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CS313 > Chapter 4 > Flashcards

Chapter 4 Flashcards

1
Q

Link-State Routing Protocols and Areas

A
  • Link-state routing protocols like OSPF and IS-IS partition routing domains into areas.
  • Areas are groups of routers that exchange link-state information.
  • There is a special area known as the backbone area (Area 0), which connects all other areas.
  • Example: Routers R1, R2, and R3 can be members of both the backbone area and nonbackbone areas (e.g., Area 1, Area 2).
  • A router that is a member of both the backbone area and a nonbackbone area is called an Area Border Router (ABR).
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2
Q

Routing Within and Between Areas

A
  • Within an area, all routers exchange link-state advertisements to develop a complete map of the area.
  • Link-state advertisements from non-ABR routers do not leave the area, which improves scalability.
  • To route between nonbackbone areas, packets travel through the backbone area (Area 0).
  • Area border routers summarize routing information from one area and advertise it into the backbone area.
  • Routers in the backbone area then summarize and advertise this information into nonbackbone areas.
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3
Q

Tradeoff Between Scalability and Routing Optimality

A
  • Dividing a domain into areas trades off routing optimality for scalability.
  • All packets between nonbackbone areas must travel through the backbone area, even if a shorter path exists.
  • This design decision ensures scalability by limiting the number of routers that need to exchange routing information.
  • Example: Even if R4 and R5 are directly connected, packets cannot flow between them if they are in different nonbackbone areas.
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4
Q

Virtual Links and Routing Flexibility

A
  • Virtual links allow network administrators to connect routers not directly connected to the backbone to routers in Area 0.
  • Example: A virtual link can connect R8 in Area 1 to R1 in Area 0, making R8 part of the backbone.
  • Virtual links improve routing optimality by allowing non-ABR routers to participate in backbone routing.
  • The cost of the virtual link is determined by routing information exchanged in the respective nonbackbone area.
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5
Q

Autonomous Systems (AS) and Routing

A
  • Autonomous systems (AS) provide hierarchical aggregation of routing information in large networks like the Internet.
  • Routing in AS is divided into:
    • Intradomain routing (within an AS)
    • Interdomain routing (between ASs or routing domains)
  • AS model allows each AS to use its own intradomain routing protocols and policies independently.
  • Interdomain routing involves sharing reachability information between ASs.
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6
Q

Challenges in Interdomain Routing

A
  • Interdomain routing requires each AS to define its own routing policies.
  • Example: Policies might prefer one path (AS X) over another (AS Y), avoid carrying traffic between specific AS pairs, and prioritize certain providers over others.
  • Complex policies need to be supported without relying on other ASs, due to competitive and confidential reasons.
  • Interdomain routing protocols must handle misconfigurations and malicious behaviors from other ASs.
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7
Q

History and Evolution of Interdomain Routing Protocols

A
  • Exterior Gateway Protocol (EGP) was the first interdomain routing protocol but had limitations with the Internet’s evolving topology.
  • Border Gateway Protocol (BGP), specifically BGP-4, replaced EGP and supports a graph model for interconnecting ASs.
  • BGP can accommodate non-tree-structured internetworks, such as multiprovider networks.
  • Today’s Internet is a complex network of interconnected ASs, mainly operated by private ISPs.
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8
Q

Types of Autonomous Systems (AS)

A
  • Stub AS: Connects to only one other AS and carries local traffic only.
  • Multihomed AS: Connects to multiple ASs but refuses to carry transit traffic.
  • Transit AS: Connects to multiple ASs and carries both transit and local traffic.
  • AS types influence routing decisions and policies within the network.
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9
Q

Goals and Challenges in Interdomain Routing

A
  • Interdomain routing aims to find loop-free paths compliant with AS policies.
  • Challenges include scale (handling 700,000+ prefixes), diverse routing policies, and trust between ASs.
  • Interdomain routing focuses on reachability rather than optimizing path costs across multiple ASs.
  • The autonomous nature of ASs complicates path cost calculations due to varying metrics and policies.
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10
Q

Basics of BGP

A
  • Each AS has one or more border routers responsible for forwarding packets between autonomous systems (AS).
  • Border routers may also function as BGP speakers, which communicate routing information with other BGP speakers in different ASs.
  • BGP is not a distance-vector or link-state protocol; it advertises complete paths as enumerated AS sequences to reach specific networks.
  • This path-vector approach is crucial for making policy decisions and preventing routing loops in complex AS networks.
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11
Q

