Internet Engineering Task Force SIP WG Internet Draft J.Rosenberg,H.Schulzrinne
draft-ietf-sip-srv-03.txtdraft-ietf-sip-srv-04.txt dynamicsoft,Columbia U. DecemberJanuary 24, 20012002 Expires: MayJuly 2002 SIP: Locating SIP Servers STATUS OF THIS MEMO This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate 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 To view the list Internet-Draft Shadow Directories, see http://www.ietf.org/shadow.html. Abstract The Session Initiation Protocol (SIP) makes use ofuses DNS procedures to allow a client to resolve a SIP URI into the IP address, port, and transport protocol of the next hop to contact. It also uses DNS to allow a server to send a response to a backup client in the event of a failure ofif the primary client.client has failed. This document describes those DNS pro- ceduresprocedures in detail. 1 Introduction The Session Initiation Protocol (SIP)  is a client-server protocol used for the initiation and management of communications sessions between users. SIP end systems are called user agents, and intermedi- ateintermediate elements are known as proxy servers. A typical SIP configuration, referred to as the SIP "trapezoid" is shown in Figure 1. In this diagram, a caller, UA1caller in domain A (UA1) wishes to call joe@B.Joe in domain B (joe@B). To do so, it communi- catescommunicates with proxy 1 in its domain (domain A). Proxy 1 forwards the request to the proxy for the domain of the called party (domain B), which is proxy 2. Proxy 2 forwards the call to the called party, UA 2. ............................ .............................. . . . . . +-------+ . . +-------+ . . | | . . | | . . | Proxy |------------- | Proxy | . . | 1 | . . | 2 | . . | | . . | | . . / +-------+ . . +-------+ \ . . / . . \ . . / . . \ . . / . . \ . . / . . \ . . / . . \ . . / . . \ . . / . . \ . . +-------+ . . +-------+ . . | | . . | | . . | | . . | | . . | UA 1 | . . | UA 2 | . . | | . . | | . . +-------+ . . +-------+ . . Domain A . . Domain B . ............................ .............................. Figure 1: The SIP trapezoid As part of this call flow, proxy 1 needs to determine a SIP server for domain B. To do this, proxy 1 makes use of DNS procedures, using both theSRV  and NAPTR  records. This document describes the specific problems that SIP uses DNS to help solve, and provides a solution. 2 Problems DNS is Needed to Solve DNS is needed to help solve severaltwo aspects of the general call flow described in the Introduction. First off,The first is for proxy 1 needsto discover the SIP server in domain B, in order to forward the call for joe@B. Specifically,The second is for proxy 2 to identify a backup for proxy 1 in the event it fails after forwarding the request. For the first aspect, proxy 1 specifically needs to deter- minedetermine the IP address, port and transport protocol for the server in domain B. Transport Protocol is particularly noteworthy. Unlike many other protocols, SIP can run over a variety of transports,transport protocols, including TCP, UDP, TLS/TCP and SCTP. Therefore, discovery of transports for a particular domain is an important part of the processing.Thus, clients need to be able to automatically determine which transport protocols are available. The proxy sending the request has a particular set of transportstransport protocols it supports (all proxies must implement both TCP and UDP)and a preference for using those tran- sports.transport protocols. Proxy 2 has its own set of transportstransport protocols it supports (the minimal overlap is UDP and TCP in this case),supports, and relative prefer- encespreferences for those transports.transport protocols. All proxies must implement both UDP and TCP, so that there is always an intersection of capabilities. Some form of DNS procedures are needed for proxy 1 to discover the available transportstransport protocols for SIP services at domain B, and the relative preferences of those transports. This information can be merged with thetransport protocols. Proxy 1 intersects its list of supported transportstransport protocols with those of proxy 2 and prefer- ences atthen chooses the protocol preferred by proxy 1, resulting in a selection of a transport.2. It is important to note that DNS processinglookups can be used multiple times throughout processing of a call. In general, an element that wishes to send a request (generally called(called a client) may need to per- formperform DNS processing to determine the IP address, port, and transport protocol of a next hop element, generallycalled a server (it can be a proxy or a user agent). Such processing could, in principle, occur at every hop between elements. Since SIP is used for the establishment of interactive communications services, the time it takes to complete a transaction between a caller and called party is important. Typically, the total delay betweentime from when a userthe caller initiates a call until the call, and when they get an indica- tion thattime the called party is beingalerted to the call, needs toshould be ............................ .............................. . . . . . +-------+ . . +-------+ . . | | . . | | . . | Proxy |------------- | Proxy | . . | 1 | . . | 2 | . . | | . . | | . . / +-------+ . . +-------+ \ . . / . . \ . . / . . \ . . / . . \ . . / . . \ . . / . . \ . . / . . \ . . / . . \ . . +-------+ . . +-------+ . . | | . . | | . . | | . . | | . . | UA 1 | . . | UA 2 | . . | | . . | | . . +-------+ . . +-------+ . . Domain A . . Domain B . ............................ .............................. Figure 1: The SIP trapezoid lessno more than a few seconds. Given that there can be multiple hops, each of which is doing DNS processinglookups in addition to other potentially time-intensive operations, the amount of time available for DNS pro- cessinglookups at each hop is limited. Scalability and high availability are important in SIP. SIP services scale up through clustering techniques. InTypically, in a morerealistic version of the network in Figure 1, proxy 2 would typicallybe a cluster of homogeneously configured proxies. DNS needs to provide the ability for domain B to configure a set of servers, along with prioritization and weights in order to provide a crude level of capacity basedcapacity-based load balancing. High availability is accomplished inSIP through detection of failuresassures high availability by having upstream elements.elements detect failures. For example, assume that proxy 2 is implemented as a cluster of two proxies, proxy 2.1 and proxy 2.2. If proxy 1 would sendsends a request to proxy 2.1 (one ofand the proxies inrequest fails, it retries the "cluster"request by sending it to proxy 2).2.2. This request would fail, and that would be detected by proxy 1. Proxy 1 would then try another of the proxies,proxy 2.2. In many cases, such as the one above,proxy 1 will not know which domains it will ultimately communicate with. That information would be known when a user actually makes a call to another user in that domain. Proxy 1 may never communicate with that domain again after the call com- pletes.completes. Proxy 1 couldmay communicate with thousands of different domains within a few minutes, and proxy 2 could receive requests from thousands of different domains within a few minutes. Because of this "many-to-many" relationship, and the possibly long intervals between communications between a pair of domains, it is not generally possible for an ele- mentelement to perpetuallymaintain dynamic availability state for the prox- iesproxies it will communicate with. When a proxy gets its first call with a particular domain, it will try the servers in that domain in some order until it finds one thatsthat is available. The identity of the avail- ableavailable server would ideally be cached for some amount of time in order to reduce call setup delays of subsequent calls. However, theThe client cannot actively "ping" thequery a failed serversserver continuously to determine when they come back alive, because of scalability concerns.it becomes available again, since this does not scale. Furthermore, the availa- bilityavailability state must eventually be flushed in order to redistribute load to recovered elements when they come back online. It is possible for elements to fail in the middle of a transaction. For example, after proxy 2 forwards the request to UA 2, proxy 1 fails. UA 2 sends its response to proxy 2, which tries to forward it to proxy 1, which is no longer available. Ideally, we would likeThe second aspect of the flow in the introduction for which DNS is needed, is for proxy 2 to use DNS procedures toidentify a backup serverfor proxy 1 that it can use to forwardsend the response.response to. This problem is more realis- ticrealistic in SIP than it is in other transactional protocols. The reason is that a SIP response can take a *long*long time to be generated, because a human user frequently needs to be consulted in order to generate that response. As such, it is not uncommon for tens of seconds to elapse between a call request and its acceptance. 3 Terminology In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALLNOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119  and indicate requirement levels for compliant SIP implementations. 4 Client Usage Usage of DNS differs for clients and for servers. This section discusses client usage. The assumption isWe assume that the client is stateful (either a UAC or a stateful proxy). Considerations for statelessStateless proxies are discussed in Section 22.214.171.124. The procedures here are invoked when a client needs to send a request to a server for which it does not already know an explicit IP address, port, and transport. This occurs when an element wishes to send a request to a serveridentified by a SIP URI, or when an ele- mentelement wishes to send a request to a specific configured server, independent of the SIP URI,URI (called an outbound proxy), but the configured serveroutbound proxy is identified by a domain name instead of a numeric IP address. Frequently, this is because the