Network%20Layer

1
Network Layer

• Dr. Yingwu Zhu

2
Network Layer

• Introduction
• Datagram networks
• IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• Whats inside a router

• Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• Routing in the Internet
• RIP
• OSPF
• BGP
• Broadcast and multicast routing

3
Network layer

• transport segment from sending to receiving host
• on sending side encapsulates segments into
  datagrams
• on receiving side, delivers segments to transport
  layer
• network layer protocols in every host, router
• Router examines header fields in all IP datagrams
  passing through it

4
Key Network-Layer Functions

• analogy
• routing process of planning trip from source to
  dest
• forwarding process of getting through single
  interchange

• forwarding move packets from routers input to
  appropriate router output
• routing determine route taken by packets from
  source to dest.
• Routing algorithms

5
Interplay between routing and forwarding
6
Network Layer

• Introduction
• Datagram networks
• IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• Whats inside a router

• Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• Routing in the Internet
• RIP
• OSPF
• BGP
• Broadcast and multicast routing

7
Datagram networks

• Connection-less service no call setup at network
  layer
• routers no state about end-to-end connections
  (why?)
• no network-level concept of connection
• packets forwarded using destination host address
• packets between same source-dest pair may take
  different paths

1. Send data
2. Receive data
8
Network Layer

• Introduction
• Datagram networks
• IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• Whats inside a router

• Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• Routing in the Internet
• RIP
• OSPF
• BGP
• Broadcast and multicast routing

9
The Internet Network layer

• Host, router network layer functions

Transport layer TCP, UDP
Network layer
Link layer
physical layer
10
The Internet Protocol (IP)
Protocol Stack
App
Transport
TCP / UDP
Data
Hdr
TCP Segment
Network
IP
Data
Hdr
IP Datagram
Link
11
The Internet Protocol (IP)

• Characteristics of IP
• CONNECTIONLESS mis-sequencing
• UNRELIABLE may drop packets
• BEST EFFORT but only if necessary
• DATAGRAM individually routed

Source
Destination
R2
D
H
R1
R3

• Architecture
• Links
• Topology

R4
Transparent
12
IP datagram format
6 for TCP

• how much overhead with TCP?
• 20 bytes of TCP
• 20 bytes of IP
• 40 bytes app layer overhead

13
IP Fragmentation Reassembly

• Problem A router may receive a packet larger
  than the maximum transmission unit (MTU) of the
  outgoing link.
• different link types, different MTUs
• Solution large IP datagram divided
  (fragmented) within net
• one datagram becomes several datagrams
• reassembled only at final destination, why?
• IP header bits used to identify, order related
  fragments

fragmentation in one large datagram out 3
smaller datagrams
reassembly
E.g., Ethernet frames carry up to 1500 bytes,
frames for some wide-area links carry no more
than 576 bytes. (MTU the max of data a
link-layer frame can carry)
14
IP Fragmentation and Reassembly

• Example
• 4000 byte datagram
• MTU 1500 bytes

1480 bytes in data field
offset 1480/8
Note the offset value is specified in units of
8-byte chunks!!!
15
Fragmentation

• Fragments are re-assembled by the destination
  host not by intermediate routers.
• To avoid fragmentation, hosts commonly use path
  MTU discovery to find the smallest MTU along the
  path.
• Path MTU discovery involves sending various size
  datagrams until they do not require fragmentation
  along the path.
• Most links use MTUgt1500bytes today.
• Try traceroute www.berkeley.edu 500 F
  andtraceroute www.berkeley.edu 1501
• -F Set the "don't fragment" bit, return error it
  is too long
• Bonus Can you find a destination for which the
  path MTU lt 1500 bytes?

