COMP 3331/9331: Computer Networks and Applications

Network Layer

COMP 3331/9331:� Computer Networks and

ApplicationsWeek 8

Network Layer Part 2Reading Guide: Chapter 4: Sections 4.3 & 4.4

Assignment Due Soon

Network Layer 2

v  HARD Deadline: 16th Sept (Fri) v  MAKE SURE YOUR CODE

WORKS ON A CSE MACHINE

v  Remember to disable all debugging output

v  Remember to submit report v  2 Bonus marks can be used to

cover lost marks in any assessable component

v  Assignment 2 to be released next week (deadline: Friday, Week 13)

3

4.1 introduction 4.2 virtual circuit and

datagram networks 4.3 what’s inside a router 4.4 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

4.7 broadcast and multicast routing

Network Layer: outline

Network Layer

4

IP addressing: introduction

v  IP address: 32-bit identifier for host, router interface

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

multiple interfaces §  host typically has one or

two interfaces (e.g., wired Ethernet, wireless 802.11)

v  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

223.1.3.2 223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 1 1

Network Layer

5

IP addressing: introduction

Q: how are interfaces actually connected? A: we’ll learn about that in the link layer

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

223.1.3.2 223.1.3.1

223.1.3.27

A: wired Ethernet interfaces connected by Ethernet switches

A: wireless WiFi interfaces connected by WiFi base station

For now: don’t need to worry about how one interface is connected to another (with no intervening router)

Network Layer

6

Subnets

v IP address: § subnet part - high order bits

§ host part - low order bits

v what’s a subnet ? § device interfaces with same subnet part of IP address

§ can physically reach each other without intervening router network consisting of 3 subnets

223.1.1.1

223.1.1.3

223.1.1.4 223.1.2.9

223.1.3.2 223.1.3.1

subnet

223.1.1.2

223.1.3.27 223.1.2.2

223.1.2.1

Network Layer

7

recipe v  to determine the

subnets, detach each interface from its host or router, creating islands of isolated networks

v  each isolated network is called a subnet

subnet mask: /24

Subnets 223.1.1.0/24

223.1.2.0/24

223.1.3.0/24

223.1.1.1

223.1.1.3

223.1.1.4 223.1.2.9

223.1.3.2 223.1.3.1

subnet

223.1.1.2

223.1.3.27 223.1.2.2

223.1.2.1

Network Layer

8

how many? 223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2 223.1.2.1

223.1.2.6

223.1.3.2 223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.0

223.1.7.1 223.1.8.0 223.1.8.1

223.1.9.1

223.1.9.2

Subnets

Network Layer

Original Internet Addressesv  First eight bits: network address (/8) v  Last 24 bits: host address

Assumed 256 networks were more than enough!

Network Layer 9

v  Three main classes

§  Class A

§  Class B

§  Class C

Next Design: “Classful” Addressing

network host 0

0

~2M nets 254 hosts

8

network host 1

16 0

0

network host 1

24 0

1 0

~16K nets ~65K hosts

126 nets ~16M hosts

Problem: Networks only come in three sizes! Network Layer 10

11

Today’s addressing: CIDR

CIDR: 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

11001000 00010111 00010000 00000000

subnet part

host part

200.23.16.0/23

200.23.17.255

200.23.16.0

/23

IP address

Network Layer

12

Subnet Address v  Subnet Mask

§  Used in conjunction to with the network address to indicate how many higher order bits are used for the network part of the address (i.e. network prefix)

•  Bit-wise AND §  223.1.1.0/24 is equivalent to

223.1.1.0 with subnet mask 255.255.255.0

v  Broadcast Address §  host part is all 111’s §  E.g. 223.1.1.255

v  Subnet Address §  Host part is all 0000’s §  E.g. 223.1.1.0

v  Both of these are not assigned

223.1.1.0/24 223.1.2.0/24

223.1.3.0/24 Host B Dot-decimal

address Binary

IP address 223.1.1.2 11111101.00000001.00000001.00000010

Subnet Mask 255.255.255.0 11111111.11111111.11111111.00000000

Network Part 223.1.1.0 11111101.00000001.00000001.00000000

Host Part 0.0.0.2 00000000.00000000.00000000.00000010

B: 223.1.1.2

Network Layer

13

IP addresses: how to get one? Q: How does a host get IP address? v  hard-coded by system admin in a file

