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No question: Moodle | 1 external routing bgp picture from : anderson, s., salamatian, l., bischof, z. s., dainotti, a. and barford, p., 2022, october. igdb : connecting the physical and logical layers of the internet. in proceedings of the 22nd acm internet measurement conference ( pp. 433 - 448 ). contents a. bgp at a high level 1. inter -... | EPFL COM 407 Moodle |
No question: Moodle | information ) finally stored? • inside a local forwarding table • directly into the packet headers domains — terminology ard = autonomous routing domain = routing domain under a single administrative entity as = autonomous system = ard with a number ( “ as number ” ), used in bgp routes • as number is 32 bits, written ... | EPFL COM 407 Moodle |
No question: Moodle | inter - domain routing context the internet is too large + heterogeneous ( i. e. it is split into various domains ) to be run by one routing protocol. we use hierarchical routing instead : - within domains, we use an igp ( = internal gateway protocol ), e. g. rip, ospf ( standard ), igrp ( cisco ) with ospf : large dom... | EPFL COM 407 Moodle |
No question: Moodle | ##3 d c n4 d n5 b best paths in e n5 n3 n4 e selects one of the 2 available routes according to some criterion bgp advertisement policies … … implement domains ’ business agreements ( e. g. customer - provider relationships, shared - cost peering ) via : import ( what to accept ) and export rules ( what to advertize to... | EPFL COM 407 Moodle |
No question: Moodle | ##p3 to find a route to c2 that transits via isp1 isp 1 isp 3 isp 2 c1 c2 c3 n2 isp2 c2 : n2 provider customer shared cost isp1 - isp2 and isp1 - isp3 are peers ; isp2 - isp3 are not peers nor customers / providers. all apply the rule “ routes coming from peers or providers are not propagated to peers or providers ”. w... | EPFL COM 407 Moodle |
No question: Moodle | ##gp router [ bgp - 4, rfc 4271 ] : - receives and stores candidate routes from its bgp neighbor peers, after applying import policy rules - applies the decision process to select at most one route per destination prefix and keeps all other accepted routes as backup - exports the selected routes to bgp neighbors, after... | EPFL COM 407 Moodle |
No question: Moodle | avoid redundant traffic - prepend own as number over i - bgp - modify the “ next - hop ” attribute of a route [ see also later ] • know about all inter - domain link subnets via igp d1 d2 d4 d5 d3 a b g h c d e f x : n1 x : n1 e - bgp e - bgp i - bgp r bgp sessions over tcp connections physical links say what is always... | EPFL COM 407 Moodle |
No question: Moodle | bgp e - bgp i - bgp r bgp sessions over tcp connections physical links go to web. speakup. info or download speakup app join room 46045 solution answer d. the route was learnt by from, i. e. via internal bgp ( i - bgp ). therefore it should not be re - advertized over i - bgp. there is no need since all other routers i... | EPFL COM 407 Moodle |
No question: Moodle | attributes : • destination ( subnet ) prefix • path to the destination ( as - path or an authenticated bgpsec _ path ) • next - hop ( modified by e - bgp, left unchanged by i - bgp ) • origin : route learnt from igp, bgp, static • other attributes : local - pref, atomic - aggregate ( = route cannot be dis - aggregated ... | EPFL COM 407 Moodle |
No question: Moodle | weight ( cisco proprietary ) 1. highest local - pref 2. shortest as - path 3. lowest med, if taken seriously by this network 4. e - bgp > i - bgp ( = if route is learnt from e - bgp, it has priority ) 5. shortest path to next - hop, according to igp 6. lowest bgp identifier ( router - id of the bgp peer from whom route... | EPFL COM 407 Moodle |
No question: Moodle | accepts the updates and stores them in adj - rib - in • [ decision process : ] r1 designates these routes as best routes • [ export filters : ] r1 puts updates into adj - rib - out, which will cause them to be sent to other bgp neighbors / peers from r3 10. 1 / 16 as = y next - hop = 1. 1. 1. 2 best from r3 10. 2 / 16 ... | EPFL COM 407 Moodle |
No question: Moodle | = 2. 2. 2. 1 10. 2 / 16 as = y next - hop = 2. 2. 2. 1 which of the two new routes ( in red ) are promoted by the decision process to “ best routes ” assuming weight, local _ pref and med are empty? r2 — > r1 step 2 answer d r1 applies again its decision process. now it has several possible routes to each prefix. the f... | EPFL COM 407 Moodle |
