Lecture 8: TCP Friendliness, DCCP, NATs,and STUNCongestion Control• TCP dynamically adapts its rate in response tocongestion• AIMD causes flows to converge to fair goodput• But how do losses (e.g., bit errors) affect goodput?• What about UDP?Chiu Jain Phase PlotsFlow A rate (bps)Flow B rate (bps)FairA=BEfficientA+B=Coverloadunderloadt1t2t3t4t5t6Responding to Loss• Set threshold tocwnd2• On timeout- Set cwnd to 1- Causes TCP to enter slow start• On triple duplicate ACK (Reno)- Set cwnd tocwnd2- Retransmit missing segment- Causes TCP to stay in congestion avoidanceAnalyzing TCP Simply• Assume all segments are MSS long• Assume a packet loss rate p• Assume a constant RTT• Assume p is small (no timeouts)Analysis• Window size W cuts toW2after a loss• Grows to W afterW2RTTs• Goodput =34· W · MTU ·1RTTWindow Size• p =1(W2+(W2+1)+...+W )• p ≈138W2• W ≈q83·p• Goodput =34·q83·p· MTU ·1RTT• Goodput =1.22·MTURTT ·√p• Constant factor changes based on delayed acks,etc.TCP Friendliness• Don’t want other protocols to disrupt TCP• UDP happily shuts down TCP flows• “TCP friendliness:” obeying TCP congestioncontrol as per prior goodput equation- Does not imply acting like TCP- E.g., does not require abrupt window changesDCCP• Datagram Congestion Control Protocol (DCCP)provides congestion control for unreliabledatagrams (RFC 4340)• Connection-oriented protocol- Request-response-ack establishment- Close-reset or CloseReq-Close-reset teardown• Counts packets, not bytesDCCP SegmentSequence Numbers• Every DCCP packet uses a new sequence number- Data- Acknowledgements- Control traffic• Acknowledgements are for last packet received- Not cumulative acknowledgements- Does not succinctly describe connection history- Options can give packet vectorsSynchronization• DCCP uses sequence number windows to protectfrom attacks• Large bursts of losses cause packets to fall pastwindows• Need to resynchronizeSynchronization ExchangeSynchronization on Reset ProblemSynchronization on Reset SolutionDCCP options• Data offset field ≥ generic header size• Optional header fields- Padding (0x00)- Mandatory (0x01), reset if not possible- Change/Confirm L/R (0x20, 0x21, 0x22, 0x23)- Ack VectorCongestion Control• Defines Congestion Control IDs (CCIDs)• Negotiated with change/confirm L/R options• Each half-connection can have differentcongestion control• CCID 2: TCP congestion control (AIMD) (RFC4941)• CCID 3: TCP-friendly congestion control (RFC4942)Ack Vector• 0: Received, 1: Received ECN, 2: Reserved, 3: Notyet received+--------+--------+--------+--------+--------+--------|0010011?| Length |SSLLLLLL|SSLLLLLL|SSLLLLLL| ...+--------+--------+--------+--------+--------+--------Type=38/39 \___________ Vector ___________...0 1 2 3 4 5 6 7+-+-+-+-+-+-+-+-+|Sta| Run Length|+-+-+-+-+-+-+-+-+CCID 2• Uses TCP congestion control- Maintains a cwnd, slow-start, etc.• Adds congestion control to acks- Sender specifies an AckRatio, R- Ratio of data to ack packets (TCP with delayed ACKs is 2)- On detecting ack losses, double R- AftercwndR2−Rlossless congestion windows, decrement RCCID 3• Uses TCP-friendly congestion control• Uses a sending rate, rather than a congestionwindow• Receiver sends feedback once per RTT, reportingloss rate• If sender hears no feedback, halves sending rate• Security issue with loss rate reporting: report lossintervals, rather than just a loss rate, verifiablewith ECN noncesDCCP Today• Numerous implementations• IETF Standards Track• Well suited to VoIP, Internet Gaming, etc.• Sees very little useNAT• Network Address TranslatorNAT(128.34.22.8)Client A(10.0.0.101)NAT(76.18.117.20)Client B(10.1.1.9)Session A-S10.0.0.101:123418.181.0.31:22Session B-S10.1.1.9:541118.181.0.31:22Server(18.181.0.31)Session B-S76.18.117.20:1000118.181.0.31:22Session A-S128.34.22.8:610118.181.0.31:22Motivations and Complications• There are only 232IP addresses• Firewalls for security• Breaks end-to-end (node does not know itsexternal IP)• Node might not even know if it’s behind a NAT• NAT needs to be able to dynamically assignmappingsTypes of NAT (RFC 3849)• Full Cone: no ingress filter (single local-externalmapping)• Restricted Cone: ingress filter on address• Port Restricted: ingress filter on address/port• Symmetric: different mappings for differentexternal destinations• Teminology is imperfect (static port mappings,etc.)How a NAT Works• Maps between global and local (IP,port) pairs• Requires knowledge of transport packet format• UDP datagram, TCP SYN- Can shut down TCP mapping on FIN+ACK- UDP requires timeouts (> 2 minutes, unless IANA saysotherwise)• RFC 4787/BCP 127 defines recommendedbehaviorsNAT Problems• Incoming TCP connections• E.g., Skypeclientssupernodesbootstrapsuper nodecallrelayTCP Through NATs• Server socket doesn’t initiate traffic: NAT can’t setup mapping• Rendezvous servers (as in Skype)• Connection reversal through rendezvous if onlyone is behind a NAT (rendezvous server asksun-NAT node to open a port so NAT node canconnect)TCP ReversalMore NAT Problems• Port mapping: 0-1023 should map to 0-1023• Port parity: even port → even port, odd port → oddport (RFC 3550: RTP uses even, RTCP uses odd))• Hairpinning: two nodes behind a NATcommunicate with external IPsSTUN (RFC 3849)• “Simple Traversal of User Datagram Protocol(UDP) Through Network Address Translators(NATs)”• Enables a node to- Determine if it is behind a NAT, and if so, what kind- Obtain a public IP address/port pair• Client-server protocol, requires no changes toNATs• STUN server coordinatesSTUN Binding Requests• Node sends BR to STUN server• STUN sends a response that has the address andport it sees- If different than node’s local address and port, it’s behind aNAT• Node can probe to see what kind of NAT- Ask STUN server to respond from different address: noresponse, address restricted; different response, symmetric- Ask STUN server to respond from different port: noresponse, port restricted• When does STUN not work?NAT Hole-Punching• Problem with STUN: doesn’t work when twonodes are behind same NAT• Two nodes A, B communicate with server S• A and B report their local IP address to server• Server tells the other the address/port pair (L, G)• A tries to send UDP packets to (LB, GB) using LA• B tries to send UDP packets to (LA, GA) using LBNAT
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