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

Chapter 4 Network Layer
Computer
Networking: A Top
Down Approach
6th edition
Jim Kurose, Keith Ross Addison- Wesley March 2012
Network Layer 4-1

Chapter 4: network layer
chapter goals:
 understand principles behind network layer services:
 network layer service models  forwarding versus routing
 how a router works
 routing (path selection)
 broadcast, multicast
 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
 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
physical
application
transport
network
data link
physical
network
network
data link
physical
data link
network
data link
physical
network
network
data link
data link
physical
physical
network
network
data link physical
data link
physical
net
work
data link
physical
application
network
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
 forwarding: move packets from router’s input to appropriate router output
 routing: determine route taken by packets from source to dest.
 routing algorithms
analogy:
 routing: process of planning trip from source to dest
 forwarding: process of getting through single interchange
Network Layer 4-5

Network layer: data plane, control plane
Data plane Control plane
 local, per-router function  network-wide logic
 determines how datagram  determines how datagram is
arriving on router input port is forwarded to router output port
 forwarding function
routed among routers along end-end path from source host to destination host
 two control-plane approaches: • traditional routing algorithms:
implemented in routers
• software-defined networking (SDN): implemented in (remote) servers
values in arriving packet header
0111
3
1
2
Network Layer: Data Plane 4-6

Interplay between routing and forwarding
routing algorithm
local forwarding table
header value
0100 0101 0111 1001
output link
3 2 2 1
routing algorithm determines end-end-path through network
forwarding table determines local forwarding at this router
value in arriving packet’s header
0111
3
1 2
Network Layer 4-7

Per-router control plane
Individual routing algorithm components in each and every router interact in the control plane
Routing Algorithm
control plane
data plane
values in arriving packet header
0111
3
1
2
Network Layer: Control Plane 5-8

Logically centralized control plane
A distinct (typically remote) controller interacts with local control agents (CAs)
Remote Controller
control plane
data plane
CA
CA
CA
CA
CA
values in arriving packet header
0111
3
1
2
Network Layer: Control Plane 5-9

Network service model
Q: What service model for “channel” transporting datagrams from sender to receiver?
example services for individual datagrams:
 guaranteed delivery
 guaranteed delivery with less than 40 msec delay
example services for a flow of datagrams:
 in-order datagram delivery
 guaranteed minimum bandwidth to flow
 restrictions on changes in inter-packet spacing
Network Layer 4-10

Network layer service models:
Network Service Architecture Model
Internet best effort ATM CBR
ATM VBR ATM ABR ATM UBR
Guarantees ?
Loss Order Timing
Congestion feedback
no (inferred via loss)
no congestion no congestion yes
no
Bandwidth
none no no no
constant yes yes yes rate
guaranteed yes yes yes rate
guaranteed no yes no minimum
none no yes no
Network Layer 4-11

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 4-12

Connection, connection-less service
 datagram network provides network-layer connectionless service
 virtual-circuit network provides network-layer connection service
 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 4-13

Datagram networks
 no call setup at network layer
 routers: no state about end-to-end connections
 no network-level concept of “connection”
 packets forwarded using destination host address
application
transport
application
1. send datagrams
2. receive datagrams
transport
network
network
data link
data link
physical
physical
Network Layer 4-14

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 4-15

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
Network Layer 4-16

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
Link interface
11001000 00010111 00010*** *********
0
11001000 00010111 00011000 *********
1
11001000 00010111 00011*** *********
2
otherwise
3
examples:
DA: 11001000 00010111 00010110 10100001 DA: 11001000 00010111 00011000 10101010
Network Layer 4-17

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 4-18

Router architecture overview
two key router functions:
 run routing algorithms/protocol (RIP, OSPF, BGP)
 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 4-19

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:
 given datagram dest., lookup output port using forwarding table in input port memory (“match plus action”)
 goal: complete input port processing at ‘line speed’
 queuing: if datagrams arrive faster than forwarding rate into switch fabric
Network Layer 4-20

Switching fabrics
 transfer packet from input buffer to appropriate output buffer
 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
 three types of switching fabrics
memory
memory bus crossbar
Network Layer 4-21

Switching via memory
first generation routers:
 traditional computers with switching under direct control of CPU
 packet copied to system’s memory
 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 4-22

