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No question: Moodle | b non encrypted macsec header ( sectag ) fcs encrypted 4 b 16 b campus network with access control servers wifi base station with access control functions bridge lisa bart bart, an epfl student, connects his pc to epfl wifi, which uses per - user access control, authentication and encryption. bart configures his pc to ... | EPFL COM 407 Moodle |
No question: Moodle | authentication data / key. so, when trying to verify the authenticity of lisa ’ s frames, the base station will see this mismatch, and it will discard the frames. ( = campus network with access control servers wifi base station with access control functions bridge lisa bart m1 m2 m3 1 bart, an epfl student, connects hi... | EPFL COM 407 Moodle |
No question: Moodle | is the nat that must connect to the epfl wifi network. since we assume that bart could connect to epfl, it means that the nat box succeeded to connect to epfl. in this case, bart can connect and lisa also. the epfl network sees bart's nat and thinks that bart and lisa are programs on bart's nat. an example where this o... | EPFL COM 407 Moodle |
No question: Moodle | of the exact match. • the table size and lookup time are prohibitive, if network becomes very large. • so, we prefer routers for large networks. • switches are plug - and - play devices, independent from ip numbering plans and whether ipv4 or ipv6 is used ( as opposed to routers ). • historically, routers were in softw... | EPFL COM 407 Moodle |
No question: Moodle | about them now ) c. interesting. ( i had limited hands - on experience with networks and i am happy to dive into these details ) d. other how do you find the labs so far? a. completely useless, i am losing my time b. somewhat useful, they help me catch up with things i already knew c. useful, they help me understand in... | EPFL COM 407 Moodle |
No question: Moodle | forwarding tables? - do they all use the same routing protocol / policies, even if they belong to different operators? igp, bgp • can we avoid congestion? - can tcp sources use all the available capacity, without suffering significant losses? - also, can we ensure that all sources take a “ fair share ” of the available... | EPFL COM 407 Moodle |
No question: Moodle | ##d. m3u ip multicast • recall : - unicast = source sends data to a unicast ip address, and a single destination receives it - multicast = source sends data to a multicast ip address, but multiple destinations receive it • multicast delivers packets to multiple recipients without sending multiple copies from the source... | EPFL COM 407 Moodle |
No question: Moodle | 255 ) - ipv6 : ff00 : : / 8, bits 13 - 16 determine the “ scope ” : ff02 / 16 = same subnet, ff05 / 16 = same domain • any source multicast ( asm ) group : - the group is identified by the multicast address ; - any source can send to this group • source specific multicast ( ssm ) group : - the group is identified by ( ... | EPFL COM 407 Moodle |
No question: Moodle | at r1 by using e. g. the multicast routing protocol pim - sm ; this results in a tree being built. data sent by s now reach a. 4. b joins the multicast address m. 5. r4 informs the rest of the network that m has a member at r4 ; the multicast routing protocol adds branches to the tree. data sent by s now reach both a a... | EPFL COM 407 Moodle |
No question: Moodle | is more complicated ; it uses one multicast router as rendez - vous point ( rp ) : destination routers create a tree from rp, using rpf ; router closest to source sends source packets to rp ; if there exists an interested receiver in the domain, rp creates a tree from source ( using rpf ) otherwise drops ; destinations... | EPFL COM 407 Moodle |
No question: Moodle | • bier routers rely on unicast routing in order to determine how to deliver a packet to the set of bfers indicated in the bier header - if several bfers are reached via different next - hops, bier routers replicate packet - then send only a single copy to each next hop for all bfers that are reached through this next h... | EPFL COM 407 Moodle |
No question: Moodle | r5 id = 5 ip dest = m to { 1, 3, 4 } 2 s ip header bier header ip dest = m to { bfers } group membership information is distributed by an external infrastructure called “ multicast flow overlay ”, for example : a special set of servers used to control the distribution of the video content, or a system to manage multica... | EPFL COM 407 Moodle |
