1. Overview The Internet (IP) Protocol Datagram format IP fragmentation
ICMP: Internet Control Message Protocol
NAT: Network Address Translation
Routing in the Internet
Intra-AS routing: RIP and OSPF
Inter-AS routing: BGP
Multicast Routing
Some slides are in courtesy of J. Kurose and K. Ross

2. IP addressing: CIDR Classful addressing:
inefficient use of address space, address space exhaustion
e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts 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 network portion of address
network
part
host
/23

3. 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
Organization 7
/23
“Send me anything
with addresses
beginning
/16”
ISPs-R-Us

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

5. 32 bit destination IP address
IP datagram format
ver
length
32 bits
data
(variable length,
typically a TCP
or UDP segment)
16-bit identifier
Internet
checksum
time to
live
32 bit source IP address
IP protocol version
number
header length
(bytes)
max number
remaining hops
(decremented at
each router)
for
fragmentation/
reassembly
total datagram
length (bytes)
upper layer protocol
to deliver payload to
head.
len
type of
service
“type” of data
flgs
fragment
offset
upper
layer
32 bit destination IP address
Options (if any)
E.g. timestamp,
record route
taken, specify
list of routers
to visit.
how much overhead with TCP?
20 bytes of TCP
20 bytes of IP
= 40 bytes + app layer overhead

6. 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

7. IP Fragmentation and ReassemblyID =x
offset =0
fragflag
length
=4000 =1
=1500
=185
=370
=1040
One large datagram becomes
several smaller datagrams
Example
4000 byte datagram
MTU = 1500 bytes
1480 bytes in data field
offset =
1480/8

8. ICMP: Internet Control Message Protocol
used by hosts, routers, gateways to communication 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
Ping, traceroute uses ICMP

9. Getting a datagram from source to dest.
forwarding table in A
Dest. Net. next router Nhops
IP datagram:
misc
fields
source
IP addr
dest
data A B E
datagram remains unchanged, as it travels source to destination
addr fields of interest here

10. Getting a datagram from source to dest.
forwarding table in A
misc
fields
Dest. Net. next router Nhops
data
Starting at A, send IP datagram addressed to B:
look up net. address of B in forwarding table
find B is on same net. as A
link layer will send datagram directly to B inside link-layer frame
B and A are directly connected A B E

11. Getting a datagram from source to dest.
forwarding table in A
misc
fields
Dest. Net. next router Nhops
data
Starting at A, dest. E:
look up network address of E in forwarding table
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

12. Getting a datagram from source to dest.
forwarding table in router
Dest. Net router Nhops interface
misc
fields
data
Arriving at , destined for
look up network address of E in router’s forwarding table
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 !!! (hooray!) A B E

13. NAT: Network Address Translation
rest of
Internet
local network
(e.g., home network)
10.0.0/24
All datagrams leaving local
network have same single source NAT IP address: ,
different source port numbers
Datagrams with source or
destination in this network
have /24 address for
source, destination (as usual)

14. NAT: Network Address Translation
Motivation: local network uses just one IP address as far as outside word is concerned:
no need to be allocated range of addresses from ISP: - just one IP address is used 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).

15. 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

16. NAT: Network Address Translation
NAT translation table
WAN side addr LAN side addr
1: host
sends datagram to
, 80
2: NAT router
changes datagram
source addr from
, 3345 to
, 5001,
updates table
, , 3345
…… ……
S: , 3345
D: , 80 1
S: , 80
D: , 3345 4
S: , 5001
D: , 80 2
S: , 80
D: , 5001 3
4: NAT router
changes datagram
dest addr from
, 5001 to , 3345
3: Reply arrives
dest. address:
, 5001

17. 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 shortage should instead be solved by IPv6

18. Overview The Internet (IP) Protocol Datagram format IP fragmentation
ICMP: Internet Control Message Protocol
NAT: Network Address Translation
Routing in the Internet
Intra-AS routing: RIP and OSPF
Inter-AS routing: BGP
Multicast Routing
Some slides are in courtesy of J. Kurose and K. Ross

19. Architecture of Dynamic Routing
IGP
EGP (= BGP)
AS 1
IGP
IGP = Interior Gateway Protocol
Metric based: OSPF, IS-IS, RIP,
EIGRP (cisco)
AS 2
EGP = Exterior Gateway Protocol
Policy based: BGP
The Routing Domain of BGP is the entire Internet

20. The Gang of Four OSPF RIP IS-IS BGP Link State Vectoring EGP IGP
Used in upper-tier ISPs
Lower-tier ISPs and enterprise networks

21. 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 router (per link)
Advertisements disseminated to entire AS (via flooding)
Periodically broadcast even when there is no link cost change
Carried in OSPF messages directly over IP (rather than TCP or UDP

22. 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)
Integrated uni- and multicast support:
Multicast OSPF (MOSPF) uses same topology data base as OSPF
Hierarchical OSPF in large domains.

23. Hierarchical OSPF

24. 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.