Network Working Group P. Srisuresh Request for Comments: 3303 Kuokoa Networks Category: Informational J. Kuthan Fraunhofer Institute FOKUS J. Rosenberg dynamicsoft A. Molitor Aravox Technologies A. Rayhan Ryerson University August 2002 Middlebox communication architecture and framework Status of this Memo This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved.
AbstractA principal objective of this document is to describe the underlying framework of middlebox communications (MIDCOM) to enable complex applications through the middleboxes, seamlessly using a trusted third party. This document and a companion document on MIDCOM requirements ([REQMTS]) have been created as a precursor to rechartering the MIDCOM working group. There are a variety of intermediate devices in the Internet today that require application intelligence for their operation. Datagrams pertaining to real-time streaming applications, such as SIP and H.323, and peer-to-peer applications, such as Napster and NetMeeting, cannot be identified by merely examining packet headers. Middleboxes implementing Firewall and Network Address Translator services typically embed application intelligence within the device for their operation. The document specifies an architecture and framework in which trusted third parties can be delegated to assist the middleboxes to perform their operation, without resorting to embedding application intelligence. Doing this will allow a middlebox to continue to provide the services, while keeping the middlebox application agnostic.
path of an application instance is outside the scope of this document. Further, any communication amongst middleboxes is also outside the scope of this document. This document describes the framework in which middlebox communication takes place and the various elements that constitute the framework. Section 2 describes the terms used in the document. Section 3 defines the architectural framework of a middlebox for communication with MIDCOM agents. The remaining sections cover the components of the framework, illustration using sample flows, and operational considerations with the MIDCOM architecture. Section 4 describes the nature of MIDCOM protocol. Section 5 identifies entities that could potentially host the MIDCOM agent function. Section 6 considers the role of Policy server and its function with regard to communicating MIDCOM agent authorization policies. Section 7 is an illustration of SIP flows using a MIDCOM framework in which the MIDCOM agent is co-resident on a SIP proxy server. Section 8 addresses operational considerations in deploying a protocol adhering to the framework described here. Section 9 is an applicability statement, scoping the location of middleboxes. Section 11 outlines security considerations for the middlebox in view of the MIDCOM framework.
middleboxes may continue to embed application intelligence for certain applications and depend on MIDCOM protocol and MIDCOM agents for the support of remaining applications. NAT-TERM] for the definition of Transparent routing, various NAT types, and the associated terms in use. Two types of NAT are most common. Basic-NAT, where only an IP address (and the related IP, TCP/UDP checksums) of packets is altered and NAPT (Network Address Port Translation), where both an IP address and a transport layer identifier, such as a TCP/UDP port (and the related IP, TCP/UDP checksums), are altered. The term NAT in this document is very similar to the IPv4 NAT described in [NAT-TERM], but is extended beyond IPv4 networks to include the IPv4-v6 NAT-PT described in [NAT-PT]. While the IPv4 NAT [NAT-TERM] translates one IPv4 address into another IPv4 address to provide routing between private V4 and external V4 address realms, IPv4-v6 NAT-PT [NAT-PT] translates an IPv4 address into an IPv6 address, and vice versa, to provide routing between a V6 address realm and an external V4 address realm. Unless specified otherwise, NAT in this document is a middlebox function referring to both IPv4 NAT, as well as IPv4-v6 NAT-PT.
Applications such as FTP, SIP, and RTSP use a control session to establish data sessions. These control and data sessions can take divergent paths. While a proxy can intercept both the control and data sessions, it might intercept only the control session. This is often the case with real-time streaming applications such as SIP and RTSP.
