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Network Layer (IP)

Last updated May 1, 2022

Layer 3, the layer below the Transport Layer and layer above the Link Layer

1. Unit: Packet (datagram)
2. Responsibilities: Routes packet through routers to destination machine (not necessary if two devices are on the same network)
3. Two main functions
1. Forwarding: move packets from router’s input to appropriate router output (process of getting through a single interchange)
2. Routing: determine route taken by packets from source to destination (process of planning trip from source to destination)

# Packet Definition

2. Gateway Next Hop: address of the entry point
3. AS Path: Sequence of AS’s a packet would need to travel through

# Network Tiers

The structure of the internet is organized into entities called autonomous systems (ASs).

Each AS is

• assigned a range/collection of IP addresses

• responsible for routing to addresses it “owns”

• responsible for routing to addresses that are not its responsibility

• Peering vs Transit

• Transit: AS pays for the right to transit traffic across another AS
• Peering: mutual exchange of traffic between networks
• Tier 1 Networks

• A network that can exchange traffic with other Tier 1 networks without paying any fees (transit-free) for the exchange of traffic in either direction
• Tier 2 Networks

• A network that peers for free with some networks, but still purchases IP transit or pays for peering to reach at least some portion of the Internet
• Tier 3 Networks

• A network that solely purchases transit/peering from other networks to participate in the Internet. Everybody else

# Routing

Both IGP and EGP run at the application layer

# Internal Gateway Protocols (IGP)

• routing within a single AS, under the control of a single administrative entity
• this gives other routers complete info about the entire network
• every so often, each router uses Dijkstra’s to find shortest path to all routers, then it updates its forwarding table
• OSPF (open-shortest-path-first)
• most used IGP in the internet
• uses link-state protocol (each router has complete topological map of the entire AS)
• supports extensions such as areas (support hierarchy and scaling)
• Distance Vector
• every so often, each router tells its neighbours about the cost of its best routes to the networks it knows about
• a receiving router checks if any of the broadcasted routes would shorten their path to destination
• if so, it updates its routing table to route through the first router

# External Gateway Protocol (EGP)

• routing between different AS, no control over the routing policies of other AS (External Gateway Protocols - EGP)
• BGP
• the protocol that all ASs use for inter-AS routing
• packets are not routed to specific destination address, but to CIDRized prefixes, with each prefix representing a subnet or collection of subnets
• enables each router to
• obtain prefix information from neighbouring ASs
• determine “best” routes to the prefixes