No question: Moodle | 1, 3, 4 ) to r7. 2. from unicast routing, r7 finds that r3 and r4 are reached via the same next - hop ( r8 ) but r1 requires a different next - hop. therefore, r7 duplicates the packet and creates 2 packets, one with bier header ( 1 ), sent to r6, and one with bier header ( 3, 4 ), sent to r8. 3a. r6 forwards the packe... | EPFL COM 407 Moodle |
No question: Moodle | packet is forwarded to r3 ’ s west lan interface where b can receive it. bier : packet replication • for each destination bfer, bier router pre - computes ( based on the ip forwarding table ) a “ forwarding bit mask ” that indicates the set of destination bfers that are reached by the same next - hop. • to forward a pa... | EPFL COM 407 Moodle |
No question: Moodle | destination set. 3. now, so it breaks. s = s s1 = { 1, 2 } s ′ = s ∩s1 = { 1 } s ′ ′ = { 1, 2 } = { 3, 4 } s ′ ′ { 3, 4 } { 3, 4 } { 3, 4 } = ∅ r1 id = 1 r2 id = 2 r3 id = 3 r4 id = 4 r7 1 r6 r8 3a ip dest = m to { 1 } ip dest = m to { 3, 4 } ip dest = m to { 1, 3, 4 } 3b ip dest = m to { 3, 4 } 4b 4a ip dest = m to { ... | EPFL COM 407 Moodle |
No question: Moodle | ##tring = 00001 • set intersection becomes a bitwise and with mask • set difference becomes a bitwise and with the bit - inverted mask • more complicated mechanisms exist, if the # of bfers is large [ rfc 8279 ] bier index forwarding table at r7 dest. bfer forwarding bit mask next - hop 1 0 0011 r6 2 0 0011 r6 3 0 1100... | EPFL COM 407 Moodle |
No question: Moodle | ? r1 id = 1 r2 id = 2 r3 id = 3 r4 id = 4 r7 1 r6 r8 3a ip dest = m to { 1 } ip dest = m to { 3, 4 } ip dest = m to { 1, 3, 4 } 3b ip dest = m to { 3, 4 } 4b 4a ip dest = m to { 1 } ip dest = m to { 3 } 6b r5 id = 5 2 bier table contains static info, no per - flow state r1 id = 1 r2 id = 2 r3 id = 3 r4 id = 4 r7 1 r6 r... | EPFL COM 407 Moodle |
No question: Moodle | dynamic ) per - flow information cached by bier • bier forwarding table, called bit index forwarding table, is automatically derived from router ’ s unicast ip forwarding table. • inside a bier domain, multicast packets have an additional header and the ip destination address is not used ( tunneling ). multiple bier do... | EPFL COM 407 Moodle |
No question: Moodle | 33 - 33 - xx - xx - xx - xx ipv6 multicast ip dest address 229. 130. 54. 207 ip dest address ( hexa ) e5 - 82 - 36 - cf ip dest address ( bin ) … - 10000010 - … keep last 23 bits ( bin ) … - 00000010 - … keep last 23 bits ( hexa ) 02 - 36 - cf mac address 01 - 00 - 5e - 02 - 36 - cf 1st bit of hextet is 0 mac multicast... | EPFL COM 407 Moodle |
No question: Moodle | / or w / o bier makes attacks easier ( e. g. denial of service, witty worm ) mitigations : limit multicast rate and number of groups ; control which multicast group is allowed ( access lists ) ssm is safer as routers and destination can reject unwanted sources igmp / mld is not secure and has the same problems as arp /... | EPFL COM 407 Moodle |
No question: Moodle | i. i don ’ t know a. in order to send to a multicast group a system must first join the group with igmp or mld b. in order to receive from a multicast group a system must first join the group with igmp or mld c. a system can know whether a packet is multicast by analyzing the ip destination address. go to web. speakup.... | EPFL COM 407 Moodle |
No question: Moodle | only ingress routers ( such as r5 on the figure ) need to cache per - flow information. the destination mac address is … a. a group address derived from the last 23 bits of the ipv6 target address b. a group address derived from the last 24 bits of the ipv6 target address c. a group address derived from the last 32 bit... | EPFL COM 407 Moodle |
No question: Moodle | problem • bier uses a different forwarding principle, based on bistrings ( which represent sets of destinations ). the transport layer : tcp and udp machine b application transport network data link physical machine a application network data link physical transport series of links, switches routers, lans, … contents 1... | EPFL COM 407 Moodle |
No question: Moodle | . the role of a program that waits for requests to come e. the role of a program that allows users to access large amounts of resources f. none of the above g. i don ’ t know go to web. speakup. info or download speakup app join room 46045 solution answer d formally, a server is a role at the transport layer, where the... | EPFL COM 407 Moodle |
No question: Moodle | . the example shows a packet sent by the name resolver process at host a, to the domain name server ( dns ) process at host b. the udp header contains the source and destination ports. the destination port number is used to contact the name server process at b ; the source port is not used directly ; it will be used in... | EPFL COM 407 Moodle |