§  Windows: control-panel->network->configuration->tcp/ip->properties

§  UNIX: /etc/rc.config v  DHCP: Dynamic Host Configuration Protocol:

dynamically get address from as server §  “plug-and-play”

Network Layer

14

DHCP: Dynamic Host Configuration Protocol goal: allow host to dynamically obtain its IP address from network server when it

joins network §  can renew its lease on address in use §  allows reuse of addresses (only hold address while

connected/“on”) §  support for mobile users who want to join network (more

shortly) DHCP overview:

§  host broadcasts “DHCP discover” msg §  DHCP server responds with “DHCP offer” msg §  host requests IP address: “DHCP request” msg §  DHCP server sends address: “DHCP ack” msg

Network Layer

15

DHCP client-server scenario

223.1.1.0/24

223.1.2.0/24

223.1.3.0/24

223.1.1.1

223.1.1.3

223.1.1.4 223.1.2.9

223.1.3.2 223.1.3.1

223.1.1.2

223.1.3.27 223.1.2.2

223.1.2.1

DHCP server

arriving DHCP client needs address in this network

Network Layer

16

DHCP server: 223.1.2.5 arriving client

DHCP discover

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

src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 654 lifetime: 3600 secs

DHCP request src: 0.0.0.0, 68 dest:: 255.255.255.255, 67 yiaddrr: 223.1.2.4 transaction ID: 655 lifetime: 3600 secs

DHCP ACK src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 655 lifetime: 3600 secs

DHCP client-server scenario

Network Layer

17

DHCP: more than IP addresses

DHCP can return more than just allocated IP address on subnet: §  address of first-hop router for client §  name and IP address of DNS sever §  network mask (indicating network versus host portion

of address)

Network Layer

18

v  connecting laptop needs its IP address, addr of first-hop router, addr of DNS server: use DHCP

router with DHCP server built into router

v  DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in 802.1 Ethernet

v  Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server

v  Ethernet demuxed to IP demuxed, UDP demuxed to DHCP

168.1.1.1

DHCP UDP IP Eth Phy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP UDP IP Eth Phy

DHCP

DHCP

DHCP

DHCP DHCP

DHCP: example

Network Layer

19

v  DCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server

v  encapsulation of DHCP server, frame forwarded to client, demuxing up to DHCP at client

DHCP: example

router with DHCP server built into router

DHCP

DHCP

DHCP

DHCP

DHCP UDP IP Eth Phy

DHCP

DHCP UDP IP Eth Phy

DHCP

DHCP

DHCP

DHCP

v  client now knows its IP address, name and IP address of DSN server, IP address of its first-hop router

Network Layer

20

DHCP: Wireshark output (home LAN)

Message type: Boot Reply (2) Hardware type: Ethernet Hardware address length: 6 Hops: 0 Transaction ID: 0x6b3a11b7 Seconds elapsed: 0 Bootp flags: 0x0000 (Unicast) Client IP address: 192.168.1.101 (192.168.1.101) Your (client) IP address: 0.0.0.0 (0.0.0.0) Next server IP address: 192.168.1.1 (192.168.1.1) Relay agent IP address: 0.0.0.0 (0.0.0.0) Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Server host name not given Boot file name not given Magic cookie: (OK) Option: (t=53,l=1) DHCP Message Type = DHCP ACK Option: (t=54,l=4) Server Identifier = 192.168.1.1 Option: (t=1,l=4) Subnet Mask = 255.255.255.0 Option: (t=3,l=4) Router = 192.168.1.1 Option: (6) Domain Name Server Length: 12; Value: 445747E2445749F244574092; IP Address: 68.87.71.226; IP Address: 68.87.73.242; IP Address: 68.87.64.146 Option: (t=15,l=20) Domain Name = "hsd1.ma.comcast.net."

