Carrier Wave! Signals! Modulation! Internet Protocol Stack!

Transcription

1 Internet Protocol Stack! Carrier Wave! application: supporting network applications!! HTTP, SMTP, FTP, etc.! transport: endhost-endhost data transfer!! TCP, UDP! network: routing of datagrams from source to destination!! IP, routing protocols! link: data transfer between neighboring network elements!! Ethernet, WiFi! physical: bits on the wire! application transport network link physical Observation: a continuous, oscillating signal propagates further (with less signal loss) than other signals! Hence to send data long distance, we use a continuous sine wave as a carrier wave! Data is carried by modifying the carrier wave, a process called modulation! Two types of modulation:! 1.! Amplitude Modulation (AM): not as robust! 2.! Frequency Modulation (FM): more robust! Modem: modulator-demodulator! Modulation! Signals! Other types of modulation:! 1.!Binary Frequency-Shift Keying (BFSK): uses 2 fixed-amplitude carrier signals, each representing " 1 and 0! 2.!Binary Phase-Shift Keying (BPSK): both frequency and amplitude of the carrier signal are fixed, 1 and 0 are different phases of the signal! 3.!Differential Phase-Shift Keying (DPSK): phase shift occurs at each bit transition, the phase difference is relative to the last bit! Halsall! Bit vs. Baud!! bit transmitted as electrical or optical signal!! bit rate: number of bits per second!! baud rate: signal/voltage level changes per second!! each level can represent multiple bits!! for binary signalling, bit rate == baud rate!! for M-ary signalling, bit rate! baud rate!! example: 4-ary signalling! Example: RS-232!! negative voltage (-15V) represents a 1!! positive voltage (+15V) represents a 0!! bit rate == baud rate!! 7 bits/character!! to allow asynchronous communication: " 1 start bit, 1 stop bit! voltage sec -3 7 baud/sec = 14 bps time

2 Modulation! Other types of modulation:! 1.!Quadrature Phase-Shift Keying (QPSK): each phase of the signal represents 2 bits, giving 4 values! 2.!8PSK: each phase represents 3 bits, giving 8 values" Reducing phase differences makes the signal more prone to noise and interference! 3.!Quadrature Amplitude Modulation (QAM): introduces amplitude in addition to phase variations: 16QAM carries 4 bits per symbol! 4.!Orthogonal FDM (OFDM): uses orthogonal sub-frequencies, instead of mathematical codes, to carry signal; uses Inverse FFT to create composite waveform, decoded with FFT! Halsall! Dahlman et al.! Wireless Link Characteristics! Differences from wired link.!! decreased signal strength: radio signal attenuates as it propagates through matter (path loss)!! interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well!! multipath propagation: radio signal reflects off objects, ground, arriving at destination at slightly different times!. make communication across (even a point to point) wireless link much more difficult! Wireless Link Characteristics! SNR: signal-to-noise ratio! BER: bit error rate!! larger SNR easier to extract signal from noise (a good thing )! SNR versus BER tradeoffs!! given physical layer: increased power! increased SNR! decreased BER!! given SNR: choose physical layer that meets BER requirement, giving highest throughput!! SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate)!! all the latest standards have rate adaptation, including , , and LTE! Fundamental trade-off: power vs. range vs. rate! BER SNR(dB) QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) Multi-antenna (MIMO) Techniques! With bandwidth reaching Shannon s limit, future gain in bandwidth will come from smarter antenna use:! Beamforming: generating interfering patterns from multiple antennae such that the intended signal is strengthened, in the direction intended (a.k.a., smart antenna and adaptive antenna system (AAS))! Spatial multiplexing: transmitting multiple data streams simultaneously on the same frequency using multiple antennae; streams received over multiple antennae and separated using various detection algorithms" (usually MIMO is used to refer to spatial multiplexing in particular)! Chen & Guizani!

