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Versions: 00
Network Working Group B. Frank
Internet-Draft Tridium, Inc
Intended status: Standards Track September 11, 2009
Expires: March 15, 2010
Chopan - Compressed HTTP Over PANs
draft-frank-6lowapp-chopan-00.txt
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Abstract
This document describes a method for compressing HTTP messages into a
binary format to be transmitted using UDP over 6LoWPAN wireless
networks.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3
1.2. Security Considerations . . . . . . . . . . . . . . . . . 3
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Datagram Format . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Format Notation . . . . . . . . . . . . . . . . . . . . . 5
2.2. Request Format . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Response Format . . . . . . . . . . . . . . . . . . . . . 6
2.4. Compressed Headers . . . . . . . . . . . . . . . . . . . . 7
2.5. Mime Type Codes . . . . . . . . . . . . . . . . . . . . . 9
2.6. Example . . . . . . . . . . . . . . . . . . . . . . . . . 9
3. UDP Transmission . . . . . . . . . . . . . . . . . . . . . . . 11
4. Transaction-Id . . . . . . . . . . . . . . . . . . . . . . . . 12
5. Caching . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Cache Control . . . . . . . . . . . . . . . . . . . . . . 13
5.2. ETag Validation . . . . . . . . . . . . . . . . . . . . . 14
5.3. Interception Proxy Caching . . . . . . . . . . . . . . . . 15
5.4. Sleeping Nodes . . . . . . . . . . . . . . . . . . . . . . 16
5.5. Cache Refresh . . . . . . . . . . . . . . . . . . . . . . 17
5.6. Caching non-GET Methods . . . . . . . . . . . . . . . . . 18
6. HTTP to Chopan Gateways . . . . . . . . . . . . . . . . . . . 20
7. Security . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
8. Normative References . . . . . . . . . . . . . . . . . . . . . 23
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 24
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1. Introduction
The Pervasive Internet is a vision that everyday devices with
microprocessors are woven into the fabric of the Internet. One of
the critical emerging technologies in this domain is 6LoWPAN which
enables low cost, low power devices to communicate using the Internet
Protocol. 6LoWPAN is the first step towards building the Pervasive
Internet. Chopan defines the next step: integrating 6LoWPAN devices
with the World Wide Web to leverage the massive investment in
existing URI and HTTP infrastructure.
Chopan is derived from HTTP with these changes:
o UDP: utilizes UDP packets instead of TCP as the underlying
transport protocol
o Binary compression: HTTP headers are compressed into a binary
format to save bandwidth and buffer space
o Interception Caches: transparent caching is used to minimize PAN
traffic and manage sleeping nodes
o Gateways: may be used to translate between full HTTP and Chopan to
interoperate with the existing Web infrastructure
1.1. Requirements notation
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 [RFC2119].
1.2. Security Considerations
Discussed in Section 7.
1.3. Terminology
6LoWPAN: IPv6 for Low power Personal Area Networks described in
[RFC4944].
Compression: translation from of a TCP/HTTP text based message into a
compressed binary UDP/Chopan message (gateway functionality).
Decompression: translation from of a binary UDP/Chopan message into a
TCP/HTTP text based message (gateway functionality).
Gateway: a node which transparently translates between HTTP and
Chopan messages.
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HTTP: Hyper Text Transfer Protocol described in [RFC2616].
PAN: Personal Area Network - an IP sub-network with constrained
bandwidth and/or constrained computing devices. This specification
is designed for low power PANs running 6LoWPAN, but Chopan is an
ideal solution for any network with bandwidth or computing
restraints.
Interception Proxy Cache: a node which transparently intercepts HTTP
requests to an origin server and returns cached responses on its
behalf.
Origin Server: the server on which the master version of resource
resides.
Resource: an abstract unit of information identified with a URI and
transported over a network using a MIME typed representation.
Sleeping Nodes: battery powered network nodes which spend most of
their time in a hibernation state to converse power.
TCP: Transmission Control Protocol described in [RFC0793].
UDP: User Datagram Protocol described in [RFC0768].
