RFC 5305
IS-IS Extensions for Traffic Engineering
RFC 5305: IS-IS Extensions for Traffic Engineering ✨
okay bestie, this is the one about giving IS-IS some main character energy for traffic engineering 🚦💅 it's giving network spec drama. 🎀
Here's the title block, all the facts up front:
- Network Working Group, Request for Comments: 5305 🦋
- Title: IS-IS Extensions for Traffic Engineering
- Authors: T. Li (Redback Networks, Inc.) and H. Smit 👩💻
- Category: Standards Track
- Date: October 2008 📅
- Obsoletes: RFC 3784 (so this one replaces it, no cap) 💖
Status of This Memo ✨
ok so this part is the official vibe check. This document is an Internet standards track protocol for the Internet community, and it's asking everyone to talk about it and suggest improvements 🗣️ For the standardization state and status of this protocol, check the current edition of the "Internet Official Protocol Standards" (STD 1). Distribution of this memo is unlimited 🌈
Abstract 🌸
this doc extends the Intermediate System to Intermediate System (IS-IS) protocol so it can support Traffic Engineering (TE) 🚦 IS-IS routers get to place new info into Link State Protocol Data Units (LSPs), and that info describes extra details about the state of the network that are useful for traffic engineering computations. literally so iconic 😭
Table of Contents 🧚
1. Introduction ....................................................2
1.1. Requirements Language ......................................3
2. Introducing Sub-TLVs ............................................3
3. The Extended IS Reachability TLV ................................3
3.1. Sub-TLV 3: Administrative Group (color, resource class) ....6
3.2. Sub-TLV 6: IPv4 Interface Address ..........................6
3.3. Sub-TLV 8: IPv4 Neighbor Address ...........................6
3.4. Sub-TLV 9: Maximum Link Bandwidth ..........................7
3.5. Sub-TLV 10: Maximum Reservable Link Bandwidth ..............7
3.6. Sub-TLV 11: Unreserved Bandwidth ...........................7
3.7. Sub-TLV 18: Traffic Engineering Default Metric .............8
4. The Extended IP Reachability TLV ................................8
4.1. The up/down Bit ...........................................10
4.2. Expandability of the Extended IP Reachability TLV
with Sub-TLVs .............................................11
4.3. The Traffic Engineering Router ID TLV .....................11
5. IANA Considerations ............................................12
5.1. TLV Codepoint Allocations .................................12
5.2. New Registries ............................................13
5.2.1. Sub-TLVs for the Extended IS Reachability TLV ......13
5.2.2. Sub-TLVs for the Extended IP Reachability TLV ......15
6. Security Considerations ........................................15
7. Acknowledgements ...............................................15
8. References .....................................................15
8.1. Normative References ......................................15
8.2. Informative References ....................................151. Introduction 🎀
the IS-IS protocol is specified in ISO 10589 [ISO-10589], with extensions for supporting IPv4 specified in [RFC1195]. Each Intermediate System (IS) (router) advertises one or more IS-IS Link State Protocol Data Units (LSPs) with routing information 📦 Each LSP is a fixed header plus a bunch of tuples, and each tuple has a Type, a Length, and a Value. Those tuples are commonly called TLVs, and they're a great way to encode info in a flexible, extensible format 💜
this doc designs new TLVs to replace the existing IS Neighbor TLV and IP Reachability TLV 🔄 They also add more info about the characteristics of a particular link to an IS-IS LSP ✨ These characteristics are needed for traffic engineering [RFC2702]. The secondary goals are increasing the dynamic range of the IS-IS metric and improving how IP prefixes are encoded 🍓
the router ID is useful for traffic engineering because it describes one single address that can always be used to reference a particular router 🧚
Mechanisms and procedures to migrate to the new TLVs are not discussed in this document. 🙈
