INTERNET-DRAFT Mingui Zhang Intended Status: Informational Xudong Zhang Expires: December 16, 2012 Donald Eastlake Huawei June 14, 2012 TRILL IS-IS MTU Negotiation draft-zhang-trill-mtu-negotiation-03.txt Abstract The IETF TRILL protocol provides least cost pair-wise layer 2 data forwarding by using IS-IS link state routing. This document defines a new link MTU size negotiation mechanism to update the TRILL documents "Routing Bridges (RBridges): Base Protocol Specification" and "Routing Bridges (RBridges): Adjacency". Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. 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/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Copyright and License Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents Mingui Zhang Expires December 16, 2012 [Page 1] INTERNET-DRAFT MTU Negotiation June 14, 2012 carefully, as they describe your rights and restrictions with respect to this document. 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 described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Content . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2. Conventions used in this document . . . . . . . . . . . . . 3 2. Issues of Current Link MTU Testing . . . . . . . . . . . . . . 3 2.1. Campus Wide Dependence . . . . . . . . . . . . . . . . . . 4 2.2. Inconstant Consequences of Wrong Configuration . . . . . . 4 3. TRILL IS-IS MTU Negotiation . . . . . . . . . . . . . . . . . . 5 3.1. Determination of Link-Wide Lz . . . . . . . . . . . . . . . 6 3.2. Link MTU Size Testing Algorithm . . . . . . . . . . . . . . 6 3.3. Re-determining Campus-Wide Sz . . . . . . . . . . . . . . . 7 3.4. Relationship between Port MTU and Sz . . . . . . . . . . . 8 3.5. LSP Synchronization . . . . . . . . . . . . . . . . . . . . 8 4. Determining Traffic Link MTU Size . . . . . . . . . . . . . . . 9 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 9 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 9 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 7.1. Normative References . . . . . . . . . . . . . . . . . . . 10 7.2. Informative References . . . . . . . . . . . . . . . . . . 10 Author's Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 Mingui Zhang Expires December 16, 2012 [Page 2] INTERNET-DRAFT MTU Negotiation June 14, 2012 1. Introduction The base TRILL protocol includes the way RBridges determine the minimum inter-RBridge link size for the whole campus (campus-wide Sz), for the proper operation of TRILL IS-IS. [RFC6327] defines the diagram of state transitions of an adjacency. If MTU testing is in effect, the "link MTU size is successfully tested (A6)" is an event causing transition between "2-way" state and "Report" state of an adjacency. RBridges use what they believe to be the campus-wide Sz to do link MTU size testing and a successfully tested link MTU size X must be not less than the value of campus-wide Sz [RFC6325]. This document analyzes the possible issues caused by the definition that link MTU size testing depends on the collection of campus-wide Sz. In order to break the global dependence on campus-wide Sz, link- wide Lz, which is the minimum acceptable inter-RBridge link MTU size for a link, is used to replace the role of campus-wide in link MTU size testing. Based on link-wide Lz, a new link MTU size testing algorithm is designed for adjacent RBridges to determine the most suitable size of their link MTU. There are PDUs which are limited to a local link, such as CSNPs and PSNPs. These PDUs should not be confined by the campus-wide Sz. Instead, these PDUs should be formatted not greater than the value of link-wide Lz. 1.1. Content Section 2 analyzes the issues caused by the dependence on campus-wide Sz for link MTU size testing. Section 3 defines a new IS-IS MTU negotiation mechanism to update [RFC6325]. Section 4 describes how link traffic MTU can be determined. 1.2. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. 2. Issues of Current Link MTU Testing Link MTU size testing is defined in Section 4.3.2 of [RFC6325]. If the link MTU size is smaller than campus-wide value of Sz, which is the smallest value of originatingLSPBufferSize advertised by any RBridge in its LSP (but not less than 1470)[RFC6325], the link will Mingui Zhang Expires December 16, 2012 [Page 3] INTERNET-DRAFT MTU Negotiation June 14, 2012 not be included in the global topology. If the link MTU size X of an adjacency is successfully tested (X >= campus-wide Sz), its state will move from "2-way" to "Report", which is defined in [RFC6327]. The link MTU size testing depends on the believed value of campus- wide Sz, which can be problematic. Some issues causes by this dependence are given in the following subsections. 