Networking Working Group L. Ginsberg Internet-Draft P. Psenak Intended status: Standards Track S. Previdi Expires: April 16, 2016 Cisco Systems October 14, 2015 Segment Routing Conflict Resolution draft-ginsberg-spring-conflict-resolution-00.txt Abstract In support of Segment Routing (SR) routing protocols advertise a variety of identifiers used to define the segments which direct forwarding of packets. In cases where the information advertised by a given protocol instance is either internally inconsistent or conflicts with advertisements from another protocol instance a means of achieving consistent forwarding behavior in the network is required. This document defines the policies used to resolve these occurrences. Requirements Language 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]. 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/. 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." This Internet-Draft will expire on April 16, 2016. Ginsberg, et al. Expires April 16, 2016 [Page 1] Internet-Draft sr-conflict-resolution October 2015 Copyright Notice Copyright (c) 2015 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 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. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. SR Global Block Inconsistency . . . . . . . . . . . . . . . . 3 3. Segment Identifier Conflicts . . . . . . . . . . . . . . . . 5 3.1. Conflict Types . . . . . . . . . . . . . . . . . . . . . 6 3.1.1. Prefix Conflict . . . . . . . . . . . . . . . . . . . 6 3.1.2. SID Conflict . . . . . . . . . . . . . . . . . . . . 7 3.2. Processing conflicting entries . . . . . . . . . . . . . 8 3.2.1. Ignore conflicting entries . . . . . . . . . . . . . 8 3.2.2. Preference Algorithm . . . . . . . . . . . . . . . . 8 3.2.3. Candidate Preference Algorithm . . . . . . . . . . . 9 3.2.4. Example Behavior . . . . . . . . . . . . . . . . . . 9 3.2.5. Other Preference Factors to consider . . . . . . . . 10 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 5. Security Considerations . . . . . . . . . . . . . . . . . . . 11 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 7.1. Normative References . . . . . . . . . . . . . . . . . . 11 7.2. Informational References . . . . . . . . . . . . . . . . 12 Appendix A. Alternate Prefix Conflict Algorithm . . . . . . . . 12 Ginsberg, et al. Expires April 16, 2016 [Page 2] Internet-Draft sr-conflict-resolution October 2015 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 1. Introduction Segment Routing (SR) as defined in [SR-ARCH] utilizes forwarding instructions called "segments" to direct packets through the network. Depending on the forwarding plane architecture in use, routing protocols advertise various identifiers which define the permissible values which can be used as segments, which values are assigned to specific prefixes, etc. Where segments have global scope it is necessary to have non-conflicting assignments - but given that the advertisements may originate from multiple nodes the possibility exists that advertisements may be received which are either internally inconsistent or conflicting with advertisements originated by other nodes. In such cases it is necessary to have consistent resolution of conflicts network-wide in order to avoid forwarding loops. The problem to be addressed is protocol independent i.e., segment related advertisements may be originated by multiple nodes using different protocols and yet the conflict resolution MUST be the same on all nodes regardless of the protocol used to transport the advertisements. The remainder of this document defines conflict resolution policies which meet these requirements. All protocols which support SR MUST adhere to the policies defined in this document. 