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
 32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers
bus
Network Layer 4-23

Switching via interconnection network
 overcome bus bandwidth limitations
 banyan networks, crossbar, 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 60 Gbps through the interconnection network
crossbar
Network Layer 4-24

Output ports
switch fabric
This slide in HUGELY important!
datagram buffer
queueing
link layer
protocol (send)
line termination
 buffering required when datagrams arrive from fabric faster than the transmission rate
 scheduling discipline chooses among queued datagrams for transmission
Datagram (packets) can be lost due to congestion, lack of buffers
Network Layer 4-25
Priority scheduling – who gets best performance, network neutrality

Output port queueing
switch fabric
switch fabric
at t, packets more from input to output
one packet time later
 buffering when arrival rate via switch exceeds output line speed
 queueing (delay) and loss due to output port buffer overflow!
Network Layer 4-26

How much buffering?
 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
 recent recommendation: with N flows, buffering
equal to
RTT.C
N
Network Layer 4-27

Input port queuing
 fabric slower than input ports combined -> queueing may occur at input queues
 queueing delay and loss due to input buffer overflow!
 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 4-28

Scheduling mechanisms
 scheduling: choose next packet to send on link  FIFO (first in first out) scheduling: send in order of
arrival to queue
 real-world example?
 discard policy: if packet arrives to full queue: who to discard?
• tail drop: drop arriving packet
• priority: drop/remove on priority basis • random: drop/remove randomly
packet arrivals
queue link
packet departures
(waiting area)
(server)
Network Layer: Data Plane 4-29

Scheduling policies: priority
priority scheduling: send highest priority queued packet
 multiple classes, with different priorities
 class may depend on marking or other header info, e.g. IP source/dest, port numbers, etc.
 real world example?
high priority queue (waiting area)
arrivals
departures
classify link
low priority queue (server) (waiting area)
2 1345
arrivals
packet in service
departures
1
3
2
4
5
1324 5
Network Layer: Data Plane 4-30

Scheduling policies: still more
Round Robin (RR) scheduling:
 multiple classes
 cyclically scan class queues, sending one complete
packet from each class (if available)  real world example?
2 1345
arrivals
packet in service
departures
1
3
2
4
5
1334 5
Network Layer: Data Plane 4-31

Scheduling policies: still more
Weighted Fair Queuing (WFQ):
 generalized Round Robin
 each class gets weighted amount of service in
each cycle
 real-world example?
Network Layer: Data Plane 4-32

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 4-33

The Internet network layer
host, router network layer functions:
transport layer: TCP, UDP
IP protocol
• addressing conventions
• datagram format
• packet handling conventions
routing protocols
• path selection
• RIP, OSPF, BGP
forwarding table
ICMP protocol
• error reporting • router
“signaling” link layer
physical layer
network layer
Network Layer 4-34

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?
 20 bytes of TCP
 20 bytes of IP
 =40bytes+app layer overhead
Network Layer 4-35

IP fragmentation, reassembly
 network links have MTU (max.transfer size) – largest possible link-level frame
 different link types, different MTUs
 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 4-36
…
…

IP fragmentation, reassembly
length =4000
ID =x
fragflag =0
offset =0
example:
 4000 byte datagram  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 4-37

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 4-38

IP addressing: introduction
 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
 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)
 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 4-39

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 4-40

Subnets IP address:
subnet part – high order bits
host part – low order bits
what’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 4-41

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
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 4-42

Subnets
how many?
223.1.1.2
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 4-43

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 4-44

IP addresses: how to get one?
Q: How does a host get IP address?
 hard-coded by system admin in a file
 Windows: control-panel->network->configuration- >tcp/ip->properties
 UNIX: /etc/rc.config
 DHCP: Dynamic Host Configuration Protocol:
dynamically get address from as server  “plug-and-play”
Network Layer 4-45

