Notes On Chapter Twenty-Six -- Internet Routing and Routing Protocols

• 26.0 Study Guide
  
  
  • Know what static routing and dynamic routing are.
  • Understand that every Internet host requires at least a rudimentary routing capability.
  • Understand what Autonomous Systems are.
  • Understand why it is not OK for an Internet router to broadcast routing information to all the other Internet routers.
  • Understand what Interior Gateway Protocols (IGPs) are.
  • Understand what Exterior Gateway Protocols (EGPs) are.
  • Know what policy constraints are, and why they are necessary.
  • Understand how EGPs and IGPs work together on the Internet - in other words be able to explain what is going on in Figure 26.3.
  • Understand that IGPs use routing metrics, but that EGPs do not, and why.
  • Know the basics of what BGP is and how it works. Know that it's an EGP.
  • Know the basics about what RIP is and how it works. Know that RIP is an IGP.
  • Know the basics of what OSPF is and how it works. Know that OSPF is an IGP.
  • Be able to compare and contrast RIP and OSPF. Know advantages/disadvantages of one versus the other.
  • Understand how an internet topology translates into an OSPF graph, as illustrated by Figure 26.6.
  • Know what multicast routing is and the reasons why it is difficult to implement.
  
  
• 26.1 Introduction
  
  
  • Propagation of routing information
  • Forwarding table updates
  • Routing update protocols
  • Interior and exterior routing protocols
  
  
• 26.2 Static vs Dynamic Routing
  
  
  • When a router boots, it typically loads a forwarding table from a file on secondary storage.
  • With static routing, the contents of the forwarding table remain unchanged unless an administrator (a person) modifies it.
  • With dynamic routing, the router executes route propagation software that communicates with route propagation software on other routers, learns about changing network conditions and updates its forwarding table accordingly.
  • The goal is to keep packets moving along optimal routes from source to destination.
  
  
• 26.3 Static Routing in Hosts and a Default Route
  
  
  • Refer to Figure 26.1.
  • Every Internet host that sends an IP datagram is responsible to forward it to its first-hop destination.
  • Therefore every Internet host needs to have a forwarding table.
  • Most hosts use just a simple static table with two entries:
    • one entry specifying direct delivery for the directly-connected network, and
    • an entry specifying a default router able to forward datagrams to the rest of the Internet.
  
  
  
  
• 26.4 Dynamic Routing and Routers
  
  
  • Refer to Figure 26.2.
  • Most Internet routers need to perform dynamic routing.
  • Fundamentally, that is because the Internet is dynamic. A router needs to maintain next-hop information for reaching every network in the Internet. Congestion comes and goes. Network routers and links go up and down.
  • Suppose a router R is connected to a few networks, and suppose each of those networks is connected to several other routers. To do a good job, R needs to know at all times, and for every network N in the Internet, which is the best next-hop destination to use for N.
  • This information can change too rapidly for human administrators to keep up with it by manually altering forwarding tables.
  • As an example of how dynamic routing can work, consider that router R₁ in figure 26.2 might communicate with router R₂ across Network 3. Router R₁ might learn from router R₂ about the existence of Network 2 through the exchange of routing information with router R₂, and router R₁ might update its forwarding table to show that the next hop for packets addressed to Network 2 should be router R₂.
  
  
  
  
• 26.5 Routing in the Global Internet
  
  
  • Too much traffic would be generated if every Internet router exchanged routing information with every other Internet router.
  • To avoid excessive amounts of routing information messages, the Internet uses a routing hierarchy.
  • Routers in specific groups exchange routing information and representatives of the groups report the information to routers outside the group.
  • Regulators of the Internet have given organizations a lot of discretion to choose their own routing hierarchies and protocols.
  
  
• 26.6 Autonomous System Concept
  
  
  • The term autonomous system (AS) is often used to refer to groups of routers that exchange information and use representatives to communicate to the rest of the Internet routers.
  • Typically an autonomous system is a connected set of networks and routers controlled by a single administrative authority, such as an ISP, or a public or private organization.
  
  
• 26.7 The Two Types of Internet Routing Protocols
  
  
  • An Internet routing protocol is either an interior gateway protocol or an exterior gateway protocol. (Routers are also known as Gateways.)
    
    
  • 26.7.1 Interior Gateway Protocols (IGPs)
    • For exchanging routing information within an autonomous system, routers use interior gateway protocol software.
    • Usually IGP software is easy to install and operate, but has limited scalability.
    
    
  • 26.7.2 Exterior Gateway Protocols (EGPs)
    • When a router belonging to one autonomous system exchanges routing information with a router belonging to a different autonomous system, they use an exterior gateway protocol.
    • EGPs are typically more complex to install and operate but scale well.
    • Routing information is communicated in summary form.
    • Managers can configure the software to put policy constraints on the information released outside the organization. (A basic idea is to be cautious about advertising routes to outsiders if you don't want outsiders to use those routes. )
    
    
  • 26.7.3 Illustration of How IGPs and EGPs Are Used
    • Refer to Figure 26.3.
    • Routers in Autonomous System 1 use an interior gateway protocol to exchange routing information among themselves. The same goes for routers in Autonomous System 2.
    • Routers 1 and 4 use an exterior gateway protocol to exchange summary routing information regarding the Autonomous Systems to which they belong.
    • When router 1 receives new information from router 4, router 1 passes some of it along to routers 2 and 3, so they can update their routing tables with information about routes to networks that are connected to Autonomous System 2. Router 4 behaves the same way, giving information to routers 5 and 6 about networks connected to Autonomous System 1.
    
