Скачать презентацию Transporting Voice by Using IP Chapter 2 Скачать презентацию Transporting Voice by Using IP Chapter 2

f42fa758f8c1108401c3c0d3a5e9402a.ppt

  • Количество слайдов: 76

Transporting Voice by Using IP Chapter 2 Transporting Voice by Using IP Chapter 2

Internet Overview n A collection of networks n The private networks n n The Internet Overview n A collection of networks n The private networks n n The public networks n n n LANs, MANs, WANs Institutions, corporations, business and government May use various communication protocols ISP: Internet Service Providers Using Internet Protocol To connect to the Internet n Using IP Internet Telephony 2

Internet Overview n Interconnecting networks n Routers provides the connectivity Internet Telephony 3 Internet Overview n Interconnecting networks n Routers provides the connectivity Internet Telephony 3

Overview of the IP Protocol Suite n IP n n n A routing protocol Overview of the IP Protocol Suite n IP n n n A routing protocol for the passing of data packets Must work in cooperation with higher layer protocols The OSI seven-layer model n n The top layer: usable information passed The information must be n n n Packaged appropriately Routed correctly And it must traverse some physical medium Internet Telephony 4

The IP suite and the OSI stack n TCP n n Reliable, error-free, in-sequence The IP suite and the OSI stack n TCP n n Reliable, error-free, in-sequence delivery UDP n No sequencing, no retransmission Internet Telephony 5

Internet Standards and the Process n The Internet Society n n A non-profit, non-governmental, Internet Standards and the Process n The Internet Society n n A non-profit, non-governmental, international, professional membership organization Keep the Internet alive and growing “to assure the open development, evolution, and the use of Internet for the benefit of all people throughout the world” The tasks n n Support the development and dissemination of Internet standards Support the RD related to the Internet and internetworking Assists developing countries Form a liaison for Internet development Internet Telephony 6

n IAB n n The Internet Architecture Board The technical advisory group n n n IAB n n The Internet Architecture Board The technical advisory group n n n provides overall architectural advice to IESG, IETF & ISOC appoints IRTF chair Technical guidance, oversee the Internet standards process Internet Telephony 7

n IETF, formed in 1986 n Comprises a huge number of volunteers n n n IETF, formed in 1986 n Comprises a huge number of volunteers n n Develop Internet standards n n n “We reject kings, presidents and voting. We believe in rough consensus and running code”, Dave Clark standards only when people use them Detailed technical work Working groups (>100) n avt, megaco, iptel, sip, sigtran Internet Telephony 8

n IESG, The Internet Engineering Steering Group n n Area Directors + IETF Chair n IESG, The Internet Engineering Steering Group n n Area Directors + IETF Chair Manage the IETF’s activities Approve an official standard Can a standard advance? Internet Telephony 9

n IANA, The Internet Assigned Numbers Authority n Unique numbers and parameters used in n IANA, The Internet Assigned Numbers Authority n Unique numbers and parameters used in Internet standards n n IP addresses n n mostly delegated to IP 5 regional Address registries domain names n n n Be registered with the IANA deals with top level domains (TLDs) mostly delegated to DNS name registries functions split with the creation of ICANN n Internet Corporation for Assigned Names and Numbers Internet Telephony 10

n Internet Research Task Force (IRTF) n Focused on long term problems in Internet n Internet Research Task Force (IRTF) n Focused on long term problems in Internet n n n n n Anti-Spam Crypto Forum Delay-Tolerant Networking End-to-End Host Identity Protocol Internet Measurement IP Mobility Optimizations Network Management Peer-to-Peer Routing Internet Telephony 11

Top Level View of Organization n Internet Society IAB IRTF IASA IANA RFC IESG Top Level View of Organization n Internet Society IAB IRTF IASA IANA RFC IESG area “the IETF” Internet Telephony 12

The Internet Standards Process n The process n n RFC 2026 First, Internet Draft The Internet Standards Process n The process n n RFC 2026 First, Internet Draft n n The early version of spec. Can be updated, replaced, or made obsolete by another document at any time IETF’s Internet Drafts directory Six-month life-time Internet Telephony 13

n RFC n n n Proposed standard n n Request for Comments An RFC n RFC n n n Proposed standard n n Request for Comments An RFC number A stable, complete, and well-understood spec. Has garnered significant interest May be required to implement for critical protocols Draft standard n n n Two independently successful implementations Interoperability be demonstrated Requirements not met be removed Internet Telephony 14

n A standard n n n The IESG is satisfied The spec. is stable n A standard n n n The IESG is satisfied The spec. is stable and mature Significant operational experience A standard (STD) number Not all RFCs are standards n Some document Best Current Practices (BCPs) n n Others applicability statements n n Processes, policies, or operational considerations How a spec be used, or different specs work together STD 1 list the various RFCs Internet Telephony 15

