Skip to main content

5.3. Tunnel Header Field Descriptions

5.3. Tunnel Header Field Descriptions​

Inner Header (IH): The inner header is the header on the datagram received from the originating host. The source and destination IP addresses are EIDs [RFC0791] [RFC2460].

Outer Header: (OH) The outer header is a new header prepended by an ITR. The address fields contain RLOCs obtained from the ingress router's EID-to-RLOC Cache. The IP protocol number is "UDP (17)" from [RFC0768]. The setting of the Don't Fragment (DF) bit 'Flags' field is according to rules listed in Sections 5.4.1 and 5.4.2.

UDP Header: The UDP header contains an ITR selected source port when encapsulating a packet. See Section 6.5 for details on the hash algorithm used to select a source port based on the 5-tuple of the inner header. The destination port MUST be set to the well-known IANA-assigned port value 4341.

UDP Checksum: The 'UDP Checksum' field SHOULD be transmitted as zero by an ITR for either IPv4 [RFC0768] or IPv6 encapsulation [UDP-TUNNELS] [UDP-ZERO]. When a packet with a zero UDP checksum is received by an ETR, the ETR MUST accept the packet for decapsulation. When an ITR transmits a non-zero value for the UDP checksum, it MUST send a correctly computed value in this field. When an ETR receives a packet with a non-zero UDP checksum, it MAY choose to verify the checksum value. If it chooses to perform such verification, and the verification fails, the packet MUST be silently dropped. If the ETR chooses not to perform the verification, or performs the verification successfully, the packet MUST be accepted for decapsulation. The handling of UDP

checksums for all tunneling protocols, including LISP, is under active discussion within the IETF. When that discussion concludes, any necessary changes will be made to align LISP with the outcome of the broader discussion.

UDP Length: The 'UDP Length' field is set for an IPv4-encapsulated packet to be the sum of the inner-header IPv4 Total Length plus the UDP and LISP header lengths. For an IPv6-encapsulated packet, the 'UDP Length' field is the sum of the inner-header IPv6 Payload Length, the size of the IPv6 header (40 octets), and the size of the UDP and LISP headers.

N: The N-bit is the nonce-present bit. When this bit is set to 1, the low-order 24 bits of the first 32 bits of the LISP header contain a Nonce. See Section 6.3.1 for details. Both N- and V-bits MUST NOT be set in the same packet. If they are, a decapsulating ETR MUST treat the 'Nonce/Map-Version' field as having a Nonce value present.

L: The L-bit is the 'Locator-Status-Bits' field enabled bit. When this bit is set to 1, the Locator-Status-Bits in the second 32 bits of the LISP header are in use.

x 1 x x 0 x x x

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |N|L|E|V|I|flags|            Nonce/Map-Version                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Locator-Status-Bits                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

E: The E-bit is the echo-nonce-request bit. This bit MUST be ignored and has no meaning when the N-bit is set to 0. When the N-bit is set to 1 and this bit is set to 1, an ITR is requesting that the nonce value in the 'Nonce' field be echoed back in LISP- encapsulated packets when the ITR is also an ETR. See Section 6.3.1 for details.

V: The V-bit is the Map-Version present bit. When this bit is set to 1, the N-bit MUST be 0. Refer to Section 6.6.3 for more details. This bit indicates that the LISP header is encoded in this case as:

0 x 0 1 x x x x

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |N|L|E|V|I|flags|  Source Map-Version   |   Dest Map-Version    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 Instance ID/Locator-Status-Bits               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

I: The I-bit is the Instance ID bit. See Section 5.5 for more details. When this bit is set to 1, the 'Locator-Status-Bits' field is reduced to 8 bits and the high-order 24 bits are used as an Instance ID. If the L-bit is set to 0, then the low-order 8 bits are transmitted as zero and ignored on receipt. The format of the LISP header would look like this:

x x x x 1 x x x

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |N|L|E|V|I|flags|            Nonce/Map-Version                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 Instance ID                   |     LSBs      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

flags: The 'flags' field is a 3-bit field reserved for future flag use. It MUST be set to 0 on transmit and MUST be ignored on receipt.

