Network Layer Review Flashcards

The network layer is responsible for transporting segments from sending to receiving host.
Sender: Encapsulates segments into datagrams and passes them to the link layer.
Receiver: Delivers segments to the transport layer protocol.
Network layer protocols exist in every Internet device: hosts, routers.
Routers: Examine header fields in all IP datagrams passing through them and move datagrams from input ports to output ports to transfer datagrams along the end-to-end path.

Data Plane: Per-router function that determines how a datagram arriving on a router input port is forwarded to the router output port.
Control Plane: Network-wide logic that determines how a datagram is routed among routers along the end-to-end path from source to destination.
Two control-plane approaches:
- Traditional routing algorithms: Implemented in routers.
- Software-Defined Networking (SDN): Implemented in remote servers.

Individual routing algorithm components in each router interact in the control plane.

When looking for a forwarding table entry for a given destination address, use the longest address prefix that matches the destination address.

Deals with adding and dropping packets when buffers are full.
- Tail drop: Drop the arriving packet.
- Priority: Drop/remove packets based on priority.

Deciding which packet to send next on a link.
- First Come, First Served (FCFS): Packets transmitted in order of arrival to the output port. Also known as First-In-First-Out (FIFO).
- Priority: Arriving traffic is classified and queued by class; packets are sent from the highest priority queue that has buffered packets.
- Round Robin (RR): Server cyclically scans class queues, sending one complete packet from each class (if available) in turn.
- Weighted Fair Queueing

CIDR (Classless Inter-Domain Routing): Subnet portion of address of arbitrary length.
Address format: a.b.c.d/x, where x is the number of bits in the subnet portion of the address.

A subnet is a network consisting of device interfaces that can physically reach each other without passing through an intervening router.
IP addresses have structure with a subnet part (common high order bits for devices in the same subnet) and a host part (remaining low order bits).

Goal: Host dynamically obtains IP address from a network server when it joins the network.
Can renew its lease on the address in use.
Allows reuse of addresses (only hold address while connected).
Supports mobile users who join/leave the network.
DHCP Overview:
- Host broadcasts DHCP discover msg (optional).
- DHCP server responds with DHCP offer msg (optional).
- Host requests IP address: DHCP request msg.
- DHCP server sends address: DHCP ack msg.

Example sequence of messages between a DHCP client and server, including DHCP discover, offer, request, and ACK messages.

DHCP can return more than just the allocated IP address on the subnet:
- Address of first-hop router for client.
- Name and IP address of DNS server.
- Network mask (indicating network versus host portion of address).

All devices in a local network share just one IPv4 address as far as the outside world is concerned.
Datagrams leaving the local network have the same source NAT IP address but different source port numbers.

Host 10.0.0.1 sends a datagram to 128.119.40.186, 80
NAT router changes datagram source address from 10.0.0.1, 3345 to 138.76.29.7, 5001, and updates the NAT translation table.
Reply arrives, destination address: 138.76.29.7, 5001

Initial motivation: 32-bit IPv4 address space would be completely allocated.
Tunneling: IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers (