INTERNET DRAFT Weibin Zhao Service Location Group Henning Schulzrinne draft-zhao-slp-da-interaction-04.txt Columbia University Expires: November 22, 2000 May 22, 2000 mSLP - Mesh-enhanced Service Location Protocol draft-zhao-slp-da-interaction-04.txt 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 The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This document presents mSLP - Mesh-enhanced Service Location Protocol, which enhances SLPv2 with a scheme for the interaction of Directory Agents (DAs). mSLP proposes to use a fully meshed peering DA architecture. Peer DAs exchange service registration information, and maintain the same consistent data for the shared scopes. mSLP provides a reliable directory service for an SLP system. It also greatly simplifies the implementation of service registration for a thin-client Service Agent (SA). mSLP is backward compatible with SLPv2, and incremental deployment is supported. 1. Introduction In the Service Location Protocol (RFC 2608 [1]), Directory Agents (DAs) are introduced for caching service advertisements from Service Agents (SAs), and answering queries from User Agents (UAs). They W. Zhao, H. Schulzrinne Expires: November 22, 2000 [Page 1]
Internet Draft DA Interaction May 22, 2000 exist to enhance the performance and scalability of SLP. When multiple DAs are present, how should they interact with each other? This is an open issue in SLPv2. This document presents mSLP - Mesh-enhanced Service Location Protocol, which enhances SLPv2 with a scheme for the interaction of DAs. mSLP proposes that if DAs are needed in an SLP system, a fully meshed peering DA architecture should be used, i.e., more than one DA should be present for each scope, and they should maintain a fully meshed peer relationship (similar to IBGP [2]). Peer DAs exchange their data for the shared scopes when they set up a peer relationship, and continue to exchange new service registration information during the entire peering period. As a result, they maintain the same consistent data for the shared scopes. mSLP provides a reliable directory service for an SLP system. It also greatly simplifies the implementation of service registration for a thin-client SA. The scalability of SLP is thereby enhanced. mSLP is backward compatible with SLPv2, and incremental deployment is supported. The rest of this document is organized as follows: Section 2 defines the terminology. Section 3 reviews the current DA message flows in SLPv2. Section 4 defines the mesh-enhancement extension. In Section 5, we describe the DA peer relationship. In Section 6, we present the message forwarding control among DAs. We discuss our design in Section 7, list constants in Section 8, and give security considerations in Section 9. 2. Terminology Peer DAs If two DAs have one or more scopes in common within one administrative domain, they are peers. Peer DAs coordinate with each other and store the same consistent data for the shared scopes. Peering TCP connection A persistent TCP connection that is kept between a pair of peer DAs for the entire peering period. It provides a reliable communication channel for the peer DAs to exchange messages. Therefore, a DA implementation is not burdened by managing message retransmissions. Its closing can be an indication of the termination of the peer relationship. Mesh-enhanced DA A DA who maintains a peering TCP connection to each of its peers and forwards service registration information to its peers according to the rules given in this W. Zhao, H. Schulzrinne Expires: November 22, 2000 [Page 2]