IPR (Patents) n RFC 2026 revised IETF IPR rules n now use standards sequence IPR (Patents) n RFC 2026 revised IETF IPR rules n now use standards sequence to check IPR issues n n n used to require “fair & non-discriminatory” licensing some standards blocked using old process require multiple implementations based on multiple licenses to progress to Draft Standard or Internet Standard but a worry about “submarine patents” IPR working group n n clear up fuzzy language in RFC 2026 produced RFC 3978 and RFC 3979 Internet Telephony 16

IPR, contd. n IETF IPR (patent) rules in RFC 3979 n n n require IPR, contd. n IETF IPR (patent) rules in RFC 3979 n n n require disclosure of your own IPR in your own submissions & submissions of others “reasonably and personally” known IPR WG takes IPR into account when choosing solution n RFC 3669 gives background and guidance push from open source people for RF (royalty-free)only process consensus to not change to mandatory RF-only n but many WGs tend to want RF or IPR-free Internet Telephony 17

IP n RFC 791 n n Amendments, RFCs 950, 919, and 920 Requirements for IP n RFC 791 n n Amendments, RFCs 950, 919, and 920 Requirements for Internet hosts, RFCs 1122, 1123 Requirements for IP routers, RFC 1812 IP datagram n n Data packet with an IP header Best-effort protocol Internet Telephony 18

IP Header n Version n n 4 Header Length Type of Service Total Length IP Header n Version n n 4 Header Length Type of Service Total Length Identification, Flags, and Fragment Offset n n n A datagram can be split into fragments Identify data fragments Flags n n n a datagram can be fragmented or not Indicate the last fragment TTL n A number of hops (not a number of seconds) Internet Telephony 19

n Protocol n n n The higher-layer protocol TCP, 6; UDP, 17 Source and n Protocol n n n The higher-layer protocol TCP, 6; UDP, 17 Source and Destination IP Addresses Internet Telephony 20

IP Routing n n Based on the destination address Routers n n n Can IP Routing n n Based on the destination address Routers n n n Can contain a range of different interfaces Determine the best outgoing interface for a given IP datagram Routing table n n n Destination IP route mask the longest match Internet Telephony 21

Populating routing tables n Issues n n The correct information in the first place Populating routing tables n Issues n n The correct information in the first place Keep the information current in a dynamic environ The best path? Protocols n n RIP, Routing Information Protocol IGRP, Internet Gateway Routing Protocol OSPF, Open Short Path First BGP, Border Gateway Protocol, RFC 1771 Internet Telephony 22

TCP n n n n In sequence, without omissions and without errors End-to-end confirmation, TCP n n n n In sequence, without omissions and without errors End-to-end confirmation, packet retransmission, Flow control RFC 793 Break up a data stream in segments Attach a TCP header Sent down the stack to IP At the destination, checks the header for errors n n Send back an ack The source retransmits if no ack within a given period Internet Telephony 23

n The TCP header Internet Telephony 24 n The TCP header Internet Telephony 24

The TCP Header n TCP Port Numbers n n n Identifying a specific instance The TCP Header n TCP Port Numbers n n n Identifying a specific instance of a given application A unique port number for a particular session Well-known port numbers n n n IANA, 0 -1023 23, telnet; 25, SMTP Many clients and a server n n n TCP/IP Source address and port number + Destination address and port number A socket address Internet Telephony 25

n Sequence and acknowledge numbers n n n Identify individual segments Actually count data n Sequence and acknowledge numbers n n n Identify individual segments Actually count data octets transmitted A given segment with a SN of 100 and contains 150 octets of data n n n The ack number will be 250 The SN of the next segment is 250 Other header fields n n Data offset: header length (in 32 -bit words) URG: 1 if urgent data is included, use urgent pointer field ACK: 1, an ACK PSH: a push function, be delivered promptly Internet Telephony 26

n n RST: reset; an error and abort a session SYN: Synchronize; the initial n n RST: reset; an error and abort a session SYN: Synchronize; the initial messages FIN: Finish; close a session Window n n The amount of buffer space available for receiving data Checksum n 1's complement of 1's complement sum of all 16 -bit words in the TCP header, the data, and a pseudo header Internet Telephony 27

n Urgent Pointer n n n An offset to the first segment after the n Urgent Pointer n n n An offset to the first segment after the urgent data Indicates the length of the urgent data Critical information to be sent to the user application ASAP. Internet Telephony 28

