Network Layer 4

1
Network Layer (4)
2
Classless Addressing

• Addresses allocated in contiguous blocks
• Number of addresses assigned always power of 2
• Network portion of address is of arbitrary length
• Address format a.b.c.d/x
• x is number of bits in network portion of address

3
Classless addressing

• Example.
• Cambridge 194.24.0.0/21 194.24.0.0 --
  194.24.7.255
• Edinburgh 194.24.8.0/22 194.24.8.0 --
  194.24.11.255
• (Available) 194.24.12.0/22 194.24.12.0 --
  194.24.15.255
• Oxford 194.24.16.0/20 194.24.16.0
  -- 194.24.31.255

4
IP Addressing
first 24 bits are network address
5
IP Addressing
Interconnected system consisting of six networks
6
CIDR

• A router keeps routing table with entries
• IP address, 32-bit mask, outgoing line
• When an IP packet arrives, the router checks its
  routing table to find the longest match.

7
CIDR

• Example.
• Cambridge 194.24.0.0/21 194.24.0.0 --
  194.24.7.255
• Edinburgh 194.24.8.0/22 194.24.8.0 --
  194.24.11.255
• (Available) 194.24.12.0/22 194.24.12.0 --
  194.24.15.255
• Oxford 194.24.16.0/20 194.24.16.0
  -- 194.24.31.255
• When a packet addressing to 194.24.17.1 arrives,
  where should it be sent to?

8
CIDR Entry aggregation

• How does a router in Tallahassee route packet to
  C,E and O, assuming that he has only two outgoing
  links?
• All to New York.
• Can he reduce the size of his routing table?

C
E
N
O
H
T
9
CIDR Entry Aggregation

• From 194.24.0.0 to 194.24.31.255, all to N.
• So aggregate the three entries into one
  194.24.0.0/19.
• The N router can do the same thing.

C
E
N
O
H
T
10
CIDR

• If later the free address space 194.24.12.0/22
  194.24.12.0 -- 194.24.15.255 is assigned to
  Pittsburgh and has to go through Houston, what
  should the router at Tallahassee do?

C
E
N
P
O
H
T
11
CIDR

• When a packet arrives addressing 194.24.15.8, the
  router checks the routing table and there will be
  two matches 194.24.12.0/22 and 194.24.0.0/19.
  Pick the longest match.

12
NAT Network Address Translation

• IP address is a scarce resource.
• So, give a company only one or a few IP addresses
  used by the gateway router.
• Within the company, each machine has an unique IP
  address, chosen from
• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16
• These addresses can only appear within a company
  but never on the outside Internet

13
NAT

• Whenever a machine wants to send a packet to the
  outside, the packet will be sent to the NAT box.
• The NAT box will convert the internal IP address
  to the real IP address of the company, and pass
  the packet to the gateway router.
• When there is a packet destined for an internal
  machine arrived at the router, what should the
  router and NAT box do?
• For IP packets carrying TCP or UDP, use port
  number. Other protocols are much more
  compliated.

14
NAT

• For IP packets carrying TCP or UDP, use port
  number.
• When an outgoing packet arrives at the NAT box,
• The IP address is replaced
• The source port number is replaced
• Header checksum is recomputed
• When a reply came for this process, use the
  replaced source port number as index to find the
  correct IP address and original port number.

15
ICMP

• ICMP Internet Control Message Protocol
• Each ICMP message is encapsulated in an IP packet
• Treated like any other datagram, but no error
  message sent if ICMP message causes error
• Some interesting messages
• Time exceeded When an IP packet arrived at a
  router is dropped because the TTL field becomes
  0, the router will send an ICMP TIME EXCEEDED
  message back to the source. Used in traceroute.
• Echo and Echo reply ping.

16
Address Resolution

• IP address is virtual
• Not understood by underlying the hardware of
  physical networks
• IP packets need to be transmitted by the
  underlying physical network
• Address resolution
• Translating IP address to physical address
• Address Resolution Protocol (ARP)

17
ARP Example
18
ARP Cache

• Each computer maintains a cache table
• IP address ? hardware address mapping
• Only about computers on the same network
• Exchanges ARP messages
• To resolve IP addresses with unknown hardware
  addresses

19
ARP Protocol

• When a node sends an IP packet
• To another node on the same physical network
• Look up destination address in the ARP table
• If not found
• Broadcast a request to the local network
• Whose IP address is this?

