detnet02 unsync networks - Institute of Computer ... · • Sum of AVB traffic may not exceed 75% of the port ... – SkewMax determines duplicate-elimination interval 27. Peter

Unsynchronized Networks

Peter Puschner, Institut für Technische Informatik Wilfried Steiner, TTTech AG

Peter Puschner, TU Wien 2

Ethernet Basics

Peter Puschner, TU Wien

Ethernet Devices • Today we mainly know two Ethernet devices:

– End Stations and Bridges • End stations are also called “end systems” or “end points” or “network interface cards” • Bridges are also called switches

– Note, “bridge” is the correct technical term while “switch” is a marketing brand. – However, as bridge and switch are today mostly used synonymously we use both terms also in this tutorial.

• End stations are connected to bridges through ports and communication links.

End Station 1

End Station 2

End Station 3

End Station 4

Bridge A1

Bridge B1 Bridge B2

Communication LinkMulti-hop Communication Link

Port

CM

SM Synchronization Master

CompressionMaster

SC Synchronization Client

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Closer Look at an End Station* • PMC

–  PCI Mezzanine Card –  Peripheral Component Interconnect

• Data Link Layer –  OSI Layer 2 –  Media Access Control (MAC) –  e.g., IEEE 802.3 Ethernet

• Physical Layer –  OSI Layer 3 –  e.g., IEEE 802.3, 802.11, 802.15

Media Independent Interface (MII)

*TTTech’s TTEPMC Card

This is the area of this tutorial


Ethernet Frame Format

•  Some important aspects: –  Frames contain address information regarding their source and their destination. –  The destination address may be either unicast, multicast, or broadcast. –  The 802.1Q header is more prominently known as VLAN tag. –  The Payload is between 46 and 1500 octets.

•  We will not discuss Jumbo Frames in this tutorial (can discuss in Q/A).

–  Ethernet uses a 4 octets CRC called the Frame Check Sequence.

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NIC

SWITCH

NIC

NICNIC

NICSWITCH

NIC

NIC

NIC

NIC

SWITCH

NIC

NIC

X

X

Asynchronous Communication §  Transmission Points in Time are not predictable à Transmission Latency and Jitter accumulate à Number of Hops has a significant impact

Ethernet = Unsynchronized Communication

X

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Basic Operation • CSMA/CD (Carrier-Sense Multiple-Access / Collision Detection)

–  All end stations are connected to a physical bus (no bridges). –  In case multiple end stations start to transmit at about the same

point in time – the signals collide on the wire. –  End points realize this collision and send a jamming signal. –  Retry of transmission after random timeout.

• Switched Ethernet –  All end stations are connected to bridges. Bridges can be connected

to each other. –  Physical collisions cannot happen any more – but “logical collisions”

remain. –  Multiple end stations may send messages to the same receiver. –  As the bridge has limited frame buffer, this buffer may overflow and

frames may be lost.


Operation – Basic Switch

Best Effort

Basic Switch

3 4

2 1

7 8

6 5


Operation – Basic Switch

Best Effort

Basic Switch

Best-effort frame delivery (standard Ethernet traffic) is NOT guaranteed !

3 6 7 4

5 1 2

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Selection of Standards and Solutions

• IEEE 802.3: “Ethernet” • IEEE 802.1Q: “IEEE Standard for Local and metropolitan area networks--Media Access Control (MAC) Bridges and Virtual Bridged Local Area Networks”


Ethernet and Real-Time Communication


NIC

SWITCH

NIC

NICNIC

NICSWITCH

NIC

NIC

NIC

NIC

SWITCH

NIC

NIC

X

X

Asynchronous Communication §  Transmission Points in Time are not predictable à Transmission Latency and Jitter accumulate à Number of Hops has a significant impact

Ethernet = Unsynchronized Communication

X

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Priorities • Frames with a high priority can overtake frames with a lower priority.

Best Effort

Basic Switch + Priorities

. . .Prio High

Prio Low L1 L2 L3

H1 H2

Best Effort

Basic Switch + Priorities

. . .Prio High

Prio Low L3

H1 H2 L1 L2

Problems with priorities: •  High priority frames may “starve” low priority frames. •  Too many high priority frames:

à performance of high priority frames becomes insufficient.

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Traffic Shaping I: Credit-Based Shaping

frame frame

frame

frame

frame frame frame frame

Class A Queue

Queue with lower priority

Class A Queue transmit allowed

Class A Queue transmit

output port

low credit

high credit

Class A queued frames

t0 t1 t2 t3 t4 t5 t6

Class A credit

t7

t

idle slope

send slope

frame

frame

t8


Traffic Shaping I: Credit-Based Shaping

• Credit-based shaping is realized in the IEEE 802.1Q Audio/Video Bridging Standard.

• The aim is to guarantee 2ms network latency for SR Class A traffic over seven hops (=six bridges), considering several assumptions, e.g.,

–  100 Mbit/sec network –  SR Class A may be sent with a period of 125us –  Limited number of AVB streams

•  Sum of AVB traffic may not exceed 75% of the port transmit rate. •  75% of 125us = 93.75us •  Minimum Ethernet frame size is 6.72us à int(93.75us/6.72us) = 13 frames max. per port

• The credit-based shaper operates on one or many outgoing queues per port in the bridge.