3 Data Link Layer! The Data Link layer can be further subdivided into:! 1.! Logical Link Control (LLC): error and flow control! 2.! Media Access Control (MAC): framing and media access! different link protocols may provide different services, e.g., Ethernet doesn t provide reliable delivery (error recovery)! application transport network LLC MAC physical Multiple Access Problem! Broadcast channel of rate R bps, shared medium!! if two users send at the same time, collision results in no packet being received (interference)!! if no users send, channel goes idle!! thus, want to have only one user send at a time! Media Access Control:!! determines who gets to send next!! what to do if more than one hosts " send at the same time and there s collision! Duplex mode:!! half duplex: only one end can send at a time!! full duplex: both ends can send simultaneously! MAC Protocols: a Taxonomy! Three broad classes:! 1.! Channel Partitioning!! divide channel into smaller pieces (time slots, frequency, code)!! allocate piece to node for exclusive use!! TDMA, FDMA, CDMA! 2.! Random Access!! channel not divided, allow collisions!! recover from collisions!! CSMA/CD, CSMA/CA! 3.! Polling, reservation, taking turns!! Nodes take turns, but nodes with more to send can take longer turns!! Token ring, WiMAX! WiMAX: Downlink, Uplink Scheduling! Transmission frame!! down-link subframe: base station to node!! uplink subframe: node to base station! pream. DL- MAP UL- MAP DL burst 1 DL burst 2 downlink subframe DL burst n Initial maint. request conn. SS #1 SS #2 SS #k uplink subframe base station tells mobiles who will get to receive (DL map)! and who will get to send (UL map), and when! WiMAX standard provide mechanism for scheduling, but not the scheduling algorithm itself!

4 TDMA: Time Division Multiple Access!! channel divided into N time slots, one per user!! access to channel in "rounds"!! each station gets fixed length slot (length = packet transmission time) in each round! frequency! Example:! 4 users! time!! inefficient with low duty cycle users and at light load: unused slots go idle!! example: 6-station LAN, 1,3,4 have packets, slots 2,5,6 idle! FDMA: Frequency DMA!! channel spectrum divided into frequency bands!! each station assigned fixed frequency band! frequency! time!! unused transmission time in frequency bands go idle!! example: 6-station LAN, 1,3,4 have packets, frequency bands 2,5,6 idle! frequency bands time Example:! 4 users! FDMA, TDMA, CDMA! code! CDMA: Code DMA! Unique code (c m ) assigned to each user (m); i.e., code set partitioning! All users share same frequency, but each user has own chipping sequence (i.e., code) to encode data! Encoded signal for bit i for user m (Z i,m ) = (original data bit i, d i ) * (chipping sequence, c m )! Decoding: dot-product of encoded signal and chipping sequence divided by c m =M Allows multiple users to coexist and transmit simultaneously with minimal interference (if codes are orthogonal or have low cross correlation)!

5 CDMA Encode/Decode! CDMA with Multiple Users! sender! data! bits! code! d 1 = d 0 = slot 1! slot 0! Z i,m = d i c m channel output Z i,m 1-1 slot 1! channel! output! slot 0! channel! output! receiver! received! input! code! slot 1! slot 0! D i = M! Z i,m c m m=1 c m d 1 = -1 slot 1! channel! output! d 0 = 1 slot 0! channel! output! CDMA: Two-Sender Case! Dot Product Review!! b a! The dot product of two vectors a and b is a scalar value a b cos(!)! Another way to compute a b is as:"! b a b =! a x a y # x # 2 % & " $ b " % y $ & = ' a b i i = a x b x + a y b y i=0! If a and b are orthogonal ( a! b ),! = 90 0," a b = 0

6 CDMA Decoding! Receiver extraction works because each bit of code (c m,i ) is equally likely to be -1 or +1! So if the wrong code (c m,i ) is used to decode, "c m c m is likely to be 0! If the right code is used, the sign of "c m c m determines whether data is 1 or -1! Instead of low-cross correlation random code per sender, can use orthogonal codes which guarantees "c m c m = 0 if the wrong code is used to decode! Wireless Link Characteristics! Differences from wired link.!! decreased signal strength: radio signal attenuates as it propagates through matter (path loss)!! interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well!! multipath propagation: radio signal reflects off objects, ground, arriving at destination at slightly different times!. make communication across (even a point to point) wireless link much more difficult! CDMA: Issues! LAN architecture!! Code choice: Barker (802.11), Walsh (cdmaone), Orthogonal Variable Spreading Factor (WCDMA)!! Power control: powerful signal interferes with others!! open-loop: observe received signal, works for TDD, not so well for FDD!! closed-loop: requires fast " feedback time! AP! Internet! hub, switch! or router! Wireless host communicates with base station/access point (AP)! Basic Service Set (BSS) (aka cell ) in infrastructure mode contains:!! wireless hosts!! access point (AP): base station!! RAKE receiver:!! takes advantage of multipathing: multiple copies of streams arrive at the receiver as signal is bounced off the environment; with RAKE receiver, late arriving copies used to help strengthen the signal!! enables soft handoff: a mobile receives copies from multiple base stations during handoff! Kwok & Lau! BSS 1! AP! BSS 2! Ad hoc mode contains hosts only!