combination enables the agents to facilitate traversal of the middlebox by the application's packets. A MIDCOM agent may interact with one or more middleboxes. Only "In-Path MIDCOM agents" are considered in this document. In- Path MIDCOM agents are agents which are within the path of those datagrams that the agent needs to examine and/or modify in fulfilling its role as a MIDCOM agent. "Within the path" here simply means that the packets in question flow through the node that hosts the agent. The packets may be addressed to the agent node at the IP layer. Alternatively they may not be addressed to the agent node, but may be constrained by other factors to flow through it. In fact, it is immaterial to the MIDCOM protocol which of these is the case. Some examples of In-Path MIDCOM agents are application proxies, gateways, or even end-hosts that are party to the application. Agents not resident on nodes that are within the path of their relevant application flows are referred to as "Out-of-Path (OOP) MIDCOM agents" and are out of the scope of this document. POL-TERM]; and also acts as a policy repository, holding MIDCOM related policy profiles in order to make authorization decisions. [POL-TERM] defines a PDP as "a logical entity that makes policy decisions for itself or for other network elements that request such decisions"; and a policy repository as "a specific data store that holds policy rules, their conditions and actions, and related policy data". A middlebox and a MIDCOM PDP may communicate further if the MIDCOM PDP's policy changes or if a middlebox needs further information. The MIDCOM PDP may, at anytime, notify the middlebox to terminate authorization for an agent. The protocol facilitating the communication between a middlebox and MIDCOM PDP need not be part of the MIDCOM protocol. Section 6 in the document addresses the MIDCOM PDP interface and protocol framework independent of the MIDCOM framework. Application specific policy data and policy interface between an agent or application endpoint and a MIDCOM PDP is out of bounds for this document. The MIDCOM PDP issues addressed in the document are focused at an aggregate domain level as befitting the middlebox. For example, a SIP MIDCOM agent may choose to query a MIDCOM PDP for the administrative (or corporate) domain to find whether a certain user is allowed to make an outgoing call. This type of application
specific policy data, as befitting an end user, is out of bounds for the MIDCOM PDP considered in this document. It is within bounds, however, for the MIDCOM PDP to specify the specific end-user applications (or tuples) for which an agent is permitted to be an ALG. POL-TERM], which defines a filter as "A set of
terms and/or criteria used for the purpose of separating or categorizing. This is accomplished via single- or multi-field matching of traffic header and/or payload data". 5-Tuple specification of packets in the case of a firewall and 5- tuple specification of a session in the case of a NAT middlebox function are examples of a filter. POL-TERM], which defines a policy action as "Definition of what is to be done to enforce a policy rule, when the conditions of the rule are met. Policy actions may result in the execution of one or more operations to affect and/or configure network traffic and network resources". NAT Address-BIND (or Port-BIND in the case of NAPT) and firewall permit/deny action are examples of an Action.
+---------------+ +--------------+ | MIDCOM agent | | MIDCOM agent | | co-resident on| | co-resident | | Proxy Server | | on Appl. GW | +---------------+ +--------------+ ^ ^ | | +--------+ MIDCOM | | | MIDCOM | Protocol | | +-| PDP | | | / +--------+ +-------------+ | | / | MIDCOM agent| | | / | co-resident | | | / | on End-hosts|<-+ | | / +-------------+ | | | | v v v v +-------------------------------------------+ | Middlebox Communication |Policy | | Protocol (MIDCOM) Interface |Interface | +----------+--------+-----------+-----------+ Middlebox | | | | | Functions | Firewall | NAT | VPN | Intrusion | | | | tunneling | Detection | +----------+--------+-----------+-----------+ Middlebox | Middlebox function specific policy rule(s)| Managed | and other attributes | Resources | | +-------------------------------------------+ Figure 1: MIDCOM agents interfacing with a middlebox Firewall ACLs, NAT-BINDs, NAT address-maps and Session-state are a few of the middlebox function specific policy rules. A session state may include middlebox function specific attributes, such as timeout values, NAT translation parameters (i.e., NAT-BINDS), and so forth. As Session-state may be shared across middlebox functions, a Session-state may be created by a function, and terminated by a different function. For example, a session-state may be created by the firewall function, but terminated by the NAT function, when a session timer expires. Application specific MIDCOM agents (co-resident on the middlebox or external to the middlebox) would examine the IP datagrams and help identify the application the datagram belongs to, and assist the middlebox in performing functions unique to the application and the middlebox service. For example, a MIDCOM agent, assisting a NAT middlebox, might perform payload translations, whereas a MIDCOM agent
assisting a firewall middlebox might request the firewall to permit access to application specific, dynamically generated, session traffic.
