1. CS 5565 Network Architecture and Protocols
Lecture 16
Godmar Back

2. Announcements Midterm April 1 (Wednesday) Project 2A due Apr 8
Required Reading:
DCCP by Koehler et al, SIGCOMM 2006
CS 5565 Spring 2009
2/23/2019

3. Handling multiple clients using multiple execution contexts
A/B:
# grows &
shrinks A B
C/D:
fixed # C D
Q.: When would you use which?
CS 5565 Spring 2009
2/23/2019

4. Network Layer

5. The Network Layer transports 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
CS 5565 Spring 2009
2/23/2019

6. Key Network-Layer Functions
forwarding: move packets from router’s input to appropriate router output
routing: determine route taken by packets from source to dest.
Routing algorithms
analogy:
routing: process of planning trip from source to dest
forwarding: process of getting through single interchange
CS 5565 Spring 2009
2/23/2019

7. Interplay between Routing and Forwarding1 2 3
0111
value in arriving
packet’s header
routing algorithm
local forwarding table
header value
output link
0100
0101
1001
CS 5565 Spring 2009
2/23/2019

8. 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
CS 5565 Spring 2009
2/23/2019

9. Network Layer Service Models:
Guarantees ?
Network
Architecture
Internet
ATM
Service
Model
best effort
CBR
VBR
ABR
UBR
Congestion
feedback
no (inferred
via loss) no
congestion
yes
Bandwidth
none
constant
rate
guaranteed
minimum
Loss no
yes
Order no
yes
Timing no
yes
CS 5565 Spring 2009
2/23/2019

10. Datagram vs VC Networks
Datagram network provides network-layer connectionless service
VC network provides network-layer connection service
Analogous to the transport-layer services, but different in:
Service: host-to-host
No choice: network provides one or the other
Implementation: in the core
CS 5565 Spring 2009
2/23/2019

11. Virtual Circuits
“source-to-dest path behaves much like telephone circuit”
performance-wise
network actions along source-to-dest path
call setup, teardown for each call before data can flow
each packet carries VC identifier (not destination host address)
every router on source-dest path maintains “state” for each passing connection
link, router resources (bandwidth, buffers) may be allocated to VC
CS 5565 Spring 2009
2/23/2019

12. Virtual Circuits: Signaling Protocols
used to setup, maintain teardown VC
used in ATM, frame-relay, X.25
not used in today’s Internet (at least not end-to-end)
application
transport
network
data link
physical
application
transport
network
data link
physical
5. Data flow begins
6. Receive data
4. Call connected
3. Accept call
1. Initiate call
2. incoming call
CS 5565 Spring 2009
2/23/2019

13. VC Implementation A VC consists of:
Path from source to destination
VC numbers, one number for each link along path
Entries in forwarding tables in routers along path
Packet belonging to VC carries a VC number.
VC number must be changed on each link.
New VC number comes from forwarding table
CS 5565 Spring 2009
2/23/2019

14. Forwarding Tables in VCN12 22 32 1 2 3
VC number
interface
number
Forwarding table in
northwest router:
Incoming interface Incoming VC # Outgoing interface Outgoing VC #
… … … …
Routers maintain connection state information!
CS 5565 Spring 2009
2/23/2019

15. Datagram Networks no call setup at network layer
routers: no state about end-to-end connections
no network-level concept of “connection”
packets forwarded using destination host address
packets between same source-dest pair may take different paths
application
transport
network
data link
physical
application
transport
network
data link
physical
1. Send data
2. Receive data
CS 5565 Spring 2009
2/23/2019

16. Internet vs ATM Internet ATM data exchange among computers
“elastic” service, no strict timing req.
“smart” end systems (computers)
can adapt, perform control, error recovery
simple inside network, complexity at “edge”
many link types
different characteristics
uniform service difficult
ATM
evolved from telephony
human conversation:
strict timing, reliability requirements
need for guaranteed service
“dumb” end systems
telephones
complexity inside network
CS 5565 Spring 2009
2/23/2019

17. Router Architectures; Packet Forwarding & Classification

18. Router Architecture Overview
Two key router functions:
run routing algorithms/protocol (RIP, OSPF, BGP)
forwarding datagrams from incoming to outgoing link
CS 5565 Spring 2009
2/23/2019

19. Input Port Functions Decentralized switching: Physical layer:
bit-level reception
Decentralized switching:
given datagram dest., lookup output port using forwarding table in input port memory
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
CS 5565 Spring 2009
2/23/2019

