Learning HTTP/2

Frames
As mentioned before, HTTP/2 is a framed protocol. Framing is a method for wrapping
all the important stuff in a way that makes it easy for consumers of the protocol
to read, parse, and create. In contrast, HTTP/1 is not framed but is rather text delimited.
Look at the following simple example:
GET / HTTP/1.1
Host: www.example.com
Connection: keep-alive
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_4)...
Accept-Encoding: gzip, deflate, sdch
Accept-Language: en-US,en;q=0.8
Cookie: pfy_cbc_lb=p-browse-w; customerZipCode=99912|N; ltc=%20;...
Parsing something like this is not rocket science but it tends to be slow and error
prone. You need to keep reading bytes until you get to a delimiter, in this case,
while also accounting for all of the less spec compliant clients that just send . A
state machine looks something like this:
loop
while( ! CRLF )
read bytes
end while
if first line
parse line as the Request-Line
else if line is empty
break out of the loop # We are done
else if line starts with non-whitespace
parse the header line into a key/value pair
else if line starts with space
add the continuation header to the previous header
end if
end loop
# Now go on to ready the request/response based on whatever was
# in the Transfer-encoding header
Writing this code is very doable and has been done countless times. The problems
with parsing an h1 request/response are:
• You can only have one request/response on the wire at a time. You have to parse
until done.
• It is unclear how much memory the parsing will take. What buffer are you reading
a line into? What happens if that line it too long? Grow and reallocate?
28 | Chapter 3: The Protocol