CMPE 150: Introduction to Computer Networks FINAL REVIEW. Venkatesh Rajendran. Spring 2003 UCSC CMPE150 1

CMPE 150: Introduction to Computer Networks FINAL REVIEW Venkatesh Rajendran Spring 2003 UCSC CMPE150 1

Class Final Exam Final exam Three hours 40 50 questions -- comprehensive Multiple choice as the midterm Scantron bring your sheet and pencils Wednesday June 11 th Six short days.. Tick, tick, tick 8:00 11:00am Spring 2003 UCSC CMPE150 2

Principles of Computer Communication Protocol specification: The description of the protocol is complete and accurate. Safety: A protocol does what it is supposed to do, all the time. Liveness: A protocol does not leave any deadlocks. Efficiency: A protocol makes efficient use of available resources. Fairness: Fair or contractual use of resources Simplicity is desirable, but not necessary. Spring 2003 UCSC CMPE150 3

Layering Model Purpose is to divide and conquer complex software and hardware needed to implement services Partition services and functions needed in system into layers Each layer of service is provided by peer protocol entities Communication can be point-to-point or multipoint Layer N packets NODE A Layer-N Protocol Entity interface (virtual communication) protocol Layer-N Protocol Entity NODE B Layer-(N - 1) Protocol Entity Layer-(N - 1) Protocol Entity Spring 2003 UCSC CMPE150 4

Protocol Correctness A protocol must be safe and live Safety: Protocol provides the desired service all the time Liveness: Protocol has no deadlocks (no process waits forever for an event to occur) Proving one may depend on the other Spring 2003 UCSC CMPE150 5

Protocol Performance Average delay Time between transmission of an information bit and reception of the bit at the receiver Throughput or capacity Number of information bits sent divided by the time between transmission of first bit and delivery of the last bit Computations will make strong assumptions; in most cases, results of analytical model provide only a rough approximation Most effective for comparative analysis Spring 2003 UCSC CMPE150 6

Basic Network Services S 1,2 1 2 2 2 1 1,2 D Data may take different paths to destination 1 1 Shared network resources Connection-oriented service: Reliable data transfer: In-order delivery, no duplicates or missing data. Flow control: Do not congest the receiver(s). Congestion control: Do not congest the network(s). Spring 2003 UCSC CMPE150 7

Basic Network Services S 1,2,3 2,1,3 D Connectionless service: Shared network resources No delivery guarantees needed from the network. Any connection-oriented service to application is provided by end-to-end protocol. Spring 2003 UCSC CMPE150 8

Circuit Switching S D call request call accept DATA call termination termination ack Portion of physical resource is assigned to a single connection. Delay and signaling overhead in establishing and ending connections. Spring 2003 UCSC CMPE150 9

S Message Switching D message Message from sender is sent on a store-and-forward basis. Message has a header used for forwarding. Resources shared among different calls. Spring 2003 UCSC CMPE150 10

Statistical Multiplexing Share the same communication channel among multiple connections without fixed allocations of the resource to those connections. S1 m2 m1 m2 m2 m1 m1 m2 m1 D1 m2 m1 S2 m2 m1 Link is shared based on the statistics of each connection or flow. D2 Limitation: Entire message must be received at a switch before it can be forwarded Spring 2003 UCSC CMPE150 11

S Packet Switching D packet 1 packet 2 packet 3 packet 4 Resources are shared among connections Packets from the same connection can be processed concurrently Connection setup delay can be avoided using datagrams Spring 2003 UCSC CMPE150 12

Packet Switching Information is organized into packets A packet consists of a header and a payload Header specifies the control information needed to transport the packet from origin to destination Packets are forwarded from source to destination using routing tables There are two basic approaches to packet switching: datagrams virtual circuits Spring 2003 UCSC CMPE150 13

Datagrams a c 1 a->b c->d a->e 2 3 a->e a->b 5 c->d a->e 6 a->b a->e 4 d 7 e b To b go to 2 next To d go to 3 next To e go to 2 next To 4 go to 3 next. Routing table specifies next hop to each destination Packets are forwarded based on the routing table Each packet is routed independently Spring 2003 UCSC CMPE150 14

Virtual Circuits 2 VC1 6 e a c 1 3 VC3 5 VC2 4 d 7 b Virtual circuits are established and terminated much like circuits in circuit switching. Statistical multiplexing using packets, rather than FDM or TDM is used to share links among connections. Spring 2003 UCSC CMPE150 15

Transmission Media We consider the physical layer as a black box We are interested in the characteristics and services provided by the transmission media that impact the link layer and higher layers. Parameters: Bandwidth Delay or latency: average and variance (aka jitter) Storage capacity (bandwidth-delay product) Reliability and security Order of delivery Type of sharing or access Spring 2003 UCSC CMPE150 16

Bandwidth We think of the bandwidth of a network or link as the number of information bits that can be transmitted over it in a certain period of time (e.g., bits per second). The bandwidth of a link is really the frequency range tolerated by the channel without major attenuation. Telephone line is 3000 Hz (300Hz to 3300 Hz) Available bandwidth depends on the rate at which channel can change stored energy. We can model waveforms as sums of sine waves of different frequencies. Channel attenuates and delays each frequency component differently, causing distortion. Spring 2003 UCSC CMPE150 17

Sources of Packet Delay A transmission time of packet over each link B nodal processing queueing delay nodal processing queueing delay propagation delay of each link Spring 2003 UCSC CMPE150 18

Sources of Packet Delay Nodal processing: Checking for bit errors. Determining output link. Queueing delay: Time waiting at output link for transmission. Depends on congestion level of router. Transmission delay: Time to send bits into link: L/R, where R = link bandwidth (bps) and L = packet length (bits) Propagation delay: Time for each bit to traverse a link: d/s, where d = length of physical link and s = propagation speed in medium (~2x10 8 m/sec) Spring 2003 UCSC CMPE150 19

Bandwidth-Delay Product The amount of data stored in the link. Think of a link as a pipe; the latency is the length of the pipe and the bandwidth is its diameter. The BD product gives the volume of the pipe. Example: A channel of 50 ms latency and just 45 Mbps bandwidth can hold 2.25 million bits (the same as the memory of a PC of early 80s!). We are moving to Gigabit networks... Big bandwidth and big distances require: Big aggregation and big memories at hosts New reliable transmission algorithms Migration from client-server to client-content models. Spring 2003 UCSC CMPE150 20

Other Parameters Reliability: We will assume that information is transmitted correctly across a link or network with a given likelihood. Security: We will likely not cover this aspect in much detail :( Order of delivery: We will assume FIFO and non- FIFO delivery of packets or messages, depending on the protocol and transmission media. Access: We will consider point-to-point and broadcast links. Spring 2003 UCSC CMPE150 21

Functions at The Link Layer MAC: (Medium Access Control) Framing and synchronization Error checking Naming within LANs (with MAC addresses) Sharing of medium Flow control (in some cases) LLC: (Logical Link Control) Retransmission strategy Link management (deciding when a link exists or does not) Flow control (in some cases) Spring 2003 UCSC CMPE150 22

Framing of Bits The objective is for the receiver to understand the packets (frames) sent by the sender when bits may get corrupted over the channel. Three approaches: Character- or byte-oriented framing: BISYNC, IMP-IMP, SLIP, and PPP Bit-oriented framing: HDLC Clock-based framing: SONET Spring 2003 UCSC CMPE150 23

Character-Oriented Framing A frame starts and ends with a predefined sequence of control bytes or characters, and the occurrence of such sequence in the packet payload is avoided by byte stuffing. Consider the ARPANET IMP-IMP protocol: SYN SYN DLE STX packet payload (header and data) DLE ETX CRC CRC CRC SYN = synchronization DLE = data link escape STX = packet start Character stuffing: If DLE occurs in packet data, sender substitutes DLE with DLE DLE Receiver substitutes DLE DLE in packet payload with DLE In PPP, flag is 01111110 and such pattern in payload is preceded by 01111101 Read Section 5.8 in textbook summarizing PPP Spring 2003 UCSC CMPE150 24

