DigiComm II-1 TAPAS EB/IAB Meeting Newcastle, 5/9/02 Real inter-domain paths are unlikely to offer explicit SLA although limited local SLAs are avail. Instead, model/measure inter-domain implicit SLA Assuming Internet Exchanges are not under- provisioned, can expect SLA between 2 ISPs to be Worst of 2 (or in general, n) ISPs Net SLAs current: 90%ile delay, availability, NOT b/w though, some offer 90%ile loss too…

DigiComm II-2 Given Network SLA Above Transport Service, we still need an SLA Lets say it specifies Throughput, delay, delay jitter, residual loss Availability So the IP level has some packet loss probability – however this is NOT the important factor for the transport protocol in today’s network – the congestion level (provisioning) is – see Gibbens/Kelly/Macfadyen et al

DigiComm II-3 Transport SLA All transport protocols today are being designed to be “TCP friendly” – this means that their target throughput (transmission rate) is typically equation driven (this applies to multicast too): Rate = 2/3 * mtu / rtt * sqrt(p) Mtu = packet size Rtt = round trip time (can assume roughly 2*delay) P = packet loss probability Net is engineered for expected set of source rates (assuming TCP mix of mice &elephants, and long term balanced matrix of source/destination pairs) So for given bottlenecks, router incurs loss to cause share

DigiComm II-4 Loss Concealment Cost (this is not an option!) We have 2 transport techniques for loss concealment (whether random noise induced, or much more commonly, congestive packet loss – note that congestive loss doesn’t mean congestion (c.f. fast retransmit) _ only persistent loss does….occasional loss is a rate feedback mechanism Retransmission (TCP, PGM) Forward erasure/error correction (ALC, and sometimes PGM)

DigiComm II-5 Costs of Loss Concealment Retransmitted packet still has independent loss probability – hence expected mean delay for packets is (assuming binomial back-off for subsequent retransmits!): E(d_) = Sum over I to infinity of rtt*2^I * (1-p)^I Luckily for us, this converges for small p! Can compute delay variance similarly…. FEC has no delay penalty, but takes a fixed delay at source to add redundancy and takes a percentage hit from the network overhead of transmitting, so its nice if you want good delay properties E(txput) = 1 + p + epsilon

DigiComm II-6 Factors for API to transport TCP masks loss (almost for ever, at least 4 minutes!) PGM, or DCCP don’t (or SCTP) and have a well specified interface for signalling the inability to recover from loss…..and can allow this to be time bounded (I.e. to “fail” a transaction…for example) Some work we need to do: Look at distribution of rates and loss in multicast! Look at solving equations above for multicast and for slow start and for non-independent packet loss (take some measurements!) Maxemchuk’s protocol does better but need external clock synch

DigiComm II-7 Conclusions/Consequences Need distributed algorithm spec for delay and delay variance and residual loss acceptability Can then, given network SLA, chose a transport protocol and a configuration, and derive Transport SLA to meet this Has a cost Can be done on a pure best effort (but provisioned) network If network offers differentiation, can choose to trade-off IP SLA cost (e.g. $!) and Transport SLA costs (delay for loss concealment, or excess load from FEC) to meet Application requirements Of course if we had global deployment of ECN…

DigiComm II-8 Pragmatic TAPAS Choices No IP multicast No IP QoS Yes, provisioning (ie delay and loss %ile SLAs) Yes, new transport work Maybe consider partial ECN deployment…

DigiComm II-9 Cambridge TAPAS Research Topics Solve Equations above Relate to network calculus and to predicate routing or similar tools for solving for Transport API SLA for a set of users, on a set of ISPs Re-do for multicast (subject to selection of appropriate multicast transport!) Expose transport APIng SLAng to higher level Look at monitoring architecture and interface!