reply

Message type: Boot Request (1) Hardware type: Ethernet Hardware address length: 6 Hops: 0 Transaction ID: 0x6b3a11b7 Seconds elapsed: 0 Bootp flags: 0x0000 (Unicast) Client IP address: 0.0.0.0 (0.0.0.0) Your (client) IP address: 0.0.0.0 (0.0.0.0) Next server IP address: 0.0.0.0 (0.0.0.0) Relay agent IP address: 0.0.0.0 (0.0.0.0) Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Server host name not given Boot file name not given Magic cookie: (OK) Option: (t=53,l=1) DHCP Message Type = DHCP Request Option: (61) Client identifier Length: 7; Value: 010016D323688A; Hardware type: Ethernet Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Option: (t=50,l=4) Requested IP Address = 192.168.1.101 Option: (t=12,l=5) Host Name = "nomad" Option: (55) Parameter Request List Length: 11; Value: 010F03062C2E2F1F21F92B 1 = Subnet Mask; 15 = Domain Name 3 = Router; 6 = Domain Name Server 44 = NetBIOS over TCP/IP Name Server ……

request

Network Layer

21

DHCP: further details

v  DHCP uses UDP and port numbers 67 (server side) and 68 (client side)

v  Usually the MAC address is used to identify clients §  DHCP server can be configured with a “registered list” of

acceptable MAC addresses

v  DHCP offer message includes ip address, length of lease, subnet mask, DNS servers, default gateway

v  DHCP security holes §  DoS attack by exhausting pool of IP addresses §  Masquerading as a DHCP server §  Authentication for DHCP - RFC 3118

Network Layer

22

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

CIDR: Addresses allocated in contiguous prefix chunks

Recursively break down chunks as get closer to host

12.0.0.0/8

12.0.0.0/15

12.253.0.0/16

12.2.0.0/16 12.3.0.0/16

: :

12.3.0.0/22 12.3.4.0/24

: :

12.3.254.0/23

12.253.0.0/19 12.253.32.0/19 12.253.64.0/19 12.253.64.108/30 12.253.96.0/18 12.253.128.0/17

: : :

: Network Layer 23

24

Hierarchical addressing: route aggregation

“Send me anything with 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 7 Internet

Organization 1

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

200.23.20.0/23 Organization 2

. . .

. . .

hierarchical addressing allows efficient advertisement of routing information:

Network Layer

25

Quiz: What should we do if organization 1 decides to switch to ISPs-R-Us

Network Layer

A: Move 200.23.18.0/23 to ISPs-R-Us (and break up Fly-By-Night’s/20 block). B: Give new addresses to Organization 1 (and force them to change all their addresses) C: Some other solution


200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0


Organization 1

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


. . .

. . .

26

ISPs-R-Us has a more specific route to Organization 1


200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0


Organization 1

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


. . .

. . .

Hierarchical addressing: more specific routes

Network Layer

Longest prefix matching

27

Example: continued

v  But how will this work? v  Routers in the Internet will have two entries in

their tables §  200.23.16.0/20 (Fly-by-Night-ISP) §  200.23.18.0/23 (ISPs-R-Us)

v  Longest prefix match

White Paper on IP addresses linked to page - Very informative

IP address

200.23.31.255

200.23.16.0

/20

200.23.19.255

200.23.18.0

/23

Network Layer

28

More on IP addresses

v  IP addresses are allocated as blocks and have geographical significance

v  It is possible to determine the geographical location of an IP address

http://www.geobytes.com/IpLocator.htm

Source: www.xkcd.com

Network Layer

29

IP addressing: the last word... Q: how does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers http://www.icann.org/

