobject). What is this, and

    how is it

    >>     supposed to be used? The document is silent on this matter, and I

    have seen

    >>     modules with problems around numeric index leafs much like this

    earlier. Are

    >>     these numbers stable, i.e. remains the same forever?

    >>

    >> [TS]: The list of explicit route objects defines a TE path (e.g.

    hops), where the index is used as a key to identify a specific entry in

    the list. A lower index implies the TE path traverses this entry

    earlier. We will clarify the description and remove mention of

    “subobject” in the next update to the document.

    >

    > [janl] Very good re description changes.

    >

    > I'm a little worried that numeric indices will end up in the same

    quagmire+impasse where ACLs have been the last decades. Isn't there a

    risk that people will want to insert entries between two consecutive

    indices? Is "renumber" an operation that someone might ask for on these

    indices?

    > [TS]: The list is ordered by the key (index).. We’ve added a note in

    the description that entries with lower index are visited first by the

    path. It is understandable that the list may have to be reconfigured if

    an entry can not be fit in between two existing ones. Note, a change in

    the configured ERO list is generally handled as non-destructive event to

    an existing/provisioned tunnel (e.g. handled as make-before-break).

    >

    >       leaf index {

    >         type uint32;

    >         description

    >           "Route object entry index. A lower index indicates

    >            path traverses the hop earlier than the higher index

    >            hop(s)";



    [janl2] Thanks for the clarification. My strong advice in the situation

    you describe would be to change the lists with a uint32 index key to an

    ordered-by user list with a string key.

[TS2]: we discussed this amongst the authors. We have agreed that ordered-by user is a good option to define ordering of the entries in the list. We also agreed to leave the uint32 index as a distinguisher of entries only – as opposed to a string key.



    Several decades' experience attempting to automate management of ACLs

    keyed by integers speak quite strongly against that solution. The IETF

    standard YANG modules for ACLs have now abandoned this approach and

    adopted ordered-by user lists with string keys.



    Let's say a manager has pushed a path to a set of devices. Now it needs

    to configure a re-route around a network problem. This means replacing

    one of the configured legs with five legs. Now, if they are keyed by

    integers, this may make it necessary to renumber rules to make this

    re-route fit. Of course an "experienced" manager may leave some room in

    between rules so that this renumbering doesn't happen so often. The

    problem is that if it could ever happen, whoever programs the manager

    still needs to write (and test) the code for this. Then there are

    references to these rules (exclude, include) that also need to be

    updated if this happens. More code, more test. And if a human goes in

    and makes this change manually on a device, the manager no longer has

    any clue as to what happened. All it sees is a bunch of new rules with

    new rule names; and a bunch of rules with the same names, many of which

    have different content; it has no way of learning the intent. What has

    been added, changed and removed? When nothing is stable, reconciliation

    of a change becomes pretty much impossible. ACL management is

    notoriously inefficient today, largely due to this seemingly small (and

    completely unnecessary) problem with the management approach.



    The fix is simple. Make sure rule identities do not also double as

    sequencing information. If identifiers are also used to convey important

    configuration information, so that existing identifiers must change

    whenever the intent is adjusted, that leads to an expensive solution. In

    YANG, there is a mechanism that is made for this situation. That's

    order-by user lists. In such lists, the order of entries is controlled

    by the user (operator, programmer) without relying on the key value(s)

    to determine that order. The user can insert any number of rules before

    or after any other, or first or last in the list. The user can move

    entries around without changing their names.



    This makes the device interface significantly easier to use for both

    operators and programmers. While integer rule names are still possible,

    the string type is better as some folks might falsely believe the an

    integer key would be used as a sorting metric.



    To fix this in the model, the lists currently keyed by leaf index would

    look something like this instead:



      list example-route-state {

        ordered-by user;

        key name;

        uses record-route_state;

        ...

      }

[TS2]: thanks for the detailed explanation and sharing your experience on this. We agreed “ordered-by-user” is acceptable to define the order of items in the list and index key is just to distinguish the different entries with no additional ordering meaning. We’ve added ordered-by-user to the route-object lists using index to address this.



