Automatic Repeat Request and the Data Link Layer

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(Mäkelänkatu 91) 00601 Helsinki, Finland tel. +358 9 478 1011 fax. +358 9 478 1030

Automatic Repeat Request and

the Data Link Layer

Reliable Communications with Retransmission

• End to End

— Eg. TCP

• Hop to Hop

— Eg. SMTP, X.25, HDLC

• ARQ

— Automatic Repeat Request

— A technique, abstract concept

— Used in many protocols for reliable transmission

Basic ARQ

• Data (SDUs) is divided/packaged to packets (PDUs) that contain a header and checksum, these are called information frames

• There are also empty packets called control frames

• And there is a timeout mechanism

• Problem: what if a frame is received and acknowledged after the timeout at sender’s end?

Sender Packets in transit

packet 1

packet gets lost waiting for

acknowledgment retransmission

Receiver

packet arrives acknowledgment sent

ARQ Control Frames

• ACK, acknowledgment

• NAK, negative acknowledgment

• ENQ, enquiry

ARQ Sequence Numbers

• It is possible for the sender and receiver to get out of syncronization

— A problem that all protocols must address

• Sender and receiver can be synchronized by having a sequence number in each frame

• In theory one bit sequence number would be sufficient for stop-and-wait ARQ

— Stop-and-wait means that only one frame is in transmission at one time

— One bit sequence number is not sufficient if the network may duplicate frames

• Larger sequence numbers allow multiple frames to be in transit

ARQ Stop-and-Wait Frame Loss

• Information frames are ACKed, control frames not

• When a frame is lost:

• 1) Sender retransmits after timeout or

• 2) ENQ is replied with the last frame sent

— Sender sends ENQ after timeout

— Reciever sends last ACK

Go-Back-N ARQ Frame Loss

• A sufficiently large sequence number and a sliding window are used

• Receiver ACKs only frames in sequence

• When an information frame is lost, it and all frames sent after it must be retransmitted

— Frame loss is recognized either from timeout or the receiver sends a NAK when it receives a frame out of sequence

— The receiver requires a buffer the size of one frame

• If the ACK control frame is lost, a later ACK can replace it

• This increases the efficiency of bandwidth usage compared to stop-and-wait ARQ

Selective Repeat ARQ

• When the receiver recieves a frame out of secuence, it sends a NAK for the missing frame and that frame only is resent

— More complex for the receiver, requires a larger receive buffer

• This is more efficient for channels with large error rates than Go-Back-N ARQ

The Data Link World

• Traditionally the data communications world has been built on

— Local Area Networks

> Ethernet, Token Ring, ATM over physcal media

— Point to point connections between LANs

> HDLC, PPP, SLIP, ATM, Frame Relay, ISDN over telecoms infrastructure

• Also

— Campus networks

> FDDI, HIPPI, ATM, optical rings

— Mobile wireless networks

> GPRS, UMTS, WLAN

— Plenty of other solutions

> Cable modems, Bluetooth

HDLC

• High-level Data Link Control

• Data link layer protocol

• Point-to-point and point-to-multipoint connections

— Unbalanced (master/slave) point-to-point and multipoint

— Balanced point-to-point (each side has dual roles)

• Encapsulates network layer packets (SDUs) to frames (PDUs)

• Provides both connnection oriented and connectionless service

HDLC Frame Format

• Flag is 01111110

— Bit stuffing is used in other data, every 11111 is followed by 0

> This is bit-limited protocol, not byte limited

• Adress is receiver adress

• Control is 8 bits, 16 in extended mode for longer sequences

• Information is paylod data

• CRC is the checksum in ITU-CRC, 16 or 32 bits

Flag Address Control Information (data) CRC Flag

HDLC Control Byte for Information Frame

• Starts with bit 0

• Secuence numbers can be extended with an additional byte to 7 bits from 3

— Receive ACKs may be sent piggypacked in information frames or in control frames

• Poll/Final bit is used for master/slave communications

— In unbalanced mode the secondaries (slaves) may not send without receiving first a frame with the P/F bit on form the primary (master)

