Audio/Video Transport Working Group Tmima Koren Internet Draft Cisco Systems July 16, 2001 Stephen Casner Expires March 2002 Packet Design draft-ietf-avt-crtp-enhance-02.txt John Geevarghese Telseon Bruce Thompson Patrick Ruddy Cisco Systems Compressing IP/UDP/RTP headers on links with high delay, packet loss and reordering Status of this memo This document is an Internet Draft and is in full conformance with all provisions of Section 10 of RFC 2026. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsolete by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at: http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at: http://www.ietf.org/shadow.txt This draft is a work item of the IETF Audio/Video Transport working group. The working group mailing list is avt@ietf.org. Subscribe via the web at http://www.ietf.org/mailman/listinfo/avt. Copyright (C) The Internet Society (1999-2001). All Rights Reserved. Abstract This document describes a header compression scheme for point to point links with packet loss and long delays. It is based on CRTP, the IP/UDP/RTP header compression described in [RFC2508]. CRTP does not perform well on such links: packet loss results in context corruption and due to the long delay, many more packets are discarded before the context is repaired. To correct the behavior of CRTP over such links, a few extensions to the protocol are specified here. The extensions aim to reduce context corruption by changing the way the compressor updates the context at the decompressor: updates are repeated and include updates to full and differential context parameters. With these extensions, CRTP performs well over links with packet loss, packet reordering and long delays. The IPCP option 'IP header compression' (described in RFC 2509) is also extended to negotiate using the extended CRTP. 1.0 Introduction RTP header compression (CRTP) as described in RFC 2508 was designed to reduce the header overhead of IP/UDP/RTP datagrams by compressing the three headers. The IP/UDP/RTP headers are compressed to 2-4 bytes most of the time. CRTP was designed for reliable point to point links with short delays. It does not perform well over links with high rate of packet loss, packet reordering and long delays. An example of such a link is a PPP session that is tunneled using an IP level tunneling protocol such as L2TP. Packets within the tunnel are carried by an IP network and hence may get lost and reordered. The longer the tunnel, the longer the round trip time. Another example is an IP network that uses layer 2 technologies such as ATM and Frame Relay for the access portion of the network. Layer 2 transport networks such as ATM and Frame Relay behave like point to point serial links in that they do not reorder packets. In addition, Frame Relay and ATM virtual circuits used as IP access technologies often have a low bit rate associated with them. These virtual circuits differ from low speed serial links in that they may span a larger physical distance than a point to point serial link. Speed of light delays within the layer 2 transport network will result in higher round trip delays between the endpoints of the circuit. In addition, congestion within the layer 2 transport network may result in an effective drop rate for the virtual circuit which is significantly higher than error rates typically experienced on point to point serial links. CRTP is widely deployed and has relatively low computational complexity. It is desirable to extend its usage over such links. This can be achieved with a few simple extensions to the protocol. 1.1 CRTP Operation During compression of an RTP stream, a session context is defined. For each context, the session state is established and shared between the compressor and the decompressor. Once the context state is established, compressed packets may be sent. The context state consists of the full IP/UDP/RTP headers, a few first order differential values, a link sequence number, a generation number and a delta encoding table. The headers part of the context is set by the FULL_HEADER packet that always starts a compression session. The first order differential values (delta values) are set by sending COMPRESSED_RTP packets that include updates to the delta values. The context state must be synchronized between compressor and decompressor for successful decompression to take place. If the context gets out of sync, the decompressor is not able to restore the compressed headers