| Routing Area Working Group | H. Gredler, Ed. |
| Internet-Draft | Juniper Networks, Inc. |
| Intended status: Standards Track | S. Amante |
| Expires: November 16, 2013 | Level 3 Communications, Inc. |
| T. Scholl | |
| Amazon | |
| L. Jalil | |
| Verizon | |
| May 15, 2013 |
Advertising MPLS labels in IGPs
draft-gredler-rtgwg-igp-label-advertisement-05
Historically MPLS label distribution was driven by session oriented protocols. In order to obtain a particular routers label binding for a given destination FEC one needs to have first an established session with that node.
This document describes a mechanism to distribute FEC/label mappings through flooding protocols. Flooding protocols publish their objects for an unknown set of receivers, therefore one can efficiently scale label distribution for use cases where the receiver of label information is not directly connected.
Application of this technique are found in the field of backup (Bypass, R-LFA) routing, Label switched path stitching, egress protection, explicit routing and egress ASBR link selection.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
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This Internet-Draft will expire on November 16, 2013.
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MPLS label allocations are predominantly distributed by using the LDP [RFC5036], RSVP [RFC5151] or labeled BGP [RFC3107] protocol. All of those protocols have in common that they are session oriented, which means that in order to learn the Label Information database of a particular router one needs to have a direct control-plane session using the given protocol.
There are a couple of practical use cases where the consumer of a MPLS label allocation may not be adjacent to the router having allocated the label. Bringing up an explicit session using existing label distribution protocols between the non-adjacent label allocator and the label consumer is the existing remedy for this dilemma.
For LDP protection routing LDP next next hop labels [NNHOP] have been proposed to provide the 2 hop neighborhood labels. While the 2 hop neighborhood provides good backup coverage for the typical network operator topology it is inadequate for some sparse for example ring like topologies.
Depending on the application, retrieval and setup of forwarding state of such >1 hop label allocations may only be transient. As such configuring and un-configuring the explicit session is an operational burden and therefore should be avoided.
The use cases described in this document are equally applicable to IPv4 and IPv6 carried over MPLS. Furthermore the proposed use of distributing MPLS Labels using IGP prototocols adheres to the architectural principles laid out in [RFC3031].
It may not be immediate obvious, however introduction of Remote LFA [I-D.ietf-rtgwg-remote-lfa] technology has implied important changes for an IGP implementation. Previously the IGP had a one-way communication path with the LDP module. The IGP supplies tracking routes and LDP selects the best neighbor based upon FEC to tracking routes exact matching results. Remote LFA changes that relationship such that there is a bi-directional communication path between the IGP and LDP. Now the IGP needs to learn about if a label switched path to a given destination prefix has been established and what the ingress label for getting there is. The IGP needs to push that label for the tracking routes of destinations beyond a remote LFA neighbor.
Since the IGP is now aware of label switched paths and it does create forwarding state based on label information it makes sense to distribute label switched paths by the IGP as well.
This section lists example use cases which illustrate IGP distribution of MPLS label switched paths.
Deployment of Loop free alternate backup technology [RFC5286] results in backup graphs whose coverage is highly dependent on the underlying Layer-3 topology. Typical network deployments provide backup coverage less than 100 percent (see RFC 6571 Section 4.3 for Results [RFC6571]) for IGP destination prefixes.
By closer examining the coverage gaps from the referenced production network topologies, it becomes obvious that most topologies lacking backup coverage are close to ring shaped topologies [coverage-gap-analysis].
Remote LFA [I-D.ietf-rtgwg-remote-lfa] has introduced the notion of a "remote" LFA neighbor. This helper router which is both in P and Q space could forward the traffic to the final destination. Router 'H' is in P space, however due to the actual metric allocation router 'H' is not in Q space.
