Networking Fundamentals and Challenges in Data Centers

 
14-760:
ADV. REAL-WORLD NETWORKS
 
SWITCHING FABRIC, ROUTERS, ROUTING
LECTURE 3 * SPRING 2019(KESDEN)
 
REMEMBER SOCRATIVE
 
https://api.socrative.com/rc/FX5GFK
 
WHERE ARE WE?
 
Last class
Reviewed the basics of networking. Hopefully a refresher from prior class(es)
Today
Examine the role of switches and routers vis-à-vis data center and Internet design
Much of this will likely be a review for some (but not nearly all)
Next class
Look at the internals architecture of switches and routers.
This is likely totally new to most.
 
TODAY
 
Definitions
Data Center Traffic Patterns
Data Center Fabric
Managing Parallelism Within Networks
Routing and Routers/Internet Switches (This is probably mostly review)
Looking forward to next class
 
DEFINITIONS FOR TODAY
 
“Routing” (Network-layer packet switching)
The hop-to-hop management of the movement of traffic among networks
“Switching” (Link-layer frame switching)
The movement of frames within a local network
Note:
There is nothing wrong with using the word “switching” at other-than the link layer, e.g. network layer switch,
application layer switch, etc.
Such references describe switches that make decisions based upon information present at other layers of the
network stack
That just doesn’t happen to be the way I am using the words today, so I define them here for today’s
discussion.
 
TODAY
 
Definitions
Data Center Challenges and Traffic Patterns
Data Center Fabric
Managing Parallelism Within Networks
Routing and Routers/Internet Switches (This is probably mostly review)
Looking forward to next class
 
DC NETWORKING CHALLENGES
 
Within a Data Center:
Distances are short, the speed of light is less of a factor
Serialization/Deserialization delay (bits/second) is a factor
Managing bandwidth becomes complex as it is easy to add wire, but hard to use it.
vs. Long haul:
Speed of light is significant source of latency, unlikely to be resolved soon
Additional bandwidth can typically readily be achieved with parallelism, e.g. more fiber
 
TYPES OF TRAFFIC
 
Ingress – Traffic entering from the outside, e.g. requests entering a data center
Most commonly from outside requests.
Can be from queries to related off-site services.
Egress – Traffic leaving to the outside, e.g. responses, streaming video, database results, etc.
Most commonly from providing service.
Can be queries to related off-site services
East-West – Traffic within a data center that crosses higher level switches
Most commonly from communication within integrated services
Incast – Traffic within a data center that becomes concentrated
Most commonly from parallel query concentrating results
 
TYPES OF TRAFFIC: INGRESS
 
Core
 
Aggregation
 
Leaf
 
TYPES OF TRAFFIC: EGRESS
 
Core
 
Aggregation
 
Leaf
 
TYPES OF TRAFFIC: EAST-WEST
 
Core
 
Aggregation
 
Leaf
 
TYPES OF TRAFFIC: INCAST
 
Core
 
Aggregation
 
Leaf
 
TODAY
 
Definitions
Data Center Challenges and Traffic Patterns
Data Center Fabric
Managing Parallelism Within Networks
Routing and Routers/Internet Switches (This is probably mostly review)
Looking forward to next class
 
NETWORK SWITCHES
 
Ports connect to network segments
Hosts and uplink at leaf level
Other switches at other levels
Switches have to move messages (frames, packets, etc) from one port to another
Hardware solutions: Crossbar, etc
Shared memory solutions: input and output queues
Limited throughput (processor time, shared memory, crossbar, etc)
Limited number of ports, limited throughput per port, limited aggregate throughput
Commodity-level capacities are relatively affordable
Trying to buy more capacity is expensive with limited gain
 
Since, beyond a certain point, we can’t make switches wider and/or faster, we need to
“fan out”, most commonly with a tree topology
Venerable 3-tier network is a straight-forward example:
 
DC TOPOLOGY:
VENERABLE 3-TIER
 
Core
 
Aggregation
 
Leaf
 
Can add a redundant core for increased throughput and resilience
 
DC TOPOLOGY:
VENERABLE 3-TIER WITH REDUNDANT CORE
 
Core
 
Aggregation
 
Leaf
 
Scales nicely, but …
Higher up gets over-subscribed since everything passes through
Over-subscription increases with scale
Request-to-stream and host-to-host cases generate bottlenecks
 