No question: Moodle | 16 as = y next - hop = 2. 2. 2. 1 10. 1 / 16 as = y next - hop = 2. 2. 2. 1 10. 2 / 16 as = y next - hop = 2. 2. 2. 1 r2 — > r1 another fundamental example • 3 bgp routers in as x. • an igp ( e. g. ospf ) also runs on r1, r21 and r22. • assume : - all link costs are equal to 1. - r3 and r4 advertise only their directly... | EPFL COM 407 Moodle |
No question: Moodle | 1 / 16 as = y r3 — > r1 step 2 • r1 accepts the updates and stores it in adj - rib - in • r1 designates this route as best route • r1 does not put route into adj - rib - out to r21 because i - bgp is not repeated over i - bgp r1 does not put route into adj - rib - out to r3 this would create an as - path loop from r3 1... | EPFL COM 407 Moodle |
No question: Moodle | . 3. 3. 1 10. 2 / 16 as = y next - hop = 3. 3. 3. 1 will the decision process promote the new route to “ best route ” assuming that weight, local _ pref, med are empty? r21 — > r1 solution answer a the decision process now has to choose between two routes with same destination prefix 10. 2 / 16. both were learnt from i... | EPFL COM 407 Moodle |
No question: Moodle | packets customer 1 customer 2? → a. r12 - r11 - r21 b. r12 - r22 - r21 c. it depends on the configuration of bgp at isp1 and isp2 d. both in parallel e. i don ’ t know go to web. speakup. info or download speakup app join room 46045 solution answer c : it depends on the configuration. if configuration is as in “ fundam... | EPFL COM 407 Moodle |
No question: Moodle | learnt by igp, e. g. : redistribute ospf into bgp • if igp = ospf, in principle, only internal prefixes should be redistributed • such bgp routes have attribute origin = igp. • when originated, the bgp next - hop of such a route is its igp next - hop. 2. aggregation ( of routes ) routes usually overlap • expected to be... | EPFL COM 407 Moodle |
No question: Moodle | . the two prefixes are contiguous and can be aggregated as 2001 : baba : bebe / 47 actual advertizements : as3 sends to as4 the update 2001 : baba : bebe / 47 as - path = 3 { 1 2 } as4 sends the update 2001 : baba : bebe / 47 as - path = 4 3 { 1 2 } solution 2001 : baba : bebe / 48 2001 : baba : bebf / 48 2001 : baba :... | EPFL COM 407 Moodle |
No question: Moodle | path = 1 2001 : baba : bebf / 48, as - path = 2 2001 : baba : bebf / 48, as - path = 2 2001 : baba : bebe / 47, as - path = 3 { 1 2 } assume the decision process in as4 designates both routes as best. which path does a packet from as4 to 2001 : baba : bebf / 48 follow? a. as4 - as3 - as2 b. as4 - as2 c. i don ’ t known... | EPFL COM 407 Moodle |
No question: Moodle | as - path = 1 2001 : baba : bebf / 48, as - path = 2 2001 : baba : bebf / 48, as - path = 2 2001 : baba : bebe / 47, as - path = 3 { 1 2 } assume the link as2 - as4 breaks … • at as4 : keepalive mechanism detects that the border router at as2 is unreachable • related adj - rib - in, adj - rib - loc routes are declared ... | EPFL COM 407 Moodle |
No question: Moodle | ( e. g. : ospf ) • typically only routes learnt by e - bgp are redistributed ( unless bgp redistribute - internal is used ) • igp propagates the routes to all routers in domain • works with ospf, might not work with other igps ( table too large for igp ) example ( re - distribution ) assume all routers run bgp apart fr... | EPFL COM 407 Moodle |
No question: Moodle | y as z e - bgp e - bgp r4 r1 r2 r5 r6 18. 1 / 16 i - bgp igp ( ospf ) igp ( ospf ) 2. 2. 2. 2 how routes learnt by bgp are written into forwarding tables? there are two possible ways : 1. redistribution of bgp into igp : routes learnt by bgp are passed to igp ( e. g. : ospf ) • typically only routes learnt by e - bgp a... | EPFL COM 407 Moodle |
No question: Moodle | 18. 1 / 16? a. at r6 : 2. 2. 2. 2, at r2 : 2. 2. 2. 2 b. at r6 : 2. 2. 2. 2, at r2 : the ip address of r1 - east c. at r6 : 2. 2. 2. 2, at r2 : the ip address of r6 - south d. none of the above e. i don ’ t know as x as y as z e - bgp e - bgp r4 r1 r2 r5 r6 18. 1 / 16 i - bgp igp ( ospf ) igp ( ospf ) 2. 2. 2. 2 exampl... | EPFL COM 407 Moodle |
No question: Moodle | problem is usually solved via recursive table lookup. ( see next slides ) recursive table lookup why? a bgp router injects a route into its forwarding table = it copies the bgp next - hop into the forwarding table ’ s next - hop. so, the forwarding table may indicate next - hops which are not “ on - link ” ( i. e. with... | EPFL COM 407 Moodle |
No question: Moodle | table at r1 is looked up first, next - hop 2. 2. 2. 2 is found ; - a second lookup for 2. 2. 2. 2 is done ( or has been done in advance ) ; - packet is sent to r1 ( interface 2. 2. 1. 1 ) over eth0. example ( injection, cont'd ) to next hop interface 18. 1 / 16 2. 2. 2. 2 n / a 2. 2. 2. 2 2. 2. 1. 1 eth0 forwarding tab... | EPFL COM 407 Moodle |
