From 3c3ed1c6541dda6590ee3150068d9056a6b5cea7 Mon Sep 17 00:00:00 2001 From: Nemo Date: Fri, 24 Jul 2020 22:21:25 +0530 Subject: [PATCH] HPACK talk at RFCs We Love --- hpack/index.html | 203 +++++++++++++++++++++++++++++++++++++++++++++ hpack/index.md | 208 +++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 411 insertions(+) create mode 100644 hpack/index.html create mode 100644 hpack/index.md diff --git a/hpack/index.html b/hpack/index.html new file mode 100644 index 0000000..80f5727 --- /dev/null +++ b/hpack/index.html @@ -0,0 +1,203 @@ +
+

HPACK (RFC 7541)

+

- Header Compression for HTTP/2

+
+
+

http/1.1

+

Request

+
GET /path HTTP/1.1
+Host: www.example.com
+Accept-Encoding: gzip, deflate,br
+
+
+
+

http/1.1

+

Response

+
HTTP/1.1 200 OK
+Accept-Ranges: bytes
+Content-Length: 438
+Content-Type: text/html; charset=UTF-8
+Content-Encoding: gzip
+
+
+
+

PROS

+
    +
  • Easy to support, works both ways
    • +
+
+
+

CONS

+
    +
  • Different algorithms have different implementations
    • +
  • Does not support header compression
    • +
  • Multiple security issues, via chosen-plaintext attacks.
    • +
+
+
+

Unrelated: There's also TLS compression (don't use it)

+
+
+

hpack

+
+
+

core ideas

+
    +
  1. most web requests will have fairly similar headers.
    2. +
  2. should be fast.
    3. +
  3. reduce complexity or implementation issues.
    4. +
  4. ignore encoding, focus on a correct decoding.
    5. +
  5. "streaming"
    6. +
+
+
+

important-to-note

+
    +
  1. Headers can be repeated
    2. +
  2. Headers are an Ordered List
    3. +
+
+
+

Static Table

+

Generated by frequency of header fields on popular websites.

+
+-------+-----------------------------+---------------+
+| Index | Header Name                 | Header Value  |
++-------+-----------------------------+---------------+
+| 1     | :authority                  |               |
+| 2     | :method                     | GET           |
+| 3     | :method                     | POST          |
+| 4     | :path                       | /             |
+| 5     | :path                       | /index.html   |
+| 6     | :scheme                     | http          |
+| 7     | :scheme                     | https         |
+| 8     | :status                     | 200           |
+| 9     | :status                     | 204           |
+| 10    | :status                     | 206           |
+| 11    | :status                     | 304           |
+
+
+
+

Index Address Space

+
   <----------  Index Address Space ---------->
+   <-- Static  Table -->  <-- Dynamic Table -->
+   +---+-----------+---+  +---+-----------+---+
+   | 1 |    ...    | s |  |s+1|    ...    |s+k|
+   +---+-----------+---+  +---+-----------+---+
+                          ^                   |
+                          |                   V
+                   Insertion Point      Dropping Point
+
+

Representation

+

A header field(=name+value) can be represented as:

+

an index

+

is a reference to either:

+
    +
  • the static table
    • +
  • the dynamic table
    • +
+
+
+

or a literal (1)

+

the header field name can be represented

+
    +
  • literally
    • +
  • a reference to either tables
    • +
+
+
+

or a literal (2)

+

the header value is represented literally and

+
    +
  1. ADD it to the dynamic table
    2. +
  2. DON'T ADD it to the dynamic table
    3. +
  3. NEVER ADD it to the dynamic table
    4. +
+
+
+

a literal

+

can be presented in two forms:

+
    +
  1. directly
    2. +
  2. using a Huffman code
    3. +
+
+
+

decisions while parsing

+
    +
  • do a table lookup +
      +
    • static table
      • +
    • dynamic table
      • +
    +
    • +
  • read it literally +
      +
    • directly
      • +
    • using the huffman code
      • +
    +
    • +
+

Almost is encoded as a integer or as a string

+
+
+

literal representation

+
   0   1   2   3   4   5   6   7
+ +---+---+---+---+---+---+---+---+
+ | H |    String Length (7+)     |
+ +---+---------------------------+
+ |  String Data (Length octets)  |
+ +-------------------------------+
+
+

H=1 -> Huffman Encoded
+H=0 -> Encoded as a "string"

+

index representation

+

This is simple table lookup

+
   0   1   2   3   4   5   6   7
+ +---+---+---+---+---+---+---+---+
+ | 1 |        Index (7+)         |
+ +---+---------------------------+
+
+
+
+

