Overlapping Hash TrieA Longest Prefix First Search Scheme for IPv4/IPv6 LookupAuthor: Qiong Sun, Zhenqi_ng Li, Yan MaPublisher: Communication Tecnology, 2006. ICCT 06. Iternational Conference onPresenter: Chen-Yu Chang

OutlineRelated work

Introduction

OHT of IPv4/IPv6OHT constructionPrefixs of corresponding data structureImportant parametersIP lookup in OHTOHT updating operation

Performance evaluation

Related workLPFSTPuts the routing entry with longer prefix on the upper level of the tree

This scheme can stop immediately as matching

OutlineRelated work

Introduction

OHT of IPv4/IPv6OHT constructionPrefixs of corresponding data structureImportant parametersIP lookup in OHTOHT updating operation

Performance evaluation

IntroductionAlthough LPFST scheme can stop immediately when meeting a match, the bottleneck that choosing the entry in numerous long-prefixes within a few memory accesses is still unsettled and so that increased performance from LPFS is very limited compared to SPFS scheme.

OHT can not only find out the longest matching prefix within a great many long-prefixes immediately taking advantage of hash, but can also support fast update.

OutlineRelated work

Introduction

OHT of IPv4/IPv6OHT constructionPrefixs of corresponding data structureImportant parametersIP lookup in OHTOHT updating operation

Performance evaluation

OHT construction(1/8)According to the routing table, we created a binary trie as follow.

OHT construction(2/8)SegmentThere is no prefix shorter than 2 in the routing table.so we firstly constructed a segment at length 2 .(SL for segment- length).

OHT construction(3/8)Secondly, we carve out a two stride sub-triebetween length 3(TSL for trie start_length) and 4(TEL for trie end length)Sub-trie

OHT construction(4/8)Overlap the next two-stride prefixes of length 5 and 6 onits own sub-trie. As it is a longest prefix first search scheme,we overlap the prefixes of length 6 on trie length(TL) 3(say prefix G overlap on D) and the ones of length 5 on TL4 ( F overlap on the node under C).Overlapping

OHT construction(5/8)Finally, we put prefixes longer than 6(TEL) in hashtables overlapping on segment.Overlapping

OHT construction(6/8)ACBDEFGIHJAs one segment node can have more than one length, we introduce another structure: hash index link to indicate the length of its hash table and they are organizedin length-descending order.778Index

OHT construction(7/8)Data Structure of OHT developed from ordinary trie.7Hash tableHash indexSegmentSub-trie

OHT construction(8/8)We should keep in mind that every TL is overlapped by the long-prefixes with fixed corresponding length (CL), for instance, CL3 is 6 on TL3 and CL4 is 5 on TL4, and they has the following equation: ( CLn = 2*TEL + 1 TLn)5 = CL3 = 2*4+1-46 = CL3 = 2*4+1-3

Prefixs of corresponding data structureIn conclusion, we classify all the prefixes into 4 categories based on their lengths and every class has its data structure to deal with (summarized in tablel).

Important parameters(1/3)For the routing table in above example, the parameters like SL, TSL, TEL can be chosen easily, but for real routing table, these parameters should be defined based on the analysis of real routing table. The conclusion of prefix distribution is summarized in table2.

Important parameters(2/3)In accordance, we choose these parameters with the following guidelines1. SL should be the length less than which there are no prefixes. Only in this way can a segment be formed without prefix-expansion.

2. CLTSL should be the length greater than which there are few distinct length prefixes, so that the length of hash index link on segment will not be long.

Important parameters(3/3)The table3 shows these parameters we choose for IPv4 and IPv6.

IP lookup in OHTEx1:11101101Ex2:00110001Ex3:10001010

OHT updating operationwhile for deleting or modifying, we could execute the similar algorithm of insert(x, length) to find the node and change the next hop information either to a default one or to the modified one. It is quite simple and flexible.

OutlineRelated work

Introduction

OHT of IPv4/IPv6OHT constructionPrefixs of corresponding data structureImportant parametersIP lookup in OHTOHT updating operation

Performance evaluation

Performance comparision of IPv4