DC TOPOLOGY:
VENERABLE 3-TIER, ANALYSIS
 
1-2 Switch Throughput
 
W
 Switch Throughput
 
W 
2
 Switch Throughput
 
DC TOPOLOGY:
CLOS NETWORK
 
By Piggly (talk) (Uploads) - Transferred from en.wikipedia to Commons.,
Public Domain, https://commons.wikimedia.org/w/index.php?curid=61536102
 
Allocating an input port,  and associated
throughput,
Allocates path whole way through.
NxN connectivity with switches with
less than NxN connectivity
Basically a way to make a large NxN
switch
An expensive solution and not likely to
need all throughput capacity
simultaneously
 
DC TOPOLOGY:
LEAF AND SPINE
 
Type of Clos network
Essentially folded, but still N-to-N connections
Derived from old phone company architecture,
    invented in 1950s.
Sometimes Called “Folded Clos”
All paths are same length from edge to edge
Need to pick path, as can choose any middle router
Very redundant
Can implement at layer-2 or layer-3
 
DC TOPOLOGY:
FAT-TREES WITH SKINNY SWITCHES: GOALS
 
Use all commodity switches
Full throughput from host-to-host
Compatible with usual TCP/IP stack
Better energy efficiency per unit throughput from more smaller switches than fewer bigger
switches
As you look at the next slide note that, given 
K
 ports/switch, we are using:
K/2
 ports up + 
K/2
 ports down, or
K
 ports up, or
K
 ports down
 
FAT TREE (K=4)
K PORTS/SWITCH
Note the replacement
of aggregation layer
switches with 2 layers of
K/2 K-port switches
(K/2)
2
 core routers
K pods
K-port switches support K
3
/4 servers
(K/2)
2
servers
per pod
 
Base Image Credit: Mysore, et al, "PortLand: a scalable fault-tolerant layer 2 data center network fabric", SIGCOM,
October 2009, Pages 39-50.
 
DC TOPOLOGY:
FAT TREE DETAILS
 
K-ary fat free: three layers (core, aggregation, edge)
Each pod consists of (K/2)
2
 servers and 2 layers of K/2 K-port switches.
Each edge switch connects (K/2) servers to (K/2) aggregator switches
Each aggregator switch connects (K/2) edge and (K/2) core switches
(K/2)
2
 core switches, each ultimately connecting to K pods
Providing K different roots, not 1. Trick is to pick different ones
K-port switches support K
3
/4 servers/host:
(K/2 hosts/switch * K/2 switches per pod * K pods)
 
DC TOPOLOGY:
WHAT IS ACTUALLY USED?
 
Redundant 3-Tier architectures
Often used for general IT data centers
They scale well
Clos Networks
Too expensive
Not likely to be sufficiently utilized
Fat Tree Networks
Look good on paper and very practical
Might use them, if we needed to
Leaf-And-Spine
Right now, port density seems to be supporting their use
Not clear that next generation of switches won’t trade bandwidth for port density and push us back to Fat Tree or other
 
DC TOPOLOGY:
WHAT IS ACTUALLY USED?
 
Note that there are many other possibilities, e.g.
Grid/Cube-like architectures: Mesh, Torus, Hypercube, Butterfly, Dragonfly, HyperX, etc
Note also that different parts of a data center, with different requirements may be
networked differently, e.g. commodity file storage, vs bulk streaming, vs app servers, etc.
 
TODAY
 
Definitions
Data Center Traffic Patterns
Data Center Fabric
Managing Parallelism Within Networks
Routing and Routers/Internet Switches (This is probably mostly review)
Looking forward to next class
 
PARALLELISM WITHIN NETWORKS:
TRADITIONAL THINKING
 
Parallelism is for redundancy
Run self-healing (periodically updating)a Spanning Tree Protocol (STP) among
switches to agree a network tree without cycles
Avoid frames being forwarded forever in loops
Uses redundancy to select an alternate path in the event of hard failure
 
PARALLELISM WITHIN NETWORKS:
BANDWIDTH-MOTIVATED THINKING
 
Parallelism is for bandwidth
Have parallelism where needed to support traffic patterns
Find ways to use bandwidth without causing problems
Avoid looping
Use all available paths
 
PARALLELISM WITHIN NETWORKS:
WHERE IS BANDWIDTH NEEDED?
 
Depends upon usage pattern
If most activity is rack-local
At the rack switch
3-Tier might be fine
If most of the activity is east-west
Need more capacity at higher level(s), e.g. core and/or aggregation, etc
If most of activity is concentrating at egress (or ingress)
Need more core upstream capacity (supported by core)
 
PARALLELISM WITHIN NETWORKS:
HOW TO USE PARALLELISM?
 