Figure 2 below illustrates a scenario where the in-path MIDCOM agents
interface with the middlebox. Let us say, the MIDCOM PDP has pre- configured the in-path proxies as trusted MIDCOM agents on the middlebox and the packet filter implements a 'default-deny' packet filtering policy. Proxies use their application-awareness knowledge to control the firewall function and selectively permit a certain number of voice stream sessions dynamically using MIDCOM protocol. In the illustration below, the proxies and the MIDCOM PDP are shown inside a private domain. The intent however, is not to imply that they be inside the private boundary alone. The proxies may also reside external to the domain. The only requirement is that there be a trust relationship with the middlebox. +-----------+ | MIDCOM | | PDP |~~~~~~~~~~~~~| +-----------+ \ \ +--------+ \ | SIP |___ \ ________| Proxy | \ Middlebox \ / +--------+.. | +--------------------+ | : | MIDCOM | | | | RTSP +---------+ :..|........| MIDCOM | POLICY | SIP | ____| RTSP |.....|........| PROTOCOL | INTER- | | / | Proxy |___ | | INTERFACE | FACE | | | +---------+ \ \ |--------------------| | | \ \______| |__SIP | | \________| |__RTSP | | ---| FIREWALL |--->-- +-----------+ /---| |---<-- +-----------+| Data streams // +--------------------+ +-----------+||---------->----// | |end-hosts ||-----------<----- . +-----------+ (RTP, RTSP data, etc.) | . Outside the Within a private domain | private domain Legend: ---- Application data path datagrams ____ Application control path datagrams .... Middlebox Communication Protocol (MIDCOM) ~~~~ MIDCOM PDP Interface | . private domain Boundary | Figure 2: In-Path MIDCOM Agents for middlebox Communication
figure 1. The MIDCOM PDP is a logical entity which may reside physically on a middlebox or on a node external to the middlebox. The protocol employed for communication between the middlebox and the MIDCOM PDP is unrelated to the MIDCOM protocol. Agents are registered with a MIDCOM PDP for authorization to invoke services of the middlebox. The MIDCOM PDP maintains a list of agents that are authorized to connect to each of the middleboxes the MIDCOM PDP supports. In the context of the MIDCOM Framework, the MIDCOM PDP does not assist a middlebox in the implementation of the services it provides. The MIDCOM PDP acts in an advisory capacity to a middlebox, to authorize or terminate authorization for an agent attempting connectivity to the middlebox. The primary objective of a MIDCOM PDP is to communicate agent authorization information, so as to ensure that the security and integrity of a middlebox is not jeopardized. Specifically, the MIDCOM PDP should associate a trust level with each agent attempting to connect to a middlebox and provide a security profile. The MIDCOM PDP should be capable of addressing cases when end-hosts are agents to the middlebox.
establishment of keys to protect subsequent traffic. Two-way authentication is often required to prevent various active attacks on the MIDCOM protocol and secure establishment of keying material. Security services such as authentication, data integrity, confidentiality and replay protection may be adapted to secure MIDCOM messages in an untrusted domain. Message authentication is the same as data origin authentication and is an affirmation that the sender of the message is who it claims to be. Data integrity refers to the ability to ensure that a message has not been accidentally, maliciously or otherwise altered or destroyed. Confidentiality is the encryption of a message with a key, so that only those in possession of the key can decipher the message content. Lastly, replay protection is a form of sequence integrity, so when an intruder plays back a previously recorded sequence of messages, the receiver of the replay messages will simply drop the replay messages into bit-bucket. Certain applications of the MIDCOM protocol might require support for non-repudiation as an option of the data integrity service. Typically, support for non-repudiation is required for billing, service level agreements, payment orders, and receipts for delivery of service. IPsec AH ([IPSEC-AH]) offers data-origin authentication, data integrity and protection from message replay. IPsec ESP ([IPSEC- ESP]) provides data-origin authentication to a lesser degree (same as IPsec AH if the MIDCOM transport protocol turns out to be TCP or UDP), message confidentiality, data integrity and protection from replay. Besides the IPsec based protocols, there are other security options as well. TLS based transport layer security is one option. There are also many application-layer security mechanisms available. Simple Source-address based security is a minimal form of security and should be relied on only in the most trusted environments, where those hosts will not be spoofed. The MIDCOM message security shall use existing standards, whenever the existing standards satisfy the requirements. Security shall be specified to minimize the impact on sessions that do not use the security option. Security should be designed to avoid introducing and to minimize the impact of denial of service attacks. Some security mechanisms and algorithms require substantial processing or storage, in which case the security protocols should protect themselves as well as against possible flooding attacks that overwhelm the endpoint (i.e., the middlebox or the agent) with such processing. For connection oriented protocols (such as TCP) using security services, the security protocol should detect premature closure or truncation attacks.