No question: Moodle | , then ip layer fragments it - at the ip layer of the source, info about fragments added inside ip header - not visible to the transport layer - if a fragment / piece is lost then the entire message is considered lost application layer should handle these, if necessary how is udp implemented in practice? via a socket l... | EPFL COM 407 Moodle |
No question: Moodle | udpclient < destaddr > bonjour les amis % %. / udpserv & % what socket to open? ipv4 or ipv6? • transport layer is not affected by the choice of ip ( no udpv6, nor tcpv6 ) • but, there are ipv4 and ipv6 sockets • an application program has to choose ipv4 or ipv6, or better support both how? use socket. getaddrinfo ( ) ... | EPFL COM 407 Moodle |
No question: Moodle | ##v6 socket can be dual - stack • in some machines, ipv6 sockets can be bound to both ipv6 and ipv4 addresses of the local host • how? the correspondents ’ ipv4 addresses are mapped to ipv6 addresses - using the ipv4 - mapped ipv6 address format - i. e., by appending the ipv4 address to prefix : : ffff : 0 : 0 / 96 • s... | EPFL COM 407 Moodle |
No question: Moodle | • socket 5 is bound to local address 2001 : baba : : b0b0 and port 32456 ; receives all data to 2001 : baba : : b0b0 udp port 32456 • socket 3 is bound to local address 11. 22. 33. 44 and port 32456 ; receives all data to 11. 22. 33. 44 udp port 32456 • socket 4 is bound to local address 11. 22. 33. 44 and port 32654 ;... | EPFL COM 407 Moodle |
No question: Moodle | id = 4 send / receive buffers port = 32456 port = 32654 application program udp ip ipv4 socket id = 5 ipv6 socket ipv6 packet ipv4 packets port = 32456 udp datagrams address = 11. 22. 33. 44 address = 2001 : baba : : b0b0 r s r s common send / receive buffers for 2 addresses recap - udp on the sending side : os sends t... | EPFL COM 407 Moodle |
No question: Moodle | reliable, in - order delivery what does this mean? tcp guarantees that all data is delivered in order and without loss, unless the connection is broken how does tcp achieve this? • uses sophisticated mechanisms to detect reordering and loss : - per - byte sequence numbers — > data is numbered - a connection - setup pha... | EPFL COM 407 Moodle |
No question: Moodle | • however, the maximum segment size in this example is 1000 bytes. so, tcp may also merge 2 blocks of data in one segment if this data happens to be available at the send buffer of the socket. • packets 3, 4 and 7 are lost. • b returns an acknowledgement in the ack field. the ack field is cumulative, so ack 8501 means ... | EPFL COM 407 Moodle |
No question: Moodle | ##00 1 2 3 4 5 6 7 8 9 deliver bytes 8001 : 8500 deliver bytes 8501 : 10000 deliver bytes 10001 : 10500 a b 10 tcpmaxdupacks set to 1 at a cumulative + selective ack 2 data blocks are merged, because here : mss = 1000 in addition to the ack field, most tcp implementations also use the sack field ( selective acknowledge... | EPFL COM 407 Moodle |
No question: Moodle | is sent. this is what the slide ’ s title means by “ optimized segmentation ”. • when receiving packet 8, b can deliver bytes 9001 : 10000 because packet 5 was received and kept in the receive buffer. tcp receiver uses a receive buffer = re - sequencing buffer to store incoming packets before delivering them to applica... | EPFL COM 407 Moodle |
No question: Moodle | window edge = smallest non - ack ’ ed sequence number upper window edge = lower _ edge window _ size + window usable part of the window, seq numbers that the sender can send note : here we assume that no more data exists in the socket ; so only segment 0 is sent, even though segments 0, 1, 2, 3 could be sent, too. wind... | EPFL COM 407 Moodle |
No question: Moodle | . [ 0 ; 3 ] s = 0 0 ; retransmission buffer s = 1 s = 2 s = 3 s = 0 s = 4 0 ; 1 0 ; 2 0 ; 2 ; 3 0 ; 0 ; resequencing buffer 1 1 ; 2 1 ; 2 ; 3 0 ; 1 ; 2 ; 3 deliver 0... 3 4 deliver 4 a = - 1, sack = 1 a = - 1, sack = 1 - 3 a = 3 a = - 1, sack = 1 - 2 4 ; a fixed - size ( static ) window cannot prevent receive - buffer ... | EPFL COM 407 Moodle |
No question: Moodle | ##coming lecture ] = 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 s = 1 ack = - 1, window = 2 s = 0 s = 2 s = 3 s = 4 ack = 0, window = 2 s = 5 s = 6 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 ack = 2, window = 4 0 1 2 3 4 5 6 7 8 9 10 11 12 ack = 0, window = 4 0... | EPFL COM 407 Moodle |