2. SR Global Block Inconsistency In support of an MPLS dataplane routing protocols advertise an SR Global Block (SRGB) which defines a set of label ranges reserved for use by the advertising node in support of SR. The details of how protocols advertise this information can be found in the protocol specific drafts e.g., [SR-OSPF] and [SR-IS-IS]. However the protocol independent semantics are illustrated by the following example: Ginsberg, et al. Expires April 16, 2016 [Page 3] Internet-Draft sr-conflict-resolution October 2015 The originating router advertises the following ranges: Range 1: (100, 199) Range 2: (1000, 1099) Range 3: (500, 5990 The receiving routers concatenate the ranges and build the Segment Routing Global Block (SRGB) as follows: SRGB = (100, 199) (1000, 1099) (500, 599) The indexes span multiple ranges: index=0 means label 100 ... index 99 means label 199 index 100 means label 1000 index 199 means label 1099 ... index 200 means label 500 ... Note that the ranges are an ordered set - what labels are mapped to a given index depends on the placement of a given label range in the set of ranges advertised. For the set of ranges to be usable the ranges MUST be disjoint. The question then arises what receiving routers should do if they receive an SRGB which includes overlapping ranges. In such a case the following rule is defined: Each range is examined in the order it was advertised. If it does not overlap with any advertised range which preceded it the advertised range is used. If the range overlaps with any preceding range it MUST NOT be used and all ranges advertised after the first encountered overlapping range also MUST NOT be used. Consider the following example: Ginsberg, et al. Expires April 16, 2016 [Page 4] Internet-Draft sr-conflict-resolution October 2015 The originating router advertises the following ranges: Range 1: (100, 199] Range 2: (1000, 1099) Range 3: (100, 599) Range 4: (2000, 2099) Range 3 overlaps with Range 1. Only Ranges #1 and #2 are usable. Ranges #3 and #4 are ignored. Note that Range #4 is not used even though it does not overlap with any of the other ranges. 3. Segment Identifier Conflicts In support of an MPLS dataplane Segment identifiers (SIDs) are advertised and associated with a given prefix. SIDs may be advertised in the prefix reachability advertisements originated by a routing protocol. SIDs may also be advertised by a Segment Routing Mapping Server (SRMS). A generalized mapping entry can be represented using the following definitions: Pi - Initial prefix Pe - End prefix L - Prefix length Lx - Maximum prefix length (32 for IPv4, 128 for IPv6) Si - Initial SID value Se - End SID value R - Range value Mapping Entry is then the tuple: (Pi/L, Si, R) Pe = (Pi + ((R-1) << (Lx-L)) Se = Si + (R-1) Note that the SID advertised in a prefix reachability advertisement can be more generally represented as a mapping entry with a range of 1. Conflicts in SID advertisements may occur as a result of misconfiguration. Conflicts may occur either in the set of advertisements originated by a single node or between advertisements originated by different nodes. When conflicts occur, it is not possible for routers to know which of the conflicting advertisements is "correct". If a router chooses to use one of the conflicting Ginsberg, et al. Expires April 16, 2016 [Page 5] Internet-Draft sr-conflict-resolution October 2015 entries forwarding loops and/or blackholes may result unless it can be guaranteed that all other routers in the network make the same choice. Making the same choice requires that all routers have identical sets of advertisements and that they all use the same selection algorithm. 3.1. Conflict Types Various types of conflicts may occur. 3.1.1. Prefix Conflict When different SIDs are assigned to the same prefix we have a "prefix conflict". Consider the following set of advertisements: (192.0.2.120/32, 200, 1) (192.0.2.120/32, 30, 1) The prefix 192.0.2.120/32 has been assigned two different SIDs - 200 by the first advertisement - 30 by the second advertisement. Prefix conflicts may also occur as a result of overlapping prefix ranges. Consider the following set of advertisements: (192.0.2.1/32, 200, 200) (192.0.2.121/32, 30, 10) Prefixes 192.0.2.121/32 - 192.0.2.130/32 are assigned two different SIDs - 320 through 329 by the first advertisement - 30 through 39 by the second advertisement. The second example illustrates a complication - only part of the range advertised in the first advertisement is in conflict. It is logically possible to isolate the conflicting portion and try to use the non-conflicting portion(s) at the cost of increased implementation complexity. The algorithm defined here does NOT attempt to support use of a partial range. A variant of the overlapping prefix range is a case where we have overlapping prefix ranges but no actual SID conflict. (192.0.2.1/32, 200, 200) (192.0.2.121/32, 320, 10) Although there is prefix overlap between the two entries the same SID is assigned to all of the