nl: router

Network Layer 4-1

Network Layer

Goals: understand principles behind network

layer services: routing (path selection) dealing with scale how a router works advanced topics: IPv6, mobility

instantiation and implementation in the Internet

Network Layer 4-2

Topics: Datagram vs Virtual Circuit Router IP: Internet Protocol

Datagram format, IPv4 addressing ICMP IPv6

4.5 Routing algorithms Link state Distance Vector Hierarchical routing

4.6 Routing in the Internet RIP, OSPF, BGP

Network Layer 4-3

Network layer transport segment from sending to receiving

host on sending side encapsulates segments into

datagrams on rcving side, delivers segments to transport

layer network layer protocols in every host, router Router examines header fields in all IP

datagrams passing through it

networkdata linkphysical








application

transportnetworkdata linkphysical

application


Network Layer 4-4

Key Network-Layer Functions

forwarding: move packets from router’s input to appropriate router output

routing: determine route taken by packets from source to dest.

Routing algorithms

analogy:

routing: process of planning trip from source to dest

forwarding: process of getting through single interchange

Network Layer 4-5

1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-6

Connection setup 3rd important function in some network arch.: Virtual circuits network provides network-layer conn

service used in ATM, frame-relay, X.25 Signaling protocols used to setup, maintain teardown VC

application


application


1. Initiate call 2. incoming call

3. Accept call4. Call connected5. Data flow begins 6. Receive data

Network Layer 4-7

VC implementation

A VC consists of:1. Path from source to destination2. VC numbers, one number for each link along

path3. Entries in forwarding tables in routers along

path Packet belonging to VC carries a VC

number. VC number must be changed on each

link. New VC number comes from forwarding table

Network Layer 4-8

Forwarding table in VC

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC # Outgoing interface Outgoing VC #

1 12 3 222 63 1 18 3 7 2 171 97 3 87… … … …

Forwarding table innorthwest router:

Routers maintain connection state information!Forwarding table is modified whenever there’s conn setup or teardown(happen at a microsecond timescale in a tier-1 router)

Network Layer 4-9

Network service model

Q: What service model for “channel” transporting datagrams from sender to rcvr?

a service model defines the characteristics of end-to-end transport of packets between

Example services for individual datagrams:

guaranteed delivery Guaranteed delivery with

less than certain delay (e.g. 40 msec)?

Example services for a flow of datagrams:

In-order datagram delivery

Guaranteed minimum bandwidth to flow

Restrictions on changes in inter-packet spacing

Network Layer 4-10

Case study: ATM ABR congestion control

two-byte ER (Explicit Rate) field in RM cell congested switch may lower ER value in cell sender’ send rate thus minimum supportable rate on path

across all switches EFCI (Explicit Forward Congestion Indication) bit in data cells: set to

1 in congested switch to indicate congestion to destination host. when RM arrives at destination, if most recently received data

cell has EFCI=1, sender sets CI bit in returned RM cell

Network Layer 4-11

Network layer service models:

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees ?

CBR: constant bit rateVBR: variable bit rateABR: available bit rateUBR: unspecified bit rate

Network Layer 4-12

Datagram or VC network: why?

Internet data exchange among

computers “elastic” service, no strict

timing req. “smart” end systems

can adapt, perform control, error recovery

simple inside network, complexity at “edge”

Additional func built in higher levels

many link types different characteristics uniform service difficult

VC network (e.g. ATM) evolved from telephony human conversation:

strict timing, reliability requirements

need for guaranteed service

“dumb” end systems telephones complexity inside

network (e.g. network-assisted congestion control)

Network Layer 4-13

Topics:

Router IP: Internet Protocol




Network Layer 4-14

Router Architecture Overview

Two key router functions:

run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link E.g. Cisco 12K, Juniper M16, Foundry SuperX