§  allocates addresses §  manages DNS §  assigns domain names, resolves disputes

v  Regional Internet Registries (RIR) act as intermediaries §  RIPE NCC (Riseaux IP Europiens Network Coordination Center)

for Europe, Middle East, Africa §  APNIC (Asia Pacific Network Information Center) for Asia and

Pacific §  ARIN (American Registry for Interent Numbers) for the

Americas, Caribbean, sub-Saharan Africa §  LACNIC (Latin America and Caribbean) Network Layer

Made-up Example in More Detail

v  ICANN gives APNIC several /8s v  APNIC gives Telstra one /8, 129.0/8

§  Network Prefix: 10000001 v  Telstra gives UNSW a /16, 129.94/16

§  Network Prefix: 1000000101011110 v  UNSW gives CSE a /24, 12.197.242/24

§  Network Prefix: 100000010101111011110010 v  CSE gives me a specific address 129.94.242.51

§  Address: 10000001010111101111001000110011

30 Network Layer

Quiz: Header Fields

v  Which of the following fields is not part of either a TCP or UDP header?

A.  Source port B.  Source IP address C.  Receive window D.  Length E.  Checksum

31 Network Layer

Quiz: DHCP

v  What transport protocol does DHCP use? A.  UDP B.  TCP C.  IP D.  HTTP

32 Network Layer

Quiz: IP Addressing

v  How many IP addresses belong to the subnet 128.119.254.0/25 ? What are the IP addresses at the two end-points of this range ?

33

Answer: 27 = 128 addresses (126 are usable)

10000000 01110111 11111110 00000000

subnet part

host part

128.119.254.0/25 IP address

128.119.254.127

128.119.254.0

/25

Network Layer

Quiz: Subnets

Network Layer 34

v  How many subnets are there in this network?

223.1.2.2 223.1.2.1

223.1.2.6

223.1.3.2 223.1.3.1

223.1.3.27

223.1.8.0 223.1.8.1

Quiz: Subnets

v  The two subnets 128.119.245.129/25 and 128.119.245.4/26 have overlapping IP addresses.

A.  True B.  False

Network Layer 35

10000000 01110111 11111110 10000001

subnet part

host part

128.119.254.129/25

10000000 01110111 11111110 00000100

subnet part

host part

128.119.254.4/26

Network Layer 4-36

Private Addresses

v  Defined in RFC 1918: - 10.0.0./8 (16,777,216 hosts) -  172.16.0.0/12 (1,048,576 hosts) -  192.168.0.0/16 (65536 hosts)

v  These addresses cannot be routed - Anyone can use them -  Often used for NAT

Network Layer 37

Network Layer

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 of Internet

datagrams with source or destination in this network have 10.0.0/24 address for source, destination (as usual)

all datagrams leaving local network have same single

source NAT IP address: 138.76.29.7,different source

port numbers 38

Network Layer

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


39

Network Layer

10.0.0.1

10.0.0.2

10.0.0.3

S: 10.0.0.1, 3345 D: 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 table WAN 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, 5001 D: 128.119.40.186, 80 2

2: NAT router changes datagram source addr from 10.0.0.1, 3345 to 138.76.29.7, 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 router changes datagram dest addr from 138.76.29.7, 5001 to 10.0.0.1, 3345


40

Network Layer

Local network uses just one IP address as far as outside world is concerned: §  range of addresses not needed from ISP: just one

IP address 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

NAT Advantages

41

42

Quiz: NAT

v  Devices inside the local network are not explicitly addressable or visible by outside world.

