CSCD58 W INTER 2018 W EEK 6 - N ETWORK L AYER P ART 1 Brian - - PowerPoint PPT Presentation

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CSCD58 W INTER 2018 W EEK 6 - N ETWORK L AYER P ART 1 Brian - - PowerPoint PPT Presentation

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Admin VC vs Datagram IP DHCP Break CIDR NAT ICMP IPv6 CSCD58 W INTER 2018 W EEK 6 - N ETWORK L AYER P ART 1 Brian Harrington University of Toronto Scarborough February 13, 2018 Admin VC vs Datagram IP DHCP Break CIDR NAT ICMP

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Admin VC vs Datagram IP DHCP Break CIDR NAT ICMP IPv6

CSCD58 WINTER 2018

WEEK 6 - NETWORK LAYER PART 1 Brian Harrington

University of Toronto Scarborough

February 13, 2018

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ADMIN

  • Assignments
  • Midterm after reading week (Feb 27)
  • In class
  • Covering everything up-to transport layer (i.e., not this

week’s material)

  • No lecture or tutorials that week
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CONNECTION VS CONNECTION-LESS

  • Recall: In Transport layer we had 2 options:
  • UDP: Connectionless - just send data out
  • TCP: Connection-oriented: establish a connection first
  • In Network Layer, we also have 2 options:
  • Datagram: Let the data find its own way
  • Virtual Circuit (VC): Establish end-to-end pathway first
  • Actually... we don’t have any choice. Network is either

Datagram or VC, implemented in network core.

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VIRTUAL CIRCUIT NETWORKS

  • Try to emulate behaviour of circuit switched network
  • Set up call before (tear down after)
  • Each VC path segment has unique number
  • Packet only has to hold VC number (not source or dest)
  • Routers keep state of each VC
  • Can allocate resources (bandwidth, buffers, etc) to each

VC = predictable, reliable circuit-switching-like performance

  • Example
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VIRTUAL CIRCUIT

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DATAGRAM

  • No call setup
  • Routers don’t need to keep any state info
  • At each point, packets are forwarded using host address
  • Routers can’t know all 4 billion + IP addresses
  • List range instead
  • Example
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DATAGRAM EXAMPLE

  • 11001000 00010111 00010110 10100001
  • 11001000 00010111 00011000 10101010
  • 11001000 00010111 00001011 00110101
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DATAGRAM OR VC?

  • VC
  • Smart network core, dumb end systems
  • Complexity inside network
  • Can provide timing, reliability guarantees
  • Used in Asynchronous Transfer Mode (ATM) networks
  • Network World 2001: “I give Ethernet six months before the

whole thing is gone”... “This is just not going to work, ATM is going to dominate”

  • Datagram
  • Smart end systems, dumb core
  • “Complexity at the edge”
  • Elastic/flexible
  • Most importantly? Cheap!
  • Internet standard
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FORMAT OF IP DATAGRAM

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FORMAT OF IP DATAGRAM

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IP FRAGMENTATION

  • Not everyone can carry packets of the same size
  • Ethernet: 1500 bytes
  • WLAN (802.11): 7891 bytes
  • Some WANs: 576 bytes
  • Sometimes we’ll need to fragment packets and put them

back together later

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IP FRAGMENTATION

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IP FRAGMENTATION

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IP: INTERNET PROTOCOL

  • Note: Focusing on IP v4 (we’ll discuss IPv6 later)
  • Each interface identified by 32-bit number
  • Interface: connection between host/router and physical link
  • routers usually have many interfaces
  • hosts usually have 1-2 (e.g,. ethernet + wifi)
  • 223.1.2.1 = 11011111 00000001 00000010 00000001
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SUBNETS

  • subnet: group of device interfaces with same higher order

part of IP address

  • interfaces on a subnet can talk to each other without

intervening router

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SUBNETS

  • subnet: group of device interfaces with same higher order

part of IP address

  • interfaces on a subnet can talk to each other without

intervening router

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SUBNETS

  • Pop quiz: how many subnets? What are their masks?
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DHCP: DYNAMIC HOST CONFIGURATION PROTOCOL