UTF-8: Encoding of Unicode characters compatible with ASCII described
in [RFC2279]
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2. Datagram Format
Chopan uses a customized binary encoding for HTTP requests and
responses to achieve message compression into a UDP packet. A two
byte magic number is used to identify the packet as a Chopan message
- "h6" for requests and "H6" for responses. Both requests and
responses allow for zero or more compressed headers.
Any bytes after the headers in the packet are considered the message-
body. The length of the message-body is implied by the packet length
(the Content-Length header MAY be omitted). The entire message MUST
fit with in a single UDP packet. When running over 6LoWPAN, messages
SHOULD fit into a single 802.15.4 frame to avoid fragmentation.
2.1. Format Notation
Message formats are described as a data structure using the following
primitive types:
o u1: an unsigned 8-bit byte
o u2: an unsigned 16-bit integer in network byte order
o str: UTF-8 [RFC2279] encoded text, followed by a null terminator
(0x00) byte
o x[]: an sequence of type x which contains zero or more occurrences
o x|y: either x OR y
2.2. Request Format
A normal HTTP request is composed of a request-line, a set of
request-headers, and the message-body. This information is
compressed in the following binary format:
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request
{
u2 magic 0x6836 - ASCII "h6"
method method
str uri
header[] headers
u1 zero byte end of headers
u1[] message-body
}
method
{
u1 method-code
utf optional string value only if method-code is 0x80
}
The HTTP request-line contains three pieces of information: the
method, URI, and version. The URI is encoded as a null-terminated
UTF-8 string. Standard request methods are encoded into a byte as
follows:
Method Code ASCII Char
------------ ---- ----------
DELETE 0x44 D
GET 0x47 G
HEAD 0x48 H
OPTIONS 0x4F O
POST 0x50 P
PUT (Update) 0x55 U
TRACE 0x54 T
str value 0x80 -
Most standard methods are encoded into a single byte, for example
"GET" is encoded into the ASCII byte 'G'. If the method code is
0x80, then it is followed by a null terminated UTF-8 string.
2.3. Response Format
A normal HTTP response is composed of a status-line, response-
headers, and the message-body. This information is compressed in the
following binary format:
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response
{
u2 magic - ASCII "H6"
u1 status-code
header[] headers
u1 zero byte end of headers
u1[] message-body
}
The HTTP status code is compressed into a single byte where the top
3-bits represent the 100s decimal digit, and the bottom 5-bits
represent the last two decimal digits. Example of binary mappings:
1xx -> 0x2X, b001x_xxxx
2xx -> 0x4X, b010x_xxxx
3xx -> 0x6X, b011x_xxxx
4xx -> 0x8X, b100x_xxxx
5xx -> 0xAX, b101x_xxxx
200 -> 0x40 // OK
404 -> 0x84 // Not Found
415 -> 0x4F // Unsupported Media Type
416 -> 0x50 // Requested range not satisfiable
417 -> 0x51 // Expectation Failed
2.4. Compressed Headers
Standardized HTTP request and response headers are compressed using
predefined binary codes. Compressed headers are encoded as follows:
header
{
u1 header-code (high bit determines encoding of value)
u2|str value (u2 or str based on header-code high bit)
}
Headers are encoded using a 8-bit header-code which represents the
header name. If the high bit (0x80) is clear in the header-code,
then the value is encoded as an unsigned 16-bit integer. If the high
bit is set, then the value is encoded as a null terminated UTF-8
string. The u2 value encoding allows compression on a header-by-
header basis. Refer to the table below how each header utilizes a u2
value.
If an HTTP header name does not have a standard binary encoding, then
it MAY be stripped at the proxy gateway, otherwise it can be passed
using its string name. Uncompressed header names are encoded as
follows:
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uncompressed-header
{
u1 header-code is 0x7f (u2 val) or 0xff (str val)
str name encoded as null terminated string
u2|str value
}
The following table defines the header codes for standard HTTP
headers. Each code has the high bit clear indicating a u2 value.
