程序代写代做代考 algorithm C dns DHCP graph assembly Chapter 4 Network Layer

Chapter 4 Network Layer
copyright J.F Kurose and K.W. Ross, All Rights Reserved
Network Layer 4-1

Chapter 4: network layer
chapter goals:
v understand principles behind network layer services:
§ network layer service models § forwarding versus routing
§ how a router works
§ routing (path selection)
§ broadcast, multicast
v instantiation, implementation in the Internet
Network Layer 4-2

Chapter 4: outline
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 4-3

Network layer
v transport segment from sending to receiving host
v on sending side encapsulates segments into datagrams
v on receiving side, delivers segments to transport layer
v network layer protocols in every host, router
v router examines header fields in all IP datagrams passing through it
application
transport
network
data link
physical
network
data link
data link
network
data link
physical
physical
network
network
physical
network
physical
physical
network
network
data link
physical
network
net
data link
data link
data link physical
work
data link
physical
application
transport
data link
network
network
physical
data link
network
data link
physical
data link physical
physical
Network Layer 4-4

Two key network-layer functions
v forwarding: move packets from router’s input to appropriate router output
v routing: determine route taken by packets from source to dest.
§ routing algorithms
analogy:
v routing: process of planning trip from source to dest
v forwarding: process of getting through single interchange
Network Layer 4-5

Interplay between routing and forwarding
routing algorithm
local forwarding table
header value
0100 0101 0111 1001
output link
3 2 2 1
0111
routing algorithm determines end-end-path through network
forwarding table determines local forwarding at this router
value in arriving packet’s header
3
1 2
Network Layer 4-6

Connection setup
v 3rd important function in some network architectures:
§ A TM, frame relay, X.25
v before datagrams flow, two end hosts and intervening routers establish virtual connection
§ routers get involved
v network vs transport layer connection service:
§ network: between two hosts (may also involve intervening routers in case of VCs)
§ transport: between two processes
Network Layer 4-7

Network service model
Q: What service model for “channel” transporting datagrams from sender to receiver?
example services for individual datagrams:
v guaranteed delivery
v guaranteed delivery with less than 40 msec delay
example services for a flow of datagrams:
v in-order datagram delivery
v guaranteed minimum bandwidth to flow
v restrictions on changes in inter-packet spacing
Network Layer 4-8

Network layer service models:
Network Service Architecture Model
Internet best effort ATM CBR
ATM VBR ATM ABR ATM UBR
Guarantees ?
Congestion feedback
no (inferred via loss)
no congestion no congestion yes
no
Bandwidth Loss none no
constant yes rate
guaranteed yes rate
guaranteed no minimum
none no
Order Timing no no
yes yes yes yes yes no
yes no
Network Layer 4-9

Connection, connection-less service
v datagram network provides network-layer connectionless service
v virtual-circuit network provides network-layer connection service
v analogous to TCP/UDP connecton-oriented / connectionless transport-layer services, but:
§ service: host-to-host
§ no choice: network provides one or the other § implementation: in network core
Network Layer 4-11

Virtual circuits
“source-to-dest path behaves much like telephone circuit”
§ performance-wise
§ network actions along source-to-dest path
v call setup, teardown for each call before data can flow v each packet carries VC identifier (not destination host
address)
v every router on source-dest path maintains “state” for each passing connection
v link, router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service)
Network Layer 4-12

VC implementation
a VC consists of:
1. path from source to destination
2. VC numbers, one number for each link along path
3. entries in forwarding tables in routers along path
v packet belonging to VC carries VC number (rather than dest address)
v VCnumbercanbechangedoneachlink. § new VC number comes from forwarding table
Network Layer 4-13

VC forwarding table
12
1
22
3
32
forwarding table in northwest router:
Incoming interface
Incoming VC # Outgoing interface
Outgoing VC #
VC number
interface number
2
1 12 3 22
2 63 1 18
3 7 2 17
1 97 3 87 …………
VC routers maintain connection state information!
Network Layer 4-14

Virtual circuits: signaling protocols
v used to setup, maintain teardown VC v used in ATM, frame-relay, X.25
v not used in today’s Internet
application
transport
5. data flow begins
4. call connected 1. initiate call
6. receive data
3. accept call 2. incoming call
network
data link
physical
application
transport
network
data link
physical
Network Layer 4-15

Datagram networks
v no call setup at network layer
v routers: no state about end-to-end connections
§ no network-level concept of “connection”
v packets forwarded using destination host address
application
transport
application
1. send datagrams
2. receive datagrams
transport
network
data link
network
physical
data link
physical
Network Layer 4-16

Datagram forwarding table
routing algorithm
local forwarding table
dest address output link
address-range 1 address-range 2 address-range 3 address-range 4
3 2 2 1
4 billion IP addresses, so rather than list individual destination address
list range of addresses (aggregate table entries)
IP destination address in arriving packet’s header
1 32
Network Layer 4-17

