RE: Composite Link Requirements as WG document
"So, Ning" <ning.so@verizonbusiness.com> Mon, 07 December 2009 19:13 UTC
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Subject: RE: Composite Link Requirements as WG document
Date: Mon, 07 Dec 2009 19:12:35 +0000
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From: "So, Ning" <ning.so@verizonbusiness.com>
To: PAPADIMITRIOU Dimitri <Dimitri.Papadimitriou@alcatel-lucent.com>, "John G. Scudder" <jgs@juniper.net>, rtgwg@ietf.org
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Dimitri, Both of your questions are related to the motivation of the draft. From carrier/operator's point of view, we have thought long and hard about this, and we have detailed our motivations in the previous drafts, although not in this one because we thought they were well understood since we have presented it numerous times in several WGs and meetings. I have copied and pasted what we had in the last draft below. I think it addressed your questions in detail. Appendix A. Problem Statements Two applications are described here that encounter problems when multiple parallel links are deployed between two routers in today's IP/MPLS networks. A.1. Incomplete/Inefficient Utilization An MPLS-TE network is deployed to carry traffic on RSVP-TE LSPs, i.e. traffic engineered flows. When traffic volume exceeds the capacity of a single physical link, multiple physical links are deployed between two routers as a single backbone trunk. How to assign LSP traffic over multiple links and maintain this backbone trunk as a higher capacity and higher availability trunk than a single physical link becomes an extremely difficult task for carriers today. Three methods that are available today are described here. 1. A hashing method is a common practice for traffic distribution over multiple paths. Equal Cost Multi-Path (ECMP) for IP services and IEEE-defined Link Aggregation Group (LAG) for Ethernet traffic are two of the widely deployed hashing based technologies. However, two common occurrences in carrier networks often prevent hashing being used efficiently. First, for MPLS networks carrying mostly Virtual Private Network (VPN) traffic, the incoming traffic are usually highly encrypted, so that hashing depth is severely limited. Second, the traffic in an MPLS-TE network typically contain a certain number of traffic flows that have vast differences in the bandwidth requirements. Furthermore, the links may be of different speeds. In those cases hashing can cause some links to be congested while others are partially filled because hashing can only distinguish the flows but not the flow rates. A TE based solution better applies for these cases. IETF has always had two technology tracks for traffic distribution: TE-based and non-TE based. A TE based solution provides a natural compliment to non-TE based hashing methods. 2. Assigning individual LSPs to each link through constrained routing. A planning tool can track the utilization of each link and assignment of LSPs to the links. The lag time between measurement and action may be large relative to the fluctuations in LSP utilization, and the availability and performance of the planning tool are also important operational factors. Furthermore, the flexibility in response to failure scenarios is limited as summarized in the following. To gain high availability, FRR [RFC4090] is used to create a bypass tunnel on a link to protect traffic on another link or to create a detour LSP to protect another LSP. If BW is reserved for the bypass tunnels or the detour LSPs, the network will need to reserve a large amount of capacity for failure recovery, which reduces the capacity to carry other traffic. If BW is not reserved for the bypass tunnels and the detour LSPs, the planning tool can not assign LSPs properly to avoid the congestion during link failure when there are more than two parallel links. This is because during the link failure, the impacted traffic is simply put on a bypass tunnel or detour LSPs which does not have enough reserved bandwidth to carry the extra traffic during the failure recovery phase. An alternative is to use prioritized queuing so that premium traffic sees good performance during a congestion interval. The LSP sources will receive an RSVP-TE path error and reoptimize the new LSP . After a configurable period of time the LSPs will reoptimize and all traffic will experience good performance. However, it will not work well when support is needed for both RSVP-TE signaled and LDP signaled traffic. 3. Facility protection, also called 1:1 protection. Dedicate one link to protect another link. Only assign traffic to one link in the normal condition. When the working link fails, switch traffic over the protection link. This requires 50% capacity for failure recovery. This works when there are only two links. Under the multiple parallel link condition, this causes inefficient use of network capacity because there is no protection capacity sharing. In addition, due to traffic burstiness, having one link fully loaded and another link idle increases transport latency and packet loss, which lowers the link performance quality for transport. None of these methods satisfies carrier requirement either because of poor link utilization or poor performance. This forces carriers to go with the solution of deploying single higher capacity link. However, a higher capacity link can be expensive as compared with parallel low capacity links of equivalent aggregate capacity; a high capacity link can not be deployed in some circumstances due to physical impairments; or the highest capacity link may not large enough for some carriers. An LDP network can encounter the same issue as an MPLS-TE enabled network when multiple parallel links are deployed as a backbone trunk. An LDP network can have large variance in flow rates where, for example, the small flows may be carrying stock tickers at a few kbps per flow while the large flows can be near 10 Gbps per flow carrying machine to machine and server to server traffic from individual customers. Those large traffic flows often cannot be broken into micro flows. Therefore, hashing would not work well for the networks carrying such flows. Without per-flow TE information, this type of network has even more difficulty to use multiple parallel links and keep high link utilization. A.2. Inefficiency/Inflexibility of Logical Interface Bandwidth Allocation Using logically-separate routing instances in some implementations further complicates the situation. Dedicating separate physical backbone links, or, in the case