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 4-46

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 4-47

DHCP client-server scenario
DHCP server: 223.1.2.5
DHCP discover
arriving client
src : 0.0.0.0, 68
Broadcast: is there a
dest.: 255.255.255.255,67 DHCPysiaedrdvr:er0o.0u.0t.0there? transaction ID: 654
DHCP offer
src: 223.1.2.5, 67
Broadcast: I’m a DHCP
dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4
DHCP request
server! Here’s an IP
transaction ID: 654
address you can use
lifetime: 3600 secs
src: 0.0.0.0,68
dest:: 255.255.255.255, 67
Broadcast: OK. I’ll take
yiaddrr: 223.1.2.4
that IP address!
transaction ID: 655 lifetime: 3600 secs
DHCP ACK
src: 223.1.2.5, 67 dest: 255.255.255.255, 68
Broadcast: OK. You’ve
yiaddrr: 223.1.2.4
got that IP address!
transaction ID: 655 lifetime: 3600 secs
Network Layer 4-48

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 4-49

DHCP: example
DHCP
DHCP
 connecting laptop needs its IP address, addr of first-hop router, addr of DNS server: use DHCP
 DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in 802.1 Ethernet
 Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server
 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 4-50

DHCP: example
DHCP
DHCP
UDP
DHCP
IP
DHCP
Eth
DHCP
Phy
DHCP
DHCP
UDP
DHCP
IP
DHCP
Eth
router with DHCP server built into router
 DCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server
 encapsulation of DHCP server, frame forwarded to client, demuxing up to DHCP at client
 client now knows its IP address, name and IP address of DSN server, IP address of its first-hop router
DHCP
Phy
DHCP
Network Layer 4-51

DHCP: Wireshark output (home LAN)
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 ……
Message type: Boot Reply (2) Hardware type: Ethernet Hardware address length: 6 Hops: 0
Transaction ID: 0x6b3a11b7
reply
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.”
Network Layer 4-52

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 4-53

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
.
. Fly-By-Night-ISP
“Send me anything with addresses beginning 200.23.16.0/20”
. .
200.23.30.0/23
Organization 7
. .
Internet
ISPs-R-Us
“Send me anything with addresses beginning 199.31.0.0/16”
Network Layer 4-54

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
.
. Fly-By-Night-ISP
“Send me anything with addresses beginning 200.23.16.0/20”
. .
200.23.30.0/23
Organization 1
200.23.18.0/23
Organization 7
. .
Internet
ISPs-R-Us
“Send me anything
with addresses beginning 199.31.0.0/16 or 200.23.18.0/23”
Network Layer 4-55

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 4-56

NAT: network address translation
rest of Internet
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
138.76.29.7
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)
Network Layer 4-57

NAT: network address translation
motivation: 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
 devices inside local net not explicitly addressable, visible by outside world (a security plus)
Network Layer 4-58

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
Network Layer 4-59

NAT: network address translation
NAT translation table
WAN side addr
LAN side addr
138.76.29.7, 5001
……
10.0.0.1, 3345
……
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
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
1
4
10.0.0.1 10.0.0.2
10.0.0.3
138.76.29.7
3
4: NAT router
changes datagram
dest addr from
138.76.29.7, 5001 to 10.0.0.1, 3345
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
Network Layer 4-60

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, e.g., P2P applications
 address shortage should instead be solved by IPv6
Network Layer 4-61

NAT traversal problem
 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) ?
10.0.0.1
10.0.0.4
NAT router
client
 only one externally visible NATed address: 138.76.29.7
 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
138.76.29.7
Network Layer 4-62

NAT traversal problem
 solution 2: Universal Plug and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATed host to:
 learn public IP address (138.76.29.7)
 add/remove port mappings (with lease times)
i.e., automate static NAT port map configuration
10.0.0.1
IGD
NAT router
Network Layer 4-63

NAT traversal problem
 solution 3: relaying (used in Skype)
 NATed 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 4-64

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 4-65

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 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 4-66

Traceroute and ICMP
 source sends series of UDP segments to dest
 first set has TTL =1
 second set has TTL=2, etc.  unlikely port number
 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
 when ICMP messages arrives, source records RTTs
stopping criteria:
 UDP segment eventually arrives at destination host
 destination returns ICMP “port unreachable” message (type 3, code 3)
 source stops
3 probes 3 probes
3 probes
Network Layer 4-67

IPv6: motivation
 initial motivation: 32-bit address space soon to be completely allocated.
 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 4-68

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 4-69

Other changes from IPv4
 checksum: removed entirely to reduce processing time at each hop
 options: allowed, but outside of header, indicated by “Next Header” field
 ICMPv6: new version of ICMP
 additional message types, e.g. “Packet Too Big”  multicast group management functions
Network Layer 4-70