7 802.11: Channels, Association! b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies!! AP admin chooses frequency for AP!! interference possible: channel can be same as that chosen by neighboring AP!! Host: must associate with an AP!! scans channels, listening for beacon frames containing AP s name (SSID) and MAC address!! selects AP to associate with!! may perform authentication!! will typically run DHCP to get IP address in AP s subnet! : Passive/Active Scanning! AP 1! BBS 1! 1 H1! 1 2 BBS 2! AP 2! Passive Scanning:! (1)!Beacon frames sent from APs! (2)!Association Request frame sent from H1 to selected AP! (3)!Association Response frame sent from selected AP to H1! 3 AP 1! BBS 1! 2 1 H1! BBS 2! AP 2! Active Scanning:! (1)!Probe Request frame broadcast from H1! (2)!Probe Response frames sent from APs! (3)!Association Request frame sent from H1 to selected AP! (4)!Association Response frame sent from selected AP to H1! Wireless Network Characteristics! Multiple wireless senders and receivers create additional problems (beyond multiple access):! A! C! B! Hidden terminal problem!! B, A hear each other!! B, C hear each other!! A, C can not hear each other!! A, C unaware of their interference at B! A! B! C! A s signal! strength! space! C s signal! strength! Signal fading:!! B, A hear each other!! B, C hear each other!! A, C can not hear each other interfering at B! IEEE : Multiple Access! Collision: two or more nodes transmitting at same time! : CSMA - sense before transmitting!! don t collide with ongoing transmission by another node! : no collision detection!!! difficult to receive (sense collisions) when transmitting due to weak received signals (fading)!! can t sense all collisions in any case: hidden terminal, fading!! goal: avoid collisions: CSMA/C(ollision)A(voidance)! A! C! B! A! B C! A s signal! strength! space! C s signal! strength!

8 IEEE MAC Protocol: CSMA/CA! sender:! 1.! if sense channel idle for DIFS time, transmit entire frame (no CD)! 2.! if sense channel busy start DIFS+random backoff time (why the random part?)!! timer counts down while channel idle!! transmit when timer expires! 3.! if no ACK, increase random backoff interval, repeat 2! receiver:!! if frame received OK, return ACK after " SIFS time (why we need ACK?)! DIFS! sender! data! ACK! receiver! SIFS! Use of Short Inter-Frame Spacing and Distributed IFS act as a way to prioritize transmission (ACKs get to go first)! Collision Avoidance! Allow sender to reserve channel rather than relying on random access of data frames: avoid collisions of long data frames! Sender first transmits small request-to-send (RTS) packets to AP using CSMA!! RTSs may still collide with each other (but they re short)! AP broadcasts clear-to-send (CTS) in response to RTS! RTS heard by all nodes!! sender transmits data frame!! other nodes defer transmissions! Avoid data frame collisions completely by using small reservation packets!! But usually not done on deployed systems... (why?)! RTS-CTS Exchange! Wireless Network Characteristics! time! A! B! AP! RTS(A)! RTS(A)! CTS(A)! DATA (A)! ACK(A)! reservation collision CTS(A)! ACK(A)! RTS(B)! defer! D! A! C! B! Exposed terminal problem:!! B sends to A!! C can send to D without interfering with B s transmission to A!! Solution: if C hears B s RTS but not A s CTS, C can send" (A can be an AP)!

