Network Working Group G. Chen Internet-Draft China Mobile Intended status: Informational February 18, 2013 Expires: August 22, 2013 Analysis of CGN Port Allocation Method draft-chen-sunset4-cgn-port-allocation-00 Abstract The document enumerated methods of port assignment in CGN contexts. The analysis categorized the different methods with several key features. Corresponding to those features, the uses of existing protocols are also described. The potential concerns and workaround have been discussed. It's expected the document could provide a informative base line to help operators choosing a proper method. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as Chen Expires August 22, 2013 [Page 1] Internet-Draft cgn-port February 2013 described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Port Allocation Management . . . . . . . . . . . . . . . . . . 3 2.1. NAT vs NAPT . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2. Dynamic vs Static . . . . . . . . . . . . . . . . . . . . . 4 2.3. Centralized vs Distributed . . . . . . . . . . . . . . . . 5 3. Discussions . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 6.1. Normative References . . . . . . . . . . . . . . . . . . . 6 6.2. Informative References . . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7 Chen Expires August 22, 2013 [Page 2] Internet-Draft cgn-port February 2013 1. Introduction With the depletion of IPv4 address, CGN has been adopted by ISPs to expand IPv4 spaces. Relying upon the mechanism of multiplexing multiple subscribers' connections over a smaller number of shared IPv4 addresses, CGN mapped IP addresses from one address realm to another, providing transparent routing to end hosts. [I-D.ietf-behave-lsn-requirements] defined the term of CGN. Several proposals including DS-Lite, NAT64, NAT444 would likely fall into the scope. [RFC6269] has provided a thoughtful analysis on the issues of IP sharing. It was point out that those IP sharing bring the impacts to law enforcement since the information of source address would be lost during the translation. Network administrators have to log the mapping status for each connection in order to identify a specific user associated with an IP address. It would post a challenge to operators, since it requires additional storage resource and data inspection process for indentifying the real users. It's desirable to compact the logging information by a rational port allocation. Those allocation policies should consider the tradeoff between port utilization and log storage compression. The document is trying to enumerate the several dimensions for assigning the port information. It's expected administrator could use those factors to determine their own properties. 2. Port Allocation Management This section lists several factors to allocate the port information for CGN equipments. It likely that each allocation model would have an exemplified case. The relevant issues and potential workarounds have also been described for each aspect. 2.1. NAT vs NAPT CGN may not do Network Address Port Translation (NAPT), but only Network Address Translation (NAT). In those cases, there is no concern about port assignment. Those translation methods would relieve the demands of log information storage, since NAT does not have to administer address management with session flows. Furthermore, there is no requirement to maintain log when CGN performing stateless translations. Some existing practices are listed below from two aspects. o Stateful NAT The stateful NAT can be implemented either by static address translation or dynamic address translation. Chen Expires August 22, 2013 [Page 3] Internet-Draft cgn-port February 2013 In the case of static address assignment, one-to-one address mapping for hosts between a private network address and an external network address would be pre-configured on the NAT operation. Those cases normally occurred when a server deployed in a internal domain. The static configuration ensure the stable inbound connectivity. The static method is also easier for Lawful interception system to derive the mapped address, since the mapping didn't change with time. Dynamic address assignment would periodically free the binding so that the global address could be recycled for later uses. Addresses could be more efficiently used by time-division manner. It only requires systems maintaining mappings for per-customer, other than per-session flow. This method is usually adopted to reduce the log burden in some protocols. o Stateless NAT The stateless NAT is performed in compliant with [RFC6145]. Public IPv4 address is required to be inserted in IPv6 address. Therefore, CGN could directly extract the address and no need to record mapping states. The lawful interception could likely indentify the IPv4 address through received IPv6 address. It's a protocol to eliminate the log information storage. There are two potential concerns for those technologies. First off, the static one-to-one mapping may didn't address the issue of IPv4 depletion. Secondly, it introduced the dependency of IPv4/IPv6. That would create new limitations since the change of IPv4 address would cause renumbering of IPv6 addresses. Whereas, that is useful for the IDC migration where there is IPv6 servers pools to receive inbound connections from IPv4 users externally[I-D.anderson-siit-dc]. 