Transition from IPv4 to IPv6
 not all routers can be upgraded simultaneously
 no “flag days”
 how will network operate with mixed IPv4 and IPv6 routers?
 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 4-71

Tunneling
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 4-72

Tunneling
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
src:B dest: E
flow: X src: A dest: F
data
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 4-73

IPv6: adoption
 US National Institutes of Standards estimate [2013]:  ~3% of industry IP routers
 ~11% of US gov’t routers
 Long (long!) time for deployment, use
 20 years and counting!
 think of application-level changes in last 20 years: WWW, Facebook, …
 Why?
Network Layer 4-74

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 4-75

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 4-76

Graph abstraction
5
v3w u231z
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
2
5
Network Layer 4-77

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 4-78

Routing algorithm classification
Q: global or decentralized information?
global:
 all routers have complete topology, link cost info
 “link state” algorithms
decentralized:
 router knows physically- connected neighbors, link costs to neighbors
 iterative process of computation, exchange of info with neighbors
Q: static or dynamic? static:
 routes change slowly over time
dynamic:
 routes change more quickly
 periodic update
 in response to link
cost changes
 “distance vector” algorithms
Network Layer 4-79

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 4-80

A Link-State Routing Algorithm
Dijkstra’s algorithm
 net topology, link costs
known to all nodes
 accomplished via “link state
broadcast”
 all nodes have same info
 computes least cost paths
notation:
 c(x,y): link cost from node x to y; = ∞ if not direct neighbors
 D(v): current value of cost of path from source to dest. v
from one node (‘source”)  p(v): predecessor node
to all other nodes
 gives forwarding table for that node
 iterative: after k iterations, know least cost path to k dest.’s
along path from source to v
 N’: set of nodes whose least cost path definitively known
Network Layer 4-81

Dijsktra’s Algorithm
1
2
3
4
5
6
7
8
9
10 addwtoN’
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
11
12
13
14
15 until all nodes in N’
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 */
Network Layer 4-82

Dijkstra’s algorithm: example
Step 0
D(v) D(w) D(x) D(y) D(z) N’ p(v) p(w) p(x) p(y) p(z)
u 7,u 3,u 5,u ∞ ∞ uw 6,w 5,u11,w ∞
1 2 3 4 5
uwx 6,w uwxv
uwxvy uwxvyz
11,w 14,x
10,v 14,x 12,y
x
9
notes:
5
4
7
 construct shortest path tree by tracing predecessor nodes
 tiescanexist(canbebroken u arbitrarily)
8
3
w
y z
7
v
3
4
2
Network Layer 4-83

Dijkstra’s algorithm: another example
Step N’ D(v),p(v) D(w),p(w)
D(x),p(x) D(y),p(y) D(z),p(z) 1,u ∞ ∞
0 u 1 ux 2 uxy 3 uxyv 4 uxyvw 5 uxyvwz
2,u 5,u 2,u 4,x 2,u 3,y
3,y
5 v3w
2
u
1
2,x
∞
4,y 4,y 4,y
5 231z
x1y2
Network Layer 4-84

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 4-85

Dijkstra’s algorithm, discussion
algorithm complexity: n nodes
 each iteration: need to check all nodes, w, not in N  n(n+1)/2 comparisons: O(n2)
 more efficient implementations possible: O(nlogn)
oscillations possible:
 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 4-86

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 4-87

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 4-88

Bellman-Ford example
5 v3w5
2
u
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
node achieving minimum is next
hop in shortest path, used in forwarding table
231z xy2
1
1
Network Layer 4-89

Distance vector algorithm
 Dx(y) = estimate of least cost from x to y
 x maintains distance vector Dx = [Dx(y): y є N ]
 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): y є N ]
Network Layer 4-90

Distance vector algorithm
key idea:
 from time-to-time, each node sends its own distance vector estimate to neighbors
 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
 under minor, natural conditions, the estimate Dx(y) converge to the actual least cost dx(y)
Network Layer 4-91

Distance vector algorithm
iterative, asynchronous:
each local iteration caused by:
 local link cost change
 DV update message from
neighbor
distributed:
 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 4-92