9 Frame: Addressing! Frame: Addressing! frame address address address seq address duration payload CRC control control 4 Address 1: MAC address! of wireless host or AP! to receive this frame! Address 2: MAC address! of wireless host or AP! transmitting this frame! Address 3: MAC address! of router interface to which AP is attached! Address 3: used only in ad hoc mode! Internet! router! H1! R1! AP! R1 MAC addr AP MAC addr! dest. address! source address! frame! AP MAC addr H1 MAC addr R1 MAC addr! address 1! address 2! address 3! frame! Frame: More! frame control Protocol version duration address 1 Type duration of reserved! transmission time (RTS/CTS)! Subtype address 2 To AP address 3 From AP seq control More frag address 4 frame seq #! (for reliable ARQ)! 1 1 Power Retry mgt payload More data CRC WEP Rsvd : Power Management! Power Management!! node-to-ap: I am going to sleep until next beacon frame!! AP knows not to transmit frames to this node!! node wakes up before next beacon frame!! beacon frame: contains list of mobiles with AP-tomobile frames waiting to be sent!! node will stay awake if it has frame incoming; otherwise sleep again until next beacon frame! frame type! (RTS, CTS, ACK, data)!

10 Wired Equivalent Privacy (WEP):! Authentication:!! host requests authentication from access point!! access point sends 128 bit nonce!! host encrypts nonce using shared symmetric key!! access point decrypts nonce, authenticates host! No key distribution mechanism! Authentication: knowing the shared key is enough! WEP Data Encryption! Host/AP share 40 bit symmetric key (semi-permanent)! Host appends 24-bit initialization vector (IV) to create 64-bit key! 64 bit key used to generate stream of keys, k i IV! k i IV used to encrypt i-th byte, d i, in frame:! c i = d i XOR k i IV! IV and encrypted bytes, c i sent in frame, IV sent in the clear! Sender-side WEP encryption! Breaking WEP Encryption! Security vulnerability:!! 24-bit IV, one IV per frame! IV s eventually reused!! IV transmitted in plaintext! IV reuse detected!! Since shared key is not changed, this gives only 2 24 unique keys!! Key reuse is > 99% probable after only 12,000 frames!! With 1K frame size, at 11 Mbps transmission, this means only a few seconds of transmission! Attack:!! Trudy causes Alice to encrypt known plaintext d 1 d 2 d 3 d 4!! Trudy sees: c i = d i XOR k i IV!! Trudy knows c i and d i, so can compute k i IV!! Trudy knows encrypting key sequence k 1 IV k 2 IV k 3 IV!! next time IV is used, Trudy can decrypt!! i: Improved Security! A 2004 amendment to the original standard, incorporated into the 2007 updated standard!! replaces WEP!! subsumes WPA (Wi-Fi Protected Access)!! a.k.a. WPA2 or RSN (Robust Security Network)! Provides key distribution! Uses authentication server separate from access point!

11 802.11i Encryption Key Distribution! Cellular Networks: the Last Mile! MH: mobile host 1 Discovery of security capabilities AP: access point wired network 2 MH and AS mutually authenticate, together generate Master Key (MK). AP servers as pass through 3 MH derives Pairwise Master Key (PMK) 4 MH, AP use PMK to derive Temporal Key (TK) used for message encryption, integrity 3 AS derives same PMK, sends to AP AS: Authentication server Three techniques for sharing mobile-to-base station radio spectrum:!! Combined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slots!! CDMA: code division frequency bands multiple access!! Taking turn by scheduling! time slots What is Mobility?! : Mobility Within Same Subnet! Spectrum of mobility, from the network perspective:! no mobility! mobile wireless user, using same AP! mobile user, (dis)connecting from network using DHCP! high mobility! mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone)! H1 remains in same IP subnet: IP address can remain same! switch: which AP is associated with H1?!! self-learning (Ch. 5): switch will see frame from H1 and remember which switch port can be used to reach H1! BBS 1! AP 1! router! hub or! switch! AP 2! H1! BBS 2!