Mask the code with 0x80 to obtain the str value code:
Header Code Notes
------------------ ---- -------------------------------------------
End-Of-Headers 0x00 zero indicates no more headers
Uncompressed 0x7F name string, u2/string value
Accept 0x01 u2 val: mime type code
Accept-Charset 0x02
Accept-Encoding 0x03
Accept-Language 0x04
Accept-Ranges 0x05
Age 0x06 u2 val: delta age in seconds
Allow 0x07
Authorization 0x08
Awake-Time 0x09 u2 val: seconds, used with check-in request
Cache-Control 0x0A u2 val: max-age in seconds
Connection 0x0B unsupported
Content-Encoding 0x0C
Content-Language 0x0D
Content-Length 0x0E u2 val: bytes; omit to imply by packet size
Content-Location 0x0F
Content-MD5 0x10
Content-Type 0x11 u2 val: mime type code
Cookie 0x12
Date 0x13
ETag 0x14 u2 val: etag is 4 digit upper case hex str
Expect 0x15 u2 val: uncompressed code 100 is 0x64
Expires 0x16 should be avoided (use max-age)
From 0x17
Host 0x18
If-Match 0x19 u2 val: etag is 4 digit upper case hex str
If-Modified-Since 0x1A should be avoided (use max-age)
If-None-Match 0x1B u2 val: etag is 4 digit upper case hex str
If-Range 0x1C
If-Unmodified-Since 0x1D should be avoided (use max-age)
Last-Modified 0x1E should be avoided (use age, max-age)
Location 0x1F
Max-Forwards 0x20 u2 val: number of hops
Pragma 0x21 obsolete
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Proxy-Authenticate 0x22
Proxy-Authorization 0x23
Range 0x24
Referer 0x25
Retry-After 0x26 u2 val: seconds, used with 202 responses
Server 0x27
Set-Cookie 0x28
Sleep-Time 0X29 u2 val: seconds, used for check-in requests
TE 0x2A
Transaction-Id 0x2B u2 val: same as 4 digit upper case hex str
Trailer 0x2C unsupported
Transfer-Encoding 0x2D
Upgrade 0x2E
User-Agent 0x2F
Vary 0x30
Via 0x31
Warning 0x32 u2 val: uncompressed code 111 is 0x6F
WWW-Authenticate 0x33
2.5. Mime Type Codes
The Accept and Content-Type headers may be compressed into an
unsigned 16-bit type code using the following table:
Mime Type Code Notes
------------------------ ------ -------------------------------
application/octet-stream 0xA001 used for arbitrary binary files
text/plain 0xB001 charset implied to be UTF-8
text/html 0xB002 charset implied to be UTF-8
text/xml 0xB003 charset implied to be UTF-8
text/csv 0xB004 charset implied to be UTF-8
NOTE: we also need to give thought to what kind of information models
we use and how they are represented with existing or new MIME types.
For example we might want to use ASN.1 MIBs, binary oBIX, etc...
2.6. Example
Assume the following HTTP request:
GET /pt07 HTTP/1.1
Host: sensor2086.acme.com
Accept: text/plain
If-None-Match: "3A7F"
Cache-Control: max-age=900
The HTTP request above would be compressed into the following
sequence of hexadecimal bytes:
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68 36 47 2F 70 74 30 37 00 01 B0 01 1B 3A 7F 0A 03 84 00
^ ^ ^ ^ ^ ^ ^
| | | | | | +- End
| | +- URI | | +- Cache-Control
| +- GET | +- If-None-Match
+- magic +- Accept
Note that we stripped the Host header and compressed Accept, If-None-
Match, and Cache-Control into two byte header values.
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3. UDP Transmission
One of the primary characteristics of Chopan is the ability to
transmit HTTP requests and responses over UDP. Since HTTP was
originally designed to be run over TCP, we must make some design
trade-offs to layer the protocol over an unreliable packet based
transport.
Chopin follows the standard HTTP request/response model. A client
makes a Chopan request to a server with a request message. When the
server receives the request, it sends the client back a response
message.
Both the request and response messages MUST fit within one UDP
packet, as such large message bodies are not supported. However, the
Range header may be used to chunk the transfer of resources which do
not fit a single UDP packet. When running over 6LoWPAN, messages
SHOULD fit into a single 802.15.4 frame to avoid fragmentation.