accurately. The decompressor invalidates the context and sends a CONTEXT_STATE packet to the compressor indicating that the context has been corrupted. To resume compression, the compressor must reestablish the context. During the time the context is corrupted, the decompressor discards all the packets received for that context. Since the context repair mechanism in CRTP involves feedback from the decompressor, context repair takes at least as much time as the round trip time of the link. If the round trip time of the link is long, and especially if the link bandwidth is high, many packets will be discarded before the context is repaired. On such links it is desirable to minimize context invalidation. 1.2 How do contexts get corrupted? As long as the fields in the combined IP/UDP/RTP headers change as expected for the sequence of packets in a session, those headers can be compressed, and the decompressor can fully restore the compressed headers using the context state. When the headers don't change as expected it's necessary to update some of the full or the delta values of the context. For example, the RTP timestamp is expected to increment by delta RTP timestamp (dT). If silence suppression is used, packets are not sent during silence periods. Then when voice activity resumes, packets are sent again, but the RTP timestamp is incremented by a large value and not by dT. In this case an update must be sent. If a packet that includes an update to some context state values is lost, the state at the decompressor is not updated. The shared state is now different at the compressor and decompressor. When the next packet arrives at the decompressor, the decompressor will fail to restore the compressed headers accurately since the context state at the decompressor is different than the state at the compressor. 1.3 Preventing context corruption Note that the decompressor fails not when a packet is lost, but when the next compressed packet arrives. If the next packet happens to include the same context update as in the lost packet, the context at the decompressor may be updated successfully and decompression may continue uninterrupted. If the lost packet included an update to a delta field such as the delta RTP timestamp (dT), the next packet can't compensate for the loss since the update of a delta value is relative to the previous packet which was lost. But if the update is for an absolute value such as the full RTP timestamp or the RTP payload type, this update can be repeated in the next packet independently of the lost packet. Hence it is useful to be able to update the absolute values of the context. The next chapter describes several extensions to CRTP that add the capability to selectively update absolute values of the context, rather than sending a FULL_HEADER packet, in addition to the existing updates of the delta values. This enhanced version of CRTP is intended to minimize context invalidation and thus improve the performance over lossy links with a long round trip time. 1.4 Specification of Requirements The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 2. Enhanced CRTP This chapter specifies the changes in this enhanced version of CRTP. They are: - Extensions to the COMPRESSED_UDP packet to allow updating the differential RTP values in the decompressor context and to selectively update the absolute IP ID and RTP values. This allows context sync to be maintained even with some packet loss. - A 'headers checksum' to be inserted by the compressor and removed by the decompressor when the UDP checksum is not present so that validation of the decompressed headers is still possible. This allows the decompressor to verify that context sync has not been lost after a packet loss. An algorithm is then described to use these changes with repeated updates to achieve robust operation over links with packet loss and long delay. 