• It guarantees “fairness” properties wrt. lower priority traffic than AVB traffic, i.e., it is guaranteed that bursts of AVB traffic will be interrupted and low priority non-AVB (standard Ethernet) traffic will be served.


Traffic Shaping I: Rate-Constrained Traffic

Switch/RouterRec

eiver

Sender

Rate-Constrained Traffic (RC)

min. duration min. duration min. duration


Traffic Shaping I: Rate-Constrained Traffic

• Rate-constrained traffic is implemented in ARINC 664-p7. • It operates on a per stream basis

–  in ARINC 664-p7 called Virtual Link (VL) • Strong scientific foundation of latency analysis and several implementations of tools.

–  e.g., network calculus, trajectory approach, response-time analysis • Latency is typically calculated as a function of:

–  Number, size, and rate of frames –  Network topology –  Switch model (e.g., switching delay)

• In the process of calculating the latency often the required buffer sizes in the bridges are derived. • à If done right, then it buffer overflows can be excluded and latencies can be guaranteed.


AFDX / ARINC 664 AFDX … Avionics Full Duplex Switched Ethernet •  Quality of Service

–  Bandwidth guarantee –  Transmission jitter and latency –  Bit Error Ratio (BER)

•  Weight •  Cost (development, deployment)

builds on ARINC 429, MIL-STD 1553

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AFDX Characteristics •  Serial data transfer •  Based on Ethernet IEEE802.3 •  10-100 Mbit/s •  Medium: copper or optic fiber •  Traffic control

–  Bandwidth guarantees for Virtual Links

•  Reliability –  Dual redundancy for each AFDX channel

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AFDX Network Architecture

•  two independent redundant networks •  at least 20 ports per switch

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Switch

Switch

End System

End System

End System

End System


AFDX System Components

•  Each port (ES, switch) consists of Rx and Tx port •  Cable contains two twisted-wire pairs

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AFDX End

System

Partition 1

AFDXSwitch

Controllers

Sensors

Actuators

Partition 2

Partition 3

Avionics Computer System AFDX Network

Avionics Subsystem


AFDX Communication Ports •  Communication ports

–  end points of communication –  Supported by OS API

•  Sampling Ports –  Buffer stores a single message –  New message overwrites buffer, non-consuming read

•  Queuing Ports –  Stores a up to a max. number of messages –  FIFO queue

•  Operations: send_msg(port_ID, msg), recv_msg(port_ID, msg)

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Virtual Link (VL) •  Defines logical communication link •  determines frame routing

–  Must originate at a single defined End System –  Delivers packets to a fixed set of End Systems –  Carries messages from one or more comm. ports

•  16-bit Virtual Link ID •  Uses Ethernet Destination Address field

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Constant Field: 32 bits Virtual Link ID 0000 0011 0000 0000 0000 0000 0000 0000 16-bit unsigned integer


Virtual Link Scheduling •  Traffic shaping by ES’s VL scheduler •  VL scheduler multiplexes all VLs of ES •  Bandwidth Allocation Gap (BAG)

–  Per VL –  Defines minimum gap between frames –  Range 1-128 ms, power of 2

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frame frame

BAG BAG

max. jitter max. jitter


Sub Virtual Links •  VLs regulate flow onto physical link •  Sub-VLs regulate flow into VL •  VL must be able to handle 4 Sub-VL queues •  Sub-VL queues are served in round-robin

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AFDX Frame Structure

or

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Preamble 7 bytes

IFG 12 bytes

MAC Dest 6 bytes

SFD 1 byte

MAC Src 6 bytes

Type IPv4 2 bytes

FCS 4 bytes

SN 4 bytes

IP Hdr 20 bytes

UDP Hdr 8 bytes

AFDX Payload up to 1471 bytes

Padding 0-16 bytes

Payload 1-17 bytes


Reliability Support •  Integrity Checking

–  Per network and VL –  Uses Sequence Numbers (SN) of messages –  Sender: consecutive SNs per VL, SN=0 on startup –  Receiver accepts:

•  SN = 0: reset •  SN = SN_old + 1 oder SN = SN_old + 2 •  Other frames are discarded

•  Redundancy Management –  Discard duplicates received from IC –  SkewMax determines duplicate-elimination interval

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AFDX Switch •  Switching function

–  Filtering and policing –  Only valid frames are forwarded to right ports –  Uses static configuration tables

•  Monitoring function –  Logs all operations and events –  Communicates with Network Management Function

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AFDX Frame Filtering Only valid frames are forwarded •  Valid VL identifier •  Use VL ID to forward to allowed destination ports •  FCS validity •  Ethernet frame size alignment •  Ethernet frame size range •  Adherence to MTU of VL

(MTU … maximum transfer unit, max. number of bytes transmitted in VL frame; Lmax)

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AFDX Traffic Policing Checks adherence to specified limits of bandwidth use •  Non-complying traffic is discarded •  Byte-based policing

–  Checks bandwidth use of VL in bits/s

•  Frame-based policing –  Checks use of VL in frames/s

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Documents

detnet02 unsync networks - Institute of Computer ... · • Sum of AVB traffic may not exceed 75% of the port ... – SkewMax determines duplicate-elimination interval 27. Peter