a prior version of this document was published as [RFC3784] with Informational status. This version is on the standards track 👑
1.1. 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] 🫶
2. Introducing Sub-TLVs 🌈
this doc introduces a new way to encode routing info in IS-IS, called a sub-TLV 💅 Sub-TLVs are basically like regular TLVs. They use the same concepts, but TLVs live inside IS-IS packets while sub-TLVs live inside TLVs. So TLVs add extra info to IS-IS packets, and sub-TLVs add extra info to particular TLVs 🧩
each sub-TLV has three fields: a one-octet Type field, a one-octet Length field, and zero or more octets of Value. The Type field tells you what kind of items are in the Value field, and the Length field tells you how long the Value field is, in octets 🔥 A sub-TLV can hold multiple items, and you can work out how many items there are from the length of the whole sub-TLV, as long as you know how long each item is 📏
unknown sub-TLVs are to be ignored and skipped upon receipt. no drama, just vibes 🙃
the sub-TLV type space is managed by the IETF IS-IS WG [ISIS-WG]. New type values get allocated after review on the IETF IS-IS mailing list 📬 That will normally require publishing extra documentation about how the new type is used. If the IS-IS working group has disbanded, the review shall be performed by a Designated Expert assigned by the responsible Area Director 🫡
3. The Extended IS Reachability TLV 💖
the extended IS reachability TLV is TLV type 22 ✨
ok so the existing IS reachability (TLV type 2, defined in ISO 10589 [ISO-10589]) holds info about a series of IS neighbors. For each neighbor there's a structure with the default metric, the delay, the monetary cost, the reliability, and the 7-octet ID of the adjacent neighbor 🤔 Of all that, the default metric is the one people actually use 💀
the default metric is currently one octet. One bit says whether the metric is internal or external, and another bit that was originally unused was later defined by [RFC5302] as the up/down bit. The remaining 6 bits store the actual metric, which gives a possible metric range of 0-63. That limitation is one of the restrictions we want to lift 🙅♀️
the other three metrics (delay, monetary cost, and reliability) aren't commonly implemented, so they're basically unused overhead in the TLV 😴 The neighbor is identified by its system ID, typically 6 octets, plus one octet for the pseudonode number. So the existing TLV eats 11 octets per neighbor: 4 octets for metric and 7 octets for neighbor identification. To show multiple adjacencies, this structure gets repeated inside the IS reachability TLV. The TLV is limited to 255 octets of content, so a single TLV can describe up to 23 neighbors. The IS reachability TLV can be repeated within the LSP fragments to describe more neighbors 🧚♀️
the proposed extended IS reachability TLV has a new data structure, which looks like this:
7 octets of system ID and pseudonode number
3 octets of default metric
1 octet of length of sub-TLVs
0-244 octets of sub-TLVs, where each sub-TLV consists of a
sequence of
1 octet of sub-type
1 octet of length of the Value field of the sub-TLV
0-242 octets of valueso if no sub-TLVs are used, the new encoding needs 11 octets and can contain up to 23 neighbors 🎀 The encoding allows for 255 octets of sub-TLVs, but that max can't fit in the overall IS reachability TLV, so the practical maximum is 255 octets minus the 11 octets above, which is 244 octets. There's no defined mechanism for extending the sub-TLV space, so wasting sub-TLV space is discouraged 🙏
the metric octets are encoded as a 24-bit unsigned integer. The Metric field in the new extended IP reachability TLV is encoded as a 32-bit unsigned integer instead. These different sizes were chosen so it's very unlikely that the cost of an intra-area route has to be chopped off to fit in the Metric field of an inter-area route 🍰