Bit-Oriented Framing The same procedure is used with bits, bits are stuffed to break the occurrence of control flags. Bit stuffing consists of adding a bit after the occurrence of a bit pattern equal to the control flag used to frame the packet. Assume the control flag is 01111110 If flag pattern occurs in payload, sender must transmit something different and receiver must be able to get original data. HOW?...Sender inserts a 0 after 5 1 s, and receiver deletes any 0 received after five 1 s in a bit sequence, and the frame is ended if after 5 1 s 10 is detected. Original data: flag..1011010100111111011000101111110011. payload with bit stuffing flag 01111110..101101010011111010 110001011111010011. 01111110 Receiver then deletes stuffed 0 in any sequence...1111101 and obtains original data:..1011010100111111011000101111110011. Spring 2003 UCSC CMPE150 25

CRC CRC codes detect errors by adding a few bits (redundancy bits or CRC bits) to each packet. The attractive features of CRC are that only a few redundancy bits are needed to protect many bits of information, and it can be implemented with very simple hardware. A message of m bits is transmitted with r redundancy bits as a transmitted string T = M.R With the message bits being the most significant bits transmitted, and given that R occupies r bit positions, we have T M 2 r + R r+m-1 r M r-1 0 bits R Spring 2003 UCSC CMPE150 26

CRC The procedure to choose R to protect M is remarkably simple. A string G of r+1 bits called the generator is agreed upon. R is chosen so that T = A xg for some A => If the received string T is not a multiple of G, then an error has been detected! T equals the (modulo 2) addition of T and an error pattern E, such that a 0 in the pattern indicates no error in that bit position. Now we have: R is the remainder of dividing by G the shifted M T M Spring 2003 UCSC CMPE150 27 2 also M r 2 = r + A G T ' M R 2 = + r A G; R + R + E therefore,

CRC Therefore, to protect M, the sender: Computes R by dividing M (shifted r bit positions) by G Transmits M. R When the receiver obtains T : It divides T by G Decides that there is an error if the remainder of the division is not 0 The trick is then choosing a G for which the likelihood that T contains an error string E that is divisible by G is very small. Spring 2003 UCSC CMPE150 28

Contention-Based MAC Protocols No coordination: Stations transmit at will when they have data to send (e.g., ALOHA) Carrier sensing (listen before transmit): Stations sense the channel before transmitting a data packet (e.g., CSMA). Listen before and during transmission: Stations listen before transmitting and stop if noise is heard while transmitting (CSMA/CD). Collision avoidance (floor acquisition): Stations carry out a handshake to determine which one can send a data packet (e.g., MACA, FAMA, IEEE802.11, RIMA). Collision resolution: Stations determine which one should try again after a collision. Spring 2003 UCSC CMPE150 29

ALOHA Protocol The first protocol for multiple access channels; the first analysis of such protocols (Norm Abramson, Univ. of Hawaii, 1970). Originally planned for systems with a central base station or a satellite transponder. Two frequency bands; Up link and down link (413MHz, 407MH at 9600bps) Central node retransmits every packet it receives! Spring 2003 UCSC CMPE150 30

ALOHA Protocol Population is a large number of bursty stations. Each station transmits a packet whenever it receives it from its user; no coordination with other stations! Central node retransmits all packets (good or bad) on down link. Stations decide to retransmit based on the information they hear from central node Spring 2003 UCSC CMPE150 31

Throughput of ALOHA Protocol packet overlaps with start of packet from node i packet overlaps with end of packet from node i interfering frame node i frame interfering frame time t0-1 t0 t0 + 1 N nodes in the system The probability of a station starting a packet in a given time slot is p. Node i transmits in time slot starting at t0, i.e., time slot 2. The packet from node i is successful if no other station transmits in the time slots 1 and 2. Spring 2003 UCSC CMPE150 32

Throughput of ALOHA Protocol packet overlaps with start of packet from node i packet overlaps with end of packet from node i interfering frame node i frame interfering frame time t0-1 t0 t0 + 1 Node i s frame is vulnerable from any arrival in the time interval (t0-1, t0+1] Highest throughput when we have one packet for each 2-packet time period Spring 2003 UCSC CMPE150 33

Slotted ALOHA The throughput of ALOHA can be improved by reducing the time a packet is vulnerable to interference from other packets. Slotted ALOHA works in a slotted channel providing discrete time slots. Stations can start transmitting only at the beginning of time slots. The time synchronization needed for slotting is accomplished at the physical layer, and some synchronization is required in many cases anyway. Spring 2003 UCSC CMPE150 34

Throughput of Slotted ALOHA The vulnerability period of a packet is a slot time: arrivals i time Any arrivals in prior slot collide with packet i We double the capacity of the channel (to about 36%)because we reduce in half the vulnerability period of a packet. Spring 2003 UCSC CMPE150 35

CSMA: Carrier Sense Multiple Access The capacity of ALOHA or slotted ALOHA is limited by the large vulnerability period of a packet. By listening before transmitting, stations try to reduce the vulnerability period to one propagation delay. This is the basis of CSMA (Kleinrock and Tobagi, UCLA, 1975) Same assumptions made for ALOHA are made now for CSMA. Spring 2003 UCSC CMPE150 36

CSMA Protocol no Packet ready Channel Busy? yes Assume non-persistent carrier sensing. Requires a maximum propagation delay much smaller than packet lengths! transmit wait for a round-trip time delay packet transmission k times yes positive ack? no compute random backoff integer k Spring 2003 UCSC CMPE150 37

CSMA Throughput Because prop. delay is much smaller than packet length, slotted and pure CSMA have very similar performance. When MAC protocol requires small prop delays, we can use slotted version to predict performance of unslotted version. 1 Analytical Results Reminder: These results are only an upper bound on performance, because we did not take into account the effect of ACKs sent from receivers! S (Throughput) 0.9 0.8 0.7 0.6 0.5 0.4 Slotted Aloha Pure CSMA Slotted CSMA 0.3 0.2 Pure Aloha 0.1 0 10-3 10-2 10-1 10 0 10 1 10 2 10 3 10 4 10 5 Spring 2003 UCSC CMPE150 Offered Load: G 38

CSMA/CD: CSMA with Collision Detection CSMA improves on the performance of ALOHA tremendously. The remaining limitation is that, once a packet is sent, feedback occurs a roundtrip time after the entire packet is transmitted. The solution to improve on the performance of CSMA is to listen to the channel while a packet is being sent. This is called collision detection. R.M Metcalfe and D.R. Boggs, Ethernet: Distributed Packet Switching for Local Computer Networks, Comm. ACM, Vol. 19, 1976 (Xerox PARC). Spring 2003 UCSC CMPE150 39

CSMA/CD Protocol Packet ready Non-persistent transmission strategy Collision detection serves as a NACK! Channel busy? no yes delay packet transmission k times Assumption are: All stations hear one another Propagation delay is much smaller than packets transmit no Collision detected? yes abort transmission compute random backoff integer k send jamming signal Station listens to channel while transmitting; Collision is detected when signals sent and heard differ. Jamming signal sent to ensure all stations know of the collision. Spring 2003 UCSC CMPE150 40

CSMA/CD collision detection Spring 2003 UCSC CMPE150 41

Persistence after Carrier Sensing After detecting carrier, a station can persist trying to transmit after the channel is idle again. Persistence can be done with some probability; in which case we have a p-persistent strategy (Ethernet uses a 1-persistent strategy) Persistence can be limited, in which case a station persists trying to transmit only if the channel becomes idle within a given timeout that is much smaller than the duration of a data packet. Can you think why these approaches are desirable? Spring 2003 UCSC CMPE150 42

Collision Avoidance Collision avoidance emulates collision detection in networks where stations are half duplex. First protocol was proposed by Kleinrock and Tobagi (Split Reservation Multiple Access). Many protocols have been proposed since then: MACA, MACAW, FAMA, RIMA. The objective of collision avoidance protocols is to eliminate the hidden-terminal problem of CSMA: R S, R, and N hear one another, and R, N, and H hear one another S N H N hears S s transmission However, S and H cannot hear each other s transmissions to R, and cause interference at the receiver R. Spring 2003 UCSC CMPE150 43