Datagram forwarding table
Destination Address Range
Link Interface
11001000 00010111 00010000 00000000
through
11001000 00010111 00010111 11111111
0
11001000 00010111 00011000 00000000
through
11001000 00010111 00011000 11111111
1
11001000 00010111 00011001 00000000
through
11001000 00010111 00011111 11111111
2
otherwise
3
Q: but what happens if ranges don’t divide up so nicely?
Network Layer 4-18

Longest prefix matching
longest prefix matching
when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address.
Destination Address Range
11001000 00010111 00010*** *********
11001000 00010111 00011000
*********
Link interface
0
1
2
11001000 00010111 00011*** *********
otherwise
3
examples: Received a packet with the following destination address
DA: 11001000 00010111 00010110 10100001 which interface? DA: 11001000 00010111 00011000 10101010 which interface?
Network Layer 4-19

Datagram or VC network: why?
Internet (datagram)
v data exchange among computers
§ “elastic” service, no strict timing req.
v many link types
§ different characteristics
§ uniform service difficult
v “smart” end systems (computers)
§ can adapt, perform control, error recovery
§ simple inside network, complexity at “edge”
ATM (VC)
v evolved from telephony v human conversation:
§ strict timing, reliability requirements
§ need for guaranteed service v “dumb” end systems
§ telephones
§ complexity inside network
Network Layer 4-20

Router architecture overview
two key router functions:
v run routing algorithms/protocol (RIP, OSPF, BGP) v forwarding datagrams from incoming to outgoing link
forwarding tables computed, pushed to input ports
routing, management control plane (software)
forwarding data plane (hardware)
high-seed switching
fabric
router input ports
router output ports
routing processor
Network Layer 4-22

Input port functions
link layer
protocol (receive)
lookup, forwarding
queueing
line termination
switch fabric
physical layer:
bit-level reception
data link layer:
e.g., Ethernet see chapter 5
decentralized switching:
v given datagram dest., lookup output port using forwarding table in input port memory (“match plus action”)
v goal: complete input port processing at ‘line speed’
v queuing: if datagrams arrive faster than forwarding rate into switch fabric
Network Layer 4-23

Switching fabrics
v transfer packet from input buffer to appropriate output buffer
v switching rate: rate at which packets can be transfer from inputs to outputs
§ often measured as multiple of input/output line rate § N inputs: switching rate N times line rate desirable
v three types of switching fabrics
memory
memory bus crossbar
Network Layer 4-24

Switching via memory
first generation routers:
vtraditional computers with switching under direct control of CPU
vpacket copied to system’s memory
v speed limited by memory bandwidth (2 bus crossings per datagram)
system bus
input port (e.g., Ethernet)
memory
output port (e.g., Ethernet)
Network Layer 4-25

Switching via a bus
v datagram from input port memory to output port memory via a
shared bus
vbuscontention: switchingspeed limited by bus bandwidth
v 32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers
bus
Network Layer 4-26

Switching via interconnection network
v overcome bus bandwidth limitations
v banyan networks, crossbar, other interconnection nets initially developed to connect processors in multiprocessor
v advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric.
v Cisco 12000: switches 60 Gbps through the interconnection network
crossbar
Network Layer 4-27

Output ports
switch fabric
datagram buffer
queueing
link layer
protocol (send)
line termination
v buffering required when datagrams arrive from fabric faster than the transmission rate
v scheduling discipline chooses among queued datagrams for transmission
Network Layer 4-28

Output port queueing
switch fabric
switch fabric
at t, packets more one packet time later from input to output
v Speed up factor of N: Fabric faster than N ports
v buffering when arrival rate via switch exceeds output line
speed
v queueing (delay) and loss due to output port buffer overflow! Network Layer 4-29

How much buffering?
v RFC 3439 rule of thumb: average buffering equal to “typical” RTT (say 250 msec) times link capacity C
§ e.g., C = 10 Gpbs link: 2.5 Gbit buffer
v recent recommendation: with N flows, buffering
equal to
RTT.C
N
v In practice because memory is cheap router buffers are large
Network Layer 4-30

Input port queuing
v fabric slower than input ports combined -> queueing may occur at input queues
§ queueing delay and loss due to input buffer overflow!
v Head-of-the-Line (HOL) blocking: queued datagram at front
of queue prevents others in queue from moving forward
switch fabric
switch fabric
output port contention: only one red datagram can be transferred.
lower red packet is blocked
one packet time later: green packet
experiences HOL blocking
Network Layer 4-31

Chapter 4: outline
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 4-32

The Internet network layer
host, router network layer functions:
transport layer: TCP, UDP
routing protocols
• path selection
• RIP, OSPF, BGP
forwarding table
IP protocol
• addressing conventions
• datagram format
• packet handling conventions
ICMP protocol
• error reporting • router
“signaling” link layer
physical layer
network layer
Network Layer 4-33