Because UDP is unreliable, there is no guarantee that a server
receives a request, nor that a client receives the response. If a
client does not receive a response to its request after a reasonable
amount of time, then it SHOULD retry the request up to three times
before timing out. It is therefore possible that the server might
receive the same request multiple times. A request is "retry safe"
if it can be retried multiple times by the client without
compromising server state. Idempotent methods like GET and HEAD MUST
be retry safe. Methods such as PUT and DELETE should also be retry
safe since they atomically modify or delete the resource. Methods
like POST are typically not retry safe unless coupled with another
mechanism. In the next section we examine an extension to HTTP for
making requests retry safe with the Transaction-Id header.
UDP does not guarantee message order. Therefore, it is the client's
responsibility to impose message ordering if required. Message
ordering can be maintained by waiting for a response, before sending
the next request. When message ordering is not required, the client
MAY have multiple simultaneous outstanding requests. This can
increase throughput on networks with high latency. If performing
concurrent requests, clients SHOULD use the Transaction-Id header to
match responses to requests.
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4. Transaction-Id
Due to the unreliable nature of UDP, requests and responses do not
have guaranteed delivery or ordering. This can particularly cause
problems when a non-idempotent request is received successfully by
the server, but the response packet is dropped. In this case the
client's expected behavior is to retry the request which might cause
the server to receive the same request multiple times. For methods
such as POST which are not implicitly retry-safe, we define a new
header called Transaction-Id.
Transaction-Id is a unique identifier generated by the client. The
tuple of the client's IP address, port number, and Transaction-Id
should be globally unique within the transaction's temporal window.
Any retries initiated by the client MUST include the same transaction
id in the retry requests.
When a server receives a request with a Transaction-Id header, it
MUST pass the identifier back to the client via the response's
Transaction-Id header. The server MAY also choose to utilize the
Transaction-Id to implement "at-most-once" semantics. It is a server
local matter to decide how to apply the transaction id for a given
HTTP method and resource.
If a client attempts to request a method on the resource which
requires a Transaction-Id header and fails to specify one, then the
server SHOULD respond with 400 Bad Request.
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5. Caching
6LoWPAN networks are typified by a gateway device which acts as a
router or bridge between the PAN and the external IP network. Often
the external IP network is physically connected by high a bandwidth
technology such as Ethernet or WiFi. The PAN itself typically has
low bandwidth and is composed of resource constrained nodes. Often
times the nodes in a PAN are battery powered, and spend most of their
time sleeping.
Because of this physical architecture, it is desirable for the more
capable nodes in the PAN to serve as caches for the more constrained
devices. Effective use of caching enables Chopan to optimize both
bandwidth on the PAN and power on constrained devices. In the case
of a sleeping node, it allows proxies to immediately return cached
representations of resources.
5.1. Cache Control
HTTP [RFC2616] defines a sophisticated caching model in sections 13
and 14.9. This model has multiple caching features, often with
overlapping functionality. Since Chopan is targeted for resource
constrained devices, this specification recommends use of a subset of
the HTTP caching model based on resource age and max-age.
It is expected that most resources accessed by Chopan are
representations of sensor data. The nature of the sensor data
determines its cache life. For example a temperature sensor in a
room is likely to change very slowly, so it might have a cache life
of fifteen minutes. But a temperature sensor in an oven might have a
cache life of only ten seconds before it is considered stale data.
Chopan uses existing HTTP caching features to give both the client
and server a say in cache management.
When an origin server publishes a resource representation via a GET
request, it SHOULD specify the Age header. For example if a resource
represents a sensor, and that sensor was read 4 seconds ago, then the
Age header should be set to 4 seconds. If the resource has an age
less than 1 second, then set the Age header to 0. The Age header
SHOULD be compressed into a two byte value if less than 18.2 hours.
In cases when the origin server has knowledge about the cache life of
a given resource, it SHOULD set the Cache-Control header with a Max-
Age directive. Note that the two byte value encoding of Cache-
Control is implied to be Max-Age as a number of seconds. When the
server specifies Max-Age, it is directing upstream proxies and
clients how long to cache the resource. For example if a resource
specifies an Age of 4 seconds, and a Max-Age of 30 seconds, then the
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resource should be cached for 26 seconds before it is considered
stale.