2.1. Campus Wide Dependence Sz:1800 Sz:1800 Sz:1800 +---+ +---+ +--+ +---+ |RB1|(2000)-(2000)|RB2| (2000)-(1700)|B1|(1700)-(2000)|RB3| +---+ ^ +---+ +--+ +---+ (2000) | ^ |<---- Report | (2000) Report +---+ |RB4| +---+ | Sz:1600 v Sz:1800 Sz:1800 Sz:1800 +---+ +---+ +--+ +---+ |RB1|(2000)-(2000)|RB2| (2000)-(1700)|B1|(1700)-(2000)|RB3| +---+ ^ +---+ +--+ +---+ | ^ Report | 2-way Figure 2.1: Adjacency campus wide dependence Take Figure 2.1 as an example, all the adjacencies are in report states. After RB4 leaves the campus and its LSPs age out, RB2 and RB3 find the campus-wide Sz grows. They test the MTU according to the new value of campus-wide Sz 1800. Since RB2 and RB3 is connected by a low-end bridge whose port MTU is 1700. The test will not be successful. This adjacency has to return to 2-way state. The state of an adjacency can be determined by another remote adjacency. The stability of the campus-wide Sz in such a badly configured campus can be poor resulting in maintenance problems. (In a well-configured campus all RBridges should be configured with the same originatingLSPBufferSize.) 2.2. Inconstant Consequences of Wrong Configuration Take Figure 2.2 as an example, the originatingLSPBufferSize value of RB3 is falsely configured to be greater than its port MTU. The link Mingui Zhang Expires December 16, 2012 [Page 4] INTERNET-DRAFT MTU Negotiation June 14, 2012 MTU testing is successful because the campus-wide Sz 1600 is smaller than the two port MTUs of the adjacency between RB2 and RB3. The adjacency will be in "Report" state. However, when RB4 leaves the campus and the campus-wide Sz is updated to 1800, the link MTU test of link RB2-RB3 cannot be successful. Sz:1600 Sz:1800 Sz:1800 Sz:1800 +---+ +---+ +---+ +---+ |RB4|(2000)-(2000)|RB1|(2000)-(2000)|RB2|(2000)-(1700)|RB3| +---+ ^ +---+ ^ +---+ ^ +---+ | | | Report Report Report | v Sz:1800 Sz:1800 Sz:1800 +---+ +---+ +---+ |RB1|(2000)-(2000)|RB2|(2000)-(1700)|RB3| +---+ ^ +---+ ^ +---+ | | Report 2-way Figure 2.2: Inconstant consequences of wrong configuration 3. TRILL IS-IS MTU Negotiation In order to solve the problems depicted in Section 2, this draft introduces a new value "Lz" which is the acceptable inter-RBridge link size required by each RBridge. Link-wide Lz is the minimum Lz required by all RBridges on a specific link. It is used in link MTU size testing to replace the role of campus-wide Sz. A successfully tested link MTU size X is not necessarily greater than the value of campus-wide Sz any more. Section 3.2 will define how to test this X value based on the value of link-wide Lz. Although the introduction of Lz changes the way link MTU size is tested, the adjacency state diagram defined in [RFC6327] does not change. "Link MTU size is successfully tested (A6)" is still an event causing transition between "2-way" state and "Report" state of an adjacency. The maximum size of PDUs exchanged only between neighbors instead of the whole campus should be confined by link-wide Lz instead of the campus-wide Sz. CSNPs and PSNPs are such kind of PDUs. They are exchanged just on the link after a DRB is selected on the link. As for campus-wide Sz, RBridges continue to propagate their Mingui Zhang Expires December 16, 2012 [Page 5] INTERNET-DRAFT MTU Negotiation June 14, 2012 originatingLSPBufferSize across the campus through the advertisement of LSPs. Each RBridge should format their "campus-wide" PDUs, such as LSPs, not greater than what they believe to be the campus-wide Sz. 3.1. Determination of Link-Wide Lz RBridges on a LAN link should exchange their assumption on the value of "Lz" through IIH using the originatingSNPBufferSize (Sz) contained in the PORT-TRILL-VER sub-TLV (see Section 6). The smallest value of the Lz collected on a link, but not less than 1470, is the link-wide Lz. It is different from the campus-wide Sz which is determined by having each RBridge in the campus advertise its own desired value of Sz in LSPs as defined in Section 4.3.1 of [RFC6325]. With IIH, an RBridge gets the value of Lz from its neighbor not later than the time when the adjacency moves to 2-way state. An RBridge should be aware of what size of PDUs will be accepted by its neighbor without exceeding its originatingSNPBufferSize. Lz:1800 Lz:1800 +---+ | +---+ |RB1|(2000)-|-(2000)|RB2| +---+ | +---+ | Lz:1800 | +---+ +--+ |RB3|(2000)-(1700)|B1| +---+ +--+ | Figure 3.1: Link MTU has to be tested Sometimes, even all RBridges on a specific LAN link have reached consensus on the value of