Unsynchronized Networks Peter Puschner, Institut fr Technische - - PowerPoint PPT Presentation

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 Link Multi-hop Communication Link Port

CM SM

Synchronization Master Compression Master

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

Peter Puschner, TU Wien

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 NIC NIC NIC SWITCH NIC NIC NIC NIC SWITCH NIC NIC

X X

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

Ethernet = Unsynchronized Communication

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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.

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Operation – Basic Switch

Best Effort Basic Switch

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Operation – Basic Switch

Best Effort Basic Switch

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

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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”

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Ethernet and Real-Time Communication

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NIC SWITCH NIC NIC NIC NIC SWITCH NIC NIC NIC NIC SWITCH NIC NIC

X X

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

Ethernet = Unsynchronized Communication

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Priorities

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

Best Effort Basic Switch + Priorities

. . .

L1 L2 L3 H1 H2

Best Effort Basic Switch + Priorities

. . .

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

• utput 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

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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.

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Traffic Shaping I: Rate-Constrained Traffic

Switch/Router Receiver Sender

Rate-Constrained Traffic (RC)

• min. duration
• min. duration
• min. duration

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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
• f 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.

Peter Puschner, TU Wien

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

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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

AFDX Switch Controllers Sensors Actuators

Partition 2 Partition 3

Avionics Computer System AFDX Network Avionics Subsystem

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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

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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

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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

• r

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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

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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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