to prevent overflow inside a traffic engineering Shortest Path First (SPF) implementation, all metrics greater than or equal to MAX_PATH_METRIC SHALL be considered to have a metric of MAX_PATH_METRIC 🚫 It's easiest to pick MAX_PATH_METRIC so that MAX_PATH_METRIC plus a single link metric doesn't overflow the number of bits for internal metric calculation. We assume that's 32 bits, so MAX_PATH_METRIC was chosen to be 4,261,412,864 (0xFE000000, 2^32 - 2^25) 🔢
if a link is advertised with the maximum link metric (2^24 - 1), this link MUST NOT be considered during the normal SPF computation 🙅 This lets a link get advertised for purposes other than building the normal Shortest Path Tree. An example is a link that's available for traffic engineering but not for hop-by-hop routing 🦋
certain sub-TLVs are established here 🌸
+------------+----------------+-------------------------------------+
| Sub-TLV | Length | Name |
| type | (octets) | |
+------------+----------------+-------------------------------------+
| 3 | 4 | Administrative group (color) |
| | | |
| 6 | 4 | IPv4 interface address |
| | | |
| 8 | 4 | IPv4 neighbor address |
| | | |
| 9 | 4 | Maximum link bandwidth |
| | | |
| 10 | 4 | Maximum reservable link bandwidth |
| | | |
| 11 | 32 | Unreserved bandwidth |
| | | |
| 18 | 3 | TE Default metric |
| | | |
| 250-254 | | Reserved for Cisco specific |
| | | extensions |
| | | |
| 255 | | Reserved for future expansion |
+------------+----------------+-------------------------------------+each of these sub-TLVs is described below. Unless stated otherwise, multiple occurrences of the information are supported by multiple inclusions of the sub-TLV 💜
3.1. Sub-TLV 3: Administrative Group (color, resource class) 🎨
okay so the administrative group sub-TLV is a 4-octet bit mask that the network administrator assigns 💅 each set bit means one administrative group is assigned to the interface. so cute and simple 🎀
by convention, the least significant bit is called group 0 and the most significant bit is called group 31 🌈
this sub-TLV is OPTIONAL, so you can skip it 🫶 and it SHOULD show up once at most in each extended IS reachability TLV 🍓
3.2. Sub-TLV 6: IPv4 Interface Address 🦋
this sub-TLV holds a 4-octet IPv4 address for the interface that the (main) TLV is describing ✨ it can show up multiple times, bestie 💖
⚠️ implementations MUST NOT inject a /32 prefix for the interface address into their routing or forwarding table 😭 because it can cause forwarding loops when they're talking to systems that don't support this sub-TLV 💀
if a router implements the basic TLV extensions from this document, it MAY add or omit this sub-TLV when it describes an adjacency 🧚 but if a router does traffic engineering, it MUST include this sub-TLV 👑
3.3. Sub-TLV 8: IPv4 Neighbor Address 🌸
this one holds a single IPv4 address for a neighboring router on this link 🫶 it can also appear multiple times 🔥
🚫 implementations MUST NOT inject a /32 prefix for the neighbor address into their routing or forwarding table 😭 same vibes as before: it can lead to forwarding loops with systems that don't support this sub-TLV 💅
if a router implements the basic TLV extensions in this document, it MAY add or omit this sub-TLV from the description of an adjacency 🦋 but if it does traffic engineering, it MUST include this sub-TLV on point-to-point adjacencies 👑
3.4. Sub-TLV 9: Maximum Link Bandwidth 🚀
this sub-TLV has the maximum bandwidth that can be used on this link in this direction (from the system that originated the LSP to its neighbors) ✨ it's super useful for traffic engineering 🍓
the maximum link bandwidth is encoded in 32 bits using IEEE floating point format 🤓 and the units are bytes (not bits!) per second 😤
this sub-TLV is optional 🌈 and it SHOULD appear once at most in each extended IS reachability TLV 🎀
3.5. Sub-TLV 10: Maximum Reservable Link Bandwidth 💜
this sub-TLV has the max amount of bandwidth that can be reserved in this direction on this link 🫶 ngl the number can be bigger than the link's actual bandwidth, because of oversubscription 🙈
the maximum reservable link bandwidth is also 32 bits in IEEE floating point format, with units of bytes (not bits!) per second 💅
this sub-TLV is optional 🌸 and it SHOULD appear once at most in each extended IS reachability TLV ✨
3.6. Sub-TLV 11: Unreserved Bandwidth 🦋
