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Questions and Answers
ICMP is used primarily for establishing direct communication between two devices.
ICMP is used primarily for establishing direct communication between two devices.
False
An echo request in ICMP is primarily used for the 'ping' command.
An echo request in ICMP is primarily used for the 'ping' command.
True
In ICMP, a Type 3 message indicates a destination network is unreachable.
In ICMP, a Type 3 message indicates a destination network is unreachable.
True
The source sends only one UDP segment with a TTL of 1 during the traceroute process.
The source sends only one UDP segment with a TTL of 1 during the traceroute process.
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ICMP messages are carried in IP datagrams.
ICMP messages are carried in IP datagrams.
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Routers in the same autonomous system (AS) run different routing protocols.
Routers in the same autonomous system (AS) run different routing protocols.
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A gateway router is located at the center of its own autonomous system.
A gateway router is located at the center of its own autonomous system.
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The forwarding table is configured by both intra-AS and inter-AS routing algorithms.
The forwarding table is configured by both intra-AS and inter-AS routing algorithms.
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Inter-AS routing is responsible for propagating reachability information to all routers in an autonomous system.
Inter-AS routing is responsible for propagating reachability information to all routers in an autonomous system.
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Intra-AS routing sets entries for both internal and external destinations in the forwarding table.
Intra-AS routing sets entries for both internal and external destinations in the forwarding table.
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In distance vector routing, a node can advertise an incorrect link cost to its neighbors.
In distance vector routing, a node can advertise an incorrect link cost to its neighbors.
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The count to infinity problem can be completely solved by using poisoned reverse in distance vector protocol.
The count to infinity problem can be completely solved by using poisoned reverse in distance vector protocol.
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Link state protocols require O(nE) messages to converge, where n is the number of nodes and E is the number of links.
Link state protocols require O(nE) messages to converge, where n is the number of nodes and E is the number of links.
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In hierarchical routing, all routers are assumed to be identical.
In hierarchical routing, all routers are assumed to be identical.
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The convergence time for distance vector algorithms is constant and does not vary.
The convergence time for distance vector algorithms is constant and does not vary.
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Router 1d uses the inter-AS protocol to determine which gateway packets should be forwarded to for subnet x.
Router 1d uses the inter-AS protocol to determine which gateway packets should be forwarded to for subnet x.
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Hot potato routing involves sending packets towards the farthest router for improved efficiency.
Hot potato routing involves sending packets towards the farthest router for improved efficiency.
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AS1 learns about subnet x's reachability from both AS3 and AS2 simultaneously.
AS1 learns about subnet x's reachability from both AS3 and AS2 simultaneously.
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The intra-AS routing information is irrelevant in determining the least cost path to a gateway.
The intra-AS routing information is irrelevant in determining the least cost path to a gateway.
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Router 1d must rely solely on its intra-AS routing information to build its forwarding table for subnet x.
Router 1d must rely solely on its intra-AS routing information to build its forwarding table for subnet x.
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A global routing algorithm has incomplete knowledge of the network topology.
A global routing algorithm has incomplete knowledge of the network topology.
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Dynamic routing algorithms change routes quickly in response to link cost changes.
Dynamic routing algorithms change routes quickly in response to link cost changes.
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In distance vector routing, all routers share their complete topology with one another.
In distance vector routing, all routers share their complete topology with one another.
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Dijkstra’s algorithm can compute least cost paths only when the link costs are known to all nodes.
Dijkstra’s algorithm can compute least cost paths only when the link costs are known to all nodes.
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Static routing algorithms automatically adjust to changes in the network.
Static routing algorithms automatically adjust to changes in the network.
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A forwarding table determines the end-to-end path through the network.
A forwarding table determines the end-to-end path through the network.
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Connection setup is a function that is essential in some network architectures like ATM and X.25.
Connection setup is a function that is essential in some network architectures like ATM and X.25.
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The network layer connection service is defined between individual processes.
The network layer connection service is defined between individual processes.
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In-order datagram delivery is one of the example services provided for the flow of datagrams.