agent might be unable to process the control stream to permit the data sessions. When a middlebox notices an incoming MIDCOM session, and the middlebox has no prior profile of the MIDCOM agent, the middlebox will consult its MIDCOM PDP for authenticity, authorization, and trust guidelines for the session. figure 3 below, we consider SIP applications (Refer [SIP]) to illustrate the operation of the MIDCOM protocol. Specifically, the application assumes that a caller, external to a private domain, initiates the call. The middlebox is assumed to be located at the edge of the private domain. A SIP phone (SIP User Agent Client/Server) inside the private domain is capable of receiving calls from external SIP phones. The caller uses a SIP Proxy, node located external to the private domain, as its outbound proxy. No interior proxy is assumed for the callee. Lastly, the external SIP proxy node is designated to host the MIDCOM agent function. Arrows 1 and 8 in the figure below refer to a SIP call setup exchange between the external SIP phone and the SIP proxy. Arrows 4 and 5 refer to a SIP call setup exchange between the SIP proxy and the interior SIP phone, and are assumed to be traversing the middlebox. Arrows 2, 3, 6 and 7 below, between the SIP proxy and the middlebox, refer to MIDCOM communication. Na and Nb represent RTP/RTCP media traffic (Refer [RTP]) path in the external network. Nc and Nd represent media traffic inside the private domain. _________ --->| SIP |<-----\ / | Proxy | \ | |_________| | 1| |^ ^| 4| | || || | |8 2||3 7||6 |5 ______________ | || || | _____________ | |<-/ _v|____|v___ \->| | | External | Na | | Nc | SIP Phone | | SIP phone |>------->| Middlebox |>------>| within | | |<-------<|___________|<------<| Pvt. domain| |____________| Nb Nd |____________| Figure 3: MIDCOM framework illustration with In-Path SIP Proxy As for the SIP application, we make the assumption that the middlebox is pre-configured to accept SIP calls into the private SIP phone. Specifically, this would imply that the middlebox implementing firewall service is pre-configured to permit SIP calls (destination
TCP or UDP port number set to 5060) into the private phone. Likewise, middlebox implementing NAPT service would have been pre- configured to provide a port binding, to permit incoming SIP calls to be redirected to the specific private SIP phone. I.e., the INVITE from the external caller is not made to the private IP address, but to the NAPT external address. The objective of the MIDCOM agent in the following illustration is to merely permit the RTP/RTCP media stream (Refer [RTP]) through the middlebox, when using the MIDCOM protocol architecture outlined in the document. A SIP session typically establishes two RTP/RTCP media streams - one from the callee to the caller and another from the caller to the callee. These media sessions are UDP based and will use dynamic ports. The dynamic ports used for the media stream are specified in the SDP section (Refer [SDP]) of the SIP payload message. The MIDCOM agent will parse the SDP section and use the MIDCOM protocol to (a) open pinholes (i.e., permit RTP/RTCP session tuples) in a middlebox implementing firewall service, or (b) create PORT bindings and appropriately modify the SDP content to permit the RTP/RTCP streams through a middlebox implementing NAT service. The MIDCOM protocol should be sufficiently rich and expressive to support the operations described under the timelines. The examples do not show the timers maintained by the agent to keep the middlebox policy rule(s) from timing out. MIDCOM agent Registration and connectivity between the MIDCOM agent and the middlebox are not shown in the interest of restricting the focus of the MIDCOM transactions to enabling the middlebox to let the media stream through. MIDCOM PDP is also not shown in the diagram below or on the timelines for the same reason. The following subsections illustrate a typical timeline sequence of operations that transpire with the various elements involved in a SIP telephony application path. Each subsection is devoted to a specific instantiation of a middlebox service - NAPT (refer [NAT-TERM], [NAT- TRAD]), firewall and a combination of both NAPT and firewall are considered.
