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Questions and Answers
What is the purpose of the 'Longest Prefix Matching Rule'?
What happens when a large IP datagram is sent over a link layer with a smaller MTU?
What is the unit of measurement for the fragment offset field in an IPv4 datagram?
What is the purpose of a subnet in IPv4?
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What is the notation '/24' in an IP address?
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What is CIDR?
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What is the destination IP address of a DHCP discovery message?
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Why are all DHCP messages broadcast?
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What is the purpose of the fragment offset field in an IPv4 datagram?
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What happens to a large IP datagram that is sent over a link layer with a small MTU?
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Study Notes
Distance Vector Algorithm
- A distance vector contains estimates of path costs from node x to y
- Each node x maintains information for each direct neighbor: cost c(x,v) from x to v
- Node x's distance vector: Dx = [Dx(y): y in N]
Internet Approach to Scalable Routing
- Routers are aggregated into regions known as Autonomous Systems (AS)
- Intra-AS routing (Interior Gateway Protocols): within the same AS
- Inter-AS routing: among different ASes (e.g. ISPs)
- Border Gateway Protocol (BGP) is used for inter-AS routing
Border Gateway Protocol (BGP)
- Provides each AS a means to exchange routes with other ASes (eBGP) and distribute reachability information to routers within the AS (iBGP)
- Determines "good" routes to other networks based on:
- Local preference value attribute
- Shortest AS-PATH
- Closest NEXT-HOP router
- Additional criteria
Subnetting
- A single prefix is used to advertise multiple subnets (address aggregation or supernetting)
- ISP gets IP address from ICANN
- Subnet gets IP address from ISP
Network Address Translation (NAT)
- NAT router behaves to the outside world as a single device having a public IP address
- NAT router hides details of the home network
- NAT router has a private IP address for the home network
IPv4 Checksum
- Header Checksum field is used to detect bit errors in received IP datagrams
- At the sender, the checksum is calculated by summing the 2-byte numbers in the header using 1's complement arithmetic
From IPv4 to IPv6
- IPv4 address depletion: 32-bit address allows only 2^32 unique IP addresses
- IPv6: 128-bit address
- Tunneling: IPv6 datagram is carried as payload in IPv4 datagram among IPv4 routers
Network Layer - Control Plane
- Routing algorithms determine "good" paths from sending host to receiving host through network of routers
- Global (Link-State) algorithms: OSPF
- Decentralized (Distance Vector) algorithms
Graph Abstraction of the Network
- Graph: G = (N, E) where N is the set of routers and E is the set of links
Dijkstra's Algorithm
- Used for routing algorithms
Distance Vector Algorithm
- Each node maintains distance vector Dx = [Dx(y): y in N]
IPv4 - Fragmentation and Reassembly
- Data link layer frames carry IP datagrams
- Maximum Transmission Unit (MTU): maximum amount of data that a link layer frame can carry
- Large IP datagram is fragmented into smaller IP datagrams to fit into the link layer frame MTU
- Fragment offset field is 13 bits and determines the size of the fragments (except the last)
Subnetting
- Subnet: devices that have the same subnet part in their IP addresses
- CIDR notation is also called subnet mask
- Subnet portion (Prefix) can be of arbitrary length (classless)
How to Get an IP Address?
- Hosts get IP address using DHCP (Dynamic Host Configuration Protocol)
- DHCP process:
- Discovery: src=0.0.0.0 , dest = 255.255.255.255
- Offer: broadcast to 255.255.255.255
- Request: broadcast
- ACK: broadcast
- All DHCP messages are broadcast because client initially doesn't know DHCP server's IP
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Description
This quiz covers the basics of distance vector algorithm, including its application in routing protocols such as OSPF and BGP, used in internet routing and autonomous systems.