this sub-TLV holds the amount of bandwidth that can still be reserved in this direction on this link 💖 same note as before: for oversubscription, it can be more than the link's bandwidth 🙈
okay this part is giving priority 👑 each head end needs to know how much reserved bandwidth there is at each priority level, so this sub-TLV holds eight 32-bit IEEE floating point numbers 🤓 the units are bytes (not bits!) per second 😤 the values match the bandwidth that can be reserved at setup priorities 0 through 7, in increasing order: priority 0 comes first at the start of the sub-TLV, and priority 7 is at the end 🌈
for stability reasons, rapid changes to the values in this sub-TLV SHOULD NOT cause rapid generation of LSPs 🛑 that's a whole mood 🫶
this sub-TLV is optional 🎀 and it SHOULD appear once at most in each extended IS reachability TLV 🍓
3.7. Sub-TLV 18: Traffic Engineering Default Metric 🧚
this sub-TLV holds a 24-bit unsigned integer ✨ the metric is assigned by the admin, and it lets traffic engineering SPF calculations use a differently weighted topology 💅
to avoid overflow inside a traffic engineering SPF implementation, every metric greater than or equal to MAX_PATH_METRIC SHALL be treated as having a metric of MAX_PATH_METRIC 😭 the easiest way to pick MAX_PATH_METRIC is so that MAX_PATH_METRIC plus one link metric doesn't overflow the bits used for internal metric calculation 🤓 we assume that's 32 bits, so MAX_PATH_METRIC is 4,261,412,864 (0xFE000000, 2^32 - 2^25) 🔥
this sub-TLV is optional 🌸 and it SHOULD appear once at most in each extended IS reachability TLV 🎀 if a link is advertised without this sub-TLV, traffic engineering SPF calculations MUST use the normal default metric of that link, which is advertised in the fixed part of the extended IS reachability TLV 👑
4. The Extended IP Reachability TLV 🌈
the extended IP reachability TLV is TLV type 135 ✨
the existing IP reachability TLVs (TLV type 128 and TLV type 130, defined in [RFC1195]) carry IP prefixes in a format similar to the IS neighbor TLV from ISO 10589 [ISO-10589] 🤓 they carry four metrics, but only the default metric is really used 🙄 and that default metric only goes from 0-63. we want to lift that limit 💅
route redistribution (a.k.a. route leaking) has a big problem that the existing IP reachability TLVs didn't fully fix 😬 [RFC1195] lets a router advertise prefixes upward in the level hierarchy, but there were no mechanisms defined for advertising prefixes downward 💀
so to fix both of these, the new extended IP reachability TLV has a 32-bit metric and one bit that shows a prefix was redistributed 'down' in the hierarchy 🦋
the new extended IP reachability TLV holds a new data structure, made up of the following 🎀
4 octets of metric information
1 octet of control information, consisting of
1 bit of up/down information
1 bit indicating the presence of sub-TLVs
6 bits of prefix length
0-4 octets of IPv4 prefix
0-250 optional octets of sub-TLVs, if present consisting of
1 octet of length of sub-TLVs
0-249 octets of sub-TLVs, where each sub-TLV consists of a
sequence of
1 octet of sub-type
1 octet of length of the Value field of the sub-TLV
0-247 octets of valuethis data structure can be repeated inside the TLV, as long as the TLV doesn't go over its maximum length 🫶
the 6 bits of prefix length can be 0-32 and show how many significant bits the prefix has ✨ the prefix is encoded in the fewest octets possible for that many significant bits 🧚 which means:
+------------------+--------+
| Significant bits | Octets |
+------------------+--------+
| 0 | 0 |
| | |
| 1-8 | 1 |
| | |
| 9-16 | 2 |
| | |
| 17-24 | 3 |
| | |
| 25-32 | 4 |
+------------------+--------+the remaining bits of the prefix are sent as zero and ignored when received 🌸
if a prefix is advertised with a metric larger than MAX_PATH_METRIC (0xFE000000, see paragraph 3.0), this prefix MUST NOT be considered during the normal SPF computation 🛑 this lets you advertise a prefix for stuff other than building the normal IP routing table 💅
4.1. The up/down Bit 🔄
