1. 2017 session 1 TELE3118: Network Technologies Week 4: Network Layer Data Plane: Basics, Addressing
Some slides have been adapted from:
Computer Networking: A Top Down Approach, 7th (global) edition. Jim Kurose, Keith Ross. Pearson, April All material copyright J.F Kurose and K.W. Ross, All Rights Reserved.
Computer Networks, 5th edition. Andrew S. Tanenbaum, David J. Wetherall, Pearson, 2010.
Network Layer

2. Why a network layer? Why not one big flat LAN? IP provides:
Different LAN protocols
Flat address space not scalable
IP provides:
Global addressing
Forwarding of packets from source to destination across multiple LANs
Route selection across WAN
Why a single IP?
Maximise interoperability
Minimise service interfaces
Why a narrow IP?
Least common network functionality
Best-effort, no assurances
“hourglass model”
(Steve Deering)
Network Layer

3. Network layer transport segment from sending to receiving host
application
transport
network
data link
physical
transport segment from sending to receiving host
on sending side encapsulates segments into datagrams
on receiving side, delivers segments to transport layer
network layer protocols in every host, router
router examines header fields in all IP datagrams passing through it
network
data link
physical
application
transport
network
data link
physical
Network Layer: Data Plane

4. Network layer: data plane, control plane
forwarding: move packets from router’s input to appropriate router output
local, per-router function
Control plane
routing: determine route taken by packets from source to destination
network-wide logic
Two approaches:
traditional routing algorithms: distributed across routers
software-defined networking (SDN): centralized in (remote) servers 1 2 3
0111
values in arriving
packet header
Network Layer: Data Plane

5. Addressing IP address: IP versus Ethernet 4-bytes = 32-bits
Dot-notation
Globally unique
Hierarchical:
network + host
IP versus Ethernet
Hierarchical vs. flat
Portability: “location” vs. “identification” =
223 1 1 1
Network Layer

6. Subnets IP address: What’s a subnet ? 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
LAN
network consisting of 3 subnets
Network Layer

7. Subnets
How many?
Network Layer

8. IP addressing: “class-full”to
network
host B to 10
network
host to C
110
network
host to D
1110
multicast address
32 bits
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
Network Layer

9. IP addressing: “class-less”
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
/23
Network Layer

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
UNIX: /etc/rc.config
DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server
“plug-and-play”
Network Layer: Data Plane

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/“on”)
support for mobile users who want to join network (more shortly)
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

12. DHCP client-server scenario
/24
arriving DHCP
client needs
address in this
network
/24
/24
Network Layer: Data Plane

13. 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?
DHCP offer
src: , 67
dest: , 68
yiaddrr:
transaction ID: 654
lifetime: 3600 secs
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
Broadcast: OK. I’ll take that IP address!
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

14. DHCP: more than IP addresses
DHCP can return more than just allocated IP address on subnet:
network mask (indicating network versus host portion of address)
address of first-hop router for client
name and IP address of DNS sever
Network Layer: Data Plane

15. DHCP: example
connecting laptop needs its IP address, addr of first-hop router, addr of DNS server: use DHCP
DHCP
UDP IP
Eth
Phy
DHCP
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

16. DHCP: example
DCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server
DHCP
DHCP
UDP IP
Eth
Phy
encapsulation of DHCP server, 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
Network Layer: Data Plane

17. DHCP: Wireshark output (home LAN)
Message type: Boot Reply (2)
Hardware type: Ethernet
Hardware address length: 6
Hops: 0
Transaction ID: 0x6b3a11b7
Seconds elapsed: 0
Bootp flags: 0x0000 (Unicast)
Client IP address: ( )
Your (client) IP address: ( )
Next server IP address: ( )
Relay agent IP address: ( )
Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a)
Server host name not given
Boot file name not given
Magic cookie: (OK)
Option: (t=53,l=1) DHCP Message Type = DHCP ACK
Option: (t=54,l=4) Server Identifier =
Option: (t=1,l=4) Subnet Mask =
Option: (t=3,l=4) Router =
Option: (6) Domain Name Server
Length: 12; Value: E F ;
IP Address: ;
IP Address: ;
IP Address:
Option: (t=15,l=20) Domain Name = "hsd1.ma.comcast.net."
reply
Message type: Boot Request (1)
Hardware type: Ethernet
Hardware address length: 6
Hops: 0
Transaction ID: 0x6b3a11b7
Seconds elapsed: 0
Bootp flags: 0x0000 (Unicast)
Client IP address: ( )
Your (client) IP address: ( )
Next server IP address: ( )
Relay agent IP address: ( )
Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a)
Server host name not given
Boot file name not given
Magic cookie: (OK)
Option: (t=53,l=1) DHCP Message Type = DHCP Request
Option: (61) Client identifier
Length: 7; Value: D323688A;
Option: (t=50,l=4) Requested IP Address =
Option: (t=12,l=5) Host Name = "nomad"
Option: (55) Parameter Request List
Length: 11; Value: 010F03062C2E2F1F21F92B
1 = Subnet Mask; 15 = Domain Name
3 = Router; 6 = Domain Name Server
44 = NetBIOS over TCP/IP Name Server ……
request
Network Layer: Data Plane

18. IP addresses: how to get one?
Q: How does network get subnet part of IP addr?
A: gets allocated portion of its provider ISP’s address space
ISP's block /20
Organization /23
Organization /23
Organization /23
… … ….
Organization /23
Network Layer

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

20. 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
Network Layer: Data Plane