of sharing of a single common link, dedicating a portion of the link, to each routing instance is not efficient. Assume that each routing instance must have at least the capacity of a single link, and also must have equal access to unused capacity. Then, for example, if there are 2 routing instances and 3 parallel links and half of each link bandwidth is assigned to a routing instance, neither routing instance can support an LSP with bandwidth greater than half the link bandwidth. The same problem is also present in the case of the sharing of a single common link using the dedicated logical interface and link bandwidth method. An alternative in dealing with multiple parallel links is to assign a logical interface and bandwidth on each of the parallel physical links to each routing instance, which improves efficiency as compared to dedicating physical links to each routing instance. Note that the traffic flows and LSPs from these different routing instances effectively operate in a Ships-in-the-Night mode, where they are unaware of each other. Inflexibility results if there are multiple sets of LSPs (e.g., from different routing instances) sharing one link or a set of parallel links, and at least one set of LSPs can preempt others, in this case more efficient sharing of the link set between the routing instances is highly desirable. A.3. Additional Functions Required Beyond Link Bundling A link bundle [RFC4201] is a collection of TE links. It is a logical construct that represents a way to group/map the information about certain physical resources that interconnect routers. The purpose of link bundle is to improve routing scalability by reducing the amount of information that has to be handled by OSPF/IS-IS. Each physical link in the link bundle is an IGP link in OSPF/IS-IS if numbered link is used. A link bundle only has the significance at the router control plane. The mapping of LSP to component link in a bundle is determined at LSP setup time and this mapping does not change due to newly configured LSP/LDP traffic on the same component link. A link bundle only applies to RSVP-TE signaled traffic, which implies no mix of RSVP-TE and LDP traffic on a same interface. CTG can handle RSVP-TE and LDP signaled traffic. Link bundling does not support groups of links with different characteristics (e.g., bandwidth, latency). Furthermore, link bundling only supports RSVP-TE signaled LSPs and not LDP signaled LSPs. Ning So Lead Engineer Enterprise Data Network and Traffic Planning 972-729-7905 -----Original Message----- From: rtgwg-bounces@ietf.org [mailto:rtgwg-bounces@ietf.org] On Behalf Of PAPADIMITRIOU Dimitri Sent: Sunday, December 06, 2009 4:11 AM To: John G. Scudder; rtgwg@ietf.org Cc: alia.atlas@bt.com; ZININ Alex Subject: RE: Composite Link Requirements as WG document Hi, Two comments on this document: "Unlike a link bundle [RFC4201], the component links in a composite link can have different properties such as cost or capacity." Component link "capacity" heterogeneity is "allowed" in 4201 I can understand the potential limitation of identical TE metrics (for each component) unfortunately limited explanation are given to sustain why it should be addressed. "This document describes a framework for managing aggregated traffic over a composite link." [...] "To achieve the better component link utilization and avoid component link congestion, the document describes some new aspects on the traffic flow assignment to component links." The document is specifically dealing to TE control but does not explain why "measuring" component link capacity is assumed impossible (cf. 4201 Section 4) ? Bottom-line: not clear if this document is intended to improve applicability of bundling TE control in IP/MPLS or if the current bundling approach is not fulfilling expected functionality (and this document would outline the why/what). The issue is that the document describes "modeling" but does not provide an answer to this question. Thanks, -dimitri. > -----Original Message----- > From: rtgwg-bounces@ietf.org [mailto:rtgwg-bounces@ietf.org] > On Behalf Of John G. Scudder > Sent: Tuesday, November 10, 2009 10:08 AM > To: rtgwg@ietf.org > Cc: alia.atlas@bt.com; ZININ Alex > Subject: Composite Link Requirements as WG document > > Folks, > > At today's meeting we received a request to adopt draft-so-yong-mpls- > ctg-requirement-00 as a working group document. There was > reasonably > strong support in the room for doing so. Please respond to the > mailing list with your discussion, support or opposition (please do > this even if you did so in person). The deadline for comments is > November 30. > > Note that accepting the document simply means that the working group > would begin working on requirements. It does not imply blanket > acceptance of the document as it now stands. > > Thanks, > > --John > _______________________________________________ > rtgwg mailing list > rtgwg@ietf.org > https://www.ietf.org/mailman/listinfo/rtgwg > _______________________________________________ rtgwg mailing list rtgwg@ietf.org https://www.ietf.org/mailman/listinfo/rtgwg
- Composite Link Requirements as WG document John G. Scudder
- RE: Composite Link Requirements as WG document Yong Lucy
- RE: Composite Link Requirements as WG document Mcdysan, David E
- RE: Composite Link Requirements as WG document So, Ning
- Re: Composite Link Requirements as WG document Greg Mirsky
- Re: Composite Link Requirements as WG document Andrew G. Malis
- Re: Composite Link Requirements as WG document Andrew G. Malis
- RE: Composite Link Requirements as WG document Dean Cheng
- Re: Composite Link Requirements as WG document Mach Chen
- Re: Composite Link Requirements as WG document Dan Li
- RE: Composite Link Requirements as WG document Yuji KAMITE
- Re: Composite Link Requirements as WG document Raymond Key
- Re: Composite Link Requirements as WG document Simon Delord
- Re: Composite Link Requirements as WG document Lizhong Jin
- RE: Composite Link Requirements as WG document PAPADIMITRIOU Dimitri
- RE: Composite Link Requirements as WG document Yong Lucy
- RE: Composite Link Requirements as WG document So, Ning
- RE: Composite Link Requirements as WG document PAPADIMITRIOU Dimitri
- RE: Composite Link Requirements as WG document So, Ning
- Extended comment period for Composite Link Requir… John G. Scudder
- Re: Extended comment period for Composite Link Re… Lou Berger
- RE: Extended comment period for Composite Link Re… So, Ning
- Re: Extended comment period for Composite Link Re… Lou Berger
- Re: Extended comment period for Composite Link Re… John G. Scudder