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**Traffic Engineering for ISP Networks**

Jennifer Rexford Computer Science Department Princeton University

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**Internet Service Provider (ISP) backbones**

Outline Internet Service Provider (ISP) backbones Wide-area network with multiple Points of Presence Shortest-path link-state routing between edge routers Optimization: Tuning routing to the traffic Optimizing routing given a topology and traffic matrix Local search to select the integer link weights Tomography: Inferring the traffic matrix Estimating traffic matrix from routing and link loads Conclusion and ongoing work

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**Example Backbone: Abilene Internet2 Newtork**

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**Points-of-Presence (PoPs)**

Inter-PoP links Long distances High bandwidth Intra-PoP links Short cables between racks or floors Aggregated bandwidth Links to other networks Wide range of media and bandwidth Inter-PoP Intra-PoP Other networks

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**Routing Inside an Internet Service Provider**

Routers flood information to learn the topology Routers determine “next hop” to reach other routers… By computing shortest paths based on the link weights Routers forward packets via the “next hop” link(s) 2 1 3 1 3 2 1 5 4 3

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**Optimization: Tuning Routing to the Traffic**

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**Link Weights Control the Flow of Traffic**

Routers compute paths Shortest paths as sum of link weights Operators set the link weights To control where the traffic goes 2 1 3 1 3 2 1 3 5 4 3

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**Heuristics for Setting the Link Weights**

Proportional to physical distance Cross-country links have higher weights than local ones Minimizes end-to-end propagation delay Inversely proportional to link capacity Smaller weights for higher-bandwidth links Attracts more traffic to links with more capacity Tuned based on the offered traffic Network-wide optimization of weights based on traffic Directly minimizes key metrics like max link utilization

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**Why Are the Link Weights Static?**

Strawman alternative: load-sensitive routing Link metrics based on traffic load Flood dynamic metrics as they change Adapt automatically to changes in offered load Reasons why this is typically not done Delay-based routing unsuccessful in the early days Oscillation as routers adapt to out-of-date information Most Internet transfers are very short-lived Research and standards work continues… … but operators have to do what they can today

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**Big Picture: Measure, Model, and Control**

Network-wide “what if” model Topology/ Configuration Offered traffic Changes to the network measure control Operational network

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**Traffic Engineering in an ISP Backbone**

Topology Connectivity and capacity of routers and links Traffic matrix Offered load between points in the network Link weights Configurable parameters for Interior Gateway Protocol Performance objective Balanced load, low latency, service level agreements … Question: Given the topology and traffic matrix in an IP network, which link weights should be used?

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**Key Ingredients of Our Approach**

Measurement Topology: monitoring of the routing protocols Traffic matrix: widely deployed traffic measurement Network-wide models Representations of topology and traffic “What-if” models of shortest-path routing Network optimization Efficient algorithms to find good configurations Operational experience to identify key constraints

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**Formalizing the Optimization Problem**

Input: graph G(R,L) R is the set of routers L is the set of unidirectional links cl is the capacity of link l Input: traffic matrix Mi,j is traffic load from router i to j Output: setting of the link weights wl is weight on unidirectional link l Pi,j,l is fraction of traffic from i to j traversing link l

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**Multiple Shortest Paths With Even Splitting**

0.5 0.25 1.0 Values of Pi,j,l

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**Defining the Objective Function**

Computing the link utilization Link load: ul = Si,j Mi,j Pi,j,l Utilization: ul/cl Objective functions min(maxl(ul/cl)) min(Sl f(ul/cl)) f(x) 1 x

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**Complexity of the Optimization Problem**

NP-hard optimization problem No efficient algorithm to find the link weights Even for the simple convex objective functions Why can’t we just do multi-commodity flow? E.g., solve the multi-commodity flow problem… … and the link weights pop out as the dual Because IP routers cannot split arbitrarily over ties What are the implications? Have to resort to searching through weight settings

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**Optimization Based on Local Search**

Start with an initial setting of the link weights E.g., same integer weight on every link E.g., weights inversely proportional to link capacity E.g., existing weights in the operational network Compute the objective function Compute the all-pairs shortest paths to get Pi,j,l Apply the traffic matrix Mi,j to get link loads ul Evaluate the objective function from the ul/cl Generate a new setting of the link weights repeat

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**Making the Search Efficient**

Avoid repeating the same weight setting Keep track of past values of the weight setting … or keep a small signature (e.g., a hash) of past values Do not evaluate a weight setting if signatures match Avoid computing the shortest paths from scratch Explore weight settings that changes just one weight Apply fast incremental shortest-path algorithms Limit the number of unique values of link weights Do not explore all 216 possible values for each weight Stop early, before exploring the whole search space