The INVITE from the caller (external) is assumed to include the SDP payload. You will note that the MIDCOM agent requests the middlebox to permit the Private-to-external RTP/RTCP flows before the INVITE is relayed to the callee. This is because, in SIP, the calling party must be ready to receive the media when it sends the INVITE with a session description. If the called party (private phone) assumes this and sends "early media" before sending the 200 OK response, the firewall will have blocked these packets without this initial MIDCOM signaling from the agent. SIP Phone SIP Proxy Middlebox SIP Phone (External) (MIDCOM agent) (FIREWALL (private) | | Service) | | | | | |----INVITE------>| | | | | | | |<---100Trying----| | | | | | | | Identify end-2-end | | | parameters (from Caller's | | | SDP) for the pri-to-Ext | | | RTP & RTCP sessions. | | | (RTP1, RTCP1) | | | | | | | |+Permit RTP1, RTCP1 +>| | | |<+RTP1, RTCP1 OKed++++| | | | | | | |--------INVITE---------------------->| | | | | | |<-----180 Ringing--------------------| |<--180Ringing----| | | | |<-------200 OK-----------------------| | | | | | Identify end-2-end | | | parameters (from callee's | | | SDP) for the Ext-to-Pri | | | RTP and RTCP sessions. | | | (RTP2, RTCP2) | | | | | | | |+Permit RTP2, RTCP2 +>| | | |<+RTP2, RTCP2 OKed++++| | | | | | |<---200 OK ------| | | |-------ACK------>| | | | |-----------ACK---------------------->| | | | | |<===================RTP/RTCP==========================>|
| | | | |-------BYE------>| | | | |--------------------------BYE------->| | | | | | |<----------200 OK--------------------| | | | | | |++Cancel permits to | | | | RTP1, RTCP1, RTP2, | | | | and RTCP2 +++++++++>| | | |<+RTP1, RTP2, RTCP1 & | | | | RTCP2 cancelled ++++| | | | | | |<---200 OK-------| | | | | | | Legend: ++++ MIDCOM control traffic ---- SIP control traffic ==== RTP/RTCP media traffic section 7.1), the INVITE from the caller (external) is assumed to include the SDP payload. You will note that the MIDCOM agent requests the middlebox to create NAT session descriptors for the private-to-external RTP/RTCP flows before the INVITE is relayed to the private SIP phone (for the same reasons as described in section 7.1). If the called party (private phone) sends "early media" before sending the 200 OK response, the NAPT middlebox will have blocked these packets without the initial MIDCOM signaling from the agent. Also, note that after the 200 OK is received by the proxy from the private phone, the agent requests the middlebox to allocate NAT session descriptors for the external-to- private RTP2 and RTCP2 flows, such that the ports assigned on the Ma for RTP2 and RTCP2 are contiguous. The RTCP stream does not happen
with a non-contiguous port. Lastly, you will note that even though each media stream (RTP1, RTCP1, RTP2 and RTCP2) is independent, they are all tied to the single SIP control session, while their NAT session descriptors were being created. Finally, when the agent issues a terminate session bundle command for the SIP session, the middlebox is assumed to delete all associated media stream sessions automagically. SIP Phone SIP Proxy Middlebox SIP Phone (External) (MIDCOM agent) (NAPT (Private) IP Addr:Ea | Service) IP addr:Pa | | IP addr:Ma | | | | | |----INVITE------>| | | | | | | |<---100Trying----| | | | | | | | |++ Query Port-BIND | | | | for (Ma, 5060) +++>| | | |<+ Port-BIND reply | | | | for (Ma, 5060) ++++| | | | | | | |++ Query NAT Session | | | | Descriptor for | | | | Ea-to-Pa SIP flow+>| | | |<+ Ea-to-Pa SIP flow | | | | Session Descriptor+| | | | | | | Determine the Internal | | | IP address (Pa) | | | of the callee. | | | | | | | Identify UDP port numbers | | | on Ea (Eport1, Eport1+1) | | | for pri-to-ext RTP & RTCP | | | sessions (RTP1, RTCP1) | | | | | | | |++Create NAT Session | | | | descriptors for | | | | RTP1, RTCP1; Set | | | | parent session to | | | | SIP-ctrl session ++>| | | |<+RTP1, RTCP1 session | | | | descriptors created+| | | | | | | | |..redirected..| | |--------INVITE--------|------------->| | | | |
