SOURCE ROUTING IN COMPUTER NETWORK S. Carl A. Sunshin e. Information Sciences Dept. The Rand Corporatio n 1700 Main Stree t Santa Monica, Ca.

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1 SOURCE ROUTING IN COMPUTER NETWORK S Carl A. Sunshin e Information Sciences Dept. The Rand Corporatio n 1700 Main Stree t Santa Monica, Ca Introductio n As plans for network interconnection develop, the problems o f internet routing and addressing become increasingly important. In on e popular model of internet addressing, a hierarchical form of network an d local (within network) address is used, with the source providing onl y the destination address while the intermediate network(s) and/or Gateway s between networks take care of routing packets to that destination b y various paths. This and related techniques requiring some form o f routing table and knowledge at intermediate nodes are more full y discussed in [1,2,3]. This paper considers another technique for internet routing in whic h the source of internet packets specifies the complete internet route. When the entire route accompanies each internet packet, no routing decisions o r tables are required at Gateways, but the packet format is complicated an d overhead increases. In particular, the packet must carry a varying numbe r of intermediate addresses depending on the path and destination [4]. Thi s overhead may be reduced by setting up a fixed route with connection table s [1] when a connection is established. Global Namin g The primary advantage of source routing is the elimination of comple x routing responsibilities from intermediate nodes. Instead, responsibilit y for routing falls on the source nodes which must be able to construc t complete routes to any desired destination. Source routing also eliminate s the need for global agreement on network names, since the name of eac h destination becomes equivalent to a path specification for reaching th e destination node. The connection of names and path specifications is particularl y apparent in an addressing scheme outlined by Steve Crocker (persona l communication). In Crocker ' s proposal, local networks are represented by a single switch connecting all local Hosts (see figure 1). In addition, som e lines from the switch may go to other switches (nets), providing networ k interconnection. The path to any destination is the series of switc h addresses (lines) traversed to reach that destination. A local path is on e address long, while paths to remote Hosts require an additional addres s 29

2 element for each switch traversed. Figure 1 gives several examples of pat h specifications. If most traffic is local, then such variable length pat h addressing may result in a shorter average address length since only a local address is needed for local traffic. If each local net (switch) has a globally agreed name, the n individual Hosts may be specified by their net name and local Host number, independent of the path(s) available to reach them. However, such globa l address agreements are not necessary if source routing is used, since an y Host may still be addressed by specifying a path to it. This simplifie s addition of new networks, or replacement of a single Host by a network, because the new nodes may be addressed by adding one more address elemen t to existing path specifications. For example, if Host 4 of the lower ne t in figure 1 is replaced by a network of Hosts, then a path from Host A t o Host E is (8,5,4,2). With a hierarchical address space and routing by Gateways, additio n of a new network requires global agreement on the network name, an d insertion of a new row for that network in all Gateway routing tables. With source addressing, only sources accessing a new net (switch) need t o know the new topology. A disadvantage of specifying destinations by their path names is tha t the " name " of a destination depends on the location of the source. Tw o different Hosts talking to the same third party may have different paths t o and hence different names for the same destination. This situation i s similar to dialing a special prefix from an 'inside" phone line, or th e regular 3-digit prefix from an ' outside " phone line to reach the sam e phone. Return Path Constructio n Crocker ' s single switch network model applies most clearly to loo p networks and other fully connected nets (broadcast transmission). Farbe r and Vittal [4] have proposed a similar source routing approach fo r interconnection of multiple DCS type loop networks. In addition t o specifying its destination, each packet normally identifies its source. Crocker and Farber have described similar means for progressivel y converting the destination path specification into a return path to th e source as the packet traverses successive path elements. These algorithm s work essentially as follows : Each time the packet enters a switch, th e address of the entry port is appended to the end of the path specification. Whenever the packet leaves a switch at an exit port, the correspondin g address is removed from the head of the path specification. Thus th e address length is constant at all points external to the switches. Fo r example, figure 1 shows seven points on the path between Hosts A and D. The path specification at each point is : (i) (8,5,1) Star t (ii) (8,5,1,6) Entry port adde d (iii) (5,1,6) First destination reache d (iv) (5,1,6,6) Entry port adde d (v) (1,6,6) Second destination reached 3 0

3 (vi) (1,6,6,5) Entry port adde d (vii) (6,6,5) Final destination reache d When the packet reaches its final destination, the resulting pat h specification is the path back to the source in reverse order. If the lin e between switches has the same address (name) in both switches, then the two address transformations described above become a simple cyclic shift (e.g. point iv to point vi). If an error occurs at some intermediate point, th e partially transformed path specification may still be reversed to correctl y return an error message to the source. The above example assumed that all path elements were the same size. It is quite possible for some switches (networks or integrated combination s of networks) to have larger numbers of ports, and to consume larger addres s elements. Two approaches to mixed size address elements are possible : I n the first, the end of each address element is explicitly indicated by a special marker of some sort. In the second, a universally agreed basi c unit of address implicitly defines the end of address elements. Thi s allows switches to consume any integral number of address units, but the y must then append the exit port address units in reverse order to the tai l of the path specification as the packet leaves the switch. As an example, consider a switch that consumes three (decimal) digi t addresses followed by switches handling single digit addresses. In th e first approach, a packet arriving at port 123 with address (456,7,8 ) (explicitly marked address elements) would leave the switch at port 45 6 with address (7,8,123). In the second approach, the universal unit o f address is a digit, and the packet arriving with address (45678) leaves th e switch with address (78321). It is even possible for the same switch t o consume different sized address elements. So long as the entry and exi t ports have the same size address, the path specification will remain a constant size between switches. Conclusion s Source routing simplifies routing at intermediate points by placin g all responsibility for route selection at the source. When the sourc e corresponds to a human user (perhaps accessing remote computing service s from a terminal), the user establishes the initial route using whateve r criteria he desires, and may update the route in response to observe d performance. Unfortunately, sources communicating with many destination s may need to know the topology and performance of much of the internetwor k system in order to construct successful routes. Typically less informatio n is available to evaluate alternative routes, and changes must be infrequen t (particularly for short addressing with its essentially fixed route), s o non-optimal routes will result. Although shortest path routing may b e reasonably amenable to source specification, other routing criteria such a s bandwidth, delay, or cost may be highly dynamic and more difficult t o project from the source. A combination of hierarchical routing fo r " established" portions of a system, and source Specified routing for newl y added or dynamically changing portions may prove advantageous and merit s further investigation. 31

4 Reference s [1] C. A. Sunshine, "Interconnection of Computer, Networks, " to appear i n Computer Networks Journal, volume 1, number 3. [2] A. Belloni, M. Bezzotti, and G. Le Moli, " Routing and Internetworking, " Alta Frequenza 44, 4, April 1975, pp Also INWG Protocol Not e #10, August [3] J. M. McQuillan, " Adaptive Routing Algorithms for Distributed Compute r Networks, " BBN Report No. 2831, May (PhD Thesis, Harvar d University ) [4] D. J. Farber and J. J. Vittal, "Extendability Considerations in th e Design of the Distributed Computer System (DCS)," Proc. Nationa l Telecommunications Conf., Atlanta, Georgia, Novwember

5 Figure 1 Source Routin g (I ) (III ) (IV ) O Hos t 0 Switch Path from Host A 0 to Host B is 7 ) to Host C is (8, 1 ) to Host D is (8, 5, 1 ) to Host E is (8, 5, 4, 2) 33