IP datagram format
IP protocol version number header length (bytes)
“type” of data
max number remaining hops
(decremented at each router)
upper layer protocol to deliver payload to
32 bits
total datagram length (bytes)
for fragmentation/ reassembly
ver
head. len
16-bit identifier
time to live
type of service
upper layer
length
flgs
fragment offset
32 bit source IP address
32 bit destination IP address
options (if any)
data (variable length,
typically a TCP or UDP segment)
header checksum
e.g. timestamp, record route taken, specify list of routers to visit.
how much overhead?
v 20 bytes of TCP
v 20 bytes of IP
v =40bytes+app layer overhead
Network Layer 4-34

IP fragmentation, reassembly
v network links have MTU (max.transfer size) – largest possible link-level frame
§ different link types, different MTUs
v 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 datagram out: 3 smaller datagrams
reassembly
Network Layer 4-35
…
…

IP fragmentation, reassembly
length =4000
ID =x
fragflag =0
offset =0
example:
v 4000 byte datagram v MTU = 1500 bytes
1480 bytes in data field
offset = 1480/8
one large datagram becomes several smaller datagrams
length =1500
ID =x
fragflag =1
offset =0
length
ID
fragflag
offset =185
=1500
=x
=1
length =1040
ID =x
fragflag =0
offset =370
Network Layer 4-36

Chapter 4: outline
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 4-37

IP addressing: introduction
v IP address: 32-bit identifier for host, router
223.1.1.1
interface
223.1.1.2
223.1.1.4
223.1.2.1 223.1.2.9
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
223.1.1.3
223.1.3.27
223.1.3.1
223.1.2.2
223.1.3.2
223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1 1
Network Layer 4-38

IP addressing: introduction
223.1.1.1
223.1.1.2
A: wired Ethernet interfaces connected by Ethernet switches
For now: don’t need to worry about how one interface is connected to another (with no intervening router)
Q: how are interfaces actually connected?
223.1.2.1 223.1.2.9
A: we’ll learn about that in chapter 5, 6.
223.1.1.4
223.1.1.3
223.1.3.27
223.1.3.1
223.1.2.2
223.1.3.2
A: wireless WiFi interfaces connected by WiFi base station
Network Layer 4-39

Subnets vIP address:
§subnet part – high order bits
§host part – low order bits
vwhat’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
223.1.1.4 223.1.2.9
223.1.2.1
223.1.1.3
223.1.3.1
223.1.2.2 223.1.3.27
subnet
223.1.3.2
network consisting of 3 subnets
Network Layer 4-40

Subnets
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
223.1.1.0/24
223.1.1.1
223.1.1.2
223.1.1.4 223.1.2.9
223.1.2.0/24
223.1.2.1
223.1.1.3
223.1.3.1
223.1.2.2 223.1.3.27
subnet
223.1.3.2
223.1.3.0/24
subnet mask: /24
Network Layer 4-41

Subnets 223.1.1.2
how many?
223.1.1.1 223.1.1.4 223.1.1.3
223.1.9.2
223.1.7.0
223.1.9.1
223.1.2.6
223.1.8.1
223.1.2.2
223.1.8.0
223.1.3.1
223.1.7.1
223.1.3.27 223.1.3.2
223.1.2.1
Network Layer 4-42

IP 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
subnet part
host part
11001000 00010111 00010000 00000000 200.23.16.0/23
Network Layer 4-43

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 4-44

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 [optional]
§ DHCP server responds with “DHCP offer” msg [optional] § host requests IP address: “DHCP request” msg
§ DHCP server sends address: “DHCP ack” msg
Network Layer 4-45

DHCP client-server scenario
223.1.1.0/24
223.1.1.1
DHCP server
223.1.2.9
223.1.2.1
223.1.1.2 223.1.1.4
arriving DHCP client needs address in this network
223.1.1.3
223.1.3.1
223.1.3.27
223.1.2.2
223.1.2.0/24
223.1.3.2
223.1.3.0/24
Network Layer 4-46

DHCP client-server scenario
DHCP server: 223.1.2.5
DHCP discover
arriving client
src : 0.0.0.0, 68
dest.: 255.255.255.255,67 yiaddr: 0.0.0.0 transaction ID: 654
DHCP request
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
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
Network Layer 4-47

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 4-48

DHCP: example
DHCP
DHCP
v connecting laptop needs its IP address, addr of first-hop router, addr of DNS server: use DHCP
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
UDP
DHCP
IP
DHCP
Eth
DHCP
Phy
DHCP
DHCP
168.1.1.1
router with DHCP server built into router
DHCP
UDP
DHCP
IP
DHCP
Eth
DHCP
Phy
Network Layer 4-49

DHCP: example
router with DHCP server built into router
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
v client now knows its IP address, name and IP address of DSN server, IP address of its first-hop router
DHCP
DHCP
UDP
DHCP
IP
DHCP
Eth
DHCP
Phy
DHCP
DHCP
UDP
DHCP
IP
DHCP
Eth
DHCP
Phy
DHCP
Network Layer 4-50