+-----+
| D |
+-----+
/ \
/ M1 \ M4 >= (M1 + M2 + M3)
/ \
+-----+ +-----+
| PLR | | H |
+-----+ +-----+
\ /
\ M2 / M3
\ /
+-----+
| E |
+-----+
Figure 1: Coverage gap analysis
The protection router (PLR) evaluates for a primary path to destination 'D' if {E -> H -> D} is a viable backup path. Because the metric M4 {H -> D} is higher than the sum of the original primary path and the path from router 'H' to the PLR, this particular path would result in a loop and therefore is rejected.
Now consider that router 'H' would advertise a label for FEC 'D', which has the semantics that H will POP the label and forward to the destination node 'D'. This is done irrespective of the underlying IGP metric 'M4' it is a 'strict forwarding' label. The PLR router can now construct a label stack where the outermost label provides transport to router 'H'. The next label on the MPLS stack is the IGP learned 'strict forwarding label' label. Note that the label 'strict forwarding' semantics are similar to a 1-hop ERO (Explicit route object). The Remote 'LFA' calculation would need to get changed, such that even if a node is not in PQ space, but rather in P space, it may get used as a backup neighbor if it advertises a strict forwarding label to the final destination. A recursive version of the algorithm is applicable as well as long a node in P space has some non looping LSP path to the final destination. The PLR router can now program a backup path irrespective of the undesirable underlying layer-3 topology.
Using existing tunnels for backup routing has been previously described in [I-D.bryant-ipfrr-tunnels]. Section 5.2.3 'Directed forwarding' describes an option to insert a single MPLS label between the tunnel and the payload. Traffic may thereby be directed to a particular neighbor. The mechanism described in this document, is an MPLS specific manifestation of 'Directed forwarding'.
In the topology described in Figure 2. router 'S' is facing a dilemma. Router S receives a BGP route from all of its 4 upstream routers. Using existing mechanism the provider owning AS1 can control the loading of its direct links *to* its ASBR1 and ASBR2, however it cannot control the load of the links beyond the ASBRs, except manually tweaking the eBGP import policy and filtering out a certain prefix. It would be more desirable to have visibility of all four BGP paths and be able to control the loading of those four paths using Weighted ECMP. Note that the computation of the 'Weight' percentage and the component doing this computation (Router embedded or SDN) is outside the scope of this document.
If all the ASes would be under one common administrative control then the network operator could deploy a forwarding hierarchy by using [RFC3107] to learn about the remote-AS BGP nexthop addresses and associated labels. An ingress router 'S' would then stack the transport label to its local egress ASBR and the remote ASBR supplied label. In reality it is hard to convince a peering AS to deploy another protocol just in order to easier control the egress load on the WAN links for the ingress AS.
A 'strict forwarding' paradigm would solve this problem: An Egress ASBR (e.g. ASBR 1 and 2) allocates a strict forwarding label toward all of its peering ASes and advertises it into its local IGP. The forwarding state of all those labels is to POP off the label and forward to the respective interface. The ingress router 'S' then builds a MPLS label stack by combining its local transport label to ASBR1 or ASBR2 with the IGP learned label pointing to the remote-AS ASBR.
-------------traffic-flow--------->
<-----------signaling-flow---------
:
: AS3
: +-------+
AS1 _:___+ ASBR3 |
/ : +-------+
+-------+ :
| ASBR1 | : AS4
+-------+ : +-------+
/ \_:___+ ASBR4 |
/ : +-------+
/ :
+-----+ :
| S | :
+-----+ : AS5
\ : +-------+
\ _:___+ ASBR5 |
\ / : +-------+
+-------+ :
| ASBR2 | : AS6
+-------+ : +-------+
\_:___+ ASBR6 |
: +-------+
:
Figure 2: Egress ASBR Link selection
ASBR {1,2} may want to periodically check the liveliness state to the endpoint of the label (ASBR {3,4,5,6}) which they are advertising. BFD Echo mode [RFC5880] is suitable technology to ensure liveliness state of undirectional links.