12 How do You Contact a Mobile Friend! Mobility: Approaches! Consider friend frequently changing addresses, how do you find her?! search all phone books?! call her parents?! expect her to let you! know where he/she is?! I wonder where Alice moved to? Let routing handle it: routers advertise permanent not! address of mobile-nodes-in-residence via usual routing scalable! table exchange.! to millions of!! routing tables indicate where each mobile located! mobiles!! 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! Boeing Connexion Mobility Service! Mobility: Vocabulary! home network: permanent home of mobile! (e.g., /24)! home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote! Permanent address: address in home network, can always be used to reach mobile! e.g., ! wide area

13 Mobility: More Vocabulary! correspondent: wants to communicate with mobile! Permanent address: remains constant (e.g., )! Care-of-address: address in visited network.! (e.g., 79, )! wide area visited network: network in which mobile currently resides (e.g., /24)! foreign agent: entity in visited network that performs mobility functions on behalf of mobile.! Mobility: Registration! home 2! wide area foreign agent contacts home agent home: this mobile is resident in my network! End result:! Foreign agent knows about mobile! Home agent knows location of mobile! visited 1! mobile contacts foreign agent on entering visited Mobility via Indirect Routing! home! correspondent addresses packets using home address of mobile! home agent intercepts packets, forwards to foreign agent! 1! wide area 2! foreign agent receives packets, forwards to mobile! 4! 3! visited! mobile replies directly to Indirect Routing! Mobile uses two addresses:!! permanent address: used by correspondent (hence mobile location is transparent to correspondent)!! care-of-address: used by home agent to forward datagrams to mobile! Foreign agent functions may be done by mobile itself! Moving between networks: suppose mobile user moves to another! registers with new foreign agent!! new foreign agent registers with home agent!! home agent update care-of-address for mobile!! packets continue to be forwarded to mobile (but with new care-of-address)! mobility, changing foreign networks is transparent: on going connections can be maintained!! Triangle routing:! correspondent-home-network-mobile!! inefficient when correspondent, mobile are" in same

14 Mobility via Direct Routing! home! correspondent requests, receives foreign address of mobile! correspondent forwards to foreign agent! 2! 1! wide area foreign agent receives packets, forwards to mobile! 3! 5! 4! visited! mobile replies directly to Mobility via Direct Routing" Comments! overcome triangle routing problem! non-transparent to correspondent: correspondent must get care-of-address from home agent!! what if mobile changes visited network?! Accommodating Mobility " with Direct Routing! Anchor foreign agent: FA in first visited Data always routed first to anchor FA! When mobile moves: new FA arranges to have data forwarded from old FA (chaining)! correspondent wide area 1! correspondent agent anchor foreign agent 5! 4! 3! new foreign agent foreign net visited at session start 2! new foreign network Mobile IP [RFC3220]! Has many features we ve seen:!! home agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet)! three components to standard:!! indirect routing of datagrams!! agent discovery!! registration with home agent!

15 Components of! Cellular Network Architecture! MSC! wired public telephone! MSC! MSC! MSC! MSC! different cellular networks,! operated by different providers! GSM: Handoff within Common MSC! old BS! VLR! Mobile! Switching! Center! old! routing! new! routing! new BS! Handoff goal: route call via new base station (without interruption)! Reasons for handoff:!! stronger signal to/from new BS (continuing connectivity, less battery drain)!! load balance: free up channel in current BS!! GSM doesn t mandate why to perform handoff (policy), only how (mechanism)! Handoff initiated by old BS! old BS GSM: Handoff with Common MSC! VLR Mobile Switching Center new BS 1.! old BS informs MSC of impending handoff, provides list of 1 + new BSs 2.! MSC sets up path (allocates resources) to new BS 3.! new BS allocates radio channel for use by mobile 4.! new BS signals MSC, old BS: ready 5.! old BS tells mobile: perform handoff to new BS 6.! mobile, new BS signal to activate new channel 7.! mobile signals via new BS to MSC: handoff complete. MSC reroutes call 8.! MSC-old-BS resources released! Handling Mobility in" Cellular Networks! Home network: network of cellular provider you subscribe to (e.g., Sprint PCS, Verizon)!! home location register (HLR): database in home network containing permanent cell phone #, profile information (services, preferences, billing), information about current location (could be in another network)! Visited network: network in which mobile currently resides!! visitor location register (VLR): database with entry for each user currently in! could be home