No question: Moodle | bgp. thus r1 does not have any route to 18. 1 / 16 in its forwarding table. the packet cannot be forwarded by r1 ( “ destination prefix not found ” ). 52 in practice, simple injection implies that all routers need to run bgp the “ injection - only ” bgp setup : • all routers run bgp ( in addition to igp ) even if not b... | EPFL COM 407 Moodle |
No question: Moodle | ( internal prefixes of as x ). • scion ( alternative to bgp ) uses a similar mechanism. as x as y as z e - bgp e - bgp r4 r1 r2 r5 r6 18. 1 / 16 i - bgp igp igp 2. 2. 2. 2 2. 2. 20. 1 to next - hop flags 18. 1 / 16 2. 2. 2. 2 insert next - hop as source routing header at r2 alternative : bgp with mpls alternative to re... | EPFL COM 407 Moodle |
No question: Moodle | . 1 / 16 2. 2. 2. 2 mpls label 23 at r2 injection conflicts in frr and cisco, bgp always injects routes into forwarding table, even if these routes are redistributed into igp. this may cause injection conflicts : • a route may be injected into the forwarding table by both igp ( e. g. ospf ) and bgp. to solve the confli... | EPFL COM 407 Moodle |
No question: Moodle | into ospf because it was learnt with i - bgp and only e - bgp is redistributed, as we assumed. • at : r6 injects 18. 1 / 16 from bgp into its forwarding table ; in r6 ’ s forwarding table we see : 18. 1 / 6, admin dist = 20, next - hop = 2. 2. 2. 2 then r6 redistributes 18. 1 / 16 from bgp into ospf with ospf cost = 20... | EPFL COM 407 Moodle |
No question: Moodle | pref = 10 r1 chooses the largest preference as x e - bgp r1 r2 r6 i - bgp i - bgp e - bgp e - bgp pref = 10 pref = 100 local - pref example : link as2 - as4 is expensive as 4 sets local - pref to : • 100 to all routes received from as 3 • 50 to all routes received from as 2 r1 receives the route as2 as1 10. 1 / 16 over... | EPFL COM 407 Moodle |
No question: Moodle | as3 as best route to 10. 1 / 16 because of the local - pref attribute r3 advertises only its best route to as5, i. e. 10. 1 / 16 as - path = as4 as3 as1 r1 injects in forwarding table the next - hop corresponding to the r2 - as3 link and therefore the packet to 10. 1. 1. 1 goes via as3 answer c is not possible because ... | EPFL COM 407 Moodle |
No question: Moodle | ##3 1. 1. 1. 1 2. 2. 2. 2 63 r1 has 2 routes to 10. 2 / 16 : one via r3, learnt from r3 by e - bgp ( med = 50 ), one via r4, learnt from r2 by i - bgp ( med = 10 ). the decision process at r1 prefers … a. the route via r2 b. the route via r3 c. both d. i don ’ t know 64 solution answer a r1 prefers the route via r2 bec... | EPFL COM 407 Moodle |
No question: Moodle | best route to 10. 1 / 16 the route learnt from r2 r2 is informed of the route suppression by i - bgp r2 has now only 1 route to 10. 1 / 16 and 1 route to 10. 2 / 16 ; traffic to 10. 1 / 16 now goes to r2 med allows as y to be dual homed and use closest link – other links are used as backup packet to 10. 1. 2. 3 67 conv... | EPFL COM 407 Moodle |
No question: Moodle | ##s • if a2 receives ( 2 ) dest = 2001 : 1 : 1 : : / 48, as - path = bcd from b2 before receiving a route to 2001 : 1 : 1 : : / 48 from d, a2 stores it and will prefer it over any route to 2001 : 1 : 1 : : / 48 received later from d. a2 ’ s best route is dest = 2001 : 1 : 1 : : / 48, as - path = bcd, next - hop = b2s t... | EPFL COM 407 Moodle |
No question: Moodle | i don ’ t know solution 69 answer b bgp sends modifications to neighbors, including additions and withdrawals of best routes. 70 route flap damping ( or dampening ) why? route flap : a route is successively withdrawn, updated, withdrawn, updated etc. caused e. g. by unstable bgp routers ( crash, reboot, crash, reboot..... | EPFL COM 407 Moodle |
No question: Moodle | of traffic ( e. g provider should use r1 - r3 for all traffic to 10. 1 / 16 • stub domains ( e. g., epfl ) can use a private as number - - not usable in the global internet, used only between client and provider ( e. g., switch ) • provider translates this number to his own when exporting routes to the outside world. •... | EPFL COM 407 Moodle |
No question: Moodle | bgp this architecture results in fewer ibgp internal peerings ( no mesh, but hierarchy ), and avoids loops cluster _ id attribute associated with the advertisement as z e - bgp e - bgp i - bgp i - bgp i - bgp i - bgp i - bgp e - bgp rr rr rr i - bgp cluster 1 cluster 2 cluster 3 75 an interconnection point 76 avoiding ... | EPFL COM 407 Moodle |
No question: Moodle | as usual d. i don ’ t know isp 1 isp 3 isp 2 c1 c2 c3 128. 178 / 15 128. 178 / 16 as - path = isp3 128. 179 / 16 as - path = isp3 78 solution answer a or b • if aggregation is not done by isp1, the routes to 128. 178 / 16 and 128. 178 / 15 are different. by longest prefix match, all traffic to 128. 178 / 16 ( and to 12... | EPFL COM 407 Moodle |