A: This is an advantage B: This is a disadvantage

Network Layer

Network Layer

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

LAN-side address! v  NAT is controversial:

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

•  NAT possibility must be taken into account by app designers, e.g., P2P applications

§  address shortage should instead be solved by IPv6


43

44

NAT: Practical Issues v  NAT modifies port # and IP address

§  Requires recalculation of TCP and IP checksum v  Some applications embed IP address or port numbers in their message payloads

§  DNS, FTP (PORT command), SIP, H.323 §  For legacy protocols, NAT must look into these packets and translate the

embedded IP addresses/port numbers §  Duh, What if these fields are encrypted ?? (SSL/TLS, IPSEC, etc) §  Q: In some cases why may NAT need to change TCP sequence number??

v  If applications change port numbers periodically, the NAT must be aware of this

v  NAT Traversal Problems §  E.g: How to setup a server behind a NAT router? §  How to talk to a Skype user behind a NAT router? §  Possible workarounds in next few slides

Network Layer

Network Layer

NAT traversal problem v  client wants to connect to

server with address 10.0.0.1 §  server address 10.0.0.1 local to

LAN (client can’t use it as destination addr)

§  only one externally visible NATed address: 138.76.29.7

v  solution1: statically configure NAT to forward incoming connection requests at given port to server §  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

?

45

Network Layer

NAT traversal problem v  solution 2: Universal Plug and Play

(UPnP) Internet Gateway Device (IGD) Protocol. Allows NATed host to: v  learn public IP address

(138.76.29.7) v  add/remove port mappings

(with lease times)

i.e., automate static NAT port map configuration

10.0.0.1

NAT router

IGD

46

Network Layer

NAT traversal problem v  solution 3: relaying (used in Skype)

§  NATed client establishes connection to relay §  external client connects to relay §  relay bridges packets between to connections

138.76.29.7

client

1. connection to relay initiated by NATed host

2. connection to relay initiated by client

3. relaying established

NAT router

10.0.0.1

47

Network Layer 48

NAT: Devil in the details

v  Despite the problems, NAT has been widely deployed v  Most protocols can be successfully passed through a NAT, including

VPN v  Modern hardware can easily perform NAT functions at > 100 Mbps v  IPv6 is still not widely deployed commercially, so the need for NAT is

real v  After years of refusing to work on NAT, the IETF has been developing

“NAT control protocols” for hosts v  Lot of practical variations

§  Full-cone NAT, Restricted Cone NAT, Port Restricted Cone NAT, Symmetric NAT, …..

•  The devil is in the detail v  External link under lecture notes for further reading (not examinable)

Network Layer








49

Network Layer

ICMP: internet control message protocol

v  used by hosts & routers to communicate network-level information §  error reporting:

unreachable host, network, port, protocol

§  echo request/reply (used by ping)

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

datagrams v  ICMP message: type, code

plus first 8 bytes of IP datagram causing error

Type Code description 0 0 echo reply (ping) 3 0 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 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header

50

Network Layer

Traceroute and ICMP v  source sends series of

UDP segments to dest §  first set has TTL =1 §  second set has TTL=2, etc. §  unlikely port number

v  when nth set of datagrams arrives to nth router: §  router discards datagrams §  and sends source ICMP

messages (type 11, code 0) §  ICMP messages includes

name of router & IP address

v  when ICMP messages arrives, source records RTTs

stopping criteria: v  UDP segment eventually

arrives at destination host v  destination returns ICMP

“port unreachable” message (type 3, code 3)

v  source stops

3 probes

3 probes

3 probes

51

Network Layer

IPv6: motivation v  initial motivation: 32-bit address space soon to be

completely allocated. v  additional motivation:

§  header format helps speed processing/forwarding §  header changes to facilitate QoS

IPv6 datagram format: §  fixed-length 40 byte header §  no fragmentation allowed

52

https://www.google.com/intl/en/ipv6/statistics.html

Network Layer

IPv6 datagram format

priority: identify priority among datagrams in flow (traffic class) flow Label: identify datagrams in same “flow.” (concept of“flow” not well defined). next header: identify upper layer protocol for data

data

destination address (128 bits)

source address (128 bits)

payload len next hdr hop limit flow label pri ver

32 bits 53

Network Layer

Other changes from IPv4

v  checksum: removed entirely to reduce processing time at each hop

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

v  ICMPv6: new version of ICMP §  additional message types, e.g. “Packet Too Big” §  multicast group management functions