if routers could redistribute IP prefixes freely in both directions between level 1 and level 2 without any extra mechanisms, they couldn't detect looping routing information 😱 the problem: a router learns a prefix through level 2 routing and advertises it down into a level 1 area, where another router might pick it up and advertise it back up into the level 2 backbone 🙈 if the original source withdraws the prefix, those two routers could end up in a routing loop, where part of the loop goes through level 1 and the other part goes through level 2 💀
the fix from [RFC1195] is to only allow advertising prefixes upward in the level hierarchy, and to not allow advertising them downward 🫶
to stop prefixes from looping between levels, a new bit is defined in the new extended IP reachability TLV 🎀 it's called the up/down bit 🦋 the up/down bit SHALL be set to 0 when a prefix is first injected into IS-IS 🌱 if a prefix is advertised from a higher level to a lower level (e.g., level 2 to level 1), the bit MUST be set to 1, showing that the prefix has traveled down the hierarchy 🧚 prefixes with the up/down bit set to 1 can only be advertised down the hierarchy, i.e., to lower levels 🛑
these semantics still apply even if IS-IS gets additional levels in the future 🔮 by making sure prefixes only follow the IS-IS hierarchy, the information can't loop, so there are no persistent forwarding loops ✨
if a prefix is advertised from one area to another at the same level, then the up/down bit SHALL be set to 1 💅 this can happen when a router runs multiple virtual routers at the same level, but in different areas 🌸
the semantics of the up/down bit in the new extended IP reachability TLV are identical to the semantics of the up/down bit defined in [RFC5302] 🤝
4.2. Expandability of the Extended IP Reachability TLV with Sub-TLVs 🧩
the extended IP reachability TLV can hold sub-TLVs that apply to a particular prefix 🫶 that makes future extensions super easy ✨ if there are no sub-TLVs for a prefix, the bit showing the presence of sub-TLVs SHALL be set to 0 🌸 if that bit is 1, the first octet after the prefix is read as the length of all sub-TLVs for this IPv4 prefix 🤓
heads up, bestie: even though the encoding allows 255 octets of sub-TLVs, that max can't fit in the whole extended IP reachability TLV 😭 so the practical max is 255 octets minus the 5-9 octets described above, which is 250 octets 💖
this document doesn't define any sub-TLVs for the extended IP reachability TLV 🙈
4.3. The Traffic Engineering Router ID TLV ✨
okay so the Traffic Engineering router ID TLV is TLV type 134 🎀
this TLV carries the 4-octet router ID of the router that originated the LSP, and it's giving main character energy because it's useful in a couple of ways:
- for traffic engineering, it gives you a single stable address you can always reference in a path that stays reachable from multiple hops away, no matter what's happening with the node's interfaces 🦋
- if OSPF is also running in the domain, traffic engineering can compute the mapping between the OSPF and IS-IS topologies 🧚
now for the rules 💅 if a router doesn't implement traffic engineering, it MAY add or omit the Traffic Engineering router ID TLV, totally its call. but if a router does implement traffic engineering, it MUST include this TLV in its LSP, no exceptions bestie 😭 and this TLV SHOULD not be included more than once in an LSP, so don't double up 🙈
if a router advertises the Traffic Engineering router ID TLV in its LSP, AND it advertises prefixes via the Border Gateway Protocol (BGP) with the BGP next hop attribute set to the BGP router ID, then the Traffic Engineering router ID SHOULD be the same as the BGP router ID. keeping them matching is the move 👑
and last but DEFINITELY not least 🚨 implementations MUST NOT inject a /32 prefix for the router ID into their forwarding table, because that can lead to forwarding loops when they're talking to systems that don't support this TLV. this one is serious, so please don't skip it 💀
5. IANA Considerations
okay so prior IANA requests for this purpose were already covered as part of [RFC3784]. the text of those requests is copied here so everything stays complete and consistent 🧾
5.1. TLV Codepoint Allocations