URI is contained in the Request-URI of a request to be sent. The procedures defined here MUSTin no way affect this URI (i.e., the URI is not rewritten with the result of the DNS looksup), they only result in an IP address, port and transport protocol where the request can be sent. The procedures here MUST be done exactly once per transaction. That is, once a server has successfully been contacted (success is defined below), all retransmissions of the request and the ACK for non-2xx responses MUST be sent to the same server.host. Furthermore, a CANCEL for a particular request MUST be sent to the same serverhost that the request was delivered to. Note that, becauseBecause the ACK request for 2xx responses constitutes a different transaction, there is no requirement that it be delivered to the same server that received the original request (indeed, if that server did not record-route, it will most definitely not get the ACK). If the request is being delivered to an outbound proxy, a temporary URI, used for purposes of this specification, is constructed. That URI is of the form sip:<proxy>, where <proxy> is the domain of the outbound proxy. The first step is to identify the TARGET. TheWe defined TARGET is set toas the value of the maddr parameter of the URI, if present, otherwise, the host value of the hostport construction.component of the URI. It representsidentifies the domain to be contacted. 3.1We determine the transport protocol, port and IP address of a suitable instance of TARGET in Sections 4.1 and 4.2. 4.1 Selecting a Transport Next,Protocol First, the client selects a transport is selected.protocol. If the URI specifies a transport,transport protocol in the transport parameter, that transport protocol MUST be used. Otherwise, if no transport protocol is specified, but the TARGET is a numeric IP address, the client SHOULD use UDP. Otherwise, if no transport protocol is specified, and the target is not a numeric IP address, the client SHOULD perform a NAPTR query. Thisquery isfor the domain in the SIP URI. The services relevant for the task of transport protocol selection are those with NAPTR service "SIP+D2T", which providesfields with values "SIP+D2x", where x is a letter that corresponds to a transport protocol supported by the domain. This specification defines D2U for UDP, D2T for TCP, D2S for SCTP and D2L for TLS over TCP. We also establish an IANA registry for NAPTR service name to transport protocol mappings. These NAPTR records provide a mapping from a domain to a transportthe SRV record for contacting that domain. The transport is ofa server with the form of an SRV record, usingspecific transport protocol in the "S"NAPTR flag.services field. The resource record will contain a replacement value (not aand an empty regular expression),expression, which is the SRV record for athat particular transport.transport protocol. If the server supports multiple transports,transport protocols, there will be multiple NAPTR records, each with a different orderservice value. TheAs per RFC 2915 , the client MUST discard any records that contain an SRV value with awhose services fields indicate transport protocols not supported by the client, but otherwise follow the processing rules of .client. The NAPTR processing in RFC 2915 will result is that thein selection of a transport protocol (and an SRV record along with it) with most preferred transport protocol of the server that is supported by the client will get used.client. As an example, consider foo.com.example.com. A client wishes to contact a SIP server in foo.com.example.com. It performs a NAPTR query for that domain, and the following records are returned: ;; order pref flags service regexp replacement IN NAPTR 90 50 "s" "SIP+D2T" "" _sip._tcp.foo.com_sip._tcp.school.edu IN NAPTR 100 50 "s" "SIP+D2T""SIP+D2U" "" _sip._udp.foo.com_sip._udp.example.com IN NAPTR 110 50 "s" "SIP+D2T""SIP+D2S" "" _sip._tls.foo.comtls-sip.example.com This indicates that the server supports TCP, UDP, and TLS, in that order of preference.order. If the client supports UDP and TLS, UDP will be used, based on an SRV lookup of _sip._udp.foo.com. Somehow this doesn't seem right, since the client needs to look at the replacement values to discard entries. Perhaps the query should instead be done for sip.<domain>, and the service field is "TCP+D2T" or "UDP+D2T"?