link-wide Lz, it does not mean that these RBridges can safely exchange PDUs between each other. Take Figure 3.1 as an example. RB1, RB2 and RB3 are three RBridges on the same LAN link and their Lz is 1800, so the link-wide Lz of this LAN link is 1800. There is a intermediate bridge (say B1) between RB2 and RB3 whose port MTU size is 1700. If RB2 sends PDUs formatted in the size of 1800, it will be discarded by B1. Therefore the link MTU size has to be tested. After the link MTU size of an adjacency is successfully tested, these CSNP and PSNP PDUs will be formatted no greater than the tested link MTU size and will be safely transmitted on this link. 3.2. Link MTU Size Testing Algorithm A link MTU size testing method given by the last paragraph of Section 4.3.2 of [RFC6325]. The following Binary Search algorithm in which Mingui Zhang Expires December 16, 2012 [Page 6] INTERNET-DRAFT MTU Negotiation June 14, 2012 link-wide Lz is used in the testing instead of campus-wide Sz. Step 0: RB1 sends an MTU-probe padded to the size of link-wide Lz. 1) If RB1 successfully receives the MTU-ACK to the probe of the value of link-wide Lz from RB2, then link MTU size is set to the size of link-wide Lz and stop. 2) RB1 tries to send an MTU-probe padded to the size 1470. a) If RB1 fails to receive an MTU-ACK from RB2 after k tries (where k is a configurable parameter whose default is 3), RB1 sets the "failed minimum MTU test" flag for RB2 in RB1's Hello and stop. b) Link MTU size <-- 1470, X1 <-- 1470, X2 <-- link-wide Lz, X <-- [(X1 + X2)/2] (Operation "[...]" returns the fraction-rounded- up integer.). Repeat Step 1. Step 1: RB1 tries to send an MTU-probe padded to the size X. 1) If RB1 fails to receive an MTU-ACK from RB2 after k tries, then: X2 <-- X and X <-- [(X1 + X2)/2] 2) If RB1 receives an MTU-ACK to a probe of size X from RB2 then: link MTU size <-- X, X1 <-- X and X <-- [(X1 + X2)/2] 3) If X1 >= X2 or Step 1 has been repeated n times (where n is a configurable parameter whose default is 5), stop. Else go to Step 1. Since the execution of the above algorithm can be resource consuming, it is recommended that the DRB takes the responsibility to do the testing. If the testing is finished and the tested link MTU size is smaller than the original link-wide Lz or the minimum Sz that has been advertised to the DRB, the DRB SHOULD send the tested link MTU size as its local originatingSNPBufferSize in IIH and originatingLSPBufferSize in LSP number zero (shorted as LSP0). This will trigger other RBridges on the link to update their link-wide Lz and campus-wide Sz to be the size of the tested link MTU. Then CSNPs, PSNPs and LSPs (including those used for LSP database synchronization) can be rightly resized and successfully exchanged on the link. 3.3. Re-determining Campus-Wide Sz Mingui Zhang Expires December 16, 2012 [Page 7] INTERNET-DRAFT MTU Negotiation June 14, 2012 RBridges may join in or leave the campus from time to time. The campus-wide Sz can become outdated. Section 4.3.1 of [RFC6325] does not define when to re-determine the campus-wide Sz. The following suggestions are given for campus-wide Sz re-determination. 1) When a new RBridge whose Sz is smaller than current campus-wide Sz joins in the campus, it MUST report its Sz in an LSP which will cause other RBridges update their campus-wide Sz. Then the LSPs in the campus will be resized to be no greater than the new campus- wide Sz. 2) When an RB whose originatingLSPBufferSize is right at the campus- wide Sz leaves the campus, and its LSPs are purged from the remaining campus after reaching MaxAge [ISO10589]. The campus-wide Sz ought to be recomputed as well. Frequent LSP "resizing" is harmful to the stability of the whole campus, so it should be dampened. Within the two kinds of resizing actions, only the upward resizing will be dampened. When an upward resizing event happens, a timer is set (this is a configurable parameter whose default value is 300 seconds). Before this timer expires, all subsequent upward resizing will be dampened. In a well-configured campus with all RBridges configured to have the same originatingLSPBufferSize, no resizing will be necessary. 3.4. Relationship between Port MTU and Sz When port MTU size is smaller than the local originatingSNPBufferSize (Sz) of an RBridge, this port should be explicitly disabled from the TRILL campus. On the other hand, when an RBridge receives an LSP with size greater than its local Sz or the campus-wide Sz, this LSP should be normally processed rather than discarded. If the size of an LSP is larger than the MTU size of a port over which it is to be propagated, no attempt shall be made to propagate this LSP over the port and an LSPTooLargeToPropagate alarm shall be generated [ISO10589]. 