No question: Moodle | the internet routing registry ( irr ). ases can read routing registries and use them to verify the routes received from bgp peers not cryptographic, best effort. 80 origin validation : roa owner of an address block creates a ( cryptographically signed ) route origin authorization ( roa ) that contains as number and ip ... | EPFL COM 407 Moodle |
No question: Moodle | scion ( https : / / scion - architecture. net, ethz, adrian perrig ) is an alternative to bgp ( and to ip ) that uses source routing and systematic encryption. 82 c. illustrations : the switch network www. switch. ch 83 roa signed and valid number of announced prefixes seen by hurricane electric : bgp. he. net / report... | EPFL COM 407 Moodle |
No question: Moodle | ##m computer communication review. vol. 18. no. 4. acm, 1988. example 1 : congestion due to greedy sources assume : two flows and. sources are greedy ( i. e. send as much as they want ) ; loss may happen loss is proportional to submitted traffic and links can be fully utilized s1 →d1 s2 →d2 s1 c1 = 100 kb / s s2 c2 = 1... | EPFL COM 407 Moodle |
No question: Moodle | s2 : 10 kb / s the problem was that s2 sent too much ( but it did not know ) s1 s2 d1 d2 x4, 1 = 10 x5, 2 = 10 c1 = 100 kb / s c2 = 1000 kb / s c3 = 110 kb / s c4 = 100 kb / s c5 = 10 kb / s example 2 : congestion collapse assume : each source sends traffic 2 hops away at rate e. g., source, at node, sends traffic to a... | EPFL COM 407 Moodle |
No question: Moodle | for i i λ + λ ′ λ < c 2 λ ′ ′ = λ λ > c 2 + ′ ′ = + ′ ( 1 ) " = + ′ ′ ( 2 ) ′ " link ( i - 1 ) link i link ( i + 1 ) node i node i + 1 source i λ ′ λ λ ′ ′ λ λ λ ′ we obtain, for : > 2 " = 2 ( −1 + 1 + ) for large offered traffic load, the limit of throughput is 0 c / 2 = 10 λ λ ′ ′ so, as λ — > + ∞, throughput — > 0 c... | EPFL COM 407 Moodle |
No question: Moodle | mb / s d. none of the above e. i don ’ t know go to web. speakup. info or download speakup app join room 46045 solution answer c total throughput maximize subject to over the max can be obtained by linear programming, or directly here by inspection : • because, and • is achieved with and therefore the max is 20 mb / s ... | EPFL COM 407 Moodle |
No question: Moodle | called pareto - efficient ( or pareto - optimal ), iff increasing the rate of a flow must be at the expense of decreasing the rate of some other flow i. e. is pareto - efficient iff : for any other feasible every flow has a bottleneck link = for every flow there exists a link, used by, which is saturated, i. e. its con... | EPFL COM 407 Moodle |
No question: Moodle | none i. i don ’ t know = 1, = 0. 5, = 8, = 1 = 1, = 1, = 8, = 1 = 1, = 1, = 2, = 7 2 gb / s 9 gb / s flow 1 flow 0 flow 2 10 gb / s flow 3 go to web. speakup. info or download speakup app join room 46045 solution answer f ( b and c ) allocation a : flow 1 does not have a bottleneck. its rate can be increased unilateral... | EPFL COM 407 Moodle |
No question: Moodle | = 1 = 8 = 1 = 1 = 1 = 8 solution allocation c : link 1 is bottleneck for sources 0 and 1 link 3 is bottleneck for sources 0, 2 and 3 every flow has a bottleneck. none can be increased unilaterally. allocation c is pareto - efficient. observation : link 2 is not saturated in this pareto - efficient allocation. = 1, = 1,... | EPFL COM 407 Moodle |
No question: Moodle | or download speakup app join room 46045 solution maximize subject to with the solution is mb / s answer b = = = 2 10 = 1 egalitarianism is not always pareto - efficient • egalitarianism gives mb / s to all but, we could give more to without hurting anyone, allocation is not pareto - efficient • a better allocation woul... | EPFL COM 407 Moodle |
No question: Moodle | / s mb / s mb / s x0 = 1 x1 = 9 x2 = 1 go to web. speakup. info or download speakup app join room 46045 solution allocation a increase ( e. g. and decrease ( and ( ; this does not contradict fairness because and are larger than so, there exists one increase that does not contradict fairness a is not max - min fair allo... | EPFL COM 407 Moodle |
No question: Moodle | in our case ), the unique max min fair allocation is obtained by the water - filling algorithm : 1. mark all flows as non frozen 2. do 3. increase the rate of all non frozen flows to the largest possible common value 4. mark flows that use a saturated link as frozen 5. until all flows are frozen water - filling : examp... | EPFL COM 407 Moodle |