Network Layer 4-15

Input Port Functions

Decentralized switching: given datagram dest., lookup output

port using forwarding table in input port memory

goal: complete input port processing at ‘line speed’

queuing: if datagrams arrive faster than forwarding rate into switch fabric

Physical layer:bit-level reception

Data link layer:e.g., Ethernetsee chapter 5

Network Layer 4-16

Three types of switching fabrics

Network Layer 4-17

Switching Via MemoryFirst generation routers: traditional computers with switching under direct control of CPUpacket copied to system’s memory speed limited by memory bandwidth (2 bus crossings per datagram)

InputPort

OutputPort

Memory

System Bus

Recent development: Processors in input line cards perform lookup and storing packets into memory: shared mem multiprocessorsE.g. Cisco’s Catalyst 8500

Network Layer 4-18

Switching Via a Bus

datagram from input port memory to output port memory via a shared

bus bus contention: switching speed

limited by bus bandwidth 1 Gbps bus, Cisco 1900: sufficient

speed for access and enterprise routers (not regional or backbone)

E.g. 1Gbps bw supports up to 10 T3 (45- Mbps) links

Network Layer 4-19

Switching Via An Interconnection Network overcome bus bandwidth limitations A crossbar switch is an interconnection network

consisting of 2n buses that connect n input to n output ports.

Advanced design: fragmenting datagram into fixed length cells at the input port, switch cells through the fabric and assemble at output ports.

Cisco 12000: switches 60 Gbps through the interconnection network

Omega

Network Layer 4-20

Output Ports

Buffering required when datagrams arrive from fabric faster than the transmission rate Queueing and Buffer management

Scheduling discipline chooses among queued datagrams for transmission

Network Layer 4-21

Output port queueing

buffering when arrival rate via switch exceeds output line speed (switching fabric speed: rate of moving pkt from in-ports to out-ports)

queueing (delay) and loss due to output port buffer overflow! Buffer size = RTT times Link Capacity

A packet scheduler at output port must choose among queued to transmit using FIFO or more sophisticated such as weighted fair queuing (WFQ) that shares the outgoing link fairly among different end-to-end connections.

Network Layer 4-22

Input Port Queuing If fabric slower than input ports combined then queueing

may occur at input queues. It can be eliminated if the switching fabric speed is at least n times as fast as the input line speed, where n is the number of input ports

Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward. Only occurs at input ports. As soon as the packet arrival rate on the input lines reaches 58% of their capacity, the input queue will grow to unbounded length, due to HOL blocking

queueing delay and loss due to input buffer overflow!

Network Layer 4-23

Active Queue Management Drop-Tail policy

Drop arrival packets due to overflow Random Early Detection (RED)

Maintain a weighted average for the length of the output queue

If queue length < Threshold_min, admit it If queue length > Threshold_max, drop it Otherwise, drop it with a probability (a function

of the average queue length) RED drops packets before the buffer is full

in order to provide congestion signals to senders

Router Processor Execute routing protocols Maintain the routing information and

forwarding tables Perform network management functions

Network Layer 4-24

CISCO 12000 Gigabit Router Processor (GRP)

Network Layer 4-25

Forwarding table

Destination Address Range Link Interface 11001000 00010111 00010000 00000000 through 0 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000 through 1 11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000 through 2 11001000 00010111 00011111 11111111

otherwise 3

packets forwarded using destination host addressThe tables are modified by routing alg anytime (every 1~5 minutes)

packets between same source-dest pair may take diff paths

Network Layer 4-26

Longest prefix matching

DA: 11001000 00010111 00011000 10101010

Examples

DA: 11001000 00010111 00010110 10100001 Which interface?

