Re: [bess] Encoding a 20 bit label in a 24 bit field.

"Ali Sajassi (sajassi)" <> Mon, 22 October 2018 22:32 UTC

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From: "Ali Sajassi (sajassi)" <>
To: Greg Mirsky <>, Stephane Litkowski <>
CC: "Jakob Heitz (jheitz)" <>, "" <>, BESS <>
Thread-Topic: [bess] Encoding a 20 bit label in a 24 bit field.
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Date: Mon, 22 Oct 2018 22:32:37 +0000
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Subject: Re: [bess] Encoding a 20 bit label in a 24 bit field.
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Hi Greg,
The text that Stephane mentioned is from “MVPN for PMSI tunnel attribute” and not from proposed text for RFC 7432 errata.

Hi Stephane,
I think the provided text in Errata below is sufficient and we don’t need to say “The lower order 4 bits SHOULD be sent as 0 and ignored on receipt." In case of penultimate hop pop and need to use EXP bits for QoS.


From: Greg Mirsky <>
Date: Monday, October 22, 2018 at 3:07 PM
To: Stephane Litkowski <>
Cc: Cisco Employee <>, "Jakob Heitz (jheitz)" <>, "" <>, BESS <>
Subject: Re: [bess] Encoding a 20 bit label in a 24 bit field.

Hi Stephane,
should note that "setting the MPLS Label field to zero" may be also interpreted as a valid label, IPv4 Explicit Null Label.


On Mon, Oct 22, 2018 at 6:47 AM <<>> wrote:

Does anyone disagree with the additional Jakob's statement proposal ?
" The lower order 4 bits SHOULD be sent as 0 and ignored on receipt."

As an example, in MVPN for PMSI tunnel attribute, we have the following statement which does not tell anything about the lower order bits:
" If the MPLS Label field is non-zero, then it contains an MPLS label
   encoded as 3 octets, where the high-order 20 bits contain the label
   value.  Absence of an MPLS Label is indicated by setting the MPLS
   Label field to zero."


-----Original Message-----
From: BESS [<>] On Behalf Of Ali Sajassi (sajassi)
Sent: Tuesday, October 16, 2018 19:30
To: Jakob Heitz (jheitz); Zhuangshunwan; BESS
Subject: Re: [bess] Encoding a 20 bit label in a 24 bit field.

Yes, just the encoding of label value needs to be clear.


On 10/15/18, 6:24 PM, "BESS on behalf of Jakob Heitz (jheitz)" <<> on behalf of<>> wrote:

    How about:
    The lower order 4 bits SHOULD be sent as 0 and ignored on receipt.


    -----Original Message-----
    From: Zhuangshunwan <<>>
    Sent: Monday, October 15, 2018 6:02 PM
    To: Jakob Heitz (jheitz) <<>>; BESS <<>>
    Subject: RE: Encoding a 20 bit label in a 24 bit field.

    It is good to make this explicit. This ambiguity has led to some unnecessary interworking problems.

    Should we also need to explicitly define the "bottom of stack" bit in the low-order bit of the 3-octet label field?


    -----Original Message-----
    From: BESS [<>] On Behalf Of Jakob Heitz (jheitz)
    Sent: Tuesday, October 16, 2018 4:21 AM
    To: BESS <<>>
    Subject: [bess] Encoding a 20 bit label in a 24 bit field.

    We have proposed the following erratum for RFC 7432.



    -----Original Message-----
    From: RFC Errata System <<>>
    Sent: Friday, October 12, 2018 12:37 PM
    To: Ali Sajassi (sajassi) <<>>;<>;<>;<>;<>;<>;<>;<>;<>;<>; Giles Heron (giheron) <<>>;<>
    Cc: Krishnamoorthy Arumugham (karumugh) <<>>;<>;<>
    Subject: [Technical Errata Reported] RFC7432 (5523)

    The following errata report has been submitted for RFC7432, "BGP MPLS-Based Ethernet VPN".

    You may review the report below and at:

    Type: Technical
    Reported by: Krishnamoorthy Arumugham <<>>

    Section: 7

    Original Text
    Clarifications to following sub-sections:
    Section 7.1
    Section 7.2
    Section 7.5

    Corrected Text
    Section 7.1:
    Add below text to the section 7.1 regarding the encoding of MPLS label:

    "The value of the 20-bit MPLS label is encoded in the high-order 20 bits of the 3 bytes MPLS Label field."

    Section 7.2:
    Add below text to the section 7.2 regarding the encoding of both the MPLS label fields:

    "The value of the 20-bit MPLS label is encoded in the high-order 20 bits of the 3 bytes MPLS Label field for both MPLS Label1 and MPLS Label2."

    Section 7.5:
    Add below text to the section 7.5 regarding the encoding of ESI Label fields:

    "The value of the 20-bit MPLS label is encoded in the high-order 20 bits of the ESI Label field."

    MPLS label is a 20-bit value and is stored in a 3 bytes field in a packet. The 20-bit MPLS label value is generally stored in higher order 20 bits of the 3 byte label field. The exact encoding to be followed for storing MPLS label values are not explicitly mentioned in the RFC 7432 under section 7.1, 7.2 and 7.5 for different types of EVPN routes. This lead to ambiguity in different implementations. Hence a clarification is required.

    This erratum is currently posted as "Reported". If necessary, please
    use "Reply All" to discuss whether it should be verified or
    rejected. When a decision is reached, the verifying party
    can log in to change the status and edit the report, if necessary.

    RFC7432 (draft-ietf-l2vpn-evpn-11)
    Title               : BGP MPLS-Based Ethernet VPN
    Publication Date    : February 2015
    Author(s)           : A. Sajassi, Ed., R. Aggarwal, N. Bitar, A. Isaac, J. Uttaro, J. Drake, W. Henderickx
    Category            : PROPOSED STANDARD
    Source              : Layer 2 Virtual Private Networks
    Area                : Routing
    Stream              : IETF
    Verifying Party     : IESG

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