Link State Routing In particular OSPF dr. C. P. J. Koymans Informatics Institute University of Amsterdam (version 1.2, 2010/03/02 13:33:33) Monday, March 1, 2010 Link State Protocols Basic ideas Problems and pitfalls OSPF Rationale Parameters Databases Network representation and architecture OSPF packet details OSPF packet header The different kinds of OSPF packets Link State Advertisement packets Link State Advertisement header The different kinds of Link State Advertisements Link State Protocol Link State Packets A Link State Protocol Builds on complete information about the network topology Uses Dijkstra s Shortest Path Tree algorithm Replaces a distance vector protocol in the case of more complex topologies Has a faster convergence time than distance vector protocols Links State Packets (LSPs) represent the state of a router and its links to the rest of the network This representation suffices for point to point links Broadcast networks (LANs) and NBMA networks are represented by virtual nodes inside the topology Designated routers (DRs) speak on their behalf
Non-broadcast networks Learn to know your neighbours NBMA Non-Broadcast Multiple Access Full mesh of connectivity Connectivity via elected DR Point-to-Multipoint A subset of the collection of all point-to-point links No full mesh of connectivity No DR is elected Easy on point to point links Just send out greetings (hello packets) Both ends will become adjacent A little more advanced on broadcast networks Send out hellos using multicast Not all neighbours will become adjacent LSP generation LSP distribution Periodic announcements with a long period (like 30 minutes) Triggered updates when there are changes, which can be Detection of a new neighbour (link or node coming up) Detection of a link or node failure Change of a link cost LSPs are distributed throughout the network Distribution can t use the routing database for distant nodes Why? What is the difference with RIP here? Ordinary flooding is problematic It would need a TTL to prevent loops It could cause exponential multiplication of packets Smart flooding recognizes identical LSPs Causing propagation to be tree-like
Most recent LSP problem Pitfalls LSPs may arrive out of order Therefore we need a mechanism to recognize older packets What about using timestamps? Timestamps cause trouble if clocks are not synchronised or out of order! What about using sequence numbers? Sequence numbers need an aging procedure to protect from stale information, for instance when a router reboots and starts from scratch Sequence numbers may wrap Sequence number ordering is not a total ordering, not even a partial ordering a < b < c < d < a Intermezzo ARPANET lockup The devilish dice There are three dice, A, B and C such that A is better than B B is better than C C is better than A A:114444 B:333333 C:222255 S xcba P cba cba cba cba Q cba cba R
Lockup solution OSPF advantages (1) One could use a very large sequence number space and wait for timeouts after overflow One could use an age (or ttl) field and always increase the age (decrease the ttl) by at least one and furthermore increase (decrease) it periodically OSPF Introduces hierarchical routing Supports load balancing Supports subnets Supports unnumbered interfaces/networks Supports point-to-point, broadcast, NBMA and point-to-multipoint networks Supports virtual links for backbone connectivity OSPF advantages (2) OSPF fun :) OSPF Has built in authentication Uses efficient multicast for flooding Uses metrics built on cost, per interface Is easily extendable for multicast routing (MOSPF) IETF T-shirt IS IS = 0 Old time IETF versus ISO controversy But OSPF in fact really builds on BBN s research on SPF Early versions of OSI s IS-IS
Some OSPF facts Timers and Overflow OSPF builds directly upon IP OSPF uses protocol type 89 Current OSPF version is 2 RFC 2328 (Moy, 1998) RFC 2740 (OSPF for IPv6) version 3 OSPF uses LSA (Link State Advertisement) terminology in stead of LSP (Link State Packet) These must be the same for all OSPF neighbours HelloInterval (default 10 sec) RouterDeadInterval (default 40 sec) In case of database overflow external routing information is dropped first Consistent LSA Databases (Backup) Designated Router LSA s must be acknowledged LSA s (and their acks) are queued/flagged for transmission LSA s must time out at about the same time by using triggered distribution of age MaxAge packets which must be honored but only if there is already an LSA for this node in the local database A Designated Router (DR) and a Backup Designated Router (BDR) is elected on every multi-access network using Hello packets The (B)DR represents the network as a (virtual) node and acts on its behalf The DR/BDR election process is sticky The priority of routers can be configured