LISP Nonce: The LISP 'Nonce' field is a 24-bit value that is randomly generated by an ITR when the N-bit is set to 1. Nonce generation algorithms are an implementation matter but are required to generate different nonces when sending to different destinations. However, the same nonce can be used for a period of time to the same destination. The nonce is also used when the E-bit is set to request the nonce value to be echoed by the other side when packets are returned. When the E-bit is clear but the N-bit is set, a remote ITR is either echoing a previously requested echo-nonce or providing a random nonce. See Section 6.3.1 for more details.

LISP Locator-Status-Bits (LSBs): When the L-bit is also set, the 'Locator-Status-Bits' field in the LISP header is set by an ITR to indicate to an ETR the up/down status of the Locators in the source site. Each RLOC in a Map-Reply is assigned an ordinal value from 0 to n-1 (when there are n RLOCs in a mapping entry). The Locator-Status-Bits are numbered from 0 to n-1 from the least significant bit of the field. The field is 32 bits when the I-bit is set to 0 and is 8 bits when the I-bit is set to 1. When a Locator-Status-Bit is set to 1, the ITR is indicating to the ETR that the RLOC associated with the bit ordinal has up status. See Section 6.3 for details on how an ITR can determine the status of the ETRs at the same site. When a site has multiple EID-Prefixes that result in multiple mappings (where each could have a different Locator-Set), the Locator-Status-Bits setting in an encapsulated packet MUST reflect the mapping for the EID-Prefix that the inner-header source EID address matches. If the LSB for an anycast Locator is set to 1, then there is at least one RLOC with that address, and the ETR is considered 'up'.

When doing ITR/PITR encapsulation:

o The outer-header 'Time to Live' field (or 'Hop Limit' field, in the case of IPv6) SHOULD be copied from the inner-header 'Time to Live' field.

o The outer-header 'Type of Service' field (or the 'Traffic Class' field, in the case of IPv6) SHOULD be copied from the inner-header 'Type of Service' field (with one exception; see below).

When doing ETR/PETR decapsulation:

o The inner-header 'Time to Live' field (or 'Hop Limit' field, in the case of IPv6) SHOULD be copied from the outer-header 'Time to Live' field, when the Time to Live value of the outer header is less than the Time to Live value of the inner header. Failing to perform this check can cause the Time to Live of the inner header to increment across encapsulation/decapsulation cycles. This check is also performed when doing initial encapsulation, when a packet comes to an ITR or PITR destined for a LISP site.

o The inner-header 'Type of Service' field (or the 'Traffic Class' field, in the case of IPv6) SHOULD be copied from the outer-header 'Type of Service' field (with one exception; see below).

Note that if an ETR/PETR is also an ITR/PITR and chooses to re-encapsulate after decapsulating, the net effect of this is that the new outer header will carry the same Time to Live as the old outer header minus 1.

Copying the Time to Live (TTL) serves two purposes: first, it preserves the distance the host intended the packet to travel; second, and more importantly, it provides for suppression of looping packets in the event there is a loop of concatenated tunnels due to misconfiguration. See Section 9.3 for TTL exception handling for traceroute packets.

The Explicit Congestion Notification ('ECN') field occupies bits 6 and 7 of both the IPv4 'Type of Service' field and the IPv6 'Traffic Class' field [RFC3168]. The 'ECN' field requires special treatment in order to avoid discarding indications of congestion [RFC3168]. ITR encapsulation MUST copy the 2-bit 'ECN' field from the inner header to the outer header. Re-encapsulation MUST copy the 2-bit 'ECN' field from the stripped outer header to the new outer header. If the 'ECN' field contains a congestion indication codepoint (the value is '11', the Congestion Experienced (CE) codepoint), then ETR decapsulation MUST copy the 2-bit 'ECN' field from the stripped outer header to the surviving inner header that is used to forward the

packet beyond the ETR. These requirements preserve CE indications when a packet that uses ECN traverses a LISP tunnel and becomes marked with a CE indication due to congestion between the tunnel endpoints.

Last updated on