TCP Connections n n An examples After receiving n n n 100, 200, 300 TCP Connections n n An examples After receiving n n n 100, 200, 300 ACK 400 Closing a connection n > FIN < ACK, FIN > ACK Internet Telephony 29

UDP n n n Pass individual pieces of data from an application to IP UDP n n n Pass individual pieces of data from an application to IP No ACK, inherently unreliable Applications n n n A quick, one-shot transmission of data, request/response DNS If no response, the AP retransmit the request The AP includes a request identifier The source port number is optional Checksum n The header, the data and the pseudo header Internet Telephony 30

n UDP header Internet Telephony 31 n UDP header Internet Telephony 31

Voice over UDP, not TCP n Speech n n n Small packets, 10 – Voice over UDP, not TCP n Speech n n n Small packets, 10 – 40 ms Occasional packet loss is not a catastrophe Delay-sensitive n n 5 % packet loss is acceptable if evenly spaced n n n Resource management and reservation techniques A managed IP network In-sequence delivery n n TCP: connection set-up, ack, retransmit → delays Mostly yes UDP was not designed for voice traffic Internet Telephony 32

The Real-Time Transport Protocol n RTP: A Transport Protocol for Real-Time Applications n n The Real-Time Transport Protocol n RTP: A Transport Protocol for Real-Time Applications n n UDP n n RFC 1889 RTP – Real-Time Transport Protocol RTCP – RTP Control Protocol Packets may be lost or out-of-sequence RTP over UDP n n n A sequence number A time stamp for synchronized play-out Does not solve the problems; simply provides additional information Internet Telephony 33

n RTCP n n n A companion protocol Exchange messages between session users # n RTCP n n n A companion protocol Exchange messages between session users # of lost packets, delay and inter-arrival jitter Quality feedback RTCP is implicitly open when an RTP session is open n n Not mandatory E. g. , RTP/RTCP uses UDP port 5004/5005 Internet Telephony 34

RTP Payload Formats n RTP carries the actual digitally encoded voice n n n RTP Payload Formats n RTP carries the actual digitally encoded voice n n n RTP header + a payload of voice samples UDP and IP headers are attached Many voice- and video-coding standards n A payload type identifier in the RTP header n n Specified in RFC 1890 New coding schemes have become available See table 2 -1 A sender has no idea what coding schemes a receiver could handle Internet Telephony 35

n Separate signaling systems n n n Capability negotiation during the call setup SIP n Separate signaling systems n n n Capability negotiation during the call setup SIP and SDP A dynamic payload type may be used n n Also support new coding scheme in the future The encoding name is also significant n n n Unambiguously refer to a particular payload specification Should be registered with the IANA RED, Redundant payload type n n n Voice samples + previous samples May use different encoding schemes Cope with packet loss Internet Telephony 36

RTP Header Format Internet Telephony 37 RTP Header Format Internet Telephony 37

The RTP Header n Version (V) n n Padding (P) n n 2 The The RTP Header n Version (V) n n Padding (P) n n 2 The padding octets at the end of the payload The payload needs to align with 32 -bit boundary The last octet of the payload contains the count Extension (X) n 1, contains a header extension Internet Telephony 38

n CSRC Count (CC) n n Marker (M) n n n The number of n CSRC Count (CC) n n Marker (M) n n n The number of contributing source indentifiers RFC 1890 The first packet of a talkspurt, after a silence period Payload Type (PT) n RED is an exception n n RFC 2198, the addition of headers Sequence number n n A random number at the beginning Incremented by one for each RTP packet Internet Telephony 39

n Timestamp n n n 32 -bit The instant at which the first sample n Timestamp n n n 32 -bit The instant at which the first sample The receiver n n n Synchronized play-out Calculate the jitter The clock freq depends on the encoding n n n E. g. , 8000 Hz, TS=1/10 samples; TS=11 Support silence suppression The initial timestamp is a random number Internet Telephony 40