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
y∞∞∞ y z∞∞∞ z
cost to
cost to
x y z
node y
table x y z
201 710
cost to
2 y 1 x∞∞∞ xz
y201 7
z
∞∞∞
node z table
x ∞∞ ∞
y z
cost to
xyz
∞∞∞ 710
time
Network Layer 4-93
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 2 y 1
x∞∞∞ x027 x023 x z
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
node z table
cost to
cost to cost to
y201 z∞∞∞
y201 y201 7 z710 z310
xyz x ∞∞ ∞
xyz xyz
y∞∞∞ z710
x 0 2 7 x 0 2 3 y201 y201
z310 z310 time
Network Layer 4-94
from from
from
from from from
from
from from

Distance vector: link cost changes
link cost changes:
 node detects local link cost change  updates routing info, recalculates
distance vector
 if DV changes, notify neighbors
1
x
4
y 1 50
z
“good news travels fast”
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 4-95

Distance vector: link cost changes
link cost changes:
 node detects local link cost change
 bad news travels slow – “count to
infinity” problem!
 44 iterations before algorithm
stabilizes: see text
poisoned reverse:
60
4
y 1 50
x
z
 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)
 will this completely solve count to infinity problem?
Network Layer 4-96

Comparison of LS and DV algorithms
message complexity
 LS: with n nodes, E links, O(nE) msgs sent
 DV: exchange between neighbors only
robustness: what happens if router malfunctions?
LS:
 convergence time varies speed of convergence
 LS: O(n2) algorithm requires O(nE) msgs
 may have oscillations
 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 4-97

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 4-98

Hierarchical routing
our routing study thus far – idealization  all routers identical
 network “flat”
… not true in practice
scale: with 600 million destinations:
 can’t store all dest’s in routing tables!
 routing table exchange would swamp links!
administrative autonomy
 internet = network of networks
 each network admin may want to control routing in its own network
Network Layer 4-99

Hierarchical routing
 aggregate routers into regions, “autonomous systems” (AS)
 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:
 at “edge” of its own AS
 has link to router in another AS
Network Layer 4-100

Interconnected ASes
3c 3a 3b AS3
2a
2c
2b AS2
 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 4-101

Inter-AS tasks
 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
3c
3b 3a AS3
1c 1a
2c
2a
other networks
job of inter-AS routing!
other networks
1b AS1
2b AS2
1d
Network Layer 4-102

Example: setting forwarding table in router 1d
 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
 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
3b 3a AS3
x
1c 1a
2c
2a
other networks
other networks
1b AS1
2b AS2
1d
Network Layer 4-103

Example: choosing among multiple ASes
 now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2.
 to configure forwarding table, router 1d must determine which gateway it should forward packets towards for dest x
 this is also job of inter-AS routing protocol!
3c
3b 3a AS3
x
1c 1a
2c
2a
other networks
other networks
1b AS1
2b AS2
1d
?
Network Layer 4-104

Example: choosing among multiple ASes
 now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2.
 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!
 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 4-105

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 4-106

Intra-AS Routing
 also known as interior gateway protocols (IGP)  most common intra-AS routing protocols:
 RIP: Routing Information Protocol
 OSPF: Open Shortest Path First
 IGRP: Interior Gateway Routing Protocol (Cisco proprietary)
Network Layer 4-107

RIP ( Routing Information Protocol)
 included in BSD-UNIX distribution in 1982
 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)
u
v
AB CD
from router A to destination subnets: subnet hops
u1 v2 w2
w
z
xx3 yy3 z2
Network Layer 4-108

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 4-109

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 4-110

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 4-111

RIP table processing
 RIP routing tables managed by application-level process called route-d (daemon)
 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 4-112

OSPF (Open Shortest Path First)
 “open”: publicly available
 uses link state algorithm
 LS packet dissemination
 topology map at each node
 route computation using Dijkstra’s algorithm
 OSPF advertisement carries one entry per neighbor  advertisements flooded to entire AS
 carried in OSPF messages directly over IP (rather than TCP or UDP
 IS-IS routing protocol: nearly identical to OSPF
Network Layer 4-113

OSPF “advanced” features (not in RIP)
 security: all OSPF messages authenticated (to prevent
malicious intrusion)
 multiple same-cost paths allowed (only one path in RIP)
 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)
 integrated uni- and multicast support:
 Multicast OSPF (MOSPF) uses same topology data
base as OSPF
 hierarchical OSPF in large domains.
Network Layer 4-114

