Interconnected computing clouds
Given that cloud computing is an abstraction of services, interconnected computing clouds, also called hybrid clouds, are a logical progression, abstracting multiple clouds.
As one moves up the abstraction hierarchy, from Infrastructure as a Service to Software as a Service, the interfaces involve become increasingly application-specific. In the moderate term, cloud interconnection is likely to be vendor-driven, using proprietary interfaces.
Standards for interconnection
- Virtual machine (VM) image distribution (e.g., Desktop Management Forum (DTMF) Open Virtualization Format (OVF) 
- VM provisioning and control (e.g., the proprietary API to Amazon Elastic Computing Cloud)
- Inter-cloud VM exchange; interfaces are lacking here
- Persistent storage (e.g., Azure Storage, S3, EBS, GFS, Atmos)
- VM service level agreements (SLA); definitions are lacking for machine readable uptime, resource guarantees, storage redundancy
- Secure VM configuration, such as Security Content Automation Protocol (SCAP)
VMware, the largest virtualization vendor, has offered its vCloud API to the Desktop Management Forum, which they say is responsive to open standards. An industry analyst, Chris Wolf of the Burton Group, said that making the API available without the infrastructure is marketing, not interoperability.
Linking PaaS and SaaS
IaaS interconnection conceptually is easier than interconnection at higher levels of abstraction, more likely to provide business-to-business rather than user-to-service functionality. SaaS linkage is sometimes called Enterprise Service Bus. Vendors in this space, such as Rearden Commerce and Ariba, are brokers between customers and service providers; Rearden's product is an automated personal assistant that goes to approved service providers. Ariba offers "spend management" SaaS.
By their value-added nature, it is much harder to standardize interfaces at the higher levels of cloud service. There are possible approaches, however, such as
- Supported programming languages
- APIs for cloud services
- SaaS-specific authentication / authorization
- Formats for data import and export (e.g., XML schemas)
- Separate standards may be needed for each application space
Security may be one of the first places for open standards in IaaS, PaaS, and SaaS. 
Open interface possibilities include:
- Identity and Access Management (IAM)
- IdM federation (SAML, WS-Federation, Liberty ID-FF)
- Strong authentication standards (HOTP, OCRA, TOTP)
- Entitlement management (XACML)
- Data Encryption (at-rest, in-flight), Key Management
- Public Key Infrastructure (PKI), PKCS, KEYPROV (CT-KIP, DSKPP), EKMI
- Records and Information Management (ISO 15489)
- Electronic legal discovery with the Electronic Discovery Reference Model (EDRM).
Citrix and Signacert are building a security system for cloud interconnection, using a whitelist repository of trust information. 
- Peter Mell, Tim Grance (7 October 2009), Effectively and Securely Using the Cloud Computing Paradigm, NIST Information Technology Laboratory, pp. 48-49
- Open Virtualization Format Specification, Desktop Management Forum, 2007
- Jon Brodkin (31 August 2009), "VMware cloud initiative raises vendor lock-in issue", Network World, p. 1, 19
- Mell and Grace, October 2009, p. 50
- Electronic Discovery Reference Model
- "Citrix to Establish Virtual Infrastructure Security Validation in Collaboration with SignaCert.", Business Wire, 9 September 2009