n Synchronization Source (SSRC) n n n 32 -bit identifier The entity setting the n Synchronization Source (SSRC) n n n 32 -bit identifier The entity setting the sequence number and timestamp Chosen randomly, independent of the network address Meant to be globally unique within the session May be a sender or a mixer Contributing Source (CSRC) n n An SSRC value for a contributor 0 -15 CSRC entries Internet Telephony 41

n RTP Header Extensions n n Accommodate the common requirements of most media streams n RTP Header Extensions n n Accommodate the common requirements of most media streams For payload formats that requires Internet Telephony 42

Mixers and Translators n Mixers n n Enable multiple media streams from different sources Mixers and Translators n Mixers n n Enable multiple media streams from different sources to be combined An audio conference n n n Could also change the data format The SSRC is the mixer n n If the capacity or bandwidth of a participant is limited More than one CSRC values A translator n n Manage communications between entities that does not support the same coding scheme The SSRC is the participant, not the translator Internet Telephony 43

The RTP Control Protocol (RTCP) n RTCP n n A companion control protocol of The RTP Control Protocol (RTCP) n RTCP n n A companion control protocol of RTP Periodic exchange of control information n n A third party can also monitor session quality n n For quality-related feedback Using RTCP and IP multicast Five type of RTCP packets n n Sender Report: transmission and reception statistics Receiver Report: reception statistics Internet Telephony 44

n Source Description (SDES) n n One or more Must contain a canonical name n Source Description (SDES) n n One or more Must contain a canonical name n n BYE n n Separate from SSRC which might change When both audio and video streams exist n Have different SSRCs n Have the same CNAME for synchronized play-out Regular names, such as email address or phone number The end of participation APP n Convey application-specific functions Internet Telephony 45

n Two or more RTCP packets will be combined n n n SRs and n Two or more RTCP packets will be combined n n n SRs and RRs should be sent as often as possible New receivers in a session must receive CNAME An SR/RR must contain an SDES packet A SDES packet is sent with an SR/RR even if no data to report An example RTP compound packet Internet Telephony 46

RTCP Sender Report n SR n n Header Info Sender Info Receiver report blocks RTCP Sender Report n SR n n Header Info Sender Info Receiver report blocks Option n Profile-specific extension Internet Telephony 47

n Resemble to a RTP packet n Version n n Padding bit n n n Resemble to a RTP packet n Version n n Padding bit n n 2 Padding octets? RC, report count n n The number of reception report blocks 5 -bit n n n If more than 31 reports, an RR is added PT, payload type SSRC of sender Internet Telephony 48

n NTP Timestamp n Network Time Protocol Timestamp n n n The time elapsed n NTP Timestamp n Network Time Protocol Timestamp n n n The time elapsed in seconds since 00: 00, 1/1/1900 (GMT) 64 -bit n 32 MSB: the number of seconds n 32 LSB: the fraction of a seconds (200 ps) A primary time server n n n NTP, RFC 1305 Station WWV, Fort Collins, Colorado, by NIST Station WWVB, Boulder, Colorado, by NIST Internet Telephony 49

n RTP Timestamp n n Sender’s packet count n n Corresponding to the NTP n RTP Timestamp n n Sender’s packet count n n Corresponding to the NTP timestamp Use the same units and has the same offset as used for RTP timestamps For better synchronization Cumulative within a session Sender’s octet count n Cumulative Internet Telephony 50

RR blocks n SSRC_n n n Fraction lost n n n Fraction of packets RR blocks n SSRC_n n n Fraction lost n n n Fraction of packets lost since the last report issued by this participant By examining the sequence numbers in the RTP header Cumulative number of packets lost n n The source identifier Since the beginning of the RTP session Extended highest sequence number received n n n The sequence number of the last RTP packet received 16 lsb, the last sequence number 16 msb, the number of sequence number cycles Internet Telephony 51

n Interarrival jitter n n Last SR Timestamp (LSR) n n n An estimate n Interarrival jitter n n Last SR Timestamp (LSR) n n n An estimate of the variance in RTP packet arrival The middle 32 bits of the NTP timestamp used in the last SR received from the source in question Used to check if the last SR has been received Delay Since Last SR (DLSR) n The duration in units of 1/65, 536 seconds Internet Telephony 52