shared prefixes by the two entries. It is possible to utilize both entries but it complicates the implementation of the database required to support this. See Ginsberg, et al. Expires April 16, 2016 [Page 6] Internet-Draft sr-conflict-resolution October 2015 Appendix A for a more complete discussion of this case. An alternative is to ensure at the nodes which originate these advertisements that no such overlap is allowed to be configured. Such overlaps can then be considered as a conflict if they are received. This allows a simpler and more efficient implementation of the database. This is the approach assumed in this document. Given two mapping entries: (P1/L1, S1, R1) and (P2/L2, S2, R2) a prefix conflict exists if all of the following are true: 1)The prefixes are in the same address family. 2)L1 == L2 3)((P1 < P2) && (P1e >= P2)) || ((P2 < P1 ) && (P2e >= P1)) 3.1.2. SID Conflict When the same SID has been assigned to multiple prefixes we have a "SID conflict". Consider the following example: (192.0.2.1/32, 200, 1) (192.0.2.222/32, 200,1) SID 200 has been assigned to 192.0.2.1/32 by the first advertisement. The second advertisement assigns SID 200 to 192.0.2.222/32. SID conflicts may also occur as a result of overlapping SID ranges. Consider the following set of advertisements: (192.0.2.1/32, 200, 200) (192.1.2.1/32, 300, 10) SIDs 300 - 309 have been assigned to two different prefixes. The first advertisement assigns these SIDs to 192.0.2.101/32 - 192.0.2.110/32. The second advertisement assigns these SIDs to 192.1.2.1/32 - 192.1.2.10/32. The second example illustrates a complication - only part of the range advertised in the first advertisement is in conflict. It is logically possible to isolate the conflicting portion and try to use the non-conflicting portion(s) at the cost of increased implementation complexity. The algorithm defined here does NOT attempt to support use of a partial range. Ginsberg, et al. Expires April 16, 2016 [Page 7] Internet-Draft sr-conflict-resolution October 2015 SID conflicts are independent of address-family and independent of prefix len. A SID conflict occurs when a mapping entry which has previously been checked to have no prefix conflict assigns one or more SIDs that are assigned by another entry which also has no prefix conflicts. 3.2. Processing conflicting entries Two general approaches can be used to process conflicting entries. 1. Conflicting entries can be ignored 2. A standard preference algorithm can be used to choose which of the conflicting entries will be used The following sections discuss these two approaches in more detail. Note: This document does not discuss any implementation details i.e. what type of data structure is used to store the entries (trie, radix tree, etc.) nor what type of keys may be used to perform lookups in the database. 3.2.1. Ignore conflicting entries In cases where entries are in conflict none of the conflicting entries are used i.e., the network operates as if the conflicting advertisements were not present. Ikplementation requires identifying the conflicting entries and ensuring that they are not used. The occurrence of conflicts is easily diagnosed from the behavior of the network as the forwarding of traffic which would, in the absence of conflicts, utilize segments no longer does so. Which prefixes are impacted is easily seen and therefore the entries which are misconfigured are easily identified. Unintended traffic flow will never occur. The downside of ignoring conflicting entries is that forwarding of all packets with destinations covered by the conflicting entries will always be negatively impacted. 3.2.2. Preference Algorithm For entries which are in conflict properties of the advertisement (e.g. prefix value, prefix length, SID value, etc.) are used to determine which of the conflicting entries are used in forwarding and which are ignored. Ginsberg, et al. Expires April 16, 2016 [Page 8] Internet-Draft sr-conflict-resolution October 2015 This approach requires that conflicting entries first be identified and then evaluated based on the preference rule. Based on which entry is preferred this in turn may impact what other entries are considered in conflict i.e. if A conflicts with B and B conflicts with C - it is possible that A does NOT conflict with C. Hence if as a result of the evaluation of the conflict between A and B, entry B is not