    
    
    
  • 26.7.4 Optimal Routes, Routing Metrics and IGPs
    • Ideally routers cooperate to keep datagrams moving along optimal paths. However the definition of "optimal" is elusive. Is it least delay? Highest throughput? Least jitter?
    • Currently, the typical Internet router works with a very simplified approximation to the concept of "optimal path." Usually the length of a path is measured as a combination of hop-count and administrative cost.
    • The idea of the administrative cost metric is for administrators to artificially inflate hop-counts in some forwarding tables in order to prevent certain paths from being used by certain hosts.
    • IGPs use routing metrics.
    • EGPs DO NOT use routing metrics. To use routing metrics, EGPs would have to compare and resolve routing information from different autonomous systems, compiled using different IGPs. EGPs don't try to address such problems. They just communicate reachability information.
  
  
• 26.8 Routes and Data Traffic
  
  
  • Refer to Figure 26.4.
  • "When routing advertisement flows out, data will begin to flow in."
  
  
  
  
• 26.9 The Border Gateway Protocol
  
  
  • Border Gateway Protocol (BGP) is the Internet's most widely used exterior gateway protocol.
  • BGP characteristics:
    • Routing Among Autonomous Systems: BGP gives routes as paths of autonomous systems.
    • Provision for Policies: Managers can configure BGP to restrict which routes it advertises to outsiders.
    • Facilities for Transit Routing: BGP classifies each AS as a transit or stub system.
      • A transit system agrees to pass traffic through to another autonomous system.
      • A stub system does not.
    • Reliable Transport: Routers executing BGP software use TCP connections to communicate reliably.
  • The Internet relies heavily on BGP. Tier-1 ISPs use BGP.
  
  
• 26.10 The Routing Information Protocol (RIP)
  
  
  • RIP was among the first interior gateway protocols used on the Internet.
  • RIP Characteristics:
    • Designed for Routing within an Autonomous System
    • Measures distance using a Hop Count Metric (The directly-connected net is counted as one hop away.)
    • RIP communicates using UDP, which is Unreliable Transport.
    • RIP uses Broadcast or Multicast Delivery.
    • RIP has Support for CIDR and Subnetting. It sends a mask with each destination address.
    • RIP has Support for Default Route Propagation.
    • RIP uses a Distance Vector Algorithm.
    • There is a Passive Version for Hosts. Hosts don't advertise routes, but they can listen to RIP and use the info to update their routing tables. This allows hosts to select among multiple routers that may be connected to their network.
    • There is a version of RIP for IPv6 - RIP next generation (RIPng).
    
    
  • Entries in a RIP forwarding table are basically of the form:
    
    (destination network, distance)
    
    where distance is the number of hops to the destination.
  • RIP is very "plug and play." Mainly all a manager has to do is start it up on each router. The routers talk to each other, and after a short time they all have the information they need.
  
  
• 26.11 RIP Packet Format
  
  
  • Refer to Figure 26.5.
  • This is the format of the RIP update message used with IPv4
  • Notice that it has the form of a list of sets of route information.
  • Each route is represented with the following information: family, route tag, IP address, address mask, next hop, and distance.
  • The family field is almost always just a 16-bit code that says the type of address family used is IP.
  • The route tag is something used to group entries.
  • See RFC 1723 for more details.
  
  
  
  
• 26.12 The Open Shortest Path First Protocol (OSPF)
  
  
  • A disadvantage of RIP and other distance-vector protocols is that it generates a lot of datagrams. Every router on a network broadcasts information to all the other directly-connected routers.
  • Thus RIP becomes less and less efficient as the number of routers and networks that can be reached increases.
  • Also routes tend to propagate slowly.
  • The Internet Engineering Task Force (IETF) designed the Open Shortest Path First Protocol (OSPF) to satisfy demand for an IGP that scales to large autonomous systems better than RIP.
  • OSPF Characteristics:
    • Routing within Autonomous Systems: OSPF is an IGP.
    • CIDR Support: OSPF sends masks with addresses.
    • Authenticated Message Exchange: Route info reveived is authenticated.
    • Imported Routes: For example a router can learn a route from BGP and propagate it with OSPF.
    • Link-State Algorithm: messages are of the form "The link between X and Y is up (or down)" and OSPF uses Dijkstra's algorithm to calculate shortest paths.
    • Support for Metrics: An administrator can assign a cost to each route.
    • Extension to IPv6: Version 3 of OSPF (OSPFv3) propagates routes to IPv6 destinations.
    • Support for Multi-Access Networks: Rather than allow all routers on a directly-connected multi-access network (e.g. an Ethernet) to broadcast routes, OSPF optimizes by designating a single router to do it.
  