[I-D.minto-2547-egress-node-fast-protection] describes how PE routers advertising their labeled routes could get protected from node-failures. This is a local repair technology being dependent upon successful construction of a LFA path from any PLR to the 'protector PE' in a network.
>>>>>>>>CTX-label>>>>>>
// \\
// \\
+------+ +------+ +------+ +------+ +------+ +------+
| CE1 +---+PEingr+---+PEprot+---+ P +---+ PLR +-X-+PEegr +
+------+ +------+ +------+ +------+ +------+ +------+
\\ \ / //
>>>>>>>>>>>>>>>>>>>>>>primary-LSP>>>>>>>>
\ /
\ +------+ /
\___+ CE2 +____/
+------+
Figure 3: Backup Context advertisement
Assume a primary LDP LSP from the 'ingress PE' router to the 'egress PE' router. Now consider the FRR calculation from the 'PLR' router if its direct link to the 'egress PE' router fails (X) or the entire 'egress PE' goes down. The 'PLR' cannot find a LFA path to local-repair the traffic to the 'protector PE'. This is because the 'protector PE' router has not yet converged, and hence would want to forward the traffic to the original PE egress router, such that a temporal forwarding loop would be established.
Using IGP advertisement of MPLS Labels the 'protector PE' router can advertise a Label which identifies backup traffic such that arriving traffic, can be forwarded using a context specific forwarding table, rather than the main LSP transit table. The advertised context label is a unidirectional pointer to the 'egress PE' router. The LFA calculation of the PLR gets augmented such that it considers advertised labels pointing to the original tail-end of the LSP. The network learns thereby an egress LSP point which is is as good as the original egress LSP point.
IGP advertised strict forwarding labels can be utilized for constructing simple EROs via virtue of the MPLS label stack. In a classical traffic engineering problem [ero-label-stacking] is illustrated. The best IGP path between {S,D} is {S, R3, R4, D}. Unfortunately this path is congested. It turns out that the links {S, R1}, {R1, R4} and {R2, R4} do have some spare capacity. In the past a C-SPF calculation would have passed the ERO {S, R1, R4, R2, D} down to RSVP for signaling.
One of the functions that RSVP-TE provides, is that it keeps track of all the reservations over a particular link, enabling support for such traffic engineering features as bandwidth constraints, LSP priorities, and LSP preemption. However, support for these features with RSVP-TE has a cost associated with it, as it does require a node to maintain control and data plane state for all the individual point-to-point LSPs traversing the node (modulo the LSPs that rely on the LSP hierarchy). This is a use case for constructing explicitly routed paths, without the need to maintain per LSP control/data plane state on the nodes traversed by the LSP. This use case assumes that either support for bandwidth constrains, LSP priorities, and LSP preemption is not needed, or that such support is provided by means outside of this document.
+----+ +----+
| R1 +---------+ R2 |
+----+ 2 +----+
/ \ | \
/ 2 \ | \ 2
/ \ | \
+-----+ \ | +-----+
| S | \ 5 | 5 | D |
+-----+ \ | +-----+
\ \ | /
\ 1 \ | / 1
\ \ | /
+----+ +----+
| R3 +---------+ R4 |
+----+ 1 +----+
Figure 4: Explicit Routing using Label stacking
Consider now every router along the path does advertise a strict forwarding label for its direct neighbor. Router S could now construct a couple of paths for avoiding the hot links without explicitly signaling them.
Note that not every hop in the ERO needs to be unique label in the label stack. This is undesired as existing forwarding hardware technology has got upper limits how much labels can get pushed on the label stack. In fact an existing tunnel (for example LDP tunnel {S, R1, R2} can be reused for certain path segments.
In a network that is protecting nodes and links using IGP advertised labels, it is critical to perform fast restoration using local-repair, with packet forwarding restoration times comparable to RSVP Fast Re-Route (FRR) [RFC4090] or Loop Free Alternates [RFC5286].