  • Hosts should be mobile
  • Want to dynamically obtain IP address from local network

server when you join a network

  • Each host leases an address (can renew periodically)
  • Allows reuse of addresses (when lease expires, address is

up-for-grabs

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DHCP

  • Basic Steps:
  • host broadcasts: DHCP discover [optional]
  • DHCP server responds: DHCP offer [optional]
  • host requests: DHCP request
  • DHCP server sends: DHCP ack
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DHCP

  • 255.255.255.255 = broadcast to entire subnet
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CIDR: CLASSLESS INTERDOMAIN ROUTING

  • So how do we get IP addresses in the first place?
  • ISP gets allocated a block by ICANN (Internet Corporation

for Assigned Names and Numbers) http://www.icann.org/

  • Organization (could also be ISP) gets a sub-block

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

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CIDR

  • Hierarchical Addressing
  • Allows efficient advertising of routing information
  • But what if an organization wants to move?
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CIDR

  • No problem, just go with whoever has the most specific

route

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NAT: NETWORK ADDRESS TRANSLATION

  • Not every interface needs a unique IP
  • Not enough addresses to go around
  • IPv4 - 232 ∼ 4.2 billion addresses
  • Too much work to keep track of every update
  • Solution: “hide” network behind a single ISP
  • To the rest of the world: whole network has 1 address
  • Inside network: We’ve got loads of room
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NAT: NETWORK ADDRESS TRANSLATION

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NAT: NETWORK ADDRESS TRANSLATION

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NAT: NETWORK ADDRESS TRANSLATION

  • Benefits of NAT
  • Only need 1 IP address from ISP
  • Can change addresses of local devices without notifying
  • utside world
  • Can change ISPs without affecting local network
  • Devices inside network not directly accessible/visible to
  • utside world (good for security)
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NAT: NETWORK ADDRESS TRANSLATION

  • Idea: Translate each local IP + port number into public port

number

  • Outgoing:

nat_sourceip +nat_sourceport → externalip +externalport

  • Incoming:

externalip +externalport → nat_sourceip +nat_sourceport

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NAT: NETWORK ADDRESS TRANSLATION

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NAT: NETWORK ADDRESS TRANSLATION

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NAT: NETWORK ADDRESS TRANSLATION

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NAT: NETWORK ADDRESS TRANSLATION

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NAT: NETWORK ADDRESS TRANSLATION

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ICMP: INTERNET CONTROL MESSAGE PROTOCOL

  • Simple messages sent at

network level

  • Sort of like a pseudo-layer

between Transport and Network layers

  • Useful for a lot of low-level

functions

Type Code description echo reply (ping) 3

  • dest. network unreachable

3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 source quench (congestion control - not used) 8 echo request (ping) 9 route advertisement 10 router discovery 11 TTL expired 12 bad IP header

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ICMP & TRACEROUTE

  • Traceroute is implemented in ICMP
  • Less overhead to use bandwidth/mess up timings
  • Idea: use TTL field
  • When TTL expires, router sends back ICMP 11-0:

TTL expired, with router info attached

  • Send out UDP segment with TTL = 1, get info of first router
  • Send out UDP segment with TTL = 2, get info of second

router

  • Eventually, reach destination, pick unlikely port#. Host will

send back ICMP 3-3: Dest port unreachable

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IPV6

  • IPv4’s 32-bit address space “soon to be” used up
  • (actually practically exhausted in 2011)
  • Updated header format to improve speed
  • Better processing, faster forwarding, get rid of unused fields
  • Can add QoS info
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IPV6

  • fixed length header
  • no fragmentation allowed
  • datagrams form a flow
  • Not really well defined
  • No checksum (let TCP handle that)
  • Options indicated by next_hdr field (held inside data)
  • Major difference: 128bit addressing = 2128 ≈ 3.4x1028

addresses

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IPV6

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IPV6

  • 1996 - defined (RFC 1883)
  • 2011 - IPv4 exhausted, plans for IPv6 to take over bulk of

networking traffic by the end of the year

  • 2017 - We’ve finally reached a milesone...
  • (of 20% adoption)