DHCP: Wireshark output (home LAN)
Message type: Boot Reply (2) Hardware type: Ethernet Hardware address length: 6 Hops: 0
Transaction ID: 0x6b3a11b7
reply
Message type: Boot Request (1) Hardware type: Ethernet Hardware address length: 6 Hops: 0
Transaction ID: 0x6b3a11b7
request
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.”
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 ……
Network Layer 4-51

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
Organization 0 Organization 1 Organization 2
… Organization 7
11001000 00010111 00010000 00000000
11001000 00010111 00010000 00000000 11001000 00010111 00010010 00000000 11001000 00010111 00010100 00000000
….. …. 11001000 00010111 00011110 00000000
200.23.16.0/20
200.23.16.0/23 200.23.18.0/23 200.23.20.0/23
…. 200.23.30.0/23
Network Layer 4-52

Hierarchical addressing: route aggregation
hierarchical addressing allows efficient advertisement of routing information:
Organization 0
200.23.16.0/23
Organization 1
200.23.18.0/23
Organization 2
200.23.20.0/23
. Organization 7 .
200.23.30.0/23
.
. Fly-By-Night-ISP
ISPs-R-Us
“Send me anything with addresses beginning 200.23.16.0/20”
Internet
“Send me anything with addresses beginning 199.31.0.0/16”
Network Layer 4-53

Hierarchical addressing: more specific routes
ISPs-R-Us has a more specific route to Organization 1
Organization 0
200.23.16.0/23
Organization 2
200.23.20.0/23
. Organization 7 .
200.23.30.0/23
Organization 1
200.23.18.0/23
.
. Fly-By-Night-ISP
ISPs-R-Us
“Send me anything with addresses beginning 200.23.16.0/20”
Internet
“Send me anything
with addresses beginning 199.31.0.0/16 or 200.23.18.0/23”
Network Layer 4-54

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
Network Layer 4-55

NA T: network
rest of Internet
address translation
local network (e.g., home network) 10.0.0/24
10.0.0.4
10.0.0.1 10.0.0.2
10.0.0.3
all datagrams leaving local network have same single
source NAT IP address: 138.76.29.7,different source port numbers
datagrams with source or destination in this network have 10.0.0/24 address for source, destination (as usual)
138.76.29.7
Network Layer 4-56

NA T: network address translation
motivation: local network uses just one IP address as far as outside world is concerned:
§rangeofaddressesnotneededfromISP: justone 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
§ devices inside local net not explicitly addressable, visible by outside world (a security plus)
Network Layer 4-57

NA T: network address translation
implementation: NA T 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 NA T translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair
§ incoming datagrams: replace (NA T IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table
Network Layer 4-58

NA T: network address translation
2: NAT router changes datagram source addr from 10.0.0.1, 3345 to 138.76.29.7, 5001, updates table
2
1: host 10.0.0.1 sends datagram to 128.119.40.186, 80
NAT translation table
WAN side addr
138.76.29.7, 5001
……
LAN side addr
10.0.0.1, 3345
……
S: 10.0.0.1, 3345
D: 128.119.40.186, 80
S: 138.76.29.7, 5001 D: 128.119.40.186, 80
10.0.0.4
138.76.29.7
3
1
4
10.0.0.1 10.0.0.2
10.0.0.3
S: 128.119.40.186, 80 D: 10.0.0.1, 3345
S: 128.119.40.186, 80 D: 138.76.29.7, 5001
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
Network Layer 4-59

NA T: network address translation
v 16-bit port-number field:
§ 60,000 simultaneous connections with a single
LAN-side address! v NA T 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
Network Layer 4-60

NA T 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)
client
138.76.29.7
10.0.0.1
10.0.0.4
NAT router
?
§ only one externally visible NATed address: 138.76.29.7
v solution1: statically configure NA T 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
Network Layer 4-61

NA T 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 NA T port map configuration
10.0.0.1
IGD
NAT router
Network Layer 4-62

NA T traversal problem
v solution 3: relaying (used in Skype)
§ NA Ted client establishes connection to relay § external client connects to relay
§ relay bridges packets between to connections
2. connection to relay initiated
by client
client
1. connection to relay initiated
by NATed host
138.76.29.7
10.0.0.1
3. relaying established
NAT router
Network Layer 4-63

Chapter 4: outline
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 4-64

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
3 0
3 1
3 2
3 3
3 6
3 7
4 0
echo reply (ping)
dest. network unreachable dest host unreachable dest protocol unreachable dest port unreachable
dest network unknown dest host unknown
source quench (congestion control – not used)
echo request (ping)
route advertisement
router discovery
8 0
9 0
10 0
110 TTL expired 120 bad IP header
Network Layer 4-65

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
Network Layer 4-66

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
Network Layer 4-67

IPv6 datagram format
priority: identify priority among datagrams in flow flow Label: identify datagrams in same “flow.”
(concept of“flow” not well defined). next header: identify upper layer protocol for data
ver
pri
flow label
payload len
source address (128 bits)
destination address (128 bits)
data
next hdr
hop limit
32 bits
Network Layer 4-68