Internet Draft DA Interaction May 22, 2000 document. A mesh-enhanced DA MUST carry the "mesh- enhanced" attribute in its DAAdvert messages. Mesh-aware SA An SA who understands the "mesh-enhanced" attribute in a DAAdvert message and uses the mesh-enhanced DA capability accordingly. 3. DA Message Flows in SLPv2 This section reviews the current DA message flows in SLPv2. Figure 1 illustrates SA registrations with DAs. For each service registration (SrvReg) and deregistration (SrvDeReg) message, a DA replies with a service acknowledgment (SrvAck) message. +----+ +----+ | | --- SrvReg/SrvDeReg --> | | | SA | | DA | | | <------- SrvAck ------- | | +----+ +----+ Figure 1. SA Registrations Figure 2 shows UA queries with DAs. A DA replies with a service reply (SrvRply) message to a service request (SrvRqst) message, a service type reply (SrvTypeRply) message to a service type request (SrvTypeRqst) message, and an attribute reply (AttrRply) message to an attribute request (AttrRqst) message. +----+ +----+ | | ----- SrvTypeRqst/SrvRqst/AttrRqst ----> | | | UA | | DA | | | <---- SrvTypeRply/SrvRply/AttrRply ----- | | +----+ +----+ Figure 2. UA Queries Figure 3 depicts DA discovery. A DA replies with a unicast DA advertisement (DAAdvert) message to a multicast SrvRqst message which has "service:directory-agent" as service type. +-------+ Multicast SrvRqst +----+ | | ----- (service:directory-agent) ----> | | | UA/SA | | DA | | | <-------- Unicast DAAdvert ---------- | | +-------+ +----+ Figure 3. DA Discovery W. Zhao, H. Schulzrinne Expires: November 22, 2000 [Page 3]
Internet Draft DA Interaction May 22, 2000 Figure 4 shows DA advertisements which are multicast periodically. +----+ +-------+ | | | | | DA | ---- Multicast DAAdvert ---> | UA/SA | | | | | +----+ +-------+ Figure 4. DA Advertisement From Figure 1 to 4, we can see that SLPv2 does not define the message flows among DAs. We will define these flows for the interaction of DAs and refine above flows in the following sections. 4. Mesh-enhancement Extension We define a new SLP extension - "mesh-enhancement extension" for specifying the interaction operations of mesh-enhanced DAs. This extension is always six bytes long: five-byte extension header and one-byte extension data denoted as "Action-ID". Figure 5 illustrates its format. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Mesh-Enhancement Extension ID | Next Extension Offset (NEO) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NEO, contd. | Action-ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5. Mesh-Enhancement Extension The Action-ID is used to specify the requested operations from the other party or to indicate status information to the other party. Table 1 lists the defined Action-IDs. Action-ID Abbreviation 0 No_Action 1 Mesh_Forward_Rqst 2 Data_Dump_Rqst 3 Peer_Conn_Indication 4 Peer_Conn_Keepalive Table 1. Actions in Mesh-Enhancement Extension No_Action: null operation. It is used to turn off other interaction operations. W. Zhao, H. Schulzrinne Expires: November 22, 2000 [Page 4]
Internet Draft DA Interaction May 22, 2000 Mesh_Forward_Rqst: mesh forwarding request. It requests that the receiving DA of this message SHOULD forward the message to all peer DAs in the registration scope. Data_Dump_Rqst: data dump request. It requests that the receiving DA of this message SHOULD dump all the service registration data in the shared scopes to the requesting DA. Peer_Conn_Indication: peer connection indication. It indicates that this is a peering TCP connection. Peer_Conn_Keepalive: peer connection keepalive. It indicates that this TCP connection is alive, the other party SHOULD not close it. 5. Peer Relationship We use a set of fully meshed peering TCP connections among peer DAs. A peer relationship has three stages: setting up, keeping and tearing down. We describe each stage in detail next. 5.1. Setting Up a Peer Relationship In SLP, each DA periodically sends DA Advertisement (DAAdvert) messages to the administratively scoped SLP multicast [3] address (239.255.255.253 is the default address in the IPv4 local scope). All DAs listen to this address. Figure 6 illustrates the DAAdvert messages from mesh-enhanced DAs. +-----+ +-----------+ | | | | | DA1 | ------- Multicast DAAdvert ------> | DA2/UA/SA | | | [attr = mesh-enhanced] | | +-----+ +-----------+ Figure 6. Mesh-enhanced DA Advertisement A mesh-enhanced DA MUST maintain a peer list. It adds an entry to the list whenever it discovers a new peer, and removes an entry when it finds the corresponding peer is down. Each entry in this peer list SHOULD include peer URL, shared scopes, boot timestamp, and peering TCP connection ID. The boot timestamp is to identify a rebooted peer. The peering TCP connection is used for message forwarding. When a mesh-enhanced DA learns about a new peer, it MUST create a peering TCP connection to the peer at the port 427 if such connection does not yet exist. Then it sends a DAAdvert message with the mesh- enhancement extension (Action-ID = Peer_Conn_Indication) through this W. Zhao, H. Schulzrinne Expires: November 22, 2000 [Page 5]
