Chapter 4: Network Layer (Data Plane)

Network Layer Functions

• Logical communication between hosts.

• Transport segment from sending host to receiving host.

• Forwarding

• Routing

Sender (Network Layer)

Encapsulates segments into datagrams and passes them to the link layer.

Receiver (Network Layer)

Delivers segments to a transport layer protocol.

Router

Moves datagrams from input ports to output ports to transfer datagrams along an end-end path.

Forwarding

Move packets from a router’s input link to appropriate router output link.

Routing

Determine route taken by packets from source to destination.

Data Plane

• Local, per-router function.

• Determines how datagram arriving on router input port is forwarded to router output port.

Control Plane

• Network-wide logic.

• Determines how datagram is routed among routers along end-end path from source host to destination host.

2 Control-Plane Approaches

• Traditional Routing Algorithms

• Software-Defined Networking (SDN)

Traditional Routing Algorithms

Routing algorithms implemented in routers.

Software-Defined Networking (SDN)

Routing algorithms implemented in (remote) servers.

Per-Router Control Plane

Individual routing algorithm components in each and every router interact in the control plane

Software-Defined Networking (SDN) Control Plane

Remote controller computes, installs forwarding tables in routers.

Example Services for Individual Datagrams

• Guaranteed delivery.

• Guaranteed delivery with less than 40 msec delay

Example Services for a Flow of Datagrams

• In-order datagram delivery

• Guaranteed minimum bandwidth to flow

• Restrictions on changes in inter-packet spacing

Guarantees the Internet Service Model Doesn’t Offer

• Successful datagram delivery to destination.

• Timing or order of delivery.

• Bandwidth available to end-end flow.

Input Port Queuing

Needed if datagrams arrive faster than forwarding rate into switch fabric.

Destination-Based Forwarding

Forward based only on destination IP address (traditional).

Generalized Forwarding

Forward based on any set of header field values; Match bits in arriving packet, and take action.

Longest Prefix Matching

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

Switching Fabrics

Router component that transfers packets from input link to appropriate output link.

Switching Rate

Rate at which packets can be transfered from inputs to outputs.

3 Major Switching Fabric Types

• Memory

• Bus

• Interconnection Network

Switching via Memory

Packet copied to system’s memory.

Switching via a Bus

Datagram from input port memory to output port memory via a shared bus.

Switching via Interconnection Network

• Exploits Parallelism

  • fragment datagram into fixed length cells on entry

  • switch cells through the fabric, reassemble datagram at exit

HOL Blocking

Queued datagram at front of queue prevents others in queue from moving forward.

Output Port Queuing

Required when datagrams arrive from fabric faster than link transmission rate.

Packet Scheduling

Deciding which packet to send next on link.

Packet Scheduling Algorithms

• FCFS / FIFO

• Priority

• Round-Robin

• WFQ

FCFS / FIFO Scheduling

Packets transmitted in order of arrival to output port.

Priority Scheduling

• Arriving traffic classified, queued by class.

• Send packet from highest priority queue that has buffered packets.

Round-Robin Scheduling

• Arriving traffic classified, queued by class.

• Server cyclically, repeatedly scans class queues, sending one complete packet from each class (if available) in turn.

WFQ

• Each class, i, has weight, wi, and gets weighted amount of service in each cycle.

Order on Protecting and Promoting an Open Internet Rules

• No Blocking

• No Throttling

• No Paid Prioritization

IP Protocol Features

• Datagram format

• Addressing

• Packet handling conventions

Internet Control Message Protocol (ICMP) Features

• Error reporting

• Router “signaling”

IP Address

32-bit identifier associated with each host or router interface.

Interface

Connection between host/router and physical link.

Subnet

Device interfaces that can physically reach each other without passing through an intervening router.

Dynamic Host Configuration Protocol (DHCP)

Protocol which enables host to dynamically obtain IP address from network server when it “joins” network.

Hierarchical Addressing

Allows efficient advertisement of routing information.

Network Address Translation (NAT)

All devices in local network share just one IPv4 address as far as outside world is concerned.

Advantages of NAT

• Just one IP address needed from provider ISP for all devices

• Can change addresses of host in local network without notifying outside world

• Can change ISP without changing addresses of devices in local network

• Security: devices inside local net not directly addressable, visible by outside world

Tunneling

IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers (“packet within a packet”).

Packet-Handling Rules for Generalized Forwarding

• Match: pattern values in packet header fields

• Actions: for matched packet: drop, forward, modify matched packet or send matched packet to controller

• Priority: disambiguate overlapping patterns

• Counters: #bytes and #packets

Middlebox

Device used for anything outside of fowarding.

Examples of Middleboxes

• NAT

• Firewalls

• Load balancers

• Caches

Network Functions Virtualization (NFV)

Programmable services over white box networking, computation, and storage.

3 Cornerstone Beliefs of Internet Architectural Principles

• Simple connectivity

• IP protocol: that narrow waist

• Intelligence and complexity at network edge