Prefix Match Link Interface 11001000 00010111 00010 0 11001000 00010111 00011000 1 11001000 00010111 00011 2 otherwise 3

Forwarding table with 4 entries and using longest prefix match:

Network Layer 4-27

Lookup in an IP Router

Unicast destination address based lookup

Dstn Addr

Next Hop

--------

---- ----

--------

Dstn-prefix Next HopForwarding Table

Next Hop Computation

Forwarding Engine

Incoming

Packet

HEADER

Need to be as fast as line speed!! e.g OC48 link runs at 2.5Gbps, packet=256bytes 1 million lookups/sLow storage : ~100K entriesFast updates: few thousands per second, but ideally at lookup speed

Network Layer 4-28

Route Lookup Using CAM

PriorityEncoder

Location 0

1

2

3

4

5

6

103.23.122.7 P1

P1 103.23.122/23 171.3.2.22

P2

P3

P4

P5

103.23/16

101.1/16

101.20/13

100/9

171.3.2.4

120.33.32.98

320.3.3.1

10.0.0.111

Prefix Next-hop1

1

0

0

0

0

0

To find the longest prefix cheaply, need to keep entries sorted in order of decreasing prefix lengthsK. pagiamtzis, Intro to CAM pagiamtzis.com/cam/camintro.html

Content-Address Memory: Fully associative mem: Cisco 8500

Exact match (fixed-length) search op in a single clock cycle

Network Layer 4-29

Topics:

Router IP: Internet Protocol




Network Layer 4-30

The Internet Network layer

forwardingtable

Host, router network layer functions:

Routing protocols•path selection•RIP, OSPF, BGP

IP protocol•addressing conventions•datagram format•packet handling conventions

ICMP protocol•error reporting•router “signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer

Network Layer 4-31

IP datagram format

ver Datagram length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

Header checksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length (bytes)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength, bytes)

upper layer protocolto deliver payload to6 for tcp, 17 for udp

head.len

type ofservice

“type” of data flgsfragment

offsetupper layer

32 bit destination IP address

Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit.

how much overhead with TCP?

20 bytes of TCP 20 bytes of IP = 40 bytes +

app layer overhead

Network Layer 4-32

IP Fragmentation & Reassembly network links have MTU

(max.transfer size) - largest possible link-level frame. different link types,

different MTUs large IP datagram divided

(“fragmented”) within net one datagram becomes

several datagrams “reassembled” only at

final destination IP header bits used to

identify, order related fragments

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly

Network Layer 4-33

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte IP

datagram MTU = 1500 bytes (4000-20 bytes

header)=3980 bytes of data to be fragmented

3 fragments (1480+1480+1020=3980)

amount of data in all but last fragment must be multiples of 8 1480 bytes in

data fieldoffset =1480/8

Network Layer 4-34

IP Addressing: introduction IP address: 32-bit

identifier for host, router interface, in dotted-decimal notation

interface: connection between host/router and physical link router’s typically have

multiple interfaces host typically has one

interface IP addresses associated

with each interface

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

Network Layer 4-35

Subnets (aka IP networks) IP address:

subnet part (high order bits)

host part (low order bits)

What’s a subnet ? device interfaces with

same subnet part of IP address

can physically reach each other without intervening router

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

network consisting of 3 subnets

subnet

To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet.

Network Layer 4-36

SubnetsHow many? 223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2223.1.2.1

223.1.2.6

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.0

223.1.7.1223.1.8.0223.1.8.1

223.1.9.1

223.1.9.2

Network Layer 4-37

IP addressing: CIDRCIDR: Classless InterDomain Routing

subnet portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in subnet

portion of address. Notation /x is subnet mask. The high order x bits are

the network prefix.

Before CIDR, classful addressing: A (/8), B(/16), C(/24). Replaced by CIDRized address.

11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23

Network Layer 4-38

IP addresses: how to get one?

Q: How does host get IP address?

hard-coded by system admin in a file Wintel: control-panel->network->configuration->tcp/ip-

>properties UNIX: /etc/rc.config

DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server “plug-and-play”

5: DataLink Layer 5-39

DHCP (Dynamic Host Configuration Protocol)

The DHCP relay agent (implemented in the IP router) records the subnet from which the message was received in the DHCP message header for use by the DHCP server.