NEW:

        list path-route-object {

          key index;

          ordered-by user;

          description

            "List of route objects either returned by the computation

             engine or actually used by an LSP";

          leaf index {

            type uint32;

            description

              "Route object entry index. The index is used to

               identify an entry in the list. The order of entries

               is defined by the user without relying on key values";







    Where the record-route_state looks like this:



      grouping record-route_state {

        description

          "The record route grouping";

        leaf name {

          type string;

          description

            "Record route hop name. The name is used to

             identify an entry in the list. Records listed

             earlier in the list means the path traverses it earlier";

        }



    I'm not sure I understand the author's intent perfectly, but my

    impression is that some of the lists keyed by index are meant to refer

    to route-objects in other lists. Like list route-object-include-object

    entries would only make sense when they point to (use the same index as)

    an existing route-object. Is that so? Any such reference should be

    modeled using the leafref type, and a path pointer to the list that

    contains relevant entries. E.g. like this:



        list route-object-include-object {

          key name;

          description

            "List of explicit route objects to be included

             in path computation";

          leaf name {

            type leafref {

              path "/path/to/the/rule/name"; // fill in which list we're

    pointing to here

            }

            description

              "Route object entry name. Points out a route object

               in list xxxx that will be included in the path.";

          }



    Let me know if this makes sense. Happy to discuss and/or explain

    further.



    >>     #2: There are few leafs (5) with default values given, and none

    with mandatory.

    >>     Probably needs to increase before we get to last call.

    >> [TS]: In general, the team tried to avoid setting defaults unless it

    is strictly dictated by another RFC/standard. The team will review again

    to see if any was missed and will take action.

    >

    > [janl] That's all fine. Just remember to describe what happens when a

    leaf has no value every time there is no default or mandatory. Easy to

    forget, experience shows. Missing this causes non-interoperability in

    the standard.

    > [TS]: the team has opted to assign defaults to all optional leafs in

    the latest revision of the draft.



    [janl2] Very good.



    >>     #3: There are several choices in the module that are meant to be

    augmented with

    >>     additional cases. In many instances, this is explicitly spelled

    out, very good.

    >>     If this is meant to happen in all choices, it would be nice to

    point this out

    >>     in every instance. Also, if there are any specific assumptions or

    >>     considerations to keep in mind when augmenting in a new

    technology, please note

    >>     that in the description as well.

    >> [TS]: yes, we will update as necessary.

    >

    > [janl] Great!

    >

    >>     Issues and nits:

    >>

    >>     #4: Unclear data type

    >>     419:

    >>       typedef admin-group {

    >>         type binary {

    >>           length 4;

    >>

    >>     What is the format of this binary? If this is always a 4-byte

    binary, wouldn't

    >>     a numeric type be more user friendly, e.g. uint32?

    >> [TS]: the standard defines admin group as 4-octet binary (9 -

    Administrative group (4 octets) ), and for extended-admin-group as

    series of those. We are also defining a union to encompass both as

    admin-groups below. Do you still have concerns on this?

    >>

    >>   typedef admin-groups {

    >>     type union {

    >>       type admin-group;

    >>       type extended-admin-group;

    >>     }

    >>     description "TE administrative group derived type";

    >>   }

    >

    > [janl] I'm have two concerns. The first is that I don't know what the

    format of the type is. How do I know how to construct one of these

    values, or interpret an existing one? Is this a randomly chosen value?

    The other concern is how an operator would enter this value. The binary

    type is not the most user friendly of types. A numeric representation in

    the management interface might make better sense.

    > [TS]: RFC3630 and RFC5305 defines admin-groups (aka affinity colors)

    as binary flags that are user specified (e.g. bit-position X is for RED,

    and bit-position Y is for GREEN). So, binary type would be closest to

    this representation. On other hand, RFC7591 states that a binary value

    is represented as a JSON string -- base64 encoding

    (https://tools.ietf.org/html/rfc7951#section-6.6

    <https://tools.ietf.org/html/rfc7951#section-6.6>). Let us know if there

    are still concerns on this.



    [janl2] The YANG binary type is mostly used for large binary chunks that

    an operator would never type, e.g. a 2048-bit RSA key or a new software

    image. As I understand, the admin-group type is used for something an

    operator might configure or read. If so, I would suggest changing this

    type. If this is a 4-octet value, an uint32 would work nicely, would it

    not? "2149847551" Or a dotted-quad? "128.36.17.255" Or maybe you could

    invent a more human friendly interface, e.g. a string of zeros and ones

    with dashes at strategic positions?

    "10000000-00100100-00010001-11111111"



[TS2]: OK, we’ve reviewed this and we agreed hexstring type is easier for users to pass/respresent such information. We’ve made this change to existing leafs with binary type to use hexstring.



