Publish/Subscribe Internetworking Transport Abstractions & Congestion Control Somaya Arianfar & Pasi Sarolahti 26.9.2011

Publish/Subscribe Internetworking Transport Abstractions & Congestion Control

Somaya Arianfar & Pasi Sarolahti26.9.2011

Publish/Subscribe InternetworkingTransport &Congestion Control

© Pasi Sarolahti 2011

Agenda (for the general transport part)

• Traditional transport abstractions– Reflections from how things are done today

• Rethinking the transport concepts (layering etc.)• Reliable Multicast Transport• Peer-to-Peer Transport• HTTP as a Pub/Sub Transport• Open Issues

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The OSI Layer Model

3

Application

Presentation

Session

Transport

Network

Data link

Physical

Program

Application

Presentation

Session

Transport

Network

Data link

Physical

Program Data

Data

Data

Data

Data

Data

Data

Data

DataData

DataD

ata

Data

DataD

ata

Data

DataD

ata

Data

DataD

ata

Data

DataD

ata

Data

Data

DataData

DataD

ata

DataD

ata

Data

DataData

Data



PURSUIT Functional Model

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API APIAPI

Current Internet Transport Protocols

Reliable Byte Stream

Unreliable Datagrams

Reliable Messages

TCP UDP SCTPDCCP

- Segmentation- Flow multiplexing - Retransmissions- Congestion ctrl

- Flow multiplexing

- Flow multiplexing- Cong. ctrl

- Flow multiplexing - Retransmissions- Congestion ctrl- Lots of features…- Extensible

Internet Protocol



IETF and the Transport Area

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Congestion control and related transport protocols

Interconnecting Content Distribution Networks

Network File System, network storage

Peer-to-peer communication

TCP extensions and maintenance

Reliable multicast transport

QoS, new transport protocols, traffic engineering

DCCP

LEDBAT CONEX

PCN

CDNI

NFSV4STORM

PPSP DECADE ALTO

TCPM MPTCP

RMT

TSVWG



Classical Example: TCP

• Service model: Reliable byte stream– No message boundaries

• Send(1024 bytes) may result in Receive(512 B) + Receive(512 B)

– One-to-one association

• Segmentation to packets– Depends on network path MTU

• Reliable: retransmissions based on acknowledgments and timer

• Congestion control: based on RTT measurements and packet loss events

• Much intelligence at sender, some intelligence at receiver– No intelligence in network

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Another Example: DCCP(Datagram Congestion Control Protocol)

• Service Model: Unreliable datagrams– Send(1024 B) results in Receive(1024 B) – or no Receive at all– One-to-one association

• Unreliable: no retransmissions• Congestion control: based on RTT measurements and

packet loss events– Requires acknowledgment mechanism– …and connection state at the sender

• Feature negotiation (e.g., for congestion control algorithm)• Much intelligence at sender, some intelligence at receiver

– Very different from UDP (for unreliable datagrams)

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Connection-oriented vs. Connectionless

• Two ends of the connection need to agree on– Protocol parameters

• e.g., type of acknowledgments, maximum segment size

– Shared “secrets”• e.g., TCP’s initial sequence number

• Reliable connection establishment method needed– Typically: three-way handshake

• Connection-oriented is easy with 1-to-1 connections– How to do it between multiple senders and receivers?

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Middleboxes and Connectivity

• NATs, Firewalls and other types of intermediaries have become common– These operate on packets’ transport headers (or deeper)– Typically support only the most common protocols

• TCP and UDP• One difficulty: pseudoheader checksum calculation and NATs

• Difficult challenge for deployment of new protocols– Unknown protocols often lost in transit

• Common trick is to encapsulate packets inside UDP• Additional problem: short lease times

– Idle periods lead to dropped connections

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Rethinking Layering

• Transport Layer– Communication abstractions,

reliability, etc.