Hierarchical OSPF
boundary router
backbone router
backbone
area border routers
area 3
area 1
internal routers
area 2
Network Layer 4-115

Hierarchical OSPF
 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.
 area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers.
 backbone routers: run OSPF routing limited to backbone.
 boundary routers: connect to other AS’s.
Network Layer 4-116

Internet inter-AS routing: BGP
 BGP (Border Gateway Protocol): the de facto inter-domain routing protocol
 “glue that holds the Internet together”  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.
 allows subnet to advertise its existence to rest of Internet: “I am here”
Network Layer 4-117

BGP basics
 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
 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 2b
AS2
AS3
1c
1a AS1 1d
2a 1b
other networks
other networks
Network Layer 4-118

BGP basics: distributing path information
 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
 when router learns of new prefix, it creates entry for prefix in its forwarding table.
3b 3a
eBGP session iBGP session
2a
AS2
AS3
1c 1a
2c 2b
other networks
other networks
1b AS1
1d
Network Layer 4-119

Path attributes and BGP routes
 advertised prefix includes BGP attributes  prefix + attributes = “route”
 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)
 gateway router receiving route advertisement uses import policy to accept/decline
 e.g., never route through AS x  policy-based routing
Network Layer 4-120

BGP route selection
 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 4-121

BGP messages
 BGP messages exchanged between peers over TCP connection
 BGP messages:
 OPEN: opens TCP connection to peer and authenticates
sender
 UPDATE: advertises new path (or withdraws old)
 KEEPALIVE: keeps connection alive in absence of UPDATES; also ACKs OPEN request
 NOTIFICATION: reports errors in previous msg; also used to close connection
Network Layer 4-122

Putting it Altogether:
How Does an Entry Get Into a Router’s Forwarding Table?
 Answer is complicated!
 Ties together hierarchical routing (Section 4.5.3)
with BGP (4.6.3) and OSPF (4.6.2).  Provides nice overview of BGP!

How does entry get in forwarding table?
routing algorithms
local forwarding table
prefix output port
138.16.64/22 124.12/16 212/8 …………..
3 2 4 …
entry
Assume prefix is in another AS.
Dest IP
3
1 2

How does entry get in forwarding table?
High-level overview
1. Router becomes aware of prefix
2. Router determines output port for prefix
3. Router enters prefix-port in forwarding table

Router becomes aware of prefix
3c
3b
3a BGP message
2c 2b
AS2
AS3
1c
1a AS1 1d
2a 1b
other networks
other networks
 BGP message contains “routes”
 “route” is a prefix and attributes: AS-PATH, NEXT-
HOP,…
 Example: route:
 Prefix:138.16.64/22 ; AS-PATH: AS3 AS131 ; NEXT-HOP: 201.44.13.125

Router may receive multiple routes
3c
3b
3a BGP message
2c 2b
AS2
AS3
1c
1a AS1 1d
2a 1b
other networks
other networks
 Router may receive multiple routes for same prefix  Has to select one route

Select best BGP route to prefix
 Router selects route based on shortest AS-PATH
 Example:
 AS2 AS17 to 138.16.64/22
 AS3 AS131 AS201 to 138.16.64/22
 What if there is a tie? We’ll come back to that!
select

Find best intra-route to BGP route
 Use selected route’s NEXT-HOP attribute
 Route’s NEXT-HOP attribute is the IP address of the
router interface that begins the AS PATH.
 Example:
 AS-PATH: AS2 AS17 ; NEXT-HOP: 111.99.86.55
 Router uses OSPF to find shortest path from 1c to 111.99.86.55
3c
3b 3a AS3
1c 1a
111.99.86.55
2c
2a
other networks
other networks
1b AS1
2b AS2
1d

Router identifies port for route
 Identifies port along the OSPF shortest path  Adds prefix-port entry to its forwarding table:
 (138.16.64/22 , port 4)
3c
router 3a port
AS311c4 2a2c
23 2b 1a 1b AS2
3b
other networks
other networks
AS1 1d

Hot Potato Routing
 Suppose there two or more best inter-routes.
 Then choose route with closest NEXT-HOP  Use OSPF to determine which gateway is closest  Q: From 1c, chose AS3 AS131 or AS2 AS17?
 A: route AS3 AS201 since it is closer
3c
3b 3a AS3
1c 1a
2c
2a
other networks
other networks
1b AS1
2b AS2
1d

How does entry get in forwarding table? Summary
1. 2.
Router becomes aware of prefix
 via BGP route advertisements from other routers
Determine router output port for prefix
 Use BGP route selection to find best inter-AS route  Use OSPF to find best intra-AS route leading to best
inter-AS route
 Router identifies router port for that best route
Enter prefix-port entry in forwarding table
3.