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
Network Layer 4-69

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 header fields
IPv4 source, dest addr
IPv6 header fields
IPv6 source dest addr
UDP/TCP payload
IPv6 datagram IPv4 datagram
IPv4 payload
Network Layer 4-70

T unneling
A B
IPv6 IPv6
IPv4 tunnel E connecting IPv6 routers
F
logical view:
physical view:
IPv6 IPv6
ABCDEF
IPv6 IPv6 IPv4 IPv4 IPv6 IPv6
Network Layer 4-71

T unneling
A B
IPv6 IPv6
IPv4 tunnel E connecting IPv6 routers
F
logical view:
physical view:
IPv6 IPv6
ABCDEF
IPv6
IPv6 IPv4
IPv4 IPv6 IPv6
flow: X src: A dest: F
data
src:B dest: E
Flow: X Src: A Dest: F
data
src:B dest: E
flow: X src: A dest: F
data
Flow: X Src: A Dest: F
data
A-to-B: IPv6
B-to-C: IPv6 inside IPv4
B-to-C: E-to-F: IPv6 inside IPv6
IPv4
Network Layer 4-72

Chapter 4: outline
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 4-73

Interplay between routing, forwarding
routing algorithm
local forwarding table
dest address
address-range 1 address-range 2 address-range 3 address-range 4
output link
3 2 2 1
routing algorithm determines end-end-path through network
forwarding table determines local forwarding at this router
IP destination address in arriving packet’s header
1 32
Network Layer 4-74

Graph abstraction
5 v3w
5 u231z
2
1xy2 graph: G = (N,E) 1
N = set of routers = { u, v, w, x, y, z }
E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }
aside: graph abstraction is useful in other network contexts, e.g., P2P, where N is set of peers and E is set of TCP connections
Network Layer 4-75

Graph abstraction: costs
2
u
1
5
v
2
x
3
3 1
w
1
y
5 2
z
c(x,x’) = cost of link (x,x’) e.g., c(w,z) = 5
cost could always be 1, or inversely related to bandwidth, or inversely related to congestion
cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp)
key question: what is the least-cost path between u and z ? routing algorithm: algorithm that finds that least cost path
Network Layer 4-76

Routing algorithm classification
Q: global or decentralized information?
global:
v all routers have complete topology, link cost info
v “link state” algorithms
decentralized:
v router knows physically- connected neighbors, link costs to neighbors
v iterative process of computation, exchange of info with neighbors
Q: static or dynamic? static:
v routes change slowly over time
dynamic:
v routes change more quickly
§ periodic update
§ in response to link
cost changes
v “distance vector” algorithms
Network Layer 4-77

A Link-State Routing Algorithm
Dijkstra’s algorithm
v net topology, link costs
known to all nodes
§ accomplished via “link state
broadcast”
§ all nodes have same info
v computes least cost paths from one node (‘source”) to all other nodes
§ gives forwarding table for that node
v iterative: after k iterations, know least cost path to k dest.’s
notation:
v c(x,y): link cost from nodextoy; =∞ifnot direct neighbors
v D(v): current value of cost of path from source to dest. v
v p(v): predecessor node along path from source to v
v N’: set of nodes whose least cost path definitively
known
Network Layer 4-79

Dijsktra’s Algorithm
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Initialization:
N’={u}
for all nodes v
if v adjacent to u then D(v) = c(u,v)
else D(v) = ∞
Loop
find w not in N’ such that D(w) is a minimum addwtoN’
update D(v) for all v adjacent to w and not in N’ :
D(v) = min( D(v), D(w) + c(w,v) )
/* new cost to v is either old cost to v or known shortest path cost to w plus cost from w to v */
until all nodes in N’
Network Layer 4-80

Dijkstra’s algorithm: example D(v) D(w) D(x) D(y) D(z)
Step N’ p(v) p(w) p(x) p(y) p(z) 0 u 7,u 3,u 5,u ∞ ∞ 1uw6,w 5,u11,w∞
2 uwx 6,w
11,w 14,x
10,v 14,x 12,y
uwxv 4 uwxvy 5 uwxvyz
notes:
3
x
9
v construct shortest path tree by tracing predecessor nodes
v tiescanexist(canbebroken u arbitrarily)
8
5
4
7
3
w
y z
7
v
3
4
2
Network Layer 4-81

Dijkstra’s algorithm: another example
Step N’ 0 u 1 ux 2 uxy 3 uxyv 4 uxyvw 5 uxyvwz
D(v),p(v) D(w),p(w) 2,u 5,u 2,u 4,x 2,u 3,y 3,y
D(x),p(x) D(y),p(y) D(z),p(z) 1,u ∞ ∞
2,x 4,y 4,y 4,y
5 v3w
2
u
1
5 231z
x1y2
Network Layer 4-82