Goals:
Must avoid cycles
Usage should be divided among paths
Ways to divide usage
Each use, e.g. network session, picks different path
Uses divide their traffic among parallel paths
 
USING PARALLELISM WITHIN NETWORKS:
SOLVABLE AT DIFFERENT LEVELS (EXAMPLES)
 
MPTCP – TCP that shares multiple paths for single session
Creates many sub-sessions and multiplexes overarching session among them
PORTland network – Link-Layer solutions
Path is based upon sender’s location in network
 
TODAY
 
Definitions
Data Center Traffic Patterns
Data Center Fabric
Managing Parallelism Within Networks
Routing and Routers/Internet Switches 
(This is probably mostly review)
Looking forward to next class
 
ROUTING
 
As we leave the domain of a single network, we enter the domain of internetworking
Many techniques for global connectivity are possible, but “packet switching” is the most
common for Internet traffic as it crosses providers
Other types of switching, including (and perhaps especially) virtual circuits, e.g. ATM, are
commonly used within provider networks.
Circuit switching is a fading technology, e.g. POTS, ISDN.
Packet switching involves a “router” making hop-by-hop decisions about how to “route” a
packet.
 
ROUTERS
 
May Also be called “Internet Switches” or “Network-layer switches”
May also perform other roles
Rule-based Firewalls
Rule-based load-balancing
Network Address Translation (NAT)
Virtual Private Networks and Tunneling
 
ROUTING PROTOCOLS
 
Routers exchange information with each other about the state of the network
Routers collect this into 
routing tables
Algorithms reduce the 
routing tables 
to simple 
forwarding tables
 that the switch can quickly
consult w.r.t. which port should be the destination port for a particular incoming packet.
 
DATA PLANE VS CONTROL PLANE
 
Routing algorithms run in the 
Control Plane
The control plane decides the rules for forwarding, but it does not do the forwarding
The 
Data Plan
 is where the forwarding actually occurs
Packets move in and out via the data plane, which consults the distilled tables provided by the
control plane
 
TODAY
 
Definitions
Data Center Traffic Patterns
Data Center Fabric
Managing Parallelism Within Networks
Routing and Routers/Internet Switches (This is probably mostly review)
Looking forward to next class
 
NEXT CLASS
 
Routers and switches need to sustain a very, very high throughput.
They need to very rapidly
Source a packet
Decide what to do with it
Sink it to another port
This could involve buffering it at the input port, for a decision, and/or at the output port
This could involve a computer-like shared memory system or a more purpose-built hardware
switch
 
NEXT CLASS, 
CONT.
 
Next class, we are going to take a look a the architectures, where the points of
contention are within them, discuss their performance and relative costs, &c.
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Today's lecture covers definitions of routing and switching in networking, focusing on data center challenges, traffic patterns, fabric management, and parallelism within networks. The session also explores the role of switches and routers in data center and Internet design, emphasizing the importance of efficient data center networking to overcome challenges related to distances, speed of light, bandwidth management, and types of traffic.

  • Networking fundamentals
  • Data center challenges
  • Routing
  • Switching
  • Traffic patterns

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  1. 14-760: ADV. REAL-WORLD NETWORKS SWITCHING FABRIC, ROUTERS, ROUTING LECTURE 3 * SPRING 2019(KESDEN)

  2. REMEMBER SOCRATIVE https://api.socrative.com/rc/FX5GFK

  3. WHERE ARE WE? Last class Reviewed the basics of networking. Hopefully a refresher from prior class(es) Today Examine the role of switches and routers vis- -vis data center and Internet design Much of this will likely be a review for some (but not nearly all) Next class Look at the internals architecture of switches and routers. This is likely totally new to most.

  4. TODAY Definitions Data Center Traffic Patterns Data Center Fabric Managing Parallelism Within Networks Routing and Routers/Internet Switches (This is probably mostly review) Looking forward to next class

  5. DEFINITIONS FOR TODAY Routing (Network-layer packet switching) The hop-to-hop management of the movement of traffic among networks Switching (Link-layer frame switching) The movement of frames within a local network Note: There is nothing wrong with using the word switching at other-than the link layer, e.g. network layer switch, application layer switch, etc. Such references describe switches that make decisions based upon information present at other layers of the network stack That just doesn t happen to be the way I am using the words today, so I define them here for today s discussion.