RTCP Receiver Report n RR n n Issued by a participant who receives RTP RTCP Receiver Report n RR n n Issued by a participant who receives RTP packets but does not send, or has not yet sent Is almost identical to an SR n n PT = 201 No sender information Internet Telephony 53

RTCP Source Description Packet n Provides identification and information regarding session participants n n RTCP Source Description Packet n Provides identification and information regarding session participants n n Header n n Must exist in every RTCP compound packet V, P, SC, PT=202, Length Zero or more chunks of information n n An SSRC or CSRC value One or more identifiers and pieces of information n Email address, phone number, name Defined in RFC 1889 A unique CNAME n E. g. , [email protected] Internet Telephony 54

n RTCP BYE Packet n n Indicate one or more media sources are no n RTCP BYE Packet n n Indicate one or more media sources are no longer active Application-Defined RTCP Packet n n For application-specific data For non-standardized application Internet Telephony 55

Calculating Round-Trip Time n n Use SRs and RRs E. g. n n n Calculating Round-Trip Time n n Use SRs and RRs E. g. n n n Report A: A, T 1 → B, T 2 Report B: B, T 3 → A, T 4 RTT = T 4 -T 3+T 2 -T 1 RTT = T 4 -(T 3 -T 2)-T 1 Report B n n LSR = T 1 DLSR = T 3 -T 2 Internet Telephony 56

Calculation Jitter n The mean deviation of the difference in packet spacing at the Calculation Jitter n The mean deviation of the difference in packet spacing at the receiver n n Si = the RTP timestamp for packet I Ri = the time of arrival D(i, j) = (Rj-Sj) - (Ri- Si) The Jitter is calculated continuously n J(i) = J(i-1) + (| D(i-1, i) | - J(i-1))/16 Internet Telephony 57

Timing of RTCP Packets n RTCP provides useful feedback n n n Regarding the Timing of RTCP Packets n RTCP provides useful feedback n n n Regarding the quality of an RTP session Delay, jitter, packet loss Be sent as often as possible n n n Consume the bandwidth Should be fixed at 5% An algorithm, RFC 1889 n n Senders are collectively allowed at least 25% of the control traffic bandwidth The interval > 5 seconds 0. 5 – 1. 5 times the calculated interval A dynamic estimate the avg RTCP packet size Internet Telephony 58

IP Multicast n An IP diagram sent to multiple hosts n n n Conference IP Multicast n An IP diagram sent to multiple hosts n n n Conference To a single address associated with all listeners Multicast groups n n Multicast address Join a multicast group n n Inform local routers Routing protocols n n Support propagation of routing information for multicast addresses Minimize the number of diagrams sent Internet Telephony 59

n IPv 4 n n n Multicast addresses: 224. 0. 0. 0 – 239. n IPv 4 n n n Multicast addresses: 224. 0. 0. 0 – 239. 255 224. 0. 0. 1 all hosts on a local subnet 224. 0. 0. 2 all routers on a local subnet 224. 0. 0. 5 all routers supporting OSPF 224. 0. 0. 9 all routers supporting RIP v. 2 IGMP, Internet Group Message Protocol n n Advertise membership in a group to routers RFC 1112, 2236 Internet Telephony 60

IP Version 6 n The explosive growth of the Internet n n n Expanded IP Version 6 n The explosive growth of the Internet n n n Expanded address space, 128 bits Simplified header format Improved support for headers and extensions Flow-labeling capability n n IPv 4 address space, 32 -bit Real-time and interactive applications Better support at the IP level for real-time ap Authentication and privacy n At the IP level Internet Telephony 61

IP Version 6 Header Internet Telephony 62 IP Version 6 Header Internet Telephony 62

IP Version 6 Header n Version n n Traffic Class, 8 -bit n n IP Version 6 Header n Version n n Traffic Class, 8 -bit n n 6 For the quality of service Flow Label, 20 -bit n n n Label sequences of packets A flow : = source address, destination address, flow label 0, no special handling Internet Telephony 63

n Payload Length, 16 -bit unsigned integer n n n The length of payload n Payload Length, 16 -bit unsigned integer n n n The length of payload in octets Header extensions are part of the payload Next Header, 8 -bit n The next higher-layer protocol n n n The existence of IPv 6 header extensions Hop Limit, 8 -bit unsigned integer n n Same as the IPv 4 The TTL field of the IPv 4 header Source and Destination Addresses, 128 -bit Internet Telephony 64