Collision Avoidance Because of hidden terminals, the vulnerability of a data packet is just as in pure ALOHA, twice its length. With collision avoidance, stations exchange small control packets to determine which sender can transmit to a receiver. The collision avoidance dialogue can be controlled by the sender or the receiver. In sender-initiated collision avoidance we have: RTS (S to R) -> CTS (R to S) -> DATA (S to R) -> ACK (R to S) In receiver-initiated collision avoidance we can have: RTR (R to S) -> DATA (S to R) -> ACK (R to S) Spring 2003 UCSC CMPE150 44

Example of CSMA/CA: Floor Acquisition Multiple Access Stations use carrier sensing to send any packet. The CTS lasts much longer than an RTS (CTS Dominance) to force the interfering sources to detect carrier (from the receiver) and back off. S to R RTS 2τ R to S RTS CTS H to R noise is heard RTS S R H CTS RTS CTS time RTS from S arrives at R with no collisions. RTS from H must start within one prop. delay from CTS from R to S. H must hear noise from CTS and backs off! Spring 2003 UCSC CMPE150 45

Collision Resolution and Backoff Strategies Used to stabilize the system by preventing traffic loads that exceed its capacity. Collision resolution: Let packet that collide resolve when each is transmitted and block new traffic from entering the system. Backoff strategies: Increase the time between retransmissions when traffic load (that creates collisions) increases. Spring 2003 UCSC CMPE150 46

Collision Resolution and Backoff Strategies Backoff strategy in Ethernet: After experiencing the n th collision of a frame, pick a value, K, randomly from the set {0, 1, 2,, 2^m -1 } with m= min{n, 10}. Wait K * 512 bit times before attempting a retransmission. Goal is to reduce offered load to the channel; however, it provides no assurance that a retransmission will be sent ahead of another new transmission from other nodes. Spring 2003 UCSC CMPE150 47

TDMA TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle Spring 2003 UCSC CMPE150 48

FDMA FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle. time frequency bands Spring 2003 UCSC CMPE150 49

Channel Partitioning (CDMA) CDMA (Code Division Multiple Access) unique code assigned to each user; i.e., code set partitioning used mostly in wireless broadcast channels (cellular, satellite, etc) all users share same frequency, but each user has own chipping sequence (i.e., code) to encode data encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping sequence allows multiple users to coexist and transmit simultaneously with minimal interference (if codes are orthogonal ) Spring 2003 UCSC CMPE150 50

CDMA Encode/Decode Spring 2003 UCSC CMPE150 51

Basic Scheme: Token Passing A token granting the right to transmit is circulated among stations. Station with something to send receiving token changes the token into a start of packet and sends its packet. The token is sent back to the system when the sender is done. Two transmission strategies: Release after transmission (RAT): Sender releases the token immediately after transmitting its packet. Release after reception (RAR): Sender waits until it hears the last bit of its own transmission before releasing the token. Token Passing protocols can be used in any network topology; however, token management is simpler in rings. Spring 2003 UCSC CMPE150 52

Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver, sender clock rates Spring 2003 UCSC CMPE150 53

Unreliable, Connectionless Service Connectionless: No handshaking between sending and receiving adapter. Unreliable: receiving adapter doesn t send acks or nacks to sending adapter. Stream of datagrams passed to network layer can have gaps. Gaps will be filled if app is using TCP. Otherwise, app will see the gaps. Spring 2003 UCSC CMPE150 54

Ethernet uses CSMA/CD No slots Adapter doesn t transmit if it senses that some other adapter is transmitting, that is, carrier sense Transmitting adapter aborts when it senses that another adapter is transmitting, that is, collision detection Before attempting a retransmission, adapter waits a random time, that is, random access Spring 2003 UCSC CMPE150 55

Ethernet Technologies: 10Base2 10: 10Mbps; 2: under 200 meters max cable length. Thin coaxial cable in a bus topology. Repeaters used to connect up to multiple segments. Repeater repeats bits it hears on one interface to its other interfaces: physical layer device only! Has become a legacy technology. Spring 2003 UCSC CMPE150 56

10BaseT and 100BaseT 10/100 Mbps rate; latter called fast ethernet T stands for Twisted Pair Nodes connect to a hub: star topology ; 100 m max distance between nodes and hub nodes Hubs are essentially physical-layer repeaters: bits coming in one link go out all other links no frame buffering no CSMA/CD at hub: adapters detect collisions provides net management functionality hub Spring 2003 UCSC CMPE150 57

Gbit Ethernet Use standard Ethernet frame format Allows for point-to-point links and shared broadcast channels In shared mode, CSMA/CD is used; short distances between nodes to be efficient Uses hubs, called here Buffered Distributors Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now! Spring 2003 UCSC CMPE150 58

CSMA/CD Technology Issues IEEE802.3 and Ethernet are based on CSMA/CD. CSMA/CD is used over buses and star topologies. The most popular topology now (more than 80% of installed base) is the star topology with hubs or switches. A hub acts just like a station executing CSMA/CD, and only one transmission can succeed. A switch is different! and is the future. CPU RT Switch stores concurrently transmitted packets. No collisions. Higher throughput Limited by the switch architecture. Spring 2003 UCSC CMPE150 59

IEEE 802.11 Wireless LAN 802.11b 2.4-5 GHz unlicensed radio spectrum up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code widely deployed, using base stations 802.11a 5-6 GHz range up to 54 Mbps 802.11g 2.4-5 GHz range up to 54 Mbps All use CSMA/CA for multiple access All have base-station and ad-hoc network versions Spring 2003 UCSC CMPE150 60

Base-Station Approach Wireless host communicates with a base station base station = access point (AP) Basic Service Set (BSS) (a.k.a. cell ) contains: Wireless hosts Access point (AP): base station BSS s combined to form distribution system (DS) Spring 2003 UCSC CMPE150 61

Ad Hoc Network approach No AP (i.e., base station) wireless hosts communicate with each other to get packet from wireless host A to B may need to route through wireless hosts X,Y,Z Applications: laptop meeting in conference room, car interconnection of personal devices battlefield IETF MANET (Mobile Ad hoc Networks) working group Spring 2003 UCSC CMPE150 62

Summary of Bluetooth Low-power, small radius, wireless networking technology 10-100 meters omnidirectional not line-of-sight infared Interconnects gadgets 2.4-2.5 GHz unlicensed radio band up to 721 kbps Interference from wireless LANs, digital cordless phones, microwave ovens: frequency hopping helps MAC protocol supports: error correction ARQ Each node has a 12-bit address Spring 2003 UCSC CMPE150 63

Logical Link Control MAC protocol provides best effort service. Even when ACKs are used in the MAC, the LLC layer can decide when to retransmit. LLC bridges the gap between service expected by network layer and service provided by MAC layer. LLC uses the header information in MAC frames. Example: PPP (LLC) over Ethernet (MAC) Spring 2003 UCSC CMPE150 64

Types of Service LLC Can Provide Unacknowledged connectionless service: Datagram transmission, no connection exists, no error checking, framing is the only service provided (e.g., SLIP). Acknowledged connectionless service: No connections, each frame is ACKed individually. This service can be provided as part of the MAC itself (e.g., CSMA/CA protocols) Connection-Oriented Service: Data exchanged within a connection, provides net layer with a virtual reliable packet stream (e.g., HDLC, PPP) Spring 2003 UCSC CMPE150 65

Generic ARQ Scheme SENDER INITIATED SENDER SEQ. # PACKET CRC RECEIVER TIMEOUT acknowledge packet if no errors ACK retransmit if no ACK time time Spring 2003 UCSC CMPE150 66

Requirements in ARQ Sender labels each packet it sends using a linear sequence-number space. Receiver ACKs each packet it receives without errors and numbers each ACK with the sequence number of the corresponding packet. Sender times out after not receiving an ACK for the packet within some finite amount of time, and retransmits the packet then. Sender sends up to a certain number of un-acked packets. Spring 2003 UCSC CMPE150 67

Stop-and-Wait ARQ Sender transmits packets labeled 1, 2,.. Receiver ACKs every packet received correctly and ACK specified the packet being acknowledged (or next expected packet). Receiver passes copy of packet correctly received to the network layer and drops packets with errors. Sender retransmits copy of packet if no ACK arrives within a timeout interval. Sender and receiver are initialized to start sending and receiving packet 1. Spring 2003 UCSC CMPE150 68