Internet Draft DA Interaction May 22, 2000 channel to notify the peer that this is a peering TCP connection (Figure 7). Thus, the peer can also use this channel to forward messages in opposite direction. +-----+ +-----+ | | [Action-ID = Peer_Conn_Indication] | | | DA1 | <------------ DAAdvert (TCP) ------------- | DA2 | | | | | +-----+ +-----+ Figure 7. Peering Connection Indication After the peering TCP connection is established or identified, peer DAs begin to forward new service registration information to each other via this channel. At the same time, a mesh-enhanced DA SHOULD decide whether it needs to get the data from its new peers. For example, when a newly booted DA joins a peering DA set of three DAs, it needs to get a copy of the existing registration data from one of the three DAs, but not from all of them, which incurs a lot of redundant transmissions. (Note: How to choose a DA from the peering DA set is implementation dependent. There are a variety of choices for doing this. The new DA can randomly choose one DA from the peering DA set or just use the first DA it found. Other criteria can also be used for the selection such as the nearest DA or the most lightly loaded DA. The choice only affects performance.) To obtain the existing data, the new DA sends a DAAdvert message with the mesh-enhancement extension (Action-ID = Data_Dump_Rqst) to the chosen peer (Figure 8). On the other end, when a mesh-enhanced DA receives the "Data_Dump_Rqst" in a DAAdvert message, it dumps all the data of the shared scopes to the requesting DA. Each data record is sent as a SrvReg message, with a re-calculated new lifetime (= old lifetime - elapsed time). After exchanging their data in both directions, peer DAs have the same consistent data for the shared scopes. +-----+ [Action-ID = Data_Dump_Rqst] +-----+ | | --------- DAAdvert (TCP) --------> | | | DA1 | | DA2 | | | <--------- SrvReg (TCP) ---------- | | +-----+ (data of shared scopes) +-----+ Figure 8. Dumping Data 5.2. Keeping a Peer Relationship In SLPv2, a DA could close an idle TCP connection after CONFIG_CLOSE_CONN seconds (5 minutes at least). To keep a peering TCP connection alive, a mesh-enhanced DA SHOULD send a DAAdvert message with the mesh-enhancement extension (Action-ID = Peer_Conn_Keepalive) W. Zhao, H. Schulzrinne Expires: November 22, 2000 [Page 6]
Internet Draft DA Interaction May 22, 2000 via the channel every CONFIG_DA_KEEPALIVE (see Section 9) seconds (Figure 9). +-----+ +-----+ | | | | | DA1 | ------------ DAAdvert (TCP) ------------> | DA2 | | | [Action-ID = Peer_Conn_Keepalive] | | +-----+ +-----+ Figure 9. DA Keepalive 5.3. Tearing Down a Peer Relationship A mesh-enhanced DA SHOULD tear down a peer relationship when it finds that the peer has closed the peering TCP connection; when it receives a multicast DAAdvert message from the peer with a DA stateless boot timestamp set to 0 meaning the peer is going to shutdown; or when it has not received the keepalive DAAdvert message from the peer for more than CONFIG_DA_KEEPALIVE seconds. In the last case, there may be a network partition and peer DA states get inconsistent. To tear down a peer relationship, a DA stops forwarding any service registration information to this peer, closes TCP connection with this peer, and removes this peer from its peer list. 6. Message Forwarding Control During the peering period, peer DAs forward new service registration information from SAs to each other. The message forwarding rules are as follows: (1) Explicit Forwarding: A message is forwarded only when it explicitely requests to be forwarded. A mesh-aware SA needs to attach the mesh-enhancement extension