15-441 computer networking

Hui Zhang, Fall 2012 1

15-441 Computer Networking

Data and Control Planes


Goals of This Lecture

Show how everything fit together Review data plane components

- various switching architectures

- Head of line blocking

- Scheduling

- Header lookup

Discuss Control plane- Control processor

- Routing protocol

- Signaling protocol

- Interconnection of Multiple control protocols


Review: Structure of A Generic Communication Switch

Switches- circuit switch

- Ethernet switch

- ATM switch

- IP router

Line Cards

Switch Fabric

Control Processor

Line Cards

Switch fabric- high capacity interconnect

Line card- address lookup in the

data path (forwarding) Control Processor

- load the forwarding table (routing or signaling)



Data plane: how each packet is processed? - Forwarding: header lookup and forward the packet to

right output port

- Switching: move packets from input port to output port

- Scheduling: manage buffer and bandwidth resource

Control plane: how forwarding tables are computed

- Router: routing protocols

- ATM and phone switches: signaling & routing protocols

- Ethernet switch: learning and spanning tree


Review: Data Plane Switch

Input and output interfaces are connected through an interconnect

A interconnect can be implemented by

- Shared memory

- Shared bus

- Crossbar or other switching fabric

input interface output interface

Inter-connect


Review: Head-of-line Blocking The packet/cell at the head of an input queue cannot

be transferred, thus blocking the following packets/ cells

Cannot betransferred

because output buffer overflow

Cannot be transferred because is blocked by red cell

Output 1

Output 2

Output 3

Input 1

Input 2

Input 3


Review: Solution to Avoid Head-of-line Blocking

For each input card, maintain at each input N virtual queues, i.e., one per output

Output 1

Output 2

Output 3

Input 1

Input 2

Input 3


Input Interface

Packet forwarding: decide to which output interface to forward each packet based on the information in packet header

- Examine packet header

- Lookup in forwarding table

- Update packet headerinput interface output interface

Inter-connect


Circuit Switching

Input PortsOutput Ports

Switch

Connects (electrons or bits) ports to ports


AABB

DD

CC

EE

FF

VC7VC4

VC3VC3

VC3VC5

VC3 VC5

VC2

VC2

Virtual Circuit Switching: Label Swapping

Table at Node A

INLINK

IN VC OUTLINK

OUTVC

CA 7 AB 4

CA 2 AB 5

DA 3 AB 3


IP Routing

Packet with destination address 12.82.100.101 is sent to interface 2, as 12.82.100.xxx is the longest prefix matching packet’s destination address

……

312.82.xxx.xxx

1128.16.120.xxx

1

2128.16.120.111

12.82.100.101

12.82.100.xxx 2


Patricia Tries

Use binary tree paths to encode prefixes

Advantage: simple to implement Disadvantage: one lookup may take O(m), where m

is number of bits (32 in the case of IPv4)

001xx 2 0100x 310xxx 101100 5

0 1

0

1 0

1

1

0

0

0

0

2

3

5

1


Review: Addressing and Look-up

Flat address- Ethernet: 48 bit MAC

address

- ATM: 28 bit VPI/VCI

- DS-0: timeslot location Limited scalability High speed lookup

Hierarchical address- IP <network>.<subnet>.<host>

- Telephone: country.area.home Scalable Easy lookup if boundary is fixed

- telephony Difficult lookup if boundary is

flexible- longest prefix match for IP


Output Interface

Buffer management: decide when and which packet to drop Scheduler: decide when and which packet to transmit

- FIFO, Round Robin, Strict Priority

1

2

SchedulerBuffer


Example: FIFO router

Most of today’s routers Drop-tail buffer management: when buffer is full

drop the incoming packet First-In-First-Out (FIFO) Scheduling: schedule

packets in the same order they arrive



Data plane: how each packet is processed? - Forwarding: header lookup and forward the packet to

right output port

- Switching: move packets from input port to output port

- Scheduling: manage buffer and bandwidth resource

Control plane: how forwarding tables are computed

- Router: routing protocols

- ATM and phone switches: signaling & routing protocols

- Ethernet switch: learning and spanning tree


Signaling Protocol for Circuit-Switched and Virtual-Circuit-Switched Networks:

Signaling protocol establishes/tears down circuit - Signaling message are routed

- Signaling protocol fills the forwarding table

- What tables are maintained by a Circuit Switch processor?

- Are there routing protocols? Parameters used for establishing Virtual Circuits

- Source and destination Addresses

- Traffic Characteristics

- QoS Parameters

- Others? Parameters can be stored in forwarding table to help

forwarding decision


Three Classes of Routing Protocols

Distance vector (RIP)- Distributed path computation

- Keep only local link data

- Bellman-Ford algotrithm

Link state (OSPF, IS-IS)- Local path computation

- Distribute all link data -- each node has whole topology

- Dijkstra’s algorithm

Path vector (BGP)- Distributed path computation

- Distribute routes (prefix + path) to every node --- each node stores all routes subject to policy


Components of IP Control Plane (I)

Forwarding Information Base

LocalRIB

OSPFRIB

BGPRIB

RIPRIB

Route Selection

PP

RIPRIB

OSPFRIB

A B

OS

PF R

IP

BG

P

110 120

Machine : routing control processor Data structure: RIB (Routing Information Base) Protocol Instance Module: software module that

exchanges messages with protocol instance modules running on other routers


Components of IP Control Plane (II)


LocalRIB

OSPFRIB

BGPRIB

RIPRIB

Route Selection

PP

RIPRIB

OSPFRIB

A B

OS

PF R

IP

BG

P

110 120

Protocol: define the interaction (message formats, message semantics, timing) among distributed set of protocol instance modules. The goal is to generate a consistent set of local RIBs at each protocol module


Components of IP Control Plane (III)


LocalRIB

OSPFRIB

BGPRIB

RIPRIB

Route Selection

PP

RIPRIB

OSPFRIB

A B

OS

PF R

IP

BG

P

110 120

Local RIB: routing table entries set by configuration files Route redistribution: inject routes from one protocol

instance module to another protocol instance module (e.g OSPF to BGP)

Route selection: for each prefix, select one among multiple routes (each computed by one protocol instance)


Excerpts from a Router Configuration File interface Ethernet0 ip address 6.2.5.14 255.255.255.128 interface Serial1/0.5 point-to-point ip address 6.2.2.85 255.255.255.252 ip access-group 143 in frame-relay interface-dlci 28

router ospf 64 redistribute connected subnets redistribute bgp 64780 metric 1 subnets network 66.251.75.128 0.0.0.127 area 0 router bgp 64780 redistribute ospf 64 match route-map 8aTzlvBrbaW neighbor 66.253.160.68 remote-as 12762 neighbor 66.253.160.68 distribute-list 4 in

access-list 143 deny 1.1.0.0/16access-list 143 permit any

route-map 8aTzlvBrbaW deny 10 match ip address 4

route-map 8aTzlvBrbaW permit 20 match ip address 7

ip route 10.2.2.1/16 10.2.1.7

15-441 computer networking

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