1. Hierarchical Routing Our routing study thus far – an idealization
all routers are identical
the network is “flat”
… not true in practice
Why?
administrative autonomy
internet = network of networks
each network admin may want to control routing in its own network
scale: with 55 million+ destination hosts:
can’t store all destinations in routing tables!
routing table exchange would swamp links!
4: Network Layer

2. Hierarchical Routing gateway routers
aggregate routers into regions, called “autonomous systems” (AS)
routers in same AS run same routing protocol
“intra-AS” routing (i.e., within an AS) 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 (i.e., between AS) protocol with other gateway routers
4: Network Layer

3. Intra-AS and Inter-AS routingb C A B d
A.a
A.c
C.b
B.a c
Gateways:
perform inter-AS routing amongst themselves
perform intra-AS routers with other routers in their AS c
network layer
inter-AS, intra-AS routing in
gateway A.c
data link layer
physical layer
4: Network Layer

4. Intra-AS and Inter-AS routing
between
A and B a b C A B d c
A.a
A.c
C.b
B.a
Host h2
Host h1
Intra-AS routing
within AS B
Intra-AS routing
within AS A
We’ll examine specific inter-AS and intra-AS Internet routing protocols shortly (section 4.5)
4: Network Layer

5. The Internet Network layer
Host, router network layer functions… three major components:
Transport layer: TCP, UDP
IP protocol
addressing conventions
datagram format
packet handling conventions
Routing protocols
path selection
RIP, OSPF, BGP
Network
layer
routing
table
ICMP protocol
error reporting
router “signaling”
Link layer
Physical layer
4: Network Layer

6. IP Addressing: introduction
IP address: 32-bit identifier for host or router interface
interface: connection between host or router and the physical link
routers typically have multiple interfaces
hosts typically have only one
IP addresses are associated with the interface, not the host or the router
dotted-decimal notation: =
223 1 1 1
4: Network Layer

7. IP Addressing IP address:
network part (high order bits)
host part (low order bits)
What’s a network ? (from the IP address perspective)
device interfaces with the same network part of their IP address
hosts can physically reach each other without an intervening router
LAN
Example: network consisting of 3 IP networks (for IP addresses starting with 223, the first 24 bits are the network address – more later)
4: Network Layer

8. IP Addressing How to find the networks?
Detach each interface from routers
create “islands of isolated networks
Interconnected
system consisting
of six networks
4: Network Layer

9. IP Addresses
Given the notion of a “network”, let’s look closer at IP addresses:
“classful” addressing -
class A to
network
host (24 bits)
27 = 127 networks
224 = 16.8 million+ hosts
214 = 16,384 networks
216 = 65,536 hosts
221 = 2 million+ networks
28 = 256 hosts
24 = 16 networks
228 = million+ hosts B to 10
network
host (16 bits) to C
110
network
host (8 bits) to D
1110
multicast address (28 bits)
32 bits
What is the address space size (number of hosts) for each class?
4: Network Layer

10. IP addressing: CIDR classful addressing:
inefficient use of address space, address space exhaustion
e.g., class B network is allocated enough addresses for 65K hosts, even if only 2K hosts exist in that network
CIDR: Classless InterDomain Routing
network portion of address of arbitrary length
address format: a.b.c.d/x, where x is # bits in the network portion of an address
network
part
host
/23
4: Network Layer

11. IP addresses: how to get one?
Hosts (host portion):
hard-coded by system admin in a file
DHCP: Dynamic Host Configuration Protocol: dynamically get address (RFC 2131): “plug-and-play”
host broadcasts “DHCP discover” msg
DHCP server responds with “DHCP offer” msg
host requests IP address: “DHCP request” msg
DHCP server sends address: “DHCP ack” msg
4: Network Layer

12. IP addresses: how to get one?
Network (network portion):
get allocated portion of ISP’s address space:
ISP's block /20
Organization /23
Organization /23
Organization /23
… … ….
Organization /23
4: Network Layer

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

14. Hierarchical addressing: more specific routes
ISPs-R-Us has a more specific route to Organization 1
(longest prefix matching):
Organization 0
/23
“Send me anything
with addresses
beginning
/20”
Organization 2
/23 .
Fly-By-Night-ISP .
Internet
Routing
Hierarchy
Organization 7
/23
“Send me anything
with addresses
beginning /16
or /23”
ISPs-R-Us
Organization 1
/23
4: Network Layer