BGP Path Advertisement

A
  • BGP speakers advertise reachability information for networks assigned to their customers.
  • Example: AS 2 advertises networks 128.96, 192.4.153, 192.4.32, and 192.4.3 as reachable directly from AS 2.
  • Backbone networks then advertise paths to these networks, indicating the sequence of ASs to reach them, such as (AS 1, AS 2) and (AS 1, AS 3).
  • BGP’s path enumeration prevents routing loops by detecting and avoiding paths that lead back to the originating AS.
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12
Q

AS Numbers and Loop Prevention

A
  • AS numbers in BGP must be unique to prevent routing loops.
  • AS numbers are 32 bits long and centrally assigned to ensure uniqueness.
  • Unique AS numbers are critical for BGP speakers to correctly identify and avoid routing loops in the AS path advertisements.
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13
Q

BGP Route Selection and Advertisement

A
  • A BGP speaker selects the best route to a destination based on its local policies.
  • BGP speakers are not obliged to advertise all routes; they can choose not to advertise routes to certain prefixes, implementing policies such as not providing transit.
  • Route cancellations in BGP are achieved through withdrawn route messages, a form of negative advertisement.
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14
Q

BGP Communication and Reliability

A
  • BGP runs over TCP for reliable communication.
  • TCP ensures that once information is sent from one BGP speaker to another, it does not need to be retransmitted unless changes occur.
  • BGP speakers exchange keepalive messages to confirm connectivity and the validity of routes; absence of keepalives indicates route invalidity.
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15
Q

Common AS Relationships and Policies

A
  • Autonomous Systems (ASs) have different relationships reflecting common connectivity needs and business models.
  • Three primary relationships are:
    • Provider-Customer
    • Customer-Provider
    • Peer
  • These policies ensure that traffic is routed efficiently and economically, aligning with the business interests of each AS.
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16
Q

Provider-Customer Relationship

A
  • Description: Providers connect their customers to the Internet.
  • Policy:
    • Advertise all routes to its customer.
    • Advertise routes learned from its customer to everyone else.
  • Function: Ensures connectivity for customers and helps in routing traffic efficiently.
17
Q

Customer-Provider Relationship

A
  • Description: Customers receive traffic from and send traffic to the Internet through their provider.
  • Policy:
    • Advertise its own prefixes and routes learned from its customers to its provider.
    • Advertise routes learned from its provider to its customers.
    • Do not advertise routes between providers.
  • Function: Provides customers with access to the Internet and ensures efficient routing of traffic.
18
Q

Peer Relationship

A
  • Description: Symmetrical peering between autonomous systems that view themselves as equals.
  • Policy:
    • Advertise routes learned from its customers to its peer.
    • Advertise routes learned from its peer to its customers.
    • Do not advertise routes from its peer to any provider or vice versa.
  • Function: Minimizes costs by exchanging traffic between peers and optimizing routing.
19
Q

Summary of AS Relationships

A
  • Provider-Customer Relationship:
    • Providers connect customers to the Internet.
    • Policy: Advertise all routes to customers; advertise customer routes to everyone else.
  • Customer-Provider Relationship:
    • Customers receive and send traffic through their provider.
    • Policy: Advertise own prefixes and customer routes to provider; advertise provider routes to customers; do not advertise routes between providers.
  • Peer Relationship:
    • Peers are equals and exchange traffic without payment.
    • Policy: Advertise customer routes to peers; advertise peer routes to customers; do not advertise routes to or from other providers.
20
Q

Hierarchical Structure and Tier-1 Providers

A
  • Hierarchical structure:
    • Stub networks at the bottom, customers of providers.
    • Providers with other providers as customers higher up.
    • Tier-1 providers at the top, having customers and peers but not being customers of other providers.
  • Tier-1 providers are critical for global Internet connectivity, as they provide extensive reach without the need for transit from other providers.
21
Q

Business Policies in AS Relationships

A
  • AS relationships are based on business needs and traffic exchange efficiency.
  • Provider-Customer relationship focuses on connectivity for customers.
  • Customer-Provider relationship ensures customers have access to and from the Internet.
  • Peering relationships are symmetrical and minimize costs by exchanging traffic between peers.
22
Q

Default Route Injection in Stub AS

A
  • Description: In a stub AS that connects to other ASes at a single point, the border router is the only exit point for routes outside the AS.
  • Process:
    • Border router injects a default route into the intradomain routing protocol.
    • This default route matches any destination that is not explicitly advertised within the AS.
  • Function: Provides a simple and effective way for a stub AS to reach networks outside its domain.
23
Q