Clients MAY also specify the Cache-Control header with a Max-Age
directive on requests. In this case, the client is directing the
maximum amount of staleness which may be tolerated. For example if a
client requests a resource with a Max-Age of 10 seconds, and the
resource has an age of 8 seconds, then the server may respond with
the cached resource. If however the resource has an age older then
10 seconds, then the server should refresh its cache. In the case of
a proxy cache, this means contacting the origin server. In the case
of the origin server, it may require polling the sensor.
A resource is considered stale if its Age is greater than either Max-
Age specified by the server or the Max-Age specified by the client.
If a server node has a cached version of a resource which is stale,
it SHOULD always attempt to refresh its cache. If the cache cannot
be refreshed immediately because of normal operation (for example the
origin server is a sleeping node), then the stale resource should be
returned and the Warning header SHOULD be specified with the 110
status code (response is stale). If cache refresh fails abnormally
(for example the origin server cannot be contacted), then the stale
resource SHOULD be returned and the Warning header specified with the
111 status code (revalidation failed).
5.2. ETag Validation
Key to any caching strategy is cache validation, the mechanism used
by a client or proxy to refresh its cache with the origin server.
Often even though a cached resource has expired, the original
resource hasn't been modified. But in order to avoid re-transmitting
the entire resource the client and server must define a mechanism to
validate the cached copy. In HTTP this validation may be negotiated
using either timestamps or entity tags. Chopen discourages the use
of timestamps because often nodes do not support time clocks.
Instead entity tags SHOULD be used for cache validation.
An entity tag is an opaque hash of a given resource's version. It is
defined by the origin server using the ETag header. If possible, a
two byte etag should be used to allow for optimal compression. If an
etag was specified for a cached resource, then clients and proxies
SHOULD specify the If-None-Match header on cache refresh. The server
SHOULD respond with a 304 (Not Modified) response if the etag has
been not modified.
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5.3. Interception Proxy Caching
In Chopan, caching is done transparently to the client via
"interception caching". Interception caching is a commonly used
technique used to insert HTTP caches between clients and origin
servers, without requiring client configuration. Clients send
packets to the origin server directly, but as these packets are
routed into the PAN, one of the routing nodes processes the message
directly on behalf of the origin server. This architecture requires
that routing nodes in the PAN are actively examining the packets
before they are routed to their destination address.
The downside to using interception caching, is that technically it
breaks the encapsulation of the IP stack - routing nodes must become
aware of an application level protocol. The upside to this design,
is that client nodes do not have be explicitly configured to know
about the proxies for every PAN. Since PANs have the potential to
add billions of new nodes to the Internet, it seems reasonable to
trade-off the purity of IP routing within the PAN to maintain the
simplicity of the Internet at large.
Interception caches SHOULD use a combination of the destination port
and the packet's magic two byte marker to sniff Chopan packets. By
default we assume Chopan runs on UDP port 80, although proxies SHOULD
make this configurable.
The lifecycle of an interception cache request:
1. The client sends a request to the origin server
2. The interception proxy traps the request
3. If the request can be immediately fulfilled by a cached
representation of that resource in the proxy, then the proxy
responds directly to client on behalf of the origin server using
the origin server's IP address
4. If the proxy has no cached representation of the resource (or the
cache has expired), then it makes its own request to the origin
server for the resource to update its cache, then performs step 3
to return the cached resource to the client
5. Cache might also be actively refreshed periodically (see Cache
Refresh)
This lifecycle assumes that the origin server is a powered device
which is awake during normal operation. If the origin server is a
battery powered device then the origin server is mostly likely
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sleeping. This use case is discussed further in Sleeping Nodes
section.
Interception proxies SHOULD be transparent to the client. However,
when a proxy communicates directly with the origin server it has a
choice to forward the client's original packet (with the client's IP
address), or to initiate a new request (with the proxy's IP address).
