1. DHCP and NAT

2. IP address: how to get one?
Q: How does a router get IP address?
hard coded by system admin
every interface/port is assigned a unique IP address reflecting network assignment
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: Data Plane

3. 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
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: Data Plane

4. DHCP client-server scenario
Server sees request and responds with an IP address
DHCP
server
/24
arriving DHCP
client needs
address in this
network
/24
/24
Network Layer: Data Plane

5. DHCP: more than IP addresses
DHCP returns 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)
DCHP may not be on same subnet, in that case, the local router will forward the request to the DHCP server on behalf of the client, i.e., acting as agent. Uses IP Address Helper - see:
There maybe more that one DHCP server on the network, all will respond, client picks one, usually first response that comes in
Network Layer: Data Plane

6. DHCP client-server scenario
DHCP server:
DHCP discover
src : , 68
dest.: ,67
yiaddr:
transaction ID: 654
arriving
client
Broadcast: is there a DHCP server out there?
Client sends broadcast message over UDP to port 67 (reserved for DHCP) on the server, from port 68, asking for IP address
DHCP offer
src: , 67
dest: , 68
yiaddrr:
transaction ID: 654
lifetime: 3600 secs
DHCP server responds offering an IP address that the client can use from pool of addresses it has available.
Broadcast: I’m a DHCP server! Here’s an IP address you can use
DHCP request
src: , 68
dest:: , 67
yiaddrr:
transaction ID: 655
lifetime: 3600 secs
Note: there could be several DHCP servers that might respond to client.
Client chooses one to bind with and responds to that server accepting its IP address
Broadcast: OK. I’ll take that IP address!
Server sends final ACK binding that IP address and blocking for x secs
DHCP ACK
src: , 67
dest: , 68
yiaddrr:
transaction ID: 655
lifetime: 3600 secs
Broadcast: OK. You’ve got that IP address!
Network Layer: Data Plane

7. DHCP client-server summary
DHCP server:
arriving
client
DHCP discover
DHCP offer
DHCP request
src: , 68
dest:: , 67
DHCPRequest
yiaddrr:
transaction ID: 655
DHCP serverID:
lifetime: 3600 secs
DHCP ACK
src: , 67
dest: , 68
DHCPAck
src: , 68
dest: , 67
DHCPDiscover
yiaddr:
transaction ID: 654
src: , 67
dest: , 68
DHCPOffer
yiaddr:
transaction ID: 654
DHCP serverID:
lifetime: 3600 secs
Network Layer: Data Plane

8. DHCP: more than IP addresses
DHCP can return more than just allocated IP address on subnet:
its address of its (DHCP server)
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: Data Plane

9. DHCP: example
DHCP
UDP IP
Eth
Phy
DHCP
DHCP
connecting laptop needs its IP address, addr of first-hop router, addr of DNS server: use DHCP
DHCP
DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in Ethernet
DHCP
DHCP
UDP IP
Eth
Phy
DHCP
Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server
router with DHCP
server built into
router
Ethernet demuxed to IP demuxed, UDP demuxed to DHCP
Network Layer: Data Plane

10. DHCP: example
DHCP
DCP server formulates DHCP OFFER containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server
DHCP
UDP IP
Eth
Phy
encapsulation of DHCP OFFER, frame forwarded to client, demuxing up to DHCP at client
DHCP
UDP IP
Eth
Phy
DHCP
DHCP
router with DHCP
server built into
router
client now knows its IP address, name and IP address of DSN server, IP address of its first-hop router
DHCP
A great “plug and play” service
Q: Do you see a drawback?
Network Layer: Data Plane

11. NAT: network address translation
rest of
Internet
local network
(e.g., home network)
/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)
Network Layer: Data Plane

12. 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: Data Plane

13. 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: Data Plane

14. 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
* Check out the online interactive exercises for more examples:
Network Layer: Data Plane

15. 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
address shortage should be solved by IPv6
violates end-to-end argument
NAT possibility must be taken into account by app designers, e.g., P2P applications
NAT traversal: what if client wants to connect to server behind NAT?
Network Layer: Data Plane