> -----Original Message----- > From: Xuxiaohu > Sent: Friday, June 13, 2014 3:52 PM > To: [email protected] list; OSPF List > Subject: Encoding inconsistency between ISIS and OSPFv2 extensions for SR > > Hi all, > > There are some encoding inconsistencies between OSPFv2 extensions and ISIS > extensions for SR as follows: > > 1. In ISIS-SR, the prefix-sid advertisement is piggybacked on the IP > reachability > advertisement. In OSPF-SR, the prefix-sid advertisement is piggybacked on OSPF
s/ piggybacked on OSPF/ carried in OSPF > Extended Prefix LSA which is used to advertise other attributes associated > with > the prefix, rather than the reachability. IMHO, the OSPF encoding is more > flexible since the label distribution and the reachability advertisement are > independent. As a result, the route summary on area boundaries at least can be > enabled as before. Besides, the prefix-SID sub-TLV can be used to advertise a > range of prefix/SID pairs (see item2). In fact, ISIS allows us to do the same > way > as OSPF with a much lower cost (see section 3 of > http://tools.ietf.org/html/draft-xu-isis-global-label-sid-adv-00). Of course, > it > seems that you co-authors prefer to the piggyback way. The piggyback way here just means the current way used by the ISIS-SR. Best regards, Xiaohu > 2. In ISIS-SR, the Prefix-SID Sub-TLV can only be used to advertise an SID > for a > single prefix. And it relays on the SID/Label Binding TLV to advertise a > range of > prefix/SID pairs. In contrast, In OSPF-SR, the prefix-sid sub-TLV can be used > to > specify a range of addresses and their associated Prefix SIDs. By the way, in > the > 4.3. SID/Label Binding sub-TLV. it has the following text: "Range Size: > usage is > the same as described in Section 4.2." Did you co-authors want to propose two > ways (i.e., prefix-sid sub-TLV and SID-Label Binding sub-TLV) to achieve the > same > goal (i..e, advertise a range of prefix/SID pairs)? > 3. In ISIS-SR, the range of SID/Label values is advertised by the SR > Capability > sub-TLV. Meanwhile, the data-plane capability is advertised by such sub-TLV as > well. In OSPF-SR, the range of SID/Label values is advertised by the label/sid > range sub-TLV. I wonder what the special purpose of the data-plane capability > advertisement is in the ISIS-SR case. > > 4. In ISIS-SR, the Prefix-SID Sub-TLV can be sub-TLV of the SID/Label > Binding-TLV > and its' the the prefix-sid sub-TLV which is used to associate prefix-sids > with the > range of prefixes advertised by the SID/Label Binding TLV. In OSPF-SR, the > prefix-sid sub-TLV and the SID/label binding sub-TLV are now at the same > level of > the sub-TLV hierarchy(i.e., both of them are sub-TLVs of the OSPF Extended > Prefix TLV ). As a result. it's the SID/Label sub-TLV, rather than the > prefix-sid > sub-TLV which is used to associate prefix-sids with the range of prefixes. > 5. In ISIS-SR, "The SID/Label Sub-TLV (Type: TBD, suggested value 1) contains > the > SID/Label value as defined in Section 2.3. It MAY be present in the SID/Label > Binding TLV" . However, in OSPF-SR, "SID/Label sub-TLV as described in Section > 2.1. This sub-TLV MUST appear in the SID/Label Binding Sub-TLV". In other > words, the sid/label sub-TLV is optional to the SID/Label binding sub-TLV in > the > ISIS-SR case while the sid/label sub-TLV is mandatory to the SID/label binding > TLV in the OSPF-SR case. > > 6. In ISIS-SR, the prefix-SID sub-TLV doesn't contain the MT-ID field since > the > MT-ID field is already contained in the parent TLV of the prefix-SID sub-TLV. > In > OSPF, the MT-ID field is contained in the Prefix SID Sub-TLV since the parent > TLV > of the prefix-sid sub-TLV doesn't contain that MT-ID field. IMHO, it's better > to > contain the MT-ID in the parent prefix-specific TLV of the prefix-SID > sub-TLV. In > other words, why not contain the MT-ID in the OSPF Extended Prefix TLV, > instead of the prefix-sid sub-TLV (see section 3 of > http://tools.ietf.org/html/draft-xu-ospf-global-label-sid-adv-00)? > > Anyway, although it is unavoidable for us to define extensions to both ISIS > and > OSPF for the same thing due to the fact that both protocols have been widely > used, could we try our best to keep the encodings of ISIS and OSPF as > consistent > as possible for the same functionality? In this way, once someone has read the > ISIS extension draft, he or she can easily think of what has been done in the > OSPF extension draft accordingly, and vice verse. > > Best regards, > Xiaohu _______________________________________________ OSPF mailing list [email protected] https://www.ietf.org/mailman/listinfo/ospf