Dijkstra’s algorithm: example (2) resulting shortest-path tree from u:
vw u
z xy
resulting forwarding table in u:
destination
v x
y w z
link
(u,v) (u,x)
(u,x) (u,x)
(u,x)
Network Layer 4-83

Dijkstra’s algorithm, discussion
algorithm complexity: n nodes
v each iteration: need to check all nodes, w, not in N v n(n+1)/2 comparisons: O(n2)
v more efficient implementations possible: O(nlogn)
oscillations possible:
v e.g., support link cost equals amount of carried traffic:
1 A 1+e 2+e A 0 0 A 2+e 2+e A 0
D00B D1+e1B D00B D1+e1B
0 C e 11
e
initially
0 C 0 given these costs,
find new routing…. resulting in new costs
1 C 1+e given these costs,
find new routing…. resulting in new costs
0 C 0 given these costs,
find new routing…. resulting in new costs
Network Layer 4-84

Chapter 4: outline
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 4-85

Distance vector algorithm
Bellman-Ford equation (dynamic programming)
let
dx(y) := cost of least-cost path from x to y
then
dx(y) = min {c(x,v) + dv(y) } v
cost from neighbor v to destination y cost to neighbor v
min taken over all neighbors v of x
Network Layer 4-86

Bellman-Ford example
2
u
1
5 v3w5
clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3
B-F equation says:
du(z) = min { c(u,v) + dv(z), c(u,x) + dx(z),
c(u,w) + dw(z) } = min {2 + 5,
1 + 3, 5+3} =4
2
x
3 1
y
z
2
1
node achieving minimum is next
hop in shortest path, used in forwarding table
Network Layer 4-87

Distance vector algorithm
v Dx(y) = estimate of least cost from x to y
§ x maintains distance vector Dx = [Dx(y): y є N ]
v node x:
§ knows cost to each neighbor v: c(x,v)
§ maintains its neighbors’ distance vectors. For each neighbor v, x maintains
Dv =[Dv(y):yєN]
Network Layer 4-88

Distance vector algorithm
key idea:
v from time-to-time, each node sends its own distance vector estimate to neighbors
v when x receives new DV estimate from neighbor, it updates its own DV using B-F equation:
Dx(y) ← minv{c(x,v) + Dv(y)} for each node y ∊ N
v under minor, natural conditions, the estimate Dx(y) converge to the actual least cost dx(y)
Network Layer 4-89

Distance vector algorithm
iterative, asynchronous:
each local iteration caused by:
v local link cost change
v DV update message from
neighbor
distributed:
v each node notifies neighbors only when its DV changes
§ neighbors then notify their neighbors if necessary
each node:
wait for (change in local link cost or msg from neighbor)
recompute estimates if DV to any dest has
changed, notify neighbors
Network Layer 4-90

Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2
Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)}
= min{2+1 , 7+0} = 3
node x
table x y z
x027 x023
yz ∞ ∞ ∞ yz ∞∞∞
cost to
cost to
x y z
node y
table x y z
201 710
cost to
2 y 1 xy∞∞∞ x7z
z
201 ∞∞∞
node z table
x∞∞∞
y z
cost to
xyz
∞∞∞ 710
time
Network Layer 4-91
from from
from
from

Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2
cost to
x y z
x027 x023 x023
y∞∞∞ y201 y201 z∞∞∞ z710 z310
node y cost to cost to cost to table x y z x y z x y z
x∞∞∞ x027 x023 y201 y201 y201 z∞∞∞ z710 z310
Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)}
= min{2+1 , 7+0} = 3
node x
table x y z
cost to
cost to
xyz
2 y 1
x z
7
node z table
x∞∞∞ x027 x023
y∞∞∞ y201 y201
z710 z310 z310 time
cost to
cost to cost to
xyz
xyz xyz
Network Layer 4-92
from from
from
from from from
from
from from

Distance vector: link cost changes
link cost changes:
v node detects local link cost change v updates routing info, recalculates
distance vector
v if DV changes, notify neighbors
“good news travels fast”
1
x
4
y 1 50
z
t0 : y detects link-cost change, updates its DV, informs its neighbors.
t1 : z receives update from y, updates its table, computes new least cost to x , sends its neighbors its DV.
t2 : y receives z’s update, updates its distance table. y’s least costs do not change, so y does not send a message to z.
Network Layer 4-93

Distance vector: link cost changes
link cost changes:
v node detects local link cost change v bad news travels slow – “count to
infinity” problem!
v 44 iterations before algorithm
stabilizes: see text
poisoned reverse:
60
4
y 1 50
x
z
v If Z routes through Y to get to X :
§ Z tells Y its (Z’s) distance to X is infinite (so Y won’t route
to X via Z)
v will this completely solve count to infinity problem?
Network Layer 4-94