In-order datagram delivery is one of the example services provided for the flow of datagrams.
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Guaranteed delivery within 40 msec delay is a type of network service model.
Guaranteed delivery within 40 msec delay is a type of network service model.
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Tunneling allows IPv6 datagrams to be carried as payload in IPv6 datagrams among IPv4 routers.
Tunneling allows IPv6 datagrams to be carried as payload in IPv6 datagrams among IPv4 routers.
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The US National Institutes of Standards estimate that around 11% of industry IP routers have adopted IPv6.
The US National Institutes of Standards estimate that around 11% of industry IP routers have adopted IPv6.
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Deployment and use of IPv6 has been ongoing for more than 20 years.
Deployment and use of IPv6 has been ongoing for more than 20 years.
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An IPv4 tunnel connects IPv6 routers by carrying IPv6 packets inside IPv4 packets.
An IPv4 tunnel connects IPv6 routers by carrying IPv6 packets inside IPv4 packets.
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In the physical view of tunneling, only IPv4 routers are involved in the communication.
In the physical view of tunneling, only IPv4 routers are involved in the communication.
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CIDR allows for a subnet portion of an IP address to be of arbitrary length.
CIDR allows for a subnet portion of an IP address to be of arbitrary length.
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DHCP requires that a host manually configures its IP address to join a network.
DHCP requires that a host manually configures its IP address to join a network.
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A DHCP client sends a 'DHCP offer' message to request an IP address from the server.
A DHCP client sends a 'DHCP offer' message to request an IP address from the server.
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The structure of an IP address in CIDR format is a.b.c.d/x, where x represents the number of bits in the host part of the address.
The structure of an IP address in CIDR format is a.b.c.d/x, where x represents the number of bits in the host part of the address.
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A DHCP server can renew the lease of an IP address when a host remains connected to the network.
A DHCP server can renew the lease of an IP address when a host remains connected to the network.
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Study Notes
Computer Networking: A Top-Down Approach
- The book is titled "Computer Networking: A Top-Down Approach" by Jim Kurose and Keith Ross, 6th edition, Addison-Wesley, March 2012.
- The provided materials are PowerPoint slides intended for educational purposes.
- The authors make the slides freely available, but request that users cite the source and note copyright when using them, especially on webpages.
Chapter 4: Network Layer
- Chapter 4 focuses on the principles behind network layer services, including network layer service models, forwarding versus routing, how a router works, routing (path selection), broadcast, multicast, instantiation, and implementation in the Internet.
- The chapter's outline covers introductions to virtual circuits and datagram networks, what's inside a router, Internet protocol (IP), routing algorithms, routing in the internet, and broadcast/multicast routing.
- Key topics include datagram format, IPv4 addressing, ICMP, and IPv6, providing a comprehensive overview of network layer functions.
Two Key Network-Layer Functions
- Forwarding: the process of moving packets from a router's input to its appropriate output.
- Routing: the process of determining the path packets take from source to destination. Routing algorithms are used to compute these paths.
Interplay Between Routing and Forwarding
- Routing algorithms determine the end-to-end path through a network.
- Forwarding tables determine how packets are forwarded locally at each router.
- Packet headers contain information used by forwarding tables to direct packets to their next destination.
Connection Setup
- Connection setup is a crucial function in some network architectures, like ATM, frame relay, and X.25.
- Before datagrams travel, end hosts and intervening routers negotiate a virtual connection.
Network Service Models
- Network service models describe the way networks handle individual or flow datagrams.
- Examples include guaranteed delivery and delivery with less than 40 ms delay for individual datagrams.
- Flow-level service options include in-order data delivery and guaranteed minimum bandwidth.
Network Layer Service Models
- This table presents various network architectures and their service models, including guarantees and congestion handling methods.
- Key examples include an internet, Internet architecture, ATM architecture and its services (CBR, VBR, ABR, UBR).
Virtual Circuits
- Virtual circuits behave like telephone circuits.
- They establish a dedicated path before data transmission begins.