2.2. Dynamic vs Static When the case comes to port assignment, there are two methods for port allocations. o Dynamic assignment CGN normally do the dynamic assignment. In respect to the received connections, ports can be allocated to each sessions. NAT64, DS-Lite and NAT444 would do the dynamic approach by default, since it achieves maximum port utilization. One downside for this approach is CGN has to record log information for each session. That would potential increase the log volume. There is a statistic from field trials that the average number of connections per customer per day at approximately 10,000 connections. If log system is required to store information for 180 days, the testing shown that the amount of data records would achieve 20T. Chen Expires August 22, 2013 [Page 4] Internet-Draft cgn-port February 2013 o Static assignment The static assignment make a bulk of port reservation for a specific address. The bulk of port could be either a contiguous or non- contiguous port range for sake of attacks defense. [I-D.donley-behave-deterministic-cgn]has described a deterministic NAT to reserve a port range for each specific IP address. That is a significant improvement for lightening log volume. However, a trade- off should be made when administor has to consider the port utilization. For the administor who prioritize the port utilization, dynamic assignment maybe a suitable solution for them. Another consideration is using Address-Dependent Mapping or Address and Port- Dependent Mapping[RFC4787] to increase the port utilization. This feature has already been implemented as vendor-specific features. Whereas, it should be noted that REQ-7, REQ-12 in[I-D.ietf-behave-lsn-requirements] may reduce the needs. 2.3. Centralized vs Distributed The port allocation can be managed as a centralized way on CGN or distributed to downstream devices(e.g, CPE connected with CGN) . o Centralized Assignment A centralized method would make port assignment when traffic come to CGN box. The allocation policy is enforced on CGN. CGN make either a dynamic or static port assignment to the received session. o Distributed Assignment CGN could also delegate the pre-allocated port range to customer edge devices. That can be achieved through additional out-band provisioning signals(e.g.[I-D.ietf-pcp-base] ,[I-D.tsou-pcp-natcoord][I-D.ietf-softwire-map-dhcp]). The distributed model normally performed A+P for static port assignment. CGN should hold the corresponding mapping in accordance with assigned ports. Those methods would shift CGN port computation into downstream devices. The detailed benefits to CGN was documented in [I-D.ietf-softwire-stateless-4v6-motivation]. 3. Discussions With demands of reducing log volume, there are several approaches of port assignment described in the aforementioned section. It could be found that a trade-off between maximum port utilization and log volume always exist regarding justifications of different solutions. In respect to difference of port assignment, the granularity of log Chen Expires August 22, 2013 [Page 5] Internet-Draft cgn-port February 2013 could be ranked as per-session, per-port-bulk, per-customer and None. With the reduction of log volume, port utilization is likely decreased. Therefore, the decision should be made if there is a quantitative statistic to evaluate what is gain from reducing log volume and loss from decreasing port utilization. Those data analysis is planned to be added after further lab testing. Operators could choose the proper method considering following: o Average connectivities per customer per day o Peak connectivities per day o The amount of public IPv4 address in CGN o Application demands for specific ports o The parallel processing capabilities of CGN o The tolerance of Log volume 4. Security Considerations TBD 5. IANA Considerations This document makes no request of IANA. 6. References 6.1. Normative References [I-D.ietf-pcp-base] Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P. Selkirk, "Port Control Protocol (PCP)", draft-ietf-pcp-base-29 (work in progress), November 2012. [I-D.ietf-softwire-map-dhcp] Mrugalski, T., Troan, O., Bao, C., Dec, W., and L. Yeh, "DHCPv6 Options for Mapping of Address and Port", draft-ietf-softwire-map-dhcp-01 (work in progress), August 2012. [RFC4787] Audet, F. and C. Jennings, "Network Address Translation (NAT) Behavioral Requirements for Unicast UDP", BCP 127, Chen Expires August 22, 2013 [Page 6] Internet-Draft cgn-port February 2013 RFC 4787, January 2007. [RFC6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation Algorithm", RFC 6145, April 2011. 6.2. Informative References [I-D.anderson-siit-dc] Anderson, T., "Stateless IP/ICMP Translation in IPv6 Data Centre Environments", draft-anderson-siit-dc-00 (work in progress), November 2012. [I-D.donley-behave-deterministic-cgn] Donley, C., Grundemann, C., Sarawat, V., Sundaresan, K., and O. Vautrin, "Deterministic Address Mapping to Reduce Logging in Carrier Grade NAT Deployments", draft-donley-behave-deterministic-cgn-05 (work in progress), January 2013. [I-D.ietf-behave-lsn-requirements] Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A., and H. Ashida, "Common requirements for Carrier Grade NATs (CGNs)", draft-ietf-behave-lsn-requirements-10 (work in progress), December 2012. [I-D.ietf-softwire-stateless-4v6-motivation] Boucadair, M., Matsushima, S., Lee, Y., Bonness, O., Borges, I., and G. Chen, "Motivations for Carrier-side Stateless IPv4 over IPv6 Migration Solutions", draft-ietf-softwire-stateless-4v6-motivation-05 (work in progress), November 2012. [I-D.tsou-pcp-natcoord] Sun, Q., Boucadair, M., Deng, X., Zhou, C., Tsou, T., and S. Perreault, "Using PCP To Coordinate Between the CGN and Home Gateway", draft-tsou-pcp-natcoord-09 (work in progress), November 2012. [RFC6269] Ford, M., Boucadair, M., Durand, A., Levis, P., and P. Roberts, "Issues with IP Address Sharing", RFC 6269, June 2011. Chen Expires August 22, 2013 [Page 7] Internet-Draft cgn-port February 2013 Author's Address Gang Chen China Mobile 53A,Xibianmennei Ave., Xuanwu District, Beijing 100053 China Email: phdgang@gmail.com Chen Expires August 22, 2013 [Page 8]