DOCSLIB.ORG

Rethinking Acks at the Transport Layer

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Rethinking Acks at the Transport Layer Rethinking ACKs at the Transport Layer Ana Custura Tom Jones Gorry Fairhurst University of Aberdeen University of Aberdeen University of Aberdeen Abstract—Acknowledgements are a core component of trans- ACK Use-case TCP SCTP DCCP RTCP QUIC port protocols. They can send control information and be used Connection es- x x x x to receive feedback about transmission progress, to measure tablishment responsiveness and capacity of the network path, and numerous Param. Reneg. x x x other purposes. This paper takes a long look at the way acknowledgements are used across different transports and what Path estimation x x x x x ACK information is actually needed in new protocol designs. Loss detection x x x x x TCP’s use of ACKs has inspired the design of other Internet ACK Vector x x x transports and was used as a model for QUIC. CC x x x ? x Index Terms—ACK, transport protocols ACK-clocking x x Pacing ? ? x x x I. INTRODUCTION TABLE I: A summary of ACK functions by transport protocol. The design of any Internet transport protocol needs to operate safely over a wide range of path characteristics and be robust to changes in the set of devices forming the network path. Most transports rely on feedback from the remote This coordinates protocol state at endpoints (e.g., TCP SYN endpoint to inform the local endpoint about the success of options; SCTP INIT; DCCP feature negotiation). transmission across the path. Acknowledgments (ACKs) are ACKs have a role in estimating path characteristics. After often sent as separate packets. They can have any size, but an endpoint has started communicating, ACKs can be used to are typically much smaller than data packets. Some transports adjust the Round Trip Time (RTT) to track potential change therefore allow the ACK information to be coalesced with in path characteristics [8]. other information sent by a remote endpoint. Many transports Most reliable transports trigger loss recovery after ACKs send data in one direction and ACKs in the opposite direction. fail to confirm delivery by reception. Loss could be detected Many transports allow ACKs to be observed in the network. by observing the time-ordering of received ACKs (as in TCP Some devices intentionally reduce the volume/rate of ACKs DupACK, RFC 5681) or by utitilising a timer [8]. Care is to improve performance for asymmetric paths [9]. This can needed to avoid interpreting ACK loss (e.g. under persistent lead to network device ossification when connectivity becomes congestion) or ACK reordering, as as a signal of data loss. reliant on seeing ACKs. In response, protocols, such as Internet flows need to implement safeguards to avoid in- QUIC [14] have been designed to encrypt and authenticate appropriate impact on other flows that share resources along their ACKs. This prevents in-network modification, and moti- a path. Reception of ACKs indicate data has successfully vates our examination of how this will impact performance. traversed the network. Congestion control (CC) algorithms can The remainder of this paper explores how ACKs are used by estimate a safe transmission rate from the rate of reception Internet transport protocols. It examines the usage of ACKs in of ACKs, or the volume of data acknowledged per RTT. In QUIC and suggests the present design will face performance general, CC benefits from frequent feedback of ACKs. limits when used over asymmetric paths. It concludes with To promote more fair sharing of capacity, senders can limit recommendations for the design of an appropriate ACK policy bursts in transmission (e.g. SCTP, RFC 4960). ACKs can help for future transports such as QUIC. to pace the forward transmission. This could be explicit (e.g. II. FUNCTIONS OF ACKNOWLEDGMENTS each ACK releases new data) or implicit (i.e., ACKs drive a rate limiter or burst-mitigation method). This needs at least Although the most familiar use-case of ACKs is to perform multiple updates per RTT, but can benefit from additional retransmission or repair, the feedback of ACKs provides a understanding the timing of data received. range of important transport functions [8]. ACKs are used to complete setup of a new flow by determin- ing whether the remote endpoint accepts the communication. III. ACK POLICIES OF COMMON TRANSPORT PROTOCOLS This ensures both endpoints associate packets with the flow and understand how to process packets. Many transports sup- This section examines how ACKs are used in different port negotiation, allowing endpoints to agree (acknowledge) transports. Table I enumerates the protocols examined and a set of features and parameters to configure the transport. identifies relevant ACK functions. Annex to ISBN 978-3-903176-28-7© 2020 IFIP 731 A. Transmission