No question: Moodle | or download speakup app join room 46045 solution answer a the max - min fair allocation via water - filling gives the same rate to all flows but this seems “ not fair enough ” in terms of resource usage! actually, one could claim that should be penalized and get 5x less, because it uses 5x more resources ( answer d ). ... | EPFL COM 407 Moodle |
No question: Moodle | i. e. ), the new allocation is feasible ( = within constraints ). the total rate of change is. so, we could change the allocation and obtain a positive total rate of change. a is not proportionally fair + = 1 … 4 0 < 2 2 + 4 2 > 0 let and ;. if is small enough ( i. e. ), the new allocation is feasible. the total rate o... | EPFL COM 407 Moodle |
No question: Moodle | techniques to solve this, but here we can also do a direct solution … = + + + + subject to + + + + c c c c we have to solve the optimization problem : observe : at the maximum point, we must have equality in all constraints otherwise we can increase and increase u ( i. e. find a better maximum ). therefore, for any cho... | EPFL COM 407 Moodle |
No question: Moodle | e. i don ’ t know = 1, = 9, = 1 = 0. 909, = 9, = 1. 010 = 1. 009, = 8. 991, = 0. 999 = 0. 909, = 9. 091, = 1. 010 go to web. speakup. info or download speakup app join room 46045 solution answer d. why? • we saw earlier that a is not proportionally fair • b is not pareto - efficient ( you can increase only ) — therefor... | EPFL COM 407 Moodle |
No question: Moodle | ). ( ) ( ) = : proportional fairness ( ) = : large max min fairness ( ) = 1 −1 ⇒ ≈ rate utility recap sources should adapt their rate to the state of the network in order to avoid inefficiencies and congestion collapse. this is called “ congestion control ”. such control mechanism should target a form of fairness that ... | EPFL COM 407 Moodle |
No question: Moodle | digital equipment corporation, august 1987. a simple network model network sends a one - bit feedback : if — > positive feedback if — > negative feedback sources reduce rate if, increase otherwise : what form of increase / decrease laws should one pick? ( ) = 0 ( ) ( ) = 1 ( ) > ( + 1 ) ( ) = 1 capacity rate ( ) feedba... | EPFL COM 407 Moodle |
No question: Moodle | or proportionally fair allocations here ( in the example with 1 link ) they are the same we will now analyze the impact of each of the four coefficients and.,, zoom on 2 sources using a single link = + ( ) = 0 ( ) = 0 ( ) = 1 target rate of source 1 rate of source 2 zoom on 2 sources ; say what is true a. 1 = additive ... | EPFL COM 407 Moodle |
No question: Moodle | in a more complex network setting, what type of fairness does aimd achieve? a. max - min b. proportional c. none of the above d. i don ’ t know fairness of aimd answer c aimd with : additive increase, multiplicative decrease, and one update per time unit implements utility fairness, with utility of flow given by, where... | EPFL COM 407 Moodle |
No question: Moodle | . 25 target rate time rate of this source source 1 with slow start time rate ( all 3 sources ) source 3 source 3 actual rate target rate end of slow start end of slow start 3 sources u1 = 0. 5, v1 = 0, u0 = 0, v0 = 0. 01 ( unit : mb / s ) 3rd source starts with rate v0 source 1 source 1 with slow start without slow sta... | EPFL COM 407 Moodle |
No question: Moodle | from network ( according to the decbit principle ) this avoids congestion collapses and ensures some sort of fairness many congestion control algorithms popular ones are : tcp reno ( the most mature and well explored, widely used until recently ) tcp cubic ( widespread today in linux servers ) data center tcp ( microso... | EPFL COM 407 Moodle |
No question: Moodle | its sending rate? • tcp reno adjusts the sliding window size • based on the approximation : w = min ( cwnd, offeredwindow ) offeredwindow = window advertized by other end in tcp ’ s window field cwnd = controlled by tcp congestion control rate ≈ w rtt slow start by adjusting cwnd … … multiplicative increase : ( slow st... | EPFL COM 407 Moodle |
No question: Moodle | increment cwnd by 1 mss multiplicative decrease : ( after detecting loss — negative feedback ) - ssthresh = 0. 5 cwnd - cwnd = 1mss ( if timeout ) or something else ( if fast retransmit ) [ see fast recovery ] × × cwnd = 1 mss 2. 5 2. 9 2 3. 83 aimd by adjusting cwnd … example of congestion - window evolution without f... | EPFL COM 407 Moodle |
No question: Moodle | w rtt × when loss is detected by 3 duplicate acks at any phase : ssthresh = 0. 5 × current - cwnd ssthresh = max ( ssthresh, 2 × mss ) cwnd = ssthresh + 3 × mss cwnd = min ( cwnd, 64k ) goto fast recovery when in fast recovery, for each duplicate ack received : cwnd = cwnd + mss ( exp. increase ) cwnd = min ( cwnd, 64k... | EPFL COM 407 Moodle |