  6. TODAY Definitions Data Center Challenges and Traffic Patterns Data Center Fabric Managing Parallelism Within Networks Routing and Routers/Internet Switches (This is probably mostly review) Looking forward to next class

  7. DC NETWORKING CHALLENGES Within a Data Center: Distances are short, the speed of light is less of a factor Serialization/Deserialization delay (bits/second) is a factor Managing bandwidth becomes complex as it is easy to add wire, but hard to use it. vs. Long haul: Speed of light is significant source of latency, unlikely to be resolved soon Additional bandwidth can typically readily be achieved with parallelism, e.g. more fiber

  8. TYPES OF TRAFFIC Ingress Traffic entering from the outside, e.g. requests entering a data center Most commonly from outside requests. Can be from queries to related off-site services. Egress Traffic leaving to the outside, e.g. responses, streaming video, database results, etc. Most commonly from providing service. Can be queries to related off-site services East-West Traffic within a data center that crosses higher level switches Most commonly from communication within integrated services Incast Traffic within a data center that becomes concentrated Most commonly from parallel query concentrating results

  9. TYPES OF TRAFFIC: INGRESS Core Aggregation Leaf

  10. TYPES OF TRAFFIC: EGRESS Core Aggregation Leaf

  11. TYPES OF TRAFFIC: EAST-WEST Core Aggregation Leaf

  12. TYPES OF TRAFFIC: INCAST Core Aggregation Leaf

  13. TODAY Definitions Data Center Challenges and Traffic Patterns Data Center Fabric Managing Parallelism Within Networks Routing and Routers/Internet Switches (This is probably mostly review) Looking forward to next class

  14. NETWORK SWITCHES Ports connect to network segments Hosts and uplink at leaf level Other switches at other levels Switches have to move messages (frames, packets, etc) from one port to another Hardware solutions: Crossbar, etc Shared memory solutions: input and output queues Limited throughput (processor time, shared memory, crossbar, etc) Limited number of ports, limited throughput per port, limited aggregate throughput Commodity-level capacities are relatively affordable Trying to buy more capacity is expensive with limited gain

  15. DC TOPOLOGY: VENERABLE 3-TIER Since, beyond a certain point, we can t make switches wider and/or faster, we need to fan out , most commonly with a tree topology Venerable 3-tier network is a straight-forward example: Core Aggregation Leaf

  16. DC TOPOLOGY: VENERABLE 3-TIER WITH REDUNDANT CORE Can add a redundant core for increased throughput and resilience Core Aggregation Leaf

  17. DC TOPOLOGY: VENERABLE 3-TIER, ANALYSIS Scales nicely, but Higher up gets over-subscribed since everything passes through Over-subscription increases with scale Request-to-stream and host-to-host cases generate bottlenecks 1-2 Switch Throughput W Switch Throughput W 2 Switch Throughput

  18. DC TOPOLOGY: CLOS NETWORK Allocating an input port, and associated throughput, Allocates path whole way through. NxN connectivity with switches with less than NxN connectivity Basically a way to make a large NxN switch An expensive solution and not likely to need all throughput capacity simultaneously By Piggly (talk) (Uploads) - Transferred from en.wikipedia to Commons., Public Domain, https://commons.wikimedia.org/w/index.php?curid=61536102

  19. DC TOPOLOGY: LEAF AND SPINE Type of Clos network Essentially folded, but still N-to-N connections Derived from old phone company architecture, invented in 1950s. Sometimes Called Folded Clos All paths are same length from edge to edge Need to pick path, as can choose any middle router Very redundant Can implement at layer-2 or layer-3

  20. DC TOPOLOGY: FAT-TREES WITH SKINNY SWITCHES: GOALS Use all commodity switches Full throughput from host-to-host Compatible with usual TCP/IP stack Better energy efficiency per unit throughput from more smaller switches than fewer bigger switches As you look at the next slide note that, given K ports/switch, we are using: K/2 ports up + K/2 ports down, or K ports up, or K ports down

  21. Note the replacement of aggregation layer switches with 2 layers of K/2 K-port switches (K/2)2 core routers FAT TREE (K=4) K PORTS/SWITCH K pods (K/2)2 servers per pod K-port switches support K3/4 servers Base Image Credit: Mysore, et al, "PortLand: a scalable fault-tolerant layer 2 data center network fabric", SIGCOM, October 2009, Pages 39-50.