15. IP addressing: the last word...
Q: How does an ISP get a block of addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers (RFC 2050)
non-profit organization
allocates addresses via regional registries (mid-2000)
ARIN - North and South America, part of Africa
RIPE – Europe and surrounding countries
APNIC – Asia Pacific region
manages DNS
assigns domain names, resolves disputes
4: Network Layer

16. Getting a datagram from source to dest.
routing table* in A
dest. net. next router #hops
IP datagram:
misc.
fields
source
IP addr
dest
data A B E
addresses remain unchanged, as the datagram travels from source to destination
address fields of interest here (provided by the source host A)
* Note – more on this later
4: Network Layer

17. Getting a datagram from source to dest.
misc
fields
data
dest. net. next router #hops
Starting at A, given an IP datagram addressed to B:
look up network address of B
find B is on same network as A
link layer will send datagram directly to B inside link-layer frame
B and A are directly connected A B E
4: Network Layer

18. Getting a datagram from source to dest.
misc
fields
dest. net. next router #hops
data
Starting at A, destination E:
look up network address of E
E on different network
A, E not directly attached
routing table: next hop router to E is
link layer sends datagram to router inside link-layer frame
datagram arrives at
continued….. A B E
4: Network Layer

19. Getting a datagram from source to dest.
network router #hops interface
dest next
misc
fields
data
Arriving at , destined for
look up network address of E
E on same network as router’s interface
router, E directly attached
link layer sends datagram to inside link-layer frame via interface
datagram arrives at !!! A B E
4: Network Layer

20. Next Lesson: IP Structure and Routing in the Internet
4: Network Layer

21. 32 bit destination IP address
IP datagram format
IP protocol version
number
32 bits
total datagram
length (bytes)
header length
(bytes)
head.
len.
type of
service
ver.
datagram length
for
fragmentation/
reassembly
fragment
offset
“type” of data
16-bit identifier
flgs
max number of
remaining hops
(decremented at
each router)
time to
live
upper
layer
header
checksum
32 bit source IP address
32 bit destination IP address
upper layer protocol
to deliver payload to
(e.g. TCP, UDP, …
see RFC 1700)
Options (if any)
e.g. timestamp,
record route
taken, specify
list of routers
to visit (field is
rarely used in
practice).
data
(variable length,
typically a TCP
or UDP segment)
4: Network Layer

22. IP Fragmentation & Reassembly
network links have MTU (Max. Transfer Unit) 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 and order related fragments
fragmentation:
in: one large datagram
out: 3 smaller datagrams
reassembly
4: Network Layer

23. IP Fragmentation and ReassemblyID =x
offset =0
More bit
length
=3980
One large datagram becomes
several smaller datagrams ID =x
offset =0
More bit =1
length
=1480 ID =x
offset
=1480
More bit =1
length ID =x
offset
=2960
More bit =0
length
=1020
Note: Offset is actually
specified as number of 8-byte (64-bit) units.
4: Network Layer

24. Transport and application layer in the network core?
data link
physical
application
transport
Application and
transport layer
uses in the core
routing table
updates/broadcasts
router management
router error reporting
application
transport
network
data link
physical
4: Network Layer

25. ICMP: Internet Control Message Protocol
Type Code description
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*
TTL expired
bad IP header …
used by hosts, routers, gateways to communicate network-level information
error reporting: unreachable host, network, port, protocol
echo request/reply (used by ping)
network-layer “above” IP:
ICMP messages are carried in IP datagrams
ICMP message: type, code, and checksum, plus header and first 8 bytes of IP datagram causing error/ response
(See RFC 792, 1296*)
4: Network Layer

26. Routing in the Internet
The Global Internet consists of Autonomous Systems (AS) interconnected with each other:
Stub AS: small corporation
Multi-homed AS: large corporation (no transit)
Transit AS: provider
Two-level routing:
Intra-AS: administrator is responsible for choice
Inter-AS: unique standard
4: Network Layer

27. Internet AS Hierarchy Inter-AS border (exterior gateway) routers
Intra-AS (interior gateway) routers
4: Network Layer