20
ARP Response

• The target node responds to sender (unicast?)
• With its physical address
• Adds the requester into its ARP table (why?)
• On receiving the response
• Requester updates its table
• Other nodes upon receiving the request
• Refresh the requester entry if already there
• No action otherwise (why?)
• Table entries deleted if not refreshed for a while

21
DHCP

• DHCP Dynamic Host Configuration Protocol
• A new machine asks for an IP address
• Broadcast DHCP DISCOVER packet
• A DHCP relay agent got this packet and relay it
  to the DHCP server
• The DHCP server assigns an IP address
• Periodically renew

22
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

• special routers in AS
• run intra-AS routing protocol with all other
  routers in AS
• also responsible for routing to destinations
  outside AS
• run inter-AS routing protocol with other gateway
  routers

23
Intra-AS and Inter-AS routing

• Gateways
• perform inter-AS routing amongst themselves
• perform intra-AS routing with other routers in
  their AS

b
a
a
C
B
d
A
network layer
inter-AS, intra-AS routing in gateway A.c
link layer
24
Intra-AS and Inter-AS routing
Host h2
Intra-AS routing within AS B
Intra-AS routing within AS A
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

26
Intra-AS Routing

• Also known as Interior Gateway Protocols (IGP)
• Most common IGPs
• RIP Routing Information Protocol
• OSPF Open Shortest Path First
• IGRP Interior Gateway Routing Protocol (Cisco
  proprietary)

27
OSPF

• Represents the network as a graph, and runs the
  shortest path algorithm to find the path to any
  router.
• Divide the network into areas for scalability.
• The backbone area is called area 0
• Within one area, a router has the same link state
  database as all other routers. Routers belonging
  to two areas keeps two databases. Link state in
  one area is not told to other areas.
• Route local area ? backbone ? local area

28
OSPF

• Each router knows the shortest path to reach
  routers within his area.
• Backbone routers also accept information from
  area border routers to compute the shortest path
  to reach other routers. Then advertise this
  information to the border routers, who tells
  routers inside the area. To be able to select
  the best exit router in an area

29
OSPF

• To learn the link state, use flooding
• Inefficient to talk to every router on the same
  LAN
• So, select a designated router and let it to be
  adjacent to all other routers on the same LAN.
  Only exchange link state between the adjecent
  routers
• Messages include
• HELLO, LINK STATE UPDATE, LINK STATE ACK,
  DATABASE DESCRIPTION, LINK STATE REQUEST

30
Inter-AS routing
31
Internet Inter-AS routing BGP

• BGP (Border Gateway Protocol) the de facto
  standard
• Path Vector protocol
• similar to Distance Vector protocol
• each Border Gateway broadcast to neighbors
  (peers) entire path (I.e, sequence of ASs) to
  destination
• E.g., Gateway X may send its path to dest. Z
• Path (X,Z) X,Y1,Y2,Y3,,Z

32
Internet Inter-AS routing BGP

• BGP messages exchanged using TCP.
• 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

33
Internet Inter-AS routing BGP

• Suppose gateway X send its path to peer gateway
  W
• W may or may not select path offered by X
• cost, policy (dont route via competitors AS),
  loop prevention reasons.
• If W selects path advertised by X, then
• Path (W,Z) W, Path (X,Z)
• Note X can control incoming traffic by
  controlling its route advertisements to peers
• e.g., dont want to route traffic to Z ? dont
  advertise any routes to Z

34
BGP an example
3210 4210 7610
128.186.0.0/16
35
BGP

• Line FG goes down.
• Node F wants to find path to D.
• B BCD
• E EFGCD
• I IFGCD
• F can quickly decide to ignore the paths from I
  and E.

C
D
B
A
G
F
H
E
J
I
36
Virtual Circuit

• Destination information is large and the table is
  large
• Consider 32 bit IP address. A full table will
  have 4G entries.
• If an IP packet is 1250 byte long and the link
  speed is 10Gbps, how much time do you have for
  this lookup?
• (1. You dont have to implement the full table.
  2. You can also use pipeline.)

37
Virtual Circuit

• Circuit means a path between the source and the
  destination.
• Real circuit switching has a physical path set up
  between the source and the destination, like
  telephone network
• When you dial, a request is sent to the network,
  network finds if there are free links on the path
  and reserve that link for you.
• Virtual circuit is different used in packet
  switching networks.
• No real path set up, because it is packet
  switching (although link bandwidth can be
  reserved).
• But still has the connection phase. The purpose
  is to let the routers know how to route the
  packets of this virtual circuit.

38
Virtual Circuits
H2
B
D

• When setting up the virtual circuit, a VC
  identifier is picked. The router knows where to
  forward a packet with a certain VC identifier.
• Each packet will carry the VC identifier, which
  is much shorter than the full destination
  address, so allows more efficient table lookup.
• Resources can also be reserved. QoS.
• A practical problem in a distributed environment
  different stations may pick the same VC
  identifier.
• Labels can be swapped without causing confusion.

A
F
H3
C
H1
E
As Table In Out H1, 1 C, 1 H2, 1 C, 2
Cs Table In Out A, 1 E, 1 A, 2 D,
1