_sip._udp.example.com. It is STRONGLY RECOMMENDED thatnot necessary for the domain suffixes in the replace- mentreplacement field (i.e., foo.com above)to match the domain of the original query. Without that,query (i.e., example.com above). However, for backwards compatibility betweenwith RFC 2543 and this specification will not be possible. THis2543, a domain MUST maintain SRV records for the domain of the original query, even if the NAPTR record is becausein a different domain. As an example, even though the SRV record for TCP is _sip._tcp.school.edu, there MUST also be an SRV record at _sip._tcp.example.com. RFC 2543 clientswill go directly tolook up the SRV records usingfor the domain suffixes.directly. If these are non- existent,do not exist because the NAPTR replacement usedpoints to a different suffix, communication will not take place. Indomain, the event thatclient will fail. If no NAPTR records are found, the client constructs SRV recordsqueries for those transportstransport protocols it supports, and does a query for each. Queries are done using the service identifier "_sip". If the query is successful, it means that theA particular transport is sup- ported.supported if the query is successful. The client MAY use any transport protocol it desires which is sup- portedsupported by the server. This is a change from RFC 2543, which used to merge the priority values across different SRV records. 3.24.2 Determining portPort and IP Once the transport protocol has been determined, the next step is to determine the IP address and port. If TARGET is a numeric IP address, usethe client uses that address. If the URI also contains a port, useit uses that port. If no port is specified, useit uses the default port for the particular transport.transport protocol. If the TARGET was not a numeric IP address, but a port is present in the URI, first checkthe cache to determine if a server has been pre- viously contacted successfully for that TARGET and port. If one has been, use that server. Otherwise, performclient performs an A or AAAA record lookup of the domain name. The result will be a list of IP address, each of which can be contacted at the specific port from the URI and tran- sporttransport protocol determined previously. Processing then proceeds as described in Section 126.96.36.199 of this document. There is a weird case where, where the URI had a domain name and a port. SRV records will potentially be used to determine the transport,transport protocol, based on the algorithms above, but A records used for the actual lookup. That seems odd. If the TARGET was not a numeric IP address, and no port was present in the URI, first checkthe cache to see if a server had been previ- ously contacted successfully for that TARGET. If one had been, use that. Otherwise, performclient performs an SRV query using the service identifier "_sip" and the transport protocol as determined from Section 3.1,4.1, as specified in RFC 2782 . The procedures of RFC 2782, as described in the Sec- tionSection titled "Usage rules" are followed, augmented by the additional procedures of Section 188.8.131.52 of this document. This is a change. Previously, if the port was explicit, but with a value of 5060, SRV records were used. Now, A records will be used. A result of this is that the URL comparison rules need to change to reflect that sip:user@foosip:email@example.com and sip:user@foo:5060sip:firstname.lastname@example.org:5060 are NOT equivalent any longer. I think this should not cause any serious interoperability issues, but further consideration is needed. 3.34.3 Details of RFC 2782 processProcess RFC 2782 spells out the details of how a set of SRV records are sorted and then tried. However, it only states that the client should "try to connect to the (protocol, address, service)" without giving any details on what happens in the event of failure. Those details,details are described here for SIP. The client client MAY maintain a table indicating the status of a particular host (that is, whether it was ever successfully contacted, or whether attempts to contact it resulted in a failure). The table is indexed with the caseIP address, port, and transport for a particular host. If a particular host is listed with a status of "failed", that entry SHOULD be discarded after one hour, so that the host can be used once more if it has recovered. When processing the list of SIP,SRV entries (or A records, depending on how the URI was resolved), the client MAY remove any entries for hosts which are described here.marked as "failed" in the table. The remaining entries are then tried according to RFC 2782. For SIP requests, failure occurs if the transaction layer reports a 503 error response or a transport failure of some sort (generally, due to ICMP errors or TCP connection failures). Failure also occurs if the transaction layer times out without ever having received ANYany response, provisional or final (i.e., timer B or timer F fires). If a failure occurs, the client SHOULD create a new request, which is identical to the previous, but has a different value of the Via branch ID than the previous (and therefore constitutes a new SIP transaction). That request is sent to the next element in the list as specified by rfc2782. A server has been contacted "successfully" if a request sent to that server generates any kind of response, provisional or final. A map- pingRFC 2782. 