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**Incorporating Operational Realities**

Minimize number of changes to the network Changing just 1 or 2 link weights is often enough Tolerate failure of network equipment Weights settings usually remain good after failure … or can be fixed by changing one or two weights Limit dependence on measurement accuracy Good weights remain good, despite random noise Limit frequency of changes to the weights Joint optimization for day and night traffic matrices

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**Application to AT&T’s Backbone Network**

Performance of the optimized weights Search finds a good solution within a few minutes Much better than link capacity or physical distance Competitive with multi-commodity flow solution How AT&T changes the link weights Maintenance done every night from midnight to 6am Predict effects of removing link(s) from the network Reoptimize the link weights to avoid congestion Configure new weights before disabling equipment

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**Example from My Visit to AT&T’s Operations Center**

Amtrak repairing/moving part of the train track Need to move some of the fiber optic cables Or, heightened risk of the cables being cut Amtrak notifies us of the time the work will be done AT&T engineers model the effects Determine which IP links go over the affected fiber Pretend the network no longer has these links Evaluate the new shortest paths and traffic flow Identify whether link loads will be too high

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**Same process applied to other cases**

Example Continued If load will be too high Reoptimize the weights on the remaining links Schedule the time for the new weights to be configured Roll back to the old weight setting after Amtrak is done Same process applied to other cases Assessing the network’s risk to possible failures Planning for maintenance of existing equipment Adapting the link weights to installation of new links Adapting the link weights in response to traffic shifts

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**Conclusions on Traffic Engineering**

IP networks do not adapt on their own Routers compute shortest paths based on static weights Service providers need to adapt the weights Due to failures, congestion, or planned maintenance Leads to an interesting optimization problems Optimize link weights based on topology and traffic Optimization problem is computationally difficult Forces the use of efficient local-search techniques Results of the local search are pretty good Near-optimal solutions that minimize disruptions

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**Robust link-weight assignments**

Ongoing Work Robust link-weight assignments Link/node failures Range of traffic matrices More complex routing models Hot-potato routing BGP routing policies Interaction between ASes Inter-AS negotiation for joint optimization Grappling with scalability and trust issues

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**Tomography: Inferring the Traffic Matrix**

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**Computing the Traffic Matrix Mi,j**

Hard to measure the traffic matrix IP networks transmit data as individual packets Routers do not keep traffic statistics, except link utilization on (say) a five-minute time scale Need to infer the traffic matrix Mi,j from Current topology G(R,L) Current routing Pi,j,l Current link load ul Link capacity cl

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**Inference: Network Tomography**

From link counts to the traffic matrix Sources 5Mbps 3Mbps 4Mbps 4Mbps Destinations

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**Tomography: Formalizing the Problem**

Ingress-egress pairs p is a ingress-egress pair of nodes (i,j) xp is the (unknown) traffic volume for this pair Mi,j Routing Plp is proportion of p’s traffic that traverses l Links in the network l is a unidirectional edge ul is the observed traffic volume on this link Relationship: u = Px (work backwards to get x)

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**Tomography: One Observation Not Enough**

Linear system of n nodes is underdetermined Number of links e is around O(n) Number of ingress-egress pairs c is O(n2) Dimension of solution sub-space at least c - e Multiple observations are needed k independent observations (over time) Stochastic model with Poisson iid counts Maximum likelihood estimation to infer matrix Doesn’t work all that well in practice…

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**Approach Used at AT&T: Tomo-gravity**

Gravitational assumption Ingress point a has traffic via Egress point b has traffic veb Pair (a,b) has traffic proportional to via * veb 9 6 3 20 14 7 21 10

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**Approach Used at AT&T: Tomo-gravity**

Problem with gravity model Gravity model ignores the load on the inside links Gravity assumption isn’t always 100% correct Resulting traffic matrix might not satisfy the link loads Combining the two techniques Gravity: find a traffic matrix using the gravity model Tomography: find the family of traffic matrices consistent with all link load statistics Tomo-gravity: find the tomography solution that is closest to the output of the gravity model Works extremely well (and fast) in practice

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**Managing IP networks is challenging**

Conclusions Managing IP networks is challenging Routers don’t adapt on their own to congestion Routers don’t reveal much information about traffic Measurement provides a network-wide view Topology Traffic matrix Optimization enables the network to adapt Inferring the traffic matrix from the link loads Optimizing the link weights based on the traffic matrix

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**New Research Direction: Design for Manage-ability**

Two main parts of network management Control: optimization Measurement: tomography Two research approaches Bottom up: do the best with what you have Top down: design systems that are easier to manage Design for manage-ability “If you are both the professor and the student, you create exam questions that are easy to answer.” – Mung Chiang

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