21. IP addressing: the last word...
Q: how does an ISP get block of addresses?
A: ICANN: Internet Corporation for Assigned Names and Numbers
allocates addresses
manages DNS
assigns domain names, resolves disputes
Network Layer: Data Plane

22. 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)
Network Layer: Data Plane

23. 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

24. 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

25. 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

26. 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

27. Router architecture overview
high-level view of generic router architecture:
routing, management
control plane (software)
operates in millisecond
time frame
routing
processor
forwarding data plane (hardware) operttes in nanosecond timeframe
high-seed
switching
fabric
router input ports
router output ports
Network Layer: Data Plane

28. Input port functions decentralized switching:
lookup,
forwarding
queueing
line
termination
link
layer
protocol
(receive)
switch
fabric
physical layer:
bit-level reception
data link layer:
e.g., Ethernet
see chapter 5
decentralized switching:
using header field values, lookup output port using forwarding table in input port memory (“match plus action”)
goal: complete input port processing at ‘line speed’
queuing: if datagrams arrive faster than forwarding rate into switch fabric
Network Layer: Data Plane

29. Input port functions decentralized switching:
lookup,
forwarding
queueing
line
termination
link
layer
protocol
(receive)
switch
fabric
physical layer:
bit-level reception
decentralized switching:
using header field values, lookup output port using forwarding table in input port memory (“match plus action”)
destination-based forwarding: forward based only on destination IP address (traditional)
generalized forwarding: forward based on any set of header field values
data link layer:
e.g., Ethernet
see chapter 5
Network Layer: Data Plane

30. Destination-based forwarding
forwarding table
Destination Address Range
through
otherwise
Link Interface 1 2 3
Q: but what happens if ranges don’t divide up so nicely?
Network Layer: Data Plane

31. Longest prefix matching
when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address.
Destination Address Range
*** *********
*********
*** *********
otherwise
Link interface 1 2 3
examples:
DA:
which interface?
DA:
which interface?
Network Layer: Data Plane

32. Longest prefix matching
longest prefix matching: often performed using ternary content addressable memories (TCAMs)
content addressable: present address to TCAM: retrieve address in one clock cycle, regardless of table size
Cisco Catalyst: can up ~1M routing table entries in TCAM
Network Layer: Data Plane

33. Switching fabrics
transfer packet from input buffer to appropriate output buffer
switching rate: rate at which packets can be transfer from inputs to outputs
often measured as multiple of input/output line rate
N inputs: switching rate N times line rate desirable
three types of switching fabrics
memory
memory
bus
crossbar
Network Layer: Data Plane

34. Switching via memory first generation routers:
traditional computers with switching under direct control of CPU
packet copied to system’s memory
speed limited by memory bandwidth (2 bus crossings per datagram)
input
port
(e.g.,
Ethernet)
memory
output
system bus
Network Layer: Data Plane

35. Switching via a bus datagram from input port memory
to output port memory via a shared bus
bus contention: switching speed limited by bus bandwidth
32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers
bus
Network Layer: Data Plane

36. Switching via interconnection network
overcome bus bandwidth limitations
banyan networks, crossbar, other interconnection nets initially developed to connect processors in multiprocessor
advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric.
Cisco 12000: switches 60 Gbps through the interconnection network
crossbar
Network Layer: Data Plane

37. Input port queuing
fabric slower than input ports combined -> queueing may occur at input queues
queueing delay and loss due to input buffer overflow!
Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward
one packet time later: green packet experiences HOL blocking
switch
fabric
switch
fabric
output port contention:
only one red datagram can be transferred. lower red packet is blocked
Network Layer: Data Plane

38. Output port queueing
datagram
buffer
queueing
switch
fabric
link
layer
protocol
(send)
line
termination
buffering required when datagrams arrive from fabric faster than the transmission rate
scheduling discipline chooses among queued datagrams for transmission
Datagram (packets) can be lost due to congestion, lack of buffers
Priority scheduling – who gets best performance, network neutrality
Network Layer: Data Plane

39. Scheduling mechanisms
scheduling: choose next packet to send on link
FIFO (first in first out) scheduling: send in order of arrival to queue
real-world example?
discard policy: if packet arrives to full queue: who to discard?
tail drop: drop arriving packet
priority: drop/remove on priority basis
random: drop/remove randomly
packet
arrivals
packet
departures
queue
(waiting area)
link
(server)
Network Layer: Data Plane

40. Scheduling policies: priority
high priority queue
(waiting area)
low priority queue
arrivals
classify
departures
link
(server)
priority scheduling: send highest priority queued packet
multiple classes, with different priorities
class may depend on marking or other header info, e.g. IP source/dest, port numbers, etc.
real world example? 2 1 3 4 5
arrivals 1 3 2 4 5
packet in service
departures 1 3 2 4 5
Network Layer: Data Plane

41. Scheduling policies: still more
Round Robin (RR) scheduling:
multiple classes
cyclically scan class queues, sending one complete packet from each class (if available)
real world example? 1 2 3 4 5
arrivals
departures
packet in service
Network Layer: Data Plane

42. Scheduling policies: still more
Weighted Fair Queuing (WFQ):
generalized Round Robin
each class gets weighted amount of service in each cycle
real-world example?
Network Layer: Data Plane

43. Chapter 4: done!
Question: how do forwarding tables (destination-based forwarding) or flow tables (generalized forwarding) computed? Answer: by the control plane
Network Layer: Data Plane

44. Network Layer