4.4 Consideration for Stateless Proxies The process of the tuple (TARGET, input TRANSPORT, input PORT) toprevious sections is highly stateful. When a specificserver (IP address, transport, port) that wasis contacted successfully SHOULD be cachedsuccessfully, all requests for a duration equal to the TTL ofthe A recordtransaction, as well as CANCEL requests for that server itself. Note, in the above tuple, input TRANSPORT and input PORT refer to the transport and port values from the URI itself, if present. If a client attempts to contact the server listed in the cache, but the request fails, the server MUST be removed from the cache, and the entire DNS processing must restart by following the procedures in Section 3.1 again. 3.4 Consideration for Stateless Proxies The process of the previous sections is highly stateful. When a server is contacted successfully, all requests for the transaction (plus a CANCEL for that transaction) MUST go totransaction, MUST go to the same server. The identity of the successfully contacted server is a form of transac- tiontransaction state. This presents a challenge for stateless proxies, which still need to meet the requiretmentrequirement for sending all requests in the transaction to the same server. The requirement is not difficult to meet in the simple case where there were no failures when attempting to contact a server. Whenever the stateless proxy receives the request, it performs the appropriate DNS queries as described above. Unfortunately, the procedures of RFC 2782 and RFC 2915 are not guaranteed to be deterministic. This is because records that contain the same priority and weight (in the case of SRV) or order and preference (in the case of NAPTR) have no specified order. The stateless proxy MUST define a deterministic order to the records in that case, using any algorithm at its dispo- sal.disposal. One suggestion is to alphabetize them, for example. To make lifeprocessing easier for stateless proxies, it is RECOMMENDED that domain adminis- tratorsadministrators make the weights of SRV records with equal priority different (for example, using weights of 1000 and 1001 if two servers are equivalent, rather than assigning both a weight of 1000), and simi- larlysimilarly for NAPTR records. If the first server is contacted success- fully, things are fine.successfully, the proxy can remain stateless. However, if the first server is not contacted successfully, and a subsequent server is, the proxy cannot remain stateless for this transaction. This is becauseIf it were stateless, a retransmission could very well go to a different server if the failed one recovers between retransmissions. As such, whenever a proxy does not success- fullysuccessfully contact the first server, it SHOULD act as a stateful proxy. 4Unfortunately, it is still possible for a stateless proxy to deliver retransmissions to different servers, even if it follows the recommendations above. This can happen if the DNS TTLs expire in the middle of a transaction, and the entries had changed. This is unavoidable. Network implementors should be aware of this limitation, and not use stateless proxies that access DNS if this error is deemed critical. 5 Server Usage RFC 2543bis defines procedures for sending responses from a server back to the client. Typically, for unicast requests, the response is sent back to the source IP address where the request came from, using the port contained in the Via header. However, it is important to provide failover support when the client element fails between send- ingsending the request and receiving the response. The procedures here are invoked when a server sends a response to the client and that response fails. "Fails" is defined here as any response which causes an ICMP error message to be returned, or when the transport connection the request came in on closes before the response can be sent. In these cases, the server examines the value of the sent-by con- structionconstruction in the topmost Via header. If it contains a numeric IP address, the server attempts to send the response to that address, using the transport protocol from the Via header, and the port from sent-by, if present, else the default for that transport.transport protocol. If, however, the sent-by field contained a domain name and a port number, the server queries for A records with that name. It tries to send the response to each element on the resulting list of IP addresses, using the port from the Via, and the transport protocol from the Via. As in the client processing, the next entry in the list is tred if the one before it results in a failure. If, however, the sent-by field contained a domain name and no port, the server queries for SRV records using the service identifier "_sip" and the transport protocol from the topmost Via header. The resulting list is sorted as described in , and the response is sent to the topmost element on the new list described there. If that results in a failure, the next entry on the list is tried. 