Multicast usage Hierarchical routing On LANs every router becomes adjacent to the Designated Router Backbone is area 0 Limbs (non-backbone) are areas not equal to 0 R Update DR Acknowledgement Update S ABRs Backbone (area 0) Multicast to AllDRouters (224.0.0.6) Multicast to AllSPFRouters (224.0.0.5) 1 2 3 4 Virtual Links Inter Area Summaries Area 5 is not physically connected to the backbone A virtual link extends the backbone to the new ABR and behaves as an unnumbered point to point link ABRs Backbone (area 0) 1 2 3 4 5 IASs are injected by ABRs IASs make use of a hub and spoke topology Summary information is spread RIP -like There are no loops or slow convergence Virtual links maintain the hub and spoke topology These virtual spokes are in fact paths through their transit area treated as unnumbered links in the backbone
Router roles Area Border Router (ABR) Backbone router Has at least one interface inside area 0 Internal router All interfaces are completely within a single area Area Border Router (ABR) Has an interface inside area 0 and one or more other areas Autonomous System Boundary Router (ASBR) Participates in another (external) routing protocol Attaches to multiple areas Runs multiple copies of the basic algorithm, once for each area Summarizes area data (destination networks) towards the backbone Receives summarized data from the backbone about other areas including the backbone itself Can aggregate summary data Autonomous System Boundary Router (ASBR) Stub(by) area Can be part of any area Interfaces with other routing protocols BGP, RIP, IS-IS,... Injects external routes into OSPF ASBR notion is independent of backbone, internal or ABR router A stub(by) area is an area into which no external routing information is injected by the ABRs It uses a default route for all external destinations A default route is injected by all ABRs A totally stubby area is a stubby area into which not even inter-area summaries are injected
OSPF packet header (24 bytes) OSPF packet header fields 0 7 8 15 16 31 Version Type Packet length Router ID Area ID Checksum AuType Authentication OSPF packet header fields Version 2 Type 1 5 (see next slide) Packet length Total length, including this header Router ID ID of packet originating router Area ID The area a packet belongs to Checksum One s complement checksum AuType Null, Simple or Crypto Authentication Pointer to message digest (Crypto) OSPF packet types OSPF Hello packet (20 + N 4 bytes) OSPF packet types Type Meaning 1 Hello 2 Database Description 3 Link State Request 4 Link State Update 5 Link State Acknowledgement 0 15 16 23 24 31 Network Mask HelloInterval Options Rtr Pri RouterDeadInterval Designated Router Backup Designated Router Neighbor #1. Neighbor #N Repeated for each living neighbor
OSPF Hello packet fields OSPF DD packet (2 + N 20 bytes) Hello fields 0 15 16 23 24 31 Network Mask HelloInterval Options Rtr Pri RouterDeadInterval Designated Router Backup Designated Router Neighbor subnet mask of link interval in seconds between hellos multiple metrics (T 1 ); no stub area (E) router priority, used for DR election interval to consider a silent neighbor dead IP address of designated router IP address of backup designated router (living) neighbor IDs Interface MTU Options 00000 I M M S DD sequence number LSA header #1. LSA header #N Partial list of database 1 deprecated because of lack of experience with TOS OSPF DD packet fields OSPF LS request packet (N 12 bytes) DD fields Interface MTU Detection of MTU mismatch Options Same as for Hello packets I Init bit (first packet) M More bit (more packets follow) MS Master/Slave bit DD sequence number Used for ordering DD packets LSA header Link state database instance identity 2 0 31 LS type Link State ID Advertising Router Multiple occurrences 2 Including age and sequence number