Control Protocol (TCP) individual received packets. Like TCP, new connections start TCP is the most popular transport protocol and currently with AR 1:2. A DCCP sender can adjust transport parameters carries the majority of web and streaming video traffic. It during a connection, using DCCP feature negotiation [17]. uses a sliding window to provide reliable data reception using This allows a sender to change the AR. a cumulative ACK. This has influenced the design of later CCID2 (RFC 4341) defines an approximately TCP-friendly protocols. In TCP, ACK packets are used to perform all method that can adapt the AR, proving it does not exceed functions detailed in Section II. cwnd/2 and must be >= 2 for a cwnd of 4 or more packets. We define the ACK Ratio (AR) as the number of packets For each cwnd of data with l lost or marked ACK, the AR 2 received to the number of ACKs sent. Early versions of TCP is doubled; and is decremented for each cwnd =(AR − AR) sent an ACK for each received data packet, resulting in AR consecutive cwnds of data with no ACK loss or marking. This 1:1. While this provides ample information, it contributes 50% does not adapt to path delays, such as queues building; and of packets sent. A TCP receiver can delay sending an ACK for is unable to account for the cost of link ACK transmission. up to two times the Maximum Segment Size of data [2] (AR An AR more than 1:2 would increase burst sizes, unless the 1:2). RFC 5681 requires Delayed ACKs to be sent within 500 sender performs pacing (RFC 4341). ms of the arrival of a packet. However, many current receivers use a smaller delay, e.g. 200ms or 40ms. D. The Real Time Protocol (RTP) A TCP CC establishing the path capacity grows the con- gestion control (CC) window cwnd [2] exponentially during RTP (RFC 3550) is a connection-less transport for real slow-start, where each received ACK increases the cwnd and time media that is often sent at a target rate. RTP Control the sending rate is controlled by the ACK rate. Delaying ACKs Protocol (RTCP) provides control and signals statistics (e.g., can reduce the rate of cwnd growth. loss rate and delay variance) and can be used to provide Since Appropriate Byte Counting (RFC 3465), CC operates loss recovery/repair and CC. Reports can be encrypted. RTCP on the cumulative acknowledged bytes, not on individual reports are often regularly sent, e.g. a randomised default of ACKs. Delayed ACKs increase the time to reach the capacity five seconds. To reduce ACK frequency, a single packet can where the delay is significant compared to the RTT. For this carry multiple RTCP messages. The RTCP extended report reason, Delayed ACKs After Slow Start (DAASS) [1] can (XR) packet (RFC 3611) supports an ACK vector for less revert to an AR of 1:1 during slow start. However, a TCP frequent feedback (e.g., once per RTT, or each 50-200ms). receiver does not in general know which CC is used, so cannot know when slow start has finished. There is therefore E. QUIC no standard algorithm for implementing DAASS. The Linux QUIC is a new connection-oriented protocol being specified kernel implements a form of DAASS (TCP QUICKACK) by the IETF [14]. Packets have monotonically increasing num- using AR 1:1 for at least the first 8 packets1. bers to eliminate ACK ambiguity, and provides more precise ACK Delay is suspended during TCP loss recovery. Out RTT estimation [15]. Successful transmission is tracked by of sequence packets are reported using the selective acknowl- feedback of cumulative ACK frames. In contrast to TCP and edgement (SACK) option (RFC 2018). An ACK carries up to DCCP, QUIC encrypts all header fields including ACKs. 3 SACK blocks, each describing gaps in the received window. IETF QUIC [14] currently recommends sending an ACK SACK improves efficiency after loss, but increases the volume for each alternate ACK-eliciting packet and implementing of ACKs while a receiver is awaiting retransmission. ACK delay. This mimics the recommended policy for TCP B. The Stream Control Transport Protocol (SCTP) (AR 1:2). The default delay is 25 milliseconds (the value SCTP (RFC 4960) offers connection-oriented multi- used by emerging implementations such as Chromium and streaming, and supports a number of modes, from reliable Quicly). A QUIC receiver shares the ACK delay with the streams like TCP, to partial reliability. Although based on sender to improve the precision of RTT calculations. QUIC datagrams, methods resemble TCP. ACKs confirm connection employs a CC similar to TCP, but adapted to QUIC [12]. establishment and shutdown, and are used to provide relia- Early specifications for QUIC allowed continuing to sending bility. The SCTP initialisation (and INIT ACK) can be larger an ACK Frame for every subsequently received packet for up than for TCP. SCTP follows TCP guidelines on delayed ACKs, to 1/8 of an RTT after reordering.