54

Network Layer

Transition from IPv4 to IPv6 v  not all routers can be upgraded simultaneously

§  no “flag days” §  how will network operate with mixed IPv4 and

IPv6 routers? v  tunneling: IPv6 datagram carried as payload in IPv4

datagram among IPv4 routers

IPv4 source, dest addr IPv4 header fields

IPv4 datagram IPv6 datagram

IPv4 payload

UDP/TCP payload IPv6 source dest addr

IPv6 header fields

55

Network Layer

Tunneling

physical view: IPv4 IPv4

A B

IPv6 IPv6

E

IPv6 IPv6

F C D

logical view:

IPv4 tunnel connecting IPv6 routers E

IPv6 IPv6

F A B

IPv6 IPv6

56

Network Layer

flow: X src: A dest: F data

A-to-B: IPv6

Flow: X Src: A Dest: F data

src:B dest: E

B-to-C: IPv6 inside

IPv4

E-to-F: IPv6

flow: X src: A dest: F data

B-to-C: IPv6 inside

IPv4

Flow: X Src: A Dest: F data

src:B dest: E

physical view: A B

IPv6 IPv6

E

IPv6 IPv6

F C D

logical view:

IPv4 tunnel connecting IPv6 routers E

IPv6 IPv6

F A B

IPv6 IPv6

Tunneling

IPv4 IPv4

57

Quiz: NAT

v  A host with a private IP address 192.168.0.2 opens a TCP socket on its local port 4567 and connects to a web server at 34.5.6.7. The NAT’s public IP address is 22.33.44.55. Which of the following mapping entries could the NAT create as a result?

A.  [22.33.44.55, 3333]à[192.168.0.2, 80] B.  [34.5.6.7, 80] à [22.33.44.55, 4567] C.  [192.168.0.2, 80]à[34.5.6.7, 4567] D.  [22.33.44.55, 3967]à[192.168.0.2, 4567]

Network Layer 58

192.168.0.2

NAT router

22.33.44.55

34.5.6.7

Quiz: NAT

v  A host with a private IP address 192.168.0.2 opens a TCP socket on its local port 4567 and connects to a web server at 34.5.6.7. The NAT’s public IP address is 22.33.44.55. Suppose the NAT created the mapping [22.33.44.55, 3967]à[192.168.0.2, 4567] as a result. What are the source and destination port numbers in the SYNACK response from the server?

A.  80, 3967 B.  4567, 80 C.  3967, 80 D.  3967, 4567

Network Layer 59

192.168.0.2

NAT router

22.33.44.55

34.5.6.7

Quiz: IPv6

v  Which of the following is not true? A.  IPv6 increases the size of the IP address space

from 2^32 to 2^128. B.  IPv6 removes checksums and fragmentation

compared to IPv4. C.  IPv6 has fixed length headers. D.  IPv6 adds reliability at the network layer.

Network Layer 60

61








Network Layer

IP Routers

v  Core building block of the Internet infrastructure

v  $120B+ industry

v  Vendors: Cisco, Huawei, Juniper, Alcatel-Lucent (account for >90%)

Network Layer 62

Router definitions

1

2

3

4 5 …

N-1

N

•  N = number of external router “ports” •  R = speed (“line rate”) of a port •  Router capacity = N x R

R bits/sec

Network Layer 63

Networks and routers

Optus IBM

UoW

UNSW

core

core

edge (ISP)

edge (enterprise)

home, small business

Network Layer 64

Examples of routers (core)

72 racks, 1MW

Cisco CRS •  R=10/40/100 Gbps •  NR = 322 Tbps

Juniper T4000 •  R= 10/40 Gbps •  NR = 4 Tbps

Network Layer 65

Examples of routers (edge)

Cisco ASR 1006 •  R=1/10 Gbps •  NR = 40 Gbps

Juniper M120 •  R= 2.5/10 Gbps •  NR = 120 Gbps

Network Layer 66

Examples of routers (small business)

Cisco 3945E •  R = 10/100/1000 Mbps •  NR < 10 Gbps

Network Layer 67

What’s inside a router?