2.1 Extended COMPRESSED_UDP packet It is possible to accommodate some packet loss between the compressor and decompressor using the "twice" algorithm in RFC 2508 so long as the context remains in sync. This requires reliably communicating both the absolute value and the delta value whenever the delta value changes. For many environments, sufficient reliability can be achieved by repeating the update with each of several successive packets. The COMPRESSED_UDP packet satisfies the need to communicate the absolute values of the differential RTP fields, but it is specified in RFC 2508 to reset the delta RTP timestamp. That limitation can be removed with the following simple change: RFC 2508 describes the format of COMPRESSED_UDP as being the same as COMPRESSED_RTP except that the M, S and T bits are always 0 and the corresponding delta fields are never included. This enhanced version of CRTP changes that specification to say that the T bit may be nonzero to indicate that the delta RTP timestamp is included explicitly rather than being reset to zero. A second change adds another byte of flag bits to the COMPRESSED_UDP packet to allow only selected individual uncompressed fields of the RTP header to be included in the packet rather than carrying the full RTP header as part of the UDP data. The additional flags do increase computational complexity somewhat, but the corresponding increase in bit efficiency is important when the differential field updates are communicated multiple times in successive COMPRESSED_UDP packets. With this change, there are flag bits to indicate inclusion of both delta values and absolute values, so the flag nomenclature is changed. The original S, T, I bits which indicate the inclusion of deltas are renamed dS, dT, dI, and the inclusion of absolute values is indicated by S, T, I. The M bit is absolute as before. A new flag P indicates inclusion of the absolute RTP payload type value and, as in the COMPRESSED_RTP packet, a four-bit CC field copies the absolute value of the CC field in the RTP header. The last of the three changes to the COMPRESSED_UDP packet deals with updating the IP ID field. For this field, the COMPRESSED_UDP packet as specified in RFC 2508 can already convey a new value for the delta IP ID, but not the absolute value which is only conveyed by the FULL_HEADER packet. Therefore, a new flag I is added to the COMPRESSED_UDP packet to indicate inclusion of the absolute IP ID value. The I flag replaces the dS flag which is not needed in the COMPRESSED_UDP packet since the delta RTP sequence number always remains 1 in the decompressor context and hence does not need to be updated. The format of the flags/sequence byte for the original COMPRESSED_UDP packet is shown here for reference: +---+---+---+---+---+---+---+---+ | 0 | 0 | 0 |dI | link sequence | +---+---+---+---+---+---+---+---+ The new definition of the flags/sequence byte plus an extension flags byte for the COMPRESSED_UDP packet is as follows, where the new F flag indicates the inclusion of the extension flags byte: +---+---+---+---+---+---+---+---+ | F | I |dT |dI | link sequence | +---+---+---+---+---+---+---+---+ : M : S : T : P : CC : (if F = 1) +...+...+...+...+...............+ dI = delta IP ID dT = delta RTP timestamp I = absolute IP ID F = additional flags byte M = marker bit S = absolute RTP sequence number T = absolute RTP timestamp P = RTP payload type CC = number of CSRC identifiers When F=0, there is only one flags byte, and the only available flags are: dI, dT and I. In this case the packet includes the full RTP header. As in RFC 2508, if dI=0, the decompressor does not change deltaI. If dT=0, the decompressor sets deltaT to 0. Some example packet formats will illustrate the use of the new flags. First, when F=0, the 'traditional' COMPRESSED_UDP packet which carries the full RTP header as part of the UDP data: 0 1 2 3 4 5 6 7 +...............................+ : msb of session context ID : (if 16-bit CID) +-------------------------------+ | lsb of session context ID | +---+---+---+---+---+---+---+---+ |F=0| I |dT |dI | link sequence | +---+---+---+---+---+---+---+---+ : : + UDP checksum + (if nonzero in context) : : +...............................+ : : + "RANDOM" fields + (if encapsulated) : : +...............................+ : delta IPv4 ID : (if dI = 1) +...............................+ : delta RTP timestamp : (if dT = 1) +...............................+ : : + IPv4 ID + (if I = 1) : : +...............................+ | UDP data | : (uncompressed RTP header) : When F=1, there is an additional flags byte and the available flags are: dI, dT, I, M, S, T, P, CC. In this case the packet does not include the full RTP header, but includes selected fields from the RTP header as specified by the flags. As in RFC 2508, if dI=0 the decompressor does not change deltaI. However, in contrast to RFC 2508, if dT=0 the decompressor KEEPS THE CURRENT deltaT in the context (DOES NOT set deltaT to 0). An enhanced COMPRESSED_UDP packet is similar in contents and behavior to a COMPRESSED_RTP packet, but it has more flag bits, some of which correspond to absolute values for RTP header fields. COMPRESSED_UDP with individual RTP fields, when F=1: 0 1 2 3 4 5 6 7 +...............................