- Routers maintain state information for each connection.
- Packets carry a virtual circuit identifier rather than the destination host address.
Virtual Circuit Implementation
- A virtual circuit consists of a path from source to destination, virtual circuit numbers assigned to links along the path, and entries in the forwarding tables of routers.
- Packets belonging to a particular virtual circuit carry the VC number.
Virtual Circuits: Signaling Protocols
- Signaling protocols are used to establish, maintain, and terminate virtual circuits.
- The protocols are used in ATM, frame relay, and X.25, but not today's Internet.
Datagram Networks
- Unlike virtual circuits, datagram networks don't establish a dedicated path.
- Packets are forwarded based on the destination host address.
- Routers have no information about the end-to-end connection.
Datagram Forwarding Table
- To route all 4 billion possible IP addresses, aggregate addresses into ranges using a forwarding table consisting of destination address ranges and their corresponding output links.
Longest Prefix Matching
- Routers use longest prefix matching to find the most specific entry in a forwarding table that matches a destination address.
Datagram or Virtual Circuit Network: Why?
- Datagram networks (like the Internet) offer elasticity and flexibility because they don't need formal connection setup.
- Virtual circuit networks (like ATM) enforce strict requirements on performance and resources, offering guaranteed service.
Router Architecture Overview
- Routers perform two key functions: running routing algorithms and forwarding datagrams.
- Routing algorithms (e.g., RIP, OSPF, BGP) compute paths to destinations.
- Forwarding algorithms move datagrams from input ports to output ports.
- Routing processes and data forwarding can be separated.
- Components include a routing processor, high-speed switching fabric, input ports, and output ports.
Input Port Functions, Switching Fabrics
- Input ports receive bits from the physical layer, encapsulate into datagrams, look up forwarding tables, and queue datagrams.
- Switching fabrics transfer packets from input buffers to appropriate output buffers.
- Various fabric types exist, each with different speed characteristics.
Switching via Memory, Bus Interconnection Network
- The simplest switching fabric method is switching via memory.
- Datagrams are copied to system memory, and this method is speed-limited by memory bandwidth.
- Bus-based switching transfers datagrams via a shared bus, but switching speed is limited by bus contention.
- Interconnection networks (e.g., banyan switches and crossbars) exceed the speed limitations of memory and bus-based networks.
Output Ports
- Output ports accept a datagram from the switching fabric and transmit it to the outgoing link.
- Buffering is required to handle situations when fabric transfer rate exceeds outgoing link rate.
- Scheduling of datagrams is critical to ensure efficient output.
How Much Buffering?
- RFC 3439 provides a rule of thumb for buffering requirements.
- Appropriate buffering can minimize congestion and packet loss in routers.
Input Port Queuing
- Input ports must be able to handle situations when datagrams arrive faster than forwarding rate.
- Head-of-the-Line (HOL) blocking can occur when a queued datagram prevents other datagrams from being processed.
IP Datagram Format, IP Fragmentation, Reassembly
- The IP datagram format describes the structure of IP packets.
- Fragmentation is used when a datagram exceeds the network links' Maximum Transmission Unit (MTU) size.
- The IP protocol handles fragmentation and reassembly in datagrams to accommodate their transmission.
- Mechanisms are in place to correctly handle fragment order.
IP Addressing: Introduction, Subnets, and CIDR
- An IP address is a 32-bit identifier for hosts and routers.
- Subnets group devices with identical IP address portions to avoid intervening routers when reaching other devices within the same subnet.
- Classless Inter-Domain Routing (CIDR) uses variable-length subnet masks for addressing.
IP Addresses, DHCP, DHCP Client-Server Scenario
- Hosts obtain IP addresses dynamically using DHCP.
- DHCP dynamically allocates IP addresses, providing for better management at both the host and network levels.
- The DHCP client-server scenario describes the exchange of messages between clients and servers to obtain IP addresses.
- DHCP can return additional information, such as the IP address of the first-hop router and the IP address of the DNS server.