16
Network Layer

• Introduction
• Virtual circuit and datagram networks
• Whats inside a router
• IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• IPv6

• Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• Routing in the Internet
• RIP
• OSPF
• BGP
• Broadcast and multicast routing

17
IP Addresses

• IP (Version 4) addresses are 32 bits long
• Every interface has a unique IP address
• A computer might have two or more IP addresses
• A router has many IP addresses
• IP addresses are hierarchical
• They contain a network ID and a host ID
• E.g. SeattleU addresses start with 172.17
• IP addresses are assigned statically or
  dynamically (e.g. DHCP)
• IP (Version 6) addresses are 128 bits long

18
IP Addresses
Originally there were 5 classes
24
1
7
CLASS A
Host-ID
0
Net ID
16
2
14
CLASS B
Host-ID
10
Net ID
8
3
21
CLASS C
110
Net ID
Host-ID
4
28
CLASS D
1110
Multicast Group ID
5
27
CLASS E
11110
Reserved
A
B
C
D
0
232-1
19
IP AddressesExamples
Class A address www.mit.edu 18.181.0.31
(18lt128 gt Class
A) Class B address www.seattleu.edu 172.17.
72.14 (128lt171lt12864
gt Class B)
20
IP Addressing

• Problem
• Address classes were too rigid. For most
  organizations, Class C were too small and Class B
  too big. Led to inefficient use of address space,
  and a shortage of addresses.
• Organizations with internal routers needed to
  have a separate (Class C) network ID for each
  link.
• And then every other router in the Internet had
  to know about every network ID in every
  organization, which led to large address tables.
• Small organizations wanted Class B in case they
  grew to more than 255 hosts. But there were only
  about 16,000 Class B network IDs.

21
IP Addressing

• Two solutions were introduced
• Subnetting within an organization to subdivide
  the organizations network ID.
• Classless Interdomain Routing (CIDR) in the
  Internet backbone was introduced in 1993 to
  provide more efficient and flexible use of IP
  address space.
• CIDR is also known as supernetting because
  subnetting and CIDR are basically the same idea.

22
Subnetting
16
2
14
CLASS B e.g. Company
Host-ID
10
Net ID
16
16
2
14
2
14
e.g. Site
Host-ID
0000
Host-ID
1111
10
Net ID
10
Net ID
Subnet ID (20)
Subnet Host ID (12)
Subnet ID (20)
Subnet Host ID (12)
16
16
2
14
2
14
e.g. Dept
10
Net ID
Host-ID
Host-ID
1111011011
10
Net ID
000000
Subnet ID (26)
Subnet Host ID (6)
Subnet ID (22)
Subnet Host ID (10)
23
Subnetting

• Subnetting is a form of hierarchical routing.
• Subnets are usually represented via an address
  plus a subnet mask or netmask.
• e.g.
• zhuy_at_cs1 /sbin/ifconfig eth0
• eth0 Link encapEthernet HWaddr
  00219B8F646D
• inet addr10.124.72.20
  Bcast10.124.72.255 Mask255.255.255.0
• Netmask 255.255.255.0 the first 24 bits are the
  subnet ID, and the last 8 bits are the host ID.
• Can also be represented by a prefix length,
  e.g. 171.64.15.0/24, or just 171.64.15/24.

24
IP Addressing
223.1.1.1

• IP address 32-bit identifier for host, router
  interface
• interface connection between host/router and
  physical link
• routers typically have multiple interfaces
• host typically has one interface
• IP addresses associated with each interface

223.1.2.9
223.1.1.4
223.1.1.3
223.1.1.1 11011111 00000001 00000001 00000001
223
1
1
1
25
Subnets
223.1.1.1

• IP address
• subnet part (high order bits)
• host part (low order bits)
• Whats a subnet ?
• device interfaces with same subnet part of IP
  address
• can physically reach each other without
  intervening router

223.1.2.1
223.1.1.2
223.1.2.9
223.1.1.4
223.1.2.2
223.1.1.3
223.1.3.27
subnet
223.1.3.2
223.1.3.1
network consisting of 3 subnets
26
Subnets

• Recipe
• 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 /24
27
Subnets
223.1.1.2

• How many?

