Status of This Memo

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Table of Contents

1. Introduction

The SIP Geolocation specification [RFC6442] describes the "Geolocation" SIP header field, which is used to indicate that the SIP message is conveying location information. [RFC6442] specifies that SIP intermediaries should not add locationValues to a SIP request that already contains a locationValue. [RFC6442] also states that if a SIP intermediary adds location, it is fully responsible for addressing the concerns of any 424 (Bad Location Information) SIP response it receives. However, some communications architectures, such as 3GPP [TS23-167] and ETSI [M493], prefer to use information provided by edge proxies or acquired through the use of core-network nodes before using information provided solely by user equipment (UE). These solutions don't preclude the use of UE-provided location but require a means of being able to distinguish the identity of the node adding the locationValue to the SIP message from that provided by the UE.¶

[RFC6442] stipulates that the order of locationValues in the Geolocation header field is the same as the order in which they were added to the header field. Whilst this order provides guidance to the recipient as to which values were added to the message earlier in the communication chain, it does not identify which node added the locationValue. Knowing the identity of the entity that added the location to the message allows the recipient to choose which location to consider first rather than relying solely on the order of the locationValues in the Geolocation header field.¶

This document extends the Geolocation header field of [RFC6442] by allowing an entity adding the locationValue to identify itself using a hostname. This is done by defining a new geoloc-param header field parameter, "loc-src". How the entity adding the locationValue to the header field obtains the location information is out of scope of this document. Please note that the "loc-src" parameter field does not alter the subject of the locationValue.¶

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3. Rationale

The primary intent of the "loc-src" parameter in this specification is for use in emergency calling. There are various architectures defined for providing emergency calling using SIP-based messaging. Each has its own characteristics with corresponding pros and cons. All of them allow the UE to provide location information; however, many also attach other sources of location information to support veracity checks, to provide backup information, or to be used as the primary location.¶

This document does not comment on these various architectures or on the rationale for including multiple locationValues. It does recognize that these architectures exist and that there is a need to identify the entity adding the location information.¶

The "loc-src" parameter adds the location source generating the locationValue to allow recipients to make informed decisions about which of the multiple values to use.¶

The "loc-src" parameter is applicable within a single private administrative domain or between different administrative domains where there is a trust relationship between the domains. Thus, it is intended to use this parameter only in trust domains where Spec(T) as described in [RFC3325] exists.¶

The "loc-src" parameter is not included in a SIP message sent to another network if there is no trust relationship. The "loc-src" parameter is not applicable if the administrative domain manages emergency calls in a way that does not require any generation of the location.¶

The functional architecture to support emergency caller location described within ETSI [M493] is an example of an architecture where it makes sense to use this parameter.¶

4. Mechanism

The mechanism adds a geoloc-param parameter to the locationValue defined in [RFC6442] that identifies the hostname of the entity adding the locationValue to the Geolocation header field. The Augmented BNF (ABNF) [RFC5234] for this parameter is shown in Figure 1.¶

       location-source = "loc-src" EQUAL hostname
       hostname = <defined in RFC 3261>

Only a fully qualified host name is valid. The syntax does not support IP addresses, and if an entity conforming to this specification receives a Geolocation header field with a "loc-src" parameter containing an IP address, it MUST remove the parameter.¶

A SIP intermediary conformant to this specification adding a locationValue to a Geolocation header field SHOULD also add a "loc-src" header field parameter so that it is clearly identified as the node adding the location. A User Agent (UA) MUST NOT insert a "loc-src" header field parameter. If a SIP intermediary receives a message from an untrusted source with the "loc-src" parameter set, then it MUST remove the "loc-src" parameter before passing the message into a trusted network.¶

5. Example

The following example shows a SIP INVITE message containing a Geolocation header field with two locationValues. The first locationValue points to a Presence Information Data Format Location Object (PIDF-LO) in the SIP body using a content-indirection (cid:) URI per [RFC4483], and this is provided by the UE. The second locationValue is an https URI provided by a SIP intermediary, which identifies itself using the "loc-src" parameter.¶

   INVITE sip:bob@biloxi.example.com SIP/2.0
   Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bK74bf9
   Max-Forwards: 70
   To: Bob <sip:bob@biloxi.example.com>
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   Call-ID: 3848276298220188511@atlanta.example.com
   Geolocation: <cid:target123@atlanta.example.com>,
        <https://lis.example.com:8222/y77syc7cuecbh>;
                 loc-src=edgeproxy.example.com
   Geolocation-Routing: yes
   Accept: application/sdp, application/pidf+xml
   CSeq: 31862 INVITE
   Contact: <sip:alice@atlanta.example.com>
   Content-Type: multipart/mixed; boundary=boundary1
   Content-Length: ...

6. Privacy Considerations

This document doesn't change any of the privacy considerations described in [RFC6442]. While the addition of the "loc-src" parameter identifies the entity that added the location in the signaling path, this addition provides little more exposure than adding a proxy identity to the Record-Route header field (privacy defined in [RFC3323]).¶

7. Security Considerations

This document introduces the ability of a SIP intermediary to insert a host name indicating that they added the specific locationValue to the Geolocation header field. The intent is for this field to be used by the location recipient in the event that the SIP message contains multiple locationValues. As a consequence, this parameter should only be used by the location recipient in a trusted network. Adding this parameter in an untrusted network serves solely to give location information to untrusted parties and is NOT RECOMMENDED.¶

As already stated in [RFC6442], securing the location hop by hop, using TLS, protects the message from eavesdropping and modification in transit but exposes the information to all SIP intermediaries on the path as well as the endpoint. The "loc-src" parameter is applicable within a single private administrative domain or between different administrative domains where there is a relationship between the domains. If such a trust relationship is not given, it is strongly recommended to delete the location information.¶

The use of this parameter is not restricted to a specific architecture, but using multiple locations and loc-src may end in compatibility issues. [RFC6442] already addresses the issue of multiple locations. To avoid problems of a possible corruption of the location information including the "loc-src" parameter when using an untrusted relationship, it is strongly recommended to delete location information when passed to another domain out of the trust domain.¶

8. IANA Considerations

Authors' Addresses