First consider the timing of events assuming control-plane convergence as the sole repair mechanism. In Figure 5 a link failure scenario is illustrated. The best IGP path between {S,D} is {S, R3, R4, D}. When the directly adjacent link between R3 to R4 experiences a failure, (e.g.: fiber cut), the length of time to restore packet forwarding, from S to D, is dependent on several factors:
The overall length of IGP convergence time, is largely dependent on the slowest router programming changed forwarding state. This is inherent unpredictable due to the CPU load and overall scheduling state in the affected systems, hence control-plane as the sole repair technology is ineffective. In contrast, local-repair technology helps to minimize transient packet loss. In local-repair technology a backup path is programmed ahead of time. Once the link fails a forwarding plane may immediately change forwarding state (= local-repair) to the backup path. This keeps the traffic flowing until the control-plane calculates and installs the new primary path and backup path tuples forwarding state for a given destination in the network.
In the below example, the IGP calculates using C-SPF and pre-establishes a FRR Bypass LSP along {R3, R1, R2, R4} to provide Link Protection of the R3 to R4 link. When that link fails, R3 will local-repair traffic along the {R3, R1, R2, R4} Bypass LSP while simultaneously signaling in the Control Plane to the Head-End LSR, S, that the R3 to R4 link has failed. This allows time for S to run C-SPF to calculate a new, optimal forwarding path around the link failure; signal a new LSP through intermediate LSRs; and, finally, S may perform "make-before-break" to start forwarding traffic on the new LSP.
Note that the algorithmic complexity of a single-destination C-SPF is much less compared to the the all-destination, per-neighbor forward SPF and per-neighbor reverse SPF a router doing Remote LFA [I-D.ietf-rtgwg-remote-lfa] calculations.
+----+ +----+
| R1 +---------+ R2 |
+----+ 2 +----+
/ | | \
/ 2 | | \ 2
/ | | \
+-----+ | | +-----+
| S | | 4 | 5 | D |
+-----+ | | +-----+
\ | | /
\ 1 | | / 1
\ | | /
+----+ +----+
| R3 +---------+ R4 |
+----+ 1 +----+
Figure 5: Protection LSPs using Label stacking
For construction of the Bypass LSP a constrained-SPF (C-SPF) calculation is commenced. The C-SPF calculation computes an alternative path to R4, without transiting the {R3, R4} link. Furthermore the backup path MUST not violate any SRLGs with respect to the {R3, R4} link. A possible backup path result for R3 is {R3, R1, R2, R4}.
Next R3 needs to construct the label stack for this particular Bypass LSP. Assume that each router along the Bypass LSP has advertised a label binding for reaching its direct neighbor.
Now R3 can construct the the label-stack fully describing the bypass LSP: For the last hop from R2 to R4, label 204 is pushed on the stack For the penultimate hop from R1 to R2, label 102 is pushed on the stack Since the first hop of the Bypass LSP is a local choice, there is no need to encode an actual label (label 301), but rather program a nexthop forwarding action to R1.
RSVP headends learn about all their bypasses using RESV messages. When stacking IGP advertised labels, there is no direct comparable concept of a 'single head-end' node. All one-hop LSPs are in fact head-end nodes of their own and since there is no end-to-end signaling there is also no way about learning the bypasses that transit nodes have set up. IGP advertised labels hence mandate that all Bypass LSPs needs to be signaled to the rest of the network, such that the edge routers can have full insight (and control) what links may get utilized during local-repair. This is necessary, such that an edge router who may wants to enforce path policy constraints (e.g. end-to-end delay, hop count, path diversity, SRLG) can prefer or avoid certain paths (and their Bypasses) for path construction.
