[Users] TLS requirements and some brainstorming (long email)

Hi all! There are several scenarios where TLS will be used to interconnect SIP proxies. (open)ser's TLS implementation should be generic enough to handle all the useful scenarios. Thus, to better understand the requirements, first I present some examples where (open)ser+TLS will be useful. (I do not propose which of the following interconnect models are good or bad. However, openser should be capable to handle all of them, best in a mixed mode). Enterprise scenario: A company uses TLS to interconnect their SIP proxies via public Internet. The proxies import the companies selfsigned CA-cert as trusted CAs. The proxies trust other proxies as soon as their cert is validated using the root CA. This is already possible using openser 1.0.0 (= or ser+experimental TLS) Federation scenario: Some ITSPs form a federation. The federation-CA signs the certs of the ITSPs. Here, the validation is like in the enterprise scenario. (open)ser validates against the federations CA-cert. This works with openser 1.0.0 as long as the ITSP is only in one federation, or uses different egress/ingress points for each federation. If the ITSP is member of two federations and uses one egress/ingress proxy, it has to decide which certificate it should present to the peer. The originating proxy could choose the proper client certificate for example by using a table like (or having the certificate as blob directly in the DB): dst_domain certificate sip.atlanta.com /etc/openser/federationAcert.pem sip.biloxy.com /etc/openser/federationBcert.pem sip.chicago.com /etc/openser/federationAcert.pem Presenting the proper server certificate, is more difficult. The server does not know if the incoming TLS request belongs to a member of fedA, fedB or someone else. Thus, presenting the wrong certificate will lead to the clients rejecting the certificate due to failed validation. One solution would be sending the "trusted_ca_keys" (TLS extension) in Client Hello. Unfortunatelly this is not supported in openssl (and gnutls). Any workaround for this? Anyway, in this scenario it is important to have the certificate parameters (Subject, Issuer) available in the routing logic to make routing decisions based on the TLS authenticaten and adding them to the CDRs (e.g. via AVPs and extra accounting) Bilateral scenario: An ITSP has bilateral trust relationships. Each ITSP has its own CA which signs the certs of this ITSP. If another ITSP wants to trust this ISTP it only has to import the others CA-cert. This works already with openser 1.0.0, but exporting the cert parameters for extra accounting will be useful. Hosted SIP scenario: An ITSP hosts multiple SIP domains for its customers. If the server has to offer a certificate which includes the proper SIP domain, the server_name extension is needed to indicate the requested domain in the client_hello request. Then the server will present the proper certificate and domain validation (Subject domain == SIP domain) in the client will succeed. This will work fine with initial (out-of-dialog) requests as they usually will include the SIP domain in the request URI. There will be problems for responses and in-dialog requests as usually the Record-Route and Via headers only includes IP addresses. Thus, the SIP proxy either has to insert the SIP domain into Via and Record-Route, or the domain validation should only be done for in-dialog requests. This leads to the problem of domain validation. The TLS connection will be set up after all the routing logic, somewhere inside t_relay. Thus, if we want domain validation, it will be inside t_relay. Maybe we can use a certain flag to indicate if domain-validation should be done (on a per-transaction basis). This might cause problems if there is already a TLS connection to the requested destination, but without domain validation or validation against a different domain (virtual domain hosting). How to solve this? I can't propose a solution to all scenarios. But I think I showed that the certificate selection and validation should be very flexible, e.g. by choosing the proper client certificate for each transaction and different routing in the server depending on the presented client certificate and the cerfiticate signer (e.g. based on a whitelist). Further we have to take care to add certifcates and CA-certs during runtime, e.g. using a FIFO command "tls_reload". This should also drop all existing TLS connections. Having a maximum connection time after which we force re-validation will also be useful. Also (open)ser should allow to import CRL (certificate revocation lists) (shouldn't be a problem with openssl) or usage of OCSP (Online Certificate Status Protocol). Now I'm ready for some discussions :-) regards klaus