1

2

N

1

2

N

Linecards (input)

Interconnect (Switching) Fabric

Route/Control Processor

Linecards (output)

Processes packets on their way in

Processes packets before they leave

Transfers packets from input to output ports

Input and Output for the same port are on one physical linecard

Network Layer 68


1

2

N

1

2

N

Linecards (input)

Interconnect (Switching) Fabric


Linecards (output)

(1) Implement IGP and BGP protocols; compute routing tables (2) Push forwarding tables to the line cards

Network Layer 69


1

2

N

1

2

N

Linecards (input)

Interconnect Fabric


Linecards (output)

Constitutes the data plane

Constitutes the control plane

Network Layer 70

Input Linecards

v  Tasks §  Receive incoming packets (physical layer stuff) §  Update the IP header

•  TTL, Checksum, Options (maybe), Fragment (maybe) §  Lookup the output port for the destination IP address in the

Forwarding Table using Longest Prefix Matching §  Queue the packet at the switch fabric

v  Challenge: speed! §  100B packets @ 40Gbps à new packet every 20 nano secs!

v  Typically implemented with specialized hardware §  ASICs, specialised “network processors” §  “exception” processing often done at control processor

Network Layer 71

Output Linecard

l  Packet classification: map each packet to a “flow” l  Flow (for now): set of packets between two particular endpoints

l  Buffer management: decide when and which packet to drop l  Scheduler: decide when and which packet to transmit

1

2

Scheduler

flow 1

flow 2

flow n

Classifier

Buffer management

Network Layer 72

Output Linecard

l  Packet classification: map each packet to a “flow” l  Flow (for now): set of packets between two particular endpoints

l  Buffer management: decide when and which packet to drop l  Scheduler: decide when and which packet to transmit

l  Used to implement various forms of policy l  Deny all e-mail traffic from ISP-X to Y (access control) l  Route IP telephony traffic from X to Y via Z (policy) l  Ensure that no more than 50 Mbps are injected from ISP-X (QoS)

Network Layer 73

Simplest: FIFO Router

v  No classification v  Drop-tail buffer management: when buffer is full drop the

incoming packet v  First-In-First-Out (FIFO) Scheduling: schedule packets in the

same order they arrive

1

2

Scheduler Buffer

Network Layer 74

Packet Classification

v  Classify an IP packet based on a number of fields in the packet header, e.g., §  source/destination IP address (32 bits) §  source/destination TCP port number (16 bits) §  Type of service (TOS) byte (8 bits) §  Type of protocol (8 bits)

v  In general fields are specified by range §  classification requires a multi-dimensional range search!

1

2 Scheduler

flow 1

flow 2

flow n

Classifier

Buffer management Network Layer 75

Scheduler v  One queue per “flow” v  Scheduler decides when and from which queue to send a

packet v  Goals of a scheduling algorithm:

§  Fast! §  Depends on the policy being implemented (fairness, priority, etc.)

1

2

Scheduler

flow 1

flow 2

flow n

Classifier

Buffer management

Network Layer 76

Priority Scheduler

Example: Priority Scheduler

v  Priority scheduler: packets in the highest priority queue are always served before the packets in lower priority queues

High priority

Medium priority

Low priority

Network Layer 77

Example: Round Robin Scheduler

v  Round robin: packets are served from each queue in turn

Fair Scheduler

High priority

Medium priority

Low priority

Network Layer 78

BurstyLossFromDrop-TailQueuingv  TCPdependsonpacketloss

§  Packetlossisindica<onofconges<on§  AndTCPdrivesnetworkintolossbyaddi<verateincrease

v  Drop-tailqueuingleadstoburstyloss§  Iflinkiscongested,manypacketsencounterfullqueue§  Thus,losssynchronisa<on:

