chapters 4,5: network layermckinley/422/f19/lectures/set4-2.pdf · cse 422 -mckinley network layer...

CSE 422 - McKinley Network Layer

Chapters 4,5: Network Layerr Introduction (forwarding and routing)r Review of queueing theoryr Router design and operationr IP: Internet Protocol

m IPv4 (datagram format, addressing, NAT, ICMP)m IPv6

r Routing algorithms m Link state, Distance Vector

r Routing in the Internetm Autonomous Systemsm Routing protocols (OSPF, BGP)

r Generalized Forwarding & SDN


Router Architecture Overview

Two key router functions:r run routing algorithms/protocol (RIP, OSPF, BGP)r forwarding datagrams from incoming to outgoing link


Input Port Functions

Decentralized switching:r given datagram destination, lookup output

port using forwarding table in input port memory

r goal: complete input port processing at �line speed�

r queueing: if datagrams arrive faster than forwarding rate into switch fabric

Physical layer:bit-level reception

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


Three types of switching fabrics

(or, more generally, aninterconnection network)


Switching Via MemoryFirst generation routers:r were simply computers, with “switching” under direct control of CPUr packet copied to system�s memory, routed, then copied outr speed limited by memory bandwidth (2 bus crossings per datagram)

InputPort

OutputPort

Memory

System Bus


Switching Via a Busr datagram from input port

memoryto output port memory via a shared bus

r bus contention: switching speed limited by bus bandwidth

r 32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers


Switching Via Interconnection Network

r overcome bus bandwidth limitationsr Banyan networks, other interconnection networks

initially developed to connect processors in multiprocessor

r advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric.

r Cisco 12000: switches 60 Gbps through the interconnection network


Output port queueing

r buffering when arrival rate via switch exceeds output line speed

r queueing (delay) and loss due to output port buffer overflow!


Input Port Queueingr if the switching fabric is slower than input ports

combined speed, queueing may occur at input queues r Head-of-the-Line (HOL) blocking: queued datagram

at front of queue prevents others in queue from moving forward

r queueing delay and loss due to input buffer overflow!


Chapters 4,5: Network Layerr Introduction (forwarding and routing)r Review of queueing theoryr Router design and operationr IP: Internet Protocol

m IPv4 (datagram format, addressing, NAT, ICMP)m IPv6

r Routing algorithms m Link state, Distance Vector

r Routing in the Internetm Autonomous Systemsm Routing protocols (OSPF, BGP)

r Generalized Forwarding & SDN

CSE 422 - McKinley

Internet Protocol (IP)r IP is a DoD standard, designed from the

beginning with internetworking in mindr IP features

m unreliable -delivery of packets is not guaranteed

m connectionless -packets are routed and handled independently, even if part of the same message

m best-effort delivery -packets are only discarded when underlying components fail or an overload condition occurs

r Defines basic unit of data transfer, the IP datagram

Network Layer


The Internet Network layerThe picture can't be displayed.

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


IP (v4) datagram format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifierheader

checksumtime to

live32 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 to

head.len

type ofservice

�type� of data flgs fragmentoffset

upperlayer

32 bit destination IP address

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

how much overhead with TCP?r 20 bytes of TCPr 20 bytes of IPr = 40 bytes + app layer

overhead

CSE 422 - McKinley

Internet Addressing

r To support a universal communication service, a globally accepted addressing method is required.

r IPv4 address properties m unique 32-bit number for each host m an IP address comprises two components m netid identifies a network (subnet) m hostid identifies a host on that network

Network Layer


IP Address Allocation

Q: How does an ISP get block of addresses?A: ICANN: Internet Corporation for Assigned

Names and Numbers Non-profit organization that:

m allocates addressesm manages DNSm assigns domain names, resolves disputes

CSE 422 - McKinley

Traditional IPv4 Addressing

32 Bits

Range of host addresses

1.0.0.0 to 127.255.255.255

128.0.0.0 to 191.255.255.255

192.0.0.0 to 223.255.255.255

224.0.0.0 to 239.255.255.255

240.0.0.0 to 247.255.255.255

Class

0 Network Host

10 Network Host

110 Network Host

1110 Multicast address

11110 Reserved for future use

A

B

C

D

E

Network Layer

CSE 422 - McKinley

Addressing Conventionsr Normally, the 32-bit IP addresses are written as

four decimal numbers separated by decimal points m 10000000 00001010 00000010 00011110 m Is written as 128.10.2.30

r A network address with hostid = 0...0 refers to a network.

r A network address with hostid = 1...1 is a (directed) broadcast address, referring to all hosts on a specified network. Where possible (e.g., Ethernet), IP broadcasting takes advantage of a hardware broadcast facility.

Network Layer

CSE 422 - McKinley

Addressing Conventionsr A network address with netid = 1...1 and

hostid =1...1 is a broadcast address for the local network. Local broadcast is used in startup before a host knows its IP address or the IP address of the local network.

r 127.0.0.0 is reserved for loopback. Using this network address, a packet will not get to the network.

r Question: If an IP address identifies a host, how do we assign a single IP address to a gateway/router node, which by definition belongs to multiple networks?

Network Layer


IPv4 Addressingr IP address: 32-bit

identifier for host or router interface

r interface: connection between host/router and physical linkm router�s typically have

multiple interfacesm host typically has one

interfacem 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

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Subnetting (traditional)r Some bits in the hostid part of the IP

address are actually used to specify a particular physical network, such as a LAN on a campus

Network Layer

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Subnetting (traditional)r All gateways on rest of the Internet route

packets as if there were one physical network behind G

r The partitioning of the local address part is an autonomous decision

r A subnet mask indicates which bits in the hostid are to be used for network addressing

r 11111111 11111111 11111111 00000000 11111111 11111111 00011000 01000000

r Although not required to do so, managers would usually select contiguous bits for the subnetwork and use the same mask for all physical networks at a particular site.