20. Forwarding Tables Destination Address Range Link Interface
through
through
through
otherwise
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21. Longest Prefix Matching
Prefix Match Link Interface
otherwise
Examples
DA:
Which interface?
DA:
Which interface?
DA:
Which interface?
CS 5565 Spring 2009
2/23/2019

22. Packet Classification
Service
Example
Packet Filtering
Deny all traffic from ISP3(on interface X) destined to E2
Policy Routing
Send all voice-over-IP traffic arriving from E1 (on interface Y) and destined to E2 via a separate ATM network.
Accounting & Billing
Treat all video traffic to E1(via interface Y) as highest priority and perform accounting for the traffic sent this way.
Traffic Rate Limiting
Ensure that ISP2 does not inject more than 10Mbps of traffic and 50Mbps of total traffic on interface X
Traffic Shaping
Ensure that no more than 50Mbps of web traffic is injected into ISP2 on interface X.
Source [Gupta & McKeown 2001]
CS 5565 Spring 2009
2/23/2019

23. Performance Metrics for Packet Classification (Gupta/McKeown)
Search speed
10 Gbps will have Mp/s for min-TCP
Low storage requirements
The smaller, the faster (SRAM)
Ability to handle large real-life classifiers
Fast updates
Scalability in number of header fields used for classification
Flexibility in specification
Not just prefixes: ranges, operators, wildcards, etc.
CS 5565 Spring 2009
2/23/2019

24. Example for 2 Fields
Priority
Geometry problem: N number of regions, d number of dimensions; regions are prioritized
Best worst case time O(log N) time with O(Nd) space
Best worst case space O(N) with O((log N)d-1) time
CS 5565 Spring 2009
2/23/2019

25. Packet Classification Solutions
Basic data structures
Linear search, caching, hierarchical tries, set-pruning tries
Geometry-based structures
Grid-of-tries, AQT (area-based quadtree), FIS (fat-inverted segment tree)
Heuristics
RFC (recursive flow classification), hierarchical cuttings, tuple-space search
Hardware
Ternary CAM (content-addressable memory), bitmap-intersection
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2/23/2019

26. Three Types of Switching Fabrics
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27. 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)
Newer routers: processing done via local processors on line cards
Input
Port
Output
Memory
System Bus
CS 5565 Spring 2009
2/23/2019

28. 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 (not regional or backbone)
CS 5565 Spring 2009
2/23/2019

29. Switching Via An Interconnection Network
Overcome bus bandwidth limitations
Banyan/Butterfly networks, other interconnection nets initially developed to connect processors in multiprocessor machines
Advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric.
Cisco 12000: switches Gbps through the interconnection network
See Cisco Whitepaper for more information & background
CS 5565 Spring 2009
2/23/2019

30. Output Ports
Buffering required when datagrams arrive from fabric faster than the transmission rate
Possibility of queueing (delay) and loss due to output port buffer overflow!
Scheduling discipline chooses among queued datagrams for transmission
CS 5565 Spring 2009
2/23/2019

31. Scheduling Disciplines
Single or multiple queues
If multiple: Flow-based or Class-based
FCFS, RR (Round-Robin), WRR
Various priority schemes:
expedited packets, assured forwarding
WFQ (Weighted Fair Queuing)
proportional sharing of link between queues
HFSC (Hierarchical Fair Service Curve)
hierarchical extension, better queuing delay bounds
CS 5565 Spring 2009
2/23/2019

32. HFSC Decouple latency and bandwidth allocations Source: [Zhang]
CS 5565 Spring 2009
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33. Active Queue Management (AQM)
When should packets be dropped?
Goal: avoid congestion
Simplest policy: drop-tail
If queue is full, drop new arrivals
More sophisticated:
Random Early Detection (RED)
Before queue fills up, mark some packets randomly for drop
Idea: force TCP congestion control to throttle rate
Lots of research in this area
CS 5565 Spring 2009
2/23/2019

34. Input Port Queuing
Fabric slower than input ports combined  queueing may occur at input queues
Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward
queueing delay and loss due to input buffer overflow!
CS 5565 Spring 2009
2/23/2019

35. Summary Basics of Network Layer Next: Routing Algorithms
Routing (path selection) vs Forwarding (switching)
Service models
Datagram Networks vs VC Networks
Basics of routers
Packet classification, Packet Scheduling, AQM
Next: Routing Algorithms
CS 5565 Spring 2009
2/23/2019