Automatic Repeat Request Data Link Layer Traffic Management

Automatic Repeat Request Data Link Layer Traffic Management

Reliable Communications with Retransmission How to transport data units over an unreliable data link in a reliable way? End to End E.g.. TCP Hop to Hop E.g.. SMTP, X.25, HDLC Answer: ARQ Automatic Repeat Request ARQ is an abstract concept, not a protocol itself 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 Sender 1. A packet is sent 3. A packet is re-sent after a timeout Packets in transit 2. A packet is lost Receiver 4. Acknowledgment is sent Problem: what if a frame is received and acknowledged after the timeout at sender s end?

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 Control Frames ACK, acknowledgment NAK, negative acknowledgment ENQ, enquiry

ARQ Stop-and-Wait Frame Loss Handling One frame is sent at a time Rule: the information (data transporting) 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 Receiver sends last ACK sent Enables re-synchronization

Go-Back-N ARQ Frame Loss Handling 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 The sender has to have a buffer that holds all frames that have been transmitted but not ACKed 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 Latency is a problem for stop-and-wait ARQ

Selective Repeat ARQ Frame Loss Handling When the receiver receives a frame out of sequence, 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

Data Link Protocols

The Data Link Layer Traditionally the data communications world has been built on Point to point connections between LANs HDLC, PPP, SLIP, ATM etc. over telecoms infrastructure Telecoms infra: Sonet, SDH, PDH Local Area Networks Ethernet, Token Ring, ATM over physcal media 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 Address Control Information (data) CRC Flag Flag is 01111110 Bit stuffing is used for other data, every 11111 is followed by 0 HDLC is bit-limited protocol, not byte limited Address is the receiver address Control is 8 bits, 16 in extended mode for longer sequences Information is payload data CRC is the checksum in ITU-CRC, 16 or 32 bits

HDLC Control Byte for Information Frame 1 2-4 5 6-8 0 Send sequence Poll/Final Receive Sequence Starts with bit 0 Sequence 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)

HDLC Control Byte for Supervisory Frame 1-2 3-4 5 6-8 10 Control field Poll/Final Receive Sequence 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

HDLC Control Byte for Unnumbered Frame 1-2 3-4 5 6-8 11 Message Poll/Final Message Starts with bits 11 Used to set up connections and master/slave relationships

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) Done by hand (permanent virtual circuit) or by using PNNI protocol (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 After route creation each switch along the route knows where to send a packet with a certain VPI/VCI header The VPI/VCI header may change along the route Each ATM switch needs only to be aware of the next hop

How the ATM works (cont.) The ATM data unit is called a cell 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

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) Or to find configuration parameters for this host (DHCP) Unicasts are used for normal traffic

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 provides 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 services 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 Old de facto-standard, description in RFC 1055

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 A simplified PPP state diagram: Up Opened Dead Established Authenticate Success/ None Fail Fail Down Terminate Closing Network Session up/down events not shown (from hardware or manually) In Network state different network layer protocols may open or close while PPP stays connected: different possible states not shown

Some Other Data Link Protocols ISDN Frame Relay FDDI GPRS + UMTS WLAN Bluetooth Cable modems And the mighty IP runs over everything

Congestion Control Paul Ferguson and Geoff Huston, Quality of Service: Delivering QoS on the Internet and in Corporate Networks, Wiley, 1998

The Problem On a network with multiple hosts, traffic arrives in bursts to the routers Often there is only a single link for the traffic to go on This is a Quality of Service issue (QoS) Here we discuss only the basic case of one quality class

Traffic Shaping Individual flows can be shaped Packets are put on wait Packets are discarded It is also possible to shape future traffic Does not necessarily compete with queuing but rather supports it Can be done by the sender or along the transmission path

Filtering of Bursts Simplest traffic shaping method is to filter traffic bursts Leaky bucket can be used Excess traffic burst are filtered Packets can be discarded Basically a buffer and some logic Limits the outflow of data Used originally in ATM networks Potentially creates latency Can be a problem for some real time audio and video applications

Leaky Bucket Fixed queue size When the queue is full extra incoming packets get discarded Constant exit rate

Token Bucket A token is required for forwarding of packets Based on bucket, which can hold B tokens R tokens arrive the bucket per each second Extra tokens are stored in the bucket if they are not needed Bursts are allowed to certain limit Regulates also the interval of outgoing packets Allows short bursts to go through When the bucket is full of tokens Longer bursts are held back When the token bucket is empty, the outgoing rate is same as token creation date

Token Bucket R tokens per second B tokens in bucket

Random Early Detection (RED) When the buffer in the router starts to fill, arriving packets are dropped randomly Likelihood of a packet getting discarded is directly related to how full the buffer is A discarded packet sends a message to the higher level protocol used Suits well with TCP protocol Individual TCP protocol sessions drop speed Affects future traffic Happens gradually

Random Early Detection Router queue Likelihood of an incoming packet getting discarded: (qlen - minth) maxth - minth maxth qlen minth

Conclusions Routers (and switches) have important effect on the QoS properties of the network This should be taken in consideration in implementations Different queues Individual traffic flows can be shaped Leaky bucket, token bucket, RED