BGP routing policy
W
B
A
C
legend: X
Y
provider network
customer network:
 A,B,C are provider networks
 X,W,Y are customer (of provider networks)  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 4-133

BGP routing policy (2)
legend: X
Y
 B advertises path BAW to X
provider network
customer network:
W
B
A
C
 A advertises path AW to B
 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 4-134

Why different Intra-, Inter-AS routing ?
policy:
 inter-AS: admin wants control over how its traffic routed, who routes through its net.
 intra-AS: single admin, so no policy decisions needed scale:
 hierarchical routing saves table size, reduced update traffic
performance:
 intra-AS: can focus on performance
 inter-AS: policy may dominate over performance
Network Layer 4-135

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 4-136

Broadcast routing
 deliver packets from source to all other nodes  source duplication is inefficient:
duplicate
duplicate
R1 creation/transmission R1
duplicate
R2
R3 R4 R3 R4
R2
source in-network duplication duplication
 source duplication: how does source determine recipient addresses?
Network Layer 4-137

In-network duplication
 flooding: when node receives broadcast packet, sends copy to all neighbors
 problems: cycles & broadcast storm
 controlled flooding: node only broadcasts pkt if it hasn’t broadcast same packet before
 node keeps track of packet ids already broadacsted
 or reverse path forwarding (RPF): only forward packet
if it arrived on shortest path between node and source
 spanning tree:
 no redundant packets received by any node
Network Layer 4-138

Spanning tree
 first construct a spanning tree
 nodes then forward/make copies only along
spanning tree
A
B c
D E
G
(a) broadcast initiated at A
A
B c
D E
G
(b) broadcast initiated at D
F
F
Network Layer 4-139

Spanning tree: creation
 center node
 each node sends unicast join message to center
node
 message forwarded until it arrives at a node already belonging to spanning tree
AA
3
cB cB
4
DD E5E
2
F
F
1
GG
(a) stepwise construction of (b) constructed spanning spanning tree (center: E) tree
Network Layer 4-140

Multicast routing: problem statement
goal: find a tree (or trees) connecting routers having
local mcast group members
 tree: not all paths between routers used
 shared-tree: same tree used by all group members
 source-based: different tree from each sender to rcvrs
legend
group member
not group member
router with a group member
router without group member
shared tree
source-based trees
Network Layer 4-141

Approaches for building mcast trees
approaches:
 source-based tree: one tree per source  shortest path trees
 reverse path forwarding
 group-shared tree: group uses one tree  minimal spanning (Steiner)
 center-based trees
…we first look at basic approaches, then specific protocols adopting these approaches
Network Layer 4-142

Shortest path tree
 mcast forwarding tree: tree of shortest path routes from source to all receivers
 Dijkstra’s algorithm s: source
R1
1
R2
3
LEGEND
router with attached group member
router with no attached group member
link used for forwarding, i indicates order link added by algorithm
2
5
6
R7
R4
4
R5
R3
i
R6
Network Layer 4-143

Reverse path forwarding
 rely on router’s knowledge of unicast shortest path from it to sender
 each router has simple forwarding behavior:
if (mcast datagram received on incoming link on shortest path back to center)
then flood datagram onto all outgoing links else ignore datagram
Network Layer 4-144

Reverse path forwarding: example
s: source
R1 R2
R4
R5 R7
LEGEND
router with attached group member
router with no attached group member
datagram will be forwarded
datagram will not be forwarded
R3
R6
 result is a source-specific reverse SPT
 may be a bad choice with asymmetric links
Network Layer 4-145