223.1.1.1
223.1.1.4
223.1.1.3
223.1.7.0
223.1.9.2
223.1.9.1
223.1.7.1
223.1.8.0
223.1.8.1
223.1.2.6
223.1.3.27
223.1.2.1
223.1.2.2
223.1.3.2
223.1.3.1
28
Classless Interdomain Routing (CIDR)Addressing

• The IP address space is broken into line
  segments.
• Subnet portion of address of arbitrary length
• Each line segment is described by a prefix.
• A prefix is of the form x/y where x indicates the
  prefix of all addresses in the line segment, and
  y indicates the length of the segment.
• e.g. The prefix 128.9/16 represents the line
  segment containing addresses in the range
  128.9.0.0 128.9.255.255.

128.9.0.0
65/8
128.9/16
0
232-1
216
128.9.16.14
29
Classless Interdomain Routing (CIDR)Addressing
128.9/16
0
232-1
128.9.16.14
30
Classless Interdomain Routing (CIDR)Addressing

• Prefix aggregation
• If a service provider serves two organizations
  with prefixes, it can (sometimes) aggregate them
  to form a shorter prefix. Other routers can refer
  to this shorter prefix, and so reduce the size of
  their address table.
• E.g. ISP serves 128.9.14.0/24 and 128.9.15.0/24,
  it can tell other routers to send it all packets
  belonging to the prefix 128.9.14.0/23.
• ISP Choice
• In principle, an organization can keep its prefix
  if it changes service providers.

31
Size of the Routing Table at the core of the
Internet

• Source http//www.cidr-report.org/

32
IP addresses how to get one?

• Q How does host get IP address?
• hard-coded by system admin in a file
• Wintel control-panel-gtnetwork-gtconfiguration-gttcp
  /ip-gtproperties
• UNIX /etc/rc.config
• DHCP Dynamic Host Configuration Protocol
  dynamically get address from as server
• plug-and-play

33
IP addresses how to get one?

• Q How does network get subnet part of IP addr?
• A gets allocated portion of its provider ISPs
  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
34
Hierarchical addressing route aggregation
Hierarchical addressing allows efficient
advertisement of routing information
Organization 0
Organization 1
Send me anything with addresses beginning
200.23.16.0/20
Organization 2
Fly-By-Night-ISP
Internet
Organization 7
Send me anything with addresses beginning
199.31.0.0/16
ISPs-R-Us
35
Hierarchical addressing more specific routes
ISPs-R-Us has a more specific route to
Organization 1
Organization 0
Send me anything with addresses beginning
200.23.16.0/20
Organization 2
Fly-By-Night-ISP
Internet
Organization 7
Send me anything with addresses beginning
199.31.0.0/16 or 200.23.18.0/23
ISPs-R-Us
Organization 1
Longest prefix match
36
IP addressing the last word...

• Q How does an ISP get block of addresses?
• A ICANN Internet Corporation for Assigned
• Names and Numbers
• allocates addresses
• manages DNS
• assigns domain names, resolves disputes

37
NAT Network Address Translation
rest of Internet
local network (e.g., home network) 10.0.0/24
10.0.0.1
10.0.0.4
10.0.0.2
138.76.29.7
10.0.0.3
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, differe
nt source port numbers
38
NAT Network Address Translation

• 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).