One of the shortcomings of existing traffic-engineering solutions is that existing label switched paths cannot get advertised and shared by many ingress routers in the network. In the example network [advertising-path-segments] a LSP with an ERO of {R4, R2, R6} has been established in order to utilize two unused north / south links. The only way to attract traffic to that LSP is to advertise the LSP as a forwarding adjacency. This causes loss of the original path information which might be interesting for a potential router which might wants to use this LSP for backup purposes. A computing router would need to have all underlying fate-sharing and bandwidth utilization information.
+----+ +----+ +----+
| R1 +---------+ R2 +---------+ R5 |
+----+ 2 +----+ 2 +----+
/ \ | \ \
/ 2 \ | \ \ 2
/ \ | \ \
+----+ \ | \ +----+
| S | \ 5 | 5 \ 5 | D |
-----+ \ | \ +----+
\ \ | \ /
\ 1 \ | \ / 1
\ \ | \ /
+----+ +----+ +----+
| R3 +---------+ R4 |---------+ R6 |
+----+ 1 +----+ 1 +----+
Figure 6: Advertising path segments
The IGP on R4 can now advertise the LSP segment by advertising its ingress label and optionally pass the original ERO, such that any upstream router can do their fate-sharing computations. Potential ingress routers now can use this LSP as a segment of the overall LSP. Furthermore ingress routers can combine label advertisements from different routers along the path. For example router S could stacks its LDP path to R2 {S, R1, R2} plus the IGP learned RSVP LSP {R4, R5, R6} plus a strict forwarding label {R6, D}.
Consider Figure 7. There is a LSP {S, R1, R2, D} which seeks link-protection against failure of the {R1, R2} link using R-LFA.
+----+ +----+
| R1 +----X----+ R2 |
+----+ 1 +----+
/ >>>>LSP-A->D>>>> \
/ 1 // \\ \ 1
/ // \\ \
+-----+ // \> +-----+
| S | | D |
+-----+ +-----+
\ /
\ 2 / 4
\ /
+----+ +----+
| R3 +---------+ R4 |
+----+ 3 +----+
Figure 7: Avoidance of T-LDP for obtaining infrastructure labels
The Remote LFA Calculations results in the following Node sets.
The PLR router (R1) needs to obtain the label-bindings from R4 towards the final destination D in order to push the two LSPs {R1, S, R3, R4} and {R4, D}. State of the art is to establish a targeted LDP session between PLR (R1) and the R-LFA Neighbor (R4). It would be desirable to avoid dynamic bringup of T-LDP sessions. Rather the IGP should supply the corresponding Label Bindings. Furthermore it would be desirable to apply some form of message compression, such that (unlike T-LDP) not per-FEC label bindings need to be exchanged. Applying Label Block style encoding [RFC4761] would be a suitable technology to compress the messaging overhead.
Many thanks to Yakov Rekhter, Ina Minei, Stephane Likowski and Bruno Decraene for their useful comments.
This memo includes no request to IANA.
This document does not introduce any change in terms of IGP security. It simply proposes to flood existing information gathered from other protocols via the IGP.
| [RFC4090] | Pan, P., Swallow, G. and A. Atlas, "Fast Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090, May 2005. |
| [I-D.ietf-rtgwg-remote-lfa] | Bryant, S., Filsfils, C., Previdi, S., Shand, M. and S. Ning, "Remote LFA FRR", Internet-Draft draft-ietf-rtgwg-remote-lfa-01, December 2012. |
| [I-D.bryant-ipfrr-tunnels] | Bryant, S., Filsfils, C., Previdi, S. and M. Shand, "IP Fast Reroute using tunnels", Internet-Draft draft-bryant-ipfrr-tunnels-03, November 2007. |
| [I-D.minto-2547-egress-node-fast-protection] | Jeganathan, J. and H. Gredler, "2547 egress PE Fast Failure Protection", Internet-Draft draft-minto-2547-egress-node-fast-protection-01, October 2012. |
| [NNHOP] | Chen, E., Shen, N. and A. Tian, "Discovering LDP Next-Nexthop Labels", November 2005. |