Specific Route Injection by Border Routers

A
  • Description: Border routers inject specific routes learned from external ASes into the intradomain routing protocol.
  • Process:
    • Border router learns specific network prefixes (e.g., 192.4.54/24) from BGP.
    • Injects these specific routes into the intradomain protocol with associated costs.
  • Function: Allows routers within the AS to learn how to reach specific external network prefixes efficiently.
24
Q

Interior BGP (iBGP) in Backbone Networks

A
  • Description: In backbone networks, where a large number of prefixes are learned from BGP, iBGP is used to distribute this information internally.
  • Process:
    • iBGP redistributes BGP-learned routes to all routers within the AS.
    • Maintains a mesh of iBGP sessions among all routers in the AS.
  • Function: Enables all routers in the AS to determine the best exit (border) router for reaching any external prefix.
25
Q

Integrating Interdomain and Intradomain Routing

A
  • Description: Combining BGP (interdomain) with an intradomain protocol (IGP) to provide complete routing information within an AS.
  • Process:
    • Border routers exchange BGP information (eBGP) with external ASes.
    • iBGP is used to distribute this information to all routers within the AS.
    • Each router also maintains an IGP to learn internal routes within the AS.
    • Routers use both BGP-learned information and IGP-learned routes to determine the best path to each external prefix.
  • Function: Provides comprehensive routing capabilities, ensuring efficient routing of packets both within and outside the AS.
26
Q

Introduction to IPv6

A
  • Purpose: Address limitations of IPv4, such as address exhaustion.
  • Design Goals: Simplify address assignment, improve routing efficiency, enhance security, and support new services.
27
Q

Address Format

A
  • Address Length: 128 bits (compared to 32 bits in IPv4).
  • Representation: Eight groups of four hexadecimal digits separated by colons (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334).
  • Zero Compression: Omitting leading zeros and replacing consecutive zeros with “::” (e.g., 2001:db8::8a2e:370:7334).
28
Q

Address Types

A
  • Unicast: Identifies a single interface. Types include:
    • Global Unicast: Globally unique addresses routable on the internet.
    • Link-Local: Used for communication within a single link (e.g., FE80::/10).
    • Unique Local: Used within a site or organization (e.g., FC00::/7).
  • Multicast: Identifies multiple interfaces, typically in the same group.
  • Anycast: Identifies multiple interfaces, but packets are delivered to the nearest one.
29
Q

Header Format

A
  • Simplified Header: Compared to IPv4, the IPv6 header is streamlined to improve processing efficiency.
  • Fields:
    • Version: Indicates the protocol version (6 for IPv6).
    • Traffic Class: Used for traffic prioritization.
    • Flow Label: Identifies flows of packets for special handling.
    • Payload Length: Length of the payload following the header.
    • Next Header: Indicates the type of the next header (similar to the Protocol field in IPv4).
    • Hop Limit: Replaces the Time to Live (TTL) field in IPv4.
    • Source and Destination Addresses: 128-bit addresses of the sender and receiver.
30
Q

Extension Headers

A
  • Purpose: Provide additional functionalities and options.
  • Types:
    • Hop-by-Hop Options: Processed by every node along the path.
    • Destination Options: Processed by the destination node.
    • Routing Header: Specifies a list of intermediate nodes to be visited.
    • Fragment Header: Supports packet fragmentation.
    • Authentication Header (AH): Provides packet integrity and authentication.
    • Encapsulating Security Payload (ESP): Provides confidentiality, data integrity, and authentication.
31
Q

IPv6 Address Autoconfiguration

A
  • Stateless Address Autoconfiguration (SLAAC):
    • Process: Hosts generate their own addresses using a combination of locally available information and router advertisements.
    • Router Advertisements: Routers periodically send advertisements to announce their presence and provide network information.
  • Stateful Configuration (DHCPv6):
    • Purpose: Provides additional configuration options such as DNS server addresses.
    • Process: Similar to DHCP in IPv4, where a server assigns addresses and other network information to hosts.
32
Q

Transition Mechanisms

A
  • Dual Stack: Allows IPv4 and IPv6 to coexist on the same network infrastructure.
  • Tunneling: Encapsulates IPv6 packets within IPv4 packets to traverse IPv4 networks.
  • Translation: Converts IPv6 packets to IPv4 packets and vice versa.

CS313 (9 decks)

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