• Flow Regulation Layer– Congestion control, etc.– Adaptability to special

algorithms and path conditions– Multipath communication

• Endpoint Layer– Logical endpoints (i.e., ports)– Extension of network layer

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B. Ford and J. Iyengar, “Breaking Up the Transport Logjam”In Proc. of ACM HotNets-VII, October 2008



Structured Streams

• Addresses some of TCP problems– Head-of-Line blocking– Integrated congestion control

• Channel Protocol– Congestion control– Security associations

• Stream Protocol– Hierarchical model

• E.g., “www” root stream distributed into streams of web pages and subobjects

– Stream is associated with channel

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B. Ford, “Structured Streams: a New Transport Abstraction”In Proc. of ACM SIGCOMM ‘07, August 2007



Host-Centric Networking vs.Content-Centric Networking• Host networking

– Resources are located based on topological address

• IP addresses• Routing is “easy”

– DNS is used to map host names to IP addresses

– Communication associations are between hosts

• E.g. TCP connections

– Security model usually based on hosts

• Content networking– Resources are located by

content-based name• Often flat, topology-

independent namespace• Routing is difficult• Sometimes host identities not

even known

– No host-based communication abstractions

• Content distribution, content migration becomes easier

– Security model based on content

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But…

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How do we implement transport for publish/subscribe networks?

Picture from: http://www.clker.com



Multicast Transport

• Special IP address range reserved for “channels”– UDP packets– Receivers join and leave channels using IGMP or MLD protocols– Source specific multicast limits the allowed sources of data

• Multicast not globally deployed– But in limited use e.g. in IPTV distribution

• Many channels do not need reliability– Real-time A/V streaming, etc.

• How to guarantee reliability in multicast session?– In a session of 100 subscribers, what if one receiver misses data?

• What is the feedback mechanism?• What to retransmit, when and where?

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Reliable Multicast Transport

• Designated receivers (RMTP)– Separate acknowledgments,

aggregated over tree of designated receivers

• Network coding– Add redundancy along data– For the price of overhead, can

re-construct individual packets

• Data Carousel– Repeated transmission of

same data

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A. Dimakis, K. Ramchandran, Y Wu and C. Suh,“A Survey on Network Codes for Distributed Storage”,In Computing Research Repository, April 2010.http://arxiv.org/abs/1004.4438

J.C. Lin and S. Paul,“RMTP: A Reliable Multicast Transport Protocol”,In Proc. of IEEE Infocom ‘96, March 1996.



Peer-to-Peer Transport (BitTorrent)

• Peers exchange 256 KB chunks– Controlled by tracker– Exchanged in “random” order

• Old version: on top of TCP– Each peer runs multiple TCP

connections– TCP congestion control

• New version: dedicated transport protocol– Delay-based congestion

control– Yields to other traffic

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tracker



Content-orientedness and TCP(Warning: research in progress…)

• Target: get some of the benefits of content-centric networking to TCP– mainly caching

• New TCP option– Content label – identifies

content in TCP payload

• Benefits:– Packet-based caching in

current infrastructure

• Challenges– Interactions with some TCP

details

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cachesenderreceiver



HTTP as a Pub/Sub Transport

• Much of the Internet traffic use HTTP– Accessible from most locations behind middleboxes– Lots of apps: real-time video, Email, version control, file storage

• Content-centric protocol methods– GET: fetch a resource from a server– PUT: push content to a server– HTTP chunking enables datagram-like service (e.g., for video)

• Pub/Sub could be implemented with variation of GET– S-GET: Remember the GET request at HTTP server (like

named pipe)

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L. Popa, A. Ghodsi, I. Stoica,“HTTP as the Narrow Waist of the Future Internet”In Proc. of ACM HotNets-IX, October 2010



Open(?) Issues with Publish/Subscribe Transport• How to know Maximum

Transmission Unit?– What is the packet size?

• When to retransmit (if retransmissions are needed)?

• Request per packet vs. request per “flow”?– How to do flow control?

• How to build feedback channel?

• Reliability? Buffering?

• Reliable transport on top of pub/sub, or pub/sub over reliable transport?

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Publish/Subscribe Internetworking Transport Abstractions & Congestion Control Somaya Arianfar & Pasi Sarolahti 26.9.2011