5 Security Considerations The authors do not believe that this specification introduces any additional security issues beyond those already described in RFC 2782 and RFC 2915.6 Registration of NATPR D2T Resolution Service Name: Domain NameConstructing SIP URIs In many cases, and element needs to Transport * Mnemonic: D2T * Number of Operands: 1 * Type of Each Operand: Each operand isconstruct a domain * Format of Each Operand: Each operand isSIP URI for inclusion in a domain nameContact header in standard format * Algorithm: Opaque * Output: Onea REGISTER, or more SRV record keys * Error Conditions: o No overlapin transport between client and server * Security Considerations: 7 Author's Addresses Jonathan Rosenberg dynamicsofta Record-Route header in an INVITE. According to , these URIs have to have the property that they resolve to the specific element that inserted them. However, if they are constructed with just an IP address, for example: sip:184.108.40.206 sip:email@example.com;maddr=220.127.116.11 then should the element fail, there is no way to route the request or response through a backup. SRV provides a way to fix this. Instead of using an IP address, a domain name that resolves to an SRV record can be used: sip:server23.provider.com sip:firstname.lastname@example.org;maddr=server23.provider.com The SRV records for a particular target can be set up so that there is a single record with a low value for the priority field, and this record points to the specific element that constructed the URI. However, there are additional records with higher priority that point to backup elements that would be used in the event of failure. This allows the constraint of  to be met while allowing for robust operation. 7 Security Considerations The authors do not believe that this specification introduces any additional security issues beyond those already described in RFC 2782 and RFC 2915. 8 Registration of NATPR D2X Resolution Service Name: Domain Name to Transport Protocol * Mnemonic: D2X, where X is managed by an IANA registration process * Number of Operands: 1 * Type of Each Operand: Each operand is a domain * Format of Each Operand: Each operand is a domain name in standard format * Algorithm: Opaque * Input String: The domain name from the SIP URI being used to generate the NAPTR query. * Output: One or more SRV record keys * Constraints: All records MUST only use the S flag. The P flag is expressly forbidden. * Error Conditions: o No overlap in transport protocol between client and server * Security Considerations: none 9 IANA Considerations The usage of NAPTR records described here requires well known values for the service fields for each transport supported by SIP. The table of mappings from service field values to transport protocols is to be maintained by IANA. New entries in the table MAY be added at any time when new transport protocols become available. Such additions are subject to expert review. The registration MUST include the following information: Service Field: The service field being registered. An example for a new fictitious transport protocol called NCTP might be "SIP+D2N". Protocol: The specific transport protocol associated with that service field. This MUST include the name and acronym for the protocol, along with reference to a document that describes the transport protocol. For example - "New Connectionless Transport Protocol (NCTP), RFC5766". Name and Contact Information: The name, address, email address and telephone number for the person performing the registration. The following values are to be placed into the registry: Services Field Protocol SIP+D2T TCP SIP+D2U UDP SIP+D2L TLS over TCP (RFC 2246) SIP+D2S SCTP (RFC 2960) 10 Changes Since -03 o Added IANA registration process. o Included text discussing the problem of DNS TTL expiration for stateless proxies. o Clarified that maintenance of the table of availability for servers is not a cache, and it is totally unrelated to DNS processing. o Changed the construction of the services field in NAPTR to include the transport protocol, so its SIP+D2X, where X depends on the transport protocol. o Relaxed the constraint that the domain suffix in the NAPTR records equal that of the target. o Added a section on how to construct URIs for insertion into Contact and Record-Route headers. 11 Acknowledgements The authors would like to thank Patrik Faltstrom for his useful comments. 12 Author's Addresses Jonathan Rosenberg dynamicsoft 72 Eagle Rock Avenue First Floor East Hanover, NJ 07936 email: email@example.com 8Henning Schulzrinne Columbia University M/S 0401 1214 Amsterdam Ave. New York, NY 10027-7003 email: firstname.lastname@example.org 13 Bibliography  J. Rosenberg, H. Schulzrinne, et al. , "SIP: Session initiation protocol," Internet Draft, Internet Engineering Task Force, Oct. 2001. Work in progress.  A. Gulbrandsen, P. Vixie, and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)," Request for Comments 2782, Internet Engineering Task Force, Feb. 2000.  M. Mealling and R. Daniel, "The naming authority pointer (NAPTR) DNS resource record," Request for Comments 2915, Internet Engineering Task Force, Sept. 2000.  S. Bradner, "Key words for use in RFCs to indicate requirement levels," Request for Comments 2119, Internet Engineering Task Force, Mar. 1997. Full Copyright Statement Copyright (c) The Internet Society (2002). All Rights Reserved. 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