Re: [mpls] wg last call on draft-ietf-mpls-ldp-igp-sync-01.txt
"Luyuan Fang (lufang)" <lufang@cisco.com> Fri, 02 May 2008 01:37 UTC
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Thread-Topic: [mpls] wg last call on draft-ietf-mpls-ldp-igp-sync-01.txt
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From: "Luyuan Fang (lufang)" <lufang@cisco.com>
To: bruno.decraene@orange-ftgroup.com
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Subject: Re: [mpls] wg last call on draft-ietf-mpls-ldp-igp-sync-01.txt
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Bruno, Thanks for the discussion again. Please see in-line. Luyuan > -----Original Message----- > From: bruno.decraene@orange-ftgroup.com > [mailto:bruno.decraene@orange-ftgroup.com] > Sent: 14 April 2008 06:18 > To: Luyuan Fang (lufang) > Cc: loa@pi.se; mpls@ietf.org > Subject: RE: [mpls] wg last call on > draft-ietf-mpls-ldp-igp-sync-01.txt > > Hi Luyuan, > > Thanks for your detailed answers. > More in-line. > > Bruno > > > De : Luyuan Fang (lufang) [mailto:lufang@cisco.com] > > > > Hi Bruno, > > > > Thank you for the comments. Please see in-line. > > > > > -----Original Message----- > > > > > > Hi Markus, Alia, Luyuan, all, > > > > > > A few comments: > > > > > > 1) May be the document should talk about its interaction with > > > graceful restart. (I guess this document should not be > used when GR > > > is used, unless GR is aborted or failed.). > > > > > > > Very good point. Let's try to look the relationship between LDP-IGP > > synch (LDP-IGP) and Graceful Restart (GR). In fact, LDP-IGP is also > > used when GR is used. > > > > There are LDP GR [RFC 3478] and IGP GR - OSPF GR [RFC 3623] > and ISIS > > GR [RFC 3847]. > > > > A. LDP GR and LDP-IGP synch: > > > > LDP GR provides a mechanism to minimize the impact on MPLS labeled > > traffic caused by LSR's control plane restart. LDP GR is taking the > > proactive approach. > > > > LDP-IGP synch is reactive, it provides the exchange and > synch action > > between LDP and IGP when LDP is already failed. It prevents router > > sending traffic to the path where IGP is up and LDP is down. > > > > There are many occasions you don't have a chance to > gracefully restart. > > E.g. bring up a new link; operational error - missed to > configure LDP; > > implementation error causes label corruption, etc. > > > > Both LDP GR and LDP-IGP synch provide labeled traffic > protection under > > different conditions. > > Agreed: both GR and LDP-IGP sync have value and can be > enabled on the same router at the same time. > > My point was that for a given failure, only one of mechanism > should take care of the failure. So if GR handle the failure, > eg LDP session failure, LDP-IGP sync should not change the > IGP link cost. > > > B. IGP GR and LDP-IGP synch > > > > IGP GR or non-stop forwarding (NSF) is to keep IGP router > stay on the > > forwarding path even as its IGP is restarted. > > > > An IGP instance performing NSF should honor the LDP-IGP synch > > procedure to check the LDP-IGP synch parameter on the > standby routing device. > > Do you mean: what should we do if between two neighbor > routers, IGP GR is working but is still underway while LDP GR failed? > I didn't think of this specific interaction but the subject > of interaction between IGP and LDP GR procedures seems broad. > I don't have detailed knowledge of IS-IS GR, but in general > to stay in the safe area, it seems that the IGP topology > should not change during the IGP GR procedure. So in this > case, what should be the LDP-IGP sync behavior? > > Note that these GR interactions are not specific to the > IGP-LDP sync draft. Eg: what should we do if IGP GR succeed > and BGP GR failed? (for a node proving connectivity between > two ASBRs of the same AS where tunneling (MPLS or IP) is not used). > > so we can probably keep this out of this draft > > > IGP GR protects traffic during the switch over. LDP-IGP protects > > labeled traffic from being blackholed when LDP is faulty for some > > reason while IGP is still up. > > > > > > > > 2) " While the link could still be