Load more

Recommended publications
  • The Transport Layer: Tutorial and Survey SAMI IREN and PAUL D
    The Transport Layer: Tutorial and Survey SAMI IREN and PAUL D. AMER University of Delaware AND PHILLIP T. CONRAD Temple University Transport layer protocols provide for end-to-end communication between two or more hosts. This paper presents a tutorial on transport layer concepts and terminology, and a survey of transport layer services and protocols. The transport layer protocol TCP is used as a reference point, and compared and contrasted with nineteen other protocols designed over the past two decades. The service and protocol features of twelve of the most important protocols are summarized in both text and tables. Categories and Subject Descriptors: C.2.0 [Computer-Communication Networks]: General—Data communications; Open System Interconnection Reference Model (OSI); C.2.1 [Computer-Communication Networks]: Network Architecture and Design—Network communications; Packet-switching networks; Store and forward networks; C.2.2 [Computer-Communication Networks]: Network Protocols; Protocol architecture (OSI model); C.2.5 [Computer- Communication Networks]: Local and Wide-Area Networks General Terms: Networks Additional Key Words and Phrases: Congestion control, flow control, transport protocol, transport service, TCP/IP 1. INTRODUCTION work of routers, bridges, and communi- cation links that moves information be- In the OSI 7-layer Reference Model, the tween hosts. A good transport layer transport layer is the lowest layer that service (or simply, transport service) al- operates on an end-to-end basis be- lows applications to use a standard set tween two or more communicating of primitives and run on a variety of hosts. This layer lies at the boundary networks without worrying about differ- between these hosts and an internet- ent network interfaces and reliabilities.
    [Show full text]
  • Solutions to Chapter 2
    CS413 Computer Networks ASN 4 Solutions Solutions to Assignment #4 3. What difference does it make to the network layer if the underlying data link layer provides a connection-oriented service versus a connectionless service? [4 marks] Solution: If the data link layer provides a connection-oriented service to the network layer, then the network layer must precede all transfer of information with a connection setup procedure (2). If the connection-oriented service includes assurances that frames of information are transferred correctly and in sequence by the data link layer, the network layer can then assume that the packets it sends to its neighbor traverse an error-free pipe. On the other hand, if the data link layer is connectionless, then each frame is sent independently through the data link, probably in unconfirmed manner (without acknowledgments or retransmissions). In this case the network layer cannot make assumptions about the sequencing or correctness of the packets it exchanges with its neighbors (2). The Ethernet local area network provides an example of connectionless transfer of data link frames. The transfer of frames using "Type 2" service in Logical Link Control (discussed in Chapter 6) provides a connection-oriented data link control example. 4. Suppose transmission channels become virtually error-free. Is the data link layer still needed? [2 marks – 1 for the answer and 1 for explanation] Solution: The data link layer is still needed(1) for framing the data and for flow control over the transmission channel. In a multiple access medium such as a LAN, the data link layer is required to coordinate access to the shared medium among the multiple users (1).
    [Show full text]
  • Is QUIC a Better Choice Than TCP in the 5G Core Network Service Based Architecture?