Selective Repeat ARQ Motivation: SWP leaves sender idle for long periods of time waiting for ACKs. Solution: Allow sender to transmit multiple packets while waiting for the ACK of a given packet. Have a pipeline of packets! Requirements: Sender and receiver can buffer a number (W ) of packets Sender labels packets using consecutive numbers 1, 2,. Receiver buffers packets received without error, ACKs them, and delivers packets to network layer in the correct order (e.g., if packet P1 is in error and P2 and P3 are received correctly, the receiver buffers them until it receives P1 correctly) Sender buffers copies of transmitted packet until it receives the corresponding ACK. Sender retransmits a packet when its timeout expires with no ACK. ACK refers to the sequence number of the packet it acknowledges. Spring 2003 UCSC CMPE150 69

Sequence Numbering in SRP Assume that window is W and packets are numbered modulo 2W (from 0 to 2W -1) Assume that, at time T, packet labeled n is passed to network layer at the receiver (and is the packet with the highest number that can be passed to net layer). R time sender sent packet n <= => sender received all ACKs up to ACK(n-W), because window is W => Smallest sequence number pending an ACK at the sender is n-w+1 n T => n+1 has not been received! (o.w., it would have been sent to network layer) => ACK(n+1) not sent to sender => Largest sequence number sent by sender is n+w Given n, the possible range of sequence numbers of packets at the sender is {n-w+1, n+w} and using modulo 2W sequence number space works correctly Spring 2003 UCSC CMPE150 70

Go-Back-N (GBN) ARQ SRP requires sender and receiver to have a buffer, which is not an issue today. With GBN, the receiver discards any packet it receives out of order; therefore, it does not need a buffer. Receiver accepts only those packets received in order. Receiver ACKs a packet received correctly with the sequence number of the last packet received in sequence. The sender starts a timer for each packet it transmits, and after the timeout of a packet expires, it retransmits the packet and all the packets sent after that packet. Sender can have up to W packets waiting for ACKs. Spring 2003 UCSC CMPE150 71

Interconnecting with hubs Backbone hub interconnects LAN segments Extends maximum distance between nodes Individual segment collision domains become one large collision domain! If a node in CS and a node EE transmit at same time: collision Cannot interconnect 10BaseT & 100BaseT Spring 2003 UCSC CMPE150 72

Internetworking with Bridges Bridges are used to interconnect LANs at the link layer. Frame forwarding from one LAN to another is based on the destination s link-level address (MAC address) without making any changes to the frame. A MAC address is a name, and for a bridge the address of the destination is the adjacent LAN over which the frames to the destination should be forwarded. Plug-and-play, self-learning bridges do not need to be configured. Spring 2003 UCSC CMPE150 73

Traffic Isolation with Bridges Bridge installation breaks LAN into LAN segments Bridges filter packets: Same-LAN-segment frames not usually forwarded onto other LAN segments LAN segments become separate collision domains collision domain bridge collision domain = hub = host LAN segment LAN segment LAN (IP network) Spring 2003 UCSC CMPE150 74

Internetworking with Bridges To which LAN segment should the bridge forward a frame? A routing problem! There are two types of bridges that have been used: Transparent Source routing Spring 2003 UCSC CMPE150 75

Transparent Bridges: Summary The purpose of transparent bridges is to keep the packet forwarding functionality transparent to the hosts. Transparent bridges establish and manage a spanning tree of the network to eliminate packet looping. The address of a station is always the LAN over which packets from that station came last; this is a dynamic process. If no address is known, a bridge broadcasts packets for a station over all its ports (or those in the spanning tree). Spring 2003 UCSC CMPE150 76

Bridges in Mesh Topologies Alternative paths from source to destination LANs are desirable for increased reliability. Disabled Spring 2003 UCSC CMPE150 77

Spanning Tree Algorithm (STA) The objective is to define a single spanning tree in the internet over which packets flow without looping. Basis of operation (Perlman 1992, part of IEEE standard): Elect distributedly a single bridge as the root of the tree Calculate distance (in hops) on a shortest path to root Elect a designated bridge for each LAN (e.g., closest to the root in the LAN) Allow only designated bridge to forward packets to and from its LAN root A distributed election process is used to build the spanning tree! Spring 2003 UCSC CMPE150 78

STA Operation Each bridge has multiple MAC addresses (one per port) A bridge has a bridge-wide ID (one of the MAC addresses) HELLOs: messages used to build tree, sent to all bridges of a LAN HELLO specifies: Root ID: The MAC address of the bridge assumed to be the root Transmitting bridge ID: MAC address of bridge sending HELLO Cost: Length (in hops) of path from bridge to root A bridge starts by considering itself the proposed root Bridge starts election process by sending HELLO = own ID, 0, own ID Spring 2003 UCSC CMPE150 79

STA Operation Bridges adopt the smallest HELLO they hear: Minimum root ID Smallest distance to root Minimum reporting bridge ID Bridge compares its own HELLO with its neighbors HELLOs, and chooses the smallest Its root port becomes the port to neighbor bridge with smallest HELLO Bridge composes a new HELLO, adding 1 to the distance to adopted root Bridge 20 must adopt HELLO from neighbor 94 over port x: smallest root ID and smallest distance to root! x 20 y [10,4,94] z [10,20,15] [10,5,20] [21,2,30] [10,5,20] Spring 2003 UCSC CMPE150 80

STA Operation Bridge sends new HELLO over all ports from which larger HELLOs were received. Bridge knows if it is the designated bridge for a LAN if it does not hear a smaller HELLO than its own. Its root port is the port from which the smallest HELLO was received. Bridge puts its root port and all ports for which it is the designated bridge in forwarding state. Bridge puts all other ports in blocking state. Data packets, control packets, and learning of addresses take place only over ports in forwarding state (over the spanning tree). Spring 2003 UCSC CMPE150 81

Example of STA Operation [10,4,94] [10,4,50] root port x 20 z v [21,2,30] [10,5,20] y forwarding mode forwarding mode [10,20,15] [10,5,20] Spring 2003 UCSC CMPE150 82

Example 20 5 4 root 3 7 10 20 5 4 3 7 10 Spring 2003 UCSC CMPE150 83

Bridges vs. Routers Both store-and-forward devices Routers: network layer devices (examine network layer headers) Bridges are link layer devices Routers maintain routing tables, implement routing algorithms Bridges maintain bridge tables, implement filtering, learning and spanning tree algorithms Spring 2003 UCSC CMPE150 84

Routers vs. Bridges Bridges + and - + Bridge operation is simpler requiring less packet processing. + Bridge tables are self learning. - All traffic confined to spanning tree, even when alternative bandwidth is available. Spring 2003 UCSC CMPE150 85

Routers vs. Bridges Routers + and - + arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing protocols) + provide protection against broadcast storms - require IP address configuration (not plug and play) - require higher packet processing bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts) Spring 2003 UCSC CMPE150 86

Ethernet Switches Essentially a multi-interface bridge layer 2 (frame) forwarding, filtering using LAN addresses Switching: A-to-A and B-to- B simultaneously, no collisions large number of interfaces often: individual hosts, starconnected into switch Ethernet, but no collisions! Spring 2003 UCSC CMPE150 87

Network Layer The main functions at the network layer are addressing, routing, congestion control, and admission control. Addressing consists of identifying where a destination is with respect to the network topology. Routing consists of (a) computing paths from sources to destinations and (b) forwarding packets along such paths. Congestion control consists of limiting the amount of data a source can sent into the network. Admission control consists of limiting the number of sources allowed to send data into the network, and in a way is part of system-wide congestion control. Spring 2003 UCSC CMPE150 88

Routing Algorithms Most books and papers classify routing algorithms into distance-vector and link-state algorithms. Distance-Vector Algorithm: Routers exchange their distances to known destinations; a router uses the distance vectors received from its neighbors to compute its own distances. Computation is distributed. Link-State Algorithm: Routers exchange information about the state of the links in the network; a router uses this information to compute its distances to destinations. Computation is local. This is a very limiting view! Spring 2003 UCSC CMPE150 89