to a SrvReg or SrvDeReg message and set the Action-ID to Mesh_Forward_Rqst if it wants the message to be forwarded by a mesh-enhanced DA. This is for the compatibility with SLPv2, where SAs need to register with all existing DAs. To avoid unnecessary forwarding, the explicit forwarding rule is used. (2) One-hop Forwarding: A SrvReg or SrvDeReg message is forwarded only once by a mesh-enhanced DA to all its peers in the registration scope. Before forwarding a message, a mesh-enhanced DA sets the Action-ID in the mesh-enhancement extension to No_Action. In this way, a forwarded message will never be forwarded again. Since the peering DA set is in a fully connected mesh, one-hop forwarding is enough to ensure that a message can reach all peer DAs. Figure 10 shows how a SrvReg/SrvDeReg message is forwarded. W. Zhao, H. Schulzrinne Expires: November 22, 2000 [Page 7]
Internet Draft DA Interaction May 22, 2000 [Action-ID = +----+ Mesh_Forward_Rqst] +-----+ [Action-ID = No_Action] +-----+ | | -- SrvReg/SrvDeReg -> | | --- SrvReg/SrvDeReg --> | | | SA | | DA1 | | DA2 | | | <------ SrvAck ------ | | <------- SrvAck ------- | | +----+ +-----+ +-----+ Figure 10. Forwarding Registration (3) Handling SrvAck: A DA always replies with a SrvAck message when it receives a SrvReg or SrvDeReg message. So a mesh-enhanced DA SHOULD process SrvAck messages from other DAs. 7. Design Discussion In this section, we discuss several important design issues. 7.1 Meshed Peering TCP Connections The fully meshed peering DA architecture is built on top of a set of fully meshed peering TCP connections. We choose to use this set of fully meshed peering TCP connections mainly because it greatly facilitates message forwarding control. Any service registration information received by a DA only needs one-hop forwarding to reach all other DAs in the peering DA set. We anticipate a small number of DAs for each service scope, and 2-4 is the recommended value. There is no need to have separate DA for each scope. A DA can serve multiple scopes, and a peering TCP connection is used for all shared scopes between each pair of peer DAs. 7.2 Reliability The fully meshed peering DA architecture provides maximum robustness. It ensures that no single failure point exists in the SLP directory service. All service registrations are kept by at least two DAs. If one DA is down, at least one other peer DA can continue to provide the service information for the scope. Moreover, the peering DA architecture ensures the data consistency among peer DAs automatically. It also enables a DA to recover from crash with much less effort since a rebooted DA can get the existing data from its peering DA set. This is done through DA coordination, no SA involvement is needed. 7.3 Simplifying SA Registrations and Scalability The fully meshed peering DA architecture greatly simplifies SA registrations. A mesh-aware SA only needs to register with one mesh-enhanced DA in the registration scope. The information will be W. Zhao, H. Schulzrinne Expires: November 22, 2000 [Page 8]
Internet Draft DA Interaction May 22, 2000 propagated automatically within the peering DA set. Thus, a mesh- aware SA does not need to implement the complicated algorithm to register with all existing DAs and to re-register when new mesh- enhanced DAs are discovered, or old mesh-enhanced DAs are found to have rebooted. With mesh-enhanced DAs and simplified SAs, the overall SLP system scalability can be enhanced. 7.4 Forwarding UA Queries A further extension to the interaction of SLP DAs is to forward UA queries besides SA registrations. It works as follows: When a mesh- enhanced DA receives a UA query which is not in its scope, it forwards the query to another DA which supports the scope. This can simplify UA implementation since UAs do not need to keep track of DA scopes. A UA can send its queries to any mesh-enhanced DA. However, this adds much complexity to the mesh-enhanced DA implementation. First, a mesh-enhanced DA needs to keep track of all DAs of all scopes, not only the DAs that share some scopes with it. Second, a mesh-enhanced DA needs to forward the query to another DA, and it also needs to forward the reply from another DA back to the UA. We did not include this extension in our scheme mainly due to its complexity. However, for a thin-client UA implementation, it might deserve further consideration. 