39
NAT Network Address Translation

• 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

40
NAT Network Address Translation
NAT translation table WAN side addr LAN
side addr
138.76.29.7, 5001 10.0.0.1, 3345

10.0.0.1
10.0.0.4
10.0.0.2
138.76.29.7
10.0.0.3
4 NAT router changes datagram dest addr
from 138.76.29.7, 5001 to 10.0.0.1, 3345
3 Reply arrives dest. address 138.76.29.7,
5001
41
NAT Network Address Translation

• 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
• 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

42
Network Layer

• Introduction
• Virtual circuit and datagram networks
• Whats inside a router
• IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• IPv6

• Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• Routing in the Internet
• RIP
• OSPF
• BGP
• Broadcast and multicast routing

43
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 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
44
An asideError Reporting (ICMP) and traceroute

• Internet Control Message Protocol
• Used by a router/end-host to report some types of
  error
• E.g. Destination Unreachable packet cant be
  forwarded to/towards its destination.
• E.g. Time Exceeded TTL reached zero, or
  fragment didnt arrive in time. Traceroute uses
  this error to its advantage.
• An ICMP message is an IP datagram, and is sent
  back to the source of the packet that caused the
  error.

45
Traceroute and ICMP

• Source sends series of UDP segments to dest
• First has TTL 1
• Second has TTL2, etc.
• Unlikely port number
• When nth datagram arrives to nth router
• Router discards datagram
• And sends to source an ICMP message (type 11,
  code 0)
• Message includes name of router IP address

• When ICMP message arrives, source calculates RTT
• Traceroute does this 3 times
• Stopping criterion
• UDP segment eventually arrives at destination
  host
• Destination returns ICMP host unreachable
  packet (type 3, code 3)
• When source gets this ICMP, stops.

It would be fun if you design a traceroute tool
on your own!
46
Network Layer

• Introduction
• Datagram networks
• IP Internet Protocol
• Datagram format
• IPv4 addressing
• ICMP
• Whats inside a router

• Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• Routing in the Internet
• RIP
• OSPF
• BGP
• Broadcast and multicast routing

47
Router Architecture Overview

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

48
Input Port Functions
Physical layer bit-level reception

• 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

Data link layer e.g., Ethernet
49
Three types of switching fabrics
50
Switching Via Memory

• First generation routers
• traditional computers with switching under
  direct control of CPU
• packet copied to systems memory
• speed limited by memory bandwidth (2 bus
  crossings per datagram)

51
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)

52
Switching Via An Interconnection Network

• overcome bus bandwidth limitations
• Banyan networks, other interconnection nets
  initially developed to connect processors in
  multiprocessor
• Advanced design fragmenting datagram into fixed
  length cells, switch cells through the fabric.
• Cisco 12000 switches Gbps through the
  interconnection network

53
Output Ports

• Buffering required when datagrams arrive from
  fabric faster than the transmission rate
• Scheduling discipline chooses among queued
  datagrams for transmission

54
Output port queueing

• buffering when arrival rate via switch exceeds
  output line speed
• queueing (delay) and loss due to output port
  buffer overflow!

55
Input Port Queuing

• Fabric slower than input ports combined -gt
  queueing may occur at input queues
• Head-of-the-Line (HOL) blocking queued datagram
  at front of queue prevents others in queue from
  moving forward
• queueing delay and loss due to input buffer
  overflow!

56
How a Router Forwards Datagrams
128.17.20.1
e.g. 128.9.16.14 gt Port 2
R2
Prefix
Port
Next-hop
3
65/8
128.17.16.1
R1
R3
1
128.9/16
2
128.17.14.1
2
128.9.16/20
2
128.17.14.1
3
128.9.19/24
7
128.17.10.1
128.9.25/24
2
128.17.14.1
R4
128.9.176/20
1
128.17.20.1
142.12/19
3
128.17.16.1
128.17.16.1
Forwarding/routing table
57
How a Router Forwards Datagrams

• Every datagram contains a destination address.
• The router determines the prefix to which the
  address belongs, and routes it to theNetwork ID
  that uniquely identifies a physical network.
• All hosts and routers sharing a Network ID share
  same physical network.

58
Forwarding Datagrams

• Is the datagram for a host on a directly attached
  network?
• If no, consult forwarding table to find next-hop.