Shortest-Path Routing Problem: Compute the path of minimum length from each router to each destination Notation: G(N, E) is the network of N nodes and E links i P j N i q i (i, k) p i l k k D i j h2 = l i hop h P i. ( h i, h + 1 i j hx ) j Spring 2003 UCSC CMPE150 90

Bellman-Ford Algorithm BF iterates on the number of hops away from a node. Step 1: Initialize source node S with a 0 distance to itself and all other nodes with an infinite distance. Step 2: Set H = 1 Step 3: Label all nodes H hops away from S with the smallest distance from S to the nodes. Step 4: Stop if all nodes have been covered and no label can be reduced by increasing H. Else, set H = H+1 and repeat Step 3 0 S 1 5 Spring 2003 UCSC CMPE150 91 A 2 B 10 2 1 C D 10 Link costs are the same in both link directions 2 2 E

Bellman-Ford Algorithm H = 4: 1 5 A 10 C 1 4 2 S 2 2 10 E 6 0 5 2 B 1 D 3 4 No more nodes can be reached and no label can be reduced Spring 2003 UCSC CMPE150 92

Distributed Bellman-Ford Algorithm (DBF) The objective of DBF is to have a distributed implementation of BF, so that routers can compute distances to destinations distributedly. To accomplish this, the computation of a distance to a destination starts at the destination itself. The iteration of DBF is on the number of hops away from a destination. DBF operates independently for each destination. Destination starts by stating the distance to itself is 0 The neighbors of the destination receive this information, process it and send their own updates. Distances propagate throughout the network. Spring 2003 UCSC CMPE150 93

DBF Information maintained at each router: Distance Table: Distance to each destination reported by each neighbor Link-Cost Table: Cost of link to each adjacent node Routing Table: Distance and successor (next hop) to each destination Information exchanged among routers: Vector of one or more entries, each entry stating the distance to a destination Services assumed: Update messages are exchanged reliably, a node knows who its neighbors are Spring 2003 UCSC CMPE150 94

Example of DBF Operation For simplicity, we will assume synchronous operation in all cases! d 4 3 2 1 c b a 1 1 1 1 j 0 1 1 2 2 3 3 4 4 time Spring 2003 UCSC CMPE150 95

Counting to Infinity in DBF The problem with DBF is that it does not have a termination detection mechanism! 4 3 2 1 d c b a j 3 = 2+1 X 4 4 = 3+1 5 5 5 5 6 6 6 6 7 7 7 7. etc time Spring 2003 UCSC CMPE150 96

Ad Hoc Solutions (do not work) Counting to N takes too long! Alternatives include: Split horizon: Does not report routes through a successor to the successor itself. Hold-down timer: After distance to destination increases, send update stating new distance through current successor, wait for a long period of time before computing new successor and shortest distance and then act as in DBF. Poisoned reverse: After distance increase, report an infinite distance and then correct the distance. (Or in general, reporting infinity to the successor of destinations routed through it). Next-hop information: Communicate the distance and next hop to each destination (used in RIP v2) Spring 2003 UCSC CMPE150 97

Looping in DBF 5, B A 10 2, j C 1 2 6, A S 2 2 10 X0, j j 5 2 B 1 D 3, D 2, j Spring 2003 UCSC CMPE150 98

Looping in DBF 5, B A 10 5,B C 5 1 6, A S 2 2 10 5 B 1 D 4 3, D 4, B Spring 2003 UCSC CMPE150 99

Looping in DBF 5, B A 10 5,B C Erroneous paths persist as long as they appear to be the shortest paths. 6, A S 1 2 2 10 Similar looping could occur if the cost of the links to j increased drastically (e.g., to 20). DBF cannot be used with link costs that have a large variance! 5 5 B D etc 1 3, D 4, B Spring 2003 UCSC CMPE150 100

Traditional Link-State Algorithm (LSA) Developed as a result of DBF s looping and nontermination problems. Two components: Topology map distribution Local shortest path computation Each router runs a local shortest-path algorithm (Dijkstra s) using the topology stored locally. Flooding is used to replicate the topology map at every router. Each router is responsible for reporting the state of outgoing links to the rest of the network. Two link-state updates per link reach every router. Spring 2003 UCSC CMPE150 101

Shortest-Path First (SPF) Algorithm Step 1: Initialize Set SPF = { root }, where root is router running SPF Distance to root = 0 and distance to other nodes = cost of link or infinity Step 2: Find next node for SPF set: Find a node x not in SPF set such that: distance to/from root = Min{distance to node outside of SPF set} Augment SPF set with x Stop if SPF set contains all nodes Step 3: Change minimum distance: For each node y outside SPF set do: dist. to y = Min{ dist. to y, dist. to z in SPF + cost of (z, y) } Repeat Step 2 Spring 2003 UCSC CMPE150 102

SPF Example SPF ={S, A, B, D} Labels do not change as we continue to expand SPF set 1 1 A 10 5 C 2 SPF ={S, A, B, D, C} SPF ={S, A, B, D, C, E} Stop after covering E since all nodes are covered by SPF set. 0 S 5 2 B 2 1 D 10 2 E 6 3 4 Note that iteration is on the next node that can be covered with the next shortest path; hence complete topology must be known by router. Spring 2003 UCSC CMPE150 103

Flooding of Link States Information Stored at Routers: Each router maintains all the nodes and all the links in the network in a topology graph. Each link in the graph has a cost, a sequence number, and an age. Spring 2003 UCSC CMPE150 104

Flooding of Link States Information Exchanged: Each router is responsible for communicating the latest state of each adjacent outgoing link. The router sends a link state update (LSU) to report changes on an adjacent outgoing link. A sequence number is used to identify the latest LSU. An LSU also specified the age of the LSU, and the age of an LSU is decremented each time it is forwarded and while it is in storage. We assume that LSUs are exchanged reliably between any two routers and that a router knows who its neighbors are! Spring 2003 UCSC CMPE150 105

IP Internetworking Based on Cerf s catenet model V.G. Cerf, The Catenet Model for Internetworking, IEN 48, July 1978. Basic premises: Heterogeneous transmission media Heterogeneous hardware and OS in hosts and gateways Common protocol for network interconnection runs in all gateways and hosts! Common protocol used for data transfer and signaling Common address space used to identify where a host or router is in the internetwork An address states at which network a node attaches to the internetwork Spring 2003 UCSC CMPE150 106

Service Model: Theory and Practice The Internet Protocol (IP) evolved from the catenet model. Theory: Datagram Delivery is assumed, so that packets can get lost, out of order, and multiple copies can be delivered. Practice: TCP needs in-order delivery of packets to work efficiently, and (as we will see) Internet routing protocols provide a single path for each destination and do not adapt very rapidly. Too many destinations! Spring 2003 UCSC CMPE150 107

IPv4 Datagram Format IP protocol version number header length (words) type of data max number remaining hops (decremented at each router) upper layer protocol to deliver payload to how much overhead with TCP? 20 bytes of TCP 20 bytes of IP = 40 bytes + app layer overhead ver head. len 16-bit identifier time to live type of service upper layer 32 bits flgs length fragment offset Internet checksum 32 bit source IP address 32 bit destination IP address Options (if any) data (variable length, typically a TCP or UDP segment) total datagram length (bytes) for fragmentation and reassembly e.g. timestamp, record route taken, specify list of routers to visit. Spring 2003 UCSC CMPE150 108

IPv4 Addresses IP addresses are global and, unlike MAC addresses, they are hierarchical. IP address has a network part and a host part and specifies host@network A host has an address for each network to which it attaches. IP addresses are denoted using the dotted-decimal notation: Each byte of the address is written in its decimal form and is separated by a dot from the other bytes, e.g., 5.7.2.1 => 00000101 00000111 00000010 00000001 Spring 2003 UCSC CMPE150 109

IPv4 Addresses (past) 0 8 16 24 31 Class A 0 network host 126 16 million Class B 10 network host 16,382 65,534 Class C 110 network host 2 million 254 Class D 1110 multicast address Class E 11110 reserved address Spring 2003 UCSC CMPE150 110