Network Layer 4-40

IP addresses: how to get one?

Q: How does network get subnet part of IP addr?

A: gets allocated portion of its provider ISP’s address space

ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. ….

Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23

Network Layer 4-41

Hierarchical addressing: route aggregation

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16”

200.23.20.0/23Organization 2

...

...

Hierarchical addressing allows efficient advertisement of routing information.

Two example businesses

Network Layer 4-42

Hierarchical addressing: more specific routesAssume ISPs-R-Us has been acquired by FBN-ISP and Org1 be transferredto ISPs-R-Us:

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16or 200.23.18.0/23”

200.23.20.0/23Organization 2

...

...

Network Layer 4-43

IP addressing: the last word...

Q: How does an ISP get a block of addresses?

A: ICANN: Internet Corporation for Assigned

Names and Numbers: www.icann.org allocates address space Top-level domain name system management manages DNS root servers Protocol identifier assignment assigns domain names, resolves disputes

http://www.icann.org/

Network Layer 4-44

NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

10.0.0.4

138.76.29.7

local network(e.g., home network)

10.0.0/24

rest ofInternet

Datagrams with source or destination in this networkhave 10.0.0/24 address for

source, destination (as usual)

All datagrams leaving localnetwork have same single source

NAT IP address: 138.76.29.7,different source port numbers

Network Layer 4-45


Motivation: local network uses just one IP address as far as outside world is concerned: no need to be allocated range of addresses from

ISP: - just one IP address is used for all devices can change addresses of devices in local network

without notifying outside world can change ISP without changing addresses of

devices in local network devices inside local net not explicitly

addressable, visible by outside world (a security plus).

Network Layer 4-46


Implementation: NAT router must:

outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #). . . remote clients/servers will respond using (NAT IP

address, new port #) as destination addr.

remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair

incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table

Network Layer 4-47


10.0.0.1

10.0.0.2

10.0.0.3

S: 10.0.0.1, 3345D: 128.119.40.186, 80

1

10.0.0.4

138.76.29.7

1: host 10.0.0.1 sends datagram to 128.119.40.186, 80

NAT translation tableWAN side addr LAN side addr

138.76.29.7, 5001 10.0.0.1, 3345…… ……

S: 128.119.40.186, 80 D: 10.0.0.1, 3345

4

S: 138.76.29.7, 5001D: 128.119.40.186, 80

2

2: NAT routerchanges datagramsource addr from10.0.0.1, 3345 to138.76.29.7, 5001,updates table

S: 128.119.40.186, 80 D: 138.76.29.7, 5001

3

3: Reply arrives dest. address: 138.76.29.7, 5001

4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345

Network Layer 4-48


16-bit port-number field: 60,000 simultaneous connections with a

single LAN-side address! NAT is controversial:

routers should only process up to layer 3 but NAT router need to change the transport port.

violates end-to-end argument• NAT possibility must be taken into account by app

designers, eg, P2P applications address shortage should instead be solved by

IPv6

Skype through NAT

NAT prevents a connection from being initiated from outside.

How can Alice call Bob, both residing behind NAT (NAT traversal) ?? Alice sign-in with its super-peer (Sa) Bob sign-in with its super-peer (Sb) Alice calls Bob: Alice SaSbBob If Bob takes the call, Sa and Sb select a non-

NAT super-peer for voice relay See chapter 2 (4th ed) for details

Network Layer 4-49

Network Layer 4-50

Recap: Internet Network layer

forwardingtable

Host, router network layer functions:

Routing protocols•path selection•RIP, OSPF, BGP

IP protocol•addressing conventions•datagram format•packet handling conventions

ICMP protocol•error reporting•router “signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer

Network Layer 4-51

ICMP: Internet Control Message Protocol used by hosts & routers to

communicate network-level information error reporting:

unreachable host, network, port, protocol

echo request/reply (used by ping)

network-layer “above” IP: ICMP msgs carried in IP

datagrams ICMP message: type, code,

different fields depending on the type/code. If it’s a reply type then it would have IP Header and first 8 bytes of IP datagram data.