used for IP > forwarding, it is not > > > useful for traffic with packets carrying a label stack of > more than > > > one label or when the IP address carried in the packet is out of > > > the > > > RFC1918 space." > > > > > > It seems that the use of RFC 1918 IP address is just an > example. One > > > could find others (eg BGP free core). > > > Proposition to change to " While the link could still be > used for IP > > > forwarding, it is not useful for MPLS forwarding (e.g. > > > MPLS VPN, BGP free core.)" > > > > > > > Agreed. We can used both examples. > > > > How about we say "While the link could still be used for IP > > forwarding, it is not useful for MPLS forwarding, e.g., > MPLS VPN; BGP > > route free core; IP address carried in the packet is out of the > > RFC1918 space, etc."? > > Fine, thanks. > > > > > > > 3) "The LDP protocol itself has currently no means to > indicate to a > > > service depending on it whether there is an uninterrupted label > > > switched path available to the desired destination or not." > > > > > > Eventually, it could be useful to say why LDP ordered mode + FEC > > > withdraw is not an existing mean. > > > > LDP is not a routing protocol. IGP directs the traffic to > the shortest > > path regardless there is an established LSP along the path or not. > > Under the LDP down and IGP up condition: When LDP Ordered > Control is > > used, the labeled packets drop happens at the LSR of the ingress of > > network as LSP cannot be established on the shortest path > selected by > > IGP; When Independent Control is used, the labeled packets drop > > happens at the LSR where LDP failed (could be a mid point > in the LSP) as the LSP is broken. > > We have seen both blackholing cases in live networks. For > more details > > on LDP blackholing problems and solution, also see "LDP Failure > > Detection and Recovery", by L. Fang, A. Atlas, F. Chiussi, K. > > Kompella, and G. Swallow, IEEE Communications Vol.42 No.10, > October 2004. > > > > > Or may be you want to say that LDP has currently no way > to correct > > > the issue, for example by using a different IGP path. > > > > > > > Yes, thanks. We can re-word this similar as you suggested: LDP has > > currently no way to correct the issue. LDP is not a routing > protocol, > > it cannot re-direct traffic to an alternate IGP path. > > Fine, thanks. > > > > > > 4) "In the case of OSPF this cost is LSInfinity (16-bit value > > > 0xFFFF) as proposed in [RFC3137]." > > > > > > Could you also indicate the metric to use for IS-IS? > > > Especially since: > > > - further in the doc you recommend that all > implementation use the > > > same metric to avoid asymmetric link cost > > > - RFC 3784 seems to indicate that the max metric link (2^24 - > > > 1) is already reserved > > > > > > > Very good point. > > We shall add the ISIS max metric in the draft. The max metric value > > for ISIS using wide metrics is 0xFFFFFE per RFC 3784. > > Thanks. > > > > > > 5) "It is recommended that both sides of a link implement this > > > mechanism to be effective and to avoid asymmetric link > costs which > > > could cause problems with IP multicast forwarding." > > > > > > Avoiding asymetric link cost during all the procedure > would require > > > that both routers sharing the LDP session have the same way to > > > detect when "LDP is considered fully operational". This > is currently > > > not the case. > > > > Understood. It is a recommendation for better performance, not a > > requirement. > > > > > As the document propose to wait "some time", the document could > > > possibly propose a default value for this timer and warn > that if the > > > operator wish to change that value, it should change it > on both side > > > of the LDP session. > > > > > > > We prefer not to set the hold-down timer with a default value. The > > reason is that the LIB tables size can vary largely, as well the > > equipment capability/speed. If the default value is too small, this > > function would not work; if it is too big, it slows down the > > convergence time. But the hold-down time guidelines through > equipment > > testing would be helpful. > > You're right but this discussion could apply to lot of > default timers. Eg GR timers. Yet most specification defines > default values for their timers. > > > LDP End-of-LIB (draft-asati-mpls-ldp-end-of-lib-01.txt) is > currently a > > work in progress. When End-of-LIB is implemented, it can signal the > > end of IGP hold-down time, no longer need for the configured nor > > default timers. > > OK, given this, I guess it's not a big deal not to define > default value for the timers. May be the document could say > that such LDP sync mechanism (LDP End of LIB) should > preferably be used (over timers) once they are available. (in > order to avoid asymmetric routing) > Agreed, we planned to mention LDP End of LIB work. > > > Also, as there is a risk of transient asymetric link cost > (because > > > the end of the procedure is not stricly defined) could > you possibly > > > provide more precision on the problems that could be faced by IP > > > multicast? > > > > > > > Yes, transient asymmetric cost is always a risk, it is not just for > > this problem, it is rather a common problem. e.g. it is same as in > > BGP. We would treat it as out of scope for this draft. > > Ok but this document: > - Specifically states that asymmetric links cost could cause > problem with multicast ("asymmetric link costs [which] could > cause problems with IP multicast forwarding") > - In the short term (waiting for LDP end-of-LIB) does not > fully avoid asymmetric link cost. Quite the contrary, > compared to doing nothing, it creates new opportunity for this. > > So could the draft just list (or reference a document > describing) multicast issues with IGP asymmetric cost? > Otherwise, as a service provider proving IP, MPLS VPN and > multicast on the same AS, while LDP end-of-LIB is not > available, I wonder if I should use this draft or not as the > draft says it improves MPLS VPN availability but could create > issues with multicast. > I have checked with several Multicast experts. The conclusion is that IGP asymmetric link cost does not cause problems for multicast. So we will remove that line. > > > Thanks, > > > Regards, > > > Bruno > > > > > > > Thanks, > > Luyuan > > > > > > > > From Loa Andersson > > > > > > > > Working group, > > > > > > > > this is to initiate a two week working group last call on: > > > > > > > > draft-ietf-mpls-ldp-igp-sync-01.txt > > > > > > > > Please send your comments to the working group mailing list > > > or to the > > > > working group chairs before April 11. > > > > > > > > Loa and George > > > > > > > > -- > > > > Loa Andersson > > > > > > > > Principal Networking Architect > > > > Acreo AB phone: +46 8 632 77 14 > > > > Isafjordsgatan 22 mobile: +46 739 81 21 64 > > > > Kista, Sweden email: > loa.andersson@acreo.se > > > > loa@pi.se > > > > _______________________________________________ > > > > mpls mailing list > > > > mpls@ietf.org > > > > https://www.ietf.org/mailman/listinfo/mpls > > > _______________________________________________ > > > mpls mailing list > > > mpls@ietf.org > > > https://www.ietf.org/mailman/listinfo/mpls > > > > _______________________________________________ mpls mailing list mpls@ietf.org https://www.ietf.org/mailman/listinfo/mpls
- [mpls] wg last call on draft-ietf-mpls-ldp-igp-sy… Loa Andersson
- Re: [mpls] wg last call on draft-ietf-mpls-ldp-ig… bruno.decraene
- Re: [mpls] wg last call on draft-ietf-mpls-ldp-ig… Luyuan Fang (lufang)
- [mpls] Ended: wg last call on draft-ietf-mpls-ldp… Loa Andersson
- Re: [mpls] wg last call on draft-ietf-mpls-ldp-ig… bruno.decraene
- Re: [mpls] Ended: wg last call on draft-ietf-mpls… Luyuan Fang (lufang)
- Re: [mpls] wg last call on draft-ietf-mpls-ldp-ig… priyanka gupta
- Re: [mpls] wg last call on draft-ietf-mpls-ldp-ig… gaurav gupta
- Re: [mpls] wg last call on draft-ietf-mpls-ldp-ig… Luyuan Fang (lufang)
- Re: [mpls] wg last call on draft-ietf-mpls-ldp-ig… Luyuan Fang (lufang)
- Re: [mpls] wg last call on draft-ietf-mpls-ldp-ig… Luyuan Fang (lufang)
- Re: [mpls] wg last call on draft-ietf-mpls-ldp-ig… bruno.decraene