Transcript 3rd Edition: Chapter 4

Chapter 4
Network Layer
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Computer Networking:
A Top Down Approach
5th edition.
Jim Kurose, Keith Ross
Addison-Wesley, April
2009.
Thanks and enjoy! JFK/KWR
All material copyright 1996-2009
J.F Kurose and K.W. Ross, All Rights Reserved
Network Layer
4-1
Review: TCP
 Reliable data transfer: acks
 Pipelined protocol: in-flight packets
 Cumulated acks (single timer)
 Flow control (receiver window size)
 Congestion control (congestion window
size): AIMD
 TCP's two phase operations: Slow Start +
Congestion Control
Network Layer
4-2
Chapter 4: Network Layer
Chapter goals:
 understand principles behind network layer
services:
IP addresses (+ getting an IP address via DHCP)
 Routing algorithms
 Network of networks (BGP, dealing with scales)
 ICMP
 NAT (network address translation)

Network Layer
4-3
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
 packets between same source-dest pair may take
different paths
application
transport
network
data link 1. Send data
physical
application
transport
network
2. Receive data
data link
physical
Network Layer
4-4
The Internet Network layer
Host, router network layer functions:
Transport layer: TCP, UDP
Network
layer
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
4-5
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
how much overhead
with TCP?
 20 bytes of TCP
 20 bytes of IP
 = 40 bytes + app
layer overhead
32 bits
ver
head. type of
len service
16-bit identifier
time to
live
upper
layer
total datagram
length (bytes)
length
fragment
flgs
offset
header
checksum
for
fragmentation/
reassembly
32 bit source IP address
32 bit destination IP address
E.g. timestamp,
record route
taken, specify
list of routers
to visit.
Options (if any)
data
(variable length,
typically a TCP
or UDP segment)
Application
TCP/UDP
IP
Ethernet
IP Fragmentation and 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
4-7
IP Addressing
 IP address: 32-bit
identifier for host,
router interface
 interface: connection
between host/router
and physical link



router’s typically have
multiple interfaces
host typically has one
interface
IP addresses
associated with each
interface
interface
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
223.1.2.9
223.1.3.27
223.1.2.2
223.1.3.2
223.1.3.1
223.1.1.1 = 11011111 00000001 00000001 00000001
223
1
1
Network Layer
1
4-8
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
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
223.1.2.9
223.1.3.27
223.1.2.2
subnet
223.1.3.1
223.1.3.2
network consisting of 3 subnets
Network Layer
4-9
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 4-10
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
 Linux (ubuntu): /etc/network/interface
 DHCP: Dynamic Host Configuration Protocol:
dynamically get address from as server
 “plug-and-play”

Network Layer
4-11
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
an “on”)
Support for mobile users who want to join network (more
shortly)
DHCP overview:
1. host broadcasts “DHCP discover” msg
2. DHCP server responds with “DHCP offer” msg
3. host requests IP address: “DHCP request” msg
4. DHCP server sends address: “DHCP ack” msg
Network Layer 4-12
Graph abstraction of a network
5
2
u
v
2
1
x
3
w
3
1
• c(x,x’) = cost of link (x,x’)
5
1
y
z
2
Graph: G = (N,E)
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) }
- 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)
Question: What’s the least-cost path between u and z ?
Routing algorithm: algorithm that finds least-cost path
Network Layer 4-13
Routing algorithms
Global or decentralized
information?
Global:
 all routers have complete topology,
link cost info
 “link state” algorithms (OSPF)
Decentralized:
 router knows physically-connected
neighbors, link costs to neighbors
 iterative process of computation,
exchange of info with neighbors
 “distance vector” algorithms (RIP)
Static or dynamic?
Static:
 routes change slowly over
time
Dynamic:
 routes change more quickly


periodic update
in response to link cost
changes
Network Layer 4-14
Interplay between routing, forwarding
routing algorithm
local forwarding table
header value output link
0100
0101
0111
1001
3
2
2
1
value in arriving
packet’s header
0111
1
3 2
Network Layer 4-15
Hierarchical routing for scalability
Our routing study thus far - idealization
 all routers identical
 network “flat”
… not true in practice
scale: with 200 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 4-16
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
.
.
.
.
.
.
Fly-By-Night-ISP
“Send me anything
with addresses
beginning
200.23.16.0/20”
Internet
200.23.30.0/23
ISPs-R-Us
“Send me anything
with addresses
beginning
199.31.0.0/16”
Network Layer 4-17
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
.
.
.
.
.
.
Fly-By-Night-ISP
“Send me anything
with addresses
beginning
200.23.16.0/20”
Internet
200.23.30.0/23
ISPs-R-Us
Organization 1
200.23.18.0/23
“Send me anything
with addresses
beginning 199.31.0.0/16
or 200.23.18.0/23”
Network Layer 4-18
Hierarchical routing for scalability
 aggregate routers into
regions, “autonomous
systems” (AS)
 routers in same AS run
same routing protocol