7.5 Compatibility mSLP is backward compatible with SLPv2. It only defines a new attribute ("mesh-enhanced") for the DAAdvert message and a new SLP extension (mesh-enhancement extension) which is used by DAAdvert, SrvReg and SrvDeReg messages. In mSLP, DAs are enhanced with very little added complexity, and the changes are almost transparent to UAs and SAs. UAs can be kept unchanged. SAs can be greatly simplified by using the mesh-forwarding capability for the service registrations. mSLP supports incremental deployment of mesh-enhanced DAs. A mesh- enhanced DA can set up peer relationships with both mesh-enhanced DAs and non-mesh-enhanced DAs. In the latter case, the message forwarding only goes in uni-direction from the mesh-enhanced DA to a non-mesh- enhanced DA. A mesh-aware SA only needs to register with one mesh- enhanced DA. However, it still needs to take care of newly found non-mesh-enhanced DAs and rebooted non-mesh-enhanced DAs since these non-mesh-enhanced DAs can not get the existing data from other DAs. Therefore, uniformly mesh-enhanced DAs can provide a much easier job for mesh-aware SAs. We summarize the operations for mesh-aware SAs, non-mesh-aware SAs, mesh-enhanced DAs and non-mesh-enhanced DAs as follows: W. Zhao, H. Schulzrinne Expires: November 22, 2000 [Page 9]
Internet Draft DA Interaction May 22, 2000 Non-mesh-aware SA It needs to discover all DAs in its scope and register with all of them. It does NOT use the mesh-enhancement extension. Mesh-aware SA It identifies mesh-enhanced DAs from the "mesh-enhanced" attribute in the DAAdvert messages. It only needs to discover one mesh-enhanced DA and register with it using the mesh- enhancement extension (Action-ID = Mesh_Forward_Rqst). If there are no mesh-enhanced DAs in its scope, it operates in the same way as non-mesh-aware SA. Non-mesh-enhanced DA It accepts SrvReg and SrvDeReg messages from SAs and mesh- enhanced DAs normally. It dose NOT forward them. Mesh-enhanced DA It carries the "mesh-enhanced" attribute in its DAAdvert messages. It accepts SrvReg and SrvDeReg messages from SAs and mesh-enhanced DAs. It forwards the messages from mesh-aware SAs to all peer DAs (both mesh-enhanced and non-mesh-enhanced) in the registration scope. It also processes SrvAck messages from mesh-enhanced DAs. 8. Constants Mesh-enhancement Extension ID 0x0006 (Section 4) CONFIG_DA_KEEPALIVE 290 seconds (Section 5.2) Note: CONFIG_DA_KEEPALIVE < CONFIG_CLOSE_CONN, which has a default value of 300 seconds [1]. 9. Security Considerations The DA authentication is more important in mSLP, since mesh-aware SAs will have to trust the mesh-enhanced DA they are sending the SrvReg and SrvDeReg messages to forward them. DAs SHOULD use standard SLPv2 authentication to authenticate other DAs in the mesh. DAs SHOULD also perform authentication before accepting SrvReg and SrvDeReg messages to prevent illegitimate modification or elimination of legitimate service registrations. mSLP is just as secure as SLPv2. 10. References [1] E. Guttman, C. Perkins, J. Veizades, M. Day, "Service Location Protocol Version 2", RFC 2608, June 1999. W. Zhao, H. Schulzrinne Expires: November 22, 2000 [Page 10]
Internet Draft DA Interaction May 22, 2000 [2] Y. Rekhter, T. Li, "A Border Gateway Protocol 4 (BGP-4)", RFC 1771, March 1995 [3] D. Meyer, "Administratively Scoped IP Multicast", RFC 2365, July 1998 11. Acknowledgments Erik Guttman provided valuable comments and suggestions for this draft. 12. Authors' Addresses Weibin Zhao Department of Computer Science Columbia University 1214 Amsterdam Avenue, MC 0401 New York, NY 10027-7003 email: zwb@cs.columbia.edu Henning Schulzrinne Department of Computer Science Columbia University 1214 Amsterdam Avenue, MC 0401 New York, NY 10027-7003 email: hgs@cs.columbia.edu W. Zhao, H. Schulzrinne Expires: November 22, 2000 [Page 11]