used the conflict between B and C will not be considered. As at least some of the traffic continues to be forwarded after the conflict is detected, the presence of the conflict may be harder to diagnose based on traffic flowthan when using the ignore policy. The upside of the preference algorithm is that in some cases forwarding of traffic may continue to be correct despite the existence of the conflict. If the preference algorithm happens to prefer the intended configuration traffic will still be successfully delivered. Whether this will occur is a random outcome since the preference algorithm cannot know which of the conflicting entries is the "correct" entry. 3.2.3. Candidate Preference Algorithm The following algorithm is proposed. Evaluation is made in the order specified. 1. IPv4 entry wins over IPv6 entry 2. Smaller prefix length wins 3. Smaller starting address (considered as an unsigned integer value) wins 4. Smaller starting SID wins 5. Smaller range wins 6. non-attached entries preferred over attached entries (SRMS attached flag) 3.2.4. Example Behavior Consider the following simple case. The following mapping entries exist: 1. (192.0.2.1/32, 100, 200) 2. (192.0.2.200/32, 150, 300) !Prefix conflict with entry #1 Ginsberg, et al. Expires April 16, 2016 [Page 9] Internet-Draft sr-conflict-resolution October 2015 3. (193.3.3.3/32, 400, 100) !SID conflict with entry #2 Using the Ignore conflicts behavior we would not use any of the above entries. Using a preference rule which favors smaller prefixes, entries #1 and #3 would be used. o Entry #1 would be used as 192.0.200.1 is less than 192.0.2.200. o Entry #3 would be used because once Entry #2 has been excluded, entry #3 no longer conflicts with any entry which is being used. (An example of lack of transitivity of conflicts) If we now add 4. (192.0.1.1/32, 50, 100) ! Prefix conflict with #1 Using ignore policy still none of the entries would be used. Using a preference rule which favors smaller prefixes , entries #4 and #2 would be used. o Entry #4 would be used in preference to entry #1 because 192.0.1.1 < 192.0.2.1. o Entry #2 would be used because once Entry #1 is excluded entry #2 no longer has a prefix conflict with any active entry. o Entry #3 would NOT be used because once Entry #2 becomes active entry #3 loses due to the SID conflict with Entry #2 since the latter has a smaller prefix. 3.2.5. Other Preference Factors to consider Prefix to SID mapping is based on a variety of sources. o SIDs can be configured locally for prefixes assigned to interfaces on the router itself o SIDs can be received in prefix reachability advertisements from protocol peers. These advertisements may originate from peers local to the area or be leaked from other areas and/or redistributed from other routing protocols o SIDs can be received from SRMS advertisements - these advertisements can originate from routers local to the area or leaked from other areas Ginsberg, et al. Expires April 16, 2016 [Page 10] Internet-Draft sr-conflict-resolution October 2015 SIDs configured locally for prefixes associated with interfaces on the router itself are only used by the originating router in prefix advertisements - they are not installed in the forwarding plane locally. Therefore, they do not need to be considered in conflict resolution. For other sources, It may seem intuitive to assign priority based on point of origination (e.g. intra-area preferred over inter-area, prefix reachability advertisements preferred over SRMS advertisements, etc.). However, any such policy makes it more likely that inconsistent choices will be made by routers in the network and increase the likelihood of forwarding loops or blackholes. The algorithms defined in this document assume that prefix reachability advertisements are part of the set of entries considered when determining conflicts and conflict resolution and no preference is associated with prefix reachability advertisements over SRMS advertisements. It is common to use the identity of the advertising source router (e.g. router ID) as a tie breaker. However, in the case of SID advertisements it is possible that the source ID is not known. For example, when leaking SRMS advertisements the source ID may appear to be the Area Border Router (ABR) which performed the leaking. But this means that the relative preference of the SIDs associated with the leaked advertisements will have a different priority in different areas. Therefore router ID is not used in the algorithms discussed above. 