Inter-Domain Path Computation in MPLS Authors: Faisal Aslam, Zartash Afzal Uzmi, Adrian Farrel, and Michal Pioro Zartash Afzal Uzmi Department of Computer Science LUMS, Lahore, Pakistan IEEE ICC 2007 – Glasgow Wednesday, June 27, 2007

June 27, 2007 Inter-Domain Path Computation in MPLS 2 Outline Background Information –Inter-Domain Routing –Path Computation and Setup in MPLS –Visibility and Crankback Signaling Path Computation Schemes –PCE based Computation –Per-Domain Path Computation Description of CWS Scheme –Key Parameters –Properties Simulation Scenarios and Results Conclusions

June 27, 2007 Inter-Domain Path Computation in MPLS 3 Definition of a Domain A network under single administration –An Autonomous System (AS) –An IGP Area Domains connected through domain border routers (DBR) Inter-domain link Intra-domain link Interior node/router Domain border router SRC DST Goal: Set up an explicit (traffic engineered) path from SRC to DST

June 27, 2007 Inter-Domain Path Computation in MPLS 4 Setting up an Intra-Domain Path Given a set of constraints –Minimum bandwidth –Maximum delay Source/Ingress computes a path –Stores the path in an explicit route object (ERO) –Sends the setup request along the computed path –A label switched path (LSP) is established Example: –Src: 1, Dst: 3, bandwidth: 40  1  2  3 –Src: 1, Dst: 3, bandwidth: 52  1  4  5 

June 27, 2007 Inter-Domain Path Computation in MPLS 5 Setting up an Inter-Domain Path A source router knows everything it needs to know for routing in its own domain –Intra-domain path can be optimally computed Complete information about other domains may not be available. Why? –Security and confidentiality –Scalability (of routing and signaling protocols) SRC DST Inter-domain paths may not be optimal

June 27, 2007 Inter-Domain Path Computation in MPLS 6 SRC knows which of the two paths is better SRC knows traversal of domains to reach DST SRC only knows one or more DBRs to go to DST Information Scenarios How good is an inter-domain path? –information available at the source Information Scenarios –Complete visibility –Partial visibility –Local domain visibility SRC DST

June 27, 2007 Inter-Domain Path Computation in MPLS 7 Crankback Signaling What if a path can not be established? –Not enough resources to satisfy the constraint Use crankback signaling –Send PathErr messages to let upstream know –Include reason within the message –Go around elements that are unable to satisfy the constraint An upstream router “tries” another path on receiving PathErr from downstream

June 27, 2007 Inter-Domain Path Computation in MPLS 8 Crankback Example Goal: Path setup (A  D) Assume: violation on link BC Node B sends crankback (PathErr) information to Node A Node A tries to go around Node B and tries Node E now A B C E D

June 27, 2007 Inter-Domain Path Computation in MPLS 9 Per-Domain Path Computation Basic idea –Source computes a loose route Due to absence of complete visibility –Each domain border router (DBR) computes route through its own domain –Crankback signaling facilitates avoiding elements Who can refine the loose route? –Source can specify the nodes (routing points) along the computed loose route –Each routing point refines the route as it is being established

June 27, 2007 Inter-Domain Path Computation in MPLS 10 Per-Domain Path Computation Under limited visibility –Source computes path –Source creates ERO containing the path –Source sends path setup request –Source uses crankback signaling For selecting domain exit point –Each intermediate routing point Also uses crankback signaling If the constraint can not be met at a point –Send crankback signals upstream –Upstream will try another border router If the constraints are met –Path is reserved!

June 27, 2007 Inter-Domain Path Computation in MPLS 11 Limitation of Per-Domain Method Selects first available path Selected path could be the worst possible (constraints still satisfied) Proposal (CWS): –Establish the first available path –Start sending traffic over –Keep looking for better paths

June 27, 2007 Inter-Domain Path Computation in MPLS 12 CWS Scheme Starts sending traffic on first found path –Same setup delay as in other schemes Continue quest for better paths When a new “better” path is found –Switch traffic onto new path –Use make-before-break mechanism –Still keep looking for even better paths

June 27, 2007 Inter-Domain Path Computation in MPLS 13 CWS Key Attributes KT Flag –Part of the Path message Travels from source to destination –Value is set when crossing the domain –Indication to downstream routers SKT Flag –Normally, PathErr is generated by downstream routers when a path can not be found –When KT flag is set, PathErr is always generated by the downstream SKT set if a path was successfully found SKT unset for normal crankback method

June 27, 2007 Inter-Domain Path Computation in MPLS 14 CWS: Crankback Extension LNDC parameter –Limit on number of domains crossed –Initially set by admin (could be -1) –Length of the shortest path found so far NDC parameter (number of domains crossed so far) –Always less than or equal to LNDC NDC and LNDC together make search faster –Stop search from a branch if NDC exceeds LNDC NDC and LNDC both are part of the Path message

June 27, 2007 Inter-Domain Path Computation in MPLS 15 CWS –Preserving Confidentiality IDPA attribute –Inter-domain path attribute If n is the number of DBRs in a domain –Total number of intra-domain paths is maxp = n x (n-1) –Choose P maxp to represent all possible intra-domain paths such that: P maxp = 2P maxp-1 -1 Choose IDPA values judiciously

June 27, 2007 Inter-Domain Path Computation in MPLS 16 Simulation Setup Inter-domain networks –Net1: 20 POPs, 46 links (800 units capacity) –Net2: COST266 (28 POPs, 37 links, 500 units) Intra-domain network (in each POP) –15-node, 28-link topology at each POP –For NET1: link capacity (U~[ ] units) –For NET2: each node-pair connected randomly Traffic: 2000 requests arrive one-by-one –SRC and DST chosen randomly –Request size: uniform between 5 and 25 units –Infinite call holding time Criteria for evaluation –Number of LSPs placed –Number of domains crossed

June 27, 2007 Inter-Domain Path Computation in MPLS 17 Simulation Results: NET2 Number of domains crossed compared with standard scheme: NET1: 18% fewerNET2: 20% fewer

June 27, 2007 Inter-Domain Path Computation in MPLS 18 Conclusions Inter-domain visibility is limited –Per-domain computation may lead to highly suboptimal path CWS scheme keeps searching for better paths (using KT and SKT flags) Confidentiality is maintained using IDPA attribute CWS crosses fewer routing domains on average when compared with the traditional per-domain scheme CWS accommodates more LSP requests when compared with the standard per- domain scheme