Comparison of LS and DV algorithms
message complexity
v LS: with n nodes, E links, O(nE) msgs sent
v DV: exchange between neighbors only
robustness: what happens if router malfunctions?
LS:
§ convergence time varies speed of convergence
v LS: O(n2) algorithm requires O(nE) msgs
§ may have oscillations
v DV: convergence time varies
§ may be routing loops
§ count-to-infinity problem
§ node can advertise incorrect link cost
§ each node computes only its own table
DV:
§ DV node can advertise incorrect path cost
§ each node’s table used by others
• error propagate thru network
Network Layer 4-95

Chapter 4: outline
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 4-96

Hierarchical routing
our routing study thus far – idealization v all routers identical
v network “flat”
… not true in practice
scale: with 600 million destinations:
v can’t store all dest’s in routing tables!
v routing table exchange would swamp links!
administrative autonomy
v internet = network of networks
v each network admin may want to control routing in its own network
Network Layer 4-97

Hierarchical routing
v aggregate routers into regions, “autonomous systems” (AS)
v routers in same AS run same routing protocol
§ “intra-AS” routing protocol
§ routers in different AS can run different intra- AS routing protocol
gateway router:
v at “edge” of its own AS
v has link to router in another AS
Network Layer 4-98

Interconnected ASes
3c 3a 3b AS3
2a
2c
2b AS2
v forwarding table configured by both intra- and inter-AS routing algorithm
§ intra-AS sets entries for internal dests
§ inter-AS & intra-AS sets entries for external dests
1c
1a1d 1bAS1
Intra-AS Routing algorithm
Inter-AS Routing algorithm
Forwarding table
Network Layer 4-99

Inter-AS tasks
v suppose router in AS1 receives datagram destined outside of AS1:
§ router should forward packet to gateway router, but which one?
AS1 must:
1. learn which dests are reachable through AS2, which through AS3
2. propagate this reachability info to all routers in AS1
job of inter-AS routing!
3c
3b
3a
AS31c 2a2c
1a 1b 2b AS1 1d AS2
other networks
other networks
Network Layer 4-100

Example: setting forwarding table in router 1d
v suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c), but not via AS2
§ inter-AS protocol propagates reachability info to all internal routers
v router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1c
§ installs forwarding table entry (x,I)
3c
x
3b
3a
AS31c 2a2c
1a 1b 2b AS1 1d AS2
other networks
other networks
Network Layer 4-101
…

Example: choosing among multiple ASes
v now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2.
v to configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x
§ this is also job of inter-AS routing protocol!
v hot potato routing: send packet towards closest of two routers.
determine from forwarding table the interface I that leads to least-cost gateway. Enter (x,I) in
forwarding table
learn from inter-AS protocol that subnet x is reachable via
multiple gateways
hot potato routing: choose the gateway that has the smallest least cost
use routing info from intra-AS protocol to determine costs of least-cost paths to each
of the gateways
Network Layer 4-103

Intra-AS Routing
v also known as interior gateway protocols (IGP) v most common intra-AS routing protocols:
§ RIP: Routing Information Protocol
§ OSPF: Open Shortest Path First
§ IGRP: Interior Gateway Routing Protocol (Cisco proprietary)
Network Layer 4-105

RIP ( Routing Information Protocol)
v included in BSD-UNIX distribution in 1982
v distance vector algorithm
§ distance metric: # hops (max = 15 hops), each link has cost 1
§ DVs exchanged with neighbors every 30 sec in response message (aka advertisement)
§ each advertisement: list of up to 25 destination subnets (in IP addressing sense)
from router A to destination subnets:
subnet hops u1
v2
u
zyy3 z2
v
AB
w
w2 CDx x3
Network Layer 4-106

RIP: example
w
A
x
z y
DB
C
routing table in router D
destination subnet next router # hops to dest
wA2 yB2 zB7
x — 1 …. …. ….
Network Layer 4-107

RIP: example
A-to-D advertisement
dest next hops
w-1 x-1 zC4 …. ……
w
A
x
z y
DB
C
routing table in router D
destination subnet next router # hops to dest
wA2 yBA2
5
zB7
x — 1 …. …. ….
Network Layer 4-108

RIP: link failure, recovery
if no advertisement heard after 180 sec –> neighbor/link declared dead
§ routes via neighbor invalidated
§ new advertisements sent to neighbors
§ neighbors in turn send out new advertisements (if tables changed)
§ link failure info quickly (?) propagates to entire net
§ poison reverse used to prevent ping-pong loops (infinite distance = 16 hops)
Network Layer 4-109

RIP table processing
v RIP routing tables managed by application-level process called route-d (daemon)
v advertisements sent in UDP packets, periodically repeated
routed routed
transport (UDP)
network (IP)
link
physical
forwarding table
transprt (UDP)
forwarding table
network (IP)
physical
link
Network Layer 4-110