3.5. LSP Synchronization The DRB of a LAN link is elected as early as in the "Detect" state of an adjacency. The DRB begins to send out CSNP to synchronize the LSP database of the RBridges attached to this LAN link when the adjacency between this RBridge and the DRB moves to 2-way state. If a non-DRB RBridge receives this CSNP and finds that LSPx is not in its LSP database, it will send out PSNP to request LSPx from the DRB. If a non-DRB receives this CSNP and finds that LSPx is not in the LSP database of the DRB, it will also send out LSPx to the DRB. DRB and non-DRB on a link should start to synchronize LSP database Mingui Zhang Expires December 16, 2012 [Page 8] INTERNET-DRAFT MTU Negotiation June 14, 2012 using CSNPs and PSNPs with a neighbor when the adjacency between them moves to the 2-way state [RBclr]. The CSNPs and PSNPs should be formatted in chunks of size at most the link-wide Lz. Since the link MTU size has not been tested in the 2-way state, link-wide Lz may be greater than the prospective tested link MTU size. In that case, an CSNP or PSNP may be discarded if its size is greater than the prospective tested link MTU size. After the link MTU size is successfully tested, the adjacencies will begin to format these PDUs in the size no greater than it, therefore these PDUs will finally successfully get through. 4. Determining Traffic Link MTU Size Campus-wide Sz and link-wide Lz are used to confine the size of TRILL IS-IS PDUs. They are different from the MTU size restricting the size of TRILL data frames. The size of TRILL data frames is restricted by the physical links and devices. It is possible that a TRILL data frame successfully get through the campus but its size is greater than the campus-wide Sz or link-wide Lz. The algorithm defined in link MTU size testing can also be used in TRILL traffic MTU size testing, only that the link-wide Lz used in that algorithm should be replaced with the port MTU of the RBridge sending MTU probes. The successfully tested size X can be advertised as an attribute of this link using MTU sub-TLV defined in [RBisisb]. Unlike RBridges, end stations do not participate the exchange of ISIS PDUs of TRILL, therefore they can not grasp the link traffic MTU size from a TRILL campus automatically. An operator may collect these values using network management tools such as TRILL ping or TraceRoute. Then the path MTU is set as the smallest tested link MTU on this path and end stations should not generate native frames that may exceed this path MTU. 5. Security Considerations This document raises no new security issues for IS-IS. 6. IANA Considerations The Lz value of an IS is included in PORT-TRILL-VER sub-TLV as originatingSNPBufferSize and sent in IIH (TBD) [RBisisb]. If originatingSNPBufferSize is missing from an IIH, it is assumed that its originating IS is implicitly advertising its Lz value as 1470 octets. 7. References Mingui Zhang Expires December 16, 2012 [Page 9] INTERNET-DRAFT MTU Negotiation June 14, 2012 7.1. Normative References [RFC6325] R. Perlman, D. Eastlake, et al, "RBridges: Base Protocol Specification", RFC 6325, July 2011. [RFC6327] D. Eastlake, R. Perlman, et al, "Routing Bridges (RBridges): Adjacency", RFC 6327, July 2011. [RBclr] D. Eastlake, M. Zhang, et al, "TRILL: Clarifications, Corrections, and Updates", draft-ietf-trill-clear-correct- 03.txt, working in progress. [RBisisb] D. Eastlake, A. Banerjee, et al, "Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS", draft-eastlake-isis-rfc6326bis-07.txt, working in progress. 7.2. Informative References [ISO10589] ISO, "Intermediate system to Intermediate system routeing information exchange protocol for use in conjunction with the Protocol for providing the Connectionless-mode Network Service (ISO 8473)," ISO/IEC 10589:2002. Mingui Zhang Expires December 16, 2012 [Page 10] INTERNET-DRAFT MTU Negotiation June 14, 2012 Author's Addresses Mingui Zhang Huawei Technologies Co.,Ltd Huawei Building, No.156 Beiqing Rd. Z-park ,Shi-Chuang-Ke-Ji-Shi-Fan-Yuan,Hai-Dian District, Beijing 100095 P.R. China Email: zhangmingui@huawei.com Xudong Zhang Huawei Technologies Co.,Ltd Huawei Building, No.156 Beiqing Rd. Z-park ,Shi-Chuang-Ke-Ji-Shi-Fan-Yuan,Hai-Dian District, Beijing 100095 P.R. China Email: zhangxudong@huawei.com Donald E. Eastlake, 3rd Huawei Technologies 155 Beaver Street Milford, MA 01757 USA Phone: +1-508-333-2270 EMail: d3e3e3@gmail.com Mingui Zhang Expires December 16, 2012 [Page 11]