Reverse path forwarding: pruning
 forwarding tree contains subtrees with no mcast group members
 no need to forward datagrams down subtree
 “prune” msgs sent upstream by router with no
downstream group members
s: source
R1 R2
LEGEND
R4
P
router with attached group member
router with no attached group member
prune message
links with multicast forwarding
R3 P R6
R5 P
R7
Network Layer 4-146

Shared-tree: steiner tree
 steiner tree: minimum cost tree connecting all routers with attached group members
 problem is NP-complete
 excellent heuristics exists
 not used in practice:
 computational complexity
 information about entire network needed
 monolithic: rerun whenever a router needs to join/leave
Network Layer 4-147

Center-based trees
 single delivery tree shared by all
 one router identified as “center” of tree
 to join:
 edge router sends unicast join-msg addressed to center
router
 join-msg “processed” by intermediate routers and forwarded towards center
 join-msg either hits existing tree branch for this center, or arrives at center
 path taken by join-msg becomes new branch of tree for this router
Network Layer 4-148

Center-based trees: example
suppose R6 chosen as center: LEGEND
R1
3
2
R5
router with attached group member
router with no attached group member
1
R4
R2 R3
1
path order in which join messages generated
R6
R7
Network Layer 4-149

Internet Multicasting Routing: DVMRP
 DVMRP: distance vector multicast routing protocol, RFC1075
 flood and prune: reverse path forwarding, source- based tree
 RPF tree based on DVMRP’s own routing tables constructed by communicating DVMRP routers
 no assumptions about underlying unicast
 initial datagram to mcast group flooded everywhere
via RPF
 routers not wanting group: send upstream prune msgs
Network Layer 4-150

DVMRP: continued…
 soft state: DVMRP router periodically (1 min.) “forgets” branches are pruned:
 mcast data again flows down unpruned branch
 downstream router: reprune or else continue to receive
data
 routers can quickly regraft to tree
 following IGMP join at leaf  odds and ends
 commonly implemented in commercial router
Network Layer 4-151

Tunneling
Q: how to connect “islands” of multicast routers in a “sea” of unicast routers?
physical topology logical topology
 mcast datagram encapsulated inside “normal” (non- multicast-addressed) datagram
 normal IP datagram sent thru “tunnel” via regular IP unicast to receiving mcast router (recall IPv6 inside IPv4 tunneling)
 receiving mcast router unencapsulates to get mcast datagram
Network Layer 4-152

PIM: Protocol Independent Multicast
 not dependent on any specific underlying unicast routing algorithm (works with all)
 two different multicast distribution scenarios :
dense:
sparse:
 # networks with group members small wrt # interconnected networks
 group members “widely dispersed”
 bandwidth not plentiful
Network Layer 4-153
 group members densely packed, in “close” proximity.
 bandwidth more plentiful

Consequences of sparse-dense dichotomy:
dense
 group membership by routers assumed until routers explicitly prune
 data-driven construction on mcast tree (e.g., RPF)
sparse:
 no membership until routers
explicitly join
 receiver- driven construction of mcast tree (e.g., center- based)
 bandwidth and non-group-  bandwidth and non-group- router processing profligate router processing conservative
Network Layer 4-154

PIM- dense mode
flood-and-prune RPF: similar to DVMRP but…  underlying unicast protocol provides RPF info
for incoming datagram
 less complicated (less efficient) downstream flood than DVMRP reduces reliance on underlying routing algorithm
 has protocol mechanism for router to detect it is a leaf-node router
Network Layer 4-155

PIM – sparse mode
 center-based approach
 router sends join msg to
rendezvous point (RP)
 intermediate routers update state and forward join
R1
R4
 after joining via RP, router can switch to source- specific tree
 increased performance: less concentration, shorter paths
R2
R3
join
join join
R5
rendezvous point
R6
all data multicast from rendezvous point
R7
Network Layer 4-156

PIM – sparse mode
sender(s):
 unicast data to RP, which distributes down RP-rooted tree
R1
R4
 RP can extend mcast tree upstream to source
 RP can send stop msg if no attached receivers
 “no one is listening!”
R2
R3
join
join join
R6
R5
all data multicast from rendezvous point
R7
rendezvous point
Network Layer 4-157

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
 understand principles behind network layer services:
 network layer service models, forwarding versus routing how a router works, routing (path selection), broadcast, multicast
 instantiation, implementation in the Internet
Network Layer 4-158