Static Huffman Code

+
                                                code
+                  code as bits                 as hex   len
+sym              aligned to MSB                aligned   in
+                                               to LSB   bits
+(  0)  |11111111|11000                             1ff8  [13]
+(  1)  |11111111|11111111|1011000                7fffd8  [23]
+(  2)  |11111111|11111111|11111110|0010         fffffe2  [28]
+(  3)  |11111111|11111111|11111110|0011         fffffe3  [28]
+
+
+
+

dynamic table updates

+
    +
  • You can add entries to the table
    • +
  • You can set the table size dynamically
    • +
+
+

This mechanism can be used to completely clear entries from the
+dynamic table by setting a maximum size of 0, which can subsequently
+be restored.

+
+
+
\ No newline at end of file diff --git a/hpack/index.md b/hpack/index.md new file mode 100644 index 0000000..a995b30 --- /dev/null +++ b/hpack/index.md @@ -0,0 +1,208 @@ +# HPACK (RFC 7541) +\- Header Compression for HTTP/2 + +--- + +# http/1.1 + +## Request + +``` +GET /path HTTP/1.1 +Host: www.example.com +Accept-Encoding: gzip, deflate,br +``` + +--- + +# http/1.1 + +## Response + +``` +HTTP/1.1 200 OK +Accept-Ranges: bytes +Content-Length: 438 +Content-Type: text/html; charset=UTF-8 +Content-Encoding: gzip +``` + +--- + +# PROS + +- Easy to support, works both ways + +--- + +# CONS +- Different algorithms have different implementations +- Does not support header compression +- Multiple security issues, via chosen-plaintext attacks. + +--- + +Unrelated: There's also TLS compression (don't use it) + +--- + +# hpack + +--- + +# core ideas + +1. most web requests will have fairly similar headers. +2. should be fast. +3. reduce complexity or implementation issues. +4. ignore encoding, focus on a correct decoding. +5. "streaming" + +--- + +# important-to-note + +1. Headers can be repeated +2. Headers are an Ordered List + +--- + +# Static Table + +Generated by frequency of header fields on popular websites. + +``` ++-------+-----------------------------+---------------+ +| Index | Header Name | Header Value | ++-------+-----------------------------+---------------+ +| 1 | :authority | | +| 2 | :method | GET | +| 3 | :method | POST | +| 4 | :path | / | +| 5 | :path | /index.html | +| 6 | :scheme | http | +| 7 | :scheme | https | +| 8 | :status | 200 | +| 9 | :status | 204 | +| 10 | :status | 206 | +| 11 | :status | 304 | +``` + +--- + +# Index Address Space + +``` + <---------- Index Address Space ----------> + <-- Static Table --> <-- Dynamic Table --> + +---+-----------+---+ +---+-----------+---+ + | 1 | ... | s | |s+1| ... |s+k| + +---+-----------+---+ +---+-----------+---+ + ^ | + | V + Insertion Point Dropping Point +``` + +# Representation + +A header field(=`name+value`) can be represented as: + +## an index +is a reference to either: + +- the static table +- the dynamic table + +--- + +## or a literal (1) + +the header field name can be represented + +- literally +- a reference to either tables + +--- + +# or a literal (2) + +the header value is represented literally and + +1. **ADD** it to the dynamic table +2. **DON'T ADD** it to the dynamic table +3. **NEVER ADD** it to the dynamic table + +--- + +# a literal + +can be presented in two forms: + +1. directly +2. using a Huffman code + +--- + +# decisions while parsing + +- do a table lookup + - static table + - dynamic table +- read it literally + - directly + - using the huffman code + +Almost is encoded as a integer or as a string + +--- + +# literal representation + + +``` + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | H | String Length (7+) | + +---+---------------------------+ + | String Data (Length octets) | + +-------------------------------+ +``` +H=1 -> Huffman Encoded +H=0 -> Encoded as a "string" + +# index representation + +This is simple table lookup + +``` + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 | Index (7+) | + +---+---------------------------+ +``` + +--- + +# Static Huffman Code + +``` + code + code as bits as hex len +sym aligned to MSB aligned in + to LSB bits +( 0) |11111111|11000 1ff8 [13] +( 1) |11111111|11111111|1011000 7fffd8 [23] +( 2) |11111111|11111111|11111110|0010 fffffe2 [28] +( 3) |11111111|11111111|11111110|0011 fffffe3 [28] +``` + +--- + +# dynamic table updates + +- You can add entries to the table +- You can set the table size dynamically + +>This mechanism can be used to completely clear entries from the + dynamic table by setting a maximum size of 0, which can subsequently + be restored. \ No newline at end of file