OSPF LS request packet fields OSPF LS Update packet LS request fields LS type Link State ID Advertising Router Link type of the LSA ID of (link type specific part of) the LSA Router ID of originating router 0 31 # LSA s (N) LSA #1 N occurrences Together these entries uniquely identify an LSA, but not an LSA instance. The Database Description packets refer to a specific LSA instance, but without actual data. LSA #N OSPF LS Update packet fields OSPF LS Acknowledgement packet (N 20 bytes) LS Update fields # LSA s Number of LSA s inside the update packet LSA #i A complete Link State Advertisement 0 31 LSA header #1 N occurrences Link State Updates contain complete and specific instances of Link State Advertisements with all relevant data. LSA header #N
OSPF LS Acknowledgement packet fields Link State Advertisements LS Acknowledgement fields LSA header #i A complete Link State Header Link State Acknowledgements again refer to specific instances, but contain no actual data. Multiple LSA s may be found inside a Link State Update packet Every LSA consists of LSA Header (20 bytes) LSA type specific content LSA header (20 bytes) LSA header fields 0 15 16 23 24 31 LS age Options LS type Link State ID Advertising Router LS sequence number LS Checksum Length LSA header fields LS age Options LS type Link State ID Advertising Router LS sequence number LS Checksum Length Time in seconds since the LSA was originated Same as for Hello packets Link type of the LSA (see next slide) ID of (link type specific) part of the LSA Router ID of originating router Used for most recent check of LSA packets The Fletcher checksum of the LSA (without age) Length in bytes of the LSA, including the header
LS types NSSA LS Types Type Meaning 1 Router LSA 2 Network LSA 3 Network Summary LSA 4 AS Boundary Router Summary LSA 5 AS External LSA 7 NSSA LSA (see next slide) NSSA stands for Not So Stubby Area Support for certain external routes throughout the area itself NSSA uses its own special type (7) These LSA s are translated at the ABR into ordinary external LSA s (type 5) for the rest of the OSPF domain It is common to inject a type 7 default route from the NSSA border router(s) into the NSSA. Link State IDs Router LSA (LS type = 1) without header Link State IDs Type Meaning 1 ID of originating router 2 IP address of the network s DR 3 The destination s IP network address 4 ID of described ASBR 5 The destination s IP network address repeated # links times 0 7 8 1516 31 0 V E B 0 # links Link ID Link Data Type # TOS metric } TOS 0 TOS metric # TOS times 3 Originated by every router Flooded throughout the area(s) 3 For backward compatibility
Router LSA fields Router LSA type field Router LSA fields V Router is virtual link endpoint E Router is AS Boundary Router B Router is Area Border Router # links Number of router links described Link ID ID of the connected network Link Data Extra information on network Type Type of connected network # TOS Number of extra TOSs (usually 0) metric Cost of link Router LSA type field Type Meaning 1 Point-to-point link 2 Transit network 3 Stub network 4 Virtual link Router LSA Link ID field Router LSA Link Data field Router LSA Link ID field Type Meaning 1 ID of neighbor router 2 IP address of designated router 3 IP (sub)network number 4 ID of neighbor router Router LSA Link Data field Type Meaning 1 Originating router s interface IP address 2 Originating router s interface IP address 3 Connected (sub)network mask 4 Originating router s interface IP address
Network LSA (LS type = 2) without header (4 + N 4 bytes) Network LSA fields 0 31 Network Mask Attached Router } Repeated for each attached router Network LSA fields Network Mask Attached Router (Sub)network mask Router ID of router(s) on network Originated by Designated Router Flooded throughout the area Summary LSA (type = 3, 4) without header Summary LSA fields 0 7 8 31 Network Mask 4 0 metric TOS TOS metric } for each desired TOS 5 Summary LSA fields Network Mask 6 Address mask for the advertised destination metric Cost to advertised destination Originated by Area Border Router Flooded throughout the area(s) 4 Only relevant for type 3 5 For backward compatibility 6 Only relevant for type 3
AS External LSA (LS type 5) without header AS External LSA fields 0 7 8 31 Network Mask E 0 metric Forwarding address External Route Tag E TOS TOS metric Forwarding address External Route Tag for each desired TOS 7 AS External LSA fields Network Mask Address mask for the advertised destination E External cost is higher than internal cost metric Cost to advertised destination Forwarding address Comparable to Next Hop in RIP External Route Tag Comparable to Route Tag in RIP Originated by AS Boundary Router Flooded throughout the whole AS 7 For backward compatibility