IPv4 Addressing Problems There were too few networks left due to the class structure used in IP address assignments! There are many more IP devices and appliances coming. Routing tables cannot have millions of entries. Solutions: Aggregation of addresses without classes (subnetting, and now CIDR) New and bigger global address space (IPv6) Locally unique addresses (NAT and other techniques) Spring 2003 UCSC CMPE150 111

IP Addressing: CIDR Classful addressing: Inefficient use of address space, address space exhaustion. A class B address has enough addresses for 65K hosts, even if only a few more than 256 hosts are located in that network CIDR: Classless InterDomain Routing Eliminate the strict assignment of address portion in class-full addressing. Enable a network portion of address of arbitrary length. CIDR Address Format: a.b.c.d/x, where x is # bits in network portion of address network part host part 11001000 00010111 00010000 00000000 200.23.16.0/23 Spring 2003 UCSC CMPE150 112

Assigning Blocks of Addresses to ISPs ICANN: Internet Corporation for Assigned Names and Numbers Allocates IP address space Manages DNS (domain name system) Assigns domain names and resolves disputes Spring 2003 UCSC CMPE150 113

Internet Control Protocols In addition to packet forwarding and keeping routing tables correct, sending IP packets requires a number of control protocols: Application has the name of an intended destination. An IP address has to be found for that name; The application typically calls a resolver in the Domain Name System (DNS) or uses a static hosts file (e.g., /etc/hosts) Host determines if destination IP address is the same or different. If different, packet is sent to an attached (default) router. If same subnet, the IP address must be converted to a MAC address using a protocol (ARP). Destination router must also map IP address to MAC address using ARP. Errors may have to be reported to the source of an IP packet using a protocol (ICMP). Spring 2003 UCSC CMPE150 114

Fragmentation Packet length is in bytes and includes header; maximum length is then 65,535 bytes MAC protocol my not support such long packets, and an IP packet may have to be fragmented. Ethernet accepts frames of up to 1500 bytes and FDDI of up to 4500 bytes Each fragment is a self-contained datagram. Fragmentation is handled with: The packet ID, which is the same for all fragments The offset, which states the byte (position) of the fragment A flag indicating that there a more fragments for the same ID coming. Spring 2003 UCSC CMPE150 115

IPv4 Header TTL (time to live indicates how long the packet can stay in the network; it is specified in hops and is decremented each time the packet is forwarded. Default is 64 hops; nodes can play with the field to limit the scope Protocol specifies the type of payload Checksum is computed considering the entire header as a sequence of 16-bit words, adding them up with 1 s complement arithmetic and taking the 1 s complement of the result. This checksum is NOT as powerful as a CRC but is simple to do in software. Why this way? Because it is done at each hop (software) What if we process headers in hardware? Spring 2003 UCSC CMPE150 116

Error Reporting In general, errors can be reported to the origin of a packet or to intermediate relays or both. In the IP Internet, errors are reported to the source using ICMP (internet control message protocol). The choice stems from using IP for all signaling and user data transfer in the Internet. ICMP messages are encapsulated in IP. An IP packet specifies the source and destination and not the relays (options are not supported in general) Spring 2003 UCSC CMPE150 117

Address Resolution Protocol Goal: Enable a host to build a table of mappings between IP addresses and MAC addresses in a dynamic manner. Mappings are called ARP cache or ARP table. Approach: ARP is designed assuming a fully connected, broadcast link layer (LAN) and the requestor is responsible for persisting. Hosts and routers broadcast requests and responses and listen to requests and responses from any other node in the LAN. Different approach would be needed in a multihop LAN. Spring 2003 UCSC CMPE150 118

Dynamic Host Configuration Host must be assigned an IP address, because it is not committed to hardware as a MAC address. Configuring hosts with proper IP addresses is involved. DHCP (dynamic host configuration protocol) is a solution to this configuration and management problem. DHCP is intended to support manual, automatic and dynamic configurations DHCP is designed to work with no pre-configured addresses of servers and across networks. Spring 2003 UCSC CMPE150 119

DHCP: Dynamic Host Configuration Protocol Goal: Allow host to dynamically obtain its IP address from network server when it joins network. Can renew its lease on address in use Allows reuse of addresses only hold address while connected and on Support for mobile users who want to join network (more shortly) DHCP overview: host broadcasts DHCP discover msg DHCP server responds with DHCP offer msg host requests IP address: DHCP request msg DHCP server sends address: DHCP ack msg Spring 2003 UCSC CMPE150 120

NAT: Network Address Translation rest of Internet 138.76.29.7 10.0.0.4 local network (e.g., home network) 10.0.0/24 10.0.0.1 10.0.0.2 10.0.0.3 All datagrams leaving local network have same single source NAT IP address: 138.76.29.7, different source port numbers Datagrams with source or destination in this network have 10.0.0/24 address for source, destination (as usual) Spring 2003 UCSC CMPE150 121

NAT Motivation Local network uses just one IP address as far as outside word is concerned No need to be allocated range of addresses from ISP: - just one IP address is used for all devices Can change addresses of devices in local network without notifying outside world Can change ISP without changing addresses of devices in local network Devices inside local net not explicitly addressable, visible by outside world (a security plus). Spring 2003 UCSC CMPE150 122

Functions of NAT Router Outgoing datagrams: Replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #)... remote clients/servers will respond using (NAT IP address, new port #) as destination addr. Remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair Incoming datagrams: Replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table Spring 2003 UCSC CMPE150 123

NAT Issues 16-bit port-number field: 60,000 simultaneous connections with a single LANside address! NAT is controversial: Routers should only process up to layer 3 Violates end-to-end argument NAT possibility must be taken into account by app designers, e.g., P2P applications Should address shortage be solved using IPv6 instead? Spring 2003 UCSC CMPE150 124

NAT Issues Violates the architectural model Every IP address uniquely identifies a single machine (now many use 10.0.0.1) Move from connectionless to connection oriented NAT router must maintain state for each connection if it crashes, all connections are destroyed NAT violates protocol layering. If TCP/UDP change. Different header, etc. NAT will fail.. NAT removes the independence of the layers Transport protocols other than TCP/UDP will likely fail at a NAT router Some applications place IP address in the data for the receiver to use. NAT will not find and translate these 16-bit port-number field: Maximum of 60,000 simultaneous connections with a single Ip address RFC 2993 discusses these and other problems Spring 2003 UCSC CMPE150 125

Internet Routing Protocols Intra-domain routing: RIP, OSPF, EIGRP Single-path routing protocols, static link costs Performance (shortest path) Inter-domain routing: Border Gateway Protocol (BGP) Single path Policy based Spring 2003 UCSC CMPE150 126

RIP (v1) Based on DBF Used in small internets Problems: Counting to infinity and looping, single-path routing, link cost should be 1 or infinity Update specifies only a destination network and a distance to it; hence, no variable subnet masks are allowed in local internet and a static subnetting convention must be used for all routers Router sends its routing table to its neighbors every 30 sec. or when it must update its routing table. Runs on top of UDP. Spring 2003 UCSC CMPE150 127

RIPv2 Adds the next hop to a destination and subnet mask in each update. Variable subnets are allowed. Performance does not improve much. Spring 2003 UCSC CMPE150 128

OSPF: Open Shortest Path First Dijkstra s SPF used to compute shortest paths locally based on topology map. Flooding is used to disseminate topology maps. Sequence numbers and age fields are used to validate link-state updates. Runs on top of IP and implements its own reliable transmission of link-state updates. Designated routers are used to reduce overhead within a LAN, and areas connected by a backbone are used to reduce overhead across LANs. HELLOs used to identify neighbors. Spring 2003 UCSC CMPE150 129

BGP BGP (Border Gateway Protocol): the de facto standard for Internet inter-as routing. Path Vector protocol: Similar to Distance Vector protocol Each Border Gateway broadcast to neighbors (peers) entire path (i.e., sequence of AS s) to destination BGP routes to networks (ASs), not individual hosts e.g., Gateway X may send its path to dest. Z: Path (X,Z) = X,Y1,Y2,Y3,,Z Spring 2003 UCSC CMPE150 130

Routing in Ad Hoc Networks The problem addressed is host and router mobility. IETF MANET Working Group is defining which approach to evolve into a standard. Proposals: On-demand routing approaches: DSR, AODV, TORA, and many others. Table-driven and hybrid approaches: STAR and approaches based on topology broadcast. Spring 2003 UCSC CMPE150 131