    DEGREE PROJECT IN INFORMATION AND COMMUNICATION TECHNOLOGY, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020 Is QUIC a Better Choice than TCP in the 5G Core Network Service Based Architecture? PETHRUS GÄRDBORN KTH ROYAL INSTITUTE OF TECHNOLOGY SCHOOL OF ELECTRICAL ENGINEERING AND COMPUTER SCIENCE Is QUIC a Better Choice than TCP in the 5G Core Network Service Based Architecture? PETHRUS GÄRDBORN Master in Communication Systems Date: November 22, 2020 Supervisor at KTH: Marco Chiesa Supervisor at Ericsson: Zaheduzzaman Sarker Examiner: Peter Sjödin School of Electrical Engineering and Computer Science Host company: Ericsson AB Swedish title: Är QUIC ett bättre val än TCP i 5G Core Network Service Based Architecture? iii Abstract The development of the 5G Cellular Network required a new 5G Core Network and has put higher requirements on its protocol stack. For decades, TCP has been the transport protocol of choice on the Internet. In recent years, major Internet players such as Google, Facebook and CloudFlare have opted to use the new QUIC transport protocol. The design assumptions of the Internet (best-effort delivery) differs from those of the Core Network. The aim of this study is to investigate whether QUIC’s benefits on the Internet will translate to the 5G Core Network Service Based Architecture. A testbed was set up to emulate traffic patterns between Network Functions. The results show that QUIC reduces average request latency to half of that of TCP, for a majority of cases, and doubles the throughput even under optimal network conditions with no packet loss and low (20 ms) RTT. Additionally, by measuring request start and end times “on the wire”, without taking into account QUIC’s shorter connection establishment, we believe the results indicate QUIC’s suitability also under the long-lived (standing) connection model.
    [Show full text]
  • Medium Access Control Layer
    Telematics Chapter 5: Medium Access Control Sublayer User Server watching with video Beispielbildvideo clip clips Application Layer Application Layer Presentation Layer Presentation Layer Session Layer Session Layer Transport Layer Transport Layer Network Layer Network Layer Network Layer Univ.-Prof. Dr.-Ing. Jochen H. Schiller Data Link Layer Data Link Layer Data Link Layer Computer Systems and Telematics (CST) Physical Layer Physical Layer Physical Layer Institute of Computer Science Freie Universität Berlin http://cst.mi.fu-berlin.de Contents ● Design Issues ● Metropolitan Area Networks ● Network Topologies (MAN) ● The Channel Allocation Problem ● Wide Area Networks (WAN) ● Multiple Access Protocols ● Frame Relay (historical) ● Ethernet ● ATM ● IEEE 802.2 – Logical Link Control ● SDH ● Token Bus (historical) ● Network Infrastructure ● Token Ring (historical) ● Virtual LANs ● Fiber Distributed Data Interface ● Structured Cabling Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer 5.2 Design Issues Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer 5.3 Design Issues ● Two kinds of connections in networks ● Point-to-point connections OSI Reference Model ● Broadcast (Multi-access channel, Application Layer Random access channel) Presentation Layer ● In a network with broadcast Session Layer connections ● Who gets the channel? Transport Layer Network Layer ● Protocols used to determine who gets next access to the channel Data Link Layer ● Medium Access Control (MAC) sublayer Physical Layer Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer 5.4 Network Types for the Local Range ● LLC layer: uniform interface and same frame format to upper layers ● MAC layer: defines medium access ..
