Midterm Review. Congestion Mgt, CIDR addresses,tcp processing, TCP close. Routing. hierarchical networks. Routing with OSPF, IS-IS, BGP-4

Transcription

1 Midterm Review Week 1 Congestion Mgt, CIDR addresses,tcp processing, TCP close Week 2 Routing. hierarchical networks Week 3 Routing with OSPF, IS-IS, BGP-4 Week 4 IBGP, Prefix lookup, Tries, Non-stop routers, Onion Routers

2 Prefix lookup Prefix problem Airline example from Varghese ch 11 Not just a match, but longest prefix match Fixed prefixes like phone numbers are easier. Class A, B, C networks implemented this, CIDR undid it.

3 Prefix Lookup Problems About 453,143 IP4 prefixes, 900k flows Caching not effective 12,704 IP 6 prefixes 50% are minimum size TCP packets Prefix lookup speed should match minimum packet size to avoid queueing Prefixes from 8-30 bits 30 memory lookups in worst case

4 Prefix Notation Binary wildcard format UofC * Slash format \16 Netmask Netmask

5 Prefix Lookup Problems Updates Network instability can lead to needing updates as often as in milliseconds Incremental Updates or 2 match tables Need SRAM for wirespeed lookup IP V6 and Multicast may require deeper lookups

6 Avoiding Lookups Virtual Circuits--Frame Relay and PSTN Threaded Indices more flexible as routes can be recomputed Pass route indices with destination cost Cisco TagSwitching and MPLS Doesn t deal with hierarchical routing

7 Flow Switching Perform normal route lookup on first packet On subsequent packets, create virtualcircuit like flow, to fast switch packets An elegant solution, but short term TCP connection on backbone do not favor it For example, httpd TCP gets

8 Nonalgorithmic Techniques Bigger/faster caches More cache memory on chips Content Addressable Memory Search and update in one memory cycle 5 nsec search of 100k prefixes Prefix format review

9 Tries Create a bit tree for prefix map Unibit Tries, have one bit at each node compress one way branches Multibit Tries Avoid RAM/DRAM latency with stride For example, remove odd nodes

10 Fixed Stride Tries Reduce latency of memory seeks, but expanding memory use (time for space) Empty strides variable strides can reduce memory waste Optimal stride is h*d minimized Find by recursive search

11 Update of Tries Incremental Update possible Keep 1 bit Trie in slow memory Expand or contract trie nodes as necessary

12 Level Compressed Tries Pick largest root Trie with no empties Repeat for subnodes Finally write as an array, starting with root trie where pointers become array offsets Avoids empty elements, but height grows LC tree produced by optimal VS Trie

13 Lulea-Compressed Tries Use string compression tricks with bitmap representing repeats Start with fixed stride leaf pushed trie Compress results Efficient Storage, slow on insert

14 Binary Search Binary searches are appealing with log2n Binary matching great for exact matching Extend for variable, by creating an array of N hash tables for each prefix length Naively, start with longest prefix However, produces linear search

15 Binary search Search on Lengths Pick media length, return match or no match Binary search on prefix tables If match is found at length m, search greater than m, similarly no match at m, lengths less than m

16 Nonstop Forwarding(NSF) Routers with separate data and control hw Control CPU (RP) can crash, while forwarding continues Method for managing backup RP startup while still forwarding packets Planned and unplanned failures Graceful restart

17 NSF Operation Restarting RP sends grace period request Neighbors continue to advertise restarting router in LSA even though no LSAs are received from that router Routing continues with topology in effect before restarting router crashed Exception is if there are other router LSA changes, then go OSPF Limits loops

18 NSF Operation Planned control failure: grace LSA only flooded to directly connected neighbors Unplanned control failure: Must send grace LSA before hellos or SPFs, so send multiple grace LSAs to directly connected neighbors

19 During Grace Period No new LSAs or changes to tables OSPF calculations, but no table updates OSPF designated router (DR) Smart router (and backup BDR) Multiaccess networks (Ethernet)

20 Exiting Graceful Restart All adjacencies have been reestablished Knows adjacencies from before restart Inconsistent LSA is received from neighbor Neighbor may not support grace period Grace period ends

21 Helpful Neighbors Receives Graceful restart Must be adjacent, unexpired grace, no topology changes Exit helper mode Restarting node finishes Grace period terminates Network topology change occurs

22 TOR Routers

23 TOR Routers Compared to Anonymous Web Proxy, TOR is IP based, so it can work with any application SOCKS aware, so works with any

24 Questions?