No question: Moodle | 10 11 12 s = 7 1 unit of data = 1000 bytes 1 packet = 1000 bytes receive buffer size = 4000 bytes 1 unit of data, 1 packet = 1000 bytes ack = 4, window = 2 s = 1 ack = - 1, window = 2 s = 0 s = 2 s = 3 s = 4 ack = 0, window = 2 s = 5 s = 6 ack = 2, window = 4 ack = 0, window = 4 ack = 6, window = 0 ack = 6, window = 4 ... | EPFL COM 407 Moodle |
No question: Moodle | 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3... | EPFL COM 407 Moodle |
No question: Moodle | ##1 sack 9001 : 10001 win 3000 251 : 400 ( 150 ) ack 10001 win 2500 10001 : 10500 ( 500 ) ack 401 win 14247 1 2 3 4 5 6 7 8 9 10 bytes... : 8500 are available and consumed bytes 8501 : 10000 are available a b app consumes bytes 8501 : 10000 ( 0 ) ack 10001 win 4000 11 retransmission after timeout the picture shows a sa... | EPFL COM 407 Moodle |
No question: Moodle | to read the data. later, the application reads ( and consumes ) the data 8501 : 10000. this frees some buffer space on the receiving side of b ; the window can now be increased to 4000. at line 10, b sends an empty tcp segment with the new value of the window. note that numbers on the figure are rounded for simplicity.... | EPFL COM 407 Moodle |
No question: Moodle | ≥ sender side : • data accumulates in send buffer until tcp decides to create a segment receiver side : • data accumulates in receive buffer until potential losses are repaired and data is put in order, so that application can consume it no boundaries between bytes : • several small messages written by a ’ s app may be... | EPFL COM 407 Moodle |
No question: Moodle | page download is delayed until the loss is repaired. head - of - the line blocking may also occur for a real - time streaming app and is probably even worse : with tcp, the loss of one packet delays all subsequent packets until the loss is repaired, whereas the live application might prefer to skip the lost packet and ... | EPFL COM 407 Moodle |
No question: Moodle | ack closed application active open passive open application close : active close fin _ wait _ 1 connection setup data transfer connection release recall : tcp connections involve only two hosts ( routers in between are not involved ) before transmitting useful data, tcp requires a connection setup phase : - used to agr... | EPFL COM 407 Moodle |
No question: Moodle | src port dest port sequence number ack number hlen window flags rsvd urgent pointer checksum segment data ( if any ) tcp header ( 20 bytes + options ) ip header ( 20 or 40 b + options ) < = mss bytes 32 bits indicates the next expected seq num from the other host tcp data tcp hdr ip data = tcp segment ip hdr prot = tcp... | EPFL COM 407 Moodle |
No question: Moodle | tcp to deliver the data immediately. if it is not set, tcp may pack together several pdus into one sdu. this is because of the streaming service of tcp : tcp accepts and delivers contiguous sets of bytes, without any “ message ”. the push bit is used by telnet after every end of line. • the urgent bit indicates that th... | EPFL COM 407 Moodle |
No question: Moodle | socket ( conn ) is created ; this will be used by the server to send / receive data. this example is simplistic : - client sends one message to server and quits ; - server responds to this single client - reality is different, see next slide 1 2 client s = socket. socket ( ) ; server s s1 = socket. socket ( ) s. connec... | EPFL COM 407 Moodle |
No question: Moodle | used for storing incoming syn packets ; • accept ( ) blocks until a syn packet is received for this local port number. it creates a new socket ( in pink ) and returns the file descriptor ( conn ) to be used to interact with this new socket • recv ( ) blocks until one block of data is ready to be consumed on this port n... | EPFL COM 407 Moodle |
No question: Moodle | ( ) ; conn. recv ( ) conn. close ( ) ; conn. recv ( ) conn. close ( ) ; conn. recv ( ) conn. close ( ) ; conn. recv ( ) conn. close ( ) ; conn. recv ( ) conn. close ( ) ; tcp sockets ( practical scenario ) how the operating system views tcp sockets? tcp ip r id = 3 id = 4 port = 32456 address = 128. 178. 151. 84 r id =... | EPFL COM 407 Moodle |
No question: Moodle | example : multicast ( tcp does not support multicast ip addresses ) why both tcp and udp? most applications use tcp rather than udp, as this avoids re - inventing error recovery in every application but some applications do not need error recovery in the way tcp does it ( i. e. by packet retransmission ) for example : ... | EPFL COM 407 Moodle |
No question: Moodle | ##mbps ; can we increase this? = > negotiate a scaling factor of the window during connection setup ( rfc1323 ) • how to detect packet loss? • how to avoid syn flooding ( denial of service attack )? • how to avoid the delay of the 3 - way handshake? [ we will see only the last three in detail ; see textbook section 4. ... | EPFL COM 407 Moodle |