1 2-4 5 6-8

0 Send sequence Poll/Final Receive Sequence

HDLC Control Byte for Supervisory Frame

• Starts with bits 10

• Control 00 is RR, Receive Ready (ACK)

• Control 01 is REJ, Reject (NAK)

• Control 10 is RNR, Receive Not Ready

— Sent for eg. buffers full, used for flow control

• Control 11 is SREJ, Selective Reject

— Used to request retransmission of a single frame

1-2 3-4 5 6-8

10 Control field Poll/Final Receive Sequence

HDLC Control Byte for Unnumbered Frame

• Starts with bits 10

• Used to set up connections and master/slave relationships

1-2 3-4 5 6-8

11 Message Poll/Final Message

ATM, Asynchronous Transfer Mode

• Standardized by the ATM Forum, an industry consortium

• When introduced, a competitor for Internet Protocol

> ATM to the desktop

• Currently mostly used as flexible telecoms infrastructure

— Customer connections can be configured immediately from a central control location

— Enables allocation and control of bandwidth (not at the ATM level but at the endpoints)

• ATM network consists of ATM switches that are connected to other switches and ATM nodes using point to point connections

— Different from traditional LAN topologies

How the ATM works

• Two entities want to communicate

• A route over the ATM network is created

— The routing is a separate function from data transmission (switching)

— Currently often done by hand (permanent virtual circuit)

— PNNI protocol exists for creating routes (Private Network to Network Interface)

• The route is named using using a VPI/VCI pair and configured in each switch of the ATM network

— Virtual Path Identifier

— Virtual Channel Identifier

How the ATM works (cont.)

• The ATM packet, called a cell, is exactly 53 bytes long

• The header is 5 bytes long and contains the

— VPI/VCI address (VPI is 8 or 12 bits, VCI is 16 bits)

— Payload type (3 bits)

— Priority (1 bit)

— Header checksum (8 bits)

• The header contains all needed information for the switches to transmit the packet

• The rest of 48 bytes is the payload (data)

— Now how do we use the 48 bytes?

ATM Addressing

• The VPI/VCI pair need to be unique only in each switch along the path

— It may be changed along the switching path

• 20 octet (byte) ATM addresses exist

— 13 bytes of prefix (network address)

— 6 byte ID (can be Ethernet address)

• Other 20 byte addressing schemes exist also

• These are not really used very much

AAL, ATM Adaptation Layer

• The different AAL layers provide ATM services to higher level protocols

• AAL1 provides a constant rate bitstream

— No error detection, missing cells are reported

— Suits for audio and video

• AAL2 was intended for packet data, but is dead

• AAL3/4 is a connection oriented or connectionless stream or packet data service with or without reliability

— Supports multiplexing several connections/packets over a single VPI/VCI

• AAL5 is a more simple and efficient version of the AAL3/4

— The AAL usually selected for IP traffic

The Ethernet

• IEEE 802.3 standard

• A limited distance LAN protocol and cabling standard

• Several physical cabling and bandwidth options

— Coaxial cable 10Base5 and 10Base2, mostly historical

> All nodes connect to the same coax

— Twisted pair 10BaseT, 100BaseT

> Nodes are connected using a hub or switch

— Also optical and wireless Ethernet and gigabit speeds

How the Ethernet Uses the Shared Media?

• 1-persistent CSMA/CD

— Carrier Sense Multiple Access / Collision Detection

• Each node waits for a free moment to send

• If two or more send simultaneously they detect the collision and each stops sending and waits a random interval

• All nodes listen to all traffic and pick their own as directed by the MAC address

Ethernet Addressing

• Each network card has a unique hardware address (MAC)

— In some hardware can be changed with software

— The MAC address usually has a manufacturer ID part and an unique part

— Duplicate MAC addresses in the same LAN happen, but extremely rarely

• Ethernet frames start with a 48-bit destination address followed by a 48-bit source address

• Hosts can send unicasts or broadcasts

— The broadcast is used to find who else is on the same LAN

— Or to find who has a certain IP address on this LAN (ARP)

Hubs and switches

• Twisted pair Ethernet hosts use a physical star configuration

• A hub repeats all traffic to all stations

• A switch learns which MAC addresses are in which physical ports and transmit only necessary traffic