Gateway router
 Direct link to router in
another AS
“intra-AS” routing
protocol
routers in different AS
can run different intraAS routing protocol
Network Layer 4-19
Interconnected ASs
3c
3a
3b
AS3
1a
2a
1c
1d
1b
Intra-AS
Routing
algorithm
2c
AS2
AS1
Inter-AS
Routing
algorithm
Forwarding
table
2b
 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
Network Layer 4-20
Internet inter-AS routing: BGP
 BGP (Border Gateway Protocol):
the de facto standard
 BGP provides each AS a means to:
1. Obtain subnet reachability information from
neighboring ASs.
2. Propagate reachability information to all ASinternal routers.
3. Determine “good” routes to subnets based on
reachability information and policy.
 allows subnet to advertise its existence to
rest of Internet: “I am here”
Network Layer 4-21
BGP basics
 pairs of routers (BGP peers) exchange routing info
over TCP connections (called BGP sessions)
 BGP sessions need not correspond to physical
links.
 when AS2 advertises prefix “200.23.16.0/23” to AS1:
 AS2 promises it will forward datagrams towards
that prefix.
 AS2 can aggregate prefixes in its advertisement
eBGP session
3c
3a
3b
AS3
1a
AS1
iBGP session
2a
1c
1d
1b
2c
AS2
2b
Network Layer 4-22
Distributing reachability info
 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.
3c
3a
3b
AS3
Any dest w/
IP addr AS1
should be
routed to 1c
Any dest w/
IP addr AS1
should be
routed to 2a
eBGP session
1a
AS1
iBGP session
2a
1c
1d
1b
2c
AS2
2b
Network Layer 4-23
Path attributes & 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)
 when gateway router receives route
advertisement, uses local import policy to
accept/decline.
Network Layer 4-24
BGP route selection
 router may learn about more than 1 route
to some prefix. Router must select route.
 elimination rules:
1.
2.
3.
4.
local preference value attribute: policy
decision
shortest AS-PATH
closest NEXT-HOP router: hot potato routing
additional criteria
Network Layer 4-25
Why different Intra- and 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 4-26
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 4-27
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
138.76.29.7
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)
Network Layer 4-28
NAT: 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
NAT translation table
WAN side addr
LAN side addr
1: host 10.0.0.1
sends datagram to
128.119.40.186, 80
138.76.29.7, 5001 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
138.76.29.7
S: 128.119.40.186, 80
D: 138.76.29.7, 5001
3: Reply arrives
dest. address:
138.76.29.7, 5001
3
1
10.0.0.4
S: 128.119.40.186, 80
D: 10.0.0.1, 3345
10.0.0.1
10.0.0.2
4
10.0.0.3
4: NAT router
changes datagram
dest addr from
138.76.29.7, 5001 to 10.0.0.1, 3345
Network Layer 4-29
NAT: Network Address Translation
 16-bit port-number field:

60,000 simultaneous connections with a single
LAN-side address!
 NAT is controversial:
 routers
should only process up to layer 3
 violates end-to-end argument
• NAT possibility must be taken into account by app
designers, eg, P2P applications
 address
IPv6
shortage should instead be solved by
Network Layer 4-30
NAT traversal problem
 client wants to connect to
server with address 10.0.0.1


server address 10.0.0.1 local
Client
to LAN (client can’t use it as
destination addr)
only one externally visible
NATted address: 138.76.29.7
 solution 1: statically
configure NAT to forward
incoming connection
requests at given port to
server

10.0.0.1
?
138.76.29.7
10.0.0.4
NAT
router
e.g., (123.76.29.7, port 2500)
always forwarded to 10.0.0.1
port 25000
Network Layer 4-31
NAT traversal problem
 solution 2: Universal Plug and
Play (UPnP) Internet Gateway
Device (IGD) Protocol. Allows
NATted host to:
 learn public IP address
(138.76.29.7)
 add/remove port mappings
(with lease times)
10.0.0.1
IGD
10.0.0.4
138.76.29.7
NAT
router
i.e., automate static NAT port
map configuration
Network Layer 4-32
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
3. relaying
established
1. connection to
relay initiated
by NATted host
138.76.29.7
10.0.0.1
NAT
router
Network Layer 4-33
Network Layer 4-34