The Network Layer · Outline Basic network-layer architecture of a datagram network Introduction to...

The Network Layer

Antonio Carzaniga

Faculty of InformaticsUniversity of Lugano

April 21, 2009

Outline

Basic network-layer architecture of a datagram network

Introduction to forwarding

Introduction to routing

General architecture of a router

Switching fabric and queuing

Internet network-layer protocol

The Internet protocol (IP)

Fragmentation

Application Level

browser

server

Application Level

browser

server

Application Level

browser

server

GET /carzaniga/ HTTP/1.1

Host: www.inf.unisi.ch

Application Level

browser

server

HTTP/1.1 200 OK

Application Level

browser

server

GET /carzaniga/anto.png HTTP/1.1

Host: www.inf.unisi.ch

Application Level

browser

server

HTTP/1.1 200 OK

Transport Level

browser

server

Transport Level

browser

server

Transport Level

browser

server

Network Layer

browser

server

Network Layer

browser

server

Network Layer

browser

server

Router

Fundamental component of the network layer

Router

A node in a graph

Router

A node in a graph

A finite set of input/output (physical) connections◮ a.k.a., interfaces or ports

Focus: “Datagram” Networks

Packet-switched network

◮ information is transmitted in discrete units called datagrams

Connectionless service

◮ a datagram is a self-contained message

◮ treated independently by the network

◮ no connection setup/tear-down phase

“Best-effort” service

◮ delivery guarantee: none

◮ maximum latency guarantee: none

◮ bandwidth guarantee: none

◮ in-order delivery guarantee: none

◮ congestion indication: none

Datagram Network

Potentially multiple paths for the same source/destination

Datagram Network

Potentially asymmetric paths

Datagram Network

Forwarding

A sends a datagram to B

Forwarding

to: B. . .

The datagram is forwarded towards B

Forwarding

to: B. . .

The datagram is forwarded towards B

Forwarding

to: B. . .

Forwarding

to: B. . .

Forwarding

to: B. . .

forwarding

dest. output

. . . . . .

B port 4

. . . . . .

Forwarding

to: B. . .

forwarding

dest. output

. . . . . .

B port 4

. . . . . .

Forwarding

to: B. . .

forwarding

dest. output

. . . . . .

B port 4

. . . . . .

Forwarding

to: B. . .

forwarding

dest. output

. . . . . .

B port 4

. . . . . .

Forwarding

Input: datagram destination

Forwarding

Output: output port

Forwarding

Output: output port

Simple design: “forwarding table”

Forwarding

Output: output port

Issues

Forwarding

Output: output port

Issues

◮ how big is the forwarding table?

Forwarding

Output: output port

Issues

◮ how fast does the router have to forward datagrams?

Forwarding

Output: output port

Issues

◮ how fast does the router have to forward datagrams?

◮ how does the router build and maintain the forwarding table?

Routing

router k

Router Functions

routing

communications

with neighbors:

routing protocol

routing

Router Functions

routing

communications

with neighbors:

routing protocol

routing

forwarding

Router Functions

routing

communications

with neighbors:

routing protocol

routing

forwarding

forwardinginput packets

from input ports

output packets

to output ports

Anatomy of a Router

routing

processor

input port

output port

switch

fabric

Anatomy of a Router

routing

processor

input port

output port

switch

fabric

data link

processing

lookup

forwarding

queuing

Anatomy of a Router

routing

processor

input port

output port

switch

fabric

Anatomy of a Router

routing

processor

input port

output port

switch

fabric

queuing data link

processing

Queuing

Where does queuing occur?

Queuing

Input ports

◮ queuing may occur here if the switching fabric is slower than

the aggregate speed of all the input lines. I.e., TS < nTin

Queuing

Input ports

◮ queuing may occur here if the switching fabric is slower than

the aggregate speed of all the input lines. I.e., TS < nTin

Output ports

◮ queuing may occur here because of the limited throughput of

the output link. I.e., Tout < min(TS ,nTin)

Queuing

What happens when packets queue up in a router?