16 GSM: Indirect Routing to Mobile! GSM: Handoff Between MSCs! home MSC consults HLR,! gets roaming number of! mobile in visited mobile! user! HLR! 2! 4! home! home! Mobile! Switching! Center! VLR! Mobile! Switching! Center! visited! 3! 1! call routed! to home Public switched! telephone! network! home MSC sets up 2 nd leg of call! to MSC in visited MSC in visited network completes! call through base station to mobile! home Home MSC! anchor MSC! MSC! PSTN! MSC! MSC! Anchor MSC: first MSC visited during call!! call remains routed through anchor MSC! New MSCs add on to end of MSC chain as mobile moves to new MSC! IS-41 allows optional path minimization step to shorten multi-msc chain! Session Initiation Protocol (SIP)! Comes from IETF! SIP long-term vision!! All telephone calls and video conference calls take place over the Internet!! People are identified by names or addresses, rather than by phone numbers!! You can reach the callee, no matter where the callee roams, no matter what IP device the callee is currently using! Included as part of 3GPP s IP Multimedia Subsystem (IMS)! SIP Services! Setting up a call!! provides mechanisms for caller to let callee know she wants to establish a call!! provides mechanisms so that caller and callee can agree on media type and encoding!! provides mechanisms to end call! Determine current IP address of callee!! maps mnemonic identifier to current IP address! Call management!! add new media streams during call!! change encoding during call!! invite others!! transfer and hold calls!

17 Setting up a Call to an IP Address!! Alice s SIP invite message indicates her port number & IP address, and encoding that Alice prefers to receive (PCM ulaw)"! Bob s 200 OK message indicates his port number, IP address & preferred encoding (GSM)"! SIP messages can be sent over TCP or UDP; here sent over RTP/UDP" Setting up a Call (more)! Codec negotiation:!! suppose Bob doesn t have PCM ulaw encoder!! Bob will instead reply with 606 Not Acceptable Reply and list encoders he can use!! Alice can then send a new INVITE message, advertising an appropriate encoder! Rejecting the call!! Bob can reject with replies busy, gone, payment required, forbidden! Media can be sent over RTP or some other protocol!! Default SIP port number is 5060! Name Translation and User Location! Caller wants to call callee, but only has callee s name or address! Need to get IP address of callee s current host:!! user moves around!! DHCP protocol!! user has different IP devices (PC, PDA, car device)! Result can be based on:!! time of day (work, home)!! caller (don t want boss to call you at home)!! status of callee (calls sent to voic when callee is already talking to someone)! Service provided by SIP servers:!! SIP registrar server!! SIP proxy server! Example of SIP Message! INVITE sip:bob@domain.com SIP/2.0 Via: SIP/2.0/UDP From: sip:alice@hereway.com To: sip:bob@domain.com Call-ID: a2e3a@pigeon.hereway.com Content-Type: application/sdp Content-Length: 885 c=in IP m=audio RTP/AVP 0 Notes:! HTTP message syntax! sdp = session description protocol! Call-ID is unique for every call!! Here we don t know Bob s IP address, intermediate SIP servers will be necessary!! Alice sends and " receives SIP messages " using the SIP default " port number 5060!! Alice specifies in Via:" header that SIP client " sends and receives " SIP messages over UDP!

18 SIP Registrar! When Bob starts SIP client, client sends SIP REGISTER message to Bob s registrar server (similar function needed by Instant Messaging)! Register Message:! REGISTER sip:domain.com SIP/2.0 Via: SIP/2.0/UDP From: sip:bob@domain.com To: sip:bob@domain.com Expires: 3600 SIP Proxy! Alice sends invite message to her proxy server!! contains address sip:bob@domain.com! Proxy responsible for routing SIP messages to callee!! possibly through multiple proxies! Callee sends response back through the same set of proxies! Proxy returns SIP response message to Alice!! contains Bob s IP address! Note: proxy is analogous to local DNS server! Example! Caller jim@umass.edu " with places a! call to keith@upenn.edu " (1) Jim sends INVITE" message to umass SIP" proxy (2) Proxy forwards" request to upenn " registrar server" (3) upenn server returns" redirect response," indicating that it should " try keith@eurecom.fr! (4) umass proxy sends INVITE to eurecom registrar (5) eurecom registrar forwards INVITE to , which is running keith s SIP client (6-8) SIP response sent back (9) media sent directly between clients! Note: also a SIP ack message, which is not shown!