| |<-----180Ringing---------------------| | | | | |<--180Ringing----| | | | |<-------200 OK-----------------------| | | | | | Identify UDP port numbers | | | on Pa (Pport2, Pport2+1) | | | for ext-to-pri RTP & RTCP | | | sessions (RTP2, RTCP2) | | | | | | | |++Create consecutive | | | | port BINDs on Ma | | | | for (Pa, Pport2), | | | | (Pa, Pport2+1) ++++>| | | |<+Port BINDs created++| | | | | | | |++Create NAT Session | | | | descriptors for | | | | RTP2, RTCP2; Set | | | | parent session to | | | | SIP-ctrl session ++>| | | |<+RTP2, RTCP2 session | | | | descriptors created+| | | | | | | Modify the SDP | | | parameters in "200 OK" | | | with NAPT PORT-BIND | | | for the RTP2 port on Ma. | | | | | | |<---200 OK ------| | | | | | | |-------ACK------>| | | | | | | | Modify IP addresses | | | appropriately in the SIP | | | header (e.g., To, from, | | | Via, contact fields) | | | | |..redirected..| | |-----------ACK--------|------------->| | | | | | | | | |<===================RTP/RTCP============|=============>| | | | | |-------BYE------>| | | | | | | | |----------------------|-----BYE----->| | | | | | |<----------200 OK--------------------|
| | | | | |+++Terminate the SIP | | | | Session bundle +++>| | | |<++SIP Session bundle | | | | terminated ++++++++| | | | | | |<---200 OK-------| | | | | | | Legend: ++++ MIDCOM control traffic ---- SIP control traffic ==== RTP/RTCP media traffic
| |<+ Port-BIND reply | | | | for (Ma, 5060) ++++| | | | | | | |++ Query NAT Session | | | | Descriptor for | | | | Ea-to-Pa SIP flow+>| | | |<+ Ea-to-Pa SIP flow | | | | Session Descriptor+| | | | | | | Determine the Internal | | | IP address (Pa) | | | of the callee. | | | | | | | Identify UDP port numbers | | | on Ea (Eport1, Eport1+1) | | | for pri-to-ext RTP & RTCP | | | sessions (RTP1, RTCP1) | | | | | | | |++Create NAT Session | | | | descriptors for | | | | RTP1, RTCP1; Set the| | | | parent session to | | | | point to SIP flow++>| | | |<+RTP1, RTCP1 session | | | | descriptors created+| | | | | | | |++Permit RTP1 & RTCP1 | | | | sessions External to| | | | middlebox, namely | | | | Ma to Ea:Eport1, | | | | Ma to Ea:Eport1+1 | | | | sessions ++++++++++>| | | |<+Ma to Ea:Eport1, | | | | Ma to Ea:Eport1+1 | | | | sessions OKed ++++++| | | | | | | | |..redirected..| | |--------INVITE--------|------------->| | | | | | |<-----180Ringing---------------------| | | | | |<--180Ringing----| | | | |<-------200 OK-----------------------| | | | | | Identify UDP port numbers | | | on Pa (Pport2, Pport2+1) | | | for ext-to-pri RTP & RTCP | | | sessions (RTP2, RTCP2) | |
| | | | | |++Create consecutive | | | | port BINDs on Ma | | | | for (Pa, Pport2), | | | | (Pa, Pport2+1) ++++>| | | |<+Port BINDs created | | | | on Ma as (Mport2, | | | | Mport2+1) ++++++++++| | | | | | | |++Create NAT Session | | | | descriptors for | | | | RTP2, RTCP2; Set the| | | | parent session to | | | | point to SIP flow++>| | | |<+RTP2, RTCP2 session | | | | descriptors created+| | | | | | | Modify the SDP | | | parameters in "200 OK" | | | with NAPT PORT-BIND | | | for RTP2 port on Ma. | | | | | | | |++Permit RTP2 & RTCP2 | | | | sessions External | | | | middlebox, namely | | | | Ea to Ma:Mport2, | | | | Ea to Ma:Mport2+1 | | | | sessions ++++++++++>| | | |<+Ea to Ma:Mport2, | | | | Ea to Ma:Mport2 | | | | sessions OKed ++++++| | | | | | |<---200 OK ------| | | | | | | |-------ACK------>| | | | | |..redirected..| | |-----------ACK--------|------------->| | | | | | | | | |<===================RTP/RTCP============|=============>| | | | | |-------BYE------>| | | | | | | | |----------------------|-----BYE----->| | | | | | |<----------200 OK--------------------| | | | | | |+++Terminate the SIP | |
| | Session bundle +++>| | | |<++SIP Session bundle | | | | terminated ++++++++| | | | | | | |++Cancel permits to | | | | sessions External | | | | middlebox, namely | | | | Ma to Ea:Eport1, | | | | Ma to Ea:Eport1+1 | | | | Ea to Ma:Mport2, | | | | Ea to Ma:Mport2+1 | | | | sessions ++++++++++>| | | |<+Removed permits to | | | | sessions listed ++++| | | | | | |<---200 OK-------| | | | | | | Legend: ++++ MIDCOM control traffic ---- SIP control traffic ==== RTP/RTCP media traffic