Queuing

Scheduling: deciding which packets to process

Queuing

◮ first-come-first-served

Queuing

◮ weighted fair queuing: the router tries to be balance traffic

evenly among the different end-to-end connections. Essential

to implement quality-of-service guarantees

Queuing

Deciding when to drop packets, and which packets to drop

Queuing

◮ drop tail: drop arriving packets when queues are full

Queuing

◮ drop tail: drop arriving packets when queues are full

◮ active queue management: a set of policies and algorithms to

decide when and how to drop or mark packets in the attempt

to prevent congestion

Internet Network Layer

Routing: defining paths and compiling forwarding tables

◮ RIP

◮ OSPF

◮ BGP

◮ RIP

◮ OSPF

◮ BGP

◮ RIP

◮ OSPF

◮ BGP

◮ addressing

◮ datagram format

◮ fragmentation and packet handling

◮ RIP

◮ OSPF

◮ BGP

◮ addressing

◮ datagram format

◮ RIP

◮ OSPF

◮ BGP

◮ addressing

◮ datagram format

◮ error reporting

◮ signaling

IPv4 Datagram Format

vers. hlen

vers. hlen type of service

vers. hlen type of service datagram length

identifier flags fragmentation offset

time-to-live

time-to-live protocol

time-to-live protocol header checksum

source address

destination address

source address

destination address

options (if any)

source address

destination address

options (if any)

Fragmentation

routing

processor

input port

output port

switch

fabric

Fragmentation

routing

processor

input port

output port

switch

fabric

Fragmentation

routing

processor

input port

output port

switch

fabric

Fragmentation

routing

processor

input port

output port

switch

fabric

Fragmentation

routing

processor

input port

output port

switch

fabric

MTU = 1500b

size = 1000b

Fragmentation

routing

processor

input port

output port

switch

fabric

MTU = 1500b

size = 1000b

MTU = 512b

Fragmentation

routing

processor

input port

output port

switch

fabric

MTU = 1500b

size = 1000b

MTU = 512b

How does the router handle cases where the size of an input

datagram exceeds the maximum transmission unit (MTU) of

the output link?

Fragmentation

routing

processor

input port

output port

switch

fabric

MTU = 1500b

size = 1000b

MTU = 512b

How does the router handle cases where the size of an input

datagram exceeds the maximum transmission unit (MTU) of

the output link?

The datagram is fragmented

Fragmentation

input datagram

header

Fragmentation

input datagram

header

Fragmentation

input datagram

header

fragment 1

header

fragment 2

header

Fragmentation

input datagram

header

fragment 1

header

fragment 2

header

The destination reassembles fragmented datagrams

Fragmentation

input datagram

header

fragment 1

header

fragment 2

header

◮ not intermediate routers

◮ this is mainly to push complexity out of the network

Fragmentation

input datagram

header

fragment 1

header

fragment 2

header

◮ a datagram may have to be fragmented further along the path

Fragmentation

input datagram

header

fragment 1

header

fragment 2

header

The fragmentation scheme must ensure that the destinationhost

◮ can recognize two fragments of the same original datagram

◮ can figure out if and when all the fragments have been received

Fragmentation

input datagram

header

fragment 1

header

fragment 2

header

The fragmentation scheme must ensure that the destinationhost

◮ can recognize two fragments of the same original datagram

◮ can figure out if and when all the fragments have been received

The fragmentation scheme must ensure that the intermediate

routers can fragment a datagram to whatever level necessary

Fragmentation

Initial (non-fragmented) datagram format (datasize = 1000)

Fragmentation

◮ sender host assigns a 16-bit identifier to the datagram (e.g.,

Fragmentation

◮ the fragment offset is set to 0, indicating that this packet

contains data starting at position 0 of the original datagram

◮ fragment offset is actually the offset in units of 8 bytes

(remember it’s only 13 bits. . . )

Fragmentation

◮ the “more fragments” flag is set to 0, indicating that no (more)

fragments have been sent

Fragmentation

◮ the “more fragments” flag is set to 0, indicating that no (more)

fragments have been sent

identifier fragment more header total

offset fragments length length

789 0 0 20 1020

Fragmentation

Fragmentation to an MTU of 512

Fragmentation

◮ sender must split the datagram into 3 fragments:

Fragmentation

789 0 1 20 508

Fragmentation

789 0 1 20 508

789 61 1 20 508

Fragmentation

789 0 1 20 508

789 61 1 20 508

789 122 0 20 44

The Network Layer · Outline Basic network-layer architecture of a datagram network Introduction to...

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