• Manyflowsloseoneormorepackets• Inresponse,manyflowsdividesendingrateinhalf

79 Network Layer

SlowFeedbackfromDropTailv  Feedbackcomeswhenbufferiscompletelyfull

§  …eventhoughthebufferhasbeenfillingforawhile

v  Plus,thefillingbufferisincreasingRTT§  …makingdetec<onevenslower

v  MightbebeOertogiveearlyfeedback§  Andget1-2connec<onstoslowdownbeforeit’stoolate

80 Network Layer

RandomEarlyDetec<on(RED)v  BasicideaofRED

§  Routerno<cesthatqueueisgeUngbacklogged§  …andrandomlydropspacketstosignalconges<on

v  Packetdropprobability§  Dropprobabilityincreasesasqueuelengthincreases§  Else,setdropprobabilityasfunc<onofavgqueuelengthand<mesincelastdrop

81 Average Queue Length

Dro

p Pr

obab

ility

0

1

Network Layer

Proper<esofREDv  Dropspacketsbeforequeueisfull

§  Inthehopeofreducingtheratesofsomeflows

v  Dropspacketinpropor<ontoeachflow’srate§  High-rateflowshavemorepackets§  …and,hence,ahigherchanceofbeingselected

v  Dropsarespacedoutin<me§  WhichshouldhelpdesynchronisetheTCPsenders

v  Tolerantofburs<nessinthetraffic§  Bybasingthedecisionsonaveragequeuelength

82

Buffer management is known as Active Queue Management Network Layer

ProblemsWithREDv  Hardtogettunableparametersjustright

§  Howearlytostartdroppingpackets?§  Whatslopeforincreaseindropprobability?§  What<mescaleforaveragingqueuelength?

v  REDhasmixedadop<oninprac<ce§  Ifparametersaren’tsetright,REDdoesn’thelp§  Hardtoknowhowtosettheparameters

v  Manyothervaria<onsinresearchcommunity§  Nameslike“Blue”(self-tuning),“FRED”…

83 Network Layer

Switching

1

2

N

1

2

N

Linecards (input)

Interconnect Fabric


Linecards (output)

Network Layer 84

Shared Memory (1st Generation)

Route Table CPU

Buffer Memory

Line Interface

MAC

Line Interface

MAC

Line Interface

MAC

Limited by rate of shared memory

Shared Backplane

(* Slide by Nick McKeown, Stanford Univ.) Network Layer 85

Shared Bus (2nd Generation)

Route Table CPU

Line Card

Buffer Memory

Line Card

MAC

Buffer Memory

Line Card

MAC

Buffer Memory

Fwding Cache

Fwding Cache

Fwding Cache

MAC

Buffer Memory

Limited by shared bus

(* Slide by Nick McKeown) Network Layer 86

Point-to-Point Switch (3rd Generation)

Line Card

MAC

Local Buffer Memory

CPU Card

Line Card

MAC

Local Buffer Memory

Switched Backplane

Fwding Table

Routing Table

Fwding Table

(*Slide by Nick McKeown) Network Layer 87

© Nick McKeown 2006

NxR

3rd Gen. Router: Switched Interconnects

This is called an “output queued” switch Network Layer

88



This is called an “input queued” switch Network Layer

89

Two challenges with input queuing

1) Need an internal fabric scheduler!

Network Layer 90


1 x R

Fabric Scheduler


91

Two challenges with input queuing

1)  Need an internal fabric scheduler! 2)  Must avoid “head-of-line” blocking

Network Layer 92

Head of Line Blocking

HoL blocking limits throughput to approximately 58% of capacity Network Layer 93

“Virtual Output Queues”

Network Layer 94


1 x R

Fabric Scheduler


95

Reality is more complicated

v  Commercial (high-speed) routers use §  combination of input and output queuing §  complex multi-stage switching topologies (Clos, Benes) §  distributed, multi-stage schedulers (for scalability)

Network Layer 96

Documents

COMP 3331/9331: Computer Networks and Applications