No question: Moodle | ack = 201, win = 1 ’ 000 ack = 201, win = 1 ’ 000 ack = 901, win = 1 ’ 000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 ack = 201, win = 1 ’ 000 ack = 201, win = 1 ’ 000 tcpmaxdupacks = 3 during congestion avoidance : cwnd ←cwnd + mss2 cwnd mss = 100 at time 1, the sender is in “ congestion avoidance ” mode. the ... | EPFL COM 407 Moodle |
No question: Moodle | 907. at times 15, 16 and 18, the congestion window is increased by 1 mss, i. e. 100 bytes, by application of the fast recovery algorithm. at time 15, this allows to send one fresh packet, which occurs at time 17. at time 19 the lost packet is acked, the source exits the fast recovery mode and enters congestion avoidanc... | EPFL COM 407 Moodle |
No question: Moodle | 1 ’ 000 ack = 201, win = 1 ’ 000 ack = 901, win = 1 ’ 000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 ack = 201, win = 1 ’ 000 ack = 201, win = 1 ’ 000 go to web. speakup. info or download speakup app join room 46045 solution answer c the congestion window is 407, the advertised window is 1000, and the last ack ... | EPFL COM 407 Moodle |
No question: Moodle | ##p source does not know the cause of a loss. side - effect : a flow that experiences accidental losses on its wireless access link may never manage to get its fair share on another bottleneck link down its path, because it will be constantly reducing its sending rate “ thinking that it experiences congestion ”. soluti... | EPFL COM 407 Moodle |
No question: Moodle | destination using one tcp connection each, rtts are 60ms and 140ms. bottleneck is link « router - destination ». who gets more? a. gets a higher throughput b. gets a higher throughput c. both get the same d. i don ’ t know router destination 10 mb / s, 20 ms 1 mb / s 10 ms 10 mb / s, 60 ms s1 s2 go to web. speakup. inf... | EPFL COM 407 Moodle |
No question: Moodle | uses more resources. the mechanic of tcp reno that is close to proportional fairness leads to this source having less rate, which is desirable in view of the theory of fairness. 2. if this flow has simply a larger rtt, then things are different. the mechanics of additive increase leads to this source having less rate, ... | EPFL COM 407 Moodle |
No question: Moodle | = 1. 22 go to web. speakup. info or download speakup app join room 46045 solution if processing time is negligible and router drops packets in the same proportion for all flows, then throughput is proportional to 1 / rtt, thus i. e. answer c. 1 = 1 = 7 3 time ack numbers s1 s2 tcp reno — shortcomings • rtt bias – not n... | EPFL COM 407 Moodle |
No question: Moodle | control ( in its original version ) or reno ’ s congestion control ( in the standard version ). another method ( e. g. for videoconferencing application ) is to control the rate by computing the rate that tcp reno would obtain. e. g. : tfrc ( tcp - friendly rate control ) protocol application adapts the sending rate ( ... | EPFL COM 407 Moodle |
No question: Moodle | * 0. 5 tcp cubic modifies reno why? increase tcp rate faster on lfns how? cubic keeps the same slow start, fast recovery phases as reno, but : • during congestion avoidance, the increase is not additive but cubic • multiplicative decrease with factor 0. 7 ( instead of 0. 5 ) say congestion avoidance is entered at time ... | EPFL COM 407 Moodle |
No question: Moodle | term mss per rtt ( instead of 1 ) and multiplicative decrease. formally :, where and is computed s. t. this hypothetical reno has the same loss - throughput formula as standard reno : cubic ’ s throughput ≥ reno ’ s throughput with equality when rtt or bandwidth - delay product is small ( i. e. when in non - lfns ) = 0... | EPFL COM 407 Moodle |
No question: Moodle | = 100 ms 8. tackling the bufferbloat syndrome with ecn and aqm using loss as congestion feedback has a major drawback = self - inflicted delay : increased latencies and buffers are not well utilized due to bufferbloat syndrome. from : n. cardwell, y. cheng, c. s. gunn, s. h. yeganeh, and v. jacobson, “ bbr : congestion... | EPFL COM 407 Moodle |
No question: Moodle | data is not delivered any faster since the bottleneck does not serve packets any faster and the throughput stays the same for the sender : the amount of inflight data is larger, but the round - trip time increases by the corresponding amount. excess data in the buffer is useless for throughput gain and a queuing delay ... | EPFL COM 407 Moodle |