IP version 6 addresses n XXXX: XXXX: XXXX n n E. g. , 1511: IP version 6 addresses n XXXX: XXXX: XXXX n n E. g. , 1511: 1: 0: 0: 0: FA 22: 45: 11 n n n Leading zeros are omitted = 1511: 1: : FA 22: 45: 11 0: 0: AA 11: 50: 22: F 77 = : : AA 11: 50: 22: F 77 n n X is a hexadecimal character “: : ” can appears only once Should not generally require significant changes to up-layer protocols n But the checksum calculation in UDP and TCP includes a pseudo header Internet Telephony 65

IP version 6 special addresses n The all-zeros address, : : n n The IP version 6 special addresses n The all-zeros address, : : n n The loopback address, : : 1 n n On a virtual internal interface IPv 6 address with embedded IPv 4 address (type 1) n n n An unspecified address; a node does not yet know its address 96 -bit zeros + 32 -bit IPv 4 address : : 140. 113. 17. 5 IPv 6 address with embedded IPv 4 address (type 2) n n n 80 -bit zeros + FFFF + 32 -bit IPv 4 address 0: 0: 0: FFFF: 140. 113. 17. 5 : : FFFF: 140. 113. 17. 5 Internet Telephony 66

IP Version 6 Header Extensions n Extension Headers n Are not acted upon by IP Version 6 Header Extensions n Extension Headers n Are not acted upon by intermediate nodes n n With one exception Next Header n n The type of payload carried in the IP datagram The type of header extension Each extension has its own next header field An example Internet Telephony 67

Header extension n Use the next header field Internet Telephony 68 Header extension n Use the next header field Internet Telephony 68

Hop-by-hop extension n The exception header extension n n Next header Length n n Hop-by-hop extension n The exception header extension n n Next header Length n n Examined and processed by every intermediate node If present, must immediately follow the IP header Of variable length in units of eight octets, not including the first eight octets Options n TLV (Type-Length-Value) format n n 8 -bit, 8 -bit (in units of octets), variable length Type [0: 2] of special significance Internet Telephony 69

n Hop-by-hop header extension Internet Telephony 70 n Hop-by-hop header extension Internet Telephony 70

n Type[0: 1] n n n 00 skip this option and continue processing the n Type[0: 1] n n n 00 skip this option and continue processing the header 01 discard the packet 10 discard the packet and send an ICMP Parameter Problem, Code 2 message to the originator of the packet 11 do above only if the destination address in the IP header is not a multicast address Type[2] n n 1, the option data can be changed 0, cannot Internet Telephony 71

n Destination options extension n n Has the same format as the hop-by-hop extension n Destination options extension n n Has the same format as the hop-by-hop extension Only the destination node examine the extension Header type = 60 Routing Extension n n A routing type field to enable various routing options Only routing type 0 is defined for now Specify the nodes that should be visited Next header Length Internet Telephony 72

n Routing Extension Internet Telephony 73 n Routing Extension Internet Telephony 73

n n Routing type Segments Left n n n The number of nodes that n n Routing type Segments Left n n n The number of nodes that still need to be visited A list of IP addresses needs to be visited Is this type of header analyzed by intermediate node? n n n Yes or no A->Z, 3, (B, C, D) A->B, 3, (C, D, Z) A->C, 2, (B, D, Z) by B A->D, 1, (B, C, Z) by C A->Z, 0, (B, C, D) by D Internet Telephony 74

Interoperation IPv 4 and IPv 6 will coexist for a long time n n Interoperation IPv 4 and IPv 6 will coexist for a long time n n IPv 4 nodes IPv 6 nodes via IPv 4 networks IPv 4 nodes via IPv 6 networks IPv 4 -compatible nodes with IPv 4 -compatible addresses at the boundaries of IPv 6 networks n IPv 6 in IPv 4 packets Internet Telephony 75

Interoperation IPv 4 and IPv 6 n IPv 4 -compatible nodes with IPv 4 Interoperation IPv 4 and IPv 6 n IPv 4 -compatible nodes with IPv 4 -compatible addresses at the boundaries of IPv 6 networks n IPv 6 in IPv 4 packets Internet Telephony 76