    [Show full text]
  • Chapter 3 Transport Layer
    Chapter 3 Transport Layer A note on the use of these Powerpoint slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: Computer § If you use these slides (e.g., in a class) that you mention their source (after all, we’d like people to use our book!) Networking: A Top § If you post any slides on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this Down Approach material. 7th edition Thanks and enjoy! JFK/KWR Jim Kurose, Keith Ross All material copyright 1996-2016 Pearson/Addison Wesley J.F Kurose and K.W. Ross, All Rights Reserved April 2016 Transport Layer 2-1 Chapter 3: Transport Layer our goals: § understand principles § learn about Internet behind transport transport layer protocols: layer services: • UDP: connectionless • multiplexing, transport demultiplexing • TCP: connection-oriented • reliable data transfer reliable transport • flow control • TCP congestion control • congestion control Transport Layer 3-2 Chapter 3 outline 3.1 transport-layer 3.5 connection-oriented services transport: TCP 3.2 multiplexing and • segment structure demultiplexing • reliable data transfer 3.3 connectionless • flow control transport: UDP • connection management 3.4 principles of reliable 3.6 principles
    [Show full text]
  • Guidelines for the Secure Deployment of Ipv6
    Special Publication 800-119 Guidelines for the Secure Deployment of IPv6 Recommendations of the National Institute of Standards and Technology Sheila Frankel Richard Graveman John Pearce Mark Rooks NIST Special Publication 800-119 Guidelines for the Secure Deployment of IPv6 Recommendations of the National Institute of Standards and Technology Sheila Frankel Richard Graveman John Pearce Mark Rooks C O M P U T E R S E C U R I T Y Computer Security Division Information Technology Laboratory National Institute of Standards and Technology Gaithersburg, MD 20899-8930 December 2010 U.S. Department of Commerce Gary Locke, Secretary National Institute of Standards and Technology Dr. Patrick D. Gallagher, Director GUIDELINES FOR THE SECURE DEPLOYMENT OF IPV6 Reports on Computer Systems Technology The Information Technology Laboratory (ITL) at the National Institute of Standards and Technology (NIST) promotes the U.S. economy and public welfare by providing technical leadership for the nation’s measurement and standards infrastructure. ITL develops tests, test methods, reference data, proof of concept implementations, and technical analysis to advance the development and productive use of information technology. ITL’s responsibilities include the development of technical, physical, administrative, and management standards and guidelines for the cost-effective security and privacy of sensitive unclassified information in Federal computer systems. This Special Publication 800-series reports on ITL’s research, guidance, and outreach efforts in computer security and its collaborative activities with industry, government, and academic organizations. National Institute of Standards and Technology Special Publication 800-119 Natl. Inst. Stand. Technol. Spec. Publ. 800-119, 188 pages (Dec. 2010) Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately.
    [Show full text]
  • QUIC Record Layer
    A Security Model and Fully Verified Implementation for the IETF QUIC Record Layer Antoine Delignat-Lavaud∗, Cédric Fournet∗, Bryan Parnoy, Jonathan Protzenko∗, Tahina Ramananandro∗, Jay Bosamiyay, Joseph Lallemandz, Itsaka Rakotonirinaz, Yi Zhouy ∗Microsoft Research yCarnegie Mellon University zINRIA Nancy Grand-Est, LORIA Abstract—Drawing on earlier protocol-verification work, we investigate the security of the QUIC record layer, as standardized Application Application by the IETF in draft version 30. This version features major HTTP/2 HTTP/3 differences compared to Google’s original protocol and early IETF drafts. It serves as a useful test case for our verification TLS QUIC methodology and toolchain, while also, hopefully, drawing atten- tion to a little studied yet crucially important emerging standard. TCP UDP We model QUIC packet and header encryption, which uses IP IP a custom construction for privacy. To capture its goals, we propose a security definition for authenticated encryption with Fig. 1: Modularity of current networking stack vs. QUIC semi-implicit nonces. We show that QUIC uses an instance of a generic construction parameterized by a standard AEAD-secure scheme and a PRF-secure cipher. We formalize and verify the it is possible to combine both features in a single message, security of this construction in F?. The proof uncovers interesting saving a full network round-trip. limitations of nonce confidentiality, due to the malleability of short From a security standpoint, a fully-integrated secure trans- headers and the ability to choose the number of least significant port protocol offers the potential for a single, clean security bits included in the packet counter.
