Network Layer I

Question 1

what is the Network layer

Answer 1

• Network layer protocol provides for logical communication between hosts.
• The network layer can be decomposed into two interacting parts, the data plane
  and the control plane.

Question 2

what is the data plane

Answer 2

• The data plane functions include per-router functions in the network layer that
  determine how a datagram arriving on one of a router’s input links is forwarded
  to one of that router’s output links
• Traditionally, the control plane routing protocols and the data plane forwarding
  functions have been implemented together, monolithically, within a router
• Software-defined networking (SDN) explicitly separates the data plane and the
  control plane by implementing the control plane functions as a separate service,
  e.g. in a remote “controller.

Question 3

What is the control plane

Answer 3

• The control plane functions include the network-wide logic that controls how a
  datagram is routed among routers along an end-to-end path from the source
  host to the destination host (using routing algorithms).
• Traditionally, the control plane routing protocols and the data plane forwarding
  functions have been implemented together, monolithically, within a router
• Software-defined networking (SDN) explicitly separates the data plane and the
  control plane by implementing the control plane functions as a separate service,
  e.g. in a remote “controller.

Question 4

How the network layer protocol runs

Answer 4

• The network layer protocols run in every host and
  router.
• On the sending side, the network layer takes
  segments from the transport layer, encapsulates each
  segment into a datagram, and then sends the
  datagrams to its nearby router.
• On the receiving side, the network layer receives the
  datagrams from its nearby router, extracts the
  transport layer segments, and delivers the segments
  up to the transport layer

Question 5

Two key network layer functions

Answer 5

Forwarding
Routing

Question 6

Forwarding

Answer 6

• When a packet arrives at a router’s input link, the router must move the packet
  to the appropriate output link.
• For example, a packet arriving from a host to the nearby router must be
  forwarded to the next router on a path. Forwarding is the most common and
  important one function in the data plane. Other functions include blocking
  packets from exiting a router (if, for example, the packet originated at a known
  malicious sending host, or is destined to a forbidden destination host) or
  duplicating packets and sent over multiple outgoing links

Question 7

Routing

Answer 7

• The network layer must determine the route or path taken by packets as they
  flow from a sender to a receiver.
• The algorithms that calculate these paths are referred to as routing algorithms.
• Routing is implemented in the control plane of the network layer

Question 8

Forwarding and Routing

Answer 8

• Forwarding refers to the router-local action of transferring a packet from an input link
  interface to the appropriate output link interface. Forwarding takes place at very short
  timescales (typically a few nanoseconds), and thus is typically implemented in
  hardware.
• Routing refers to the network-wide process that determines the end-to-end paths
  that packets take from source to destination. Routing takes place on much longer
  timescales (typically seconds), and is often implemented in software.
• A key element in every network router is its forwarding table. A router forwards a
  packet by examining the value of one or more fields in the arriving packet’s header,
  and then using these header values to index into its forwarding table. The value stored
  in the forwarding table entry for those values indicates the outgoing link interface at
  that router to which that packet is to be forwarded

Question 9

how the router’s forwarding
tables are configured?

Answer 9

Traditional approach
SDN approach

Question 10

SDN approach

Answer 10

• The routing algorithm that determines the contents of the routers’ forwarding
  tables runs in a physically separated remote controller that computes and
  distributes the forwarding tables to be used by each and every router.
• The routing device performs forwarding only, while the remote controller
  computes and distributes forwarding tables. The remote controller might be
  implemented in a remote data centre with high reliability and redundancy, and
  might be managed by the ISP or some third party.
• How might the routers and the remote controller communicate?
• By exchanging messages containing forwarding tables and other pieces of
  routing information.

Question 11

Traditional approach

Answer 11

• The routing algorithm that determines the contents of the routers’ forwarding
  tables runs in each and every router and both forwarding and routing functions
  are contained within a router.
• The routing algorithm function in one router communicates with the routing
  algorithm function in other routers to compute the values for its forwarding
  table.
• How is this communication performed?
• By exchanging routing messages containing routing information according to a
  routing protocol.

Question 12

Router architecture

Answer 12

• A router consists of four router components.
  Input ports. An input port performs the physical layer function of terminating an incoming
  physical link at a router.
• An input port also performs link layer functions needed to interoperate with the link
  layer at the other side of the incoming link.
• It also performs a lookup function where the forwarding table is consulted to determine
  the router output port to which an arriving packet will be forwarded via the switching
  fabric.
• Control packets (for example, packets carrying routing protocol information) are
  forwarded from an input port to the routing processor.
• The number of ports supported by a router can range from a relatively small number in
  enterprise routers to hundreds of high speed ports in a router at an ISP’s edge.

Question 13

Switching fabric

Answer 13

The switching fabric connects the router’s input ports to its output ports.
This switching fabric is completely contained within the router

Question 14

Output ports

Answer 14

An output port stores packets received from the switching fabric and
transmits these packets on the outgoing link by performing the necessary link-layer and
physical-layer functions.

Question 15

Routing processor

Answer 15

• In traditional routers, it executes the routing protocols, maintains routing tables and
  attached link state information, and computes the forwarding table for the router.
• In SDN routers, the routing processor is responsible for communicating with the remote
  controller in order to (among other activities) receive forwarding table entries computed
  by the remote controller, and install these entries in the router’s input ports.
• The routing processor also performs the network management functions

Question 16

Routing algorithms

Answer 16

• The goal of a routing algorithm is to determine “good” paths (routes) from senders to
  receivers through the network of routers. A path is the sequence of routers packets will
  traverse in going from source to destination. A good path is the one that has the least cost
  (fastest, least congested, etc)

Question 17

Routing algorithms can be classified as

Answer 17

• Centralised: all routers have the complete
  topology of a network (“link state” algorithms),
  or
• Decentralised: a router knows only the physical
  connected neighbouring routers (“distance
  vector” algorithms).
• Static: routes change very slowly over time.
• Dynamic: change the routing paths as the
  network traffic loads or topology change.
• Load-sensitive: link costs vary dynamically to
  reflect the current level of congestion in the
  underlying link.
• Load insensitive: a link’s cost does not explicitly
  reflect its current (or recent past) level of
  congestion.