Type Code description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header

Taxonomy 1-52

Recap: “Real” Internet delays and routes What do “real” Internet delay & loss look like? Traceroute program (in Unix) or Tracert (MS-DOS):

provides delay measurement from source to router along end-end Internet path towards destination. For all i: sends three packets that will reach router i on path

towards destination router i will return packets to sender sender times interval between transmission and reply.

3 probes

3 probes

3 probes

Network Layer 4-53

Traceroute and ICMP

Source sends series of UDP segments to dest First has TTL =1 Second has TTL=2, etc. Unlikely port number

(depends on implementation) When nth datagram arrives

to nth router: Router discards datagram And sends to source an ICMP

message (type 11, code 0 which means TTL expired)

Message includes name of router& IP address

When ICMP message arrives, source calculates RTT

Traceroute does this 3 timesStopping criterion UDP segment eventually

arrives at destination host Destination returns ICMP

“host unreachable” packet (type 3, code 3) if port is sent. When source gets this ICMP, stops.

Other Tracert implementation stops when ping reply is received from destination.

Network Layer 4-54

IPv6 Initial motivation: 32-bit address space

soon to be completely allocated. Expanded addressing capabilities: 128 bit

Additional motivation: header format helps speed

processing/forwarding• fixed-length 40 byte header• no fragmentation allowed

header changes to facilitate QoS• Flow label and priority

These are also three most important changes

Network Layer 4-55

IPv6 Header (Cont)Priority (8-bits): identify priority among datagrams in flowFlow Label (20-bits): identify datagrams in same “flow.” (concept of“flow” not well defined).Next header (8-bits): identify upper layer protocol for data (similar to Upper-layer protocol in IPv4)

Traffic Class is similar to Type Of Service in IPv4

Similar to TTL in IPv4 (8-bits)

Network Layer 4-56

Other Changes from IPv4

Checksum: removed entirely to reduce processing time at each hop

Options: allowed, but outside of header, indicated by “Next Header” field

ICMPv6: new version of ICMP additional message types, e.g. “Packet Too

Big” multicast group management functions

Network Layer 4-57

Transition From IPv4 To IPv6

Not all routers can be upgraded simultaneous no “flag days” How will the network operate with mixed IPv4 and

IPv6 routers? Two proposed solutions:

Dual-stack approach: IPv6 to IPv4 and vice versa translation of datagrams at routers that can understand IPv6 and IPv4. Some fields data will be lost.

Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers

Network Layer 4-58

TunnelingA B E F

IPv6 IPv6 IPv6 IPv6

tunnelLogical view:

Physical view:A B E F

IPv6 IPv6 IPv6 IPv6

C D

IPv4 IPv4

Flow: XSrc: ADest: F

data


data


data

Src:BDest: E


data

Src:BDest: E

A-to-B:IPv6

E-to-F:IPv6

B-to-C:IPv6 inside

IPv4

D-to-E:IPv6 inside

IPv4

Network Layer 4-59

Summary

Network Layer Services: forwarding, routing and connection setup in some networks

Best effort network Service Models Router Architecture Overview

Input/Output ports and queuing Switching via Memory/Bus/Interconnected network

Network Layer Functions: forwarding via routing protocols, routing via IP error reporting via ICMP

IP Datagram Format IP Fragmentation and Reassembly IP Addressing: subnets, CIDR, assignments, Hierarchical addressing Network Address Translation (NAT) Internet Control Message Protocol (ICMP) usage IPv6 motivation, datagram format and transition to IPv4 through

Tunneling

nl: router

Documents

internet network layer

network layer services

layer network layer

bgp network layer

network layer goals

forwarding table network

key networklayer functions

virtual circuits network