4. IANA Considerations None. 5. Security Considerations TBD 6. Acknowledgements The authors would like to thank Martin Pilka for sharing his knowledge of algorithm implementation and complexity. 7. References 7.1. Normative References Ginsberg, et al. Expires April 16, 2016 [Page 11] Internet-Draft sr-conflict-resolution October 2015 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . 7.2. Informational References [SR-ARCH] "Segment Routing Architecture, draft-ietf-spring-segment- routing-05(work in progress)", September 2015. [SR-IS-IS] "IS-IS Extensions for Segment Routing, draft-ietf-isis- segment-routing-extensions-05(work in progress)", June 2015. [SR-OSPF] "OSPF Extensions for Segment Routing, draft-ietf-ospf- segment-routing-extensions-05(work in progress)", June 2015. Appendix A. Alternate Prefix Conflict Algorithm It is possible when encountering overlapping prefix ranges to allow use of such entries when there is no actual SID conflict. This case can be avoided if configuration of such entries is blocked at the source - which allows a far simpler implementation when processing received entries. The latter model is what is described in the body of this document. What is described below is the algorithm and consequences of trying to use overlapping ranges when the SID assignments do not conflict. The algorithm used to determine if two entries are in conflict is as follows. Given two mapping entries: (P1/L1, S1, R1) and (P2/L2, S2, R2) a prefix conflict exists if all of the following are true: 1)The prefixes are in the same address family. 2)L1 == L2 3)(P1 < P2) && (P1e >= P2) && (((P2-P1)>>(Lx-L)) + S1) != S2) || (P2 < P1 ) && (P2e >= P1) && (((P1-P2)>>(Lx-L)) + S2) != S1) Ginsberg, et al. Expires April 16, 2016 [Page 12] Internet-Draft sr-conflict-resolution October 2015 From an implementation standpoint, the complexity comes in supporting lookup of the SID for a given prefix in the presence of overlapping ranges which are in use. Consider the following simple example: Entry #1: (192.0.2.1/32, 100, 250) Entry #2: (192.0.2.101/32, 200, 10) Entry #3: (192.0.2.201/32, 300, 100) Using the conflict detection algorithm described in this section there is no conflict in the three ranges since all prefixes which overlap between the entries are assigned the same SID in all ranges. If however we want to find the SID for the prefix 192.0.2.150, here are the steps one might go through: 1)Find the entry with the largest value of starting prefix which is less than or equal to 192.0.2.150. In the example this would be Entry #2 above. 2)Determine if the range covers 192.0.2.150. In the example the answer to this would be "no". If we were not required to support overlapping entries at this point we would be done and conclude that no SID was assigned. But because we know that we may have overlapping entries we have to walk backwards (conceptually) and look at all of the entries with starting prefixes (of prefix length 32) which are less than 192.0.2.150 and check if their range is large enough to include that prefix. In the presence of a large # of entries this would be very slow. To avoid this problem we could build a local entry which contained all of overlapping entries (in this example all three of the entries shown) and use that in our actiuve database to do the lookups. In this example we would generate: (192.0.2.1/32, 100, 300) Then we could use steps #1 and #2 above and be confident in the answer we get. However, since we are no longer using the entries we received in our active database we would need to maintain a link between the generated entry and the set of overlapping entries we received which caused its generation so that if one of those entries was removed or modified we could properly update our active database. Ginsberg, et al. Expires April 16, 2016 [Page 13] Internet-Draft sr-conflict-resolution October 2015 Authors' Addresses Les Ginsberg Cisco Systems 510 McCarthy Blvd. Milpitas, CA 95035 USA Email: ginsberg@cisco.com Peter Psenak Cisco Systems Apollo Business Center Mlynske nivy 43 Bratislava 821 09 Slovakia Email: ppsenak@cisco.com Stefano Previdi Cisco Systems Via Del Serafico 200 Rome 0144 Italy Email: sprevidi@cisco.com Ginsberg, et al. Expires April 16, 2016 [Page 14]