+ : msb of session context ID : (if 16-bit CID) +-------------------------------+ | lsb of session context ID | +---+---+---+---+---+---+---+---+ |F=1| I |dT |dI | link sequence | +---+---+---+---+---+---+---+---+ | M | S | T | P | CC | +---+---+---+---+---------------+ : : + UDP checksum + (if nonzero in context) : : +...............................+ : : : "RANDOM" fields : (if encapsulated) : : +...............................+ : delta IPv4 ID : (if dI = 1) +...............................+ : delta RTP timestamp : (if dT = 1) +...............................+ : : + IPv4 ID + (if I = 1) : : +...............................+ : : + RTP sequence number + (if S = 1) : : +...............................+ : : + + : : + RTP timestamp + (if T = 1) : : + + : : +...............................+ : RTP payload type : (if P = 1) +...............................+ : : : CSRC list : (if CC > 0) : : +...............................+ : : : RTP header extension : (if X set in context) : : +-------------------------------+ | | / RTP data / / / | | +-------------------------------+ : padding : (if P set in context) +...............................+ Usage for the enhanced COMPRESSED_UDP packet: It is useful for the compressor to periodically refresh the state of the decompressor to avoid having the decompressor send CONTEXT_STATE messages in the case of unrecoverable packet loss. Using the flags F=0 and I=1, dI=1, dT=1, the COMPRESSED_UDP packet refreshes all the context parameters. When compression is done over a lossy link with a long round trip delay, we want to minimize context invalidation. If the delta values are changing frequently, the context might get invalidated often. In such cases the compressor may choose to always send absolute values and never delta values, using COMPRESSED_UDP packets with the flags F=1, and any of S, T, I as necessary. 2.2 CRTP Headers Checksum RFC 2508, in Section 3.3.5, describes how the UDP checksum may be used to validate header reconstruction periodically or when the 'twice' algorithm is used. When a UDP checksum is not present (has value zero) in a stream, such validation would not be possible. To cover that case, this enhanced CRTP provides an option whereby the compressor MAY replace the null UDP checksum with a 16-bit headers checksum (HDRCKSUM) which is subsequently removed by the decompressor after validation. A new flag C in the FULL_HEADER packet, as specified below, indicates when set that all COMPRESSED_UDP and COMPRESSED_RTP packets sent in that context will have HDRCKSUM inserted. The compressor MAY set the C flag when UDP packet carried in the FULL_HEADER packet originally contained a checksum value of zero. If the C flag is set, the FULL_HEADER packet itself MUST also have the HDRCKSUM inserted. If a packet in the same stream subsequently arrives at the compressor with a UDP checksum present, then a new FULL_HEADER packet MUST be sent with the flag cleared to re- establish the context. The HDRCKSUM is calculated in the same way as a UDP checksum except that it does not cover all of the UDP data. That is, the HDRCKSUM is the 16-bit one's complement of the one's complement sum of the pseudo-IP header (as defined for UDP), the UDP header, and the first 12 bytes of the UDP data which are assumed to hold the fixed part of an RTP header. The extended part of the RTP header and the RTP data will not be included in the HDRCKSUM. The HDRCKSUM is placed in the COMPRESSED_UDP or COMPRESSED_RTP packet where a UDP checksum would have been. The decompressor MUST zero out the UDP checksum field in the reconstructed packets. For a non-RTP context, there may fewer than 12 UDP data bytes present. The IP and UDP headers may still be compressed into a COMPRESSED_UDP packet. For this case, the HDRCKSUM is calculated over the pseudo-IP header, the UDP header, and the UDP data bytes that are present. If the number of data bytes is odd, then a zero padding byte is appended for the purpose of calculating the checksum, but not transmitted. The HDRCKSUM does not validate the RTP data. If the link layer is configured to deliver packets without checking for errors, then errors in the RTP data will not be detected. Over such links, the compressor SHOULD add the HDRCKSUM if a UDP checksum is