NEW:



  typedef admin-group {

    type yang:hex-string {

      /* 01:02:03:04 */

      length "1..11";

    }

    description

      "Administrative group/Resource class/Color representation in

       hex-string type.";

    reference "RFC3630 and RFC5305";

  }



    If you leave this as type binary, at least don't expect operators to be

    able to enter these values with reasonable effort. The binary type is

    not meant for this.



    I noticed you also modeled the leaf as-number as type binary. This is

    probably not a good idea. Suggest using uint16 or uint32.



[TS2]: fixed this to use:

NEW:

          leaf as-number {

            type inet:as-number;

            mandatory true;

            description "The AS number";

          }



    >>     #5: identifier in container with optional leafs

    >>     1496:

    >>       grouping te-topology-identifier {

    >>

    >>     The name suggests this is used as an identifier, but all the

    leafs are

    >>     optional. This is not typical. They are also in a container,

    precluding them

    >>     from being used as list keys. Is that as intended?

   >> [TS]: this container is not meant to be reused for a list, rather

    contains ID(s) that together can be used to lookup a specific topology

    that the TE tunnel is using. We can clarify this in the description

    >

    > [janl] Hmm, ok. Still not sure what happens if you omit some value(s),

    but I guess I should wait to see your clarification.

    > [TS]: we have addressed this by specifying defaults for the optional

    leafs.



    [janl2] Nice.



    >>     #6: Optional -id leafs again

    >>     1700:

    >>               leaf node-id {

    >>               leaf link-tp-id {

    >>     1768:

    >>             leaf address {

    >>     1783:

    >>             leaf node-id {

    >>             leaf link-tp-id {

    >>

    >>     Leafs that appear to be used as identifiers are optional

    >> [TS]: such IDs identify a specific node, link, or address in the

    network but in our applications we do not to use them as keys in such

    lists. For example list explicit-route-objects defines a separate index

    leaf to be used as key -- since, a path may contain the same node-id or

    address repeated multiple times (looping path)..

    >

    > [janl] So what happens if you omit specifying some of them?

    > [TS]: This was reworked and added a mandatory to leafs that are must,

    and added defaults to optional leafs.



    [janl2] ok.



    >>     #7: binary length in bits?

    >>     1731:

    >>                leaf as-number {

    >>                 type binary {

    >>                   length 16;

    >>     1773:

    >>             leaf ip-flags {

    >>               type binary {

    >>                 length 8;

    >>     1805:

    >>               leaf label-flags {

    >>                 type binary {

    >>                   length 8;

    >>

    >>     It appears to me the modeler might have thought the length is

    given in bits.

    >>     The value of length is in bytes, however.

    >> [TS]: good catch, thanks. We'll correct it to indicate number of

    octets as RFC6020 defines.

    >>

    >>     #8: Must expression copy paste

    >>     1852:

    >>         container label-end {

    >>           must "not(../label-end/te-label/direction) or "

    >>             + "not(te-label/direction) "

    >>             + "or ../label-end/te-label/direction =

    te-label/direction" {

    >>

    >>     This must expression appears to have been copied from

    label-start. In any case,

    >>     it always evaluates to true and has no effect.

    >> [TS]: thanks, we can see the bug in the expression as is, and this

    will be fixed/updated.

    >>

    >>         container label-end {

    >>           must "not(../label-start/te-label/direction) or "

    >>             + "not(te-label/direction) "

    >>             + "or ../label-start/te-label/direction =

    te-label/direction" {

    >

    > [janl] This revised must statement is superfluous. The must statement

    on container label-start already covers this case.

    > [TS]: addressed



    [janl2] This may not work as intended anymore. The must expression is

    fine in itself, but now the direction leaf has a default (forward),

    which means it will always exist with one value or another. The case

    that the   not(.... /direction)   expressions test for will therefore

    never happen. Only the last part (direction = direction) will be

    relevant, and I'm not sure that's what you wanted.



[TS2: I’ve updated the check to accommodate the default case.