DHCP: Example and more than IP Addresses
- The client-server scenario provides an example of how DHCP facilitates the allocation of IP addresses.
- DHCP can provide more than just IP addresses; additional information like default gateway and DNS server addresses is also relayed.
IP Addressing; the last word, Hierarchical Addressing, Route Aggregation
- ICANN (Internet Corporation for Assigned Names and Numbers) manages IP address allocation.
- Hierarchical addressing groups similar networks for easier routing and management.
- Route aggregation summarizes address ranges in routing tables.
NAT (Network Address Translation)
- NAT translates private IP addresses to a public IP address to allow multiple devices in a private network to access the internet with just one public IP address from the internet service provider (ISP).
- NAT implementation, including how it handles outgoing and incoming datagrams, is important to maintain a consistent and robust communication topology.
- NAT is commonly used in home networks to extend the number of connections to the internet.
- Potential disadvantages may include having to deal with port numbers and how to handle simultaneous connections.
NAT Traversal Problems, Solutions
- NAT traversal problems occur when a client inside a NATed network wants to connect with a server outside the private network.
- Solution 1: configure NAT to explicitly map incoming connections from specific ports (destination ports) to specific local addresses (source addresses).
- Solution 2: use UPnP (Universal Plug and Play) which automates the mapping of ports and servers.
ICMP (Internet Control Message Protocol)
- ICMP is used for error reporting (e.g., unreachable host, destination unreachable) and other network management functions.
- ICMP messages are encapsulated within IP datagrams to provide connectivity between systems communicating under the same network.
Traceroute and ICMP
- Traceroute uses ICMP messages to determine the path taken by network packets between two systems.
- Traceroute sends UDP echo requests with incrementing Time-To-Live (TTL) values.
- ICMP responses help pinpoint routers along the path.
IPv6: Motivation, Datagram Format
- IPv6 was designed to address the limitations of IPv4 address space expansion.
- The IPv6 datagram format is fixed-length, reducing fragmentation needs and enhancing efficiency.
IPv6 Datagram Format
- The IPv6 datagram format includes a priority field, a flow label, and a next header field.
- The datagram format is more efficient and flexible for modern, high-speed networks.
Other Changes from IPv4
- IPv6 includes checksum removal for faster processing, and options are placed exteriorly to the fixed header.
- ICMPv6 provides some new message types and expands on existing ones for better management of networks.
Transition from IPv4 to IPv6
- Transition from IPv4 to IPv6 requires using tunneling techniques.
- Routers need to communicate and carry datagrams for successful transition.
Tunneling
- An IPv6 datagram is carried as a payload within an IPv4 datagram (tunneling) to allow IPv6 communication across a network of IPv4 routers.
IPv6 Adoption
- IPv6 adoption has been slower than expected due to several factors.
- It's important to understand that many applications and infrastructures now use IPv6 and there's extensive use of IPv6.
Graph Abstraction, Graph Abstraction Costs, Routing Algorithm Classification
- Networks can be abstracted as graphs, where nodes represent routers and edges represent links with costs. Least-cost paths can be determined using routing algorithms.
- Routing algorithms are categorized by whether or not they use a global view of the network topology or rely on decentralized information.
- Routing algorithms are further categorized as static or dynamic based on whether the paths remain constant over time or change as network conditions evolve.
Dijkstra's Algorithm
- Dijkstra's algorithm computes the shortest path between a source and all other nodes in a graph with non-negative edge weights.
- It's an iterative algorithm that incrementally expands the set of nodes whose least cost paths are known.
Dijkstra's Algorithm: Example, Resulting Shortest Path Tree, Discussion
- Examples illustrate how Dijkstra's algorithm computes shortest paths in different network topologies.
- It constructs a shortest path tree and generates a forwarding table for a given node.
- Some discussions highlight that the algorithm's efficiency can vary based on the network and implementation details.
- Possible oscillations and the impact of link cost changes should be considered to determine the best algorithm performance.
Distance Vector Algorithm
- The Bellman-Ford algorithm computes the least-cost path between nodes in a graph with possibly negative edge weights.