OSPF (Open Shortest Path First)
v “open”: publicly available
v uses link state algorithm § LS packet dissemination
§ topology map at each node
§ route computation using Dijkstra’s algorithm
v OSPF advertisement carries one entry per neighbor v advertisements flooded to entire AS
§ carried in OSPF messages directly over IP (rather than TCP or UDP
v IS-IS routing protocol: nearly identical to OSPF
Network Layer 4-111

OSPF “advanced” features (not in RIP)
v security: all OSPF messages authenticated (to prevent
malicious intrusion)
v multiple same-cost paths allowed (only one path in RIP)
v for each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort ToS; high for real time ToS)
v integrated uni- and multicast support:
§ Multicast OSPF (MOSPF) uses same topology data
base as OSPF
v hierarchical OSPF in large domains.
Network Layer 4-112

Hierarchical OSPF
boundary router
backbone router
backbone
area border routers
area 3
area 1
internal routers
area 2
Network Layer 4-113

Hierarchical OSPF
v two-level hierarchy: local area, backbone.
§ link-state advertisements only in area
§ each nodes has detailed area topology; only know direction (shortest path) to nets in other areas.
v area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers.
v backbone routers: run OSPF routing limited to backbone.
v boundary routers: connect to other AS’s.
Network Layer 4-114

Internet inter-AS routing: BGP
v BGP (Border Gateway Protocol): the de facto inter-domain routing protocol
§ “glue that holds the Internet together” v BGP provides each AS a means to:
§ eBGP: obtain subnet reachability information from neighboring ASs.
§ iBGP: propagate reachability information to all AS- internal routers.
§ determine “good” routes to other networks based on reachability information and policy.
v allows subnet to advertise its existence to rest of Internet: “I am here”
Network Layer 4-115

BGP basics
v BGP session: two BGP routers (“peers”) exchange BGP messages:
§ advertising paths to different destination network prefixes (“path vector” protocol)
§ exchanged over semi-permanent TCP connections
v when AS3 advertises a prefix to AS1:
§ AS3 promises it will forward datagrams towards that prefix § AS3 can aggregate prefixes in its advertisement
3c
3b
3a BGP message
2c
2a
AS2
AS3
1c 1d
other networks
other networks
1a AS1
1b
2b
Network Layer 4-116

BGP basics: distributing path information
v using eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1.
§ 1c can then use iBGP do distribute new prefix info to all routers in AS1
§ 1b can then re-advertise new reachability info to AS2 over 1b-to- 2a eBGP session
v when router learns of new prefix, it creates entry for prefix in its forwarding table.
3b 3a
AS3 1c
1a AS1 1d
eBGP session iBGP session
1b
2c 2b
other networks
other networks
2a
AS2
Network Layer 4-117

Path attributes and BGP routes
v advertised prefix includes BGP attributes § prefix + attributes = “route”
v two important attributes:
§ AS-PATH: contains ASs through which prefix
advertisement has passed: e.g., AS 67, AS 17
§ NEXT-HOP: indicates specific internal-AS router to next- hop AS. (may be multiple links from current AS to next- hop- AS)
v gateway router receiving route advertisement uses import policy to accept/decline
§ e.g., never route through AS x § policy-based routing
Network Layer 4-118

BGP route selection
v router may learn about more than 1 route to destination AS, selects route based on:
1. local preference value attribute: policy decision
2. shortest AS-PATH
3. closest NEXT-HOP router: hot potato routing
4. additional criteria
Network Layer 4-119

BGP messages
v BGP messages exchanged between peers over TCP connection
v BGP messages:
§ OPEN: opens TCP connection to peer and authenticates
sender
§UPDATE:advertisesnewpath(orwithdrawsold)
§ KEEPALIVE: keeps connection alive in absence of UPDATES; also ACKs OPEN request
§ NO TIFICA TION: reports errors in previous msg; also used to close connection
Network Layer 4-120

BGP routing policy
W
B
A
C
legend: X
Y
provider network
customer network:
v A,B,C are provider networks
v X,W,Y are customer (of provider networks) v X is dual-homed: attached to two networks
§ X does not want to route from B via X to C §..so X will not advertise to B a route to C
Network Layer 4-121

BGP routing policy (2)
legend: X
Y
v B advertises path BAW to X
provider network
customer network:
W
B
A
C
v A advertises path AW to B
v Should B advertise path BAW to C?
§ No way! B gets no “revenue” for routing CBAW since neither W nor
C are B’s customers
§ B wants to force C to route to w via A
§ B wants to route only to/from its customers!
Network Layer 4-122

Why different Intra-, Inter-AS routing ?
policy:
v inter-AS: admin wants control over how its traffic routed, who routes through its net.
v intra-AS: single admin, so no policy decisions needed scale:
v hierarchical routing saves table size, reduced update traffic
performance:
v intra-AS: can focus on performance
v inter-AS: policy may dominate over performance
Network Layer 4-123

Chapter 4: done! 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
v understand principles behind network layer services:
§ network layer service models, forwarding versus routing how a router works, routing (path selection), broadcast, multicast
v instantiation, implementation in the Internet
Network Layer 4-124

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