Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 1

routing at the network layer, related topics

1. IP addressing, masking, and IP packet format

2. DNS, ARP, RARP, DHCP, and ICMP

3. support for VPNs & tunnels

4. routing over internets

- intradomain (e.g., DV, OSPF)

- interdomain (e.g., BGP)

Chapter 4: internetworking

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 2

summary: packet switching review efficient

can send from any input that is ready

general multiple types of applications

accommodates bursty traffic addition of queues

store and forward packets are self contained units can use alternate paths – reordering

contention (i.e., no isolation) congestion delay

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 3

internetwork (Internet)

• network of networks

• network: “physical” network

• “logical” network: interconnected physical networks

• “router”: boundary node

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 4

a simple internetwork

R2

R1

H4

H5

H3H2H1

Network 2 (Ethernet)

Network 1 (Ethernet)

H6

Network 4(point-to-point)

H7 R3 H8

Network 3 (FDDI)

R1 R2 R3

H1 H8

ETH FDDI

IP

ETH

TCP

FDDI PPP PPP ETH

IP

ETH

TCP

IP IP IP

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 5

Internet Protocol (IP)

• example system– best known– largest (scale)– still, only an example (versions, “alt-IP”)

• service model (host-to-host)– global address scheme– “best effort” delivery (“unreliable”)

• loss, disorder, redundancy

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 6

IP packet format

basic header: 20B

Version HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

SourceAddr

DestinationAddr

Options (variable) Pad(variable)

0 4 8 16 19 31

Data

32b “words”frag/defragword

demuxfor header

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 7

IP packet header fields version number (4 bits)

indicates the version of the ip protocol necessary to know what other fields to expect typically “4” (for IPv4), and sometimes “6” (for IPv6)

header length (4 bits) number of 32-bit words in the header typically “5” (for a 20-byte IPv4 header) can be more when “IP options” are used

type-of-service (8 bits) allow packets to be treated differently based on needs e.g., low delay for audio, high bandwidth for bulk transfer

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 8

IP Packet header fields (continued) total length (16 bits)

number of bytes in the packet maximum size is 63,535 bytes (216 -1) … though underlying links may impose harder limits

fragmentation information (32 bits) packet identifier, flags, and fragment offset supports dividing a large IP packet into fragments … in case a link cannot handle a large IP packet

Time-To-Live (TTL) (8 bits) used to identify packets stuck in forwarding loops … and eventually discard them from the network

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 9

fragmentation & reassembly of IP packets

H1 R1 R2 R3 H8

ETH FDDI

PPP IP (376)

PPP IP (512)

PPP IP (512) (512)

ETH IP

ETH IP

(512)ETH IP

(376)

IP (1400) IP (1400)

R1 R2 R3

max txn unit (MTU)each frag is also a datagramexample: PPP 532B max payload

*

* not reassembled here ...

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 10

IP header (a)

Ident = x

Start of header

Rest of header

1400 data bytes

Offset = 00

(b)

Ident = x

Start of header

Rest of header

512 data bytes

Offset = 01

Ident = x

Rest of header

512 data bytes

Offset = 641

Start of header

Ident = x

Start of header

Rest of header

376 data bytes

Offset = 1280

offset of data only count by 8B

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 11

time-to-live (TTL) field potential robustness problem

forwarding loops can cause packets to cycle forever confusing if the packet arrives much later

time-to-live field in packet header TTL field decremented by each router on the path packet is discarded when TTL field reaches 0… …and “time exceeded” message is sent to the source

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 12

application of TTL in traceroute

time-to-live field in IP packet header source sends a packet with a TTL of n each router along the path decrements the TTL “TTL exceeded” sent when TTL reaches 0

traceroute tool exploits this TTL behavior

sourcedestination

TTL=1

Time exceeded

TTL=2

Send packets with TTL=1, 2, … and record source of “time exceeded” message

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 13

IP packet header fields (continued) Protocol (8 bits)

identifies the higher-level protocol e.g., “6” for the Transmission Control Protocol (TCP) e.g., “17” for the User Datagram Protocol (UDP)

important for demultiplexing at receiving host indicates what kind of header to expect next can call proper protocol handler routine

IP header IP header

TCP header UDP header

protocol=6 protocol=17

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 14

IP packet header fields (continued) Checksum (16 bits)

sum of all 16-bit words in the IP packet header if any bits of the header are corrupted in transit … the checksum won’t match at receiving host receiving host discards corrupted packets

Sending host will retransmit the packet, if needed

134+ 212

= 346

134+ 216

= 350

Mismatch!

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 15

IP packet header (continued) two IP addresses

source IP address (32 bits) destination IP address (32 bits)

destination address unique identifier for the receiving host allows each node to make forwarding decisions

source address unique identifier for the sending host recipient can decide whether to accept packet enables recipient to send a reply back to source

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 16

what if the source lies? source address should be the sending host

but, who’s checking, anyway? you could send packets with any source you want

why would someone want to do this? launch a denial-of-service attack

send excessive packets to the destination … to overload the node, or the links leading to the node

evade detection by “spoofing” but, the victim could identify you by the source address so, you can put someone else’s source address in the packets

also, an attack against the spoofed host spoofed host is wrongly blamed spoofed host may receive return traffic from the receiver

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 17

Ethernet v/s IP addressing

recall Ethernet addresses:1. are 48 bit & globally unique

2. are flat (not hierarchical!)

3. belong to network adaptor (e.g., Ethernet card)

4. once assigned to a card, cannot be changed

IP addresses1. are 32 bit & globally unique (except when using NAT)

2. are hierarchical (network part, host part)

3. belong to a computer/node/station

4. once assigned to a node, may be changed

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 18

some relevant questions to ask

1. how many IP addresses does a host have?

2. how many IP addresses does a router have?

3. how is routing different from forwarding?

4. how do we map IP addresses to Ethernet addresses?

5. how do logical names resolve to IP addresses? ...

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 19

the DNS system hierarchy (from ch 9)

edu com

princeton■ ■ ■

mit

cs ee

ux01 ux04

physics

cisco yahoo nasa nsf arpa navy acm ieee

Gov mil org net uk fr

■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■

Princeton

name server

Cisco

name server

CS

name server

EE

name server...

Root

name server

... zones

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 20

IP address classes

note: If host field is all zeros, then address belongs to network, not host

2b

1b

b/2

b/4

b/4

27-2 (126) 224-2 (16m)

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 21

special IP addresses

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 22

creating subnets

Original host address space

subnet masks – used by routers for routing ... why?

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 23

router implementationrouters handle variable-size packets (unlike simple switches)but, switching fabrics handle fixed-size 'cells'

-- thus, ports must frag/defrag packets into fixed size cells(what is another term for port?)

packet forwarding has- centralized control (= single processing engine), or- distributed control (= multiple engines,

typically, one per line card)

recent development: network processing unit (NPU) - IP address lookup, CRC, checksum, frag/defrag,

forwarding, ...

next slides adapted from: J. Rexford, Princeton

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 24

inside a high-end router

SwitchingFabric

Processor

Line card

Line card

Line card

Line card

Line card

Line card

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 25

router physical layout

Juniper T series

Cisco 12000

Crossbar

Linecards

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 26

line cards (interface cards, adaptors)

interfacing physical link switching fabric

packet handling packet forwarding decrement time-to-live buffer management

link schedulingpacket filteringrate limitingpacket markingmeasurement

to/from link

to/from switch

lookup

Rec

eive

Transm

it

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 27

switching fabric deliver packet inside the router

from incoming interface to outgoing interface a small network in and of itself

must operate very quickly multiple packets going to same outgoing interface switch scheduling to match inputs to outputs

implementation techniques bus, crossbar, interconnection network, … running at a faster speed (e.g., 2x) than links dividing variable-length packets into cells

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 28

packet switching

R1Link 1

Link 2

Link 3

Link 4

Link 1, ingress Link 1, egress

Link 2, ingress Link 2, egress

Link 3, ingress Link 3, egress

Link 4, ingress Link 4, egress

ChooseEgress

ChooseEgress

ChooseEgress

ChooseEgress

“4”

“4”

Don Montgomery, CSE 4344, School of Engineering, Southern Methodist University Chapter 4, slide 29

router processor so-called “loopback” interface

IP address of the CPU on the router

control-plane software implementation of the routing protocols creation of forwarding table for the line cards

interface to network administrators command-line interface for configuration transmission of measurement statistics

handling of special data packets packets with IP options enabled packets with expired time-to-live field