not present, and the decompressor SHOULD validate each reconstructed packet to make sure that at least the headers are correct. This ensures that the packet will be delivered to the right destination. If only HDRCKSUM is available, the RTP data will be delivered even if it includes errors. This might be a desirable feature for applications that can tolerate errors in the RTP data. The same holds for the extended part of the RTP header. Here is the format of the FULL_HEADER length fields with the new flag C to indicate that a header checksum will be added in COMPRESSED_UDP and COMPRESSED_RTP packets: For 8-bit context ID: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|1| Generation| CID | First length field +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 |C| seq | Second length field +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ C=1: HDRCKSUM will be added For 16-bit context ID: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|1| Generation| 0 |C| seq | First length field +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ C=1: HDRCKSUM will be added +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | CID | Second length field +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2.3 CRTP operation in 'N' mode The 'N' mode is a method of operation where the compressor tries to keep the decompressor in sync by sending changes multiple times. The 'N' is a number that represents the quality of the link between the hosts, and it means that the probability of more than N adjacent packets getting lost on this link is small. For every change in a full value or a delta value, if the compressor includes the change in N+1 consecutive packets, there is a very good chance that the compressor and decompressor can stay in sync using the 'twice' algorithm. CONTEXT_STATE packets should also be repeated N+1 times (using the same sequence number). It is up to the implementation to find a scheme to derive an appropriate N for a link. This scheme may be used at any time and does not require negotiation. Some short notations: FH FULL_HEADER CR COMPRESSED_RTP CU COMPRESSED_UDP Here is an example to demonstrate the usage of the N scheme. In this stream the audio codec sends a sample every 10 milliseconds The first talkspurt is 1 second long. Then there are 2 seconds of silence, then another talkspurt. We also assume in this example that the IP ID field does not increment at a constant rate because the host is generating other uncorrelated traffic streams at the same time and therefore the delta IP ID changes for each packet. When there is no loss on the link, we can use COMPRESSED_RTP packets in the following sequence: seq Time pkt updates and comments # type 1 10 FH 2 20 CR dI dT=10 3 30 CR dI 4 40 CR dI ... 100 1000 CR dI 101 3010 CR dI dT=2010 102 3020 CR dI dT=10 103 3030 CR dI 104 3040 CR dI ... In the above sequence, if a packet is lost we cannot recover ('twice' will not work due to the unpredictable IP ID) and the context must be invalidated. Here is the same example in 'N' mode, when N=2. Note that the compressor only sends the absolute IP ID (I) and not the delta IP ID (dI). seq Time pkt CU flags updates and comments # type F I dT dI M S T P 1 10 FH 2 20 FH repeat constant fields 3 30 FH repeat constant fields 4 40 CU 1 1 1 0 M 0 1 0 I T=40 dT=10 5 50 CU 1 1 1 0 M 0 1 0 I T=50 dT=10 repeat update T & dT 6 60 CU 1 1 1 0 M 0 1 0 I T=60 dT=10 repeat update T & dT 7 70 CU 1 1 0 0 M 0 0 0 I 8 80 CU 1 1 0 0 M 0 0 0 I ... 100 1000 CU 1 1 0 0 M 0 0 0 I 101 3010 CU 1 1 0 0 M 0 1 0 I T=3010 T changed, keep deltas 102 3020 CU 1 1 0 0 M 0 1 0 I T=3020 repeat updated T 103 3030 CU 1 1 0 0 M 0 1 0 I T=3030 repeat updated T 104 3040 CU 1 1 0 0 M 0 0 0 I 105 3050 CU 1 1 0 0 M 0 0 0 I ... This second example is the same sequence, but assuming the delta IP ID is constant. First the basic CRTP for a lossless link: seq Time pkt updates and comments # type 1 10 FH 2 20 CR dI dT=10 3 30 CR 4 40 CR ... 100 1000 CR 101 3010 CR dT=2010 102 3020 CR dT=10 103 3030 CR 104 3040 CR ... For the equivalent sequence in 'N' mode, the more efficient COMPRESSED_RTP packet can still be used once the deltas are all established: seq Time pkt CU flags updates and comments # type F I dT dI M S T P 1 10 FH 2 20 FH repeat constant fields 3 30 FH repeat constant fields 4 40 CU 1 1 1 1 M 0 1 0 I dI T=40 dT=10 5 50 CU 1 1 1 1 M 0 1 0 I dI T=50 dT=10 repeat updates 6 60 CU 1 1 1 1 M 0 1 0 I dI T=60 dT=10 repeat updates 7 70 CR 8 80 CR ... 