On-Demand Routing Routers maintain routing-table entries for only those destinations that they need to reach. To reach a destination with an unknown route, source sends a flood search packet, just like in source routing bridges. Flood search packet reaches the destination one or multiple times. Destination sends a reply one or multiple times with the desired routing information. DSR uses source routes in flood searches and replies; AODV uses destination sequence numbers. Spring 2003 UCSC CMPE150 132

Multicast: one sender to many receivers Multicast: act of sending datagram to multiple receivers with single transmit operation analogy: one teacher to many students Spring 2003 UCSC CMPE150 133

IP Multicast Architecture Based on Steve Deering s original proposal (SIGCOMM 88 Proceedings; his PhD thesis) It consists of three basic components: Group addressing based on globally unique identifiers (IP multicast addresses) Separation of senders and receivers with anonymous receiver affiliation A tree-based routing and group structure Spring 2003 UCSC CMPE150 134

IP Multicast Architecture (contd..) 128.59.16.12 128.119.40.186 multicast group 226.17.30.197 128.34.108.63 128.34.108.60 multicast group concept: use of indirection hosts addresses IP datagram to multicast group routers forward multicast datagrams to hosts that have joined that multicast group Spring 2003 UCSC CMPE150 135

IP Multicast Addresses 28 bits for groups ~ 250 million groups Two kinds of supported addresses: Permanent, e.g., 224.0.0.1 all systems on LAN 224.0.0.2 all routers on LAN Temporary - must be created before they can be addressed (join and leave) Spring 2003 UCSC CMPE150 136

IGMP: Internet Group Management Protocol host: sends IGMP report when application joins mcast group router: sends IGMP query at regular intervals host belonging to a mcast group must reply to query query report Spring 2003 UCSC CMPE150 137

Distance Vector Multicast Routing Protocol (DVMRP) DVMRP: distance vector multicast routing protocol, RFC1075 flood and prune: reverse path forwarding, source-based tree RPF tree based on DVMRP s own routing tables constructed by communicating DVMRP routers no assumptions about underlying unicast odds and ends commonly implemented in commercial routers Mbone routing done using DVMRP Spring 2003 UCSC CMPE150 138

PIM: Protocol Independent Multicast not dependent on any specific underlying unicast routing algorithm (works with all) two different multicast distribution scenarios : Dense: group members densely packed, in close proximity. bandwidth more plentiful Sparse: # networks with group members small wrt # interconnected networks group members widely dispersed bandwidth not plentiful Spring 2003 UCSC CMPE150 139

Dense Consequences of Sparse-Dense group membership by routers assumed until routers explicitly prune data-driven construction on mcast tree (e.g., RPF) bandwidth and nongroup-router processing profligate Dichotomy: Sparse: no membership until routers explicitly join receiver- driven construction of mcast tree (e.g., centerbased) bandwidth and nongroup-router processing conservative Spring 2003 UCSC CMPE150 140

Mobility Management Each network is identified by the network prefix part of the IP address. When a node moves from one network to another network, the IP address should change according to the network prefix of the new network. All the connections are identified by the IP address and the port number of the host. Spring 2003 UCSC CMPE150 141

Mobility Approaches Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence not via usual routing table exchange. scalable to millions of mobiles routing tables indicate where each mobile located no changes to end-systems Let end-systems handle it: indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remote direct routing: correspondent gets foreign address of mobile, sends directly to mobile Spring 2003 UCSC CMPE150 142

Mobility: Registration home network visited network 2 wide area network foreign agent contacts home agent home: this mobile is resident in my network 1 mobile contacts foreign agent on entering visited network End result: Foreign agent knows about mobile Home agent knows location of mobile Spring 2003 UCSC CMPE150 143

Mobility Support via Indirect Routing home network correspondent addresses packets using home address of mobile home agent intercepts packets, forwards to foreign agent 1 wide area network 2 foreign agent receives packets, forwards to mobile 4 3 visited network mobile replies directly to correspondent Spring 2003 UCSC CMPE150 144

Mobility via Direct Routing home network correspondent forwards to foreign agent foreign agent receives packets, forwards to mobile 4 visited network correspondent requests, receives foreign address of mobile 2 1 wide area network 3 4 mobile replies directly to correspondent Spring 2003 UCSC CMPE150 145

Mobile IP RFC 3220 Has many features we have discussed: Home agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet) Three components to standard: Agent discovery Registration with home agent Indirect routing of datagrams Spring 2003 UCSC CMPE150 146

Services: Transport Protocols Reliable or unreliable transport from source process to end process(es) Multiplexing and demultiplexing Flow control Avoid overflowing receiver s buffer Congestion control Avoid overflowing the network bottleneck Examples: UDP and TCP Spring 2003 UCSC CMPE150 147

Why Multiplexing IP delivers packets from source host to destination host. However, multiple processes run in the hosts! Applications require communication among processes, not just host computers. Example: Multiple telnet sessions, email, ftp sessions, and www can all be running concurrently in the same host. Ports are defined as the addresses of processes inside a host. How do we identify processes uniquely and efficiently? Spring 2003 UCSC CMPE150 148

Transport Protocols Transport protocols used today are point to point! UDP used for: Remote file server (NFS), name translation (DNS), intra-domain routing (RIP), network management (SNMP), multimedia applications and telephony. TCP used for: Electronic mail (SMTP), file transfer (FTP), remote login (Telnet), web (HTTP) No standard multipoint e-t-e protocol yet! Spring 2003 UCSC CMPE150 149

User Datagram Protocol (UDP) Provides best effort e-t-e delivery of segments among processes: No guarantees for delivery, ordering, duplicates Small overhead, no connection state, no flow control, no congestion control. Header specifies the minimum needed for multiplexing and framing. Spring 2003 UCSC CMPE150 150

TCP Flow Control vs. Congestion Control Reliable vs. Unreliable Communication TCP history service provided applications error recovery congestion control proposed enhancements Spring 2003 UCSC CMPE150 151

Services Provided Layer 4 - Transport layer End-to-end flow control Reliable byte stream In-order packet delivery (buffering) Connection-oriented Socket <host address, port> uniquely identifiable connection Spring 2003 UCSC CMPE150 152

Connection establishment client SYN(port,ISN=10) SYN(ISN=35,ACK=11) server connection request (SYN) connection granted (SYNACK) ACK = 36 ACK A Three-way handshake How to choose ISN? Spring 2003 UCSC CMPE150 153

Flow Control vs. Congestion Congestion control Control Global issue: concerns all routers and hosts on path from Source to Destination make sure every subnet can handle the traffic 1Mbps Router 1Mbps Router 1Mbps Senders Receiver Spring 2003 UCSC CMPE150 154

Flow Control vs. Congestion Control Flow Control Involves two endpoints Make sure sender doesn t transmit faster than receiver can absorb packets Server 1Gbps File transfer PC 1Mbps Spring 2003 UCSC CMPE150 155

TCP Flow Control Transmission Window a.k.a. congestion window (cwnd) Sliding window maintained by sender Conservation of packets Receiver s window set through socket API controlled by the receiver advertised to the sender in field of TCP header (RAW) cwnd 1 2 3 4 5 6 7 8 9 10 11 12... Sent and ACKed Can t send Sent, not ACKed Send ASAP Spring 2003 UCSC CMPE150 156

TCP Congestion Control Slow Start Due to Van Jacobson (SIGCOMM 88) Algorithm used at beginning of connection and after a timeout leads to exponential growth in the amount of outstanding data in network cwnd doubles every RTT!! Algorithm: When an ACK is received before timeout: cwnd = cwnd + 1 for each ack d segment Spring 2003 UCSC CMPE150 157

Congestion Avoidance CA is flow control imposed by the sender: Introduce a new variable, ssthresh initialized to 65,535 (max. window) If timeout, set ssthresh = cwnd/2 and cwnd =1 Re-enter slow start, until cwnd = ssthresh When cwnd = ssthresh, then grow cwnd linearly until it reaches RAW: ACK is received (before a timeout) then cwnd = cwnd + 1/cwnd Hence, cwndincreases by 1 segment every RTT Spring 2003 UCSC CMPE150 158