  
• 26.13 An Example OSPF Graph
  
  
  • Refer to Figure 26.6.
  • OSPF calculates paths in graphs like the one in Figure 26.6 (b).
  • Note that the routers correspond to nodes in the graph, and the networks correspond to some of the edges in the graph.
  • Note that not all networks turn up as edges in the graph, and
  • a single network may correspond to several edges in the graph.
  • The way the algorithm works, each router in 26.6 (b) periodically checks that it can reach each of the routers to which it is directly connected by one of the edges.
  • Then the router broadcasts that link-state information to other routers.
  • Each router uses link-state information to recompute shortest paths.
  
  
  
  
• 26.14 OSPF Areas
  
  
  • A hierarchical approach allows OSPF to scale better than RIP.
  • OSPF allows managers to subdivide their networks into OSPF areas.
  • Routers within a single area periodically exchange link-state information. If they broadcast, it is not forwarded across the area boundary.
  • A single representative router from each area is designated to communicate with one or more representatives of other areas.
  • Area representatives exchange summarized routing information.
  
  
• 26.15 Intermediate System - Intermediate System (IS-IS)
  
  
  • IS-IS is an IGP similar to OSPF.
  • It was originally proprietary to DEC, and used in OSI networks.
  • A version that works with IP routing is now available.
  • The IETF added many features to OSPF, and now the lower overhead of IS-IS has made it popular.
  • The development of a version of OSPF that could handle IPv6 contributed significantly to the increase in overhead.
  • IS-IS is gaining in popularity among large ISPs.
  
  
• 26.16 Multicast Routing
  
  
  • 26.16.1 IP Multicast Semantics
    • Hosts are allowed to join or leave an IP multicast group at any time, by sending a message to a local router.
    • Member hosts receive one copy of each datagram sent to an IP multicast group. If there are multiple applications on the host that have requested membership, then the host is required to furnish each of those application with a copy of each multicast datagram.
    • There's no method provided for discovering the identities of members of an IP multicast group, or for learning the size of the group.
    • An arbitrary application may send a datagram to any IP multicast group at any time. It is not required to join the group.
    
    
  • 26.16.2 IGMP
    • A host uses Internet Group Multicast Protocol (IGMP) to communicate with a router when joining or leaving an IP multicast group.
    
    
  • 26.16.3 Forwarding and Discovery Techniques
    • Internet routers have the responsibility to propagate multicast routing information.
    • Dynamic group membership and support for anonymous senders make the routing problems hard.
    • Because members can join and leave at will, the system has to support routes that change quickly and continually.
    • Because senders are arbitrary, routing information has to be globally available.
      
      
    • Multicast protocol approaches to datagram forwarding:
      
      
      • Flood-and-Prune
        • This works well for small multicast groups all attached to contiguous LANs connected by routers, e.g. within a company "intranet".
        • Initially routers cooperate to flood - to forward each multicast datagram to all networks (via hardware multicast), while avoiding loops, and also exchanging information about group membership.
        • When routers learn that there are no members of the group on a connected network, they prune it - they stop forwarding the group datagrams to it.
        
        
      • Configuration-and-Tunneling
        • This works well when the group is geographically dispersed - a few members at each of a few sites.
        • A router at each site is configured to know about other sites.
        • When the router receives one of the datagrams for the group, it forwards it via hardware multicast locally.
        • It then consults its configuration information and sends a copy to each remote site on its list - using IP-in-IP tunneling, to encapsulate the multicast datagrams in unicast datagrams.
        
        
      • Core-Based Discovery
        • This technique helps provide "smooth growth" of an IP multicast group.
        • A core unicast address is assigned to the multicast group. If router R₁ receives a multicast datagram to forward, it encapsulates it and forwards it to the group's core unicast address.
        • Routers along the way examine the datagram.
        • When it arrives at a router R₂ that participates in the group, R₂ removes the datagram from its encapsulation and forwards it to members of the group using multicast routing.
        • If R₁ wants to forward a request to join the group to R₂, it also sends to the core unicast address.
        • When R₂ gets the request, it adds a new route to its multicast forwarding table and starts forwarding multicast datagrams to R₁.
        • A basic idea here is to grow the set of routers receiving the multicast as a tree.
    
    
  • 26.16.4 Multicast Protocols
    • Refer to Figure 26.7.
    • Notwithstanding twenty years of research and experimentation, there is still no Internet-wide multicast routing facility.
    • Proposed Protocols:
      • Distance Vector Multicast Routing Protocol (DVMRP): a type of configuration-and-tunneling protocol
      • Core Based Trees (CBT): a core-based-discovery protocol
      • Protocol Independent Multicast - Sparse Mode (PIM-SM): a core-based-discovery protocol
      • Protocol Independent Multicast - Dense Mode (PIM-SM): a flood-and-prune protocol
      • Multicast Extension to the Open Shortest Path First Protocol (MOSPF): builds on OSPF and uses link-state requests within an organization.
    • Despite a couple of decades of research, there is still no practical Internet-wide multicast routing standard.