No question: Moodle | ##ed towards the source 5. packets with ece are forwarded towards the source 6. packets with ece are received by source. 7. source applies multiplicative decrease of the congestion window. source sets the congestion window reduced ( cwr ) flag in tcp header. the receiver continues to set the ece flag until it receives ... | EPFL COM 407 Moodle |
No question: Moodle | ecn capable ect ( 0 ) and ect ( 1 ) historically used at random ; today used to differentiate congestion control ( tcp cubic vs dctcp ) 11 = used by routers to signal that congestion is experienced ( ce ) if congested, router marks ect ( 0 ) or ect ( 1 ) packets ; but discards non ect packets 2 bits in tcp header but a... | EPFL COM 407 Moodle |
No question: Moodle | th - max max - p 1 q ( marking prob ) but … active queue management does not require ecn aqm can also be applied even if ecn is not supported e. g. with red, a packet is dropped with probability computed by the red curve - packet may be discarded even if there is some space available! expected benefits in this case : -... | EPFL COM 407 Moodle |
No question: Moodle | room with lots of racks of pcs and switches where many distributed apps are running : e. g. youtube, cff. ch, switchdrive, etc what is special about data centers? • most traffic is tcp • very small latencies ( 10 - 100 s ) • lots of bandwidth • various traffic patterns coexist : - internal traffic ( distributed computi... | EPFL COM 407 Moodle |
No question: Moodle | : multiplicative decrease by 50 % or 30 % is still abrupt throughput inefficiency ⇒ data center tcp why? improve performance for jumbo flows when ecn is used how? avoid brutal multiplicative decrease of 50 % ( of reno ) or 30 % ( of cubic ) instead, tcp source estimates prob of congestion from ecn echoes • ecn echo is ... | EPFL COM 407 Moodle |
No question: Moodle | words, dctcp competes unfairly with other tcps ; this is why it cannot be deployed outside data centers ( or other controlled environments ). inside data centers, care must be given to separate the dctcp flows ( i. e. the internal flows ) from other flows. this can be done with class - based queuing [ see next ]. × 0. ... | EPFL COM 407 Moodle |
No question: Moodle | bandwidth and the min rtt separately 2. controls directly the rate ( not the window ) using pacing ( = implementing a packet spacer ) that tries to keep amount of inflight data close to bottleneck bandwidth minrtt ( optimal operating point ) × n. cardwell, y. cheng, c. s. gunn, s. h. yeganeh, and v. jacobson, “ bbr : c... | EPFL COM 407 Moodle |
No question: Moodle | offered window • there is also a startup phase ( similar to cubic and reno ) with exponential increase of rate • no reaction to losses or ecn ( ) ( ) = 1. 25 ; 0. 75, ; 1 ; 1 ; 1 ; 1 ; 1 ; 1 2 × figure from : ware, r., mukerjee, m. k., seshan, s. and sherry, j., 2019, october. modeling bbr's interactions with loss - ba... | EPFL COM 407 Moodle |
No question: Moodle | the exponential startup behavior from cubic by doubling the sending rate with each round - trip. once the measured bandwidth does not increase further, bbr assumes to have reached the bottleneck bandwidth. since this observation is delayed by one rtt, a queue was already created at the bottleneck. bbr tries to drain it... | EPFL COM 407 Moodle |
No question: Moodle | . org / post / bbrs _ basic _ beauty / performance of bbrv1 but … bbrv1 takes no feedback from network – no reaction to loss or ecn • [ hock et al, 2017 ] find that bbrv1 ’ s estimated bottleneck bandwidth ignores how many flows are competing fairness issues with : - bbr flows of different rtts and - bbr versus other t... | EPFL COM 407 Moodle |
No question: Moodle | a scheduler such as weighted round robin ( wrr ) or deficit round robin ( drr ). wrr and drr have one queue per class. at every round, queues are visited in sequence. wrr serves packets of class in one round. drr serves bits of class in one round. used in enterprise or industrial networks to support non - congestion - ... | EPFL COM 407 Moodle |
No question: Moodle | mb / s and class 2 may borrow the remaining capacity, the available capacities for class 2 are : 9 mb / s, 8 mb / s and 8 mb / s. 10 mb / s 10 mb / s 10 mb / s pc1 pc2 s1 class 2 low prio 9 mb / s available 7. 5 mb / s guaranteed 8 mb / s available 7. 5 mb / s guaranteed 8 mb / s available 7. 5 mb / s guaranteed soluti... | EPFL COM 407 Moodle |
No question: Moodle | s available 7. 5 mb / s guaranteed the future of congestion control in the past, most tcp versions have relied on loss or ecn as negative signal. some versions also relied on delay only ( tcp vegas ) or use delay as well as loss ( pcc ). congestion control today wants to also achieve “ per - flow fairness ”. but each f... | EPFL COM 407 Moodle |