Putting it together: Slow start and congestion avoidance The algorithm: If cwnd < ssthresh do slow start Else if cwnd >= ssthresh do congestion avoidance Spring 2003 UCSC CMPE150 159

Graph of CA and SS Spring 2003 UCSC CMPE150 160

cwnd TCP Congestion Control: Underdamped Feedback System! ssthresh ssthresh/2 rt times Spring 2003 UCSC CMPE150 161

Problems with CA and SS Slow start is an attempt to discover the network bandwidth Discovery proceeds by filling network queues in intermediate routers. Once queues are full, routers drop packets. Once loss discovered, it s too late! TCP sender reduces window when loss is discovered Queue level oscillates between full and cwnd/2 What sort of problems does this introduce?? Spring 2003 UCSC CMPE150 162

Fast Retransmit and Fast Recovery When third duplicate ACK is detected: Retransmit missing segment Set ssthresh = cwnd/2 Set cwnd = ssthresh + 3 For each additional duplicate ACK: increment cwnd by one segment transmit a new packet if possible why? With each ACK, we know a data pkt left the network When new ACK arrives: cwnd = ssthresh Spring 2003 UCSC CMPE150 163

Nagle s Algorithm Solution proposed in RFC 896 in 1984 For Silly Window Syndrome Solution: only one outstanding small segment is allowed no additional small segments can be sent until the previous is ACKed Once the ACK arrives, several small segments will have been buffered and can now be transmitted Solution is self-clocking based on ACK arrival Sometimes Nagle s alg. Should be turned off X window system server need to send mouse movements for real-time applications Spring 2003 UCSC CMPE150 164

Improvements to TCP SACK: include information in the ACK which indicates missing packets in the window Vegas: use rate control instead of arrival of ACKs to pace data into network Spring 2003 UCSC CMPE150 165

TCP-SACK Goal: Improve TCP error recovery mechanism Selectively acknowledge lost data within the transmission window Uses sequence number ranges example: ACK = 1000, SACK = 1040:1080 Limited by max. size of TCP header to 3 distinct ranges Important when there are multiple losses per window multiple losses often results in a timeout Significance performance improvements in wired networks Does this approach improve congestion control? Spring 2003 UCSC CMPE150 166

Vegas - Congestion Avoidance Vegas attempts to be proactive to congestion Tries to avoid losses by preventing congestion Uses a rate-based approach instead of window-based Keeps transmission rate within a calculated range Spring 2003 UCSC CMPE150 167

Vegas - Important terms for CA Base RTT RTT of a segment in uncongested network minimum RTT ever observed generally first RTT measurement taken window small at beginning of connection Expected Throughput cwnd/basertt Actual Throughput Time one segment Note # of packets transmitted during 1 RTT interval Actual Throughput = #packets/rtt Spring 2003 UCSC CMPE150 168

Vegas - CA algorithm Basic idea: Try to keep the expected throughput close to the ideal throughput Let Diff = Expected - Actual Rules for window adjustment: Diff < α, Diff > β, increase linearly decrease linearly α < Diff < β, hold window steady Where α & β are expressed in KB/s (ie, 10 KB/s and 30KB/s) less than the Expected Rate. Spring 2003 UCSC CMPE150 169

Vegas - CA Algorithm Region A: increase window not pushing hard enough Region B: keep window constant Region C: decrease window pushing system too hard Spring 2003 UCSC CMPE150 170

Domain Name System (DNS) Basic function: translation of names (ASCII strings) to network (IP) addresses and viceversa. Example: zephyr.isi.edu <-> 128.9.160.160 Try the nslookup program (even in Windoze) % nslookup zephyr.isi.edu (or any other name you whish to resolve) Spring 2003 UCSC CMPE150 171

How is it used? Client-server model. Client DNS (running on client hosts), or resolver. Application calls resolver with name. Resolver contacts local DNS server (using UDP) passing the name. Server returns corresponding IP address. Spring 2003 UCSC CMPE150 172

DNS Name Space A Tree-based Hierarchy: int com edu gov mil org net us ca ibm ucsc eng sales cse cats soe Spring 2003 UCSC CMPE150 173

Domain names: DNS Names Concatenation of all domain names starting from its own all the way to the root separated by a dot. (Reverse order of IP addresses) Refers to a tree node and all names under it. Case insensitive. Components up to 63 characters. Full name less than 255 characters. Spring 2003 UCSC CMPE150 174

Name Space Management Domains are autonomous. Organizational boundaries. Each domain manages its own name space independently of other domains. Delegation: When creating new domain: register with parent domain. For name uniqueness. For name resolution. Spring 2003 UCSC CMPE150 175

Name Resolution Application wants to resolve name. Resolver sends query to local name server. Resolver configured with list of local name servers. Select servers in round-robin fashion. If name is local, local name server returns matching authoritative RRs. Authoritative RR comes from authority managing the RR and is always correct. Cached RRs may be out of date. Spring 2003 UCSC CMPE150 176

Recursive Resolution Recursive query: Each server that does not have information forwards it to someone else. Response finds its way back. Alternative is Iterative query: Name server not able to resolve query, sends back the name of the next server to try. Some servers use this method. More control for clients. Spring 2003 UCSC CMPE150 177

Electronic Mail The First Killer App Non-interactive. Deferred mail (e.g., destination temporarily unavailable). Spooling: Message delivery as background activity. Mail spool: temporary storage area for outgoing mail. Spring 2003 UCSC CMPE150 178

SMTP Simple Mail Transfer Protocol How messages are transferred over a TCP/IP internet. Defines commands used to exchange mail between mail clients and servers. Problems reported to user by e-mail. Spring 2003 UCSC CMPE150 179

POP3 & IMAP POP3 (Post Office Protocol v3) & IMAP (Internet Message Access Protocol) User invokes POP3/IMAP client; connects to server through TCP. Requires authentication (user id and passwd). Commands to retrieve and delete messages from permanent mailbox. POP3 downloads all messages, IMAP more sophisticated read headers before selective download of mail. Mail server needs to run SMTP and POP3/IMAP. Spring 2003 UCSC CMPE150 180

Web and HTTP Web page consists of objects Object can be HTML file, JPEG image, Java applet, audio file, Web page consists of base HTML-file which includes several referenced objects Each object is addressable by a URL Example URL: www.someschool.edu/somedept/pic.gif host name path name Spring 2003 UCSC CMPE150 181

HTTP Overview (continued) Uses TCP: client initiates TCP connection (creates socket) to server, port 80 server accepts TCP connection from client HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server) TCP connection closed HTTP is stateless server maintains no information about past client requests aside Protocols that maintain state are complex! past history (state) must be maintained if server/client crashes, their views of state may be inconsistent, must be reconciled Spring 2003 UCSC CMPE150 182

HTTP Connections Nonpersistent HTTP At most one object is sent over a TCP connection. HTTP/1.0 uses nonpersistent HTTP Persistent HTTP Multiple objects can be sent over single TCP connection between client and server. HTTP/1.1 uses persistent connections in default mode Spring 2003 UCSC CMPE150 183

Method types HTTP/1.0 GET POST HEAD asks server to leave requested object out of response HTTP/1.1 GET, POST, HEAD PUT uploads file in entity body to path specified in URL field DELETE deletes file specified in the URL field Spring 2003 UCSC CMPE150 184

Cookies: keeping state Many major Web sites use cookies Four components: 1) cookie header line in the HTTP response message 2) cookie header line in HTTP request message 3) cookie file kept on user s host and managed by user s browser 4) back-end database at Web site Example: Susan access Internet always from same PC She visits a specific e- commerce site for first time When initial HTTP requests arrives at site, site creates a unique ID and creates an entry in backend database for ID Spring 2003 UCSC CMPE150 185

Cookies (continued) What cookies can bring: authorization shopping carts recommendations user session state (Web e-mail) aside Cookies and privacy: cookies permit sites to learn a lot about you you may supply name and e-mail to sites search engines use redirection & cookies to learn yet more advertising companies obtain info across sites Spring 2003 UCSC CMPE150 186

Spring 2003 UCSC CMPE150 187