Signaling and context specific Header information is sent in-band, within the same data stream for applications such as HTTP embedded applications, sun-RPC (embedding a variety of NFS apps), Oracle transactions (embedding oracle SQL+, MS ODBC, Peoplesoft) etc. H.323 is an example of an application that sends signaling in both dedicated stand-alone sessions, as well as in conjunction with data. H.225.0 call signaling traffic traverses middleboxes by virtue of static policy, no MIDCOM control needed. H.225.0 call signaling also negotiates ports for an H.245 TCP stream. A MIDCOM agent is required to examine/modify the contents of the H.245 so that H.245 can traverse it. H.245 traverses the middlebox and also carries Open Logical Channel information for media data. So, the MIDCOM agent is once again required to examine/modify the payload content needs to let the media traffic flow. The MIDCOM architecture takes into consideration, supporting applications with independent signaling and data sessions as well as applications that have signaling and data communicated over the same session. In the cases where signaling is done on a single stand-alone session, it is desirable to have a MIDCOM agent interpret the signaling stream and program the middlebox (that transits the data stream) so as to let the data traffic through uninterrupted.
establishing a "pin-hole" to permit a TCP/UDP session (the port parameters of which are dynamically determined) through a firewall or retain an address/port bind in the NAT device to permit sessions to a port. These requirements are met by current generation middleboxes using adhoc methods, such as embedding application intelligence within a middlebox to identify the dynamic session parameters and administering the middlebox internally as appropriate. The objective of the MIDCOM architecture is to create a unified, standard way to exercise this functionality, currently existing in an ad-hoc fashion, in some of the middleboxes. By adopting MIDCOM architecture, middleboxes will be able to support newer applications they have not been able to support thus far. MIDCOM architecture does not, and must not in anyway, change the fundamental characteristic of the services supported on the middlebox. Typically, organizations shield a majority of their corporate resources (such as end-hosts) from visibility to the external network by the use of a De-Militarized Zone (DMZ) at the domain edge. Only a portion of these hosts are allowed to be accessed by the external world. The remaining hosts and their names are unique to the private domain. Hosts visible to the external world and the authoritative name server that maps their names to network addresses are often configured within a DMZ (De-Militarized Zone) in front of a firewall. Hosts and middleboxes within DMZ are referred to as DMZ nodes. Figure 4 below illustrates the configuration of a private domain with a DMZ at its edge. Actual configurations may vary. Internal hosts are accessed only by users inside the domain. Middleboxes, located in the DMZ may be accessed by agents inside or outside the domain.
\ | / +-----------------------+ |Service Provider Router| +-----------------------+ WAN | Stub A .........|\|.... | +---------------+ | NAT middlebox | +---------------+ | | DMZ - Network ------------------------------------------------------------ | | | | | +--+ +--+ +--+ +--+ +-----------+ |__| |__| |__| |__| | Firewall | /____\ /____\ /____\ /____\ | middlebox | DMZ-Host1 DMZ-Host2 ... DMZ-Name DMZ-Web +-----------+ Server Server etc. | | Internal Hosts (inside the private domain) | ------------------------------------------------------------ | | | | +--+ +--+ +--+ +--+ |__| |__| |__| |__| /____\ /____\ /____\ /____\ Int-Host1 Int-Host2 ..... Int-Hostn Int-Name Server Figure 4: DMZ network configuration of a private domain.