No question: Moodle | udp ). traditional tcp uses : • the window to control the amount of traffic : additive increase or cubic ( as long as no loss ) ; multiplicative decrease ( following a loss ). • loss as congestion signal. too much buffer is as bad as too little buffer — bufferbloat provokes large latency for interactive flows. • ecn ca... | EPFL COM 407 Moodle |
No question: Moodle | may follow different routes / paths - so, packets may be dropped, delayed or reordered recap : most important protocol = ip ( internet protocol ) recall ip : subnets and subnet masks • subnet < — a lan, i. e. a set of devices : • connected at the data - link layer • sharing the same ip - address prefix, e. g. : 128. 17... | EPFL COM 407 Moodle |
No question: Moodle | 3. ipv6 addresses 4. nats 5. host configuration 6. hop limit and ttl 7. arp ( connection with mac layer ) textbook chapter 5 : the network layer recall : goal of ip = interconnect all systems in the world using ip addresses network interface must have an ip address rule # 1 in detail : 1. assign addresses based on a st... | EPFL COM 407 Moodle |
No question: Moodle | to : b. d. h2 longest prefix match ( = closest matching table entry ) benefit : addresses can be aggregated, tables can be compressed to … outgoing interface b. * 2 a. * 0 to … outgoing interface a. * 1 b. d. * 2 b. * 3 r2 ’ s forwarding table to … outgoing interface a. * 1 b. d. * 1 b. c. * 0 to … outgoing interface b... | EPFL COM 407 Moodle |
No question: Moodle | ethernet concentrator c. there is no destination ip address in the packet since communication is inside the subnet and does not go through a router 1 go to web. speakup. info or download speakup app join room 46045 solution answer a the ip address is always present in the ip header if we use tcp / ip, even if communica... | EPFL COM 407 Moodle |
No question: Moodle | hex < — > decimal is best done by a calculator = 128 + 64 + 8 = 200 special cases to remember = = = = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 } 200 { 0, 1 } 1100 1000 { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,,,,,, } 1100 1100 15dec 1111 255dec the mask 255. 255. 254. 0 means that the subnet is made of the first … a. 16 bits b. 18 bits c.... | EPFL COM 407 Moodle |
No question: Moodle | solution answer c router ’ s north interface and h are in the same subnet so h must have a subnet prefix of 23 bits. we have : - router north ’ s subnet prefix : 2. 4. 8 / 23 = 0000 0010 0000 0100 0000 100 - so a is correct because : 2. 4. 8. 2 = 0000 0010 0000 0100 0000 1000 0000 0010 - b is also correct because : 2. ... | EPFL COM 407 Moodle |
No question: Moodle | to - ipv4 relay routers 169. 254. 0. 0 / 16 link local addresses ( can be used only by systems in same lan ) 224 / 4 multicast 240 / 4 reserved “ for experimental / future use ” until recently 255. 255. 255. 255 / 32 link local ( lan ) broadcast ipv4 packet format header 20 bytes ( + options, if any ) payload higher la... | EPFL COM 407 Moodle |
No question: Moodle | with larger addresses of : 128 bits ( unique addresses ) otherwise, it offers essentially the same services as ipv4 but ipv6 is incompatible with ipv4 ; routers and hosts must handle separately a can talk to w, b can talk to w, a and b cannot communicate at the network layer →≈4 ⋅109 ≈3 ⋅1038 application tcp ipv6 ipv4 ... | EPFL COM 407 Moodle |
No question: Moodle | ##f9 a few ipv6 global unicast addresses the block 2000 / 3 ( i. e. 2xxx and 3xxx ) is for global / public unicast addresses 2001 : 620 : : / 32 switch 2001 : 620 : 618 : : / 48 epfl 2001 : 620 : 8 : : / 48 ethz 2a02 : 1200 : : / 27 swisscom 2001 : 678 : : / 29 provider independent address 2001 : : / 32 teredo ( tunnel... | EPFL COM 407 Moodle |
No question: Moodle | multicast ( see ndp later ) ff02 : : 1 / 128 link local broadcast ff02 : : 2 / 128 multicast to all link - local routers ( in same lan ) epfl private ipv6 public and private prefixes are structured an epfl public address : 2001 : 620 : 618 : 1a6 : a00 : 20ff : fe78 : 30f9 an epfl private address : fd24 : ec43 : 12ca : ... | EPFL COM 407 Moodle |
No question: Moodle | 7 00 : 00 : c0 : b8 : c2 : 8d in - inr 128. 178. 156. 1 2001 : 620 : 618 : 1ad : : 1 00 : 00 : 0c : 02 : 78 : 36 128. 178. 79. 1 2001 : 620 : 618 : 1ab : : 1 00 : 00 : 0c : 17 : 32 : 96 ed2 - in 182. 1 2001 : 620 : 618 : 1ac : : 1 in - inj 128. 178. 182. 3 2001 : 620 : 618 : 1ac : : 5 182. 5 2001 : 620 : 618 : 1ac : : ... | EPFL COM 407 Moodle |
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