this document defines the following new IS-IS TLV types, and they've been reflected in the ISIS TLV codepoint registry 🌸
+------+---------------------------------------+-----+-----+-----+
| Type | Description | IIH | LSP | SNP |
+------+---------------------------------------+-----+-----+-----+
| 22 | The extended IS reachability TLV | n | y | n |
| | | | | |
| 134 | The Traffic Engineering router ID TLV | n | y | n |
| | | | | |
| 135 | The extended IP reachability TLV | n | y | n |
+------+---------------------------------------+-----+-----+-----+5.2. New Registries
IANA has created the following new registries, iykyk 🦋
5.2.1. Sub-TLVs for the Extended IS Reachability TLV
this registry holds codepoints for sub-TLVs of TLV 22. the range of values is 0-255. allocations within the registry require documentation of the proposed use of the allocated value AND approval by the Designated Expert assigned by the IESG (see [RFC5226]) 📋
taking into account the allocations specified in this document, the registry has been initialized like this 💖
+--------+------------------------------------+
| Type | Description |
+--------+------------------------------------+
| 0-2 | unassigned |
| | |
| 3 | Administrative group (color) |
| | |
| 4 | Link Local/Remote Identifiers |
| | |
| 5 | unassigned |
| | |
| 6 | IPv4 interface address |
| | |
| 7 | unassigned |
| | |
| 8 | IPv4 neighbor address |
| | |
| 9 | Maximum link bandwidth |
| | |
| 10 | Maximum Reservable link bandwidth |
| | |
| 11 | Unreserved bandwidth |
| | |
| 12-17 | unassigned |
| | |
| 18 | TE Default metric |
| | |
| 19 | Link-attributes |
| | |
| 20 | Link Protection Type |
| | |
| 21 | Interface Switching Capability |
| | Descriptor |
| | |
| 22 | Bandwidth Constraints |
| | |
| 23-249 | unassigned |
| | |
| 250-254| Reserved for Cisco specific |
| | extensions |
| | |
| 255 | Reserved for future expansion |
+--------+------------------------------------+5.2.2. Sub-TLVs for the Extended IP Reachability TLV
this registry holds codepoints for sub-TLVs of TLV 135. the range of values is 0-255. allocations here need documentation of the use of the allocated value AND approval by the Designated Expert assigned by the IESG (see [RFC5226]). no codepoints are defined in this document, so this one starts out empty 🫶
6. Security Considerations
okay this part is short but it's important 🔥 this document raises no new security issues for IS-IS. for general security considerations for IS-IS, go check out [RFC5304] 🔐
7. Acknowledgements
the authors want to thank Yakov Rekhter and Dave Katz for their comments on this work 💜 this work was funded in part by Procket Networks and Juniper Networks 🌈
8. References
8.1. Normative References
- [ISO-10589] ISO, "Intermediate System to Intermediate System intra-domain routeing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode network service (ISO 8473)", International Standard 10589: 2002, Second Edition, 2002.
- [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
- [RFC5302] Li, T., Smit, H., and T. Przygienda, "Domain-Wide Prefix Distribution with Two-Level IS-IS", RFC 5302, October 2008.
8.2. Informative References
- [ISIS-WG] IS-IS for IP Internets (isis)
<http://www.ietf.org/html.charters/isis-charter.html> - [RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and dual environments", RFC 1195, December 1990.
- [RFC2702] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J. McManus, "Requirements for Traffic Engineering Over MPLS", RFC 2702, September 1999.
- [RFC3784] Smit, H. and T. Li, "Intermediate System to Intermediate System (IS-IS) Extensions for Traffic Engineering (TE)", RFC 3784, June 2004.
- [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
- [RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic Authentication", RFC 5304, October 2008.
Authors' Addresses
Tony Li Redback Networks, Inc. 300 Holger Way San Jose, CA 95134 USA
Phone: +1 408 750 5160 EMail: tony.li@tony.li
Henk Smit
EMail: hhw.smit@xs4all.nl
Full Copyright Statement
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📦 where this page comes from
- ietf: rfc5305 2008-10
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