Proxy's SHOULD initiate new requests using the proxy's own IP
address. This means that origin servers are effectively responding
directly to the proxy with no knowledge of the original client
request. The disadvantage of this model is that it breaks end-to-end
communication principles of the Internet. However this model
provides significant advantages:
o On 6LoWPAN it keeps IP addressing to intra-PAN nodes which results
in better compression (since we don't need to pass through the
external IP address);
o It ensures that the response gets routed directly back to the
proxy for caching;
o Gateways which are translating TCP/HTTP into UDP/Chopan do not
have UDP packets from the client to begin with (rather they are
translating from a TCP stream)
o Sleeping nodes which require active cache refresh must be polled
directly by the proxy
5.4. Sleeping Nodes
PANs commonly include battery powered nodes which spend most of their
time sleeping to conserve power. These nodes periodically wake up to
check sensors, perform computation, and catch up on network
communications. Because of their nature, sleeping nodes do not make
for reliable origin servers. Chopan handles this use case by
fronting all sleeping nodes with interception caches. This allows
all requests for resources on the sleeping nodes to be transparently
brokered by proxies. Proxies then synchronize their caches with the
sleeping nodes periodically during a "check-in" process.
The lifecycle for interception caching of sleeping nodes follows the
standard interception model detailed above. However, when a request
is made for a resource the proxy doesn't have in its cache, the
request cannot be immediately fulfilled. In this case the proxy
SHOULD return a 202 Accepted response indicating that background
processing is required before the request can be completed (waiting
for the sleeping node to wake up). The Retry-After header SHOULD be
set indicating the number of seconds before the request should be
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tried again. The retry time should be based on the time it will take
the sleeping node to wake up, check-in, and give the proxy a chance
to refresh its cache. The Retry-After header can be estimated from
the Awake-Time and Sleep-Time headers (see below).
Sleeping nodes MUST be configured to check-in with their proxy or
proxies when they wake up. This is done by sending a POST request to
the "/ci" URI of each proxy. When a proxy node receives a check-in
request, it SHOULD respond with 200 OK response. The sleeping node
SHOULD use standard retry/timeout mechanism to ensure that the
check-in is received by the proxy. After the sleeping node has
checked-in, then the proxy SHOULD poll for all the resources in its
cache which require refreshing. This will include all new pending
resources which resulted in 202 responses. After the sleeping node
has given the proxy a chance to refresh its cache, it can go back to
sleep.
Sleeping nodes SHOULD specify the Awake-Time and Sleep-Time headers
in their check-in request. The Awake-Time header specifies how long
the node expects to stay awake to give the proxy a chance for cache
refresh. The Sleep-Time indicates how long the node expects to sleep
before the next check-in. A proxy should expect the next check-in
after the sum of Awake-Time and Sleep-Time has elapsed - this period
can then be used for estimating the proxy's Retry-After header.
5.5. Cache Refresh
Chopan proxies can take an active or a passive approach to cache
refresh. In a passive model, stale resources are allowed to expire
and are eventually flushed from the cache. New requests for the
resources are forwarded to the origin server, and the response is
used to refresh the cache. On the other hand, the proxy can actively
poll origin servers to refresh cached resources independent of client
requests.
For sleeping nodes, proxies MUST actively refresh their cache. This
is required because there are only limited windows of opportunity
while the node is awake for the proxy to refresh resources.
When the origin server is a powered node, either active or passive
cache refresh may be used. Using active refresh to proactively keep
caches refreshed can potentially decrease the latency of external
requests.
Cached resources can be in one of the following states:
o Pending: these are resources on sleeping nodes which have resulted
in a 202 response. Eventually we expect to poll the node on
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check-in and turn them into fresh resources or invalid resources.
o Fresh: these are resources with an Age less than both the client's
and server's configured Max-Age.
o Stale: these are resources with an Age which exceeds either the
client's or server's Max-Age. The proxy may continue to maintain
stale resources in the cache for some period of time.
o Flushed: resources may be flushed from a cache at any time.
Normally stale resources are flushed after a timeout period.
However LRU caches may flush fresh resources if buffer space is
exceeded.
o Invalid: some proxies may maintain a cached representation of a
resource to indicate an error condition. This is helpful when a
proxy receives a request for a sleeping node and returns 202, then
after the check-in discovers the origin server returns 404 for the
resource. In this case the proxy SHOULD temporarily cache an
error return so that the client's next poll will receive a 404
instead of another 202.