    [Show full text]
  • Congestion Control Tuning of the QUIC Transport Layer Protocol Spring 2018
    Congestion Control Tuning of the QUIC Transport Layer Protocol Spring 2018 Wendi Qu Director: Llorenç Cerdà-Alabern Departament d'Arquitectura de Computadors Degree: Bachelor Specialization: Information Technologies Facultat d’Informatica de Barcelona (FIB) Universitat Politecnica de Catalunya (UPC) - BarcelonaTech April 2018 UNIVERSITAT POLITÈCNICA DE CATALUNYA (UPC) Abstract The QUIC protocol is a new type of reliable transmission protocol based on UDP. Its establishment is mainly to solve the problem of network delay. It is efficient, fast, and takes up less resources. The QUIC gathers the advantages of both TCP and UDP. The first part of this thesis studies the development background of the QUIC protocol in terms of characteristics and perspectives of what they can do and how they work. Because it adds the congestion control algorithm used by TCP based on the UDP protocol, we have conducted further research and analysis of the Cubic algorithm to investigate the impact of its parameters on the behavior. The second part includes performance and fairness tests for QUIC and TCP implementations. The simulation framework Mininet is used to perform these tests using controlled network properties. In this process we verified the reliability of the mininet. This work shows how Mininet builds a test system to analyze the implementation of the transport protocol. QUIC's tests show that the performance of QUIC has improved, and the test of fairness have identified specific areas that may require further analysis. In the third part, we test the influence of the parameter on the behavior of the algorithm in the congestion control algorithm. We present an initial experimental evaluation of the newly proposed Cubic-TCP algorithm.
    [Show full text]
  • Computer Network Transport Layer.Pdf
    The Transport Layer Chapter 6 Transport Layer • It is the heart of the whole protocol hierarchy. • Its task is to provide reliable, cost-effective data transport from the source machine to the destination machine, independently of the physical network or networks currently in use. • It provides service to the application layer. • Transport layer makes use of the services provided by the network layer. • The hardware and/or software within the transport layer that does the work is called the transport entity. • It provides both connectionless and connection oriented service. • Connections have three phases: establishment, data transfer, and release. Services Provided to the Upper Layers The relationship of network, transport, and application layers Transport Service Primitives The primitives for a simple transport service Transport Service Primitives • To see how these primitives might be used, consider an application with a server and a number of remote clients. To start with, the server executes a LISTEN primitive, typically by calling a library procedure that makes a system call to block the server until a client turns up. When a client wants to talk to the server, it executes a CONNECT primitive. The transport entity carries out this primitive by blocking the caller and sending a packet to the server. Encapsulated in the payload of this packet is a transport layer message for the server's transport entity. Transport Service Primitives Nesting of TPDUs, packets, and frames. Elements of Transport Protocols • Addressing • Connection establishment • Connection release • Error control and flow control • Multiplexing • Crash recovery Addressing How a user process in host 1 establishes a connection with a mail server in host 2 via a process server.
    [Show full text]
  • Medium Access Control Sublayer
    Telematics Chapter 5: Medium Access Control Sublayer User Server watching with video Beispielbildvideo clip clips Application Layer Application Layer Presentation Layer Presentation Layer Session Layer Session Layer Transport Layer Transport Layer Network Layer Network Layer Network Layer Prof. Dr. Mesut Güneş Data Link Layer Data Link Layer Data Link Layer Computer Systems and Telematics (CST) Physical Layer Physical Layer Physical Layer Distributed, embedded Systems Institute of Computer Science Freie Universität Berlin http://cst.mi.fu-berlin.de Contents ● Design Issues ● Metropolitan Area Networks ● Network Topologies (()MAN) ● The Channel Allocation Problem ● Wide Area Networks (WAN) ● Multiple Access Protocols ● Frame Relay ● Ethernet ● ATM ● IEEE 802.2 – Logical Link Control ● SDH ● Token Bus ● Network Infrastructure ● Token Ring ● Virtual LANs ● Fiber Distributed Data Interface ● Structured Cabling Prof. Dr. Mesut Güneş ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer 5.2 Design Issues Prof. Dr. Mesut Güneş ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer 5.3 Design Issues ● Two kinds of connections in networks ● Point-to-point connections OSI Reference Model ● Broadcast (Multi-access channel, Application Layer Random access channel) Presentation Layer ● In a network with broadcast Session Layer connections ● Who gets the channel? Transport Layer Network Layer ● PtProtoco ls use dtdtd to determ ine w ho gets next access to the channel Data Link Layer ● Medium Access Control (()MAC) sublay er Phy sical Laye r Prof. Dr. Mesut Güneş ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer 5.4 Network Types for the Local Rang e ● LLC layer: uniform interface and same frame format to upper layers ● MAC layer: defines medium access - LLC IEEE 802.2 Logical Link Control ..
