1. Control-Data Plane Separation
Part I
Lecture 3, Computer Networks (198:552)
Fall 2019

2. Edge and core: a useful distinction
Edge: data origins or sinks (“endpoints”)
Your laptop, mobile phone, Google’s servers
Core: machines processing & transmitting data
Your WiFi router, Rutgers’s firewall, Verizon’s routers
Varies by context: one person’s core is another’s edge
The Internet

3. Today, we’ll focus on the functions of the core of the network.

4. Structure of the network core
The Internet

5. Structure of the network core
Verizon
AT&T
Rutgers
Sprint
Comcast

6. Structure of the network core
Verizon
AT&T
Rutgers
Sprint
Comcast
Autonomous systems (ASes)

7. Structure of the network core
Verizon
AT&T
Rutgers
Sprint
Independently administered set of routers

8. Moving pkts through Internet
Verizon
AT&T
Rutgers
Sprint
Routing protocols inside and between ASes

9. What’s a protocol?
Informally, a set of rules to communicate over a network
Messages: how to structure a network conversation
Actions: what to do when you are told something (or not)
For example: what you should say next
But could be many other (complex) things
“How are you?”
“I’m good, how are you?”

10. Control and Data Planes

11. Two key functions of the network core
Forwarding: move packets from a router’s input to appropriate output port
happens per packet
Routing: determine route taken by packets from source to destination
happens slower than per packet

12. An analogy: taking a trip
Forwarding: getting through a single interchange
Routing: planning the trip from source to destination

13. Core: Split into data and control planes
Data plane: handles individual packets
Local, per-router function
Forwarding, “drop”, “buffer”, …
“Mark”, “schedule”, “measure”, …
Control plane: handle events
Network-wide logic
Compute how to move data end to end
Need to track the topology of the net
Split is motivated by need for high-speed packet processing

14. The Data Plane
What’s inside a router?

15. What do routers look like?
Access routers
Core router
Data center top-of-rack switch

16. Control & Data Planes inside a router
Traditionally:
Individual routing algorithm components in each and every router interact in the control plane
Control plane
per route-change processing
(~ a few seconds)
Routing
Algorithm
data
plane
control
Data plane
per-packet processing
(~ tens of nanoseconds)
0111 1 2 3
values in arriving
packet header

17. Router architecture overview
forwarding
processor
Control plane
high-speed
switching
fabric
Data plane
router input ports
router output ports

18. Destination-based Forwarding in the Internet
Packet
payload
header
Router
Destination Address
Route Lookup Data Structure
Outgoing Port
Forwarding Table
Dest-network
Port /8 3
/16 1
/19 7

19. We’ll look into the internals of routers in much more detail later.
Output Ports
packet
buffer
queueing
Outgoing
network interface
switch
fabric
Buffering required when pkts arrive from fabric faster than the outgoing transmission rate
Implication: if buffers filled up, packets are dropped
Scheduling discipline chooses among pkts queued for transmission
Implication: Who gets priority is chosen by the scheduler

20. The Control Plane
Routing Protocols

21. Routing enables forwarding
Each router creates & looks packets up in its own forwarding table
But the computation of the table is itself distributed
Three aspects of a routing protocol:
What outcome it computes
What algorithm it runs
How the protocol learns the location of endpoints

22. An example: OSPF Open Shortest Path First (OSPF) used within an AS
Verizon
AT&T
Rutgers
Sprint
Open Shortest Path First (OSPF) used within an AS

23. Edge weights set by the network administrator
What OSPF computes
Shortest path(s) between each pair of nodes
Separate shortest-path tree rooted at each node
Path(s) with minimum sum of link metrics
Disadvantages
All nodes need to agree on the link metrics
Multipath routing is limited to “equal cost multipath”
Edge weights set by the network administrator

24. How OSPF solves the shortest path problem
Compute: path costs to all nodes
From a given source u to all other nodes
Cost of the path through each outgoing link
Next hop along the least-cost path to s 2 1 3 1 4 u 2 1 5 4 3 6 s

25. Dijkstra’s algorithm Once each router knows all nodes and link costs:
Each node computes shortest paths to other nodes
Initialization
Loop
S = {u} for all nodes v if (v is adjacent to u) D(v) = c(u,v) else D(v) = ∞
add w with smallest D(w) to S
update D(v) for all adjacent v:
D(v) = min{D(v), D(w) + c(w,v)}
until all nodes are in S

26. OSPF route computation example3 2 1 4 5 3 2 1 4 5 3 2 1 4 5 3 2 1 4 5

27. OSPF route computation example (cont.)3 2 1 4 5 3 2 1 4 5 3 2 1 4 5 3 2 1 4 5

28. OSPF: Shortest-path tree
Shortest-path tree from u
Forwarding table at u 3 2 1 4 5 u v w x y z s t
link v
(u,v) w
(u,w) x
(u,w) y
(u,v) z
(u,v) s
(u,w) t
(u,w)

29. How OSPF on one router learns about other routers
Each router sends out its own address and neighborhood link costs over all of its links: link state advertisement
Each router forwards advertisements from others
A process known as link state flooding
As long as the neighboring router and the link to it are alive, the link cost is included in the flooded message
OSPF is a link state protocol: if the state of the link changes (up/down/cost change), the entire network will know