5.6. Caching non-GET Methods
In most circumstances, clients make GET requests to retrieve
representations of resources. In this case, proxies are caching the
response which contains that resource representation. However
clients may also perform POST, PUT, or DELETE requests. In the case
where the origin server is a powered node, these requests SHOULD
always be immediately forwarded to the origin server.
However in the case of sleeping nodes, the proxy MUST cache the
request itself until the node wakes up and checks-in. Without this
functionality it would be impossible to perform these HTTP methods on
sleeping nodes. Non-GET methods to sleeping nodes MUST use a
Transaction-Id to associate the request with a specified client IP
address, port number, and transaction id.
Let's consider a transaction for a resource POST on a sleeping node:
1. Client POSTs to origin server with a unique transaction id
2. Proxy transparently intercepts the request, caches it, and
returns 202
3. Upon check-in the proxy forwards the request then caches the
response
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4. Client waits for Retry-After, then resubmits POST request using
same transaction id
5. Proxy transparently intercepts the request and returns cached
response with the transaction id
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6. HTTP to Chopan Gateways
Chopan leverages the HTTP standard in order to provide
interoperability with the World Wide Web. Interoperability is
achieved by using standard HTTP external to the PAN and using Chopan
internal to the PAN. Nodes which perform HTTP-Chopan translation are
called Chopan gateways:
o Requests into the PAN are translated from HTTP to Chopan.
o Requests from inside the PAN to the external network are
translated from Chopan to HTTP.
o Requests from inside the PAN to other nodes inside the PAN are
Chopan end-to-end
Diagram of gateway translations:
<= External | PAN =>
Client -> [HTTP] -> Gateway -> [Chopan] -> Server
Server <- [HTTP] <- Gateway <- [Chopan] <- Client
Gateway translations SHOULD be performed transparently. Clients
external the PAN assume they are communicating HTTP directly to the
origin server. Gateways intercept these HTTP requests and translate
them into Chopan requests. Likewise responses are translated from
Chopan back to HTTP.
Because Chopan recommends that translation happens transparently,
this means that the gateway must be sniffing incoming packets for
TCP/HTTP requests. This design has all the same issues as detailed
in Interception Proxy Caching. It is expected that in most
implementations the gateway will also perform interception caching,
although this specification does not require it.
HTTP to Chopan is referred to as compression, and Chopan to HTTP is
referred to as decompression. During the compression process the
text format of requests and responses is encoded into Chopan's binary
message format. Each HTTP header is examined and mapped into its
binary encoding. Depending on the quality of the PAN link layer, the
compression process may strip out HTTP headers, according to these
priorities:
o Content type and cache-control headers SHOULD never be stripped
o Standard headers with u2 value encodings or short strings SHOULD
be maintained
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o Standard headers without u2 value encodings or with longer strings
MAY be stripped
o Non-standard headers SHOULD be stripped (assuming typical PAN
constraints)
The Chopan compression and stripping of headers is a gateway to
origin server matter. This does not free the gateway from faithfully
implementing the full HTTP specification and abiding by its
conventions. In the cases where HTTP headers or functionality is
reduced to meet Chopan constraints, the gateway should compensate so
that the client's perspective is communication with a fully compliant
HTTP origin server.
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7. Security
Ideally Internet protocols implement an end-to-end security model
between the two endpoint nodes. However it is difficult to implement
end-to-end session based security with unreliable packet protocols
and sleeping nodes. Rather Chopan, recommends that the security
strategy is divided between internal and external PAN nodes.
Internally all PAN nodes should be fully trusted using link layer
security such as the AES encryption specified by 802.15.4.
External to the PAN, the gateway should utilize full TCP/HTTP to
enable the well known security mechanisms associated with those
protocols. This includes TLS/HTTPS and the various HTTP
authentication mechanisms.
NOTE: A lot more to think about here...
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8. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", RFC 768,
August 1980.
[RFC0793] Postel, J., "Transmission Control Protocol", RFC 793,
September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
[RFC2616] Fielding, R., "Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2616, June 1999.
[RFC4944] Montenegro, G., "Transmission of IPv6 Packets over IEEE
802.15.4 Networks", RFC 4944, September 2007.
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Author's Address
Brian Frank
Tridium, Inc
Richmond, VA
US
Email: brian.tridium@gmail.com
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