59
Inside a router
Link 1, ingress
Link 1, egress
Choose Egress
Link 2, ingress
Link 2, egress
Choose Egress
Link 3, ingress
Link 3, egress
Choose Egress
Link 4, ingress
Link 4, egress
Choose Egress
60
Inside a router
Forwarding Table
Link 1, ingress
Link 1, egress
Forwarding Decision
Link 2, ingress
Link 2, egress
Choose Egress
Link 3, ingress
Link 3, egress
Choose Egress
Link 4, ingress
Link 4, egress
Choose Egress
61
Forwarding in an IP Router

• Lookup packet DA in forwarding table.
• If known, forward to correct port.
• If unknown, drop packet.
• Decrement TTL, update header Checksum.
• Forward packet to outgoing interface.
• Transmit packet onto link.

Question How is the address looked up in a real
router?
62
Making a Forwarding DecisionClass-based
addressing
IP Address Space
Class A
Class B
Class C
D
Class A
Routing Table
Class B
212.17.9.4
Exact

Match
Class C
212.17.9.0
Port 4
212.17.9.0
Exact Match There are many well-known ways to
find an exact match in a table.
63
Direct Lookup
Next-hop, Port
IP Address
Memory
Data
Address
Problem With 232 addresses, the memory would
require 4 billion entries.
64
Associative LookupsContents addressable memory
(CAM)
Associative Memory or CAM

• Advantages
• Simple
• Disadvantages
• Slow
• High Power
• Small
• Expensive

Port Number
Network Address
Port Number
Search Data
Hit?
32
Search data is compared with every entry in
parallel
65
Hashed Lookups
Search Data
Hashing Function
16
32
Data
Address
Memory
66
Lookups Using HashingAn example
Memory
1
2
3
4
Search Data
Associated Data
1
2
16
Hashing Function
Hit?
32
1
2
3
Linked list of entrieswith same hash key.
67
Lookups Using Hashing

• Advantages
• Simple
• Expected lookup time can be small
• Disadvantages
• Non-deterministic lookup time
• Inefficient use of memory

68
Trees and Tries
Binary Search Tree
lt
gt
lt
gt
lt
gt
69
Search TriesMultiway tries reduce the number of
memory references
16-ary Search Trie
0000, ptr
1111, ptr
1111, ptr
0000, 0
1111, ptr
0000, 0
000011110000
111111111111
Question Why not just keep increasing the degree
of the trie?
70
Classless AddressingCIDR
128.9/16
0
232-1
128.9.16.14
Most specific route longest matching prefix
Question How can we look up addresses if they
are not an exact match?
71
Ternary CAMs
Associative Memory
Value
Mask
Port
10.1.1.32
1
255.255.255.255
255.255.255.0
Port
10.1.1.0
2
255.255.255.0
10.1.3.0
3
10.1.0.0
4
255.255.0.0
255.0.0.0
10.0.0.0
4
Priority Encoder
Note Most specific routes appear closest to top
of table
72
Longest prefix matches using Binary Tries
Example Prefixes
0
1
a) 00001
b) 00010
c) 00011
g
d) 001
e) 0101
i
f
d
f) 011
g) 10
h
h) 1010
e
i) 111
j
j) 111100
a
b
c
k) 11110001
k
73
Lookup Performance Required
Line
Line Rate
Pktsize40B
Pktsize240B
T1
1.5Mbps
4.68
Kpps
0.78
Kpps
OC3
155Mbps
480
Kpps
80
Kpps
OC12
622Mbps
1.94
Mpps
323
Kpps
OC48
2.5Gbps
7.81
Mpps
1.3
Mpps
OC192
10
Gbps
31.25
Mpps
5.21
Mpps
74
Discussion

• Why was the Internet Protocol designed this way?
• Why connectionless, datagram, best-effort?
• Why not automatic retransmissions?
• Why fragmentation in the network?
• Must the Internet address be hierarchical?
• What address does a mobile host have?
• Are there other ways to design networks?