1. CS/ECE 438: Communication Networks
Fall 2017
4. Network Layer

2. Chapter 4: Network Layer
application
transport
network
link
physical

3. Chapter 4: Network Layer
Kurose & Ross 6th Edition:
Chapter 4: Network Layer
Kurose & Ross 7th Edition:
Chapter 4: Network Data Plane
Chapter 5: Network Control Plane
This Course: Hybrid
-Combine in one chapter
-But follow 7th Ed. (Mostly!)
application
transport
network
link
physical

4. Chapter 4: Network Layer
our goals:
understand principles behind network layer services:
Network layer service models
Forwarding versus routing
How a router works
Routing (Path Selection)
Broadcast, Multicast
Dealing with Scale
Advanced Topics: IPv6, Mobility, SDN Controllers
Instantiation, Implementation in the Internet
Network Management

5. Chapter 4: Outline Data Plane Control Plane Overview of Network Layer
What’s Inside a Router?
IP: Internet Protocol
Routing Protocols
Intra-AS Routing in the internet: OSPF
Routing among ISPs: BGP
The SDN Control Plane
ICMP: The Internet Control Message Protocol
Network Management and SNMP

6. 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

7. Two key network-layer functions
forwarding: move packets from router’s input to appropriate router output
routing: determine route taken by packets from source to destination
routing algorithms
analogy: taking a trip
forwarding: process of getting through single interchange
routing: process of planning trip from source to destination

8. Network layer: data plane, control plane
local, per-router function
determines how datagram arriving on router input port is forwarded to router output port
forwarding function
Control plane
network-wide logic
determines how datagram is routed among routers along end-end path from source host to destination host
two control-plane approaches:
traditional routing algorithms: implemented in routers
software-defined networking (SDN): implemented in (remote) servers 1 2 3
0111
values in arriving
packet header

9. Per-router control plane
Individual routing algorithm components in each and every router interact in the control plane
Routing
Algorithm
data
plane
control
values in arriving
packet header
0111 1 2 3

10. Logically centralized control plane
A distinct (typically remote) controller interacts with local control agents (CAs)
Remote Controller CA
data
plane
control
values in arriving
packet header 1 2
0111 3

11. Network service model
Q: What service model for “channel” transporting datagrams from sender to receiver?
example services for individual datagrams:
guaranteed delivery
guaranteed delivery with less than 40 msec delay
example services for a flow of datagrams:
in-order datagram delivery
guaranteed minimum bandwidth to flow
restrictions on changes in inter-packet spacing

12. Network layer service models:
Guarantees ?
Network
Architecture
Internet
Service
Model
best effort
Congestion
feedback
no (inferred
Via loss
Bandwidth
none
Loss no
Order no
Timing no

13. Chapter 4: Outline Overview of Network Layer What’s Inside a Router?
IP: Internet Protocol
Routing Protocols
Intra-AS Routing in the internet: OSPF
Routing among ISPs: BGP
The SDN Control Plane
ICMP: The Internet Control Message Protocol
Network Management and SNMP

14. 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

15. 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”)
goal: complete input port processing at ‘line speed’
queuing: if datagrams arrive faster than forwarding rate into switch fabric
data link layer:
e.g., Ethernet
see chapter 5

16. 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

17. 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?

18. 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?

19. Longest prefix matching
we’ll see why longest prefix matching is used shortly, when we study addressing
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

20. Switching fabrics
transfer packet from input buffer to appropriate output buffer
switching rate: rate at which packets can be transferred 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

21. 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

22. 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

23. 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

24. Input Port Queueing Packets may queue at input ports. Why?
Answer 1: Slow switching fabric
switch
fabric

25. Input port queuing Answer 2: Output port contention
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 X
switch
fabric
output port contention:
only one red datagram can be transferred. lower red packet is blocked

26. Reducing Input Queueing
Why?
Reduce HOL blocking
Avoid packet drops at input queues
Save on queue memory
How?
Increase switch fabric speed
Increase inbound capacity of output ports

27. Output ports
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

28. Output port queueing
at t, packets move
from input to output
one packet time later
switch
fabric
buffering when arrival rate via switch exceeds output line speed
queueing (delay) and loss due to output port buffer overflow!