No question: Moodle | : smoothed rtt rttvar : ℓ1 −error bound srtt = ( 1 −α ) ⋅srtt + α ⋅rtt rttvar = ( 1 −β ) ⋅rttvar + β ⋅ | srtt −rtt | rto = srtt + η ⋅rttvar α = 1 / 8, β = 1 / 4, η = 4 example 0 2 4 6 8 10 12 14 1 11 21 31 41 51 61 71 81 91 101 111 121 131 141 seconds seconds rto sampledrtt • initial value is at 6 ( default for linux )... | EPFL COM 407 Moodle |
No question: Moodle | ##mit principle : when duplicate acks are received, declare a loss ( recall : duplicate ack = a tcp segment where the ack value repeats a previously received ack value ) • tcpmaxdupacks is set by the operating system ( default value is 3 ) • which data is lost is inferred from the sack blocks n = p1 p2 p3 p4 ack = 1000... | EPFL COM 407 Moodle |
No question: Moodle | not detected, if ( forward or reverse ) channel is broken • isolated / single packets, that are lost, are not detected • last / trailing packets ( of a block of application data ), that are lost, may not be detected loss probe avoids the latter issue : when the probe timeout pto = min { rto, max { 2 * smoothedrtt, 10ms... | EPFL COM 407 Moodle |
No question: Moodle | configurable “ reordering ( time ) window ” syn cookies ( = specific initial seq numbers ) why? mitigate impact of syn flood attack : lots of bogus syn packets from ( spoofed ) source addresses sent to a server in detail : whenever a tcp server receives a syn packet, it keeps state about this connection ( source ip add... | EPFL COM 407 Moodle |
No question: Moodle | = y + 1 • server verifies that hash is valid ; if so creates socket, using the mss recovered from cookie • if syn was bogus, no ack follows and damage is reduced to the time for computing the hash, but server is always up ( syn flood is avoided! ) server does not implement syn cookies. if ack ( 3 ) is lost, server will... | EPFL COM 407 Moodle |
No question: Moodle | cases, the client application sends data when the syn ack is received. so even if ack ( 3 ) is lost, the next segment of data serves as an ack and the client will detect this loss. - some applications that do not have this property, e. g. ssh or mysql. - such applications would hang on the client side if ack ( 3 ) is l... | EPFL COM 407 Moodle |
No question: Moodle | someone that has connected before • server can send data already in syn - ack tfo tcp client tfo tcp server syn mac in tcp option get / hello. htm syn ack server data ack ack more data 5. secure transport • tcp has no security • needs to be complemented with a security layer such as transport layer security ( tls ) ( t... | EPFL COM 407 Moodle |
No question: Moodle | for encryption, the other for decryption. one of the keys must be secret ( private ), the other one is public. the private key cannot be computed from the public key other than by brute - force ( in principle ). this eases the problem of key distribution but encryption / decryption is computationally intensive. in all ... | EPFL COM 407 Moodle |
No question: Moodle | tag a digital certificate for bekb contains … a. the public key of bekb b. the identity of bekb ( official name as of registre du commerce ) c. the public key of the issuing ca d. a and b e. a and c f. b and c g. all h. none i. i do not know go to web. speakup. info or download speakup app join room 46045 solution answ... | EPFL COM 407 Moodle |
No question: Moodle | sockets ) are transformed / “ wrapped ” sockets. 1 2 client s = socket. socket ( ) ; server s ss. connect ( s, 5003 ) ss. send ( … ) ss. close ( ) s1. listen ( ) ss = ssl. wrap _ socket ( s, ca _ cert ) create context object with certificate and keys s1 = socket. socket ( ) s1. bind ( 5003 ) sconn = context. wrap _ soc... | EPFL COM 407 Moodle |
No question: Moodle | a tls socket sconn. this requires the server to provide its pair of keys and its certificate. the client can send data only when the handshake is completed ( at point 3 ). with https / tls1. 3, how many rtts are required before data transfer can occur? a. 1 b. 2 c. 3 d. 4 e. 0 f. i do not know go to web. speakup. info ... | EPFL COM 407 Moodle |
No question: Moodle | higher layers? ” • “ and also, the structure of the course is very messy. goes from ip layer to mac layer to ipv4 to ipv6 to ip layer again like uehh ” • “ i think that the course material lack of structure, informations is thrown in a sense i don't understand. ” • “ quizzes don't explain the solutions or no hint, i do... | EPFL COM 407 Moodle |
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