  22. DC TOPOLOGY: FAT TREE DETAILS K-ary fat free: three layers (core, aggregation, edge) Each pod consists of (K/2)2 servers and 2 layers of K/2 K-port switches. Each edge switch connects (K/2) servers to (K/2) aggregator switches Each aggregator switch connects (K/2) edge and (K/2) core switches (K/2)2 core switches, each ultimately connecting to K pods Providing K different roots, not 1. Trick is to pick different ones K-port switches support K3/4 servers/host: (K/2 hosts/switch * K/2 switches per pod * K pods)

  23. DC TOPOLOGY: WHAT IS ACTUALLY USED? Redundant 3-Tier architectures Often used for general IT data centers They scale well Clos Networks Too expensive Not likely to be sufficiently utilized Fat Tree Networks Look good on paper and very practical Might use them, if we needed to Leaf-And-Spine Right now, port density seems to be supporting their use Not clear that next generation of switches won t trade bandwidth for port density and push us back to Fat Tree or other

  24. DC TOPOLOGY: WHAT IS ACTUALLY USED? Note that there are many other possibilities, e.g. Grid/Cube-like architectures: Mesh, Torus, Hypercube, Butterfly, Dragonfly, HyperX, etc Note also that different parts of a data center, with different requirements may be networked differently, e.g. commodity file storage, vs bulk streaming, vs app servers, etc.

  25. TODAY Definitions Data Center Traffic Patterns Data Center Fabric Managing Parallelism Within Networks Routing and Routers/Internet Switches (This is probably mostly review) Looking forward to next class

  26. PARALLELISM WITHIN NETWORKS: TRADITIONAL THINKING Parallelism is for redundancy Run self-healing (periodically updating)a Spanning Tree Protocol (STP) among switches to agree a network tree without cycles Avoid frames being forwarded forever in loops Uses redundancy to select an alternate path in the event of hard failure

  27. PARALLELISM WITHIN NETWORKS: BANDWIDTH-MOTIVATED THINKING Parallelism is for bandwidth Have parallelism where needed to support traffic patterns Find ways to use bandwidth without causing problems Avoid looping Use all available paths

  28. PARALLELISM WITHIN NETWORKS: WHERE IS BANDWIDTH NEEDED? Depends upon usage pattern If most activity is rack-local At the rack switch 3-Tier might be fine If most of the activity is east-west Need more capacity at higher level(s), e.g. core and/or aggregation, etc If most of activity is concentrating at egress (or ingress) Need more core upstream capacity (supported by core)

  29. PARALLELISM WITHIN NETWORKS: HOW TO USE PARALLELISM? Goals: Must avoid cycles Usage should be divided among paths Ways to divide usage Each use, e.g. network session, picks different path Uses divide their traffic among parallel paths

  30. USING PARALLELISM WITHIN NETWORKS: SOLVABLE AT DIFFERENT LEVELS (EXAMPLES) MPTCP TCP that shares multiple paths for single session Creates many sub-sessions and multiplexes overarching session among them PORTland network Link-Layer solutions Path is based upon sender s location in network

  31. TODAY Definitions Data Center Traffic Patterns Data Center Fabric Managing Parallelism Within Networks Routing and Routers/Internet Switches (This is probably mostly review) Looking forward to next class

  32. ROUTING As we leave the domain of a single network, we enter the domain of internetworking Many techniques for global connectivity are possible, but packet switching is the most common for Internet traffic as it crosses providers Other types of switching, including (and perhaps especially) virtual circuits, e.g. ATM, are commonly used within provider networks. Circuit switching is a fading technology, e.g. POTS, ISDN. Packet switching involves a router making hop-by-hop decisions about how to route a packet.

  33. ROUTERS May Also be called Internet Switches or Network-layer switches May also perform other roles Rule-based Firewalls Rule-based load-balancing Network Address Translation (NAT) Virtual Private Networks and Tunneling

  34. ROUTING PROTOCOLS Routers exchange information with each other about the state of the network Routers collect this into routing tables Algorithms reduce the routing tables to simple forwarding tables that the switch can quickly consult w.r.t. which port should be the destination port for a particular incoming packet.

  35. DATA PLANE VS CONTROL PLANE Routing algorithms run in the Control Plane The control plane decides the rules for forwarding, but it does not do the forwarding The Data Plan is where the forwarding actually occurs Packets move in and out via the data plane, which consults the distilled tables provided by the control plane

  36. TODAY Definitions Data Center Traffic Patterns Data Center Fabric Managing Parallelism Within Networks Routing and Routers/Internet Switches (This is probably mostly review) Looking forward to next class

  37. NEXT CLASS Routers and switches need to sustain a very, very high throughput. They need to very rapidly Source a packet Decide what to do with it Sink it to another port This could involve buffering it at the input port, for a decision, and/or at the output port This could involve a computer-like shared memory system or a more purpose-built hardware switch

  38. NEXT CLASS, CONT. Next class, we are going to take a look a the architectures, where the points of contention are within them, discuss their performance and relative costs, &c.

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