Section 6 discusses the middlebox to MIDCOM PDP interactions in view of policy decisions. Authentication refers to confirming the identity of an originator for all datagrams received from the originator. Lack of strong credentials for authentication of MIDCOM messages between an agent and a middlebox can seriously jeopardize the fundamental service rendered by the middlebox. A consequence of not authenticating an agent would be that an attacker could spoof the identity of a "legitimate" agent and open holes in the firewall. Another would be
that it could otherwise manipulate the state on a middlebox, creating a denial-of-service attack by closing needed pinholes or filling up a NAT table. A consequence of not authenticating the middlebox to an agent is that an attacker could pose as a middlebox and respond to NAT requests in a manner that would divert data to the attacker. Failing to submit the required/valid credentials, once challenged, may indicate a replay attack, in which case a proper action is required by the middlebox such as auditing, logging, or consulting its designated MIDCOM PDP to reflect such failure. A consequence of not protecting the middlebox against replay attacks would be that a specific pinhole may be reopened or closed by an attacker at will, thereby bombarding end hosts with unwarranted data or causing denial of service. Integrity is required to ensure that a MIDCOM message has not been accidentally or maliciously altered or destroyed. The result of a lack of data integrity enforcement in an untrusted environment could be that an imposter will alter the messages sent by an agent and bring the middlebox to a halt or cause a denial of service for the application the agent is attempting to enable. Confidentiality of MIDCOM messages ensure that the signaling data is accessible only to the authorized entities. When a middlebox agent is deployed in an untrusted environment, lack of confidentiality will allow an intruder to perform traffic flow analysis and snoop the middlebox. The intruder could cannibalize a lesser secure MIDCOM session and destroy or compromise the middlebox resources he uncovered on other sessions. Needless to say, the least secure MIDCOM session will become the achilles heel and make the middlebox vulnerable to security attacks. Lastly, there can be security vulnerability to the applications traversing a middlebox when a resource on a middlebox is controlled by multiple external agents. A middlebox service may be disrupted due to conflicting directives from multiple agents associated with different middlebox functions but applied to the same application session. Care must be taken in the protocol design to ensure that agents for one function do not abruptly step over resources impacting a different function. Alternately, the severity of such manifestations could be lessened when a single MIDCOM agent is responsible for supporting all the middlebox services for an application, due to the reduced complexity and synchronization effort in managing the middlebox resources.
[SIP] Rosenberg, J., Shulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., Schooler, E., "SIP: Session Initiation Protocol", RFC 3261, June 2002. [SDP] Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998. [H.323] ITU-T Recommendation H.323. "Packet-based Multimedia Communications Systems," 1998. [RTP] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 1889, January 1996. [RTSP] Schulzrinne, H., Rao, A. and R. Lanphier: "Real Time Streaming Protocol (RTSP)", RFC 2326, April 1998. [FTP] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9, RFC 959, October 1985. [NAT-TERM] Srisuresh, P. and M. Holdrege, "IP Network Address Translator (NAT) Terminology and Considerations", RFC 2663, August 1999. [NAT-TRAD] Srisuresh, P. and K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)", RFC 3022, January 2001. [NAT-PT] Tsirtsis, G. and P. Srisuresh, "Network Address Translation - Protocol Translation (NAT-PT)", RFC 2766, February 2000. [IPsec-AH] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402, November 1998. [IPsec-ESP] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload (ESP)", RFC 2406, November 1998. [TLS] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999. [POL-TERM] Westerinen, A., Schnizlein, J., Strassner, J., Scherling, M., Quinn, B., Herzog, S., Huynh, A., Carlson, M., Perry, J. and S. Waldbusser, "Terminology for Policy-Based Management", RFC 3198, November 2001.
Full Copyright Statement Copyright (C) The Internet Society (2002). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society.