Hi Eric, sorry for coming back late to you. I’ll go through our suggestions asap.
Thanks. s. > On Oct 22, 2015, at 10:16 PM, Eric C Rosen <[email protected]> wrote: > > I'd like to make some suggestions for textual changes to sections 3.1 and > 4.3 of draft-ietf-idr-prefix-sid. The main purpose of these suggestions is > to clarify the use of the Originator SRGB TLV, and to remove what I > think is an excessive and distracting amount of repetition about the > inadvisability of allowing different nodes to use different SRGBs. > > My proposal for the text of these sections follows. In addition to the > changes I mentioned above, some typos in the original text are fixed, and > there is a suggestion (explained below in brackets) for slightly modifying > the text about the AFI/SAFIs with which the Prefix-SID attribute may be > used. A few other explanations can be found in brackets below inside the > proposed text. > > -------------------- > > 3.1. MPLS Prefix Segment > > In this document, we specify "MPLS Prefix Segments" only for BGP routes > that have an AFI/SAFI of 1/4 or 2/4. The applicability of MPLS prefix > segments to other AFI/SAFIs is outside the scope of this document. > > [The original text said "A Multiprotocol BGP labeled IPv4/IPv6 Unicast > ([RFC3107]) session type is required", I don't think that is quite precise. > If a session has multiple AFI/SAFIs, including 1/4, I don't think we want to > say that the attribute can be placed in any UPDATE on that session. Also, > it's not quite accurate to say that RFC3107 is restricted to 1/4 and 2/4; > RFC3107 doesn't mention the AFI. And we may want to leave it open that the > Prefix Segment notion may eventually be applied somehow to SAFI-128 routes.] > > The BGP Prefix Segment is realized on the MPLS dataplane in the > following way: > > As described in [I-D.ietf-spring-segment-routing-msdc] the > operator assigns a globally unique "index", L_I, to a locally > sourced prefix of a BGP speaker N which is advertised to all other > BGP speakers in the SR domain. > > According to [I-D.ietf-spring-segment-routing], each BGP speaker > is configured with a label block called the Segment Routing Global > Block (SRGB). (While it is recommended to use the same SRGB across > all the nodes within the SR domain, the SRGB of a node is a local > property and could be different on different speakers). > > The index L_I is a 32 bit offset in the SRGB. Each BGP speaker > derives its local MPLS label, L, by adding L_I to the start value > of its own SRGB, and programs L in its MPLS dataplane as its > incoming/local label for the prefix. (See section 5.1 for more > details.) > > [Added reference to section 5.1.] > > The outgoing label for the prefix is found in the NLRI of the > Multiprotocol BGP labeled IPv4/IPv6 Unicast prefix advertisement. > The index L_I is only used as a hint to derive the local/incoming > label. > > Section 4.1 of this document specifies the Label-Index TLV of the BGP > Prefix-SID attribute; this TLV can be used to advertise the label index > of a given prefix. > > If the BGP speakers are not all configured with the same SRGB, and if > traffic-engineering within the SR domain is required, each node may be > required to advertise its local SRGB. One way of advertising the local > SRGB is to use the segment routing extensions of BGP-LS > (draft-gredler-idr-bgp-ls-segment-routing-ext-00.txt). An alternative > option is to use the Originator SRGB TLV of the prefix-SID attribute, as > specified in Section 4.3 of this document. > > [Rearranged last paragraphs slightly to improve flow, imo.] > > > > 4.3. Originator SRGB TLV > > The Originator SRGB TLV is an optional TLV and has the following > format: > > 0 1 2 3 > 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 > +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ > | Type | Length | Flags | > +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ > | Flags | > +-+-+-+-+-+-+-+-+ > > +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ > | SRGB 1 (6 octets) | > | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ > | | > +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ > > +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ > | SRGB n (6 octets) | > | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ > | | > +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ > > [I can rarely get the ascii art to render correctly, but the diagram above > is supposed to be unchanged from what appears in the draft.] > > where: > > o Type is 3. > > o Length is the total length of the value portion of the TLV: 2 + > multiple of 6. > > o Flags: 16 bits of flags. None are defined in this document. > Flags SHOULD be clear on transmission and MUST be ignored at > reception. > > o SRGB: 3 octets of base followed by 3 octets of range. Note that the > SRGB field MAY appear multiple times. If the SRGB field appears > multiple times, the SRGB consists of multiple ranges. The meaning of > an SRGB with multiple ranges is explained in Section 3.2 ("SID/Label > Range TLV") of [I-D.ietf-ospf-segment-routing-extensions]. > > [Added some text about the semantics of the SRGB field appearing multiple > times, with reference to a document that actually explains it.] > > When a BGP speaker attaches a Prefix-SID attribute to a given route, the > Originator SRGB TLV MUST NOT be included in the attribute unless the > following conditions hold: > > - The prefix field of the route's NLRI contains a host address (i.e., a > /32 IPv4 address or a /128 IPv6 address). > > - The value of the Originator SRGB TLV specifies the SRGB of the node > that is identified by the prefix field of the NLRI. > > [This paragraph and bullet items are added in order to make clear just what > the semantics of the TLV are.] > > If a BGP route is received that contains a Prefix-SID attribute with an > Originator SRGB TLV, but the prefix field of the NLRI does not contain a > host address, the attribute SHOULD be regarded as malformed. If a > Prefix-SID attribute contains more than one SRGB TLV, it SHOULD be > regarded as malformed. See section 7 for the treatment of a malformed > Prefix-SID attribute. > > When a route carrying the Prefix-SID attribute is propagated, the > Originator SRGB TLV (if present) MUST NOT be changed. > > ----------------- > > > > _______________________________________________ > Idr mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/idr _______________________________________________ spring mailing list [email protected] https://www.ietf.org/mailman/listinfo/spring
