Scalable Robust Alerting Protocol (SCRAP)
draft-barnes-atoca-delivery-00.txt

Abstract

Emergency alerts need to be delivered reliably from one source to many recipients at once. TCP is unsuitable for this style of delivery, because the large number of acknowledgements would likely cause network congestion. This document defines a UDP-based protocol for delivering alerts that supports fragmentation and retransmission for reliability, and allows the sender of a datagram to control whether acknowledgements are sent.

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Status of this Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on April 26, 2012.

Copyright Notice

Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Open Questions
• 2. Definitions
• 3. Packet Format
• 4. URI Format
• 5. Sender Processing
• 6. Receiver Processing
• 7. IANA Considerations
• 8. Security Considerations
• 9. Acknowledgements
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Author's Address

1. Introduction

[TODO]

1.1. Open Questions

Should we include hash value in URI format? Or leave that to the metadata/configuration protocol?

Should we randomize the order in which fragments are transmitted in order to deal with correlated loss?

2. Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

3. Packet Format

[Sent over UDP. Default port is XXX.]

[Payload format: Alert ID (2 octets / 16 bits); Fragment number (1 octet / 8 bits); Number of fragments in alert (1 octet / 8 bits); Alert data (remaining octets)]

[Content of reassembled packets MUST be an ESCAPE-formatted alert]

4. URI Format

[Specifies the format/source of an alert that will be sent]

[alert-src:[host/IP]:[srcport]?:[dstport]]

5. Sender Processing

[Choose an alert ID]

[Divide payload into fragments that will fit within an MTU]

[Attach headers to fragments]

[Transmit fragments in order]

[Re-transmit the sequence to deal with loss:]

[Probability of receipt given loss (p=P(success)), #fragments (F), #retransmissions (R), : q = (1 - (1-p)^R)^F]

[Number of retransmissions given loss (1-p), resilience (q), #fragments (F): R = log(1-q^(1/F)) / log(1-p)]

6. Receiver Processing

[Maintain a set of alert buffers identified by alert ID (possibly an empty set)]

[Alert buffer contents: alert id; fragments to be received; list of fragment numbers; list of fragments]

[When an alert packet arrives...]

[If there is a buffer for its ID, add it to the buffer; if the buffer is complete, re-assemble, validate ESCAPE, and deliver alert to higher layer]

[If there is not a buffer for its ID, then allocate a new one and add the fragment.]

[If all fragments for an alert do not arrive within T1 milliseconds, discard the buffer; default T1=5000]

7. IANA Considerations

[Default port number]

8. Security Considerations

[This protocol provides no security protections; security provided by ESCAPE.]

[Main concern is DOS, mitigated by buffer timeouts; at worst, have to buffer 2^32 octets, if all buffers full for all alert IDs. MAY impose limits on buffer size / number of buffers active simultaneously. ]

9. Acknowledgements

[TODO]

10. References

10.1. Normative References

10.2. Informative References

Author's Address