- Each node maintains a distance vector with estimates of least costs to other nodes.
- Nodes periodically exchange distance vectors to update their estimates.
Distance Vector Algorithm: Key Idea, Link Cost Changes
- Distance vector algorithms are decentralized, iterative, and asynchronous.
- Involves nodes communicating their distance vectors to their neighbors with updates whenever changes occur.
- Key idea is iterative exchange of distance vectors converging over time to the least cost between every pair of network links or nodes.
- Link cost changes are handled by nodes updating their distance vectors and notifying neighbors of changes.
Broadcast Routing, Flooding, Controlled Flooding, Spanning Tree
- Broadcast routing efficiently sends a packet to all nodes in a network (unicast, multicast or broadcast).
- Flooding is a simple but inefficient broadcast method, and it can get slow or cause broadcast storms.
- Controlled flooding prevents cycles of messages by only forwarding packets on the shortest path from source to destination.
- A spanning tree is built (constructed stepwise) to avoid routing loops and duplicated messages that can flood the network.
Multicast Routing: Problem Statement, Approaches
- Multicast routing finds the optimal tree or trees to transmit data to a group of receivers.
- Approaches include source-based trees (based on the source computing a set of paths) and group-shared trees (where all receivers in the group share a single tree).
- Multicast trees must be properly constructed and maintained to avoid redundancy and ensure message delivery as efficiently and reliably as possible.
Shortest Path Tree, Reverse Path Forwarding, Pruning, Shared-Tree: Steiner Tree
- Shortest path trees are calculated for source-based multicast (and typically using Dijkstra's algorithm); they are effective when receivers are close together and bandwidth is ample to handle all connections.
- Reverse path forwarding (RPF) is a simple source-based method where routers only forward multicast datagrams on links corresponding to the shortest unicast path back to the source.
- Pruning is an optimization where subtrees containing no group members are removed from the forwarding tree to reduce message traffic.
- Steiner trees find the minimum cost trees connecting all group members.
Center-Based Trees, Center-Based Trees: Example
- Center-based trees build a single tree rooted at a central router.
- Routers send join messages to the center, which builds the forwarding tree.
Internet Multicasting Routing: DVMRP, PIM, Sparse, Dense
- Different protocols, like DVMRP, PIM, have various mechanisms for multicast routing.
- PIM (Protocol Independent Multicast) provides distinct methods (sparse, dense) for handling the distribution of multicast messages across a network.
- PIM is useful for efficiently distributing data across networks.
Hierarchical OSPF
- Hierarchical OSPF organizes routers within an autonomous system (AS) into areas.
- Link-state advertisements are limited to a local area, then summarized and aggregated for efficient routing update distribution across the network.
Internet Inter-AS Routing: BGP (Border Gateway Protocol)
- BGP is the protocol used to exchange routing information between autonomous systems (ASes) in the internet.
- BGP is crucial for the operation of wide-area networks.
- BGP defines messages for creating a session, advertising routes, updating routes (and removing them), and handling errors and errors within networks.
BGP Messages, BGP Route Selection, How Does an Entry Get in Forwarding Table?
- BGP messages are exchanged via TCP.
- BGP message types (OPEN, UPDATE, KEEPALIVE, NOTIFICATION) facilitate communications between routing tables.
- Route selection in BGP depends on factors like AS-PATH attribute shortest path length and NEXT-HOP attribute.
- Router entry creation involves awareness of the prefix, determining the appropriate output port, and adding the prefix-port pair to the forwarding table.
Putting it Altogether
- This section summarizes the processes that contribute to establishing a finalized forwarding table in a router. The process is quite complex, requiring communication and decisions among various routing algorithms and protocols. This section emphasizes that the forwarding table contains destination, output port attributes.
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Description
This quiz covers key concepts related to the Internet Control Message Protocol (ICMP) and routing in computer networks. Topics include the function of ICMP messages, the traceroute process, and the differences between intra-AS and inter-AS routing. Test your knowledge on how routers communicate and manage routing tables.