NEW:

    container label-start {

      must "(not(../label-end/te-label/direction) and" +

                    " not(te-label/direction))"

        + " or "

        +  "(../label-end/te-label/direction = te-label/direction)"

        + " or "

        +  "(not(te-label/direction) and" +

                    " (../label-end/te-label/direction = 'forward'))"

        + " or "

        +  "(not(../label-end/te-label/direction) and" +

                    " (te-label/direction = 'forward'))" {

        error-message

          "label-start and label-end must have the same direction.";

      }



<snip>

    container label-end {

      must "(not(../label-start/te-label/direction) and" +

                    " not(te-label/direction))"

        + " or "

        +  "(../label-start/te-label/direction = te-label/direction)"

        + " or "

        +  "(not(te-label/direction) and" +

                    " (../label-start/te-label/direction = 'forward'))"

        + " or "

        +  "(not(../label-start/te-label/direction) and" +

                    " (te-label/direction = 'forward'))" {

        error-message

          "label-start and label-end must have the same direction.";

      }



    >>     #9: Unclear bit field

    >>     1885:

    >>         leaf range-bitmap {

    >>           type binary;

    >>           description

    >>             "When there are gaps between label-start and label-end,

    >>              this attribute is used to specify the positions

    >>              of the used labels.";

    >>         }

    >>

    >>     Need more information on how to interpret this leaf. Which bits

    map to what,

    >>     and what does the bit field values 0 and 1 indicate?

    >> [TS]: ]: Each bit-position in the bitmap will map to an offset from

    the label-start. For example, if label-start=16000 and bitmap=0x11, then

    labels={16000, 16004} are relevant. We will clarify this in the next

    update.

    >

    > [janl] Very good. Since you are using type binary and refer to the

    contents using integers, you should also specify how you map those

    integer values to bit positions in the binary. E.g. big-endian?

    > [TS]: We have clarified this in the description of the leaf that it is

    big-endian representation



    [janl2] Good that you added a description, but it doesn't make me much

    wiser, I'm afraid. Even if there is a reference to RFC7951 (which is

    only applicable to RESTCONF, btw), the new description doesn't describe

    what the values mean (even when encoded according to 7951s6.6).



    Try to describe the algorithm I, as operator or network automation

    programmer, should use to figure out what values to use here. The

    encoding is also important, but if this is something operators should

    enter, the binary type isn't what you want. If you pick a different

    type, the encoding will probably fall out automatically.



[TS2]:  I added the below description and example to describe the algorithm.



NEW:

    leaf range-bitmap {

      type yang:hex-string;

      description

        "When there are gaps between label-start and label-end,

         this attribute is used to specify the positions

         of the used labels. This is represented in big-endian as

         hex-string.



         Each bit-position in the range-bitmap hex-string maps to a

         label in the range derived from the label-start.



         For example, assuming label-start=16000 and range-bitmap=0x01000001,

         then:

          - bit-position(0) is set, and the corresponding mapped label

            from the range is: 16000 + (0 * label-step) or

            16000 for default label-step=1.

          - bit-position(24) is set, and the corresponding mapped label

            from ihe ranage is: 16000 + (24 * label-step) or

            16024 for defautl label-step=1";

    }

  }



Regards,

Tarek



    Best Regards,

    /jan





    >

    > Regards,

    > Tarek

    >>     #10: Canonical representation

    >>     67:

    >>       typedef te-bandwidth {

    >>

    >>     The type is based on a string with a pattern allowing hex

    characters and an

    >>     upper or lowercase P. Since the pattern allows multiple

    representations of the

    >>     same underlaying value (0x1p10 presumably means the same as

    0x1p0xa and

    >>     0x1P0XA) the question comes up if there is a canonical

    representation of this

    >>     value, e.g. using all lowercase and all hex, or if the string

    must be

    >>     remembered exactly as given by the client. The description could

    answer this

    >>     question.

    >> [TS]: We will update the description to indicate more strict

    canonical form (all upper case).

    >>

    >>     #11: Mix of upper and lowercase

    >>     The module specifies many enumeration and identity values. Some

    are all

    >>     lowercase. Some are all uppercase. The principle of least

    astonishment suggests

    >>     to pick one and stick with it. YANG recommendations suggest to

    use all

    >>     lowercase when in doubt.

    >>

    >>       typedef te-link-direction {

    >>       typedef te-label-direction {

    >>       typedef te-hop-type {

    >>       identity LSP_METRIC_TYPE {

    >>       identity LSP_METRIC_RELATIVE {

    >>       identity LSP_METRIC_ABSOLUTE {

    >>       identity LSP_METRIC_INHERITED {

    >> [TS]: we will update to all lower-case to align with YANG

    recommendations.

    >

    > [janl] Great!

    >

    > /jan









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