29. How much buffering? 𝑅𝑇𝑇×𝐶 𝑁
RFC 3439 rule of thumb: average buffering equal to “typical” RTT (say 250 msec) times link capacity C
e.g., C = 10 Gpbs link: 2.5 Gbit buffer
recent recommendation [Appenzellet’04]: with N flows, buffering equal to:
𝑅𝑇𝑇×𝐶 𝑁

30. Scheduling mechanisms
scheduling: choose next packet to send on link
FIFO (first in first out) scheduling: send in order of arrival to queue
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)

31. 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. 2 1 3 4 5
arrivals 1 3 2 4 5
packet in service
departures 1 3 2 4 5

32. 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? 2 1 3 4 5
arrivals 1 3 2 4 5
packet in service
departures 1 2 4 5 3

33. Scheduling policies: still more
Weighted Fair Queuing (WFQ):
generalized Round Robin
each class gets weighted amount of service in each cycle

34. Chapter 4: Outline Overview of Network Layer What’s Inside a Router?
IP : Internet Protocol
Routing Protocols
Intra-AS Routing in the internet: OSPF
Routing among ISPs: BGP
The SDN Control Plane
ICMP: The Internet Control Message Protocol
Network Management and SNMP
Data Format and Fragmentation
IPv4 Addressing
Network Address Translation
IPv6

35. The Internet network layer
host, router network layer functions:
transport layer: TCP, UDP
IP protocol
addressing conventions
datagram format
packet handling conventions
routing protocols
path selection
RIP, OSPF, BGP
network
layer
forwarding
table
ICMP protocol
error reporting
router “signaling”
link layer
physical layer

36. 32 bit destination IP address
IP datagram format
IP protocol version
number
ver
length
32 bits
data
(variable length,
typically a TCP
or UDP segment)
16-bit identifier
header
checksum
time to
live
32 bit source IP address
head.
len
type of
service
flgs
fragment
offset
upper
layer
32 bit destination IP address
options (if any)
total datagram
length (bytes)
header length
(bytes)
“type” of data
for
fragmentation/
reassembly
max number
remaining hops
(decremented at
each router)
upper layer protocol
to deliver payload to
e.g. timestamp,
record route
taken, specify
list of routers
to visit.
how much overhead?
20 bytes of IP
20 bytes of TCP
= 40 bytes + app layer overhead

37. 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 …

38. IP fragmentation, reassemblyID =x
offset =0
fragflag
length
=4000
example:
4000 byte datagram
MTU = 1500 bytes ID =x
offset =0
fragflag =1
length
=1500
=185
=370
=1040
one large datagram becomes
several smaller datagrams
1480 bytes in data field
offset =
1480/8

39. IP fragmentation, reassembly
Path MTU discovery
MSS clamping
Send large packet with Don’t Fragment (DF) flag set
If arrives at router with smaller MTU, packet dropped
ICMP “packet too big” sent back, with MTU
Fails if ICMP packets are blocked
Router adds/alters TCP maximum segment size (MSS) option to all flows
Breaks layering guarantees

40. Chapter 4: Outline Overview of Network Layer What’s Inside a Router?
IP : Internet Protocol
Routing Protocols
Intra-AS Routing in the internet: OSPF
Routing among ISPs: BGP
The SDN Control Plane
ICMP: The Internet Control Message Protocol
Network Management and SNMP
Data Format and Fragmentation
IPv4 Addressing
Network Address Translation
IPv6

41. IP addressing: introduction
IP address: 32-bit identifier for host, router interface
interface: connection between host/router and physical link
Router’s typically have multiple interfaces
host typically has one or two interfaces (e.g., wired Ethernet, wireless )
IP addresses associated with each interface =
223 1 1 1

42. IP addressing: introduction
Q: how are interfaces actually connected?
A: we’ll learn about that in next chapters.
A: wired Ethernet interfaces connected by Ethernet switches
A: wireless WiFi interfaces connected by WiFi base station
For now: don’t need to worry about how one interface is connected to another (with no intervening router)

43. 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
subnet
network consisting of 3 subnets

44. Subnets
/24
/24
/24
subnet
recipe
to determine the subnets, detach each interface from its host or router, creating islands of isolated networks
each isolated network is called a subnet
subnet mask: /24

45. Subnets
how many?