28. Intra-AS Routing Also known as Interior Gateway Protocols (IGP)
Most common IGPs:
RIP: Routing Information Protocol (legacy)
OSPF: Open Shortest Path First (common)
EIGRP: Enhanced Interior Gateway Routing Protocol (proprietary – Cisco Systems)
4: Network Layer

29. RIP ( Routing Information Protocol)
Distance vector algorithm
Included in BSD-UNIX Distribution in 1982
RFC 1058 (version 1), RFC 1723 (version 2)
Distance metric: # of hops (max = 15 hops)
Can you guess why?
Distance vectors: exchanged every 30 seconds via Response Message (also called advertisement)
Each advertisement: routing info for maximum of 25 destination nets within the AS
4: Network Layer

30. RIP (Routing Information Protocol)z w x y A D B C
Destination Network Next Router Num. of hops to dest.
w A 2
y B 2
z B 7 x
… … …
Routing table in D
4: Network Layer

31. RIP: Link Failure and Recovery
If no advertisement heard after 180 sec --> neighbor/link declared dead
routes via that neighbor are 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)
4: Network Layer

32. RIP Table processing example (BSD UNIX)
RIP routing tables managed by application-level process called route-d (BSD UNIX daemon)
advertisements sent in UDP packets, periodically repeated
4: Network Layer

33. RIP Table example (continued)
Router: giroflee.eurocom.fr
Destination Gateway Flags Ref Use Interface
UH lo0
U fa0
U le0
U qaa0
U le0
default UG
Three attached class C networks (LANs)
Router only knows routes to attached LANs
Default router used to “go up” to next logical level
Route multicast address:
Loopback interface (for debugging)
4: Network Layer

34. Problems/limitations with RIP
Good for small systems, but doesn’t scale well
Count-to-infinity problem… poisoned reverse only
Comparatively slow convergence
1979 – RIP version 2, link state algorithm
1988 – IETF initiates work on replacement
1990 – OSPF became new standard
4: Network Layer

35. 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
However….
OSPF advertisement carries only one entry per neighbor router
Advertisements disseminated to entire AS (via flooding)
4: Network Layer

36. OSPF “advanced” features (not in RIP)
Security: all OSPF messages are authenticated (to prevent malicious intrusion); TCP connections used
Multiple same-cost paths allowed (only one path in RIP)
For each link, multiple cost metrics for different Types Of Service (e.g., satellite link cost set “low” for best effort; high for real time)
Integrated uni- and multicast support:
Multicast OSPF (MOSPF) uses same topology data base as OSPF
Hierarchical OSPF in large domains.
4: Network Layer

37. Hierarchical OSPF
4: Network Layer

38. Hierarchical OSPF Two-level hierarchy: local area and backbone.
link-state advertisements only in local area
each node 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 ASs. (Note: synonymous with the “gateway routers” we discussed in section 4.3)
4: Network Layer

39. EIGRP (Enhanced Interior Gateway Routing Protocol)
CISCO proprietary; successor of RIP (mid 80’s)
uses Distance Vector, like RIP
several cost metrics (delay, bandwidth, reliability, load etc)
uses TCP (!) to exchange routing updates
Loop-free routing via a distributed update routing algorithm (called DUAL) based on diffused computation
4: Network Layer

40. Inter-AS routing
4: Network Layer

41. Internet inter-AS routing: BGP
BGP (Border Gateway Protocol): the de facto standard
Path Vector protocol:
similar to Distance Vector protocol
each Border Gateway broadcasts to neighbors (peers) the entire path (I.e, sequence of ASs) to destination
E.g., Gateway X may send its path to destination Z:
Path (X,Z) = X,Y1,Y2,Y3,…,Z
4: Network Layer

42. Internet inter-AS routing: BGP
Suppose: gateway X send its path to peer gateway W
W may or may not select a path offered by X
cost, policy (don’t route via competitors AS), loop prevention reasons.
If W selects a 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., don’t want to route traffic to Z -> don’t advertise any routes to Z
4: Network Layer

43. 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 message; also used to close connection
4: Network Layer

44. Why different Intra- and Inter-AS routing ?
Policy:
Inter-AS: admin wants control over how its traffic is routed, who routes through its net.
Intra-AS: single admin, so no policy decisions needed
Scale:
hierarchical routing saves table size, reduces update traffic
Performance:
Intra-AS: can focus on performance
Inter-AS: policy may dominate over performance
4: Network Layer