Network Layer


Subnetting (generalized)r IP address:

m subnet part (high order bits)

m host part (low order bits)

r What�s a subnet ?m device interfaces with

same subnet part of IP address

m 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


Subnets 223.1.1.0/24 223.1.2.0/24

223.1.3.0/24

Procedurer To determine the

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

Subnet mask notation: address/#bits

CSE 422 - McKinley

Traditional Addressing Problem

Network Layer


IP addressing: CIDRCIDR: Classless InterDomain Routing

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

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23


IP addresses: how to get one?Q: How does a host get IP address?

r Static IP: hard-coded by system admin in a filem Windows:

• control-panel->network->configuration->tcp/ip->propertiesm Traditional UNIX:

• /etc/rc.configm Linux:

• /etc/network/interfacesr DHCP: Dynamic Host Configuration Protocol:

dynamically get address from as serverm �plug-and-play�


DHCP: Dynamic Host Configuration ProtocolGoal: allow host to dynamically obtain its IP address from

network server when it joins networkCan renew its lease on address in useAllows reuse of addresses (only hold address while connected and �on�)

Support for mobile users who want to join network (more shortly)DHCP overview:

m host broadcasts �DHCP discover� msg [optional]m DHCP server responds with �DHCP offer� msg

[optional]m host requests IP address: �DHCP request� msgm DHCP server sends address: �DHCP ack� msg


DHCP client-server scenario

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

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork


DHCP client-server scenarioDHCP server: 223.1.2.5 arriving

client

time

DHCP discover

src : 0.0.0.0, 68 dest.: 255.255.255.255,67yiaddr: 0.0.0.0transaction ID: 654

DHCP offersrc: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 654Lifetime: 3600 secs

DHCP requestsrc: 0.0.0.0, 68 dest:: 255.255.255.255, 67yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

DHCP ACKsrc: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs


Other DHCP functionality

DHCP can return more than just allocated IP address on subnet:m address of first-hop router for clientm name and IP address of DNS severm network mask (indicating network versus host

portion of address)


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



r Motivation: local network uses just one IP address as far as outside world is concerned:m range of addresses not needed from ISP: just one IP

address for all devicesm can change addresses of devices in local network

without notifying outside worldm can change ISP without changing addresses of

devices in local networkm devices inside local net not explicitly addressable,

visible by outside world (a security plus).


NAT: Network Address TranslationImplementation: NAT router must:

m 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.

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

m 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



10.0.0.1

10.0.0.2

10.0.0.3

S: 10.0.0.1, 3345D: 128.119.40.186, 80

110.0.0.4

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 addr138.76.29.7, 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, 802

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 arrivesdest. address:138.76.29.7, 5001

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



r 16-bit port-number field: m 60,000 simultaneous connections with a single LAN-

side address!r Problem with this approach???r NAT is controversial:

m routers should only process up to layer 3m violates end-to-end argument

• NAT possibility must be taken into account by app designers, eg, P2P applications

m address shortage should instead be solved by IPv6


NAT traversal problemr client wants to connect to

server with address 10.0.0.1m server address 10.0.0.1 local

to LAN (client can�t use it as destination addr)

m only one externally visible NATted address: 138.76.29.7

r One solution: statically configure NAT to forward incoming connection requests at given port to serverm e.g., (123.76.29.7, port 2500)

always forwarded to 10.0.0.1 port 25000

10.0.0.1

10.0.0.4

NAT router

138.76.29.7

Client ?


NAT traversal problemr Yet another: relaying (used in Skype)

m NATed client establishes connection to relaym External client connects to relaym relay bridges packets between to connections

138.76.29.7Client

10.0.0.1

NAT router

1. connection torelay initiatedby NATted host

2. connection torelay initiatedby client

3. relaying established


IP Fragmentation & Reassemblyr network links have MTU

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

different MTUs r large IP datagram divided

(�fragmented�) within netm one datagram becomes

several datagramsm �reassembled� only at

final destinationm IP header bits used to

identify, order related fragments

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly


IP Fragmentation and ReassemblyID=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

Exampler 4000 byte

datagramr MTU = 1500 bytes

1480 bytes in data field

offset =1480/8

CSE 422 - McKinley

ICMP: Internet Control Message Protocol

r IP is actually a large distributed systemr ICMP isused by hosts & routers to

communicate network-level informationmerror reporting: unreachable host,

network, port, protocolmecho request/reply (used by ping)

r Technically a network-layer �above� IP:mICMP msgs carried in IP datagrams

Network Layer


ICMP CodesType 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

CSE 422 - McKinley

ICMP message content

r ICMP message: type, code plus IP header and first 8 bytes of payload in datagram causing error. Why the payload???

Network Layer


Traceroute and ICMP

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

r When nth datagram arrives to nth router:m Router discards datagramm And sends to source an

ICMP message (type 11, code 0)

m Message includes name of router& IP address

r When ICMP message arrives, source calculates RTT

r Traceroute does this 3 times

Stopping criterionr UDP segment eventually

arrives at destination host with bogus port number

r Destination returns ICMP �host unreachable� packet (type 3, code 3)

r When source gets this ICMP, stops.

chapters 4,5: network layermckinley/422/f19/lectures/set4-2.pdf · cse 422 -mckinley network layer...

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