T6b

Interior vs. Exterior Protocols

  • Interior Protocols:
    • Can be Link State (LS) or Distance Vector (DV).
    • May have access to perfect information.
    • Scalability needs are limited.
    • Focus on finding the shortest path.
    • Changes need to propagate quickly.
  • Exterior Protocols:
    • Distance Vector-ish (DV).
    • Perfect information is impossible.
    • Scalability is crucial.
    • "Shortest" is more flexible, considering policies.
    • Changes propagate more carefully.

Hierarchical Routing

  • Scaling Problems:
    • Growing routing tables.
    • Increasing routing computation.
    • Expanding forwarding tables.
  • Network Aggregation:
    • LAN prefixes already aggregate subnets.
    • No need to advertise every single host on a LAN.
    • Treat a group of subnets as a larger subnet.
    • Example: Adjacent /24s within a /16 (150.203.aaa.bbb).
    • Geographical aggregation.

Routing to a Region

  • Aggregation:
    • Aggregate nodes/subnets to hide internal complexity.
    • Results in shorter tables.
  • Downside:
    • Less optimal paths.
    • Example: Full path 1A to 5C [1B] = 5 hops vs. Hierarchical path 1A to 5C [1C] = 6 hops.

Routing to a Region (Outside View)

  • Outside a Region:
    • Routers know one route to reach the region.
    • All hosts are aggregated, reducing communication and computation.
  • Inside a Region:
    • More than one route in/out of a region may exist.
    • Local router makes decisions on how to enter/exit the region.
    • A region provides context and designates border routers.
    • Internal arrangements are made within the region.

Policy-Based Routing and Routing Policies

  • Core of the Internet:
    • Multiple ISPs interconnect via Internet Exchange Points (IXP).
    • All entities are running a business (or a country).
      *Example visual representation of the routing environment among interconnected entities

Policy Routing

  • Adding Human Needs:
    • Introduce policies to protocols based on factors like:
      • Money
      • Politics
      • "Security"
    • Example: National Research and Education Networks (NRENs) have an R&E traffic policy, including:
      • Wholesale purchase.
      • Not competing with commercial providers.
      • Social good.

National Research and Education Networks (NRENs)

  • Diagram illustrating peering and transit relationships among various entities (Microsoft, Google, Amazon, Universities, Cultural institutions, Instruments, GÉANT).

Costs and Shortest Path

  • Local Priority:
    • Each ISP aims to offload traffic quickly.
    • Technical Term: Hot Potato Routing.
    • Leads to sub-optimal shortest paths.
    • Asymmetric paths.
    • Hierarchy is (consciously) broken for business reasons.

Common Policies: Transiting and Peering

  • ISPs (Transiting):
    • Take your traffic and pass it through their network to the Internet.
    • They take Internet traffic and pass it through to you.
    • You pay them.

Common Policies: Transiting and Peering (cont.)

  • ISPs (Peering):
    • Take your traffic and pass it through to a specific network.
    • Return replies.
    • You cannot reach the Internet through them.
    • Mutual benefit; maybe no money exchanged.
    • Examples: CDN, cloud provider, NREN.

Border Gateway Protocol (BGP)

  • Key Protocol:
    • Main Internet routing protocol today.
    • Uses TCP-based message exchanges.
    • Manually configured, adjacent devices.
  • Key Concepts:
    • Aggregates nodes within an Autonomous System (AS) - a region, a business, an ISP.
    • Identifies Border Routers (or Gateways) which run BGP.
    • Edge: separates interior and exterior routing protocols (Intradomain vs. Interdomain, Enterprise vs. Wide-Area).
    • Builds BGP tables which populate forwarding/routing tables.

BGP: Distance Vector (DV) vs. Link State (LS)

  • Path Vector:
    • BGP is more of a path vector protocol than a distance vector protocol.
  • Announcements:
    • IP Prefix(es), Next Hop.
    • Path: list of AS’s to transit.
    • Allows loop detection and removal.
    • No explicit distance indications.
    • Multi-protocol capable.

IXPs

  • Links to relevant resources provided

BGP Route Advertisements Example

  • Diagram illustrating BGP route advertisements with prefixes, AS paths, and next hops for different Autonomous Systems

BGP Example Advertisement

  • Listing of BGP advertisement examples including Rank, AS, Description, prefix, and AS Path. Example:
    • 128.6.0.0/16 4608 7575 11537 10466 46

Policy Implementation

  • Configuration:
    • Configure route advertisements.
    • Border routers advertise available paths with policy constraints.
    • Advertisements are sent only to AS’s that may use them.
  • Actions:
    • Can weight or prepend AS paths.
    • Filter out paths that cannot be used.
    • Offer transit/peering selectively.
    • Provide faster/slower paths based on $$$.
  • Listening:
    • Border routers listen for available paths and pick the most suitable one.
  • Criteria:
    • Loop detection/avoidance.
    • Shortest, cheapest, trustworthy, friendly, safe, politically/contractually-suitable.
    • Human-driven optimization.
    • Advertise the chosen path as appropriate.

BGP Example

  • Diagram illustrating businesses and their BGP relationships; AS1 is selling transit to AS2, AS3, and AS4; AS2 and AS3 are peering, as are AS3 and AS4; AS2 is selling transit to customer A.

BGP Advertisements

  • Example Advertisements:
    • AS2 to AS1: I have a Customer: [A, (AS2), router2U]
    • AS1 to AS2: I offer Transit: [B,(AS1,AS3), router1L] and [C,(AS1,AS4), router1L]
    • AS3 to AS2: I offer Peer: [B,(AS3), router3L]
    • AS2 to AS3: [A,(AS2), router2R]
    • Letters indicate U=up L=left R=right

Customer A View

  • AS2 (and Customer A):
    • Hears one option for reaching C: (AS1, AS4).
    • (Could) hear two options for reaching B: Transit (AS1, AS3) and Peer (AS3).
    • Peering traffic is usually free.

In Closing

  • Routing is complex and hard.
  • DV, LS, and BGP are very important.
  • The Internet is large and complex.
  • Policies:
    • Policies are important; the Internet is also a business.
  • Edge Case:
    • Connecting interior and exterior routing/gateway protocols is literally an edge case (not discussed).
  • Performance Challenges:
    • Scalability, convergence, reliability, trustworthiness, optimization.
    • All in a globally distributed, real-time, 24/7 system with billions of customers.