100 1000 CR 101 3010 CU 1 0 0 0 M 0 1 0 T=3010 T changed, keep deltas 102 3020 CU 1 0 0 0 M 0 1 0 T=3020 repeat updated T 103 3030 CU 1 0 0 0 M 0 1 0 T=3030 repeat updated T 104 3040 CR 105 3050 CR ... 3. Negotiating usage of enhanced-CRTP RFC 2509 [IPCPHC] specifies how the use of CRTP is negotiated on PPP links using the IP Compression Protocol option of IPCP: IPCP option 2: IP compression protocol protocol 0x61: indicates RFC 2507 header compression sub-option 1: enables use of COMPRESSED_RTP, COMPRESSED_UDP and CONTEXT_STATE as specified in RFC 2508 To use the enhanced CRTP defined in this document, a new sub-option 2 is added. The new sup-option 2 is negotiated instead of, not in addition to, sub-option 1. Description Enable use of Protocol Identifiers COMPRESSED_RTP and CONTEXT_STATE as specified in RFC 2508 plus COMPRESSED_UDP with additional flags as defined in this document, and enable use of the C flag with the FULL_HEADER Protocol Identifier as defined in this document to indicate use of HDRCKSUM with COMPRESSED_RTP and COMPRESSED_UDP packets. 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 2 Length 2 4. Security Considerations Because encryption eliminates the redundancy that this compression scheme tries to exploit, there is some inducement to forego encryption in order to achieve operation over a low-bandwidth link. However, for those cases where encryption of data and not headers is satisfactory, RTP does specify an alternative encryption method in which only the RTP payload is encrypted and the headers are left in the clear. That would allow compression to still be applied. A malfunctioning or malicious compressor could cause the decompressor to reconstitute packets that do not match the original packets but still have valid IP, UDP and RTP headers and possibly even valid UDP check-sums. Such corruption may be detected with end-to-end authentication and integrity mechanisms which will not be affected by the compression. Constant portions of authentication headers will be compressed as described in [IPHCOMP]. No authentication is performed on the CONTEXT_STATE control packet sent by this protocol. An attacker with access to the link between the decompressor and compressor could inject false CONTEXT_STATE packets and cause compression efficiency to be reduced, probably resulting in congestion on the link. However, an attacker with access to the link could also disrupt the traffic in many other ways. A potential denial-of-service threat exists when using compression techniques that have non-uniform receiver-end computational load. The attacker can inject pathological datagrams into the stream which are complex to decompress and cause the receiver to be overloaded and degrading processing of other streams. However, this compression does not exhibit any significant non-uniformity. 5. Acknowledgements The authors would like to thank Van Jacobson, co-author of RFC 2508, and the authors of RFC 2507, Mikael Degermark, Bjorn Nordgren, and Stephen Pink. The authors would also like to thank Dana Blair, Francois Le Faucheur, Tim Gleeson, Matt Madison, Hussein Salama, Mallik Tatipamula, Mike Thomas, Alex Tweedly, Herb Wildfeuer, and Dan Wing. 6. References [CRTP] S. Casner, V. Jacobson, "Compressing IP/UDP/RTP Headers for Low-Speed Serial Links", RFC2508, February 1999. [IPHCOMP] M. Degermark, B. Nordgren, S. Pink, "IP Header Compression", RFC2507, February 1999. [IPCPHC] M. Engan, S. Casner, C. Bormann, "IP Header Compression over PPP", RFC2509, February 1999. [KEYW] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC2119, BCP 14, March 1997. [RTP] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC1889, January 1996. 7. Authors' Addresses Tmima Koren Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134-1706 United States of America Email: tmima@cisco.com Stephen L. Casner Packet Design 2465 Latham Street, Third Floor Mountain View, CA 94040 United States of America Email: casner@acm.org John Geevarghese Telseon Inc. 480 S. California Palo Alto, CA 94306 United States of America Email: geevjohn@hotmail.com Bruce Thompson Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134-1706 United States of America Email: brucet@cisco.com Patrick Ruddy Cisco Systems, Inc. 3rd Floor, 96 Commercial Street Edinburgh EH6 6LX Scotland Email: pruddy@cisco.com 8. Copyright Copyright (C) The Internet Society 1999-2001. All Rights Reserved. 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