— Security point: switches are harder to eavesdrop, but can usually be made to enter a fail- safe mode, where they act as a hub

• Switches provide better performance than hubs

• Both may be chained within certain limits

Extending Ethernet

• The physical reach of the Ethernet can be extended using repeaters, bridges and other devices

• However having a large (many hosts) Ethernet is very vulnerable to problems

— Broadcast storms can cripple the whole network (as happened to FUNET once)

• There is a reason for the higher level protocols like IP

Lan topologies

• Lans can be

— Buses

— Stars

— Rings

— Other

> A switch with connections to local hubs, a star of stars

— Wireless

> Raises interesting problems, A can see B, B can see C, A can not see C...

LAN cabling

• Expensive, difficult to change afterwards

— The reason for 10BaseT was the ability to use existing telephone cabling

• Since the analog signals are RF, the quality of cabling and connectors is very important

— CAT 5 minimum

• The physical routing of the cabling creates security considerations

• The physical maximum length of a single hub to workstation and overall cabling length creates installation considerations

Token Ring

• Instead of collision detection, there is a token (data) circulating in the network

• The node having the token at the moment has the permission to send, all others must wait for their turn

• The protocol must be able to handle situations like the node dieing, while it posesses the token

— Usually if there is no token seen within a certain time period, a new token is created

• IBM Token Ring network was physically a star, logically a ring

ATM LANs

• ATM uses only point to point connections

• IP on a LAN assumes that local network addresses can be reached directly and traffic to other addresses must be sent to the router

• There are several workarounds that make it possible to build an IP LAN using ATM

— Most common is an ATM LAN Emulation server that emulates broadcasts and other serv- ices

• But the 100 Mbps switched Ethernet killed the need for ATM LANs

Point-to-point links

• Need to connect 2 networks or computers with a dedicated link

— dial-up hosts and modem pools, inter-office routing

• Endpoints might be single computers, routers or bridges

• Dial-up connections, on-demand routing

— Dial-up access to networks with telephone/ISDN lines

— Routing between offices over leased lines

— Encapsulating of network routes over different networks (tunneling)

SLIP overview

• Simple packet framing protocol

— Framing is done with END (octal 300) and ESC (octal 333) special characters. Sender just transfers data packet and END

— Occurrances of END and ESC characters in data bytes are escaped with two byte sequences (ESC+octal 334 nad ESC+octal 335, respectively)

• Static IP addresses for endpoints

— Every dial-up client needs own IP address

• No type field for packets

— only one protocol over one SLIP connection

PPP overview

• Specification in RFCs 1661,1662, 1663 and others

• A protocol capable of multiplexing different network protocols over a single point-to-point link

— For example IP, IPX, XNS and AppleTalk concurrently

— HDLC-like

• Carefully designed for compatibility with most commonly used hardware

• Independent protocols for link and network control

— Link encapsulation options, authentication and link quality control configured with link control protocol, with reasonable default values

— Extensible with new network protocols: each network protocol has its own network control packets and configuration options

Practical PPP examples

• Dial-up Internet connections

— Physical connection with modem or ISDN

— Dynamic IP addresses and network configuration for clients

— Easy to install and reliable

— Supported for almost all client platforms

• Simple VPN (Virtual private network) over TCP/IP and ssh

— Secure Shell connection from distant location to intranet over untrusted networks

— PPP runs over SSH connection with link ends in secure networks

— Packets from distant location to intranet are routed over PPP to intranet. Cheap, dirty and practical VPN solution

… Practical PPP examples

• PPP over Ethernet

— Specified in RFC 2516

— Used in some DSL and cable modem configurations

— Makes it possible to decouple providing

— Integrates with existing authentication/billing systems

Description of a PPP Session

• Simplified PPP state diagram:

— Session up/down events not shown (from hardware or manually)

Dead

Terminate

Establish

Network

Authenticate SUCCESS/NONE

FAIL

DOWN

FAIL

OPENED UP

CLOSING

Some Other Data Link Protocols

• ISDN

• Frame Relay

• FDDI

• GPRS + UMTS

• WLAN

• Bluetooth

• Cable modems

• And the mighty IP runs over everything