    [Show full text]
  • TS 102 636-3 V1.1.1 (2010-03) Technical Specification
    ETSI TS 102 636-3 V1.1.1 (2010-03) Technical Specification Intelligent Transport Systems (ITS); Vehicular Communications; GeoNetworking; Part 3: Network architecture 2 ETSI TS 102 636-3 V1.1.1 (2010-03) Reference DTS/ITS-0030004 Keywords addressing, ITS, network, point-to-multipoint, point-to-point, protocol ETSI 650 Route des Lucioles F-06921 Sophia Antipolis Cedex - FRANCE Tel.: +33 4 92 94 42 00 Fax: +33 4 93 65 47 16 Siret N° 348 623 562 00017 - NAF 742 C Association à but non lucratif enregistrée à la Sous-Préfecture de Grasse (06) N° 7803/88 Important notice Individual copies of the present document can be downloaded from: http://www.etsi.org The present document may be made available in more than one electronic version or in print. In any case of existing or perceived difference in contents between such versions, the reference version is the Portable Document Format (PDF). In case of dispute, the reference shall be the printing on ETSI printers of the PDF version kept on a specific network drive within ETSI Secretariat. Users of the present document should be aware that the document may be subject to revision or change of status. Information on the current status of this and other ETSI documents is available at http://portal.etsi.org/tb/status/status.asp If you find errors in the present document, please send your comment to one of the following services: http://portal.etsi.org/chaircor/ETSI_support.asp Copyright Notification No part may be reproduced except as authorized by written permission. The copyright and the foregoing restriction extend to reproduction in all media.
    [Show full text]
  • The Network Layer
    TheThe NetworkNetwork LayerLayer • Concerned with getting packets from the source all the way to the destination: Routing through the subnet, load balancing, congestion control. • Protocol Data Unit (PDU) for network layer protocols = packet • Types of network services to the transport layer: – Connectionless: Each packet carries full destination address. – Connection-oriented: • Connection is set up between network layer processes on the sending and receiving sides. • The connection is given a special identifier until all data has been sent. • Internal organization of the network layer (in the subnet): – Datagram: Packets are sent and routed independently with each carrying the full destination address (TCP/IP) – Virtual circuit: A virtual circuit is set up to the destination using a circuit number stored in tables in routers along the way. Packets only carry the virtual circuit number. All packets follow the same route (ATM). EECC694 - Shaaban #1 Final Review Spring2000 5-11-2000 RoutingRouting AlgorithmsAlgorithms To decide which output line an incoming packet should be transmitted on. • Static Routing (Nonadaptive algorithms): – Shortest path routing: • Build a graph of the subnet with each node representing a router and each arc representing a communication link. • The weight on the arcs represents: a function of distance, bandwidth, communication costs mean queue length and other performance factors. • Several algorithms exist including Dijkstra’s shortest path algorithm. – Selective flooding: Send the packet on all output lines going in the right direction to the destination. – Flow-based routing: Based on known capacity and link loads. EECC694 - Shaaban #2 Final Review Spring2000 5-11-2000 StaticStatic Routing:Routing: ShortestShortest PathPath RoutingRouting • First five steps of an example using Dijktra’s algorithm EECC694 - Shaaban #3 Final Review Spring2000 5-11-2000 StaticStatic Routing:Routing: Flow-BasedFlow-Based RoutingRouting • A routing matrix is constructed; used when the mean data flow in network links is known and stable.
    [Show full text]